Functionally Graded Materials VIII

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Authors: F.Z. Mezahi, Abdelhamid Harabi, S. Zouai, S. Achour, Didier Bernache-Assollant
Abstract: In this work, the hydroxyapatite was prepared from cortical bone after calcination at 700°C during 1 hour. In order to improve mechanical properties of HA, 5 wt% of the ZrO2 (stabilized with 12.0 wt% CeO2), Al2O3 and TiO2 were added to HA powder as a reinforcing phase. All the powders were sintered at 1300°C for 2 hours. The XRD was utilized to identify the phases composition. It was found that the initial calcined powder is hydroxyapatite with the following chemical composition Ca5(PO4)3OH. In addition, the phenolphthalein test has put into evidence the existence of free CaO. For powders containing ZrO2, the XRD spectra has showed a little percent of formed b-tricalcium phosphate (b-TCP); the HA was decomposed to (b-TCP) and CaO which forms with ZrO2 the calcium zirconate (CaZrO3). Nevertheless, in powders containing TiO2, the XRD spectra showed that a partial decomposition of HA to b-TCP was occurred with formation of calcium titanium oxide (CaTiO3). However, for powders containing Al2O3, a nearly full decomposition of HA to b-TCP was occurred, the free Al2O3 was present. Finally, it has been found that HA composites containing a large amount of b-tricalcium phosphate are resorbale than HA composites containing a small amount of b-TCP and they were not well densified. The microhardness values of HA- ZrO2 composites were greater than those of HA- Al2O3 and HATiO2 composites.
Authors: Bilge Saruhan, Mathias Stranzenbach
Authors: A. Witvrouw, A. Mehta
Abstract: It is difficult to meet all the different material and economical requirements posed to a MEMS structural layer that can be integrated with the electronics on the same substrate using a single layer process. Therefore a multilayer process, which uses a combination of a CVD crystallization layer and a high-growth rate PECVD bulk layer was developed. High-quality films with excellent electrical and mechanical properties can be obtained at low temperature (#450°C) and high deposition rates (~100 nm/min). Fine-tuning of the stress gradient is accomplished by the use of a top stress compensation layer, whose optimal thickness was estimated from an evaluation of the stress gradient profile over thickness. These layers have been used for processing a 10 µm thick poly-SiGe gyroscope on top of a standard 0.35 µm CMOS process.
Authors: Christoph Häusler, Herbert Balke
Abstract: The Hilbert problems and their solutions of the near tip field for the permeable and the impermeable electrode-ceramic interfacial cracks are derived with the aid of the modified Stroh formalism. In addition, a linear extrapolation method for numerical calculation of field intensity factors is given. This extrapolation method is based on a special combination of the field quantities.
Authors: S. Wenmackers, P. Christiaens, W. Deferme, M. Daenen, K. Haenen, M. Nesládek, Patrick Wagner, V. Vermeeren, L. Michiels, Martin Van de Ven, M. Ameloot, J. Wouters, L. Naelaerts, Z. Mekhalif
Abstract: Synthetic diamond is regarded as a promising material for biosensors: it forms a stable platform for genetic assays and its biocompatibility opens the possibility for in vivo sensing. In this study the use of a thymidine linker for covalent DNA attachment was evaluated. Contact angle measurements provided a qualitative test of the initially oxidized surface. X-ray photoemission spectroscopy was used for further analysis of the oxides and for monitoring the effect of subsequent chemical treatments. The presence of FITC-labelled DNA was confirmed by confocal fluorescence microscopy. Enzyme linked immunosorbent assays indicated that this DNA was merely adsorbed on the diamond surface instead of covalently bound.
Authors: Cestmir Drasar, Eckhard Müller
Abstract: Recently, there is a strong interest in developing superior thermoelectric materials with the aim to improve the performance of a thermoelectric device. However, the performance of a thermoelectric generator (TEG) can be considerably improved also by applying a graded composition along the temperature gradient inside the thermoelectric device so that at each position the respective material achieves its maximum thermoelectric performance (TE FGM principle). Combining the high efficiency of Bi2Te3 (used at low temperatures) and general durability of FeSi2 (applied up to high temperatures) will result in a thermoelectric device with enhanced efficiency operating in air at a wide temperature range. The challenge is to contact these dissimilar materials without any negative impact on the performance of TEG. Besides providing a good electrical and thermal contact between Bi2Te3 and FeSi2 the junctions have to remain mechanically and chemically stable over long term. A Bi2Te3-SiO2 composite interlayer was used to adjust the different coefficient of thermal expansion (CTE) of FeSi2 (≈ 7·10-6 K-1) and Bi2Te3 (≈ 19·10-6 K-1). Due to low chemical stability of the Bi2Te3/FeSi2 contact at elevated temperatures (1000 h @ 300°C) a contacting material (diffusion barrier) based on Ni, Zn, Ti, and ZnTe was tested. Some contacts show excellent chemical and mechanical stability, though the electrical properties of the contacts do not meet the requirements (e.g. ZnTe is a wide gap semiconductor with high electrical resistivity). According to very recent studies at the Zn-based diffusion barriers a very thin layer of undoped ZnTe growing at the Bi2Te3/Zn interface causes the deterioration of the contact resistance. Ideas solving this problem are outlined and discussed.
Authors: Yoshihiro Obata, Kazutoshi Takeuchi, Kouichi Akaeda, Kozo Kanayama
Abstract: Compressed wood has different grading structure in an annual ring from one of natural wood. This paper treats the relationship between grading structures and effective thermal conductivity of natural and compressed woods. The Lorentz function and the power function are assumed as grading patterns of thermal conductivity. The grading thermal conductivity shows smaller effective thermal conductivity than the homogeneous wood with same average density. The sharper grading pattern gives much smaller effective thermal conductivity. The grading pattern of compressed wood is assumed as a model with locally compressed region. The calculated effective thermal conductivity by the model agrees with the measured thermal conductivity.
Authors: Ik Jin Kim, Hae Jin Lee
Abstract: The problem with zeolite crystals synthesized by conventional methods is that they are extremely small: their size typically varies 2 and 8 µm, To better define the structure of a zeolite, scientists need to grow crystal that are 100 to 1000 times larger. In this work, large crystals zeolite NaX with a uniform size of 50µm were grown by a continuous crystallization method from seed crystals (5-10µm) formed in a mother solution with the 3.5Na2O : Al2O3 : 2.1SiO2 : 1000H2O composition. In order to grow zeolite NaX crystals to an appropriate size by the continuous method, the mother solution was fed into an autoclave a solution with various seed contents (3 - 20wt%); the autoclave has at 90°C and the solution was added after 7, 12, 16, 19 and 24 days or at 100°C after 7 and 9 days.
Authors: Yoshihisa Uchida, Shuntaro Higa, Yoshiyuki Uchida, Niichi Hayashi
Abstract: Mechanical properties of TiO2-Kaolin Functionally Graded Materials (FGMs) that combined TiO2 and Korean kaolin (Al2Si2O5(OH)4) in a graded distribution were experimentally investigated. TiO2 is an excellent photocatalysis material, however, it does not have the mechanical strength sufficiently. Therefore, FGMs were used as the photocatalysis materials with mechanical strength. To improve the mechanical properties, Korean kaolin was selected as a binder. Bulk FGMs were produced by vacuum filtration and compression involving wet filtration, mechanical compression and sintering. By this process, it is easy to control the thickness of FGMs. The porosity in FGM depends on the content of TiO2. The porosity in FGM decreases with increasing pressure of the compressor. The shrinkage decreases with increasing compression pressure at all ratios of TiO2 and kaolin. The materials have almost no shrinkage at 10 MPa. The bending strength and the Young’ modulus depends on content of TiO2. The bending strength of material was found to be approximately 2.5 MPa for 10 MPa compression pressure at TiO2 side. The bending strength and the Young’ modulus increases with increasing compression pressure. These results indicate that kaolin particles are useful to improve the strength of FGM. These properties of FGM, such as porosity, shrinkage, mechanical strength, etc., can be controlled by content of TiO2 and compression pressure. Based on an optimal design of production conditions, FGMs were developed for the nitrogen oxides (NOx) removal. The NOx reduction increases with increasing compression pressure. NO of 11.7 mg/m2h was removed under present conditions. The sintering temperature ensured the optimal removal of NOx was 800 oC. After NOx removal test, the FGMs were found to be free from erosion due to photocatalysis. And kaolin particles do not interfere in photocatalysis effect of TiO2. It can be concluded that TiO2-Kaolin FGM have a possibility for application to the air purification.
Authors: Frans M.M. Snijkers, Steven Mullens, Anita Buekenhoudt, Jan Luyten, W. Vandermeulen
Abstract: Ceramic foams can be used as filters, dust collectors, light weight components and catalyst carriers. They can be produced by a variety of techniques. The performance of ceramic foams will be strongly improved when their mechanical properties are improved. For this reason, we produced ceramic foams both by a modified reaction bonded (RB) replica technique and by gel casting. With both methods, reticulated foam structures with enhanced mechanical strength were obtained. Zeolites are a special type of materials that are characterized by high catalytic properties. They can be brought on a structured carrier by dip and slurry coating. Nevertheless, in situ coating has as main advantage that the support is used as the base for nucleation. This results in the formation of a chemical bond between the zeolite crystals and the support. The goal of this contribution is twofold: at first we demonstrate how Al2O3 foams with improved mechanical strength can be produced both by the modified RB-alumina replica technique and by gel casting. Secondly, it is shown that these ceramic foams can be coated with (silicalite) zeolite crystals by insitu crystallization from a precursor sol. The two-layer material combinations have been characterized with FESEM, XRD, CT (computer assisted tomography), IA (Image Analysis) and by mechanical tests.

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