This contribution reports on luminescence properties of divalent ytterbium in alpha-SiAlON at room temperature. Ytterbium-doped alpha-SiAlON powders, with the compositions of (M(1-2x/v)Yb(x))(m/v)Si(12-m-n)Al(m+n)O(n)N(16-n) (M = Ca, Li, Mg, and Y, v is the valency of M, 0.002 < or = x < or = 0.10, 0.5 < or = m = 2n < or = 3.5), were synthesized by sintering at 1700 degrees C for 2 h under 0.5 MPa N2. A single, intense, broad emission band, centered at 549 nm, is observed due to the electronic transitions from the excited state 4f(13)5d to the ground state 4f14 of Yb2+. The luminescence of Yb2+ in alpha-SiAlON occurs at relatively low energy, which is attributable to the large crystal field splitting and nephelauxetic effect due to the nitrogen-rich coordination of Yb2+. The dependence of luminescence properties on the Yb2+ concentration, chemical composition, and annealing is discussed. It is suggested that this novel green phosphor could be applied in white light-emitting diodes (LEDs) when combined with a red phosphor and a blue LED.

Introducing carbon nanotubes (CNTs) into polymer or ceramic matrices has been a promising approach to obtain ultra-strong, extra-toughened materials as well as multifunctional composites. Most of the previous work on CNT composites has focused on strengthening and toughening of matrix materials at ambient conditions. However, so far there is a lack of information on the mechanical behavior of these composites at elevated temperature. Recently, single-walled CNTs were found to undergo a superplastic deformation with an appealing 280% elongation at a high temperature (Huang et al 2006 Nature 439 281). This discovery implies the high probability for the potential usage of CNTs as reinforcing agents in engineering high-temperature ceramics with improved ductility. Here, for the first time, we demonstrate that a small addition of boron nitride nanotubes (BNNTs) can dramatically enhance the high-temperature superplastic deformation (SPD) of engineering ceramics. More specifically, 0.5 wt% addition of BNNTs leads to an inspiring brittle-to-ductile transition in Al 2 O 3 ceramics even at a moderate temperature (1300 • C). For Si 3 N 4 ceramics, 0.5 wt% addition of BNNTs could also decrease the true stress by 75% under the same deformation conditions. In contrast, addition of micro-sized or nano-sized BN powders has no or a negative effect on the superplasticity of these ceramics. The underlying SPD-enhancement mechanism is discussed in terms of the inhibition of static and dynamic grain growth of the matrix and the energy-absorption mechanism of BNNTs. The unraveled capability of BNNTs to enhance the SPD behavior will make BNNTs promising components in cost-effective complex ceramics with good comprehensive mechanical properties.

Rare-earth-doped Ca-a-SiAlON phosphors, with the compositions of (Ca 1À3/2x RE x ) m/2 Si 12ÀmÀn Al m1n O n N 16Àn (RE 5 Ce, Sm, and Dy, 0.5rm 5 2nr3.0), were prepared by sintering at 17001C for 2 h under 10 atm N 2 . The concentration of rare earths varied from 3 to 30 at.% with respect to Ca. The photoluminescence (PL) properties were investigated as functions of the composition of the host matrix (i.e., m) and the concentration of rare earths (i.e., x). The results show that the emission properties can be optimized by tailoring m and x. The Ce 31 luminescence originating from the 4f-5d interconfigurational transitions is greatly affected by the environment surrounding the Ce 31 ions, which differs from the Sm 31 or Dy 31 luminescence arising from the 4f-4f intraconfigurational transitions. X-ray diffraction and scanning electron microscopy were used to explain the composition and concentration dependence of PL properties. Lighting Co. Ltd., Tokyo, Japan). The emission spectrum was corrected for the spectral response of a monochromator and Hamamatsu R928P photomultiplier tube (Hamamatsu Photonic K.K., Hamamatsu, Japan) by a light diffuser and tungsten 2883 J ournal

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Transparent ceramics (TCs) are promising for high-power (hp) white light-emitting diode (WLED) and laser diode (LD) lighting. However, comfortable warm white light has not been achieved only using a single TC in hp-WLEDs/ LDs. Herein, highly transparent Gd 3 Al 4 GaO 12 :Ce 3+ (GAGG:Ce 3+ ) TCs (transmittance, T = 55.9−80.2%) were prepared via a solid-state reaction. Ce 3+ as a doped activator center in grains plays a positive role in luminescence based on the microstructural investigations by scanning electron microscopy and the cathodoluminescence system. T decreases upon increasing the Ce 3+ concentration and/or the ceramic thickness, whereas the luminous efficacy of hp-WLEDs/LDs goes up. For blue hp-LEDs driven at 350 mA or LDs of 2 W, warm white light with a low correlated-color temperature of ∼3000 K was achieved by a single GAGG:Ce 3+ TC, benefiting from its broad emission band (full width at half maximum, FWHM = 133−137 nm) and abundant red components (peaking at about 568−574 nm). The color-rendering index of hp-WLEDs reaches 78.9. These results are much better than the performance of the traditional Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ) TC, indicating that GAGG:Ce 3+ TCs are promising color converters for hp-WLEDs/LDs with a comfortable warm white light. KEYWORDS: Gd 3 Al 4 GaO 12 :Ce 3+ , transparent ceramic, high-power white LEDs, laser-driven lighting, warm white light

We report the preparation of nanosized silicon nitride (Si3N4) ceramics via high‐energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β‐Si3N4 and sintering additives of 5‐mol% Y2O3 and 2‐mol% Al2O3 was prepared by high‐energy mechanical milling. After milling, the powder mixture was mostly transformed into a non‐equilibrium amorphous phase containing a large quantity of well‐dispersed nanocrystalline β‐Si3N4 particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano‐Si3N4 grains with an average diameter of about 70 nm, which led to noticeable high‐temperature ductility and elevated hardness.

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