Papers by Keyword: Zirconium Diboride

Abstract: The ultra-dispersive powders of pre-ceramic precursors for boron carbide based composites were obtained by relatively low-temperature (at 200 – 1000 °C) synthesis from liquid charges containing available compounds such as salts and oxides. Boron carbide matrix ceramics were compacted by their reactive spark plasma sintering (SPS) at 1500 – 1700 °C. It is noted that the X-ray diffraction (XRD) peaks corresponding (m)ZrO₂ and WC phases presented in the synthetic pre-ceramic precursors disappear after the SPS is conducted at 1500 °C. It is established that the addition of tungsten and cobalt compounds promotes both the low-temperature synthesis of ceramic components and sintering processes of their powders. Energy dispersive X-ray (EDX) analysis showed that the ceramics contain a small amount of Co (0.8 – 2 wt.%). The density of samples of cobalt-containing ceramics B₄C–ZrB₂–W₂B₅–Co is higher compared to that of cobalt-free ceramics B₄C–ZrB₂–W₂B₅.

Abstract: Silicon carbide (SiC)-containing zirconium diboride (ZrB₂) composites have become an important class of ultra-high temperature ceramic materials for the thermal protection systems of re-entry hypersonic vehicles with sharp leading edge profiles. Previous studies in ZrB₂-SiC composites showed that nano-sized SiC particles-containing ZrB₂ composites had a greater strength and a better oxidation resistance compared to ZrB₂-beased composites with micron-sized SiC particles. However, it is difficult for obtaining a homogenous microstructural ZrB₂-based composite with nano-sized SiC particles because of agglomerates of the SiC particles. In this study, homogenously dispersed nano-sized SiC particles-containing ZrB₂ composites were prepared using polymer-derived SiC-dispersed ZrB₂ composite powders followed by hot pressing at different temperatures between 1750°C and 1900°C. The microstructure of the resulting composites was characterized by field emission scanning electron microscopy. Four-point flexural strength of the obtained composites was measured at room temperature. The effects of the sintering temperatures and SiC content on the microstructure and the flexural strength of the composites were discussed.

Abstract: Zirconium diboride is widely applied because of some excellent performances. The oxidation kinetics of ZrB₂-YAG-Al₂O₃ composite materials were researched, which helps to improve the performance of ultra-high-temperature composite materials. The results show the oxidation weight gain is decreased with increasing the content of YAG-Al₂O₃ and the molar ratio of Al₂O₃. The oxidation weight gain is increased with prolonging the oxidation time under 1300°Cæ, the oxidation weight gain ratio is decreased with prolonging the oxidation time. The effecting tendency of oxidation weight gain is not abvious with varying the contend of YAG-Al₂O₃ upon 1300°Cæ, however, the effecting tendency of oxidation weight gain is very abvious with varying the molar ratio of Al₂O₃.

Abstract: Zirconium diboride nanopowders were synthesized by sol–gel method using different zirconium sources of zirconium carbonate and zirconium nitrate based on different gelling processing. Both of zirconium source can be applied in the synthesis of good performance ZrB₂ powders while the in-suit sol-gel using zirconium carbonate tend to form the spherical ZrB₂ powders about 50nm and the traditional sol-gel using zirconium nitrate prefer to form worm ZrB₂ powders about 200nm. The influences of B/Zr molar ration of zirconium carbonate and zirconium nitrate of sol-gel method on the phase constitution was investigated. And the gel mechanism was discussed to explain the different phase constitution, morphology of final products.

Abstract: The ZrB₂ powders with different morphology were prepared by pressureless reactions, using ZrO₂, B₂O₃, B₄C, and graphite as raw materials. Three kinds of chemical reaction system were employed. The ratio of raw materials and reaction temperature were adjusted to prepare ZrB₂ powders of different morphology and particle size. The phase composition and purity of the as-prepared powders were analyzed by XRD, while the morphology and particle size were analyzed by SEM. The ZrB₂ powders were purified by removing impurities at 600 °C in a muffle furnace in air atmosphere. The results showed that in the reaction systems of ZrO₂-B₂O₃-C and ZrO₂-B₄C-C, the ZrB₂ could be generated at 1500 °C. The morphology of the as-prepared ZrB₂ powders were particles, rod-like or near spherical for ZrO₂-B₂O₃-C system and particles for ZrO₂-B₄C-C system. In the reaction system of ZrO₂, B₂O₃, B₄C, and C with a mole ratio of 3:2:1:8, the ZrB₂ powders with high purity could be produced at 1700 °C. The ZrB₂ powder was near spherical. After heat treatment, the particle size and morphology changed to some extent.

Abstract: In the present study, we report on the synthesis and carbothermal reduction of ultra-fine zirconium diboride powders by using inorganic-organic hybrid precursors of Zirconium (IV) nitrate pentahydrate, boric acid and citric acid as sources of zirconia, boron oxide and carbon, resoectively. The effect of molar ratio of reactants and reaction temperatures on the as-synthesized precursors were investigated. The thermodynamic change in the ZrO₂-B₂O₃-C system was mainly studied by thermogravimetric and differential scanning calorimetry. The phase compositions and crystalline state of the products after heat treatment was determined by X-ray diffraction and the crystallite size and morphology of the synthesized powders were characterized by scanning electron microscopy. It was found that the as-synthesized precursor with B/Zr molar ratio of 3.5 can transform into zirconium diboride and zirconium carbide by heating in an argon atmosphere with temperatures as low as 1400°C and the synthesized powders exhibited near-spherical morphology with a samll average crystallite size of about 200nm and dispersed relatively uniformly. Moreover, with the reaction temperature increased, the purity of the zirconium diboride powders are higher. The mixture was finally transformed into pure zirconium diboride at 1600°C. However, the grain sizes increased significantly and tended to be aggregated with the reaction temperature increased to 1600°C. The synthesized ZrB₂ powders showed a porous structure and the grain sizes on the exterior is larger than the interior because of the higher heat treatment temperature. The finally single ultra-fine ZrB₂ grain sizes were distributed from 190nm to 690nm in two-dimensions and have a larger specific surface area of 88.14m²/g.

Abstract: Zirconium diboride (ZrB₂) powders were synthesized by sol-gel carbonthermal reduction method using zirconium oxychloride (ZrOCl₂·8H₂O), boric acid (H₃BO₃), source (C₁₂H₂₂O₁₁) and citrate (as chelation agent). The Zirconium diboride precursors were calcined at 1600¡æ for 2h, 3h and 4h under argon atmosphere. The precursors and sintered samples were characterized by TG-DSC, XRD and SEM. Results revealed that the ZrB₂ powders prepared at 1600°C for 4h exhibited quasi-tabular morphology with a small average crystalline size about 20μ. With respect to the conventional solid state method, the Sol-gel method route guaranteed a faster, easier and energy-saving process for obtaining nanopowders.

Abstract: In this study, oxidation behavior of ZrB₂-MoSi₂-SiC composite was investigated in the hot-pressed 5-20 vol% SiC-containing ZrB₂-20 vol% MoSi₂-based composites which were exposed to dry air between 1100°C and 1500°C up to 10 hours. The effects of SiC additive on the oxidation behavior were assessed. Experimental results showed that the weight gain due to oxidation exposure in air increased with increasing exposure temperature and exposure time. Parabolic oxidation behavior was observed for all the compositions composites. On the other hand, the weight gain decreased with increasing amount of SiC added. The addition of SiC improved the oxidation resistance of the composites, and the improvement was enhanced with increasing amount of SiC added. In addition, X-ray diffraction was used to identify major crystalline phases present in both the as-received and the post-oxidized composites. The oxidized sample surface was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructure of the post-oxidized composites consisted of two characteristic regions: oxidized reactive region and unreactive bulk material region. Furthermore, the oxidized reactive region divided into an outermost dense silica-rich scale layer and oxidized reactive mixture layer. The improvement of the oxidation resistance due to the addition of SiC is associated with the presence of the thicker dense outermost scale layer which inhibited inward diffusion of oxygen through it.

Abstract: Zirconium diboride nano-powders were synthesized by novel sol-gel technology using zirconium oxychloride, boric acid and sucrose as row materials. Different sol network modifiers, such as propylene oxide (PO) and citric acid (CA), were used to control the formation of zirconia precursor sol, respectively. PO could stabilize the sol by protonation and ring-opening reactions, and CA could form the sol network by chelation, then transformed to gel network. Gel was dried, ground, and exposed to carbonthermal reduction heat treatment (1500°C, 2h, flowing 95%argon mix with 5% hydrogen) to prepare ZrB2 nano-powders. In this study, effects of sol network modifiers on preparation procedure, powder characteristics were performed by using Infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The purity of ZrB2 will improve by increasing the molar ratio of B/Zr, the photomicrograph revealed nearly pure ZrB2 nano-powders with spherical shape of 50-200nm and well dispersed were obtained with a molar ratio of B/Zr of 3.0 at 1500°C for 2h using PO as chemical modifier.

Abstract: Zirconium diboride is widely applied because of some excellent performances. The results show the A1(OH)3-Y(OH)3/ZrB2 composite powders is prepared by a co-precipitation method, the shell-core structure A12O3-Y2O3/ZrB2 composite powders is prepared by sintering A1(OH)3-Y(OH)3/ZrB2 composite powders. The coating and density ZrB2-YAG-Al2O3 multi-phase ceramics is sintered at 1700°C, 20MPa for 4min by using Al2O3-Y2O3-80wt%ZrB2 composite powders, the oxidation weight is increased with increasing the oxidation temperature, however, the oxidation weight is decreased with increasing the YAG-Al2O3 content. ZrB2 reacted with O2 to form B2O3, and B2O3 is reacted with Al2O3 to form Al18B4O33, which is melted and coated on the surface of ceramics to form a protective layer for the oxidation resistance of ceramics at high temperature.