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1.
Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO 3/SrCe 0.95Yb 0.05O 3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO 3) and 1200 °C (SrCe 0.95Yb 0.05O 3−δ). Dense SrCeO 3 and SrCe 0.95Yb 0.05O 3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO 3 and 1–2 μm for SrCe 0.95Yb 0.05O 3−δ. The electrical conductivity of SrCe 0.95Yb 0.05O 3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere. 相似文献
2.
Layered -titanate materials, Na xM x/2Ti 1−x/2O 2 (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na 0.4Ni 0.2Ti 0.8O 2 were ca. 7×10 3 S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm −1 K −1 at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na 0.4Ni 0.2Ti 0.8O 2 and Na 0.4Co 0.2Ti 0.8O 2 was about 1.9×10 −4 and 1.2×10 −4 K −1 around 700 °C, respectively. 相似文献
3.
Five Si 3N 4---Al 2O 3 ceramic grades were prepared by hot pressing at 1650°C. Backscattered electron micrographs revealed four different phases. Quantitative electron probe microanalysis allowed the identification of these phases as X-sialon, -Al 2O 3, O'-sialon and β-sialon. The maximum solubility of Al 2O 3 in Si 3N 4 and in Si 2N 2O at 1650°C was determined as well as the chemical composition of X-sialon. Based on these results, a slightly revised phase diagram is proposed. The general features describing the microstructure of the various phases have been investigated by transmission electron microscopy. The influence of the various phases on the mechanical properties was investigated; hardness, fracture toughness and elastic modulus were measured. The oxidation behaviour has been studied in air at 1300°C and 1450°C. The metastable phase diagram of Al 2O 3---SiO 2 in the absence of mullite can be used to predict the oxidation products and relative amounts formed in the oxide layers. 相似文献
4.
Alumina composites have been prepared with particle inclusions of 0–30 wt% titanium carbonitride and/or 0–5 wt% nickel, or nickel plus molybdenum metal. The metal was added in three different ways; as metal powder, as metal oxide, or as the intermetallic compound Ti 2Ni. Pressureless sintering at 1750°C gave densities varying from 90% of theoretical density to full density. All materials were post-HIPed to full density at 1600°C before measurement of mechanical properties. Addition of metal alone increased the fracture toughness from 3·0 to 3·7 MPam1/2, but decreased the Vickers hardness, HV 10, from 1650 to 1500. The simultaneous addition of hard titanium carbonitride inclusions compensated for the decrease in hardness and gave a further increase in fracture toughness. The alumina composites with 5 wt% metal and 30 wt% Ti(C,N) inclusions had a hardness of 1800 and a fracture toughness of about 5 MPam1/2. 相似文献
5.
β-sialon and Nd 2O 3-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO 2 (3 mol% Nd 2O 3) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO 2(Nd 2O 3) or ZrN were seen in both the HIPed β-sialon/ZrO 2(Nd 2O 3) composites and the HIPed Nd 2O 3-stabiIized α-sialons with ZrN additions. 相似文献
6.
Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF 2 and YF 3 as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F 2 and Ca 12Al 14O 32F 2. Liquid CaYAlO 4 mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F 2 at a relatively low temperature results in homogeneous YF 3 distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity. 相似文献
7.
Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba 2Si 12−xAl xO 2+xN 16−x ( x = 20.2). Appropriate amount of the precursor powders (BaCO 3, -Si 3N 4, AlN, Al 2O 3) with a targeted composition of BaAlSi 5O 2N 7 was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘ c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m 1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms. 相似文献
8.
Platelike CaTiO 3 particles with an orthorhombic perovskite structure have been synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of the layer-structured CaBi 4Ti 4O 15 at 950 °C. The CaTiO 3 particles inherited and retained the shape of the precursor particles with a thickness of approximately 0.3 μm, and a width of 2–6 μm. XRD analysis showed that in the TMC reaction, the crystallographic {0 0 1} plane of CaBi 4Ti 4O 15 is converted into the {1 0 0} plane of CaTiO 3. Using the platelike CaTiO 3 particles as templates in the templated grain growth method, dense {1 0 0} grain-oriented CaTiO 3 ceramics having a {1 0 0} orientation could be fabricated at sintering temperatures between 1350 and 1500 °C. The maximum orientation factor reached 99.7% at 10% of template. It was found that texturing improves microwave dielectric low-loss properties, providing a 1.55 times higher Qf value of 9310 GHz in textured ceramics compared to that of 6005 GHz in non-textured ceramics. 相似文献
9.
Partial oxidation of methane to synthesis gas was carried out using supported iridium–nickel bimetallic catalysts, in order to reduce loading levels of iridium and nickel, and to avoid carbon deposition on nickel-based catalysts by adding iridium. The performance of supported iridium–nickel bimetallic catalysts in synthesis gas formation depended strongly upon the support materials. La 2O 3 gave the best performance among the support materials tested. Ir(0.25 wt%)–Ni(0.5 wt%)/La 2O 3 afforded 36% conversion of methane (CH 4/O 2=5) to give CO and H 2 with the selectivities of above 90% at 800°C, and those at 600°C were 25.3% conversion of methane and CO and H 2 selectivities of about 80%, respectively. Reduced monometallic Ir(0.25 wt%)/La 2O 3 and Ni(0.5 wt%)/La 2O 3 catalysts did not produce synthesis gas at 600°C. A higher conversion of methane was obtained by synergistic effects. The product concentrations of CO, H 2, and CO 2, and CH 4 conversion were maintained in high values, even increasing the space velocity of feed gas over Ir–Ni/La 2O 3 catalyst, indicating that rapid reaction takes place. As a by-product, a small amount of carbon deposition was observed, but carbon formation decreased with increasing the space velocity. On the other hand, with reduced monometallic Ni(10 wt%)/La 2O 3 catalyst, yield of synthesis gas and carbon decreased with increasing the space velocity. 相似文献
10.
A series of ceramics samples, Na 5+xYAl xSi 4-xO 12, has been prepared by a solid state reaction with the starting materials of SiO 2, Y 2O 3, Al 2O 3 and Na 2CO 3. Their crystalline structure and morphology have been studied by the determination of XRD, IR, TG, DTA and SEM. Their conductivity has been measured by means of the complex impedance method. The dependence of the conductivity and density of the samples on the amount of the added Al 2O 3 and the reaction between the conductivity and the temperature have been discussed. When x = 0, the density of the sintering sample is 90% T.D., and the conductivity is 1·48 x 10 −1 (ωcm) −1 at 300°C; when x = 0·1, the density is up to 97% T.D., and the conductivity up to 1·74 x 10 −1 (ω cm) −1 at 300°C. 相似文献
11.
A series of CuO–ZnO/Al 2O 3 solids were prepared by wet impregnation using Al(OH) 3 solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O 2 at temperatures ranged between 125°C and 200°C. The results showed that CuO interacts with Al2O3 to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl2O4 at 600°C while stimulates its production at 800°C. The treatment of CuO/Al2O3 solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol−1 in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature. 相似文献
12.
The required sintering temperature of Pb(Zr 0·52Ti 0·48)O 3 ceramics (abbreviated as PZT 52/48) can be lowered to about 1000°C by incorporating Li 2CO 3, Na 2CO 3 or Bi 2O 3. A dielectric constant of about 1000 and a planar coupling factor of between 45% and 65% are obtained in PZT 52/48 ceramics sintered at 1025°C, with added Li 2CO 3 and Bi 2O 3. The optimal amount of the additives, which can be deduced from the densification, the dielectric and piezoelectric properties of the sintered PZT 52/48 ceramics, is 0·375 wt% of Li 2CO 3 together with an equal mole fraction of Bi 2O 3. A planar coupling factor of 65% is obtained. This is explained, with the aid of X-ray diffraction (XRD) analysis, by a maximum c/ a ratio and consequently by a large spontaneous polarization. The PZT 52/48 ceramics sintered with Li 2CO 3 and Bi 2O 3 under the optimal conditions can have ε 33T of about 1000, kp higher than 60%, Qm around 100 and tan δ less than 2·0%. 相似文献
13.
Dense sialon ceramics along the tie line between Si 3N 4 and Nd 2O 3·9AlN were prepared by hot-pressing at 1800°C. The materials were subsequently heat-treated in the temperature range 1300–1750°C and cooled either by turning off the furnace (yielding a cooling rate (T cool) of ∼50°C/min) or quenching (T cool≥ 400°C/min). It was found necessary to use the quenching technique to reveal the true phase relationships at high temperature, and it was established that single-phase α-sialon forms for 0.30 x 0. 51 in the formula Nd xSi 12–4S x Al 4.5 x O 1. 5 x , N 16–1.5 x . The α-sialon is stable only at temperatures above 1650°C, and it transforms at lower temperatures by two slightly different diffusion-controlled processes. Firstly, an α-sialon phase with lower Nd content is formed together with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed α-sialon decomposes to the more stable β-sialon and either the melilite phase or a new phase of the composition NdAl(Si 6-zAl z)N 10-zO z. Nd-doped α-sialon ceramics containing no crystalline intergranular phase show very high hardness (HV10 = 22. 5 GPa) and a fracture toughness ( K lc= 4.4 MPa·m 1/2) at room temperature. The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both the fracture toughness and hardness of the materials. 相似文献
14.
Spherical shaped spray-dried admixtures of chemical pure and very fine -Al 2O 3 and quartz powders with mullite composition (72 wt% Al 2O 3, 28 wt% SiO 2) were used as starting materials. The spray-dried powders (10–100 μm) were melted in a nitrogen plasma flame and subsequently quenched in water thus producing spherical, hollow, and porous particles (≤ 100 μm). The as-quenched spherules consist of mullite glass, some residual -Al 2O 3 and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to >90 wt.%. 27Al and 29Si MAS NMR studies show that the rapidly quenched mullite glass is composed of a network of (SiO)-tetrahedra and (AlO)-octahedra, -pentahedra, and -tetrahedra. The frequency distribution of (AlO)-structural units is similar to those in metakaolinite, type I (polymer) mullite precursors, and in other melt-quenched aluminium-silicate glasses suggesting strong structural similarities of these phases. This has been supported by the exothermic mullite crystallization process taking place at ≈ 980 °C in all cases. 相似文献
15.
Dense α-sialon materials were produced by hot isostatic pressing (HIP) and post-hot isostatic pressing (post-HIP) using compositions with the formula Y x (Si 12–4.5 x , Al 4.5 x )-(O 1.5 x ,N 16–1.5 x ) with 0.1 ≤ x ≤ 0.9 and with the same compositions with extra additions of yttria and aluminum nitride. X-ray diffraction analyses show how the phase content changes from large amounts of β-sialon ( x = 0.1) to large amounts of α-sialon ( x = 0.4) and increasing amounts of mellilite and sialon polytypoids ( x = 0.8). Samples HIPed at 1600°C for 2 h contained unreacted α-silicon nitride, while those HIPed at 1750°C for 1 h did not. This could be due to the fact that the time is to short to achieve equilibrium or that the high pressure (200 MPa) prohibits α-sialon formation. Sintering at atmospheric pressure leads to open porosity for all compositions except those with excess yttria. Therefore, only samples with excess yttria were post-HIPed. Microstructrual analyses showed that the post-HIPed samples had the highest α-sialon content. A higher amount of α-sialon and subsequently a lower amount of intergranular phase were detected at x = 0.3 and x = 0.4 in the post-HIPed samples in comparison to the HIPed. The hardness (HV10) and fracture toughness ( K IC) did not differ significantly between HIPed and post-HIPed materials but vary with different x values due to different phase contents. Measurements of cell parameters for all compositions show a continuous increase with increasing x value which is enhanced by high pressure at high x values. 相似文献
16.
Results of a study of the behaviour of aqueous suspensions of carbothermally prepared β-sialon are presented. The influence of an oxidation treatment, attrition milling, sintering additive (Y 2O 3) and addition of a deflocculant (Dolapix CE 64) on the stabilisation behaviour of the suspensions was studied. Obtained results show that the surface properties of β-sialon particles are affected, as expected, by the oxidation treatment (isoelectric point, pH iep ≈ 2.1) and by attrition milling of the power (pH iep ≈ 4.6). In the presence of Y 2O 3 the suspension of β-sialon becomes less stable (pH iep ≈ 3.4 ), the viscosity is increased and the stable pH range is reduced. This behaviour is explained in terms of surface composition of β-sialon, the solubility of yttria in acidic solution and the possibility of precipitation of yttrium-hydroxy complexes on β-sialon surface at higher pH while increasing the pH. Addition of the deflocculant increases the absolute zeta potential and changes the suspension behaviour from shear thickening to shear thinning. Low viscosity suspensions with 60 wt% β-sialon and 10 wt% Y 2O 3 can be prepared at pH>10, such that they can be used for slip casting. 相似文献
17.
A new brownmillerite-related compound. Ca 2Cr 2O 5, has been prepared. It has been indexed according to an orthorhombic lattice a = 5·750 Å, b = 14·398 Å and c = 5·483 Å. A series of experiments was performed in order to find the appropriate firing temperature. The total conductivity was measured by a four-point method in the range of 690–911°C. Impedance spectroscopy was also employed in the temperature range 343–785°C. Conductivity measurements at different oxygen pressures at 500°C suggest that Ca 2Cr 2O 5 is a predominantly ionic conductor at P o2 = 1–10 −2atm. 相似文献
18.
The durability of plasma-sprayed metals bonded with a polyimide adhesive has been studied. Metal adherend surfaces were prepared for adhesive bonding by plasma-spraying inorganic powders on aluminum and titanium. The plasma-sprayed materials included Al 2O 3, AlPO 4. MgO, and SiO 2 on aluminum, and TiO 2, TiSi 2, MgO, and SiO 2 on titanium. The coatings were sprayed at two different thicknesses. Durability studies of samples prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to the conditions; low temperature, - 20°C; relative humidity at elevated temperature, 70% RH at 66°C; elevated temperature (160°C) in air, high temperature (160°C) in vacuum (130 torr, 0.2 atm.), and room temperature. Crack growth rate and mode of failure were determined. The results of the durability tests indicate that thin coatings (25 μm) of plasma-sprayed materials perform better than thicker (150 μm) coatings. The crack growth rate for thin coatings (25 μm) of Al 2O 3, AlPO 4, SiO 2, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarly, the durability performance for titanium samples prepared with a 25 μm-thick TiO 2, TiSi 2, and SiO 2 plasma-sprayed coatings was equivalent to that for a Turco®-prepared titanium surface. Although the evaluation of durability as a function of surface chemistry was an objective of the study, it was not possible to evaluate the effect, since most failures occurred within the adhesive (cohesive failure) during the environmental tests. That failure occurred in the adhesive indicates that the coating-adherend and the coating-adhesive interactions are sufficiently robust to prevent interfacial failure under the experimental conditions investigated. 相似文献
19.
Dispersing La 2O 3 on δ- or γ-Al 2O 3 significantly enhances the rate of NO reduction by CH 4 in 1% O 2, compared to unsupported La 2O 3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La 2O 3 precursor used, the pretreatment, and the La 2O 3 loading. The most active family of catalysts consisted of La 2O 3 on γ-Al 2O 3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m 2) occurred between the addition of one and two theoretical monolayers of La 2O 3 on the γ-Al 2O 3 surface. The best catalyst, 40% La 2O 3/γ-Al 2O 3, had a turnover frequency at 700°C of 0.05 s −1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La 2O 3/Al 2O 3 catalysts exhibited stable activity under high conversion conditions as well as high CH 4 selectivity (CH 4 + NO vs. CH 4 + O 2). The addition of Sr to a 20% La 2O 3/γ-Al 2O 3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO =4 to the latter Sr-promoted La 2O 3/Al 2O 3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La 2O 3. Dispersing La 2O 3 on SiO 2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La 2O 3. This is presumably due to water sensitivity and silicate formation. The La 2O 3/Al 2O 3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
20.
Optically clear, inclusion-free mullite fibers up to 80 mm in length and 80×70 μm 2 in cross-section were grown from an alumino silicate melt by the internal crystallization method (ICM). Microprobe analysis reveals a high chemical homogeneity at 76.5±0.5 wt.% Al 2O 3 and 23.5±0.5 wt.% SiO 2, which is close to the 2/1-mullite composition. Areas of slightly decreased Al 2O 3 content (≈74.5 wt.%) occur rarely, whereas areas of increased Al 2O 3 content were not observed. Polarized infrared-reflection micro-spectroscopy using spot sizes of 60 μm diameter on sections cut parallel and perpendicular to the fiber axis show the single crystal character of the fibers. Optical microscopy shows that the fibers consist of a mosaic of single crystal areas up to about 5 mm in length. The c axis of the single crystal individuals are misalligned up to θ ≈±3° with respect to the fiber axis. These orientational misfits are believed to be the reason for the development of the mosaic type microstructure during the growth process. 相似文献
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