The role of the SnO2 interphase in an alumina/glass composite: a fractographic study

The role of tin dioxide (SnO₂) interphase for the alumina/glass composite system was investigated using fractography. Alumina (Al₂O₃) and glass form a strong chemical bond which is undesirable for toughness in a ceramic matrix composite. SnO₂ interphase was incorporated to prevent this strong bond between alumina and glass. SnO₂ was deposited on Al₂O₃ substrates via chemical vapour deposition and bonded with glass. The role of the interphase was then studied by characterizing the fracture surfaces of the bend test and special composite disc samples loaded in diametral compression. Bend tests results showed that the SnO₂ interphase and/or the SnO₂/Al₂O₃ interface acted as a plane of weakness. Secondary cracking at 90° to the major crack direction was observed along this plane of weakness, which appears to be in accord with the Cook and Gordon model. Crack deflection and secondary cracking were also observed in the SnO₂ region of the compression samples. These results indicate the suitability of SnO₂ interphase for the alumina/glass composite system.     

Acoustic microscopy of ceramic-fibre composites

The microstructure of Nicalon-reinforced borosilicate glass was studied using scanning acoustic microscopy (SAM) and scanning electron microscopy (SEM), Acoustic micrographs exhibited high contrast, and the formation of Rayleigh-wave-interference fringes both within and around fibres allowed thein-situ-fibre and matrix elastic moduli to be estimated. The matrix was found to have undergone partial devitrification resulting in the formation of cristobalite. The presence of a high volume fraction of reinforcing fibres was found to have little effect on cristobalite formation and distribution. The thermal expansion mismatch between -cristobalite and the devitrified glass matrix and the /-cristobalite phase change were found to give rise to extensive microcracking around individual cristobalite grains and partial fibre/matrix debonding.     

Synthesis,microstructure and mechanical properties of Al2O3 reinforced Ni3Al matrix composite

A new method to synthesize alumina reinforced Ni₃Al intermetallic matrix composites has been described. The powder mixture of nickel and aluminium was mechanically alloyed. The powder mixture was excessively heated during mechanical alloying and then exposed to atmosphere for oxidation. The oxidized powder mixture was transformed into alumina reinforced nickel aluminide matrix composite on subsequent pulse current processing. Alumina reinforcements were generated in the nickel aluminide matrix by in situ precipitation. The microstructure of the composite showed that the alumina reinforcements were 50–150 nm in size. The fine alumina reinforcements were homogeneously distributed in the matrix phase. The mechanical properties of the alumina reinforced nickel aluminide matrix composite fairly exceeded the nickel aluminide alloys. This novel synthesis approach allowed the rapid and facile production of high strength alumina reinforced Ni₃Al matrix composites.     

Studies on the Composite System of Bi-2212 Glass Ceramic and MgB<Subscript>2</Subscript> Superconductors

The composites of glass ceramic Bi-2212 and MgB₂ superconductors were prepared at ambient conditions. The transmission electron microscopy images of the composite samples illustrate the presence of glass ceramic inclusions in bulk MgB₂. Temperature-dependent magnetization of the composite samples shows two superconducting transitions: one at 80 K corresponding to the Bi-2212 phase and a second one at 39 K corresponding to the MgB₂ phase, suggesting that the two superconducting phases are separated with clear boundaries. The critical current density (J _c) and pinning force values are increased in composite systems by an order of magnitude compared to that of individual samples. The pinning mechanism in the composite sample is the same as in the matrix phase. Reduced field maxima (h _max) are observed at 0.15 for composite samples. A low value of h _max for composite samples indicates the random orientation of grain boundaries and repulsive pinning force in the composite samples.     

Contact wear mechanisms of a dental composite with high filler content

The contact wear behavior of a dental ceramic composite containing 92 wt% silica glass and alumina filler particles in a polymeric resin matrix was examined. Because this composite is used for dental restorations, the tests were conducted under contact conditions that were relevant to those that exist in the mouth. Wear tests were conducted on a pin-on-disk tribometer with water as a lubricant. Results on wear volume as a function of load indicated two distinct regimes of wear. The wear volume increased slightly as the load was increased from 1 to 5 N. As the load was further increased to 10 N, the wear volume increased by one order of magnitude. At loads above 10 N (up to a maximum of 20 N), the wear volume was found to be independent of load. Examination of the wear tracks by SEM revealed that a surface film had formed on the wear tracks at all loads. Examination of these films by TEM showed that the films contained a mixture of small gamma-Al₂O₃ crystallites and glass particles. FTIR analysis of the adhered films indicated the presence of hydrated forms of silica and alumina, suggesting reaction of filler particles with water. Chemical analysis by ICP-MS of water samples collected after the wear tests confirmed the presence of Al and other elemental constituents of the filler particles. It is proposed that three simultaneous processes occur at the sliding contact: tribochemical reactions and film formation, dissolution of the reacted products, and mechanical removal of the film by microfracture. At low loads, wear occurs primarily by a tribochemical mechanism, i.e., formation and dissolution of the reaction products. At higher loads, wear occurs by a combination of tribochemical processes and mechanical detachment of the surface film.     

Synthesis, characterization and mechanical properties of nano alumina particulate reinforced magnesium based bulk metallic glass composites

Mg₆₇Zn₂₈Ca₅ bulk metallic glass reinforced with 0.66-1.5 vol% of nano alumina particulates were successfully synthesized using disintegrated melt deposition technique. Microstructural characterization revealed reasonably uniform distribution of alumina particulates in a metallic glass matrix. The reinforced particles have no significant effect on the glass forming ability of the monolithic glass matrix. Mechanical characterization under compressive loading showed improved micro hardness, fracture strength and failure strain with increase in nano alumina particulate reinforcement. The best combination of strength, hardness and ductility was observed in Mg/1.5 vol% alumina composite with fracture strength of 780 MPa and 2.6% failure strain.     

Barium borosilicate glass as a matrix for the uptake of dyes

Barium borosilicate (BBS) and sodium borosilicate (SBS) glass samples, prepared by the conventional melt-quench method, were used for the uptake of Rhodamine 6G dye from aqueous solution. The experimental conditions were optimized to get maximum uptake and was found to be 0.4 mg of dye per gram of BBS glass sample. For the same network former to modifier ratio, barium borosilicate glasses are found to have improved extent of uptake for the dye molecules from aqueous solutions compared to sodium borosilicate glasses. Based on ²⁹Si MAS NMR studies on these glasses, it is inferred that significantly higher number of non-bridging oxygen atoms present in barium borosilicate glasses compared to sodium borosilicate glasses is responsible for its improved uptake of Rhodamine 6G dye. ¹¹B MAS NMR studies have confirmed the simultaneous existence of boron in BO₃ and BO₄ configurations in both barium borosilicate and sodium borosilicate glasses. The luminescence studies have established that the dye molecule is incorporated into the glass matrix through ion exchange mechanism by replacing the exchangeable ions like Na⁺/Ba²⁺ attached with the non-bridging oxygen atoms present in the glass.     

Development of iron oxide and titania treated fly ash based ceramic and its bioactivity

The increasing accumulation of fly ash from thermal power plants poses a major problem to the environment. The present work reflects the novel utilization of this profusely available industrial waste in the form of an antibacterial hard ceramic material by treating fly ash with ferric oxide (Fe₂O₃) and titania (TiO₂) during sintering process at 1600 °C. The developed material shows more than 90% bacterial reduction against both Gram-positive and Gram-negative bacteria. The mechanism of their antibacterial action was studied by transmission electron microscopy (TEM) image analysis of the bacterial cross-section. The developed ceramic material acquires hardness due to the enhancement of the natural mullite content in the matrix. The mullite content and the crystallinity of mullite have shown their increasing trend with increasing concentration of the metal oxide during sintering process. A maximum of ~ 37% increase in mullite was obtained for 7% w/w Fe₂O₃ and TiO₂. Metal oxide lowered the activation energy of the reaction and enhanced the reaction rate of alumina (Al₂O₃)–silica (SiO₂) to form mullite which increases the hardness. The study highlights novel utilization of fly ash as a hard ceramic antibacterial product (bioceramics) for both structural and hygiene applications in an eco-friendly way.     

Effects of borosilicate glass addition on the structure and dielectric properties of ZnTiO3 ceramics

ZnTiO₃ powders and borosilicate glass were made by sol-gel method, and then mixed for co-firing at low temperatures. The results show that the borosilicate glass was liquefied to improve the density of the ceramic during sintering. However, Zn₄O(BO₂)₆ and TiO₂ were formed if too much borosilicate glass was added (over 10 wt.%). The microwave dielectric properties of the ZnTiO₃ co-fired with borosilicate glass were also improved dramatically. With 5 wt.% borosilicate glass addition, ZnTiO₃ ceramics can be sintered at 850 °C and shows excellent microwave properties: 22.2 for dielectric constant, and 52,460 for Q × f value at a frequency of 6 GHz.     

Hot-pressed phosphate glass–ceramic matrix composites containing calcium phosphate particles for nuclear waste encapsulation

Sodium aluminium phosphate (NaAlP) glass–ceramic composites were produced as potential wasteforms for the immobilization of special categories of halide-containing radioactive waste. Sintering conditions for encapsulating a simulated waste (a calcinated mixture of calcium phosphate host and various oxides) in the cold-pressed NaAlP glass–ceramic were first determined and the results were compared with similar samples prepared by hot pressing. In both cases, the conditions aimed to provide a very high-density material, via as low production temperatures as possible, in conjunction with a high waste loading (75 wt.% simulated waste to 25 wt.% glass). It was found that by hot pressing and using a NaAlP glass–ceramic containing 2 mol% B₂O₃, significantly lower temperatures could be employed compared to the cold pressing and sintering route. The lowest temperature at which a sufficiently dense hot-pressed product was achieved (86% theoretical density), that exhibited mechanical properties similar to those of borosilicate glass (e.g. Young’s modulus 67 ± 2 GPa), was 550 °C. This processing temperature is considerably lower than values reported in the literature for similar systems. As such, hot pressing can be considered as a convenient technique for the fabrication of this type of composite for waste encapsulation.     

Tensile behaviour of borosilicate glass matrix-Nicalon (silicon carbide) fibre composites

Unidirectional composites consisting of a borosilicate glass (Corning 7740) matrix reinforced with Nicalon (silicon carbide) fibres were fabricated and tested in monotonic tension at temperatures ranging from room temperature to 650 °C. The ultimate tensile strength showed little dependence on temperature up to about 425 °C and failed by longitudinal splitting. There was a significant increase in strength at 540 °C and a slight decrease in strength when tested above this temperature, and the failure involved extensive fibre pull-out. The elastic modulus (stiffness) was found to decrease progressively with increasing temperature. The matrix consists of borosilicate glass within the plies and very fine grains of alpha (low) cristobalite in the inter-ply regions. The behaviour of the composite as a whole was found to be dependent upon the behaviour of the matrix at the temperature of testing.     

Role of particle size of alumina on the formation of aluminium titanate as well as on sintering and microstructure development in sol–gel alumina–aluminium titanate composites

With a view to develop low temperature fine grained alumina–aluminium titanate composite, influence of alumina particle size on the temperature of formation of the aluminium titanate, sintering behaviour and microstructure development of alumina–aluminium titanate composite prepared through a sol–gel core shell approach is reported. The alumina matrix composite containing 20 wt% aluminium titanate has been prepared from alumina powders having different average particle size in the range 300–600 nm. The alumina particle size appears to have no significant influence on the formation temperature of in situ formed aluminium titanate. However, the microstructural analysis of the dense ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increase in the alumina particle size. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. Smaller starting alumina particle size led to finer grain size composites. The present study therefore shows that although the starting particle size of alumina has no significant role on the lowering of formation temperature of aluminium titanate, it does influence the microstructure of the composite.     

Reaction bonded niobium carbide ceramics from polymer-filler mixtures

Manufacturing of novel reaction bonded Niobium Carbide (NbC) containing ceramic composites derived from polymer/filler mixtures was investigated. Poly(methylsiloxane) filled with 40 vol.% of a mixture of metallic Niobium (Nb) (reactive filler) and alumina powder (inert filler) was pyrolysed in inert atmosphere up to 1450°C. During pyrolysis metallic niobium reacted with carbon from the decomposition products of the preceramic polymer binder to form microcrystalline composites of NbC, Al₂O₃ and a silicon oxycarbide glass. Microstructure formation of specimens prepared with different niobium to alumina ratio in the starting mixture was experimentally examined and compared to thermodynamic phase equilibria calculations. Materials of high NbC content exhibit high hardness and wear resistance.     

Preparation of xTiO2·(1 − x)Al2O3 catalytic supports by the sol-gel method: physical and structural characterisation

A series of xTiO₂·(1 – x)Al₂O₃ mixed oxides has been prepared by the sol-gel method with variable amounts of TiO₂, from pure alumina to pure titania. Textural, bulk and surface characterisation of the samples has been carried out by nitrogen physisorption (S _BET, porosity), thermal analysis (TGA, DTA), X-ray diffraction (XRD), zero point charge (ZPC) and surface acidity. The textural results show that at low titania contents, higher surface areas than those of pure alumina are obtained, and in the titania-rich samples, higher surface areas than for pure titania are stabilised. The titanium content also affects the crystallization process. Furthermore, the strength and distribution of the acid sites may be varied by changing the composition of the mixed oxide.     

基于RSM的Al2O3陶瓷膏体配方优化

为采用低成本在低温下实现复杂陶瓷零件的成型,研究Al2O3陶瓷膏体的配方对其成型过程以及成型后物理机械性能的影响.采用中心组合方法设计实验方案,考察了p H值、粘结剂和分散剂三因素对Al2O3陶瓷膏体黏度的影响,运用响应面方法(RSM)对影响Al2O3陶瓷膏体黏度的工艺参数进行详细分析,建立了Al2O3陶瓷膏体黏度的响应模型,进行响应曲面分析,根据满意度函数(DFM)确定最佳陶瓷膏体黏度值对应的工艺参数,试验表明,所建立的模型能实现相应的黏度值预测.     

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m^1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al₂O₃-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.     

一种低烧、低介、高频多层片式电感用的介质材料

利用复合结构原理,研制出“硼硅玻璃＋α－石英＋硅酸锌”系低烧低介陶瓷材料,利用ＸＲＤ、ＨＰ４１９４频谱仪、ＳＥＭ分析了该材料的晶相组成、介电性能及显微结构．结果表明:在高频下,该材料具有很低的介电常数(Ｋ＝４~５,１ＭＨｚ)和很低的介电损耗(ｔａｎδ＜０．００１,１ＭＨｚ);同时能在低于９００℃温度下烧结．该材料是一种理想的适用于高频(１ＧＨｚ以上)多层片式电感(ＭＬＣＩｓ)元件用的介质材料．同时得出:在该组成中,硼硅玻璃重烧结的过程中有方石英析出;少量硅酸锌由于锌离子进入玻璃中而转变为亚硅酸锌;硅酸锌在该组成中有助烧结的作用,该材料介电性能随频率的变化与德拜方程的描述基本吻合．     

Er3+/Yb3+ codoped oxyfluoride borosilicate glass ceramic containing NaYF4 nanocrystals for amorphous silicon solar cells

Transparent oxyfluoride borosilicate glass ceramic containing cubic NaYF₄ nanocrystals were successfully fabricated. The cubic NaYF₄ nanocrystals with average size of 30 nm were precipitated in the glass matrix, which was confirmed by the X-ray diffraction and TEM results. In comparison with the as-made glass, significant enhancement of upconversion luminescence is observed in the Er³⁺/Yb³⁺ codoped transparent glass ceramic, which may be due to the variation of coordination environment of Er³⁺ and Yb³⁺ ions after crystallization. The high transparency, intense upconversion luminescence and the simple, low-cost fabrication process make this material exhibiting potential applications in the fields of amorphous silicon solar cells.     

Multi-photon luminescence in Er/Yb:SrO⋅TiO2 glass ceramic

Er³⁺/Yb³⁺ doped strontium titanate borosilicate glass was prepared. Glass ceramic was prepared by controlled heat treatment (at 955 °C) of glass. Ti₁₀O₁₉ and Sr₃Ti₂O₇ were found as major crystalline phases. The emission spectra of glass and glass ceramic samples were investigated under 976 nm laser excitation. In glass ceramic, the intensity of the emitted radiation was much higher (≈50 times for green and ≈10 times for red emission) than in the glass. A new three photon process was found to be responsible for emission at low power which is not yet observed in Er³⁺/Yb³⁺:SrO⋅TiO₂ glass ceramic system to the best of our knowledge. The details of upconversion mechanisms e.g. Energy Transfer (ET) and Excited State Absorption (ESA) were studied by power-intensity log dependence. It is expected that Er³⁺/Yb³⁺ doped nanocrystalline (?10 nm) Sr₃Ti₂O₇ phase was responsible for the observed upconversion phenomenon in glass ceramic.     

Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

The influence of processing conditions on the morphology of calcium hexaluminate (CA₆) grains in Al₂O₃: 30 vol% CaO·6Al₂O₃ (CA₆) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA₆ grains developed a platelike morphology when CaCO₃ was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA₆ grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA₆ reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.Cement Nomenclature C CaO - A Al₂O₃