CATHODE LUMINESCENCE DEVICE

Abstract: A cathode luminescence device which is adjustable in luminance and is cheap and economical, and a phosphor powder which is used preferably for this cathode luminescence device. The device comprises a long linear cathode, an electron beam gun which causes emission of electrons from the linear cathode, makes the electrons converge in a linear form, and has intermediate electrodes for deflecting the electron beam, an anode which consists of a base and a conductive film formed on the base and to which a voltage for accelerating electrons is applied, a fluorescent film formed by applying a phosphor powder to the conductive film, and a container whose light emitting surface is transparent and whose inside is kept at a vacuum needed for cathode luminescence. The phosphor powder comprises spherical primary particles having rough surfaces and made of polycrystalline substance of Y₂O₂S.

Description: The present invention relates to a cathode luminescence apparatus, and particularly to a cathode luminescence apparatus which may be used to illuminate rooms, in liquid crystal display devices as a back light, etc., and which can be used as a pixel unit of a large scale video displays, and to a phosphor powder for the same. Conventionally, fluorescent lights are popularly used as room lights. In such fluorescent lights, the ultraviolet light produced by discharge in mercury vapor irradiates a phosphor screen formed on an inner face of a glass tube so as to be converted into visible light. Conventional fluorescent lights, however, have some disadvantages. Brightness of the phosphor screen will be increased if the intensity of the ultraviolet light is increased, but the temperature of the mercury vapor is restricted by physical conditions, so that the brightness of the fluorescent light is limited. Therefore the illumination level in rooms is controlled only by the number of fluorescent lights. To make a plane light source with fluorescent lights, a plurality of fluorescent lights must be arranged in parallel, because fluorescent lights are line light sources, so that the cost must be increased. Further, the above-mentioned phosphor screen is formed by applying onto a substrate a slurry composed mainly of a mixture of an organic binder solution, such as cellulose nitrate solution, and phosphor powders. The best fluorescent substance for the slurry application method would be one capable of dispersing well in the slurry and producing high light scattering from the phosphor screen in dry state. In connection with the above, the conventional phosphor powders contain small single crystals in quantity, have low coefficients of light scattering, and have weak strengths of adhesion onto substrates. Owing to these properties, those phosphor screens in which the conventional phosphor powders are used, are prone to the problem of pinholes. Preferred embodiments of the present invention can provide a cathode luminescence apparatus, which allows for adjusting brightness and which is economical; and a phosphor powder, which has tough strength of adhesion onto substrates and high coefficients of light scattering in a dry phosphor screen. US-A-4,608,518 discloses cathode luminescence apparatus comprising: a line electron gun having a line cathode, which is elongated in a first direction, and an intermediate electrode for controlling the passage of electrons from the line cathode; an anode for accelerating electrons emitted from the line electron gun, the anode having an electric conductive section formed on a substrate, such that in use, voltage is applied thereto for the acceleration; a phosphor screen made of phosphor particles on the electric conductive section; and a container accommodating the line electron gun and the anode, the container having a light emitting face, through which visible light is capable of passing, the container being evacuated to a vacuum for cathode luminescence. The present invention in one aspect provides such apparatus characterised in that said intermediate electrode is capable of drawing out electrons from the line cathode, of linearly focusing these electrons, and of reciprocatively scanning electrons focused in a second direction perpendicular to the first direction. With this structure, brightness of luminescence on the phosphor screen can be adjusted by the voltage applying to the intermediate electrode. Moreover, because the line electron gun focuses and reciprocatively scans electrons, a plane light source can be realized with a one line electron gun. On the other hand, the phosphor powder suitably used for the above-described apparatus is desirably composed of primary particles of Y 2 O 2 S polycrystal which are almost spherical with their surfaces made rough by etching. They may be about 4.5 µm in median diameter so as to pass, for example, through a 400-mesh sieve. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side sectional view of a cathode luminescence apparatus of an embodiment; Fig. 2 is a front view of a line electron gun of the apparatus; Fig. 3 is a perspective view of a line cathode of the electron gun; Fig. 4 is a perspective view showing a method of attaching the line cathode; Fig. 5 is a sectional view showing another example of a line cathode; Fig. 6 is an explanation view showing another example of an intermediate electrode of the line electron gun; Fig. 7 is an explanation view showing another example of the intermediate electrode of the line electron gun; Fig. 8 is an explanation view showing another example of the intermediate electrode of the line electron gun; Fig. 9 is an explanation view showing another example of the intermediate electrode of the line electron gun; Fig. 10 is an explanation view showing another example of the intermediate electrode of the line electron gun; Fig. 11 is a partial sectional view showing an anode and a phosphor screen; Fig. 12 is a front sectional view of another embodiment of the apparatus; Fig. 13 is a plan sectional view of another embodiment of the apparatus; Fig. 14 is a plan sectional view of another embodiment of the apparatus; Fig. 15 is a plan sectional view of another embodiment of the apparatus; Fig. 16 is a plan sectional view of another embodiment of the apparatus; Fig. 17 is a plan sectional view of another embodiment of the apparatus; Fig. 18 is a plan sectional view of another embodiment of the apparatus; Fig. 19 is a plan sectional view of another embodiment of the apparatus; Fig. 20 is a plan sectional view of another embodiment of the apparatus; Fig. 21 is a plan sectional view of another embodiment of the apparatus; Fig. 22 is a sectional view taken along line B-B of Fig. 21; Fig. 23 is a side sectional view of another embodiment of the apparatus; Fig. 24 is a front view of the apparatus of Fig. 23; and Fig. 25 is a graph showing the change of diameter of great circle of a Y 2 O 2 S particle, which depends on temperature of material. EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Firstly, a structure of a cathode luminescence apparatus is explained. In Fig. 1, symbol 10 shows a housing, which is an example of containers, and which has a first wall section 12, a second wall section 14 and a transparent section 16. At least the transparent section 16, whose outer face is an emitting face through which the visible light is passed outward, is made of transparent materials, e.g. glass. The first wall section 12 and the second wall section 14 may be made of anti-heat materials, e.g. opaque metals. An inner space of the housing 10 is evacuated to a proper vacuum for cathodluminescence. Symbol 18 is a reflector formed on inner faces of the first wall section 12 and the second wall section 14. The reflector 18 reflects a part of light, which is emitted from a phosphor screen toward the inner faces of the housing 10, not the transparent section 16, so as to introduce it to the transparent section 16, so that the brightness of the light emitted from the transparent section 16 can be higher. The reflector 18 may be, for example, an evaporation layer of aluminum, silver, chromium, etc.. Symbol 20 is a line electron gun, which is fixed on the inner face of the first wall section 12. The line electron gun 20 has a base board 22, a control electrode 24, a line cathode 26, a first intermediate electrode 28 and a second intermediate electrode 30. A front view of the line electron gun 20 is shown in Fig. 2. The line cathode 26 is provided to release electrons, and is elongated in a first direction (the right-left direction in Fig. 2). To uniformly release electrons in a broad area, the line cathode 26 is formed into an elongated shape. The line cathode 26 may be made of a wire resistance, e.g. tungsten, whose outer face is coated with cathode materials, e.g. an alkali earth carbonate. A mere wire resistance without the cathode material may be used for the line cathode 26 but it is apt to transform by heat expansion. Additionally, the area of wire resistance without the cathode material is smaller, so that amount of electrons to be released must be fewer. Therefore, in the present embodiment, the line cathode 26 has a structure shown in Figs. 3 and 4. In Fig. 3, symbol 32 is a ceramic core made of ceramics, e.g. aluminous ceramic, zirconic ceramic. The ceramic core 32 may be made by melting ceramics, but it is difficult to melt ceramics because of high melting temperature. Then, in the present embodiment, the ceramic core 32 is made by uniformly coating a ceramic powder on an outer face of a high melting wire resistance, which does not react to the ceramic powder, e.g. tungsten, molybdenum, zirconium, and sintering the ceramic powder. The ceramic core 32 must have enough diameter, which is capable of preventing the ceramic core 32 from hanging down and breaking. The diameter should be 0.1 mm - 3 mm, preferably 0.3 mm - 2 mm, more preferably 1.0 mm ± 0.5 mm. Symbol 34 is a wire heater for heating the line cathode 26. The wire heater 34 is a wire resistance, which is winded round the outer circumferential face of the ceramic core 32. In the line cathode 26, under operating temperature ± 10°C, heat distribution should be uniform in the axial direction so as to uniformly release electrons. To uniformly release electrons, the wire heater 34 is uniformly and densely wound round the ceramic core 32. By winding the wire heater 32, the transformation thereof can be prevented, and the surface area thereof can be broader, so that the amount of electrons to be released can be increased. Symbol 36 is a cathode material coating the outer circumferential face of the wire heater 34. The cathode material 36 is a material of oxides, e.g. an alkali earth carbonate. When the cathode material 36 is heated under the vacuum condition of at or less than 10 -5 torr, the cathode material 36 changes to an oxide. The oxide when heated releases electrons. The line cathode 26 is fixed as shown in Fig. 4. One end of the ceramic core 32 is fixed on a base 38; the other end is loosely attached for heat expansion thereon by a clasp 40 and bolts 42. The base 38 is fixed on the base board 22 ( the base 38 is not shown in Fig. 1 ). Another example of the line cathode 26 is shown in Fig. 5. In Fig. 5, the line cathode 26 has a hollow metal cylinder 40, whose outer circumferential face is coated with the cathode material, and a coil heater 42, which is accommodated in the cylinder 40. Melting point of the cylinder 40 is higher than that of the cathode material, and the cylinder 40 is formed of a thin metal plate, e.g. nickel or nickel alloy, which does not react with respect to the cathode material so as to keep the function of the cathode. Both ends or one end of the cylinder 40 may be open. The coil heater 42 is made of a wire, e.g. tungsten wire. The coil heater 42 may be coated with sintered powder, e.g. alumina, magnesia, which has an insulating function at high temperatures for preventing short circuit. The whole outer face of the cylinder or a front part thereof, from which electrons are released, may be coated with the cathode material. The cylinder 40 may be supported and fixed by a plurality of arms 44. In each line cathode 26, the length of the line cathode 26 in the first direction is equal to or slightly longer than that of the phosphor screen (described later). Having an elongated form, the line cathode 26 is capable of releasing many electrons. During operation, the line cathode 26 is continuously heated, so that electrons are continuously released therefrom. In Figs. 1 and 2, the first intermediate electrode 28 of the line electron gun 20 is provided to draw electrons from said line cathode 26. The first intermediate electrode 28 is elongated in the first direction as well as the line cathode 26, and has a slit 46a in the front part. Pulse voltage is applied to the first intermediate electrode so as to draw thermoelectrons from the line cathode 26. The electrons drawn out from the line cathode 26 irradiate the phosphor screen to produce illumination. However, this illumination is not so bright. To have enough brightness, there is provided the second intermediate electrode 30, which is capable of linearly focusing the drawn-out electrons and reciprocatively scanning electrons focused in a second direction (the up-down direction in Fig. 1) perpendicular to the first direction. Scanning angle is shown as angle A. The focusing and the scanning are executed by applying AC voltage, e.g. sine waves, to the second intermediate electrode 30. The second intermediate electrode 30 is elongated in the first direction as well as the line cathode 26, and has a slit 46b in the front part. Since the first intermediate electrode 28 and the second intermediate electrode 30 have elongated shapes, it is sometimes difficult to precisely assemble. If the locating accuracy of the first intermediate electrode 28 and the second intermediate electrode 30 is low, uneven distribution of electron density is caused, so that the phosphor screen does not illuminate uniformly. This problem can be solved by providing the control electrode 24. The control electrode 24 is provided on the base board 22 near the line cathode 26. The control electrode 24 has a half cylindrical shape having a slit in the front part, and partly surrounds the line cathode 26. Having the half cylindrical shape, even though the line cathode 26 is thick, the control electrode 24 is capable of controlling the extraction of electrons and illuminating the phosphor screen uniformly. In the embodiment shown in Figs. 1 and 2, a couple of electrodes - the first intermediate electrode 28 and the second intermediate electrode 30 - are provided as the intermediate electrode, but the intermediate electrode is not limited to above described structure. Next, intermediate electrodes having other structures will be explained. In Fig. 6, the intermediate electrode has the first intermediate electrode 28 for extracting electrons from the line cathode 26, the second intermediate electrode 30 for linearly focusing electrons extracted in the first direction, and a third intermediate electrode 50 for reciprocatively scanning electrons linearly focused in the second direction. Extraction, focusing and scanning are respectively executed by each electrode, so that higher accuracy control can be possible. Said control electrode may be added to the example shown in Fig. 6. In Fig. 7, the intermediate electrode has the simplest structure. The intermediate electrode has only one electrode 28 for extracting electrons from the line cathode 26 and focusing the electrons extracted. When voltage of zero or ± nearly zero with respect to the line cathode 26 is applied to the electrode 28, thermoelectrons are extracted from the line cathode 26. The extraction is controlled by the electrode 28, but the distribution of electrons may be uneven, so that the range of use should be limited. An example shown in Fig. 8 is capable of overcoming the disadvantage of Fig. 7. Its basic structure is similar to the example shown in Figs. 1 and 2. Namely, the intermediate electrode has the first intermediate electrode 28 for extracting electrons from the line cathode 26, and the second intermediate electrode 30 for linearly focusing electrons extracted in the first direction and reciprocatively scanning electrons linearly focused in the second direction. Pulse voltage, for example, is applied to the first intermediate electrode 28 to extract electrons; high voltage is applied to the second intermediate electrode 30 to focus and reciprocatively scan electrons. An example shown in Fig. 9 has the control electrode 24 having a plane shape. The function of the control electrode 24 is the same as the example shown in Figs. 1 and 2. This example is preferable in case of having a thin line cathode 26. An example shown in Fig. 10 has the intermediate electrode shown in Fig. 7 and the control electrode shown in Figs. 1 and 2. In each example of the intermediate electrode, the amount of electrons to be extracted can be controlled by adjusting the voltage applied to the intermediate electrode, so that the brightness of the phosphor screen can be controlled. The line electron gun 20 is capable of linearly focusing electrons extracted in the first direction by the intermediate electrode, and reciprocatively scanning electrons focused in the second direction, on the phosphor screen, so that a plane light source can be realized with one line electron gun. Note that, in this embodiment, the electrodes except the second intermediate electrode 30 which has the thickness of 5 mm or less are made of thin metal plates whose thickness is 2 mm or less, so that the total thickness of the line electron gun can be 15 mm or less. Thus a thinner apparatus can be realized. The scanning angle depends on the voltage applied to the intermediate electrode. The higher the voltage rises, the wider the scanning angle is. The scanning angle depends on the distance between the line electron gun 20 and the transparent section 16, too. Therefore, to make a thin, e.g. 3 cm, apparatus, the scanning angle should be larger. In other words, apparatus having a desired thickness can be realized by adjusting the scanning angle. Next, the phosphor screen and an anode formed on the inner face of the transparent section 16 of the housing 10 (see Figs. 1 and 2) will be explained with respect to Fig. 11. In Fig. 11, the glass forming the transparent section 16 serves as a substrate of the anode. There is formed the anode 52, which is a layer made of an optical transparent material, e.g. tin oxide, on the inner face of the transparent section 16. Voltage, e.g. 2 KV, for accelerating electrons irradiated from the line electron gun 20 is applied to the anode 52. Furthermore, when plus voltage is applied to the reflector 18, which is coated with metal, a uniform plus-electric field is formed in the housing 10, so that the scanning can be controlled easier. In this case, the electric potential of the reflector 18 which is provided at the rear side of the line cathode 26 (the left side in Fig. 1) should be the same as that of the cathode. The phosphor screen 54, which is made of a phosphor powder (particles), is formed on the inner face of the anode 52 which is an electric conductive layer. When thermoelectrons collide against the phosphor screen 54, the energy of the electrons is converted into light, so that the phosphor screen 54 emits light. When the phosphor screen 54 is formed on an electric conductive material, the brightness is increased about 50 % with respect to the phosphor screen formed on a non-conductive material. Therefore, in the example shown in Figs. 1 and 2, the phosphor screen 54 is formed on the anode 52 made of an electric conductive material. Since the substrate 16 is optically transparent and the phosphor screen 54 has a suitable thickness, e.g. 4 mg/cm 2 , the illuminated light can be effectively introduced outward through the transparent section 16. Chemical structure of the phosphor screen 54 will be explained later but brightness of the phosphor screen 54 depends on conditions of thermoelectrons, e.g. voltage applied to the anode 52, density, irradiating period. Especially by continuous irradiation whith a high density of electrons, the phosphor powder is heated. Upon reaching a certain temperature, the brightness of the phosphor powder is quickly decreased. Therefore, overheating caused by focusing electrons must be prevented. On the other hand, in case of irradiating electrons having quite high density for short period of time like pulses, the phosphor powder brightly illuminates without temperature rise. Then, in the present embodiment, the phosphor screen 54 emits light in pulses. The scanning of the focused electrons in the second direction enables the phosphor powder to be brightly illuminated without overheating. Successively, other embodiments of the apparatus will be described. Note that elements which have been explained in the former embodiment are assigned the same symbols, and explanation will be omitted. In Fig. 12, the apparatus has a container 56, whose front face and outer circumferential face are made of transparent glass like a light bulb. The apparatus has a socket section 58 as electrodes to which voltage is applied. If the shape and the size of the socket section 58 are standardized, the apparatus would be capable of wide use. In Fig. 13, the apparatus has a plurality of, e.g. four, line electron guns 20. Intermediate electrodes are mutually connected in series or in parallel, and control electrodes, are also mutually connected in series or in parallel. With this structure, each line electron gun 20 can be synchronized, so that no flickers are caused because of no time lag of illumination. In Fig. 14, one or a plurality of line electron guns 20 are accommodated in a glass housing 10, which has a curved face. The width of the apparatus is determined by the degree of vacuum resistance. A wide apparatus must be made of thick glass, so that it is heavier. To reduce the weight, a curved glass plate may be used for a housing. The phosphor screen 54 is formed on a transparent or semitransparent glass plate 60, which is provided on the front side; the line electron guns 20 are set on a glass plate 62, which is provided on the rear side. Illuminated light is introduced outward through the front glass 60. Locations of the line electron guns 20 in the housing 10 are not limited, and any locations from which the line electron guns 20 are capable of irradiating the phosphor screen 54 are allowed. In Figs. 15 and 16, the line electron guns 20, which are the same as those shown in Fig. 7 or 8, are diagonally set in the housing 10. By diagonally setting the line electron guns 20, irradiating efficiency to the phosphor screen 54 can be increased. In these embodiments, the reflectors 18 prevent uneven illumination caused by uneven electron density. Note that the structure of the line electron guns 20 is not limited to that shown in Fig. 7 or 8. In Fig. 17, a couple of line electron guns 20 are diagonally set at each end of the housing 10. As well as the embodiments shown in Figs. 15 and 16, irradiating efficiency to the phosphor screen 54 can be increased by diagonally setting the line electron guns 20. Moreover, there is also provided the reflector 18 on the inner face of the housing 10. In Fig. 18, the phosphor screens 54 are formed on the inner face of the transparent section 16, which is provided on the front side of the housing 10, and on the inner face of the first wall section 12, which is provided on the rear side thereof. The line electron guns 20 are respectively set at each end of the housing 10 to face each other. By providing two phosphor screens 54, the illumination area can be increased and the brightness of the apparatus can be increased. In Fig. 19, the phosphor screen 54 is formed on the inner face of the first wall section 12, not on the transparent section 16. Electrons are repeatedly irradiated from two line electron guns 20, which are diagonally set at each end of the housing 10. Light illuminated from the phosphor screen 54 is introduced outward through the transparent section 16, which has no phosphor screen on the inner face. Note that, in Figs. 18 and 19, the thickness of the phosphor screen 54, which is formed on the inner face of the first wall section 12, can be thicker since the phosphor screens 54 does not affect the introduction of the light. An embodiment shown in Fig. 20 is a modification of the embodiment shown in Fig. 13, with added reflectors 18. Scanning ranges of the line electron guns 20 are partly overlapped. To prevent uneven brightness, which is caused by the overlapping, the intermediate electrodes of the line electron guns 20 may be connected in parallel. With this parallel connection, electrons irradiated from the line electron guns 20 can be synchronously scanned in the same directions, so that uneven illumination can be prevented. In the apparatus of the present invention, light illuminated from the phosphor screen 54 is introduced outward through the transparent section 16, so that a plane light source can be provided which is capable of lighting with diffused light. In case of using this plane light source in houses or offices as room lights, the apparatus may be fixed on walls or ceilings. The apparatus can be also used as lamps instead of conventional lamps. In case of using as lamps, the apparatus may have the structure shown in Fig. 12, which has a socket section for wide use. Successively, examples employing the present invention will be explained. A thin apparatus is suitable for a back light of liquid crystal display units (LCD). In case of the back light of LCDs, momentary brightness of the phosphor powder is more important than mean plane-brightness. Momentary brightness of the phosphor powder, which is scanned by focused electrons, is 100 times or more as much as the mean plane-brightness. Therefore, if addressing points of an LCD and addressing points of the phosphor screen 54 are synchronized, the addressing points of the LCD can be irradiated by bright light. The apparatus is also capable of use as a pixel of a large scale monochrome or color display device. The device has multiple pixels, which are arranged in an X-Y matrix form, and selected pixels are illuminated by an X-Y addressing method so as to form images. Apparatuses of above described embodiments can be used . To display video images on the display device, video signals are applied to each control electrode 24 of each luminescence apparatus. Furthermore, a plurality of phosphor screens, which have the same color or different colors, can be formed in one luminescence apparatus. In Figs. 21 (side sectional view) and 22 (sectional view through the housing 10 taken along line B-B), the apparatus has three phosphor screens 54. Each phosphor screen 54 is partitioned by black members 64, which are made of carbon. Note that a broad phosphor screen 54 may be divided by the black members 64. Number of the line electron guns 20 is equal to that of the phosphor screens 54, and each line electron gun 20 irradiates the phosphor screen 54 corresponding thereto. Therefore, the length of the line electron guns 20 shuld be equal to or slightly longer than the phosphor screens 54 because one line electron gun 20 scans one phosphor screen 54. By controlling electric potential of each control electrode 24 of each line electron gun 20, illumination of each phosphor screen 54 can be controlled, so that the apparatus can be used as three pixels of the display device. Furthermore, if the three phosphor screens 54 are respectively coated with red, green and blue phosphor powders, they can be used as one color pixel. A color display unit can be realized by arranging multiple color pixels in an X-Y matrix form. A large display device, e.g. an electric bulletin board, will be explained with reference to Figs. 23 (side sectional view) and 24 (front view). As to one line cathode 26, which is included in one line electron gun 20 and arranged in an X-direction, the extraction range, which corresponds to one phosphor screen 54, is divided into a plurality of groups in the X-direction. Namely, a plurality of control electrodes 24, each of which corresponds to each said group, are provided to said one line electron gun 20. With this structure, one phosphor screen 54, which corresponds to one line electron gun 20, is divided into said groups, so that the extraction is independently executed group by group, and the phosphor screen 54 can be independently illuminated group by group. A plurality of intermediate electrodes 68 are, of course, provided to each control electrode 24. Furthermore, a plurality of other line electron guns 20, whose structure is the same as that of said gun 20, are also provided in a Y-direction. A transparent section 16, or a displaying face, is divided into multiple groups in the X- and Y-directions in a matrix form. Thus, one phosphor screen 54 can be illuminated group by group. The groups are partitioned by black members if 64, which are provided on the phosphor screen 54. With the X-Y matrix form of the groups, the display device can be used as an electric bulletin board. If this display device is used as a pixel or a part of a larger display device, brightness and resolving power can be higher. The phosphor screen 54 may be colored any optional colors for a color display device. In the luminescent apparatus of the present invention, the emission intensity depends not only on the energy amount given in unit time but also greatly on the phosphor powder composing the phosphor screen. For the above reason, in what follows, the phosphor powders suitably used for the apparatus will be described together with the method for their preparation. First, suitable amounts of component compounds for the fluorescent substance and flux, e.g., Y 2 O 3 , an activator compound determining the emission color such as Eu 2 O 3 , powdery sulfur, and a carbonate of alkali metal as the flux, are weighed in a dry room. The weighed mixture is mixed in a V-shaped mixer, ball mill, or otherwise, so as to form a uniform mixture. The essence of flux is sodium sulfide (NaSx) formed by the reaction of sulfur with an alkali metal salt. The presence of lithium sulfide (Li 2 S) in NaSx is indispensable for the growth of spherical particles. When the NaSx flux contains lithium (Li) ion at an adequate concentration, the speed of growth of Y 2 O 2 S crystal is increased, and polycrystals containing a large quantity of crystal growth axes grow more rapidly than single crystals. If the NaSx flux contains potassium (K) ions, plate crystals of Y 2 O 2 S grow selectively and in an amount which depends on the amount of K ions. Therefore, it is undesirable to add K beyond a certain limit (e.g., 5 wt%). The mixing of Li ions in the raw material mixture may be effected by using such Li compounds as are decomposed when heated and react with sulfur in the mixture to form Li 2 S, e.g., Li 2 CO 3 , Li 3 PO 4 , and LiOH. The optimum amount of the Li ion to be contained in NaSx is in such a wide range as to have no sensitive effect. When the amount of LiSx contained in the whole flux is in the range of 5 to 60%, spherical particles may be obtained; increase in the amount of Li causes the mixture to be tightly sintered after calcination. If the sintered strength is high, the flux is hard to remove completely. To relieve this difficulty, the content of Li 2 S should desirably be in the range of 20 to 50%, the most desirable range being between 30% and 40%. With a view to make the sintering effect mild, it is effective to add about 5% of K 2 S x . In order to obtain phosphor powder with good reproducibility, it is effective to mix the powders of raw materials uniformly. For the uniform mixing of the powdery raw materials, each particle of the powdery raw materials to be mixed should desirably have almost the same weight (mass). If the particles of the powdery raw materials to be mixed are largely different in mass from one another, the result of mixing, even if the mixing time is prolonged, displays a deviated distribution due to the difference in mass among powders, always leading to a heterogeneity. The non-uniform mixing of raw materials should be avoided because it results in a heterogeneous property in phosphor powders, forming the main factor of variation in production reproducibility. For this reason, it is necessary to make uniform the masses of powdery raw materials, such as sulfur, Na 2 CO 3 , and Li 2 CO 3 . The criterion for the mass homogenization had better refer to the particle mass of the raw material Y 2 O 3 (ca. 3.5 µm in particle size measured through sedimentation). To obtain highly emissive fluorescent substances, the particle diameter of the raw material Y 2 O 3 should be as small as possible. However, the particle diameter of Y 2 O 3 , under the restriction imposed on industrial production, is hard to be made less than the above specified 3.5 µm. All the raw material powders except Y 2 O 3 and the activator oxides, which have passed through a 60-mesh sieve, desirably a 100-mesh sieve, may be used practically. When fine powders are mixed, when raw material powders are stored in high-temperature high-humidity rooms, or when weighings are conducted in high-humidity rooms, powders are caused to aggregate as a result of absorbing moisture in the air through capillary condensation. Such aggregated particles should not be used because their use is equivalent to using large-size raw material powders. Therefore, the storage, weighing, and preparation of raw material powders are to be conducted in rooms where the relative humidity is lower than 50%. The uniformly mixed powdery raw materials are put into an alumina crucible with air included in the powders being removed sufficiently. To remove air in the powers, the crucible may be mechanically vibrated. The air in the powders should be removed not only because it is a thermal insulator which prevents raw material powders from being heated uniformly, but also because, when heated abruptly, it causes powder bumping, resulting in furnace pollution. The crucible, in which the raw material powder has been put, is sealed with a crucible lid made of alumina. The crucible is transferred into a muffle, and the muffle is heated. In this process, the temperature is to be held around 150°C for about 30 min. for the purpose of removing water from the mixed powders and air bubbles in the powders. In addition, if the muffle is held for 30 min. to 60 min. at a temperature (e.g., 600°C) slightly higher than the melting point of the alkali metal sulfide to be formed by the reaction of the alkali metal with sulfur, favorable results may be obtained since the surfaces of the particles of Y 2 O 3 and activator compounds are enveloped with the flux. After such a pretreatment has been conducted, the temperature of the muffle is raised to ca. 800°C and held there for ca. 30 min. and thus a favorable result may be obtained since the conversion of Y 2 O 3 into Y 2 O 2 S is advanced to completion without the particle diameter of the y 2 O 3 S being enlarged. After the Y 2 O 3 has thus been converted into Y 2 O 2 S, the crucible is to be heated by raising the temperature of the muffle to the temperature at which the specified particle diameter is obtained. When the heating temperature exceeds 1100°C consideration needs to be taken to prevent air from entering the muffle atmosphere from the muffle outside. Each of the spherical particles obtained consists of a polycrystal having a plurality of crystal axes inside. The curve A in Fig. 25 shows the change in the median particle diameter of spherical particles with heating temperature. It may be understood that a polycrystal is rapid in crystal growth. Therefore, to obtain spherical particles with a given particle diameter, it is necessary to control strictly the heating time within the crucible. Since the thermal conduction is not good in the powdery mixture within the crucible, a short-time heating produces fluorescent substances with wide particle size distributions. In order to obtain fluorescent substances with narrow particle size distributions, it is necessary to hold the crucible at the given temperature for a long time. Practically, when a 500-ml crucible is used, a temperature hold for 2 to 3 hours is necessary. The growth of fine single crystals is in contrast to the case of polycrystals. Fine single crystals grow with slow speeds of crystal growth. The curve B in Fig. 5 shows the dependence of the growth speed of fine single crystals on heating temperature. The sufficiently heated crucible is allowed to cool gradually down to near room temperature so that the alumina may be prevented from being damaged from thermal distortion. In the cooled crucible, we obtain a heated rod having been sintered by the solidification of the flux. If we remove only the flux from this heated rod, we may obtain a powder, an aggregate of phosphor particles. To remove the flux, the heated rod needs to be soaked in hot water at 70°C to 90°C. The flux gets dissolved into the hot water, though gradually. To remove the flux completely, it is effective to allow the fluorescent substance to be suspended for several hours in strongly stirred hot water. To get a good result, the above-described operation is to be repeated several times. Then, the surface of the particles need to be etched by use of 1-N hydrochloric or nitric acid. For the etching to be effected uniformly on all the particles, the acid solution having the fluorescent substance suspended had better be stirred as strongly as possible. In this manner, the surface of particles may be etched selectively. For example, Y 2 O 2 S phosphor powders may be etched with nitric or hydrochloric acid. To make the surface of Y 2 O 2 S fluorescent substance rough, 3 to 40% acid concentrations are available, favorable results are obtained at acid concentrations of 5 to 20%, and the most desirable result is obtained at acid concentrations of 8 to 10%. If the acid concentration is lower than 3%, the surface of the fluorescent substance cannot be etched sufficiently by the acid. The powders which have being etched are subjected to washing with pure water several times in order to get rid of the remaining acid. The etched surface of the fluorescent substance is unstable chemically and physically in that it tends to get polluted by moisture, air, carbon dioxide, and so on when exposed to air. The etched surface is stabilized by being suspended in dilute phosphoric acid or an aqueous solution of a phosphoric compound. To stabilize the surface of all powders uniformly, the suspended solution should effectively be stirred strongly with a view to prevent the sedimentation of particles. Favorable concentrations of phosphoric acid are in the range of 0.1% to 30%, more detailedly the range of 1% to 10% is desirable, and the most desirable result may be obtained in the range of 3% to 6%. In the case of using a solution of sodium phosphate, a good result may be obtained in the range of 1% to 20%, and a favorable result may be obtained in the range of 1% to 10%. After having the surface stabilized, the phosphor powders are washed several times with pure water to get rid of the remaining phosphoric acid and phosphate. After washing, water is removed by filtration, and drying is effected by using a dryer heated at 150°C. After drying, sieving is conducted at room temperature using a 400-mesh sieve. The powders passing through the sieve give phosphor powders excellent in flowing. These powders are aggregates composed of primary particles which are approximately spherical and have rough surfaces. When an application onto the phosphor screen of a cathode-ray tube is made with these powders, a phosphor screen is obtained which is highly superior in membrane property to the conventional fluorescent substances. In what follows are described concrete examples of methods of producing phosphor powders. Y 2 O 3 (100g), S (50g), Eu 2 O 3 (4.0g), Na 2 CO 3 (35g), Li 2 CO 3 (20g), and Na 2 HPO 4 (5g), all having passed through a 100-mesh sieve, are weighed out in a dry room into a glass bottle. This glass bottle is subjected to a 1-day mixing using a ball mill device. The raw material after the mixing is transferred into a 200-ml alumina crucible, which is mechanically vibrated to get rid of the air in the raw material powders. The crucible has its lid applied and is then put into an electric furnace (muffle) to be heated. The heating is conducted in such a way that the temperature is kept at 150°C for 30 min., then at 600°C for 30 min. and at 800°C for 30 min., and finally at a high temperature of 1200°C for 2 hours. At the end of final heating, the electric furnace is turned off to cool the crucible. When the temperature of the electric furnace has reached almost to room temperature, the crucible is taken out of the electric furnace. In the crucible, a hard-sintered mixture of phosphor powders and flux is found in the form of a rod. This rod is taken out of the crucible and is soaked in hot water at 80 °C in a stainless container. The flux is caused to dissolve selectively into the hot water and thus only the phosphor powders deposit on the bottom of the container. After the hot water with the flux dissolved has been taken out, the phosphor powders are again suspended in hot water at 80°C and the suspension is stirred violently. By stirring in such a way, the flux remaining on the surface of the phosphor particles may be removed. The stirring time is to be 3 hours. After this operation, the solution with the flux dissolved is taken out, and the powders are subject to rinsing with pure water twice or so. The powders after the rinsing are suspended in a 10% nitric acid solution, which is stirred strongly for 2 hours. After the stirring, the acid solution is taken out, and rinsing with pure water is conducted twice or so. Then, the powders are suspended in a 5% phosphoric acid solution, which is stirred strongly for about 1 hour. The phosphoric acid solution is taken out, and the phosphor powders are rinsed several times with pure water. The rinsing effect is augmented if we employ a filtering operation after each rinsing. The fluorescent substance which has been through rinsing is separated from its contained water by filtration and dried for 5 hours in a dryer warmed at 130°C. The dried phosphor powders are sieved with a 400-mesh sieve; in this sieving powders consisting of primary particles distributed according to the logarithmic normal distribution will come through the sieve. The median particle diameter of this fluorescent substance is 4.5 µm according to the microscopic method. And, each particle is a near-spherical particle with its surface made rough. The phosphor powder thus obtained is easily applicable onto a face plate (substrate) by means of the ordinary PVA-fluorescent substance slurry method. When Eu 2 O 3 is used as an activator, a phosphor powder capable of developing red color may be obtained; if we select known activators and use similar procedures as described above, we may obtain phosphor powders capable of developing green or white color. For a phosphor powder capable of developing blue color, we use, for example, a phosphor powder composed mainly of zinc sulfide powder. When a phosphor powder obtained in the above-described method was applied onto a face plate (substrate), we found a remarkable improvement in the points of pinhole, cross-contamination, pattern cut, and so on, and the resolving power augmented. In addition, in a cathode luminescence apparatus, the emission intensity of the phosphor screen was found to be increased. It should be noted that such phosphor powders as used in the present invention may, of course, also be used suitably as phosphor powders for cathodic tubes of television receivers, computers, etc..

Claims: A cathode luminescence apparatus, comprising: a line electron gun (20) having a line cathode (26), which is elongated in a first direction, and an intermediate electrode (28, 30, 50) for controlling the passage of electrons from the line cathode 26); an anode (52) for accelerating electrons emitted from said line electron gun (20), said anode (52) having an electrically conductive section formed on a substrate (16), such that, in use, voltage is applied thereto for the acceleration; a phosphor screen (54) made of phosphor particles on said electric conductive section (52); and a container (10) accommodating said line electron gun (20) and said anode (52), said container (10) having a light emitting face, through which visible light is capable of passing, said container (1) being evacuated to a vacuum for cathode luminescence; and characterised in that said intermediate electrode (28,30,50) is constructed and arranged so as to be capable of drawing out electrons from said line cathode (28, 30, 50), of linearly focusing said electrons and of reciprocatively scanning electrons focused in a second direction perpendicular to the first direction. Apparatus according to claim 1, wherein; said intermediate electrode has a first intermediate electrode (28) for drawing electrons from said line cathode (26), and a second intermediate electrode (3) for linearly focusing the drawn out electrons and reciprocatively scanning electrons focused in the second direction. Apparatus according to claim 1, wherein; said intermediate electrode has a first intermediate electrode (28) for drawing electrons from said line cathode (26), a second intermediate electrode (30) for linearly focusing the drawn out electrons, and a third intermediate electrode (50) for reciprocatively scanning electrons focused in the second direction. Apparatus according to claim 1, 2 or 3 wherein: said line cathode (26) has a ceramic core (32), a wire heater (34), which is wound round said ceramic core (32), and an oxide layer (36), which coats said wire heater (34). Apparatus according to claim 1, 2 or 3, wherein; said line cathode (26) has a hollow cylinder section (40), a coil heater (42), which is accommcdated in said cylinder section (4), and an oxide layer (36), which coats the outer circumferential face of said cylinder section (40). Apparatus according to claim 1, 4 or 5, wherein there are provided a plurality of said intermediate electrodes (28,30), each of which is capable of independently operating, in said container (10). Apparatus according to any of claims 1, 2, 3, 4, 5 and 6, wherein there is provided a plurality of groups, each of which comprises a said anode (52) and a said phosphor screen (54), in said container (10). Apparatus according to any of claims 1, 2, 3, 4, 5 and 6, wherein said anode (52) is divided into a plurality of parts, and said phosphor screen (54) is divided into a plurality of parts. Apparatus according to claim 7 or claim 8, wherein said groups or said parts are mutually partitioned by black members (64). Apparatus according to any preceding claim, wherein said phosphor particles comprise spherical primary particles having surfaces which have been roughened by etching, said primary particles being polycrystals of Y 2 O 2 S. The apparatus according to claim 10, wherein the median diameter of said primary particles is 4.5 µm, and said primary particles are capable of passing through a 400-mesh sieve.

Inventors: OZAWA RYUJI, OZAWA, RYUJI, OZAWA, RYUJI, MIYOTA KABUSHIKI KAISHA
Applicants: MIYOTA KK, MIYOTA KABUSHIKI KAISHA
Application year: 1992
Main classification: H01J31
IPC codes: H01J31, H01J63, C09K11