1. Field of the Invention
This invention relates to novel fluorescent compositions and low-velocity electron excited fluorescent display devices utilizing the same, and more particularly is concerned with novel fluorescent compositions which can display emission having high luminance in a high state of color purity under low-velocity electron excitation, and low-velocity electron excited fluorescent display devices containing as a fluorescent screen these fluorescent compositions.
2. Description of the Prior Art
Recently, a low-velocity electron excited fluorescent display device (hereinafter referred to as "fluorescent display device") have been widely employed as a display device for desk top electronic calculators and various kinds of measuring instruments. As is well known, the fluorescent device of this kind in general has a fundamental structure such that both an anodic plate having a fluorescent screen on one side thereof and a cathode standing face to face with the above-described fluorescent screen are enclosed in an evacuated tube wherein the fluorescent screen placed on the anodic plate is excited by low-velocity electrons emitting from the cathode to result in emission of light of certain wavelengths. Both FIGS. 1 and 2 give outlines of typical structures of fluorescent display devices, and they show a diode type display tube and a triode type display tube, respectively. As shown in both FIG. 1 and FIG. 2, one side of an anodic plate 11 made of, e.g., an aluminium plate, has a fluorescent screen 12 thereon. The other side of the anodic plate 11 is supported by a ceramic base plate 13. The diode type display tube is equipped with a cathode standing face to face with the above-described fluorescent screen 12 placed on the one side of the anodic plate 11, and emission occurs by excitation of the fluorescent screen 12 which arises from low-velocity electrons emitted from the cathode 14. In particular, the triode type display tube shown in FIG. 2 additionally has a grid electrode 15 between the cathode 14 and the fluorescent screen 12 so as to control or diverge low-velocity electrons emitted from the cathode 14. Moreover, when the surface of the fluorescent screen 12 has wide area, two or more cathodes may be additionally placed in both fluorescent display tubes shown in FIG. 1 and FIG. 2 wherein only one cathode is placed, and there is no particular limit to the number of cathodes that can be placed therein. The aforesaid anodic plate 11 having a fluorescent screen 12 on one side thereof, the ceramic base plate 13 and the cathode 14 (which are shown in FIG. 1), or the aforesaid anodic plate 11 having a fluorescent screen 12 on one side thereof, the ceramic base plate 13, the cathode 14 and the grid electrode 15 (which are shown in FIG. 2) are enclosed in a transparent container 16, made of, for example, glass, the pressure inside which is held at a high vacuum of 10.sup.-5 to 10.sup.-9 Torr.
Zinc activated zinc oxide phosphors (ZnO:Zn) have been commonly known as phosphors employed for the above-described fluorescent display devices which can emit light of high luminance under low-velocity electron excitation occurring under certain conditions, particularly under acceleration potential below 100V. Phosphors of this kind can be prepared by firing zinc oxide (ZnO) alone in a reducing atmosphere, or by firing ZnO contaminated with a slight amount of a certain zinc compound other than ZnO such as zinc sulfide (ZnS) or the like in air, and they can give forth greenish white emission of high luminance when excited by low-velocity electrons. Fluorescent display devices having the fluorescent screen made of the aforesaid (ZnO:Zn) have been commercially used as display devices for, e.g., desk top electronic calculators and various kinds of measuring instruments. However, aside from (ZnO:Zn) almost no phosphors are known which can emit light under low-velocity electron excitation, and therefore, fluorescent display devices equipped with fluorescent screens containing phosphors other than (ZnO:Zn) are rare at the present stage of this art. Emission color of (ZnO:Zn) is greenish white as described above, and therefore a fluorescent display device utilizing (ZnO:Zn) has inadequate color purity as a green emitting display. Accordingly, the present invention is aimed at providing green emitting compositions and fluorescent display devices using them which can emit green light in a high state of color purity. In addition, the present invention is also aimed at providing fluorescent compositions capable of emitting light of specific wavelengths, other than greenish ones, in high luminance and at providing fluorescent display devices utilizing them.