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排序方式: 共有46条查询结果,搜索用时 15 毫秒
1.
This paper describes a new multilayer TBC that incorporates a 10 μm-thick oxygen barrier layer of yttrium–aluminum garnet (YAG) into a typical YSZ TBC system. The thermal conductivity of as-sprayed YAG/YSZ coatings was reduced due to excessive porosity and amorphous areas in the YAG layer. After long-term heat treatments, the conductivity of the multilayer was unaffected by the presence of YAG. Sintering and recrystallization of the amorphous YAG regions occurred during high-temperature heat treatments. While YAG itself possesses excellent phase stability, its presence also improved the phase stability of zirconia near the YAG/YSZ interface, inhibiting the outward diffusion of yttrium from high-yttria t-ZrO2 The YAG layer reduced the NiCoCrAlY bond-coat oxidation rate by a factor of three during isothermalfurnace tests conducted at 1200° C. 相似文献
2.
Additive Manufacturing of Dense Ceramic Parts via Direct Ink Writing of Aqueous Alumina Suspensions 总被引:1,自引:0,他引:1
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Lisa Rueschhoff William Costakis Matthew Michie Jeffrey Youngblood Rodney Trice 《International Journal of Applied Ceramic Technology》2016,13(5):821-830
Additive manufacturing of near‐net‐shaped dense ceramic components has been established via room‐temperature direct writing of highly loaded aqueous alumina suspensions in a layer‐by‐layer fashion. The effect of alumina solid loading on rheology, specimen uniformity, density, microstructure, and mechanical properties was studied. All suspensions contained a polymer binder (~5 vol.%), dispersant, and 51–58 vol.% alumina powder. Rheological measurements indicated all suspensions to be yield‐pseuodoplastic, and both yield stress and viscosity were found to increase with increasing alumina solid loading. Shear rates ranging from 19.5 to 24.2/s, corresponding to viscosities of 9.8 to 17.2 Pa·s, for the 53–56 vol.% alumina suspensions were found to produce the best results for the 1.25‐mm tip employed during writing. All parts were sintered to >98% of true density, with grain sizes ranging from 3.2 to 3.7 μm. The average flexure strength, which ranged from 134 to 157 MPa, was not influenced by the alumina solid loading. 相似文献
3.
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
4.
Horizontal Dip‐Spin Casting of aqueous alumina‐polyvinylpyrrolidone suspensions with chopped fiber
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Valerie Wiesner Manuel Acosta Lisa Rueschhoff Jeffrey Youngblood Rodney Trice 《International Journal of Applied Ceramic Technology》2017,14(6):1077-1087
A novel ceramic processing method, called Horizontal Dip Spin Casting (HDSC), enabled fabrication of tubular ceramic parts with an aligned chopped fiber phase. HDSC was demonstrated using highly loaded aqueous alumina suspensions with >50 vol.% solids loading and ≤5 vol.% water‐soluble polymer employed as a rheological modifier. Chopped carbon fibers were added to the suspensions to attain maximum loadings of 30 vol.%. During forming, cylindrical foam molds were dipped into the suspension while being rotated radially about the long axis. Simultaneously, a doctor blade was placed at a specified distance from the foam surface to facilitate the flow of the suspension to align the fiber and control the thickness of the material that accrued on the mold. Rheological study of alumina‐PVP suspensions with and without chopped carbon fiber showed that the suspensions exhibited a yield‐pseudoplastic flow behavior. The degree of alignment of the carbon fiber phase in the green bodies was characterized for various suspension formulations, carbon fiber contents and forming speeds. Stereological characterization of green body specimens confirmed the effectiveness of HDSC to attain the desired tubular geometry with considerable fiber alignment for a suspension composition containing ≤20 vol.% chopped fibers. 相似文献
5.
Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al2O3 in the coating. 相似文献
6.
C. Favazza J. Trice A. K. Gangopadhyay H. Garcia R. Sureshkumar Ramki Kalyanaraman 《Journal of Electronic Materials》2006,35(8):1618-1620
Most metals on SiO2 have a finite contact angle and are therefore subject to dewetting during thermal processing. The resulting dewetting morphology
is determined primarily by nucleation and growth or instabilities. The dewetting mechanism implies a disordered spatial arrangement
for homogeneous nucleation, but an ordered one for instabilities such as spinodal decomposition. Here, we show that the morphology
of laser-melted ultrathin Co film (4-nm thick) can be attributed to dewetting via an instability. Dewetting leads to breakup
of the continuous Co film into nanoparticles with a monomodal size distribution with an average particle diameter of 75 nm±23
nm. These nanoparticles have short-range order (SRO) of 130 nm in their separation. This result has important implications
for nanomanufacturing with a robust spacing or size selection of nanoparticles in addition to spatial ordering. 相似文献
7.
Fibrous Monolithic Ceramics 总被引:2,自引:0,他引:2
Desiderio Kovar Bruce H. King Rodney W. Trice John W. Halloran 《Journal of the American Ceramic Society》1997,80(10):2471-2487
Fibrous monolithic ceramics are an example of a laminate in which a controlled, three-dimensional structure has been introduced on a submillimeter scale. This unique structure allows this all-ceramic material to fail in a nonbrittle manner. Materials have been fabricated and tested with a variety of architectures. The influence on mechanical properties at room temperature and at high temperature of the structure of the constituent phases and the architecture in which they are arranged are discussed. The elastic properties of these materials can be effectively predicted using existing models. These models also can be extended to predict the strength of fibrous monoliths with an arbitrary orientation and architecture. However, the mechanisms that govern the energy absorption capacity of fibrous monoliths are unique, and experimental results do not follow existing models. Energy dissipation occurs through two dominant mechanisms—delamination of the weak interphases and then frictional sliding after cracking occurs. The properties of the constituent phases that maximize energy absorption are discussed. 相似文献
8.
Zun Chen Scott Speakman Jane Howe Hsin Wang Wally Porter Rodney Trice 《Journal of the European Ceramic Society》2009,29(8):1403-1411
The phase evolution occurring during the reaction between corrosive V2O5 (Tm = 690 °C) and a plasma-sprayed 7 wt.% Y2O3–ZrO2 (YSZ) coating from 700 to 900 °C has been investigated in situ by X-ray diffraction. The temperature and time of interaction between the V2O5 and YSZ coating determines the phases observed. Between 700 and 750 °C, reaction products of ZrV2O7 and YVO4 were observed within minutes of reaching the test temperature. m-ZrO2 was observed after 220 and 60 min at 700 and 750 °C, respectively. The simultaneous formation of both ZrV2O7 and YVO4 at the beginning of the reaction along with the delay of the m-ZrO2 formation suggests similar reactivity between both Zr and Y with V2O5. The weight percent of the ZrV2O7 phase began to diminish after 150 and 60 min at 700 and 750 °C, respectively. For reaction temperatures of 800 and 900 °C, there is a rapid decrease in the amount of t′-ZrO2 and a rapid increase in the amount of m-ZrO2 with reaction time. YVO4 was also observed at these reaction temperatures. SEM and TEM microstructural observations confirmed the phases detected from the in situ XRD experiments. Reactions between YSZ and V2O5 suggest that the formation of a liquid phase due to the high solubility of both zirconia and yttria in vanadia is the dominate mechanism that damages the coating. The thermal conductivity of a plasma-sprayed YSZ coating reacted with up to 1 wt.% V2O5 did not significantly change due to the small volume affected. 相似文献
9.
Religious beliefs about mental illness represent one potential influence on the choice to utilize or avoid professional help. For example, believing that devout spirituality assures mental health and/or that mental illness indicates spiritual failure may discourage religious individuals from seeking help. Such beliefs have traditionally been attributed to Pentecostals, but no research has assessed this assumption. This study examined Pentecostal perspectives on depression's causes and treatments. Contrary to the Pentecostal theological literature, participants endorsed a variety of causal factors. Regarding cures, however, faith was endorsed as the most effective option. Implications are provided and recommendations are offered to practitioners working with Pentecostals, including consultation and collaboration with clergy and religiously sensitive psychoeducational programs. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
10.
Using a role and exchange theory framework, this study examines the commitment to their organization and to the federal service of 634 managers in 71 federal government organizations. Results indicate that certain role factors such as tenure and work overload and personal factors such as attitude toward change and job involvement are strong influences on commitment. Implications of the findings and the need for further theoretical and methodological refinements are discussed. 相似文献