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1.
The phase diagram of the system CaF2 -AlF3 was established from microscopic, powder X-ray diffraction, quench, and DTA data obtained from samples encapsulated in sealed Pt tubes and either reacted in the solid state or melted. Two compounds, CaAlF5 and Ca2 AlF7 , melted incongruently at 873° ° 3° and 845° °3° C, respectively. Previously unreported Ca2 AlF7 was successfully indexed as orthorhombic with α0 = 18.22 Å, b 0 =9.06 Å, and c 0 = 7.11 Å. The only eutectic in the system exists at 836° ° 3° C and 37.5 mol% AlF3 . 相似文献
2.
Potassium phosphate oxynitride (KPON) glasses were made by heating crystalline KPO3 at 702° to 775° C in dry ammonia. The softening temperature, thermal expansion coefficient, refractive index, and dissolution rate in water were measured as a function of nitrogen content and compared with the properties of oxynitride glasses made from LiPO3 and NaPO3 . 相似文献
3.
Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO2 -Al2 O3 solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO2 - and Al2 O3 -rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of ∼1540° C at a critical composition of ∼36 mol% Al2 O3 was considered to be thermodynamically most probable; the gap extended from ∼11 to °49 mol% Al2 O3 at 1100° C. SiO2 -rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy. 相似文献
4.
N. M. BOTROUS EL BADRAMANY E. F. MINA H. D. MERCHANT S. ARAFA R. P. POPLAWSKY 《Journal of the American Ceramic Society》1979,62(3-4):113-116
The electrical resistivity of Fe2+ 0.95 Fe3+ 2.05 O2+ 4 , Ni2+ 0.51 Fe2+ 0.46 Fe1.95 ,3+ O2− 4 , and Ni0.89 ,2+ Fe2+ 0.13 Fe3+ 2.11 O2− 4 single crystals was determined at >300°K. The dc resistivity up to 800°K shows a monotonic decrease with increased temperature, with an average activation energy of 0.065 eV independent of temperature and Fez+ concentration. The ac resistivity, at frequencies of 540 kHz to 34 MHz up to 450°K, shows temperature and frequency dependences of the relative dielectric constant similar to that found when true dielectric relaxation occurs, with the activation energy depending slightly on frequency and ferrous ion content. 相似文献
5.
Dielectric properties of A(B3+ 1/2 B5+ 1/2 )O3 (A = Ba, Ca, Sr; B3+ = La, Nd, Sm, Yb; B5+ = Nb, Ta) ceramics have been investigated at microwave frequencies. Sr(B3+ 1/2 B5+ 1/2 )O3 and Ca(B3+ 1/2 B5+ 1/2 )O3 ceramics have relative dielectric constants (ε r ) above 20 and negative temperature coefficients of resonant frequency (T f ). In the group of Ba(B3+ 1/2 B5+ 1/2 )O3 ceramics, T f changes from + 118 ppm/° to nearly zero according to the kinds of B-site ions. Among the ceramics investigated, Sr(Sm1/2 Ta1/2 )O3 ceramics have the highest Q values at microwave frequencies. For Sr(Sm1/2 Ta1/2 )O3 ceramics Q = 7000, ε r = 27.7, and T f =−62.5 ppm/° at 8.5 GHz. The microstructure of Sr(Sm1/2 Ta1/2 )O3 ceramics is composed of a matrix of the ternary compound (Sr-Sm-Ta-O system) and secondary phase grains of the binary compound (Sm-Ta-O system). 相似文献
6.
Carlos E. Bamberger Tamara J. Havedock Otto C. Kopp† 《Journal of the American Ceramic Society》1994,77(6):1659-1661
The ternary system SrO-CeO2 -TiO2 was investigated using X-ray diffractometry. The formation of a new compound, Sr2 Ce2 Ti5 O16 , was established, and its compatibilities with SrO, SrCeO3 , and SrTiO3 were studied. The results revealed the existence of a series of compounds Sr6–12x Ce6x Ti5 O16 and solid solutions Sr2+n Ce2 Ti5+n O16+3n ( n ≤ 6). 相似文献
7.
Mechanism and Kinetics for the Formation of Uranium Mononitride by the Reaction of Uranium Dioxide with Carbon and Nitrogen 总被引:1,自引:0,他引:1
The mechanism and kinetics of UN formation by reaction of a pellet of mixed UO2 and C with N2 were studied for temperatures of 1420° to 1750°C. The reaction followed the first-order rate equation; the activation energy was 83 kcal/mol. Only UN1−x C x was produced. The lattice parameter variation of UN1−x C x had a minimum and a maximum during reaction; at the maximum, UN1−x C x + C + N2 were in equilibrium. The overall reaction was divided into four stages: (1) formation of UN1−x C x from UO2 , (2) decarburization of UN1−x C x , (3) formation of UN1−x C x with the equilibrium composition, and (4) pure UN formation. The lowest reaction rate was in stage (4). 相似文献
8.
The reoxidation process in highly Ce3+ -doped BaTiO3 ceramics was studied using TEM. Samples of two different types of solid solutions, Ba1−X Ce3+ X Ti1−X/4 ( V Ti ) X/4 O3 and Ba1−X Ce3+ X Ti4+ 1− X Ti3+ X O3 , were prepared by sintering oxide mixtures in air and in a reducing atmosphere, respectively. The solid solutions were reoxidized by annealing in air at high temperatures (1000°—1100°C). As a result of internal oxidation of Ce3+ and Ti3+ , fluorite CeO2 and monoclinic Ba6 Ti17 O40 phases were precipitated in the perovskite matrix. In Ba1−X Ce3+ X Ti1−X /4( V Ti )X/4O3 solid solution precipitates nucleate heterogeneously at grain boundaries and at extended defects inside the grains, whereas in Ba1−X Ce3+ X Ti4+ 1−X Ti3+ X O3 solid solution precipitates are nucleated mainly homogeneously inside reoxidized perovskite grains. The form of the precipitates and their orientational relationship with the matrix, as well as the mechanism of internal oxidation, are discussed. 相似文献
9.
Winnie Wong-Ng Lawrence P. Cook Boris Paretzkin Michael D. Hill Judith K. Stalick 《Journal of the American Ceramic Society》1994,77(9):2354-2362
Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd2 O3 corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T c superconductor phase, and has the formula Ba2–x Nd1+x Cu3 O6+z , where × < ≅ 0.7. At the ideal compound stoichiometry of Ba2 NdCu3 O6+z , the transformation from the high- T c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba2+2x -Nd4–2x Cu2–x O10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd2 O3 and Nd2 O3 –CuOx systems are also presented. Standard X-ray diffraction patterns of BaNd2 –CuO5 and (Nd1.9 Ca0.1 )CuO4–z are provided. 相似文献
10.
Subsolidus phase relations in the La2 O3 –BaO–CuO system were studied at 950°C. Three previously reported binary compounds exist (La2 CuO4 , BaLa2 O4 , and BaCuO2 ) and five previously reported ternary phases occur (La2-x Bax CuO4-(x/2)+δ , La4-2x Ba2+2x Cu2-x O10-2x , La2-x Ba1+x Cu2 O6-(x-2) , La3-x Ba3+x Cu6 O14±δ , and La4 BaCu5 O13+δ ). Of the seven phases in the diagram, all but BaLa2 O4 , BaCuO2 , and La4 BaCu5 O13+δ were shown to exhibit significant ranges of solubility. The diagram is important in that both >30 K (La2-x Bax CuO4-(x/2)+δ ) and >90 K (La3-x Ba3+x Cu6 O14+δ , x=1) superconductors occur. 相似文献
11.
Previous solid state analyses of sintering in Ti4+ -doped-commercial alumina are shown to be in error because a liquid phase exists in the appropriate region of the Al2 O3 −TiO3 −Na2 Ophase diagram at least by 1350° C, a temperature lower than that at which "solid state" studies were conducted. It is suggested that liquid-phase sintering is a much more common occurrence than was realized formerly. 相似文献
12.
The volatility of UO2±x and the phase relations in the system uranium-oxygen were studied using thermogravimetric techniques. Chemical reactions describing the loss of uranium from UO2±x at temperatures between 1100° and 2200°C in oxygen pressures between approximately 102 and 10−6 torr are proposed. Results were obtained requiring the consideration of UO4 (g) as the uranium-bearing vapor species above UO2±x . Evidence supporting the existence of UO4 (g) included the volatilization of material with an oxygen-to-uranium ratio of 4 during the decomposition of UO2+x (0.Z > × > 0) to near-stoichiometric UO2 in vacuum above 1500°C and the dependence of the evaporation rate of the uranium dioxide on the oxygen pressure between 1200° and 1500°C. The equilibrium oxygen pressures over compositions between UO2.02 and UO2.63 in the UO2+x and U3 O8-y regions and over the boundary between these phases were measured between 1000° and 1600°C. The equilibrium oxygen-to-uranium ratio of UO2±x was less than 2 above 1700°C in vacuum. 相似文献
13.
Byung-Kook Kim Soon Ja Park Hiro-o Hamaguchi 《Journal of the American Ceramic Society》1994,77(10):2648-2652
New methods of determining the oxygen self-diffusion coefficients (D*o ) in oxides have been developed using Raman spectroscopy combined with the 16 O–18 O exchange technique. From the depth-profiles of the 18 O concentration in the 16 O–18 O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D *o was determined for 2.8 mol% Y2 O3 -containing tetragonal zirconia polycrystall (the depth-profile method). Thus-obtained results are expressed as D *O,D-P = 1.82(+0.41 −0.40 ) × 10−1 ·exp{−(139.3 ± 0.2) [kJ/mol]/ RT } [cm2 /s] in the temperature range of 700–950° C. We also determined D *o for the same sample from the Raman spectrometric monitoring of the ambient gas during the 16 O–18 O exchange reaction (the gas-monitoring method). Thus-obtained results are expressed as D *O,G-M = 1.14(+0.05 −0.04 ) × 10−2 exp{−(117.5 ± 0.4) [kJ/mol]/ RT } [cm2 /s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D *o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D *o in oxides. 相似文献
14.
Vidya S. Tiwari Neelam Singh Dhananjai Pandey 《Journal of the American Ceramic Society》1994,77(7):1813-1818
Ba1–x Cax TiO3 powders have been prepared using a novel route involving solid-state reaction in a mixture of Ba1–x Cax CO3 and TiO2 . The Ba1–x Cax CO3 precursors used in this method were prepared by a chemical coprecipitation route to ensure a uniform supply of Ba and Ca ions during thermochemical reaction with TiO2 . The compositional homogeneity of Ba1–x Cax TiO3 powder prepared by such a semiwet route is compared with those prepared by the conventional dry route, involving thermochemical reaction in a mixture of BaCO3 , CaCO3 , and TiO2 , using principles of X-ray line broadening. It is shown that the powders obtained by the semiwet route possess better compositional homogeneity, over a length scale of 1800 Å corresponding to the coherently scattering domain size, than those prepared by the conventional dry route. Microstructural studies have revealed grain sizes of the order of 1 μm and several micrometers, respectively, for ceramics prepared by the semiwet and conventional dry routes. The solid solubility limit of Ca in Ba1–x Cax TiO3 ceramics fired at 1300°C is 16 mol% for samples prepared by the semiwet route while conventionally processed ceramics had a Ca solid Solubility limit of 12 mol% at the same temperature. 相似文献
15.
Tadashi Takenaka Amar S. Bhalla L. Eric Cross Koichiro Sakata 《Journal of the American Ceramic Society》1989,72(6):1016-1023
New piezoelectric and pyroelectric ceramics consisting of antiferroelectric lead zirconate (PZ) and relaxor ferroelectric lead zinc niobate (PZN) are studied from an application view-point of the field-induced antiferroelectric-to-ferroelectric phase transition. An antiferroelectric-ferroelectric phase boundary exists in PbZrx (Zn1/3 Nb2/3 )1−x O3 (PZZN-1000x) close to x = 0.93 to 0.94 at room temperature. A new ferroelectric rhombohedral phase change, Fα –F'α , at low temperature is found and studied by the temperature dependence of the pyroelectric coefficient. Electrical poling in these ceramics is easy, and the coercive field Ec ∼8 to kV/cm is rather low. Samples with compositions in the range PZZN-86 to PZZN-92 have a large electromechanical coupling constant, k (kt and k15 ∼50% to 60%), and a low dielectric constant, ɛs (ɛT 33 /ɛ0 = 260 to 320, ɛT 11 /ɛ0 = 380). PZZN ceramics appear to be potential candidates for high-frequency ultrasonic transducers used in the thickness shear mode. The pyroelectric figure of merit (Fv ) of these ceramics is comparable to the values published for the PZT-based or PbTiO3 -based materials. 相似文献
16.
Phase equilibria of the La2 O3 -SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La2 O3 , and SrO. Stable intermediate phases are La2 CuO4 in the LaO1.5 -CuO binary and Sr2 CuO3 , SrCuO2 , and Sr14 Cu24 O41 in the CuO-SrO binary. The La2-x Sr x CuO4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La2-x Sr1+x Cu2 O6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La8-x Sr x Cu8 O20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La x Sr14-x -Cu24 O41 solid solution is stable where 0 ≤ x ≤ 6, and the La1+x Sr2-x Cu2 O5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La2 O3 -SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges. 相似文献
17.
Phase equilibria in the CaO-CuO system have been determined at 1 atm pressure in oxygen over the temperature range 800° to 1300deg; C. CaO is the stable phase at the calcium-rich end of the system. Two intermediate crystalline phases, namely Ca2 CuO3 and Ca3 Cu7 O10 , form. Caz CuO3 is stable up to 1085°± 3deg; C, where it melts incongruently to CaO + liquid (peritectic liquid has ε82% CuO). Ca3 Cu7 O10 becomes stable at 977°± 3deg; C by reaction between Ca2 CuO3 and CuO. Ca3 Cu7 O10 melts incongruently at 1046°± 3deg; C to Ca2 CuO3 + liquid (peritectic liquid has ε83% CuO). CuO is the stable copper oxide phase up to 1061°± 3deg; C at the copper-rich end of the system; at higher temperatures, Cu2 O is stable until the liquidus is reached at 1121deg; C. The binary eutectic is at 1045°± 5deg; C, in which a liquid with 83% CuO coexists with Ca3 Cu7 O10 and CuO. 相似文献
18.
Girish Harshé Dinesh Agrawal Santosh Limaye 《Journal of the American Ceramic Society》1994,77(7):1965-1968
The NaZr2 P3 O12 (NZP) family of materials is attracting increasing attention due to its low-thermal-expansion behavior. The system Ba1+x Zr4 P6–2x Si2x O24 (0 ≤ x ≤ 1), belonging to the NZP family, shows ultralow thermal expansion over a wide temperature range. It also shows anisotropy in its lattice thermal expansion. This causes microcracking as the sintered specimens are cooled, which results in degradation of the mechanical properties. In this work, the chemical stability, strength, and Young's modulus of Ba1+x Zr4 P6–2x O24 ( X = 0.25 and 0.5) ceramics at high temperatures have been determined. An attempt has been made to correlate the mechanical properties to the thermal expansion anisotropy. 相似文献
19.
Matthew J. Watson Helen M. Chan Martin P. Harmer Hugo S. Caram 《Journal of the American Ceramic Society》2005,88(12):3380-3387
The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1° C/min to 1100° C. Heat-treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30° C/mm) and compositional gradients (>5000 mol·(m3 ·mm)−1 ), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h. 相似文献
20.
Lijie Zhang Jie Chen Helen M. Chan Martin P. Harmer 《Journal of the American Ceramic Society》1989,72(10):1997-2000
In situ annealing studies of YBa2 Cu3 O6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa2 Cu3 O6+x grain boundaries. Quantitative chemical analysis using X-ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO2 in the atmosphere. 相似文献