Transparent polycrystalline nanoceramics consisting of triclinic Al₂SiO₅ kyanite and Al₂O₃ corundum

Abstract

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite (91.4 vol%) and Al2O3 corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by direct conversion from Al2O3‐SiO2 glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.

Microstructural effects on hardness and optical transparency of birefringent aluminosilicate nanoceramics

“…The quantitative chemical composition of the starting glass was determined to be homogenous for the entire glass sphere with 57.4 ± 0.5 mol% Al 2 O 3 and 42.6 ± 0.5 mol% SiO 2 . Additional details can be found elsewhere …”

“…These anvils compressed a pressure medium consisting of a 5% Cr 2 O 3 doped octahedral MgO with edge length of 18 mm (and 14 mm for the 6‐rams LVP) with cylindrical sample and resistive heater (LaCrO 3 ) inside. Further details of the high‐ PT syntheses are described elsewhere . The sample with highest transparency was reproducible synthesized in three experiments at 10 GPa and 1200°C.…”

“…Transmission electron microscopy (TEM) investigations were performed with a FEI Tecnai F30 STwin (FEG, 300 kV) installed at the Institute for Materials Science at Kiel University (Germany). Details of microstructure characterization and light transmission measurement can be found in Gaida et al…”

“…Nanoceramics composed of triclinic kyanite crystals (Δ n = 0.012‐0.016) with small amounts of trigonal α‐alumina (Δ n = 0.008) crystals were successfully fabricated by the high‐ PT crystallization from glass . These composites are fully crystallized, have an isotropic microstructure with average grain sizes of several hundred to a few tens of nanometers and are highly transparent with a real in‐line transmission (RIT) of 78% (at a light wavelength of 645 nm), which is close to that of cubic polycrystalline ceramics . Such a high transparency is remarkable, since kyanite single crystals are optical biaxial and birefringent in all directions ( n max = 0.012‐0.016).…”

“…Since these samples cover a wide range of grain sizes and very narrow grain size distribution in each sample, investigations on the material hardness will provide important information about the controls of the microstructure on the transparency‐hardness relationship of polycrystalline ceramics. In the present study, this relationship is investigated on triclinic kyanite‐based nanoceramics …”