共查询到20条相似文献,搜索用时 15 毫秒
1.
William G. Fahrenholtz Ronald E. Loehman Kevin G. Ewsuk 《Journal of the American Ceramic Society》2002,85(1):258-260
Al2 O3 -MoSi2 composites were prepared by reactive hot pressing using molybdenum, aluminum, and mullite powders as precursors. The Gibbs free energy was highly negative for the composite-forming reaction, which indicated that the products were stable relative to the reactants. After the reaction, the composites had high relative density, ∼96%. Based on the composite-forming reaction, the composites should have contained 18 vol% MoSi2 in an Al2 O3 matrix. Scanning electron microscopy revealed that the MoSi2 inclusions were elongated, with an average thickness of ∼5 μm and inclusion lengths that ranged from 5 to 50 μm. Average composite strength was 467 MPa, and toughness was 3.7 MPa·m1/2 . 相似文献
2.
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
3.
Ya-Feng Yang Hui-Yuan Wang Ru-Yi Zhao Yun-Hong Liang Qi-Chuan Jiang 《International Journal of Applied Ceramic Technology》2009,6(3):437-446
The composites synthesized with three kinds of B4 C particles mainly consist of TiC, TiB2 , and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B4 C particles are larger than that synthesized with ∼140 μm B4 C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B4 C particles, while some large pores exist in the composites synthesized with ∼140 μm B4 C particle. With the decrease of B4 C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase. 相似文献
4.
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
5.
William G. Fahrenholtz Gregory E. Hilmas Shi C. Zhang Sumin Zhu 《Journal of the American Ceramic Society》2008,91(5):1398-1404
Zirconium diboride (ZrB2 ) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB2 with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB2 with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB2 was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction. 相似文献
6.
Synthesis and Electrical Characterization of Thin Films of PT and PZT Made from a Diol-Based Sol-Gel Route 总被引:7,自引:0,他引:7
Yeur-Luen Tu Maria L. Calzada Nicolas J. Phillips Steven J. Milne 《Journal of the American Ceramic Society》1996,79(2):441-448
Lead titanate (PT) sols were prepared using propanediol, butanediol, or pentanediol solutions of lead acetate trihydrate and titanium diisopropoxide bis(acetylacetonate). Precursor sols for PbZr0.53 Ti0.47 O3 (PZT) films were prepared from propanediol solutions, with zirconium tetrapropoxide being used as the zirconium source. Films were formed by spin-coating the sols onto silicon and platinized silicon substrates; the resulting gel layers were converted to ceramic films by adopting a two-stage heating schedule with final firing temperatures of 600–700°C. Information on film crystallization, microstructure development, and electrical properties is presented for both compositions. The limiting thickness of surface-smooth crack-free single-layer films was ∼1 μm. The PT films exhibited a "linear" polarization-electric field ( P-E ) response, while the PZT films gave rise to characteristic ferroelectric P-E hysteresis loops. A 0.5 μm thick single-layer PZT film exhibited remanent polarization (Pr ) values of ∼34 μC·Cm−2 , with a coercive field ( E c ) of ∼45 kV·Cm−2 ; the relative permittivity (ɛr ) and the dissipation factor ( D ) were ∼1250 and 0.07. For a 1 μm single-layer PZT film, the respective values were P r ∼19 μC∼Cm−2 , E c ∼40 kV∼Cm−2 , ɛr ∼750, and D = 0.03. 相似文献
7.
The effect of particle size distribution on the sintering of Al2 O3 was investigated. Samples could be sintered to high relative density (∼99%), small average grain size (1 μm), and no growth of exaggerated grains using powders with either broad or narrow particle size distribution. However, the broad particle size distribution provided the advantage that powder compacts could be prepared with higher green density and, therefore, samples could be densified with less total shrinkage. 相似文献
8.
Ping-Hua Xiang Yoshiaki Kinemuchi Koji Watari Fei Cao Xian-Lin Dong 《Journal of the American Ceramic Society》2006,89(2):684-687
Highly textured Bi3 NbTiO9 ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi3 NbTiO9 plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi3 NbTiO9 ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi3 NbTiO9 precursor. Meanwhile, the orientation degree of textured Bi3 NbTiO9 ceramics increases with sintering temperature. Highly oriented Bi3 NbTiO9 (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi3 NbTiO9 ceramic exhibits anisotropic electrical properties. 相似文献
9.
Tsuyoshi Murata Kiyoshi Itatani F. Scott Howell Akira Kishioka Makio Kinoshita 《Journal of the American Ceramic Society》1993,76(11):2909-2911
Submcrometer-scale magnesium nitride (Mg3 N2 ) powder was prepared by low-pressure chemical vapor deposition, i.e., the reaction of Mg vapor with mixed NH3 –N2 gases at 800°C under a pressure of ∼1 kPa. The mixing ratios of NH3 –N2 gases were 0% NH3 –100% N2 (pure N2 ), 20% NH3 –80% N2 , 40% NH3 –60% N2 , 60% NH3 –40% N2 , 80% NH3 –20% N2 , and 100% NH3 –0% N2 (pure NH3 ). The reactions between Mg vapor and NH3 –N2 gases produced platy Mg3 N2 particles <0.2 μm and acicular particles with long-axis length of ∼0.2 μm, whereas the reaction of Mg vapor with pure NH3 gas produced spherical Mg3 N2 particles with diameters of ∼0.1 μm. 相似文献
10.
Pressureless sintering of SiC-whisker-reinforced Al2 O3 composites was investigated. In Part I of the study, the effect of the matrix (Al2 O3 ) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al2 O3 powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6-μm matrix average grain intercept size. 相似文献
11.
R. J. BRATTON 《Journal of the American Ceramic Society》1974,57(7):283-286
A translucent polycrystalline MgAl2 O4 ceramic was prepared from finely divided coprecipitated spinel in which a small amount of CaO added as a sintering aid was uniformly distributed. The CaO promotes densification through the formation of a liquid phase at the sintering temperatures. Depending on the sintering treatment, the relative density of the sintered spinel was 99.7 to ∼100% of theoretical. The in-line optical transmission was > 10% from 0.3 to 6.5 μm. Total transmission in the visible region was between 67 and 78%. 相似文献
12.
Dense, small-grained BaTiO3 ceramics, with a grain size around 1 μm and a relative sintered density >98%, were obtained at 1100°C from sol-gel-derived gel monoliths without using any sintering additives. The monolithic gels asprepared had a relative density of about 50% and consisted of ultrafine pseudo-cubic BaTiO3 particles (<50 nm). These gels, with a significantly high density compared with that of previous ones (∼30%), have been synthesized at room temperature from a sol solution with a concentration of equimolar mixture of titanium isopropoxide and barium ethoxide (0.8 mol/L), using the methanol/2-methoxyethanol mixed-solvent system. Microstructural development of the gel monoliths with increasing sintering temperature and the dielectric properties of the obtained dense BaTiO3 ceramic have been investigated. 相似文献
13.
Seongtae Kwon Edward M. Sabolsky Gary L. Messing Susan Trolier-McKinstry 《Journal of the American Ceramic Society》2005,88(2):312-317
Lead magnesium niobate–lead titanate, 0.675Pb(Mg1/3 Nb2/3 )O3 –0.325PbTiO3 (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO3 (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d 33 -coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d 33 -piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT. 相似文献
14.
Antonio Feteira Lisa J. Gillie Ralf Elsebrock Derek C. Sinclair 《Journal of the American Ceramic Society》2007,90(5):1475-1482
The crystal structure and dielectric properties of LaYbO3 ceramics prepared by the mixed-oxide route have been investigated. Rietveld refinements performed on X-ray and neutron diffraction data show the room-temperature structure to be best described by the orthorhombic Pnma space group [ a =6.02628(9) Å, b =8.39857(11) Å, and c =5.82717(7) Å; Z =4, and theoretical density, D x =8.1 g/cm3 ] in agreement with electron diffraction experiments. LaYbO3 ceramics fired at 1600°C for 4 h attain ∼97% of D x and their microstructures consist of randomly distributed equiaxed grains with an average size of ∼8 μm. Conventional transmission electron microscopy shows densification to occur in the absence of a liquid phase and reveals domain-free grains. The relative permittivity, ɛr , of LaYbO3 ceramics at radio frequencies is ∼26 in the range ∼10–300 K; however, a small dielectric anomaly is detected at ∼15 K. At room temperature and microwave frequencies, LaYbO3 ceramics exhibit ɛr ∼26, Q × f r ∼20 613 GHz (at 7 GHz), and τf ∼−22 ppm/K. Q × f r show complex subambient behavior, decreasing from a plateau value of ∼20 000 GHz between ∼300 and 200 K to a second plateau value of ∼6000 GHz at ∼90 K before decreasing to <1000 GHz at ∼10 K. The large decrease in Q × f r at low temperature may be related to the onset of antiferromagnetism at ∼2.7 K. 1 相似文献
15.
J. Martinez-Fernandez M. Jimenez-Melendo A. Dominguez-Rodriguez Arthur H. Heuer 《Journal of the American Ceramic Society》1991,74(5):1071-1081
The temperature dependence of the Vickers microhardness was studied in 3.4-mol%-Y2 O3 -partially-stabilized ZrO2 (Y-PSZ) single crystals up to 1000°C; the samples had previously been annealed at 1600°C for 150 h to develop "colony" precipitates of tetragonal ZrO2 in the cubic ZrO2 matrix. Indentation caused extensive stress-induced martensitic transformation of the colony precipitates to monoclinic symmetry in zones which extended in extreme cases up to several hundred micrometers from the indent. For indents made at 500°C and above, the M d and M f temperatures are 450° and 310°C, respectively; A s is ∼600°C ( M d is the temperature of initial transformation (the "martensite start temperature") in deformed samples; M f is the temperature at which the final transformation occurs; A s is the temperature at which the reverse (monoclinic → tetragonal) transformation begins). However, extensive transformation zones are also found for indents made at 200°, 300°, and 400°C. The dislocation density introduced during indentation is responsible for nucleating the transformation in a zone adjacent to the indent. However, the transformation zone extends further than the plastic zone around the indent, indicating extensive autocatalytic transformation. Transformation within the zone appeared to occur in individual plates with {110} habit planes. The plate dimensions (∼100 μm ×∼175 μm ×∼10 μm) are large compared to the size of the colony precipitates (∼2 μm in maximum dimension). 相似文献
16.
Ultrafine (<0.1 μm) high-purity θ-Al2 O3 powder containing 3–17.5 mol%α-Al2 O3 seeds was used to investigate the kinetics and microstructural evolution of the θ-Al2 O3 to α-Al2 O3 transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al2 O3 lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al2 O3 sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm. 相似文献
17.
Ritesh Rawal Antonio Feteira Neil C. Hyatt Derek C. Sinclair Kumaravinothan Sarma Neil McN. Alford 《Journal of the American Ceramic Society》2006,89(1):332-335
The dielectric properties of dense ceramics of the n =0 member of a newly identified homologous series Ba3+ n LaNb3 Ti n O12+3 n , where n =0, 1, and 2, are reported. Single-phase powders can be obtained from the mixed-oxide route at 1350°C and dense ceramics (>97% of the theoretical X-ray density) with uniform microstructures (3–5 μm) can be obtained by sintering in air at 1500°C. The ceramics are excellent dc insulators with a band gap >2.6 eV that resonate at microwave frequencies with a relative permittivity, ɛr ∼44, a quality factor, Q × f r , of ∼9000 at f r ∼5.5 GHz and a temperature coefficient of resonant frequency, TCf ,∼−100 ppm/K. 相似文献
18.
RICHARD A. LANGMAN ROBERT B. RUNK SIDNEY R. BUTLER 《Journal of the American Ceramic Society》1973,56(9):486-488
PLZT specimens 99.99% of theoretical density containing 7 at.% La with a 65/35 Zr/Ti atom ratio were fabricated with average grain diameters of ∼1 μm by pressure sintering. The dense, single-phase specimens were isothermally heat-treated in a low PbO activity atmosphere. For grain diameters from 1 to 5 μm, grain growth obeyed the D3 -D0 3 =k't relation, with an apparent activation energy of 86 kcal/mol. The mechanism controlling the grain-growth rate is consistent with a solid-solution impurity drag caused by an La ion concentration gradient near the grain boundaries. 相似文献
19.
Preliminary results indicate that large strains (∼80%) and strain rates (0.001 s−1 ) can be obtained without tearing (or cracking) in fine-grain ZrO2 (0.3 μm) and Al2 O3 /ZrO2 (1 μm) ceramics. Alumina develops crystallographic and morphological texture as previously reported by Heuer et al.1 相似文献
20.
Extensive grain growth was observed by scanning electron microscopy in very porous Al2 O3 compacts, even at densities <40% of theoretical. After ∼7% shrinkage at 1700°C, the grain size increased from ∼0.3 to 0.51 μm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar. 相似文献