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1.
Yoshikazu Suzuki Naoki Kondo Tatsuki Ohji 《Journal of the American Ceramic Society》2003,86(7):1128-1131
Porous CaZrO3 /MgAl2 O4 composites were synthesized in air by pressureless reactive sintering of an equimolar mixture of dolomite (CaMg(CO3 )2 ), monoclinic zirconia ( m -ZrO2 ), and α-alumina powders, with a 0.5 wt% lithium fluoride additive. The reaction behavior of the mixed powders (with/without LiF additive) was studied using high-temperature X-ray diffraction. A bulk porous composite resulted from sintering at 1300°C for 2 h (in a nearly closed container, so as to increase the LiF-doping effect), which consisted of fine grains (CaZrO3 and MgAl2 O4 , ∼0.5–1 μm) and well-grown idiomorphic ones (MgAl2 O4 octahedra ∼ 2–4 μm). The idiomorphic spinel grains were located around the inner walls of relatively large pores. The composite showed appreciably high bending strength (σf = 110 ± 8 MPa for a porosity of 31%). The porous CaZrO3 /MgAl2 O4 composites can be applied as high-temperature filters and lightweight structural components. 相似文献
2.
The composition Zn2.33 Sb0.67 O4 (or Zn7 Sb2 O12 ) exists in two polymorphic forms. The thermodynamically stable, low-temperature orthorhombic β form transforms to the high-temperature cubic α-polymorph with a spinel structure at 1225°±25°C. The transformation is fully reversible but slower in the α→β direction and therefore, it is easy to preserve the high-temperature α-polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the α-polymorph directly at low temperatures, e.g., 900°C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the α-polymorph has been confirmed by Rietveld refinement of X-ray powder diffraction data to be an inverse spinel, (Zn)[Sb2/3 Zn4/3 ]O4 , in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn. 相似文献
3.
New Strategies for Preparing NanoSized Silicon Nitride Ceramics 总被引:2,自引:0,他引:2
Xin Xu Toshiyuki Nishimura Naoto Hirosaki Rong-Jun Xie Yinchun Zhu Yoshinobu Yamamoto Hidehiko Tanaka 《Journal of the American Ceramic Society》2005,88(4):934-937
We report the preparation of nanosized silicon nitride (Si3 N4 ) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si3 N4 and sintering additives of 5-mol% Y2 O3 and 2-mol% Al2 O3 was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline β-Si3 N4 particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si3 N4 grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness. 相似文献
4.
Thermal reactions in 93% Al2 O3 -7% MgO and 95.8% Al2 O3 -4.2% MgO gels seeded with α-Al2 O3 , MgAl2 O4 , α-Fe2 O3 , and SiO2 , sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al2 O3 lowered the α-Al2 O3 crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe2 O3 and SiO2 seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures. 相似文献
5.
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
6.
Xiaodong Li Ji-Guang Li Zhimeng Xiu Di Huo Xudong Sun 《Journal of the American Ceramic Society》2009,92(1):241-244
Neodymium (Nd):Y3 Al5 O12 (YAG) ceramics of excellent transparency have been fabricated by solid-reactive sintering, using nanosized γ-Al2 O3 and Y2 O3 powders as the starting materials. Reaction sequences and sintering behaviors of the powder mixture were characterized by X-ray diffractometry and dilatometry. One feature of the solid reactions involving γ-Al2 O3 is the occurrence of hexagonal YAlO3 , which is unstable and transforms to perovskite YAlO3 (YAP) upon further heating. Because of the high reactivities of the starting nanopowders, a complete conversion of the powder mixture to YAG has been achieved by heating at 1300°C for 4 h, via Y4 Al2 O9 , hexagonal YAlO3 , and YAP phases. In-line transmittances of the 1.5 at.% Nd:YAG ceramics (doped with 0.5 wt% of tetraethyl orthosilicate) at 700 nm are 81.0% and 65.7% after vacuum sintering at 1700° and 1600°C for 5 h, respectively. 相似文献
7.
Splat and other rapid quenching techniques were applied to a systematic study of simple oxides, and four new pure oxide glasses were obtained (V2 O5 , TeO2 , MoO3 , and WO2 ). The high-temperature tetragonal form of WO3 coexists with a metastable short-range-order-only phase. The structure of a new metastable high-temperature phase, δ-Ta2 O6 , resembles δ-Nb2 O6 . Predominant phases found in quenched Al2 O3 belong to the γ-(spinel type) series. 相似文献
8.
Naoto Hirosaki Yoshinobu Yamamoto Toshiyuki Nishimura Mamoru Mitomo Junichi Takahashi Hisanori Yamane Masahiko Shimada 《Journal of the American Ceramic Society》2002,85(11):2861-2863
Phase relationships in the Si3 N4 –SiO2 –Lu2 O3 system were investigated at 1850°C in 1 MPa N2 . Only J-phase, Lu4 Si2 O7 N2 (monoclinic, space group P 21 / c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si3 N4 –SiO2 –Lu2 O3 system. The Si3 N4 /Lu2 O3 ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si3 N4 , and a new phase of Lu3 Si5 ON9 , having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si3 N4 –LuN–Lu2 O3 system. The phase diagram suggests that Lu4 Si2 O7 N2 is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications. 相似文献
9.
Daniela Domanski Guillermina Urretavizcaya Facundo J. Castro Fabiana C. Gennari 《Journal of the American Ceramic Society》2004,87(11):2020-2024
MgAl2 O4 spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al2 O3 –MgO, AlO(OH)–MgO, and α-Al2 O3 –MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl2 O4 occurred faster with γ-Al2 O3 than with AlO(OH) or α-Al2 O3 . After 140 h, the mechanochemical treatment of the γ-Al2 O3 –MgO mixture yielded 99% of MgAl2 O4 . 相似文献
10.
Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov 《Journal of the American Ceramic Society》2009,92(8):1806-1812
Polymorphic phase transitions in Ba4 Nb2 O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4 Nb2 O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba4 Nb2 O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4 Nb2 O9 grains decomposes to nanocrystalline Ba5 Nb4 O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3 . Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3 Nb2 O8 phase is observed above 1150°C. 相似文献
11.
Arian Nijmeijer Henk Kruidhof Rune Bredesen Henk Verweij 《Journal of the American Ceramic Society》2001,84(1):136-140
Supported mesoporous γ-Al2 O3 membranes deteriorate and blister in steam-containing environments at high temperatures. This deterioration led us to the development of a new type of supported γ-Al2 O3 membrane with significantly improved stability under hostile conditions. Two measures were taken to achieve this result. First, the γ-Al2 O3 itself was stabilized by an addition of 6 mol% La2 O3 to suppress pore growth of the mesoporous structure. Second, the adherence of the γ-Al2 O3 membrane to the α-Al2 O3 support was significantly improved by application of phosphate bonding between the membrane layer and the support, using an Al(H2 PO4 )3 precursor solution. Membranes applied without phosphate bonding were separated from the α-Al2 O3 support during high-temperature steam treatment, resulting in complete loss of separative properties. The newly developed membranes could be operated for 100 h at 600°C in H2 O/CH4 = 3/1 (by volume) at 2.5 MPa total pressure with no delamination or cracking in the membrane–support interface and with no significant pore growth in the γ-Al2 O3 membrane. 相似文献
12.
Carlos E. Bamberger George M. Begun Dale E. Heatherly 《Journal of the American Ceramic Society》1983,66(11):c208-c209
A new, alternative, route to the synthesis of metal phosphates, using SiP2 O7 as a reagent, was studied. The reaction of SiP2 O7 with metal oxides yields a solid mixture of silica and metal phosphates; its reaction with metal fluorides yields solid metal phosphates and gaseous mixtures of SiF4 and POF3 . 相似文献
13.
The subsolidus phase relations in the entire system ZrO2 -Y2 O3 were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y2 O3 and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y4 Zr3 O12 +hexagonalphase Y6 ZrO11 at 45 mol% Y2 O3 and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y6 ZrO11 at ∼72 mol% Y2 O3 and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y2 O3 with ideal formula Y4 Zr3 O12 , and another, a new hexagonal phase, at 75 mol% Y2 O3 with formula Y6 ZrO11 . They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO2 -Y2 O3 . 相似文献
14.
K. Thomas Jacob Kampurath P. Jayadevan Yoshio Waseda 《Journal of the American Ceramic Society》1998,81(1):209-212
The standard Gibbs energy of formation of the spinel MgAl2 O4 from component oxides, MgO and α-Al2 O3 , has been determined in the temperature range 900 to 1250 K using a solid-state cell incorporating single-crystal CaF2 as the solid electrolyte. The cell can be represented as—Pt,O2 ,MgO+MgF2 |CaF2 |MgF2 +MgAl2 O4 +α-Al2 O3 ,O2 ,Pt—The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression—capdelta G °f,ox =−23600 − 5.91 T (±150) J/mol—The 'second-law' enthalpy of formation of MgAl2 O4 obtained in this study is in good agreement with high-temperature solution calorimetric studies reported in the literature. 相似文献
15.
Solid-state reactions of equimolar mixtures of Bi2 O3 and Fe2 O3 from 625° to 830°C and their kinetics were investigated. The reaction rates were determined from the integrated X-ray diffraction intensities of the strongest peaks of the reactants and products. The activation energy for the formation of BiFeO3 was 96.6±9.0 kcal/mol; that for a second-phase compound, Bi2 Fe4 O9 , which formed above 675°C, was 99.4±9.0 kcal/mol. Specific rate constants for these simultaneous reactions were obtained. The preparation of single-phase BiFeO3 from the stoichiometric mixture of Bi2 O3 and Fe2 O3 is discussed. 相似文献
16.
Chien-Cheng Lin Yao-Wen Chang Kun-Lin Lin Kun-Fung Lin 《Journal of the American Ceramic Society》2008,91(7):2321-2327
Various Y2 O3 /ZrO2 samples were fabricated by hot pressing, whereby Y2 O3 was mutually dissolved or reacted with ZrO2 as a solid solution or Zr3 Y4 O12 . Hot-pressed samples were allowed to react with Ti melt at 1700°C for 10 min in argon. Microstructural characterization was conducted using X-ray diffraction and analytical electron microscopy. The Y2 O3 /ZrO2 samples became more stable with increasing Y2 O3 because Y2 O3 was hardly reacted and dissolved with Ti melt. The incorporation of more than 30 vol% Y2 O3 could effectively suppress the reactions in the Ti side, where only a very small amount of α-Ti and β'-Ti was found. When ZrO2 was dissolved into Ti on the zirconia side near the original interfaces, Y2 O3 reprecipitated in the samples containing 30%–70 vol% Y2 O3 , because the solubility of Y2 O3 in Ti was very low. In the region far from the original interface, α-Zr, Y2 O3 , and/or residual Zr3 Y4 O12 were found in the samples containing more than 50 vol% Y2 O3 and the amount of α-Zr decreased with increasing Y2 O3 . 相似文献
17.
Phase relations in the binary system between SiO2 -P2 O5 and SiO2 were investigated by the quenching method using sealed platinum tubes to prevent the loss of P2 O5 . The compound Si02 -P2 O5 exists in two forms, the low-temperature β form inverting sluggishly but reversibly to the high-temperature β form at 1030°C. The β form melts congruently at 1290°C. The compound 2SiO2 -P2 O5 melts incongruently at 1120°C to a silica-rich liquid and SiOa -P2 O5 . In the region between 5 and 25 mole % PO2 , reactions were so sluggish that no data could be obtained by quenching. 相似文献
18.
Silvia Schicker Daniel E. García Igor Gorlov Rolf Janssen Nils Claussen 《Journal of the American Ceramic Society》1999,82(10):2607-2612
Wet milling of Al2 O3 -aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al2 O3 and Fe2 O3 /Al/Al2 O3 powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe2 O3 /Al/Al2 O3 powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al2 O3 powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8. 相似文献
19.
Phase equilibria in the system SrO-CdO-V2 O5 in air were established from data obtained by DTA, quenching, and high-temperature solid-state reaction experiments. The SrO-V2 O5 boundary system contains 4 compounds at SrO to V2 O5 molar ratios of 4:1, 3:1, 2:1, and 1:1. A fifth compound with a molar composition of ∼10:3 with the apatite crystal structure was also found; it may, however, be a hydroxyapatite phase. The CdO-V2 O5 system contains the compounds 3CdO·V2 O5 , 2CdO·V2 O5 , and CdO·V2 O5 . The latter compound exhibits a rapid reversible polymorphic transition at 180°C. Complete solid solubility exists in the SrO-CdO system. The most probable compatibility relations were determined from the data available for the SrO-CdO-V2 O5 ternary system. Limited solid solubility exists between SrO·V2 O5 and CdO·V2 O5 , and the high-temperature CdO·V2 O5 polymorph is stabilized to room temperature by solid solution of SrO·V2 O5 . Evidence for the existence of 2 ternary compounds with limited local solid solubility is also presented. 相似文献
20.
An intimate Ba-Al-Al2 O3 -SiO2 powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl2 Si2 O8 . Barium oxidation was completed, and a binary silicate compound, Ba2 SiO4 , had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl2 O4 had formed. Diffraction peaks consistent with hexagonal BaAl2 Si2 O8 , BaAl2 O4 , β-BaSiO3 , and possibly β-BaSi2 O5 were detected after 24 h at 900°C. Diffraction peaks for BaAl2 O4 and BaAl2 Si2 O8 were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl2 O4 at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl2 Si2 O8 . The observed reaction path was compared to prior work with other inorganic precursors to BaAl2 Si2 O8 . 相似文献