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The miniaturization of high-brightness white light-emitting diodes (WLEDs) is limited by the low thermal performance of phosphors. In this study, the microstructure, optical properties, and thermal performance of Al 2 O 3 –Y 3 Al 5 O 12 :Ce3+ (Al 2 O 3 –YAG:Ce) composite ceramics fabricated by hot pressing were investigated. By promoting the growth of Al 2 O 3 grains while maintaining a high composite density, thermal performance of the composite ceramics was significantly increased. The thermal conductivity of a Al 2 O 3 –40-vol% YAG:Ce ceramic reached 21.8 W/m/K, which is close to the theoretical value. In addition, this composite ceramic exhibited the highest energy efficiency. After packaging with a high-power LED chip with dimensions of 1 mm × 1 mm, a high luminous flux of 639 lm was generated, while the reduction in output power at 250 °C was as low as 6%. This indicated excellent high-temperature stability and potential for applications in solid-state lighting. [ABSTRACT FROM AUTHOR]

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y 2 O 3 powders as well as commercial Al 2 O 3 and Yb 2 O 3 powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb3+ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO 2 formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb3+ easily entered the YAG lattice and had a high segregation coefficient. [ABSTRACT FROM AUTHOR]

A novel Al 2 O 3 -Ti(C 0.7 N 0.3 )-cBN composite was designed and prepared for the first time by hot pressing. The addition of cBN was found to be very effective in simultaneously enhancing the hardness and toughness of the Al 2 O 3 -Ti(C 0.7 N 0.3 ) ceramics. The optimum cBN content of the Al 2 O 3 -Ti(C 0.7 N 0.3 )-cBN composite was determined to be 5 vol%. The optimal properties were achieved by sintering at 1400 °C for 20 min under 40 MPa，and the Vickers hardness, bending strength, and fracture toughness were 20 GPa, 548 MPa, and 5.3 MPa m 1/2 , respectively. When turning stellite 3 using the Al 2 O 3 -Ti(C 0.7 N 0.3 )-cBN insert, the longest cutting length achieved at a cutting speed of 117 m/min was 5122 m, 73% and 45% longer than those of the Al 2 O 3 -Ti(C 0.7 N 0.3 ) and commercial A66N inserts, respectively. This indicates that the Al 2 O 3 -Ti(C 0.7 N 0.3 )-cBN composite is a promising tool material for the turning of stellite alloys. [ABSTRACT FROM AUTHOR]

(Tb 0.985 Ce 0.015 ) 3 (Al 1- x Ga x ) 5 O 12 garnet phosphors ( x = 0, 0.1, 0.2, 0.3, and 1.0) were prepared by calcining their coprecipitated precursors in air at 1500 °C, followed by reduction in hydrogen at 1200 °C. Rietveld refinement of the XRD results suggested that the Ga dopant predominantly resides at the octahedral Al site of the garnet lattice. Ga doping led to linearly expanded lattice constant, cell volume and theoretical density of the garnet compound and successively lower intensity and longer average fluorescence lifetime of the ~ 570 nm emission of Ce 3+ . Blue shifted emission and 4f( 2 F 5/2 )→5d 1 (E 2g ) excitation and red shifted 4f( 2 F 5/2 )→5d 2 (E 2g ) excitation were also observed for the Ce 3+ activator at a higher Ga content. The phenomena were interpreted by considering the band structure of the host, distortion of the CeO 8 polyhedron, and centroid shift and field splitting of the Ce 3+ 5d energy levels. [ABSTRACT FROM AUTHOR]

A series of eulytite-type Sr 3 Y 1- x (PO 4 ) 3 : x Eu 3+ ( x = 0–0.13) and Sr 3- y Y(PO 4 ) 3 : y Eu 2+ ( y = 0–0.10) phosphors were successfully synthesized via gel-combustion and subsequent calcination in O 2 and Ar/H 2 atmospheres at 1250 °C, respectively. Detailed crystal structure analysis via Rietveld refinement showed that the phosphors were crystallized in the cubic system (space group I -43 d , No. 220), in which the Eu 3+ and Eu 2+ activators reside at the Y 3+ and Sr 2+ sites, respectively. The trivalent Eu 3+ ions (CN = 6) exhibited typical narrow-band luminescence via intra-4 f 6 transitions, with the red emission at ~ 615 nm being dominant ( 5 D 0 → 7 F 2 transition, FWHM = 15.9 ± 0.2 nm). The divalent Eu 2+ ions (CN = 6 and 9) showed broad-band luminescence ranging from light-blue to blue via 4 f 6 5 d 1 → 4 f 7 transitions (FWHM = 115 ± 2 nm). The optimal Eu 3+ and Eu 2+ concentrations were determined to be 10 at% ( x = 0.10) and 7 at% ( y = 0.07), respectively, and the mechanisms of concentration quenching were discussed. The excitation/emission properties, fluorescence decay kinetics, CIE chromaticity, and particularly the rarely addressed thermal stability of the phosphors were investigated in detail. [ABSTRACT FROM AUTHOR]

Sc 2 O 3 -Y 2 O 3 solid solution (SYSS) ceramics with novel nanocrystalline structures have been successfully fabricated by spark plasma sintering (SPS) method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), FT-IR and indentation techniques. The SYSS ceramics can possess high hardness by keeping their excellent infrared transmission performance. Especially, the transmittance of as-obtained SYSS ceramics can exceed as high as 80% in the infrared region of 2–6 µm. Moreover, the Vickers hardness is increased to 9.18 GPa by increasing the addition ratio of Sc 2 O 3 to 20 at%. Here, it is very important to point out that the mechanical strength of yttria ceramics can be evidently enhanced without deteriorating their optical transmission performance by the addition of Sc 2 O 3 as well as the application of SPS technique, where the solid solution strengthening and fine grain strengthening are involved during the synthesis process, respectively. [ABSTRACT FROM AUTHOR]

(Tb 1 -x Ce x ) 3 Al 5 O 12 yellow phosphors (0≤x≤0.05) were calcined from their coprecipitated carbonate precursors, and the effects of the temperature and atmosphere (air and H 2 ) of calcination on the sequence of phase evolution and the characteristics of the powders were investigated in detail. The activation energy for the crystallite growth during calcination was estimated to be ~39 kJ/mol. The powder calcined at 1000 °C showed good reactivity and was sintered into a ceramic plate of ~97% dense (average grain size: ~1.3 µm) in a H 2 /Ar gas mixture at the relatively low temperature of 1500 °C. The phosphors simultaneously exhibit the 4f 8 →4f 7 5d 1 , 7 F 6 → 5 D 3 and 7 F 6 → 5 D 4 excitations of Tb 3+ and the 4f 1 →5d 1 excitation of Ce 3+ when monitoring the yellow emission of Ce 3+ at 560 nm, suggesting the presence of efficient Tb 3+ →Ce 3+ energy migration. The optimal Ce 3+ content for luminescence was found to be x =0.015 and 0.01 under the direct excitation of Ce 3+ and through Tb 3+ →Ce 3+ energy transfer, respectively, and concentration quenching of luminescence was analyzed to be resulted from exchange interaction. Luminescence features of the phosphors, including excitation, emission, quantum yield, fluorescence lifetime, color coordinates and color temperature, were thoroughly investigated against the processing temperature and Ce 3+ content, with an in-depth discussion on the process of energy transfer among the optically active Tb 3+ and Ce 3+ ions. The materials may find application in blue-light excited white LEDs. [ABSTRACT FROM AUTHOR]