| Combustion heat is a vital physicochemical parameter of materials.The standard combustion enthalpy and standard enthalpy of the sample can be calculated from the heat of combustion.It is also an important indicator for measuring the energy level of energetic materials.At present,the heat of combustion of materials is measured by the common water equivalent method,that is,the quantitative sample is ignited in a closed and fixed volume oxygen bomb,the temperature rise of the water in the inner cylinder is measured,and the heat of combustion is calculated according to the heat capacity of the calorimetric system.However,the method has defects of complicated operation,harsh environmental conditions,poor test accuracy,large sample amount,and the method unsuitable for the measurement of explosive samples and precious specimen.Microcalorimetry is based on Calvet principle,which uses thermopile to measure the heat released by sample directly.Microcalorimetry has advantages of high sensitivity,good stability,small sample volume,good repeatability and high precision.Therefore,this subject proposes the study on combustion heat measurement method based on Calvet principle.The main work includes the following three parts:(1)Calvet principle and the effect of thermal power on its calorimetric accuracy.The effects of heat of different powers and the same power at different time on the temperature T0 of thermopile external interface of the calorimeter and calorimetric coefficient were studied.The results show that the temperature T0 outside the thermopile does not change with the increase of power when the thermal power is less than or equal to 338 mW,and the calorimetric coefficient S value does not change with the heating time.When the power is greater than or equal to 474 mW,the temperature T0 outside the thermopile increases with the increase of power.Additionally,the results show that the temperature T0 outside the thermopile does not change with the increase of time when the thermal power is less than or equal to 352 mW,however,when the power is greater than or equal to 496 mW,the longer the time of heating,the higher the external interface temperature T0 rises,the smaller the E’ value of the thermopile is smaller than the actual E value,and the larger the S value;in addition,the high power rapid exothermic process will lead to the rapid exchange of heat between the internal and external interfaces of the thermopile.This heat will accumulate in the soaking block and cannot reach equilibrium in a short time,which makes the temperature T0 of the external interface of the measuring thermopile and the reference thermopile inconsistent,cannot be eliminated by the difference completely,and will also cause the reduced accuracy of the calorimetric result.(2)The overall design of the combustion calorimeter based on the Calvet principle.The instrument will consist of the following five parts:temperature control system,thermopile calorimetric system,electric energy calibration system,oxygen bomb and data recording and processing system.In order to reduce the rapid heat exchange caused by the rapid increase of the internal interface temperature T,the material and structure of the oxygen bomb were studied in detail.The relationship between the thermal power in different materials and structural bodies and the temperature of the soaking block and calorimetric coefficient was studied experimentally.The stainless steel body with 3 mm wall thickness and air layer was used as body of the oxygen bomb of the combustion calorimeter.In addition,in order to reduce the inconsistency of the external interface temperature T0 of the thermopile and the reference thermopile due to the rapid heat exchange of heat,the design of the temperature control system was studied by using a 10 dm3 water bath as the soaking block of the external interface of the thermopile.At the same time,the mechanical stirring accelerates the temperature of the water bath to reach equilibrium quickly,ensuring that the temperature T0 of the external interface of the measuring thermopile and the reference thermopile is consistent,and the influence of the fluctuation of the temperature T0 outside the thermopile on the calorimetric result is eliminated by the difference.(3)According to the previous design,a micro-combustion bomb which is suitable for measuring the heat of combustion based on Calvet principle was developed.The effects of samples,helium,oxygen,ignition mode,heating method and ignition wire on full combustion were studied in the micro-bomb.The results show that crucible with thin wall,shallow opening,and small mass is beneficial for the full combustion of a small amount of sample.By adjusting the ignition mode,oxygen pressure,ignition wire diameter,ignition current,and ignition time,benzoic acid samples with a mass range of 7～27 mg can be fully burned. |
