Issue No. 2, August

Abstract

Colloidal processing was applied to a commercial 5 vol% 3Y-ZrO2 nanosuspension with a particle size of 10-15 nm. The nanosuspension was concentrated by evaporation or by the newly developed method of osmotic dehydration. The viscosity and stability of concentrated suspensions were investigated. The concentrated nanosuspension prepared by osmotic dehydration was consolidated by centrifugation in non-porous moulds. The dried deposit had a relative density of 46% and pores ranged from 4 to 8 nm. This deposit was densified by pressureless presintering to closed porosity, followed by hot isostatic pressing in order to obtain transparent ceramics. After sintering, the tetragonal zirconia retained the nanocrystalline structure with an average grain size of 65 nm and an in-line transmission of 25 % (at 633 nm wavelength and 0.5 mm plate thickness).

Abstract

It was shown that fumed silica particles (FS), dispersed in polypropylene glycol (PPG), form shear thickening fluids (STF). PPGs with different molar mass were tested. The best combination of the properties (high viscosity, obtained at high shear rate) present the fluids composed of 7 nm FS and PPG 425. The highest volume fraction of FS, which was possible to disperse in PPG 425, was 25%. This fluid exhibited the highest viscosity. The highest magnitude of shear thickening effect was obtained, however, for 17.5 vol.% of the solid phase. Dynamic oscillatory shear experiments were conducted at either a constant amplitude or frequency. The constant strain amplitude tests showed, that for the frequency sweep, the systems showed viscous properties, except that of 25 vol.% of FS in PPG 425, which exhibited elastic properties in almost entire range of the frequency investigated. For the constant strain sweep, for low strains, the elastic modulus and loss modulus were hardly dependent on the strain, but for relatively high strain, this dependency was increasing. Also the complex viscosity was also growing for high strain values.

Abstract

The Solid Oxide Fuel Cell Roll (SOFCRoll) is a novel design based on a double spiral. Combining structural advantages of tubular geometries with processing advantages of thick film methods, it utilises a single cofiring process. The initial concept used separate tape cast layers which were laminated before rolling. To optimise layer thickness to function, thinner screen printed layers were combined into the tape cast structure in 2^nd generation cells. This presented several processing challenges, such as achieving dense electrolyte layers, maintaining porous electrode and current collecting layers and incorporation of integral gas channels. Performance has been promising with open circuit voltages close to 1V and cell power of over 400mW at 800°C, however cracking is still evident. Therefore further iterations are in development where thinner layers are sequentially cast, aiming to improve interfacial bonding and better match plasticity and burn out to reduce cracking. This paper reviews key aspects of understanding and development of the SOFRoll, the challenges that have been tackled and what challenges remain, along with future directions for development and potential applications for this device.

Abstract

We account for the problem of optimal control of ceramic mould manufacturing in lost wax cast processes with the aid of a mixed linear algebraic-statistical approach based on the employment of Singular Value Decomposition (SVD) and Neural Networks (NN). We consider the peculiar aspect of minimizing ceramic inclusions occurrence in equiaxed superalloy turbine components which are manufactured resorting to gravitational pouring. The optimization consists in finding optimal extrema of scalar and/or vectorial functions of the type R^k➛R^m i.e. Key Process Variable domain (KPV) vs. Target Variable domain (TV) over a large set of experimental data affected by acquisition noise leading to a typical sparse multiblock array. The goal of the work consists in the assessment of possible significant statistical multivariate correlations amongst the KPV and TV when the dimension of domain space, k, has an order of magnitude of tens, in the presence of quasi-rank deficient input matrix.

Abstract

The processing of porous ceramics spheres (PCS) has been developed for biphasic calcium phosphates (BCP), hydroxyapatite (HAp) and beta tricalcium phosphate (β-TCP) in order to be used mostly as bone fillers and drug delivery systems. The importance of the PCS is due to better accommodation of them in order to fill empty spaces and also because is more friendly to cells and bone tissue growth. Also is important to obtain a surface roughness to increase the surface area in contact with the living tissue and their fluids. There are several methods used to achieve the PCS form and most of them use suspensions based on liquids immiscibility effect or additives. The aim of this work was to achieve PCS of BCP, HAp and β-TCP with rough surface and varying size without using solutions or additives. The method developed is based on a mechanical continuous movement of the particles, relying on the normal ability of the ceramic powders to aggregate themselves while rolling in a cylindrical container for long periods. The physical forces involved in the process, gravity, particle attraction, centripetal force and shocking make the ceramic rounds with golf ball appearance on its surface. With this method it was possible obtain PCS with 30% of porosity with rough surface and size between 1 to 4 mm in diameter.

Abstract

Zirconia-alumina composites are structural ceramics which due to their high strength and toughness are interesting in biomedical and engineering applications. Reinforcement of such materials with in situ formed platelets can improve fracture toughness and reliability, the mechanisms are however not yet fully understood. In this study alumina and zirconia based composites (ZTA and ATZ) reinforced with various hexaaluminates were investigated. In ZTA materials the main effect of platelets is the improvement of toughness as the the grain size distribution of the microstructure is broadened and transformability of the zirconia dispersion is improved. Crack deflection by platelets is unimportant, toughening is commonly achieved at the expense of strength and hardness. In case of zirconia based composites results are strongly depending on the type of stabilizer (Y-TZP or Ce-TZP) used and the type of hexaaluminates formed in situ. Here platelets can cause crack deflection and crack bridging. By variation of the composite recipes a multitude of compositions can be produced which have mechanical properties tailored for individual applications.

Abstract

Mullite-gahnite composites with different phase-proportions were prepared using sol-gel process. Crystallization path was determined using differential thermal analysis (DTA). X-ray powder diffraction (XRD) was used to study the crystal phases development. The course of the thermal reactions is dominated by the intermediate formation of two spinel phases. The former phase was attributed to gahnite, while the latter to Al-Si spinel. Zn loading decreases amounts of mullite and α-alumina, while increases gahnite and amorphous phase. The observed microstructure of sintered bodies is characterized by fine gahnite particles distributed among larger mullite grains, which is highly favourable for ceramics with high mechanical requirements.

Abstract

Liquid phase sintering (LPS) is widely used as a materials processing technique for hightemperature applications. In LPS, particle-particle contact size and distribution, 3-D coordination number, connectivity, and contiguity are important microstructure parameters which, to a large extent, determine the mechanical properties of the sintered materials. These features all depend on the grain size, solid volume fraction and dihedral angle during sintering. The dihedral angle is an important parameter in LPS. It is the angle formed between the 2 solid-liquid interfaces at the intersection of a grain boundary with the liquid. A higher solid volume fraction, on the other hand, favors a larger 3-D coordination number, connectivity, and contiguity. In practice, studying the correlation between these parameters and direct measurement of them is not a trivial task. Among them, 3-D measurement of dihedral angle is believed to be the most challenging one. In the current study, phase-field modeling is employed to simulate LPS in two phase systems (solid and liquid). Simulations are performed for the different ratios of grain boundary to solid-liquid energies and the different solid volume fractions. To create initial structures with high solid volume fraction, an advanced particle packing algorithm is employed. An extended sparse bounding-box algorithm is used to speed-up the computations and makes it computationally efficient for 3-D simulations. Contiguity, connectivity, and three dimensional coordination number were measured in the self similar regime. The results were compared with empirical rules and experimental data and are used to estimate the mean 3-D dihedral angle.

Abstract

The influence of the addition of 0.25, 2 and 5 wt.% alumina on the mechanical properties and low temperature degradation (LTD) behaviour of 3 mol% yttria-coated ZrO₂ powder based Y-TZP ceramics was investigated, and compared to commercial powder based co-precipitated 3Y-TZPs with 0-0.25 wt.% Al₂O₃ addition. The ceramics were subjected to accelerated hydrothermal degradation in an autoclave in H₂O at 134°C up to 40 hrs. X-ray diffraction and Raman spectroscopy were used to assess the LTD behaviour.
Incorporating the Y₂O₃ stabilizer by means of a coating method resulted in a higher LTD resistance without compromising the higher fracture toughness, compared to the co-precipitation method. Alumina addition did not significantly influence the mechanical properties of all Y-TZPs but significantly increased the LTD resistance of the Y-TZP ceramics. The LTD resistance of 0.25 wt% Al2O3 doped TZPs was substantially higher than that of ceramics containing 2 or 5 wt.% Al₂O₃, which had a comparable susceptibility. The highest LTD resistance for the 0.25 wt.% alumina doped ceramics could be correlated to the solubility limit of alumina in zirconia.

Abstract

Structural ceramics, such as alumina, are widely used in industrial applications due to their properties such as hardness, chemical resistance and thermal stability in high temperatures. However, low fracture toughness has limited applications of these materials. One way to overcome the low fracture toughness is to control the microstructure, seeking a refined and homogeneous microstructure. In this way, the aim of this study was to investigate the effect of unconventional methods sintering based on two steps and microwaves sintering methods on the microstructure of a commercial alumina ceramic. From two-steps sintering method, the samples are first heated to a high temperature and then cooled down to a lower than one temperature to suppress grain-boundary migration. Ceramics obtained by microwave sintering method were produced after 1100, 1300 and 1350°C for 1 hour. The microwave heating method reduces the time and temperature processing, which results in energy saving. The sintered samples were characterized by apparent density, scanning electron microscopy and measurements of grain size. The results showed that two-steps sintering and microwave were effective to control the microstructure of alumina and that in two steps sintering it is essential to control the sintering steps to inhibit grain growth.

Abstract

Ceramics for dental applications have become increasingly important in the last decades. Particularly, the introduction of yttria-stabilized zirconia tetragonal polycrystalline (Y-TZP) materials as an alternative to the manufacturing of dental implants and prosthesis has provided a powerful tool to meet the demands required for these replacements in terms of biocompatibility, toughness, hardness and optical properties. Several commercial Y-TZP materials are currently available on the market and strong efforts in research and development facilities are being carried out to improve processing of Y-TZP to fully consolidate odontological pieces. Novel processing methods for ceramic powder sintering, including Y-TZP, aim to reduce processing times and production costs significantly, while maintaining or even improving the resulting microstructure and mechanical properties of the material. One of these methods includes microwave sintering. The purpose of this study is to characterize and compare the resulting properties of Y-TZP materials after conventional sintering and the non-conventional method of microwave heating. In this work one commercial material and one laboratory-synthesized Y-TZP powder are considered. The results suggest that microwave sintering results, generally, in better mechanical properties of the material in terms of hardness and fracture toughness than conventional sintering.

Abstract

In this work, the preparation of Al₂O₃-TiO₂ ceramics by high-energy ball milling varying the molar fraction in 1:1 and 3:1 was investigated. The powder mixtures were processed in a planetary mill at 250rpm and a ball-to-powder weight ratio of 5:1, for 120min. Compacts were obtained by cold pressing at 100MPa. These specimens were heated at 1000°C for 30min to promote the water evaporation, and subsequently sintered at 1500°C for 240min. Samples were characterized by relative density and X-ray diffraction. Hardness and fracture toughness were determined by Vickers Indentation Method. The crystallite sizes were lower than 420 and 560Å in Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ powders, respectively. After sintering, XRD analysis indicates Al₂TiO₅ and Al₂O₃/Al₂TiO₅ as major crystalline phases for Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ compositions, respectively. The relative density of the Al₂O₃-TiO₂ ceramics was higher than 90% in both compositions. However, hardness and fracture toughness results of 10.7GPa or 10.5GPa and 3.2MPa.m^1/2 or 2.6MPa.m^1/2 for Al₂O₃-TiO₂ and 3Al₂O₃-TiO₂ mixtures respectively, indicates that microstructure duplex composed by Al₂O₃ and Al₂TiO₅ grains lead to improvement of toughness of these ceramics.