Construction Of Continuous Helium Expansion System And Measurement Method Of Porosity Of Gas Storage Materials

Porous materials have a wide range of applications in various fields.In the energy field,shale gas predominantly exists in adsorbed phase in coal and natural porous materials such as shale and coal.The pore structure of artificial porous media affects the hydrate formation and gas storage capacity.New nanoporous materials such as metal-organic frameworks（MOFs）,covalent organic frameworks（COFs）and porous aromatic frameworks（PAFs）have been widely evaluated as gas storage materials.Understanding its porosity is of great help in evaluating its gas storage capacity.In the field of architecture,building envelope,as a typical multi-layer porous structure,its porosity and other physical parameters are the key factors affecting the properties of materials,which has attracted the attention of many scholars.In the field of chemistey,when porous metal is used as catalyst,the ion transport rate in the catalytic process can be significantly increased,thus increasing the rate of chemical reaction.To sum up,it is very important to accurately describe the porosity and other characteristics of porous materials.For materials with low porosity and low permeability,it is difficult to accurately measure the porosity.Although the application of various measurement methods has become well-established,its measurement accuracy has been recognized to a certain extent,due to the measurement principle and other factors,there are still some problems cannot be solved.In this study,a new continuous helium expansion system was constructed by referring to the multistage expansion method in the field of vacuum metrology.This system replacing the reference volume（a fixed size）of the common helium expansion system with a piston driven by a precise stepper motor.By controlling the displacement of the piston to change the volume of the piston cavity,the system can obtain multiple sets of data at one time.As a result,the measurement accuracy can be improved.In addition,the system is further analyzed from theoretical analysis and practical testing,which proves that the measurement error of the system is decreased compared with other methods.At the same time,the influence of gas selection on the measurement results of microporous samples is found,and the main conclusions are as follows:（1）Using the method of random simulation analysis,the volume measurement influence of piston stepping control error,pressure measurement error,single change of piston volume and filling rate was explored.It is found that pressure measurement uncertainty and stepper motor control error should be small.Considering the practical application,the piston step control error and pressure measurement error should be controlled within 0.10%.It is also found that the piston clearance volume has a great influence on the measurement.When the volume to be measured and the total volume ratio of the piston is in the range of 1:1-1:3,the measurement results are more accurate.（2）For the temperature distribution in the constant temperature thermotank of the continuous helium expansion system,the influence of the output power of the heating plate on the temperature distribution and temperature fluctuation in the constant temperature chamber is analyzed by using the method of simulation.Temperature fluctuation can be reduced by increasing the maximum and minimum values or decreasing its amplitude of the square wave function of heating plates output power.No matter changing the values,amplitude or period of the square wave function,the temperature difference between upper and lower in the thermotank cannot be reduced.By controlling the output power of the heating plate according to square wave function,the temperature fluctuation inside the thermotank can be controlled at±0.05K.（3）In the actual test,the nonlinear optimization method was used to analyze the measurement results from four aspects:pressure conversion check,helium leak correction,volume transformation coefficient verification,and measurement repeatability.The measurement repeatability of the system is about 2%.Before each experiment,the pressure conversion check and helium leak correction test should be carried out again to ensure the accuracy of the measurement results.The influence of the pressure conversion coefficient and helium leakage rate calculation coefficient is also discussed,and it is found that the pressure conversion coefficient has an effect of 10^-3-10^-2mm3 on the measured skeleton volume.When the helium leakage rate increases by 0.25 times,The corresponding porosity results have a decrease of about 0.3%.Metal foam,cement column,and activated carbon particles were selected as samples for measurement.The results were compared with the mercury injection method and low-temperature N₂ adsorption measurement results to prove the accuracy of the system measurement.It is found that when most of the sample pores are composed of micropores,the porosity calculation results are greatly affected by the adsorption effect,and it is difficult to ignore,so the gas with a small adsorption effect should be selected as far as possible.