共查询到20条相似文献,搜索用时 31 毫秒
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Pb0.91 La0.06 (Zr0.65 Ti0.35 )O3 (PLZT 6/65/35) is a relaxor ferroelectric near and above the temperature of the dielectric maximum (∼180°C). The relaxor state can persist to room temperature upon fast cooling. However, this relaxor state gradually changes to a normal ferroelectric over a long time period at 25°C, characterized by an elimination of relaxor-like dielectric dispersion and a significant rhombo-hedral broadening and subsequent splitting of the (220) X-ray diffraction peak. A transmission electron microscopy (TEM) bright-field image of a long-time relaxed sample revealed normal micrometersized ferroelectric domain contrast with relaxor-like "tweed" structure on the submi-crometer scale. The gradually structural evolution is discussed in terms of development of correlations between relaxor polar clusters. 相似文献
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Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca10 (PO4 )6 (OH)2 )) and β-tricalcium phosphate (β-TCP, Ca3 (PO4 )2 ) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography. 相似文献
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In earlier work, a prediction method of the immiscibility boundary of a ternary silicate glass system was developed involving two known binary immiscibility boundaries and a measured immiscibility temperature of one ternary glass composition. In the present work, the method is extended to the case where one of the two binary immiscibility boundaries is not known and is applied as an example to ternary silicate systems containing K2 O. First, the immiscibility boundary of the system K2 O-SiO2 is estimated by measuring the immiscibility temperatures of three glasses in the system K2 O-Li2 O (or Na2 O)-SiO2 . Using this result the immiscibility boundaries of the systems K2 O-Li2 O-SiO2 , K2 O-Na2 O-SiO2 , and K2 O-BaO-SiO2 are estimated. The results agree reasonably well with the experimentally determined immiscibility temperatures at selected compositions. 相似文献
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Equilibrium partial pressures of SiF4 were measured for the reactions 2SiO2 ( c )+2BeF2 ( d )⇋SiF4 ( g )+Be2 SiO4 ( c ) (log P siF4 (mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be2 SiO4 ( c ) +2BeF2 ( d )⇋SiF4 ( g ) +4BeO( c )(log P siF4 (mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF2 was present in solution with LiF as molten Li2 BeF4 . The solubility of SiF4 was low (∼0.04 mol kg-1 atm-1 ) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ Hf and Δ Gf values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO2 and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF4 generated showed an increasing positive deviation from the expression given for the equilibrium involving Be2 SiO4 and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO2 . 相似文献
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W. R. Xue Walter A. Schulze † Robert E. Newnham 《Journal of the American Ceramic Society》1990,73(6):1783-1784
The electromechanical properties of PbTiO3 ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO3 ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k t / k p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio. 相似文献
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E. F. RIEBLING 《Journal of the American Ceramic Society》1973,56(6):303-309
Binary Sb2 O3 -GeO2 glasses containing 45 mol% Sb2 O3 and ternary Sb2 O3 -B2 O3 -GeO2 glasses containing 50 mol% GeO2 were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb2 O3 (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb2 O3 . Amounts of Sb2 O3 >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb2 O3 -B2 O3 -GeO2 glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers. 相似文献
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Yoshikazu Suzuki Masahiro Tsukatsune Susumu Yoshikawa Peter E. D. Morgan 《Journal of the American Ceramic Society》2005,88(11):3283-3286
Porous Al2 O3 /20 vol% LaPO4 and Al2 O3 /20 vol% CePO4 composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE2 (CO3 )3 · x H2 O (RE = La or Ce), Al(H2 PO4 )3 and Al2 O3 with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO3 /MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, 31 P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites. 相似文献
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The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al2 O3 ceramic and 29 vol% SiCw /Al2 O3 composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced. 相似文献
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Si3 N4 /MoSi2 and Si3 N4 /WSi2 composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi2 and WSi2 ; the nitridation in a N2 -base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si3 N4 and MoSi2 or WSi2 ; no free elemental Si and Mo or W were detected by X-ray diffraction. 相似文献
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OLAF MULLER RUSTUM ROY WILLIAM B. WHITE 《Journal of the American Ceramic Society》1968,51(12):693-699
Phase equilibria were determined for the systems NiO-Cr2 O3 −O2 , MgO-Cr2 O3 ,-O2 , and CdO-Cr2 O3 −O2 from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO4 structure) and the spinel NiCr2 O4 . The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO4 and α-CdCrO4 (both with the CrVO4 structure), the high-temperature β-MgCrO4 and β-CdCrO4 (both with the α-MnMoO4 structure), and the spinels MgCr2 O4 and CdCr2 O4 . The cadmium system contains an additional phase, Cd2 CrO5 , which is primitive monoclinic. 相似文献
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Isao Yamashita Koji Tsukuma Takafumi Kusunose 《Journal of the American Ceramic Society》2009,92(9):2136-2138
A translucent alumina composite containing 1 vol% LaAl11 O18 , prepared by the hot isostatic pressing (HIP) method, displays both high translucency and high fracture toughness. Its total forward transmission at 600 nm is 75% (thickness 1 mm), and its bending strength and fracture toughness are estimated to be 574±15 MPa and 5.9±0.46 MPa·m0.5 , respectively. Its high translucency is due to the similarity of refractive index between the additive phase (LaAl11 O18 ) and the matrix (alumina). 相似文献
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Srinivasan Raghavan Hsin Wang Ralph B. Dinwiddie Wallace D. Porter Robert Vaen Detlev Stöver Merrilea J. Mayo 《Journal of the American Ceramic Society》2004,87(3):431-437
Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO2 -Y2 O3 -Ta2 O5 /Nb2 O5 systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented. 相似文献
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Xueyi Sun Jun Chen Ranbo Yu Ce Sun Guirong Liu Xianran Xing Lijie Qiao 《Journal of the American Ceramic Society》2009,92(1):130-132
Lead-free potassium sodium niobate-based piezoelectric ceramics (1− x )(Na0.5 K0.5 )NbO3 – x BiScO3 (KNN–BS) ( x =0∼0.05) have been prepared by an ordinary sintering process. Single perovskite phase of KNN–BS exhibits an orthorhombic symmetry at x <0.015 and pseudocubic symmetry at x >0.02, separating by a MPB at 0.015≤ x ≤0.02. Piezoelectric and ferroelectric properties are significantly enhanced in the MPB, which are as follows: piezoelectric constant d 33 =203 pC/N, planar coupling coefficient k p =0.36, remnant polarization P r =24.4 μC/cm2 . These solid solution ceramics look promising as a potential lead-free candidate materials. 相似文献
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The solid phases formed at 1400°C. in air in the three-component systems BaO-SrO-TiO2 , BaO-CaO-TiO2 , and SrO-CaO-TiO2 are described. Besides solid solutions of components with known structures, some new ternary compounds have been studied. The dielectric constants and loss factors of a number of specimens are given. Crystallographic data of the compounds BaCaTiO4 , Ba3 Ca2 Ti2 O9 , and Ca3 Ti2 O7 and of the solid solution series (Ba, Sr), TiO4 are presented. The preparation of the new compounds is described in detail. 相似文献
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The formation of BaTiO3 from equimolar BaCO3 and TiO2 (rutile) mixtures was studied in air and in CO2 . A small amount of BaTiO3 is formed first directly from BaCO3 and TiO2 at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO3 and Ba2 TiO4 are produced, with Ba2 TiO4 being formed in much larger amounts. In 1 atmosphere of CO2 , the intermediate Ba2 TiO4 was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba2 TiO4 reacts fast with 1 atmosphere of CO2 below about 1100°C. to produce BaTiO3 and BaCO3 相似文献
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Z. L. Dong T. J. White K. Sun L. M. Wang R. C. Ewing 《Journal of the American Ceramic Society》2005,88(1):184-190
Crystallochemical changes of (Pb5 Ca5 )(VO4 )6 F2 apatite under electron irradiation were examined by transmission electron microscopy. The apatite, a synthetic analog of vanadinite, was moderately stable towards a less intense 300-keV LaB6 source, while it changed rapidly in structure when exposed to the higher flux of a 200-keV field emission gun. The electron beam induced transformation of vanadinite proceeds sequentially by (i) migration and loss of fluorine, (ii) lead volatilization and conversion to 2–5-nm platelets of a glaserite-type structure, and (iii) the reduction of V5+ to V4+ with the removal of lead and calcium oxide that leads to single-crystal CaVO3 perovskite as the ultimate product. The phase transformations are interpreted based on the crystallographic relations among the CaVO3 perovskite, the (Pb5 Ca5 )(VO4 )6 F2 apatite and the glaserite-type structures, and compositional changes under electron irradiation. 相似文献