Year 2013, Volume 33

Abstract

The low thermal conductivity of Lanthanum hexaaluminate, abbreviated as LHA, combined with high structural reliability of alumina matrix ceramics attracted our attention to develop a new functionally graded layered LHA-Al₂O₃-composite, with a LHA and a porosity gradient along the thickness of a bulk oxide ceramic. LHA is formed by in–situ reaction during sintering of the alumina/LHA composite. The high sintering temperature required for completion of LHA formation in LHA-rich layers causes grain growth and a degradation of mechanical strength in alumina-rich layers. Therefore, microwave hybrid heating was investigated as a method to enhance the reaction rate without excessive grain growth. Comparison of conventionally and microwave assisted sintered homogenous composite ceramics with 20–80 volume percent LHA showed that utilization of microwave heating could enhance the solid–state reaction and densification in samples containing more than 20 volume percent LHA. Enhanced microwave absorption in LHA rich layers assisted the sintering of a functionally graded composite at lower temperatures, enabling LHA formation without any abnormal grain growth in alumina rich layers.

Abstract

Functionally graded zirconia toughened alumina (ZTA) ceramics have been fabricated from aqueous suspension with an open porous and aligned lamellae structure on one side and a dense layer on the other side. A novel combination of two processes has been merged to achieve such graded structures, i.e. unidirectional freeze casting and electrophoretic deposition (EPD). A custom-designed apparatus has been built in which a controlled double side cooling has been realized in conjunction with the possibility to introduce an electric field over the ceramic slurry prior to the freezing process. A square wave pulsed DC voltage has been used in the EPD process in order to avoid electrolysis of water. Suitable duty cycle of applied pulse voltage could gain bubble-free deposition. The thickness of the dense layer is controlled by tuning voltage, duty of cycle, pulse width and deposition time. It was shown that thicknesses up to 500μm could be achieved. The microstructure of the porous part is controlled by adjusting the temperature during the freezing process. Using temperatures between -1 and -25°C the channel width changed from 220 to 40μm, respectively.

Abstract

In a flow forming process of automobile wheels of Al-based alloys, plastic deformation of the rim part is performed by rollers from the periphery side, while the inner periphery is fixed on a steel mandrel and is slightly deformed. Therefore, the rim part has a strain-graded microstructure in the thickness direction. In this study, the effects of the strain-graded plastic deformation on mechanical properties of an Al-Si cast alloy have been investigated. The strain-graded plastic deformation in this study was done by hot rolling of an Al alloy plate together with a steel plate. The two plates were joined at one end in the longitudinal direction and were rolled from the joined edge at 330 oC using a roller with a roll diameter of 200 mm and a rotation speed of 66 per minute. The chemical composition of the alloy was Al-7mass%Si-0.3mass%Mg-0.3mass%Fe. The rolled Al alloy plate had a strain-graded microstructure in the thickness direction; the strain was the highest at the roller side surface and the lowest at the steel plate side surface. The rolling also brought about a Si particle size graded microstructure. The eutectic Si rods were broken by the rolling deformation and the Si particle size was the smallest at the roller side surface and the largest at the steel plate side surface. On the other hand, a normal rolling deformation of the Al alloy plate without the steel plate was also performed for comparison. The rolled sample having the strain-graded and Si particle size graded microstructure exhibited much more excellent bending strength and ductility compared with the normally rolled sample.

Abstract

Crystal structures of Li₂MO₃ (M=Sn, Ti) and TiO(OH)₂ have been studied in detail and refined using X-ray powder diffraction data. All compounds posses a high concentration of defects in the structure. The crystal structures of the Li₂MO₃ salts obtained at 700°C reveal stacking faults of LiM₂ metal layers, which leads to the appearance of short-range order in three possible space groups: C2/c, C2/m, P3₁12. The possibility to stabilize this imperfect state increases the mobility of the Li⁺ ions in the Li₂TiO₃ structure and allows the complete exchange of lithium by hydrogen in acid water solutions with formation of TiO(OH)₂. The crystal structure of TiO(OH)₂ belongs to the layered double hydroxide structure type with the 3R₁ sequence of oxygen layers and can be described as a stacking of charge-neutral metal oxyhydroxide slabs [(OH)₂OTi₂O(OH)₂].

Abstract

Zinc and copper pyrovanadates are promising materials for micro- and optoelectronics due to their negative coefficient of volume thermal expansion (NTE). Besides, solid solutions on the base of these compounds can be used to obtain grade materials with variable thermal coefficients. Thermal deformation of both Zn₂V₂O₇ and Cu₂V₂O₇ structures was studied. According to the structural data, NTE of these substances is provided by the zigzag shape of zinc (copper) chains alongside with stable distances between layers. The structural and electronic characteristics depending on temperature were studied for α-Zn₂V₂O₇ and α-Cu₂V₂O₇ by using the first principle method. Our results demonstrate that the lowest total energies corresponds to the structural parameters at 400° C and 200° C for α-Zn₂V₂O₇ and α-Cu₂V₂O₇, respectively. We predict that α-Zn₂V₂O₇ is a semiconductor with the band gap of 1,5 эВ and the bottom of conduction band is determined by the vanadium 3d states with small addition of antibonding oxygen 2р-states. For α-Cu₂V₂O₇, the lowest interband transitions correspond to energy of 1,6 eV and involve also the O2p and V 3d states.

Abstract

This work deals with the preparation and characterization of macroporous alumina ceramics and permeable laminates with a stepwise (layerwise) porosity gradient in the range of approx. 20–50 %. Layered structures are made by sequential casting and draining of ceramic suspensions containing corn starch (median size approx. 14 micrometers), using both traditional slip casting (TSC) and starch consolidation casting (SCC). In both techniques starch acts as a poreformer, which is eliminated during firing. The influence of the alumina concentration and starch content in the suspension on the porosity, pore size and pore connectivity in the individual layers is studied. It is shown that differential shrinkage of the layers in the case of SCC, caused by the different starch content, may be avoided by controlling the alumina content. The distribution of pore throat diameters (cell window sizes) is determined by mercury porosimetry, whereas the distribution of pore cavity diameters (cell sizes) is measured by microscopic image analysis.

Abstract

We introduce the concept of functional microchannel lining. As an example, we describe the composition and structure of a Ni-Al intermetallic layer lining the inner wall of the microchannel produced by a powder-metallurgical process utilizing microscopic reactive infiltration and/or diffusion. The Ni-Al lining layer is a thick film consists of multiple sub-layers and has a peculiar porous structure, in which long and thin micropores had grown along the thickness direction of the film. In our experiment, a nickel-powder compact containing shaped aluminum wire was sintered at temperatures between the melting points of nickel and aluminum. Molten aluminum migrated into the surrounding nickel powder and reacted with nickel, and thus a microchannel and a Ni-Al intermetallic lining layer were produced. In this process, nickel powder composed the device body, and the aluminum wire gave the shape of the microchannel and act as the aluminum source for the lining layer. Metallographical examinations revealed that both aluminum concentration and voidage in the Ni-Al lining layer show a graded distribution along the thickness direction of the layer. Such a porous structure is appropriate for a catalyst support used for high-temperature reactions.

Abstract

Reaction-sintered silicon carbide of 800 MPa class bending strength had been newly developed. The developed silicon carbide showed good rigidity, high thermal conductivity, and high density, like a conventional sintered silicon carbide. The developed silicon carbide is one of the most attractive materials for large-scale ceramic structures because of its low processing temperature, good shape capability, low-cost processing and high purity. We had fabricated some lightweight space mirrors, such as a high-strength reaction-sintered silicon carbide mirror of 650 mm in diameter. In this study, experiments were conducted to investigate the effect of annealing on the bending strength of high-strength reaction-sintered silicon carbide. The annealing heat treatments were carried out at 1073 K, 1273 K, and 1473 K in an air atmosphere. The maximum bending strength of 1091 MPa at room temperature was achieved by the annealing heat-treatment at 1273 K for 10 h in air. We confirmed that annealing heat treatment was effective to improve the bending strength of reaction-sintered silicon carbide by inducing compressive residual stress at the surface oxide layer.

Abstract

Different coating technologies, such as plasma spray (PS), physical vapor deposition (PVD) and chemical vapor deposition (CVD), which can fabricate the PFM and join it to heat sink materials simultaneously, were applied for the fabrication of plasma facing materials (PFM) in fusion reactor. In the Institute of Nuclear Materials, University of Science and Technology Beijing (USTB), the concept of functionally graded materials (FGM) was adopted to fabricate coatings for effectively alleviating the thermal stress generated between coatings and the substrate materials under high heat flux loading (5~20 MW/m²). In the last several years, functionally graded coatings, including B4C/Cu, W/Cu and Mo/Cu systems were successfully fabricated by atmospheric plasma spray (APS). Characterization of coatings was performed in order to assess microstructure, mechanical properties and high heat flux properties of the FGM coatings. Furthermore, a high thick tungsten coating with 4 mm on copper – chromium - zirconium (Cu, Cr, Zr) alloy substrates was fabricated by APS. The porosity of the coating is less than 2% while mean tensile strength of the coating is about 7 MPa. However, the content of oxygen in the coating is about 6 wt% by energy dispersive spectrum (EDS) analysis, thus further optimization is necessary.

Abstract

ZrN/TiN multi-layers were synthesized by ion beam sputtering technique. Microstructure and mechanical property of the ZrN/TiN multi-layers were characterized and the relationships between microstructure and hardness of the ZrN/TiN multi-layers with various bilayer thicknesses and thickness ratios were investigated. The microstructure of multi-layers have been investigated using transmission electron microscope (TEM) and X-ray diffraction (XRD).

Abstract

Three types: micro-, submicro- and nano-structured Si₃N₄-SiC composites have been obtained by High Pressure-High Temperature (HPHT) sintering. Density, Young modulus, hardness and fracture toughness have been measured. Composites obtained from sub-micron powders are characterized by better mechanical properties than composites obtained from nanopowders.

Abstract

Hydrothermal surface modifications of the titanium specimens were performed (i) at 200 ºC under 1.5 MPa pressure, and (ii) at 230 ºC under 2.5 MPa pressure. To see the effect of an ion implantation, two different aqueous hydrothermal environments were selected: (i) de-ionized water and (ii) calcium containing de-ionized water. Hydrothermally treated titanium surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and found to become rich with Ca when Cacontaining hydrothermal environment was used. The surface-modified titanium specimens were then kept immersed in 1.5X simulated body fluid (SBF) for 1, 2, 3 and 4 weeks. The biomimetically formed coatings were characterized using scanning electron microscopy (SEM) and Xray diffractometer (XRD). Crack formations and, consequently, severe peelings were observed after drying for all the coatings on the substrates that were treated hydrothermally using only de-ionized water. The Ca implanted titanium surfaces, on the contrary, were able to develop crack-free and quite cohesive coatings. Up to two weeks of immersion and after drying, no-cracks were observed in the coatings when the substrates were treated at higher temperature and under higher pressure (230 ºC and 2.5 MPa for the present investigation).