Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical proper...Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical properties of BxC-TaB2(x = 3.7, 4.9, 7.1) ceramics were comprehensively investigated via this method. Dense BxC-TaB2 ceramics with refined microstructure were obtained from submicro tantalum carbide and boron powder mixtures at 1800℃/50 MPa/5 min by spark plasma sintering. The stoichiometry of boron carbide was determined from lattice parameters and Raman shift. It was found that uniformly distributed TaB2 grains in the BxC matrix is favor of the densification process and restricting grain growth.Besides, planar defects with high density were observed from the as-formed B7.1 C grains and transient stress was considered to contribute to the densification involved with plastic deformation. Microstructural observations indicate the dissolution of oxygen in the TaB2 lattice and most of the B7.1 C/TaB2 phase boundaries were clean. Owing to the highly faulted structure and finer grain size, as-obtained BxC-TaB2 ceramics exhibit high Vickers hardness(33.3–34.4 GPa at 9.8 N) and relatively high flexural strength ranging from 440 to 502 MPa.展开更多
Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B...Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.展开更多
Developing non-precious catalysts as Pt substitutes for electrochemical hydrogen evolution reaction(HER)with superior stability in acidic electrolyte is of critical importance for large-scale,low-cost hydrogen product...Developing non-precious catalysts as Pt substitutes for electrochemical hydrogen evolution reaction(HER)with superior stability in acidic electrolyte is of critical importance for large-scale,low-cost hydrogen production from water.Herein,we report a CoCrFeNiAl high-entropy alloy(HEA)electrocatalyst with self-supported structure synthesized by mechanical alloying and spark plasma sintering(SPS)consolidation.The HEA after HF treatment and in situ electrochemical activation for 4000 cycles of cyclic voltammetry(HF-HEAa2)presents favourable activity with overpotential of 73 mV to reach a current density of 10 mA cm^(2) and a Tafel slope of 39.7 mV dec1.The alloy effect of Al/Cr with Co/Fe/Ni at atomic level,high-temperature crystallization,as well as consolidation by SPS endow CoCrFeNiAl HEA with high stability in 0.5 M H2SO4 solution.The superior performance of HF-HEAa2 is related with the presence of metal hydroxides/oxides groups on HEA.展开更多
Oxygen evolution reaction(OER),a half reaction involved in electrochemical water splitting,CO2 reduction,and metal–air batteries,restricts the efficiency of these energy conversion systems due to sluggish reaction ki...Oxygen evolution reaction(OER),a half reaction involved in electrochemical water splitting,CO2 reduction,and metal–air batteries,restricts the efficiency of these energy conversion systems due to sluggish reaction kinetics[1,2].To accelerate OER,highly efficient electrocatalysts are required.However,large-scale applications of the normally used OER catalysts(i.e.RuO2 and IrO2)are hampered by their instability and low abundance.It is highly desirable to develop earth-abundant catalysts with low cost,high activity and long-term stability.Co(Ni,Fe)(oxy)hydroxides(Co(Ni)-(O)OH)have emerged as promising OER catalysts in recent years[3].展开更多
The brittleness of ceramics restricts their engineering application.Prestressing is promising to solve the problem,yet still lacks enough attention and ex-tensive investigation.This work proposes the idea of macro-sca...The brittleness of ceramics restricts their engineering application.Prestressing is promising to solve the problem,yet still lacks enough attention and ex-tensive investigation.This work proposes the idea of macro-scale and micro-scale prestressed ceramics:to form compressive prestress in macro-or micro-scale range in the ceramics by designed additional force,which offsets the fracture stress at the crack tips,then enhances the strength of ceramics.The macro-scale prestressed ceramic has a designed long-range ordering stress distribution in a large scale,similar to the reinforced concrete and tempered glass.The micro-scale ceramic has a designed short-range ordered stress dis-tribution,similar to that in the natural biomaterials.Strategies constructing the macro-scale and micro-scale prestressed ceramics are planned.Future research interests and challenges are prospected for developing the mechan-ical properties of ceramics.展开更多
基金financially supported by the National Natural Science Foundation of China (Nos. 51521001 and 51832003)
文摘Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical properties of BxC-TaB2(x = 3.7, 4.9, 7.1) ceramics were comprehensively investigated via this method. Dense BxC-TaB2 ceramics with refined microstructure were obtained from submicro tantalum carbide and boron powder mixtures at 1800℃/50 MPa/5 min by spark plasma sintering. The stoichiometry of boron carbide was determined from lattice parameters and Raman shift. It was found that uniformly distributed TaB2 grains in the BxC matrix is favor of the densification process and restricting grain growth.Besides, planar defects with high density were observed from the as-formed B7.1 C grains and transient stress was considered to contribute to the densification involved with plastic deformation. Microstructural observations indicate the dissolution of oxygen in the TaB2 lattice and most of the B7.1 C/TaB2 phase boundaries were clean. Owing to the highly faulted structure and finer grain size, as-obtained BxC-TaB2 ceramics exhibit high Vickers hardness(33.3–34.4 GPa at 9.8 N) and relatively high flexural strength ranging from 440 to 502 MPa.
基金financially supported by the National Natural Science Foundation of China (51521001 and 51832003)the Fundamental Research Funds for the Central Universities
文摘Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.
基金National Natural Science Foundation of China(51521001,51832003)Fundamental Research Funds for the Central Universities(WUT:2019IB002)National College Students Innovation and Entrepreneurship Training Program(2019-C-B1-25).
文摘Developing non-precious catalysts as Pt substitutes for electrochemical hydrogen evolution reaction(HER)with superior stability in acidic electrolyte is of critical importance for large-scale,low-cost hydrogen production from water.Herein,we report a CoCrFeNiAl high-entropy alloy(HEA)electrocatalyst with self-supported structure synthesized by mechanical alloying and spark plasma sintering(SPS)consolidation.The HEA after HF treatment and in situ electrochemical activation for 4000 cycles of cyclic voltammetry(HF-HEAa2)presents favourable activity with overpotential of 73 mV to reach a current density of 10 mA cm^(2) and a Tafel slope of 39.7 mV dec1.The alloy effect of Al/Cr with Co/Fe/Ni at atomic level,high-temperature crystallization,as well as consolidation by SPS endow CoCrFeNiAl HEA with high stability in 0.5 M H2SO4 solution.The superior performance of HF-HEAa2 is related with the presence of metal hydroxides/oxides groups on HEA.
基金financially supported by the National Natural Science Foundation of China(51521001 and 51832003)the Fundamental Research Funds for the Central Universities(WUT:2019IB002)the Students Innovation and Entrepreneurship Training Program(2019-C-B1-25)。
文摘Oxygen evolution reaction(OER),a half reaction involved in electrochemical water splitting,CO2 reduction,and metal–air batteries,restricts the efficiency of these energy conversion systems due to sluggish reaction kinetics[1,2].To accelerate OER,highly efficient electrocatalysts are required.However,large-scale applications of the normally used OER catalysts(i.e.RuO2 and IrO2)are hampered by their instability and low abundance.It is highly desirable to develop earth-abundant catalysts with low cost,high activity and long-term stability.Co(Ni,Fe)(oxy)hydroxides(Co(Ni)-(O)OH)have emerged as promising OER catalysts in recent years[3].
基金Fundamental Research Funds for the Central Universities,Grant/Award Number:WUT:3120620196China Postdoctoral Science Foundation,Grant/Award Number:2021M702540National Key Research and Development Program of China,Grant/Award Number:2021YFA0715700。
文摘The brittleness of ceramics restricts their engineering application.Prestressing is promising to solve the problem,yet still lacks enough attention and ex-tensive investigation.This work proposes the idea of macro-scale and micro-scale prestressed ceramics:to form compressive prestress in macro-or micro-scale range in the ceramics by designed additional force,which offsets the fracture stress at the crack tips,then enhances the strength of ceramics.The macro-scale prestressed ceramic has a designed long-range ordering stress distribution in a large scale,similar to the reinforced concrete and tempered glass.The micro-scale ceramic has a designed short-range ordered stress dis-tribution,similar to that in the natural biomaterials.Strategies constructing the macro-scale and micro-scale prestressed ceramics are planned.Future research interests and challenges are prospected for developing the mechan-ical properties of ceramics.
