Research On Hydrogen Gas Sensing Characteristics Of V₈C₇ Doped SnO₂ Sensor

Dissolved gas analysis（DGA）is one of the effective technologies to ensure the safe and stable operation of power transformers.Gas sensor technology is the key to on-line monitoring technology.Hydrogen（H₂）is one of the main fault characteristic gases of dissolved gas in oil and can reflect faults such as partial discharge inside the transformer and spark discharge in oil.The SnO₂ gas sensor has certain sensitivity to dissolved gases in several oils,and has been widely used because of low cost.However,when it is used in the analysis of dissolved gas in oil,there are many problems such as high operating temperature,poor selectivity,and low sensitivity.Noble metal doping is an effective method to improve its gas-sensing performance.However,the cost of noble metals is higher.This paper uses a noble metal-like transition metal carbide V₈C₇,to prepare a double carbon coating structure V₈C₇.It was doped in pure SnO₂ powder by hydrothermal method to make a gas sensor,and the gas-sensing characteristics of H₂were studied.Based on the first principles of density functional theory,the H₂adsorption models of SnO₂ and V₈C₇ were constructed respectively,and their electronic properties were simulated and analyzed.The effect of V₈C₇ doping on the H₂gas-sensing characteristics of SnO₂-based sensors was explained from a microscopic level.The main results of the paper are as follows:（1）The V₈C₇ material with double carbon coating was prepared and characterized.The characterization results showed that the V₈C₇ crystals were interwoven to form a sheet network structure.The results of TEM and EDS mapping confirmed that the V₈C₇was wrapped by a carbon shell.The carbon shell has the confinement effect,which inhibits the growth of grains to obtain smaller grains and better morphology.（2）Pure SnO₂ and 1 wt%,2 wt%,3 wt%V₈C₇ doped SnO₂ nano powders were prepared by hydrothermal method.The characterization results show that pure SnO2and 1 wt%-3 wt%V₈C₇ doped samples are all rutile structure.V₈C₇ doping will reduce the sample particle size,and the phenomenon of agglomeration,dispersion decreases.It shows that the doping of V₈C₇ can inhibit the growth of grains.The gas-sensing test results show that the gas sensor with 1 wt%V₈C₇ doping has the best gas-sensing characteristics.The optimal operating temperature for H₂ has dropped by 50°C.At the optimal operating temperature,the sensitivity has been increased from 5.1 to 11.3,the response time and recovery time have been shortened by 3s and 9s,and the stability and selectivity are good.（3）Based on the first principle of density functional theory,the SnO₂ and V₈C₇crystal models were established respectively.Using the Build Layer tool of the Visualizer module of MS software,the heterojunction model of SnO₂ and V₈C₇ was constructed.The band diagram shows that the energy required for electrons to transition from the valence band to the conduction band decreases,which explains the reason for the drop in the operating temperature of the sensor.The adsorption models of H₂molecules on SnO₂（110）and V₈C₇（110）crystal faces were constructed respectively,and the optimal adsorption sites on SnO₂（110）and V₈C₇（110）crystal faces were calculated as Sn_5C and V_3C,respectively.And the adsorption energy of V_3C is much larger than Sn_5C（-0.766 e V vs-0.157 e V）,which explains the reason for the increased sensitivity of the sensor.