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1.
Gd2O3-doped Bi2O3 polycrystalline ceramic samples containing between 10 and 26 mol% Gd2O3 were fabricated by pressureless sintering of powder compacts. As-sintered samples were cubic (CaF2 structure). The cubic solid solutions underwent transformation to a rhombohedral phase when annealed at lower temperatures. Under certain conditions, the cubic phase fully transformed to the rhombohedral phase of the same composition, and the kinetics were thermally activated. This suggested that the cubic → rhombohedral transformation was a massive transformation. The transition temperatures for the occurrence of a massive transformation were experimentally determined by measuring the conductivity as a function of temperature, as well as by measuring growth rate of the rhombohedral precipitates as a function of temperature. The activation enthalpy for interface motion was measured to be ∼200 kJ/mol for the samples studied. The kinetics of cubic → rhombohedral transformation could be described by the Johnson-Mehl-Avrami equation.  相似文献   

2.
Cubic solid solutions in the Y2O3-Bi2O3 system with ∼25% Y2O3 undergo a transformation to a rhombohedral phase when annealed at temperatures ≤ 700°C. This transformation is composition-invariant and is thermally activated, and the product phase can propagate across matrix grain boundaries, indicating that there is no special crystallo-graphic orientation relationship between the product and the parent phases. Based on these observations, it is proposed that cubic → rhombohedral phase transformation in the Y2O3-Bi2O3 system is a massive transformation. Samples of composition 25% Y2O3-75% Bi2O3 with and without aliovalent dopants were annealed at temperatures ≤ 700°C for up to 10000 h. ZrO2 as a dopant suppressed while CaO and SrO as dopants enhanced the kinetics of phase transformation. The rate of cubic/rhombohedra1 interface migration (growth rate or interface velocity) was also similarly affected by the additions of dopants; ZrO2 suppressed while CaO enhanced the growth rate. Diffusion studies further showed that ZrO2 suppressed while CaO enhanced cation interdiffusion coefficient. These observations are rationalized on the premise that cation interstitials are more mobile compared to cation vacancies in cubic bismuth oxide. The maximum growth rate measured was ∼10−10 m/s, which is orders of magnitude smaller than typical growth rates measured in metallic alloys. This difference is explained in terms of substantially lower diffusion coefficients in these oxide systems compared to metallic alloys.  相似文献   

3.
Lead-free piezoelectric (K0.5Na0.5)NbO3– x wt% Bi2O3 ceramics have been synthesized by an ordinary sintering technique. The addition of Bi2O3 increases the melting point of the system and improves the sintering temperature of (K0.5Na0.5)NbO3 ceramics. All samples show a pure perovskite phase with a typical orthorhombic symmetry when the Bi2O3 content <0.7 wt%. The phase transition temperature of orthorhombic–tetragonal ( T O − T ) and tetragonal–cubic ( T C) slightly decreased when a small amount of Bi2O3 was added. The remnant polarization P r increased and the coercive field E c decreased with increasing addition of Bi2O3. The piezoelectric properties of (K0.5Na0.5)NbO3 ceramics increased when a small amount of Bi2O3 was added. The optimum piezoelectric properties are d 33=140 pC/N, k p=0.46, Q m=167, and T C=410°C for (K0.5Na0.5)NbO3–0.5 wt% Bi2O3 ceramics.  相似文献   

4.
The Phase relations of the system Gd2O3-Ta2O5 in the composition range 50 to100 mol% Gd2O3 was studied by solidstate reactions at 1350°, 1500°, or 1700°C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phase (W2 phase, space group C2221) with the composition of Gd3 TaO7 seems to melt incongruently; at about 2040°C, although this Gd3TaO7 Phase was previously reported to melt congruently. A new fluorite-type cubic phase (F phase, space group Fm3m ) was found for the first time above 1500°C in the system. It melts congruently with the composition of about 80mol% Gd2O3at 2318° 3°C. A phase diagram was proposed for the system Gd2O3–Ta2O5 in the Gd2O3–rich portion  相似文献   

5.
The crystal structures for a suite of substituted pollucites with the compositions CsTi x Al1– x Si2O6+0.5 x , 0 ≤ x ≤ 1, have been determined from Rietveld analysis of powder synchrotron XRD data. Our results indicate that the pollucite end member (CsAlSi2O6) has a tetragonal structure (space group I 41/ a ), whereas all other compositions are cubic (space group Ia 3 d ). The increased symmetry for the titanium-substituted structures is presumably due to the incorporation of additional O2− anions (needed for compensating the charge imbalance between Ti4+ and Al3+), which effectively holds open the expanded cubic framework. In situ cooling experiments of the substituted phase CsTi0.1Al0.9Si2O6.05 reveal a displacive transformation to the tetragonal structure at ∼230 K. This transformation is tricritical in nature and is analogous to the tetragonal-to-cubic transition in pollucite on heating.  相似文献   

6.
A structural phase transition between the cubic (space group, Fm 3 m) and tetragonal (space group, P 42 /nmc) phases in a zirconia–ceria solid solution (Zr1−xCexO2) has been observed by Raman spectroscopy. The cubic–tetragonal ( c–t" ) phase boundary in compositionally homogeneous samples exists at a composition X0 (0.8 < X0 < 0.9) at room temperature, where t " is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X'0 (0.65 < X'0 < 0.7) at room temperature. The sample with a composition between X0 and X'0 is t " ZrO2. By Raman scattering measurements at high temperatures, the tetragonal ( t" ) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr0.2 Ce0.8O2 solid solution.  相似文献   

7.
The phase equilibrium relations in the systems Y2O3-Al2O3 and Gd2O3-Fe2O3 were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd2O3-Fe2O3 but only metastable in the system Y2O3-AI2O3. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O3. A new 2:1 molar ratio cubic phase has been found in the system Y2O3-A12O3. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.  相似文献   

8.
A diffusionless cubic (c)→metastable tetragonal (t') phase transformation occurs in certain alloys in the ZrO2-Y2O3 system on quenching from elevated temperatures. Microstructural features due to this phase transformation, principally anti-phase domain boundaries (APB's) and mechanical accommodation twins, have been characterized using transmission electron microscopy. Certain differences between our interpretation and those of other workers are discussed.  相似文献   

9.
Phase relations in the system Bi2O3-WO3 were studied from 500° to 1100°C. Four intermediate phases, 7Bi2O3· WO3, 7Bi2O3· 2WO3, Bi2O3· WO3, and Bi2O3· 2WO3, were found. The 7B2O · WO3 phase is tetragonal with a 0= 5.52 Å and c 0= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi2O3· 2WO3 has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi2O3· WO3 and Bi2O3· 2WO3 are orthorhombic with (in Å) a 0= 5.45, b 0=5.46, c 0= 16.42 and a 0= 5.42, b 0= 5.41, c 0= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO3, 907°± 3°C and 70 mol% WO3, and 965°± 5°C and 10 mol% WO3.  相似文献   

10.
The subsolidus phase equilibria in the system Bi2O3-TiO2-Nb2O5 at 1100°C were determined by solid-state reaction techniques and X-ray powder diffraction methods. The system was found to contain 4 ternary compounds, i.e. Bi3TiNbO9, Bi7Ti4NbO21, a cubic pyrochlore solid solution having a compositional range of 3Bi2O3· x TiO2 (7– x )Nb2O5 where x ranges from 2.3 to 6.75, and an unidentified phase, 4Bi2O3·11TiO2·5Nb2O5.  相似文献   

11.
X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc2O3-stabilized ZrO2 as-sintered and after aging in air or in wet-forming gas at 850°C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc2O3 samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of ∼20% in electrical conductivity accompanied the aging. In 6 mol% Sc2O3 samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc2O3 samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400)t XRD peak of Sc2O3-stabilzed tetragonal ZrO2 accompanies the change(s) in the electrical conductivity.  相似文献   

12.
The high-energy ball-milling (HEM) method was used to synthesize the compositions of BiNbO4, Bi5Nb3O15, and Bi3NbO7 in a Bi2O3–Nb2O5 binary system. Reagent Bi2O3 and Nb2O5 were chosen as the starting materials. The X-ray diffraction patterns of the three compositions milled for different times were studied. Only the cubic Bi3NbO7 phase, Nb2O5, and amorphous matters were observed in powders after being milled for 10 h. After heating at proper temperatures the amorphous matters disappeared and the proleptic phases of BiNbO4 and Bi5Nb3O15 could be obtained. The Scherrer formula was used to calculate the crystal size and the results of nanopowders are between 10 and 20 nm. The scanning electron microscopy photos of Bi3NbO7 powders showed drastic aggregation, and the particle size was about 100 nm. The dielectric properties of ceramics sintered from the nanopowders prepared by HEM at 100–1 MHz and the microwave region were measured. Bi3NbO7 ceramics showed a good microwave permittivity ɛr of about 80 and a Q × f of about 300 at 5 GHz. The triclinic phase of BiNbO4 ceramics reached its best properties with ɛr=24 and Q × f =14 000 GHz at about 8 GHz.  相似文献   

13.
The mechanism of parahydrogen conversion was studied on Gd2O3 and Y2O3 powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd2O3, Gd, and Y. On Y2O3 the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.  相似文献   

14.
Single-phase, cubic solid solutions of baseline composition 25% Y2O3—75% Bi2O3 with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO2, or ThO2 was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO2 or ThO2 remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO2- and ThO2-doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice.  相似文献   

15.
Studies of the oxidation of Gd and Dy at P O2's from 10−0.3 to 10−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P 02 semiconducting domain yielded oxygen diffusion coefficients in Dy2O3 in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd2O3 which are considerably below literature values for monoclinic single-crystal Gd2O3. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10−6 to 10−7Ω−1 cm−1.  相似文献   

16.
Subsolidus phase relations in the low-Y2O3 portion of the system ZrO2-Y2O3 were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y2O3 is soluble in monoclinic ZrO2, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y2O3 below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y2O3 from ∼500° to ∼1600°C. At higher Y2O3 compositions, cubic ZrO2 solid solution occurs.  相似文献   

17.
The Bi2O3-PbO phase diagram was determined using differential thermal analysis and both room- and high-temperature X-ray powder diffraction. The phase diagram contains a single eutectic at 73 mol% PbO and 635°C. A body-centered cubic solid solution exists above ∼600°C within a composition range of 30 to 65 mol% PbO. The compounds α-Bi2O3, σ5-Bi2O3, and γ-PbO (litharge) have wide solubility ranges. Four compounds, 6Bi2O3·PbO, 3Bi2O3·2PbO, 4Bi2O3,5PbO, and Bi2O3·3PbO, are formed in this system and the previously unreported X-ray diffraction patterns of the latter three compounds are reported. Diffraction patterns for some of these mixed oxides have been observed in ZnO-based varistors grown using Bi2O3 and PbO as sintering aids.  相似文献   

18.
In the binary system PbO–LazO3 only one compound, 4PbO.La2O3, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La2O3, corresponding to half the a parameter of the C form of La2O3. The solid solutions existing between the compositions La2O3–2Pb0 and pure La2O3 have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm2O3 and Gd2O8 are quite similar to that with La2O3. The compound Sm2O3.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm2O3 remains in the B modification.  相似文献   

19.
Zirconia-rich subsolidus phase relationships in the ZrO2–Sc2O3 and ZrO2–In2O3 systems were investigated. Phase inconsistencies in the ZrO2–Sc2O3 system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.  相似文献   

20.
The subsolidus phase diagram of the system Bi2O3–ZnO–Ta2O5 in the region of the cubic pyrochlore phase has been determined at 1050°C. This phase forms a solid solution area that includes the ideal composition P, Bi3Zn2Ta3O14; possible solid solution mechanisms are proposed, supported by density measurements of Zn-deficient solid solutions. The general formula of the solid solutions is Bi3+ y Zn2− x Ta3− y O14− x − y , based on the creation of Zn2+, O2− vacancies in Zn-deficient compositions and a variable Bi/Ta ratio.  相似文献   

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