共查询到20条相似文献,搜索用时 31 毫秒
1.
Sonia Faaland Tor Grande Mari-Ann Einarsrud Per Erik Vullum Randi Holmestad 《Journal of the American Ceramic Society》2005,88(3):726-730
Mechanical stress–strain behavior of LaCoO3 , La0.8 Ca0.2 CoO3 , and La0.7 Ca0.3 CoO3 was studied under compression at 25° and 300°C. A hysteresis in the stress–strain relationship due to reorientation of ferroelastic domains (deformation twins) was observed, and a remanent strain is measured after unloading. The cohersive stress, defined as the maximum in effective elastic compliance during first loading, increases with substitution of Ca for La and decreases with increasing temperature. Domain reorientation was confirmed by X-ray diffraction of surfaces parallel and perpendicular to the loading direction. LaCoO3 can be regarded as a soft ferroelastic material while the 30% Ca-substituted material is a hard ferroelastic. The hysteresis of the stress–strain relationship was clearly dependent on both composition and temperature. 相似文献
2.
Mode I Fracture Toughness of a Small-Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects 总被引:1,自引:0,他引:1
The Mode I fracture toughness ( K I C ) of a small-grained Si3 N4 was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si3 N4 grains was <0.4 µm, with aspect ratios of 2–8. K I C at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K IC was observed through 1200°C, to ∼4.1 MPa·m1/2 , before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in K IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K IC . Crack healing was observed in specimens annealed in air. No R -curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C. 相似文献
3.
Teruhisa Horita Natsuko Sakai Tatsuya Kawada Harumi Yokokawa Masayuki Dokiya 《Journal of the American Ceramic Society》1998,81(2):315-320
The grain-boundary-diffusion coefficient ( D gb ) of strontium in La0.9 Ca0.13 CrO3-δ was determined by secondary-ion mass spectrometry (SIMS), using two different measurement modes: depth profiling from the surface and line scanning on the fracture surface. The depth profiles that were sputtered by an O2 + -primary-ion beam gave two slopes of strontium concentration profiles, which corresponded to grain (bulk) and grain-boundary diffusion. The depth profiles were fitted to an appropriate equation that gave the grain- (bulk-) and grain-boundary-diffusion coefficients ( D bulk = 6.5 × 10-20 m2 /s and D gb = 1.6 × 10-15 m2 /s in air at 1273 K, respectively). Initially, to obtain the D gb value via the SIMS line-scanning measurement, the fracture surface of La0.9 Ca0.13 CrO3-delta was measured by a low-energy O2 + -primary-ion beam. The line-scanning measurement enabled us to successfully determine the strontium concentration profiles around the grain boundary. However, the D gb value that was obtained via the line-scanning mode was 6.0 10−13 m2 /s, which was a factor of 100 greater than that which was obtained by the depth-profile mode. Comparison between the depth-profile and line-scanning modes will require additional careful examination. 相似文献
4.
Yeong-Shyung Chou Jeffrey W. Stevenson Timothy R. Armstrong Larry R. Pederson 《Journal of the American Ceramic Society》2000,83(6):1457-1464
This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La1– x Sr x Co0.2 Fe0.8 O3 ( x = 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations ( x = 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m1/2 ) for low strontium ( x = 0.2) content and a lower value (∼1.1 MPa·m1/2 ) for the other batches ( x = 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode. 相似文献
5.
Antonia Pajares Fernando Guiberteau Arturo Dominguez-Rodriguez Arthur H. Heuer 《Journal of the American Ceramic Society》1988,71(7):332-C
Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4-mol%-Y2 O3 -stabilized ZrO2 single crystals. Kc is anisotropic, with values of 1.9 and 1.1 MPa·m1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single-edge notched-beam geometry, and yielded values of 1.9 and 1.5 MPa·m1/2 . 相似文献
6.
Fracture Mechanisms in Ferroelectric-Ferroelastic Lead Zirconate Titanate (Zr: Ti=0.54:0.46) Ceramics 总被引:1,自引:0,他引:1
Fracture toughness, K IC , of a single-phase commercial lead zirconate titanate (PZT) ceramic (Zr/Ti=0.54/0.46) of tetragonal structure ( c/a =1.019) was measured using the single edge notched beam method above and below the Curie temperature. Domain switching (poling) under electrical and mechanical loading was examined using X-ray diffraction. Surface grinding, electrical poling, and mechanical poling caused crystallographic texture. Similar texture, indicative of domain switching, was also observed on fracture surfaces of some saples fractured at room temperature. At room temperature, the highest K IC measured was 1.85 MPa·m1/2 , while above the Curie temperature it was about 1.0 MPa·m1/2 . Cracks emanating from Vickers indents in poled samples were different in the poling and the transverse directions. The difference in crack sizes is explained on the basis of domain switching during crack growth. These results indicate that ferroelastic domain switching (twinning) is a viable toughening mechanism in the PZT materials tested. 相似文献
7.
Yu-ichi Yoshizawa Kiyoshi Hirao Shuzo Kanzaki 《Journal of the American Ceramic Society》2004,87(11):2147-2149
The mechanical properties of a textured alumina made by high-temperature deformation of normal-purity sintered alumina have been investigated. The textured alumina shows very high bending strength and extremely high fracture toughness. Fracture toughness of more than 10 MPa·m1/2 was measured by the single-edge precracked beam method, and even using the single-edge V-notched beam method, toughness of over 8 MPa·m1/2 was obtained. This high fracture toughness was attributed to a large number of aligned small platelike grains of the textured structure enhancing the grain bridging effect. 相似文献
8.
An all-alkoxide route to films and nano-phase powders of the La0.5 Sr0.5 CoO3 perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La1− x Sr x )CoO3 , and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a c =3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a r =5.417 Å, αr =59.94°. Ninety to 130 nm thick films of La0.5 Sr0.5 CoO3 were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer. 相似文献
9.
Tsu-Yung Jin Dah-Shyang Tsai Wen-Hung Chung Feng-Jiin Liu 《Journal of the American Ceramic Society》2008,91(7):2217-2222
The electrochemical performance and structural features of (La1− y Ca y )(Co x Fe1− x )O3 cathode prepared via a citrate acid gel route are studied when it is interfaced with the (La1.8 Dy0.2 )(Mo2− z W z )O9 electrolyte. The resistance and chemical capacitance of a low-frequency arc are extracted from the impedance results to evaluate its catalytic activity in oxygen reduction reaction (ORR). (La0.75 Ca0.25 )(Co0.8 Fe0.2 )O3 cathode exhibits the minimum area-specific resistance of 0.9 Ω cm2 and maximum capacitance of 5.7 mF/cm2 at 800°C among the compositions of x =0.1–0.9 and y =0.25. As the Co content increases, the decrease in resistance outweighs the increase in capacitance so that the product of resistance and capacitance ( RC time constant) decreases. In contrast, when varying the Ca content of the A-site, the changes in resistance and the capacitance compensate each other; hence the RC time constant is virtually unchanged with respect to the calcium content. Thus, Co is a more influential element than Ca on the ORR catalytic activity. The pore structure study reveals a small amount of Mo diffuses from the electrolyte into the cathode, and its quantity is reduced when interfaced to an electrolyte of high W content. 相似文献
10.
Michael Rozumek Peter Majewski Fritz Aldinger 《Journal of the American Ceramic Society》2004,87(4):656-661
The metastable crystal structure of strontium- and magnesium-substituted LaGaO3 (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La0.825 Sr0.175 Ga0.825 Mg0.175 O2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La0.8 Sr0.2 Ga0.8 Mg0.2 O2.8 . At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α a ,ortho = 10.81 × 10−6 K−1 , α b ,ortho = 9.77 × 10−6 K−1 , and α c ,ortho = 9.83 × 10−6 K−1 (25°–400°C), and for the cubic modification to be αcubic = 13.67 × 10−6 K−1 (500°–1000°C). 相似文献
11.
JOSEPH T. LEWANDOWSKI ROBERT A. BEYERLEIN JOHN M. LONGO RONALD A. McCAULEY 《Journal of the American Ceramic Society》1986,69(9):699-703
The K2 NiF4 -type phase in the La-Co-O system has been studied and vacancy formation on both the lanthanum and oxygen sublattices has been observed. The formula La2 CoO4 has generally been accepted for the K2 NiF4 -type phase in the La-Co-O system. However, a broad range of lattice constants are reported for this material, indicative of variations in composition. More recently, a range of stoichiometries has been suggested in the literature which is represented by the formula La2 CoO4+x . The variations in stoichiometry have previously been explained by the presence of excess oxygen or by inter-growths of Ruddlesden-Popper-type phases, (AO(ABO3 )n ). In this work, X-ray diffraction data confirmed the previous variations in lattice parameters but indicated that the material is lanthanum deficient, not rich in oxygen. Thermogravimetric analysis confirmed the existence of variable oxygen stoichiometry. These data were consistent with a range of compositions described by the general formula La1.83 CoO4-x , where 0 ≤ x ≤ 0.13. These materials contain a maximum of 50% trivalent cobalt when x = 0. In addition, thermo-gravimetric experiments in lower partial pressures of oxygen have enabled us to isolate a new, fully reduced form, La1.83 CoO3.75 , which contains only divalent cobalt. A two- phase region exists between La1.83 CoO3.75 and La1.83 CoO3.87 . 相似文献
12.
Boron carbide/titanium diboride composites with 20 and 40 vol% particulate TiB2 and various amounts of free carbon were investigated with respect to microcrack toughening. In agreement with previous work, the mere addition of TiB2 was found to raise the toughness from 2.2 MPa·m1/2 up to 3.0 and 3.5 MPa·m1/2 , respectively. A further and very significant increase of composite toughness up to 6.0 MPa·m1/2 was discovered upon the incorporation of free carbon. SEM and TEM observations reveal that this toughening is associated with microcracking at B4 C-TiB2 phase boundaries. Microcracking is triggered by thin carbon interlayers, which are located at hetero interfaces and supply a weak fracture path. 相似文献
13.
The mode I, mode II, and combined mode Imode II fracture behavior of ceria-doped tetragonal zirconia polycrystalline (Ce-TZP) ceramic was studied. The single-edge-precracked-beam (SEPB) samples were fractured using the asymmetric four-point-bend geometry. The ratio of mode I to mode II loading was varied by varying the degree of asymmetry in the four-point-bend geometry. The minimum strain energy density theory best described the mixed-mode fracture behavior of Ce-TZP with the mode I fracture toughness, K IC = 8.2 ± 0.6 MPa·m1/2 , and the mode II fracture toughness, KIIC = 8.6± 1.3 MPa·m1/2 . 相似文献
14.
Wei Chen Doru C. Lupascu Jürgen Rödel Christopher S. Lynch 《Journal of the American Ceramic Society》2001,84(3):593-597
This work describes the measurement of R -curve behavior in ferroelectric ceramics using four-point bend specimens with controlled semielliptical surface cracks. The results are compared for two compositions of lead lanthanum zirconate titanate. One exhibits ferroelastic behavior, the other electrostrictive linear elastic behavior. R -curves are measured in the crack length regime of 0.1 to 0.8 mm. The ferroelastic composition displays a toughness increase from 0.5 to 1.2 MPa·m1/2 . The linear elastic composition displays a flat R -curve. The R -curve behavior is attributed to ferroelastic toughening. 相似文献
15.
Sathi Rajachandran Nair Ram Dayal Purohit Avesh Kumar Tyagi Pankaj Kumar Sinha Beant Prakash Sharma 《Journal of the American Ceramic Society》2008,91(1):88-91
Ultrafine La(Ca)CrO3 (LCC) powder was prepared through the glycine–nitrate gel combustion process. It was shown for the first time that the use of relatively inexpensive CrO3 as a starting material for chromium has potential for the bulk preparation of sinter-active LCC powder. As-prepared powder, when calcined at 700°C, resulted in LCC along with a small amount of CaCrO4 . The calcined powder was found to be composed of soft agglomerates with a particle size of ≈70–290 nm. The cold pressing and sintering of the calcined powder at 1200°C resulted in the mono-phasic La0.7 Ca0.3 CrO3 with density ≈98% of its theoretical value. This is the lowest sintering temperature ever reported for La0.7 Ca0.3 CrO3 . The conductivity of the sintered La0.7 Ca0.3 CrO3 at 1000°C was found to be ≈57 S/cm in air. The sintering and electrical behavior achieved for La0.7 Ca0.3 CrO3 may find application as an interconnect material for high-temperature solid oxide fuel cells if problems with chemical expansion and poor conductivity in fuel can be overcome. 相似文献
16.
Hitoshi Taimatsu Keiji Wada Hiroyuki Kaneko Hiroshi Yamamura 《Journal of the American Ceramic Society》1992,75(2):401-405
The reaction of La1- x Ca x MnO3 ( x = 0, 0.1, 0.2) with ZrO2 -8 mol% Y2 O3 (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO3 depresses the reactivity with YSZ. A layer of La2 Zr2 O7 is formed at the La1- x Ca x MnO3 /YSZ interface after an induction period, and its formation is accelerated when the La1- x Ca x MnO3 phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO3 /single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed. 相似文献
17.
Keqin Huang Robin Tichy John B. Goodenough Christopher Milliken 《Journal of the American Ceramic Society》1998,81(10):2581-2585
The performance of La0.8 Sr0.2 Ga0.83 Mg0.17 O2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500-µm-thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO3 . The interlayer Sm-CeO2 was selected and sandwiched between the electrolyte and anode. Comparison of Sm-CeO2 /Sm-CeO2 + Ni and Sm-CeO2 + Ni as anodes showed that Sm-CeO2 /Sm-CeO2 + Ni gave an exchange current density 4 times higher than that of Sm-CeO2 + Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La0.6 Sr0.4 CoO3-delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-µm-thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm2 to 550 mW/cm2 at 800°C. The excellent cell performance showed that an LSGM-based thick membrane SOFC operating at temperatures 600° < T op < 800°C is a realistic goal. 相似文献
18.
The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r m , and the relationship S = A m /( r m )1/2 , where A m is the "mirror constant." The present work estimates the value of A m for Tyranno® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m1/2 . This value is within the range of 2–2.51 MPa·m1/2 previously obtained for nominally similar Nicalon® Si-C-O fibers. 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.
Yoshihiko Takano Masaru Yoshinaka Ken Hirota Osamu Yamaguchi 《Journal of the American Ceramic Society》2001,84(10):2445-2447
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 Al2 O3 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 Al2 O3 (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)/Al2 O3 = 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