Fracture Resistance of a Transparent Magnesium Aluminate Spinel

The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron-notched beam and the straight-notched beam macroflaw techniques, as well as the indentation-induced, controlled-microflaw test method, all in three-point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load-line displacement ( P-u ) curves of the chevronnotched test specimens, the crack growth resistance curves ( R -curves) and the total work-of-fracture were determined. It was observed that polycrystalline MgAl₂O₄ exhibits rising R -curve behavior which increases with increasing test temperature. The R -curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work-of-fracture and the R -curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking.     

La2O3对镁铝尖晶石透明陶瓷致密化及其性能的影响

镁铝尖晶石透明陶瓷兼具了良好的光学和力学性能,在军、民两用领域有着广泛的实际和潜在应用前景.由于其致密化速率低,在烧结过程中往往需要引入烧结助剂.稀土倍半氧化物熔点高,高温不易挥发,近些年被证实可以促进镁铝尖晶石陶瓷的致密化,但其促烧机理尚不明确.本文以高纯商业化镁铝尖晶石粉体为原料,La2 O3为烧结助剂,采用无压预...     

TiO2加入量对固相烧结合成镁铝尖晶石致密化行为的影响

以低品位菱镁矿和工业α氧化铝微粉为主要原料固相烧结合成镁铝尖晶石,探讨TiO2添加剂对合成镁铝尖晶石致密化行为的影响.用X射线衍射(XRD),扫描电镜(SEM)及相关分析软件对烧后试样的相组成和显微结构进行研究,以揭示镁铝尖晶石烧结致密化的过程机理.研究结果表明:添加剂TiO2与形成镁铝尖晶石的置换固溶作用是促进固相合成镁铝尖晶石烧结致密化的重要机理,阳离子空位的产生以及镁铝尖晶石的晶格缺陷,高温下有利于镁铝尖晶石的晶体发育和长大,在空间位阻效应的作用下,达到排除气孔使试样致密化的目的;随着TiO2加入量的增多,镁铝尖晶石的致密化程度也逐渐升高,能够有效的改善制品的烧结性能和显微结构.     

Vacuum Hot-Pressing of Magnesium Aluminate Spinel

The densification kinetics of magnesium aluminate spinel during vacuum hot-pressing were studied between 1175° and 1460° C and from 500 to 5100 psi. A phenomenologi-cal rate equation, which suggests a logarithmic relation between strain rate and porosity, excellently described the observed densification. Treating porosity as an independent variable was shown to be reasonable; it does not functionally restrict porosity as a modifier of the applied stress. The strain rate dependence on porosity decreased at a porosity of approximately 0.15. Below 1350° C the densification characteristics were similar to those reported for other oxide systems. At 1450°C an increase in the stress dependence of the densification rate and an interaction between stress and porosity suggested that plastic flow by dislocation motion was an operative mechanism during densification.     

Mechanochemical Synthesis of Magnesium Aluminate Spinel Powder at Room Temperature

MgAl₂O₄ spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al₂O₃–MgO, AlO(OH)–MgO, and α-Al₂O₃–MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl₂O₄ occurred faster with γ-Al₂O₃ than with AlO(OH) or α-Al₂O₃. After 140 h, the mechanochemical treatment of the γ-Al₂O₃–MgO mixture yielded 99% of MgAl₂O₄.     

Gelcasting of Magnesium Aluminate Spinel Powder

A stoichiometric MgAl₂O₄ spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41–45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of α-Al₂O₃ and calcined caustic MgO at 1400°C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H₃PO₄ and Al(H₂PO₄)₃. The as-treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly-acrylic acid (Duramax D-3005) as dispersing agents. The as-obtained stable suspensions were gelcast, dried, and sintered at 1650°C for 1–3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze-dried granules and sintered along with gelcast samples. Near-net-shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650°C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties.     

Oxygen Diffusion in Magnesium Aluminate Spinel

Oxygen self-diffusion in MgAl₂O₄ single crystals was investigated by annealing the samples in ¹⁸O enriched gas at 98 kPa (1 atm) and measuring the tracer concentration profiles using a proton activation technique. The diffusion coefficients in the range 1625 to 1925 K for stoichiometric crystals are represented by:    
Oxygen diffusivity in alumina-rich nonstoichiometric samples is higher than in stoichiometric samples.     

镁铝尖晶石超细粉末的制备

介绍了镁铝尖晶石(MgAl2O4)的结构、性能、用途，着重介绍了MgAl2O4超细粉制备的各种方法，并比较了它们的优缺点。     

Electrochemical Impedance Spectroscopy of Transparent Polycrystalline Magnesium Aluminate (MgAl2O4) Spinel

Transparent polycrystalline magnesium aluminate spinel is one of few materials capable of fulfilling certain demanding optical applications. However, LiF additive typically required to enhance sintering and impart transparency also degrades mechanical properties and causes scatter, precluding wider application. To shed light on the cause of altered properties, electrochemical impedance spectroscopy was performed between 500°C and 900°C, in oxidizing and reducing atmosphere, on fully dense, polycrystalline spinel compacts hot‐pressed with and without LiF. In combination with electron microscopy, chemical analysis, and secondary‐ion mass spectroscopy, bulk and grain‐boundary dielectric behavior were related to microstructure and chemistry. Decades higher conductivity than for comparable single crystals correlated with Al₂O₃‐rich stoichiometry, impurities, and small grain size, and suggested increased conduction and current‐line detouring along Mg‐depleted, impurity‐rich, field‐parallel grain boundaries. LiF addition reduced conductivity by one decade and increased the activation energy for conductivity, attributed to impurity removal, larger grain size, and point defects caused by lithium incorporation.     

Formation and Densification Behavior of Magnesium Aluminate Spinel: The Influence of CaO and Moisture in the Precursors

Different grades of stoichiometric and non-stoichiometric dense magnesium aluminate spinel (MgAl₂O₄) grains were prepared by a conventional double-stage firing process using two types of alumina and four types of magnesia raw materials. The MgAl₂O₄ spinel formation was found to be highly influenced by CaO and moisture present in the precursor oxides as confirmed by thermogravimetry (TG), differential thermal analysis (DTA), and X-ray diffraction (XRD) techniques. The Fourier transform-infrared spectroscopy (FTIR) study of the precursor oxides revealed the presence of moisture. Influence of alumina and magnesia composition on the densification behavior of MgAl₂O₄ spinels was assessed by characterizing bulk density (BD), apparent porosity (AP), water absorption (WA) capacity, and the microstructures of the stoichiometric, the magnesia-rich, and the alumina-rich spinels sintered at 1650°C for 1 h. Sintering studies indicate that to obtain dense stoichiometric spinel grains with >3.35 g/mL BD, <2.0% AP, and <0.5% WA, the spinel powder should possess a median particle size of <2 μm, CaO content of >0.9%, compact (green) density of >1.95 g/mL, and spinel content of >90%. Among various spinels synthesized, the magnesia-rich spinels exhibited superior properties in terms of high BD, low percentage of AP, and low WA capacity, whereas alumina-rich spinels showed inferior properties. Stoichiometric spinels exhibited an average grain size of 10 μm whereas alumina-rich spinels with 90% alumina had an average grain size of 20 μm. The increase in holding time at higher temperatures enhanced the sintering properties of the spinels, particularly the magnesia-rich spinels. Further, raw mixtures having >0.9% CaO exhibited better sintered properties as compared with others.     

TiO2对电熔合成镁铝尖晶石的影响

以轻烧氧化镁粉、工业氧化铝为原料合成电熔镁铝尖晶石材料,重点研究二氧化钛为添加剂对电熔合成镁铝尖晶石的物相组成,显微结构的影响,通过XRD,SEM分析试样的物相组成、晶胞参数和断口的微观形貌.研究发现:二氧化钛为外加剂可以增大镁铝尖晶石的晶胞参数、提高材料的体积密度进而改善材料的烧结性能,并且随着二氧化钛加入量的增加镁铝尖晶石的晶胞参数和晶胞体积呈现先增大后减小的变化规律.分析认为由于二氧化钛、钛酸镁固溶到镁铝尖晶石中促使镁铝尖晶石的晶胞参数和晶胞体积增大,而过量的钛酸镁位于晶界阻碍镁铝尖晶石的长大,导致镁铝尖晶石的晶胞参数和晶胞体积减小.在电熔法制备镁铝尖晶石时,二氧化钛的加入量不宜超过5wt%.     

氧化物添加剂在合成和烧结镁铝尖晶石过程中的作用

在钢铁窑炉以及水泥和石灰回转窑中镁铝尖晶石砖已经成功地替代了镁铬砖。利用软件和热力学基本数据解释添加剂在尖晶石形成过程中所起到的作用。为了使镁基耐火材料形成更好的结合结构,分析了镁铝合金粉末添加二价和四价氧化物添加剂在相对较低的温度下合成和烧结过程中所起的作用。利用X-射线衍射图谱中峰位移动的数据研究了活性阳离子在氧化镁-尖晶石和尖晶石-尖晶石的表面扩散情况。     

Synthesis of Magnesium Aluminate Spinel Platelets from α-Alumina Platelet and Magnesium Sulfate Precursors

Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al₂O₃ and 1–8 μm (average 3 μm) MgSO₄ heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al₂O₃ platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al₂O₃ platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina.     

Hardness and Depth-Dependent Microstructure of Ion-Irradiated Magnesium Aluminate Spinel

Stoichiometric polycrystalline magnesium aluminate spinel has been irradiated at 25° and 650°C with 2.4-MeV Mg⁺ ions to a fluence of 1.4 × 10²¹ ions/m² (∼35 dpa (displacement per atom) peak damage level). Microindentation hardness measurements and transmission electron microscopy combined with energy dispersive X-ray spectroscopy measurements were used to characterize the irradiation effects. The room-temperature hardness of spinel increased by about 5% after irradiation at both temperatures. There was no evidence for amorphization at either irradiation temperatures. Interstitial-type dislocation loops lying on {110} and {111} planes with Burgers vectors along 〈110〉 were observed at intermediate depths (∼1 μm) along the ion range. The 〈110〉{111} loops are presumably formed from 〈111〉{111} loops as a result of a shear on the anion sublattice. Only about 0.05% of the calculated displacements were visible in the form of loops, which indicates that spinel has a high resistance to aggregate damage accumulation. The peak damage region contained a high density of dislocation tangles. There was no evidence for the formation of voids or vacancy loops. The specimen irradiated at 650°C was denuded of dislocation loops within ∼1 μm of the surface.     

Deterioration in the Final-Stage Sintering of Magnesium Aluminate Spinel

Sintering of magnesium aluminate spinel of the MgO-excess, stoichiometric, and Al₂O₃-excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface layer is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions. Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less-densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum-sintering owing to MgO evaporation.     

Processing of nano-SiC ceramics: Densification by SPS and mechanical characterization

β-SiC nanopowders with a mean particle size of 16.6 nm were obtained by laser pyrolysis. De-agglomeration of the powder was performed in an aqueous medium under magnetic stirring and ball-milling. Subsequently, green bodies were prepared by slip-casting of slurries. In this context, non-conventional densification routes such as Spark Plasma Sintering (SPS) with an applied pressure of 70 MPa allowed to achieve high final densities (96% TD). Different microstructures were obtained by varying the sintering temperature. Finally, the mechanical properties of the samples (hardness, toughness) were determined and a correlation between the final microstructures and the mechanical behavior was established.     

涂有稀土元素的镁铝尖晶石的分析和光谱学分析

采用稀土阳离子隔离镁铝尖晶石的作法已经被考虑到，并且利用校对过的扫描电镜做了分析。在Daresbury实验室的超级扫描电镜表明铕离子沿尖晶石颗粒边界将尖晶石单层分离。电子能量损失光谱分析证明，铕存在于颗粒边界的0．5mm范围内。由于尖晶石具有较好的透光性、固有的硬度、较大的断裂韧性和较好的抗热、化学侵蚀性能而作为硬质窗玻璃的一种侯选材料。涂有稀土阳离子的尖晶石在可见光和红外线波长范围内具有较好的透光性。     

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near‐theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis–processing–property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green‐body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an exposé of the state of the art in transparent polycrystalline spinel fabrication.     

Hydrothermal Synthesis of Magnesium Aluminate Platelets

Magnesium aluminate (MgAl₂O₄) platelets were first synthesized by hydrothermal treatment of γ-AlO(OH) in a magnesium nitrate aqueous solution at 400°C. The platelets are 100–200 nm in width and 25 nm in thickness. The influence of temperature, the anions of the magnesium salt, the amount of magnesium salt, and precursor pH on the formation of such structure was investigated. The experimental results indicated that the anions of the magnesium salt and precursor pH could have a significant influence on the growth of MgAl₂O₄. MgAl₂O₄ formation followed the in situ transformation mechanism, which was the reason why the MgAl₂O₄ crystallites had a platelet morphology.