Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-␣-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-␣-SiAlON displayed a single broad emission band peaking at ؍ 550 -590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-␣-SiAlONs were assigned to the allowed transition of Eu 2؉ from the lowest crystal field component of 4f 6 5d to 8 S 7/2 (4f 7 ) ground-state level. The emission spectra of terbium-and praseodymium-doped Ca-␣-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D 4 3 7 F j (j ؍ 3, 4, 5, 6) transitions of Tb 3؉ and 3 P 0 3 3 H j (j ؍ 3, 4, 5) transitions of Pr 3؉ , respectively. In particular, the terbium-doped ␣-SiAlON ceramics showed a strong green emission among these phosphors.
Large colored β-Si3N4 single crystals were successfully grown from a Si melt in N2. The transmission optical absorption of coloring β-Si3N4 single crystal shows that impurities introduce a midgap level of ∼2.4 eV into the wide band gap of ∼5.3 eV in nondoped Si3N4. The infrared transmission spectrum and electron probe x-ray microanalysis of β-Si3N4 samples show that the solution of the Al element affects the silicon–nitrogen molecular vibration and the states within the band gap of β-Si3N4. The obtained results mean that the Al impurity acts as the radiative center and is the origin of the color in the β-Si3N4 single crystal.
Rhombohedrally distorted perovskite BiFeO 3 K 0.5 Na 0.5 NbO 3 ceramics with ferroelectricity and ferromagnetism were synthesized by solid-state reaction. The effect of K 0.5 Na 0.5 NbO 3 content on the crystal structure, electrical and magnetic properties of a BiFeO 3 K 0.5 Na 0.5 NbO 3 system was examined with a view to achieving multiferroic BiFeO 3-based ceramics. Perovskite BiFeO 3 K 0.5 Na 0.5 NbO 3 single phase was obtained by formation of solid solution with K 0.5 Na 0.5 NbO 3 , whereas pure BiFeO 3 ceramics contained a small amount of a second phase, such as Bi 36 Fe 2 O 57. The crystallographic symmetry of BiFeO 3 K 0.5 Na 0.5 NbO 3 changed from rhombohedral to cubic when the content of K 0.5 Na 0.5 NbO 3 exceeded 15 mol %. The rhombohedrally distorted BiFeO 3 K 0.5 Na 0.5 NbO 3 showed a high Curie temperature (>770°C) and weak ferromagnetism at room temperature. Dielectric properties of the resultant ceramics were improved by controlling the amount of Mn doping to facilitate ferroelectric measurements.
This letter reports the dielectric and ferroelectric properties of tungsten bronze Sr2−xCaxNaNb5O15 (SCNN, x=0.05–0.35) ceramics. Two dielectric anomalies and a diffuse ferroelectric transition behavior were appreciably observed in the compositions of x=0.05–0.25. The incorporation of smaller calcium cations into the crystal structure resulted in an increase in the Curie temperature, from 279 (x=0.05) to 297 (x=0.35), and a decrease in the permittivity, from 1353 to 543, at their respective Curie temperatures. Ferroelectricity was observed in the compositions with x=0.05–0.25, but absent in the compositions with x=0.30 and 0.35 at room temperature. The maximum spontaneous polarization Ps of 9.1 μC/cm2 and remanent polarization Pr of 3.0 μC/cm2 were achieved in the composition of x=0.15.
P-Si,N, powder containing 1 mol% of equimolar Y,O,-Ndz03 was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SNS) in 30-MPa nitrogen gas. These materials had a microstructure of "in-situ composites" as a result of exaggerated grain growth of some P-Si,N, grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa.ml'z because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of P-Si3N, powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.
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