共查询到20条相似文献,搜索用时 31 毫秒
1.
Yong Hwan Park Ji Man Ryu Moo Young Shin Kyung Hyun Ko Dong Wan Kim Kug-Sun Hong 《Journal of the American Ceramic Society》2001,84(11):2542-2546
The effects of Nb2 O5 and ZnO addition on the dielectric properties, especially the quality factor, of (Zr0.8 Sn0.2 )TiO4 (ZST) ceramics were investigated in terms of the sintered density acquired by the zinc. For ZST ceramics with 2 mol% added ZnO, the relative density of the samples decreased with >0.5 mol% addition of Nb2 O5 . On the other hand, for samples with 6 mol% added ZnO, the relative density remained >97%, even when the amount of Nb2 O5 was increased to 2.0 mol%. When >0.5 mol% Nb2 O5 was added, both the quality factor and the dielectric constant exhibited similar trends with sintered density. The ZST ceramics with 6 mol% added ZnO, especially, still manifested a quality factor >40 000 and a dielectric constant of 37, even when the amount of Nb2 O5 was increased, values that are not explainable by the previously suggested electronic defect model. 相似文献
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Chuanyong Huang Zilong Tang Zhongtai Zhang 《Journal of the American Ceramic Society》2001,84(7):1637-1638
Based on cause and effect rather than experiment data, zirconium hydroxide (Zr(OH)4 · n H2 O) is always considered as hydrous zirconia (ZrO2 · n H2 O). With the aid of nitric acid, XPS, XRD, XPF, and TGA, some differences between them have been confirmed. It is found that in contrast to zirconium hydroxide (the binding energy of zirconium = 183.6 eV), hydrous zirconia does not dissolve in nitric acid. The chemical properties of zirconium (181.8 eV) in hydrous zirconia are similar to those in zirconia (182.2 eV for the binding energy of zirconium), and the weight loss of hydrous zirconia is 21.5%, different from 32.19%, the weight loss of zirconium hydroxide. According to the experimental data, a structure for hydrous zirconia is proposed and then the different phenomena are explained. 相似文献
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Guo-Jun Zhang Zhen-Yan Deng Naoki Kondo Jian-Feng Yang Tatsuki Ohji 《Journal of the American Ceramic Society》2000,83(9):2330-2332
A ZrB2 –SiC composite was prepared from a mixture of zirconium, silicon, and B4 C via reactive hot pressing. The three-point bending strength was 506 ± 43 MPa, and the fracture toughness was 4.0 MPa·m1/2 . The microstructure of the composite was observed via scanning electron microscopy; the in-situ -formed ZrB2 and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B4 C will diffuse into the Zr and Si sites and form ZrB2 and SiC in situ , respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders. 相似文献
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Detlev F. K. Hennings B. Seriyati Schreinemacher Herbert Schreinemacher 《Journal of the American Ceramic Society》2001,84(12):2777-2782
BaTiO3 and Ba(Ti,Zr)O3 dielectric powders have been prepared from submicrometer BaCO3 , TiO2 , and ZrO2 . By use of submicrometer BaCO3 the intermediate formation of Ba2 TiO4 second phase can be widely suppressed. Monophase perovskites of BaTiO3 were already formed at 900°C and Ba(Ti,Zr)O3 at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm. 相似文献
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The low-temperature, Zr–Ti-ordered, form of zirconium titanate has been investigated using high-resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO4 to near Zr5 Ti7 O24 . Electron diffraction reveals that compositions with Zr : Ti between 5 : 7 and 1 : 1 have incommensurate superstructures, and phases close to 1 : 1 are commensurate with an a -axis repeat 2X that of the disordered structure. High-resolution images show that the a -doubling in ZrTiO4 corresponds to a new structure, one that consists of two Zr-rich, distorted octahedral layers alternated with two Ti-rich octahedral layers. The incommensurate compositions are composed of blocks of the 1 : 1 structure intercalated with blocks of the commensurate 5 : 7 structure, the latter having a tripled a -repeat and a ZTTZTT sequence of cation layers. The intercalation can be described as an "interface-modulated" structure resulting from the quasiperiodic insertion of (100) faults with displacement vector R =–1/3 aord in the ordered 5 : 7 phase. Although their spacing is variable, the faults are uniformly distributed in such a way as to produce incommensurate satellite reflections. The findings regarding the structure of ordered compositions in the ZrTiO4 –Zr5 Ti7 O24 system provide an improved framework for understanding the effects of ordering on the properties of zirconium titanate dielectric ceramics. 相似文献
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The NZP family of new low-expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr2 P3 O12 , which is the parent composition of this family, has been synthesized by the solution sol-gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120°C. The lowest temperature of formation of a single phase of NaZr2 P3 O12 with a high degree of crystallinity was found to be 600°C. 相似文献
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Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr2 (PO4 )3 ─KTi2 (PO4 )3 (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10−6 /°C with increasing x in KZr2 − x Tix (PO4 )3 (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface. 相似文献
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Suman Kumari Mishra Swapan Kumar Das Ajoy Kumar Ray P. Ramachandrarao 《Journal of the American Ceramic Society》2002,85(11):2846-2848
An investigation of the sintering behavior of ZrB2 powder with Fe and Cr (0 to 20 wt%) addition was conducted. It was observed that Fe addition helps to enhance the density of ZrB2 only up to 10 wt%. Further addition of Fe degrades the sintering by segregation of Fe-rich phases. Formation of a eutectic phase containing a Fe:Zr ratio of 92.57:7.43 was also found in Fe-added samples. The addition of Cr to a ZrB2 matrix was found to result in swelling of the samples, leading to several cracks. 相似文献
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Hiroyuki Ikawa Hiromasa Shimojima Kazuyori Urabe Osamu Fukunaga 《Journal of the American Ceramic Society》1991,74(8):1899-1904
Axial and dilatometric thermal expansions and phase transformations were studied for solid solutions having the α-PbO2 structure in the ZrTiO4 —In2 O3 —M2 O5 (M = Sb, Ta) system with nominal formulas of Zr x Ti y In z Sb z O4 and Zr x Ti y In z Ta z O4 where x + y + 2 z = 2. With increased substitution of z , the cell volume increased, the difference in the b parameters at room temperature between those quenched from 1400° and 1000°C decreased, and the thermal expansion decreased. The axial thermal expansion of ZrTi y In z · Ta z O4 with z = 0.3 was almost identical with that of HfTiO4 , and those with z = 0.4 and z = 0.45 were smaller than that of HfTiO4 . Unit-cell volumes of these compound were compared with those of single oxides to make it clear that the unit-cell volume of ZrTiO4 was small anomalously and to distinguish the normal and abnormal substitution systems. These results were explained by the working hypothesis proposed for these compounds. 相似文献
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Varanasi Srikanth Eleswarapu C. Subbarao Dinesh K. Agrawal Chi-Yuen Huang Rustum Roy Gutti V. Rao 《Journal of the American Ceramic Society》1991,74(2):365-368
Most members of the NaZr2 P3 O12 (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr4 P6 O24 contracts on heating and that of SrZr4 P6 O24 expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500°C, and Na and K compounds from 600°C. On the other hand, no acoustic emission signal is detected in Ca0.5 Sr0.5 Zr4 P6 O24 ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500°C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques. 相似文献
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E. R. Vance J. N. Watson M. L. Carter R. A. Day B. D. Begg 《Journal of the American Ceramic Society》2001,84(1):141-144
Brannerite, UTi2 O6 , can be formed only under low oxygen pressures by dry ceramic processing techniques, but the substitution of ∼0.2 and 0.3 formula units (fu) of Ca or Gd, respectively, for U allows the stabilization of the phase in air. The Ca/Gd in brannerite provides charge compensation for some U to exist in valence states >+4, as found by X-ray absorption spectroscopy of the U L III -edge. The maximum solubilities of Ca and trivalent rare earths in the air-fired samples, 0.3 and 0.5 fu, respectively, correspond to U having an average valence of +5. Ca and Gd had maximum solubilities of 0.2 and 0.45 fu, respectively, in argon-fired samples. An absorption band at 1448 nm in both air- and Ar-fired U-brannerite doped with Ca and Gd was observed using diffuse-reflectance spectroscopy and attributed to an electronic transition of U5+ . A similar band was observed in an annealed natural brannerite, which contained Ca, rare earths, and Th, although the band was present at ∼1520 nm in the unannealed, X-ray amorphous sample. In synthetic ThTi2 O6 (thorutite, having the brannerite structure), the solubility of Ca was undetectable and that of rare earths <0.1 fu. Other ionic substitutions in synthetic brannerites involved Hf, Pu, La, and Y for U, (Gd + Nb) for U + Ti, and Fe in the Ti site. 相似文献
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H. M. O'Bryan E. M. Gyorgy W. W. Rhodes 《Journal of the American Ceramic Society》1990,73(12):3698-3699
Ba2 YCu3 O7 ceramics doped with either Pr or U at 0.000 17 to 1.7 wt% levels have been prepared. For each sample J c (magn) has been measured with a vibrating sample magnetometer. No improvement in J c was found for either dopant and it is concluded that neither provides the clusters necessary to produce suitable pinning sites. 相似文献
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Ya-Dong Li Jiang-Hui Zhang Cao-Shui Xiong Hong-Wei Liao 《Journal of the American Ceramic Society》2000,83(4):980-982
A bulk ceramic sample La0.67 Sr0.33 Fe0.07 Mn0.93 O3 (LSFMO) with a rhombohedral structure has been prepared from a coprecipitated carbonate precursor in this study. Ferromagnetism and a negative, isotropic magnetoresistance (MR) as large as 11% have been observed in a ceramic sample of LSFMO. There are two resistivity transition peaks on the resistivity versus temperature curves. The resistivity peak and MR have been related to the ferromagnetic state in LSFMO. 相似文献
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Sergey Kucheiko Dong-Hun Yeo Ji-Won Choi Seok-Jin Yoon Hyun-Jai Kim 《Journal of the American Ceramic Society》2002,85(5):1327-1329
The microwave dielectric properties of CaTi1− x (Al1/2 Nb1/2 ) x O3 solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO3 . The substitution of Ti4+ by Al3+ /Nb5+ improved the quality factor Q of the sintered specimens. A small addition of Li3 NbO4 (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛr ) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τf ) of −2 ppm/K. Li3 NbO4 as a sintering additive had no harmful influence on τf of ceramics. 相似文献
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The cermet fuel (U3 O8 dispersed in Al) being considered for use in the Savannah River Site Reactors is thermodynamically unstable because of the potential for an exothermic metallothermic reduction reaction. This paper describes work performed to quantify the extent of reaction during powder metallurgy (P/M) processing of the U3 O8 —Al cermet fuel, and to determine the effect of partial reduction to U4 O9 on the metallothermic reduction reaction. During the fabrication of the U3 O8 —Al cermet fuel by the P/M technique, a significant portion of the U3 O8 is reduced to U4 O9 . The reaction between U4 O9 and Al is also exothermic; however, the maximum heat released by the reaction is substantially less than that released for the U3 O8 —Al reaction, approximately 335 J (80 cal) per gram of oxide reacted compared to 940 J (225 cal). Metallothermic reduction reactions for U3 O8 /U4 O9 /Al mixtures do not occur at the normal reactor operating temperature, ∼ 370 K (∼ 100°C) or at temperatures below the melting point of aluminum, 930 K (660°C). 相似文献