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1.
The microstructure, crystal phase, electrical conductivity, and mechanical strength of less than 7-mol%-Sc2O3-doped zirconia ceramics fabricated by comparatively low-temperature sintering at 1200–1300°C for 1 h were investigated. Zirconia ceramics having a uniform microstructure (grain size < 0.5 μm) stabilized with 6 mol% Sc2O3 showed high electrical conductivity (0.15 S/cm at 1000°C) and high fracture strength (660 MPa). With the increase of Sc2O3 content from 3.5 to 7 mol%, the grain size, fracture strength, and electrical conductivity at 1000°C changed from 0.2 to 0.5 μm, 970 to 440 MPa, and 0.07 to >0.2 S/cm, respectively. Sc2O3-doped zirconia polycrystals with high fracture strength and high electrical conductivity are promising candidates for the electrolyte material of solid oxide fuel cells.  相似文献   

2.
Pore–boundary separation in ZnO and 99.95ZnO·0.05Bi2O3 (in mol%) specimens during sintering at 1200°C was investigated. In pure ZnO specimens, pores were attached to the grain boundaries and disappeared during the final stage of sintering. In the Bi2O3-doped specimens, on the other hand, many pores were separated from the boundaries and trapped inside the grains. Observation using transmission electron microscopy showed that a thin layer of Bi2O3-rich phase existed at the boundaries in the Bi2O3-doped specimens. The pore separation in 99.95ZnO·0.05Bi2O3 specimens was explained in terms of the dihedral angle change and the high mobility of a liquid film boundary.  相似文献   

3.
The sintering behavior and electrical conductivity of high-purity 8-mol% Y2O3-stabilized ZrO2 (8YSZ) with Al2O3 additions were investigated. The addition of 1 wt% AI2O3 to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO2 enhanced the sinterability of 8YSZ. Na2O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai2O3 content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI2O3 showed the maximum conductivity of 0.16 S/cm at 1000°C.  相似文献   

4.
Powder compacts consisting of Al, Al2O3, and ZrO2 were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al3Zr. This Al3Zr formation was caused by the preferential heating of ZrO2 relative to Al and Al2O3 by microwave radiation. Then, Al3Zr was oxidized to form Al2O3 and ZrO2 at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering.  相似文献   

5.
Densification of Calcia-Stabilized Zirconia with Borates   总被引:1,自引:0,他引:1  
Densification studies of submicrometer ZrO2 powders stabilized with 6.5 wt% CaO (CSZ) showed borate additions (1 to 10 wt%) to be effective sintering aids. Estimated densities >99% of theoretical were obtained on sintering at 1200°Cfor 4 h with 2 wt% B2O3 or 5 wt% CaO·2B2O3 additions to the CSZ powders. Average grain sizes obtained were typically <1 μm. Partial development of a monoclinic ZrO2 phase was observed in the sintered samples. The amount of this phase varied from ∼7 to 75 wt% and was approximately linearly dependent on the additive concentration. The effect was most marked for the B2O3 additions. Development of the monoclinic phase was attributed to progressive leaching of Ca from the CSZ phase by B2O3, in effect partially destabilizing the ZrO2.  相似文献   

6.
ZrO2-2 mol% Y2O3 crystals with average grain sizes from 0.51 to 0.96 µm were prepared by sintering in air at 1400°C for 2 to 100 h. The tetragonal-to-monoclinic phase transformation associated with the low-temperature degradation was investigated to clarify how the presence of water directly affects the influence of grain size on transformation. The specimens were exposed to water at 80–120°C, a temperature range in which transformation by thermal activation is difficult in the absence of water. Contrary to expectations, this type of low-temperature transformation did not accelerate monotonously with increasing grain size. Instead, the amount of phase transformation first decreased, reaching a constant value, and then increased with increasing grain size. Such interesting results can be explained satisfactorily by the combined influences of grain size on the nucleation process, because of preferential dissolution of yttrium at the grain boundaries, and the intrinsic transformability of Y2O3-doped tetragonal ZrO2 grains.  相似文献   

7.
Compared to conventional sintering of Y2O3-doped ZrO2 using a slow heating rate of 10°C/min, microwave sintering and fast firing using a rapid heating rate of about 500°C/min resulted in lower final sintered densities. It is attributed to the residual chlorine in commerical zirconia powders manufactured by the chloride process. By calcining at 1100°C for 1 h to remove residual chlorines from the powder compacts, near full densities (>99% of theoretical) could be obtained by both fast firing and microwave sintering.  相似文献   

8.
The densification behavior of a 3-mol%-Y2O3-doped ZrO2 (3Y-ZrO2) has been investigated under N2 and O2 atmospheres. Powder compacts have been sintered at 1550° and 1400°C for various times. The density of the specimen sintered at 1550°C is higher in N2 than in O2, while the contrary result is obtained in the case of the specimen sintered at 1400°C. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO2 matrix. Because nitrogen solubility into the ZrO2 increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO2 and its role on the densification behavior of 3Y-ZrO2.  相似文献   

9.
Preparation of dense and phase-pure Ba2Ti9O20 is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba2Ti9O20 equilibrium phase boundary in the BaO–TiO2 system at high temperatures. In this study, the effects of B2O3 on the densification, microstructural evolution, and phase stability of Ba2Ti9O20 were investigated. It was found that the densification of Ba2Ti9O20 sintered with B2O3 was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B2O3. With the addition of 5 wt% B2O3, the ceramic yielded a pure Ba2Ti9O20 phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO2 or ZrO2. The facilities of B2O3 addition to the stability of Ba2Ti9O20 are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.  相似文献   

10.
Sinterability of undoped, MgO-doped, and TiO2-doped Al2O3 has been examined by applying reported sintering equations. The order of sinterability was MgO-doped ∼ undoped≪ TiO2-doped Al2O3 in the initial and intermediate stages of sintering, but a relative sintered density at 1600°C for 1 h occurred in the order undoped < TiO2-doped < MgO-doped AI2O3. The dispersion of thermal grooving angles increased in the order MgO-doped < undoped < TiO2-doped Al2O3, The change of sinterability by the dopants is explained in terms of mobility of mass transfer estimated from a densification rate in the initial- and intermediate-stage sintering and of dispersed driving forces of densification and grain growth qualitatively evaluated from the width of the dispersion of thermal grooving angles.  相似文献   

11.
β-sialon and Nd2O3-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% ZrO2 (3 mol% Nd2O3) 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 ZrO2(Nd2O3) or ZrN were seen in both the HIPed β-sialon/ZrO2(Nd2O3) composites and the HIPed Nd2O3-stabiIized α-sialons with ZrN additions.  相似文献   

12.
Densification and microstructure de velopment in Bi2O3-doped ZnO have been studied with a special emphasis on the effect of the Bi2O3 content. A small amount of Bi2O3 in ZnO (0.1 mol%) retarded densification, but the addition of Bi2O3 to more than 0.5 mol% promoted densification by the formation of a liquid phase above the eutectic temperature (∼740°C). The liquid phase increased grain-boundary mobility, which was responsible for the formation of intragrain pores and the decrease in the sintered density. The increase in the Bi2O3 content increased the probability of the formation of skeleton structure, which reduced the grain growth rate and the sintered density.  相似文献   

13.
CuO-doped tetragonal ZrO2 (3-mol%-Y2O3-doped tetragonal zirconia, 3Y-TZ) green bodies were consolidated from zirconia slurries with Cu2+ by a pressure filtration method. The slurries were prepared by dispersing 3Y-TZ powder in a solution of [NH4OH + NH3NO3] = 0.1 M at pH 11 and adding an appropriate amount of Cu(NO3)·3H2O solution. Green bodies with narrow pore-size distribution were obtained after cold isostatically pressing the pressure-filtrated bodies. Small amounts of CuO-doped samples were densified fully at 1200°C. The size of a grain of 0.16-mol%-CuO-doped 3Y-TZ sintered at 1200°C was 84 nm. Bulk and grain-boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO-doped 3Y-TZ was almost equal to the undoped one, but the grain-boundary conductivity decreased with CuO addition.  相似文献   

14.
Single-phase, cubic solid solutions of baseline composition 25% Y2O3—75% Bi2O3 with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO2, or ThO2 was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO2 or ThO2 remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO2- and ThO2-doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice.  相似文献   

15.
The densification behavior of ZrO2 (+ 3 mol% Y2O3)/85 wt% Al2O3 powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.  相似文献   

16.
In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al2O3–ZrO2–SiO2 glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al2O3–7.68 ZrO2–55.43SiO2 (mol%) to which 5 and 10 mol% Al2O3 were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO2 (2CaO·4SiO2·ZrO2) appeared in all studied specimens.  相似文献   

17.
Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide   总被引:1,自引:0,他引:1  
The phase change, densification, and microstructure development of ZnO doped with both Bi2O3 and Sb2O3 are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi3/2ZnSb3/2O7, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi2O3, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.  相似文献   

18.
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   

19.
The influence of Nd2O3 doping on the reaction process and sintering behavior of BaCeO3 is investigated. Formation of BaCeO3 is initiated at 800°C and completed at 1000°C. When Nd2O3 is added to the starting materials, the formation of BaCe1–xNdxO3–δ is delayed and the temperature for complete reaction is increased to 1100°C. Only a BaCe1-xNdxO3–δ solid solution with an orthorhombic crystal structure is present in the specimens for x ≤ 0.1. A secondary phase rich in Ce and Nd is formed within grains and at grain boundaries, when the Nd2O3 content is greater than the solubility limit (x ≥ 0.2). Pure BaCeO3 is difficult to sinter, even at 1500°C, and only a porous microstructure could be obtained. However, doping BaCeO3 with Nd2O3 markedly enhances its sinterability. The enhancement of the sinterability of Nd2O3-doped specimens at x ≤ 0.1 is attributed to the increase in the concentration of oxygen ion vacancies, which increases the diffusion rate. At x ≥ 0.2, the grain size is abnormally coarsened, which is caused by the formation of a liquid phase. While this liquid phase accelerates sintering, its beneficial effect on densification is counteracted by the segregation of the secondary grain-boundary phase which inhibits sintering.  相似文献   

20.
Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO2(3Y) and Al2O3 have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The microstructures are such that tetragonal ZrO2 (0.3 μm) and Al2O3 (0.5 μm) particles are located at the grain boundaries of the CoAl (8.5 μm) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO2(3Y)/Al2O3= 16/4 mol% are 3.87 MPa·m1/2 and 1080 MPa, respectively, and high strength (>600 MPa) can be retained up to 1000°C.  相似文献   

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