1. Field of the Invention
The present invention relates to a fluorescent material and a fluorescent display apparatus including the fluorescent material as a light source.
2. Discussion of Related Art
There has conventionally been known a Zn1-xCdxS fluorescent material (hereinafter, referred to as the ZnCdS fluorescent material) which is used as a red-light emitting fluorescent material that is excited by a low-voltage electron beam and is employed by a fluorescent display device such as a vacuum fluorescent display (VFD). However, recently, the use of harmful elements such as cadmium Cd is restricted in view of environmental quality, and the ZnCdS fluorescent material is subject to that restriction. In addition, if the ZnCdS fluorescent material as a sulfide is decomposed by its exposure to the electron beam, then the sulfur S scattered from the sulfide may lower an electron emitting capability of an oxide cathode as the source of electrons. As far as the present application is concerned, the “low-voltage electron beam” is defined as an electron beam that is accelerated by an electric voltage of from about 10 V to about 100 V that is preferable for VFD, unless otherwise defined.
In the above-indicated background, there has been proposed an oxide fluorescent material that does not contain cadmium Cd or sulfur S and emits a red light in response to a low-voltage electron beam. More specifically described, the oxide fluorescent material contains, as a host crystal an oxide of an alkaline earth metal and titanium Ti, and a rare earth element and a group-3 element each as an additive. The alkaline earth metal may be magnesium Mg, strontium Sr, calcium Ca, or barium Ba; the rare earth element may be cerium Ce, praseodymium Pr, europium Eu, terbium Tb, erbium Er, or thulium Tm; and the group-3 element may be aluminum Al, gallium Ga, indium In, or thallium Tl. A typical composition of the above-indicated oxide fluorescent material may be SrTiO3:Pr,Al, as disclosed by Patent Document 1 (Japanese Patent Application Publication No. 8-85788) or Patent Document 2 (Japanese Patent Application Publication No. 2003-41246). Here, the elements Pr, Al on the right-hand side of the colon “:” indicate the components or elements added to strontium titanate SrTiO3 as the host crystal. This composition preferably contains from 0.1 mol % to 2 mol % of praseodymium Pr and from 1 mol % to 50 mol % of aluminum, as taught by Patent Document 1.
In addition, there has also been proposed a CaTiO3 fluorescent material as an example of a composition wherein the alkaline earth metal is calcium Ca, although this fluorescent material is not for use in VFD. The CaTiO3 fluorescent material is disclosed by Non-Patent Document 1 (Vecht et al. “New electron excited light emitting materials” J. Vac. Sci. Technol. B 12(2), March/April 1994 p. 781-784), Non-Patent Document 2 (P. T. Diallo et al. “Improvement of the optical performances of Pr3+ in CaTiO3” Journal of Alloys and Compounds 323-324 (2001) p. 218-222), or Non-Patent Document 3 (Seung-Youl Kang et al. “The influence of Li Addition on Cathodoluminescence for CaTiO3:Pr+3” EURODISPLAY 2002 p. 777-779).