Chapter 31. Thermal Methods

Transcription

1 Chapter 31. Thermal Methods Thermal analysis: Physical property of a substance or its reaction products is measured as a function of temperature. * TGA: Thermogravimetric Analysis ( 熱重分析法 ) * DTA: Differential Thermal Analysis ( 微差熱分析法 ) * DSC: Differential Scanning Calorimetry ( 微差掃描卡計法 ) * Microthermal Analysis ( 微熱分析 ) 2015/1/6 1 1) Thermogravimetric Analysis (TGA: 熱重分析法 ) FIGURE 31-1 Thermobalance components. The balance beam is shown as A. The sample cup and holder are B; C is a counterweight. D is a lamp and photodiodes, E is a magnetic coil, and F is a permanent magnet. The computer data-acquisition, data-processing, and control systems are components G, H, and I. Component J is the printer and display unit. P.895 1

2 FIGURE 31-2 Controlled atmospheric thermogram ( 熱圖 ) for a bituminous coal sample. A nitrogen atmosphere was used for about 18 min followed by an oxygen atmosphere for 4 to 5 min. The experiment was then completed in nitrogen. P.895 FIGURE 31-3 Thermograms for some common polymeric materials. PVC = polyvinyl chloride; PMMA = polymenthyl-methacrylate; LDPE = low-density polyethylene; PFE = polytetrafluoroethylene; Pl = aromatic polypyromelitimide. P.896 2

3 FIGURE 31-4 Thermogravimetric determination of carbon black in polyethylene. P.897 FIGURE 31-5 Thermogram for decomposition of CaC 2 O 4 H 2 O in an inert atmosphere. P.897 3

4 FIGURE 31-6 Decomposition of CaC 2 O 4 H 2 O, SrC 2 O 4 H 2 O, and BaC 2 O 4 H 2 O. P.898 2) Differential Thermal Analysis (DTA: 微差熱分析法 ) The temperature difference T) between a substance (T s ) and a reference (T r ) is measured as a function of temperature while they are subjected to a controlled temperature program. Thermogram T (T s -T r ) versus sample temperature FIGURE 31-8 Schematic diagram of a typical instrument for DTA. TC = thermocouple. P.898 4

5 H = 0 FIGURE 31-7 Differential thermogram showing types of changes encountered with polymeric materials. DTA peaks result from: 1)Physical changes * Endothermic: fusion, vaporization, sublimation, absorption, desorption. * Exothermic: adsorption, crystallization. 2) Chemical reactions * Endothermic: dehydration, reduction, decomposition. * Exothermic: oxidation in air or O 2, polymerization, catalytic reactions. P.898 FIGURE 31-9 Differential thermograms for sulfur. 113 : rhombic form monoclinic form 124 : melting point of sulfur 179 : liquid phase transition 446 : boiling point of sulfur P.900 5

6 FIGURE Differential thermogram for benzoic acid. Curve A taken at atmospheric pressure. Curve B taken at a pressure of bar (200 lbs/in. 2 ). P.900 3) Differential Scanning Calorimetry (DSC): 微差掃描卡計 The most often used thermal analysis method. The difference in the heat flow between a sample and a reference is measured as a function of temperature or time: the temperature is raised at a specified rate (usually linearly) or hold at a given temperature (isothermal). DSC: calorimetric method in which differences in energy are measured. DTA: differences in temperature are measured. Types of DSC instruments: (1)Power-compensated DSC (lower sensitivity, rapid response) (2)Heat-flux DSC (3)Modulated DSC (MDSC) 6

7 (1) Power-compensated DSC: the temperatures of the sample and reference are kept equal to each other while both temperatures are increased or decreased linearly. FIGURE Power-compensated DSC sample and reference holders and heaters. A temperature program is generated by the computer system. Platinum resistance thermometers, in contact with the sample and reference holders, sense any difference between the programmed temperature and the temperatures of the sample and reference. The error signal is used to adjust the power applied to the sample and the power applied to the reference platinum resistance heaters. The DSC output signal is the difference in the power required between the sample and the reference so that both equal the programmed temperature. P.901 (1) Heat-flux DSC: the difference in heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate. FIGURE Heat-flux DSC. P.901 7

8 dh/dt = C p dt/dt + f(t, t) FIGURE Typical DSC scan for a polymeric material. Note the step transition at about 63. There is an exothermic event at approximately 90 and an endothermic event at 160. Note that the thermogram represents the sum of the heat flow due to heat capacity and kinetic processes. P.902 3) Modulated DSC (MDSC): the same heating and cell arrangement as the heat-flux DSC. A sinusoidal function is superimposed on the overall temperature program to produce a micro heating and cooling cycle as the overall temperature is steadily increased or decreased. Related to kinetic processes Associated with heat capacity component FIGURE Deconvoluted thermogram from an MDSC apparatus showing non-reversing and reversing heat flow components. P.902 8

9 * DSC Experiment * DSC Calibration: Baseline calibration, heat flow calibration, temperature calibration, heat capacity calibration. * DSC Data Analysis Applications: 1) Glass transition temperature 2) Crystallinity and crystallization rate 3) Reaction kinetics FIGURE Determination of onset temperatures for a transition and an exothermic event (crystallization). P.903 9