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  • 1
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Nucleation and growth of mullite whiskers in the La2O3−Al2O3−SiO2 system were investigated in the 1500°–1700°C temperature range. A differentiaC Bhermal analysis (DTA) showed that the mullitization temperature decreases from 1350°C to 1240°C as a result of lanthania doping. In the temperature range of 1250°–1500°C, most of the mullite grains have an Al2O3/SiO2= 1.5 composition throughout the ceramic body; however, from 1400°C upward, the number of anisotropic grains with the Al2O3/SiO2= 1.3 composition begins to increase. The concentration of alumina in the composition of the grain-boundary phase decreases as firing temperatures increase. At temperatures 〉 1500°C, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al2O3/SiO2= 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina-rich, rosace-like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 μm/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: We have investigated the potential use of Bi4Ge3O12 (BGO) laser-sintered ceramics in scintillator devices for radiation detection. Relevant results from the characterization made during the ceramic processing are presented, such as particle size analysis, X-ray diffraction studies, and thermal analysis. Furthermore, we have compared the microstructure, relative density, thermoluminescent (TL) emission, and transparency degree of ceramic bodies sintered by CO2 laser heating with a reference set of ceramics sintered in an electrical furnace, and also with single crystals grown by the Czochralski technique. It was observed that the laser-sintered BGO presented a relative density of 98% (±2%), better pore shrinkage, larger grains, and light transmission intensity 1.5 times higher than the BGO ceramics sintered in a furnace. The TL curves of all samples irradiated with UV and β−rays presented peaks at 75°, 102°, and 143°C, and provided strong indication that the intergrain defects have the same nature of the bulk defects and do not contribute with new traps in the temperature range studied. The relative areas of the total TL emission after β irradiation were 6,0:4,4:1 for the reference ceramic, laser sintered ceramic, and single crystal, respectively. For UV irradiated samples, this relation was 7,1:4,7:1. From these results, we have concluded that laser-sintered ceramics have an amount of charge traps lower than the conventionally sintered BGO samples, thus having a higher radioluminescence yield.
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  • 3
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide–silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0–8.0 h resulted in a high surface concentration of mullite whiskers. Their c-axes were aligned preferentially along the pellet surface; the maximum length was 50 μm, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm3 in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30–1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Westerville, Ohio : American Ceramics Society
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Crack-velocity (v–K) curves and crack-resistance (R) curves for unpoled ferroelectric and ferroelastic lead zirconate titanate (PZT) were determined for long cracks in compact-tension (CT) geometry using an in situ fracture device on the stage of an optical microscope. The steady-state crack length and the plateau value of R-curves measured at controlled constant velocities increased with increased velocity. The plateau value for 10−6 m/s was 1.2 MPa·m1/2 after 1.3 mm of crack extension and for 10−4 m/s was 1.4 MPa·m1/2 after 2.2 mm.
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  • 5
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A description is given of the nonohmic behavior obtained in (SnxTi1−x)O2-based systems. A matrix founded on (SnxTi1−x)O2-based systems doped with Nb2O5 leads to a low-voltage varistor system with nonlinear coefficient values of ∼9. The presence of the back-to-back Schottky-type barrier is observed based on the voltage dependence of the capacitance. When doped with CoO, the (SnxTi1−x)O2-based system presents higher nonlinear coefficient values (〉30) than does the SnO2-based varistor system.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: R-curves of lead zirconate titanate (PZT) have been measured with compact tension (CT) specimens for different poling conditions and grain sizes. Depending on poling direction the plateau value of the R-curves ranged from 1.13 to 1.54 MPa·m1/2 for a grain size of 6.4 μm and from 1.14 to 1.30 MPa·m1/2 for a grain size of 5.2 μm. Poling in the thickness direction yielded the material with the highest fracture toughness while the direction parallel to the loading direction led to the lowest fracture toughness.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: PbO-BiO1.5-GaO1.5-based glasses are good candidates for optical applications, because of some of their interesting characteristics, such as high refraction indices and high transmission in the ultraviolet (UV), visible (VIS), and infrared (IR) regions. A limited stage in the processing of these glasses is the corrosion that is caused by the melt in all currently used conventional crucibles, such as noble metals (platinum or gold) and Al2O3. The absorption of crucible material by the glass composition may reduce the transmission level, the cutoff in the UV-VIS, and IR regions, and the thermal stability. In this study, a SnO2 crucible has been tested for PbO-BiO1.5-GaO1.5 molten glass. Optical and thermal analyses show, in some cases, advantages over the use of platinum and Al2O3 crucibles. A visible cutoff value of 474 nm has been measured, and a longer melting time (850°C for 4 h) results in a significant reduction of the O-H absorption band at 3.2 μm.
    Type of Medium: Electronic Resource
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