Patent GB-1340975-A (GB)
Title: Method of manufacturing a luminescent screen
IPC: H01J29/18
Filed: 1972-01-19
Abstract: 1340975 Luminescent screens N P SOSCHIN G A KITAEV and N P KADOMSKAYA 19 Jan 1972 2517/72 Heading C4S [Also in Division H1] A method for producing a luminescent screen in which image colours depend on electron beam energy, comprises depositing on a screen at least two luminescent materials one at least of which, prior to or after deposition, is coated with a barrier layer which may be inert or of the other luminescent materials by treating with an aqueous solution containing a soluble salt of at least one element selected from Groups IIb and IIIb of the periodic system, and at least one soluble compound selected from thiourea, allylthiourea, thiosemicarbazide, selenourea, dimethylselenourea, an alkali metal selenosulphate and thioacetamide, the pH of the solution being maintained in the range -2 to 14, the barrier layer being composed of at least one element from Groups IIb and IIIb in chemical combination with S and/or Se. The soluble salts may be the halides, nitrates, sulphates or acetates of the Groups IIb, IIIb elements, the soluble salt concentration in the starting solution, and the S and Se compounds concentration being 0À01 to 5 M. The luminescent particles may be dispersed in the starting solution, and prior to treatment of the luminescent particles, a Ag, Au, Cu or Mn soluble salt may be added, and after barrier deposition, the barrier layer made luminescent by heat treatment at # 250‹C. The additional salts may be in concentration 1 Î 10<SP>-6</SP> to 1 Î 10<SP>-4</SP> M, and the particles with barrier layer may alternatively be treated with an aqueous solution of the above Ag, Au, Cu or Mn salts, then heated to make luminescent. The barriers which are luminescent may also be self actuated by heating at # 250‹ C. in hydrogen or vacuo to increase Zn or Cd ions for example in the barrier. Variation of duration and temperature of treatment controls diffusion depth so only part may be luminescent and colour variations with depth may be achieved. Preferred barrier materials are Zn, Cd, Ga, In sulphides or selenides, retaining visible light transparency. The barriers may be simple compounds of ZnS, CdS, ZnSe, Ga 2 S 3  types or on compositions having mixed anions and/or cations, e.g. (Zn x Cd 1-x )(S y Se 1-y ) or the mixed chalcogenides enabling varying the refractive index to enhance light output. Barrier thickness may be controlled by solution concentration and heating accelerates deposition. Fig. 1 may include a luminescent layer 1 (e.g. green emitting) deposited conventionally, barrier layer 3, red-emitting phosphor 4, and Al coating 5 to enhance luminous efficiency, a third luminescent layer being optional. Fig. 3 includes coated green phosphor particles with uncoated phosphor particles, deposited on the screen conventionally, e.g. from a potassium silicate solution. Third, blue phosphor particles with a different thickness barrier layer may be included. Multilayer screens may include particles of dissimilar threshold excitation values, each layer including particles of the same threshold, or for instance, a 3 colour screen of a layer of two phosphor mixture, of dissimilar thresholds, the second layer of uncoated single colour particles, or a mixture of two dissimilar particles one coated with barriers emitting a third colour, the first optionally being coated with an inert barrier. Fig. 5 (not shown) includes glass (1), phosphor (2) and luminescent barrier (9) deposited by precipitation and then treated with an aqueous solution of Ag, Au, Cu, or Mn, the depth of diffusion being governed by temperature (e.g. 250‹ C.) and duration of treatment. Barrier layers are in the range 10<SP>-12</SP> to 10<SP>-6</SP> #-cm.<SP>-1</SP> and assist charge dispersion on electron incidence. Simple or mixed Zn or Cd chalcogenides provide varying refractive index between 1À9 and 2.45 for increased brightness, thickness of 100 to 4 Î 10<SP>4</SP> Š for excitation thresholds of 50 to 20 kV. In the examples, ZnS 0À62 CdS 0À38  : Ag precipitated from potassium silicate solution to a 2À5-3 mg./cm.<SP>2</SP> coating density, is coated with a ZnS barrier layer, and superposed with Y 1À934 En 0À066 O 2 S at 1À5-2 mg./cm.<SP>2</SP>, details of solution compositions including buffer solutions, thicknesses, rate of deposition and excitation energies being disclosed for this and following examples; Fig. 6 (not shown) illustrating colour variations v. beam modulation; zinc or thiosilicate on silver activated zinc sulphide; Eu activated yttrium oxysulphide with Ag : ZnS; Tb : Y 1À98 Tb 0À02 - O 2 S on La 1À99 Pr 0À01 O 2 S with ZnSe barrier, Y 1À94 Eu 0À06 O 3  being on the backing; indium sulphide barrier on ZnS 0À8 CdS 0À2  : Cu, Al partides mixed 1 : 1 with Eu : yttrium orthovanadate; ZnS 0À8 .CdS 0À2  : Ag or with Cd sulpho-selenide of same Á as the phosphor, and mixed 3 : 1 with yttrium-aluninium garnet; Y 2 Si 2 O 7  : Ce particles coated with ZnGa 2 S 4  : Au, emission being enhanced by calcining in an inert atmosphere at 350‹ C. for 1 hour; colour variation with penetration of a particle is described for Ag : ZnS coated with silver cadmium sulphide, and calcined in air at 400‹ C. to promote Zn diffusion to the particle surface; Mn : alkaline earth and Al fluorides (3 mg./cm.<SP>2</SP>) coated with Zn and CdSe and subsequently treated with a copper solution to promote diffusion of Cu ions into the barrier layer to 75% of the thickness, the actuated barrier portion serving to emit light. Reference is made to prior art screens of mixtures of particles of different threshold excitation voltages, and screens wherein emission colour varies on passage from the external layers to the centre of the particle. Reference is made to electron penetration varying non- linearly with voltage variations, and efficiency being optimum at a particular depth of penetration.