one of the thermal methods of analysis

1. DIFFERTIAL THERMAL
ANALYSIS
SHIV

2. THERMAL ANALYSIS TECHNIQUES
• Thermal analysis comprises a group of techniques
where the properties of 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), thermo gravimetric analysis
(TGA), dilatometry (DIL), evolved gas analysis
(EGA), dynamic mechanical analysis (DMA),
dielectric analyze (DEA) etc.

3. DTA:
• DTA may be defined formally as a
technique for recording the difference
in temperature between a substance
and a reference material against either
time or temperature as the two
specimens are subjected to identical
temperature regimes in an
environment heated or cooled at a
controlled rate.

4. PRINCIPLE:
• This technique is simple. The basic principle is
• 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.

5. • 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 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)

6. • 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 finger print 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.

8. • The baseline of the DTA curve should then
exhibit discontinuities at the transition
temperatures and the slope of the curve at any
point will depend on the microstructural
constitution at that temperature.
• The area under a DTA peak can be to the
enthalpy change and is not affected by the
heat capacity of the sample.

10. INSTRUMENTATION:
Sample holder:
• This is used to contain the sample as well as reference
material.
• Material – criteria for selecting material are
• Cost, ease of fabrication and inertness towards the
reactants and products in the temperature range of
interest.
• Metallic - aluminium, nickel, stainless steel, platinum (
generally employed), and its alloys.
• Gives sharp exotherms and flat endotherms.
• Non metallic - glass, ceramic, vitreous silica or sintered
alumina.
• Gives flat exotherms and sharp endotherms.

11. • 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 sample holder and
thermocouple junction.
• These are common.
• 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.
• Sample or reference are packed in respective .

12. • Blocks with symmetrically located multiple
compartments have been designed.
• To investigate several samples simultaneously.
Furnace:
• This is device for heating the sample.
• Its actually a oven enclosed in furnace.
• In DTA apparatus, tubular furnace is preferred
for its good temp regulation and programming.
• There are inexpensive.

13. Temperature controller and recorder:
• Temperature controller:
• Three basic element required to control temperature are
sensor, control element and heater.
• Two methods:
• On – off control : if the sensor signal indicates that the temp
has become greater than the set point, the heater is turned off
• Not widely used in DTA
• Inexpensive
• Proportional control : heat in put to the system is progressively
reduced as the temperature approaches the desired value.
• These are widely used in DTA instruments

14. • Temperature programming:
• A time dependent temperature cycling of furnace is required to
produce a 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 control unit.
• Recorder:
• The signals obtained from the sensors are recorded by the
recorder. And record the DTA curve.
• Two types
• Deflection type: the recording pen is moved directly by the input
signal.
• 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 servo motor until it matches the input
signal .

15. Thermocouples :
• Thermocouple is an electrical device consisting of two
dissimilar electrical conductors forming electrical
junctions at different temperatures.
• 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.

16. Thermocouples
• Made of chromel P and alumel wires measure and control
temp upto 1100 degree C in air
• Made from pure platinum and platinum – rhodium alloys
wires measure more than 1100 degree C .
• Made from refractory materials like tungsten and
rhenium in inert gas or vacuum for upto 2100 degree
C.
Cooling device:
• its function is to maintain a suitable atmosphere in
the furnace and sample holder .

17. 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 furnace controller.
• Heart of the analysis – heating block Identical
pair of cavities for the sample, ref.material
• Whole unit is set in an oven- control pressure
• Thermocouple is place directly in contact with the
sample and another in contact with the reference
• Temp of the block is raised, the temperature of
the sample & reference follow Zero temp.
difference – no physical or chemical change
• If any reaction – difference in ΔT

18. FACTORS AFFECTING THE DTA
CURVE :
• The various factors affecting the
DTA curve are as follows:
• Environmental factors.
• Instrumental factors.
• Sample factors.

19. 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
• Static gaseous atmosphere
• 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
• 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.

20. INSTRUMENTAL FACTORS:
Sample holder:
• The geometry and material with which it is made of
affects the DTA curve.
• If material has
• High thermal conductivity – sharp exothermic peaks and flat
endothermic peaks are obtained.
• Eg. Metals
• Poor thermal conductivity - reverse is true.
• Eg. Ceramic
• the size of holder and the amount of sample should be
as small as possible for better resolution.

21. Differential temperature sensing devices:
• The thickness of thermocouple wires affect the
intensity of the peaks, 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
• Less distortion of peak heights and peak temperatures may
take place.
• But the resistance is high and may be unstable in impedance
matching.

22. 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 biflar winding is preferred.
• 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 highest accuracy, control and precision in
temperature measurement.

23. 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 20º per minute
are employed
• If the sample temperature is used as a
reference material, this minimizes the shift in
the peak temperature to higher values with
faster heating rates.

24. Recorder:
• DTA curve is greatly influenced by the
type, span, chart-speed and pen-
response of a recorder.
• If proper sensitivity is not selected,
weaker signals would not be recorder
whereas the stronger signals might
undergo damping.
• If faster charts speeds are used, DTA
peaks get flattened out.

25. SAMPLE CHARACTERISTICS:
Physical
• Packing density.
• Particle size
• Peak area decreases with increase in size.
• Peak T shifts to higher values with increase in size.
• Completion T decreases with decrease in size.
• Degree of crystallinity.
• Amount of sample influence peak area.
• As wt of the sample increases peak intensity and temperature.
• In order to maintain the heat capacity nearly constant during
heating, the sample is generally mixed with diluents. Generally,
diluents affects the area, temperature and even resolution of
the DTA peaks.

26. 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.

27. 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 not good.

28. APPLICATIONS:
• A DTA curve can be used only as a finger print 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 remains[11]
or to study archaeological materials. Using DTA one
can obtain liquidus & solidus lines of phase diagrams.

29. • Used to study the characteristic of polymeric
material.
• This technique is used for testing the purity of the
drug sample and also to test the quality control of
number of 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.