Advances in Applied Ceramics: Structural,Functional and Bioceramics

A sol-gel polymerisation/pyrolysis route has been used to produce hybrid organic-inorganic membranes. This route involved the copolymerisation of two organo-functional precursors, methyltrimethoxysilane and phenyltrimethoxysilane or diphenyldimethoxysilane. The thermal stability and structural evolution with temperature of the resulting xerogels were investigated using multinuclear solid state NMR (¹³C and ²⁹Si), thermal analysis (TGA and DTA), and FTIR. It is shown that during pyrolysis the methyl ligand is replaced with hydroxyl groups, some of which condense into siloxane linkages. The retained hydroxyl groups are thought to provide adsorption sites throughout the material onto which polar gas molecules (such as CO₂) can adsorb, and diffuse under a pressure gradient. This hypothesis is supported by the results of gas permeation trials on the membranes, and thus a direct link is established between membrane performance and material structure.

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.

A review has been undertaken of recently published data on ultrasonic velocity-porosity for a variety of porous ceramic materials, including information on pore structure. These experimental data have been compared with data calculated using a spheroidal pore model incorporating information on pore volume fraction, shape, and orientation. Good agreement is obtained between experimental and calculated values, especially when fractional porosity is less than about 0·25, even when a single ‘effective’ pore shape is employed in the calculation. The agreement improves if a different pore shape for each porosity level (point by point analysis) is used. The predictive ability of the spheroidal pore model is thus demonstrated.

The prospective use of incinerated sewage sludge ash (ISSA) as a partial replacement for traditional brick-making materials has been previously reported. However, variability in the consistency of the raw materials used in commercial brick manufacture can cause serious production problems. Consequently, to gain market confidence for ISSA in this role, it is essential to establish that its quality is not detrimentally affected either by the nature of the sewage sludge being burned, or as a result of the performance of the incineration process itself. Ash sampling was conducted at a large modern commercial sewage sludge incinerator over a six week period, followed by property characterisation and ceramic evaluation. The results confirm that this source of ISSA is satisfactorily consistent in quality over time, thus verifying its potential for use in brick manufacture on a commercial basis. Recommendations for additional research to help ensure widespread acceptance are also discussed.

Seven mixes of mullite-spinel composites composed of 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100 wt-% of presynthesised mullite and magnesium aluminate (MA) spinel were prepared and fired up to 1550°C for 2 h. The phase compositions of the fired mixes were investigated using X-ray diffractometry. The physical, mechanical, and refractory properties of the fired composite batches were determined according to DIN standard specifications. The two mixes containing 40 and 20 wt-% mullite with 60 and 80 wt-% MA spinel were considered as optimum mixes, since they combined better physical, mechanical, and refractory properties than the other compositions. Additions of 2 wt-%Cr₂O₃ to both mixes resulted in further improvements in physical and mechanical properties after firing at 1550°C owing to its solid solution in mullite and spinel.

Nanocrystalline MgAl₂O₄ and ZrO₂-MgAl₂O₄ powders were synthesised by combustion and conventional solid state reaction routes. The synthesised powders were processed, dry pressed, and sintered for 3 h at temperatures ranging from 1550 to 1625°C. The sintered pellets were then characterised in terms of phase (XRD), microstructure (SEM), relative density, apparent porosity, water absorption, hardness, three point bend strength, and fracture toughness. The XRD studies revealed that ZrO₂ was present in tetragonal form in the case of combustion synthesised powders (CSP), whereas in powders obtained by solid state reaction (SSP) it was present in the monoclinic form. This study also revealed that the addition of ZrO₂ improved the mechanical properties of sintered MgAl₂O₄ samples: 20 wt-%ZrO₂-MgAl₂O₄ composites prepared from CSPs and conventional SSPs and sintered at 1625°C for 3 h had fracture toughness of 5·96 and 4·33 MPa m^1/2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, the presence of tetragonal zirconia as a major phase, and the finer microstructure are probably responsible for the superior mechanical properties exhibited by sintered CSP materials as compared with the sintered SSPs.

A novel gel tape casting process has been developed by introducing sodium alginate into a conventional aqueous tape casting slurry. The process makes use of a binder system consisting of 1 wt-% sodium alginate and 20 vol.-% styrene acrylic latex to prepare the tape casting slurry. Sodium alginate can react with calcium iodate to form a three-dimensional network at room temperature, and with the ceramic particles held in this three-dimensional network, the slurry can be consolidated to form green tapes. This allows the need for the special drying procedure that is usually a critical step in tape casting to be avoided. Styrene acrylic latex was used to improve the flexibility of the green tapes. Compared with tapes obtained by conventional aqueous tape casting, more homogeneous microstructure was obtained by this process.

To understand freezing in roofing tiles at low temperature with the aim of preventing frost damage, the effects of microstructure, pore size, porosity, and pore distribution on flexural strength at low temperature were studied. Strength was determined by four point bending tests at room temperature and low temperatures of -10, -30, and -50°C. Porosity and pore size distribution affected low temperature strength, and freezing time and temperature showed a close correlation with strength. A relationship between porosity P and flexural strength S at low temperature ( -10, -30°C) was found, given by S = S₀ exp (-bP), where S₀ = 49-58 MPa and b = 4·7-7·4. With a decreasing frozen temperature in the range -10 to -50°C, flexural strength increased, and tiles with lower porosity showed higher strength.