Differential thermal analysis - instrumental methods of analysis

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 Δ T between the sample temperature and the reference temperature T, ( Δ T = 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.

7. 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.

8. 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.

9. 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 .

10. 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.

11. 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

12. 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 .

13. 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.

14. 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 .

15. 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

16. Factors affecting the DTA curve : The various factors affecting the DTA curve are as follows: Environmental factors. Instrumental factors. Sample factors.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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 .

22. 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.

23. 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.

24. 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.

25. 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.

26. 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 .

27. 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.