US2372359A - Luminescent target - Google Patents

Description

March 27, 1945. J. c. COOK 2,372,359

LUMINESCEN'I' TARGET Filed July 51, 1941 HIGH VOL ma: ELECTRON au/v SUCH AS FROM /0 TO I00 K/LOVOLTS F/G. 2 F/G. 3 2/ 20 22 22 C 20 22 20 r all; coon-o ,JLU/D coouro CI /AMBR CHAMBER rwonsscmr mw. BACK/N6 MATERIAL MEMBER PLURAL/TY OF LIGHT RE LECT/NG LAYERS SUCH AS AGQA O ,OR 8:0. BINZER LAYER INVENTOR J C. COOK BVW Patented Mar. 27, 1945 LUMINESCENT TARGET John C. Cook, Chatham, N. J., as'signor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 31, 1941, Serial No. 404,891-

8 Claims.

This invention relates to luminescent screens 01 coatings and more particularly to fluorescent screens or targets for cathode ray tubes.

It is an object of the present invention to provide novel fluorescent screens or coatings suitable for producing intense light under cathode ray bombardment.

In order to obtain large'television images, a projection cathode ray tube has been used. In such a tube, the voltage applied to the final accelerating member is, inmany cases, as high as 50 or 100 kilovolts or more and the power in the beam is of the order of 500 to 1000 watts. This high voltage and power is necessary to obtain suflicient light for projection. purposes where large bright images are desired. The present invention relates primarily to screens or targets for projection cathode ray tubes.

A luminescent element made in accordance with this invention comprises, in one form hereinafter more fully described, a metal backing serving the triple purpose of a support member, a means for transmitting heat away from the screen (it may be fluid cooled), and a means to conduct away charges imparted to the screen by the electron beam, a thick reflecting layer of light colored material, a binding cement (which may be of the same materialas the reflecting layer), and one or more layers of fluorescent material. The thick reflecting layer is preferably made up of a plurality of thin layers. In a modified form, alternate layers of re- 7 fleeting and fluorescent material are applied to the supporting member. A preferred material for the layer to serve as the reflecting and binding-means is magnesium oxide, which is a good binder, a transmitter of light to and from the lower layers, and a reflector of light, and which does not have a deleterious effect on the phosphors used. This material in a layer of just sufficient thickness to be continuous is a better reflector than transmitter of light. Other suitable reflectingand cementing materials are aluminum oxide and beryllium oxide.

' The depth of penetration of an electron beam in a target has been shown by several investiga vention;

(if it is too thin, however, only a fraction of the energy-of the beam will be absorbed) or it may be somewhat thicker, the limiting thickness being determined by the heat and electricity transmitting ability of the reflecting material which is not as good for very thick layers as for layers of, forexa'mple, 10 mils in thickness.

A preferred method of making a screen of this" type comprises coating a metal backing plate with a continuous coating of, for example, magnesium oxide, applying a second coating after the first coating is dry, applying a binding cement of magnesium oxide to the reflecting layer of magnesium oxide and immediately sifting uniformly over the wet surface the fluorescent powder. If desired a small amount of sodium silicate in water can be added to the binding mixture or the reflecting mixture or both, the sodium silicate resulting in a higher resistance to abrasion. Alkali silicate alone is not suitable for thick coatings because on heating, water is evolved and causes the glassy silicate to froth and swellup. Moisture can get out of the thin film of silicate or from thick ones which have a large amount of filler. Addition of the silicate, however, improves the magnesium oxide binder.

The invention will be more readily understood by referring to the following description taken.

in connection'with the accompanying drawing forming a part thereof in which:

Fig. 1 shows a cathode ray tube containing a fluorescent screen in accordance with this in- Fig. 2 is an enlarged view of screen and its supporting member; and

Fig. 3 shows a modification of the screen shown in Fig. 2. U

Referring more specifically to the drawing, Fig. 1 shows, by way of example for purposes of illustration, a cathode ray tube of the projection-televisionreceiver type. The tube' l0 comprises an evacuated container H containing ber I4 is connected'by means 'of a lead'l5' to ground which is also preferably the potential of the highest anode in the electron un I2, which anode may be a conducting coating l6 within the walls of the-tube l0, although one or more a fluorescent other gun elements may be at ground potential also. The container H is so shaped that the picture developed on the fluorescent screen l3 by the scanning of the beam of electrons generated by the gun l2 and deflected over a field thereon by any suitable deflecting means (not shown), is projected by means of a suitable lens system, represented by the single lens I1, upon a viewing screen l8.

Reference will now be made to Fig. 2 which shows in greater detail the structure of a luminescent screen made in accordance with this in-' vention. Smooth layers are shown in Fig. 2 but this is merely for simplicity in description as it will be understood that these layers, formed from successive coatings of material, are in some degree, at least, interlocking, due to the composition and structure of the particles making up the individual layers.

. The screen or target of this invention preferably comprises one or more reflecting layerson a metallic backing plate, these reflecting layers being of a material which has the following properties: (1) it has a relatively high (compared to such substances as glass which have been used in the prior art) reflectivity in the visible and ultra-violet portions of the spectrum; (2) it has a low vapor pressure in vacuum so that it is stable over a long period of timeat temperatures up to 450 C. and when exposed to electron bombardment and ultra-violet light; (3) it is inert at ordinary temperatures and with respect to the fluorescent material used, which in a preferred arrangement is a combination of metallic sulphides with suitable metal activators therein; (4)

it has good mechanipal adherence and coherence; and (5) it does-notdiscolor, evolute gases or contaminate the phosphors. Upon this layer is bound the fluorescent material by a suitable cement which may be of the same; material as the reflecting coating or a mixture of this material with other materials. I

Suitablereflecting materials which have all of the above properties are magnesium-oxide (MgO), aluminum oxide (A1203) and beryllium oxide (BeO). These materials are also suitable as binding materials to cement the fluorescent material to the reflecting coating. Other white powders could be used instead of these but these have been found to be best because (1) they are chemically very inert; (2) they have desirable reflecting properties to an-unusual degree; and (3) they all have forms which are somewhat gelatinous when suspended in water and which when dried provide a coherent-coating.

In order to utilize substantially all of the large beam energy which is provided in television projection tubes, a. relatively thick coatingv of fluorescent material. A- high reflectivity of the--bind ing material in the under reflecting layer or layers is also necessary. The thickness of the fluorescent material should be suflicient to absorb all or at least substantially all of the energy of the electrons the beam. This coating, generally varies .form a few mils to lflto mils of thickness. By way of comparison, the average size fluorescent screen crystal is around .1 mil toan edge. The most'intense fluorescence occurs on the side where the beam first strikes it; but the beam, however, penetrates to the under layers of the fluorescent material and-a portion of the resulting light is transmitted through the material and is of some importance in adding to the intensity of the final spot of light. .By far the brightest spots are those from screens which are r radiations.

adapted to be viewed from the same side that receives the electrons from the scanning beam, as in the arrangement of Fig. 1. In order to increase the amount of light obtained from the under layers of the fluorescent material, the relatively high reflecting layer 20 is used. This reflecting layer 20 is coated on a metal backing M which is preferably a heat conducting disc of copper or any other suitable material. This disc or metal backing member serves the triple purpose of a support member, a means for transmitting heat away from the screen (that is, it may be cooled by water, oil, air or any other suitable fluid caused to flow adjacent thereto or therein by any suitable means), and as a means to conduct away charges imparted to the fluorescent screen by the electron beam inasmuch at it is placed at the same potential as the conducting coating [6 in the tube Hi. The reflecting layer 2i), which may be as shown in Fig. 2 a plurality of laye'rs, is thick enough to be continuous and thus is a good reflecting surface, and thin enough so that there is a minimum heat insulation between the fluorescent particles and the metal backing layer. These requirements determine the thickness as between a lower limit of about 1 mil and an upper limit which may be as high as an inch. Below 1 mil the material tends to be discontinuous while the maximum thickness is fixed by the heat conductivity which, of course, varies with the particular material used. A white reflecting under-surface is desirable in order to redirect the light to the front of the screen. While a metal backing may be used as a reflecting material it also has certain disadvantages. Copper has been used but this tends to shift the color of the luminescence towards the red portion of the spectrum. Also, metals have a fairly high reflectivity in the visible portion of the spectrum and a very high reflectivity for infra-red (heat) radiations. White or other light colored crystalline powders typically possess a very high reflectivity for visible radiations and very low reflectivity (i e., high absorption) for the infra-red These latter proper-ties are ideal for the type of tube for which this invention is primarily adapted. Moreover, metals such as silver and aluminum tend. to discolor the fluorescent particles and are not for this reason entirely suitable for reflecting materials. Furthermore, the metals themselves are not suitable as binders.

A preferred method of making the screen in accordance with this invention, given by way of example, comprises coating the --met.al backing 1 plate 14 with a continuous coating of a three per cent suspension of magnesium oxide, allowing this coating to dry, applying a second coating 20, allowing this coating to dry, applying a third coating 20, allowing this to dry, and applying additional coatings and allowing them to dry until there is built up a reflecting coating of the desired thickness. A binding cement 21 made by suspending 3 grams of light calcined magnesium oxide in 10.0 cubic centimeters of water, is then applied to the reflecting layer or layers 21) and immediately thereafter the fluorescent powder is sifted uniformly over this wet surface. The fluorescent powder 22, which may be of any well-known material capable of very high light intensity and which is preferably a mixture of Zinc and cadmium sulphides activated with copper and manganese (in order to produce .a white luminescence), is then sifted uniformly over the cementing surfac of magnesium oxide. A suitable fluorescent powder, best known to :the trade 1 asiRatterson':5N066-rpowderp immanufacture'drby the Patterson Screen Company. Dth'ersisuitable' fluorescent powders areeealciumratungstate, synthetic i'willeiniteg or a =s'ilicate. 's-The ffluorescent powderris' cemented iima thick layen by thefine, semicolloidalMgTOi-aAlzOaaorlBeO suspension; The sulfidesparticlessare. large iiiltmiil) aandi cannot abe reducednina size so:.:- arratherl rlarge' bulk of 1 cement is: needed 5111.8, :thick: film. 'iTheseematerials .are

very: suitable -:becausei=they3 aremot vitreou aand consequentlysare:diflicultiftoidegas in'ithickffllms. The excesmffluorescent :powderirmaynberremoved after thendryingtprocessywhichilmay be atnroom exactly: thess'ame method as with a1=sulfide is -usdt As it-istp oss'ible' tozgrind si licates to 'very small fizes withoutfldestroying their fluoresce'nt properties, coatings f Zthese fmateria'ls can -be applied withsnwpermanent:binder. flhick coatingS,- -IIOWEVerl-l r8quil76"a bil'ldBROfi SUlllE sort. The nature zofzwillemitexis "such .athat no refiectoris neededias itiiisi a Whitetreflecting :powder. A thick temperatures or by baking the idzube cat 'about tends :to: oxidize. iltlence ithe raquadag-or :other conductingrzlayerrristput onrfirst. After: the screen isemounted withinbthewtube,rdriedgand the excess fluorescent.matetialremoved; ithebulbisthen bakedsat. from:300 aC::to'1400..Ceand1tperhaps even as *highias 450: C. 0193, .periodrof about on'e'shour to: removeiggascfrmmthe:glass, metal,:;-etc.-- ,zThe timeiand;temperatures.givemareinot.veryicritical,

however. v

. .The neck of: the-tube:cor'itainingv thez'gun i thcn sealed en (to tithe glass smember izcontainingithe screen: and:theftube'ibakcdf in a entle'zstream of nitrogen .zat about =-300;C.:;foi iom'rten to-sixty minutes .The gtube is then--.'eva'cuaited andzmay be, if desired, baked after:evacuationoalthough thez 'latter step isrnot essential. is then sealed into the tube which is then pumped to the required degree of vacuum, as well known in the art.

If desired, a small amount (say 3 cubic centimeters) of per cent solution of sodium or po- The electron gun tassium silicate in water can be added to 100 cubic centimeters of the binding mixture, the sodium silicate in the binderlor in the reflecting coating or both) resulting in a higher resistance to abrasion. While sodium and potassium silicates are well-known binders, they are not suitable when used alone to bind thick screens as they swell up when baked but when used in combination with magnesium oxide or other suitable binding materials they act as very good hardeners coatin'gi withiau small (amount :of :a: silicate as Qua I binderaiwilltprovideiaa satisfactory? screen but the brillianceris less than; from sulfides.

5 @"Imthes case where an oxygemcontaining: phosphjomfsuch as wilflemite or: calcium l-tu-ngsta'te, is used an in-teresting. condition-exists in that .these materials are -fair-\;reflectors. Also they can be grounds-to very .fine; powders :and will bond Well with a.- small -amou-nt.oi .anealkali: silicate in thick layers andiprovide .the ifiller porosity efiect themselves esoythe silica-te can .b .outgassed. A thick screen ofthese materialsusing nasmall amount of a.silicate binder. is superior mechanicall totthat in which the silicate issmixed with MgQ,"Tfor' example, but the maximum luminous intensity and "conversion .efliciency of thesejphospho'rs' isfnotiasgoo'das fthe sulfide .type. Thick coatings of' these "materials havebeen found in practice'Wo require a "binder, although the thin translucents'creensused for the oscillograph'type cathode 'frayftu'be," that is, where the screen is viewed from" the 'rear, do-not" require a binder.

While' in the arrangement shown in Fig. 2, a plurality of rfiecting' -layers 2 Oof' magesium; aluminum'; or: berylliumoxides is shown (and this is .the mostx;saitisfactori wvay off forming '-a-' thick coatoofimat'eriakon- *metal) -it-is' possible to make this reflecting coatingi inone thick layer. The binding cement isk applied to this-reflecting coating orethe a thick: reflecting-L coating "can also comprise i the; binding Icement, the Lfluorescent materialtbeing; in this 'casera-pp'liedtozthe reflecting coating before it is dried. In another modification, shown in Fig. 3, a thick and highly reflective screen can be Obtained by applying successive layers of binder 20 and fluorescent material 22 to the'conducting backing member l4. As both the layers '20 and 22 are translucent, there is a penetration of the light rays from the lower levels of fluorescent material out into the tube through the upper layers. The number of alternate layers of fluorescent material and reflecting material is determined by the penetration of the electron beamwhich, of course, is dependent on the square of the voltage of the beam at the screen.

The screens or targets made in accordance with this invention produce intense light and have excellent mechanical properties and the methods of preparing them are simple and easily reproducible. Various modifications may be made in the screen structures and the methods of forming them without departing from the spirit of the invention, the scope of which is indicated by ple, screens using powders made of mixtures of zinc and cadmium sulphides plu activator materials which are usually copper, silver or manthe appended claims.

What is claimed is: v

1. A target for electrons having a luminous surface which is adapted to be viewed from the same side on which it is activated comprising a supporting member, a continuous light-colored reflecting member in the form of a coating on said supporting member, and a coating of fluorescent material bound to said reflecting member by a binder comprising a moist coating of an oxide of one of the group of elements consisting of magnesium, aluminum and beryllium.

2. The combination of elements as in claim 1 and being further characterized in that said light-colored reflecting member comprises an oxide of one of the group of elements consisting of magnesium, aluminum and beryllium.

3. The combination of elements as in claim and being further characterized in that said lightcolored reflecting member comprises an oxide of one of the group of elements consisting of magnesium, aluminum and beryllium, said reflecting member being of a thickness of at leastl mil.

4. A luminous surface which is adaptedto be viewed from the same side on which it is activated comprising a supporting member, a continuous light-colored reflecting member in the form of a coating on said supporting member, and a coating of fluorescent material bound to said'r'efleoting member by a binder comprising a moist coating of an oxide of one of the group of elements con-' sisting of magnesium, aluminum and beryllium, and an alkali silicate.

5. In acathode ray tube, a target, and means for generating a cathode ray having a voltage between 10 and 100 kilovolts at said target, said target comprising a metal supporting member, a coating of light-colored non-metallic reflecting material on said supporting member, and a flucrescent coating on said reflecting material, said reflecting material and said fluorescent coating together being from 1 to 15 mils in thickness. I

6. In a cathode ray tube, a target, and means for generating a cathode ray having a voltage between 10 and 100 kilovolts at said target, said target comprising a metal supporting member, a coating of light-colored non-metallic reflecting material on said supporting member, and a flucrescent coating on said reflecting material, said reflecting material and said fluorescent coating together being from 1 to 15 mils in thickness, said reflecting material comprising one of the group and aluminum oxide.

7. In a cathode raytube, a target, and means for generating a cathode ray having a voltage between 10 and 100 kilovolts at said target, said target comprising a fluid-cooled metal supporting member, a coating of light-colored non-metallic reflecting material on said supporting memher, and a fluorescent coating on said reflecting material comprisingone of the group consisting of magnesium oxide, beryllium oxide and aluminum oxide, said reflecting material and said fluorescent coating together being-from 1 to 15 mils in thickness.

8. An element for transforming energy imparted to it by electronic bombardment into light radiated from the surface of the element receiving the electrons comprising a backing element having good electric and heat conductivity, a lightreflecting coating on a surface of said backing element, and fluorescent material on said coating capable of emitting rays both' in and-above the infra-red region of the spectrum when bombarded either by electrons or by light rays above the infra-red region, said coating being predominately of material from the group consisting of the oxides of magnesium; aluminum and beryllium, and having the property of reflecting a large portion at least of the light received by it'from the fluorescent layer in the region above the infra-red portion of the spectrum and dissipating to said conductive backing element a large portion at least of the received energy in the infra-red region, said fluorescent material being in the form of discrete particles distributed within said coating near thesurface thereof and held in place thereby, the portion of said coating underneath said fluorescent material being thick enough to be continuous.

JOHN C. COOK.

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