Reaction‐Bonded Boron Carbide/Magnesium–Silicon Composites

Reaction‐bonded boron carbide was manufactured by infiltrating porous boron carbide preforms at 1273 K with a Mg‐Si eutectic alloy. The resulting composite material consists, in addition to the original B₄C, of SiC, Mg₂Si, and a Mg‐rich complex boride/carbide Mg_x(Al,Si)_y(B,C)_z phase. The composites display high hardness (1700 HV), Young's modulus (356 MPa) and a moderate bending strength (230 MPa). The ballistic efficiency (of about 6.7), as determined by the depth of penetration method, is much higher than that of alumina and similar to that of silicon‐infiltrated reaction‐bonded composites.     

Sintering and Phase Evolution of Electroless-Nickel-Coated Alumina Powder

Alumina-nickel powders have been prepared via electroless-nickel (EN) coating of submicrometer-sized alumina powder. The EN layers contain 5.1 ± 0.4 wt% phosphorus and are very unevenly distributed on the surfaces of the alumina particles. These layers consist of amorphous and microcrystalline phases. At temperatures greaterthan equal to1300°C, the EN layers de-wet from the alumina surfaces to become discrete, round particles in the alumina matrix. The alumina-EN pellets can be sintered to reach ~95% of the theoretical density at 1500°C for 4 h in graphite crucibles. The phases of the sintered samples consist of alumina, nickel, and nickel phosphides. However, in the exterior region of the sample sintered at 1500°C for 4 h, Ni₃Al will also form.     

Synthesis and Evolution of Zirconium Carbide via Sol–Gel Route: Features of Nanoparticle Oxide–Carbon Reactions

A sol–gel route was used to synthesize nanocrystalline zirconium carbide (ZrC). The starting materials were zirconium propoxide, with carbon introduced by furfuryl alcohol (FA). A block copolymer surfactant was used to homogenize the oxide and carbon components. ZrC was produced at a low temperature of 1250°C and complete conversion achieved at 1450°C. The powder was nanocrystalline size less than 100 nm. Phase changes were studied using X‐ray diffraction and Raman spectroscopy. Rietveld analysis followed the changes in crystallite size and lattice parameters of the phases during carbothermal reduction. Morphology changes were observed using nitrogen gas sorption. High‐resolution TEM and EDS were used to image the carbide lattice, surface oxides, and graphene‐like carbons. The results indicate that nanoparticle carbothermal synthesis involves agglomeration and necking as the most viable mode of mass transport to complete the carbothermal reduction.     

氧化铝/氮化铝陶瓷显微组织研究

氮化铝具有良好的热学、电学和机械等性能,是理想的电子封装材料和高性能陶瓷基板材料.本文研究了AlN加入量和烧结温度对Al2O3/AlN复相陶瓷相组成和显微组织的影响.结果表明该陶瓷在1400～ 1550℃烧结时,AlN被部分保留,少量氧原子进入AlN晶格,烧结生成4种铅锌矿结构新相,有利于提高复相陶瓷热导率;氮化铝含量和烧结温度的提高,有利于形成大尺寸晶粒.     

抗紫外纳米ZnO/TiO2粉体的氧化铝表面改性

用液相沉积法对纳米ZnO/TiO2进行了表面改性。用TEM、XRD和FTIR对产物进行了结构表征,用静态沉淀法分析了改性前后纳米ZnO/TiO2的分散稳定性,用紫外-可见分光光度计对其紫外屏蔽性能进行了检测。结果表明,改性纳米ZnO/TiO2表面存在致密的氧化铝膜,产物经充分分散后在有机介质中或水中的稳定时间分别由改性前的2 m in和5 m in提高到2 h和1 d,紫外线透过率由改性前的大于8.5%降低到小于7%。     

Phase Evolution,Microstructure, and Mechanical Properties of Alumina–Mullite–Zirconia Composites Prepared by Iranian Andalusite

This paper investigates the effects of Iranian andalusite and short milling times on alumina–mullite–zirconia composites. Andalusite powder was added at 0, 2.5, 5, and 10 wt% to an alumina–zircon mixture and the raw materials were milled for 1 or 3 h. The sintering of samples was carried out at the temperatures of 1550°C, 1600°C, and 1650°C for 3 h. Microstructural changes, phase composition, physical properties, and mechanical strength of the sintered composites were characterized by scanning electron microscopy, X‐ray diffraction, density, and strength measurement tests. Results show that andalusite promoted the decomposition of zircon and accelerated the reaction sintering of alumina–zircon, which leads to the formation of much more mullite phase and improvements to the composites’ thermal shock resistance up to about 50%.     

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore-size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build-up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.     

Residual Strains in an Al2O3-Ni Joint Bonded with a Composite Interlayer: Experimental Measurements and FEM Analyses

A cylindrical Al₂O₃-Ni joint bonded with a 4.0 mm thick composite interlayer of 40 vol% Al₂O₃-60 vol% Ni was fabricated by powder processing, and the residual strains in the specimen were studied experimentally using neutron diffraction, X-ray diffraction, and optical fluorescence spectroscopy. Experimental measurements were compared with finite element modeling results obtained using a variety of different constitutive assumptions. The predicted residual strain distribution within the Al₂O₃ along the center of the specimen was in reasonable agreement with neutron diffraction measurements, although the magnitude of the error in the experimental measurements did not allow a distinction to be made between the various modeling assumptions examined. In contrast, the predicted peak strains within the Al₂O₃ along the specimen surface were significantly higher than those measured by X-ray diffraction or optical fluorescence spectroscopy, suggesting that strain relief occurred near the free edge during cooling. The mechanism of strain relief is believed to involve damage accumulation within the composite interlayer that was not accounted for in the constitutive models used.     

水热法制备片状氧化铝纳米粉体

在碳酸氢铵和PEG2000混合溶液中,加入硫酸铝铵和PEG400的混合溶液得到白色沉淀,将白色沉淀物与正丁醇共沸后,在900℃煅烧1h,得到γ-Al2O3粉体,粉体粒径约为20nm;在1200℃煅烧1h,得到片状α-Al2O3粉体,粉体粒径约为60nm。     

以硼硅酸乙酯作结合剂的碳化硅泥料的成型性能

根据硼硅酸乙酯结合剂的种类和数量，分析了碳化硅泥料流变性状的变化。查明了在半干法成型制品时结合剂的最佳含量范围。     

Microstructure and Composition of Alumina/Aluminum Composites Made by Directed Oxidation of Aluminum

Two Al₂O₃/Al composites, grown by the directed oxidation of molten Al alloys at 1400 and 1600 K, were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and wet chemical analysis. The materials were found to contain a continuous network of Al₂O₃, which was predominantly free of grain-boundary phases and was made up of nanometer- to micrometer-sized crystallites, a continuous network of Al alloy, and isolated inclusions of Al alloy. No crystallographic orientation was observed in the metallic phase, whereas the Al₂O₃ was oriented with its c axis parallel to the growth direction. The higher process temperature yielded a lower metal content and less connectivity of the metallic consituent.     

盐助自蔓延法制备无定型硼粉中硼含量的测定

采用多元醇强化硼酸法,以甘露醇为强化剂,酚酞为指示剂测定出盐助自蔓延法制备的无定型硼粉中三氧化二硼的含量,同时用原子吸收分光光度法测定出Mg含量,利用差减法最终得到了盐助自蔓延法制备出的无定型硼粉中的硼含量,硼含量为92.800%。这种测定硼含量的方法可以非常简单、便捷的测定出样品中硼的含量,并且此方法适宜于主量元素硼的测定。     

Microstructure and miscibility of acrylonitrile–butadiene rubber/ethylene–propylene–diene monomer blends studied by positron annihilation spectroscopy

The free‐volume properties and miscibility of ethylene–propylene–diene monomer/acrylonitrile–butadiene rubber blends with poly(vinyl chloride) used for compatibilization were investigated with positron annihilation lifetime spectroscopy and Doppler broadening of annihilation radiation. The results showed that the ortho‐positronium annihilation lifetimes and intensities as well as the S parameter had a linear relationship with a negative slope as a function of the weight percentage of acrylonitrile–butadiene rubber, which indicated the miscibility of the blend. The filling effect of silica on the free‐volume properties of an ethylene–propylene–diene monomer/acrylonitrile–butadiene rubber (75/25) blend was also examined. On the other hand, a correlation between the size and concentration of the free‐volume holes and the electrical and mechanical properties of the aforementioned blends was established. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Phase Evolution upon Aging of Air Plasma Sprayed t′‐Zirconia Coatings: II–Microstructure Evolution

The correlation between microstructural and phase evolution in aged, yttria‐partially‐stabilized zirconia, air plasma‐sprayed coatings is discussed. Freestanding coatings with the dense, vertically cracked structure were isothermally aged at 1482°C (2700°F) in air. Characterization of the resulting microstructures was conducted using transmission electron microscopy, then compared with a parallel analysis of the phase evolution via synchrotron X‐ray diffraction (XRD) described in Part I. Additional context was provided by related studies on vapor‐deposited coatings. Several salient points can be extracted from these assessments. XRD was further validated as a practical method for studying phase stability after clarification of how the possible phases are defined, including the following: (i) the nature of the t′ phase observed in XRD after phase decomposition has begun and (ii) the relationship between the Y‐rich tetragonal (t″) and Y‐rich cubic (c) phases reported to coexist via XRD. A strong relationship between the initial microstructure and the subsequent phase destabilization is also reported. As a result, phase evolution is proposed to proceed via two competing routes. The interplay between these mechanisms dictates the incubation time for monoclinic formation within a given coating.     

Microstructure and Reactions of SiCw-Reinforced Alumina with Ag-Cu-In-Ti

Brazing experiments were performed at 750°C for 2 h between Ag–Cu–In–Ti alloy and SiC_w/Al₂O₃. The first clearly nonbraze layer consists of an oxide layer of metallic composition 33Ti–31Al–22Cu–14Si. Areas adjacent to the SiC whiskers were of different composition. A thin, continuous layer on the alumina portion of the composite appears to be γ-TiO. The SiC whiskers are preferentially consumed and undergo reductions in diameter of approximately 40%. Observed "knobby" whisker morphologies may be related to SiC stacking faults, η-type phases detected near the Ag–Cu eutectic portion of the joint appear to consist of Ti-Cu-Al-Si-O and Ti₃Cu₃O.     

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

A chemical process for fabrication of Si₃N₄/BN nanocomposite was devised to improve the mechanical properties. Si₃N₄/BN nanocomposites containing 0 to 30 vol% hexagonal BN ( h -BN) were successfully fabricated by hot-pressing α-Si₃N₄ powders, on which turbostratic BN ( t -BN) with a disordered layer structure was partly coated. The t -BN coating on α-Si₃N₄ particles was prepared by reducing and heating α-Si₃N₄ particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN ( h -BN) particles were homogeneously dispersed within Si₃N₄ grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro- and nanocomposites decreased with an increase in h -BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.     

Alumina/Silicon Carbide Nanocomposites by Hybrid Polymer/Powder Processing: Microstructures and Mechanical Properties

Nanocomposites with fine, coarse, and bimodal silicon carbide (SiC) particle-size distributions were hot pressed and examined by transmission electron microscopy, scanning electron microscopy, and optical microscopy, as well as by four-point-bend and indentation tests. The finer SiC nanophase was introduced homogeneously by coating a silicon-containing polymer onto the alumina (Al₂O₃) powder, followed by a pyrolysis procedure; for the coarser SiC, nanophase conventional powder processing was used. Powder- and polymer-processed nanocomposites both had their maximum strengths at 5 vol% of SiC. High-strength nanocomposites that contained a higher volume fraction of SiC could be fabricated when the two methods were combined in a hybrid processing route. The SiC phase in the resulting hybrid materials originated from both the polymer and the SiC powder. The mechanical properties of these materials could be correlated with the fabrication route. Processing-flaw populations and calculated Griffith-flaw sizes were not only smaller, but they were also significantly different in the nanocomposites, in comparison to those in Al₂O₃ ceramics; this may explain the strength increase in Al₂O₃/SiC nanocomposite materials.