共查询到20条相似文献,搜索用时 10 毫秒
1.
William E. Lee Makan Chen Peter F. James 《Journal of the American Ceramic Society》1995,78(8):2180-2186
The fabrication and crystallization of stoichiometric and near-stoichiometric celsian, BaAl2 Si2 O8 (BAS), glass made via melt and sol-gel routes is reviewed. After considering celsian polymorphism and methods of stabilizing monoclinic celsian, some new experimental results are presented which show that melt glass crystallizes predominantly via surface nucleation, but sol-gel glass via volume nucleation. Possible reasons for this behavior and the kinetics of the crystallization process are examined. 相似文献
2.
Sai Sundar V. S. S. Vepa Arun M. Umarji 《Journal of the American Ceramic Society》1993,76(7):1873-1876
The effect of substitution of calcium on the anisotropic axial thermal expansion of cordierite was investigated by using a high-temperature X-ray diffraction technique. The compositions were prepared by the sol–gel route. In the Mg2- x Ca x -Al4 Si5 O18 system, single-phase cordierite can be prepared for x up to 0.5. Thermal expansion anisotropy (α a –α c ) of cordierites reduces progressively by the substitution of increasing amounts of Ca for Mg. 相似文献
3.
MASAYUKI FUJIMOTO YET-MING CHIANG W. DAVID KINGERY 《Journal of the American Ceramic Society》1988,71(4):197-200
In a hot-pressed and deformed MgO single crystal, precipitates of Mg1-x Cax Al2 O4 spinel upon which MgAl2 O4 spinel subsequently precipitated were observed and analyzed using transmission electron microscopy and scanning electron microscopy. This behavior is related to the respective solubility limits of CaO and Al2 O3 in MgO at the hot-pressing temperature and may be aided by impurity segregation to the dislocations. The spinel selectively precipitated at the nodes of a dislocation network which was formed during [001] hot-pressing deformation, as a result of the reaction b3 = b1 + b2 = (1/2) [011] + (1/2) = [001]. The dislocation is sessile, and the precipitates have a <100>matrix ≨ <100>spinel coherent relationship. 相似文献
4.
The eutectic composition between Y4 Al2 O9 and Y2 O3 was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y2 O3 and 29.5 mol% Al2 O3 ). Based on the current results and other published data, the pseudobinary system between Al2 O3 and Y2 O3 is revised. 相似文献
5.
Chunling Liu Sridhar Komarneni Rustum Roy 《Journal of the American Ceramic Society》1995,78(9):2521-2526
Two sol-gel routes have been used to prepare celsian (BaAl2 Si2 O8 ) precursors. The crystallization behaviors of unseeded gels and the gels seeded with 5 wt% feldspar, rutile, LiAISi2 O2 , and ZrO2 crystals as well as SrAl2 Si2 O8 gel have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric and differential thermal analyses (TGA and DTA). It was found that metastable hexacelsian, rather than the preferred monoclinic celsian, was the major crystallization product in the two unseeded gels. Seeding largely enhanced the crystallization of monoclinic celsian, but the mechanisms involved were different for seeding with different crystals. SrAl2 Si2 O8 gel and ZrO2 crystals acted as normal heterogeneous centers and had little catalytic effect on monoclinic celsian crystallization. Feldspar and rutile crystals worked as epitaxial substrates for monoclinic celsian and produced pure monoclinic celsian in some temperature regions. LiAISi3 O8 seeds lowered the glass transition temperature of the immediately adjacent gel matrix and led to the crystallization of pure monoclinic celsian. 相似文献
6.
The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P2 O5 −60 wt% CaO · MgO · 2SiO2 was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P2 O5 ) and CaO · MgO · 2SiO2 were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO2 and the bulk crystallization of apatite and the CaO · MgO · 2SiO2 -β-(3CaO · P2 O5 ) composite. 相似文献
7.
Al2 O3 addition to the melt of a BiSrCaCu2 O x composition was found by TEM observation to cause the liquid-liquid phase separation of the melt-quenched glass, and to result in preferential precipitation of superconducting Bi2 Sr2 Ca1 Cu2 O x crystals from the melt during the cooling process. 相似文献
8.
Hisanori Yamane Mamoru Omori Akira Okubo Toshio Hirai 《Journal of the American Ceramic Society》1993,76(9):2382-2384
Reversible thermal phase transition was observed for Y4 Al2 O9 at 1377°C with a hysteresis width of 78°C using differential scanning calorimetry. The enthalpy of the transition (Δ H ) was about 300 cal/mol. Powder X-ray diffraction peaks of the high-temperature phase, as well as the peaks of the low-temperature phase, were characterized with a monoclinic unit cell. Thermal hysteresis was also confirmed by the temperature dependence of the lattice parameter. The unit cell volume of the high-temperature phase was 0.5% smaller than that of the low-temperature phase at 1400°C. 相似文献
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10.
Ian MacLaren Roland Schierholz Paul A. Trusty Clive B. Ponton 《Journal of the American Ceramic Society》2007,90(10):3307-3310
Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y2 Si2 O7 containing rounded glassy Y-doped SiO2 inclusions; excess glassy SiO2 -rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO2 excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined. 相似文献
11.
The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na2 Ti2 Si2 O11 -Na2 Ti2 Si2 O9 . Both narsarukite (Na2 TiSi4 O11 ) and lorenzenite (Na2 Ti2 Si2 O9 ) melt incongruently. Narsarsukite melts at 911°±°C to SiO2 +liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na2 Ti6 O13 +liquid; Na2 Ti6 O13 reacts with liquid to form TiO2 and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C. 相似文献
12.
A solid electrolyte electrochemical cell of the type Pt|Ni:NiO a =1 ∥ZrO2 +7.5% CaO∥Ni:NiO a <1 +glass|Pt was used to measure the activities of NiO in sodium disilicate glass from 750° to 1100°C. The data indicate a solubility varying from 11 mol% (5.0 wt%) at 800° to 20 mol% (9.3 wt%) at 1100°C. From the variation in NiO activity, the activity of sodium disilicate in glass solution was estimated; from these combined data partial molar free energies and entropies of solution of NiO and Na2 Si2 O5 and free energies and entropies of mixing were calculated. A partial phase diagram for the system NiO-Na2 Si2 O5 proposed from solubility data indicates a eutectic at ∼12 mol% (5.3 wt%) NiO at 830°C. 相似文献
13.
TiO2 above 4 mol% is an effective nucleating agent for CaO–P2 O5 glass which also contains substantial SiO2 and Al2 O3 additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca2 P2 O7 , whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO4 dendrites. Metastable calcium phosphate grows on the AlPO4 dendrites and later transforms to β-Ca2 P2 O7 . 相似文献
14.
The phase development, microstructure development, and Curie temperature of PbO-BaO-TiO2 -Al2 O3 -SiO2 glass- ceramics were investigated by X-ray diffractometry, electron microscopy, and dilatometry. The primary crystalline phase was perovskite titanate, which exhibited bulk nucle- ation. When the samples were heated at low temperatures, the Curie inversion was hindered and cubic perovskite was obtained. Increased heating temperature/time resulted in tetragonal perovskite. Growth of a surface layer due to the crystallization of the secondary crystalline phase PbO- Al2 O3 -2SiO2 resulted in the coexistence of coarse and fine perovskite particles. The increase in the relative number of coarse-to-fine perovskite particles with the heating temperature/time was the major factor responsible for increased spontaneous deformation and higher Curie temperature of the perovskite phase. Substitution of barium for lead in the lead titanate phase probably occurred and is explained by the use of SiO2 instead of B2 O3 as the glass former in the present study. 相似文献
15.
Lamrath Abdul Khalam Sherin Thomas Mailadil T. Sebastian 《International Journal of Applied Ceramic Technology》2007,4(4):359-366
The columbites MgNb2 O6 , MgTa2 O6 , and corundum-type Mg4 Nb2 O9 ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb2 O6 and MgTa2 O6 and by the substitution of TiO2 for Nb2 O5 in both MgNb2 O6 and Mg4 Nb2 O9 ceramics. The Mg(Nb0.7 Ta1.3 )O6 has ɛr =29, Q u × f =67 800 GHz, and τf =0.8 ppm/°C and the MgO–(0.4)Nb2 O5 –(1.5)TiO2 composition has ɛr =34.5, Q u × f =81 300 GHz, and τf =−2 ppm/°C. 相似文献
16.
The temperature dependence of the rate of growth of Na2 B4 O7 from its melt was determined between 1° and 192°C of undercooling. The maximum rate of growth was 2000 μm/min at 57°C of undercooling. Analysis of the growth data indicated that growth could occur by a screw dislocation mechanism over the entire range of undercooling. When this mechanism was assumed, there was good correlation between the experimental data and the predictions of the Turnbull and Cohen equation. 相似文献
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18.
A novel porous Yb4 Si2 O7 N2 material with uniform open-cell network structure was fabricated from the reaction between Si3 N4 , Yb2 O3 , and SiO2 . The formation of Yb4 Si2 O7 N2 during heating was studied using X-ray diffractometry. The porous structure was characterized using scanning electron microscopy and mercury porosimeter. It is shown that the formation of Yb4 Si2 O7 N2 phase starts at ∼1150°C and completes at 1350°C for 4 h, accompanied by the development of open-cell network structure. The necks between Yb4 Si2 O7 N2 particles become much thicker with increasing temperature because of the coarsening of Yb4 Si2 O7 N2 particles, thus leading to a uniform three-dimensional network structure. Furthermore, the pore size can be well controlled by adjusting reacting temperature and altering atmosphere. 相似文献
19.
Phase equilibria in the system MnO–CoO–Cr2 O3 were investigated at 1300°C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in various phase assemblages of the system were measured by determining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 1300°C, MnO–CoO and MnCr2 O4 –CoCr2 O4 solid solutions exhibit mild positive departures from ideal behavior. The activities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of formation of the compound CoCr2 O4 from its oxide components at 1300°C was determined as −37 636 J/mol, while that for MnCr2 O4 was found as −44 316 J/mol. 相似文献
20.
Serene C. Farmer Larry H. Schoenlein A. H. Heuer 《Journal of the American Ceramic Society》1983,66(7):c107-c109
Decomposition of cubic, MgO-stabilized ZrO2 solid solutions has been studied. Precipitates of the ordered compound, Mg2 Zr5 O12 , form, but only when a two-stage heat treatment involving nucleation at a low temperature followed by growth at a higher temperature is used. Other decomposition products are also present after such a heat treatment . 相似文献