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1.
Ken Hirota Yoshihiro Nakayama Masaki Kato Shingo Nakane Toshiyuki Nishimura 《International Journal of Applied Ceramic Technology》2009,6(5):607-616
Carbon nanofiber (CNF)-dispersed B4 C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B4 C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σb ) of ∼710 MPa, a Vickers hardness ( H v ) of ∼36 GPa, a fracture toughness ( K I C ) of ∼7.9 MPa m1/2 , and high-temperature σb of 590 MPa at 1600°C in N2 . Interfaces between the CNF and the B4 C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy. 相似文献
2.
Lily L. Wang Zuhair A. Munir J. Birch Holt 《Journal of the American Ceramic Society》1995,78(3):756-764
The combustion synthesis of MgO-B4 C composites was investigated by coupling a highly exothermic Mg-B2 O3 thermite reaction with a weakly exothermic B4 C formation reaction. Unlike the case of using Al as the reducing agent, the interaction between Mg and B2 O3 depends on the surrounding inert gas pressure due to the high vapor pressure of Mg. The interaction changes from one involving predominantly gaseous Mg and liquid B2 O3 to one involving liquid Mg and liquid B2 O3 as the pressure increases. At low inert gas pressure, the initiation temperature is found to be just below the melting point of Mg (650°C). As the inert gas pressure increases, the vaporization loss of reactants is reduced, and this in turn increases the combustion temperature, which promotes greater grain growth of the product phases, MgO and B4 C. The particle size of B4 C increased from about 0.2 to 5 μm as the pressure changed from 1 to 30 atm. 相似文献
3.
Rosario A. Gerhardt Julie Runyan Catherine Sana David S. McLachlan Robert Ruh 《Journal of the American Ceramic Society》2001,84(10):2335-2342
In this third paper of the series, we discuss the electrical resistivity of BN–B4 C composites with compositions ranging from 0% to 100% B4 C. After establishing the response of samples whose compositions lie far away from the percolation region, where effective medium models apply, we focus attention on samples with compositions at or near the percolation threshold (∼60% BN–40% B4 C). The large differences in electrical properties among samples with the same nominal composition can be explained by invoking a connectivity parameter. Since the difference in the electrical resistivity of BN and B4 C is about 9 orders of magnitude, the degree of connectivity of the two components at the percolation threshold determines the resultant composite resistivity. Connectivity in these composites was quantified by taking BN peak height ratios in X-ray diffraction patterns of all samples containing 60% BN–40% B4 C. The degree of preferred orientation of the BN platelets can be correlated with systematic increases in the electrical resistivity of the composites. 相似文献
4.
Yuan Zhao Lian-Jun Wang Guo-Jun Zhang Wan Jiang Li-Dong Chen 《Journal of the American Ceramic Society》2007,90(12):4040-4042
A mixture of Zr, B4 C, and Si powders was adopted to synthesize a ZrB2 –SiC composite using the spark plasma sintering–reactive synthesis (SPS–RS) method. SPS treatments were carried out in the temperature range of 1350°–1500°C under a varying pressure of 20–65 MPa with a 3-min holding time. A dense (∼98.5%) ZrB2 –SiC composite was successfully fabricated at 1450°C for 3 min under 30 MPa. The microstructure of the composite was investigated. The in situ formed ZrB2 and SiC phases dispersed homogeneously on the whole. The grain size of ZrB2 and SiC was <5 and 1 μm, respectively. A number of in situ formed ultrafine SiC particles were observed entrapped in the ZrB2 grains. 相似文献
5.
Chung-Kook Lee Yolande Berta Robert F. Speyer 《Journal of the American Ceramic Society》1996,79(1):183-192
A glass crystallization method was utilized to synthesize nanosized BaO-6Fe2 O3 platelets from a 0.412BaO-0.258B2 O3 -0.330Fe2 O3 batch composition. Quenched ribbons were inhomogeneous, showing microclustering and ∼1 μm hematite crystals. Na2 O substitutions for BaO greatly enhanced the glass-forming tendency of quenched ribbons, though quenched-in ∼0.5 μm barium ferrite crystals were infrequently present. The improved homogeneity with Na2 O substitution was attributed to lower vapor pressure of BaO during batch melting, which increased its retention in the as-quenched ribbons. Quantities of BaO equal to or in excess of Fe2 O3 allowed iron ions to adopt stable network positions in the glass melt. With Na2 O substitution, devitrification of dispersed ∼40 nm barium ferrite particles from phase-separated regions occurred after secondary heat treatment. 5 mol% Na2 O batch substitution showed the lowest crystallinity in the as-quenched ribbons, and the highest crystallinity after secondary heat treatment. After optimum devitrification, the maximum values of saturation magnetization and coercivity were 21.22 emu/g and 2.82 kOe, respectively. 相似文献
6.
The fracture strengths of sintered Al2 O3 containing 20 and 40 vol% ZrO2 (12 mol% CeO2 )—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m1/2 , respectively), by increasing the mean grain size of the t -ZrO2 (and the Al2 O3 ) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO2 contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO2 (12 mol% CeO2 ), CeTZP, and ZrO2 (12 mol% CeO2 ) ceramics containing ≤20 vol% Al2 O3 . The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites. 相似文献
7.
Mukkuttiparambil Ayyappan Sanoj Chalappurath Pattelath Reshmi Manoj Raama Varma 《Journal of the American Ceramic Society》2009,92(11):2648-2653
ZnNb2 O6 (ZN) is a columbite-structured niobate compound showing excellent dielectric properties and comparatively low sintering temperatures (∼1200°C). Hence it is a good candidate for possible low-temperature cofired ceramics (LTCC) applications. In the present investigation, ZnNb2 O6 was synthesized in the form of micrometer-sized powder using a conventional solid-state ceramic synthesis route as well as in the form of nanosized powder by a polymer complex method. The finite size effect of ZN particles on sinterability and microwave dielectric properties of sintered pellets was evaluated. The phase formation was confirmed from the X-ray diffraction (XRD) analysis. The particle size distribution of the nanoparticles was found to be of the order of 18–20 nm by using high-resolution transmission electron microscopy analysis and 30 nm by analyzing the XRD patterns using Debye Scherrer's formula, after correcting for the instrument broadening effects. A ZN–60ZnO–30B2 O3 –10SiO2 (ZBS) composite was made by adding predetermined amounts of glasses. The microstructures of the sintered pellets of ZN and ZN–ZBS composites were examined using scanning electron microscopy and analyzed using image analysis. The nano-ZN–ZBS composites were sintered to 93% of the reported density at 925°C/2 h, with microwave dielectric properties of ɛr =22.5, Q × f ∼12 800 GHz, and τf =−69.6 ppm/°C, emerging as a potential material for possible LTCC applications. 相似文献
8.
Xiaowei Yin Nahum Travitzky Peter Greil 《International Journal of Applied Ceramic Technology》2007,4(2):184-190
A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO2 , and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d 50 ≈0.7 μm) and inter-agglomerate pores ( d 50 ≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti3 AlC2 , TiAl3 , Al, and Al2 O3 , which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa. 相似文献
9.
Seeding a mixture of boehmite (AIOOH) and colloidal ZrO2 with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO2 in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO2 was determined to be ∼0.4 μm for the 14 vol% ZrO2 —Al2 O3 composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication. 相似文献
10.
Atsushi Kitaoka Ken Hirota Masaru Yoshinaka Yoshinari Miyamoto Osamu Yamaguchi 《Journal of the American Ceramic Society》2000,83(5):1311-1313
NiAl/10-mol%-ZrO2 (3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al2 O3 , has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO2 (∼0.2 μm) and Al2 O3 (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C. 相似文献
11.
Hideaki Itoh Ryuuji Yamamoto Hiroyasu Iwahara 《Journal of the American Ceramic Society》2000,83(3):501-506
High-pressure sintering behavior in the B6 O– c -BN system was investigated using in-laboratory-synthesized B6 O and commercially available c -BN powders (with an average grain size of 0.5, 3, or 6 μm). No reaction occurred between the two components under the high-pressure (4–6 GPa) and high-temperature (1500°–1800°C) conditions that have been investigated. Well-dispersed, sintered B6 O– x ( c -BN) composites (where x = 0–60 vol%) of almost-full density were prepared by sintering at a pressure of 6 GPa and temperature of 1800°C for 20 min. The maximum Vickers microhardness (46 GPa) of these composites was attained by adding 40 vol% c -BN with an average grain size of 0.5 μm. The fracture toughness of these composites increased as the c -BN content increased; the maximum fracture toughness (1.5–1.8 MPa. m1/2 ) was observed for x = 40–60 vol%. Crack deflection along the B6 O– c -BN grain boundary contributed to increasing the fracture toughness. 相似文献
12.
Porous Cr3 C2 grains (∼300 to 500 μm) with ∼10 wt% of Cr2 O3 were prepared by heating a mixture of MgCr2 O4 grains and graphite powder at 1450° to 1650°C for 2 h in an Al2 O3 crucible covered by an Al2 O3 lid with a hole in the center. The porous Cr3 C2 grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m2 /g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr3 C2 grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics. 相似文献
13.
Lingappa Rangaraj Canchi Divakar Vikram Jayaram 《Journal of the American Ceramic Society》2009,92(2):311-317
Reactive hot pressing of Ti and BN powder mixtures is used to produce dense TiN x –TiB2 composites. The effect of excess Ti along with a small addition, ∼1 wt% Ni, on the reaction and densification of the composite was investigated. A composite of ∼99.9% relative density (RD) was produced at 1200°C at 40 MPa for 30 min with 1 wt% Ni, whereas composites produced without Ni are porous and contain residual reactants. The microstructural studies on composite samples with excess Ti produced at short durations indicate the presence of a transient (Ni–Ti) phase from which Ti is finally removed to form substoichiometric TiN x . The hardness of the dense TiN x –TiB2 composite is ∼22 GPa. The densification mechanism in this system is contrasted with the role of nonstoichiometry in the Zr–B4 C system. 相似文献
14.
Comparing the crystallization mechanism of stoichiometric and B2 O3 and P2 O5 containing glass reveals that the additives extend the gap between the glass transition and crystallization temperatures and suppress formation of μ, cordierite while promoting direct crystallization of α cordierite. Detailed TEM analysis of nucleation and growth of crystals in hot-pressed pellets of B2 O3 /P2 O5 -containing glass particles shows that nucleation occurs on unidentified heterogeneous nuclei at the sites of the previous particle surfaces. Growth of α cordierite with a cellular morphology or μ cordierite with a dendritic morphology is most likely controlled by the glass composition directly ahead of the growth front. 相似文献
15.
Yongjie Yan Hui Zhang Zhengren Huang Jianxue Liu Dongliang Jiang 《Journal of the American Ceramic Society》2008,91(4):1372-1376
Ultrafine ZrB2 –SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m2 /g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB2 +20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B4 C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB2 matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures. 相似文献
16.
Julie Runyan Rosario A. Gerhardt Robert Ruh 《Journal of the American Ceramic Society》2001,84(7):1490-1496
The McLachlan equation, which incorporates both effective medium models and percolation, was used to predict the volume fraction–conductivity relationships of insulator–conductor composites, and results were compared with experimental data. Two composite systems were investigated (BN–B4 C and BN–SiC). Both systems are anisotropic, because of the orientation of BN platelets perpendicular to the hot-pressing direction. For BN–B4 C composites, with increasing B4 C content, the ac and dc conductivities are relatively constant to ∼40% B4 C (the critical volume fraction). At this composition, the conductivity suddenly increases to a value closer to that of B4 C and then resumes a gradual increase. Little difference is seen for measurements made perpendicular or parallel to the hot-pressing direction, i.e., perpendicular or parallel to the BN platelets. Similar results are found for the BN–SiC composites, except that the critical volume fraction is ∼20% SiC in this case. The experimental curves are in good agreement with those predicted by the McLachlan equation. The parameters s and t of the McLachlan equation relate to the morphology of the phases present in the microstructure. The critical volume fraction relates to the connectivity of the phases in the composites. 相似文献
17.
Yunle Gu Luyang Chen Yitai Qian Wanqun Zhang Jianhua Ma 《Journal of the American Ceramic Society》2005,88(1):225-227
Nanocrystalline boron carbide (B4 C) was synthesized via a solvothermal reduction of carbon tetrachloride using metallic lithium as reductant in the presence of amorphous boron powder at 600°C in an autoclave. The X-ray diffraction pattern of the product powder was indexed to the hexagonal B4 C phase, with lattice constants a =5.606 and c =12.089 Å. The sample was also characterized by Raman spectrum, X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry. Transmission electron microscopy revealed that the B4 C nanocrystallites were slightly agglomerated, with a particle size of approximately 15–40 nm in diameter. 相似文献
18.
Pressureless Sintering of Boron Carbide 总被引:4,自引:0,他引:4
B4 C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B4 C–B4 C contact between particles permitted via volatilization of B2 O3 particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B4 C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B4 C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B2 O3 to form B4 C and CO gas, thereby extracting the B2 O3 coatings, permitting sintering to start at ∼1350°C. 相似文献
19.
Olivera B. Miloevi Mirjana K. Mirkovi Dragan P. Uskokovi 《Journal of the American Ceramic Society》1996,79(6):1720-1722
Spherical fine (micrometer and submicrometer in size) homogeneous BaTiO3 powders were synthesized from ethanol: water solutions of BaCl2 and TiCl4 using the spray-pyrolysis technique. Two different atomizers—twin-fluid and ultrasonic, with a resonant frequency of 2.5 × 106 Hz—were used for mist generation. Hollow spherical particles containing a certain amount of unreacted BaCl2 phase and having a mean particle diameter of 2.5 μm were obtained at 1173 K using a twin-fluid atomizing system. Decomposition of precursors and their transition to the cubic BaTiO3 phase occurred, even at 973 K in the case of the ultrasonic atomizing system. For the initial droplet size of 2.2 μm and residence time of ∼60 s, spherical BaTiO3 particles with the mean particle diameter of 0.53 μm were obtained. A BaTiO3 formation mechanism has been proposed as a reaction between TiO2 and BaCl2 rather than a reaction of oxides. 相似文献
20.
Young Sik Yoon Sang Woong Na Jaehyung Lee † Myeong-Woo Cho Eun-Sang Lee Won-Seung Cho 《Journal of the American Ceramic Society》2004,87(7):1374-1377
R -curve behavior of Si3 N4 –BN composites and monolithic Si3 N4 for comparison was investigated. Si3 N4 –BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si3 N4 . BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si3 N4 grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si3 N4 . 相似文献