Differential Thermal Analysis(pdf)

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Differential Thermal Analysis(DTA)
By- Mayuri M. More
P. E. Society’s Modern College of Pharmacy(For Ladies), Moshi,pune 412105,
Savitribai Phule University, Pune, Maharashtra, India.
Authors Affiliation
Department of QAT P. E. Society’s Modern college of Pharmacy(For Ladies), Moshi, Pune
412105,(M.S.), India.
Corresponding Author E-mail ID: mayurimore2000@gmail.com

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Abstract:
Differential thermal analysis (DTA) in which a material under investigation is typically
subjected to a programmed temperature change and thermal effects in the material are
observed. (Isothermal methods are also possible though they are less common.) The term
“differential” indicates that the difference in behavior between the material under study and a
supposedly inert reference material is examined. In this manner the temperature at which any
event either absorbs or releases heat can be found. This allows the determination of, e.g., phase
transition temperatures and the study of order‐ disorder transitions and chemical reactions.
Similarly, heat capacity measurements can be performed. DTA this methods is ideally suited
for quality control, stability, and safety studies. Measurement of heat capacity can be performed
by other methods.
In DTA, the temperature difference between a substance and a reference material is measured
as a function of temperature while the substance and reference material are subjected to a
controlled temperature program.The record is the differential thermal, or DTA, curve; the
temperature difference should be plotted on the ordinate with endothermic reactions downward
and temperature or time on the abscissa increasing from left to right.
The term “quantitative differential thermal analysis” (quantitative DTA) covers those uses of
DTA where the equipment is designed to produce quantitative results in terms of energy and/or
any other physical parameter.
Keywords: Thermal analysis, Differential thermal analysis

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Contents:
 Introduction
 Basics of DTA
 Principle
 Instrumentation
 Methodology
 DTA Curve
 Factors affecting DTA curve
 Advantages
 Disadvantages
 Applications
 Conclusion

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Introduction:
DIFFERENTIAL THERMAL ANALYSIS (DTA)
Thermal analysis is the analysis of a change in a property of a sample, which is related to an
imposed change in the temperature. The sample is usually in the solid-state and the changes
that occur on heating include melting, phase transition, sublimation, and decomposition.
In practice thermal analysis gives properties like; enthalpy, thermal capacity, mass changes,
and the coefficient of heat expansion. Solid-state chemistry uses thermal analysis for studying
reactions in the solid-state, thermal degradation reactions, phase transitions, and phase
diagrams
• Thermal analysis comprises a group of techniques where the properties of a material are
studied as they change with temperature
• To determine the thermo-physical properties several methods are commonly used:
 Differential thermal analysis (DTA),
 Differential scanning calorimetry (DSC),
 Thermogravimetric analysis (TGA),
 Dilatometry (DIL),
 Evolved gas analysis (EGA),
 Dynamic mechanical analysis (DMA),
 Dielectric analysis (DEA) etc.
History:
o Le Chatelier’s (1887) described a new technique for the study of clay and minerals by
the of their temperature-time curves.
o Robert-Austen(1899) improved the technique by introducing two thermocouples.
What is DTA?
Involves the technique of recording the difference in temperature between the Test and
Reference material time being constant for both.

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DTA, Basics:
The material under study and an inert reference are made to undergo identical thermal cycles.
Any temperature difference between sample and reference is recorded. In this technique the
heat flow to the sample and reference remain the same rather than the temperature.
The differential temperature is then plotted against time, or temperature (DTA curve or
thermogram).
Definition:
Differential thermal analysis is a technique in which the temperature difference between the
sample and a thermally inert reference substance is continuously recorded as a function of
temperature or time.
The most widely used thermal method of analysis is Differential thermal analysis (DTA). In
DTA, the temperature of a sample is compared with that of inert reference material during a
programmed change of temperature.
The temperature should be the same until the thermal event occurs, such as melting,
decomposition, or change in the crystal structure. In an endothermic event takes place within
the sample, the temperature of the sample will lag behind that of the reference and a minimum
will be observed on the curve.
On the contrary, if an exothermal event takes place, then the temperature of the sample will
exceed that of the reference and a maximum will be observed on the curve. The area under the
endotherm or exothermic is related to the enthalpy of the thermal event, ΔH.
Differential thermal analysis is also known as thermography. The measurement of change in
heat content is carried out by heating the two materials at elevated temperature or cooling to
the subnormal temperature at a predetermined rate.

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DTA modes can be used to determine the following:-
• Melting points
• Glass transition temperatures
• Crystallinity
• Moisture/ volatile content
• Thermal and oxidative stability
• Purity
• Transformation temperatures
PRINCIPLE:
• It involves the technique of recording the temperature difference (ΔT) between the test sample
and an inert reference sample under controlled and identical conditions of heating or cooling
is recorded continuously as a function of temperature or time, thus the heat absorbed or emitted
by a chemical system is determined.
• If any reaction takes place in the sample, then the temperature difference will occur between
the sample and the reference material.
• In an endothermic change (such as melting or dehydration of the sample) the temperature of
the sample is lower than that of the reference material (i.e) ΔT = ‒ ve (for the endothermic
process)
• In an exothermic change or process the sample temperature is higher than that of the reference
material. (i.e) ΔT = + ve (exothermic process).
• The difference in temperature ΔT between the sample temperature and the reference
temperature T, (ΔT = Ts - Tr) is then monitored and plotted against sample temperature to give
a differential thermogram
.
• A DTA curve can be used as a fingerprint for identification purposes.
• The shape and the size of the peak give information about the nature of the test sample.
• Sharp endothermic peaks indicate phase changes (such as melting, fusion, etc.) transition
from one crystalline form to another crystalline form.
• Broad endothermic peaks are obtained from dehydration reactions.
• Chemical reactions like oxidative reactions are exothermic reactions.

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Fig. 1. Exothermic Vs Endothermic
Fig. 2. DTA Curve

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 The thermal effects may be either endothermic or exothermic.
 Enthalpic changes are caused by physical phenomena such as fusion, crystalline
structure inversion, boiling, vaporization, sublimation, and others.
 Some enthalpic changes are also caused by chemical reactions like decomposition,
oxidation, dehydration, reduction, compensation, displacement, etc.
Fig. 3. Thermal Effect
 The endo or exothermic bands and peaks appearing on the thermogram give
information regarding the detection of enthalpic changes.

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INSTRUMENTATION:
Fig. 4. DTA Instrument

pg. 10
Fig. 5. DTA Instrument
Fig. 6. DTA Instrumentation block diagram
A differential thermal analyzer is composed of five basic components, namely :
1} Sample holder
2} Furnance
3} Temperature controller and recorder
4} Thermocouple
5} Cooling device

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Sample holder:
• This is used to contain the sample as well as reference material.
Fig. 7. Sample Holder
• Material:
Criteria for selecting material are,
o Cost, ease of fabrication, and inertness towards the reactants and products in the
temperature range of interest.
o Metallic - aluminum, nickel, stainless steel, platinum ( generally employed), and its
alloys.
o Gives sharp exotherms and flat endotherms.
o Non-metallic - glass, ceramic, vitreous silica, or sintered alumina.
o Gives flat exotherms and sharp endotherms.
o Geometry – cylindrical geometry is used.
Types of sample holders:
• Sample holders with dimples in which thermocouples junctions are inserted. The dimples are
called thermocouple wells.
• It has better contact between the sample holder and the thermocouple junction.
• Specimen holder assemblies are used in which there are identical cups supported on
thermocouple spaghettis as well as metallic or non-metallic blocks with wells.

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• Sample or reference are packed in respective.
• Blocks with symmetrically located multiple compartments have been designed.
• To investigate several samples simultaneously.
Furnace:
• This is a device for heating the sample.
• It is an oven enclosed in the furnace.
• In the DTA apparatus, a tubular furnace is preferred for its good temperature regulation and
programming.
• There are inexpensive.
Temperature controller and recorder:
• Temperature controller:
• Three basic elements required to control temperature are- sensor, a control element, and a
heater.
Two methods:
1. On-off control: if the sensor signal indicates that the temp has become greater than the
setpoint, the heater is turned off.
• Not widely used in DTA
• Inexpensive
2. Proportional control: heat input to the system is progressively reduced as the
temperature approaches the desired value.
• These are widely used in DTA instruments.
Temperature programming:
• A time-dependent temperature cycling of furnace is required to produce the desired rate of
heating or cooling and to maintain the temperature at any desired value.
• A temperature programmer is employed which transmits certain time-based instructions to
the control unit.

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Recorder:
• The signals obtained from the sensors are recorded by the recorder and record the DTA curve.
Two types:
a) Deflection type: the recording pen is moved directly by the input signal.
b) Null type: the input signal is compared with a reference or standard signal and the
difference is amplified and used to adjust the reference signal through a server motor
until it matches the input signal.
Thermocouples :
• Thermocouple is an electrical device consisting of two dissimilar electrical conductors
forming electrical junctions at different temperatures.
Fig. 8. Thermocouple

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Points to be considered while selecting a thermocouple:
• Thermoelectric coefficient.
• Temperature interval.
• Chemical compatibility with the sample.
• Chemical gaseous environment used and reproducibility of the EMF vs. temperature curve as
a function of thermal cycling.
• Availability and cost of Thermocouples.
• Made of chromel P and alumel wires measure and control temp up to 1100°C in air.
• Made from pure platinum and platinum-rhodium alloys wires measure more than 1100°C.
• Made from refractory materials like tungsten and rhenium in inert gas or vacuum for up to
2100°C.
Working of Thermocouple:
Fig. 9. Working of Thermocouple
Cooling device:
• It’s function is to maintain a suitable atmosphere in the furnace and sample holder.

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Differential temperature sensor:
• (To measure the temperature difference between the sample and reference material) the
sample and reference holder are kept inside the furnace and the temperature of the furnace and
sample holder is controlled by using the furnace controller.
• Heart of the analysis – heating block identical pair of cavities for the sample, reference
material.
• Whole unit is set in an oven- control pressure.
• Thermocouple is placed directly in contact with the sample and another in contact with the
reference.
• Temperature of the block is raised, the temperature of the sample & reference follow Zero
temperature difference – no physical or chemical change.
• If any reaction – the difference in ΔT.
DTA ANALYZER:
Fig. 10. DTA Analyzer
METHODOLOGY:
o Insert a very thin thermocouple into a disposable sample tube 2 mm in diameter and
containing 0.1- 10 mg of sample,

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o Another identical tube is either kept empty or filled with a reference substance, such as
quartz, sand, alumina.
o The two tubes are simultaneously inserted into the sample block and subsequently
heated (or cooled) at a uniform predetermined programmed rate.
REQUIREMENTS:
 Keys for successful experimental practice,
o Raw materials should be of high purity.
o The fine-grained powder should be used to achieve a greater contact area and better
equilibrium conditions.
o The time at any temperature must be sufficiently long to permit the completeness of
reactions.
Factors affecting the heat transfer, Tau lag & signaling are,
1. Crucible
 Material
 Mass
 Volume
 Heat
 Capacity
2. Sample
 Mass
 Heat capacity
 Heat conductivity
3. Atmosphere
Phase diagrams & Thermal analysis:
 Phase Diagram:
o A phase diagram show conditions at which thermodynamically distinct phases can
occur at equilibrium.

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o It is determined experimentally by recording cooling rates over a range of compositions.
o Phase transitions occur along lines of equilibrium (=phase boundaries).
o Solidus = Temp. below which the substance is stable in the solid-state.
o Liquidus = Temp. above which the substance is stable in a liquid state.
Fig. 11. Phase diagram
Experimental methods for determining phase diagrams:
a) Thermal analysis
b) High-temperature microscopy
c) High-temperature X-ray diffraction
d) Measurement of electrical conductivity as a function of temperature.
Salt mixtures: solid salts have low conductivity, melts have high.
Constructing phase diagrams by experimental methods:
From DTA curves:

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 DTA—A few of the vital aspects are :
o Pre-treatment of the specimen,
o Particle size and packing of the specimen,
o Dilution of the specimen,
o Nature of the inert diluent,
o Crystalline substances must be powdered,
o and sieved through the 100-mesh sieve.
o Either to suppress an unwanted reaction (e.g., oxidation) or to explore the study of a
reaction(e.g., gaseous reaction product) the atmosphere should be controlled
adequately.
Reference materials are used in DTA:
The ideal reference material is a substance with the same thermal mass as the sample, but with
no thermal events over the temperature range of interest.
In DTA is usually used Reference Materials are as follows,
 Alumina (Al2O3),
 Carborundum(SiC) or
 Magnesium oxide(MgO) powder as the reference material for the analysis of inorganic
compounds
DTA; Phenomena causing changes in heat/temperature:
1. Physical:
Adsorption (exothermic) Desorption (endothermic) A change in crystal structure (endo
– or exothermic) Crystallization (exothermic) Melting (endothermic) Vaporization
(endothermic) Sublimation (endothermic).
2. Chemical:
Oxidation (exothermic) Reduction (endothermic) Break down reactions (endo – or
exothermic) Chemisorption (exothermic) Solid-state reactions (endo – or exothermic).
Evaluation and interpretation of DTA curves:
Typical data obtained from DTA peak evaluation,
• Onset - melting
• Endset • Integral - enthalpy ∆ h
• Peak temp - melting
• Peak height
• Peak width

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The peak temperature is affected by heating rate & sample mass, but not by ∆h (enthalpy) and
T onset.
FACTORS AFFECTING THE DTA CURVE:
The various factors affecting the DTA curve are as follows:
Fig. 12. Factors affecting DTA curve
I. SAMPLE CHARACTERISTICS/SAMPLE FACTORS:
Physical:
• Packing density.
• Particle size.
• Peak area decreases with an increase in size.
• Peak T shifts to higher values with an increase in size.
• Completion T decreases with a decrease in size.
• degree of crystallinity.
• Amount of sample influence peak area.
• As the weight of the sample increases peak intensity and temperature.
• To maintain the heat capacity nearly constant during heating, the sample is generally mixed
with diluents. Generally, diluents affect the area, temperature, and even resolution of the DTA
peaks.

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Chemical:
• The chemical reactivity of the sample, the sample holder, thermocouple material, the ambient
gaseous environment, and added diluents greatly alter the DTA peaks.
• Therefore, one should make every effort to select these materials as inert chemically as
possible with the sample.
II. INSTRUMENTAL FACTORS:
Sample holder:
• The geometry and material with which it is made to affect the DTA curve.
• If the material has high thermal conductivity – sharp exothermic peaks and flat
endothermic peaks are obtained.
Eg. Metals
• Poor thermal conductivity - the reverse is true.
Eg. Ceramic
• The size of the holder and the amount of sample should be as small as possible for better
resolution.
Differential temperature sensing devices:
• The thickness of thermocouple wires affect the intensity of the peaks, the shape of the
peaks, and the baseline.
• If wires used are much thick.
• More distortion of peak heights and peak temperatures may take place.
• If thinner wires are used,
o Less distortion of peak heights and peak temperatures may take place.
o But the resistance is high and maybe unstable in impedance matching.
Furnace characteristics:
• The type of winding shows a direct effect on DTA curves.
• It should be uniform, hand-wound are not uniform and are not useful.
• Machine wound are uniform.
• Grooved muffled cores and time bifilar winding is preferred.

pg. 21
• The entire length of the differential thermocouple should be shielded.
Temperature programmer controller:
• On-off type controllers are not used because switching off or on or full power,
considerable noise may occur particularly at temperatures above 700ºC.
• If one has to measure small differential temperature, one should maintain the highest
accuracy, control, and precision in temperature measurement.
Thermal regime:
• The heating rate has a great influence on the DTA curves.
• Higher the heating rates, higher the peak temperature, and sharper the peaks with greater
intensity.
• Generally, heating rates of 10 to 20 °C per minute are employed.
• If the sample temperature is used as reference material, this minimizes the shift in the
peak temperature to higher values with faster heating rates.
Recorder:
• DTA curve is greatly influenced by the type, span, chart-speed, and response of a recorder.
• If proper sensitivity is not selected, weaker signals would not be recorded whereas the
stronger signals might undergo damping
• If faster charts speeds are used, DTA peaks get flattened out.
III. ENVIRONMENTAL FACTORS:
• The DTA technique is more sensitive to the gaseous environment around the sample.
• Reaction of atmospheric gases with the sample may also produce extra peaks in the curve.
• In DTA two types of gaseous environment are used,
o Static gaseous atmosphere
o The atmosphere surrounding the sample is changing in concentration chemically
due to evolved gases and physically due to convection currents.
• Studies in it are imprecise.
• Dynamic gaseous atmosphere.

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• The gases are swept past the sample in a controlled way.
• Reliable and reproducible.
• Sweep gases can be inert or reactive. But should not contain any of the product gases.
ADVANTAGES:
• instruments can be used at very high temperatures.
• instruments are highly sensitive.
• characteristic transition or reaction temperatures can be accurately determined.
DISADVANTAGES:
• uncertainty of heats of fusion, transition, or reaction estimations is 20- 50%.
• Sharp thermal changes are unable to predict.
• Precision is not good.
Today’s Instrument:
 In today's market, most manufacturers don’t make true DTA system but rather have
incorporated this technology into a thermogravimetric analysis(TGA) systems, which
provide both mass loss and thermal information.
 With today’s advancement in software, even these instruments are being replaced by a
true TGA-DSC instrument that can provide the temperature and heat flow of samples,
simultaneously with mass loss.
APPLICATIONS:
• A DTA curve can be used only as a fingerprint for identification purposes but usually, the
applications of this method are the determination of phase diagrams, heat change
measurements, and decomposition in various atmospheres.
• DTA is widely used in the pharmaceutical and food industries.
• DTA may be used in cement chemistry, mineralogical research, and in environmental studies.
• DTA curves may also be used to date bone or to study archaeological materials. Using DTA
one can obtain liquidus & solidus lines of phase diagrams.
• Used to study the characteristic of polymeric material.

pg. 23
• This technique is used for testing the purity of the drug sample and also to test the quality
control of some substances like cement, soil, glass, etc.
• Used for the determination of heat of reaction, specific heat, and energy change occurring
during melting, etc.
• Trend in ligand stability (thermal stability of the ligands) gives the information about the
ligands in the coordination sphere.
Quantitative analysis: Area of DTA peak is proportional to the total heat of reaction and hence
the weight of sample.
Quality control: DTA technique has been widely used for quality control of large number of
substances like cement, glass, soil, catalyst, explosion, resinss etc.
Conclusion:
Differential thermal analysis is formally defined as a method of determining the temperature
difference between a sample and reference materials, in practice, it can tell the user a lot about
the phase properties of the material at different temperatures. The thermal analysis gives
information about changes in material properties as a function of temperature.
Several different TA methods exist; focus on TGA – DTA. The difference in temperature ΔT
between the sample temperature and the reference temperature T, (ΔT = Ts - Tr) is then monitored
and plotted against sample temperature to give a differential thermogram. The amount of
information obtainable is of great benefit to many industries hence it’s widespread use.

pg. 24
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