Synthesis of MoSi2–Al2O3 nanocomposite by mechanical alloying

MoSi₂–Al₂O₃ nanocomposite was synthesized by mechanical alloying (MA) of MoO₃, SiO₂ and Al powder mixture. The structural evolution of the powders was studied by X-ray diffraction (XRD). Both β-MoSi₂ and -MoSi₂ were obtained after 3 h of milling. The spontaneous formation of β-MoSi₂ during milling proceeded by a mechanically induced self propagating reaction (MSR), analogous to that of the self propagating high temperature synthesis (SHS). After 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. Residual Mo and Si in the 3 and 70 h milled samples formed β-MoSi₂ and Mo₅Si₃ during heating at 1000 °C, respectively.     

A simple and an effective method for the fabrication of densified glass-ceramics of low temperature co-fired ceramics

This study reports a new, simple and effective pre-calcined method for fabrication BaO–TiO₂–B₂O₃–SiO₂ low temperature co-fired ceramics (LTCC) at a sintering temperature below 900 °C, and with dielectric losses (tan δ) lower than 2 × 10⁻³. The research results have shown that the addition of 2–5 wt% Al₂O₃ could easily eliminate the porosity of the glass-ceramics because of the excellent wetting behavior between alumina and the BaO–B₂O₃–SiO₂ glass liquid phase in the low temperature co-fired ceramic system.     

Microstructure and properties of MoSi2–MoB and MoSi2–Mo5Si3 molybdenum silicides

MoSi₂-based intermetallics containing different volume fractions of MoB or Mo₅Si₃ were fabricated by hot-pressing MoSi₂, MoB, and Mo₅Si₃ powders in vacuum. Both classes of alloys contained approximately 5 vol.% of dispersed silica phase. Additions of MoB or Mo₅Si₃ caused the average grain size to decrease. The decrease in the grain size was typically accompanied by an increase in flexure strength, a decrease in the room temperature fracture toughness, and a decrease in the hot strength (compressive creep strength) measured around 1200 °C, except when the Mo₅Si₃ effectively became the major phase. Oxidation measurements on the two classes of alloys were carried out in air. Both classes of alloys were protected from oxidation by an in-situ adherent scale that formed on exposure to high temperature. The scale, although not analyzed in detail, is commonly recognized in MoSi₂ containing materials as consisting mostly of SiO₂. The MoB containing materials showed an increase in the scale thickness and the cyclic oxidation rate at 1400 °C when compared with pure MoSi₂. However, in contrast with the pure MoSi₂ material, oxidation at 1400 °C began with a weight loss followed by a weight gain and the formation of the protective silica layer. The Mo₅Si₃ containing materials experienced substantial initial weight losses followed by regions of small weight changes. Overall, the MoB and Mo₅Si₃ additions to MoSi₂ tended to be detrimental for the mechanical and oxidative properties.     

α籽晶促进低温反应溅射沉积α-Al2O3薄膜

分别使用反应溅射Al+α-Al₂O₃(15% α-Al₂O₃,质量分数)复合靶和在金箔基体表面预植α-Al₂O₃籽晶,促进α-Al₂O₃薄膜的低温沉积。使用扫描电子显微镜(SEM)、掠入射X射线衍射(GIXRD)和能谱仪(EDS)等方法表征薄膜样品的表面形貌、相结构和元素组成。结果表明,在射频反应溅射Al+α-Al₂O₃复合靶、沉积温度为560℃条件下能在Si(100)基体上沉积出化学计量比的单相α-Al₂O₃薄膜;使用射频反应溅射Al靶、沉积温度为500℃条件下能在预植α-Al₂O₃籽晶(籽晶密度为10⁶/cm²)的金箔表面沉积出化学计量比的单相α-Al₂O₃薄膜。两种研究方案的结果均表明,α-Al₂O₃籽晶能促进低温沉积单相α-Al₂O₃薄膜。     

低温反应溅射Al+α-Al2O3复合靶沉积α-Al2O3薄膜

低温沉积α-Al₂O₃薄膜是拓展其实际工程应用的关键。本研究以Al、α-Al₂O₃和Al + 15wt% α-Al₂O₃为靶材, 用射频磁控溅射在Si(100)基体上沉积氧化铝薄膜。用掠入射X射线衍射(GIXRD)、透射电子显微镜(TEM)、能谱仪(EDS)对所沉积薄膜的相结构和元素含量进行研究, 用纳米压痕技术测量薄膜硬度。结果表明, 在550 ℃的基体温度下, 反应射频磁控溅射Al+α-Al₂O₃靶可获得单相α-Al₂O₃薄膜。靶中的α-Al₂O₃溅射至基片表面能优先形成α-Al₂O₃晶核, 在550 ℃及以上的基体温度下可抑制γ相形核, 促进α-Al₂O₃晶核同质外延生长, 并最终形成单相α-Al₂O₃薄膜。     

Al2O3基多孔隔热材料表面Al2O3/MoSi2涂层的制备及其性能

采用刷涂法在Al₂O₃基多孔隔热材料表面制备Al₂O₃/MoSi₂涂层,涂层以硅溶胶作为粘结剂,纳米Al₂O₃与Al₂O₃纤维作为耐高温组分,MoSi₂为高发射率组分。通过SEM、XRD对Al₂O₃/MoSi₂涂层微观表面结构、物相组成进行分析。研究纳米Al₂O₃与Al₂O₃纤维的质量比和MoSi₂含量对Al₂O₃/MoSi₂涂层耐温性能的影响,并对Al₂O₃/MoSi₂涂层的抗热震性能、发射率进行表征。结果表明,当纳米Al₂O₃与Al₂O₃纤维的质量比小于1∶1时,热考核后Al₂O₃/MoSi₂涂层表面无裂纹产生;当纳米Al₂O₃与Al₂O₃纤维的质量比在1∶2~1∶4之间时,Al₂O₃/MoSi₂涂层中的纤维网络较完整。MoSi₂的含量为20%时,Al₂O₃/MoSi₂涂层抗热震实验循环25次后表面保持完好,热考核后在2.5~25 μm波段的平均发射率在0.85左右,具有较高的发射率。      

Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.     

Processing and interfacial bonding strength of 2014 Al matrix composites reinforced with oxidized SiC particles

The SiC powder with a SiO₂ protective layer is used as the reinforcements for 2014 Al/SiC_p composites to suppress the reaction between the Al matrix and the SiC particle. 2014 Al/SiC_p composites were fabricated by vacuum hot pressing (VHP) and subsequent extrusion using 2014 Al powders and the SiC particles covered with a SiO₂ layer. The interfacial product was found to be Mg spinel (MgAl₂O₄) formed mainly by the chemical reactions of the SiO₂ layer covered on SiC particles with the Mg, Al in the 2014 Al alloy matrix. Also the interfacial bonding strength of the composites was investigated using push-out tests of SiC rods with the SiO₂ oxidation layer, which were processed within 2014 Al alloy.     

Influence of Mn doping on microstructure and DC-accelerated aging behaviors of ZnO–V2O5-based varistors

The microstructure, electrical properties, dielectric characteristics, and DC-accelerated aging behavior of the ZnO–V₂O₅–MnO₂ system sintered were investigated for MnO₂ content of 0.0–2.0 mol% by sintering at 900 °C. For all samples, the microstructure of the ZnO–V₂O₅–MnO₂ system consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the ZnO–V₂O₅ system was found to restrict the abnormal grain growth of ZnO. The nonlinear properties and stability against DC-accelerated aging stress improved with the increase of MnO₂ content. The ZnO–V₂O₅–MnO₂ system added with MnO₂ content of 2.0 mol% exhibited not only a high nonlinearity, in which the nonlinear coefficient is 27.2 and the leakage current density is 0.17 mA/cm², but also a good stability, in which %ΔE_1 mA = −0.6%, %Δ = −26.1%, and %Δtan δ = +22% for DC-accelerated aging stress of 0.85E_1 mA/85 °C/24 h.     

Corrosion of magnesia–chrome brick in molten MgO–Al2O3–SiO2–CaO–FetO slag

The corrosion of magnesia–chrome (MgO–Cr₂O₃) brick in molten MgO–Al₂O₃–SiO₂–CaO–Fe_tO slag has been characterized using a dynamic rotary slag corrosion testing for various test cycles at 1650 °C. The open porosity decreases from 15.3 to 4.0% for three cycles, then it gradually increases from 4.0 to 4.8% when the test is extended to nine cycles, in which the permeating depth of the slag maintains at about 20 mm. The XRD pattern of the permeated layer shows the presence of the MgO, MgCr₂O₄ and CaMgSiO₄ phases. In the interior of the permeating layer cracks are formed and corrosion starts at the pores and cracks of MgO and decreases gradually. However, at 20–40 mm beneath the permeated layer edge, different shapes of MgO particles are found.     

低负载量Pd/CeO2/γ-Al2O3催化剂用于低温催化氧化VOCs

采用直接吸附法制备了Pd负载量为0.03% (质量分数)的Pd/γ-Al₂O₃和Pd/CeO₂/γ-Al₂O₃催化剂, 并用于评价VOCs的催化氧化性能。通过X射线衍射(XRD)、N₂吸附-脱附(BET)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)、氢气程序升温还原(H₂-TPR)等对催化剂的结构和表面性能进行了表征。结果表明, 在VOCs体积分数为0.1%, 空速(GHSV)为18000 mL/(g·h)条件下, Pd/CeO₂/γ-Al₂O₃催化剂上甲苯、丙酮和乙酸乙酯实现98%转化率的温度分别为205、220和275 ℃, 比Pd/γ-Al₂O₃分别降低了15、15和20 ℃, 而且即使在较高的气体空速下, Pd/CeO₂/ γ-Al₂O₃催化剂仍能展现出优异的催化氧化性能, 且具有很好的稳定性和选择性。氧化铈的加入对材料的物理化学性质和催化活性有一定的影响, 其中Pd/CeO₂/γ-Al₂O₃含有Ce ³⁺和高含量的PdO, 活性物种主要以PdO形式均匀地分散在载体γ-Al₂O₃表面。另外, PdO与非化学计量的CeO₂之间的金属-载体相互作用增强了Pd/CeO₂/γ-Al₂O₃催化氧化性能。     

制备方法对Ru/γ-Al2O3与等离子体共活化CO2甲烷化反应的影响

等离子体与催化材料协同作用CO2甲烷化反应为CO2再利用提供了可能,但催化材料的制备方法对其结构和性能有重要影响。本研究以等体积浸渍法制备的Ru/γ-Al2O3为催化材料前驱体,分别采取H2大气压冷等离子体还原和H2热还原方法制备Ru/γ-Al2O3-P和Ru/γ-Al2O3-T催化材料。考察两种方法制备Ru/γ-Al2O3催化材料与大气压冷等离子体共同作用下CO2甲烷化反应中的催化活性,并采用不同测试方法研究制备方法对Ru/γ-Al2O3结构的影响,分析影响Ru/γ-Al2O3催化活性的结构因素,进而探究了Ru/γ-Al2O3-P和Ru/γ-Al2O3-T催化材料的制备机理。研究结果表明:载体γ-Al2O3与大气压等离子体共同作用下CO2转化率为24.8%,主要产物是CO;Ru/γ-Al2O3与大气压等离子体共同作用下的主要产物是甲烷。Ru/γ-Al2O3-T和Ru/γ-Al2O3-P催化材料的CO2转化率分别为66.9%和77.3%。Ru/γ-Al2O3-P较高的催化活性源于其表面Ru还原程度高、Ru/Al原子比高以及Ru单质在载体γ-Al2O3上分散性较好且粒径较小,说明采用大气压H2冷等离子体技术可制备高活性的负载型金属催化材料。     

Raman spectroscopy and the origin of electrooptical Kerr phenomenon in niobium alkali-silicate glasses

It is shown that magnitudes of the Kerr coefficient and permittivity of 19Na₂O–11K₂O–2B₂O₃–2CdO–(66−x)SiO₂–xNb₂O₅ glasses, where 0<x<37, correlate with the content of the Na₂O·Nb₂O₅ groupings, with that content being estimated by Raman scattering data. This allows assigning to these groupings, or to other structural entities containing such groupings (for example, crystal motifs), the responsibility for high-polarizability and Kerr sensitivity of the glasses.     

Synthesis and formation mechanisms of molybdenum silicides by mechanical alloying

The synthesis and formation of MoSi₂, Mo₅Si₃, and Mo₃Si compounds by the mechanical alloying of MoSi powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10–80 at.% Si. The formation of molybdenum silicides, especially MoSi₂, during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of MoSi₂ during mechanical alloying at 67 at.% Si (MoSi₂ stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80 at.% Si, however, the formation of MoSi₂ proceeded by the gradual formation of both the and /gb phases instead of the MSR mode. The formation of Mo₅Si₃ during mechanical alloying was characterized by a slow reaction rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to the direct formation of Mo₃Si. The Mo₃Si phase appeared only after brief annealing at temperatures of 800°C and above.     

Microstructure characterization in cryomilled Al 5083

Nanocrystalline metals and alloys processed by severe plastic deformation (SPD) generally have improved mechanical strength compared with conventionally processed materials. In this work, we survey the microstructure of an Al 5083 alloy prepared by ball-milling powders at cryogenic temperatures (cryomilling) then consolidated by hot-isostatic pressing (HIPing) and extrusion into cylindrical billets. After milling, the particles are comprised of nanocrystalline grains, which are maintained following extrusion. We identify MgO, Al₆(FeMnCr), Al(MnFe)Si, AlCrMg, Mg₂Si, and SiO₂ phases as precipitates or dispersoids in the microstructure. This synthesis method results in a yield strength that is approximately twice that of typical wrought Al 5083 alloys. We find that the microhardness is essentially unchanged after annealing at temperatures up to 0.8T_m. The influence of the components of the microstructure on the measured mechanical properties is discussed.     

Microstructures in Advanced Materials Characterized by HREM and Nanoscale Analysis

Segregation of yttrium induces the formation of Y_0.25Zr_0.75O_2-x and Y_0.5Zr_0.5O_2-y microdomains, with L1₂- and L1₀-like ordered structures, in ZrO₂–6mol%Y₂O₃ ceramics in both the sintered and annealed states. The compositions of precipitates such as χ_L, χ_S, χ_SS, and small precipitates formed inside χ_L, in Cu–11.88Al–5.06Ni–1.63Mn–0.96Ti (wt.%) shape memory alloys have been determined. Under electron beam irradiation, four types of dynamic behavior of the G.P. zones were observed in the Al–6.58Zn–2.33Mg–2.40Cu (wt.%) alloy. The G.P. zone and “G.P. zone-like” defect structures were also distinguished. Lattice distortion profile in the GaAs/In_xGa_1-xAs superlattice and two-dimensional lattice distortion around a 60° dislocation core in the InAs_xP_1-x/InP superlattice were determined.     

电解液组分对TiCP/Ti6Al4V复合材料微弧氧化膜耐蚀性及耐磨性影响

采用微弧氧化技术在 TiC_P/Ti6Al4V 复合材料表面制备陶瓷膜。在NaAlO₂和NaH₂PO₂两种溶液体系中通过添加不同添加剂 NaOH、C₁₀H₁₂CaNa₂N₂O₈·4H₂O和Na₂SiO₃, 研究电解液组分对陶瓷膜组织、耐蚀性和耐磨性的影响。结果表明: 在NaH₂PO₂电解液体系中生成的膜层由金红石型和锐钛矿型TiO₂相组成, 而在NaAlO₂体系中除了生成TiO₂外, 还生成了Al₂TiO₅和γ-Al₂O₃。添加NaOH可以加快微弧氧化反应速率, 添加NaAlO₂和Na₂SiO₃有利于提高膜层的硬度, NaH₂PO₂溶液体系中形成的膜层厚度是NaAlO₂溶液体系的2~3倍。 在NaAlO₂和NaH₂PO₂电解液体系中生成的膜层, 其耐腐蚀性能排序均为: Na₂SiO₃>C₁₀H₁₂CaNa₂N₂O₈·4H₂O>NaOH。在NaAlO₂电解液体系中生成的膜层的耐磨性能排序为: Na₂SiO₃>NaOH>C₁₀H₁₂CaNa₂N₂O₈·4H₂O, 而在NaH₂PO₂电解液体系中生成的膜层的耐磨性能排序为: Na₂SiO₃>C₁₀H₁₂CaNa₂N₂O₈·4H₂O>NaOH。TiC_P/Ti6Al4V复合材料经过微弧氧化处理后, 耐磨性和耐蚀性均优于基体, 在NaH₂PO₂+Na₂SiO₃电解液中生成的微弧氧化膜的耐蚀性最好, 耐磨性也较好, 其腐蚀电流密度较钛基复合材料基体降低约2个数量级, 因此综合性能最好。     

Phase relations, lattice thermal expansion in Ce1−xEuxO2−x/2 and Ce1−xSmxO2−x/2 systems and stabilization of cubic RE2O3 (RE: Eu, Sm)

The phase relations in CeO₂–Eu₂O₃ and CeO₂–Sm₂O₃ systems have been established under slow-cooled conditions from 1400 °C. The two-phase relations differ as the CeO₂–Eu₂O₃ system showed only two monophasic phase fields, namely F-type cubic and C-type cubic, whereas CeO₂–Sm₂O₃ system showed three phase fields namely F-type cubic, C-type cubic and a biphasic field comprising of C-type cubic and monoclinic phase. An interesting observation of this investigation is the stabilization of C-type rare-earth oxide after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at RE³⁺ site. The lattice thermal expansion behavior of F-type solid solution and C-type solid solution in CeO₂–Eu₂O₃ system was investigated by high-temperature XRD.     

大气等离子喷涂MoSi2-30Al2O3电热涂层的组织结构及性能

以MoSi₂-30Al₂O₃混合粉末为原料, 利用大气等离子喷涂技术制备MoSi₂-Al₂O₃体系电热涂层。采用XRD、SEM、通电测试、热重-差热分析等对涂层的相组成、组织形貌和热稳定性进行表征。结果表明：MoSi₂-30Al₂O₃电热涂层体系组织均匀致密, 添加Al₂O₃能改善MoSi₂的电阻率及低温抗氧化性; MoSi₂-30Al₂O₃涂层电热性能优异, 在循环加热测试中, 能稳定地加热到320 ℃并长时间保温, 辊面温度分布均匀, 中部温差控制在25 ℃之内; 循环加热过程中的氧化及热应力的弛豫会导致涂层产生裂纹及孔隙进而导致涂层电阻率升高。     

Low-temperature ionic conductivity of the solid solution in the system ZrO2–Y2O3–Yb2O3

Compositional dependence of ionic conductivity in the system ZrO₂–Y₂O₃–Yb₂O₃ was investigated in the temperature range 573–873 K using the complex impedance technique. It was shown that the conductivity decreases with increasing concentration of Yb₂O₃ in the system ZrO₂–Y₂O₃–Yb₂O₃. Analyzing the experimental data according to the classic Arrhenius equation showed that such an experimental phenomenon can be attributed to the tighter association between Yb³⁺ and oxygen vacancy, compared with that between Y³⁺ and oxygen vacancy, which hinders the migration of oxygen vacancy in the materials.