M2 high-speed steel samples were fabricated by laser additive manufacturing and tempered at different times at a temperature of 560°C. The microstructures of deposited samples were characterised by fine equiaxial grains, dendrites and inter-dendritic network-shape eutectic carbides and were composed of supersaturated martensite, retained austenite and M2C-type carbides. The content of retained austenite gradually decreased with increasing tempering times. Meanwhile, the micro-hardness of deposited samples was 688 ± 10 HV, while the first, second and third tempering times were 833 ± 13 Hv, 710 ± 6 Hv and 740 ± 7 Hv, respectively (standard deviations).Wear resistances of all samples showed an adhesive wear mechanism, and M2 HSS without tempering had a lower friction coefficient with an average of 0.52. M2 HSS after tempering twice at 560°C/2 h had a larger wear volume loss than others.

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Sycamore villus fibers were used to prepare hollow and porous carbon microtubes (CMTs) and the ZnO/CMT composite with heterojunctions by simple carbonization for the first time. Because the hollow and porous structure provided more channels to facilitate the adsorption and desorption of gas molecules, both CMTs and ZnO/CMT exhibited higher sensitivity and quicker response (<16 s) to and recovery (<2 s) from multiple target analytes. Furthermore, ZnO nanoparticles were uniformly dispersed on the CMTs by zinc acetate-assisted carbonization, which avoided the agglomeration of ZnO and formed a large number of heterojunctions, greatly improving the sensitivity of ZnO/CMT. In comparison to the pure CMTs and ZnO, the response of ZnO/CMT to the four target gases increased by 1.4∼4.3 and 9.9∼18.1 times, respectively. Their limit of detection for NH 3 was calculated as 62.5 and 8.8 ppb, respectively. After 30 days, the responses of CMTs and ZnO/CMTs to 500 ppm NH 3 decreased by 9.4 and 6.5%, respectively. This indicated that CMTs and ZnO/CMT had high sensitivity and good long-term stability. This study provides a feasible way for the gas-sensing application of biomass carbon materials with heterojunction structures.

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