Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 7 contrast during melting the temperature of the sample holder does not increase in response to the temperature programmer because heat flows from the sample holder to the sample. Therefore the true temperature scanning rate of the sample is not constant over the entire temperature range of the experiment. Temperature calibration is achieved using standard reference materials whose transition temperatures are well characterized Appendices and and in the same temperature range as the transition in the sample. The transition temperature can be determined by DTA but the enthalpy of transition is difficult to measure because of non-uniform temperature gradients in the sample due to the structure of the sample holder which are difficult to quantify. This type of DTA instrument is rarely used as an independent apparatus and is generally coupled to another analytical instrument for simultaneous measurement of the phase transitions of metals and inorganic substances at temperatures greater than 1300 K. Custom DTA Many DTA instruments are constructed by individual researchers to carry out experiments under specialized conditions such as high pressure and or high temperature. Figure shows an example of a high-pressure custom DTA instrument. The pressure medium used is dimethylsilicone up to 600 MPa or kerosene up to 1000 MPa and the pressure is increased using a mechanical or Figure . Schematic diagram of a high-pressure custom DTA apparatus courtesy of Y. Maeda Page 8 Figure . DTA heating curves of polyethylene recorded at various pressures. I II 200 III 250 and IV 350 MPa courtesy of Y. Maeda an electrical pump. The temperature range is 230-670 K at a heating rate of 1-5 K min. The pressure range of commercially available high-pressure DTA systems is 1-10 MPa to guarantee the stability and safety of the apparatus. In this case the excess pressure is generated using a purge gas CO2 N2 O2 . The DTA heating curves of polyethylene measured over
