Microstructure,Electrical And Oxidation Resistance Of AlN-CoCrFeMnNi Cermet Composites

Electronic cigarettes have developed rapidly in the tobacco field because they do not contain harmful substances such as tar,carbon monoxide,and carbon dioxide.The common working temperature of the electronic cigarettes heating wire is 300~400℃,the corresponding resistivity is about 103 μΩ·cm(estimated based on the power of the electronic cigarette battery and the geometric size of the heating wire).The traditional resistance wire material,due to the small resistance rate,the heat generated is too large,and it is difficult to maintain normal operating temperature.In order to prepare resistance wire materials that can be heated directly,we selected a high-resistivity CoCrFeMnNi high-entropy alloy(resistivity of 102 μΩ·cm),compounded it with AlN insulating ceramics with good thermal conductivity to prepare cermets that meet the requirements of standard resistance wire of electronic cigarette.In this paper,x AlNCoCrFeMnNi cermets(wt.%,x=10,20,30,40,50,60,70)were prepared by hot-pressing sintering with gas atomized CoCrFeMnNi high-entropy alloy and AlN powder as raw materials.The effect of AlN content on AlN-CoCrFeNiMn cermet composites was mainly studied effects of microstructure,mechanical properties,electrical properties,and antioxidant properties.The specific research results are as follows:(1)The optimized AlN-CoCrFeNiMn cermet hot-pressing sintering process was obtained.After the cermet composite powder was hot-pressing sintering at 60 MPa and1240℃ for 2 h,the sintered body had the highest degree of densification.With the increase of AlN content,the hardness of AlN-CoCrFeNiMn cermet first increased and then decreased.When the AlN content increased 50%,the cermet sintered body had the highest hardness,which was 489 HV.(2)In the microstructure of AlN-CoCrFeNiMn cermet sintered body,in addition to the single FCC structure Co Cr Fe Ni Mn solid solution phase and AlN phase,a small amount of Cr7C3 is also formed,and there is no obvious interfacial reaction between AlN and Co Cr Fe Ni Mn high-entropy alloy.(3)The resistivity of AlN-CoCrFeNiMn cermet increases with the increase of AlN content at room temperature.When the mass fraction of AlN is 30%,the resistivity of the cermet sintered body is 103 μΩ·cm,which meets the requirements of resistance wire for electronic cigarettes.When the temperature increases,the resistivity of the AlN-CoCrFeNiMn cermet exhibits obvious electronic conduction characteristics,the resistivity of the cermet increases with the increase of temperature.(4)The electrical conductivity of AlN-CoCrFeNiMn cermet exhibits a typical percolation phenomenon in composites.Based on the general effective medium(GEM)equation,the conductivity model of AlN-CoCrFeMnNi cermet at room temperature was established,in which the percolation threshold fc was 0.85 and the critical exponent t was 1.2.That is,when the volume fraction of AlN reaches 85%,the electrical conductivity of the cermet has a sudden change,which is reduced by 5 orders of magnitude.(5)The oxidation kinetic curves of AlN-CoCrFeNiMn cermets at working temperature(300°C and 400°C)and high temperature(600°C and 800°C)all follow the parabolic law,that is,the oxidation process is divided into two stages.That is to say,the oxidation weight gain rate is faster in the early stage of oxidation,and the oxidation rate of cermet becomes slower with the prolongation of oxidation time.The oxidation products of cermet are mainly Mn2O3 and a small amount of Fe3O4.The oxidation mechanism of AlN-CoCrFeNiMn cermet is the selective oxidation caused by the rapid outward diffusion of metallic Mn and Fe cations and the inward diffusion of oxygen ions.