This study uses molecular dynamics simulations with the BKS potential to model the structures and calculate the enthalpy variations of 13 silica polymorphs over a range of densities. The key findings are:
1) The BKS potential predicts the lattice parameters of most silica phases within 2-3% accuracy and the relative stabilities of different polymorphs in agreement with experiment.
2) Enthalpies of high-density silica polymorphs that lack experimental data, such as CaCl2-type and α-PbO2-type, are predicted.
3) The calculated enthalpies exhibit two distinct regimes - almost independent of volume for low/medium-density phases, and a steep dependence for
Crimson Publishers-In vitro Bioactivity Assessment of Novel Composites Based ...CrimsonPublishersRDMS
In vitro Bioactivity Assessment of Novel Composites Based on Calcium Aluminate Cement by Cortés-Hernández DA in Research & Development in Material Science
This document summarizes factors that influence the durability of concrete, specifically related to chloride attack. It discusses the mechanisms of fluid and ion flow through concrete, including capillary suction, diffusion, migration, and permeation. Key parameters for studying chloride penetration are identified as surface chloride concentration, apparent diffusion coefficient, chloride threshold, and chloride penetration profile. Standard tests for evaluating chloride penetration and their testing periods are also outlined. The corrosion process of steel reinforcement in concrete is described as an electrochemical reaction. A corrosion model defines the stages of initiation, propagation, and acceleration of corrosion. The cracking-corrosion-cracking feedback model represents how factors influence concrete durability over time.
This document discusses a numerical model for analyzing nucleation modeling in ductile cast iron. The model simulates heat transfer during solidification and microstructure evolution. Experiments involved casting ductile iron samples with different thicknesses and inoculants. Microstructural analysis and thermal analysis were performed. The numerical model showed good agreement with experimental results for cooling curves, cooling rates, nodule counts, and solid fraction. The model was then used to study inoculation parameters and their effects.
The document summarizes molecular dynamics simulations of hydrogen-dislocation interaction in iron nanocrystals. Key findings include:
1) Simulations of iron nano-pillars with and without hydrogen show hydrogen reduces the rate at which dislocations escape to free surfaces during deformation.
2) Models with hydrogen are able to retain a higher initial dislocation density compared to uncharged models, suggesting hydrogen acts as an obstacle to dislocation movement.
3) Statistical analysis confirms hydrogen has a significant effect on dislocation retention in the first stage of deformation.
This document contains a declaration for a thesis submitted by Ng Ping Chew in fulfillment of the requirements for a Master's degree in Civil Engineering from Universiti Teknologi Malaysia. Ng declares that the thesis titled "Comparison of Creep and Shrinkage Using Different Code of Practice" does not contain any confidential information and agrees for it to be published as open access. The declaration is certified by Ng's supervisor, Assoc. Prof. Ir. Dr. Wahid Omar.
This document studies the suitability of using incinerated sewage sludge ash (ISSA) to produce geopolymer concrete as a partial replacement for fly ash. Key findings include:
1) XRF testing showed that ISSA has high concentrations of SiO2 and Al2O3 similar to fly ash, making it a potential candidate for geopolymer concrete.
2) Concrete made with only ISSA did not develop significant strength, but strength decreased gradually as ISSA percentage increased in mixes partially replacing fly ash up to 20%.
3) Concrete with up to 15% ISSA replacement was considered average quality and absorption, allowing potential use for less strength-critical applications.
4)
Influence of phase transformation on the work hardening characteristics of Pb...iosrjce
IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) is a double blind peer reviewed International Journal that provides rapid publication (within a month) of articles in all areas of mechanical and civil engineering and its applications. The journal welcomes publications of high quality papers on theoretical developments and practical applications in mechanical and civil engineering. Original research papers, state-of-the-art reviews, and high quality technical notes are invited for publications.
Crimson Publishers-In vitro Bioactivity Assessment of Novel Composites Based ...CrimsonPublishersRDMS
In vitro Bioactivity Assessment of Novel Composites Based on Calcium Aluminate Cement by Cortés-Hernández DA in Research & Development in Material Science
This document summarizes factors that influence the durability of concrete, specifically related to chloride attack. It discusses the mechanisms of fluid and ion flow through concrete, including capillary suction, diffusion, migration, and permeation. Key parameters for studying chloride penetration are identified as surface chloride concentration, apparent diffusion coefficient, chloride threshold, and chloride penetration profile. Standard tests for evaluating chloride penetration and their testing periods are also outlined. The corrosion process of steel reinforcement in concrete is described as an electrochemical reaction. A corrosion model defines the stages of initiation, propagation, and acceleration of corrosion. The cracking-corrosion-cracking feedback model represents how factors influence concrete durability over time.
This document discusses a numerical model for analyzing nucleation modeling in ductile cast iron. The model simulates heat transfer during solidification and microstructure evolution. Experiments involved casting ductile iron samples with different thicknesses and inoculants. Microstructural analysis and thermal analysis were performed. The numerical model showed good agreement with experimental results for cooling curves, cooling rates, nodule counts, and solid fraction. The model was then used to study inoculation parameters and their effects.
The document summarizes molecular dynamics simulations of hydrogen-dislocation interaction in iron nanocrystals. Key findings include:
1) Simulations of iron nano-pillars with and without hydrogen show hydrogen reduces the rate at which dislocations escape to free surfaces during deformation.
2) Models with hydrogen are able to retain a higher initial dislocation density compared to uncharged models, suggesting hydrogen acts as an obstacle to dislocation movement.
3) Statistical analysis confirms hydrogen has a significant effect on dislocation retention in the first stage of deformation.
This document contains a declaration for a thesis submitted by Ng Ping Chew in fulfillment of the requirements for a Master's degree in Civil Engineering from Universiti Teknologi Malaysia. Ng declares that the thesis titled "Comparison of Creep and Shrinkage Using Different Code of Practice" does not contain any confidential information and agrees for it to be published as open access. The declaration is certified by Ng's supervisor, Assoc. Prof. Ir. Dr. Wahid Omar.
This document studies the suitability of using incinerated sewage sludge ash (ISSA) to produce geopolymer concrete as a partial replacement for fly ash. Key findings include:
1) XRF testing showed that ISSA has high concentrations of SiO2 and Al2O3 similar to fly ash, making it a potential candidate for geopolymer concrete.
2) Concrete made with only ISSA did not develop significant strength, but strength decreased gradually as ISSA percentage increased in mixes partially replacing fly ash up to 20%.
3) Concrete with up to 15% ISSA replacement was considered average quality and absorption, allowing potential use for less strength-critical applications.
4)
Influence of phase transformation on the work hardening characteristics of Pb...iosrjce
IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) is a double blind peer reviewed International Journal that provides rapid publication (within a month) of articles in all areas of mechanical and civil engineering and its applications. The journal welcomes publications of high quality papers on theoretical developments and practical applications in mechanical and civil engineering. Original research papers, state-of-the-art reviews, and high quality technical notes are invited for publications.
Experimental Studies on the Prediction of Corrosion Levels in Reinforced TMT ...IRJET Journal
The document summarizes experimental studies on predicting corrosion levels in reinforced TMT bars in normal vibrated concrete (NVC) and self-consolidating concrete (SCC) exposed to a marine environment. Specimens of NVC and SCC with TMT bars embedded were immersed in de-ionized water with varying concentrations of NaCl and MgSO4 for curing periods of 28, 45, 60, and 90 days. Corrosion levels in the TMT bars after curing were then predicted by measuring potential differences between points on the specimens using a saturated calomel electrode and open circuit potential method. The studies aimed to investigate corrosion levels in TMT bars in NVC and SCC exposed to the marine environment
Effect of corroded reinforcement on structuresRini Dharva
This document discusses the effect of corroded reinforcement on the aging of reinforced concrete structures. It describes the basic process of corrosion through oxidation and reduction reactions. Corrosion occurs due to carbonation, which decreases the pH of concrete, or chloride ingress from deicing salts or seawater. The service life of a structure can be divided into an initiation period and a propagation period. As corrosion increases, the bond strength between steel reinforcement and concrete decreases. The document reviews two papers, one that proposes a model and condition rating system to estimate residual service life, and another that examines deterioration in Mumbai due to chloride content, sulfate content, and pH levels.
Creep is defined as the plastic deformation under sustain load. Creep strain depends
primarily on the duration of sustained loading. It has been widely acknowledged that creep of
concrete is greatly influenced by the surrounding ambient. Creep induces the deflection of the
structural member with time. Hence the study on creep of concrete is necessary to prevent failure. A
concrete cylinder is casted for two different grades and the load of 40% of its compressive strength
is applied constantly. Creep is measured with time. A time - dependent creep is provided in a graph
showing its variation.
This document presents a simplified method for estimating the entire load profile of a fully grouted anchor bolt based on strain measurement data from two points near the loaded end. Pullout tests were conducted on anchor bolts grouted in concrete with two different grout types. Strain gauges attached to the bolts recorded data during loading. Interpolation of data from two gauges was able to estimate the full load profile along the bolt, matching the profile obtained from multiple gauges. This method provides a simplified way to determine an anchor bolt's load profile without needing data from along its entire length.
Influence of metakaolin_and_flyash_on_fresh_and_hardened__properties_of_self_...Bpdplanning Madhucon
This document summarizes a study on the effects of metakaolin and fly ash on the fresh and hardened properties of self-compacting concrete. Four concrete mixtures were designed with different binary and ternary combinations of ordinary Portland cement, metakaolin, and fly ash. The fresh properties, including slump flow, V-funnel flow time, and L-box ratio, and the compressive and split tensile strengths at various ages were tested. The results showed that the compressive strength was highest for the binary and ternary blends, while the split tensile strength was highest for the controlled concrete without additions. Incorporating metakaolin and fly ash as binary and ternary blends in self
International Refereed Journal of Engineering and Science (IRJES)irjes
International Refereed Journal of Engineering and Science (IRJES) is a leading international journal for publication of new ideas, the state of the art research results and fundamental advances in all aspects of Engineering and Science. IRJES is a open access, peer reviewed international journal with a primary objective to provide the academic community and industry for the submission of half of original research and applications
This document summarizes previous geochemical modeling studies of Carlin-type gold deposits and presents new reaction path modeling conducted by the author. Key findings of the new modeling include: 1) Wall-rock reaction with host limestone, involving iron sulphidation, best explains gold deposition; 2) Neither cooling nor fluid mixing appear to be significant depositional processes on their own; 3) Most rocks containing some iron could potentially host ore-grade gold if sufficient porosity existed during mineralization. The author proposes testing a model where a transient upwards pulse of hot hydrothermal fluid interacts with laterally flowing cooler meteoric fluid to explain deposit characteristics.
The Rheological and Empirical Characteristics of Steel Fibre Reinforced Self-...Roy Belton
Investigating whether a correlation exists between rheology and empiricism for various steel fibre self-compacting concrete mixtures with additional cementitious additions (such as PFA and GGBS).
1) The document examines the high temperature oxidation behavior of a nickel-based superalloy produced by mechanical alloying. Samples were mechanically alloyed, sintered, hot-deformed, and underwent secondary recrystallization.
2) Isothermal oxidation tests were conducted on the samples at 1000°C for up to 100 hours. SEM and XRD analysis found that a protective chromium oxide (Cr2O3) film formed on the surfaces. Weight change results indicated the samples gained a small amount of weight over time, exhibiting sub-parabolic oxidation behavior.
3) XRD analysis identified the primary oxide as Cr2O3. The intensity of Cr2O3 peaks in XRD
High Entropy Alloys are a new class of alloys discovered to perform at potentially useful applications. Eg : CoCrFeMnNi is useful for Cryogenic applications and MoNbTaWV is useful for Refractory applications.
This document analyzes the hot corrosion behavior of laser melted GP1 stainless steel powder when exposed to a molten chloride and sulphide environment. The study found that the corrosion followed a parabolic rate law, with higher corrosion rates initially. Surface examination revealed pitting and scaling. Phase analysis identified oxides like Fe2O3 and Cr2O3 as well as other reaction products like NiFe2O4 and NiCr2O4, indicating the material underwent oxidation, chloridation and sulphidation during exposure. The results provide insight into corrosion mechanisms during additive manufacturing processes involving laser powder melting.
K.E. Al-Khidir, M.S. Benzagouta
Department of Petroleum Engineering and Natural Gas, College of Engineering,
King Saud University, Saudi Arabia
К.Е. Аль-Хидир, М.С. Бензагута
Инженерный колледж, Университет короля Сауда
Abstract. With reference to the reservoir characterization regarding the Khuff Formation – Khurtam Member (KSA), better reservoir quality prediction can be assigned to
the characterization of its physical properties. These properties determination are depending on facies description. These properties can be erratic according to the diagenetic change and control on pore geometrical attributes and reservoir characterization. In this
investigation, permeability and porosity have been measured in the laboratory. Rock Quality Index (RQI), flow zone index (FZI) as well as porosity groups have been
calculated. The use of different determined physical and petrophysical parameters and their plots have defined two Flow Zone Indicators (FZI) characterized by a restricted distribution of porosity permeability and rock quality index. Different approval statements were made by correlation process. These confirmation and data support made of the considered reservoir as relative tight reservoir and tight reservoir. The use of the R35 was organized to
make an approval to different statements with reference to the considered reservoir which can be classified as relative tight reservoir and tight reservoir. The R35 was carried out to approve not only that the reservoir is relative tight and tight but it can be classified in the range within the range of nano to microporous system.
Recent Developments on Silicon Carbide Thin Films for Piezoresistive Sensors ...Mariana Amorim Fraga
The document discusses InTechOpen, a large open access book publisher. Some key details:
- InTechOpen has published over 3,150 open access books from over 104,000 international authors and editors.
- Their books have been downloaded over 109 million times and delivered to 151 countries.
- 12.2% of their authors and editors are from the top 500 universities.
- They are the world's largest science, technology, and medicine open access book publisher.
High Entropy Alloy was discovered in 1996. Being a completely new topic, it is unknown to us in all aspects. It's excellent combination of all mechanical properties is representing a new frontier in Materials Engineering field of research.
The effect of oxygen flow rate on metal–insulator transition (MIT) characteri...Pawan Kumar
Vanadium dioxide (VO2), because of its unique metal–insulator phase transion around room temperature, has enormous potential for applications in thermochromic, electrochromic, microbolometery, and non-volatile switches. However, obtaining phase-pure VO2 has been challenging due to its narrow window of thermodynamic stability. Pulsed laser deposition (PLD) is considered to be one of the most promising technique to deposit phase-pure VO2 thin films. While optimizing PLD processing parameters, (such as oxygen pressure (PO2), substrate temperature (Ts), laser energy (EL), target-substrate distance (dT-S), and laser repetition rate (RR)) is crucial, and their effects have been reported, the effect of gas flow rate (QO2) has been largely neglected. Since the QO2 and PO2 of the PLD system are intertwined at the outset, most reports are focused only on either QO2 or PO2. We report on the effect of gas flow rate under constant PO2, on the quality of the VO2 thin films. Controlling flow rate affected the resistivity contrast between the metallic and insulator phases, the temperature ranges of the transition, and the width of hysteresis. We anticipate that this study will help set the standard for obtaining high quality VO2 thin films.
This document summarizes a study on the sintering mechanism of silica gel nanoparticles during initial heating stages. Thermogravimetric analysis showed weight loss up to 600°C attributed to removal of absorbed water and hydroxyl groups. Diffuse reflectance infrared spectroscopy revealed bonding changes - bridged hydroxyl groups were replaced by free hydroxyl and water molecules with heating. Heating also increased asymmetric stretching splitting, indicating greater long-range Coulombic forces from increased siloxane bonding between particles. The study concludes initial sintering is driven by dehydration and surface hydroxyl condensation forming siloxane bonds between particles.
Corrosion studies of colmonoy - 6 in nitric acid during gadolinium removal st...RAMASUBBU VELAYUTHAM
This document summarizes a study on the corrosion of Colmonoy-6, a nickel-based alloy used in valves and bearings of nuclear power plant moderator systems, when exposed to nitric acid conditions present during gadolinium removal from the moderator. Specimens of undiluted Colmonoy-6 deposits and Colmonoy-6 coatings on stainless steel with varying levels of dilution were exposed to a nitric acid solution at 65°C, and corrosion rates were estimated through weight loss and electrochemical measurements. Microstructural analysis found the undiluted deposit and multiple layer coatings contained more carbide and boride precipitates than single layer coatings, indicating lower dilution effects.
The document describes the development of a laboratory scale conical spouted bed reactor for biomass gasification. Initial experiments involve cold flow studies to establish stable spouting conditions. Future work includes hot flow studies and thermodynamic modeling to evaluate favorable operating conditions for hydrogen-rich syngas production from fuels like glycerol and propane. Existing correlations for minimum spouting velocity were found to be inadequate, so a new correlation was developed that showed excellent agreement with experimental data. Preliminary equilibrium analysis of reforming systems indicates steam reforming produces less hydrogen than partial oxidation or auto-thermal reforming. Further experiments are needed to validate the modeling results.
1) Alpha Academy provides career advisory services to help clients succeed in obtaining jobs at top foreign consulting firms, investment banks, and trading companies through individualized coaching, video lectures, and an online community.
2) Their methods include helping clients thoroughly define career goals and target companies, create strong resumes and interview preparation, and obtain acceptance offers from places like McKinsey, BCG, Goldman Sachs, and trading firms.
3) Alpha Academy claims over 150 clients are accepted annually to trading companies, and they promise absolute acceptance rates for clients who fully commit to their training program of individual coaching, video lectures, and online community support.
This document provides an overview of the LTAP/TTAP Technology Partnership Programs meeting on November 3, 2014. It discusses the national program priorities of professional development, use of performance data, collaborative relationships, and innovation. It highlights national program efforts including relationships with FHWA offices and a joint professional development program. The document also provides data on national and Great Lakes regional program performance from 2010-2013. It discusses the 2014 FHWA Strategic Planning Committee and what is happening in specific Great Lakes region states.
Experimental Studies on the Prediction of Corrosion Levels in Reinforced TMT ...IRJET Journal
The document summarizes experimental studies on predicting corrosion levels in reinforced TMT bars in normal vibrated concrete (NVC) and self-consolidating concrete (SCC) exposed to a marine environment. Specimens of NVC and SCC with TMT bars embedded were immersed in de-ionized water with varying concentrations of NaCl and MgSO4 for curing periods of 28, 45, 60, and 90 days. Corrosion levels in the TMT bars after curing were then predicted by measuring potential differences between points on the specimens using a saturated calomel electrode and open circuit potential method. The studies aimed to investigate corrosion levels in TMT bars in NVC and SCC exposed to the marine environment
Effect of corroded reinforcement on structuresRini Dharva
This document discusses the effect of corroded reinforcement on the aging of reinforced concrete structures. It describes the basic process of corrosion through oxidation and reduction reactions. Corrosion occurs due to carbonation, which decreases the pH of concrete, or chloride ingress from deicing salts or seawater. The service life of a structure can be divided into an initiation period and a propagation period. As corrosion increases, the bond strength between steel reinforcement and concrete decreases. The document reviews two papers, one that proposes a model and condition rating system to estimate residual service life, and another that examines deterioration in Mumbai due to chloride content, sulfate content, and pH levels.
Creep is defined as the plastic deformation under sustain load. Creep strain depends
primarily on the duration of sustained loading. It has been widely acknowledged that creep of
concrete is greatly influenced by the surrounding ambient. Creep induces the deflection of the
structural member with time. Hence the study on creep of concrete is necessary to prevent failure. A
concrete cylinder is casted for two different grades and the load of 40% of its compressive strength
is applied constantly. Creep is measured with time. A time - dependent creep is provided in a graph
showing its variation.
This document presents a simplified method for estimating the entire load profile of a fully grouted anchor bolt based on strain measurement data from two points near the loaded end. Pullout tests were conducted on anchor bolts grouted in concrete with two different grout types. Strain gauges attached to the bolts recorded data during loading. Interpolation of data from two gauges was able to estimate the full load profile along the bolt, matching the profile obtained from multiple gauges. This method provides a simplified way to determine an anchor bolt's load profile without needing data from along its entire length.
Influence of metakaolin_and_flyash_on_fresh_and_hardened__properties_of_self_...Bpdplanning Madhucon
This document summarizes a study on the effects of metakaolin and fly ash on the fresh and hardened properties of self-compacting concrete. Four concrete mixtures were designed with different binary and ternary combinations of ordinary Portland cement, metakaolin, and fly ash. The fresh properties, including slump flow, V-funnel flow time, and L-box ratio, and the compressive and split tensile strengths at various ages were tested. The results showed that the compressive strength was highest for the binary and ternary blends, while the split tensile strength was highest for the controlled concrete without additions. Incorporating metakaolin and fly ash as binary and ternary blends in self
International Refereed Journal of Engineering and Science (IRJES)irjes
International Refereed Journal of Engineering and Science (IRJES) is a leading international journal for publication of new ideas, the state of the art research results and fundamental advances in all aspects of Engineering and Science. IRJES is a open access, peer reviewed international journal with a primary objective to provide the academic community and industry for the submission of half of original research and applications
This document summarizes previous geochemical modeling studies of Carlin-type gold deposits and presents new reaction path modeling conducted by the author. Key findings of the new modeling include: 1) Wall-rock reaction with host limestone, involving iron sulphidation, best explains gold deposition; 2) Neither cooling nor fluid mixing appear to be significant depositional processes on their own; 3) Most rocks containing some iron could potentially host ore-grade gold if sufficient porosity existed during mineralization. The author proposes testing a model where a transient upwards pulse of hot hydrothermal fluid interacts with laterally flowing cooler meteoric fluid to explain deposit characteristics.
The Rheological and Empirical Characteristics of Steel Fibre Reinforced Self-...Roy Belton
Investigating whether a correlation exists between rheology and empiricism for various steel fibre self-compacting concrete mixtures with additional cementitious additions (such as PFA and GGBS).
1) The document examines the high temperature oxidation behavior of a nickel-based superalloy produced by mechanical alloying. Samples were mechanically alloyed, sintered, hot-deformed, and underwent secondary recrystallization.
2) Isothermal oxidation tests were conducted on the samples at 1000°C for up to 100 hours. SEM and XRD analysis found that a protective chromium oxide (Cr2O3) film formed on the surfaces. Weight change results indicated the samples gained a small amount of weight over time, exhibiting sub-parabolic oxidation behavior.
3) XRD analysis identified the primary oxide as Cr2O3. The intensity of Cr2O3 peaks in XRD
High Entropy Alloys are a new class of alloys discovered to perform at potentially useful applications. Eg : CoCrFeMnNi is useful for Cryogenic applications and MoNbTaWV is useful for Refractory applications.
This document analyzes the hot corrosion behavior of laser melted GP1 stainless steel powder when exposed to a molten chloride and sulphide environment. The study found that the corrosion followed a parabolic rate law, with higher corrosion rates initially. Surface examination revealed pitting and scaling. Phase analysis identified oxides like Fe2O3 and Cr2O3 as well as other reaction products like NiFe2O4 and NiCr2O4, indicating the material underwent oxidation, chloridation and sulphidation during exposure. The results provide insight into corrosion mechanisms during additive manufacturing processes involving laser powder melting.
K.E. Al-Khidir, M.S. Benzagouta
Department of Petroleum Engineering and Natural Gas, College of Engineering,
King Saud University, Saudi Arabia
К.Е. Аль-Хидир, М.С. Бензагута
Инженерный колледж, Университет короля Сауда
Abstract. With reference to the reservoir characterization regarding the Khuff Formation – Khurtam Member (KSA), better reservoir quality prediction can be assigned to
the characterization of its physical properties. These properties determination are depending on facies description. These properties can be erratic according to the diagenetic change and control on pore geometrical attributes and reservoir characterization. In this
investigation, permeability and porosity have been measured in the laboratory. Rock Quality Index (RQI), flow zone index (FZI) as well as porosity groups have been
calculated. The use of different determined physical and petrophysical parameters and their plots have defined two Flow Zone Indicators (FZI) characterized by a restricted distribution of porosity permeability and rock quality index. Different approval statements were made by correlation process. These confirmation and data support made of the considered reservoir as relative tight reservoir and tight reservoir. The use of the R35 was organized to
make an approval to different statements with reference to the considered reservoir which can be classified as relative tight reservoir and tight reservoir. The R35 was carried out to approve not only that the reservoir is relative tight and tight but it can be classified in the range within the range of nano to microporous system.
Recent Developments on Silicon Carbide Thin Films for Piezoresistive Sensors ...Mariana Amorim Fraga
The document discusses InTechOpen, a large open access book publisher. Some key details:
- InTechOpen has published over 3,150 open access books from over 104,000 international authors and editors.
- Their books have been downloaded over 109 million times and delivered to 151 countries.
- 12.2% of their authors and editors are from the top 500 universities.
- They are the world's largest science, technology, and medicine open access book publisher.
High Entropy Alloy was discovered in 1996. Being a completely new topic, it is unknown to us in all aspects. It's excellent combination of all mechanical properties is representing a new frontier in Materials Engineering field of research.
The effect of oxygen flow rate on metal–insulator transition (MIT) characteri...Pawan Kumar
Vanadium dioxide (VO2), because of its unique metal–insulator phase transion around room temperature, has enormous potential for applications in thermochromic, electrochromic, microbolometery, and non-volatile switches. However, obtaining phase-pure VO2 has been challenging due to its narrow window of thermodynamic stability. Pulsed laser deposition (PLD) is considered to be one of the most promising technique to deposit phase-pure VO2 thin films. While optimizing PLD processing parameters, (such as oxygen pressure (PO2), substrate temperature (Ts), laser energy (EL), target-substrate distance (dT-S), and laser repetition rate (RR)) is crucial, and their effects have been reported, the effect of gas flow rate (QO2) has been largely neglected. Since the QO2 and PO2 of the PLD system are intertwined at the outset, most reports are focused only on either QO2 or PO2. We report on the effect of gas flow rate under constant PO2, on the quality of the VO2 thin films. Controlling flow rate affected the resistivity contrast between the metallic and insulator phases, the temperature ranges of the transition, and the width of hysteresis. We anticipate that this study will help set the standard for obtaining high quality VO2 thin films.
This document summarizes a study on the sintering mechanism of silica gel nanoparticles during initial heating stages. Thermogravimetric analysis showed weight loss up to 600°C attributed to removal of absorbed water and hydroxyl groups. Diffuse reflectance infrared spectroscopy revealed bonding changes - bridged hydroxyl groups were replaced by free hydroxyl and water molecules with heating. Heating also increased asymmetric stretching splitting, indicating greater long-range Coulombic forces from increased siloxane bonding between particles. The study concludes initial sintering is driven by dehydration and surface hydroxyl condensation forming siloxane bonds between particles.
Corrosion studies of colmonoy - 6 in nitric acid during gadolinium removal st...RAMASUBBU VELAYUTHAM
This document summarizes a study on the corrosion of Colmonoy-6, a nickel-based alloy used in valves and bearings of nuclear power plant moderator systems, when exposed to nitric acid conditions present during gadolinium removal from the moderator. Specimens of undiluted Colmonoy-6 deposits and Colmonoy-6 coatings on stainless steel with varying levels of dilution were exposed to a nitric acid solution at 65°C, and corrosion rates were estimated through weight loss and electrochemical measurements. Microstructural analysis found the undiluted deposit and multiple layer coatings contained more carbide and boride precipitates than single layer coatings, indicating lower dilution effects.
The document describes the development of a laboratory scale conical spouted bed reactor for biomass gasification. Initial experiments involve cold flow studies to establish stable spouting conditions. Future work includes hot flow studies and thermodynamic modeling to evaluate favorable operating conditions for hydrogen-rich syngas production from fuels like glycerol and propane. Existing correlations for minimum spouting velocity were found to be inadequate, so a new correlation was developed that showed excellent agreement with experimental data. Preliminary equilibrium analysis of reforming systems indicates steam reforming produces less hydrogen than partial oxidation or auto-thermal reforming. Further experiments are needed to validate the modeling results.
1) Alpha Academy provides career advisory services to help clients succeed in obtaining jobs at top foreign consulting firms, investment banks, and trading companies through individualized coaching, video lectures, and an online community.
2) Their methods include helping clients thoroughly define career goals and target companies, create strong resumes and interview preparation, and obtain acceptance offers from places like McKinsey, BCG, Goldman Sachs, and trading firms.
3) Alpha Academy claims over 150 clients are accepted annually to trading companies, and they promise absolute acceptance rates for clients who fully commit to their training program of individual coaching, video lectures, and online community support.
This document provides an overview of the LTAP/TTAP Technology Partnership Programs meeting on November 3, 2014. It discusses the national program priorities of professional development, use of performance data, collaborative relationships, and innovation. It highlights national program efforts including relationships with FHWA offices and a joint professional development program. The document also provides data on national and Great Lakes regional program performance from 2010-2013. It discusses the 2014 FHWA Strategic Planning Committee and what is happening in specific Great Lakes region states.
Qingdao Kingworld Flow Control Co.,Ltd is a Chinese company established in 2009 that produces motorcycle, bicycle, ATV, UTV, and go kart tires and tubes. The company has exported products to over 30 countries across Latin America, Africa, Europe, the Middle East, and Asia. Qingdao Kingworld prides itself on its super professional team, strong quality control, extensive sales network, and client feedback. The company is committed to providing reliable rubber tires and pursuing client and quality first policies.
El Parque Metropolitano Albarregas es un área natural protegida ubicada en Mérida, Venezuela. El parque contiene ecosistemas tropicales y de montaña que son importantes ecológicamente, pero se enfrentan a problemas como invasiones, contaminación y falta de permisos adecuados.
The document discusses the benefits of exercise for both physical and mental health. It notes that regular exercise can reduce the risk of diseases like heart disease and diabetes, improve mood, and reduce feelings of stress and anxiety. The document recommends that adults get at least 150 minutes of moderate exercise or 75 minutes of vigorous exercise per week to gain these benefits.
The document provides information about Bin Qasim Power Plant 2 (BQPS II) in Pakistan. It details the plant's capacity of 560 MW generated from 3 gas turbines and 1 steam turbine. The plant site is administered by three main departments: Electrical, Mechanical, and Instruments and Control. Key installations at the plant include natural gas compressors, gas turbines, steam turbine, generators and associated equipment. Protection systems for the gas turbines are also outlined.
This document provides an overview and demonstrations of data flow transformations in SQL Server Integration Services (SSIS). It begins with a question and answer section and an overview of split and join transformations such as the conditional split, multicast, union all, merge, and lookup transformations. Business intelligence transformations like the slowly changing dimension and term extraction transformations are also covered. The document concludes with demonstrations of the transformations in SSIS packages.
The document describes Da Tung Health Promotion Paradise, which was established in 2014 to promote health education and track the health of elderly participants. It has seen increased participation but faces limitations in staffing and an inability to track health improvements. The proposed Happy Aging Healthy Academy (H.A.H.A.) model aims to address these limitations through partnerships with medical universities, use of smart technology to track health data, and establishing a sense of purpose and achievement for elderly participants. The long term vision is to implement the successful H.A.H.A. model throughout Taiwan to enhance elderly health and decrease healthcare costs.
The Bongnjo foundation was founded in 2008 in Cameroon to promote human development and empower youth through community programs. It implements projects in the Ndu region focusing on youth football tournaments that provide HIV/AIDS education and screening to 15,000 people annually. The foundation also aims to increase civic engagement and support youth education with scholarships and entrepreneurship training to over 150 young people.
Este documento discute los derechos fundamentales en Internet y su tratamiento constitucional en América Latina. Analiza temas como la libertad de expresión, restricciones de contenido en línea, y acceso a Internet para grupos marginales. Aunque los países estudiados (México, Costa Rica, Paraguay, Colombia) han progresado en la protección de privacidad y datos personales, todavía enfrentan desafíos como falta de normatividad, vigilancia estatal excesiva, y protección inadecuada de menores y propiedad intelectual. El document
Shoaib Raza is applying for a position and has over 4 years of experience as a chartered accountant at A. F. Ferguson & Co., where he gained experience in statutory audits, reviews, internal audits, and tax matters. He is seeking a career opportunity that provides competitive work environment and opportunities for growth, and believes his skills and qualifications make him a strong candidate.
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1. A molecular dynamics study of ambient and high pressure phases of silica: Structure
and enthalpy variation with molar volume
Chitra Rajappa, S. Bhuvaneshwari Sringeri, Yashonath Subramanian, and J. Gopalakrishnan
Citation: The Journal of Chemical Physics 140, 244512 (2014); doi: 10.1063/1.4885141
View online: http://dx.doi.org/10.1063/1.4885141
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/24?ver=pdfcov
Published by the AIP Publishing
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3. 244512-2 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
coesite CaCl2−typestishovit e
alpha−PbO2−type pyrite−type
Cristobalite Tridymitealpha−quartz
ZSM−5 ZSM−11 Zeolite Beta
Faujasite EMT
(a)
(b)
(c)
FIG. 1. Various polymorphs of silica investigated in this study are shown: (a) high pressure phases; (b) medium density phases at ambient temperature and
pressure; and (c) porous phases of silica.
Zwijnenburg et al.32
have compared experimental ther-
mochemical data for silica polymorphs with simulation
studies using different force fields and with DFT calculations.
More recently, Liang and coworkers33
have reported molec-
ular dynamics studies of the structural transitions and phase
transformations in melanophlogite, a low-pressure silica
polymorph.
Experimental investigations of the thermodynamic prop-
erties of silica polymorphs are well-known. Johnson and
coworkers have studied the thermodynamic properties of
silicalite.34
Petrovic et al. have measured the enthalpy of for-
mation of a number of high-silica zeolites using high temper-
ature solution calorimetry and drop calorimetry.35
They find
that the enthalpy of the high-silica zeolitic forms are about
7–14 kJ/mol higher than the enthalpy of quartz.35,36
Spec-
troscopic (IR, Raman) studies of silica polymorphs are also
available.37,38
Navrotsky et al. have measured the heat of for-
mation of various phases of high-silica zeolites as well as
some dense forms of silica as a function of molar volume.39
They have also reported the experimentally measured entropy
of formation for some silica polymorphs.39
Figure 1 shows the different silica polymorphs inves-
tigated here. Coesite, stishovite, CaCl2-type, α-PbO2-type,
and pyrite-type silica are high-pressure polymorphs. α-quartz,
cristobalite, and tridymite are medium-density polymorphs,
while ZSM-5, ZSM-11, zeolite beta, faujasite, and EMT
(hexagonal faujasite) are low-density or porous silica poly-
morphs.
In spite of innumerable studies, there is lack of a com-
prehensive and comparative study of the low, intermediate
and high density polymorphs of silica. Is there a single in-
termolecular potential that can predict the complexity of
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4. 244512-3 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
the phase diagram and the innumerable structures of silica
correctly? How well is the thermodynamics of silica phases
at various densities reproduced?
In the present study we have chosen to use the BKS po-
tential to model 13 different phases of silica polymorphs over
a range of densities since (i) it has a simple form, (ii) we focus
on structural properties in this study, and the BKS potential is
known to model structural and thermodynamic properties of
even high-pressure phases quite well.17,22
BKS potential is known to have certain limitations—
many-body terms are not accounted for, polarization effects
are not factored in, and there is an implicit assumption that
there are no significant changes in electronic structure un-
der different temperatures and pressures.7
Also, vibrational
properties estimated using BKS are known to be only qual-
itatively in agreement with experiment.17
A major drawback
of the BKS model is that the dispersive term coefficients are
unphysically large and the partial charges used are too low.40
By far the most serious shortcoming of BKS is that it leads to
both coesite and stishovite being more stable than α-quartz—
this can probably be attributed to the large dispersive term
in the potential.22,40
This is contrary to experiment as well
as DFT studies (in the generalized gradient approximation)
which predict α-quartz to be the most stable polymorph.12
BKS is also known to overestimate the transition pressure for
the stishovite I → II transition.22
We focus on the structure and thermodynamic properties
of SiO2 polymorphs and the results are compared with the
experimental data. Simulation of silica glass is also reported
and provides interesting insights into its metastability and its
origin. Finally, metastability of silica crystalline polymorphs
is briefly discussed.
II. METHODS
A. Molecular dynamics
Molecular dynamics simulations were performed in the
NPT ensemble using a variable shape simulation cell. The
simulation cell is represented by three cell vectors, a, b, and c,
following Parrinello-Rahman.41
Unlike in the original formu-
lation, we have restricted the degrees of freedom associated
with the cell matrix, h = (a,b,c) to six by choosing a to be
along the x-axis, b to be in the xy-plane, and c in any direc-
tion. This prevents the rotation of the simulation cell observed
when the cell matrix has nine degrees of freedom.42
B. Intermolecular potential
The BKS potential was used to model all the silica
polymorphs:3
φαβ(r) = Aαβexp[−Bαβr] − Cαβ/r6
+ qαqβe2
/r, (1)
with α, β = Si, O. r is the distance between the two ions of
types α and β. Charges of +2.4 and −1.2 are assigned to the
Si and O atoms, respectively. The coefficients, Aαβ, Bαβ, and
Cαβ of the Buckingham potential are given in Table I.
The BKS potential is known to exhibit an unphysical di-
vergence to −∞ at small distances. This could be a problem
TABLE I. Intermolecular interaction potential for molecular dynamics sim-
ulation of silica polymorphs.
Interaction Aαβ B−1
αβ Cαβ
type (kJ/mol) Å (kJ/mol)Å6
O–O 1.339962d+05 0.362319 1.688493d+04
Si–O 1.737098d+06 0.205205 1.288446d+04
especially while simulating high pressure phases (pyrite-type
and α-PbO2-type silica) or while simulating an amorphous
phase at high temperature before quenching it. To circumvent
this, MD simulations were carried out on such phases using
the modified BKS potential (i.e., the original BKS potential
with an additional 30-6 Lennard-Jones-type term):43
φαβ(r) = φBKS
αβ (r) + 4 αβ
σαβ
rαβ
30
−
σαβ
rαβ
6
, (2)
where φBKS
αβ (r) is the original BKS potential and rαβ is the
separation between atoms α and β, which may be either sili-
con or oxygen atoms. Si–Si and σSi–Si are both set to zero. Si–O
and O–O are taken to be 0.2988995 and 0.10140858 kJ/mol,
respectively. σSi–O and σ−O–O are given the values 1.3136 and
1.7792 Å, respectively.43
The additional term does not alter the form of the original
BKS potential at large separations, but effectively prevents the
negative divergence at short distances. This form of the mod-
ified BKS potential has been used by several groups, specifi-
cally while simulating silica at high temperatures and/or high
pressures.44–46
Several other modifications have also been
proposed to correct for the unphysical divergence at short
distances.47–52
In the classical molecular dynamics simulations whose
results we report here, the total interaction energy has contri-
butions from atom-atom (Si–O, O–O, Si–Si) repulsive terms
which are always positive. The contribution for this comes
from the first term on the right hand side of Eq. (1). The sec-
ond contribution is the attractive dispersion interaction term
which is always negative. The sum of these two terms is what
is termed by us here as the van der Waals interaction energy,
Evdw. This is also referred to as short-range interaction en-
ergy. The electrostatic interaction due to charges on the atoms
gives rise to ECoul. This is also termed as long-range interac-
tion energy. The total interaction energy is
Epot = Evdw + ECoul, (3)
where
Evdw = Erepul + Edisp
=
α β
(Aαβexp(−Bαβr) − Cαβ/r6
). (4)
There is an additional contribution arising from the PV
term:
H = Epot + PV. (5)
At high pressures, the PV term is significant. Ab initio
calculations report energies which are typically of the order12
of around −23 eV. Ab initio energies include electronic
contributions, zero-point energy of optical modes, thermal
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5. 244512-4 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
corrections, etc., while empirical potentials include disper-
sion (fluctuating dipole-fluctuating dipole moment, and other
higher order terms), repulsion term (due to Pauli exclusion
principle arising from electron cloud overlap) and long-range
electrostatic contributions. It is therefore difficult to com-
pare the ab initio energies with the energies from empiri-
cal calculations. However, the differences in energies, for ex-
ample, between silica polymorphs should be comparable in
magnitude.
III. COMPUTATIONAL DETAILS
The DLPOLY set of programs53
was used to carry out
molecular dynamics simulations at room temperature in the
NPT ensemble using a variable shape simulation cell for each
of the silica polymorphs. The simulation cell size was cho-
sen sufficiently large so that a cut-off radius of 12 Å could be
used in the calculation of short-range interactions. In order to
get good statistics, larger system sizes were used for the high-
pressure phases. For these, a larger cut-off of 14 Å has been
employed. The simulation cell consisted of 192 SiO2 units
(for low density faujasite) to 2916 SiO2 units (for high den-
sity pyrite-type silica). Table II lists the system size used for
each of the polymorphs studied here. Note that simulations
have been carried out on large systems and for reasonably
long durations. For high pressure phases simulations have
been carried out on even larger systems. All simulations were
performed so that the conservation of energy is better than 1
in 104
.
Two different glassy phases of silica were also simulated,
each one starting from the crystalline structure of α-quartz
and tridymite. We label these Glass-A and Glass-T, respec-
tively. The modified BKS potential was used in these simu-
lations. The simulation cell consisted of 882 SiO2 units and
2016 SiO2 units for simulating Glass-A and Glass-T, respec-
tively (see the supplementary material54
for details of prepa-
ration of the amorphous silica phases).
Periodic boundary conditions were employed. Electro-
static interactions were evaluated using the Ewald summation
technique. A time step of 1 fs was used for simulating most
of the crystalline polymorphs of silica. For the high-pressure
phases – stishovite, CaCl2-type, α-PbO2-type, and pyrite-type
– the timestep had to be reduced to 0.5 fs in order to obtain
good energy conservation. The durations of the equilibration
and production runs are also listed in Table II.
IV. RESULTS AND DISCUSSION
A. Structure
Although calculations on silica polymorphs have been re-
ported previously, they have been restricted to low density
polymorphs.9
This is the first study which reports results of
both low, intermediate, and high density polymorphs using
a single potential. In general many potentials include terms
such as bonded term, three-body angle dependent, and finally
four-body dihedral angle terms. However, it is well-known
that the existence of many different phases of silica arises
from the possibility to explore a large range of angles in the
three-body term. Hence, it would be difficult for a potential
with three-body terms to account for such a large number of
phases. BKS potential, on the other hand, prescribes a simple
atom-atom “non-bonded” interaction which, as we shall see,
can account for the extraordinarily wide range of structures
that silica can exist in.
Table III shows the average values of the lattice param-
eters obtained from our variable shape NPT ensemble simu-
lations for each silica polymorph. The corresponding exper-
imental values are also indicated (see caption to Table III).
Clearly, the BKS forcefield (which we have used in this study)
is able to correctly model lattice parameters across the whole
spectrum of densities and molar volumes. It is encouraging
to see that in most cases, the deviation from the experimental
value is less than 5%. Although the BKS potential was primar-
ily parametrized for tetra-coordinated silica,3
we find that this
TABLE II. The simulation cell consists of na, nb, and nc unit cells along each of a, b, and c directions. In
addition, the number of SiO2 units in the simulation cell is tabulated for each polymorph.
Silica No. of Structure
Run length
polymorph na nb nc SiO2 units reference Equilibration Production
Pyrite-type 9 9 9 2916 55 1 ns 750 ps
α-PbO2-type 9 7 8 2016 56 1 ns 1 ns
CaCl2-type 8 8 15 1920 56 1 ns 1 ns
Stishovite 8 8 15 1920 57 1 ns 1 ns
Coesite 5 3 9 2160 58 1 ns 1 ns
Glass-A 7 7 6 882 250 ps 250 ps
α-quartz 7 7 6 882 59 1 ns 1 ns
Cristobalite 6 6 4 576 60 250 ps 250 ps
Glass-T 2 7 3 2016 250 ps 250 ps
Tridymite 2 7 3 2016 61 250 ps 250 ps
ZSM-5 2 2 2 768 62 1 ns 1 ns
ZSM-11 2 2 2 768 63 250 ps 250 ps
Zeolite beta 2 2 1 256 64 250 ps 250 ps
Faujasite 1 1 1 192 65 250 ps 250 ps
EMT 2 2 2 768 66 250 ps 250 ps
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6. 244512-5 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
TABLE III. The average values of simulation cell volume, VMD , as well as the average unit cell parameters— a , b , c , α, β, and γ as obtained from the
variable shape MD simulations are tabulated for each silica polymorph. For the unit cell edge lengths and molar volume, the percentage deviation with respect
to experimental values for the crystalline form is indicated in parentheses. For the angles between unit cell axes, the experimental value for the crystalline form
is indicated in parentheses, when it is different from the experimental value.
Silica VMD a b c α β γ
polymorph (Å3) (Å) (Å) (Å) (deg.) (deg.) (deg.)
Pyrite-type 44 758.0 (1.2) 4.048 (3.0) 3.881 (−1.3) 3.909 (−0.5) 90.0 90.0 90.0
α-PbO2-type 37 835.4 (4.6) 3.845 (3.6) 4.644 (−0.2) 4.204 (1.1) 90.0 90.0 90.0
CaCl2-type 36 350.1 (4.7) 3.861 (1.4) 3.861 (1.2) 2.540 (2.0) 90.0 90.0 90.0
Stishovite 43 026.3 (−3.6) 4.112 (−1.6) 4.112 (−1.6) 2.651 (−0.5) 90.0 90.0 90.0
Coesite 73 790.9 (−1.7) 7.001 (−1.9) 12.391 (0.2) 7.229 (0.8) 90.0 121.0 (120.0) 90.0
α-quartz 33 224.2 (1.7) 4.938 (0.5) 4.938 (0.5) 5.443 (0.7) 90.0 90.0 120.0
Cristobalite 24 639.0 (−7.6) 4.911 (−1.2) 4.911 (−1.2) 6.557 (−5.3) 90.0 90.0 90.0
Tridymite 89 170.6 (−10.5) 18.087 (−2.4) 4.688(−6.3) 23.268(−2.3) 90.0 105.6 (105.82) 90.0
ZSM-5 42 621.1 (5.0) 20.263 (1.1) 20.256 (1.9) 13.629 (1.9) 90.0 90.0 90.0
ZSM-11 43 203.2 (4.6) 20.332 (1.3) 20.333 (1.3) 13.660 (1.9) 90.0 90.0 90.0
Zeolite beta 16 931.5 (1.7) 12.679 (0.2) 12.679 (0.2) 26.769 (1.4) 90.0 90.0 90.0
Faujasite 14 273.9 (6.1) 24.739 (2.0) 24.739 (2.0) 24.739 (2.0) 90.0 90.0 90.0
EMT 57 658.4 (5.1) 17.496 (1.6) 17.496 (1.6) 28.568 (1.7) 90.0 90.0 120.0
forcefield yields reasonable results for six-coordinated high-
pressure phases as well. This is in agreement with the work of
Tse and others.17,67
We believe the atom-atom potential form
is responsible for the wonderful ability of BKS potential to
account for polymorphism in silica.
B. Thermodynamics
1. Metastability of silica polymorphs
Table IV lists the average temperature, pressure, and mo-
lar volumes obtained from the variable shape NPT ensem-
ble simulations for all the silica polymorphs. Also indicated
are the values of the enthalpies and the total energies of in-
teraction, along with the short-range and long-range energy
components. We note that the Evdw is the van der Waals con-
tribution which is the sum of the Edisp and Erepul. The former
(dispersion term) is always negative and favorable while the
latter (repulsive) is always positive and destabilizes. As we
can see, the sum of these two terms is negative and favor-
able up to stishovite but for still higher pressure polymorphs,
the energy is unfavorable. The error bars are also reported in
Table IV. The errors are small suggesting that the obtained
values are reliable.
Experimentally, α-quartz is known to be the most sta-
ble silica polymorph at ambient conditions. We find that at
TABLE IV. The total energy of interaction, as well as the short-range and long-range contributions are tabulated for all the silica polymorphs simulated in
this study (variable shape MD simulation), as a function of molar volume. The average volume, temperature, and pressure obtained from the simulation are
indicated along with the set values in parentheses. Values reported are averaged over 250 ps, except for those polymorphs marked with an asterisk for which the
averages are over a period of 1 ns.
Silica V T P Evdw ECoul Epot H
polymorph (cm3/mol) (K) (katm) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol)
Pyrite-type∗ 9.24(9.14) 299.7(300) 2674.8(2674.8) +827.30(1.73) − 5941.30(0.82) − 5114.00(1.09) − 2597.64(4.96)
α-PbO2-type∗ 11.30(10.81) 299.0(300) 1188.1(1188.1) +258.06(1.54) − 5761.90(0.93) − 5503.97(0.71) − 4132.19(4.74)
CaCl2-type∗ 11.40(10.89) 298.3(300) 1188.2(1188.1) +256.22(11.7) − 5781.77(6.85) − 5525.52(4.90) − 4142.03(36.2)
Stishovite∗ 13.49(14.02) 299.2(300) 99.1(99.0) − 167.54(5.5) − 5475.52(5.1) − 5643.23(0.48) − 5496.35(19.9)
Coesite∗ 20.57(20.58) 297.5(300) 44.4(44.4) − 79.60(1.93) − 5555.09(1.78) − 5634.72(0.25) − 5532.87(6.44)
Glass-A 22.68(22.68) 299.7(300) 0.0(0.0) − 93.45(1.54) − 5481.07(1.63) − 5574.49(6.07) − 5563.38(4.99)
α-quartz∗ 22.68(22.71) 298.1(300) 0.0(0.0) − 90.25(1.38) − 5533.79(1.38) − 5624.04(0.17) − 5612.93(3.75)
Cristobalite 25.76(25.49) 288.6(298) 0.1(0.0) − 87.08(2.26) − 5520.49(2.32) − 5607.64(0.93) − 5596.70(12.2)
Glass-T 26.64(23.87) 297.6(300) 0.0(0.0) − 86.50(1.45) − 5478.67(1.98) − 5564.98(1.63) − 5554.07(6.46)
Tridymite 26.64(27.52) 290.5(298) 0.2(0.0) − 85.56(1.82) − 5524.80(2.05) − 5610.12(1.31) − 5599.21(16.3)
Glass (Expt.) 27.27
ZSM-5∗ 33.42(33.65) 298.1(298) 0.0(0.0) − 55.46(4.32) − 5529.30(1.51) − 5584.77(0.27) − 5573.57(4.32)
ZSM-11 33.88(34.02) 373.2(373) 0.0(0.0) − 55.27(1.78) − 5525.13(1.79) − 5580.34(0.22) − 5566.41(5.15)
Zeolite beta 39.83(38.56) 298.3(298) 0.0(0.0) − 53.99(2.53) − 5518.36(2.54) − 5572.66(0.23) − 5561.33(7.19)
Faujasite 44.77(44.29) 298.8(298) 0.0(0.0) − 51.37(4.11) − 5504.69(4.12) − 5556.25(0.32) − 5544.79(12.0)
EMT 45.21(46.51) 293.0(293) 0.0(0.0) − 51.32(2.72) − 5504.82(2.72) − 5556.25(0.19) − 5545.18(8.02)
Stishovite∗ 13.81(14.02) 299.7(300) 0.0(0.0) − 207.27(3.6) − 5436.98(3.6) − 5644.27(0.09) − 5632.81(12.5)
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7. 244512-6 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
ambient conditions (300 K and 1 atm pressure), the BKS po-
tential indicates that stishovite is the most stable phase and
is marginally lower in energy (by about 20 kJ/mol) than α-
quartz. Poole and coworkers also found that stishovite is more
stable than α-quartz at 300 K and 1 atm in their simulation
study of the phase diagram using BKS potential.21
Thus, it is
clear that BKS does not predict the order of stability correctly
at room temperature and pressure. Our NVE simulations of
stishovite and α-quartz (data not shown here) also indicate
that the BKS potential favours the stishovite phase at ambient
conditions.
However, under the experimental conditions at which
each of these phases is known to exist, the order of sta-
bility could be different. Hence, we carried out simula-
tions at the corresponding pressure and temperature to see
what order of stability is indicated. The results from the
variable shape Parrinello-Rahman isothermal-isobaric sim-
ulations performed at different pressures and temperatures
showed significant changes in the order of stability. Under
these conditions, we find that α-quartz is the most stable
phase. The enthalpies of various phases at their respective
pressures and temperatures are listed in Table IV. Note that
α-quartz is the most stable phase by about 117 kJ/mol as com-
pared to stishovite.
We also attempted to study a few phases which share a
co-existence line in the phase diagram as a function of pres-
sure. We have shown in Figure 2(a) the variation in enthalpy
as a function of pressure for coesite and α-quartz. In the case
of coesite, the pressure was decreased (the arrow indicates the
direction of pressure change) from 44 katm to 1 atm, while in
the case of α-quartz the pressure was increased from 1 atm to
100 katm. Note that the lines do not intersect in the range of
pressures investigated. However, this is not surprising since
the hysteresis associated with the first-order transitions will
ensure that the transitions actually take place at much higher
or lower pressures. In the case of stishovite-α-quartz (shown
in Figure 2(b)), the two come close to intersection but this
appears to be at some negative pressure. More detailed stud-
ies are required for understanding phase transitions in silica
polymorphs and these are under progress.
0 20 40 60 80 100
-5700
-5650
-5600
-5550
-5500
-5450
-5400
H(kJ/mol)
α-quartz
coesite
0 20 40 60 80 100
P (katm)
-5700
-5650
-5600
-5550
-5500
-5450
-5400
H(kJ/mol)
α-quartz
stishovite
(a)
(b)
>
<
>
<
FIG. 2. Variation of enthalpy with pressure for α-quartz, coesite, and
stishovite.
0 10 20 30 40 50
-300
0
300
600
900
Evdw
(kJ/mol)
0 10 20 30 40 50
-6000
-5800
-5600
-5400
ECoul
(kJ/mol)
0 10 20 30 40 50
V (cm
3
/mol)
-5800
-5600
-5400
-5200
-5000
Epot
(kJ/mol)
1
2, 3
4
5 6 7, 8 9, 10 11 12, 13
FIG. 3. Plot of total intermolecular interaction energy, Epot, short-range
van der Waals interaction energy, Evdw, and Coulombic energy contri-
bution, ECoul obtained from molecular dynamics simulations in the vari-
able shape NPT ensemble at the temperatures and pressures indicated in
Table IV. The following polymorphs are shown in the figure: (1) pyrite-type
silica, (2) α-PbO2-type silica, (3) CaCl2-type silica, (4) stishovite, (5) coesite,
(6) α-quartz, (7) cristobalite, (8) tridymite, (9) ZSM-5, (10) ZSM-11, (11) ze-
olite beta, (12) faujasite, and (13) EMT. The dotted lines are high-order fitted
polynomials and serve to guide the eye. Filled symbols represent the corre-
sponding values for Glass-A and Glass-T. Error bars are also indicated for all
quantities. In all cases, the error bars are smaller than the size of the symbol
used.
2. Energy contributions as a function of molar volume
Previous calculations suggest that the differences in the
enthalpies between large pore zeolites are small.9
They fur-
ther suggest that the short-range contributions are also not
very different for these zeolites. In order to understand the
metastability of the various polymorphs that we have simu-
lated in the present study, we have listed the short-range and
the long-range contributions in Table IV. Further, Figure 3
shows the variation of the total interaction energy and the as-
sociated short-range and long-range components as a function
of molar volume for all the polymorphs studied here.
The values obtained in the present study and previ-
ous calculations show good agreement with each other.
For crystalline cristobalite, the Coulombic contribution esti-
mated from this MD study is −5520 kJ/mol. This compares
well with values of −5525 kJ/mol and −5530 kJ/mol, ob-
tained from simulation studies by Yamahara et al.68
and Li
et al.,69
respectively, for BKS potential. The values reported
by Li and coworkers69
for the Coulombic components for α-
quartz, tridymite, ZSM-11, and zeolite beta are −5540.53,
−5547.72, −5537.16, and −5526.54 kJ/mol which are com-
parable to our values of −5533.79, −5524.80, −5525.13, and
−5518.36 kJ/mol, respectively. The maximum difference is
for tridymite which is around 0.4%.
Before we discuss the metastability of the various crys-
talline polymorphs, it is interesting to understand the differ-
ence between the crystalline and the amorphous forms of
silica. There have been a few ab initio studies which have
investigated pressure-induced amorphization of silica.10,30,70
The short-range contributions to the total energy is slightly
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8. 244512-7 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
0 30 60 90 120 150 180
0
0.003
0.006
0.009
0.012
0.015
f(θ)
crystal
glass
0 30 60 90 120 150 180
0
0.002
0.004
0.006
0.008
0 30 60 90 120 150 180
θ (degrees)
0
0.002
0.004
0.006
0.008
f(θ)
0 30 60 90 120 150 180
θ (degrees)
0
0.001
0.002
0.003
0.004
0.005
α-quartz tridymite
O-Si-O
Si-O-Si
O-Si-O
Si-O-Si
FIG. 4. The distribution of different angles is shown for the glass as well as
well as the crystal of comparable density.
larger in magnitude for the glassy phase as compared to the
crystalline polymorph with similar molar volume. This means
that the amorphous phases have a lower short-range energy
as compared to the crystalline form at the same density!
In other words the glasses optimize the short-range interac-
tions better than their crystalline counterparts. Thus, Glass-A
and Glass-T are found to have short-range interaction ener-
gies of −93.45 kJ/mol and −86.50 kJ/mol, while the corre-
sponding crystalline phases – α-quartz and tridymite – are
associated with short-range energies of −90.25 kJ/mol and
−85.56 kJ/mol, respectively.
The reason for better optimization of the short-range en-
ergy in the case of glasses can be understood if we look
at the structure of the glass. In the glassy phase, it is seen
that although the tetrahedra remain intact, the interconnec-
tion between two tetrahedra are altered significantly. As is
evident from Figure 4 the distribution of Si–O–Si angles is
altered significantly. The distribution changes from bimodal
to monomodal and also extends over a much larger range of
angles. This is consistent with what is known of the “soft”
bending potential at the oxygen atom; i.e., the energetic cost
associated with distorting the Si–O–Si angle is very small.71
This suggests that the link between tetrahedral units is much
more flexible and provides for much larger choice. This is
not the case for O–Si–O angle which is broader for the glass
as compared to the crystal. From the figure it is also evident
that in tridymite much smaller angles are seen as compared to
α-quartz. We see that better optimization and larger flexibil-
ity and existence of short-range order leads to lower energies
(higher in magnitude) for the glassy phase.
In contrast, the Coulombic contribution is higher (lower
in magnitude) for the glassy phase. The crystalline polymorph
has a significantly lower Coulombic energy as compared to
the glassy phase of similar density. This is understandable
since the absence of long range order in the glassy phase leads
to lower contributions from the Coulombic component of the
interaction energy (see Table IV for the actual values).
This trend in the relative values of the Coulombic and
short-range contributions between the amorphous and the
crystalline forms is similar to that observed by Yamahara
et al.68
in their MD studies of crystalline and amorphous
cristobalite. They find that the Coulombic component of in-
teraction energy for the crystal is about 25 kJ/mol more
favourable than that for the amorphous cristobalite. In the
present study, we find that the Coulombic contributions
for crystalline α-quartz and crystalline tridymite are about
52.7 kJ/mol and 46.1 kJ/mol lower (i.e., more favourable) than
the respective amorphous phases. Yamahara et al. have used a
different silica potential and so only a qualitative comparison
is possible.
Two distinct regimes can be seen in the variation of the
enthalpy and other quantities with molar volume—a sharp de-
pendence at high-pressures (for stishovite and beyond) and
a very slight variation for lower density polymorphs. Note
that stishovite and still higher pressure phases all are six-
coordinated. Thus, it appears that the change from weak de-
pendence to strong dependence on molar volume is associated
with the change from 4-coordinated to 6-coordinated phases.
This is an important link between the thermodynamics of sil-
ica phases and the structure. The present finding suggests that
there is a thermodynamic signature of the change in coordina-
tion of silicon. This has not been observed previously, to the
best of our knowledge.
The underlying reason for this marked change in the
dependence of Epot, the total potential energy of interaction
with molar volume becomes clear if we look at Table IV and
Figure 3. From Figure 3 it is evident that the variations with
molar volume of Epot and Evdw are similar. From Table IV,
we see that at high pressures, the contribution of the short-
range interaction energy, Evdw, to the total energy becomes
significant. Thus, the steep variation of Epot with molar vol-
ume, V , seen at high pressures, is due to significant increase
in the short-range energy with decrease in molar volume. The
Coulombic contribution, on the other hand, does not exhibit
any such noticeable change. This is to be expected since there
is no significant change in long-range order. Thus, at higher
densities, the short-range interaction energy has a stronger de-
pendence on molar volume than at smaller pressures.
To understand the reasons for the sudden change in the
short-range interaction energy with molar volume, we have
further analysed the nearest neighbour structure in detail.
Figure 5 shows the dependence of the BKS potential for the
Si–O interaction on the Si–O distance. The sharp increase
in the contribution from the short-range interaction can be
understood in terms of the nearest neighbour structure. In
α-quartz, we see that there are just four oxygens at a dis-
tance of 1.61 Å. We see from Figure 5 that energy becomes
unfavourable below 1.52 Å. Further, the contribution to the
short-range energy rapidly increases with decrease in Si–O
distance. In coesite, the Si–O distance and the coordination
number of Si are very similar to α-quartz. But as the pressure
increases, first there is an increase in the coordination num-
ber of silicon. Stishovite exhibits 6 coordination with most of
the Si–O bonds longer than in α-quartz (4 at 1.76 Å and 2 at
1.81 Å as obtained from MD simulation). The increased co-
ordination leads to larger short-range contribution. With fur-
ther increase in pressure, the coordination number remains
essentially unchanged but the Si–O bond distance gradually
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9. 244512-8 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4
r (A)
-250
0
250
500
750
1000
1250
U(Si-O)(kJ/mol)
BKS potential
Modified BKS potential
pyrite−type alpha−PbO2−type CaCl2−type
alpha−quartz coesite stishovite
FIG. 5. The BKS potential for the Si–O interaction is plotted as a function
of the Si–O distance. The modified form of the potential is also indicated in
the figure.
decreases: 4 at 1.64 Å and 2 at 1.62 Å for both CaCl2- and α-
PbO2-type. In the highest pressure polymorph – pyrite-type
silica – all the six Si–O bonds are of length 1.61 Å. These
changes can be more clearly seen in Figure 6 for various high-
pressure phases of silica and α-quartz. The changes in coor-
dination numbers with distance for various phases are also
shown in this figure. Thus, the shorter Si–O bond lengths from
stishovite onwards lead to a sharp increase in the contribution
from short-range interactions. Thus, the present analysis sug-
gests that the increase in the enthalpies of the various high
pressure phases arises principally from changes in the first
0 10 20 30 40 50
V (cm
3
/mol)
-6000
-5000
-4000
-3000
-2000
H
MD
(kJ/mol)
0 10 20 30 40 50
-920
-910
-900
-890
-880
-870
-860
-850
ΔH
expt
(kJ/mol)
1
2,3
4
5
6 7
7 8
9 10
11
12
13
(a)
(b)
4 5 6 9 10 11 12 13
FIG. 7. Variation of enthalpy with molar volume for different silica poly-
morphs (a) from experimental data, and (b) from variable shape NPT ensem-
ble molecular dynamics simulations. The following polymorphs are shown
in the figure: (1) pyrite-type silica, (2) α-PbO2-type silica, (3) CaCl2-type
silica, (4) stishovite, (5) coesite, (6) α-quartz, (7) cristobalite, (8) tridymite,
(9) ZSM-5, (10) ZSM-11, (11) zeolite beta, (12) faujasite, and (13) EMT. The
dotted lines are high-order polynomials and are meant to guide the eye. Filled
symbols represent the corresponding values for Glass-A and Glass-T. Error
bars are indicated for values from MD; in all cases, the error bars are within
the size of the symbol used.
coordination shell: increase in coordination number as well
as shorter Si–O bond lengths.
Figure 7 shows (a) the experimental heats of forma-
tion from available literature, and (b) the enthalpies obtained
from our MD simulations plotted as a function of molar vol-
ume for all the polymorphs investigated in this study. There
can be no quantitative comparison between the enthalpies
of formation obtained from experiment and the enthalpies
obtained from simulation. We show the variation of H =
H(polymorph) − H(α-quartz) as a function of molar volume
from experiment and MD in Figure 8. We can see that while
the trends are similar, there are quantitative differences be-
tween them. We have not shown the H for polymorphs
beyond stishovite as there is no experimental data for high
pressure phases beyond stishovite. Experimental data show
1.4 1.5 1.6 1.7 1.8 1.9 2
r (A)
0
5
10
15
20
25
gSi-O
(r)
α-quartz
coesite
stishovite
CaCl2
-type
α-PbO2
-type
pyrite-type
1.4 1.5 1.6 1.7 1.8 1.9 2
r (A)
0
1
2
3
4
5
6
7
nSi-O
(r)
α-quartz
coesite
stishovite
CaCl2
-type
α-PbO2
-type
pyrite-type
FIG. 6. Si–O radial distribution functions and associated coordination numbers are shown for the high-pressure polymorphs studied here. The corresponding
quantities for α-quartz have also been plotted for comparison.
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10. 244512-9 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
10 20 30 40 50
V (cm
3
/mol)
0
50
100
150ΔH(kJ/mol)
ΔH from exptl data
ΔH from MD
4
5
6
7
9
11
8
12,13
10
FIG. 8. Dependence of H (=H(polymorph) − H(α-quartz)) on molar vol-
ume from experimental data and from the present variable shape NPT MD
study. The following polymorphs are shown in the figure: (4) stishovite, (5)
coesite, (6) α-quartz, (7) cristobalite, (8) tridymite, (9) ZSM-5, (10) ZSM-11,
(11) zeolite beta, (12) faujasite, and (13) EMT.
that the enthalpy of stishovite is considerably higher than
that of coesite, suggesting a steeper variation of enthalpy
with molar volume at high pressures. Our own MD simu-
lations suggest that this is indeed the case. Note, however,
that there is a qualitative agreement between the BKS derived
curve and the experimental curve suggesting that the order
of the enthalpies of the various phases is in agreement with
experiment.
Further, the additional high pressure phases (CaCl2-type,
α-PbO2-type, pyrite-type) studied here show that this steep
dependence becomes even steeper with reduction in molar
volume. From Table IV one can get an approximate esti-
mate of the magnitude of enthalpy difference between these
phases and α-quartz. Thermodynamic data from experiments
on these high pressure phases are not yet available. The
present study thus reports the enthalpies of the high pressure
phases of silica for the first time. It will be interesting to com-
pare these values with experimental data as and when they
become available.
The value of enthalpy, H, obtained from MD simulation
for α-quartz (−5613 kJ/mol) is in good agreement with those
obtained by Murashov and Svishchev who report a value of
about −5620 kJ/mol for MD simulations carried out with the
BKS potential.72
Note that the lowest enthalpy is obtained for
α-quartz which is the most stable phase as indicated by our
variable shape NPT ensemble simulations.
The enthalpy values for glass (molar volume = 27.27
cm3
/mol) are from experimental data by Piccione et al.73
In the same paper, they also report the enthalpy for crys-
talline tridymite (molar volume = 27.52 cm3
/mol) to be
−910.00 kJ/mol. This is about 8.2 kJ/mol lower than the
glassy silica with similar density. We also find that the en-
thalpies of our simulated glasses are higher than that of the
corresponding crystalline polymorphs at the same density.
Glass-A and Glass-T have enthalpies of 49.55 kJ/mol and
45.14 kJ/mol higher than that of α-quartz and tridymite, re-
spectively.
0
10
20
30
40
50
60
ΔH
q
298.15
,ΔG
q
298.15
(kJ/mol)
Enthalpy
Free energy
10 20 30 40 50
V (cm
3
/mol)
-3
-2
-1
0
1
2
3
TΔS
q
298.15
(kJ/mol)
Entropy
1
2
3
4 5
6
8
12
13
1
2
3
4
5
6
7 8 9
10
11
12
13
1
2
3. 4 5
6
8. 11
12 13
g
g
g
7 9 10
11
7
9
FIG. 9. Plot of Hq
298.15, Gq
298.15, and T Sq
298.15 vs. molar volume for
different silica polymorphs using experimental data available in the literature.
Here the Hq
298.15, Gq
298.15, and T Sq
298.15 are the formation enthalpy,
free energy, and entropy of the given polymorph minus the same quantity
for α-quartz. Data for the following polymorphs are shown in the figure: (1)
stishovite, (2) coesite, (3) α-quartz, (4) cristobalite, (5) tridymite, (6) ZSM-
23, (7) ZSM-12, (8) ZSM-5, (9) ZSM-11, (10) SSZ-24, (11) zeolite beta, (12)
faujasite, and (13) EMT. The data corresponding to glass are marked with g.
Metastability is associated with many interesting phe-
nomena such as polymorphism and superconductivity.74
The
metastability of most polymorphic forms of silica arises from
rather small differences in free energy. We have compiled
the entropic contribution as well as the free energies and en-
thalpies of formation from experimental data available in the
literature.34,35,73,75
The relative values of these quantities with
respect to α-quartz are plotted as a function of molar volume
in Figure 9 for a number of polymorphs for which data are
available. For the polymorphs shown in the figure, we note
that the entropic contribution to free energy is less than 10%.
While T S◦
298.15 varies only over a range of about 6 kJ/mol,
H◦
298.15 varies over a range of almost 60 kJ/mol (see
Figure 9). That is, the entropic contribution to metastability
is very small. Thus, although we have not computed the free
energies of the various silica polymorphs, it is likely that the
trend in free energy will be dictated largely by the trend in
the enthalpy. Hence, enthalpy as well as the total interaction
energy we have obtained here from MD are indicative of the
mestability of the various silica polymorphs. It is satisfying
to note that the results from simulation predict the order of
metastability of the various polymorphs of silica correctly, in
agreement with experiment. Further, these studies show that
there are two different regimes of enthalpy dependence on
molar volume, giving rise to two distinct types of metastabil-
ity: the one at high pressures is dictated by the short-range in-
teractions and the one at lower pressures by both short-range
and long-range interactions.
V. CONCLUSIONS
This simulation study provides a comprehensive inves-
tigation of 13 different silica polymorphs, with a focus on
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11. 244512-10 Rajappa et al. J. Chem. Phys. 140, 244512 (2014)
high-density phases. There are several interesting findings.
First, the present study indicates that the BKS potential can
predict the lattice parameters of all the 13 phases of silica in
agreement with reported X-ray diffraction values. The modi-
fied BKS was crucial in reproducing the high density phases.
Further, the predicted values of the thermodynamic enthalpies
of low density phases are in good agreement with experimen-
tal data. This study predicts the enthalpies of the high-density
polymorphs for the first time—these can be verified as and
when experimental thermodynamic data become available.
The predicted enthalpies of CaCl2-type, α-PbO2-type, and
pyrite-like phase of silica are, respectively, 4142.03, 4132.19,
and 2597.64 kJ/mol. Note that the enthalpies of CaCl2-type,
α-PbO2-type are rather close to each other. This is consistent
with the fact their molar volumes are not very different. Re-
cently, there has been growing interest in prediction of high-
pressure crystalline phases.76,77
Our study shows that amorphous phases of silica are as-
sociated with lower short-range interaction energies than their
crystalline counterparts at the same density. This interesting
behaviour suggests a possible reason why silica glasses are
so commonly seen. The crystalline phase, however, exhibits
long-range order (unlike the glassy phase) and is, therefore,
associated with a predominantly favourable long-range ener-
getic contribution.
For high-density polymorphs, the enthalpy is seen to ex-
hibit a marked change from a weak to a steep dependence on
molar volume—experimental data indicate that the enthalpy
of stishovite is significantly higher than that of coesite. The
present study reports enthalpy for polymorphs denser than
stishovite and we show that the reason for the sharp change in
dependence of the enthalpy on molar volume at higher densi-
ties has its origins in the larger contributions from short-range
interactions. We further demonstrate that the increased short-
range contribution to enthalpy at high densities arises from
(i) an increase in coordination number of silicon atom from
four to six, as well as (ii) reduced Si–O bond lengths. Since
most of these are unfavourable interactions, this leads to
the observed steep increase in energy with decreasing molar
volume.
ACKNOWLEDGMENTS
J.G. wishes to thank INSA, New Delhi, for the award of
a Senior Scientist position. C.R. thanks DST, New Delhi, for
the award of a grant (DSTO1225) under the Women Scientist
Scheme. S.Y. thanks DST, New Delhi, for a Raja Ramanna
fellowship (DST1065) and grant under nano-mission for 100
teraflop computing facility (DST1169).
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