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1.
Ziqi Sun Yanchun Zhou Jingyang Wang Meishuan Li 《Journal of the American Ceramic Society》2007,90(8):2535-2541
In this paper, the mechanical properties of bulk single-phase γ-Y2 Si2 O7 ceramic are reported. γ-Y2 Si2 O7 exhibits low shear modulus, excellent damage tolerance, and thus has a good machinability ready for metal working tools. To understand the underlying mechanism of machinability, drilling test, Hertzian contact test, and density functional theory (DFT) calculation are employed. Hertzian contact test demonstrates that γ-Y2 Si2 O7 is a "quasi-plastic" ceramic and the intrinsically weak interfaces contribute to its machinability. Crystal structure characteristics and DFT calculations of γ-Y2 Si2 O7 suggest that some weakly bonded planes, which involve Y–O bonds that can be easily broken, are the sources of the low shear deformation resistance and good machinability. 相似文献
2.
γ-Y2 Si2 O7 is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10−6 /K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y2 Si2 O7 in thin-film molten Na2 SO4 at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y2 Si2 O7 exhibited good resistance against Na2 SO4 molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na2 SO4 and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y2 Si2 O7 in Na2 SO4 molten salt with flowing air was evaluated to be 255 kJ/mol. 相似文献
3.
Ian MacLaren Roland Schierholz Paul A. Trusty Clive B. Ponton 《Journal of the American Ceramic Society》2007,90(10):3307-3310
Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y2 Si2 O7 containing rounded glassy Y-doped SiO2 inclusions; excess glassy SiO2 -rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO2 excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined. 相似文献
4.
William E. Lee Charles H. DrummondIII Gregory E. Hilmas Suresh Kumar 《Journal of the American Ceramic Society》1990,73(12):3575-3579
Near-eutectic composition Y2 O3 ─SiO2 melts were formed as bulk samples or as an intergranular phase in Si3 N4 . Upon cooling to room temperature the bulk material partially crystallized to γ-Y2 -Si2 O7 whereas the intergranular phase was glass. On heat-treating at 1500°C the bulk material transformed to γ-Y2 Si2 O7 whereas the intergranular glass crystallized first to γ-Y2 Si2 O7 and then to β-Y2 Si2 O7 . Possible reasons for the different behavior are discussed. 相似文献
5.
Ziqi Sun Xinwen Zhu Meishuan Li Yanchun Zhou Yoshio Sakka 《Journal of the American Ceramic Society》2009,92(1):54-61
The dispersion of aqueous γ-Y2 Si2 O7 suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pHIEP =10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y2 Si2 O7 suspensions. 相似文献
6.
An electroconductive TiN/Al2 O3 nanocomposite was prepared by a selective matrix grain growth method, using a powder mixture of submicrosized α-Al2 O3 , nanosized γ-Al2 O3 , and TiN nanoparticles synthesized through an in situ nitridation process. During sintering, a self-concentration of TiN nanoparticles at the matrix grain boundary occurred, as a result of the selective growth of large α-Al2 O3 matrix grains. Under suitable sintering conditions, a typical interlayer nanostructure with a continuous nanosized TiN interlayer was formed along the Al2 O3 matrix grain boundary, and the electroconducting behavior of the material was significantly improved. Twelve volume percent TiN/Al2 O3 nanocomposite with such an interlayer nanostructure showed an unprecedentedly low resistivity of 8 × 10−3 Ω·cm, which was more than two orders lower than the TiN/Al2 O3 nanocomposite without such an interlayer nanostructure. 相似文献
7.
Subsolidus phase relations were established in the system Si3 N4 -SiO2 -Y2 O3 . Four ternary compounds were confirmed, with compositions of Y4 Si2 O7 N2 , Y2 Si3 O3 N4 , YSiO2 N, and Y10 (SiO4 )6 N2 . The eutectic in the triangle Si3 N4 -Y2 Si2 O7 -Y10 (SiO4 )6 N2 melts at 1500°C and that in the triangle Si2 N2 O-SiO2 -Y2 Si2 O7 at 1550°C. The eutectic temperature of the Si3 N4 -Y2 Si2 O7 join was ∼ 1520°C. 相似文献
8.
Equilibrium relationships in the system Al2 O3 -Ce2 Si2 O7 in inert atmosphere have been investigated in the temperature range 900° to 1925°C. A simple eutectic reaction was found at 1375°C and 51 mol% Ce2 Si2 O7 . A high-low polymorphic transformation in Ce2 Si2 O7 was observed at 1274°C. New XRD patterns are suggested for both polymorphs of cerium pyrosilicate. The melting point of Ce2 Si2 O7 was found to be 1788°C. A value for ΔH°m,Ce2Si2O7 of 36.81 kJ/mol was calculated from the initial slope of the experimentally determined liquidus in equilibrium with the pyrosilicate phase. 相似文献
9.
ANTHONY R. WEST 《Journal of the American Ceramic Society》1976,59(3-4):124-127
Ternary phase relations have been studied and several modifications made to Kracek's phase equilibrium diagram. These include location of the primary phase fields of Na6 Si8 O19 and Li2 Si2 O5 . Metastable phases and polymorphs were often encountered, notably a primary phase, δ-Na2 Si2 O5 , and a new polymorph of Li2 Si2 O5 . 相似文献
10.
Crystallizing the grain-boundary glass of a liquid-phase-sintered Si3 N4 ceramic for 2 h or less at 1500° led to formation of δ-Y2 Si2 O7 . After 5 h at 1500°, the δ-Y2 Si2 O7 had transformed to β-Y2 Si2 O7 with a concurrent dramatic increase in dislocation density within β-Si3 N4 grains. Reasons for the increased dislocation density are discussed. Annealing for 20 h at 1500° reduced dislocation densities to the levels found in as-sintered material. 相似文献
11.
Mark I. Jones Hideki Hyuga Kiyoshi Hirao Yukihiko Yamauchi 《Journal of the American Ceramic Society》2004,87(4):710-713
α-SiAlONs with equiaxed and elongated microstructures stabilized with Y2 O3 and Lu2 O3 were produced by hot pressing, and the phase structure and room- and high-temperature mechanical properties were assessed. Additional liquid added to the starting composition in the form of 5 wt% rare-earth monosilicate resulted in the formation of elongated microstructures and improvements in room-temperature strength and fracture toughness. The elongated grain growth was promoted by the additional liquid phase, which crystallized to form a secondary grain-boundary phase thought to be J ' (Re4 Si2– x Al x O7+ x N2– x ). For the equiaxed and the elongated samples, those sintered with Lu2 O3 showed higher hardness than the comparable Y2 O3 -sintered materials, and, at elevated temperature, the strength retention of the elongated Lu2 O3 SiAlON was much higher than that of the Y2 O3 sample, which was attributed to properties of the residual grain-boundary phase associated with the difference in the cationic radius of the stabilizing cation. 相似文献
12.
A novel porous Yb4 Si2 O7 N2 material with uniform open-cell network structure was fabricated from the reaction between Si3 N4 , Yb2 O3 , and SiO2 . The formation of Yb4 Si2 O7 N2 during heating was studied using X-ray diffractometry. The porous structure was characterized using scanning electron microscopy and mercury porosimeter. It is shown that the formation of Yb4 Si2 O7 N2 phase starts at ∼1150°C and completes at 1350°C for 4 h, accompanied by the development of open-cell network structure. The necks between Yb4 Si2 O7 N2 particles become much thicker with increasing temperature because of the coarsening of Yb4 Si2 O7 N2 particles, thus leading to a uniform three-dimensional network structure. Furthermore, the pore size can be well controlled by adjusting reacting temperature and altering atmosphere. 相似文献
13.
Rajiv S. Mishra Charles E. Lesher Amiya K. Mukherjee 《Journal of the American Ceramic Society》1996,79(11):2989-2992
High-pressure sintering of nanocrystalline γ-A12 O3 has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al2 O3 to α-Al2 O3 transformation temperature changed from 1473 K without pressure to ∼1023 K at 1 GPa. Full density was obtained at 1273 and 1323 K in 10 min. The microhardness value of fully dense α-alumina with a grain size of 142 nm was found to be 25.3 ± 0.8 GPa. The Hall-Petch slope for the very fine grain size range is different from that of the coarse-grained alumina. 相似文献
14.
A possibility to produce microwave (MW) dielectric materials by liquid-phase sintering of fine particles was investigated. Zn3 Nb2 O8 powders with a grain size 50–300 nm were obtained by the thermal decomposition of freeze-dried Zn–Nb hydroxides or frozen oxalate solutions. The crystallization of Zn3 Nb2 O8 from amorphous decomposition products was often accompanied by the simultaneous formation of ZnNb2 O6 . Maximum sintering activity was observed for single-phase crystalline Zn3 Nb2 O8 powders obtained at the lowest temperature. The sintering of as-obtained powders with CuO–V2 O5 sintering aids results in producing MW dielectric ceramics with a density 93%–97% of the theoretical, and a Q × f product up to 36 000 GHz at sintering temperature ( T s )≥680°C. The high level of MW dielectric properties of ceramics was ensured by intensive grain growth during the densification and the thermal processing of ceramics. 相似文献
15.
An intimate Ba-Al-Al2 O3 -SiO2 powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl2 Si2 O8 . Barium oxidation was completed, and a binary silicate compound, Ba2 SiO4 , had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl2 O4 had formed. Diffraction peaks consistent with hexagonal BaAl2 Si2 O8 , BaAl2 O4 , β-BaSiO3 , and possibly β-BaSi2 O5 were detected after 24 h at 900°C. Diffraction peaks for BaAl2 O4 and BaAl2 Si2 O8 were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl2 O4 at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl2 Si2 O8 . The observed reaction path was compared to prior work with other inorganic precursors to BaAl2 Si2 O8 . 相似文献
16.
The effect of Cr and Fe in solid solution in γ-Al2 O3 on its rate of conversion to α-Al2 O3 at 1100°C was studied by X-ray diffraction. The δ form of Al2 O3 was the principal intermediate phase produced from both pure γ-Al2 O3 and that containing Fe3+ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr6+ in γ-Al2 O3 and as Cr3+ in α-Al2 O3 , with θ-Al2 O3 as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al2 O3 were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al2 O3 formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al2 O3 , within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O2- ions by synchro-shear. 相似文献
17.
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
18.
Yoshikazu Suzuki Peter E. D. Morgan Koichi Niihara 《Journal of the American Ceramic Society》1998,81(12):3141-3149
Additions of 1-20 mol% Sc2 O3 or Y2 O3 to MoSi2 eliminate glassy SiO2 , which improves mechanical properties at both ambient and high temperatures. In particular, only 1 mol% ScO3 additions dramatically enhance three-point bending strength from 521 to 1081 MPa. Vickers hardness, Young's modulus, fracture toughness, and high-temperature strength are also improved by this low level of additive. The improvement of mechanical properties is attributed to the formation of crystalline silicates: Sc2 Si2 O7 , Y2 Si2 O7 , Y2 SiO5 , and Y4 Si3 O12 , which are analyzed by XRD, SEM-EDS, and TEM-EDS methods. 相似文献
19.
Jiang Li Yubai Pan Yusong Wu Huamin Kou Jingkun Guo 《International Journal of Applied Ceramic Technology》2007,4(3):276-284
High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO2 as additives using α-Al2 O3 -seeded nanocrystalline γ-Al2 O3 powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al2 O3 sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m1/2 , respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m1/2 . 相似文献
20.
Naoto Hirosaki Yoshinobu Yamamoto Toshiyuki Nishimura Mamoru Mitomo Junichi Takahashi Hisanori Yamane Masahiko Shimada 《Journal of the American Ceramic Society》2002,85(11):2861-2863
Phase relationships in the Si3 N4 –SiO2 –Lu2 O3 system were investigated at 1850°C in 1 MPa N2 . Only J-phase, Lu4 Si2 O7 N2 (monoclinic, space group P 21 / c , a = 0.74235(8) nm, b = 1.02649(10) nm, c = 1.06595(12) nm, and β= 109.793(6)°) exists as a lutetium silicon oxynitride phase in the Si3 N4 –SiO2 –Lu2 O3 system. The Si3 N4 /Lu2 O3 ratio is 1, corresponding to the M-phase composition, resulted in a mixture of Lu–J-phase, β-Si3 N4 , and a new phase of Lu3 Si5 ON9 , having orthorhombic symmetry, space group Pbcm (No. 57), with a = 0.49361(5) nm, b = 1.60622(16) nm, and c = 1.05143(11) nm. The new phase is best represented in the new Si3 N4 –LuN–Lu2 O3 system. The phase diagram suggests that Lu4 Si2 O7 N2 is an excellent grain-boundary phase of silicon nitride ceramics for high-temperature applications. 相似文献