Thermal-analysis-methods.pdf

Thermal Analysis

Prof. Nizam M. El-Ashgar

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Introduction v Thermal Analysis is the term applied to a group of methods and techniques in which chemical or physical properties of a substance, a mixture of substances or a reaction mixture are measured as a function of temperature or time, while the substances are subjected to a controlled temperature programme. v Over a dozen thermal methods can be recognized, which differ in the properties measured and the temperature programs. v Find widespread use for both quality control and research applications on industrial products such as polymers, pharmaceuticals, clays and minerals, metals and alloys. 2

What TGA Can Tell You

• • • • • •

Thermal Stability of Materials. Oxidative Stability of Materials. Composition of Multi-component Systems. Estimated Lifetime of a Product. Decomposition Kinetics of Materials. The Effect of Reactive or Corrosive Atmospheres on Materials. • Moisture and Volatiles Content of Materials.

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The following table is a list of the main thermal analysis methods:

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While carrying out these measurements, the furnace atmosphere can either be static air or a continuous flow of gas (purging). Examples are: inert conditions (nitrogen) to inhibit oxidation, or reducing condition (e.g. purging hydrogen), etc. 5

General thermodynamic relationships Thermal analyses are usually run under conditions of constant pressure, the underlying thermodynamic equation is the GibbsHelmholtz expression: ∆G0 = ∆H0 - T∆S0 where G=free energy of the system, H=enthalpy of the system, S=entropy of the system, T=temperature in kelvins The general chemical reaction aA + bB → cC + dD Is spontaneous as written if ∆G<0, is a t equilibrium if ∆G=0, and does not proceed if ∆G >0. Thermal analysis involves the monitoring of spontaneous 6 reaction.

Differentiating the Gibbs-Helmholtz equation with respect to temperature (assuming ∆S and ∆H not vary with temperature):

d (∆G ) = − ∆S dT Show how to move from a stable situation (∆G>>0) to one where reaction will occur. ∆S > 0, an increase in temperature cause ∆G<0, ∆S < 0, decreasing the temperature will achieve the desired spontaneous reaction. Once the reaction is made to occur, thermal analysis may be used to detect the process, yielding different and 7 complementary information.

Thermal methods Generalized instrument: • Temperature programmer ↓ • Sample in furnace with controlled atmosphere ↓ x axis • temperature

↓ y axis property

• Output: Plot of the property (y axis) versus T. 8

Types of thermal event - Phase transition.

- Adsorption, desorption.

- Sublimation.

- Thermal decomposition.

- Radiolytic decomposition.

- Glass transition.

- Oxidation/ combustion.

- Heterogeneous catalysis.

- Double decomposition.

- Desolvation.

- Melting (fusion).

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Thermogravimetry (TG) v Thermogravimetric analysis (TG): is the study of weight changes of a specimen as a function of temperature. The technique is useful strictly for transformations involving the absorption or evolution of gases from a specimen consisting of a condensed phase. v A plot of mass versus temperature (thermogravimetric curves or TG curves) permits evaluation of thermal stabilities, rate of reaction, reaction processes, and sample composition. v Measurements of changes in sample mass with temperature are made using thermobalance. The balance should be in a suitably enclosed system so that the atmosphere can be controlled. 10

General considerations Suitable samples for TG are solids that undergo one of the two general types of reaction: Reactant(s) → Product(s) + Gas Gas + Reactant(s) → Product(s)

(a mass loss) (a mass gain)

Processes occurring without change in mass (e.g., the melting of a sample) obviously cannot be studied by TG. 11

Mechanisms of Weight Change in TGA • Weight Loss: – Decomposition: The breaking apart of chemical bonds. – Evaporation: The loss of volatiles with elevated temperature. – Reduction: Interaction of sample to a reducing atmosphere (hydrogen, ammonia, etc). – Desorption: a substance is released from a surface.

• Weight Gain: – Oxidation: Interaction of the sample with an oxidizing atmosphere. – Absorption: absorbing something or of being absorbed.

All of these are kinetic processes (i.e. there is a rate at which they occur).

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Optimum conditions i) Few mg sample. ii) Thin layer of sample. iii) Open sample container. iv) Inert gas flow. v) Slow heating rate.

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Instrumentation Modern commercial instruments for thermogravimetry consists of: 1- A sensitive analytical balance. 2- A furnace. 3- A purge gas system for providing an inert (or sometimes reactive atmosphere). 4- A microcomputer/microprocessor for instrument control and data acquisition and display. 5- A purge gas switching system is a common option for applications in which the purge gas must be changed during an experiment. 14

Instrumentation

LINSEIS L81

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The balance • A number of different thermobalance designs are available commercially. • Capable of providing quantitative information about samples ranging in mass from 1 mg to 100 mg. • Common type balance: has a range of 5 to 20 mg. • Sample holder of balance must be housed in the furnace. But the rest of the balance must be isolated from the furnace. • A change in sample mass causes a deflection of the beam which interposes a light shutter between a lamp and one of two photodiodes. • The resulting imbalance in the photodiode current is amplified and fed into coil which is situated between the poles of a permanent magnet. • The magnetic field generated by the current in the coil restores the beam to its original position. • The amplified photodiode current is monitored and transformed into mass or mass loss information by the data acquisition system. 17

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Balances must remain precise and accurate continuously under extreme temperature and atmosphere conditions, and should deliver a signal suitable for continuous recording.

v Null-deflection weighing mechanisms are favoured in TG as they ensure that the sample remains in the same zone of the furnace irrespective of changes in mass. v Sensitivity of balance ≈ 1µg for a 1g maximum load balance. v The output weight signal may be differentiated electronically to give a derivative thermogravimetric curve (DTG) 19

TGA Apparatus

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The Furnace vFor TGA ,T range is from ambient to 1500 oC. vThe heating and cooling rate of the furnace can be selected from >0 to 200 oC/min. vInsulation and cooling to the exterior of the furnace is required to avoid heat transfer to the balance. vNitrogen or argon are usually used to purge the furnace and prevent oxidation of the sample. vFor some analysis it is desirable to switch purge gases as the analysis proceeds (different gases used (N2, O2). vThe furnace is normally an electrical resistive heater. 21

Some basic requirements of the heating chamber are: Ø be non-inductively wound. Ø be capable of reaching 100 to 200°C above the maximum desired working temperature. Ø have a uniform hot-zone of reasonable length. Ø reach the required starting temperature as quickly as possible. Ø not affect the balance mechanism through radiation or convection. vIn order to overcome the problem of possible temperature gradient, infrared or microwave radiation have been used in some equipment.

Infrared heating: use halogen lamp, temperature up to 1400°C, heating rate can be as high as 1000°C/min, accuracy is about ±0.5°C.

Microwave heating: large sample can be used because uniform heating generated within sample but temperature measurement and 22 power control are difficult.

Constant heating rate

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Constant heating rate: lag behind of the sample temperature

During heating a temperature difference between the furnace and the sample temperature appears which means that the sample temperature lags always behind the furnace temperature.

Measurement of the melting point of Di-tert.-biphenyle at different heating rates. 24

Gradual raise of temperature

Thermal equilibrium is better reached by gradual raise of the 25 temperature.

The atmosphere Sort, pressure and flow rate of the gas in the sample chamber influence the following parameters: •Sample reaction Sample reactions with the gas (oxidation in the presence of oxygen). •Heat transitions Different heat conductivity of the gases used in an experiment. •Buoyancy ‫ طﻔو‬and current effects Different density and flow rate of the gases used in an experiment. For all thermoanalytical investigations it is very important to report the sort, the pressure and the flow rate of the gases used in the experiment. 26

Thermal decomposition temperatures for CaCO3 in different gas atmospheres 27

v Thermbalance are normally housed in glass or metal system to allow for operation at pressures ranging from high vacuum (< 104 Pa) to high pressure (>3000 kPa) of inert, oxidizing, reducing or corrosive gases. v Care must be taken to correct for buoyancy arising from the lack of symmetry in the weighing system v Thermal convection ‫ اﻟﺤﻤﻞ اﻟﺤﺮاري‬is responsible for noise in the signal of TG. v The use of dense carrier gases at high pressures in hot zones with large temperature gradients give the most noise. v Fitting of convoluted baffles ‫ﺣﻮاﺟﺰ ﻣﻠﺘﻮﯾﺔ‬ was found to be most successful in reducing thermal convection.

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Instrument control/data handling • The T recorded in a thermogram is ideally the actual T of the sample. • This T can be obtained by immersing a small thermocouple directly in the sample. Problems: This procedure is seldom followed because of catalytic decomposition of samples, potential contamination of samples and weighing errors resulting from the thermocouple leads. Another method: Problems reduced by measuring T with a small thermocouple located as close as possible to the sample container. Modern thermobalances: Usually use a computerized T control routine that automatically compares the voltage output of the thermocouple with a voltage versus T table that is stored read only memory ROM. The voltage difference is used to adjust the voltage of the heater. (achieving agreement between 29 specified T program and T of sample).

The sample v Sample form, defect content, porosity and surface properties has influence to the behaviour on heating, e.g. single crystal sample give different response from powdered sample v Large sample size cause problems like heat transfer, and gas exchange with the surrounding is reduced; in general, the use of small (~ 20 mg) specimen is preferable if sensitivity of balance permits v Sample should be powdered and spread thinly and uniformly in the container 30

Crucibles

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Decomposition temperatures of CaCO3 as function of crucibles

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Temperature measurement and calibration vPlatinum resistance thermometers or thermocouples are used for temperature measurement. vLarge difference between sample temperature (Ts) and furnace temperature (Tf) can exist, sometime as high as 30°C. Calibration is thus needed. vThe difference or lag is more marked when operating in vacuum or in fast flowing atmosphere and with high heating rate. 33

Temperature calibration for small furnace can be done by making use of the melting point or Curie points of a range of metals and alloys.

vA series of high purity wires may be suspend in the region where the specimen crucible would normally be located. If the furnace temperature is slowly raised through the melting point of a particular wire, a significant weight loss will be recorded when the wire melts. vA series of fusible wire, such as : indium (156.63°C), lead (327.5°C), zinc (419.58°C), aluminium (660.37°C), silver (961.93°C), and gold (1064.42°C) should give a reasonable calibration curve.

hanger of sample pan furnace

different metal wires thermocouple

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Calibration can also be done by placing a series of ferromagnetic materials in the specimen basket and a magnet below or above it, external to the furnace. When each material goes through its Curie temperature (ferro- to paramagnetic transition), a sharp ‘weight’ change will be indicated. Using mixtures, a multi-point temperature calibration can be performed. e.g. Curie points (in oC) of alumel 163; Ni 354; perkalloy 596; Fe 780; Hisat 1000.

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Interpretation of TG and DTG curves i.The sample undergoes no decomposition with loss of volatile products over the temperature range shown but solid phase transformation, melting ,etc can not be detected by TG, ii. The rapid initial mass loss is characteristic of desorption or drying. If it is true, then re-run the sample should result in type (i) curves, iii. Single stage decomposition, iv.Multi-stage decomposition with relatively stable intermediates : provide information on the temperature limit of stability of reactants and intermediate products and also stoichiometry, v.Multi-stage decomposition with no stable intermediate product. However heating-rate effect must be considered. At low heating rate, type (v) resemble type (iv). At high heating rate, type (iv) and (v) resemble type (iii) and lose all the details, Resolution of stages can be vi.Gain in mass due to reaction with atmosphere, e.g. improved by recording DTG or by oxidation of metals, 36 digital differentiation of TG data. vii.Oxidation product decompose again at higher temperature; this is not often encountered.

Preparing the measurement General advices: • Exact characterization of the starting materials (purity, grain size)!. • Large amount of the starting material for repeated and further measurements. • Removal of absorbed water by drying (∆m must be constant). • Use samples with narrow grain size distribution (Sieving). • For measurement in vacuum no sample with a grain size below 60 mesh (0.25 mm) (a part of the sample can be lost). 37

Applications of TG vOnly for studying thermal events accompanied by mass change vProvide valuable information for desorption, decomposition and oxidation. e.g. dehydration of CuSO4·5H2O

TG curve for CuSO4⋅5H2O

TG curve for CaSO4⋅2H2O at different water-vapour pressure 38

v knowledge of thermal stability can give information on problems like the hazards of storing explosives, shelf life of drugs, etc. v The thermal balance in a TG equipment can also be used to measure vapour pressure of a sample and magnetic susceptibility, etc. ATTN: Three factors should be noted when you get a TG curve: 1. General shape, 2. The particular temperatures at which changes in mass occur (severely affected by many experimental conditions), 3. The magnitudes of the mass changes can be used for precise quantitative analysis. 39

Analytical calculations Under controlled and reproducible conditions, quantitative data can be extracted from the relevant TG curves. Most commonly, the mass change is related to sample purity or composition. Example: A pure compound may be either MgO, MgCO3, or MgC2O4. A thermogram of the substance shows a loss of 91.0 mg from a total of 175.0 mg used for analysis. What is the formula of the compound? The relevant possible reactions are MgO → No reaction MgCO3 → MgO+CO2 MgC2O4 → MgO+CO2+CO Solution:

% Mass loss Sample=(91.0/175.0)(100%)=52.0 % Mass loss if MgCO3=(44/84.3)(100%)=52.2 % Mass loss if MgC2O4=((44+28)/112.3)(100%)=64.1 40

If the preparation was pure, the compound present is MgCO3.

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Applications • Information provided by TGA methods is more limited than that obtained with DTA and DSC. • Because here T variation must bring about a change in mass of the analyte. • TGA are largely limited to decomposition and oxidation reactions and some physical processes such as vaporization, sublimation and desorption. Most important applications: • Studying of polymers: Provides information about decomposition mechanisms for various polymeric preparations. • Decomposition patterns are characteristic for each kind of polymer and identification purposes. 44

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Some Applications TGA for : PVC = polyviny chloride. PMMA = polymethyl methacrylate. LDPE = low density polyehylene. PTFE = polytetrafluoroethylene. PI = aromatic polypyromellitimide.

Conditions: 10 mg, 5oC/min in N2

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Thermogravimetric determination of Carbon black in polyethylene

• TGA Used for quant. Analysis of polyethylene (formulated with fine carbon-black particles to inhibit degradation from exposure to sunlight). • The analysis is difficult by most other substances.

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TGA for decomposition of CaC2O4.H2O in an inert atmosphere. Rate: 5oC/min. The clearly defined horizontal regions corresponds to temperature ranges in which the indicated calcium compounds are stable. Importance: Defining thermal conditions for the gravimetric determination of pure species.

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Quantitative determination of a mixture of Ca, Sr and Ba ions The three are first precipitated as monohydrated oxalates. The mass in the temperature range: 320oC- 400oC: for the anhydrous compounds (CaC2O4, SrC2O4 and BaC2O4). 580oC- 620oC: Corresponds to the weight of the three carbonates. next two steps: The weight changes results from the loss of CO2 as first CaO and then SrO are formed. The TGA data sufficient to calculate Wt of each element presents in the sample. 49

Derivative of thermogram Plots change in mass with temperature, dm/dt, and resolves changes more clearly. Modern instruments are capable of providing both TG as well as its derivative. The derivative curve may reveal information that is not detectable in the ordinary thermogram. Example: The three peaks at 140oC, 180oC and 205oC suggest that the three hydrates lose moisture at different temperatures. However all appear to lose CO2 simultaneously in a single sharp peak at 450oC. 50

Calcium Oxalate Example 120

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12.57% Water (0.8753mg) 19.47% Carbon Monoxide (1.355mg)

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30.07% Carbon Dioxide (2.093mg)

Weight (%)

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2

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Deriv. Weight (%/min)

4

0 40

20 0

200

400

600

800

Temperature (°C)

-2 100051 Universal V3.4A TA Instruments

Derivative thermogravimetry (DTG)

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Differential Thermal analysis (DTA) Measure sample temperature relative to a reference, for the same heat transferred

Thermal events: Exothermic event and endothermic event Reaction/decomposition, Melting, Crystallization, Change in crystal structure Glass transition 53

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DTA Instrumentation

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Instrumentation

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Handling : • A few mg of the sample (S) and an inert substance R are contained in a small aluminum dishes that are located above sample and reference thermocouples in an electrically heated furnace. • The reference material is an inert substance such as alumina, silicon carbide or glass beads. • The output potential Es from the sample thermocouple passes into a microcomputer where it is made to control the current input to the furnace in such a way that the sample temperature increases linearly and at a predetermined rate. • The sample thermocouple signal is also converted to temperature Ts and is then recorded as abscissa of the differential thermogram. 59

• The output across the sample and reference thermocouples ∆E is amplified and converted to temperature difference ∆T = (Tr-Ts) which serves as the ordinate of thermogram. • Circulation of inert gas such as N2 or a reactive gas such as O2 within sample and reference. • There is a constant temperature difference ∆T between s and r since they have different heat capacities. But when the sample undergoes an endo (exo) thermic change ∆T becomes different. 60

Example

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General principles • From Figure 31-7 • The initial decrease in ∆T is due to glass transition. • The glass transition temperature Tg is the characteristic temperature at which glassy amorphous polymers become flexible. • Upon being heated to a certain temperature Tg the polymer changes from a glass to a rubber . • Such a transition involves no absorption or evolution of heat (no change in enthalpy ∆H = 0). • The heat capacity of rubber is however different from that of the glass (lowering of the baseline) No peak results during this transition (because no change in enthalpy ∆H = 0). • The two maxima are the result of exothermic processes in which heat evolved from sample ( T rise). • The minimum labeled melting is the consequence of endothermic process in which heat is adsorbed by the analyte. Crystallization of 62 amorphous polymers is an exothermic process (first exothermic peak).

• The second peak in the figure is endothermic and evolves melting of the microcrystals formed in the initial exothermic process. • The third peak is exothermic and is encountered only if the heating is performed in presence of air or oxygen. (results from the exothermic oxidation of the polymer). • The final –ve change in ∆T results from the endothermic decomposition of the polymer to produce a variety of products. As suggested in the figure differential thermal analysis peaks result from both physical changes and chemical reactions induced by T changes in the sample. Physical processes: Endothermic: include fusion, vaporization, sublimation, absorption and desorption. Exothermic: Adsorption and crystallization (generally). Chemical reactions: Endothermic: include dehydration, reduction in a gaseous atmosphere and decomposition. Exothermic: oxidation in air or oxygen, polymerization and catalytic 63 reactions.

Peak areas in DTA depend upon: - Mass of the sample (m) - The enthalpy ∆H of the chemical or physical process. - Geometry and conductivity factors. A = -kGm ∆H = -K’m ∆H Where A is the peak area, G is calibration factor that depends upon the sample geometry, and k is a constant related to the thermal conductivity of the sample. -ve for exothermic enthalpy change. If K’ is remains constant: variables can be controlled such as heating rate, particle size and placement of the sample relative to the sample thermocouple. So mass of analyte can be determined by knowing of ∆H and K’ by calibration. ∆H determined if K’ and m are known. 64

Applications of DTA

DTA finds widespread use in: 1) Determining the thermal behavior and composition of naturally occurring and manufactured products. Examples: Studying and characterization of polymers. The figure illustrates the type of physical and chemical changes in polymeric materials. Thermal transition occurs for a polymer at extended range because a pure polymer is a mixture of homologs. 2) Quantitative identification and purity assessment of materials are accomplished by comparing the DTA curve of sample to that to reference curve Impurities may be detected by depression of the M.P. 65

Key points in DTA thermogram • Sharp endotherm: crystalline rearrangemets, fusion, solid state transition. • Broader endotherm: dehydration, melting of polymers, slow reactions • Narrow exotherm: crystallization • Sharp exotherm: phase change

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The figure is a DTA of a physical mixture of 7 commercial polymers. Each peak corresponds to the melting point of one of the components. PTFE has an additional low T peak that arises from crystallization transition. DTA can identify the polymers. 67

DTA used for studies the thermal behavior of pure inorganic compounds or inorganic substances: Silicates, ferrites, clays, oxides, ceramics, catalysts and glasses.

Processes occurs: Fusion desolvation, dehydration, oxidation, reduction adsorption and solid-state reactions.

Figure: DTA of calcium oxalate. Two minima (sample cooler than ref) for two endothermic reaction (endo). The single maxima: oxidation of calcium oxalate to give carbon dioxide and calcium carbonate (exo). By using inert gas (N2) three minima are encountered (endothermic decomposition of oxalate to give carbonate and carbon monoxide.

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Generation of phase diagrams and study of phase transitions. • The Figure is DTA of sulfur. • The peak at 1130C corresponds to solid-phase change (rhombic → monoclinic) form • The peak at 1240C corresponds to the melting point of the element. • (Three forms of liquid sulfur is known) The peak at 1790C apparently involves these transitions). 0C • The peak at 446 corresponds to the boiling point of sulfaur.

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Simple and accurate determination the melting, boiling and decomposition points of organic compounds. The Figure shows thermograms of benzoic acid at atmospheric pressure (A) and at 200 psi (B). The first peak corresponds for the melting point and the second to boiling point of the acid.

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To fingerprint substances A DTA curve can be used as a fingerprint for identification purposes. ↑ ∆T

a

b

temp → DTA of (a) butter and (b) margarine

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

Factors affect results in DTA • • • • •

Sample weight Particle size Heating rate Atmospheric conditions Conditions of sample packing into dishes. 72

Differential Scanning Calorimetry In power-compensated DSC, the sample and a reference material are maintained at the same temperature throughout the controlled temperature programme. The difference in the independent energy supplies to the sample and the reference is then recorded against the programme temperature DSC can be used to study heats of reaction, kinetics, heat capacities, phase transitions, thermal stabilities, sample composition and purity, critical points, and phase diagrams.

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Comparison between DAT and DSC DTA

DSC

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Differential Scanning Calorimetry (DSC) DSC is a thermal technique in which differences in heat flow into a substance and a reference are measured as a function of sample temperature while the two are subjected to a controlled temperature program. Basic differences between DSC and DTA. Ø DSC is a calorimetric method in which differences in energy are measured. Ø DTA differences in T are recorded. Ø Temperature program of the two are similar. Ø DSC has by now become the most widely used of all thermal methods. 75

Instrumentation Two types of methods: Power compensated DSC: The sample and reference material are heated by separate heaters in such away that their temperatures are kept equal while these temperatures are increased (or decreased) linearly. Heat flux DSC: The difference in heat flow into the sample and reference is measured as the sample temperature is increased (or decreased) linearly. The two methods provide the same information but the instrumentation is fundamentally different. 76

Power compensated DSC: The instrument has two independent furnaces: • One for heating the sample. • The other for heating the reference. • Commercial design: The furnaces are small weighing about a gram each, a feature that leads to rapid rates of heating, cooling and equilibration. • The furnaces are imbedded in a large temperature-controlled heat sink. • Above the furnaces are the sample and reference holders, which have platinum resistance thermometers imbedded in them to monitor the temperatures of the materials continuously. • Two control circuits are employed in obtaining differential thermograms with the instrument one for average-temperature control and one for differential-temperature control. 77

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In the average temperature control circuit: v A programmer provides an electrical signal that is proportional to the desired average temperature of the sample and reference holders as a function of time. v This signal is compared in a computer with the average of the signals from the sample and reference detectors imbedded in the sample and reference holders. v Any differences between the programmer signal and the average platinum sensor signal is used to adjust the average temperature of the sample and reference. v The average temperature then serves as abscissa for the thermogram.

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In the differential temperature circuit: v Sample and reference signals from the platinum resistance sensors are fed into a differential amplifier via a comparator circuit that determines which is greater. v The amplifier output then adjusts the power input to the two furnaces in such a way that their temperatures are kept identical. v During the exp the sample and reference are isothermal. v A signal that is proportional to the difference in power input to the two furnaces is also transmitted to the data acquisition system. v This difference in power usually in milliwatts is the information most frequently plotted as a function of sample tempearute. 80

Heat flux DSC An electrically heated constantan disc (60% Cu, 40% Ni) thermoelectric disc enables the heat to flow into s and r, which are in small Al pans in a raised platform. The differential heat flow to s and r is monitored by the difference in output at the two chromel/constantan area thermocouples. Temperature of s is obtained from the chromel/alumel junction under the s disc.

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Exothermal dQ/dT

Differential Scanning Calorimetry (DSC)

Temperature

n

DSC measures differences in the amount of heat required to increase the temperature of a sample and a reference as a function of 82 temperature

Heat Capacity Measurement Using DSC Baseline change h occurs for substances with different Cp, or for same substance after phase change.

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h ∝ Cp where Cp is thermal capacity (= mass in g × specific heat capacity in J g-1 oC-1); h = B φ Cp B = calibration factor, found by using standard e.g. sapphire; m is sample mass g; φ is heating rate oC s-1; unit of y axis (h) is J s-1. To measure Cp for a substance, obtain DSC results for: empty pans in both holders, sample in one pan and reference in other pan. s, r in crimped Al pans (T<500 oC) standard in one pan and reference in other.

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Other uses of DSC •kinetics of curing araldite; •determination of polymer oxidation temperatures as a guide to stability; •determination of phase diagrams of liquid crystals; •determination conductivity;

of

thermal

•determination of purity of pharmaceutical products, e.g. phenacetin below

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Differential Scanning Calorimetry In power-compensated DSC, the sample and a reference material are maintained at the same temperature throughout the controlled temperature programme. The difference in the independent energy supplies to the sample and the reference is then recorded against the programme temperature DSC can be used to study heats of reaction, kinetics, heat capacities, phase transitions, thermal stabilities, sample composition and purity, critical points, and phase diagrams.

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Sample containers and sampling ∗ T<500°C : usually contained in aluminium sample pans which can be sealed either by crimping or by cold-welding for holding volatile samples ∗ T>500°C : use quartz, alumina (Al2O3), gold or graphite pans ∗ the reference material in most DSC applications is simply an empty sample pan ∗ purging of gas into the DSC sample holder is possible, e.g. N2, O2, etc. ∗ the mass of (sample+pan+lid) should be recorded before and after a run so that further information about the processes can be deduced. 87

The reference sample For all difference methods (DTA, DSC) reference samples like Al2O3 are needed to ensure that the heat flow from the furnace to the sample and from the furnace to the reference sample is identical! The thermal behavior of the reference sample is included in the measured signal. Requirements for the reference sample: •Known temperature behavior •No discontinuity in the temperature curve •If possible a similar thermal behavior as the sample (similar heat capacity) For small weights of the sample and when no precise measurements are required an experiment without a reference sample is possible. In such case an empty crucible can be used as reference. 88

Interpretation of DSC curves ∗ aim at correlating the features recorded with the thermal events taking place in the sample ∗ after baseline correction, the peak area is proportional to enthalpy change,

∆H =

AK m

where K is a constant and m is the mass of the sample K can be obtained by melting a known amount of a pure metal

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v reversibility can be monitored by cooling and reheating

Heating and cooling curves for a partially crystallized polymer. 91

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∗ Melting point of pure components are easily determined ∗ DSC curves for slow cooling of mixture

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General Applications ∗ Temperature and enthalpy changes for the thermal events enthalpy ∝ area of peak after baseline correction

Corresponding TG curve 94

∗ Detection of solid-solid phase transition and the measurement of ∆H for these transitions DSC curve of carbon tetrachloride

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v Tracing the ferromagnetic to paramagnetic transformation. vMost rewarding applications is in study of polymer

•Most solid polymers are formed by rapid cooling to low temperatures (quenching) are thus in glassy state; by heating above Tg, glass transition, with change in cp but no change in enthalpy, is observed, therefore no peak is observed, only discontinuity results 96

•degradation or oxidation of polymers can be study with DSC in isothermal mode •for recycling plastics, identification is important and DSC curves provide 'fingerprint'of the materials.

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References:

∗ Introdcution to Thermal Analysis, M.E. Brown -- Chapman and Hall ∗ Thermal Analysis - Techniques and Applications, ed. E.L. Charsley and S.B. Warrington -- Royal Society of Chemistry ∗ Thermal Analysis of Materials, Robert F. Speyer –Marcel Dekker, Inc.

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