共查询到20条相似文献,搜索用时 31 毫秒
1.
Fine-grained silicon carbide ceramics with an average grain size of 140 nm or smaller were prepared by low-temperature hot-pressing of very fine -SiC powders using Al2O3-Y2O3-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure. 相似文献
2.
V. Ya. Shevchenko V. B. Glushkova T. I. Panova L. I. Podzorova A. A. Il'icheva A. E. Lapshin 《Inorganic Materials》2001,37(7):692-697
Nanometer-sized tetragonal Zr1 – x
Ce
x
O2 – powders were prepared by hydroxide precipitation and sol–gel processing. The effects of gel-aging time and solution concentration on the phase composition and particle size of the powders were studied. The ceramics prepared by sintering ultrafine powders at 1500°C had a density of 6.0 g/cm3, open porosity of 4%, and closed porosity below 1%. 相似文献
3.
Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m1/2 to 8.3 MPa m1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness. 相似文献
4.
We have successfully prepared thermoelectric materials NaCo2O4– at lower temperature (700°C) using aqueous sol-gel methods. We find that the electrical resistivity and thermopower of NaCo2O4– depend strongly on the sintering temperature and partial pressure of oxygen. The oxygen content of NaCo2O4– is found to decrease with increasing sintering temperature. The sample prepared at the combination of 700°C and pure flowing oxygen exhibits the highest value of power factor. 相似文献
5.
Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al2O3+22 wt% SiO2 (high-Al2O3 material) and 72 wt% Al2O3+28 wt% SiO2 (low-Al2O3 material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al2O3 materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al2O3 compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al2O3/SiO2 ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al2O3 were produced from high-Al2O3 powders calcined between 600 and 1100 C. Low-Al2O3 sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al2O3 materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al2O3 compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries. 相似文献
6.
K. S. Rane V. M. S. Verenkar P. Y. Sawant 《Journal of Materials Science: Materials in Electronics》1999,10(2):133-140
Electro-magnetic properties and microstructural characterization of MgFe2O4 synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe2O3 and traces of alpha-Fe2O3, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe2O3. The sources of -Fe2O3 are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe2O3 of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe2O3 gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe2O4 obtained from commercial alpha-Fe2O3 grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe2O4 from -Fe2O3 source while the alpha-Fe2O3 source gave the lowest value, 609. The Curie temperature, Tc, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest Tc of 737 K for MgFe2O4 from alpha-Fe2O3, while lower values are found for the ferrite prepared from -Fe2O3. 相似文献
7.
Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y2O3 and/or Ce02 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/CeO2: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y2O3 and pure CeO2, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the sialon type of materials. For the mixed - sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping. 相似文献
8.
Powders of Si3N4, Al2O3 and SiO2 were mixed with Y2O3 as sintering aid and hot-pressed to form o- sialon. During sintering, the o phase preferentially precipitated in the temperature region of 1550 to 1600 °C, and the phase precipitated at temperatures of 1700 °C and above. The resultant microstructure consisted of dual phases of o and . The mechanical properties of o- sialon were improved with increasing amount of the phase. When SiO2 required for the formation of the o phase was all included in the grain boundary phase, o- sialon exhibited a three-point bending strength as much as 1400 MPa. The oxidation resistance of o- sialon is expected to improve due to the presence of the o phase, but actually was not so good due to the presence of the grain-boundary glassy phase and impurities. 相似文献
9.
-SiC was pressureless sintered to 98% theoretical density using Al2O3 as a liquid-phase forming additive. The reaction between SiC and Al2O3 which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase. 相似文献
10.
Z. Z. Huang S. U. Adikary H. L. W. Chan 《Journal of Materials Science: Materials in Electronics》2002,13(10):605-608
The processing behavior of PTCR ceramics of (Ba,Sr,Ca,Pb)TiO3 solid solution composition with additives of lanthanum oxide (La2O3) and boron nitride (BN) was studied. The ceramics can be sintered at temperatures as low as 1100 °C and possess rather low room-temperature resistivity with good PTCR effect. The sample ball milled with de-ionized water exhibits a more uniform microstructure compared to the sample ball-milled with alcohol. Particle size of less than 1 m was found to be adequate for preparing the ceramics and the finer particles (0.45 m) do not significantly improve the PTCR behavior. The performance of the PTCR sample is not sensitive to the sintering parameters such as the sintering time and cooling rate. This may be ascribed to the presence of excess BaO in the sample and the low sintering temperature, thereby eliminating the effect of Ba ion vacancies on the properties of the PTCR sample. 相似文献
11.
X. M. Chen Y. Suzuki N. Sato 《Journal of Materials Science: Materials in Electronics》1995,6(1):10-16
The microstructures and the microwave dielectric characteristics of BaO·Nd2O3·5TiO2 ceramics were determined as a function of the sintering conditions. The ceramics were confirmed to have a ternary orthorhombic major phase with minor amounts of Nd2Ti2O7 and TiO2, and nearly equiaxed fine structures and homogeneous columnar structures were obtained at sintering temperatures of 1340 C and 1370 C, respectively, while discontinuous and excess grain growth were observed at sintering temperatures 1350–1360 C and 1380 C, due to the formation of a liquid phase. The desired value of Qf was achieved in ceramics with fine-grained and/or homogeneous microstructures, and the discontinuous and excess grain growth was found to be seriously harmful to the dielectric properties. 相似文献
12.
G. D. Nipan T. N. Kol'tsova E. A. Tishchenko P. Manca 《Journal of Low Temperature Physics》1996,105(5-6):1427-1432
YxBayCuzO7– ceramics forming at isobaric conditions were studied by x-ray diffraction analysis, dynamic magnetic measurements and potentiometric titration. It was established that compositions: 0.8 x 1.2 y=2, z=3; x-1, 1.8 y 2.2, z=3; x=1, y=2, 2.7 z 3.4: are in the homogeneity range of 123. It was found that compositions with nonstoichiometric cations rations have minimum Tc. 相似文献
13.
By adjusting the : SiC phase ratios in the individual starting powders, a layered SiC consisting of surface and inner layers with distinctively different microstructures are produced by hot-pressing and subsequent annealing. The surface layer consisted of relatively fine, equiaxed -SiC grains, designed for high strength, while the inner layer consisted of elongated -SiC grains, designed for high toughness. By virtue of the common SiC phase and the same sintering aids (Al2O3-Y2O3), the interlayer interfaces are chemically compatible and strongly bonded. R-curve behavior of the layered SiC was measured and compared with the related monolithic materials. The layered SiC showed better damage tolerance than monolithic materials and stronger R-curve behavior than surface layer. This superior performance of layered SiC ceramics was attributed to the contribution of both high strength of the surface layer for small flaws and high toughness of the inner layer for larger flaws. 相似文献
14.
H. M. Al-Allak T. V. Parry G. J. Russell J. Woods 《Journal of Materials Science》1988,23(3):1083-1089
The effect of AI additions on the electrical behaviour of positive temperature coefficient of resistance (PTCR) BaTiO3 ceramic sintered in air at temperatures ranging between 1220 and 1400° C have been investigated. Two batches of material, both showing a PTCR effect, were prepared identically except that additions of AI2O3 (0.55 mol %) were made to one of them. It has been confirmed that the presence of aluminium results in an increase in the temperature at which the maximum resistivity, max, occurs as well as reducing the sintering temperature, in the presence of silicon, to 1240° C. Additionally, direct comparisons between the two materials have demonstrated that such additions result in an increase of 100% in the minimum resistivity, min, at sintering temperatures beyond 1280° C. A similar increase in max for sintering temperatures below 1360° C and a five-fold reduction in the ratio of max/min in samples sintered above 1320° C have also been attributed to the presence of aluminium. It was further found that aluminium increases the average grain size by 30% and promotes the formation of a liquid phase. 相似文献
15.
Sintering of strontium titanate in the presence of lithium salts in a reducing atmosphere 总被引:1,自引:0,他引:1
M. J. Laurent G. Desgardin B. Raveau J. M. Haussonne J. Lostec 《Journal of Materials Science》1988,23(12):4481-4486
The possibility of preparing Type III ceramics for multilayer applications has been investigated in the strontium-titanium-oxygen system, using La2O3, Nd2O3and La2/3TiO3– as dopants and lithium salts as sintering agents. The introduction of bismuth was also explored. The sintering process is carried out by adding the lithium salts to mixtures of strontium titanate and dopants which are previously calcined in a reducing atmosphere. The dielectric properties and the microstructure of the ceramics have been studied. The best characteristics have been observed for the particular nominal composition SrTiO3+0.75 mol% La2/3Ti3–+3 mol% Bi2O3+10 mol % LiNO3leading to =20000, tan < 2%,R
i= 1011 cm. These results are interpreted in terms of the formation of anionic vacancies and defects in the A sites of the perovskite ABO3and of extended defects SrO and Bi2O
2
2+
which are coherent with the perovskite matrix. 相似文献
16.
Hai-Doo Kim Im-Sik Lee Suk-Won Kang Jae-Woong Ko 《Journal of Materials Science》1994,29(15):4119-4124
An in-situ sintering reaction was designed to produce lath-like \-alumina in an -alumina matrix in order to make alumina ceramics stronger and tougher. The reaction sequence to produce \-alumina (NaMg2Al15O25) requires the formation of \-alumina (NaAl11O17) and spinel (MgAl2O4) at around 1100 dgC followed by a solid-state reaction of these two phases to give \-alumina at elevated temperatures; this reaction is complete at around 1600 °C. The in-situ sintering reaction produces near-theoretically dense alumina ceramics in which lath-like \-aluminas are homogeneously distributed. The bending strength and fracture toughness increase to 620 MPa and 5 MPam1/2, respectively; these increases are thought to be due to the suppression of grain growth as well as the crack deflection and bridging associated with lath-like \-alumina. 相似文献
17.
The white phosphor matrix 4SrO·7Al2O3 has been synthesized by firing the appropriate mixture of SrCO3, Al(OH)3 and H3BO3 in the molar ratios 1:3.5:0.135 at 1300°C for 4–7 h. The crystal structure of 4SrO·7Al2O3 has been determined as a orthorhombic Pmma space group with a=24.7451(2)Å, b=8.4735(6)Å, c=4.8808(1)Å, V=1023.41(3)Å3, Z=2, and D=3.66 g cm–3 by the Rietveld analysis. The refinement figures of merit are Rp=8.26, Rwp=11.60, Rbragg=4.44 and s=2.61 for 844 reflections with 2<119.94°. And the corresponding X-ray powder diffraction data are presented for search/match analysis. 相似文献
18.
Study on the dielectric properties of oxide-doped Ba(Ti,Sn)O3 ceramics prepared from ultrafine powder 总被引:2,自引:0,他引:2
Ultrafine BaTiO3 and BaSnO3 powders were prepared by thermal decomposition of oxalates through synthesis by the method of co-precipitation; Dy2O3, Nb2O5 and ZnO, used as dopants, were added to the main constituents, known as the tailor-made Ba(Ti, Sn)O3, according to the indicated compositions, then the batches were mixed until they became homogeneous. Careful treatment was necessary in later stages of the process of ceramic making. The dielectric properties of the ceramics were measured. It was found that high , low tan and low d/dt were characteristics of these ceramics. The results could be explained by the effects of peak shifting, broadening and grain-size inhibition. The relationship between composition, structure and properties is discussed. 相似文献
19.
Changes of density, the - phase transformation, and composition of grains and grain boundaries during sintering of Si3N4 with various sintering conditions using additives of Y2O3 and Al2O3 were investigated. The phase determination of individual Si3N4 grains was performed by convergent beam electron diffraction. The relations between densification and transformation were divided into two groups, depending on the additive compositions. Aluminium dissolution into Si3N4 grains occurred mostly during - transformation process. The concentrations of aluminium and oxygen in the grain boundaries decreased as the - transformation progressed. 相似文献
20.
Superconducting Bi-(Pb)-Sr-Ca-Cu-O ceramics were prepared through the sol-gel method using an aqueous solution of metal acetates containing acetic acid and tartaric acid. The conditions of gelation and conversion of gel to Bi2Sr2CaCu2Ox phase were studied by thermal analysis, infrared absorption spectroscopy and X-ray diffraction technique. It was found that transparent gels were obtained from the solutions containing tartaric acid, with a molar ratio of C4H6O6/Cu of about 0.4. The gel matrix was assumed to be composed of carboxylate anions and metal cations. A gel of molar composition BiSrCaCu = 2223 was decomposed to CaCO3, CaO, CuO and Bi1-x SrxOy (x = 0.2 to 0.3) at low temperatures of 270 to 600 ° C and produced Bi2Sr2CaCu2Ox phase at 800 ° C via complex intermediate states. A rapid cooling of the heated product increasedT
c (end) in the gel of molar composition BiSrCaCu = 2223, while a slow cooling increasedT
c (end) in the lead-containing gel of molar composition BiPbSrCaCu = 1.850.351.92.03.1. The latter showedT
c (onset) at 115 K andT
c (end) at 105K. 相似文献