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Nitto Denko Entering the Nano-ceramics Arena
Nano-scale YAG Phosphor Material Jointly Developed
10.18.2005
Nitto Denko Technical Corp. (NDT), a U.S. R&D subsidiary of Japan’s leading optical films and electronic materials manufacturer Nitto Denko Corporation, has successfully jointly developed a nano-scale phosphor material of yittrium-aluminum-garnet (YAG) – used for white LEDs (light-emitting diodes) -- by a physical vapor process suited to mass production, Nitto Denko announced today.
In the background of the joint development this time together with an undisclosed partner, an American venture company (VC), was Nitto Denko’s strategy to first set its sights on the creation of materials used for LEDs which are experiencing a rapid market expansion.
The achievement this time marks Nitto Denko’s strategic entry into R&D in the inorganic materials field, in particular, the nano-ceramics technology area, with the aim of further expanding its business horizons Until now, Nitto Denko has been mainly engaged in generating diverse products based on polymeric materials technologies.
Currently, powder phosphors consisting of micron-size particles (hereinafter called “bulk phosphor materials”) are used widely in cathode-ray TV tubes, plasma displays panels (PDPs), fluorescent lamps and white LEDs.
In NDT’s latest development applying the nano particles production process owned by the partner VC, the resulting YAG phosphor material shows an average particle size of 40 nanometer (nm) and at the same time an internal quantum efficiency of 70%, making the material comparable to bulk YAG phosphor materials currently being widely used for white LEDs, Nitto says.
Normally, nano-sizing of fluorescent material particles lowers the material’s luminous efficiency, but application of the latest technology permits the manufacture of YAG phosphor without the luminous efficiency deterioration. At the same time, nano-sizing not only reduces light scattering but also improves the relative surface area of the material, so that improved luminous efficiency of the resulting white LEDs can be expected, Nitto points out.
Applicable also to fluorescent materials other than YAG and expected to undergo broader use, this technology is now under study at Nitto with future commercialization in mind, in application areas such as optical and electronic devices including PDPs and FEDs (field emission displays), LCD (liquid crystal display) backlights and biomedicals, in addition to white LEDs.
Background
Conventional bulk phosphor materials are obtained by mixing together the raw material powders and sintering the resulting mixture under pressure at high temperatures exceeding 1,000°C, and thereafter mechanically milling the sintered product. But as particle size goes down with milling, the proportion of particles with surface defects increases and leads to the problem of reduced luminous efficiency.
Another obstacle has been the difficulty in the milling itself, namely, milling the material down to adequately small particles which qualify as “nano-scale”.
For example, the trend going after higher-definition images in the display application continues to require an ever smaller particle size of the phosphors. Another problem is that in white LEDs and various fluorescent color-changing sheets, phosphor particles are dispersed in the base polymer, but because particle size is larger than the wavelength of light (400 nm – 800 nm), fluorescence is scattered backward by the particles toward the front and lowers the efficiency of light extraction.
A method to cut down the loss due to backward scattering of light would be in bringing particle size down to the nano level where light scattering becomes faint. It is with such a backdrop that many research organizations are working on developing nano-scale fluorescent materials including phosphors, but because their luminous efficiencies are lower compared to bulk phosphors, sufficient benefits of having nano-scale products could not be attained.
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