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BY: S. R. KANABARGI GUIDE: DR. ASIF KARIGAR

THERMAL ANALYTICAL METHODS OF ANALYSIS

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INTRODUCTION

Scope of thermal methodsDefinition, general points

Thermal methods of AnalysisVarious methods

Applications of Thermal AnalysisHow it is applicable in analysis

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Scope of Thermal Methods Definition: Thermal analysis refers to the group of

methods in which some physical property of the sample is continuously measured as a function of temperature, whilst (at the same time) the sample is subjected to a controlled temperature change.

Generally,

Temperature – ability to transfer heat, or accept heat, from other materials.

Thermometry – science of temperature measurements.

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METHODS OF THERMAL ANALYSIS

Thermogravimetry (TG) Differential Thermal Analysis (DTA) and Differential

Scanning Calorimetry (DSC) Thermomechanical analysis Thermoacoustimetry Analysis Thermoptometry Electrothermal analysis Thermomagnetometry

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Thermogravimetry (TG):– Is related to changes in weight form, indicates whether the samples is losing weight and how much.

Differential Scanning Calorimetry (DSC) : - Related to energy changes, indicates that reaction is exothermic or endothermic (and is often capable of measuring the heat change).

Thermomechanical analysis:- Relates with dimensional changes as a function of temperature, useful in the study of metals, alloys, polymers, ceramic, and glasses.

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Thermoacoustimetry Analysis – characteristic sound waves produced on heating.

Thermoacoustimetry Analysis – characteristic sound waves produced on heating.

Thermoptometry – study if an optical characteristic of a sample.

Electrothermal analysis – electrical conductivity. Mainly used in semi conductor, polymer studies, also in purity determinations.

Thermomagnetometry – variations in the magnetic property of a material with temperature.

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THERMAL METHODS OF ANALYSIS

TG – technique in which the weight of sample is measured. It is the function of temperature.

Ex. Reactant (solid) product (solid) + gas

Gas + reactant (solid) product (solid)

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Derivative thermogravimetry (DTG): It is the method of expressing the results of TG by giving the first derivative curve as a function of temperature or time.

DTA: It is a technique in which difference in temperature (∆T) between the sample and an inert reference material is measured as a function of temperature.

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DSC: It is similar to DTA, but it differs from DTA by measuring the energy that has to be applied to maintain the constant temperature.

Evolved gas detection (EGD): It is the method of detection of evolution of gas from the sample.

Evolved gas analysis (EGA): Here, the volatile products, released by the sample on decomposition, may be analyzed.

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MULTIPLE TECHNIQUES IN THERMAL ANALYSIS

It involves the combination of more than one thermal method. The most common combination is TG and DTA, ex; simultaneous TG-DTA measurements of kaolinite

Most common combined techniques: TG-DTG, TG-DTG-DTA, TG-EGD or EGA

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APPLICATIONS Determination of thermal constant.

Ex. Specific heat, heat of conductivity, melting and freezing points of pure metals.

Phase changes and phase equilibrium.

Ex. Solid to liquid phase changes like melting points or liquid to vapour changes like boiling points.

Structural changes

Ex. Solid-solid transitions where change in crystals structure occurs, it may be endothermic or exothermic.

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Thermal stability.

Ex. Polymeric materials have been widely studied

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THERMOGRAVIMETRY INSTRUMENTATION AND TECHNIQUES

The instrument used in thermogravimetry (TG) is called thermobalance. It consist of precision balance, a furnace, controlled by a temperature programmer and a recorder.

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Points: It should provide continuous and accurate

record of sample weight as a function of temperature (T).

Capacity of modern thermobalance is probably be of the order of the gram.

It should operate a wide temperature range up to 1000 Celsius or more.

It is preferable if its upper limit is at least 100 celsius above the maximum working temperature required by the operator.

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It should ensure that the sample container is always located within a uniform hot zone inside the furnace.

The temperature recorded on the TG curve should ideally be the temperature of the sample itself.

Sample should not interact with furnace or any other parts of the equipment.

The balance should not be subject to radiation or convection effects arising from the proximity of the furnace.

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Provision should be made for the safe and efficient removal of volatiles from the furnace.

It should be versatile i.e., should be easy to operate and in multiple thermal studies (simultaneous TG and DTA measurements, great help).

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MAJOR COMPONENTS OF THERMOBALANCE

BALANCE: Primary essentials are accuracy, sensitivity, and

reproducibility. A reasonable capacity is a high degree of stability and a rapid responses are also necessary. Two most common type of balances used in thermal analysis are null point balance and deflection balance.

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Null point As weight change occurs and balance beam

starts to deviate from its normal position a sensor detects the deviation and triggers the restoring force to bring the balance beam back to the null position. The restoring force is directly proportional to the weight change.

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It consists of an electronic microbalance. It has a capacity of up to 1g, so samples up to 500

mg can placed if necessary. An electronic bridge circuit is maintained at a state

of constant electromagnetic balance. When the balance arm is deflected by the change in

sample weight an excess of current flows through one of the pair of the photocells, the current produced is proportional to the sample weight.

Amplification is passed through the coil (F) thus the balance is restored at its original position. This current is further recorded of using a meter.

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Deflection Some systems depend simply on measuring

the deflection of the beam from the norm by a suitable technique.

Beam types (fulcrum is used to measure ) and cantilever type (here, the beam itself is fixed at one end)

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Furnace Heart of the balance. It holds the sample container. This

ensures uniform heating of the sample over a wide range of temperature. In this the wire is coiled by ceramic material surrounded by an insulator, this is placed within the furnace housing. This housing has a cooling facility and furnace is connected to a programmer. Nichrome wire is usually used for furnaces operating up to about 1000 Celsius. Platinum and/or rhodium alloy is used for temperature up to about 1500 Celsius.

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Temperatures can be measured by using thermocouples like

chromel/alumel (up to 1000c) platinum/metal alloys (1500c) requirements of thermocouples—

chemically inert at high temperatures,

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Programmer Directly linked to the furnace Maintains the control of throughout the process. It is brain of the thermobalance and directs the

operation, It contains temperature sensor directly in

contact with the furnace, sending information to the programmer and thereby controlling the electrical power sent to the furnace.

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Recorder It provides constant record of the sample as a

function of temperature. There are two types potentiometric recorder X-Y recorder: it plots rate directly against

temperature. X1-X2 or adjacent recorder: it provides

independent record of weight and temperature. Diagram: summary in block form of the various

components of a modern thermobalance Diagram: schematic diagram of balance and

furnace assembly.

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SOURCES OF ERROR IN THERMOGRAVIMETRY

Buoyancy effect of sample container Random fluctuation of balance

mechanism Electrostatic effect on balance

mechanism Condensation on balance mechanism Measurement of weight by balance.

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Turbulence effects from gas flow Induction effects from furnace Measurement of temperature by

thermocouple Reaction between sample and container.

Errors from 2, 3, and 8 points can be avoided by proper arrangement of the balance.

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Buoyancy effect of sample container:

Refers to the apparent gain in weight that can occur when a crucible is heated. This may be due to decreased buoyancy of atmosphere at higher temperature and increase convection effect and possible effect heat from the furnace itself. In modern balance this effect is very minimum.

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Measurement of weight:

The balance system may be calibrated for recorded weight by adding known weights to the container and noting the readings on the chart.

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Furnace effects and turbulence effects:

The flow of gas over and around the container in the furnace may cause turbulence. The heat from the furnace may cause convection effects.

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Temperature

The actual temperature of the sample will usually lag behind the temperature recorded by the thermocouple. It may be caused due to the finite time in which a weight change is recorded, the heating rate, the gas flow, the nature of the sample container and also the characteristics of the sample.

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Reaction of the sample and container:

May cause changes in weight.

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INTERPRETATION OF TG

We can consider that the TG curve directly indicates the characteristic of that compound.

This is due to the sequence of physicochemical events that occur under particular conditions over the temperature range

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Ex:1. calcium carbonate

decomposes –single step(800oC–900o C)

forms calcium oxide (a stable solid) & gas CO2

CaCO3 (s) CaO (s) + CO2 (g)

Mr (100) (56) (44) Ex: 2 another ex, ammoniun nitrate

NH4NO3 (g) 300 oC N2O (g) + 2H20 (g)

(two volatile prods)

Mr is 100 for CaCO3 & amm. nitrate leaves no residue.

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Ex: calcium oxalate monohydrate (CaC2O4.H2O) is heated in air up to 1000 oC.

We see a well separated three thermal decompositions

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Step 1: hydrate water is lost:

CaC2O4. H2O (s) CaC2O4 (s) + H2O

Mr--146 18

W% - 12.3%

Step 2 : Anhydrous salt decomposes

CaC2O4 (s) CaCO3 (s) + CO (g)

Mr – 28

W% - 31.5%

Step 3: carbonate decomposes.

CaCO3 (s) CaO (s) + CO2 (g)

Mr -- 44

W% - 61.6%

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You will notice that these values calculated for the three steps I, II & III correspond very well to the positions of three plateaus

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