Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 127 Figure . TMA instrument which employs a balance beam mechanism in compression mode courtesy of Ulvac Sinku-Riko A multipoint temperature calibration can be achieved in one run using a selection of standard materials in the sandwich configuration shown in Figure . The drawback of this method is that the standard samples can only be used once. The thermocouple which is used to record the sample temperature is rarely placed in contact with the sample but is placed as close as possible to the sample. The sample-to-thermocouple distance should be maintained constant for all samples to minimize the effect of the atmospheric conditions in the sample chamber on the recorded sample temperature. The probe displacement is calibrated using a micrometer or standard gauges whose thickness is precisely known. The applied load is calibrated using standard masses. On completion of the calibration procedures the instrument should be run under the proposed experimental conditions without the sample and the TMA curve recorded. This curve can be used later to correct for artefacts in the data which originate in the instrument. The sample should be homogeneous and where possible the upper and lower surfaces should be parallel and smooth. The samples used in TMA are relatively large and a heating or cooling rate of 1-5 K min is recommended. Normally the chamber is maintained under dry N2 at a flow rate of 10-50 ml min. The mass of the selected probe should be taken into consideration when estimating the load applied to the sample. TMA is used to determine the linear thermal expansion coefficient a of polymers defined as dz. 1 3 dT to Page 128 Sample Mourning Block Figure . A TMA temperature calibration using tin as the standard reference material. B Sandwich configuration used to achieve a multi-point temperature calibration where L0 is the original length of the sample and dL dT is the slope of the TMA curve. The calculated value of a is temperature dependent .
