Several LED devices are known as white light-emitting devices using an LED. For example, there has been widely used a technique of obtaining a white light-emitting device by disposing a phosphor such as a YAG phosphor near a gallium nitride (GaN) blue LED (Light-Emitting Diode) chip so that blue light emitted from the blue LED chip is mixed with yellow light emitted by the secondary emission of the phosphor upon receiving the blue light. Another known technique of obtaining a white light-emitting device uses a combination of a blue LED chip that emits blue light, a phosphor that emits red light upon receiving blue light, and a phosphor that emits green light, so that the colors of the respective lights from the chip and the phosphors are mixed.
White light-emitting devices using an LED are finding various applications, and representative examples include substitutions for conventional fluorescent lamps and incandescent lamps. In addition to such common applications, applications to lighting apparatuses in which very high luminance is required such as headlights of motor vehicles have also been investigated in recent years. In such applications, since high recognition of distant objects (such as signs) is required, high performance is required also in the uniformity of the tint in the white light-emitting device and the color in the irradiation area.
In common white light-emitting devices, transparent resins in which phosphor particles are dispersed are used to encapsulate the LED chip and mounting units. However, in the applications as described above in which a higher level of color uniformity is required, a structure in which an LED chip and the like are encapsulated merely with phosphor particles being dispersed in transparent resin has had such a problem that the phosphor particles having higher specific gravity than the transparent resin settle down before the resin is cured, causing color unevenness upon luminescence.
Consequently, various methods have been proposed for reducing settling of phosphors to prevent the occurrence of color unevenness. For example, it is reported that settling and segregation of phosphors is reduced by using as an encapsulating material a silicone resin having a viscosity upon curing of 100 mPa·s to 10,000 mPa·s (see PTL 1). There are also disclosed a light-emitting device in which a lipophilic compound is added as an anti-settling agent of phosphors to a liquid light transmissive encapsulating material, the compound being prepared by adding organic cations to a laminar compound mainly including a clay mineral, and a method for manufacturing the same (see PTL 2).
According to the techniques described in these literatures, the occurrence of color unevenness due to the settling of phosphor particles can be reduced to some extent. However, phosphors are dispersed in the organic resin in all these techniques. Therefore, when used in a white light-emitting device using a high luminance LED as described above, organic resin materials are degraded due to heat of the LED itself and/or heat by the emission of light from the phosphor particles excited by the light from the LED. When organic resin materials are degraded, there are cases where resins are colored to cause a reduction in transmittance and/or cases where problems like color unevenness and surface scattering caused by the deformation of the resin. Further, even when an LED does not have high luminance, such problems may frequently occur with the lapse of time. Therefore, improvements have been required in the art.
As a technique of increasing the heat resistance of the white light-emitting LED device, a technique has been proposed in which phosphor particles are dispersed in a solution containing a metal alkoxide or a ceramic precursor composition, and then the resulting dispersion is applied to an LED chip and heated to form a phosphor particle-containing ceramic (glass) layer that encapsulates the LED chip (see PTL 3). Further, a technique of adding inorganic particles to a dispersed solution as an anti-settling agent for phosphor particles has been proposed (see PTL 4).
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