Chapter 1 Introduction P.J.Haines Oakland Analytical Services,Farnham,UK MATERIALS.HEAT AND CHANGES Whenever a sample of material is to be studied,one of the easiest tests to the quantita ative measurement a grea deal of useful information on the nature of the material. In the simplest case,the temperature of the sample may increase, without any change of form or chemical reaction taking place.In short,it gets hotter.For many other materials.the behaviour is more complex. When is heated,it elts at 0Cand then boils at 100C.When sugar is heated,it melts,and then forms brown caramel.Heating coal prod inflammable gases,tars and coke.The list is endless,since every material behaves in a characteristic way when heated. Thermal methods of analysis have developed out of the scientific study of the changes in the properties of a sample which cron heating. Calorimetric metho ods eat ch Some sample properties may be obvious to the analyst,such as colour shape and dimensions or may be measured easily,such as mass,density and mechanical strength.There are also properties which depend on the bonding,molecular structure and nature of the material.These include the ther rmodynamic properties such as heat capacity,enthalpy and n ropy and the stru tural and molecular r properties which determine the X-ray diffraction and spectrometric beha Transformations which change the materials in a system will alter one or more of these properties.The change may be physical such as melting, crystalline transition or vaporisation or it may be chemical involving a
Chapter 1 Introduction P. J. Haines Oakland Analytical Services, Farnham, UK MATERIALS, HEAT AND CHANGES Whenever a sample of material is to be studied, one of the easiest tests to perform is to heat it. The observation of the behaviour of the sample and the quantitative measurement of the changes on heating can yield a great deal of useful information on the nature of the material. In the simplest case, the temperature of the sample may increase, without any change of form or chemical reaction taking place. In short, it gets hotter. For many other materials, the behaviour is more complex. When ice is heated, it melts at 0°C and then boils at 100°C. When sugar is heated, it melts, and then forms brown caramel. Heating coal produces inflammable gases, tars and coke. The list is endless, since every material behaves in a characteristic way when heated. Thermal methods of analysis have developed out of the scientific study of the changes in the properties of a sample which occur on heating. Calorimetric methods measure heat changes. Some sample properties may be obvious to the analyst, such as colour, shape and dimensions or may be measured easily, such as mass, density and mechanical strength. There are also properties which depend on the bonding, molecular structure and nature of the material. These include the thermodynamic properties such as heat capacity, enthalpy and entropy and also the structural and molecular properties which determine the X-ray diffraction and spectrometric behaviour. Transformations which change the materials in a system will alter one or more of these properties. The change may be physical such as melting, crystalline transition or vaporisation or it may be chemical involving a 1
Chapter 1 reaction which alters the chemical structure of the material.Even biologi- cal processes such as metabolism,interaction or decomposition may be included Sometimes a change brought about by heating may be reversed by cooling a sample afterwards.A pure organic substance melts sharply,for example benzoic acid melts at 122C and it recrystallises sharply when cooled below this temperature.Ammonium chloride dissociates into ammonia and hydroge n chloride gases when heated,but these recombine At high temperatu e,calcium carbonate splits up toyield calcium oside and carbon dioxide gas.and these c cooling if the carbon dioxide is not removed.The system reaches an equilibrium state at a particular temperature. cool To raise the temperature of any system heat energy must be supplied and when sufficient energy is available the system will change into a more stable state.The mechanical properties of a material change as it is heated.Often it expands and becomes more pliable well below the melting point.These are fundam ntal mpo rtan nt chan n a molecula level,and their study enables the analyst to draw valuable conclusions about the sample,its previous history,its preparation,chemical nature and the likely behaviour during its proposed use. The temperature at which a particular event occurs,or the temperature range over which a reaction happens,are often characteristic of the nature nd history of a samp somet es of the methods used sharp transitions,such as the melting of pure materials,may b used to calibrate equipment and as the"fixed points"of thermometry and of the International Practical Temperature Scale(IPTS). For example,how does the simple.pure inorganic compound potass- ium nitrate,KNO3,behave when heated?At room temperature,say 20C,this is a whit ature to 30C constant presssure we must supply an amount of heat de pending on the specific heat capacity,Cpapproximately 1 JK-g at this temperature the mass m of the sample and the change in temperature.So,for I g heated 10C,we must supply 10J.To complicate matters,the heat capacity changes with temperature as well.When the temperature reaches 128C. the crystals chan ure 53Jg their stru and this i.Then the n crystals are heated,when C ≈1.2Jk about g the melting point of 338C.when more heat must be supplied to melt the sample.Raising the temperature above the melting point eventually
2 Chapter 1 reaction which alters the chemical structure of the material. Even biological processes such as metabolism, interaction or decomposition may be included. Sometimes a change brought about by heating may be reversed by cooling a sample afterwards. A pure organic substance melts sharply, for example benzoic acid melts at 122°C and it recrystallises sharply when cooled below this temperature. Ammonium chloride dissociates into ammonia and hydrogen chloride gases when heated, but these recombine on cooling. At high temperature, calcium carbonate splits up to yield calcium oxide and carbon dioxide gas, and these too will recombine on cooling if the carbon dioxide is not removed. The system reaches an equilibrium state at a particular temperature. heat cool CaCO, (solid) A 7 CaO (solid) + CO, (gas) To raise the temperature of any system heat energy must be supplied and when sufficient energy is available the system will change into a more stable state. The mechanical properties of a material change as it is heated. Often it expands and becomes more pliable well below the melting point. These are fundamental, important changes on a molecular level, and their study enables the analyst to draw valuable conclusions about the sample, its previous history, its preparation, chemical nature and the likely behaviour during its proposed use. The temperature at which a particular event occurs, or the temperature range over which a reaction happens, are often characteristic of the nature and history of a sample, and sometimes of the methods used to study it. Sharp transitions, such as the melting of pure materials, may be used to calibrate equipment and as the "fixed points" of thermometry and of the International Practical Temperature Scale (IPTS). For example, how does the simple, pure inorganic compound potassium nitrate, KN03, behave when heated? At room temperature, say 20°C, this is a white, crystalline solid. To raise its temperature to 30°C at constant presssure, we must supply an amount of heat depending on the specific heat capacity, C, approximately 1 J K-' g-' at this temperature, the mass w1 of the sample and the change in temperature. So, for 1 g heated 10°C, we must supply 1OJ. To complicate matters, the heat capacity changes with temperature as well. When the temperature reaches 128 "C, the crystals change their structure, and this needs more energy, about 53 J g-'. Then the new crystals are heated, when C, w 1.2 J K-' g-', until the melting point of 338"C, when more heat must be supplied to melt the sample. Raising the temperature above the melting point eventually
Introduction causes the sample to decompose to form potassium nitrite,KNO2,so that the mass of the sample is decreased by around 16%and oxygen gas is given oft. This example illustrates the importance of thermal techniques and measurements Calorimetry mea sures the amounts of heat,while appro priate thermal methods give the temperatures of phase changes,the temperatures of decomposition and the products of the reaction.Other methods will show the expansion,mass and colour changes on heating. The analysis of thermal events may be approached in two ways,which overlap considerably.Either the exp ment ay be designed to o measure thermal propertie (heat capacity,enthalpy,entropy and free energy)with high precision and accuracy at particular temperatures and conditions,or we may study properties,including thermal properties,over a wider range of temperatures using a controlled heating procedure. Which experiment is chosen depends on the sample to be analysed. There would be little point in obtain ing highly accurate heat ca a polymeric or cement ampl of complex compos ition,but on heating would be informative.Theoretical work on organic structure and kinetics might require precise knowledge of equilibrium thermal properties which could not easily be obtained using variable temperature methods.Therefore,the techniques are complementary. Since the worldwide adoption of the SI system of units it is perhap useful to stre e symbo nits to be used for the physica tiesnvovedin these methods.Themjor quisare given in Appen- dix 1A and the others may be found in the references. DEFINITIONS OF THERMAL AND CALORIMETRIC METHODS community may be found in the literature. Calorimetry is the measurement of the heat changes which occur during a process.The calorimetric experiment is conducted under par- ticular trolled conditions,for rat co nsta nt volu bo 0 temperature in an therma calorimeter.Calorimetry encompasses a very large variety of techniques including titration,flow,reaction and sorption,and is used to study reactions of all sorts of materials from pyrotechnics to pharmaceuticals. Calorimetric methods may be classified either by the principle of measurement (e.g.heat cor ethod of ensatingorh eat ac mulating),or by the operation (stati ng)or by the constructio principle(single or twin cell).These will be discussed further in Chapter 5
Introduction 3 causes the sample to decompose to form potassium nitrite, KN02, so that the mass of the sample is decreased by around 16% and oxygen gas is given off. This example illustrates the importance of thermal techniques and measurements. Calorimetry measures the amounts of heat, while appropriate thermal methods give the temperatures of phase changes, the temperatures of decomposition and the products of the reaction. Other methods will show the expansion, mass and colour changes on heating. The analysis of thermal events may be approached in two ways, which overlap considerably. Either the experiment may be designed to measure thermal properties (heat capacity, enthalpy, entropy and free energy) with high precision and accuracy at particular temperatures and conditions, or we may study properties, including thermal properties, over a wider range of temperatures using a controlled heating procedure. Which experiment is chosen depends on the sample to be analysed. There would be little point in obtaining highly accurate heat capacities on a polymeric or cement sample of complex composition, but its behaviour on heating would be informative. Theoretical work on organic structure and kinetics might require precise knowledge of equilibrium thermal properties which could not easily be obtained using variable temperature methods. Therefore, the techniques are complementary. Since the worldwide adoption of the SI system of units it is perhaps useful to stress the symbols and units to be used for the physical quantities involved in these methods. The major quantities are given in Appendix 1A and the others may be found in the references.’ ,2 DEFINITIONS OF THERMAL AND CALORIMETRIC METHODS Formal definitions are not essential, but those accepted by the scientific community may be found in the literat~re.~’~ Calorimetry is the measurement of the heat changes which occur during a process. The calorimetric experiment is conducted under particular, controlled conditions, for example, either at constant volume in a bomb calorimeter or at constant temperature in an isothermal calorimeter. Calorimetry encompasses a very large variety of techniques, including titration, flow, reaction and sorption, and is used to study reactions of all sorts of materials from pyrotechnics to pharmaceuticals. Calorimetric methods may be classified either by the principle of measurement (e.g. heat compensating or heat accumulating), or by the method of operation (static, flow or scanning) or by the construction principle (single or twin cell). These will be discussed further in Chapter 5
g Chapter 1 Thermal analysis is a group of techniques in which one (or more) property of a led temperature programme.The programme may take many forms: (a)The sample may be subjected to a constant heating(or cooling)rate (dT/dt =B),for example 10K min-1. (b)The sample may be held isothermally (B=0). (c)A. emp ature programme ”may be used where a sinusoidal or other alteration is superimposed onto the underlying heating rate. (d)To simulate special industrial or other processes,a stepwise or complex programme may be used.For example,the sample might be equilibrated at 25C for 10 min,heated at 10 K min-1 up to 00C held there for 30 m and the n cooled at 5K min-to50C. (e)The heating may be controlled by the response of the sample itself THE FAMILY OF THERMAL METHODS chemical property of the sample,or its products.The more frequently used thermal analysis techniques are shown in Table I together with the names most usually emploved for them. INSTRUMENTATION FOR THERMAL ANALYSIS AND CALORIMETRY The modern instrumentation used for any experiment in thermal analysis or calorimetry is usually made up of four major parts: .The sample and a sors to detect and measure a particular property of the sample and to measure temperature; an enclosure within which the experimental parameters (e.g.tem- perature,pressure,gas atmosphere)may be controlled; .a computer to control the experimental parameters,such as the temperatur gramme,to co data fron the sens rs and to process the e data to produce meaningful results and record This is shown schematically in Figure 1,and specific applications and instrumentation will be considered in the following chapters
4 Chapter 1 Thermal anaEysis is a group of techniques in which one (or more) property of a sample is studied while the sample is subjected to a controlled temperature programme. The programme may take many forms: (a) The sample may be subjected to a constant heating (or cooling) rate (b) The sample may be held isothermally (p = 0). (c) A “modulated temperature programme” may be used where a sinusoidal or other alteration is superimposed onto the underlying heating rate. (d) To simulate special industrial or other processes, a stepwise or complex programme may be used. For example, the sample might be equilibrated at 25°C for 10 min, heated at 10 K min-’ up to 2OO0C, held there for 30 min and then cooled at 5 K min-l to 50°C. (e) The heating may be controlled by the response of the sample itself. (dT/dt = p), for example 10 K min-’. THE FAMILY OF THERMAL METHODS Every thermal method studies and measures a property as a function of temperature. The properties studied may include almost every physical or chemical property of the sample, or its products. The more frequently used thermal analysis techniques are shown in Table 1 together with the names most usually employed for them. INSTRUMENTATION FOR THERMAL ANALYSIS AND CALORIMETRY The modern instrumentation used for any experiment in thermal analysis or calorimetry is usually made up of four major parts: The sample and a container or holder; sensors to detect and measure a particular property of the sample and to measure temperature; an enclosure within which the experimental parameters (e.g. temperature, pressure, gas atmosphere) may be controlled; a computer to control the experimental parameters, such as the temperature programme, to collect the data from the sensors and to process the data to produce meaningful results and records. This is shown schematically in Figure 1, and specific applications and instrumentation will be considered in the following chapters
Introduction 5 Table 1 Thermal methods Technique Abbreviation Property Uses Thermogravimetry or TG Mass Decompositions DTA erature reactions Differential scanning calorimetry DSC or ne actions Thermomechanical analysis TMA Deformations anica Dimensional change Expansion Dynamic mechanical analysis DMA Moduli ase changes. polymer cure Eoedcatenmlanast ctrica M ed Decompositions re Thermoptometry Optica Phase changes colour changes Sound Thermoluminesc Light emitted Thermomagnetometry Magnetic Magnetic e points Sensor(s) Compute Enclosure- Sample Sensor(s) for T etc. cesso Figure 1 Schematic of general thermal analysis or calorimetry apparatus
Introduction Table 1 Thermal methods 5 Technique Abbreviation Property Uses Thermogravimetry or (Thermogravimetric analysis) Differential thermal analysis Differential scanning calorimetry Thermomechanical analysis Dynamic mechanical analysis Dielectric thermal analysis Evolved gas analysis Thermoptometry Less frequently used methods Thermosonimetry Thermoluminescence Thermomagnetometry TG TGA DTA DSC TMA DMA DETA EGA TS TL TM Mass Temperature difference Power difference or heat flow Deformations Dimensional change Moduli Electrical Gases evolved or reacted Optical Sound Light emitted Magnetic Decompositions Oxidations Phase changes, reactions Heat capacity, phase changes, reactions Mechanical changes Expansion Phase changes, glass transitions, polymer cure as DMA Decompositions Phase changes, surface reactions, colour changes Mechanical and chemical changes Oxidation Magnetic changes, Curie points Sensor( s) Enclosure Sample - Sensor( s) for T etc. Figure 1 Schematic of general thermal analysis or calorimetry apparatus