Thermal analysis H.k.D.H. badheshia.*2

Cambridge university *2Garg g.*1 Apex college of pharmacy sitapura jaipur(raj.),*1

Email –gauravggarg95@gmail.com,ggarg9595@yahoo.com ph.-9261114682

Defination- Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used - these are distinguished from one another by the property which is measured:

Differential thermal analysis (DTA): temperature difference Differential scanning calorimetry (DSC): heat difference Thermogravimetric analysis (TGA): mass Thermomechanical analysis (TMA): dimension Dilatometry (DIL): volume Dynamic mechanical analysis (DMA) : mechanical stiffness & damping Dielectric thermal analysis (DEA): dielectric permittivity & loss factor Evolved gas analysis (EGA) : gaseous decomposition products Thermo-optical analysis (TOA) : optical properties

Thermal Analysis TechniquesThermal analysis comprises a group of techniques in which a physical property of a substanceis measured as a function of temperature, while the substance is subjected to a controlledtemperature programme. In di®erential thermal analysis, the temperature di®erence that developsbetween a sample and an inert reference material is measured, when both are subjectedto identical heat{treatments. The related technique of di®erential scanning calorimetry relieson di®erences in energy required to maintain the sample and reference at an identicaltemperature.Length or volume changes that occur on subjecting materials to heat treatment are detectedin dilatometry; X{ray or neutron di®raction can also be used to measure dimensional changes

Differential Thermal Analysis (DTA) IntroductionDTA involves heating or cooling a test sample and an inert reference under identical conditions,while recording any temperature di®erence between the sample and reference. This di®erentialtemperature is then plotted against time, or against temperature. Changes in the sample whichlead to the absorption or evolution of heat can be detected relative to the inert reference.Di®erential temperatures can also arise between two inert samples when their response to theapplied heat{treatment is not identical. DTA can therefore be used to study thermal propertiesand phase changes which do not lead to a change in enthalpy. The baseline of the DTA curveshould then exhibit discontinuities at the transition temperatures and the slope of the curveat any point will depend on the microstructural constitution at that temperature.A DTA curve can be used as a ¯nger print for identi¯cation purposes, for example, in thestudy of clays where the structural similarity of di®erent forms renders di®raction experimentsdi±cult to interpret.The area under a DTA peak can be to the enthalpy change and is not a®ected by the heatcapacity of the sample.DTA may be de¯ned formally as a technique for recording the di®erence in temperature betweena substance and a reference material against either time or temperature as the twospecimens are subjected to identical temperature regimes in an environment heated or cooledat a controlled rate.

ApparatusThe key features of a di®erential thermal analysis kit are as follows (Fig. 1):1. Sample holder comprising thermocouples, sample containers and a ceramic or metallicblock.2. Furnace.3. Temperature programmer.4. Recording system.The last three items come in a variety of commercially available forms and are not be discussedin any detail. The essential requirements of the furnace are that it should provide a stableand su±ciently large hot{zone and must be able to respond rapidly to commands from the

temperature programmer. A temperature programmer is essential in order to obtain constantheating rates. The recording system must have a low inertia to faithfully reproduce variationsin the experimental set{up.Fig. 1: Schematic illustration of a DTA cell.The sample holder assembly consists of a thermocouple each for the sample and reference,surrounded by a block to ensure an even heat distribution. The sample is contained in a smallcrucible designed with an indentation on the base to ensure a snug ¯t over the thermocouplebead. The crucible may be made of materials such as Pyrex, silica, nickel or platinum, dependingon the temperature and nature of the tests involved. The thermocouples should notbe placed in direct contact with the sample to avoid co

ntamination and degradation, althoughsensitivity may be compromised. Fig-no.1 schametic illustration of DTA cell. Interpretation and Presentation of Data

A simple DTA curve may consist of linear portions displaced from the abscissa because the heatcapacities and thermal conductivities of the test and reference samples are not identical, andof peaks corresponding to the evolution or absorption of heat following physical or chemicalchanges in the test sample.There are di±culties with the measurement of transition temperatures using DTA curves. Theonset of the DTA peak in principle gives the start{temperature, but there may be temperaturelags depending on the location of the thermocouple with respect to the reference and testsamples or the DTA block. It is wise to calibrate the apparatus with materials of preciselyknown melting points. The peak area (A), which is related to enthalpy changes in the testsample, is that enclosed between the peak and the interpolated baseline. When the di®erentialthermocouples are in thermal, but not in physical contact with the test and reference materials, it can be shown that A is given byA =mqgKwhere m is the sample mass, q is the enthalpy change per unit mass, g is a measured shape factorand K is the thermal conductivity of sample. With porous, compacted or heaped samples, thegas ¯lling the pores can alter the thermal conductivity of the atmosphere surrounding the DTAcontainer and lead to large errors in the peak area. The situation is made worse when gasesare evolved from the sample, making the thermal conductivity of the DTA{cell environmentdi®erent from that used in calibration experiments.The DTA apparatus is calibrated for enthalpy by measuring peak areas on standard samplesover speci¯ed temperature ranges. The calibration should be based upon at least two di®erentsamples, conducting both heating and cooling experiments.It is possible to measure the heat capacity CP at constant pressure using DTA:CP = K0 T2 ¡ T1mHwhere T1 and T2 are the di®erential temperatures generated when the apparatus is ¯rst runwithout any sample at all and then with the test sample in position. H is the heating rate and

the constant K0 is determined by calibration against standard substances

reference crucibles are linked by good heat{°ow path. The sample and reference are enclosed

in the same furnace. The di®erence in energy required to maintain them at a nearly identical

temperature is provided by the heat changes in the sample. Any excess energy is conductedbetween the sample and reference through the connecting metallic disc, a feature absent inDTA. As in modern DTA equipment, the thermocouples are not embedded in either of thespecimens; the small temperature di®erence that may develop between the sample and theinert reference (usually an empty sample pan and lid) is proportional to the heat °ow betweenthe two. The fact that the temperature di®erence is small is important to ensure that bothcontainers are exposed to essentially the same temperature programme.The main assembly of the DSC cell is enclosed in a cylindrical, silver heating black, whichdissipates heat to the specimens via a constantan disc which is attached to the silver block.The disc has two raised platforms on which the sample and reference pans are placed. Achromel disc and connecting wire are attached to the underside of each platform, and the resultingchromel{constantan thermocouples are used to determine the di®erential temperaturesof interest. Alumel wires attached to the chromel discs provide the chromel{alumel junctionsfor independently measuring the sample and reference temperature. A separate thermocoupleembedded in the silver block serves a temperature controller for the programmed heating cycle.An inert gas is passed through the cell at a constant °ow rate of about 40 ml min¡1).The thermal resistances of the system vary with temperature, but the instruments can be usedin the `calibrated' mode, where the ampli¯cation is automatically varied with temperature togive a nearly constant calorimetric sensitivity.Heat Flow in Heat{Flux DSC SystemsA variety of temperature lags develop between the specimens and thermocouples, since thelatter are not in direct contact with the samples. The measured ¢T is not equal to TS ¡ TRwhere TS and TR are the sample and reference temperatures respectively. TS ¡ TR may bededuced by considering the heat °ow paths in the system.The following additional notation (due to Greer and Baxter) is relevant (Fig. 2):TSP , TRP = Temperature of the sample and reference platforms, respectively, asmeasured by the thermocouples. TSP is normally plotted as the abscissa of a DSCcurve.TF = Temperature of the silver heating block.RD =Thermal resistance between the furnace wall and the sample or reference platforms(units Cmin J¡1).RS; RR = Thermal resistances between the sample (or reference) platform and thesample (or reference).

CS, CR = Heat capacity of the sample (or reference) and its container.H = Imposed heating rate.¢TR = Temperature lag of the reference platform relative to furnace.¢TS = Temperature lag of the sample platform relative to furnace.¢TL = Temperature lag of the sample relative to the sample thermocouple.

Differential scanning calorimetry

Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.

The technique was developed by E.S. Watson and M.J. O'Neill in 1960,[1] and introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. The term DSC was coined to describe this instrument which measures energy directly and allows precise measurements of heat capacity.[2]

Detection of phase transitionsThe basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to observe more subtle phase changes, such as glass transitions. It is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.[3][4][5]

DTAAn alternative technique, which shares much in common with DSC, is differential thermal analysis (DTA). In this technique it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically phase changes and other thermal processes cause a difference in temperature between the sample and reference. Both DSC and DTA provide similar information and modern DTA

developed from the Boersma DTA, which first used one furnace with fixed thermocouples. Many modern commercial DTA are called heat flux DSC.

DSC curvesThe result of a DSC experiment is a curve of heat flux versus temperature or versus time. There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak, depending on the kind of technology used in the experiment. This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition. It can be shown that the enthalpy of transition can be expressed using the following equation:

ΔH = KA

where ΔH is the enthalpy of transition, K is the calorimetric constant, and A is the area under the curve. The calorimetric constant will vary from instrument to instrument, and can be determined by analyzing a well-characterized sample with known enthalpies of transition.[4]

Applications

A schematic DSC curve demonstrating the appearance of several common features

Differential scanning calorimetry can be used to measure a number of characteristic properties of a sample. Using this technique it is possible to observe fusion and crystallization events as well as glass transition temperatures Tg. DSC can also be used to study oxidation, as well as other chemical reactions.[3][4][5][6]

Glass transitions may occur as the temperature of an amorphous solid is increased. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity; no formal phase change occurs.[3][5]

As the temperature increases, an amorphous solid will become less viscous. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (Tc). This transition from amorphous solid to crystalline solid is an exothermic process, and results in a peak in the DSC signal. As the temperature increases the sample eventually reaches its melting temperature (Tm). The melting process results in an endothermic peak in the DSC curve. The ability to determine transition temperatures and enthalpies makes DSC a valuable tool in producing phase diagrams for various chemical systems.[3]

Top: A schematic DSC curve of amount of energy input (y) required to maintain each temperature (x), scanned across a range of temperatures. Bottom: Normalized curves setting the initial heat capacity as the reference. Buffer-buffer baseline (dashed) and protein-buffer variance (solid).

Normalized DSC curves using the baseline as the reference (left), and fractions of each conformational state (y) existing at each temperature (right), for two-state (top), and three-state (bottom) proteins.

ExamplesThe technique is widely used across a range of applications, both as a routine quality test and as a research tool. The equipment is easy to calibrate, using low melting indium or zinc for example, and is a rapid and reliable method of thermal analysis.

Liquid crystalsDSC is used in the study of liquid crystals. As some forms of matter go from solid to liquid they go through a third state, which displays properties of both phases. This anisotropic liquid is known as a liquid crystalline or mesomorphous state. Using DSC, it is possible to observe the small energy changes that occur as matter transitions from a solid to a liquid crystal and from a liquid crystal to an isotropic liquid.[4]

Oxidative stabilityUsing differential scanning calorimetry to study the stability to oxidation of samples generally requires an airtight sample chamber. Usually, such tests are done isothermally (at constant temperature) by changing the atmosphere of the sample. First, the sample is brought to the desired test temperature under an inert atmosphere, usually nitrogen. Then, oxygen is added to the system. Any oxidation that occurs is observed as a deviation in the baseline. Such analysis can be used to determine the stability and optimum storage conditions for a material or compound.[3]

Safety ScreeningDSC makes a reasonable initial safety screening tool. In this mode the sample will be housed in a non-reactive crucible (often gold, or gold plated steel), and which will be able to withstand pressure (typically up to 100 bar). The presence of an exothermic event can then be used to assess the stability of a substance to heat. However, due to a combination of relatively poor sensitivity, slower than normal scan rates (typically 2-3 °/min - due to much heavier crucible) and unknown activation energy, it is necessary to deduct about 75-100 °C from the initial start of the observed exotherm to suggest a maximum temperature for the material. A much more accurate data set can be obtained from an adiabatic calorimeter, but such a test may take 2–3 days from ambient at a rate of a 3 °C increment per half hour.

Drug analysisDSC is widely used in the pharmaceutical and polymer industries. For the polymer chemist, DSC is a handy tool for studying curing processes, which allows the fine tuning of polymer properties. The cross-linking of polymer molecules that occurs in the curing process is exothermic, resulting in a positive peak in the DSC curve that usually appears soon after the glass transition.[3][4][5]

In the pharmaceutical industry it is necessary to have well-characterized drug compounds in order to define processing parameters. For instance, if it is necessary to deliver a drug in the amorphous form, it is desirable to process the drug at temperatures below those at which crystallization can occur.[4]

General chemical analysisFreezing-point depression can be used as a purity analysis tool when analysed by Differential scanning calorimetry.[7] This is possible because the temperature range over which a mixture of compounds melts is dependent on their relative amounts. Consequently, less pure compounds will exhibit a broadened melting peak that begins at lower temperature than a pure compound.[4][5]

Food scienceIn food science research, DSC is used in conjunction with other thermal analytical techniques to determine water dynamics. Changes in water distribution may be correlated with changes in texture. Similar to materials science studies, the effects of curing on confectionery products can also be analyzed.

PolymersDSC is used widely for examining polymers to check their composition. Melting points and glass transition temperatures for most polymers are available from standard compilations, and the method can show up possible polymer degradation by the lowering of the expected melting point, Tm, for example. Tm depends on the molecular weight of the polymer, so lower grades will have lower melting points than expected. The percentage crystallinity of a polymer can also be found using DSC. It can be found from the crystallisation peak from the DSC graph since the heat of fusion can be calculated from the area under an absorption peak.

Impurities in polymers can be determined by examining thermograms for anomalous peaks, and plasticisers can be detected at their characteristic boiling points.

MetalsIn the last few years this technology has been involved in metallic material study. The characterization of this kind of material with DSC is not easy yet because of the low quantity of literature about it. It is known that it is possible to use DSC to find solidus and liquidus temperature of a metal alloy, but the widest application is, by now, the study of precipitations .

Thermogravimetric analysisThermogravimetric analysis or thermal gravimetric analysis (TGA) is a type of testing that is performed on samples to determine changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change. As many weight loss curves look similar, the weight loss curve may require transformation before results may be interpreted. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent. Again, interpretation is limited without further modifications and deconvolution of the overlapping peaks may be required.

TGA is commonly employed in research and testing to determine characteristics of materials such as polymers, to determine degradation temperatures, absorbed moisture content of materials, the level of inorganic and organic components in materials, decomposition points of explosives, and solvent residues. It is also often used to estimate the corrosion kinetics in high temperature oxidation.

Simultaneous TGA-DTA/DSC measures both heat flow and weight changes (TGA) in a material as a function of temperature or time in a controlled atmosphere. Simultaneous measurement of these two material properties not only improves productivity but also simplifies interpretation of the results. The complementary information obtained allows differentiation between endothermic and exothermic events which have no associated weight loss (e.g., melting and crystallization) and those which involve a weight loss (e.g., degradation).

Analyzer

Thermogram of calcium oxalate

The analyzer usually consists of a high-precision balance with a pan (generally platinum) loaded with the sample. The pan is placed in a small electrically heated oven with a thermocouple to accurately measure the temperature. The atmosphere may be purged with an inert gas to prevent oxidation or other undesired reactions. A computer is used to control the instrument.

Analysis is carried out by raising the temperature gradually and plotting weight (percentage) against temperature. The temperature in many testing methods routinely reaches 1000°C or greater, but the oven is so greatly insulated that an operator would not be aware of any change in temperature even if standing directly in front of the device. After the data are obtained, curve smoothing and other operations may be done such as to find the exact points of inflection.

A method known as hi-resolution TGA is often employed to obtain greater accuracy in areas where the derivative curve peaks. In this method, temperature increase slows as weight loss increases. This is done so that the exact temperature at which a peak occurs can be more accurately identified. Several modern TGA devices can vent burnoff to an infrared spectrophotometer to analyze composition.Categories: Thermodynamics | Materials science

Thermal Analysis of PharmaceuticalsAs a result of the Process Analytical Technologies (PAT) initiative launched by the U.S. Food and Drug Administration (FDA), analytical development is receiving more attention within the pharmaceutical industry. Illustrating the importance of analytical methodologies, Thermal Analysis of Pharmaceuticals presents reliable and versatile characterization tools for the successful development of pharmaceutical products. It draws attention to the most widely applicable methods and demonstrates how to interpret the associated data.

References1. ̂ U.S. Patent 3,263,4842. ̂ Wunderlich, B. (1990). Thermal Analysis. New York: Academic Press. pp. 137–140.3. ^ a b c d e f Dean, John A. (1995). The Analytical Chemistry Handbook. New York: McGraw Hill,

Inc.. pp. 15.1–15.5.4. ^ a b c d e f g Pungor, Erno (1995). A Practical Guide to Instrumental Analysis. Florida: Boca

Raton. pp. 181–191.5. ^ a b c d e Skoog, Douglas A., F. James Holler and Timothy Nieman (1998). Principles of

Instrumental Analysis (5 ed.). New York. pp. 805–808.6. ̂ M. J. O'Neill (1964). Anal. Chem. 36: 1238–1245. doi:10.1021/ac60213a020.7. ̂ "DSC Purity Analysis".

http://us.mt.com/mt_ext_files/Editorial/Generic/0/stare_purity_datasheet_0x00024947000255120005b219_files/51724796.pdf. Retrieved 2009-02-05.

8. Bhadeshia H.K.D.H. “Thermal analyses techniques. Differential thermal analysis”. University of Cambridge, Material Science and Metallurgy

9. Ferrer S., Borrás J., Martín Gil J. and Martín Gil F.J. "Thermal studies on sulphonamide derivative complexes”. Thermochim. Acta, 1989, 147, 321 330; 1989, 153, 205 220; 1991, 185, 315 333.

10. ̂ Martín Gil J., Martínez Villa F., Ramos-Sánchez M.C. and Martín-Gil F.J. "Studies on beta lactam antibiotics. differential thermal analysis of cephalosporins". J. Therm. Anal. Cal., 1984, 29, 1351 7.

11. ̂ Berger K.G., Akehurst E.E. “Some applications of differential thermal analysis to oils and fats”. International Journal of Food Science & Technology, 1966, 1, 237–247.

12. ̂ Ramos Sánchez M.C., Rey F.J., Rodríguez M.L., Martín Gil F.J. and Martín Gil J. "DTG and DTA studies on typical sugars". Thermochim. Acta, 1988, 134, 55 60.

13. ̂ F.J. Rey, M.C. Ramos-Sánchez, M.L.Rodríguez, J. Martín-Gil, F.J. Martín-Gil. "DTG and DTA studies on sugar derivatives". Thermochim. Acta, 1988, 134, 67 72.

14. ̂ Rodríguez Méndez M.L., Rey F.J., Martín Gil J. and Martín Gil F.J. "DTG and DTA studies on amino acids". Thermochim. Acta, 1988, 134, 73 78.

15. ̂ Ramachandran V.S. “Applications of differential thermal analysis in cement chemistry”. Chap. V, Chemical Publishing Co., Inc., New York (1969), 92.

16. ̂ Smykatz-Kloss W. “Application of differential thermal analysis in mineralogy”. J. Therm. Anal. Cal., 1982, 23, 15-44.

17. ̂ Smykatz-Kloss W., Heil A, Kaeding L. and Roller E. “Thermal analysis in environmental studies”. In: Thermal analysis in Geosciences. Springer Berlin / Heidelberg, 1991.

18. ̂ Villanueva, PrE, Girela F. y Castellanos M. “The application of differential thermal analysis and thermogravimetric analysis to dating bone remains”. Journal of Forensic Sciences, 1976, 21,

19. ̂ Misiego-Tejeda J.C., Marcos-Contreras G.J., Sarabia Herrero F.J., Martín Gil J. and Martín Gil F.J. "Un horno doméstico de la primera Edad del Hierro de "El Soto de Medinilla" (Valladolid) y su análisis por ATD". BSAA (University of Valladolid) 1993, LIX, 89 111.

20. ̂ Kingery W.D. “A note on the differential thermal analysis of archaeological ceramics”. Archaeometry, 1974, 16, 109–112.

1. NETZSCH-Gerätebau, Germany

M C Ramos-Sánchez, F J Rey, M L Rodríguez, F J Martín-Gil, J Martín-Gil, "DTG and DTA studies on typical sugars", Themochim Acta, 134: 55-60. 1988. Elsevier Science Publishers B.V., Amsterdam.

21.