共查询到20条相似文献,搜索用时 31 毫秒
1.
Michael D. Hill John E. Blendell Mark D. Vaudin C. K. Chiang 《Journal of the American Ceramic Society》1995,78(7):1953-1957
The relationships between the microstructure of sintered YBa2 Cu3 O6+ x superconductors and processing variables (sintering time, sintering temperature, and oxygen partial pressure) were examined. Large-grained microstructures were obtained in 100 kPa oxygen sintering atmospheres, while fine-grained microstructures were obtained in 2 kPa oxygen. The formation of liquid phases below the peritectic decomposition temperature of YBa2 Cu3 O6+ x was found to have an effect on both the microstructure (as observed by optical and transmission electron microscopy) and the transport critical current density ( Jc ). The critical current density was found to be highest for sintering below the lowest invariant point, which is a function of the oxygen partial pressure. However, over the range of conditions examined here, there does not appear to be any correlation between microstructural features, such as average grain size and aspect ratio, and the transport Jc . 相似文献
2.
MIHA DROFENIK 《Journal of the American Ceramic Society》1987,70(5):311-314
The grain growth of donor-doped BaTiO3 at different oxygen partial pressures was studied. Results showed that the oxygen pressure had a pronounced influence on the grain growth and related effects. A model for the grain size anomaly during sintering of donor-doped BaTiO3 in the presence of a TiO2 -rich liquid phase is proposed. 相似文献
3.
Michael K. Cinibulk Geoff E. Fair Ronald J. Kerans 《Journal of the American Ceramic Society》2008,91(7):2290-2297
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 La2 O3 or La2 Si2 O7 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
4.
Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina 总被引:7,自引:0,他引:7
The grain-growth behavior of Al2 O3 compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO2 , or CaO + TiO2 . The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na2 O + SiO2 , CaO + SiO2 , SrO + SiO2 , or BaO + SiO2 . However, an addition of MgO to the Al2 O3 doped with CaO + SiO2 resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO2 doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains. 相似文献
5.
Shiro Torizuka Kaoru Sato Hiroaki Nishio Teruo Kishi 《Journal of the American Ceramic Society》1995,78(6):1606-1610
Compacts of TiB2 with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB2 . The oxygen content of the raw TiB2 powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO2 and B2 O3 . Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB2 samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO2 + SiC → SiO2 + TiC. Therefore, the improved sinterability of TiB2 resulted from the SiO2 liquid phase that was formed during sintering when the raw TiB2 powder had 1.5 wt% oxygen. 相似文献
6.
Sang-Ho Lee Elizabeth R. Kupp Adam J. Stevenson Julie M. Anderson Gary L. Messing Xin Li Elizabeth C. Dickey John Q. Dumm Vida K. Simonaitis-Castillo Gregory J. Quarles 《Journal of the American Ceramic Society》2009,92(7):1456-1463
This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO2 doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO2 levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO2 , especially at higher temperatures, because the SiO2 solubility limit is exceeded. 相似文献
7.
Microstructural Characterization of Porous Silicon Carbide Membrane Support With and Without Alumina Additive 总被引:1,自引:0,他引:1
Manabu Fukushima You Zhou Hiroyuki Miyazaki Yu-ichi Yoshizawa Kiyoshi Hirao Yuji Iwamoto Satoshi Yamazaki Takayuki Nagano 《Journal of the American Ceramic Society》2006,89(5):1523-1529
Porous silicon carbide (SiC) membrane supports sintered at 1500°–1800°C were prepared by cold isostatic pressing (CIP) under different pressures and using different amounts of alumina additive (0%–4%). The relationship between processing factors and pore size and microstructure was examined. Varying the sintering temperature, the CIP pressure and the amount of additive used were found to be effective for controlling pore size and microstructure. The pore size and particle size of the membrane support prepared without alumina were found to increase with increasing sintering temperature. This was attributed to surface diffusion. Densification of the undoped support did not occur, however, because of concurrent pore development. In the SiC membrane support containing 4% alumina, small particles and a pore size of around 100 nm were retained. This was because of the formation of a limited amount of SiO2 –Al2 O3 liquid phase during sintering. 相似文献
8.
The microstructure of strontium titanate internal boundary layer capacitors at various stages in their processing was studied by transmission electron microscopy of rapidly quenched and normally cooled samples. Compositions containing excess TiO2 , Al2 O3 , and SiO2 have a completely wetting liquid phase at the sintering temperature; during cooling Tin O2 n −1 , Magneli phases precipitate at multiple grain junctions. Diffused metal oxides and flux (Bi2 O3 , PbO, CuO, and B2 O3 ) rapidly penetrate as a liquid phase along boundaries in postsintering heat treatment. This liquid phase disappears during slow cooling. 相似文献
9.
Yang-Il Jung Si-Young Choi Suk-Joong L. Kang 《Journal of the American Ceramic Society》2003,86(12):2228-2230
To study the effect of oxygen partial pressure on grain growth in BaTiO3 , TiO2 -excess samples have been sintered in air with and without a prior H2 heat treatment. Without prior H2 treatment, abnormal grain growth occurs below and above the eutectic temperature ( T e ). An introduction of H2 treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T e . This H2 treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed grain-growth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size. 相似文献
10.
Advanced sintering techniques for consolidation of Si3 N4 powders in the presence of an oxygen-rich liquid phase(s) require high temperatures and usually high nitrogen pressures. A stability diagram is constructed for Si3 N4 as a function of the partial pressures of nitrogen (PN2 ) and silicon (PSi ). High PN2 (20 to 100 atm) increases the stability of Si3 N4 and the oxygen-rich liquid phase by reducing the PSi and PSi0 , respectively. The region of high sinterability is outlined for submicrometer Si3 N4 powders containing 7 wt% BeSiN2 and 7 wt% SiO2 as densification aids . 相似文献
11.
Stability of Phases in the Si-C-N-O System 总被引:4,自引:0,他引:4
HARUE WADA MING-JONG WANG TSENG-YING TIEN 《Journal of the American Ceramic Society》1988,71(10):837-840
The stability of the phases in equilibrium is calculated for the Si-C-N-O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si3 N4 , Si2 N2 O, SiO2 , Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si3 N4 composite is higher with an elevated nitrogen pressure or a reduced carbon activity. 相似文献
12.
Hsiu-Fung Cheng Tsai-Fa Lin Chen-Ti Hu I-Nan Lin 《Journal of the American Ceramic Society》1993,76(4):827-832
The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr0.2 Ba0.8 )TiO3 materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al2 O3 –SiO2 –TiO2 (AST). The ultimate PTCR properties with ρht /ρrt as great as 105.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO3 is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth. 相似文献
13.
Ying-Chieh Lee Chia-Wei Lin Wei-Hua Lu Wen-Jauh Chen Wen-Hsi Lee 《International Journal of Applied Ceramic Technology》2009,6(6):692-701
In this study, the effect of SiO2 doping on the sintering behavior, microstructure, and dielectric properties of BaTiO3 -based ceramics was investigated. Silica was added to (Ba0.96 Ca0.04 )(Ti0.85 Zr0.15 )O3 (BCTZ) powder prepared using the solid-state method. SiO2 -doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO5 ) existed in samples when SiO2 was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO2 additives increased. It was found that BCTZ ceramics sintered with SiO2 are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO2 addition (3% and 5%) due to higher formation of BaTiSiO5 . Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO5 phase on the dielectric properties of BCTZ ceramics, the BaTiSiO5 composition was synthesized from individual BaCO3 , TiO2 , and SiO2 powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO5 , in the sintered ceramics. 相似文献
14.
Joaquín Aguilar-Santillán Ricardo Cuenca-Alvarez Heberto Balmori-Ramírez Richard C. Bradt 《Journal of the American Ceramic Society》2002,85(10):2425-2431
The influence of attrition milling on the thermal decomposition of kyanite (Al2 O3 ·SiO2 ) to mullite (3Al2 O3 ·2SiO2 ) and SiO2 , and its subsequent sintering, was studied. A commercial kyanite was attrition-milled for times up to 12 h. Dilatometry confirmed that as-received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one-half of the decomposition occurs in the last 35°C. For the attrition-milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition-milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering. 相似文献
15.
Improvements in the mechanical properties of magnesia-partially-stabilized zirconia are obtained by the addition of SrO. Evidence is presented which indicates that the added SrO effectively neutralizes the detrimental effects of SiO2 contaminant by forming a glass phase which is ejected from the bulk of the ceramic during sintering. This combined effect results in the retardation of the subeutectoid decomposition reaction while minimizing retention of glass phases at the grain boundaries. 相似文献
16.
Microstructural development in the interface region of α-Al2 O3 bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO2 , residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO2 doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO2 -Al2 O3 liquid at the interface region, thereby strongly influencing α-Al2 O3 grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO2 dopant in the α-Al2 O3 bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications. 相似文献
17.
Matthew M. Seabaugh Ingrid H. Kerscht Gary L. Messing 《Journal of the American Ceramic Society》1997,80(5):1181-1188
The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al2 O3 grains after sintering at 1400°C. A CaO + SiO2 liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains. 相似文献
18.
The effect of oxygen activity on the sintering of high-purity Cr2 O3 is shown. Theoretical density was approached at the equilibrium O2 partial pressure needed to maintain the Cr2 O3 phase ( P o2 =2×10−12 atm). The presence of N2 in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O2 activity, correlated with the changing predominant volatile species in the Cr-O system. 相似文献
19.
The effects of the addition of Ta205 or Nb205 on the sintering and refractory properties of 2ZrO2 ·P2 O5 -based ceramics have been investigated. The solid solution with Nb2 O5 has a lower average thermal expansion coefficient than the pure phase. The strength of the polycrystalline ceramic is reduced when the grain size exceeds the transition grain size for microcracking. The thermal expansion of the ceramic is abruptly lowered when the grain size exceeds a value which should be termed the "second transition grain size" for additional wider microcracking. SiO2 was found most effective for inhibiting grain growth to prevent microcracking and for obtaining a strong ceramic. 相似文献
20.
Lise T. Sagdahl Mari-Ann Einarsrud Tor Grande 《Journal of the American Ceramic Society》2000,83(9):2318-2320
The sintering of LaFeO3 has been studied in the temperature interval 1100–1600°C in air. The effect of cation nonstoichiometry on densification, microstructure, and phase composition is emphasized. La2 O3 was observed to inhibit both sintering and grain growth. In Fe-excess materials, exaggerated grain growth occurred, particularly above 1430°C, where a liquid phase was formed. Postsintering swelling was observed in Fe-excess materials above 1430°C. The swelling mechanism is related to phase equilibria, which are reductive in nature and lead to the evolution of oxygen gas. The density in La-excess materials remains high up to 1600°C, but the ceramics might disintegrate in air. 相似文献