METHOD OF FORMING A PHOSPHOR SCREEN

Abstract: There is provided a method of forming a phosphor screen, comprising the steps of coating a faceplate with a resist, which exhibits adhesiveness when irradiated with light, to form a resist film, selectively exposing the resist film to light to form an adhesive surface of a predetermined pattern, supplying phosphor particles having an average particle diameter of 3 to 20 microns to the faceplate so as to allow the phosphor particles so slide along the faceplate and, thus, to be attached to the adhesive surfaces of the faceplate, and removing the excess phosphor particles from the faceplate, characterized in that the surface of the phosphor particle is coated with a fine powdery material such that the specific surface area of the coated phosphor particle determined by the BET method is l.5 to 4.0 times as large as that of the phosphor particle before the coating. According to this invention there is obtained a phosphor screen free from pin holes, to increase the density of the screen, and having a clear stripe pattern and preventing color blurring.

Description: The present invention relates to a method of form­ ing a phosphor screen for a cathode ray tube. In forming a phosphor screen for a cathode ray tube, it is known to form a powdery layer utilizing a photo adhesiveness, as disclosed in, for example, Japan­ ese Patent Disclosure (Kokai) No. 55-32332. A resist film which exhibits adhesiveness when exposed to light is used in this technique so as to allow the phosphor particles to be attached to the light-exposed region of the resist film. The phosphor particles on the non-­ exposed region of the resist film are blown away by an air stream. This basic process is applied to each of blue, green and red phosphors. Ordinary phosphors such as ZnS and Y₂O₂S are used in this technique. However, this prior art is defective in that the adhesiveness of the phosphor particles to the resist film is weak, with the result that the phosphor screen is made nonuniform in thickness during the step of blowing away the unde­ sired phosphor particles. Japanese Patent Disclosure (Kokai) No. 55-69934 discloses the idea of treating the phosphor surface with a powdery material such as Zn(OH)₂ or Zn₃(PO₄)₂ such that 0.0l to 2% by weight of the powdery material is attached to the phosphor. However, it has been clari­ fied by the experiment conducted by the present inven­ tors that the phosphor coated with the powdery material exhibits an unduly high adhesivity, leading to an insuf­ ficient flow of the phosphor particles. The phosphor particles, if forcedly caused to flow, have been found to be distributed over the entire faceplate, resulting in failure to produce a phosphor screen with a clear stripe pattern. In some cases, adhesivity of the phos­ phor particles has been found insufficient. In this case, a large number of pin holes are formed in the resultant phosphor screen, leading to an insufficient density in the phosphor screen. The above-noted difficulties are thought to be derived from, for example, the fact that the particle diameter of the powdery material used for the surface treatment of the phosphor particle is not considered in the prior art exemplified above. Naturally, the optimum weight of the powdery material depends on the particle diameter thereof, making it impossible to determine the optimum amount of the powdery material used for the surface treatment of the phosphor particles unless the particle diameter of the powdery material is also de­ fined. The optimum amount of the powdery material also depends on the method of preparing the powdery material, e.g., presence or absence of agglomeration. An object of the present invention is to provide a method of forming a phosphor screen free from pin holes, to increase the density of the screen, and having a clear stripe pattern and preventing color blurring. According to the present invention, there is pro­ vided a method of forming a phosphor screen, comprising the steps of coating a faceplate with a resist, which exhibits adhesiveness when irradiated with light, to form a resist film; selectively exposing the resist film to light to form an adhesive surface of a predeter­ mined pattern; supplying phosphor particles having an average particle diameter of 3 to 20 microns to the faceplate so as to allow the phosphor particles to slide along the faceplate and, thus, to be attached to the adhesive surface of the faceplate; and remov­ ing the excess phosphor particles from the faceplate; characterized in that the surface of the phosphor particle is coated with a fine powdery material such that the specific surface area of the coated phosphor particle determined by the BET method is l.5 to 4.0 times as large as that of the phosphor particle before the coating. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawing, in which: Fig. l shows the relationship between the thickness of the phosphor film and the ratio of the specific surface area of coated phosphor particle used in the present invention to that of the phosphor particle before the coating. Experiments In the manufacture of a phosphor screen utilizing a resist which exhibits adhesivity when exposed to light, it is important for the phosphor particles to be highly slidable along a faceplate formed of, for example, glass, because the phosphor particles are repeatedly allowed to slide along the faceplate. If forcedly caused to slide along the faceplate, the phosphor parti­ cles are then scattered over the entire faceplate. It is also important for the phosphor particles to have a high bonding strength with the adhesive region of the resist film. If the bonding strength is insuffi­ cient, the phosphor particles are incapable of with­ standing the air pressure applied in the developing step, resulting in failure to obtain a phosphor screen of a desired thickness. In extreme cases, few phosphor particles remain attached to the resist film. What should also be noted is that, in the case of using a resist film which exhibits adhesiveness when irradiated with light, the phosphor particles absorb water when attached to the adhesive region of the resist film. It follows that it is necessary to severely con­ trol the surface treating conditions. The present inventors have conducted various exper­ iments in which the surface of the phosphor particle was coated with various powdery materials, finding that the bonding strength between the coated phosphor particle and the adhesive region of the resist film depends greatly on the ratio of the coated phosphor particle to the phosphor particle before the coating in the specific surface area determined by the BET method. Fig. l is a graph showing the results of the experiments. The ratio in question is plotted on the abscissa of the graph, with the thickness of the film formed by the phosphor particles attached to the resist film being plotted on the ordinate. As seen from Fig. l, the thickness of the formed film is markedly small where the ratio in the specific surface area plotted on the abscissa is smaller than l.5, indicating that the bonding strength between the coated phosphor particle and the adhesive region of the resist film is so high that the sliding of the phos­ phor particles along the resist film is greatly restricted. Where the ratio in the specific surface area is at least l.5, the phosphor particles are allowed to slide suffi­ ciently along the resist film. However, the bonding strength between the phosphor particle and the adhesive region of the resist film is decreased with increase in the ratio in question. In other words, the ratio in the specific surface area should not be unduly large, in order to obtain a phosphor film of a reasonable thick­ ness. Specifically, Fig. l shows that it is difficult to obtain a phosphor film of a reasonable thickness, if the ratio in question is larger than 4.0. What should also be noted is that the color blurring is elim­ inated with decrease in the bonding strength noted above. It follows that the ratio of the coated phosphor parti­ cle to the phosphor particle before the coating in the specific surface area should range between l.5 and 4.0. What should also be noted is that the tendency shown in Fig. l can be obtained substantially regardless of the kind and particle size of the powdery coating material as well as the kind of the phosphor body, as seen from Fig. l. Specifically, "o" in Fig. l repre­ sents a blue phosphor of ZnS:Ag having an average parti­ cle diameter of 6 microns and coated with a powdery SiO₂ having an average particle diameter of about 0.5 micron; "Ø" represents said blue phosphor coated with a powdery SiO₂ having an average particle diameter of 9bout 0.l to 0.2 micron; "Δ" represents a red phosphor of Y₂O₂S:Eu having an average particle diameter of about 6.5 microns and coated with a powdery SiO₂ having an average parti­ cle diameter of about 0.5 micron; and "x" represents said red phosphor coated with a powdery Zn(OH)₂ having an average particle diameter of about 0.3 micron. Clearly, substantially the same relationship between the ratio in the specific surface area and the thick­ ness of the formed film is obtained regardless of the phosphor-coating material combination. The data shown in Fig. l covers phosphor particles having an average diameter of 6 microns or 6.5 microns. The tendency shown in Fig. l remains unchanged as far as the average diameter of the phosphor particles falls within the range of 3 to 20 microns. It is desirable to use a powdery coating material having a maximum particle diameter of l micron. If the particle diameter is larger than l micron, the coating material tends to peel off the phosphor body. In addi­ tion, the brightness of the formed phosphor screen is reduced. Also, it is possible to use in place of the powdery coating material a surface treating agent which forms a film such as water glass or choline-SiO₂, when uncontinuous. In some cases, in addition to a surface treating agent the phosphor particle is coated with an organic or inorganic pigment (Fe₂O₃. CoO·nAℓ₂O₃, ultramarine etc.) in order to improve the contrast of the phosphor screen. If the pigment is used as a part of the powdery coating material as defined in the present invention, it is possible to obtain substantially the same effect. Example l Green phosphor particles of ZnS:Cu having an aver­ age particle diameter of 6.5 microns were dispersed in a dispersion of powdery SiO₂ having an average particle diameter of 0.05 micron, followed by adding an aqueous solution containing a sulfate acting as a surface treat­ ing agent of the SiO₂ particles. The conditions such as the weight ratio of the phosphor particles to the SiO₂ dispersion and the stirring time of the dispersion were controlled so as to obtain phosphor particles coated with the powdery SiO₂. The coated phosphor particles were found to have a specific surface area of 0.37 m²/g as determined by the BET method. The measuring is con­ ducted under nitrogen gas and by the isothermal absorp­ tion method. On the other hand, the specific surface area of the phosphor particles before the coating was 0.22 m²/g. In other words, the specific surface area of the coated phosphor particles was l.7 times as large as that of the phosphor particles before the coating. Also, blue phosphor particles of ZnS:Ag and red phosphor particles of Y₂O₂S:Eu were similarly treated so as to obtain coated phosphor particles having a specific sur­ face area of 0.33 m²/g (blue phosphor) and 0.55 m²/g (red phosphor) as determined by the BET method. The ratio of the coated phosphor particles to the phosphor particles before the coating in the specific surface area was l.5 for the blue phosphor and 2.5 for the red phosphor. On the other hand, a faceplate was coated with re­ sist which exhibits adhesiveness upon irradiation with light, said resist consisting of 3% by weight of 4-­ dimethylaminobenzene diazonium-zinc chloride complex salt, 0.2% by weight of propylene glycol alginate, 0.0l% by weight of Pluronic L-92 (trademark of a surfactant), and the balance of water, followed by selectively expos­ ing the coated faceplate to light. Then, the coated blue phosphor particles prepared in advance were sup­ plied to the faceplate to allow the phosphor parti­ cles to slide along the faceplate several times, fol­ lowed by performing an air development so as to form a blue phosphor layer in the exposed portion of the resist film. Then, green and red phosphor layers were formed on the faceplate by the similar procedures. The resul­ tant phosphor screen was found to have a thickness of 2 to 3 mg/cm². Also, a phosphor screen free from color blurring and pin holes was obtained with a high reproducibility. Example 2 Coated phosphor particles were prepared as in Example l, except that blue, green and red phosphor particles each having an average particle diameter of 6.5 microns were dispersed in a dispersion of very fine powdery SiO₂ having an average particle diameter of 0.0l micron. The specific surface area of the coated phos­ phor particles was found to be 2 to 3 times as large as that of the phosphor particles before the coating. In Example 2, the amount of SiO₂ powder used was about one-fifth the weight in Example l. On the other hand, Example 2 was substantially equal to Example l in the thickness and quality of the phosphor screen produced. Example 3 Red phosphor particles of Y₂O₂S:Eu having an aver­ age particle diameter of 6 microns were added to an aqueous solution of ZnSO₄. While sufficiently stirring the solution, ammonia was added to the solution so as to control the pH value at 7 to 8 and, thus, to deposit Zn(OH)₂ on the surfaces of the phosphor particles. The resultant phosphor particles were washed with water and, then, dried so as to obtain phosphor particles coated with a powdery Zn(OH)₂ having an average particle diameter of 0.3 micron. The specific surface area of the coated phosphor particles determined by the BET method was twice the value before the coating. A phos­ phor screen for a color picture tube was prepared by using the coated red phosphor particles thus prepared together with the coated blue and green phosphor parti­ cles prepared in Example l. The resultant phosphor screen was found substantially equal to that obtained in Example l in thickness and quality. Example 4 Green phosphor particles of ZnS:Cu, Aℓ having an average particle diameter of 8 microns were dispersed in water. Then, water glass and aluminum sulfate were added to the dispersion so as to control the pH value at 6 to 7 and, thus, to deposit Aℓ₂O₃ particles on the sur­ faces of the phosphor particles. The average particle diameter of the Aℓ₂O₃ particles was 0.l micron. Also, the specific surface area of the coated phosphor parti­ cles determined by the BET method was 2 times as large as that of the phosphor particles before the coating. A phosphor screen for a color picture tube was prepared by using the coated green phosphor particles thus prepared, together with the coated blue and red phosphor particles prepared in Example l. The resultant phosphor screen was found substantially equal to that obtained in Example l in thickness and quality. In the examples described above, the method of the present invention is applied to the formation of a phos­ phor screen for a color picture tube. However, the pre­ sent invention can be applied to the formation of a phosphor screen for various cathode ray tubes such as a projection picture tube and a monochromatic picture tube.

Claims: 1. A method of forming a phosphor screen, comprising the steps of coating a faceplate with a resist, which exhibits adhesiveness when irradiated with light, to form a resist film; selectively exposing the resist film to light to form an adhesive surface of a predetermined pattern; supplying phosphor particles having an average particle diameter of 3 to 20 microns to the faceplate so as to allow the phosphor particles to slide along the faceplate and, thus, to be attached to the adhesive surface of the faceplate; and removing the excess phosphor particles from the faceplate; characterized in that the surface of the phosphor particle is coated with a fine powdery material such that the specific surface area of the coated phosphor particle determined by the BET method is l.5 to 4.0 times as large as that of the phosphor particle before the coating. 2. The method according to claim l, characterized in that said fine powdery material is selected from the group consisting of SiO₂, Zn(OH)₂, Aℓ₂O₃, Fe₂O₃, CoO·nAℓ₂O₃ and ultramarine.

Inventors: ITOU, TAKEO PATENT DIVISION, SAGOU, SEIJI PATENT DIVISION, WATANABE, SHINGO PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: C09K11
IPC codes: H01J9, C09K11