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1.
The strengths of as-drawn, abraded, and pristine Se, As2 Se3 , As2 S3 , and Ge30 As15 Se55 fibers are reported. Pristine strength values range from an average of 80 MPa for Se and Ge30 As15 Se55 to 100 and 180 MPa for As2 Se3 and As2 S3 , respectively. Although these values are considerably higher than those reported previously for bulk samples, in no case does the maximum observed strength reach more than about 6% of that expected from simple theoretial calculations. Several factors that can control the strength are discussed. The overall behavior is interpreted in terms of the glass structures which permit localized stress relaxation effects in regions of weak, nondirectional bonds. These effects occur at temperatures well below the glass-transition region. 相似文献
2.
Adrish Ganguly Michel W. Barsoum Roger D. Doherty 《Journal of the American Ceramic Society》2007,90(7):2200-2204
In this work, we report on the interdiffusion of Ge and Si in Ti3 SiC2 and Ti3 GeC2 , as well as that of Nb and Ti in Ti2 AlC and Nb2 AlC. The interdiffusion coefficient, D int , measured by analyzing the diffusion profiles of Si and Ge obtained when Ti3 SiC2 –Ti3 GeC2 diffusion couples are annealed in the 1473–1773 K temperature range at the Matano interface composition (≈Ti3 Ge0.5 Si0.5 C2 ), was found to be given by
Dint increased with increasing Ge composition. At the highest temperatures, diffusion was halted after a short time, apparently by the formation of a diffusion barrier of TiC. Similarly, the interdiffusion of Ti and Nb in Ti2 AlC–Nb2 AlC couples was measured in the 1723–1873 K temperature range. The D int for the Matano interface composition, viz. ≈(Ti0.5 ,Nb0.5 )2 AlC, was found to be given by
At 1773 K, the diffusivity of the transition metal atoms was ≈7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter. 相似文献
D
At 1773 K, the diffusivity of the transition metal atoms was ≈7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter. 相似文献
3.
Jau-Ho Jean Chia-Ruey Chang Cheng-Dao Lei 《Journal of the American Ceramic Society》2004,87(7):1244-1249
Effects of Ag addition on sintering of a crystallizable CaO-B2 O3 -SiO2 glass have been investigated at 700°–900°C in different atmospheres. With Ag content increasing in the range of 1–10 vol%, the softening point, the densification, the onset crystallization temperature, and the total amount of crystalline phase formed of the crystallizable glass are reduced when fired in air. A bloating phenomenon is observed when the crystallizable CaO-B2 O3 -SiO2 glass doped with 1–10 vol% Ag is fired at 700°–900°C for 1–4 h. Fired in N2 or N2 + 1% H2 , however, the above phenomena disappear completely. It is thus believed that the diffusion of Ag into the crystallizable glass, which is caused by the oxidation of Ag in air, is the root cause for the above results observed. 相似文献
4.
R. J. Cava Eva M. Vogel David W. Johnson Jr. 《Journal of the American Ceramic Society》1982,65(9):c157-c159
The reported activation volume for sodium ion transport in the fast-ion conductor Na3 Zr2 Si2 PO12 is surprisingly large. Therefore, attempts were made to alter the size of the diffusion channels through partial substitution of As, Ge, Ti, Th, and Hffor the framework cations to decrease geometric structural hinderance of the diffusion process. Sodium ion conductivity is improved at temperatures below 200°C far the Hf substitutions. The existence and electrical properties of the apparently isostructural Na3 Hf2 Si2 PO12 , which has a room-temperature conductivity of 1.0Ω10−3 Ω cm−1 , are reported. 相似文献
5.
Fumiyasu Oba Kazuyoshi Tatsumi Isao Tanaka Hirohiko Adachi 《Journal of the American Ceramic Society》2002,85(1):97-100
First-principles calculations have been conducted to investigate impurities in cubic Si3 N4 and Ge3 N4 . Impurity species suitable for n - and p -type doping are suggested, in terms of the formation and ionization energies. The suggested species are P and O as n -type dopants and Al as a p -type dopant for c -Si3 N4 , and Sb and O as n -type dopants and Al as a p -type dopant for c -Ge3 N4 . The dependence of the formation energies on the chemical potentials indicates that a proper choice of growth conditions is mandatory for suppressing the incorporation of these impurities into anti and interstitial sites, where the impurities can be charged to compensate carriers. 相似文献
6.
In order to study the micromechanism of silver migration that influences the dielectric properties and reliability of Zn0.95 Mg0.05 TiO3 +0.25TiO2 (ZMT") with 1 wt% 3ZnO–B2 O3 multilayer ceramic capacitors (MLCCs), various silver (Ag)–palladium (Pd) ratios of conductors were used as inner electrodes. It was found that the electrical resistance of a MLCC sample with pure Ag as inner electrodes was degraded drastically to compared with the Ag/Pd inner electrodes at measuring temperatures ranging from 25°C to 175°C. It may be explained that the pure Ag migrates easily into the dielectric layer along the grain boundary during co-firing. The ZMT" MLCCs exhibited increasing dielectric constant and insulation resistance considerably with increasing sintering temperature. Moreover, the results also indicate that Ag diffusion changes the dielectric properties and decreases the breakdown voltage. A ZMT" MLCC with a high Ag content in the inner electrode exhibits poor reliability, and the effect of Ag+ migration is markedly enhanced when the activation energy of the ZMT" dielectric is considerably lowered due to the excessive formation of oxygen vacancies and the semiconducting Zn2 TiO4 phase when Ag+ substitutes for Zn2+ during co-firing. 相似文献
7.
Steven A. Markgraf Shiv K. Sharma Amar S. Bhalla 《Journal of the American Ceramic Society》1992,75(9):2630-2632
Raman spectra are reported for fresnoite (Ba2 Ti(Si,Ge)2 O8 glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba2 TiSi2 O8 and Ba2 TiGe2 O8 . The Ba2 TiGe2 O8 glass spectra show correspondence with the Ba2 TiGe2 O8 crystalline Raman spectra; the v s (Ge–O–Ge) mode occurs at 518 cm−1 in the glass and at 521 cm−1 in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba2 TiGe2 O8 glass as revealed by a strong band at 824 cm−1 in the I glass spectrum. The Ba2 TiSi2 O8 glass spectra are similar to the Ba2 TiSi2 O8 crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba2 TiSi2 O8 glass has a structure similar to the crystalline phase. 相似文献
8.
The microstructure of (Sr,Ca)TiO3 capacitor-varistor materials has been investigated by employing electron microscopy techniques (TEM, STEM, HREM, EDX, and EPA). The material is found to contain (Sr,Ca)TiO3 grains (∼30 μm) having perovskite crystal structure with domains, a Na+ -diffused layer at the grain boundaries which is dependent on thermal diffusion conditions, and multiple-grain junctions in which the Ti n O2n–1 Magneli phase coexists with an amorphous intergranular phase. In addition, wider grain boundaries (10–30 nm), thin grain boundaries (∼1 nm), and clean grain boundaries which are free from intergranular phase were observed, and the effects of different grain boundaries on the diffusion of Na+ are discussed. 相似文献
9.
Osamu Yamaguchi Koji Sugiura Kiyoshi Shimizu 《Journal of the American Ceramic Society》1985,68(4):110-C
A metastable modification of Y2 GeO5 is formed from an amorphous material prepared by the simultaneous hydrolysis of yttrium and germanium alkoxides. It has an orthorhombic unit cell with a =0.6068 nm, b =1.0695 nm, and c = 1.1994 nm. The chemical structure is described by the formula Y4 (Ge2 O6 )O4 . 相似文献
10.
Kuan Zong Fung Anil V. Virkar David L. Drobeck 《Journal of the American Ceramic Society》1994,77(6):1638-1648
Cubic solid solutions in the Y2 O3 -Bi2 O3 system with ∼25% Y2 O3 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 Y2 O3 -Bi2 O3 system is a massive transformation. Samples of composition 25% Y2 O3 -75% Bi2 O3 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
11.
Pottayil Uma Devi Muralidharan Kannai V. Paulose Jacob Koshy Alathoor D. Damodaran 《Journal of the American Ceramic Society》1991,74(10):2679-2682
The percolation behavior of normal-state thermopower, resistivity, and superconductivity have been studied in YBa2 Cu3 O7-δ ─Ag (YBCO─Ag) composite systems. The normal-state resistivity and thermopower show a percolation threshold at a Ag volume fraction ( V Ag ) of 20% to 30%, whereas the superconducting network shows a threshold at a V Ag of ≅70% to 80%. The results obtained from this study show that the YBCO─Ag composite obtained from Ag2 O and YBCO powders is uniformly distributed with Ag and YBCO remaining as separate phases without changing their characteristics. The measurements of thermopower indicate that the normal-state thermopower and phonon-drag thermopower are affected by the Ag addition. The stability of YBCO is increased when it is in composite form. 相似文献
12.
Ana E. Bohé Julio Andrade-Gamboa Daniel M. Pasquevich Alfredo J. Tolley Jorge L. Pelegrina 《Journal of the American Ceramic Society》2000,83(4):755-760
The microstructure of ZrO2 fine particles produced by a novel synthesis method at 450° and 950°C has been studied. The fundamentals of the synthesis method, which involves both chemical and diffusion phenomena, are presented. The method is based on mass transport through the gaseous phase between metallic zirconium and Fe2 O3 powder. The mass-transporting chemical species are zirconium and iron chlorides. This article focuses on the microstructure and structure of ZrO2 particles formed by the reaction between gaseous ZrCl4 and solid Fe2 O3 , which is a relevant reaction step that occurs during the synthesis process. The resulting ZrO2 crystals grown on Fe2 O3 particles have been analyzed using transmission electron microscopy. Microstructural characterization has been complemented by X-ray diffractometry analysis. Tetragonal-ZrO2 is produced at 450°C and monoclinic-ZrO2 single crystals are produced at 950°C. 相似文献
13.
The system HfO2 -TiO2 was studied in the 0 to 50 mol% TiO2 region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO2 was found at 1750°± 20°C. The definite compound HfTiO4 melts incongruently at 1980°± 10°C, 53 mol% TiO2 . A metatectic at 2300°± 20°C, 35 mol% TiO2 was observed. The eutectoid decomposition of HfO2,ss ) ( T ) → HfO2,ss ( M ) + HfTiO3 4,ss ss occurred at 1570°± 20°C and 22.5 mol% TiO2 . The maximum solubility of TiO2 in HfO2,ss ,( M ) is 10 mol% at 1570°± 20°C and in HfO2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO2 -rich side and in the 10 to 30 mol% TiO2 range a second monoclinic phase M of HfO2 ( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO2 region, which are based on thermal analysis techniques. 相似文献
14.
I. D. AGGARWAL C. T. MOYNIHAN P. B. MACEDO J. J. MECHOLSKY G. R. SRINIVASAN 《Journal of the American Ceramic Society》1972,55(7):366-368
The glass-forming regions were investigated for pseudobinary systems of the composition X M m Se n -(1– X )Ge1.5 As0.5 Se3 (M m Se n =PbSe, CdSe, BaSe, ZnSe, or La2 Se3 ). Systems containing PbSe, CdSe, and BaSe contain substantial regions of glass formation, and differential scanning calorimetry, electron microscopy, and water corrosion tests were used to characterize these glasses. The glasses were generally phase-separated, and the available data suggest that for PbSe melts the region of liquid-liquid immiscibility spans almost the entire pseudobinary composition range. A few studies of glass formation were conducted for CdSe-M m Se n -Ge1.5 As0.5 Se3 pseudoternary and M m Se n -GeSe2 binary melts. 相似文献
15.
JONG-HEE PARK WAYNE E. KING STEVEN J. ROTHMAN 《Journal of the American Ceramic Society》1987,70(12):880-885
The cation diffusivities in the lattice and along dislocations and grain boundaries have been measured on sintered polycrysals of Cr2 O3 ; and Cr2 Cr2 O3 -0.09 wt% Y2 O3 at1100°C and at the pO2 corresponding to that of Cr/Cr2 O3 equilibrium at that temperature. Results for lattice and dislocation diffusivities in pure Cr2 O3 are in good agreement with previous work. The present results indicate that yttrium additions have negligible effect on lattice and dislocation diffusion. However, grain-boundary diffusion in pure Cr2 O3 is significantly slower than grain-boundary diffusion in Cr2 O3 -0.09 wt% Y2 O3 . The results are discussed in terms of their implications for the reactive-element effect in high-temperature oxidation of chromium-containing alloys. 相似文献
16.
L. J. BOWEN T. G. CARRUTHERS R. J. BROOK 《Journal of the American Ceramic Society》1978,61(7-8):335-359
The rates of densification and phase transformation undergone by α-Si3 N4 during hot-pressing in the presence of Y2 O3 , Y2 O3 −2SiO2 , and Li2 0−2Si02 as additives were studied. Although these systems behave less simply than MgO-doped Si3 N4 , the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si3 N4 phase. 相似文献
17.
The influence of Nd2 O3 doping on the reaction process and sintering behavior of BaCeO3 is investigated. Formation of BaCeO3 is initiated at 800°C and completed at 1000°C. When Nd2 O3 is added to the starting materials, the formation of BaCe1–x Ndx O3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe1-x Ndx O3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd2 O3 content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO3 is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO3 with Nd2 O3 markedly enhances its sinterability. The enhancement of the sinterability of Nd2 O3 -doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering. 相似文献
18.
The influence of crystal volume fraction on fracture toughness ( K I C ) and indentation strength was analyzed for Li2 O-Al2 O3 -CaO-SiO2 (LACS) and LACS glass-ceramics containing 0.58 mmol% AgNO3 (LACS-0.58Ag) or 0.78 mmol% AgNO3 (LACS-0.78Ag). The mean flexure strength, indentation strength, and KI C values of the LACS-0.78Ag groups increased with volume fraction of crystallinity. To achieve the greatest strength and KI C in LACS-Ag specimens, a high volume fraction of crystallinity (95%) had to be produced. However, the relationship between volume fraction of crystal phase and translucency had to be analyzed to determine the influence of crystallization on the potential esthetic results that are essential for dental applications. Addition of AgNO3 to LACS glass produced a change from surface crystallization to bulk crystallization. 相似文献
19.
The phase relations for the Sc2 O3 -Ta2 O5 system in the composition range of 50-100 mol% Sc2 O3 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 Sc5.5 Ta1.5 O12 phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO4 , and Sc2 O3 were found in the system. The Sc5.5 Ta1.5 O12 phase formed in 78 mol% Sc2 O3 at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc2 O3 and decomposed to Sc5.5 Ta1.5 O12 and ScTaO4 at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc2 O3 . The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc2 O3 . 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
20.
Sea-Fue Wang Chiang-Chuang Chung Chai-Hui Wang Jinn P. Chu 《Journal of the American Ceramic Society》2002,85(6):1619-1621
Preparation of dense and phase-pure Ba2 Ti9 O20 is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba2 Ti9 O20 equilibrium phase boundary in the BaO–TiO2 system at high temperatures. In this study, the effects of B2 O3 on the densification, microstructural evolution, and phase stability of Ba2 Ti9 O20 were investigated. It was found that the densification of Ba2 Ti9 O20 sintered with B2 O3 was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B2 O3 . With the addition of 5 wt% B2 O3 , the ceramic yielded a pure Ba2 Ti9 O20 phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO2 or ZrO2 . The facilities of B2 O3 addition to the stability of Ba2 Ti9 O20 are apparently due to the eutectic liquid phase which accelerates the migration of reactant species. 相似文献