稀有金属材料与工程最新文献

Nano tin oxide (SnO₂) was synthesized by a novel, fast and facile approach through applying electric current of high density to a line-type joint specimen consisting of a high tin-content alloy between two pieces of conductor (copper or nickel). The fabricated SnO₂ shows various morphologies, such as branch-like, pine-leaf-like, grass-like and batting-like, and has the tetragonal rutile structure confirmed by scanning electron microscopy, energy dispersive X-ray spectroscope and Raman spectrum. The present work has not only offered a facile and novel method for fabricating SnO₂ of various morphologies, but also provided an inspiriting yet practical idea as well as technical possibilities for preparation of other functional oxide materials.

Ultrafine W-20wt%Cu (W-20Cu) powders doped with 0 wt%∼0.8 wt% rare earth oxides Ce_0.8Sm_0.2O_1.9 (SDC) were synthesized by the EDTA-citrate method using ammonium metatungstate and copper nitrate as raw materials, and the resultant powders were pressed and sintered at 1250 °C in H₂ atmosphere for 2 h to obtain SDC/W-20Cu composites. Morphology of the SDC/W-20Cu powders and microstructure of the sintered SDC/W-20Cu composites were observed by SEM, and density, physical and mechanical property of sintered SDC/W-20Cu composites were also tested to investigate the influence of rare earth addition. The results show that the SDC/W-20Cu powders are irregular in shape with particle size ranging from 100 nm to 200 nm. The addition of SDC has slight influence on the electrical conductivity, but obviously refines the grain size and improves the mechanical properties of the W-20Cu composites. The sintered SDC/W-20Cu samples have relative density above 97%. Bending strength and micro-hardness HV of the SDC/W-20Cu specimens are 1128 MPa and 3180 MPa with the addition of 0.6 wt% SDC, respectively, and the highest tensile strength reaches 580 MPa and 258 MPa at room temperature and 600°C, respectively.

The CoCrAlSiY alloy coating with Si mass concentrations of 0%, 2% and 5% was prepared. The effect of Si element on the structure and properties of the alloy powder and coating was analyzed. In addition, the function of Si element on the high temperature performance of the coating was also discussed. The results show that the Si element is mainly distributed in the β phase, whose volume content and distribution have a great influence on the high temperature performance of the coatings. Furthermore, the Si element promotes the change from internal oxidation to external oxidation of the alloy and the formation of the protective oxide film. However, the much higher Si content results in a higher PBR (Pilling-Bed worth ratio) value of the oxide film, and increases the oxide film stress, which is not beneficial to the improvement of the coating thermal shock resistance at high temperatures.

A new composite thermal interface material (TIM) was synthesized by combining AlN with liquid metal (LM, Ga_68.5In_21.5Sn₁₀) and polydimethylsiloxane (PDMS), one of the most commonly used silicone oils, to enhance the interfacial heat transfer. The microstructure and chemical composition of the material were analyzed using scanning electron microscopy (SEM) with an energy dispersive spectrometer (EDS) to investigate its principle of heat dissipation. The thermal conductivity (κ) of the AlN liquid metal thermal grease (ALTG) was found to be 5.014 W/m·K, higher than that of a liquid metal/PDMS composite (LMTG) and higher than that of one of the best existing thermal grease products (X23-7762) by approximately 5% and 20%, respectively. Meanwhile, the thermal contact resistance (R) was reduced by 20% and 50%, respectively, and the viscosity remained in an appropriate range, reducing the risks of overflow during usage. An actual test on a CPU showed that ALTG could significantly reduce the operating temperature. The thermal mechanism of ALTG was studied, and a synergistic effect was suggested for the heat transfer process. The results prove the ideal heat dissipation properties of TIMs and their wide application prospects in industry.

MgO film was prepared by a sol-gel method on titanium substrate for bioactive and antibacterial surface modification of osteo-implants. X-ray diffraction analysis shows that the gel film can be crystallized by the calcination at 400 °C. The MgO film is converted to Mg(OH)₂ after ageing in air. The film is crack-free observed with scanning electron microscopy, and exhibits bioactivity by inducing the formation of apatite layer in the simulated body fluid test. It is also biocompatible with osteoblast cells and slightly antibacterial against E. Coli. The sol-gel MgO film would provide a facile surface modification method for biomedical titanium implants.

The effects of particle size of precursor powders on microstructure and superconductivity of Bi-2223 tape were investigated. Three kinds of precursor powders with different particle sizes (8, 2, <1 μm) were prepared by a spray pyrolysis method via adjusting the concentration of metal nitrates solution and ball milling. All powders were composed of Bi-2212, (Sr, Ca)_xCu_yO_δ (AEC) and CuO phase after heat treatment. Results show that AEC phase dimension and content increase with decreasing of particle size of the precursor powders. However, the powder with an average particle size of 2 μm has the minimal dimension and content of CuO phase. Among the 37-filaments Bi-2223 tapes fabricated from the three kinds of precursor powders, the one from the powder with average particle size of 2 μm achieves the highest critical current (I_c), which also has the most (Bi, Pb)₃Sr₂Ca₂CuO_x (Pb-3221) and least AEC phase compared to other tapes. The particle size of the precursor powders mainly affects dimension and content of AEC and CuO phase, which further causes difference of I_c and non-superconducting phases in Bi-2223 tape.

Stray grains were observed in an electron beam welded (EBW) joint of single crystal molybdenum (Mo) in former studies. In the present paper, finite element (FE) method, combining the theoretical analysis, was used for evaluating the stray grain formation. Temperature-dependent thermal properties of molybdenum were incorporated in the model and a 3D volumetric moving double-ellipsoid heat source was applied. The relationship of parameter Φ, describing the degree of stray grain formation, to the thermal gradient G and the growth velocity V was used. Temperature and thermal gradient data of the model were extracted from simulation results under different operating conditions, and Φ was calculated. The effects of the welding parameters, including welding power Q and welding speed S on the formation of the weld stray grain were obtained.

Welding joint of 7A52 aluminum was treated by ultrasonic impact technology (UIT) and a combined treatment of aging and ultrasonic impact (A-UIT). The effects of different treatments on the microstructure and properties of the welded joint were compared and analyzed, especially the effects of A-UIT on microstructure and the mechanism of surface hardening of welded joint. The results show that the nanometer grain layer can be successfully prepared by UIT and A-UIT on the surface of aluminum alloy welded joint. From the comparison, it can be found that the surface grain size, surface microhardness and matrix microhardness of the welded joint after A-UIT are significantly higher than those of the welded joint after UIT, which exhibits a better reinforcement. The surface hardening mechanism of welded joints after UIT is fine-grained strengthening, while that of welded joints after A-UIT is fine-grained and precipitated strengthening, which is different from that of UIT.