共查询到20条相似文献,搜索用时 468 毫秒
1.
Hisanori Tsuboi Kohei Soga Hiroyuki Inoue Akio Makishima 《Journal of the American Ceramic Society》1998,81(5):1197-1202
Eu2+ -complex-doped SiO2 gels were prepared and their fluorescence properties were studied. SiO2 gels were obtained by the reactions of silicon tetrachloride, tert -butyl alcohol, and methyl alcohol under nitrogen. Both to protect Eu2+ against oxidation and to enhance the emission intensity of Eu2+ , Eu2+ was complexed with 15-crown-5 or its derivative. Samples showed blue emission. Fluorescence lifetimes of samples varied from 690 to 955 ns depending on the Eu2+ concentration at 290 K. After drying of samples, 50-70% of Eu2+ remained in the divalent state without oxidation in the samples. It was found that Eu2+ complexes inside gels were stable against oxidation in air. 相似文献
2.
This study shows that the amount ofAl2 O3 needed to form high density Si3 N4 -15Y2 -O3 samples can be reduced by using high surface area Si3 N4 powder and high N2 overpressure (high sintering temperatures) during the sintering process. The reduction in AI2 O3 content results in improved oxidation resistance of the sintered samples. 相似文献
3.
Inna G. Talmy James A. Zaykoski Mark M. Opeka 《Journal of the American Ceramic Society》2008,91(7):2250-2257
The effect of Si3 N4 , Ta5 Si3 , and TaSi2 additions on the oxidation behavior of ZrB2 was characterized at 1200°–1500°C and compared with both ZrB2 and ZrB2 /SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB2 was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si3 N4 -modified ceramics increased with increasing Si3 N4 content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB2 , with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta5 Si3 , 30 vol% TaSi2 , 35 vol% Si3 N4 , or 20 vol% Si3 N4 with 10 mol% CrB2 . These materials exceeded the oxidation resistance of the ZrB2 /SiC ceramics below 1300°–1400°C. However, the ZrB2 /SiC ceramics showed slightly superior oxidation resistance at 1500°C. 相似文献
4.
Linus U. J. T. Ogbuji 《Journal of the American Ceramic Society》1992,75(11):2995-3000
The results of two-step oxidation experiments on chemically-vapor-deposited Si3 N4 and SiC at 1350°C show that a correlation exists between the presence of a Si2 N2 O interphase and the strong oxidation resistance of Si3 N4 . During normal oxidation, k p for SiC was 15 times higher than that for Si3 N4 , and the oxide scale on Si3 N4 was found by SEM and TEM to contain a prominent Si2 N2 O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidation k p increases over 55-fold for Si3 N4 , and 3.5-fold for SiC; the Si3 N4 and SiC oxidize with nearly equal k p 's; and, most significant, the oxide scale on Si3 N4 is found to be lacking an inner Si2 N2 O layer. The implications of this correlation for the competing models of Si3 N4 oxidation are discussed. 相似文献
5.
The compressive creep behavior and oxidation resistance of an Si3 N4 /Y2 Si2 O7 material (0.85Si3 N4 +0.10SiO2 +0.05Y2 O3 ) were determined at 1400°C. Creep re sistance was superior to that of other Si3 N4 materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10−11 kg2 ·m-4 ·s−1 indicates excellent oxidation resistance. 相似文献
6.
Mats Carlsson Mats Johnsson Mats Nygren 《Journal of the American Ceramic Society》1999,82(8):1969-1976
Ta0.33 Ti0.33 Nb0.33 C and Ta0.33 Ti0.33 Nb0.33 C x N1− x whiskers were synthesized via a carbothermal vapor-liquid-solid growth mechanism in the temperature range 900°-1450°C in Ar or N2 . The optimum temperature was 1250°C. Whiskers were obtained in a yield of 70-90 vol%. The whiskers were 0.5–1 µm in diameter and 10–30 µm in length. The starting materials that produced the highest whisker yield were: TiO2 , Ta2 O5 , Nb2 O5 , C, Ni, and NaCl. C was added to reduce the oxides, and Ni to catalyze whisker growth. NaCl was used as a source of Cl for vapor-phase transportation of Ta and Nb oxochlorides and Ti chlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl2 ( g ) according to thermodynamic calculations. The rate of formation and the chemical composition of the whiskers depended on the synthesis temperature, the choice of catalyst, and the atmosphere. At low temperatures, the whiskers were enriched in Nb and Ta, whereas the Ti content increased with increased synthesis temperature. 相似文献
7.
S. Shimada T. Onuma H. Kiyono M. Desmaison 《Journal of the American Ceramic Society》2006,89(4):1218-1225
Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O2 (dry oxidation), Ar/O2 /H2 O (wet oxidation), and Ar/H2 O (H2 O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H2 O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO2 scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H2 O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO2 and H2 with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO2 scale accompanied CO2 evolution, and the H2 O-containing oxidation produced a small amount of H2 . A piece of single crystal TiC was oxidized in 16 O2 /H2 18 O to reveal the contribution of O from H2 O to the oxidation of TiC by secondary ion mass spectrometry. 相似文献
8.
Novel Mg2 Zr5 O12 /Mg2 Zr5 O12 –ZrO2 –MgF2 Gradient Layer Coating on Magnesium Formed by Microarc Oxidation
A novel Mg2 Zr5 O12 -based coating on magnesium was formed by microarc oxidation (MAO) in a K2 ZrF6 -containing electrolyte. The structure of the coating was examined by X-ray diffraction using the grazing angle method, scanning electron microscopy, and transmission electron microscopy. The friction and wear properties of the MAO-coated and -uncoated Mg samples were evaluated in a ball-on-disk testing system. The corrosion resistance of the coating in a 3.5% NaCl solution was investigated by the potentiodynamic polarization test. The coating is relatively dense and composed of a Mg2 Zr5 O12 –ZrO2 –MgF2 inner layer and a nanocrystalline Mg2 Zr5 O12 outer layer with a maximum hardness of 1240 Hv. The friction coefficient of the coating against Si3 N4 is 0.35 under a dry-sliding condition. The corrosion resistance of the magnesium substrate is improved considerably by MAO treatment. The corrosion potential of the Mg2 Zr5 O12 -coated sample is −1.43 V with a current density as low as 7.06 × 10−8 A/cm2 . It is expected that the coating can considerably protect magnesium from wear and corrosion. 相似文献
9.
Hyoungjoon Park Hae-Won Kim Hyoun-Ee Kim 《Journal of the American Ceramic Society》1998,81(8):2130-2134
The oxidation behaviors of monolithic Si3 N4 and nanocomposite Si3 N4 -SiC with Yb2 O3 as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb2 Si2 O7 and SiO2 (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si3 N4 -SiC compared to the monolithic Si3 N4 . The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb3+ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented. 相似文献
10.
Shara S. Shoup ‡ Carlos E. Bamberger § Richard G. Haire 《Journal of the American Ceramic Society》1996,79(6):1489-1493
The compounds SrPu2 Ti4 O12 , Pu2 Ti3 O8.79 , and Pu2 Ti2 O7 , where plutonium is in the (III) oxidation state, were prepared and identified via X-ray diffraction (XRD). The solid solubility limit of Pu2 Ti2 O7 in Ln2 Ti2 O7 (Ln = Gd, Er, or Lu) was also studied via XRD; it was determined that the solubility of Pu2 Ti2 O7 increased as the radius of the lanthanide ion in the host compound decreased. Attempts to synthesize Sr2 Pu2 Ti5 O16 and Sr2 Ce2–y Puy Ti5 O16 solid solutions, where plutonium is in the (IV) oxidation state, were unsuccessful. 相似文献
11.
Composite ceramic materials based on Si3 N4 and ZrO2 stabilized by 3 mol% Y2 O3 have been formed using aluminum isopropoxide as a precursor for the Al2 O3 sintering aid. Densification was carred out by hot-pressing at temperatures in the range 1650° to 1800°C, and the resulting micro-structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si3 N4 matrix with consequent high room-temperature toughness and strength. Decomposition of the ZrO2 grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO2 incorporated some N during densification. 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.
Specimens of ZrB2 containing various concentrations of B4 C, SiC, TaB2 , and TaSi2 were pressureless-sintered and post-hot isostatic pressed to their theoretical densities. Oxidation resistances were studied by scanning thermogravimetry over the range 1150°–1550°C. SiC additions improved oxidation resistance over a broadening range of temperatures with increasing SiC content. Tantalum additions to ZrB2 –B4 C–SiC in the form of TaB2 and/or TaSi2 increased oxidation resistance over the entire evaluated spectrum of temperatures. TaSi2 proved to be a more effective additive than TaB2 . Silicon-containing compositions formed a glassy surface layer, covering an interior oxide layer. This interior layer was less porous in tantalum-containing compositions. 相似文献
14.
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 Dy2 O3 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 Gd2 O3 which are considerably below literature values for monoclinic single-crystal Gd2 O3 . 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 10−7 Ω−1 cm−1 . 相似文献
15.
An intimate Ba-Al-Al2 O3 -SiO2 powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl2 Si2 O8 . Barium oxidation was completed, and a binary silicate compound, Ba2 SiO4 , had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl2 O4 had formed. Diffraction peaks consistent with hexagonal BaAl2 Si2 O8 , BaAl2 O4 , β-BaSiO3 , and possibly β-BaSi2 O5 were detected after 24 h at 900°C. Diffraction peaks for BaAl2 O4 and BaAl2 Si2 O8 were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl2 O4 at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl2 Si2 O8 . The observed reaction path was compared to prior work with other inorganic precursors to BaAl2 Si2 O8 . 相似文献
16.
Xuguang Tong Jianbao Li Xiaozhan Yang Hong Lin Gangfeng Guo Mingsheng He 《Journal of the American Ceramic Society》2006,89(5):1730-1732
In this work, self-reinforced silicon nitrides with β-Si3 N4 seeds doped with Re2 O3 (Re=Yb, Lu) were investigated. Firstly, the two kinds of seeds were obtained by heating α-Si3 N4 powder with Yb2 O3 or Lu2 O3 , respectively. Then the self-reinforced silicon nitride ceramics were prepared by HP-sintering of α-Si3 N4 powder, Re2 O3 as additive, and the as-prepared seeds. Oxidation test was carried out at 1400°C in air for 100 h with thermogravimetry analysis (TGA) measurement. Mechanical properties, scanning electronic microscopy microstructures, and X-ray diffraction patterns were measured before and after oxidation. The results indicated that the introduction of the seeds doped with Re2 O3 (Re=Yb, Lu) could obviously increase the toughness and keep the room temperature and high-temperature strength of the ceramics at high values. After oxidation, the crystalline phase in grain boundary changed and the mechanical properties decreased. TGA showed a parabolic weight gain and the oxidation mechanism was discussed. 相似文献
17.
Wei-Hsing Tuan Shi-Ming Liu Chan-Ju Ho Tsong-Jen Yang 《Journal of the American Ceramic Society》2006,89(2):754-758
Previous studies demonstrated that the strength of zirconia (ZrO2 ) could be enhanced or reduced by respectively adding micrometer-sized alumina (Al2 O3 ) or nickel (Ni) particles. In the present study, 5 vol% micrometer-sized Al2 O3 particles and 1 vol% nanometer-sized Ni particles are incorporated into the ZrO2 matrix, which is subsequently densified by pressureless sintering. The biaxial strength of the ZrO2 /(Ni+Al2 O3 ) nanocomposite is nearly double that of the monolithic ZrO2 . The increase in strength correlated with a reduction in the critical flaw size and not with any change in toughness, which may be a result of grain boundary strengthening. 相似文献
18.
Air oxidation of (U, Pu)O2 solid solutions having Pu/(U+Pu) ratios of 0.2 and 0.25 was studied using a recording thermal balance. For Pu/(U+Pu)=0.2, the original fluorite MO2 phase oxidized first to tetragonal M3 O7 ; further oxidation at 450°C< T <600°C gave a two-phase mixture of M'3 O8 and a cubic phase having a0 ≃5.43 Å. Kinetic analysis of these oxidation stages indicated that the Erofeev nucleation model explains the data reasonably well, with Δ H ≃40 kcal/mol for the formation of M3 O7 and Δ H ≃40 kcal/mol for the oxidation of M3 O7 to M'3 O8 + the cubic phase. Oxidation above M3 O7 (O/M=2.35) resulted in mechanical disintegration of the samples because of the presence of the M'3 O8 phase. For Pu/(U+Pu) = 0.25, both isothermal and programed air oxidation of homogeneous samples up to 800°C led to a compound with a maximum O/M≃2.35, i.e. M4 O9.4 , an fcc phase having a0 =5.428 Å; these samples were not converted to powder by the oxidation. 相似文献
19.
Electron paramagnetic studies showed that Ti3+ and Fe2+ occur in mullites taken from a refractory material which was fused-cast under a reducing atmosphere. Exposure of the mullite samples to temperatures >1600°C caused oxidation of Ti3+ and Fe2+ to Ti4+ and Fe3+ , respectively. 相似文献
20.
F. F. LANGE S. C. SINGHAL R. C. KUZNICKI 《Journal of the American Ceramic Society》1977,60(5-6):249-252
Composite powders were hot-pressed to determine the phase relations in the Si3 N4 -SiO2 -Y2 O3 pseudoternary system. Four quaternary compounds, Si3 Y2 O3 N4 , YSiO2 N, Y10 Si7 O23 N4 , and Y4 Si2 O7 N2 , were identified. Studies of polyphase and single-phase materials in this system showed that these 4 compounds are unstable under oxidizing conditions. Materials within the Si3 N4 -Si2 N2 O-Y2 Si2 O7 compatibility triangle precluded the unstable compounds, and are extremely resistant to oxidation. 相似文献