Issue No. 3, December

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).

Abstract

Highly porous hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) was prepared through hydrothermal (HT) transformation of aragonitic cuttlefish bones (Seppia Officinalis L. Adriatic Sea) in the temperature range from 140°C to 220°C for 20 minutes to 48 hours. Mechanism of hydrothermal transformation of bones was investigated by DTA/TG analyzer coupled online with FTIR spectrometric gas cell equipment (DTA-TG-EGA-FTIR analysis), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). DTA-TG-EGA-FTIR analysis have shown the release of CO₂ at about 400°C, 680°C and 990°C. The first release could be attributed to organics not completely removed from the heat treated bones, and the second release to decomposition of unconverted aragonite, whereas, the third one could be attributed to CO₃ ^2– groups incorporated in the structure of HA. The interconnecting porous morphology of the starting material (aragonite) was maintained during the HT treatment. The formation of dandelion-like HA spheres with diameter from 3 to 8 μm were observed, which further transformed into nanoplates and nanorods with an average diameter of about 200-300 nm and an average length of about 8-10 μm.

Abstract

Felipe Antunes Santos¹, Claudinei dos Santos^1,2, Durval Rodrigues Jr¹, Dolores Ribeiro Lazar³, Dayane Faviero de Castro¹, Daltro Garcia Pinatti¹, Rosa Ana Conte¹
1EEL - USP (Escola de Engenharia de Lorena da Universidade de São Paulo) - Polo Urbo-Industrial, Gleba AI-6, s/n, Mondesir, PC 116, Lorena, SP, Brazil
2 UNIFOA- MeMAT (Centro Universitário de Volta Redonda - Pró-Reitoria de Pesquisa e Extensão) - V. Redonda (RJ), Brazil
3IPEN (Instituto de Pesquisas Energéticas e Nucleares) - Av. Lineu Prestes, 2242, Cidade Universitária, CEP: 05508-000, São Paulo, SP, Brazil

Abstract

Using Mechanical Activation it is possible to obtain small grain size and good homogeneity in a ceramic piece. For ZnO varistor devices Mechanical Activation appears to be a good fabrication technique, since good homogeneity and small grain sizes are advantageous microstructural features. The typical formulation is composed by ZnO, Bi₂O₃, Sb₂O₃, CoO, MnO₂ and Cr₂O₃ as raw materials, and during sintering, several dissolutions and reactions to form pyrochlore and spinel phases occur. When Mechanical Activation is applied to the entire formulation, it is difficult to know what processes are being mechanically activated due to the complexity of the system. The aim of the present work was to clarify how the mechanical activation is taking place in a typical ZnO varistor formulation. The methodology consisted in the formation of all possible combinations of two out of the five oxides above mentioned and to apply mechanical activation on the mixture of each pair of powders. The results showed that systems containing Bi₂O₃ are prone to react during mechanical activation. Also, reduction reactions were observed in MnO₂. In addition, the powder mixture corresponding to the whole formulation was milled in a planetary mill, pressed and sintered, and varistor devices were fabricated. Improvement in the non-linearity coefficient and breakdown voltage was observed.

Abstract

Hetero-epitaxial growth of calcite crystals on calcium dodecyl sulfate (Ca(DS)₂ = CDS) single crystals was studied by alternate soaking method. The calcite (006) oriented single crystals grow on the (001) surface of the CDS. The hetero-epitaxial growth mechanism is discussed by the lattice matching of the a-c planes of calcite and CDS according to the structure data of the CDS single crystal.