Takaki Masaki

Tailoring of Deep‐Red Luminescence in Ca2SiO4:Eu2+

We report a new dicalcium silicate phosphor, Ca(2-x)Eu(x)SiO4, which emits red light in response to blue-light excitation. When excited at 450 nm, deep-red emission at 650 nm was clearly observed in Ca1.2Eu0.8SiO4, the external and internal quantum efficiencies of which were 44 % and 50 %, respectively. The red emission from Ca(2-x)Eu(x)SiO4 was strongly related to the peculiar coordination environments of Eu(2+) in two types of Ca sites. The red-emitting Ca2SiO4:Eu(2+) phosphors are promising materials for next-generation, white-light-emitting diode applications.

Anomalous Orange Light-Emitting (Sr,Ba)2SiO4:Eu2+ Phosphors for Warm White LEDs

Phosphors with sufficient red emission component are necessary for warm white light-emitting diodes. In this work on (Sr,Ba)2(1-x)Eu2xSiO4 phosphors, (Sr,Ba)1.5Eu0.5SiO4 achieved 75% of an internal quantum efficiency under excitation by blue light. Surprisingly, the (Sr,Ba)1.5Eu0.5SiO4 exhibited orange emission, against the well-known traditional green-yellow emission of (Sr,Ba)2SiO4:Eu(2+). Moreover, the concentration quenching of Eu(2+) in (Sr,Ba)2SiO4 was abnormally unobvious. With the help of calculations based on the density functional theory, it was discovered that the distinct local environment of luminescence centers rather than usual explanation such as self-absorption or intensified crystal field splitting, is responsible to the interesting red shifts in excitation and emission spectra. The refinement analysis based on X-ray diffraction revealed that the unequal distribution of Eu(2+) to two crystallographic sites caused low concentration of Eu(2+) at the 9-coordination site, inhibiting the concentration quenching. The (Sr,Ba)1.5Eu0.5SiO4 phosphor has warmer emission than the commercial Y3Al5O12:Ce(3+). This study also promotes research on the effect of site occupancy and the local environment of luminescence centers.

Synthesis and the Luminescent Properties of Silicate NaAlSiO4:Eu2+ Phosphor Using SiO Powder as a Silica Source

A yellow-emitting NaAlSiO4:Eu2+ phosphor was synthesized using SiO powder as a reducing agent and a silica source material. The emission intensity of the NaAlSiO4:Eu2+ phosphor synthesized using SiO powder is higher than that of a sample produced using a conventional silicate phosphor synthesis reaction using SiO2 powder. The ratio of non-reduced Eu3+ ions in the NaAlSiO4:Eu2+ phosphor synthesized using SiO powder was decreased by the reduction effect of SiO powder.

Novel Soft Chemical Synthesis Methods of Ceramic Materials

We report novel soft chemical synthesis method, solid hydratethermal reaction (SHR) method as a new soft chemistry. This method is very simple and can synthesize the ceramic materials just by storing the mixture of raw materials added a small amount of water in a reactor at low temperature below 373 K. For example, nanosize YVO4 (under 100 nm in diameter) was obtained using the SHR method.

Synthesis and the luminescent properties of NaAlSiO4:Eu2+ phosphor using SiO powder as a silica source

A yellow-emitting silicate phosphor, NaAlSiO4:Eu2+, was synthesized through a new reducing technique, using SiO powder as a silica source, during the preparation process. The addition of SiO powder as a silica source significantly enhanced the Eu2+ content, which led to an increase in the photoluminescence emission intensity of the NaAlSiO4:Eu2+ phosphor. The optimization of the composition resulted in obtaining the maximum emission intensity for NaAlSiO4:7 mol% Eu2+.

Enhancement of thermal quenching properties of a yellow-emitting SiO2-coated Y3Al5O12:Ce3+ phosphor for white light-emitting diode applications

To improve the phosphor thermal properties of phosphor-converted white light emitting diodes, the Y3Al5O12:Ce3+ phosphor was coated with SiO2. The photoluminescence properties of the SiO2-coated Y3Al5O12:Ce3+ phosphor showed excellent thermal properties at 460 nm. The temperature dependence of photoluminescence was measured from 20 to 180 °C. The SiO2-coated phosphor showed improved thermal quenching properties compared to pristine phosphors. We concluded that the SiO2-coated phosphor limits the thermal quenching properties. This result suggests that the SiO2-coated YAG:Ce3+ phosphor could be considered as a good candidate for white light-emitting diode applications.

Synthesis of Nano-Sized Materials Using Novel Water Assisted Solid State Reaction Method

We report synthesis of nanosized oxide materials using a novel water assisted solid state reaction (WASSR) method. This novel soft chemical synthesis method is very simple and can synthesize nanoparticle materials just by storing or mixing raw materials added a small amount (typically 10wt%) of water in a reactor at low temperature below 373 K. Combinations of raw materials have a significant influence on the reaction rate.

Nanophosphors synthesized by the water-assisted solid-state reaction (WASSR) method: Luminescence properties and reaction mechanism of the WASSR method

ABSTRACT Ceramic materials have been widely used in various applications and are significantly important in our daily life. The various methods for synthesizing ceramics powder materials have been already reported, and the synthesis method is known to considerably affect the characteristics and particle morphology of ceramic powder materials. We have recently developed a novel soft chemical synthesis method, named water-assisted solid-state reaction (WASSR) method, which can be synthesized at low temperatures below 100°C. We further demonstrated the effect and availability of this method on the synthesis process for ceramic oxide materials. In this paper, we review some results for the nanophosphors synthesized by the WASSR method and discuss the reaction mechanism of this method.

Development of Water Assisted Solid State Reaction for the Ceramic Materials

We report a novel soft chemical synthesis method, water assisted solid state reaction (WASSR) method. This method is very simple and can synthesize many ceramic materials just by storing or mixing raw materials added a small amount of water in a reactor at low temperature below 373 K. For example, well-crystalline SrMoO4 was obtained using the WASSR method.