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2,408,621 Patented Oct. 1, 1946 PATENT oFFics UNITED > STATES 2,408,621 rnosrnons AND THEIR STABILIZATION C. Froelich, Cleveland, Ohio, assignor to Company, a corporation of vGeneral Electric Herman New York ' No Drawing. Application May 14, v1942, Serial No. 442,993 , 9 Claims. , (01. Ill-33.5) 1, phor mixturev havefundergone prolonged atmos pheric heating at considerably higher tempera This invention relates to ?uorescent lamps. or tures than this, it was thought that such deterio ration of the- phosphor mixture must be due to some deactivating‘reaction between these com tubes and the’like, and. particularly to lumines cent materials or phosphors. ‘The inventionv is especially. concerned with deterioration of phos ponents when heated together. However, phos phors arising from manganese or the like which phors consisting solely of zinc-beryllium silicate is used for purposes of activation with matrix (or other silicates) activated with manganese ex materials such as metal silicates and the like,’ ex hibit similar deterioration. vWhen heated as empli?ed by cadmium silicate, zinc silicate, or zinc-beryllium silicate. Other matrix materials 10 above mentioned, phosphors consisting in whole or in part of such activated silicate(s) also of‘this character are metal phosphates, ‘chloro those of cad change in color- as observed under white light: their natural pure white takes on light shades of gray or brown. The ?ner the phosphor, and the mium. The invention aims not only to prevent deterioration of phosphors by manganese of their own, but also to obviate deteriorating influence 15 greater its exposed surface, the more pronounced is the deterioration byheating; and it is also of manganese in one phosphor on an associated more pronounced when the phosphor is heated phosphor that may not itself'contain any man in pure oxygen instead of in air. Even at atem ganese. An example of such a phosphor mixture perature of only 500° C. in air, standardzinc is one of manganese-activated zinc-beryllium sil beryllium silicate phosphor‘ changes color- and icate with magnesium tungstate without any added activator, which gives a white light when 20 loses about 8—10 percent in brightness. .» » - -. As there isevery reason tobelieve that silicate excited by the 2537 A. resonance radiation ‘of the phosphors consist of a matrix of compound which mercury arc discharges ' . ‘is commonly represented, for example, by the I have explained my invention hereinafter with particular reference to the above-mentioned formula (Zn,Be)2Si04, together with manganous tungstate phosphor also undergoes atmospheric utes below, red'heat, as .in lamp tube baking or heating at fairly high temperature during manu facture, though its properties would scarcely be bending; and yet this is indubitably the case. Furthermore, it. has so far proved impossible to white phosphor mixture and to zinc-beryllium 25 oxide,‘MnO, in solid‘ solution in this matrix, and as such phosphors undergo several hours of heat silicate phosphor activated .with ‘manganese. ing in airv at relatively high temperatures (such Such zinc beryllium silicate phosphor is a mate--‘ asl100° ‘C. to 1300° C.) during manufacture, it rial of almost ceramic properties, that has been heated in the air at temperatures of about 1l00° 30 is very surprising that such phosphors should not bev perfectly, stable when heated for a few min to 1300° C. during its preparation. Magnesium prepare manganese activated silicate phosphors termed ceramic. In the manufacture of ?uorescent lamps or 235 of anything like normal ?uorescence that do not tubes of the usual'positive column discharge type, show the deterioration above described, under ?nely powdered phosphor is intimately mixed with organic binder solution (as-of nitrocellulose) heat that is otherwise desirable in lamp process. by grinding in a ball-mill. After applicationv of the resulting phosphor suspension to a lamp tube and drying out of the binder liquid or solvent, the coated tube undergoes baking for some minutes ing. By re?ring such phosphor after grinding, its stabilityas against loss of brightness by heating is improved; but it still remains subject to ob jectionable deterioration. I have found that the discoloration and loss of wall temperature of some 400° C. to 600° C.,'more ?uorescent brightness which silicate phosphors andfwhite phosphor mixtures undergo when or less. heated is due to manganese which is not, appar in an oven to burn out the binder, at'an actual In some cases, as in the bending of ?uorescent electric sign tubes, the coated lamp envelopes may be heated as high as GOO-‘625° C., ently, activatingly combined, With the silicate phosphor matrix, or included in the crystal lat In the case of ?uorescent lamps coated with tice of this matrix-An other words, is not even in solid solution in the silicate. In the ‘case of white has been found that exposure to atmospheric phosphor is also somewhat affected by such or even 650° C. ' a the above-mentioned white phosphor mixture, it 50 phosphor mixtures, the magnesium tungstate “stray” manganese of the associated silicate phosphor; ‘but the maJo'r effect is on the ‘silicate ?uorescent brightness amounting to some 15;30 55 phosphor itself. For both cases, I'have'found per cent. As both the components of this phos temperatures of some 600° C. or more (as in bending coated sign tubes) results in a loss of 2,408,621 3 4 means of obviating such phosphor deterioration. I have determined that “stray” manganese is leterious compound into something that is inert commonly present even in silicate phosphor that or innocuous, as by reducing it to a lower form has not undergone the heating incident to lamp processing, and is manifested in a demonstrable compound in other Ways. oxidizing power associated with this phosphor, A method of eliminating the deleterious man ganese compound by conversionvwithout removal position as (Zn,Be)2SiO4 with activating MnO in is to reduce this compound to a lower one in solid solution therein. A test which shows this oxidizing power is to make a slurry of the phos 10 which the manganese is divalent merely. This and not to be accounted for by its essential com can be effected by heating the phosphor in vacu phor in a weakly acid solution of starch and po_ um to about 500-609" C., or in a tassium iodide, which turns blue in response to the oxidizing action of the stray manganese com pound present in the phosphor. Titration with very dilute sodium thiosulphate or the like makes the test quantitative. The explanation seems to 10 minutes before being exhausted and charged with mercury and starting gas ( e. g., argon at a pressure of 2-4 mm. of mer cury) , and sealed off. A method of actually removing the deleterious manganese compound is to treat the phosphor with‘ dilute acid in the presence of a suitable re ducer, such as sulphur dioxide, hydrogen peroxide, etc., thus converting higher manganese oxide into a water-soluble manganous salt. All that is nec~ This oxidizing power and the dark discolora tion which the phosphor undergoes when heated to some 500-650" C. would indicate a manganese compound in which manganese is more than di 30 valent (as in MnO). Evidence is inconclusive as under heating rather points to some of the many 35 Quantitative deter minations show an amount of the compound ranging from about 0.001 to the zinc-beryllium silicate With an average of about 0.006 per cent. Such an 40 before heating. It is to the minute yet dark matter that I attribute the light gray or of the phosphor is rescent brightness. I have discovered that while it seems imprac ticable to produce phosphors free from deleterious advantage that its dissolving action on the phosphor matrix is milder than that of mineral acids, ' ' 2,408,621? mix them thoroughly for applioation‘to a lamp, this phosphor shows somewhat less improvement pairment of its fluorescence. in ?uorescent‘brightness on the lamp wall than . 6 the heating of the lamp in processing it; and‘ thus prevents discoloration‘ of the phosphor and im; would be expected from brightness tests made just before and just after the reducer treatment.’ For the convenience of those‘desiring to prac-‘ . :.;v :i ' ‘ My‘ oopending divisional-application .Serial No. 666,776, ?led May 2, 1946, claims certain methods relative to thereduction of the uncombined and super?cial manganese compounds higher valent ing it), the following illustrative particulars of than two, wherein the reduction is produced un der conditions conducive to such reduction, such the preferred wet method are given, the work be ing carried on with baths or solutions at ordinary‘ methods therein claimed constitute further in-- tice the invention (but not as limiting or de?n as in an atmosphere conducive to'reduction. The ventions over and above the subject matter of the , >. instant application. The above-mentioned di To a bath of 200 cc. of distilled‘ water contain visional application is assigned to the assignee of ing 0.03 per cent of acetic acid is added 10 cc. of a 5 per cent aqueous solution of sulphur diox 15' What I claim as new and desire to secure by ide (S02). 100 g. of zinc-beryllium silicate phos Letters Patent of the United States is: phor are suspended in 200 cc. of distilled water. room ' temperature : this making a thin slurry. The solutions are then , poured together and stirred for 5-10 minutes, and application. ' ‘ ‘ ‘ ‘ -- I l. A generator of radiation consisting essen tially of a thermally synthesized luminescent the whole is then promptly ?ltered through or dinary ?lter paper on a suction ?lter. The phos phor ?lter-cake is at once thoroughly washed with distilled water drawn through it on the ?l composition of metal silicate as matrix and man ganese as an activator wherein the manganese is present only in a divalent condition activatingly ter, until the ?ltrate no longer shows any acid combined with the silicate matrix. - 2. A generator of radiation consisting essen reaction. After drying, the phosphor is ready for incorporation with the binder for application to tially of a thermally synthesized luminescent ?uorescent lamp envelopes as usual. and manganese as an activator wherein the man _ A somewhat more convenient method of treat composition of zinc-beryllium silicate as matrix ganese is exclusively in divalent condition and activatingly combined in solid solution with the ing the phosphor with reducer and dilute acid is to pass moist sulphur dioxide gas through the 30 silicate matrix. 3. A method of assuring maximum brightness dry phosphor powder for some minutes in a suit in a ?uorescent discharge lamp in which is pro able closed vessel. and afterward to wash the vided a coating of a thermally synthesized lumi gassed powder with suitably acidulated water on nescent composition of metal silicate as matrix a ?lter, followed by thorough washing with pure ‘water. However. the wet method described above 35 and manganese in divalent condition activatingly combined with said silicate matrix, which method is at present preferred for phosphors used in or dinary ?uorescent lamps. 'Manganese-activated silicate phosphor that is comprises removing from the surface of the composition uncombined and super?cial manga treated with reducer such as S02 and dilute acid nese compound whose manganese is not held fast according to the preferred wet method herein 40 by the silicate matrix and which is normally present on the particles of the composition after before described is somewhat whiter than be its synthesis. the removing step comprising treat fore: shows a gain in ?uorescent brightness of as ing the composition with a reducing agent and much as 10 per centwhen tested directly after ward. or even more. without any apparent shift in color toward yellow or green: gives no reac that with starch ' tion to an oxygen test such as acid in the presence of water to form a soluble manganese salt. 4. A method of assuring maximum brightness and potassium iodide. which indicates absence of free manganese oxide or the like higher than MnO. both at the surfaces and in the interior of in a ?uorescent discharge lamp in which is pro-_ vided a coating of a thermally synthesized lumi nescent composition of metal silicate as matrix the silicate matrix crystals: remains 100 per cent stable when heated to 500-650’ 0.. either alone or and manganese. in divalent condition activat ingly combined with said silicate matrix. which in admixture with magnesium tungstate; and method comprises removing the uncombined and shows satisfactory maintenance of output in super?cial manganese compound by treating the luminescent composition with a reducing agent ?uorescent lamps. A method of virtually eliminating their stray 55 and acid in the presence of 'water, and washing manganese from phosphors and thus stabilizing away the resulting soluble manganese compound. them is to inactivate the phosphor against dele 5. A method of assuring maximum brightness terious reaction of the stray manganese devel in a ?uorescent discharge lamp in which is pro oped during heating, as by protectively coating vided a coating of a thermally synthesized lumi the phosphor particles. For example, the pow 60 nescent composition of metal silicate as matrix dered phosphor may be suspended in a 5 per cent solution of boric oxide (B203) in methyl alcohol, which may then be ?ltered through ?lter paper on a suction ?lter, and allowed to dry. and manganese in divalent condition activat In the case of ?uorescent lamp phosphors, this general method may be used more advantageously to treat the phosphor after it has been coated on the inside of the lamp envelope. For this pur pose, the phosphor-coated lamp envelope may be ?ushed out with a l per cent solution of boric oxide in methyl alcohol, allowed to drain, and then dried, after which the lamp may be proc essed and completed as usual. The thin‘ ?lm of boric oxide with which the phosphorparticles 6 ingly combined with said silicate matrix, which method comprises removing the uncombined and super?cial manganese compound by treating the luminescent composition in ?nely divided form with sulphur dioxide and acid in the presence of water, and washing away the resulting soluble manganese compound. 6. A method of assuring maximum brightness in a ?uorescent discharge lamp in which is pro vided a coating of a thermally synthesized lumi nescent composition of metal silicate as matrix and manganese in divalent condition activat ingly combined with said silicate matrix, which are thus coated and ensealed prevents oxidizing 75 method comprises removing the uncombined and reaction of stray manganese developed during 7 2,408,621 super?cial manganese compound Whose manga and is not held fast by the silicate matrix, and which normally forms on the particles of the composition after its synthe 9. A method of assuring maximum brightness in a ?uorescent discharge lamp in which is pro vided a coating of a thermally synthesized lumi~ sis, the removing step comprising treating the nescent composition of metal silicate as matrix composition with a reducing agent and acid in the presence of water. 7. A method according to clam 6 wherein the super?cial compound whose manganese is more and manganese in divalent condition activat ingly combined with said silicatev matrix, which method comprises removing the uncombined and super?cial manganese compound by treating the than divalent is converted into a soluble man ganese salt prior to removal. 10 luminescent composition, after being applied as a coating to an envelope, with hydrogen peroxide and acid in the presence of water, and washing 8. A method, according to claim 6 wherein the away the resulting soluble. manganese compound. super?cial compound whose manganese is higher valent is converted into compound wherein the manganese is divalent and is uncombined with 15‘ HERMAN C. FROELICH. the matrix.