共查询到20条相似文献,搜索用时 13 毫秒
1.
Do-Kyun Kwon Michael T. Lanagan Thomas R. Shrout 《Journal of the American Ceramic Society》2005,88(12):3419-3422
Polycrystalline BaTe4 O9 ceramic compound was investigated as a promising microwave dielectric compound for low-temperature cofired ceramics (LTCC) applications. The binary phase BaTe4 O9 was synthesized and subsequently densified over the temperature range of only 500°–550°C, which allows for low-temperature cofiring with aluminum metal. The dielectric properties of BaTe4 O9 ceramics sintered at 550°C for 2 h were determined in the microwave region of 12–14 GHz. The dielectric constant and Q × f product obtained were 17.5 and 54 700 GHz at 12 GHz, respectively. The temperature coefficient of resonance frequency showed a negative value of −90 ppm/°C. In terms of its evaluation for LTCC, the BaTe4 O9 composition was found to be chemically compatible and successfully cofired with highly conductive aluminum electrode, while maintaining good electrical performance. 相似文献
2.
Liang Fang Heping Su Qian Yu Hui Zhang Bolin Wu 《Journal of the American Ceramic Society》2008,91(8):2769-2771
A high dielectric constant and low-loss ceramic with composition Sr4 LaTiNb3 O15 has been prepared by the conventional solid-state ceramic route. This compound adopts an A5 B4 O15 cation-deficient hexagonal perovskite structure and crystallizes in the trigonal system with unit cell parameters a =5.6307(2), c =11.3692(3) Å, V =312.16(2) Å3 , and Z =1. The dielectric properties of dense ceramics sintered in air at 1460°C have been characterized at microwave frequencies. The results show that the material affords a relatively high dielectric constant ɛr ∼43, a high quality factor Q × f ∼44 718 GHz, and a low temperature coefficient of resonant frequency TCf ∼13 ppm/°C. 相似文献
3.
Hideo Toraya Masahiro Yoshimura Shigeyuki Somiya 《Journal of the American Ceramic Society》1982,65(9):c159-c160
Sintered monoclinic HfO2 bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction-sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C. 相似文献
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The effects of controlled porosity on the dielectric breakdown of sintered MgAl2 O4 were measured using an acoustic emission technique. Results are compared to a statistical model relating dielectric strength to a random distribution of spherical pores and the probability of pores aligned in a vertical column. Data are in good agreement with the model when only maximum pore size is considered . 相似文献
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Yuanhua Lin Ce-Wen Nan Jianfei Wang Gang Liu Junbo Wu Ning Cai 《Journal of the American Ceramic Society》2004,87(4):742-745
The dielectric properties of Na0.5 Bi0.5 TiO3 (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C). 相似文献
6.
The BiVO4 additive was found effective for low-temperature firing of ZnNb2 O6 polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO4 -doped ZnNb2 O6 samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛr = 26, and τf =−57 ppm/°C. With increasing BiVO4 addition up to 20 mol% relative to ZnNb2 O6 , while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛr , was linearly increased and the temperature coefficient of resonant frequency, τf , was slightly increased. The variations in Q · f and ɛr are surely attributable to the residual BiVO4 in the ZnNb2 O6 matrix. An unexpected slight increase in τf is probably due to the formation of the Bi(V,Nb)O4 -type solid solution. 相似文献
7.
Jing Pei Zhenxing Yue Fei Zhao Zhilun Gui Longtu Li 《Journal of the American Ceramic Society》2007,90(10):3131-3137
Tungstenbronze-type Ba4 (Nd0.7 Sm0.3 )9.33 Ti18 O54 (BNST) microwave dielectric ceramics doped with 0–10 wt% silver (Ag) particles were successfully fabricated by a citrate sol–gel method. The influence of Ag doping on the sinterability, microstructure, bulk conductivity, and dielectric properties of BNST was investigated. The desired tungstenbronze-type phase was obtained at 900°–950°C. The sintering temperature of BNST decreased to 1100°C with the aid of a small amount of Ag addition (1 wt%). No chemical reaction between the tungsenbronze phase and Ag was detected. The particle size of the powders decreased with increasing Ag content up to 1 wt% and it then increased with a further increase in the Ag content. The dense fine-grained ceramics with submicrometer grains (∼300 nm) were obtained with 1 wt% Ag addition. The submicrometer-grained ceramics had excellent dielectric properties of ɛr ∼81 and Q × f ∼11 000 GHz. Both the dielectric constant and dielectric loss significantly increased with large additions (>3 wt%) of Ag due to the percolation effect. 相似文献
8.
Microwave Dielectric Properties of Doped BaTi4 O9 总被引:1,自引:0,他引:1
Subodh G. Mhaisalkar Dennis W. Readey Sheikh A. Akbar 《Journal of the American Ceramic Society》1991,74(8):1894-1898
Polycrystalline BaTi4 O9 doped with Mn, Sn, Zr, Ca, Sr, and Pb was prepared from carbonates and oxides. Single-phase ceramics with densities exceeding 97% of theoretical were made with up to 3 mol% Mn; 6 mol% Sn, Zr, and Ca; and 8 mol% Sr and Pb. Dielectric constant, k , quality factor, Q , and temperature coefficient of frequency, τ f , of ∼37, 5675, and 15 ppm/°C, respectively, were determined at 4 GHz for undoped BaTi4 O9 . Doping did not significantly affect k and τ f . However, doping with Mn, Sn, and Pb lowered Q , whereas doping with Zr, Ca, and Sr increased Q by up to 2000. Additions of 0.5 mol% MnO2 as a second phase improved Q from 3675 to 7600. 相似文献
9.
The densification behavior of Si3 N4 containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si3 N4 . Evidence of a second-phase material between the Si3 N4 grains was obtained through etching studies. Transformation of α- to β-Si3 N4 during hot-pressing is not necessary for densification. 相似文献
10.
Ganesanpotti Subodh Manoj Joseph Pezholil Mohanan Mailadil Thomas Sebastian 《Journal of the American Ceramic Society》2007,90(11):3507-3511
TeO2 particle-filled PTFE composites were prepared by the powder processing technique. The structure and microstructure of the composites were investigated by X-ray diffraction and scanning electron microscopic methods. The effect of the ceramic content (0–0.6 volume fraction) of TeO2 on the dielectric properties of the composites was studied at 1 MHz and 7 GHz. The dielectric constant and dielectric loss increased with an increase in the TeO2 content. For 60 vol% of TeO2 , the composite has a dielectric constant of 5.4 and a loss tangent of 0.006 at 7 GHz. The measured dielectric constant (ɛr ) is compared with the effective dielectric constant calculated using different theoretical models. The observed dielectric constants are in agreement with that calculated using effective medium theory. The coefficient of thermal expansion of the composites decreases with the TeO2 content, reaching a minimum of 32 ppm/°C for 60 vol% loading. 相似文献
11.
Bo Cheng Yuan-Hua Lin Hao Yang Jinle Lan Ce-Wen Nan Xi Xiao Jinliang He 《Journal of the American Ceramic Society》2009,92(11):2776-2779
Cu-doped CaTiO3 -based polycrystalline ceramics have been prepared by the conventional solid-state sintering. Our results indicate that the dielectric constant can be enhanced greatly by increasing the Cu-doped content, which show weak frequency and temperature dependence. The fitted activation energy is almost same (∼0.10 eV) as the Cu-doped content is 0.4–0.6, which may be ascribed to the first ionization of the oxygen vacancies. The origin of the high dielectric permittivity observed in these Ca1− x Cu x TiO3 -based ceramics should be attributed to the interfacial polarization mechanism, and can be well described by the percolation theory with f c ≈0.27 and s ≈0.74. 相似文献
12.
Yaping Li Evan R. Maxey James W. Richardson Jr. 《Journal of the American Ceramic Society》2005,88(5):1244-1252
The oxygen partial pressure ( p O2 )-dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe0.2 Co0.8 O x with cubic perovskite structure have been investigated by high-temperature neutron powder diffraction: one in "static" mode and one in simulated-operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable p O2 levels. Rietveld analysis on data collected for the membrane without p O2 gradients showed that the perovskite is stable in p O2 down to ∼10−12 atm, and at ∼10−14 atm it starts to decompose into a three-phase mixture containing layered intergrowth Ruddlesden–Popper phases Sr n +1 (Fe,Co) n O x with n =2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a p O2 gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a p O2 profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900°C were estimated to be 0.4–0.9 sccm/cm2 for the ∼1 mm thick membrane containing perovskite, depending upon p O2 gradient. 相似文献
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14.
Koichiro Fukuda Iwao Maki Suketoshi Ito 《Journal of the American Ceramic Society》1997,80(6):1595-1598
Crystals of β-Ca2 SiO4 (space group P 121 / n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (αi ) found from the matrix algebra analysis was as follows: α1 = 20.492 × 10−6 + 16.490 × 10−9 ( T - 25)°C−1 , α2 = 7.494 × 10−6 + 5.168 × 10−9 ( T - 25)°C−1 , α3 =−0.842 × 10−6 − 1.497 × 10−9 ( T - 25)°C−1 . The expansion coefficient α1 , nearly along [302] was approximately 3 times α2 along the b -axis. Very small contraction (α3 ) occurred nearly along [ 01]. The volume changes upon martensitic transformations of β↔αL ' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity. 相似文献
15.
The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al2 O3 :CaAl12 O19, or A12 O3 : CA6 ) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA6 ) phase possessed a platelike morphology showed more-pronounced R -curve behavior than a composite in which the CA6 phase consisted of equiaxed grains. Toughness curves derived from the indentation-strength data exhibited a "crossover," such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA6 resulted in enhanced toughening, compared to single-phase alumina of comparable grain size, thus demonstrating the viability of the in-situ -toughening approach. A simple grain-pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation-strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A12 O3 :CA6 composite were strongly bonded to the platelet A12 O3 :CA6 composite, demonstrated high strength over the range of tested flaw sizes. 相似文献
16.
The elastic properties of polycrystalline monoclinic Gd2 O3 were determined by the sonic-resonance method. Volume-fraction porosity varied from 0.025 to 0.367 and temperature from room temperature to 1400°C. The Young's and shear moduli are linear functions of volume-fraction porosity, but the rate of their decrease with increasing porosity is less than that expected. The moduli decreased more rapidly than expected with increasing temperature. The Debye temperature is 362°K. With increasing temperature, the first Grueneisen constant, γ, decreases, whereas the second Grueneisen constant, δ, increases. 相似文献
17.
Yuichi Sato Hideyuki Kanai Yohachi Yamashita 《Journal of the American Ceramic Society》1996,79(1):261-265
The doping of silver and palladium into a 0.9Pb-(Mg1/3 Nb2/3 )O3 –0.1PbTiO3 (PMN–PT) ceramic has been investigated. It was found that Ag could be incorporated into the PMN–PT lattice, though this was almost impossible with Pd. Doping at up to 0.2 mol% of Ag and Pd reduced the maximum dielectric constant ( K max ) from 21000 to 14000 without any conspicuous change in physical properties. Order–disorder transitions in the perovskite structure contributed to the decrease in K max , since the diffuseness parameter, δ, which represents the degree of the order-disorder relationship, increased with Ag doping. However, addition of more than 0.2 mol% increased K max . Increases in grain size and in the perovskite ratio contributed to this increase. The dielectric properties of Ag/Pd-doped samples were intermediate between the Ag- and Pd-doped samples. 相似文献
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A. J. KIEHLE L. K. HEUNG P. J. GIELISSE T. J. ROCKETT 《Journal of the American Ceramic Society》1975,58(1-2):17-20
The high-temperature chemical stability of hot-pressed Si3 N4 was studied between 600° and 1450°C. Reactions were followed by X-ray diffraction and scanning electron microscopy. In air, this material begins to oxidize at 700° to 750°C; a distinct amorphous siO2 surface layer results after 24 h at 750°C-Concomitant formation of cristobalite occurs, depending on exposure time, and is enhanced as temperature is Increased. Magnesium and calcium magnesium silicates form above 1000°C. The data suggest that impurities, e.g. Mg, Ca, and Fe, greatly lower the oxidation resistance of Si3 N4 in air. 相似文献