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1.
Compositions in the system ThO2-YO1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO1.5 at 1400°C and up to 45 to 50 mole % YO1.5 at 2200°C. Density data showed that these solid solutions correspond to Th1— x Y x O2—0.5 x , having a complete cation sub-lattice filled by Th4+ and Y3+ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO2 is soluble in YO1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y2O3 phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.  相似文献   

2.
The phase equilibrium diagram of the system ThO2-Nb2O was redetermined near the composition Th2Nb2O9. This phase was found to melt incongruenlly at 1362°C, with a eutectic temperature at ∼1350°C. The peritectic and eutectic compositions must occur between 60 and ∼64 mol % ThO2. From single crystal and powder X-ray diffraction data, Th2 Nb2O9 was found to have a primitive monoclinic unit cell with a = 6.711(1), b = 25.254(5), c=7.757(1)×10−1nm, β=90.46 (1)°.  相似文献   

3.
The thermal conductivities of sintered pellets of ThO2-1.3 wt% U02 were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 1014 to 4.7 × loL7 fissions/cm3. Each fission fragment damaged a region of 2.2 × 10-16 cm3 with the reduction in conductivity saturating by about 1017 fissions/cm3. Samples having exposures from 1015 to 1016 fissions/cm3 were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.  相似文献   

4.
The effect of additives on the sintering of ThO2 and ThO2-Y2O3 compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y2O3 as a separate phase increases the rate of sintering of ThO2, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.  相似文献   

5.
The electrical conductivity and thermoelectric power of highpurity polycrystalline ThO2 in thermodynamic equilibrium with the gas phase were measured as a function of temperature from 1000° to 1600°C and as a function of oxygen partial pressure from 1 to 10−22 atm. An n -type electronic contribution to the conductivity is observed above 1400°C at low oxygen pressures. An analytic solution is presented for the oxygen pressure dependence of the total conductivity in the mixed ionicelectron hole conduction region observed at higher oxygen pressures. The activation energies for p -type and ionic conduction are 1.0 and 0.9 eV, respectively. The combined conductivity and thermal emf data give a lower limit of ∼6 cm2/V-s for the electron hole mobility.  相似文献   

6.
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   

7.
Porous Cr3C2 grains (∼300 to 500 μm) with ∼10 wt% of Cr2O3 were prepared by heating a mixture of MgCr2O4 grains and graphite powder at 1450° to 1650°C for 2 h in an Al2O3 crucible covered by an Al2O3 lid with a hole in the center. The porous Cr3C2 grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m2/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr3C2 grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.  相似文献   

8.
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   

9.
In the system TiO2—Al2O3, TiO2 (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al2O3) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al2O3. At higher temperatures, anatase solid solutions transform into TiO2 (rutile) with the formation of α-Al2O3. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.  相似文献   

10.
The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H2, CO2-CO2 and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)7O2 ranging from 52.5 to 75 mole % Y2O3. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body-centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face-centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O2. For low-yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O2.25 A tentative ternary phase diagram for the system UO2-UO3-Y2O3 is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.  相似文献   

11.
Alumina reacts with 1 atm of SiF4 below 660°± 7°C to form A1F3 and SiO2. At higher temperatures the product is a mixture of fluorotopaz and AIF3. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF4 at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF4 above mixtures of fluorotopaz and AlF3 is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF4 to give acicular mullite, 2Al2O3.1.07SiO2. Alumina and mullite are stable in the presence of 1 atm of SiF4 above 973° and 1056°C, respectively. The phase diagram of the system SiO2-Al2O3-SiF4 shows only gas-solid equilibria.  相似文献   

12.
The emf method was used to determine the lower limit of oxygen activity for essentially pure anionic conduction in Th0.85Y0.15O1.925 from 775° to 1000C. The results show that the oxygen transference number is ≥0.99 when log PO 2≥−6.5(104/ T °K)+29.  相似文献   

13.
Solid-liquid equilibria at 1750°C and subsolidus phase relations in the system Si3N4−AlN-SiO2−Al2O3 were determined for the composition region bounded by the β-Si3N4 solid solution line and silica-alumina join X-ray diffraction and optical microscopy were used to determine the phases present in specimens cooled rapidly after equilibration. The extent of a single liquid-phase region and the tie lines for the βsolid solution + liquid field at 1750°C were established from quantitative X-ray diffractometry and lattice parameter measurements of βsolid solutions in equilibrium with liquid. The results were corroborated by optical microscopy and melting behavior observations. A new composition, Si12Al18O39N8, is suggested for the x1 phase. The lowest melting temperature in the system is ≅ 1480°C and the corresponding composition is 10 eq% Al-90 eq%O.  相似文献   

14.
The phase relations for the Sc2O3-Ta2O5 system in the composition range of 50-100 mol% Sc2O3 have been studied by using solid-state reactions at 1350°, 1500°, or 1700°C and by using thermal analyses up to the melting temperatures. The Sc5.5Ta1.5O12 phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO4, and Sc2O3 were found in the system. The Sc5.5Ta1.5O12 phase formed in 78 mol% Sc2O3 at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc2O3 and decomposed to Sc5.5Ta1.5O12 and ScTaO4 at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc2O3. The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc2O3. A phase diagram that includes the four above-described phases has been proposed, instead of the previous diagram in which those phases were not identified.  相似文献   

15.
Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe2O3-CaO-SiO2 glasses. Glass transition (Tg) and crystallization temperatures (T c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe2O3. 20CaO·40SiO2 (in wt%) the maximum nucleation rate was (68.6 ± 7) × 106/mm3·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m2 and 30.8 × 103 A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m3 when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 103 A/m.  相似文献   

16.
Tentative phase relations in the binary system BnOa-A12O3 are presented as a prerequisite to the understanding of the system Li2O-B2O3-Al2O3. Two binary compounds, 2A12O3.B2O3 and 9A12O3.-2B2O3, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li2O.A12O3.B2O3 and 2Li2O. 2AI2O3. 3B203. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to LizO.-5Al2O3 and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.  相似文献   

17.
Equilibrium phase diagrams for the systems MgCl2-MgF2, CaCl2-MgF2 and NaCl-MgF2 were determined by differential thermal analysis, thermal analysis, and temperature-composition equilibrium techniques. Simple eutectics were observed at 78.0±0.5 mol% MgCl2 and 628°±2°C in the MgCl2-MgF2 system, at 87.5±0.5 mol% CaCl2 and 694°±2°C in the CaCl2-MgF2 system, and at 95.5±0.5 mol% NaCl and 786°±3°C in the NaCl-MgF2 system. The phase diagrams determined for these systems were compared with phase diagrams that were computed using Temkin's model. The phase diagrams of the CaCl2-MgF2 and NaCl-MgF2 systems were also compared with diagrams that were computed using the expression suggested by Flood et al. for reciprocal systems. The experimentally determined and computed phase diagrams agreed for the MgCl2-MgF2 system but not for the CaCl2-MgF2 and NaCl-MgF2 systems.  相似文献   

18.
Some K2O-Nb2O5-GeO2 glasses are prepared, and their crystallization behaviors are examined. 25K2O·25Nb2O5·50GeO2 glass with the glass transition temperature T g= 622°3C and crystallization onset temperature T x= 668°3C shows a prominent nanocrystallization. The crystalline phase is K3,8Nb5Ge3O20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K2O·25Nb2O5·50GeO2 glass change steeply by heat treatment at T g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.  相似文献   

19.
In the system ZrO2-Al2O3, cubic ZrO2 solid solutions containing up to 40 mol% Al2O3 crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al2O3 content. At higher temperatures, the solid solutions transform into tetragonal ZrO2 and α-Al2O3. Pure ZrO2 crystallizes in the tetragonal form at 415° to 440°C.  相似文献   

20.
A tentative phase diagram for the system Al203-Nd2O3 is presented. Three compounds were obtained: a β -A12O3-type compound, the perovskite NdAlO3, and Nd4Al2O9. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al2O3, mp 1900°C; Nd4Al2O9, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al2O3, 1750°C; 23 mol% Al2O3,1800°C. Nd4Al2O9 decomposes in the solid state at 1780°C.  相似文献   

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