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PROJECT REPORT
ON
PROCESSING OF PORCELAIN INSULATORS
submitted by
SRIDHAR PRASAD P
B.Tech (Ceramic Technology)
A.C.College of Technology
Anna University
Chennai-25.
Under the guidance of
Dr.L.N.SATAPATHY
Sr. DGM (CTI)
BHEL, Bangalore-12.
MAY/JUNE 2015
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Acknowledgement
A project report is never the sole production of the person whose name appears on the cover.
Many people have lent their technical assistance, suggestions and guidance in completion of this
project.
I am extremely thankful to Mr.B.George, AGM (HRD), BHEL (EPD) for according the
permission to undertake my project in this esteemed organization.
With profound reverence and heartfelt gratitude, I would like to express my sincere thanks to my
guide Dr. L.N.Satapathy, SDGM, CTI, BHEL CORPORATE RESEARCH AND
DEVELOPMENT for his invaluable guidance and encouragement throughout the duration of
project work which helped to improve my industrial knowledge.
I am also in debt to Dr. R.N. Das, AGM, and BHEL - CTI for his guidance. I also thank
Ms.T.Radhika, Mr.Elumalai and Ms. Shruthi Hebsur for assistance and help throughout the
project work. I would like to express my gratitude to Mr.C.Jeyakumar (Retd.) for his guidance.
I finally extend my thanks to all the section in charge, section supervisors and all other staff
members of BHEL for their constant support and encouragement.
Moreover, my special thanks to my parents and friends who are the most important contributors.
I express my thanks to all of them.
SRIDHAR PRASAD P
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BHEL – At a glance
BHEL (Bharat Heavy Electricals Limited), a Public Sector Undertaking of Government of India,
is an integrated power plant equipment manufacturer and one of the largest engineering and
manufacturing companies in India.
BHEL has 14 manufacturing plants, 4 power sector regional centers, 8 service centers, 18
regional offices and over 150 project sites spread all over India and abroad. It offers a wide
spectrum of products and services for core sectors like Power generation, Transmission,
Distribution, Industry, Transportation, Oil and gas, Defense and Non-Conventional energy
systems.
Ceramic Business Unit (CBU)is one of its Strategic Business Groups and is headquartered at
Bengaluru and is supported by:
Manufacturing units with State of the Arts Technology for ceramic products, processes and
applications at:
Electro porcelains Division (EPD), Bengaluru
Insulator Plant (IP), Jagdishpur
Advanced Research and Development facilities for ceramic products, process and applications at
:
Ceramic Technological Institute (CTI), Bengaluru (A Division of Corporate
Research& Development, Hyderabad)
Electro porcelain Division (EPD) is one of manufacturing Divisions of BHEL among the four
Bengaluru based units. EPD is today the oldest as well as largest insulator manufacturing unit in
country. Apart from being a recipient of the ISO 9001 Certificate for quality systems in design
and manufacture, recognition has come by the way of awards for national productivity, safety
and quality.
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It manufactures the full range of porcelain insulators for use in a power transmission line,
substation 25KV traction systems and in electrical apparatus up to 800KV.
EPD’s major customers include utilities, railways, power plants and electrical equipment
manufacturer both in India and abroad. EPD’s insulators are well known in world market. In
addition, it also manufactures an important product called CERALIN, a highly abrasive resistant
ceramic for use in thermal power station and for various industrial applications.
EPD Plant Details:
It is situated in Malleshwaram Complex, opposite to Indian Institute Science (IISc), Bengaluru.
The land area of 36.9 acres. Around 114 number of executives, 110 number of supervisors, 610
number of artisans, 79 supporting technical staff, roughly 279 number of office supporting staff
other than that 270 number of OSW/SSW and around 43 trainees.
Capacity of Unit : Insulators 8850 Tonnes
Ceralin 1490 Tonnes
Export Destinations : 52 Countries
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Ceramic Products:
Disc Insulators
Applications:
Transmission lines from 11KV to 800KV (AC & DC) and sub Stations
Hollow Insulators
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Applications:
Power Transformers, Instrument Transformers, Electrostatic Precipitators, Circuit
Breakers of conventional and SF6 gas Insulated types
Station Post Insulator
Applications:
Bus bars support and for isolators from 11KV to 400KV
Composite Insulators
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Applications:
a) 25KV Railway Traction Insulators
Stay arm
Bracket
9-Tonne
b) Composite long rod insulators for transmission line and
substation of rating 11KV to 400KV.
Wear Resistant Materials
Superior wear resistance, chemically inert and high thermal stability characteristics make
CERALIN suitable for wide range of applications:
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i) Power Sector : Wear prone components of Bowl/Tube Mill, Fuel
piping
ii) Steel Sector : Sinter plants, Rotary Kiln, coke oven
iii) Cement Sector : Cement & raw mills, chutes and air separators
iv) Coal Sector : Slurry pipes, Chutes & launders
Industrial Ceramics
CBU caters to the need for ceramic material for various industrial applications namely:
Ceramic Membrane based Pre-filtration system for RO-DM plant
Electronic Water Level indicators
Ceramic and Tungsten Carbide flow beans
Ceramic grinding media for coal pulverizing
Honey comb ceramics for selective catalyst reduction application in Power plant
High Alumina ceramic products for many stringent applications.
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C
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Research&Development:
CBU’s Research &Development wing along with the Ceramic & Chemical Laboratories is
equipped with various testing facilities like X-ray diffractometers, spectra and flame
photometers, SEM, multichannel dilatometers, UTM etc.
Together with the Ceramic Technological Institute (set up in 1989 with UNDP assistance) and
our R&D center at Hyderabad, research programmes are conducted continually to development
of new products and processes.
Ceramic TechnologicalInstitute (CTI)
CTI was set up by BHEL in 1989 with UNDP assistance, with objective of supporting the Indian
Ceramic Industry in modernizing technology and developing new products and processes in the
emerging field of advanced ceramics. CTI is a division of Corporate Research & Development,
Hyderabad.The Institute has state-of-the-art equipment and a well equipped pilot plant for
meeting its objectives.
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Objectives of CTI include -
To conduct applied research on various aspects of ceramic materials
To upgrade technology from laboratory to pilot plant scale prior to commercial
production
Providing consultancy and engineering services
Disseminate knowledge and experience among Indian industries.
Areas of activities of CTI are:
Beneficiation of raw material
Material testing
Refractories
Ceramic Coatings
Energy saving systems
Electronic ceramics
Wear resistant materials
Technical ceramics
Traditional Ceramics and other frontier/strategic areas.
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OBJECTIVE
Clay plays a major role in the manufacturing of porcelain insulators. This study is to provide an
insight into the nature of the material and its properties by varying the clay content in the raw
materials used for the manufacturing of porcelain insulators. Three different clays of varying
composition were used and their study examines the property such as bulk density, apparent
porosity, water absorption, thermal expansion, hardness, Modulus of Rupture, Modulus of
Elasticity and X-Ray Diffraction was carried out.
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1.INTRODUCTION
CERAMICS:
The most widely accepted definition of Ceramics is given by Kingery et al [17]-“A ceramic is a
non-metallic, inorganic solid”. Thus all inorganic semiconductors are ceramics. By definition
material cases to be a ceramic when it is melted. Ceramics are usually associated with mixed
bonding a combination of covalent, ionic and sometimes metallic.
The word ceramic is derived from the Greek word keramos, which means “potter’s clay” or
“pottery”. Its origin is a Sanskrit term meaning “to burn”. So the early Greeks used “keramos”
when describing products obtained by heating clay- containing materials. The term has long
included all products made from fired clay, eg. Bricks, fireclay refractories, sanitary ware and
tableware.
The properties of ceramics are
Hard
Wear resistant
Brittle
Refractory
Thermal insulator
Electrical insulator
Nonmagnetic
Oxidation resistant
Chemical stable
The ceramics are composed of oxides, carbides, nitrides. Silicides, borides, phosphides,
tellurides and selenides are also used to produce ceramics. Ceramic processing generally
involves high temperatures and the resulting materials are heat resistant or refractory.
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Applications of ceramics
Traditional ceramics: Construction materials, bricks, pipes, pottery, porcelain.
Engineering ceramics: Refractories, high temperature ceramics.
Hardness and wear resistance: Hard surface coating (TiN, WC)
High temperature: valves, cylinder lining, fire blankets, furnace lining.
Electro ceramics: Insulators, semiconductors, metallic, superconductors, ionic
conductors, ferromagnetic, antiferromagnetic, ferrimagnetic, paramagnetic, piezoelectric,
pyroelectric, ferroelectric, sensors, batteries, fuel cells, magnets.
PLASTICITY:
Plasticity is the outstanding property of clay-water systems. It is the property a substance has
when deformed continuously under a finite force. When the force is removed or reduced, the
shape is maintained. Mineralogical composition, particle size distribution, organic substances
and additives can affect the plasticity of clays. Several measuring techniques and devices were
proposed to determine the optical water content in the body required to allow this body to be
plastically deformed by shaping. Despite the advance in the theory of the plasticity and the
methods of evaluating the plasticity of clay-water systems are presented. Despite the advance in
the theory of the plasticity and the methods of measurement, a common procedure for all types
does not exist. The most important methods are those that stimulate the conditions of real
processing.
Plasticity of clay has been ascribed to lamination, to the presence of colloids, to the presence of
organic matter, and to the presence of inorganic salts.
Plasticity in the processing of clay-based materials is a fundamental property since it defines the
technical parameters to convert a ceramic mass into a given shape by application of pressure.
Plasticity, in this case, and particularly in clay mineral systems, is defined as “the property of a
material which allows it to be repeatedly deformed without rupture when acted upon by a force
sufficient to cause deformation and which allows when acted upon by a force sufficient to cause
deformation and which allows it to retain its shape after the applied force has been removed.
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A clay-water system of high plasticity requires more force to deform it and deforms to a greater
extent without cracking than one of low plasticity which deforms more easily and ruptures
sooner.
The plasticity of clays is related to the morphology of the plate-like clay mineral particles that
slide over the others when water is added, which acts as a lubricant. As the water content of clay
is increased, plasticity increases up to a maximum, depending on the nature of the clay. Clay
workers are accustomed to speak of “fat” or highly plastic clay such as ball clay or “lean”,
relatively non-plastic clay such as kaolin, but it is very difficult to express these terms in
measurable quantities. In the industries, plasticity is also referred to as
“extrudability”,”ductilility”.”Workability” or “consistency”
The term “consistency” referring to states of ceramic raw materials, namely dry powder,
granules, plastic body, paste and slip, which are dependent on the liquid content. Fig. 1 presents
the apparent shear resistance as a function of the water content for a typical clayish material.
When water is added to dry clay, the first effect is an increase in cohesion, which tends to reach a
maximum when water has nearly displaced all air from the pores between the particles. The
minimum amount of water necessary to make clay plastic is commonly called the “plastic limit”
(PL). Addition of water into the pores induces the formation of a fairly high yield-strength body
that, however, may crack or rupture readily on deformation.
MODULUS OF ELASTICITY:
An elastic modulus, or modulus of elasticity, is a number that measures an object or substance's
resistance to being deformed elastically (i.e., non-permanently) when a force is applied to it. The
elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic
deformation region. A stiffer material will have a higher elastic modulus. An elastic modulus has
the form
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Where stress is the force causing the deformation divided by the area to which the force is
applied and strainis the ratio of the change in some length parameter caused by the deformation
to the original value of the length parameter. If stress is measured in Pascal, then since strain is a
dimensionless quantity, the units of λ will be Pascal’s as well.
Since the strain equals unity for an object whose length has doubled, the elastic modulus equals
the stress induced in the material by a doubling of length. While this scenario is not generally
realistic because most materials will fail before reaching it, it gives heuristic guidance, because
small fractions of the defining load will operate in exactly the same ratio. Thus, for steel with a
Young's modulus of 30 million psi, a 30 thousand psi load will elongate a 1 inch bar by one
thousandth of an inch; similarly, for metric units, a load of one-thousandth of the modulus (now
measured in gigapascals) will change the length of a one-meter rod by a millimeter.
In a general description, since both stress and strain are described by second-rank tensors,
including both stretch and shear components. The elasticity tensor is a fourth-rank tensor with up
to 21 independent constants.
Specifying how stress and strain are to be measured, including directions, allows for many types
of elastic moduli to be defined. The three primary ones are:
• Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along
an axis when opposing forces are applied along that axis; it is defined as the ratio of
tensile stress to tensile strain. It is often referred to simply as the elastic modulus.
• Shear modulus or modulus of rigidity (G or ) describes an object's tendency to shear (the
deformation of shape at constant volume) when acted upon by opposing forces; it is
defined as shear stress over shear strain. The shear modulus is part of the derivation of
viscosity.
• Bulk modulus (K) describes volumetric elasticity, or the tendency of an object to deform in
all directions when uniformly loaded in all directions; it is defined as volumetric stress
over volumetric strain, and is the inverse of compressibility. The bulk modulus is an
extension of Young's modulus to three dimensions.
Three other elastic moduli are Axial Modulus, Lame’s first parameter, and P-wave modulus.
Homogeneous and isotropic (similar in all directions) materials (solids) have their (linear) elastic
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properties fully described by two elastic moduli, and one may choose any pair. Given a pair of
elastic moduli, all other elastic moduli can be calculated according to formulas in the table below
at the end of page.
In viscid fluids are special in that they cannot support shear stress, meaning that the shear
modulus is always zero. This also implies that Young's modulus is always zero.
In some English texts the here described quantity is called elastic constant, while the inverse
quantity is referred to as elastic modulus.
MODULUS OF RUPTURE:
Modulus of Rupture, also known as,flexural strengthbend strength, or fracture strength,a
mechanical parameter for brittle material, is defined as a material's ability to resist deformation
under load. The transverse bending test is most frequently employed, in which a specimen
having either a circular or rectangular cross-section is bent until fracture or yielding using a three
point flexural test technique. The flexural strength represents the highest stress experienced
within the material at its moment of rupture. It is measured in terms of stress, here given the
symbol .
X-RAY DIFFRACTION:
X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase
identification of a crystalline material and can provide information on unit cell dimensions. The
analyzed material is finely ground, homogenized, and average bulk composition is determined.
Principle:
Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional
diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-
ray diffraction is now a common technique for the study of crystal structures and atomic spacing.
X-ray diffraction is based on constructive interference of monochromatic X-rays and a
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crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce
monochromatic radiation, collimated to concentrate, and directed toward the sample. The
interaction of the incident rays with the sample produces constructive interference (and a
diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the
wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a
crystalline sample. These diffracted X-rays are then detected, processed and counted. By
scanning the sample through a range of 2θangles, all possible diffraction directions of the lattice
should be attained due to the random orientation of the powdered material. Conversion of the
diffraction peaks to d-spacing allows identification of the mineral because each mineral has a set
of unique d-spacing. Typically, this is achieved by comparison of d-spacing with standard
reference patterns.
All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are
directed at the sample, and the diffracted rays are collected. A key component of all diffraction is
the angle between the incident and diffracted rays. Powder and single crystal diffraction vary in
instrumentation beyond this.
Applications:
X-ray powder diffraction is most widely used for the identification of unknown crystalline
materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to
studies in geology, environmental science, material science, engineering and biology.
Other applications include:
• characterization of crystalline materials
• identification of fine-grained minerals such as clays and mixed layer clays that are difficult to
determine optically.
• determination of unit cell dimensions
• measurement of sample purity
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Strengths and Limitations of XRD:
Strengths:-
• Powerful and rapid (< 20 min) technique for identification of an unknown mineral
• In most cases, it provides an unambiguous mineral determination
• Minimal sample preparation is required
• XRD units are widely available
• Data interpretation is relatively straight forward.
Limitations:-
• Homogeneous and single phase material is best for identification of an unknown
• Must have access to a standard reference file of inorganic compounds (d-spacings, hkls)
• Requires tenths of a gram of material which must be ground into a powder
• For mixed materials, detection limit is ~ 2% of sample
• For unit cell determinations, indexing of patterns for non-isometric crystal systems
is complicated.
• Peak overlay may occur and worsens for high angle 'reflections'.
PORCELAIN:
Porcelain is a ceramic material made by heating materials, generally including kaolin, in a kiln
to temperatures between 1,200 and 1,400 °C (2,200 and 2,600 °F). The toughness, strength, and
translucence of porcelain arise mainly from the formation of glass and the mineral mullite within
the fired body at these high temperatures. Porcelain derives its present name from the old Italian
porcellana (cowrie shell) because of its resemblance to the translucent surface of the shell.[1]
Porcelain can informally be referred to as "china" or "fine china" in some English-speaking
countries, as China was the birthplace of porcelain making. Properties associated with porcelain
include low permeability and elasticity, considerable strength, hardness, toughness, whiteness,
translucency and resonance; and a high resistance to chemical attack and thermal shock.
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The term porcelain refers to a wide range of ceramic products that have been baked at high
temperatures to achieve vitreous, or glassy, qualities such as translucence and low porosity.
Among the most familiar porcelain goods are table and decorative china, chemical ware, dental
crowns, and electrical insulators. Usually white or off-white, porcelain comes in both glazed and
unglazed varieties, with bisque, fired at a high temperature, representing the most popular
unglazed variety.
PORCELAIN INSULATOR:
Porcelain has been used as an electrical insulating material for more than 150 years. For a long
time , it has been realized that several characteristic properties of porcelain (e.g. mechanical
strength, high-power dielectric strength and corrosion resistance) as a ceramic product cannot be
obtained in other materials.
Porcelain is distinguished by the lack of open porosity in fired boy. This ceramic is commonly
referred as a triaxial white ware primarily composed of clay, flux, and filler. Potash feldspar or
sodium feldspar is generally used as fluxes and quartz or alumina is used as fillers depending on
their end use.
Temperature, time and atmosphere in the kiln affect chemical reactions and Microstructural
development in the porcelain body.
The reactions occurring throughout the firing process.
The crystal structure of kaolinite contains hydroxyl groups, and the dehydroxylation of these
groups the metakaolin (Al2O3.2SiO2.) occurs at ~550ºC.
Al2O3.2SiO2.2H2O ~550C AL2O3.2SiO2+ 2H2O
The to β quartz inversion occurs at 573ºC. Because of the relatively great flexibility of the
packed particle network. The quartz inversion is of little consequence during the heating cycle.
α Quartz ~575ºC β Quartz
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700-1000ºC
Metakaolin transforms to a spinel type structure and amorphous silica.
3(Al2O3.2SiO2) 950~1000C AL2O3.2SiO2+ 6 SiO2
Mullite Formation
Porcelain generally contains two types of mullite: primary and secondary. The spinel is non-
equilibrium unstable phase, transforms to mullite above 1075ºC.
Spinel ~1075ºC 3Al2O3.2SiO2+4SiO2
The chemical homogeneity is necessary for the formation of mullite. Actually the formation of
mullite takes place at 980ºC, but because of non homogeneity the mullite formation is delayed to
temperatures as 1300ºC.
At ~1200ºC, the melt will be saturated with silica-quartz dissolution ends and transformation of
quartz to cristobalite begins.
Above 1200ºC, mullite crystals grow to prismatic crystals into the remains of the feldspar grains.
The primary mullite forms near the feldspar. Secondary mullite formation near to the kaolin side.
On cooling, below glass transition temperature Tg, residual stresses are developed because of
thermal expansion mismatch between the glass and the included crystalline phases (i.e., mullite
and quartz and in some cases alumina and cristobalite).
At 573ºC, the conversion from β quartz to α quartz and results in the quartz particle reduction of
2%. The strain is developed and results cracks in the glassy matrix and the quartz grains.
Finally, the β to α cristobalite conversion at 225ºC~250ºC and produces a larger volumetric
change ~5% with a higher activation energy barrier, the transformation is less severe than that of
quartz.
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Raw Materials:
The primary components of porcelain are clays, feldspar or flint, and silica, all characterized by
small particle size.
To make porcelain insulator, the raw materials—such as clay, feldspar, silica, mullite, alumina—
are first crushed using jaw crushers, hammer mills, and ball mills. After cleaning to remove
improperly sized materials, the mixture is subjected to one of four forming processes—soft
plastic forming, stiff plastic forming, pressing, or extruding—depending on the type of ware
being produced. The ware then undergoes a preliminary firing step, bisque-firingis treated, all
clays vitrify (develop glassy qualities), only at extremely high temperatures unless they are
mixed with materials whose vitrification threshold is lower. Unlike glass, however, clay is
refractory, meaning that it holds its shape when it is heated. In effect, porcelain combines glass's
low porosity with clay's ability to retain its shape when heated, making it both easy to form and
ideal for domestic use. The principal clays used to make porcelain are china clay and ball clay,
which consist mostly of kaolinate, a hydrous aluminum silicate.
Feldspar, a mineral comprising mostly aluminum silicate, and flint, a type of hard quartz,
function as fluxes in the porcelain body or mixture. Fluxes reduce the temperature at which
liquid glass forms during firing to between 1,835 and 2,375 degrees Fahrenheit (1,000 and 1,300
degrees Celsius). This liquid phase binds the grains of the body together.
Silica is a compound of oxygen and silicon, the two most abundant elements in the earth's crust.
Its resemblance to glass is visible in quartz (its crystalline form), opal (its amorphous form), and
sand (its impure form). Silica is the most common filler used to facilitate forming and firing of
the body, as well as to improve the properties of the finished product. Porcelain may also contain
alumina, a compound of aluminum and oxygen, or low-alkali containing bodies, such as steatite,
better known as soapstone.
Mullite is a solid solution phase of alumina and silica commonly found in ceramics. It rarely
occurs as a natural mineral. Being the only stable intermediate phase in the Al2O3-SiO2 system at
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atmospheric pressure, mullite is one of the most important ceramic materials. Mullite has been
fabricated into transparent, translucent and opaque bulk forms. These materials may have optical
and electronic device applications. Mullite’s temperature stability and refractory nature are
superior to corundum in certain high temperature structural applications. Another characteristic
of this alumina silicate is its temperature stable defect structure, which may indicate a potential
use in fuel cell electrolytes. It has good chemical stability, creep resistant. It is used in refractory
application because of low thermal conductivity and low thermal expansion. The volume thermal
expansion decreases with alumina content and the anisotropy of thermal expansion is reduced
simultaneously. Mullite is in the form of needles in porcelain.If the needle shaped mullite formed
during sintering, it has an effect on both the mechanical and physical properties by increasing the
mechanical strength and thermal shock resistance.
Alumina is the most abundant mineral of the earth crust apart from silicates. Alumina is one of
the most versatile engineering ceramic materials exploited for a wide range of applications. It
occurs in nature as corundum and as ruby, sapphire and several other gemstones and as an
important constituent of bauxite, which is mined and refined to produce a purified, calcined
alumina in the form of a finite white powder. Aluminium oxide exists in many forms , , , ,
, , , ; these arise during the heat treatment of aluminium hydroxide. The phase
transformation path followed by various alumina precursors before they transform to stable α
phase.
The alumina phases are γ, θ, and α phase which represent the crystalline phases that are
important in the alumina structure. For instance, γ, θ, andδ alumina called transition alumina are
metastable forms of aluminium oxide while α-alumina is the most stable Al2O3 polymorph and is
usually crystallized at high temperatures. The α-phase is stable at high temperature (i.e. melting
temperature for this phase is 2051ºC) but the other phases do not always exist in the alumina.
The polycrystalline alumina α-phase is formed at high temperature of about 1000ºC. The
properties for the α-alumina are not the same for other phases, which gives the alumina phases a
wide range for applications. The α-alumina has a hexagonal structure but the γ-alumina has a
spinal structure.
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2. LITERATURE SURVEY
2.1 PROCESSING OF PORCELAIN INSULATORS:
RAW MATERIALS:-
The different raw materials used for the manufacturing of porcelain insulators are quartz, clay,
feldspar etc.
MIXING:-
The raw materials are selected and the desired amounts weighed, they go through a series of
preparation steps. The ingredients are passed through a series of screens to remove any under or
over sized materials. Screens, usually operated in a sloped position are vibrated
electromechanically or mechanically to improve flow. If the body is to be formed wet, the
ingredients are then combined to produce the desired consistency.
BALL MILLING:-
Ball milling is the process by which materials are reduced from a large size to smaller size.
Milling involves breaking up of raw material pulverization (which involves grinding the particles
themselves to a smaller size). Milling is generally done by mechanical means, including attrition
(which is particle-to-particle collision that results in agglomerate break up or particle sharing),
compression (which applies a forces that results in fracturing), and impact (which employs a
milling medium or the particles themselves to cause fracturing).
The raw materials are mixed together which include clay, quartz and flux. The slip is then
feed by hoses and milled. Pebbles are also added to the slip to crush the quartz and other particle
to a smaller size. It is then stored in tanks.
PUG MILLING & EXTRUSION:-
The pug mill is a fast continuous mixer. In the pug mill homogeneous mixing of cakes takes
place. The vacuum system is fitted that ensures the extruded clay bodies have no entrapped air.
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Mixing materials at optimum moisture content requires the forces mixing action of the pug mill
paddles. The material comes in the form of cylinder blanks.
Fig.2.2 Pug mill with de-airing chamber and Auger extrusion
CHAIN DRYING:-
The components which are shaped by extrusion are then partially dried in chain drier where the
steam is used for drying. The shaped Insulators are dried at 30-35ºC for about 3 hours to
facilitate separation of Insulator from the mould easily. The Insulators which are dried are kept
for one day, and then it is moved to turning operation.
TUNNEL DRYING:-
Prior to firing, ceramics are heat treated at temperatures below the firing temperatures. The
parameters which affect drying are soaking time, temperature zone, humidity, source of heat.
The purpose of this thermal processing is to provide additional drying, to vaporize or decompose
organic additives and other impurities, and to remove residual crystalline and chemically bound
water. Pre-sinter thermal processing can be applied as a separate step, which is referred to as
bisque firing or by gradually raising and holding the temperature in several stages. The
temperatures in this zone are increased in steps from room temperature to 120ºC and reducing to
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100ºC at the exit. The insulators are kept in the tunnel for around 38-40 hours. At the end of the
tunnel drying the moisture of the Insulator should be less than 1% before firing stage.
GLAZING:-
After the model drying is completed, the insulators are subjected to glazing. Glazing is a process
to give required colour to the insulators like red, brown, black and white. Glazing gives a coating
of 0.3mm to 0.5mm of thickness. Glazing protects them from contamination, improved
mechanical, electrical stable characteristics and provide better appearance.
Insulators are glazed in Automatic Glazing Machines (AGM). Specified specific gravity,
viscosity and thickness are maintained to enhance the glaze impact on the insulator.
Grog (crushed ceramic insulators) is put on the pin portion and head portion of glazed insulator
to facilitate better grip while cementing the metal parts and improves mechanical properties of
the Insulators.
Stamping Identification made on the insulators based on customer requirements and standards.
Glazes generally are applied by spraying or dipping. Depending on their constituents, glazes
mature at temperatures of 600ºC to 1500ºC (1110ºF to 2730ºF).
Advantages of Glazing:
Good Looking.
Additional Strength.
Clustering surface.
Smooth surface and long life.
FIRING:-
Glazed Insulators are loaded to wagons and fired at temperature at 1250ºC in Gas Fired Tunnel
Kilns. The firing work is done in the kiln. To spread the heat uniformly blowers are installed in
the kilns. Firing is the process by which ceramics are thermally consolidated into a dense,
cohesive body comprised of fine, uniform grains. This process also is referred to as sintering or
densification. In general:
(1) Ceramics with fine particle size fire quickly and require lower firing temperatures.
(2) Dense unfired ceramics fire quickly and remain dense after firing with lower shrinkage.
27. Page 27 of 48
(3) Irregular shaped ceramics fire quickly.
Other material properties that affect firing include material surface energy, diffusion coefficients,
fluid viscosity and bond strength.
Parameters that affect firing include firing temperature, time, pressure and atmosphere. A
short firing time results in a product that is porous and has a low density; a short to intermediate
firing time results in fine-grained (i.e., having particles not larger than 0.2 millimetres), high
strength products; and long firing times result in a coarse-grained products that are more creep
resistant. Applying pressure decreases firing time and makes it possible to fire materials that are
difficult to fire using conventional methods. Oxidizing or inert atmospheres are used to fire oxide
ceramics to avoid reducing transition metals and degrading the finish of the product.
In addition to conventional firing, other methods used include pressure firing, hot
forging, plasma firing, microwave firing and infrared firing. Conventional firing is accomplished
by heating the green ceramic to approximately two-thirds of the melting point of the material at
ambient pressure and holding it for a specified time in a periodic or tunnel kiln. Periodic kilns are
heated and cooled according to prescribed schedules. The heat for periodic kilns generally is
provided by electrical element or by firing with gas or oil.
Tunnel kilns generally have separate zones for preheating, firing and cooling. The kilns
may be designed so that
(1) The air heated in the cooling zone moves into the firing zone and the combustion gases in the
firing zone is conveyed to the preheat/drying zone then exhausted, or
(2) The air heated in the cooling zone is conveyed to the preheat/drying zone and firing zone
gases are exhausted separately. The most commonly used tunnel kiln design is the roller hearth
(roller) kiln. In conventional firing, tunnel kilns generally are fired with gas, oil, coal or wood.
Following firing and cooling, ceramics are sometimes re-fired after the applications of paint,
decals or ink.
Advanced ceramics often are fired in electric resistance heated furnaces with controlled
atmospheres. For some products, separate furnaces may be needed to eliminate organic
lubricants and binders prior to firing. Ceramic products also are manufactured by pressure firing,
which is similar to the forming process of dry pressing except that the pressing is conducted at
the firing temperature. Because of its higher costs, pressure firing is usually reserved for
manufacturing ceramics that are difficult to fire to high density by conventional firing.
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INSPECTIONS:-
After firing 100% inspection is carried to identify manufacturing defects, glazing defects,
loading defects and kiln defects for analysis and to initiative corrective actions at previous
stages.
ASSEMBLY:-
The metal parts, cup and pin are joined here. To join the metal parts there is a special type of
cement known as alkreet cement is used. After assembling, the insulator goes for curing (drying)
operation. The different kinds of curing are steam curing (2.5 hours), hot water curing (3 days)
and air curing (1 day).
2.2 VARIATION OF CLAY:
Factors which cause variationsin the finished sizeofclay products are determined.
These are classified intheorderoftheirimportance, and limits are established withinwhich the
factors must be controlled to reduce these variations to a point whichisconsistentwiththe commercial
productionofmodular products. Thefactors found tohavethegreatest effectonthefinished size are
soaking temperatureandsoaking time. Those factors havinga lesser effect are
(1) Firingatmosphere,
(2) Relative amountsofclay mineral and quartz presentinthe clayinthedifferent parts of the pit,
(3) Grainsize of theclayproducedbygrindingor weathering, and
(4)De-airing treatment.
A review of literature shows that very little information is available regarding the variation of
clay in the manufacturing of porcelain insulators. Hence there is plenty of scope to carry out
studies relating to it.
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3. EXPERIMENTALDETAILS
3.1 PREPARATION OF TEST SAMPLES:
Four batches were prepared with each batch containing different raw material compositions
along with calculated amount of water. All the four batches were allowed for ball milling for 19
hours.The slip obtained is poured into the plaster moulds for removal of excess water and it is
allowed to make dough suitable for extrusion. It will be dried for one day.
The body mass will be in the form of dough isrequired for extrusion. It isprepared is extruded
into rods of diameter 12.5mm. The samples were extruded and cut to a length that fits into the
wooden stand. Around 10~15 rods were done in each sample.
The samples then dried in the electric drier in the temperatures from room temperature to 110ºC
to reduce the moisture to less than 1%. Then the samples were sintered in a gas tunnel kiln
(GTK) at a temperature of 1250-1280ºC.
3.2 DENSITY, POROSITY AND WATER ABSORPTION:-
Density, porosity and volume types are described here. Bulk (or Apparent) Density (BD) is mass
divided by Bulk Volume. Bulk Volume is the volume of solid and of open and closed pores.
Apparent Porosity (% AP) is open pore volume as a percentage of Bulk Volume. Apparent Solid
Density (or ASG) is mass divided by Apparent Solid Volume. Apparent Solid Volume is the
volume of solid and of closed porosity Real Density (or SG) is mass divided by Solid Volume only
Total Porosity (% TP) is total pore volume as a percentage of Bulk Volume
Archimedes Principle states that the buoyant force on a submerged object is equal to the
weight of the fluid that is displaced by the object.Density and porosity (AP and BD ) can be
measured of ceramic materials using the Archimedes buoyancy technique with dry weights,
soaked weights and immersed weights in water.The Apparent Porosity and Bulk Density are
calculated from the Dry, Soaked and Suspended weights as follows:
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• % AP = (Soaked Weight - Dry Weight) x 100 / (Soaked Weight – Suspended Weight)
• BD = Dry Weight / (Soaked Weight – Suspended Weight)
The method of measuring density of a ceramic material is usually based on the Archimedes
principle, the procedure is given below:
-The specimens were dried in oven at 110ºC and cooled it to room temperature and dry weight
w1 is noted.
-The specimens were boiled in distilled water for a period of 45 minutes.
-The specimen was suspended in water and its weight w2 is noted.
-The saturated weight of the specimen w3 was taken after wiping the adhering water by a damp
cloth.
The density, porosity were calculated using the below formula
Bulk Density (BD) = W1
W3-W2
True Density (TD) = W1
W1-W2
% Apparent porosity = (1-BD) * 100
TD
% Water Absorption = W2-W1 * 100
W1
Where W1= dry weight in grams
W2 = suspended weight in grams
W3 = saturated weight in grams.
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3.3 MOE:-
Modulus of Elasticity (MOE) is a function of the bond strength of a material and hence it is
considered as a material property. This test is of prime importance in assessing the spalling
resistance of the samples. MOE rods were measured by using resonance frequency method.
Yong’s Modulus was evaluated by Dynamic Elastic Properties Analyzer (DEPA) is based on an
impulse excitation technique (ASTM C1259). The rods were placed on the platform containing
sensors and were tapped by a metal rod and it displays a test specimen’s complete frequency
signature. The system uses Fast Fourier Transform for highly accurate and repeatable measuring.
Then by substituting the dimensions and weight of the rods, MOE was calculated using a
software program.
The formula used to calculate MOE is
E= 1.6408 (L3 /D4) T1Wf2 *10-6
Where E= Young’s Modulus, kgf/cm2
W= Weight of the rod, gm
L= Length of the rod, cm
f = Resonant frequency of rod, Hz
T1= Correction factor for fundamental flexural model to account for finite thickness of
Bar, Poisson’s ratio
3.4 THERMAL EXPANSION:-
To determine the thermal expansion the Orton dilatometer is used. Dilatometers are used to
measure the dimensional change of a material (ceramics, glass, metals, composites, minerals,
polymers etc.) as a function of temperature.
The instrument is of horizontal type. The main advantage of the horizontal system is the uniform
temperature zone for the sample. The sample is positioned between the end of the sample holder
and the probe rod. After positioning the sample in the sample holder, the furnace is moved
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horizontally to cover the sample and sample holder. The probe rod extends from the end of the
sample, through the sample tube and connects to the displacement sensor assembly outside the
furnace. The probe rod is under load outside the furnace to keep constant with the sample, even
when shrinking.
Computer Analysis
Every Orton dilatometer is supplied with the application software to be installed on the end
user’s PC in order to acquire, save and analyze the data generated. The Orton Dilatometer
Software is written for Windows based personal computers. The software allows the dilatometer
test to be remotely programmed and started. It can be used to monitor the test in real time, or can
be used to examine the data after the test cycle. The software imports the data through the RS232
or USB interface and stores it on the hard drive for post-testing analysis.
The coefficient of thermal expansion calculated using the formula
α = (% TE)*100
(T-T0)
Where T = Testing temperature in °C
T = Room Temperature °C
3.5MODULUS OF RUPTURE (MOR):-
The strength of the ceramic material is defined as the ability of a material to retain a shape when
subjected to various forces.
The instrument used for conducting this bending test was universal testing machine used for both
tensile and compressive strength measurement. The test specimen is supported at the ends and
load is applied at the centre (3 point loading) the bend strength is defined as the (maximum
tensile stress at failure and is referred as modulus of rupture).
The three point bending strength measurements using the Lloyd (UTM).
Sample Preparation: Test samples are prepared in sizes according to the standards-IEC 60672-
1. Sintered cylindrical bars of dimension 10 mm dia and 120 mm length are used. The samples
should have a flattened edge and smooth surface.
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Procedure:
MOR of the samples were measured using the formula
M= 8PL
Πd3
Where M= MOR of the samples (MPa)
P= Load at rupture (N)
d= Diameter of the rod (mm)
L= Span length (mm)
3.6 HARDNESS:-
Hardness can be defined as the ability of a resist permanent indentation or deformation when in
contact with an indenter under load. The contact pressure involved in deforming the test surface
which is shown directly or indirectly on one of the type of scale quantifies the hardness of the
material. A hardness test consists of pressing an indenter of known geometry and mechanical
properties in to the test material. The indenter may be spherical (Brinell Test), pyramidal
(Vickers and Knoop Test) or conical (Rockwell Test).
Vickers Hardness Testing:
The Vickers test method is similar to the Brinell principle in that a defined shaped indenter is
pressed into a material, the indenting force is removed, the resulting indentation diagonals are
measured, and the hardness number is calculated by dividing the force by the surface area of the
indentation. Vickers testing is divided into two distinct types of hardness tests: macro indentation
and micro indentation tests. These two types of tests are defined ny the forces. Micro indentation
Vickers (ASTM E 384) is from 1 to 1000gf.
Vickers Test Method:
The Vickers test is performed with different forces and indenters. The square-base pyramidal
diamond indenter is forced under a predetermined load ranging from 1 to 120kgf into the
material to be tested. After the forces have reached a static or equilibrium condition and further
34. Page 34 of 48
penetration ceases, the force remains applied for a specific time (10 to 15s for normal test times)
and is then removed. The resulting unrecovered indentation diagonals are measured and
averaged to give a value in millimetres. These length measurements are used to calculate the
Vickers hardness number (Hv).
The Vickers hardness number (formerly known as DPH for diamond pyramid hardness) is a
number related to the applied force and the surface area of the measured unrecovered indentation
produced by a square-base pyramidal diamond indenter. The Vickers indenter has included face
angles of 136º, and the Vickers hardness number (Hv) is computed from the following equation:
Hv= 2Psin (136/2)/d2 = 1.8544P/d2
Where P= Indentation load in kgf
d= Mean diagonal of the indentation in mm
Hv= Vickers hardness number
3.5 X-RAY DIFFRACTION (XRD) ANALYSIS:
Sample Preparation:
Sintered body was cleaned and fine powdered using a cleaned mortar and pistil. Around 10-15
gm of sample was taken for quantitative XRD.
X ray diffraction shows quantitatively how an X-ray technique can reveal the information
concerning the structure and properties of crystalline materials.
In XRD the diffracted beam intensity is monitored electronically by a mechanical driven
scanning radiation detector and the patterns are plotted against intensity of peaks versus 2θ. XRD
pattern depicts the relationship of intensity of peaks versus 2θ angle. Initially 2θ angles
corresponding to all the peaks were measured. The values obtained are compared with standard
ICPDS file for the material.
35. Page 35 of 48
4. RESULTS AND DISCUSSIONS
1. BULK DENSITY, APPARENT POROSITY AND WATER ABSORPTION:-
The bulk density, porosity and water absorption are measured after sintering and the values are
given:
2. HARDNESS:
Hardness was measured by Vickers Hardness Tester. The Vickers hardness Hv of the
combination was measured.
Sample Vickers Hardness
BP 3 709±10
BP 4 678±10
SAMPLE BULK DENSITY (g/cc) APPARENT POROSITY (%) WATER ABSORPTION (%)
BP 1 2.57 0.3 0.115
BP 2 2.63 0.55 0.210
BP 3 2.56 0.38 0.149
BP 4 2.56 0.31 0.495
36. Page 36 of 48
3. THERMAL EXPANSION:
The thermal expansion test was conducted for all the samples and the following results are
obtained.
BP 3
38. Page 38 of 48
4. GREEN MODULUS OF ELASTICITY: -
Young’s Modulus was evaluated by Dynamic Elastic Properties Analyzer (DEPA) is based on an
impulse excitation technique for the green sample.
The mean resonance frequency and Modulus of Elasticity of the green samples are shown below
Sample
Mean
Resonance
Frequency (Hz)
Youngs
Modulus
(GPa)
BP 11 692.426 4.161
BP 21 745.71 4.761
BP 31 775.702 5.105
BP 41 727.696 4.649
5. MODULUS OF ELASTICITY: -
Young’s Modulus was evaluated by Dynamic Elastic Properties Analyzer (DEPA) is based on an
impulse excitation technique for the fired sample. The mean resonance frequency and Modulus
of Elasticity of the green samples are shown below
Sample
Mean
Resonance
Frequency (Hz)
Young’s
Modulus
(GPa)
BP 13 2942.008 103.007
BP 21 3079.564 100.491
BP 34 2945.644 100.694
BP 48 2914.464 103.227
39. Page 39 of 48
6. MODULUS OF RUPTURE: -
The three point bend strength of the samples have been given below
45. Page 45 of 48
5. CONCLUSIONS
The present work deals with the variation of clay content in the manufacturing of porcelain
insulators. Here, three types of clay of varying composition were used. The insulator rods were
prepared by extrusion. The fired rods were tested for its flexural strength, modulus of elasticity,
hardness and thermal expansion. Based on the experimental results and associated discussions,
BP 3 and BP 4 gave better results in most properties
46. Page 46 of 48
6. SCOPE FOR FUTURE WORK
The samples can be glazed and perform various test including MOE, MOR, Vickers Hardness.
The samples can be tested for dielectric strength.
47. Page 47 of 48
REFERENCES
1. William M. Cartyand Udayan Senapati “Porcelain – Raw Materials, Processing, Phase
evolution and Mechanical Behaviour”.(Journal of American Ceramic Society)Volume
81, Issue 1, pages 3–20, January 1998.
2. Jose M. Amigo, Francisco J. Serrano, Marek A. Kojdeck, Joaquın Bastida,VicenteEsteve,
Maria Mercedes Reventos, Francisco Marti “X-ray diffraction microstructure analysis of
mullite, quartz and corundum in porcelain insulators” Journal of the European Ceramic
Society 25 (2005) 1479–1486.
3. James S Reed, “Introduction to the Principles of ceramic Processing”,1986.
4. William D. Callister, “Fundamentals of Materials science and Engineering”.
5. W E Worrall, “Clay and Ceramics Raw Materials”, second Edition 1986.
6. W D Kingery, H K Bowen and D R Uhlmann, “Introduction to Ceramics”, Second
Edition.
7. Felix Singer and Sonja S Singer, “Industrial Ceramics” 1963.
8. H. Schneider, “Thermal Expansion of Mullite,” J. Am. Ceram. Soc. 73 [7] 2073–6
(1990).
9. www.wikepedia.com.
