2,409,574 Patented Oct. 15', 1946 UNITED’ STATES PATENT OFFICE 2,409,574 LUMINESCENT MATERIAL AND 'METHOD OF MANUFACTURE Humboldt W. Leverenz, South Orange, N. .L, as signor to Radio Corporation of America, a cor poration of Delaware ‘ No Drawing. Application June 11, 1942, Serial No. 446,626 9 Claims. (Cl. 252-30113) . 1 . , dependent upon the puri?cation of the constitu ents of my phosphor. These requirements for exceedingly pure constituents apparently are due in part to the characteristics of my preferred constituents. The above-mentioned objects are attained in part by controlling the constituent content of My invention relates to luminescent materials and their preparation and particularly to the preparation of luminescent zinc compounds. It has been customary in the art to synthesize luminescent materials or phosphors of zinc and cadmium sulphides by various methods although the majority of these methods result in a product which has only moderate phosphorescent prop erties. While I have disclosed in my copending certain impurities. For example, the initial con stituents from which I synthesize my phosphor application, Serial No. 412,687, ?led September 10 material and particularly the zinc salt solution 27, 1941, a sulphide phosphor having relatively high phosphorescence of relatively long duration from which I form a sulphide should have less than the following percentage of impurities: following excitation to luminescence, even great er intensities and longer periods of phosphores cence following excitation are desirable in cer Per cent Mn 16 tain applications in which such phosphors are utilized. For example, in aircraft position and distance indicating equipment, it is quite desir F'e ________________________________ __ 0.0008 _____________________. ____________ __ 0.0005 As ___________________________________ 0.0008 Ni _________________________________ __ 0.00001 I prefer to synthesize my phosphor material able to provide a cathode ray tube having a long persistence phosphor, such persistence rendering 20 starting with zinc halide or, more preferably, zinc sulphate and I remove a large proportion of in herently contained iron and certain other metals the tube suitable for portraying the trajectory of an approaching aircraft. Sulphide phosphors by an oxidation process to form insoluble com are usually characterized by their low resistance pounds which are precipitated and removed. to heat. For example, the baking to which they are usually subjected during cathode ray tube 25 More particularly I make a solution of zinc sul phate ZnSO4-7H2O) in the proportion of ap manufacture reduces their ei?ciency and for proximately 50 grams to 33 milliliters of pure certain applications they are wholly unsuitable water and add thereto a solution of hydrogen peroxide in an amount approximating 5 milli of zinc compounds possessing high luminescent 30 liters of 30 per cent saturated solution. I then add ammonium hydroxide until a slight White properties and particularly high phosphorescence turbidity is produced. Following the addition of without undue sacrifice of fluorescence. An ammonium hydroxide I bring the solution to a other object is to provide a material which is boil as rapidly as possible to remove certain vol phosphorescent over a relatively ‘long period of time following excitation to luminescence. A 35 atile products, such as ammonia gas, and con tinue to boil until ammonia fumes are unnotice further object is to provide phosphors which are able. I then allow the solution to stand for at insensitive to heat following synthesis so that least 24 hours followed by decanting or by ?lter they can be used in applications where high tem ing to remove the precipitate. In addition to the perature processing, such as encountered in tube manufacture, has little or no effect on the phos 40 removal of iron, these steps likewise remove alu minum, nickel and copper. Centrifuging may be phor efficiency. These and other objects, fea employed to reduce the time necessary to bring tures and advantages of my invention are ob down the precipitate. tained in accordance therewith by combining a Following the removal of the above impurities, zinc sulphide and a ?uoride of zinc to form a mixed sulphide-fluoride mass which is co-crys 45 the pH of the zinc sulphate solution should be for use because of their low resistance to heat. My invention has as an object, the production between 4-7, any adjustment, if necessary, being made by adding an acid, such as sulphuric acid, following which I heat the zinc sulphate solution to approximately 90° C‘. and allow it to trickle tallized to form mixed sulphide-?uoride crystals. Further in accordance with my invention I pro vide a material in which the sulphide of zinc either may be replaced in part by zinc oxide or the zinc may be replaced in part by cadmium 50 over pure zinc for the purpose of removing all with or Without zinc oxide prior to the formation elements below zinc in the electromotive force of the mixed sulphide-?uoride crystals. Fur series of the elements. The treatment with zinc metal may be accomplished at the very outset, such as by reacting an acid such as hydrochloric thermore, in accordance with my invention I control the purity of the constituents within very narrow limits to provide a material having high ?uorescent and phosphorescent e?iciencies. I have found that such properties are exceedingly 55 or sulphuric acid with zinc metal or zinc oxide in the presence of excess zinc metal. Following such a treatment the purity of the zinc sulphate _ 2,409,574. 3 4 solution will be approximately as indicated above, preferably performed in platinum receptacles to although to further assure removal of any ex prevent formation of undesired ?uorides although this may be obviated by forming the zinc ?uoride from the ?ltered precipitated Zinc sulphide or adding it thereto following precipitation and ?l tering. I then thoroughly mix the sulphide-?uo ceedingly small amount of manganese, lead and copper, I prefer to electrolyze the solution be tween platinum gauze electrodes at' 2-2.2 VOltS' for a period ranging between 50-400 hours. To still further assure purity I then fractionally pre ride mixture with a platinum rod or spatula. If an unactivated phosphor is desired, the mixture is cipitate a small portion of the zinc as a zinc sul phide by passing a small quantity of well-puri?ed dried in an oven or over a water bath, broken up hydrogen sulphide into the zinc sulphate solu 10 and fired at a temperature of from ‘750° C. to 1600° tion following which the precipitated zinc sul C. for aperiod of from 10 minutes to 10 hours phide is removed by ?ltration and discarded. depending upon the particle size desired and the This fractional precipitation step may be repeat temperature of ?ring. However, if an activated ed to further lower the concentrations of un material is desired, the activator metal is added desirable impurities. ’ 15 to the semi-?uid paste as a soluble salt in a water In accordance with my invention I precipitate zinc sulphide and zinc ?uoride (with or without an activator, such as copper, silver or gold) from solution and then the material is dried and ?red as above. As explained below in greater detail certain ?uxing agents may be added to the acti the previously prepared zinc sulphate solution or vated or unactivated material to assist the alternatively I precipitate only a pure zinc sul 20 thermo- synthesis of the phosphor. phide and then add to this sulphide a fluoride in a Asa speci?c example of one teaching of my form which will convert a portion of the zinc invention, I add to 100 grams of the pure precipi sulphide to zinc ?uoride, or still alternatively add tated zinc sulphide and contained in a platinum zinc ?uoride to the prepared zinc sulphide prior crucible a quantity of zinc ?uoride between 0.5% to calcining or heating of the mixture to produce 25 and 5% based on the zinc sulphide by weight such the sulphide-?uoride phosphor. The amount of as 2 grams of zinc ?uoride. Alternatively I add zinc ?uoride either coprecipitated with the zinc a sufficient quantity of a substance, such as hy sulphide or added thereto as a ?uoride is relatively drogen ?uoride or ammonium ?uoride, which will critical to obtain the objects of my invention. For react with the zinc sulphide to yield the desired example, the ?uoride range is preferably between quantity of zinc ?uoride. The specific quantity 0.1% and 15% by weight based on the zinc sul of zinc ?uoride cited above, that is, 2% based on phide content. I have found that in addition the zinc sulphide content, is preferred inasmuch to the zinc ?uoride a limited quantity of zinc oxide as sulphides of this composition exhibit optimum in the resultant phosphor i's bene?cial in enhanc phosphorescence under 3650 A. radiation Without ing the phosphorescent properties thereof; and 35 undue aggregation. One or more activator metals furthermore, that while a single activator, such as selected from the group consisting of copper, sil copper, is preferably incorporated in my phosphor ver and gold may be used and I prefer to add this in known and controlled amounts, it is particu activator as a soluble salt of the metal either pre larly with the teaching of my invention to utilize ceeding or following the precipitation of the zinc a plurality of activators selected from the group 40 sulphide or following the preparation of the zinc consisting of copper, silver and gold. As indicated sulphide-zinc ?uoride mixture. Since the sul above, my phosphor consists of a mixed crystal phides of copper, silver and gold are extremely in composition and it should be understood that the soluble, it is immaterial whether their soluble salts zinc ?uoride and zinc oxide which may be present be added before or after the precipitation of the are not necessarily present as strict chemical com 45 Zinc sulphide. Thus, for example, to the purified binations in the final product. These materials zinc sulphate solution or to the wet mixture of merely symbolize the presence of ?uorine and zinc sulphide and zinc fluoride I add an aqueous oxygen or their ions in the sulphide phosphor. solution containing the activator metal or metals The resultant phosphor may be symbolized as as soluble salts. I have found for the above prep ZnS(ZnF2).e(ZnO)1/:Cu, wherein the subscripts a: 50 arations, the amount of copper may vary over and y are representative of the amount of ?uoride ‘ wide limits; for example, the amount may vary and oxide in the combined phosphor crystal com from 0.0000l% to 0.1% molal with respect to the position by weight based on the zinc sulphide zinc sulphide content, although I prefer in the content. More particularly the ?uoride and oxide above example to add 0.01 gram of copper as the content should be within limits such that x plus y ?uoride. I have found that the use of silver or is equal to 0.1% to 15% based on the zinc sul gold as an activator with the copper increases the phide content of which the value 3/ is preferably phosphorescence of the resultant material and the equal to or greater than zero but not greater amount of copper, silver and gold may vary from than 5 %. ' 0.00001% to 0.1% molal, each with respect to the As one speci?c example of preparing the ‘sul 60 zinc sulphide content. In addition I have found phide-?uoride mixture prior to calcination, I pre that in a multi-activated phosphor the quantity cipitate zinc sulphide from the acidi?ed zinc sul~ of the individual activators may decrease in the phate solution by bubbling puri?ed and thor order of silver, copper and gold, the gold being oughly washed hydrogen sulphide through the so less than the copper and the copper being less lution to form a precipitate of zinc sulphide. 85 than‘ the silver. The relative proportion of these Simultaneously with or preferably following the three activators may be one part gold, two parts formation of the zinc sulphide precipitate, I pre copper and four. parts silver. However, for vary cipitate zinc ?uoride by adding gaseous or liquid ' ing the phosphorescent characteristic these rela hydrogen fluoride or an aqueous solution of hy tive proportions may vary above and below the drogen fluoride to the zinc sulphate solution. I' 70 optimum proportion. I have found that the pe obtain somewhat greater purity by forming the riod of useful phosphorescence in a silver-gold zinc ?uoride by precipitation subsequent to the activated material increases in accordance with precipitation of the zinc sulphide especially when. the amount of gold with respect to silver and sim I decant the supernatant liquid following the for-_ ilarl‘y. with a copper-silver or copper-gold acti mation of the Zinc sulphide; The above steps are‘ 75 vator, the period of useful phosphorescence in 2,409,574 creases with the quantity of copper and gold re spectively. The use of 0.1% of silver and 005% copper by weight, to weight of zinc sulphide, gives the group of metals consisting of zinc and cad mium ’wherein the ?uoride is between 0.1% and 15% by weight based on the weight of said optimum phosphorescence under cathode ray ex-‘ 7 sulphide. 2. A luminescent phosphor essentially consist citation. ing of a thermally crystallized zinc sulphide-zinc To aid in the thermo-synthesis or calcination of ?uoride combination wherein the zinc ?uoride is my phosphor I have found that speci?c preferred between 0.1% and 15% by weight based on the ?uxing agents or catalysts may be mixed with zinc sulphide content. ‘ the sulphide-?uoride mixture prior to the cal 3. A crystalline luminescent phosphor composi cining or ?ring steps. I speci?cally avoid such 10 tion represented by the general formula commonly used fluxing agents or materials as boric acid, magnesium-?uoridevand calcium car~ bonate, which do not completely evaporate or form readily removable compounds, by using a wherein M is a metal selected from the group of sodium or potassium chloride or bromide ?uxing 15 metals consisting of zinc and cadmium, F is agent which controls the crystal growth and ' ?uorine, S is sulphur, O is oxygen, and :r and y represent the weight of ?uoride and oxide re phosphor formation upon calcination to far bet spectively in the composition and each being ter advantage than other ?uxes in the prepara tion of the conventional sulphide phosphors. The ‘ greater than zero and wherein the sum of .r and sodium or potassium chlorides (or bromides) re 20 y lies between ‘0.1% and 15% of the weight of MS. act with the zinc sulphide to form zinc chloride > 4. A crystalline phosphoras'claimed in claim 3 (or bromide) which sublimes at ‘732° C. leaving wherein the value of y is greater than zero and easily soluble sodium or potassium sulphides. I, is not greater than 5%. therefore, add to the wet zinc sulphide-zinc ?u 5. A crystalline luminescent phosphor repre oride an aqueous solution containing 2 grams of sented by the general formula ZnS(ZI1F2) :c(Zl'lO‘) 11 sodium or potassium chloride (or bromide) fol wherein m and y represent the weight of zinc lowing which I evaporate to dryness, such as in ?uoride and zinc oxide respectively in the com an oven or under an infra-red lamp or over a bined composition and wherein the sum of at and water bath. I then place the material in the pow 3/ lies between 0.1% to 15% of the weight of zinc i dered state in a clean quartz crucible although a 30 sulphide content. . platinum crucible may be used where the life of 6. The method of manufacturing a lumines cent phosphor composition comprising the steps the crucible is an economic factor in commercial preparation. I then place the crucible and its 1 of adding a fluoride to a sulphur compound of a contents in a clean electric furnace and raise the metal selected from the group of metals consist temperature of the furnace to between I750" C. 35 ing of zinc and cadmium to form a mixture there~ of, adding tosaid mixture a ?ux to aid crystal~ and 1600° C. and maintain this temperature for a lization, and crystallizing said mixture by ?ring at time interval of from 10 minutes to 10 hours, a temperature between 750° C. and 1600° C. the total heating time and temperature depend , 7. The method of manufacturing a luminescent ing upon the phosphorescence intensity and par ticle size desired in the resulting phosphor mate 40 phosphor‘ composition comprising the steps of rial, the particle size and phosphorescence inten sity increasing with the time of calcination. Low group of metals consisting’ of zinc and cadmium er temperatures, such as 750° C.-1000° C. yield to a sulphide of said metal to ‘form a mixture of smaller crystals, but higher temperatures, such as 1100° C‘.-1300° C. give higher phosphorescent said ?uoride and sulphide, adding to said mixture a flux to aid crystallization, and crystallizing said intensity. Following crystallization the material mixture as a combined ?uoride and sulphide by adding a ?uoride of a metal selected from the ?ring said mixture and said ?ux at a temper is removed from the crucible, those portions ature between 750° C. and 1600° C. which reacted therewith being detected under 8. The method of manufacturing a sulphide ultra-violet excitation and usually discarded whereupon the crystallized material in accord 50 phosphor comprising preparing a solution of a sulphide of a metal in ‘the group consisting of ance with my invention is then washed ‘with zinc and cadmium having a pH ratio between 4 vigorous‘ stirring in pure distilled water satu and 7, the said solution having a maximum im rated with hydrogen sulphide. I have found that the period of Washing should be short and that ' purity by weight with respect to the solution of: preferably the material may be washed two to - three times in pure water saturated with hydrogen sulphide. The material should not be subjected to the washing water for a period in excess of Per cent Mn _________________________________ __ 0.0008 Fe _____ _ As __________________________________ __ 0.0005 0.0008 10-100 minutes (shorter time for higher ?uoride content) following which the material is im 60 mediately dried at a temperature from. 100° C. precipitating a sulphide and a ?uoride of said to 250° C. whereupon it is ready for use. I have metal fromsaid solution, adding a ?ux to the found that my product not only has exception precipitate, drying said precipitate and ?ux, and ally high phosphorescent properties and excel ?ring the precipitate and flux at a temperature lent stability to heat but that such phosphor 65 sufficient to crystallize the dried precipitate as a mixed sulphide-?uoride phosphor. escence is not obtained ata sacri?ce of ?uores cence and further that my method of production ,9. The method of synthesizing a luminescent may provide a smooth ?owing, substantially non sulphide compound of a metal selected from the group consisting of zinc and cadmium compris aggregated, non-?occulent material which may be easily applied during the subsequent manufac 70 ing mixing with a compound of said metal a quan Ni ture of luminescent coatings. I claim: ‘ ‘ 1. A luminescent phosphor essentially consist ing of a thermally crystallized combination of a sulphide and ?uoride of a metal selected from 75 __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ 0.00001 tity of a ?uoride of an element selected from the group consisting of said metal and hydrogen and ?ring said mixture to crystallize said compound in the presence of said ?uoride. HUMBOLDT W. LEVERENZ.