Mamoru Mitomo

Characterization and properties of green-emitting β-SiAlON:Eu2+ powder phosphors for white light-emitting diodes

This letter reports a β-SiAlON:Eu2+ green phosphor with the composition of Eu0.00296Si0.41395Al0.01334O0.0044N0.56528. The phosphor powder exhibits a rod-like morphology with the length of ∼4μm and the diameter of ∼0.5μm. It can be excited efficiently over a broad spectral range between 280 and 480 nm, and has an emission peak at 535 nm with a full width at half maximum of 55 nm. It has a superior color chromaticity of x=0.32 and y=0.64. The internal and external quantum efficiencies of this phosphor is 70% and 61% at λex=303nm, respectively. This newly developed green phosphor has potential applications in phosphor-converted white LEDs.

Eu2+-doped Ca-α-SiAlON: A yellow phosphor for white light-emitting diodes

In this letter, a yellow oxynitride phosphor α-SiAlON with compositions of Ca0.625EuxSi0.75−3xAl1.25+3xOxN16−x (Ca-α-SiAlON:Eu, x=0–25) was prepared by gas pressure sintering. The diffuse reflection spectrum, photoluminescence spectrum, and chromaticity of the powder phosphors were presented. It absorbs light efficiently in the UV–visible spectral region, and shows a single intense broadband emission at 583–603nm. This phosphor may become a good candidate for creating white light, typically warm white light, when coupled to a blue light-emitting diode (λem=450nm).

2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors

Green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes (LEDs). In this letter, the authors attempt to fabricate white LEDs by combining α-sialon:Yb2+ and Sr2Si5N8:Eu2+ with a blue LED die and report their optical properties. These two phosphors lend themselves for use in 2-phosphor-converted white LEDs with promising properties: a wide range of tunable correlated color temperature (2700–6700K), acceptable color rendering index (82–83), and luminous efficacy (17–23lm∕W). These LEDs are acceptable for general lighting.

Wavelength-tunable and thermally stable Li-α-sialon:Eu2+ oxynitride phosphors for white light-emitting diodes

Eu2+-activated Li-α-sialon is a promising yellow phosphor for white light-emitting diodes (LEDs). This letter reports that the emission of Eu2+ in Li-α-sialon can be tuned widely (563–586nm) by tailoring the composition or controlling the Eu2+ concentration. The thermal stability of Li-α-sialon:Eu2+, relying a little on the composition and the Eu2+ concentration, remains high in a wide temperature range (25–200°C). Moreover, the chromaticity of Li-α-sialon:Eu2+ does not shift with changes in temperature. Using a single Li-α-sialon:Eu2+ phosphor, highly efficient white LEDs (46–55lm∕W) with different color temperatures (3000–5200K) can be fabricated.

Highly efficient white-light-emitting diodes fabricated with short-wavelength yellow oxynitride phosphors

We have already reported orangish yellow Ca–α-SiAlON:Eu2+ phosphors, and applied them to fabricate warm white light-emitting diodes (LEDs). In this letter, we report on greenish yellow Li–α-SiAlON:Eu2+ phosphors, and use them to create daylight when coupled to an InGaN blue LED chip (460nm). The newly discovered Li–α-SiAlON:Eu2+ phosphors emit at shorter wavelengths of 573–577nm under the 460nm excitation, and exhibit a smaller Stokes shift than Ca–α-SiAlON:Eu2+ does. By using this short-wavelength yellow oxynitride phosphor, bright daylight emissions from white LEDs can be generated. Thus, highly efficient white LEDs with tunable white light can be fabricated with α-SiAlON:Eu2+ phosphors, enabling them for a wider range of applications.

Fabrication of silicon carbide nanoceramics

Ultrafine silicon carbide powder with an average particle size of 90 nm was densified by hot-processing with the addition of Al 2 O 3 , Y 2 O 3 , and CaO at 1750 °C. Silicon carbide nanoceramics with an average grain size of 110 nm were prepared by liquid phase sintering at low temperature. The materials showed superplastic deformation at a strain rate of 5.0 × 10 -4 /s at 1700 °C, which is the lowest temperature published. The microstructure and deformation behavior of materials from a submicrometer powder were also investigated as a reference.

Improvement of high-temperature strength of hot-pressed sintering silicon nitride with Lu2O3 addition

The flexural strength of Lu2O3-doped hot-pressed sintering Si3N4 was studied in N2 at elevated temperatures. The high-temperature strength above 1400°C was significantly improved due to Lu2O3 addition. This was attributed to the formation of a grain boundary phase, with a higher melting point, which was extensively crystallized during the sintering process.

Control and characterization of abnormally grown grains in silicon nitride ceramics

Abstract The intrinsic grain growth behavior of β-Si 3 N 4 was investigated by annealing fine-grained β-Si 3 N 4 ceramics with 0–30 wt% β-nuclei. The development of bimodal microstructures was observed in annealed materials with 0.1–10 wt% nuclei, caused by the abnormal grain growth of nuclei due to the large driving force. This driving force was related to the difference in nuclei and matrix grain solubility in the liquid phase. The driving force for abnormal grain growth was also related to the stable morphology of β-grains of about 4 in aspect ratio. The fact that the driving force was constant in materials with 0.1–3 wt% nuclei is explained by the constant diffusion space for nuclei. Nuclei addition exceeding 10 wt% decreased the driving force because of nuclei interaction. A unimodal microstructure was developed with 30 wt% nuclei addition.

A crystallographic study of a new compound of lanthanum silicon nitride, LaSi3N5

A new compound of lanthanum silicon nitride, LaSi3N5 has been prepared by the reaction between Si3N4 and La2O3 under a 50 atm nitrogen pressure at 2000° C for 2 h. The space group is P212121, Z=4, a=7.838 Å, b=11.236 Å and c=4.807 Å, Dc=4.6 g cm−3. The crystal structure data and X-ray powder diffraction data are given. The characteristics of the structure have been noted and the similarities between LaSi3N5 and Si3N4 have been discussed in terms of the fundamental structural unit of SiN4 tetrahedra.

The effect of additives on sintering behavior and strength retention in silicon nitride with RE-disilicate

Abstract Four kinds of Si 3 N 4 –RE 2 Si 2 O 7 (RE: Nd, Sm, Y, Yb) ceramics have been fabricated by hot pressing at 1725–1750°C for 2 h in N 2 gas flow and post-annealing at 1450°C for 4 h. The effect of rare-earth oxide additions on the densification process was investigated by measuring the shrinkage of the compact. Results revealed that the rare-earth oxides affect the densification process of Si 3 N 4 –RE 2 Si 2 O 7 ceramics in which the shrinkage temperature varies with the types of rare-earth additives. The Si–RE–O–N liquid phase formation temperature can be estimated from the onset temperature of final shrinkage stage, and the temperature is in the order of Sm 2 gas flow. The high temperature strength of Si 3 N 4 –Yb 2 Si 2 O 7 or Si 3 N 4 –Y 2 Si 2 O 7 ceramics is better than that of Si 3 N 4 –Sm 2 Si 2 O 7 or Si 3 N 4 –Nd 2 Si 2 O 7 ceramics. There is a roughly linear relation between the strength retention and the liquid phase formation temperature. Present results indicate that Si 3 N 4 –RE 2 Si 2 O 7 ceramics with high strength retention can be fabricated by adopting additives with small ionic radius and high ionic charge.