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1.
Xiaowen Zhang Xiaohui Wang Fangzhi Li Yanchun Zhou 《Journal of the American Ceramic Society》2009,92(11):2698-2703
Bulk Ti3 AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti3 AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m1/2 , respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti3 AlC ceramic is about 10.1 × 10−6 K−1 in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)−1 , while the thermal conductivities are 8.19 and 15.6 W·(m·K)−1 , respectively. 相似文献
2.
Chunfeng Hu Fangzhi Li Lingfeng He Mingyue Liu Jie Zhang Jiemin Wang Yiwang Bao Jingyang Wang Yanchun Zhou 《Journal of the American Ceramic Society》2008,91(7):2258-2263
In this work, a bulk Nb4 AlC3 ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb4 AlC3 were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10−6 K−1 in the temperature range of 200°–1100°C. The thermal conductivity of Nb4 AlC3 increased from 13.5 W·(m·K)−1 at room temperature to 21.2 W·(m·K)−1 at 1227°C, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106 Ω−1 ·m−1 in a temperature range of 5–300 K. Nb4 AlC3 possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m1/2 . Most significantly, Nb4 AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C. 相似文献
3.
Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates 总被引:1,自引:0,他引:1
Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO4 , Hf1.26 Ti0.74 O4 , and ZrTiO4 (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO4 is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α a =+(8.7±0.5)×10−6 °C−1 , α b =−(5.2±0.5)×10−6 °C−1 , and α c =+ (5.3±0.5)×10−6 °C−1 . The thermal expansion of Hf1.26 Ti0.74 O4 was similar to that of HfTiO4 but that of ZrTiO4 was markedly less anisotropic. The compressibilities of HfTiO4 and ZrTiO4 also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO2 ) structure. The polymorphism can be explained qualitatively on the basis of crystal structure. 相似文献
4.
5.
Jen-Yan Hsu Nan-Chung Wu Shu-Cheng Yu 《Journal of the American Ceramic Society》1989,72(10):1861-1867
The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO2 , CaO, B2 O3 , and MgO were used as additives to Al2 O3 to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B2 O3 were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10−4 (1 MHz), insulating resistivity > 1014 ω cm, coefficient of thermal expansion ∼ 7.0 × 10−6 /°, and thermal conductivity ∼ 4.1 W/(m · K). 相似文献
6.
Takayuki Nagano Shinji Fujisaki Koji Sato Koji Hataya Yuji Iwamoto Mikihiro Nomura Shin-Ichi Nakao 《Journal of the American Ceramic Society》2008,91(1):71-76
An amorphous silica membrane with an excellent hydrogen/nitrogen (H2 /N2 ) permselectivity of >10 000 and a He/H2 permselectivity of 11 was successfully synthesized on a γ-alumina (γ-Al2 O3 )-coated α-alumina (α-Al2 O3 ) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H2 permeance of >1.0 × 10−7 mol·(m2 ·s·Pa)−1 at ≥773 K. The dominant permeation mechanism for He and H2 at 373–873 K was activated diffusion. On the other hand, that for CO2 , Ar, and N2 at 373–673 K was a viscous flow. At ≥673 K, that for CO2 , Ar, and N2 was activated diffusion. H2 permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a γ-Al2 O3 mesoporous intermediate layer. 相似文献
7.
Preparation and Characterization of Aluminum Borate 总被引:2,自引:0,他引:2
Siba P. Ray 《Journal of the American Ceramic Society》1992,75(9):2605-2609
Aluminum borate, 9Al2 O3 ·2B2 O3 or Al18 B4 O33 , was synthesized by the reaction of stoichiometric amounts of α-Al2 O3 and B2 O3 . The Al18 B4 O33 material was formed into a dense ceramic by pressureless sintering with CaO, MgO, or CaAl2 B2 O7 additives. The material was characterized by low bulk density, moderate coefficient of thermal expansion (3 × 10−6 /°C to 5 × 10−6 /°C), moderate strength (210 to 324 MPa), and low dielectric constant. 相似文献
8.
Dai Guiqin Lin Wei Zheng An Huang Qingzhen Liang Jingkui 《Journal of the American Ceramic Society》1990,73(8):2526-2527
Thermal expansion of the low-temperature form of BaB2 O4 (β-BaB2 O4 ) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L t = L 0 (1 + At + Bt 2 + Ct 3 ) are given as follows: a axis, A = 1.535 × 10−7 , B = 6.047 × 10−9 , C = -1.261 × 10−12 ; c axis, A = 3.256 × 10−5 , B = 1.341 × 10−8 , C = -1.954 × 10−12 ; and cell volume V, A = 3.107 × 10−5 , B = 3.406 × 10−8 , C = -1.197 × 10−11 . Based on α t = (d L t /d t )/ L 0 , the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V. 相似文献
9.
Magnesium aluminate whiskers were synthesized by an oxidation—reduction reaction between MgO, C, and Al in a CO and CO2 atmosphere. The oxygen partial pressure suitable for MgAl2 O4 whisker growth ranged from 10−12.1 to 10−11.5 MPa. The average whisker diameter and length formed at 1500°C for 8 h were ∼3.1 μm and ∼4 mm, respectively. The whiskers grew in the [111] direction. 相似文献
10.
Paolo Scardi Rosa Di Maggio Luca Lutterotti Paul Maistrelli 《Journal of the American Ceramic Society》1992,75(10):2828-2832
Ceria-stabilized tetragonal zirconia polycrystals were obtained by thermal treatment of amorphous powder prepared by the sol–gel method. Detailed XRD profile analysis was employed to study microstructural disorder and crystallite size and shape; in particular, no fluctuation of stoichiometry was found, the main cause of disorder being attributable to dislocations. Thermal expansion measurements were carried out by high-temperature XRD at 294, 473, 673, 873, and 1073 K using silicon as an internal standard. Thermal expansion coefficients are anisotropic and changes in the stabilizer content have little effect on them. A mean value, α a = 10.6 × 10−6 (K−1 ) and α c = 13.5 × 10−6 (K−1 ), can be assumed for Zr1− x Ce x O2 with x in the range 0.12–0.18. 相似文献
11.
S. Kamakshi Sundaram Jen-Yen Hsu Robert F. Speyer 《Journal of the American Ceramic Society》1995,78(7):1940-1946
The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC( p ) /Al2 O3 composite to molten E-glass at 1550deg; C was studied. Mo showed no tendency to oxidize as it was immersed in soda-lime silicate glass in a parallel study. MoSi2 was corroded by soluble molecular oxygen, leaving a Mo5 Si3 interface behind. The SiC( p ) /Al2 O3 composite was corroded at a more rapid rate wherein the SiC component was oxidized to form amorphous silica and CO bubbles. Based on these results, the activity of soluble molecular oxygen in E-glass was determined to be in the range of 2.4 × 10-14 to 2.0 × 10-8 . 相似文献
12.
Kyoung Hun Kim Seung Hwan Shim Kwang Bo Shim Koichi Niihara Junichi Hojo 《Journal of the American Ceramic Society》2005,88(3):628-632
M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al2 O3 reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al2 O3 and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al2 O3 and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al2 O3 and NiO, exhibited a ZT value of 0.6 × 10−3 K−1 , electrical conductivity of 1.7 × 10−4 Ω−1 ·m−1 , the thermal conductivity of 5.16 W·(m·K)−1 , and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10−3 K−1 , in the literature. 相似文献
13.
The vaporization of the system MgO-Cr2 O3 was studied in a vacuum of 10−5 torr (10−3 N/m2 ) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr2 O4 increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr2 O3 . The enthalpies of vaporization of MgCr2 O4 were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively. 相似文献
14.
An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta22 W4 O67 , Ta, WO8 , and Ta16 W18 O94 ) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm3 , are relatively soft (120 to 655 kg/mm2 hardnesses), and are electrical insulators (6× 103 to 2× 108 Ω. cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10−8 /K to −5.1× 10−6 /K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity 相似文献
15.
Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li1+ x Ti2− x Al x (PO4 )3 (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li2 O─CaO─TiO2 ─Al2 O3 –P2 O5 system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li1+ x Ti2− x Al x (PO4 )3 and β-Ca3 (PO4 )2 phases. The median pore diameter and surface area of the resulting porous Li1+ x Ti2− x Al x (PO4 )3 glass-ceramics were approximately 0.2 μm and 50 m2 /g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO3 aqueous solution was 8 × 10−5 S/cm at 300 K or 2 × 10−2 S/cm at 600 K. 相似文献
16.
La0.8 Sr0.2 Cr0.9 Ti0.1 O3 perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La0.8 Sr0.2 Cr0.9 Ti0.1 O3 shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10−6 /°C in air, 10.5 × 10−6 /°C under a He–H2 atmosphere (oxygen partial pressure of 4 × 10−15 atm at 1000°C), and 10.9 × 10−6 /°C in a H2 atmosphere (oxygen partial pressure of 4 × 10−19 atm at 1000°C). La0.8 Sr0.2 Cr0.9 Ti0.1 O3 perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect. 相似文献
17.
Mikio Fukuhara 《Journal of the American Ceramic Society》1989,72(2):236-242
The effect of Al2 O3 and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al2 O3 –(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al2 O3 to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO2 and TiC therefore must have inhibited the t - → m -ZrO2 martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO2 and Al2 O3 . In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton. 相似文献
18.
M. Lou Balmer Yali Su Hongwu Xu Eric Bitten David McCready Alexandra Navrotsky 《Journal of the American Ceramic Society》2001,84(1):153-160
The structure of Cs2 ZrSi3 O9 synthesized using a sol–gel method was determined from the Rietveld refinement of experimental powder X-ray diffraction data. The refinement confirmed that this compound was isostructural with wadeite: its structure was hexagonal (space group P 63 / m ), and its lattice parameters were a = 7.2303(2) Å and c = 10.2682(4) Å. The aqueous durability of Cs2 ZrSi3 O9 varied, depending on the solution conditions. In modified leach tests with buffered (pH 7) and unbuffered solutions, the 7-d cesium release rates were <1.2 × 10−4 g·(m2 ·day)−1 , which indicated high aqueous durability. However, in unsaturated, unbuffered solutions with a pH of 9–10, the durability was much lower, with 7-d cesium release rates of 2.2 × 10−3 g·(m2 ·day)−1 . The ability of this material to retain cesium in aqueous environments can be explained by its condensed ring structure, in which the size of the channel openings is smaller than the diameter of a Cs+ ion. However, dissolution of the network silicate occurred at high pH, which resulted in the release of cesium. 相似文献
19.
Stanislav Antolin Alan S. Nagelberg David K. Creber 《Journal of the American Ceramic Society》1992,75(2):447-454
The growth of α-Al2 O3 /metal composites by the directed oxidation of molten Al-Mg-Si alloys proceeds through four distinct stages. The first stage encompasses the early heating of the alloy ingot, melting, and continued heating to between 1123 and 1173 K. In this latter temperature range, the molten alloy surface rapidly oxidizes to form a MgO-covered MgAl2 O4 layer. During further heating and initial soak at the composite growth temperature (1373 to 1573 K), the duplex layer slowly thickens (second stage). The start of the third stage, growth initiation, is marked by the spread of a metal-rich zone over the duplex layer; this metal-rich zone is believed to be connected to the molten alloy through microcracks in the thickened MgO/MgAl2 O4 layer. Small nodules of the oxide/metal composite nucleate from the metal-rich layer. During the final rapid growth stage, the small composite nodules grow and coalesce to form a macroscopically planar growth front, which persists until growth is complete. Throughout the growth process, the external surface of the α-Al2 O3 /metal composite is covered by a thin MgO layer. Immediately under this external layer and separating it from the α-Al2 O3 is a thin layer of molten metal. 相似文献
20.
Kerry K. Richardson Doug W. Freitag David L. Hunn 《Journal of the American Ceramic Society》1995,78(10):2662-2668
Advanced ceramic composite materials that exhibit high strength and toughness with good thermal shock resistance are needed for emerging high-temperature engineering applications. A recently developed in situ reinforced barium aluminosilicate glass-ceramic shows promise of meeting many of the requirements for these types of applications with the added benefit of low-cost fabrication through densification by pressureless sintering. The material is toughened through in situ growth of rodlike β-Si3 N4 grains resulting from the α–β silicon nitride phase transformation. Microstructural development and material properties for temperatures up to 1400°C are discussed. When compared to monolithic barium aluminosilicate, barium aluminosilicate reinforced with 70% by volume of Si3 N4 shows a significant increase in flexural strength (from 80 to 565 MPa) and fracture toughness (from 1.8 to 5.74 MPa·m1/2 ) with a high resistance to thermal shock. 相似文献