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1.
DAVID KALISH EDWARD V. CLOUGHERTY KONSTANTINE KREDER 《Journal of the American Ceramic Society》1969,52(1):30-36
Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 1800°C and 120,000 psi. Bend strengths were measured on the fully dense materials from 25° to 1400° C in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700° and 1000°C and vary from 39,000 to 68,000 psi for HfB2 and 30,000 to 56,000 psi for ZrB2 . The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds. 相似文献
2.
Effect of the Machining Method on the Catalycity and Emissivity of ZrB2 and ZrB2 –HfB2 -Based Ceramics
Luigi Scatteia Davide Alfano Frederic Monteverde Jean-Louis Sans Marianne Balat-Pichelin 《Journal of the American Ceramic Society》2008,91(5):1461-1468
The emissivity and the catalytic efficiency related to atomic oxygen recombination were investigated experimentally in the range 1000–2000 K for ZrB2 and ZrB2 –HfB2 -based ceramics. In order to evaluate the effect of the machining method, two series of samples, one prepared by electrical discharge machining and the other machined by diamond-loaded tools, were tested. High emissivity (about 0.7 at 1700 K) and low recombination coefficients (on average 0.08 at 1800 K) were found for all the materials. The experimental data showed an effect of the surface machining on the catalytic behavior only on the ZrB2 -based composite; conversely, small variations were found in the recombination coefficients of ZrB2 –HfB2 -based samples for the different machining processes. The surface finish affected the emissivity at lower temperatures in both compositions, with the effect becoming negligible at temperatures above 1500 K. 相似文献
3.
Yanli Xie Thomas H. Sanders Jr. Robert F. Speyer 《Journal of the American Ceramic Society》2008,91(5):1469-1474
Zirconium diboride and a zirconium diboride/tantalum diboride mixture were synthesized by solution-based processing. Zirconium n -propoxide was refluxed with 2,4-pentanedione to form zirconium diketonate. This compound hydrolyzed in a controllable fashion to form a zirconia precursor. Boria and carbon precursors were formed via solution additions of phenol–formaldehyde and boric acid, respectively. Tantalum oxide precursors were formed similarly as zirconia precursors, in which tantalum ethoxide was used. Solutions were concentrated, dried, pyrolyzed (800°–1100°C, 2 h, flowing argon), and exposed to carbothermal reduction heat treatments (1150°–1800°C, 2 h, flowing argon). Spherical particles of 200–600 nm for pure ZrB2 and ZrB2 –TaB2 mixtures were formed. 相似文献
4.
James W. Zimmermann Gregory E. Hilmas William G. Fahrenholtz Ralph B. Dinwiddie Wallace D. Porter Hsin Wang 《Journal of the American Ceramic Society》2008,91(5):1405-1411
Thermophysical properties were investigated for zirconium diboride (ZrB2 ) and ZrB2 –30 vol% silicon carbide (SiC) ceramics. Thermal conductivities were calculated from measured thermal diffusivities, heat capacities, and densities. The thermal conductivity of ZrB2 increased from 56 W (m K)−1 at room temperature to 67 W (m K)−1 at 1675 K, whereas the thermal conductivity of ZrB2 –SiC decreased from 62 to 56 W (m K)−1 over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends. The results are compared with previously reported thermal conductivities for ZrB2 and ZrB2 –SiC. 相似文献
5.
The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB2 –SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB2 –SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB2 –SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB2 –SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO2 –SiO2 –B2 O3 (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO2 precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO2 because they have less B2 O3 , which results in a smaller amount of precipitated ZrO2 and consequently fewer convection cells. 相似文献
6.
Specimens of ZrB2 containing various concentrations of B4 C, SiC, TaB2 , and TaSi2 were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 1150°–1550°C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB2 –B4 C–SiC in the form of TaB2 and/or TaSi2 increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi2 proved to be a more effective additive than TaB2 . Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions. 相似文献
7.
Ju Li Thomas J. Lenosky Clemens J. Först Sidney Yip 《Journal of the American Ceramic Society》2008,91(5):1475-1480
Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid+liquid), and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica-rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a "solid pillars, liquid roof " scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen-rich condition is performed using first-principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature T C ∼1500°C, the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen-deficient centers, instead of molecular O2 * as in the Deal–Grove model. These network defects will lead to sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high-temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry. 相似文献
8.
Preliminary results about laser sintering of zirconium diboride (ZrB2 ), a good ultra high-temperature ceramics candidate, are presented. In order to evaluate a suitable sintering method, single pulsed laser and a concentric dual laser system were carried out. Two different ZrB2 powders having ∼15 and ∼2 μm particle size were assessed for sintering. Scanning electron microscopy images showed that the concentric dual laser sintered layer using ∼2 μm particle size of ZrB2 had relatively smooth surface morphology. X-ray diffraction results confirmed that the sintered layer mainly retained the crystalline phases as the starting powder. In addition, the rapid cooling rate of laser sintering enabled the formation of needle-like nanostructures at the sintered surface. 相似文献
9.
The fabrication temperature was the principal variable in a kinetic study of the densification of hafnium diboride in high-pressure hot-pressing. Densification studies for conventional hot-pressing were reviewed and correlated with the high-pressure hot-pressing results. The consolidation of HfB2 in the open pore region during high-pressure hot-pressing is attributed to particle rearrangement caused by grain boundary sliding and fragmentation. The final stage of densification (relative density >90%) was analyzed in terms of the Nabarro-Herring vacancy creep model. An activation energy of 22,900 cal/mole was obtained for the rate-controlling step in the creep process. 相似文献
10.
William G. Fahrenholtz 《Journal of the American Ceramic Society》2005,88(12):3509-3512
A volatility diagram was calculated for temperatures of 1000, 1800, and 2500 K to understand the oxidation of ZrB2 . Applying the diagram, it can be seen that exposure of ZrB2 to air produces ZrO2 (cr) and B2 O3 (l) over the temperature range considered. The pressure of the predominant vapor species was predicted to increase from ∼10−6 Pa at 1000 K, to 344 Pa at 1800 K, and to ∼105 Pa at 2500 K. Predictions were consistent with experimental observations that ZrB2 exhibits passive oxidation below 1200 K, but undergoes active oxidation at higher temperatures due to B2 O3 (l) evaporation. 相似文献
11.
Alex Spring Wei-Ming Guo Guo-Jun Zhang Pei-Ling Wang Vladimir D. Krstic 《Journal of the American Ceramic Society》2008,91(8):2763-2765
ZrB2 –LaB6 powder was obtained by reactive synthesis using ZrO2 , La2 O3 , B4 C, and carbon powders. Then ZrB2 –20 vol% SiC–10 vol% LaB6 (ZSL) ceramics were prepared from commercially available SiC and the synthesized ZrB2 –LaB6 powder via hot pressing at 2000°C. The phase composition, microstructure, and mechanical properties were characterized. Results showed that both LaB6 and SiC were uniformly distributed in the ZrB2 matrix. The hardness and bending strength of ZSL were 17.06±0.52 GPa and 505.8±17.9 MPa, respectively. Fracture toughness was 5.7±0.39 MPa·m1/2 , which is significantly higher than that reported for ZrB2 –20 vol% SiC ceramics, due to enhanced crack deflection and crack bridging near SiC particles. 相似文献
12.
The weight loss of Cr2 O3 in oxidizing environments (Po2 = 1 to 10−3 atm) at 1200°C was measured. Both hot-pressed and sintered Cr2 O3 pellets were investigated in O2 /Ar gas mixtures, and the dependence of the weight loss on the O2 partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O2 partial pressure dependence (rate ∝ PO2 3/4 ) corresponds to that expected for the reaction Cr2 O3 (s)+3/2O2 ⇌2CrO3 (g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate-controlling step in the oxidation/vaporization of Cr2 O3 . Evaporation coefficients for the loss of CrO3 (g) under the experimental conditions were <0.01. 相似文献
13.
Inna G. Talmy James A. Zaykoski Mark M. Opeka 《Journal of the American Ceramic Society》2008,91(7):2250-2257
The effect of Si3 N4 , Ta5 Si3 , and TaSi2 additions on the oxidation behavior of ZrB2 was characterized at 1200°–1500°C and compared with both ZrB2 and ZrB2 /SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB2 was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si3 N4 -modified ceramics increased with increasing Si3 N4 content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB2 , with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta5 Si3 , 30 vol% TaSi2 , 35 vol% Si3 N4 , or 20 vol% Si3 N4 with 10 mol% CrB2 . These materials exceeded the oxidation resistance of the ZrB2 /SiC ceramics below 1300°–1400°C. However, the ZrB2 /SiC ceramics showed slightly superior oxidation resistance at 1500°C. 相似文献
14.
The active slip planes, the Burgers vector, and the critical resolved shear stress to induce macroscopic deformation in ZrB2 single crystals were determined. Room-temperature microhardness indentation and high-temperature uniaxial compression loadings were used to induce deformation. Slip occurred in a close-packed α direction on prismatic planes at room temperature and on the basal plane at high temperatures. The high-temperature yield stress and yield drop are discussed in terms of a Widmanstaetten-type precipitate observed in the crystals. 相似文献
15.
Takashi Mizuguchi Shu-Qi Guo Yutaka Kagawa 《Journal of the American Ceramic Society》2009,92(5):1145-1148
Microstructure of the hot-pressed ZrB2 with MoSi2 additive was investigated by transmission electron microscopy (TEM). The effect of MoSi2 addition on the microstructure of the ceramic was assessed. For the pure ZrB2 , the microstructure consisted of the equiaxed ZrB2 grains and a few elongated ZrB2 grains. For the ZrB2 with MoSi2 additive, the microstructure consisted almost entirely of equiaxed ZrB2 grains. A few dislocations were present in the ZrB2 grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB2 grain boundaries in the ZrB2 with MoSi2 additive. 相似文献
16.
Narottam P. Bansal Charles H. Drummond III 《Journal of the American Ceramic Society》1993,76(5):1321-1324
The kinetics of hexacelsian-to-celsian phase transformation in SrAl2 Si2 O8 have been investigated. Phase-pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat-treated at 1026°, 1050°, 1100°, 1152°, and 1200°C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k , at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the interface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k , the apparent activation energy for this reaction was evaluated to be 527 ± 50 kj/mol (126 ± 12 kcal/mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si─O bonds. 相似文献
17.
Sea-Hoon Lee Yoshio Sakka Yutaka Kagawa 《Journal of the American Ceramic Society》2007,90(11):3455-3459
Dispersion conditions of ZrB2 powder in water were investigated using poly(ethyleneimine) (PEI) as a dispersant. Pulverization of ZrB2 powder to submicrometer size was difficult and a substantial amount of large particles remained after an intensive planatery milling for 72 h. The isoelectric point (IEP) of ZrB2 powder was measured to be pH 5.8 by electrophoresis, which shifted to pH 6.2 after milling. The application of PEI changed the IEP of the boride slurry to ∼pH 11. Well-dispersed aqueous ZrB2 slurries with a high solid loading (up to 45 vol%) were fabricated at pH 6.5–7.5 by the application of 1.5 wt% PEI. 相似文献
18.
Chikung J. Yu Don M. Sparlin Harlan U. Anderson 《Journal of the American Ceramic Society》1987,70(8):189-C
The oxidation kinetics of polycrystalline LaCrO3 were determined by measuring the time and temperature dependence of the weight and conductivity change of reduced samples. A region of fast diffusion followed by a smaller, slower diffusion "tail" was observed in the thermogravimetric measurements. This observation can be interpreted as the rapid diffusion along the grain boundaries and subsequent diffusion into the body of the grain. The absence of the "tail" in the conductivity measurements is due to the high hole mobility along the boundaries. 相似文献
19.
A. E. PALADINO 《Journal of the American Ceramic Society》1965,48(9):476-478
The oxidation kinetics were determined for single-crystal SrTiO3 by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm2 /sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10−6 exp (-26.1 ± 5.0 kcal/ RT ) cm2 /sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10−5 by virtue of the relation between the oxygen self-diffusion coefficient, D02- , and the oxygen vacancy diffusion coefficient, Do□ ; D o2- = [O□] D o□ where the oxygen ion concentration [O2- ] is taken as unity. 相似文献
20.
Alumina powders with varying iron oxide contents were prepared by coprecipitation. The powders were spheroidized by passing them through an oxygen-acetylene flame. The spheres were sized, annealed, and sintered in air and in N2 with 132 ppm O2 . Isothermal studies were combined with constant-rate-of-heating studies to identify the mechanism of sintering and to calculate the diffusion coefficients. The contribution of surface diffusion during initial-stage sintering of Fe-doped Al2 O3 was estimated by combining shrinkage and neck-growth data. The effect of Ti on the sintering rate of Fe-doped Al2 O3 was also studied. Both Fe2+ and Ti4+ ions enhanced the sintering rate of Al2 O3 . A defect model for corundum is proposed to explain the sintering data for transition-metal-ion-doped Al2 O3 . 相似文献