Search

Abstract: In this work, the pressureless sintering behavior of boron carbide (B4C) ceramics with 5–10wt.% Al–Si binary additives was studied, while separate addition of individual elemental Al and Si was also investigated for comparison. Near fully-dense B4C ceramics can be prepared at 2150–2250°C when 5–7wt.% of Al–Si with molar ratio of 9:1 is added. In contrast to individual Al and Si, the use of the binary Al–Si additives in B4C ceramics results in a refined microstructure and increasing the Al–Si molar ratio from 1:1 to 9:1 promotes the densification of the B4C ceramics. Energy dispersion spectrum (EDS) analysis demonstrates the formation of SiC at the B4C grain boundaries. [Copyright &y& Elsevier]

Abstract: Unidirectional SiO2f/SiO2 composites were processed with pressureless sintering and hot-pressing, respectively. The influences of the sintering temperature, atmosphere and pre-treatment on the properties were studied. It was found that both higher sintering temperature and air atmosphere promoted the crystallization of the silica fiber, which results in degrading the fiber properties and thus restrained the enhancement of mechanical properties of SiO2f/SiO2 composites. Flexural strength of the fibrous composites sintered with hot-pressing was higher than that for pressureless sintering. The promoted mechanical properties were attributed to the pull-out of the fiber. The in situ formed pyrolytic carbon (PyC) layer on the fiber was beneficial to the properties of the fiber, however, detrimental to the dielectric properties. [Copyright &y& Elsevier]

Monodisperse ZrO2 ceramic beads with size larger than 1 mm have been prepared by an improved micro‐droplet spray forming process, through which a compressor and a dispenser were employed to produce droplets continuously. Furthermore, the slurry recipe and drying temperature have been optimized to enhance the sphericity and smoothness of the beads. The sintered ZrO2 ceramic beads present promising mechanical performance, including a relative density of 84.6%, a crush strength of 256.2 ± 36.6 N as well as a Vickers hardness of 1344.4 ± 58.3 HV. Such procedure reveals great potential in mass production of ceramic beads. [ABSTRACT FROM AUTHOR]

Zirconia microspheres with high sphericity and hardness were successfully massively produced through a cost-effective spray granulation method by using an optimized slurry with high solid content and low viscosity which was formulated with a mixed dispersant. The influence of slurry solid content, binder addition as well as drying temperature on the shape and size of the sintered microspheres were studied. Furthermore, the formation mechanism of different granulated particles was analyzed based on the illustrated migration processes in the ZrO 2 droplets with different contents of water and additives. Under the optimized conditions, the sintered ZrO 2 spheres with tetragonal phase exhibit high sphericity with highest nano-hardness of 9.5 GPa, indicating it is possible to be used in the engineering fields. Such spray granulation method could directly serve for the industrial production of zirconia and other ceramic materials since it is a stable process with high yield, good granulating quality and strong controllability. [Display omitted] • ZrO 2 microspheres with high sphericity have been produced by spray granulation. • ZrO 2 slurry with high solid content and low viscosity has been obtained. • The formation mechanism of particles with different shapes has been demonstrated. • The sintered ZrO 2 microspheres can achieve a high nano hardness of 9.5 GPa. [ABSTRACT FROM AUTHOR]

High-strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC1− x, Zr3 Al3 C5 and Zr- Al compound such as AlZr3 and Al- C- Zr. The intermediate product AlZr3 below 1600°C and remained Al- C- Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C-1900°C) could be attributed to the formation of nonstoichiometric ZrC1− x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC0.85- Zr3 Al3 C5 and ZrC0.80- Zr3 Al3 C5 based ceramics had 3-point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2-18.3 GPa and 3.3-3.5 MPa m1/2, respectively. [ABSTRACT FROM AUTHOR]

Two different modes of placing powders in a graphite die were used in a spark plasma sintering system: loading a conductive green body (zirconium nitride, ZrN, or titanium nitride, TiN) into a non-conductive bed (boron nitride) or loading the ZrN or TiN powders into the die directly. The ceramics obtained, which had completely different morphologies, have been characterized in this work. Through experimental and theoretical analysis, possible formation mechanisms of microstructural differences have been proposed. [Copyright &y& Elsevier]

Abstract: Using nano-sized starting powders, TiN ceramics were hot pressed under different sintering atmospheres, which were nitrogen, argon and vacuum (5Pa). The effects of sintering atmosphere on the densification process and microstructures of TiN ceramics were investigated. At 1500°C, ceramics densified under vacuum demonstrated higher density compared with nitrogen and argon, owing to the high nitrogen vacancy concentration which came from the dissociation of TiN under vacuum circumstance. At 1600°C, however, the densification process under vacuum will be hindered because of the decomposition of TiN. Combining a lower sintering temperature, smaller grain size and higher grain boundary concentration, the argon atmosphere is more suitable for fabrication of TiN ceramics as nuclear materials. [Copyright &y& Elsevier]

Abstract: The densification of hot-pressed ZrN ceramics doped with Zr or Ti have been investigated at 1500–1700°C. It is shown that either Zr or Ti additive can facilitate the densification process. ZrN with 20mol% Zr or Ti (named ZNZ and ZNT) sintered at 1700°C can achieve above 98% relative densities whereas densification temperature up to 2000°C is necessary for pure ZrN. The densification improvements are attributed to solid solution of Zr or Ti into ZrN to form non-stoichiometric ZrN1−x or (Zr, Ti)N1−x . The microstructures and mechanical properties of ZNZ and ZNT samples have been examined. Large grain size and flat fracture surface existed in ZNT sample sintered at 1700°C, which lead to poor toughness as low as 2.3MPam1/2. On the contrary, the fracture toughness of ZNZ sample sintered at 1700°C was up to 5.9MPam1/2, attributed to fine and uniform grain size distribution. [Copyright &y& Elsevier]

Composites of ZrC- SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC1− x, SiC, and ZrSi. Low-temperature densification of ZrC− SiC ceramics is attributed to the formed nonstoichiometric ZrC1− x and Zr- Si liquid phase. Adding 5 wt% Si to ZrC, the three-point bending strength of formed ZrC0.8-13.4 vol% SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m1/2, respectively. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, ZrO2 removing reactions of Groups IV–VI transition metal carbides (MCs, M=Hf, Nb, Ta, W, Ti and V) in ZrB2 based ultra-high temperature ceramics (UHTCs) are investigated. Distinct roles of various MCs were observed during this process. According to thermodynamic analysis and experiment verifications, the sequence for oxide removing ability of different MCs is WC>VC>NbC>TaC>HfC>TiC. The importance of this study concerns the establishment of a map of reactivity of the transition metal carbides against ZrO2, in order to choose the proper additive for the densification of ZrB2-composites.Utilizing right reactions and controlling proper sintering atmospheres, high density ZrB2–SiC ceramic could be eventually obtained by pressureless sintering at temperatures (around 2000–2200°C). [ABSTRACT FROM AUTHOR]