Architecture:Thermoanalytical Techniques.pdf
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1、1 1 1.0INTRODUCTION Thermal analysis has been defined by the International Confedera- tion of Thermal Analysis (ICTA) as a general term which covers a variety of techniques that record the physical and chemical changes occurring in a substance as a function of temperature.12 This term, therefore, en
2、com- passes many classical techniques such as thermogravimetry (TG), evolved gas analysis (EGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), and the modern techniques, such as thermo- mechanical analysis (TMA) as well as dynamic mechanical analysis (DMA), and di
3、latometry, just to name a few. The application of thermal analysis to the study of construction materials stems from the fact that they undergo physicochemical changes on heating. 1 Thermoanalytical Techniques 2Chapter 1 - Thermoanalytical Techniques 2.0CLASSICAL TECHNIQUES Ever since the invention
4、of DSC, there has been much confusion over the difference between DTA and DSC. The exact ICTA definition of DTA is a method that monitors the temperature difference existing between a sample and a reference material as a function of time and/or temperature assuming that both sample and reference are
5、 subjected to the same environ- ment at a selected heating or cooling rate.12 The plot of T as a function of temperature is termed a DTA curve and endothermic transitions are plotted downward on the y-axis, while temperature (or time) is plotted on the x-axis. DSC, on the other hand, has been define
6、d as a technique that records the energy (in the form of heat) required to yield a zero temperature difference between a substance and a reference, as a function of either temperature or time at a predetermined heating and/or cooling rate, once again assuming that both the sample and the reference m
7、aterial are in the same environment.12 The plot obtained is known as a DSC curve and shows the amount of heat applied as a function of temperature or time. As can be seen from the above definitions, the two techniques are similar, but not the same. The two yield the same thermodynamic data such as e
8、nthalpy, entropy, Gibbs free energy, and specific heat, as well as kinetic data. It is only the method by which the information is obtained that differentiates the two techniques. A brief history on the development and a comparison of the two techniques are given.* 2.1Differential Thermal Analysis a
9、nd Differential Scanning Calorimetry A little over a hundred years ago, two papers were published by Le Chtelier dealing with the measurement of temperature in clays; the first entitled On the Action of Heat on Clays and the second On the Constitution of Clays.2021 The experiment described in these
10、papers was not a truly differential one since the difference in temperature between the clay and reference material was not measured. The apparatus consisted of a Pt-Pt/ 10%-Rh thermocouple embedded in a clay sample, which in turn was packed into a 5 mm diameter Pt crucible. The crucible was then pl
11、aced in *For a more detailed history, comparison, and theoretical description, consult the refer- ences listed in Refs. 319. 3 a larger crucible, surrounded with magnesium oxide and inserted into an oven. Le Chtelier used a heating rate of 120 K min-1 and recorded the electromotive force of the ther
12、mocouple on a photographic plate at regular time intervals. As long as no phase change occurred in the clay, the temperature rose evenly and the lines on the plate were evenly spaced. If, however, an exothermic transformation took place, then the temperature rose more rapidly, and, therefore, the li
13、nes were unevenly spaced and closer together. An endothermic transition, on the other hand, caused the mea- sured temperature to rise more slowly, and the spacing between the lines was much larger. To ensure that the measured temperatures were correct, he calibrated his instrument with the aid of bo
14、iling points of known materials such as water, sulfur, and selenium, as well as the melting point of gold. Since Le Chteliers experiment does not fit the ICTA definition of DTA, his main contribution to the development of DTA was the automatic recording of the heating curve on a photographic plate.
15、True differential thermal analysis was actually developed twelve years later (in 1899) by Roberts-Austen.22 Roberts-Austen connected two Pt-Pt/10%-Ir thermocouples in parallel which, in turn, were connected to a galvanometer. One thermo- couple was inserted into a reference sample consisting of a Cu
16、-Al alloy or of an aluminum silicate clay (fireclay). The other thermocouple was embedded into a steel sample of the same shape and dimensions as the reference. Both the sample and reference were placed in an evacuated furnace. A second galvanometer monitored the temperature of the refer- ence. The
17、purpose of the experiments was to construct a phase diagram of carbon steels and, by extension, railway lines. Since his method was a true differential technique, it was much more sensitive than Le Chteliers. The DTA design used today is only a slight modification of Roberts-Austens, and the only ma
18、jor improvements are in the electronics of temperature control and in the data processing, which is now handled by computers (see Fig. 1). It took about fifty years for the DTA technique to be considered not only qualitative, but also as a quantitative means of analyzing and charac- terizing materia
19、ls. Moreover, it was only then that the Roberts-Austen setup was modified by Boersma.23 The modification was in the placement of the thermocouples. Rather than placing the thermocouples into either the sample or the reference, Boersma suggested that they be fused onto cups and that sample and refere
20、nce be placed into these cups. This modification eliminated the necessity of diluting the sample with reference materials and reduced the importance of sample size. The vast majority of todays DTA Section 2.0 - Classical Techniques 4Chapter 1 - Thermoanalytical Techniques instruments are based on th
21、e Boersma principle in that only the crucibles are in contact with the thermocouples. Boersmas DTA configuration, Fig. 1b, can be considered as the missing link between differential thermal analysis and differential scanning calorimetry. Some even feel that this configuration is, in fact, a DSC inst
22、rument. This is the major reason behind the confusion as to the differences between DTA and DSC. Figure 1. Schematic diagrams of different instruments used in thermal analysis to detect energy changes occurring in a sample: (a) conventional DTA, (b) Boersma23 DTA, (c) power-compensation DSC, and (d)
23、 heat-flux DSC. The two most crucial differences between the two techniques are: (a) in DSC, the sample and reference have their own heaters and tempera- ture sensors as compared to DTA where there is one common heater for both; (b) DTA measures T versus temperature, and, therefore, must be calibrat
24、ed to convert T into transition energies, while DSC obtains the transition energy directly from the heat measurement. The confusion is also partly due to the fact that there are at least three different types of DSC instruments: a DTA calorimeter, a heat-flux type (Fig. 2c), and a power compensation
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