Design And Performance Research Of Precision Device For Measuring Heat Capacity In The Medium Temperature Region

Heat capacity is a basic thermodynamic parameter,which is an important parameter to study and understand lattice vibration,structural transformation,magnetism,superconductivity and other phenomena of materials.It is also an important parameter to achieve the goals of energy saving,thermal management and thermal design simulation.At present,the main instruments for measuring heat capacity are physical property measurement system（PPMS）,differential scanning calorimeter（DSC）,high-temperature drop calorimeter and adiabatic calorimeter.Among them,the adiabatic calorimenter has higer accuracy in measuring heat capacity in the medium temperature region of condensed matter than other instruments mentioned above.In addition,But,traditional adiabatic calorimeter needs a large sample mass which is about（50-100）g and there is a lack of research on the repeatability of adiabatic calorimeter.However,some new materials are expensive and difficult to prepare a large number of samples,which seriously limits the exploration of thermodynamic properties of new materials and hinders the development and application of new materials.In response to the above problems,this paper mainly established a small sample-size adiabatic calorimeter in the middle temperature region,and the accuracy,precision and repeatability of the instrument were studied.The main research work are as follows:Based on the theory of heat transfer and structural characteristics of the adiabatic calorimeter,the factors affecting the adiabatic environment of the sample cell and stability of the adiabatic calorimeter were analyzed.The structure of the main parts of the adiabatic calorimeter were designed.The main material of the sample cell was silver.The heating wire was non-inductively wound on the surface of the sample cell.The platinum resistance thermometer was matched with the inlet well in the center of the sample cell through the thermowell.The thermocouples of E type were installed in a pair of thermocouple sleeves symmetrically welded on the bottom of the sample cell.The sample cell was sealed with soft metal knife edge with silver gasket and Monel alloy as knife edge material.The instrument has two layers of adiabatic shields and three layers of radiation shields.The lower covers were fixed by the heat shields fixing component.The vacuum chamber was sealed by flange.In order to reduce the radiation heat transfer of the sample cell,Comsol Multiphysics simulation software was used to study the radiation heat flux and temperature of the sample cell in a heating cycle,so as to determine a more reasonable space position of the radiation shields.In addition,the simulation results also showed that:compared with the 0-layer shield,the 3-layer shield reduced the radiant heat flux of the sample cell by about 77%.Meanwhile,the temperature variability of the sample cell was improved by 62%in the equilibrium period.Finally,after processing,assembling and debugging,the adiabatic calorimeter for small sample-size in medium temperature zone was established.The performance of the self-developed instrument was studied based on the heat capacity measurement results of copper and-Al₂O₃（SRM 720）.The heat capacity of the sample cell was calculated by combining the experimental data of copper heat capacity and the copper heat capacity data recommended by NIST in the temperature range of 300 to 570 K.The results show that the accuracy,precision and repeatability of the instrument were within±0.5%,±0.3%and±0.15%,respectively,according to the experimental values of heat capacity of-Al₂O₃ and NIST recommended values in temperature range from 300 to 570 K.