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This paper reports the texturing behavior of hexagonal boron nitride (h-BN) ceramics prepared by slip casting, followed by spark plasma sintering, using either irregularly shaped or plate-like h-BN as the starting materials. In this study, well-dispersed h-BN ethanol-based suspensions with 30 vol% solid loading were prepared by adding 2.5 wt% polyetherimide dispersant. It was found that both kinds of h-BN exhibited a substantially strong orientation during slip casting, which was featured by the alignment of the c-planes of the BN crystals perpendicular to the casting direction. Analysis and experimental results revealed that the texture development resulted from ethanol flow during slip casting and intrinsical surface properties of h-BN. Consequently, the orientation degree was very sensitive to the thickness of green bodies, which decreased when the thickness exceeded a certain value, and finally disappeared. Densities of ~92.0% and 88.9% of the theoretical for the irregularly shaped and plate-like h-BN, respectively, were achieved after sintering at 1800°C/50 MPa. The thermal conductivity of the as-prepared BN ceramics showed a strong dependence on the grain orientation, being substantially higher along the BN layers than across them. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy study of pressureless sintered ZrB2-20 vol% SiC composites with WC (5-10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO2 impurities were removed from ZrB2 grain surface were confirmed. Three new formed phases, (W, Zr)ssB, (Zr,W)ssC, and (W, Zr)ssSi2, apart from the plate-like ZrB2 and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)ssB and (W, Zr)ssSi2, appear in this system during the sintering process below 2200°C, and the former could well wet the ZrB2 grain boundaries. The mass transport process between ZrB2 and (W, Zr)ssB was also confirmed by scanning-transmission electron microscope analysis. The above results supports that the densification at 2200°C is assisted by liquid phase and the elongation of ZrB2 platelets resulted from the Ostwald ripening during the dissolution-diffusion-precipitation process. [ABSTRACT FROM AUTHOR]

WC powder was synthesized by a nitridation-carburization method with WO3 prepared by the citrate sol-gel route as the starting material, and the reaction mechanism was investigated. It was found that WO3 was converted to nanosized W2N after the nitridation process in NH3. Then, the W2N was transformed to W metal, W2C, and WC in sequence when it was carburized in CH4-H2 gas mixture. The WC powder obtained possessed a high specific surface area of 24.6 m²/g, and showed a particle size of 20-30 nm, which was much finer than the WC powder prepared by direct carburization of WO3 in CH4-H2 gas mixture. The reason was also discussed. [ABSTRACT FROM AUTHOR]

Phase pure hafnium diboride (HfB) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as-synthesized HfB powder and a commercial SiC, HfB monolithic, and HfB-20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993±90 MPa for the composite. Fracture toughness of the composite was 6.29±0.65 MPa m, which was significantly higher than that of the HfB monolithic and the reported values in literature. [ABSTRACT FROM AUTHOR]

Reaction bonding of silicon nitride (RBSN) technique combined with slip-casting shaping process was used to fabricate porous Si3N4/BN ceramic composites. Si/BN slurry with chemical stability, good dispersibility, and viscosity was prepared using glycerol trioleate (GTO) covering on Si surface and poly(acrylic acid) (PAA) as dispersant. The hydrolysis of Si was strongly prevented by GTO coating. The dispersibility of covered Si and BN suspensions were improved by PAA dispersant. Twenty volume percent covered Si/BN slurries with low viscosity were successfully casted. The cast bodies were dried at room temperature, debindered at 750°C and nitrided below 1450°C. The nitrided samples mainly consist of α-Si3N4, β-Si3N4, and h-BN. The composites exhibit homogeneous microstructure consisting of faceted particles, α-Si3N4 nanowires and a large amount of pores. The porosity is 52.64% and the pore size is in the range of 60–300 nm. The composites show compressive strength of 16.6±1.5 MPa. The dielectric constant of the composite is about 3.1 and the dielectric loss is below 0.5% under different frequencies. [ABSTRACT FROM AUTHOR]

Ultra-fine HfC powders with particle sizes of 380 or 225 nm were synthesized by carbothermal reduction of HfO2 with graphite or carbon black, respectively. The investigation of the process indicated that a vacuum level as high as 5 Pa was appropriate for decreasing the reaction temperature and consequently limiting the particle growth of HfO2 reactant. Type and size distribution of carbon sources demonstrated an obvious effect on the particle size of the HfC powders obtained. A model was proposed to explain this phenomenon. Carbon black with a characteristic particle size of 42 nm resulted in a HfC powder with a median particle size of 225 nm. Compared with the reported values in the literature, the synthesized HfC powder exhibited better sinterability than the commercial HfC powder. Using the synthesized powder, monolithic HfC ceramics pressureless sintered at 2400°C reached a relative density of 98.4% and an average grain size of about 4 μm. [ABSTRACT FROM AUTHOR]

Using fine tantalum carbide (TaC) powder with narrow particle-size distribution, monolithic TaC ceramics with a relative density of 97.5% was obtained by pressureless sintering at 2300°C without adding additives. The monolithic TaC ceramics had a very fine microstructure with grain size of about 2 μm. It is considered that the small particle size and uniform particle-size distribution of TaC powder are important factors for preparing dense TaC ceramics with fine microstructure. Otherwise, fast grain growth of TaC will occur. Compared with the reported values in literature, the monolithic TaC ceramics obtained demonstrated a high bending strength of 338±10 MPa. [ABSTRACT FROM AUTHOR]

Alterable roles of VC addition on the densification, microstructure development, and mechanical properties of the ZrB2–SiC ultrahigh temperature ceramics (UHTCs) were comprehensively investigated based on utilizing the thermodynamic analysis and experimental results. Chemical reactions of VC with the inevitably existed oxide (ZrO2) impurity on the particle surfaces of the ZrB2 starting powder, or with the ZrB2 matrix phase could be controlled by adjusting the atmospheres during hot pressing. Different chemical reactions during hot pressing also resulted in much different material microstructures and then different material properties. Benefiting the VC addition and proper sintering atmosphere, refined grain size of ZrB2 and improved mechanical properties were developed in ZrB2–SiC systems. By properly controlling these reactions, the microstructures and mechanical properties of ZrB2–SiC based UHTCs could be tailored. [ABSTRACT FROM AUTHOR]

A ZrB2–SiC–ZrC ceramic was produced by reactive hot pressing using Zr, Si, and B4C as raw materials. The kinetics of the reaction process was studied. The reduction of powders by ball milling is of crucial importance for the sintering. The self-propagating high-temperature synthesis reaction between the raw powders can be ignited by controlling the sintering conditions, which leads to a sintering temperature as low as 1600°C, the lowest sintering temperature reported thus far. The relative density is 97.3%, with an open porosity of 0.6%, and the mechanical properties are comparable to the composites that sintered at higher temperatures. The depletion of oxygen impurities during the sintering was discussed. [ABSTRACT FROM AUTHOR]

Fine hafnium diboride (HfB2) powders have been prepared by modified carbothermal/borothermal reduction of hafnium dioxide (HfO2) at relatively low temperatures (1500°–1600°C) for 1–2 h. The XRD patterns could be indexed as hexagonal HfB2 and no evidence of HfC, HfO2, or other impurities was observed. Glow discharge mass spectrometer analysis indicates that the synthesized HfB2 powders had high purity. The synthesized HfB2 powders had small average crystallite size (around 1 μm) and low oxygen content (<0.30 wt%). Scanning electron microscopy observation of the as-prepared powders demonstrated quasi-column morphology and laser particle size analysis showed monodispersity (polydispersity 0.005). [ABSTRACT FROM AUTHOR]

Powders of Nd/V-doped Bi4Ti3O12 with a composition Bi3.74Nd0.26Ti2.98V0.02O12.01 (BNTV), prepared by the molten salt synthesis, using chlorides as the fluxes, were successfully obtained. The influences of the molar ratio of KCl to NaCl, temperature, and soak time on the formation and morphology of BNTV were investigated. The results indicated that Na+ can enter the lattice of Bi4Ti3O12 (BIT), leading to a decrease in the cell dimensions of BIT phase. The grain size and morphology of BNTV powders were considerably affected by the molar ratio of KCl to NaCl in the fluxes, in which the platelets showed the morphology of a rectangle and no regular shape in KCl and NaCl fluxes, respectively. The grain size of BNTV increased with increasing amount of NaCl in the fluxes, but the thickness decreased. The platelets prepared in the NaCl flux were faceted along (0 0 1) plane. [ABSTRACT FROM AUTHOR]

A self-reinforced ultrahigh-temperature ceramic (UHTC) with elongated ZrB2 grains has been successfully densified by pressureless sintering using commercially available ZrB2, SiC, and WC powders as raw materials. Benefiting from the unique interlocking microstructure, this material had improved strength (518 ± 10 MPa) and higher fracture toughness (6.5 ± 0.2 MPa m1/2) compared to ZrB2-SiC ceramics prepared by pressureless sintering. This work provides a new route for tailoring the microstructure and mechanical properties of UHTCs. [ABSTRACT FROM AUTHOR]