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1.
JONG-HEE PARK WAYNE E. KING STEVEN J. ROTHMAN 《Journal of the American Ceramic Society》1987,70(12):880-885
The cation diffusivities in the lattice and along dislocations and grain boundaries have been measured on sintered polycrysals of Cr2 O3 ; and Cr2 Cr2 O3 -0.09 wt% Y2 O3 at1100°C and at the pO2 corresponding to that of Cr/Cr2 O3 equilibrium at that temperature. Results for lattice and dislocation diffusivities in pure Cr2 O3 are in good agreement with previous work. The present results indicate that yttrium additions have negligible effect on lattice and dislocation diffusion. However, grain-boundary diffusion in pure Cr2 O3 is significantly slower than grain-boundary diffusion in Cr2 O3 -0.09 wt% Y2 O3 . The results are discussed in terms of their implications for the reactive-element effect in high-temperature oxidation of chromium-containing alloys. 相似文献
2.
Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al2 03 second-phase particles and their interactions with grain boundaries in two high-conductivity Y2 03 /Yb2 03 stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al2 03 particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al2 O3 particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al2 03 acts as a scavenger for SiO2 , removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO2 -Al2 O3 -SiO2 interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al2 O3 additions, as reported in the literature. 相似文献
3.
Annealing of ZrO2 -toughened Al2 O3 (ZTA) at elevated temperatures causes growth of both the intergranular ZrO2 particles and the Al2 O3 "matrix" grains. Exaggerated ("breakaway") grain growth occurs in some, but not all, specimens. Analytical electron microscopy of two ZTA's, both of which contained a continuous amorphous (glassy) grain-boundary phase, but only one of which showed breakaway grain growth, revealed that the occurrence of breakaway grain growth could be correlated with the chemistry of the ubiquitous glassy grain-boundary phase. 相似文献
4.
Herbert K. Schmid 《Journal of the American Ceramic Society》1991,74(2):387-394
The microstructure and microchemistry of grain-boundary regions in (CeO2 + La2 O3 )-stabilized tetragonal ZrO2 polycrystals (Ce(La)-TZP) were studied by means of transmission electron microscopy (TEM). Evidence was found for the existence of crystalline and vitreous intergranular phases situated in small pockets at multiple grain junctions and in thin films along grain boundaries. In this ceramic system grain-boundary migration was observed in situ in the TEM in sample areas subjected to electron irradiation. Interfaces migrated away from their centers of curvature. Evidence was found for Ce de-alloying in the volume swept by the advancing boundaries. It is suggested that the coherency lattice strain brought about by a partial reduction of Ce, resulting in the diffusion of Ce3+ along grain boundaries to free surfaces, is the driving force for this phenomenon. 相似文献
5.
Auger electron spectroscopy was used to characterize grain-boundary chemistry in MgO, Al2 O3 , and MgAl2 O4 (spinel) hot-pressed with LiF or NaF. The presence of additives (F, Na) at the grain boundaries was confirmed; observation of grain-boundary impurities in MgO extends previous studies. 相似文献
6.
Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina 总被引:5,自引:0,他引:5
Jianxin Fang A. Mark Thompson Martin P. Harmer Helen M. Chan 《Journal of the American Ceramic Society》1997,80(8):2005-2012
Final-stage sintering has been investigated in ultrahigh-purity Al2 O3 and Al2 O3 that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al2 O3 , block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al2 O3 , surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al2 O3 grain boundaries. 相似文献
7.
Investigation of Grain-Boundary Segregation in Ceramic Oxides by Analytical Scanning Transmission Electron Microscopy 总被引:1,自引:0,他引:1
Analytical scanning transmission electron microscopy (AEM) was used to study grain-boundary solute segregation in the systems MgO-NiO, A12 O3 -Y, O3 , NiO-Cr2 O3 , and NiO-Al2 O3 . Electron beam spreading within the specimen was incorporated into a model to quantitatively measure solute segregation. Grain-boundary segregation occurred in A12 O3 -Y2 O3 and NiO-Cr2 O3 but was not detected in MgO-NiO and NiO-Al2 O3 specimens. These results and the quantitative measurements agree with equilibrium solute segregation theories. Microhard-ness measurements indicate no difference in hardness between the grain boundary and the matrix for Cr-doped NiO, a system in which segregation was detected. 相似文献
8.
α - Al2 O3 nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al2 O3 seed polyacrylamide gel method were used to sinter bulk Al2 O3 nanoceramics. The relative density of the Al2 O3 nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al2 O3 nanopowders. The densification and fast grain growth of the Al2 O3 nanoceramics occur in different temperature ranges. The Al2 O3 nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering. 相似文献
9.
The kinetics of grain growth in fully dense Al2 O3 with and without MgO solute additions were measured for high-purity samples containing no liquid phases. The MgO was found to suppress the grain-boundary migration rate by a factor of 50. Compensating lattice defects are suggested to play a role in grain-growth inhibition. Implications of these results to the sintering of Al2 O3 are discussed. 相似文献
10.
Mikito Kitayama Kiyoshi Hirao Akira Tsuge Koji Watari Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》2000,83(8):1985-1992
Dense β-Si3 N4 with various Y2 O3 /SiO2 additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y2 O3 /SiO2 ratio increased. The oxygen contents in the β-Si3 N4 crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of β-Si3 N4 that was sintered at various Y2 O3 /SiO2 additive ratios has been clarified. 相似文献
11.
Jong-Heun Lee Toshiyuki Mori Ji-Guang Li Takayasu Ikegami Manabu Komatsu Hajime Haneda 《Journal of the American Ceramic Society》2000,83(5):1273-1275
The scavenging of a resistive siliceous phase via the addition of Al2 O3 was studied, using imaging secondary-ion mass spectroscopy (SIMS), given the improved grain-boundary conductivity in 8-mol%-yttria-stabilized zirconia (8YSZ). The grain-boundary resistivity in 8YSZ decreased noticeably with the addition of 1 mol% of Al2 O3 . Strong SiO2 segregation at the grain boundaries was observed in a SIMS map of pure 8YSZ that contained 120 ppm of SiO2 (by weight). The addition of 1 mol% of Al2 O3 caused the SiO2 to gather around the Al2 O3 particles. The present observations provided direct and visual evidence of SiO2 segregation at the grain boundaries (which had a deleterious effect on grain-boundary conductivity) and the scavenging of SiO2 via Al2 O3 addition. 相似文献
12.
Wen-Yu Zhao Ping Wei Hai-Bin Cheng Xin-Feng Tang Qing-Jie Zhang 《Journal of the American Ceramic Society》2007,90(7):2095-2103
Single-phase BaCoTiFe10 O19 (BaCoTi-M) nanoparticles were prepared by a modified sol–gel process, using metallic chlorides as starting materials. The physical chemistry process of BaCoTi-M formation, the interdependences between composition, technological conditions, microstructure, and magnetic properties were studied by X-ray diffraction (XRD), Fourier transform-infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). XRD and FTIR results show that BaCoTi-M nanoparticles formed directly from γ-Fe2 O3 , spinel ferrite, and barium salts without the formation of α-Fe2 O3 and BaFe2 O4 . The lattice shrinkage of BaCoTi-M nanoparticles that occurred on increasing the calcining temperature from 973 to 1173 K under holding for 2 h or on increasing the holding time in the range 0–2 h at 1173 K was discovered by analyzing the dependences of lattice parameters on the heat-treatment conditions. The shrinkage led to a relatively higher concentration of magnetic Fe3+ cations in the unit cell, and resulted in an increase of specific saturation magnetization under the corresponding conditions. Microstructural characterization shows that the evolutions of coercivity, remnant magnetization, and squareness ratio depended on the crystal growth and the reduction of structural defect as well as a decrease of grain boundary. 相似文献
13.
M. F. YAN 《Journal of the American Ceramic Society》1980,63(7-8):443-447
Sintering and microstructural evolution were studied in Fe3 O4 as a model system for spinel ferrites. Fe3 O4 powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO2 atmosphere. The p O2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe3 O4 without significantly affecting density. The measured grain-boundary mobilities, M , of Fe3 O4 fit the equation M=M 0 ( T ) p O2 −1/2 with M 0 ( T ) = 2.5×105 exp[-(609kJ·mol-1 / RT ](m/s)(N/m2 )−l . The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step. 相似文献
14.
Parjaree Thavorniti Yuichi Ikuhara Taketo Sakuma 《Journal of the American Ceramic Society》1998,81(11):2927-2932
The microstructures of 5 wt% SiO2 -doped TZP, 5 wt% (SiO2 + 2 wt% MgO)-doped TZP, and 5 wt% (SiO2 + 2 wt% Al2 O3 )-doped TZP are characterized by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. An amorphous phase is formed at multiple grain junctions but not along the grain-boundary faces in these three materials. A small addition of MgO and Al2 O3 into the SiO2 phase results in a marked reduction in tensile ductility of SiO2 -doped TZP. This reduction seems to correlate with segregation of magnesium or aluminum ions at grain boundaries and a resultant change in the chemical bonding state. 相似文献
15.
The faceting of alumina interfaces in the presence of a glass affects both grain growth and grain-boundary mobility during liquid-phase sintering. The geometry and movement of facets that form during this sintering process are expected to play an essential role in the development of the final microstructure, in particular, by their influence on the topology of the grain boundaries which ultimately control the properties of Al2 O3 compacts. A new method for studying the interaction between Al2 O3 and a glass has been developed. A thin sample of Al2 O3 suitable for examination in a transmission electron microscope is prepared and examined and then reacted with SiO2 and CaO via the vapor phase. This experimental approach allows the faceting behavior of glass/Al2 O3 interfaces to be studied systematically without introducing unnecessary complications during subsequent sample preparation. Faceting occurs almost exclusively on the (0001) and {1 1 02} planes. The interaction between glass and certain structured grain boundaries in alumina has been studied using polycrystalline thin films. 相似文献
16.
Values of D0 /δ and Q for grain-boundary diffusion, derived from published studies of normal grain growth in Al2 O3 , BeO, CaO, MgO, SiO2 , and CaSiO3 , are fit by the linear compensation equation log D0 /δ=0.03170Q -7.6792 (r2 =0.9384). Comparison of grain-boundary diffusion coefficients derived from grain growth in oxides with those obtained by direct experimental measurement suggests that the kinetics of normal grain growth are controlled by grain-boundary diffusion of oxygen. 相似文献
17.
Xin Guo 《Journal of the American Ceramic Society》2003,86(11):1867-1873
The Al2 O3 addition to stabilized ZrO2 has been studied for more than 20 years. In this article, literature and new results on the positive and negative effects of Al2 O3 additions on the electrical properties of ZrO2 are summarized and analyzed. In particular, a comprehensive grain-boundary conduction model is proposed. The Al2 O3 addition always increases the bulk resistivity, mainly because of the formations of defect associates and insulating Al2 O3 second-phase particles. The Al2 O3 addition within the solubility limit increases the grain-boundary resistivity, as a result of increased grain-boundary space-charge potential; the Al2 O3 addition above the solubility limit, however, scavenges the silicon-rich second phase from the grain boundaries, thereby decreasing the grain-boundary resistivity. 相似文献
18.
Chong-Min Wang Junghyun Cho Helen M. Chan Martin P. Harmer Jeffrey M. Rickman 《Journal of the American Ceramic Society》2001,84(5):1010-1016
The microstructural features and tensile creep behavior of Al2 O3 doped with Nd2 O3 at levels ranging from 100 to 1000 ppm (Nd:Al atomic ratio) were systematically investigated. Compositional mapping, using both high-resolution scanning transmission electron microscopy and secondary ion mass spectroscopy revealed that, for all of the compositions studied, the Nd3+ ions were strongly segregated to the Al2 O3 grain boundaries. Microstructural observations revealed that the solubility of Nd2 O3 was between 100 and 350 ppm. Tensile creep tests were conducted over a range of temperatures (1200°–1350°C) and stresses (20–75 MPa). Both the stress and grain-size exponents were analyzed. In selected experiments, controlled grain-growth anneals were used to enable creep testing of samples of the same average grain size but different neodymium concentrations. Independent of dopant level, the neodymium additions decreased the creep rate by 2–3 orders of magnitude, compared with that of undoped Al2 O3 . The value of the apparent creep activation energy increased with increased dopant concentration and then saturated at dopant levels exceeding the solubility limit. Overall, the results of the present study were consistent with a creep-inhibition mechanism whereby oversized segregant ions reduce grain-boundary diffusivity by a site-blocking mechanism. 相似文献
19.
Hitoshi Nishimura Katsuyuki Matsunaga Tomohiro Saito Takahisa Yamamoto Yuichi Ikuhara 《Journal of the American Ceramic Society》2003,86(4):574-80
Symmetrical Σ7 tilt grain boundaries of alumina (Al2 O3 ) were studied using bicrystals. Three types of Σ7 boundaries were successfully fabricated, that is, rhombohedral twin (Σ7{1[Onemacr]02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {4[Fivemacr]10} and {2[Threemacr]10} (Σ7{4[Fivemacr]10} and Σ7{2[Threemacr]10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Σ7{1[Onemacr]02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Σ7{2[Threemacr]10} and Σ7{4[Fivemacr]10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grain-boundary planes. 相似文献
20.
Keqin Huang Robin S. Tichy John B. Goodenough 《Journal of the American Ceramic Society》1998,81(10):2576-2580
Alternating current impedance spectroscopy and electron microscopy were used to investigate the effect of grain boundaries in LSGM-based materials. The impurity LaSrGa3 O7 existing in the samples La0.8 Sr0.2 Ga1- y Mg y -O3–0.5(0.2+ y ) ( y = 0.05–0.10) gives a contribution to a perfect or depressed grain-boundary semicircle in the impedance spectroscopy. The depressed semicircle represents a constant phase element (CPE) indicating LaSrGa3 O7 is an oxide-ion insulator. The grain-boundary semicircle vanishes if a hydrogen-containing atmosphere is applied. This fact may reflect proton conduction in LaSrGa3 O7 . In contrast, the impurity LaSrGaO4 existing, for example, in the samples x = 0.20, y = 0.25–0.30 gives no grain-boundary contribution to the impedance. 相似文献