TAILIEUCHUNG - Introdution to Thermal Analysis Techniques and Applications Part 4

Tham khảo tài liệu 'introdution to thermal analysis techniques and applications part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 34 A method developed fairly recently h TA Instruments known as high resolution Hi-Re ĨG 41 at dynamic rate TG 42 . produces a marked decrease in the heating rate when the system crosses the upper threshold set for the rale. The changes in heating or cooling rates are controlled by adjustment of the values of I wo numerical parameters. referred Io as resolution and sensitivity. An example of die output from Hi-rcs TG using CuSO 5HjO 41Ị is shown in Figure . Figure 3 .8 Ht-Rcs TG and DTG of 41 . With permission of the Journal of Thermal Analysis and Calorimetry. 35 The problems of consistent and acceptable nomenclature for the variety of lechnii ues described above have been reviewed by Reading 33J. The term .w q le amrollnf ikttmil . ly V SCTA for all methods where the transformation undergone by the sample in some way influences tire course of the temperature programme it experiences. has now been proposed. Use ofSCTA methods can provide a more uniform reaction environment than usually exists in conventional TA. Pressure and temperature gradients within the sample can be a problem particularly for rapid strongly endothermic decompositions. These gradients can be minimized by using small sample masses and setting slow decomposition rates if adequate sensitivity is available. Isothermal experiments suffer particularly tn comparison with SCTA 43 because often the reliction rale cun be too rapid at the beginning of the experiment and far too slow near the end for the reaction to be complete in a realistic time. In linear rising temperature experiments the rate goes through a maximum near the midpoint of a reaction step. Low ering the heating rale IO decrease this maximum rale gives unduly long experiments. In SCTA. provided that the temperature gradients al lhe rate chosen are acceptable obtaining undistorlcd results will lake the minimum time possible. Barnes et al 44 have compared SCTA methods and supported the claims of enhanced resolution over .

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