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1.
Anorthite-glass films were grown on basal Al2O3 substrates using pulsed-laser deposition. The substrates were cleaned and annealed in air at 1400°C to produce crystallographically flat (0001) terraces. The films were deposited in an oxidizing environment. X-ray microanalysis confirmed the composition of the glass films to be close to that of anorthite (CaO·Al2O3·2SiO2). Although anorthite usually has triclinic symmetry, subsequent crystallization of these films in air at 1200°C resulted in the formation of pseudo-orthorhombic CaAl2Si2O8 ( o -anorthite), a known metastable form of the mineral. Microstructural characterization was performed using visible-light microscopy, scanning electron microscopy, and transmission electron microscopy. The films dewetted the substrate either before or after crystallization to form o -anorthite islands which had strong orientation relationships to the Al2O3 substrate. The epitaxy of the o -anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. The formation of three orientational variants is consistent with the symmetry of the basal Al2O3 surface. The dislocation network observed at the o -anorthite/Al2O3 interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacial amorphous layer. The study of anorthite-glass films is important because they are present in liquid-phase-sintered Al2O3, and may be devitrified by postsintering heat treatments.  相似文献   

2.
Hexagonal anorthite (CaAl2Si2O8) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca3Al2O6·6H2O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO2. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO2 substitution has been shown to reduce the conversion of hydrogarnet to Ca4Al6O13·3H2O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl2Si2O8 as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.  相似文献   

3.
Current state-of-the-art environmental barrier coatings (EBCs) for Si-based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al2O3·2SiO2) or mullite + BSAS ((1− x )BaO· x SrO·Al2O3·2SiO2, 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long-term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt-infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low-melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water-vapor-containing atmospheres. Both reactions can be EBC life-limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.  相似文献   

4.
The stability of lanthanum orthophosphate (LaPO4) on SiC was investigated using a LaPO4-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO4 is reduced in the presence of SiC and reacts to form La2O3 or La2Si2O7 and SiO2 as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO2 film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO4 coating. Continuous LaPO4 coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO4 coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO2 oxidation product to penetrate the coating.  相似文献   

5.
Reaction couples between α-SiC and cordierite (2MgO·2Al2O3·5SiO2═ Mg2Al4Si5O18) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O2→ SiO2+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.  相似文献   

6.
Microstructural characterization of a high-Al2O3 substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al2O3 grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO2 and reduced rutile (TiO2- x ). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al2O3 grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl2Si2O8), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α-cristobalite (SiO2), and α-cordierite (Mg2Al4Si5O18). In addition, small rutile particles precipitated within the Al2O3 grains.  相似文献   

7.
SiC whiskers were added to reinforce a gel-derived SiO2–Al2O3–Cr2O3 matrix. Alumina and chromia were added to induce reaction sintering of the composites. A maximum of ∼90% of theoretical density was achieved after firing in argon for 2 h at 1400°C. The optimum amounts of alumina, chromia, and SiC were 26.5, 1.6, and 4.1 vol%, respectively. It was shown that the chromium acetate hydroxide, used as a precursor for chromia, acted as a gelling agent and as a sintering additive. The effect of producing gaseous products such as SiO, CO and/or CrO3 became significant above 1400°C where mullite formation also occurred. However, the measured mechanical properties were remarkable, considering the experimental conditions applied.  相似文献   

8.
A glass with the eutectic composition 3MnO1.5–2SiO2 was used to simulate the formation of a liquid phase during sintering of BaTiO3. Two oxide additives (Mn2O3 and SiO2) performing various functions of the properties of BaTiO3 were investigated for their crystallization and thermal characteristics at temperatures ≤1400°C. The wetting behavior of the glass, the dissolution of BaTiO3 in glass melt, the identification of newly formed phases, and the sequential reaction kinetics of the glass/BaTiO3 system, especially when isothermally treated at 1150°C, were investigated by electron microscopy with quantitative X-ray energy dispersive spectroscopic (Q-EDS) analysis. The evolution of the interfacial reaction of the glass/BaTiO3 at 1150°C is reported and discussed.  相似文献   

9.
The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H2 with various P H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO2 layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO2 layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO2 inherent in the fibers.  相似文献   

10.
Mullite (3Al2O3·2SiO2) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm−1. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO2 gradually forms during prolonged heating or at higher temperatures.  相似文献   

11.
The melt infiltration method was used to fabricate a SiC-mullite composite at high temperature. Mullite was successfully obtained from a SiO2 and Al2O3 powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO2 and leaving Al2O3 and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.  相似文献   

12.
Stoichiometric albite glass (NaAlSi3O8) was seeded with 5 wt% crystalline anorthite (CaAl2Si2O8) to make albite glass-ceramics. The epitaxial crystallization of the albite glass to the glass-ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO4), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.  相似文献   

13.
Diffusion bonding of dense Si3N4 containing 10% Ce2O3, 7% A12O3, and 2% Y2O3 was attempted through hot pressing at temperatures of 1000° to 1400°C. Physically vapor-deposited (PVD) carbon and paraffin-wax candle soot were tried as interlayers. The interfaces were analyzed through optical and scanning electron microscopy, and through X-ray photoelectron spectroscopy. The bonding temperature could be reduced from 1400° (without interlayers) to 1100°C with PVD carbon or candle soot. From C Is and Si 2p spectra, the formation of SiC at the interface through the reaction of C with the silicon oxynitride layer has been identified.  相似文献   

14.
Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, TH, and methyldiethoxysilane, DH. Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of TH/DH= 1, the formation of a carbon-rich glass was observed, whereas a ratio of TH/DH= 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although TH/DH= 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, TH/DH= 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC x O2(1-x)→ x SiC + (1 - x )SiO2, when annealed at temperatures exceeding 1200°C.  相似文献   

15.
The structure of mullite, which has a composition ranging from 3Al2O3·2SiO2 to Al2O3·2SiO2, contains ordered oxygen vacancies. Sillimanite, Al2O3·SiO2, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al2O3·2SiO2) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al2O3·SiO2) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation.  相似文献   

16.
Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al2O3 and up to 1620°C for pure molten B2O3. Large expansion coefficient decreases and a slight activation energy increase for B2O3 above 1400°C suggested a tightening of its structure. The addition of Al2O3 reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO2 additions) and the increased expansion coefficients accompanying Al2O3 additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO2 and GeO2 additions at 1300°C. Microclustering of aluminum-bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO2 and GeO2 additions.  相似文献   

17.
With the addition of 1 wt% of MgO–Al2O3–SiO2 glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al2O3 spinel and an amorphous phase that contains Al2O3 and SiO2 (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al2O3 spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.  相似文献   

18.
The development of the carbon-rich interphase in Nicalon SiC fiber/Li2O-Al2O3–SiO2 glass matrix composites was examined as a function of processing parameters with the use of high-resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot-pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon-rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon-rich interphase followed a diffusion-controlled model, with an activation energy of 25.4 kcal/mol.  相似文献   

19.
The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si3N4/BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B2O3 liquid phase. With increasing exposure temperature, the Si3N4 cells began to oxidize, forming crystalline Y2Si2O7, SiO2, and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B2O3 liquid. After exposure at 1400°C, large Y2Si2O7 crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained.  相似文献   

20.
Submicrometer SiO2-Al2O3 powders with compositions of 46.5 to 76.6 wt% Al2O3 were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al2O3 than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO2-Al2O3 system. Particle size decreased with increasing Al2O3 content. The sintered densities depended upon the amount of SiO2-rich glassy phase formed during sintering and the green density expressed as a function of particle size.  相似文献   

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