A novel CVD process for the preparation of ultrafine rhenium powders was investigated using ammonium perrhenate as starting materials. In the process, volatile rhenium oxides, such as ReO4 and Re2O7, were vaporized un...A novel CVD process for the preparation of ultrafine rhenium powders was investigated using ammonium perrhenate as starting materials. In the process, volatile rhenium oxides, such as ReO4 and Re2O7, were vaporized under a controlled oxidizing atmosphere via the pyrolysis of ammonium perrhenate, and carried into reduction zone by carrier gas, and there reduced into rhenium powders by hydrogen gas. Thermodynamic calculations indicated that Re207 could be prevented from further decomposition through controlling the oxygen partial pressure higher than 10 1.248 Pa in the pyrolysis of ammonium perrhenate. This result was further validated via DSC-TGA analysis of ammonium perrhenate. The typical rhenium powders prepared by the CVD method proposed show irregular polyhedron morphology with particle size in the range of 100-800 nm and a Ds0 of 308 nm. The specific surface area and oxygen content were measured to be 4.37 m^2/g and 0.45%, respectively.展开更多
Owing to the absence of metal binder, binderless cemented carbides have higher wear, corrosion, and oxidation resistance. WC-0.3VC- 0.5Cr3C2 powders with an average particle size of 200 nm and a little amount of activ...Owing to the absence of metal binder, binderless cemented carbides have higher wear, corrosion, and oxidation resistance. WC-0.3VC- 0.5Cr3C2 powders with an average particle size of 200 nm and a little amount of active element were consolidated by spark plasma sintering. The sintered microstructure revealed that the average WC grain size was 0.24μm, which was almost consistent with the initial free powder. The results of XRD showed that W2C phase was formed. Nearly complete densification of ultrafine binderless cemented carbide was achieved by sintering at 1400℃ for 120 s under 50 MPa. The resulting hardness and the fracture toughness were 28.18 GPa and 6.05 MPa·m1/2, respectively.展开更多
用对比实验方法系统研究了原材料组成、养护制度以及加载龄期等因素对强度等级为C50的复合超细粉煤灰(compound ultra fine fly ash,CUFA)混凝土和C50普通混凝土徐变的影响规律,并分析了相应的影响机理。结果表明:相同条件下,CUFA混凝...用对比实验方法系统研究了原材料组成、养护制度以及加载龄期等因素对强度等级为C50的复合超细粉煤灰(compound ultra fine fly ash,CUFA)混凝土和C50普通混凝土徐变的影响规律,并分析了相应的影响机理。结果表明:相同条件下,CUFA混凝土徐变比普通混凝土的显著降低;与碎卵石相比,用碎石能够降低CUFA混凝土的徐变度,但骨料品种对普通混凝土的徐变度影响不明显。养护制度对CUFA混凝土的徐变度有较大影响,蒸汽养护条件下CUFA混凝土的徐变度显著降低。适量CUFA的掺入对蒸汽养护混凝土微观结构的改善作用较标准条件养护的混凝土更加显著,较大幅度地降低了混凝土的干燥徐变。CUFA混凝土的徐变度无论是在10d还是28d加载,其徐变度约在持荷60～90d后趋于稳定。展开更多
According to the principles of simultaneous equilibrium and mass balance,a series of thermodynamic equilibrium equations of Cu2+-C2O42--NH3-NH4+-H2O system at ambient temperature were deduced theoretically and the log...According to the principles of simultaneous equilibrium and mass balance,a series of thermodynamic equilibrium equations of Cu2+-C2O42--NH3-NH4+-H2O system at ambient temperature were deduced theoretically and the logarithm concentration versus pH value(lg[Cu 2+ ]T—pH)diagrams at different solution compositions were drawn.The results show that when pH is below 5.0,copper ion reacts with C2O42-directly and the morphology of copper precursor powder is of pie-shape;when pH is above 5.0,copper ion coordinates with ammonia,and the precipitation proceeds slowly accompanying with the release of copper ions from the multi-coordinated2+ 3Cu(NH) n (n=1,2,···,5)and the morphologies of copper precursor powder are respectively of rod aggregation shape(when 5.0<pH<8.0)and of rod-shape(when pH>8.0).Some experiments were performed to confirm the relation between the total concentration of copper ion and pH value.It is shown that the thermodynamic mathematical model is correct and the calculated values are basically accurate.展开更多
文摘A novel CVD process for the preparation of ultrafine rhenium powders was investigated using ammonium perrhenate as starting materials. In the process, volatile rhenium oxides, such as ReO4 and Re2O7, were vaporized under a controlled oxidizing atmosphere via the pyrolysis of ammonium perrhenate, and carried into reduction zone by carrier gas, and there reduced into rhenium powders by hydrogen gas. Thermodynamic calculations indicated that Re207 could be prevented from further decomposition through controlling the oxygen partial pressure higher than 10 1.248 Pa in the pyrolysis of ammonium perrhenate. This result was further validated via DSC-TGA analysis of ammonium perrhenate. The typical rhenium powders prepared by the CVD method proposed show irregular polyhedron morphology with particle size in the range of 100-800 nm and a Ds0 of 308 nm. The specific surface area and oxygen content were measured to be 4.37 m^2/g and 0.45%, respectively.
文摘Owing to the absence of metal binder, binderless cemented carbides have higher wear, corrosion, and oxidation resistance. WC-0.3VC- 0.5Cr3C2 powders with an average particle size of 200 nm and a little amount of active element were consolidated by spark plasma sintering. The sintered microstructure revealed that the average WC grain size was 0.24μm, which was almost consistent with the initial free powder. The results of XRD showed that W2C phase was formed. Nearly complete densification of ultrafine binderless cemented carbide was achieved by sintering at 1400℃ for 120 s under 50 MPa. The resulting hardness and the fracture toughness were 28.18 GPa and 6.05 MPa·m1/2, respectively.
文摘用对比实验方法系统研究了原材料组成、养护制度以及加载龄期等因素对强度等级为C50的复合超细粉煤灰(compound ultra fine fly ash,CUFA)混凝土和C50普通混凝土徐变的影响规律,并分析了相应的影响机理。结果表明:相同条件下,CUFA混凝土徐变比普通混凝土的显著降低;与碎卵石相比,用碎石能够降低CUFA混凝土的徐变度,但骨料品种对普通混凝土的徐变度影响不明显。养护制度对CUFA混凝土的徐变度有较大影响,蒸汽养护条件下CUFA混凝土的徐变度显著降低。适量CUFA的掺入对蒸汽养护混凝土微观结构的改善作用较标准条件养护的混凝土更加显著,较大幅度地降低了混凝土的干燥徐变。CUFA混凝土的徐变度无论是在10d还是28d加载,其徐变度约在持荷60～90d后趋于稳定。
文摘According to the principles of simultaneous equilibrium and mass balance,a series of thermodynamic equilibrium equations of Cu2+-C2O42--NH3-NH4+-H2O system at ambient temperature were deduced theoretically and the logarithm concentration versus pH value(lg[Cu 2+ ]T—pH)diagrams at different solution compositions were drawn.The results show that when pH is below 5.0,copper ion reacts with C2O42-directly and the morphology of copper precursor powder is of pie-shape;when pH is above 5.0,copper ion coordinates with ammonia,and the precipitation proceeds slowly accompanying with the release of copper ions from the multi-coordinated2+ 3Cu(NH) n (n=1,2,···,5)and the morphologies of copper precursor powder are respectively of rod aggregation shape(when 5.0<pH<8.0)and of rod-shape(when pH>8.0).Some experiments were performed to confirm the relation between the total concentration of copper ion and pH value.It is shown that the thermodynamic mathematical model is correct and the calculated values are basically accurate.
