Phase Relations in the Na2O–Al2O3–Nb2O5and CaO–Al2O3–Nb2O5Systems

Phase relations in the Na₂O–Al₂O₃–Nb₂O₅and CaO–Al₂O₃–Nb₂O₅systems were studied. The Na₂O system was found to contain neither ternary compounds nor niobate–aluminate solid solutions. In the CaO system, a ternary compound of composition 4CaO · Al₂O₃·Nb₂O₅was identified (cubic structure, a= 7.628 Å, Z= 2, _meas= _x= 4.43 g/cm³).     

Auto ignition processing of nanocrystalline α-Al2O3

This paper reports the results of an investigation aiming at synthesizing nanocrystalline α-alumina using the “Auto Ignition” processing technique. The process utilizes in-situ exothermic reactions occurring between a suitable oxidizer (e.g., aluminum nitrate) and a fuel (urea). Due to generation of high temperatures for a short time, the resulting crystalline phase(s) is(are) able to nucleate but cannot grow. This work systematically examines and characterizes the resulting powders obtained from various ratios of the oxidizer and fuel. Both X-ray diffraction and transmission electron microscopy results confirm that under certain conditions it is possible to obtain pure α-alumina in nanocrystalline size. It is expected that sinterability of such powders will be better because the question of γ to α phase transformation does not arise.     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Chemiomechanical effects on crack propagation: Polycrystalline α-Al2O3

The role of environment on microcracks in Al₂O₃ is important as the material has applications in the biomedical field, such as artificial bones. The indentation method of producing radial cracks and measuring hardness is good in that it is reproducible. The radial crack lengths in -Al₂O₃ are found to be sensitive towards the environment, and the load applied on the indentor. Long-time Vickers' microhardness and radial crack-length measurements in the presence of bio-simulating, aqueous and non-aqueous environments on the surface of -Al₂O₃ are analysed. In each environment considered, the crack propagation showed a linear variation with the applied load, and the microhardness decreased similarly with time. Evidence was sought to show that the presence of water in bio-simulating liquid helps crack propagation under load. The results are explained in terms of possible interactions between the molecules of Al₂O₃ and the chemical environment and surface energies in the presence of that environment.     

Formation of mixed TiC/Al2O3 layers and αa- and κ-Al2O3 on cemented carbides by chemical vapour deposition

Al₂O₃ and Ti(C, O) were codeposited as a mixed chemical vapour deposition (CVD) layer from AlCl₃-TiCl₄-CH₄-CO₂-H₂ gas mixtures on cemented carbides and pure alumina substrates. A thermodynamical approach of this CVD system is presented. The coatings were described by SEM and X-ray diffraction analysis. They consist of large facetted-Al₂O₃ crystals containing some titanium and surrounded by a fine grained Ti(C, O) matrix. Carbon diffusing from the cemented carbide substrate can considerably influence the morphology and the composition of the mixed coating.Methane in a AlCl₃-CO₂-H₂ environment stabilizes the-Al₂O₃ phase which can be deposited as a compact layer without whisker formation on a WC-Co substrate even without a TiC underlayer.     

Microwave dielectric properties of CaO–La2O3–Nb2O5–TiO2 ceramics

A series of ceramics with a general formula Ca_1+xLa_4?xNb_xTi_5?xO₁₇ (0 ≤ x ≤ 4) were fabricated using the solid-state ceramic route. The phase, microstructure, and microwave dielectric properties varied distinctly with composition or the value of x. X-ray diffraction results showed that the two end member phases, CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇, crystallized into single phases with orthorhombic and monoclinic crystal structure, respectively. For intermediate compounds with x = 1, 2, and 3, mixture phases CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇ coexisted and a trace amount of second phase was detected. The ceramics showed high ε _r in the range of 45–52, relatively high quality factors with Q × f in the range of 9,870–15,680 GHz and τ _f value in the range between ?38 and ?126.4 ppm/°C. τ _f of CaLa₄Ti₅O₁₇ can be tuned to a near-zero value by addition of suitable amount of TiO₂.     

Microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics with Li2O–V2O5 additions

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 3:1:3 has been investigated. The compound is composed of two phases, the Li₂TiO₃ and “M-phase” solid solution phase. The microwave dielectric ceramic has low sintering temperature (∼1100 °C) and good microwave dielectric properties of a relatively high permittivity (∼51), high Q × f value up to 8700, and small temperature coefficient (∼37 ppm/°C). The low-amount doping of 0.83Li₂O–0.17V₂O₅ (LV) can effectively lower the sintering temperature from 1100 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 1 wt.% LV-doped ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 51.3, Q × f = 7235 GHz, τ _f  = 22 ppm/°C, which suggests that the ceramics can be applied in microwave LTCC devices.     

The K2O·Al2O3-Al2O3 system

The phase relationships in the system K₂O · AL₂O₃-Al₂O₃ between 1200 and 1700° C have been experimentally established. The homogeneity range of potassium -alumina is limited by the 83 and 91 mol % Al₂O₃ compositions. The eutectic point between the K₂O-Al₂O₃ and -alumina was found to be at 1450° at about 62 mol % Al₂O₃ composition. An X-ray diffraction pattern analysis of potassium -alumina is shown.     

In situ fabrication of α-Al2O3 and Ni2Al3 reinforced aluminum matrix composites in an Al–Ni2O3 system

α-Al₂O₃ ceramic particles and Ni₂Al₃ intermetallic compound reinforced aluminum matrix composites were successfully fabricated via exothermic dispersion (XD) reaction in an Al–Ni₂O₃ system. Thermodynamic analysis indicated that the reaction between Al and Ni₂O₃ could occur spontaneously due to its negative Gibbs free energy. The reaction characteristic was discussed by using X-ray diffraction (XRD) method and differential scanning calorimetry (DSC) analysis. The results showed that the reactions of the Al–Ni₂O₃ system consisted of two steps as following: (1) the Al firstly reacted with Ni₂O₃ to form the stable α-Al₂O₃ particles and active Ni atoms; (2) the active Ni atoms further reacted with Al to form Ni₂Al₃. The values of activation energy of the two step reactions were around 457.3 and 282.4 kJ/mol, respectively. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) revealed that the Ni₂Al₃ blocks were uniformly distributed throughout the matrix, while the α-Al₂O₃ particles were slightly segregated in the matrix. The strength of the composite is controlled by the strength of Ni₂Al₃ phase, and the tensile strength and the elongation rate of the composite with 30 vol.% reinforcement volume fraction are 210 MPa and 8%, respectively.     

Electron emission measurements and the defect structure of α-Al2O3

In this article, electron emission is used to study the defect structure of alumina. The need of a direct measurement of the position of the Fermi level (or the electron concentration in the conduction band) is shown by discussing the actual electrical data on alumina. The emission has been measured over a large temperature range (1400 to 2400 K) and the emission of a technical polycrystalline alumina is reported up to the melting temperature under a controlled oxygen partial pressure. Additional results are reported for titanium- and iron-doped polycrystalline aluminas. The results are discussed from two points of view. First the quantitative data concerning the work function are taken into account and the contribution of the surface layer is discussed. Secondly, the dependency of the electron emission on the oxygen partial pressure is explained by the defect chemistry of the oxide. The absence of variation of the electron concentration in a certain range of is due to a self compensation between donor and acceptor impurities.     

High-purity disperse α-Al2O3 nanoparticles synthesized by high-energy ball milling

The preparation of disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution, high purity, and high yield is essential for producing Al₂O₃ nanocrystalline ceramic of fine grains which may exhibit a good toughness. In this work, micron-sized α-Al₂O₃ particles were directly ball-milled and subsequently washed with hydrochloric acid at room temperature. Fracture of large α-Al₂O₃ particles and cold welding of fine α-Al₂O₃ nanoparticles occur simultaneously during ball milling. It leads to the reduction of particle size with increasing milling duration below 80?h and reaches to a dynamic equilibrium with a minimal average particle size of 6.4?nm for milling durations over 80?h. Using the optimized high-energy ball milling parameters, we prepared high-purity disperse equiaxed α-Al₂O₃ nanoparticles with an average particle size of 8?nm and a purity of 99.96% (mass percent) in a high yield. After fractionated coagulation separations, disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution were obtained. Finally, Al₂O₃ nanocrystalline ceramic with a relative density of 99.8% and an average grain size of 34?nm was sintered from the disperse fine equiaxed α-Al₂O₃ nanoparticles with an average particle size of 4.8?nm and a size distribution of 2–10?nm by pressureless two-step sintering.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

γ-Al2O3载体研究进展

氧化铝催化剂载体领域中,γ-Al2O3应用最为广泛.综述了γ-Al2O3载体的主要研究方向方面的进展:制备工艺低成本化、孔结构控制、提高载体稳定性和制备超细γ-Al2O3,并指出结合制备方法的改善,开发出大比表面积、适宜孔径分布及热稳定性和抗水合性良好的γ-Al2O3载体,同时发展纳米γ-Al2O3以满足实际生产的需要.     

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。     

Nanocrystalline α-Al2O3 coatings obtained by reactive thermal anodic evaporation in arc discharge at low temperature

Al₂O₃ nanocrystalline coatings are obtained for the first time by anodic thermal evaporation of aluminum in an arc discharge in an oxygen–argon mixture at a temperature of 600°C on stainless-steel substrates with a Cr₂O₃ sublayer. The effect of the surface state of the samples and ion energy on the phase composition, microstructure, and properties of the coating is assessed. The α-Al₂O₃ phase is formed in the range of the bias potential of 25–200 V, with the growth of which the microcrystallite size decreases from 60 to 15 nm and the hardness of the coating increases from 8 to 20 GPa.

     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

B2O3-Al2O3-SiO2透明玻璃陶瓷研究

玻璃陶瓷属于一类多晶陶瓷材料.通过调整玻璃基质和晶相组成可以制备出具有良好的力学、热学、电学和光学性能的玻璃陶瓷材料.采用传统的熔融和退火技术制备出含B2O3-Al2O3-SiO2-Li2O-K2O组分的硼铝硅玻璃,并通过成核和长晶工艺最终制备出透明玻璃陶瓷.配合料在铂金坩埚中于1450℃下熔融2h,然后经两步热处理制度控制晶核的生成和晶粒的长大.采用差热分析技术确定成核和长晶温度.采用X射线衍射技术对不同热处理制度下的玻璃陶瓷样品进行分析,以确定最佳成核和长晶条件.采用扫描电子显微镜分析玻璃陶瓷形态,晶粒尺寸及其在残余玻璃相中的分布.采用UV-Vis-Nir分光光度计测定玻璃陶瓷样品的透过率.     

纳米η-Al2O3双组元法合成Na-β″-Al2O3固体电解质

以纳米η-Al2 O3为原料,氧化镁(MgO)为稳定剂,采用双组元法制备Na-β″-Al2 O3固体电解质.通过Archimedes法,SEM和三点抗弯法研究试样的致密性、显微结构和力学性能;采用XRD和交流阻抗谱仪研究试样的β″-Al2 O3相含量和离子电导率.结果表明:双组元法有利于提高试样的结构均匀性;纳米η-Al2 O3比高纯α-Al2 O3更易于合成Na-β″-Al2 O3固体电解质;适量MgO的加入有利于提高试样中β″-Al2 O3相的含量和试样的致密性,减小试样的晶界电阻,提高试样的离子电导率;过多的Mg O掺杂量导致晶内气孔尺寸的长大,反而增大了试样的晶粒电阻,导致试样的离子电导率降低.当MgO的加入量为2％(质量分数)时,试样在300℃时的离子电导率最大,为0.0396 S·cm-1.