Nitto Denko develops nano-scale YAG phosphor for white LEDs

Working with an unnamed VC partner, Nitto Denko has developed a process to produce nanometer-scale YAG phosphor materials.

Nitto Denko Technical Corp. (NDT), a US-based R&D subsidiary of the Japan-based materials firm Nitto Denko Corporation, has developed a phosphor material, yttrium-aluminum-garnet (YAG), in nano-scale form. YAG is commonly used as the down-conversion phosphor in white LEDs.

The nano-scale YAG material has been developed using a physical vapor process suited to mass production. Nitto Denko has teamed with an undisclosed partner, an US-based venture company.

Currently, powder phosphors consisting of micron-size particles (referred to as "bulk" phosphor materials) are used widely in cathode-ray TV tubes, plasma displays panels, fluorescent lamps and white LEDs.

In NDT’s latest development applying the nano-particle production process owned by the partner VC, the resulting YAG phosphor material shows an average particle size of 40 nm and at the same time an internal quantum efficiency of 70%.

Nitto says that this makes the material comparable to bulk YAG phosphor materials currently being widely used for white LEDs.

When the phosphor particles in white LEDs are reduced to the scale of tens of nanometers, this increases the surface area of the material, so that a resulting increase in the luminous efficiency of the white LEDs can be expected.

Also, particles on a nano-scale are much smaller than the wavelength of light (400-800 nm), and this significantly reduces the effect of light scattering by the particles. With bulk phosphor materials, light scattering reduces the extraction efficiency of the white LED.

However, until now, the process of making nano-scale phosphors has been difficult to control. Phosphor powders are mixed together, sintered and then mechanically milled. Usually, the milling process leads to an increase in the proportion of particles with surface defects and a resulting problem of reduced luminous efficiency.

The technology is also applicable to fluorescent materials other than YAG and is expected to undergo broader use in a range of applications. Nitto Denko has filed patent applications in the area.

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