Materials Science Forum Vols. 561-565

Abstract: Aiming for the design of simple thermal coatings including Ir as a diffusion barrier layer with B2 aluminides bond coat such as CoAl, diffusion behavior and phase equilibria at the Ir/CoAl interface have been investigated to understand thermal and chemical stability of the interface. Diffusion couples were prepared with various conditions of Ir, plate, powder and film deposited to examine the effects on diffusion behavior. Moreover, the interdiffusion coefficient of binary Ir-M in the Ir solid solution, DIr-M was determined using Boltzmann-Matano method, where M is selected form elements used in heat resistant alloys. The potential of Ir as a diffusion barrier has been evaluated particularly against diffusion of Al.

Abstract: The microstructures and mechanical properties of three powder metallurgy Ti-48Al-2Cr- 2Nb-xW alloys (where x=0, 0.5, and 1 atomic percent (at.%)) are presented. The results indicate that a solution heat treatment combined with controlled cooling generate a fully lamellar (FL) microstructure without the formation of detrimental Widmanstätten or massively transformed γ phases. Aging causes coarsening of the FL microstructure in the alloys containing 0%W and 0.5%W, while almost no coarsening occurs in the 1%W sample. The addition of W to the base composition results in the formation of precipitates at the lamellar interfaces and grain boundaries during aging which helps stabilize the FL microstructure. The amount of W and the aging time affect the room temperature hardness values and tensile properties.

Abstract: This paper reports the significant effects of addition of 30 nm SiC, polytitanocarbosilane and SiC fabric to enhance the mechanical reliability of SiC. The flexural strengths of dense SiC hot-pressed with 800 nm particles (average strength 565 MPa for Y2O3-Al2O3 additives and 640 MPa for Yb2O3-Al2O3 additives) were enhanced to average strength 735-820 MPa by the addition of 30 nm SiC particles (25 vol%). Addition of polytitanocarbosilane (3 vol%, precursor of SiC fiber) to the bimodal SiC powder compact with Y2O3-Al2O3 additives provided more excellent mechanical properties of average strength 910 MPa, fracture toughness 5.2 MPa·m1/2 and Weibull modulus 11.3. SiC fabric and SiC (60 vol%) - Al2O3 (40 vol%) sheet of 60 micrometer thick were alternatively laminated and bonded to the surfaces of dense SiC under the pressure of 5 MPa. The SiC fabric prevented the propagation of the cracks formed by Vickers indentor and showed a significant nonlinear stress-strain curve. As a result, no change in the strength was measured before and after the introduction of cracks.

Abstract: The cubic pyrochlore phase Bi1.5ZnNb1.5O7 nanopowder was successfully synthesized by the hydrothermal method (HTM) from the starting materials: Bi(NO3)3·5H2O, ZnO, Nb2O5 and the mineralizer: KOH. The XRD patterns prove that the cubic pyrochlore phase Bi1.5ZnNb1.5O7 nanopowder can be obtained by HTM, and TEM photographs show that the powders present the regularly granular shape, when the hydrothermal reactions were conducted at synthesis temperatures 140~220°C and reaction time for 6~48h. The crystalline sizes of the powders were calculated by the Scherrer equation to be about 43~49nm. The crystalline sizes decreased both with the increase in synthesis temperature and the prolonged reaction time until they reached to the minimum size about 43nm at 220°C for 24h.However, they tended to increase when the reaction time was above 24h.

Abstract: BaBi4Ti4O15 ceramics was prepared by molten salt synthesis method (MSS) and conventional solid phase process (CS). The phase of reaction products and the microstructure were detected by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the evident anisotropic flake structure of BaBi4Ti4O15 powder prepared by MSS was obtained at 850°C and the level of anisotropy of BaBi4Ti4O15 powder decreased with increasing temperature. After calcined at 950°C, sintering in the range of 1100~1170°C, the sintering temperature and the maximum permittivity of BaBi4Ti4O15 ceramics prepared by MSS at same frequency was lower than those prepared by CS. Additionally, the apparent degree of orientation was obtained and the lotgering factor (f) calculated according to the XRD results showed a value of 0.63.

Abstract: A trend in the development of WC based cemented carbides and zirconia based ceramic composites is grain size refinement and more narrow grain size distributions of the starting powder, in order to accomplish higher hardness and abrasive wear resistance. The current work reports the results of dry sliding wear experiments on laboratory-made electrically conductive ZrO2-WC composites and commercially available WC-Co based cemented carbides, which have been manufactured and finished by rough cutting wire EDM with consecutive execution of gradually finer EDM regimes. Tribological data are obtained using a small-scale pin-on-plate test rig. Wear tracks are analyzed by surface scanning topography and scanning electron microscopy, revealing that the outer extensions of the wear tracks exhibit some differences in wear behavior compared to the central parts.

Abstract: Water debinding is a new technique for ceramic injection molding. Compared with conventional thermal debinding, water-debinding has many advantages, such as environment friendly, high debinding rate, and suitable for molding large sized ceramic part. In this paper, binder system, based on polyethylene glycol(PEG) and polyvinyl butyryl(PVB), was mixed with 3Y-TZP(ZrO2) powder, and feedstock with 54vol% of ceramic powders was obtained. Shear viscosity of the feedstock was 300Pa·s at 190oC, when shear rate was 100/s, similar to the feedstock based on paraffin wax(PW). SEM showed that the feedstock and sintered parts were both homogeneous. Water debinding dynamics was also studied. For the 5mm x 6mm x 42mm sample, 65% of PEG could be extracted within 2 hours in water at 40oC, and the bending strength of sintered compacts was above 900MPa. The samples with thickness from 9mm to 12mm were made and the green parts could be debinded in water without any defects. The results showed water debinding method could increase the debinding rate and was effective to fabricate large sized ceramic parts.

Abstract: We report an investigation of millimeter-wave processing of yttria for fabrication of transparent, high-strength polycrystalline laser hosts for high energy laser (HEL) applications. Advantages of polycrystalline, compared to single-crystal laser host materials, include lower processing temperature, higher gain with flexibility of higher dopant concentrations, cheaper fabrication, and larger-size devices. Millimeter-wave processing is an alternative method to solve the problems of both conventional vacuum and low-frequency microwave sintering, such as low heating rate, poor coupling and thermal gradients. A major component of the millimeter-wave processing facility is a 20-kW, continuous-wave, 83-GHz gyrotron oscillator. Yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria sample has also been achieved using the millimeter-wave sintering process [1]. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency will be discussed.

Abstract: Barium titanate nanopowders were prepared from the modified oxalate co-precipitation method. Barium chloride dihydrate and potassium titanyl oxalate dihydrate were used as the starting precursors with the mole ratio of Ba:Ti as 1:1. Barium titanyl oxalate complex powders were obtained after adjusting the final of pH of the solution to 1. The precipitates were filtered, washed and dried at 100 oC for 5h. The precipitate powders were calcined at 400-700 oC for 2h. The phase transition was studied by X-ray diffractometry (XRD). Cubic structure of barium titanate was obtained after calcinations at 700 oC for 2h. The morphology and chemical composition were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The range of particle size was 50-100 nm with irregular in shape. The elemental composition of barium, titanium and oxygen showed the characteristic X-ray energy values.

Abstract: A Bi2-xSbxTe3(x=0-1.5) thick film thermoelectric element was fabricated using centrifugal deposition and its Sb content-dependent thermoelectric properties were investigated. When the Sb content was low (x=0.5), two types of fine structure along the direction of the thickness were observed. Pole figure measurements revealed that the vicinity of the film surface was composed of single crystal layers oriented along the c-axis and the vicinity of the interface with the substrate was composed of randomly orientated layers. As the content of Sb increased, the degree of orientation improved, and at x=1.5 the entire film was close to a single crystal. A Bi0.5Sb1.5Te3 thick film showed p-type thermoelectric properties and a thermoelectric power factor of 3.5 ×10-3W/mK2. It was thus demonstrated that centrifugal deposition can be used to fabricate thermoelectric elements with high efficiency.