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Calcium hexaluminate (CA6) was incorporated into the matrix of magnesio aluminate spinel-alumina (MA-A) via infiltration of a porous preform fabricated from α-Al2O3 and MgO powders with a saturated calcium acetate solution and subsequent firing, forming CA6/(MA-A) functionally composites with graded fracture toughness. Actually, the porous preform was partially and perpendicularly immersed (1/4 of its length) in the solution. Owing to the capillary action, the calcium acetate solution was absorbed into the porous preform, and the different absorption distance led to the graded solution concentration in the height direction of the porous preform. The in-situ formation of CA6 conferred graded microstructures, as well as improved mechanical properties on the resultant composites. The CA6 content decreased gradually along the solution absorption direction, i.e., from one end [CA6/(MA-A) region] immersed in solution to the other end [MA-A region], reducing evidently the formation of layered structure along the direction, while increasing gradually the formation of spherical alumina particles. The CA6/(MA-A) region had a better toughness that could prevent the crack propagation and improve the spalling resistance. Meanwhile, the MA-A region could provide structural support, because of the higher Vickers hardness and density. [ABSTRACT FROM AUTHOR]

Calcium hexaluminate (CA6) was incorporated into the matrix of magnesio aluminate spinel-alumina (MA-A) via infiltration of a porous preform fabricated from a-Al2O3 and MgO powders with a saturated calcium acetate solution and subsequent firing, forming CA6/(MA-A) functionally composites with graded fracture toughness. Actually, the porous preform was partially and perpendicularly immersed (1/4 of its length) in the solution. Owing to the capillary action, the calcium acetate solution was absorbed into the porous preform, and the different absorption distance led to the graded solution concentration in the height direction of the porous preform. The in-situ formation of CA6 conferred graded microstructures, as well as improved mechanical properties on the resultant composites. The CA6 content decreased gradually along the solution absorption direction, i.e., from one end [CA6/(MA-A) region] immersed in solution to the other end [MA-A region], reducing evidently the formation of layered structure along the direction, while increasing gradually the formation of spherical alumina particles. The CA6/(MA-A) region had a better toughness that could prevent the crack propagation and improve the spalling resistance. Meanwhile, the MA-A region could provide structural support, because of the higher Vickers hardness and density. [ABSTRACT FROM AUTHOR]

Preparation of nanomaterials with various morphologies and exploiting their novel physical properties are of vital importance in nanoscientific field. Similarly to the III-N compound semiconductors, Si3N4 nanostructures also could be potentially used for making optoelectronic devices. In this paper, we report on an improved Fe-catalyzed chemical vapour deposition method for synthesizing ultra-long α-Si3N4 nanobelts along with a few nanowires and nanobranches on a carbon felt substrate. The ultra-long α-Si3N4 nanobelts grew via a combined VLS-base and nanobranches via a combined double-stage VLS-base and VS-tip mechanism, as well as nanowires via VLS-tip mechanism. The three individual nanostructures showed variant optical properties as revealed by a cathodoluminescence spectroscopy. A single α-Si3N4 nanobelt or nanobranch gave a strong UV-blue emission band as well as a broad red emission, whereas a single α-Si3N4 nanowire exhibited only a broad UV-blue emission. The results reported would be useful in developing new photoelectric nanodevices with tailorable or tunable properties. [ABSTRACT FROM AUTHOR]