Luminous Glass for Emitting Warm White Light by Near-UV LED

Warm light for dinner table. White LED with long-term weatherability and high heat resistance at low cost.

Dr. Atsuo Yasumori 
Department of Material Science and Technology, Faculty of Fundamental Science and Technology

In recent years, light-emitting diodes (LEDs) have found in various applications in flat-panel displays or for high-intensity and low-power illumination, for example.

Of these, white LEDs have been attracting attention as a new illumination source to replace common, accessible fluorescent lamps and incandescent lamps. Many of existing white LEDs are a combination of a blue LED with yellow phosphors dispersed in an organic polymer or a combination of a near-UV LED with phosphors of three colors, RGB. However, the color in the former case is more blue than red, so there is a problem in that, as an illumination light, the color gives a cold impression compared with the color of electric bulbs. The latter requires some ingenuity in controlling the dispersion state of the three-color phosphors, thus increasing cost. In addition, in both cases, because of the low heat resistance or UV resistance of the organic polymer for dispersing phosphors, it is difficult to use a high-intensity LED light source.

In order to obtain a warm (yellow to orange) white light demanded for general interior illumination, Professor Yasumori invented a colorless, transparent fluorescent glass with long-term weatherability and high heat resistance.

Specifically, by utilizing the phase-separated structure of a borosilicate or silicate glass, transition metal clusters that exhibit warm white luminescence upon irradiation with near-UV light are dispersed uniformly in the glass without using rare and expensive rare earth ions generally used for light emission; the invention is thus achieved.

Examples of metals and metal ions forming the transition clusters to be used include copper (Cu), and silver (Ag). Of these, Professor Yasumori directed his attention to copper in consideration of both the color of emitted light and cost.

Copper ions in a glass have various optical properties depending on the valence, such as coloring of glass and light emission in different colors. In particular, monovalent copper ion clusters in a silicate-based glass have been reported to exhibit yellow to orange luminescence upon irradiation with near-UV light. Specifically, an alkali metal borosilicate glass having such copper ion clusters dispersed therein is practically colorless and transparent, and emits a yellow light when irradiated with UV light at a wavelength of approximately 350 to 390 nm, for example.

The phase separation of a glass is a phenomena in which, for example, when an alkali metal borosilicate glass of specific composition is placed in a certain temperature for a certain period of time, the phase in the glass is divided into two phases, a glass phase containing a large amount of silica and another glass phase containing large amounts of alkali metal ions and boric acid, and this forms a fine, nanometer-scale structure. Utilizing the properties of such a phase-separated glass, reportedly, monovalent copper ions can be stabilized in the form of clusters (Cu+-Cu+) and can also be uniformly dispersed in the glass.

The glass phase separation technology has already been widely applied to heat-resistant glasses, such as Pyrex(R) glass, for example. Therefore, it is likely that such a light-emitting glass can be offered to a market not only with long-term weatherability and high heat resistance against UV light, but also at low cost.

The main features of the light-emitting glass are that it is colorless and transparent; an expensive rare earth or a glass of special or harmful composition is not used; and the material is highly stable and low cost. Accordingly, it is also possible to produce a large-sized light-emitting glass. Therefore, prospective applications are not limited to general-purpose white LED illumination chips, and various applications will be possible, such as allowing an illumination device or the entire ceiling, wall, or floor surface to perform surface emission. Thus, even in global markets, a huge demand is expected.

In the future, by increasing the amount of copper ion clusters dispersed in the phase-separated glass or by controlling the phase-separated structure of the glass to utilize the multiple scattering of the near-UV excitation light or emitted yellow-orange light at the interface, even higher-intensity luminescence may be possible.