LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR
669
T H E LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR HERMAN C. FROELICH Lamp Development Laboratory, General Electrzc Company, Nela Park, Cleveland, Ohio Received August 81, 1943
Yellow-fluorescing zinc silicates have been prepared by two rather extreme methods. The first and older method (3, 7) involves the fusion of preformed, crystalline, and green-fluorescing zinc silicate a t temperatures between about 1400” and 1550°C. and subsequent controlled quenching in cold air, water, or the like. The second and more recent method (1) utilizes the fluxing action of halide salts, which are added in fairly large amounts and act as “catalysts” for the phosphor formation at far lower temperatures (around 850°C.). Quenched phosphors mag be made from mixtures containing zinc oxide and silica over a wide range of proportions which may even exceed the stoichiometric orthosilicate ratio of 2: 1 toward products with excbs zinc oxide; however, the most uniform and brightest phosphors are obtained with materials having the eutectic composition or around 1mole of zinc oxide for each mole of silica. The second method, on the other hand, yields good products only if the silica is present in considerable excess, namely, between 2 and 4 moles of silica for each mole of zinc oxide. The brightness of the best quenched phosphors amounts to about 65 per cent of standard green zinc silicate under A2537 excitation. Lowtemperature, fluxed yellow zinc silicate seldom exceeds about 45 per cent. Both types of materials are inactive toward long-wave ultraviolet but give a brilliant yellow under cathode-ray bombardment, where normal zinc silicate gives a bright green. Fonda has presented rather conclusive evidence that the lom-temperature preparations of yellow phosphor in particular are amorphous in character. Microscopic examination revealed their isotropic nature. X-ray diffraction patterns show only the lines of those (crystalline) constituents which are present in excess over their respective amounts required to form orthosilicates. Thus, samples made a t low temperatures with excess quartz as a source of silica gave h zinc oxide gave only zinc oxide only a typical quartz pattern; samples ~ i t excess lines; and products made with amorphous silica gave again a different pattern, showing only weak lines of cristobalite, the crystalline modification of silica into n-hich amorphous silica is converted upon heating to some 800°C. for a short time. Rooksby and hlcKeag (6) conclude, from a single pattern of their low-temperature zinc silicate, that it is identical with a crystalline p-modification of yellowfluorescing zinc silicate (ZnzSi04) which they obtained by fusion. Their pattern for the low-temperature phosphor reveals only one-half the number of lines of corresponding character and spacing compared with Fonda’s; it may equally well be interpreted as a but slightly distorted cristobalite pattern. They give no contrasting pattern for low-temperature zinc silicate made with quartz or excess zinc oxide and disregard the importance of these preparations in elucidating the nature and interpretation of the phosphors made from amorphous silica. Strongly supporting, although indirect, evidence for the amorphous habitus of
670
nmwm c. momicH
the yellow phosphor has been given by Pabst (5;see also 8), who investigated with precision the kinetics of zinc silicate formation for 2: 1 and 1:1 compositions. His x-ray patterns of heat-treated mixtures disclose sharp lines pertaining only to zinc oxide and normal crystalline zinc silicate lattices. The crystallization of zinc silicate begins a t about 775°C. and at a very slow rate. Over 140 hr. of heat treatment a t 800°C. are required to produce as little as 10 per cent crystalline zinc silicate. Ten per cent was determined to be near the approximate lower limit of sensitivity of the x-ray method with Cu K, radiation, which was also used by Fonda and by Rooksby and h1cKeag. Inasmuch as the formation of yello\r. phosphor is complete after minutes or a few hours of heating a t the most, it is evident that no detectable amounts of crystalline silicate can be formed. On the other hand, if the oxide mixture is heated excessively long in order to obtain diffraction patterns with typical though weak silicate lines, then a green-fluorescing phosphor is produced, as expected. These results, as well as thermodynamic considerations, rule out the possibility that both the 1 o ~ and the high-temperature forms of yellow zinc silicate phosphor can be crystalline; they do not preclude an amorphous nature for both. Thus it may be considered established that zinc silicate forms a stable, lowtemperature amorphous modification in ortho proportions which upon prolonged heating is converted into the normal crystalline hexagonal form, while a t high temperatures, short of the melting point, only the crystalline modification results. When manganese activates the amorphous form, a yellow phosphor is produced; when it activates the crystalline form, the fluorescent color is green. The interesting correlation between fluorescent color and the coordination number of manganese, Le., its function as a network former or network modifier, respectively, have recently been pointed out by Linwood and Weyl (4). A third method has been developed in this laboratory which permits the production of bright yellow zinc silicate phosphors a t low temperatures without fluxes. I t consists in firing a mixture of zinc oxide, silica, and preformed manganese silicate (MnSiO3) in an atmosphere of steam under careful exclusion of oxygen, and is based upon the following chemical considerations: The formation of a zinc silicate phosphor is commonly regarded a~ the solution of “manganese oxide” in a zinc silicate matrix, a process brought about through diffusion during the heat treatment of an intimate mixture of the ingredient oxides. In reality, however, the process is not so simple but takes place in a number of separate steps either concurrently or successively, depending upon the individual rate coefficients. The reaction will be considered for material which is completely dehydrated and which contains the manganese in the form of that oxide which is stable in air a t the temperature of phosphor formation. This stable compound is the oxide Mnz03,possibly contaminated with trifling amounts of the other oxides, but hereafter simply called Mn203. The pure lower oxide (MnO) is not stable in air a t some 80O0C.,and if it is formed from other MnOyielaing compounds, it is rapidly oxidized to Mn20a. Thus the reacting mixture actually consists of ZnO, Mnz03, and Si02. When subjected to heat treatment it responds in a different manner than a mixture of ZnO, MnO, and SiOI if the latter were stable in air.
LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR
671
The reaction between zinc oxide and silica proceeds with increasing velocity from very low temperatures on. The primary reaction product is an amorphous orthosilicate complex which begins to crystallize from about 775°C. on. At 800OC. it takes roughly 1 week for 10 per cent crystalline material to form; a t 1200OC. or higher the speed of crystallization is so great that no amorphous material can be detected. On the other hand, MnZOa and Si02 do not react to any extent a t some SOOOC. They combine only with extreme sluggishness a t high temperatures in air, but the reaction does not go to completion during reasonable heating periods. A mixture of these two oxides remains dark brown after several days' heating in air at 800OC.; it retains practically the same oxidizing factor which it had prior to the calcination, thus indicating the absence of a reaction. Very little, if any, MnSiOa is formed, the product remaining a mechanical mixture of MnzOa and SiOz. Yet it is certain that the manganese in silicate phosphors is present as the lower oxide, MnO, or a silicate of MnO. (For the purposes of this discussion it is immaterial whether the silicate actually formed is MnSiO,, or MndiOl.) Thus i t is evident that the phosphor formation must involve a reduction of the higher oxide MnlOa to the lower oxide MnO. The overall equation for the phosphor formation should therefore be correctly formulated: 2nZn0
+ MnzOa + (n + 1)SiOz = nZn2SiOa.MnzSiOl+ 40%
where n>> 1. The experimental observations indicate that zinc oxide or zinc silicate acts as a catalyst for the decomposition of MnzOa with liberation of oxygen gas. Several methods are known to enforce a reaction between MnZOs and SiOz a t very high temperatures. At low temperatures only one method gave good results: namely, heating the oxide mixture in an atmosphere of steam and a reducing gas such as hydrogen.' The reduction of Mn208in hydrogen begins above about 3OO0C.,while the steam functions as a non-persistent catalyst for both the reduction and the reaction between silica and freshly formed manganous oxide. The brown mixture of MnzOs and SiOz turns perfectly white after a short firing time a t 800°C. There is good evidence that this low-temperature manganese silicate is also amorphous. If the reduction and reaction are carried out a t higher temperatures, such as llOO°C., a pinker and crystalline product results. Significantly, this crystalline manganese silicate does not produce a yellow phosphor with either of the low-temperature methods; the phosphors turn out green. Synthetic manganese silicates are rather unstable products. When reheated in air they are attacked by oxygen and disintegrate according to the following equations:
+ +OS= M n ~ +q 2Si02 = MnzOs + Si02 MnzSiO, +
2MnSiOa or
$02
1 Thirr method was used in this laboratory prior to the publication of the paper by K. K. Kelley (J. Am. Chem. Soo. 66, 782 (1943)).
672
H E R 3 I A 3 C. FROELICH
Dark brown products form again from previously white material at 800°C.: and at higher temperatures the attack by oxygen is but slightly less severe. Low-temperature manganese silicate disintegrates faster than the crystalline high-temperature form. This explains why a mixture of zinc oxide, silica, and preformed manganese silicate, heated in air to some 8OO0C., does not produce a good phosphor. The tendency of manganese silicate to oxidize and disintegrate is so strong that it prevails eren in high-tenipernture phosphors (1 100-1300°C.) of zinc silicate ryhich contain the manganese silicate dissolved throughout their lattices. JThen these phosphors are reheated in air to temperatures as low as 500°C., some superficial disintegration sets in, xhich is manifest by a slight brolvning of the material, a slight loss of fluorescent brightness, and a strongly positive test for high valent manganese. In atmospheres of more strongly oxidizing character, such as ozonized oxygen or air, the attack occurs already a t temperatures below 70°C. The reluctance of Mn203and SiOz to combine makes the mechanism of phosphor synthesis someivhat complicated. In the case of t,he binary, single activated matrix zinc silicate a minimum of five steps is involved which, hon-ever, do not necessarily occur in the order listed: ( I ) reaction betireen zinc oxide and silica to form amorphous zinc silicate in ortho proportions; ( 2 ) conversion of the amorphous form into crystalline silicate; ( 3 ) reduction of RlnzOBto MnO; (4)formation of manganese silicate; (5) complete solution of manganous oxide or manganese silicate t,hroughout the zinc silicate matrix, rvhether amorphous or crystalline, t o bring the phosphor formation to conclusion. As usual, the don-est, of the above steps is the rate-determining factor and, depending upon which rcaction occurs the fastest, end products of different' properties are obtained. The natural and fluorescent colors of the finished products are thus determined by the chosen experimental conditions, ahich control the rate of the individual reaction steps involved. For low-temperature phosphors the fluorescence is yellon. Tvhen the formation of manganous oxide and manganese silicate is faster than their solution in zinc silicate, which in turn must occur faster than the formation of zinc silicate. The fluorescence is, of course, green n.hen the formation of crystalline zinc silicate is the fastest step. This is true for mixtures heated in air a t all temperatures below the fusion point; it does not' imply, homever, that all such mixtures eventually turn out with equal brightness. When the synthesis of more complicated phosphors such as zinc beryllium silicates is considered, a fourth component, BeO, adds at least one more ratedetermining step. In this case it can be demonstrated that the solution of beryllium oxide in zinc silicate phosphor is the sloivest process, because the phosphor colors develop always from green toTvard orange-red and never in the reverse order. The degree of redness is a direct measure of the solubility of beryllium oxide in the phosphor matrix. Once the limit of solubility for a given temperature has been reached, no further deepening of the fluorescent color occurs no matter how long the heating or how much more beryllium oxide is added (2). The solubility is considerably belov proportions of beryllium oxide which would give a stoichiometric (1, I ) : (1) orthosilicate, ZnBeSiOa, with 15 per cent beryllium oxide.
LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR
673
The foregoing considerations have shown that the absence of a reaction between Mn203and SiOz is responsible for the fact that a bright yellow phosphor is not formed when a mixture of zinc oxide, silica, and any manganese oxide is brought to reaction in dry air or oxygen a t some 800°C. On the other hand, they also point the way toward a feasible method of preparation. Obviously, the experimental conditions must be chosen such that manganous oxide or manganese silicate can form and remain unoxidized until the phosphor formation is complete; furthermore, the solution of (amorphous) manganese silicate in amorphous zinc silicate must be complete before the latter begins to crystallize. Numerous variations of the implicit principle have taught that it is most convenient to control the rate of solution of preformed low-temperature manganese silicate in freshly formed zinc silicate. This method has produced uniformly bright powders. Thus steps 3 and 4 have been removed as variables in the process of phosphor synthesis. Step 1 proceeds rapidly. If a good gaseous catalyst is found for step 5 , the reaction is adequately controlled and the crystallization of zinc silicate becomes by far the slowest reaction, which no longer interferes with the development of a bright yellow phosphor. Steam has been found to be a good and non-persistent catalyst for step 5. Accordingly, a good method for the preparation of yellow phosphor is as follows: Manganese carbonate (or oxide) and silica are well mixed in approximately molar proportions, preferably with a slight excess of silica, and heated in steam and hydrogen a t 800-850°C. until the product is white. This requires about 1 hr.-more or less depending upon the batch size. After cooling in hydrogen a weighed amount of this silicate is ball milled with zinc oxide and silica and then heated in a brisk current of steam, under careful exclusion of air, a t 800-850°C., until maximum brightness has been attained. The proportions should be 1-1.5 moles of zinc oxide for each mole of silica, and sufficient manganese silicate to give about 3 per cent of manganous oxide in the finished product. Instead of steam alone, a mixture of steam and pure nitrogen, carbon dioxide, or other neutral gas may be used. The phosphor is cooled in the neutral atmosphere and is then stable in air at room temperature. Finely divided, amorphous, and partly hydrated silica has produced the brightest phosphors. The presence of steam is essential for the phosphor formation; powders heated in dry nitrogen did not fluoresce. From 1.5 to 2 hr. of heating time are required at 85OOC. to develop full brightness in the phosphors; in this respect they are somewhat inferior to fluxed products, which are finished after about 4 to 1 hr. However, they have certain advantages. The brightness of steamed phosphors is considerably improved and approaches that of quenched products; they contain no fluxes which are difficult or impossible to remove; and their particle size is exceedingly fine. The fact that lorn-temperature yellow phosphor may now be prepared in a so much brighter form speaks also against Rooksby and McKeag’s conclusion that the low-temperature form is merely a case of less perfect development of the high-temperature form and that “the yellow- fluorescence is most intense in instances of most perfect crystal development”. It would indeed be an exception to all practical experience if most perfect crystals were obtained fr0.m a melt
674
HERMAN C. FORELICH
chilled during a second or less. Quenching is more nearly an ideal condition for the development of most imperfect crystals, or for the prevention of crystallization altogether. Phosphors containing 1 per cent of manganous oxide have the same color as those with 3 per cent but are somewhat less bright. Phosphors with 5 per cent of manganous oxide are redder. This is due to a reduction in brightness of the green component of the fluorescent light, rather than to an increase of red or to a substantial shift of the peak.2 The response of 5 per cent phosphors to cathode rays is very much weaker than that of 1 to 3 per cent materials. Figure 1 shows the
\. \.
\
FIG.1. Spectral sensitivity of yellow zinc silicate phosphor
spectral sensitivity of low-temperature, steamed, yellow zinc silicate toivard ultraviolet light of various wave lengths, with a sharp peak at A2537. Figure 2 gives the emission curves for steamed and fluxed yellow zinc silicates, both expressed in relative per cent of their peak emission under A2537 excitation. In absolute terms the curve for the steamed phosphor would be much higher. Noteworthy is the consistent gain in brightness of the red part of the spectrum. An x-ray pattern of the steamed material was taken by Dr. Fonda, and again it
* These percentages refer to the total weight of the calcined product. It would be more correct to express them in terms of the amount of zinc silicate contained in the phosphor without counting the inactive excess silica.
675
LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR
waa confirmed that the phosphor is amorphous, with only a few weak lines of cristobalite showing. So far it has not been possible to make a bright low-temperature yellow phosphor in stoichiometric ortho proportions according to the above method. Orthosilicate compositions were slow to develop fluorescence and then turned out mostly weak green. Linwood and Weyl (4)have given a plausible explanation why an excess of silica may be helpful or necessary in forcing the manganesematrix complex to fluoresce yellow-red. Significantly, yellow phosphors could not be obtained from precipitated zinc silicates (from zinc salt and alkali silicate); all products turned out to be green fluorescing.
----
STEAMED FLUXED
\
5000
I
t
1
I
I
I
e00
5400
5600
5800
6000
6200
FIQ.-2.
I
6400
i.
Emission of yellow zinc silicate phosphor
When steamed phosphors are reheated in air to some 700°C., they also lose some of their fluorescent brightness. The loss is roughly inversely proportional to the heating time used for the phosphor synthesis. However, a good phosphor withstands all temperatures encountered in the processing of a fluorescent lamp, and high lumen outputs have been obtained with it. Quenching did not increase the brightness or temperature stability of steamed phosphors. The formation of red Einc silicate phosphors has not been observed by the steam-firing method. While samples of incipient fluorescence appeared to be reddish rather than yellow, a matching color could be obtained by mechanical dilution of bright yellow phosphor with inactive silica.
676
HERMAR- C. FORELICH
Several attempts have been made to obt,ain this yellolv phosphor in a one-step firing process. Only fair results were had with a mixture of zinc oxide, silica, and manganous oxide (to give 1 to 3 per cent MnO, with ratios up to 1.5:1) fired in steam and hydrogen a t 800°C. A small amount (1 per cent) of boric acid may be added as a catalyst, vhich slowly volatilizes in whole or in part and produces more uniform material. Zinc oxide, of course, is easily reduced to t,he metal, which distills out of the reaction product and condenses in the cooler parts of the furnace. (The vapor pressure of zinc is roughly one-third of an atmosphere a t 800°C.) However, according to the reversible reaction ZnO
+ H2
= Zn
+ H,O
-
27 Cal.
the reduction may be suppressed if only a small amount of hydrogen is used along with plenty of steam. The amount of hydrogen must be just sufficient to assure the formation and stability of manganous oxide. Kevertheless, some loss of zinc is unavoidable. Thus a batch composition with 1.5 moles of zinc oxide will, a t the end of the firing, be reduced to 1.3: 1 or lower. Even these products are more basic than the eutectic. Both the uniformity and the brightness of phosphors synthesized in steam and hydrogen are improved if a double firing scheme is employed or if preformed manganese silicate, as well as preformed zinc silicate, are used. The latter is obtained by calcining a mixture of zinc oxide and silica at 750°C. for several hours. The method offers no obvious advantage over the steam-nitrogen firing, except that it often yields brighter phosphors. The brightness of fluxed materials may likewise be improved if the firing is carried out in a neutral atmosphere, either in the presence or in the absence of steam. I n this case the activator had best be added in the form of a compound of bivalent manganese, such as manganous chloride. I t may be ment'ioned here that the general principle outlined for the production of yellow zinc silicate phosphor is also applicable to other types of phosphors rvhich contain bivalent manganese as the activator. A simple method has been used for the rapid determination of the mole ratios of zinc oxide to silica in samples of unknown history. A mighed amount of sample, usually about 0.2 g., is suspended in 25 cc. of water and 25 cc. of 0.2 N sulfuric acid is added with vigorous shaking. dlthough very dilute, the acid decomposes both amorphous and crystalline zinc silicate phosphors completely. The excess acid may be back-titrated with 0.2 N sodium hydroxide and methyl red indicator, which gives a sharp end point. The difference represents the equivalent of acid used for the neutralization of ZnO MnO. If the amount of manganous oxide is taken into account, the percentage of zinc oxide may be computed easily. Thus a determination of the mole ratios takes but a few minutes. It is difficult to explain the action of halide catalysts on the basis of the foregoing. It might be inferred from the above that they catalyze the decomposition of the higher manganese oxides into manganous oxide and oxygen. It can be shown experimentally that this is not the case. Neither manganese oxides alone nor mixtures with silica turn white or liberate oxygen when heated in potassium
+
LOW-TEMPERATURE YELLOW ZINC SILICATE PHOSPHOR
677
chloride, cadmium chloride, or a mixture of the two. Characteristically, a yellow phosphor is obtained with these fluxes when zinc oxide and silica or precalcined, low-temperature, amorphous zinc silicate are used; crystalline zinc silicate which was precalcined a t 1000°C. or higher gave only a green phosphor with manganous oxide and flux. In order to establish the crystalline or amorphous nature of the various zinc silicate phosphors, another method is proposed. No doubt the heats of formation and the free energies of the different products are not equal. Since they are all easily decomposed by dilute sulfuric acid, precision determinations of the heat of decomposition in a calorimeter should give a reliable indication as to the identity of the various materials. The most valuable information on the lattice construction of these phosphors, however, will eventually be obtained by precise Fourier analysis of their diffraction patterns, a method which has given such good results on other silicates. SUMMARY
It has been shomn that one of the most important steps in the formation of a zinc silicate phosphor is the production of the oxide or silicate of manganese in its bivalent state. If this step is eliminated as a variable in the phosphor preparation, such as by preformation of manganese silicate at low temperatures, yellow phosphors of high brightness may be obtained. Steam is a good gaseous catalyst for the reaction, which must be carried out in the complete absence of oxygen. Rooksby and McKeag’s interpretation of the low-temperature forms of yellow zinc silicate is shown to be erroneous. Acknowledgment is due Miss I. Monte, who assisted in the preparation of the samples, to Dr. G. R. Fonda for the x-ray pattern, and to Dr. B. T. Barnes for the sensitivity and emission curves. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8)
FONDA,G. R . : J. Phys. Cheni. 44,851 (1940); British patent 544,444. FONDA, G. R . : J. Phys. Chem. 46, 282 (1941). H . W.: U. S. patent 2,129,096 (1938). LEVERENZ, LINWOOD, S. H . , AND WEYL,W. A . : J. Optical Roc. Am. 32,443 (1942). PABST,A , : Z. physik. Chem. A142, 227 (1929). ROOKSBY, H . P . , AND MCKEAG, A. H . : Trans. Faraday SOC.27,308 (1941). SCHLEEDE, A,, AND GRUHL,A , : Z. Elektrochem. 29,411 (1923). TAMXANN, G . : Z. angew. Chem. 39, 869 (1926).
