VISCOSITY AND VAPOR CONCENTRATION
559
The fluidity of the ideal solution is given in terms of the partial pressures of the constituents and the absolute temperature. I t is of interest to learn whether or not equation 7 is applicable to solutions consisting of liquid components. Mixtures of benzene and carbon tetrachloride are known to approach ideality (2). Equation 7 was found applicable to data covering a temperature range of 0' to 40°C. for these mixtures (table 4). However, in the case of mixtures of ethyl alcohol and water (20' to 75°C.) the equation does not fit when the weight per cent of alcohol is less than about 50 per cent. The curves are then similar to those for water, which is t o be expected, as the mole fraction of water present is about 0.75 or higher. SUMMARY
Empirical and theoretical expressions have been given showing the relationship between viscosity or fluidity and vapor pressure or vapor concentration. A semi-empirical equation has been deduced for the variation of fluidity with absolute temperature. Theoretical relationships have been given concerning the fluidity of solutions. REFERENCES (1) ANDRADE: Nature 126, 309 (1930). DRWCKER: Z. physik. Chem. 92, 287 (1918). DUNN:Trans. Faraday SOC.22, 401 (1926). DE GUZMAN: Anales soc. expafi. fis. qufm. 11, 353 (1913). IYER:Indian J. Phys. 6(14), 371 (1930). BENDALL AND MONROE:J. Am. Chem. SOC.39, 1799 (1917). LEDERER:Kolloid-Beihefte 34, 270 (1931). MAXWELL: Phil. Mag. 36, 129 (1868). RAMAN:Kature 111, 532, 600 (1923). SAEPPARD: Nature 126, 489 (1930). (2) BINGHAM: Fluidity and Plasticity, 1st edition. McCraw-Hill Book Co., Inc., New York (1922). (3) BINGAAM AND STEARNS:J. Chem. Phys. 2, 107 (1934). (4) HILDEBRAND: International. Critical Tables, Vol. IV, p. 19. hlcGraw-Hill Book Co., Inc., New York (1928). (5) PORTER: Phil. Mag. 23, 458 (1912). (6) RELLSTAB : Uber die Transpiration hornologer Fliissigkeiten. Inaugural dissertation, Bonn, 1868. (7) SOUDERS: J. Am. Chem. SOC.69, 1252 (1937).
CHARACTERISTICS O F SILICATE PHOSPHORS
563
mium chloride was most effective. When two parts by weight of it mere mixed with the reaction mixture and fired a t 850"C., the fluorescence rose to 75 per cent of its normal value after 70 min. The chloride by this time had completely evaporated. When the reaction mixture was fired a t 850°C. for the same period without the chloride, a product resulted whose fluorescence was only 3 per cent of normal. The catalytic effect persisted, though in lesser degree, with smaller amounts of the chloride, and was still well observable with only 1 per cent. Its basic effect lay in an acceleration of the reaction between the two oxides, as was demonstrated by firing them a t 850°C., before the addition of manganese but in the presence of cadmium chloride. The product was of course devoid of fluorescence.
FIQ.1
FIG.2
FIG.1. Heat of diffusion for mixtures containing zinc oxide, silica, and 0.4 per cent manganese FIQ.2. Spectral distribution of Zn&iO4.SiO2.Mn prepared with cadmium chloride a t 850°C. Excited by 2536 A.
When manganese oxide was added and the firing continued at 850°C., fairly strong fluorescence, 15 per cent of normal, developed after half an hour. A blank mixture of zinc oxide, silica, and manganese dioxide gave no observable fluorescence whatever under the same firing. The catalytic action of cadmium chloride eonsists, as in the case of a non-volatile flux, in dissolving away the silicate barrier between reacting particles. The low fluorescence of the final product is a result also of its transition through a solution phase. Both these effects were shown by an experiment with a normal phosphor which had acquired fully saturated fluorescence. This phosphor dissolved readily in the molten chloride. The product, on cooling, was non-fluorescent. The fluorescence reappeared after volatilization of the chloride, but was reduced 20 per cent below its original value. The highest fluorescence obtainable from firing
564
GORTON R. FONDA
a mixture of oxides in the presence of the chloride was likewise 20 per cent below the normal value. This represents therefore an equilibrium value that was approached from each side,-solution of the silicate phosphor and synthesis of the phosphor from its constituent oxides. I n neither case could the fluorescence be raised to normal by further firing a t any temperature. The cause for the lower fluorescence lies in a reduction in grain size of the finished phosphor, an effect due to the solvent action of the flux, which will be discussed later. In calculating the speed of reaction, use has been made of the fact that the fluorescence increases as the reaction progresses toward completion. At all periods of the uncatalyzed reaction, and a t all temperatures of firing, the fluorescence, regardless of its intensity, had the same spectral distribution with only slight changes in the peak value. When a flux was present, the same situation held for temperatures of 1000°C. and higher. At 850°C., however, the first fluorescence that developed, as a result of an incipient reaction, had a distribution whose peak was shifted towards the red sufficiently to change the color. This effect was most marked in the case of cadmium chloride, no doubt because it is the strongest catalyst. Cadmium silicate, which has a pink fluorescence, was not formed in this reaction. This was shown by firing a mixture of silica, manganese, and cadmium chloride. The resulting product was non-fluorescent. The curves of figure 2 represent the variations in spectral distribution that resulted from firing the mixture of oxides with variable' amounts of the chloride. In each case the firing was continued long enough to volatilize the chloride completely. The intensity of fluorescence obtained was proportional to the amount of chloride introduced, for the mixture reacted at an appreciable rate only while the chloride was present. The distribution, obtained from the use of 5 per cent cadmium chloride, has a peak a t 5800A. and its fluorescence is pink. This is in contrast to the phosphor obtained with 200 per cent cadmium chloride, whose peak is a t the normal position of 5270 A. and whose fluorescence is the normal green. The x-ray patterns of both were the same as that of the regular silicate phosphor. I n this respect it is distinct from the red and yellow zinc silicates prepared by Gruhl (5) by firing above 14OO0C. and quenching, for the crystal patterns of his products were altered, indicating that zinc silicate crystallizes in other modifications above 140OOC. 11. VARIATIONS IN COMPOSITION
Manganese concentration A peak in the fluorescence intensity occurred at an optimum concentration of about 0.5 per cent manganese. As the manganese content rose, this peak was shifted slightly toward the red. These changes are brought out in table 2 and in the curve of figure 3.
565
CHARACTERISTICS OF SILICATE PHOSPHORS
The trend of the results has a definite significance. As Dr. Clarence Zener' has pointed out, it would be natural to expect an increase in fluorescence with increase in the concentration of manganese, until a disturbing influence sets in. Such an influence evidently begins a t about 0.4per cent manganese, and can be ascribed to the loss in energy of an excited manganese atom by collision with neighboring manganese atoms, as a consequence of the mobility allowed by thermal vibration. This would involve TABLE 2 Dependence of. .fluorescence characteristics at 36°C. u in manganese content
-
YANCIANEOE
FLVORESCENCE
per csnt
per cent
0.01 0.1 0.4 0.6 0.9 1.4 2.0
12 80 96 98
100 96
90
2.3 4.5 5.0
9.0
44 34 3
P Z A L IN FLUORESCENCE
5270 5280 5290
5290 5310 5310 5330 5350
FIQ.3 FIG.4 FIQ. 3. Variation in fluorescence at 25°C. of ZnSiOs activated with manganese FIQ.4. Variation in fluorescence of ZnSiOa with temperature for various percentages of manganese
the emission of some of the energy of an excited electron in the infrared and a consequent shift of its emission in the visible toward the red. The actual occurrence of the latter is shown by the results of table 2. This view is supported by measurements on the variation of fluorescence with temperature, made in collaboration with Dr. Zener. The results are shown in the curves of figure 4 for various specific concentrations of man-
* Dr. Zener's paper on this topic will be published in the near future,
566
GORTON R. FONDA
ganese. The general increase in fluorescence that accompanies decreases in temperature is in accord with the conception of thermal vibration as an important factor. This view is strengthened by an observation of Dr. Zener’s that the poisonous action of iron becomes less pronounced as the temperature is decreased. With 0.4 per cent iron the fluorescence, amounting only to 2 per cent at room temperature, had risen to 8 per cent a t 77°K. This relation between fluorescence and the concentration, C , of the activator (or of a dye, in the case of fluorescent solutions) has frequently been found to be expressed by the empirical equation of Bruninghaus (1)
F
= AW-0
The results of table 2 can be expressed by this equation only over the range 1.4 to 9.0 per cent manganese. For lower values of manganese content the fluorescence becomes increasingly higher than would be in accord with the equation. X-ray examination produced evidence bearing upon the condition of. the manganese within the silicate. Diffraction patterns were obtained through the courtesy of L. L. Wyman and E. T. Asp of this laboratory. On using CuK, radiation and the method of back reflection, the same pattern was found for all zinc silicate phosphors, namely, a hexagonal structure, as shown in the photographs of figure 5. For manganese contents of 2 per cent and 5 per cent, however, the lattice spacing became increasingly greater. This can be observed for beams reflected through angles close to 180°,as represented in the photographs by the arc segments close to the central hole. It is evident that the manganese enters into solid solution with the zinc silicate, an atom of manganese being substituted for an occasional atom of zinc. The stretching of the lattice is a consequence of the larger ionic radius of the manganese, 0.91 A., as compared with 0.83 for zinc (4). A similar situation was found for the cadmium silicate phosphor, as illustrated in the x-ray diffraction patterns of figure 6. Here also the same pattern persisted in the presence of 5 per cent of the activator manganese. In this case, however, a reverse effect of the manganese is shown, -namely, a contraction of the lattice. This is evidence that the manganese has formed a solid solution likewise with the cadmium silicate, substituting for an occasional cadmium atom. The contraction is due to the smaller ionic radius of manganese as compared with that of cadmium, which is 1.03 A.
Silica content The concentration of silica proved to be important in its bearing upon the intensity of fluorescence, for it disclosed an unexpected effect; namely,
3NTON N. E'ONDA
569
CHARACTERISTICS OF SILICATE PHOSPHORS
that the fluorescence is not reduced as the silicate is diluted with excess silica. The thermal diagram for the system of zinc oxide and silica has been worked out by Bunting (2). It discloses only one compound, the orthosilicate ZnzSi04, having the composition of the nat‘ural mineral, willemite, with a melting point of 1507’C. At 1432OC. there is a eutectic having the composition ZnzSi04.SiOz. This has frequently been called the metasilicate, and its composition has been expressed by the formula ZnSiOs. Such a compound, however, appears not to exist, in the light of Bunting’s work and of our own x-ray study, described below. It should therefore be considered as an agglomerate including 1 mole of silicate and 1 mole of free silica. When the fluorescent orthosilicate was diluted with silica, the fluorescence of the mixture was found to decrease linearly, as would be expected. Firing of such mixtures brought about only insignificant increases TABLE 3 Variation i n j’luorescence w i t h silica content f o r 0.4 per cent manganese
I
COMPOSITION
Molar per cent ZnO
67 50 33 11
1
FLUOREBCENCE
I
Molea free
0 1 3 15
sio2
I ZnrSiO,
I. Mechanical mixture. Mn f r e e SiO;
+
102 71 17
1
11. Fired mixture of
oxids
102 98 86 72
in fluorescence. On the other hand, the products obtained from firing mixtures of oxides rose almost to normal fluorescence, despite the presence of large excesses of free silica. This is brought out in table 3. Two series of mixtures were involved. The first (I of the table) was made up of the completed phosphor, ZnpSi04.0.4 per cent Mn, mixed with additional silica. The second (11) was prepared by mixing zinc oxide and silica in the desired proportion, adding 0.4 per cent manganese, and firing. The fluorescence remained approximately the same in mixture I1 whether the 0.4 per cent manganese was calculated for the orthosilicate content or for the combined content of the two oxides. The reason for the continued high level of fluorescence of the products obtained by firing the mixture of oxides lies in the physical condition of the excess silica rather than in its incorporation into unique compounds. X-ray diffraction patterns were obtained both with MoK, and CuK, radiation, the latter by the method of back reflection. The same crystalline pattern persisted throughout, identical with that from precipitated zinc orthosilicate as well as with that from natural willemite. There was
570
QORTON E. FONDA
no change in lattice parameter, such as would have resulted from a solid solution of silica in the silicate. On the contrary, the d8raction linea of free silica appeared in the pattern obtained for the phosphor with 33 molar per cent zinc oxide. They were absent for the composition of 50 per cent zinc oxide, containing & mole of free silica, but this is not surprising, because the x-ray scattering power of silica is much less than that of the compound Zn8iOl. The conclusion is therefore that any excess silica is present as such. The condition of some of it is undoubtedly that of particles coated with a shell of silicate. It is probable, however, that some of it also is more intimately associated with the silicate and present as small colloidal groups scattered in a diffuse state throughout the mass, particularly a t coarse lattice defects of the crystal. Such a view is, of course, speculative, but it is in accord with the localized segregations of colloidal sodium produced by Rexer (9) in crystals of sodium chloride. It would account, too, for the prolonged phosphorescence that occurs at an optimum concentration of one excess mole of silica per mole of compound. It is also in accord with results observed on grinding. When phosphors having a large excess of silica, 66 molar per cent or greater, were ground, the ultimate fluorescence attained was of the same order as that found for unfired mixtures of orthosilicate and silica. The effect of the grinding was obviously just to disintegrate the mass and to cause a dilution of the silicate with silica which had hitherto been intimately dispersed within the crystals. Refiring of such a mixture had no effect whatever in restoring its fluorescence. 111. O R I G I N OF FLUORESCENCE
From a consideration of the experiments that have been described on the characteristics of fluorescent zinc silicate, it would appear that the phenomenon of fluorescence is dependent upon the presence of manganese. This is borne out by the observations that fluorescence of appreciable magnitude does not arise in the absence of manganese, that manganese when present enters into the silicate molecule, substituting for a zinc atom, and that the resulting fluorescence is proportional to the manganese concentration up to an optimum value. The manner in which light may be emitted by excitation of the manganese can best be approached theoretically by a consideration of the energy states within a crystal. Unlike the states in a gas, these cannot be represented by discrete levels, because of the high concentration of atoms in a solid. The energy of valence electrons is so strongly influenced by the location of atomic nuclei and by the mutual presence and movement of other electrons, that the possible states are extended into bands, each one of which is characterized by a
CHARACTERISTIC8 OF SILICATE PHOSPHOR8
571
finite range in energy. Furthermore, there are wide energy gaps between allowed bands. Seitz and Johnson (11) have outlined a mechanism for the occurrence of fluorescence by excitation of impurity atoms in a crystal. It considers the localized, discrete energy levels that are introduced by the presence of a foreign atom a t such low concentration that its proportion to the main atom of the crystal is far from a combining ratio. These discrete IeveLs are found, if a t all, within the forbidden zones, the gaps between the allowed energy bands. Excitation involves the ejection of an electron from one level to another, such, for instance, as the raising of an electron from one of these localized impurity levels to an upper unfilled band of the crystal. The spectrum of the fluorescence emitted by its return is continuous and of longer wave length than that of the exciting light by reason of several possible losses, including collisions with the lattice which cause some of the emission to be diverted to the infrared. IV. ABNORMAL DECREASES IN FLUOREBCENCE
Effect of grain size I n the discussion of reaction velocity, too large a size of the oxide particles was shown to be a factor which might prevenc completion of the reaction and consequently limit the fluorescence of the product. Even after a complete reaction, however, the brightness of the fluorescence has been found to depend upon the crystal size of the finished phosphor. Phosphors that exhibit greatest brightness have an average crystal size of about 4-5 p with a maximum of about 13 p and a minimum of about 1.5 p. Aggregates of larger size, obtained by sintering the phosphor, have necessarily a reduced fluorescence, because the exposed surface of the powder per unit area covered by it becomes lower as the particle size is increased. The grain size itself has been reduced in four ways: (1) by grinding in an agate mortar, (2) by reducing the temperature of formation from 1250' to llOO'C., (3) by preparation a t 850°C. with cadmium chloride as catalyst, and (4) by fusion. In each case the fluorescence was lowered, even when the phosphor had the composition of the orthosilicate, a homogeneous compound devoid of free silica. The relation between grain size and fluorescence i s shown in table 4. It is noteworthy that a refiring of the ground phosphor led to increases in both grain size and fluorescence, but failed to bring either to their original values. There is a distinct tendency for the fluorescence to vary with the fineness of the material. The coarsest material has the highest fluorescence and the finest material has the lowest. Phosphors of intermediate crystal size show fluorescence whose brightness lies midway a t values varying
572
GORTON R. FONDA
according to the method of preparation. This relationship is in accord with the condition of the manganese atoms as affected by such changes in crystal size. As has already been pointed out, the fluorescence arises presumably from excitation of the manganese atoms. This process is possible, however, only for those atoms that find themselves incorporated within the interior of a crystal. The energy states of surface atoms are so differeht that the same phenomenon would not be expected. The smaller the crystal size, the greater will be the number of manganese atoms that occ.ur on the surface of a crystal, rather than in its interior. This is more than a matter of probability of arrangement. The larger size of the manganese ion, together with its low percentage, implies that it produces localized stresses throughout the lattice. When a phosphor is ground, cracks will therefore occur most readily along planes including manganese atoms, and, consequently, more manganese atoms will find themselves on
TABLE
4
Relation between grain size and fluorescence of zinc orthosilicate with 0.4 per cent manganese
I-
DISTRIBUTION I N QBA1N BIZE
TBEATYENT
1
BEWTlYE
7-l4r p s r cant per unf p e r cent p e r cent
Prepared at 1250°C.. . . . . . . . . . . . . . . . . . . . . . Prepared at 125OOC.; ground.. . . . . . . . . . . . . Prepared at 1250'C. ; ground and refired. . , Prepared at 1100°C........................ Prepared at 850°C. with CdCll as catalyst. Prepared at 1600°C. by fusion... . . . . . . . . . .
15 0 0 0 0 0
35 0 8 5 0 5
35 5 22 80
20 80
15 95 70 15 80
15
I
102 32 81 76 70 67
external crystal surfaces than would be calculated from a probability rule of random distribution of cracks. A similar situation holds in the crystallization from a fused mass, for again boundaries between grains will form most readily along planes including the foreign atoms of manganese. When the ground silicate was refired, the product proved to be highly sintered, in contrast to the pulverous condition of the original phosphor after its preparation a t the same temperature; this is evidence of the much greater fineness of the ground product. The refiring produced a little grain growth-sufficient to raise the fluorescence somewhat-but both remained below the values characteristic of the original. The particularly low grain size of the product formed a t 85OoC. under the influence of the cadmium chloride flux is in accord with the trend observed a t higher temperatures, where a reduction in grain size accompanied a reduction in the temperature of preparation. The relation between fluorescence and grain size was confirmed by
CHARACTERISTICS OF SILICATE PHOSPHORS
573
microscopic examination of fluorescing particles. Those below 1.5 were by contrast so low in fluorescence as to appear almost dead. In another experiment samples of coarse and fine particles were collected by sedimentation of a normal phosphor in alcohol. At a 1 mm. thickness the fine particles were noticeably less bright than the coarse. Similar conclusions have been drawn by Riehl and Ortmann (10)from relations found between phosphorescence and grain size.
Efect o j conversion to glass Zinc silicate could not be converted into a glass, but this was done readily in the case of cadmium silicate by heating above its fusion temperature in a platinum crucible and cooling rapidly. In the absence of manganese, a clear white glass was formed. In the presence of 0.4 per cent manganese the glass was still clear, but was discolored a deep brown, in contrast to the white color of the crystalline product, and was devoid of fluorescence. Although the original powdered phosphor gave an x-ray diffraction pattern of lines characteristic of its crystalline structure, the glass showed no line pattern whatever. The loss of fluorescence on conversion to a glass has already been observed by Curie for zinc borate phosphors (3). He ascribed it to the change in condition from a crystalline to a glassy state. This is difficult to reconcile with the occurrence of a wide variety of fluorescent glasses the emitted light of which is dependent upon the presence of a trace of activating metal. Although in some cases such a metal forms fluorescent salts, such as the rare earths and the uranyJ salts, yet many of them are common metals whose salts are devoid of fluorescence. It seems therefore more reasonable to conclude that the conversion of a phosphor, such as cadmium silicate, into a glass is accompanied by a decomposition of the fluorescent double silicate of cadmium and manganese into a conglomerate of cadmium silicate and manganese silicate which is non-fluorescent, just as any other mixture of these compounds would be. This view seems justified by the change in color, for manganese silicate is a deep brown. It seems justified also by the diversity in size of the metal ions, cadmium and manganese. Further evidence is shown by the behavior of the glass when refired below its melting point. When the temperature was 100°C. below the normal firing temperature, the mass crystallized and developed a low fluorescence. Even after 65 hr. of firing, however, the fluorescence remained low and some of the brown discoloration persisted. This is hardly in accord with what one would expect if the return to a fluorescent phosphor involved simply the transformation from the glassy to the crystalline state. It would seem rather to denote a reaction involving the diffusion of two separate silicates to form the double silicate characteristic of the phosphor. When the glass was refired directly a t the normal firing
574
GORTON R. FONDA
temperature, it was reconverted into the normal whitc phosphor and developed a relatively bright fluorescence. This fluorescence was still from 20 to 30 per cent below the original value. Examination showed the grain size to be of the order of that noted in table 4 for fused zinc silicate. The reduction is therefore the normal one that is dependent upon the reduction in grain size. It is not surprising to find a similar effect produced by the action of a flux capable of converting a phosphor into a glass a t temperatures below its normal melting point. This could be done with zinc silicate by firing at 1000°C. with a sufficient amount of sodium borate, zinc borate, or zinc phosphate. The glass obtained was non-fluorescent. With sodium borate a glass was obtained even a t 50 per cent mixtures. It was violet in color, denoting as before a decomposition of the phosphor as the cause for the loss of fluorescence. When this glass was fired for 15 hr. a t 750"C., it devitrified into a crystalline product and developed the characteristic fluorescent spectrum of zinc silicate with an intensity 9 per cent of normal. V. EXCITATION
The two silicate phosphors, together with some sulfide phosphors, were examined for their luminous characteristics by Frank B. Quinlan and the author. The spectral distribution was found with a spectrophotometer, and the efficiency of excitation was determined under radiation from a monochromator, measuring the radiant energy of a line with a thermopile and the fluorescence lumens in a 4.5-cm. sphere with a photovoltaic cell and correcting filter. The lumens were converted into energy units by calibration of the sphere for visible light of various wave lengths. The efficiency represents, therefore, the percentage which the fluorescence energy within the visible range bears to the radiant energy. The results are given in table 5. The silicates contain 0.4 per cent manganese. The two sulfides are commercial products and presumably contain copper as an activator. The uranyl salt is potassium uranyl sulfate. The spectral distribution and excitation of the silicate phosphors are shown graphically in figures 7 and 8. No data were obtained on the absorption of ultraviolet light by the phosphors. It is interesting, however, to consider the quantum yield in terms of the quanta of radiant energy necessary to yield one quantum of fluorescence energy. This is done in table 6 for those wave lengths a t which the efficiency was particularly high. In order to have a photographic record of the efficiency of excitation, the Duclaux and Jeantet method was resorted to (6). A phosphor was coated on the outer surface of a plate of glass. This was placed over a photographic plate with a No. 16 Wratten filter in between. The combination absorbed all radiation below 5461 A., except that of 3125 A., a
5 75
CHARACTERISTICS O F SILICATE PHOSPHORS
small amount of which was transmitted. Consequently the photographic plate recorded only the fluorescent light, and displayed a pattern of lines corresponding in position with the ultraviolet lines that were capable of exciting fluorescence. Their relative intensity is a measure of the degree of excitation. The attached photographs in figure 9 bring out, for instance, the strong contrast between the two silicates, excited only by the far ultraviolet, and the group of the sulfide and uranyl salts excited most strongly by the near. TABLE 5 Percentage of jluoresceiice energy i n visible in relation to energy of ezciting line LINE
A. 2225 2378 2399 2483 2536 2652 2699 2753 2803 2893 2967 3126 3341 3651 4047
per cent
ZnCdS
I
per cent
pcr Tt
per cent
1 ~
UOrSOa pcr cent
8
m 22 30 36 25 22 19 15 6 1 0 0 0 1
4
21 24 16
15 17
,
,
1:
~
14 18 19
I
13 9 7 6 1 0 1
1
1 1 I
2
20
8
3 25
14 17 18 42 57
43
64 67
~
17
22 18 21 26 39
TAWE 6 Quanta of radiant energy for quantum of fluorescence energy
ZnlSiO,. . . . . . . . . . . . . . . . . . . . . . . . CdSiOs. . . . . . . . . . . . . . . . . . . . . . .
ZnS.. . . . . . . . . . . . . . . . . . . . . . . . . ZnCdS . . . . . . . . . . . . . . . . . . . . UOnSO, . . . . . . . . . . . . . . . . . . . . .
1.3 1.8 2.8 4.2 2.5
~
1.1
, ~
1.4 2.6
1.1 1.2 1.9
SUMMARY
A zinc silicate phosphor, as well as related ones, can be prepared by firing mixtures of the constituent oxides, with manganese as activator. The reaction is a diffusion phenomenon having a heat of diffusion of 20 to 24 Cal. Its speed depends upon the particle size of the oxides and can be accelerated by the addition of those inert substances capable of dissolving away the barrier shell of silicate as formed.
CHARACTERISTICS O F SILICATE PHOSPHORS
577
Fluorescence depends upon the presence of manganese. At an optimum concentration of 0.4 per cent manganese it shows a peak a t all temperatures and remains almost const,ant from 100°C.to 77°K. For any concentration above 0.4 per cent manganese the fluorescence increases continuously as the temperature is reduced, approaching the value of the optimum a t 77°K. The fluorescence is a t a peak for the composition of the orthosilicate but is reduced only slightly when prepared with large excesses of silica. Such phosphors are disintegrated readily by grinding and are simply conglomerates. X-ray examination shows a persistence of the orthosilicate structure throughout. It shows also a stretching of the lattice in the presence of manganese, denoting that the latter enters into the silicate structure by substitution for an occasional zinc atom. Reductions in fluorescence resulting from fusion or from grinding, even when followed by refiring, are due to permanent reductions in crystal size. The complete loss of fluorescence produced by conversion into a glass is a consequence of a decomposition of the double silicate into its constituents. The fluorescence is partially restored by refiring a t a temperature below the fusion point. The efficiency of excitation was measured between 2225 A. and 4047 A. for some silicate and sulfide phosphors. Several of them showed a quantum efficiency of nearly unity for specific wave lengths. The author is indebted to William H. Tomb, Elwood M. Douthett, and Robert I. Reed for their assistance in preparing the phosphors. REFERENCES (1) BRUNINGHAUS, L.:Compt. rend. 149, 1375 (1909). ’ (2) BUNTING, E. N.:J. Research Natl. Bur. Standards 4, 134 (1930). (3) CURIE,M.:Compt. rend. 203, 996 (1936). V. M.:Trans. Faraday SOC.26, 282 (1929). (4) GOLDSCHMIDT, (5) GRUHL: Handbuch der Physik 23, 571 (1926). (6) HEYNE,G., AND PIRANI,M.: Z. techn. Physik 14,31 (1933). (7) J.4NDER1 w.:z. anorg. Chem. 189, 1 (1927);200, 245 (1931);226, 225 (1936). (8) LENARD,P., SCHMIDT, F., AND TOMASCHEK, R.: Handbuch der Experimental Physik, Vol. XXIII,Part I, p. 456 (1928). (9) REXER,E.: Z. Physik 76, 735 (1932). H . : Ann. Physik 29, 556 (1937). (10) RIEHL,K., AND ORTYANN, (11) SEITZ,F.,AND JOHNSON, R. P . : J. Applied Physics 8, 255 (1937). 12) CYTERHOEVEN, W . , AND HESS,K . W . : Elektrische Gasentladungslampen, p. 322. .I Springer, Berlin (1938).
