Differential Thermal Analysis

Khawar yaqoob

M13-330

Thermal analysis When a material is heated its structural and chemical composition

can undergo changes such as fusion, melting, crystallization, oxidation, decomposition, transition, expansion and sintering.

Using Thermal Analysis such changes can be monitored in every atmosphere of interest. The obtained information is very useful in both quality control and problem solving.

Techniques for thermal analysis

Thermogravimetric Analysis (TGA)

Differential Thermal Analysis (DTA)

Differential Scanning Calorimetry (DSC)

Differential Thermal Analysis (DTA) In Differential Thermal Analysis, the temperature difference that

develops between a sample and an inert reference material is measured, when both are subjected to identical heat – treatments

This is a comparison method Analytical method for recording the difference in temperature

(∆T) b/w a substance and an inert reference material as a function of temperature or time

Any transformation – change in specific heat or an enthaply of transition can be detected by DTA

In DTA both test sample & an inert reference material (alumina) – controlled heating or cooling programming

Differential Thermal Analysis (DTA)DTA is used by analysts for analyzing certain properties like:• Glass transition temperature• Chemical reactions• Crystalline phase changes• Decomposition of glass batch materials

∆T VS Temp.Sharp Endothermic – changes in crystallanity or fusion

Broad endotherms - dehydration reaction

Physical changes usually result in endothermic curves

Chemical reactions are exothermic

Phenomena causing changes in temperature

Apparatus

Differential thermal analysis mainly consist of

1. Sample holder comprising thermocouples, sample containers and a ceramic or metallic block.

2. Furnace.

3. Temperature programmer.

4. Recording system.

DTA apparatus

Diagram

Procedure 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 Temperature 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 ∆T

Graphical Representation

Factors affect results in DTA

Factors that influence DTA curve

DTA

Advantages: • Instruments can be used at very high temperatures• Instruments are highly sensitive• Characteristic transition or reaction temperatures can be

accurately determined• Accurate Low detection limit (up to 10-7 g) Reliable data Easy to use Rather

cheap Minimal sample preparation

Disadvantages:

• Uncertainty of heats of fusion,• Transition or reaction estimations is 20-50%• Destructive Limited range of samples Time consuming Usually not qualitative

Applications

Quantitative identification and purity assessment of materials are accomplished by comparing the DTA curve of sample to that of a reference curve

Impurities may be detected by depression of the M.P

Applications

Pharm industry Food industry Cosmetic industry

Polymer industry Research and Development Ceramic industry

Limitations

ΔT determined not so accurate (2,3 0C) Very small value of ΔT cannot be determined and

quantified Due to heat variation between sample and reference

makes it less sensitive