8 78
HERMAX C. FROELICH AND GORTON R. FONDA
EXAGGERATED 'PHOSPHORESCEXCE OF ZINC SILICATE PHOSPHORS HERMAN C. FROELICH L a ~ l pDevelopment Laboratory, General Electric C o . , Nela Park, Cleveland, Ohio AND
GORTOX R. FOSDA Research Laboralmy, General Electric Co., Seheneclady, New I-oo,.!~
Received April SO, 1912 INTRODUCTION
The phosphorescence of zinc silicate activated with manganese shows a twostage decay (1). The initial stage is exponential and relatively rapid. The second stage is much slower and determines tjhe decay rate from about 30 millisec. onward. It is essentially of a bimolecular character but is so complex that no one equation gives expression to its entire course. Under the influence of various factors, the second stage may be made to occur at an earlier period in decay and consequently to lie a t a relatively higher level, at least with reference to the initial fluorescence from which decay started. Some of the features causing such changes have already been discussed previously. They include ( a ) the use of a colloidal gel for the original mixture rather than osides precipitated separately, (b) the presence of an excess of some neutral ingredient, such as silica, and (c) reduction in the content of manganese. More recently two other factors have been found which have the effect of giving still greater prominence to the second stage of decay. The first c0nsist.s in quenching the phosphor from an elevated temperature. The second factor, of more importance and scope, consists in the introduction of arsenic oside as a specific agent for intensifying the phosphorescence emitted as the result of manganese activation (2). DECAY OF PHOSPHORESCEXCE
Quenching was carried out on 4-g. samples heated to llOOoc'. It mas more effective when the phosphor was chilled in water rather than in liquid air. In the latter case the cooling was noticeably slower, owing apparently tQ an insulating layer of hot air that collected around the powder. Its effect was noteworthy only when the phosphor was devoid of free silica and had the composition of the orthosilicate, ZnBiO,. Its effect was hardly noticeable in the case of phosphors having the more usual composition, which includes 0.5 to 1 mole of excesa silica. There was no effect on the silicates containing arsenic, the characteristics of which are described below. The intensity of fluorescence was not appreciably altered by the quenching. 1 The phenomenon of exaggerated phosphorescence in silicates was discovered while one of us (H. C. F.) waa employed in the Research Laboratory of the Harshaw Chemical Company, Cleveland, Ohio. The authors wish to thank the Harshaw Chemical Company for permission t o publish this joint report on work carried out in the various laboratories.
EX4GGERATED PHOSPHORESCEKCE OF ZINC SILICATE PHOSPHORS
879
For the experiments with arsenic oxide, a series of phosphors was made having the coniposition of 1.1 moles of zinc oxide per mole of silica. To assure uniformity, they were prepared from the same mixture of zinc nitrate, purified especially from copper and iron, and ethyl silicate, purified by distillation. An amount of manganese nitrate was added equivalent to 0.5 per cent manganese with reference to the mixture of oxides. To portions of this batch, variable amounts of arsenic pentoxide were added from an aqueous solution.
FIG. 1. intensity. SiOt.Mn SiOz.Mn SiOz.Mn
Decay in phosphorescence of zinc silicate phosphors as percentage of initial A , ZnzSi04.Mn;B, Zn*SiO,.SiO?.Mn; C, ZnzSi04.Mn(quenched); D, Zn2SiOd. 0.002 per cent AsnO5;E, ZnnSi04.SiO~.Mn 0.005 per cent AsrOs; F , ZnzSi04. 0 01 per cent AsxOs; G , Zn?Si04.SiOz.Mn 0.05 per cent A s t 0 5 ;H, ZntSiOi, 0.50 per cent AszOa.
+ + +
+ +
The individual portions were gelled, dried, converted into oxides, and fired 9 hr. at 1100°C. Their phosphorescence was measured a t 25%. by the method previously used (1) and the results are plotted in figure 1, in terms of the percentage of the initial intensity. The initial stage of decay was exponential throughout, following the same uniform rate. The distinctive feature of the phosphors to which arsenic had been added lay in the fact that the second stage of decay became observable a t an earlier instant and therefore a t a higher level of intensity x-ith reference to the initial value. As shown in the curves and in the
880
HERMAN C. FROELICH AND GORTON R. FONDA
results of table 1, this effect became more pronounced as the amount of arsenic waa increased. With 0.5 per cent of arsenic pentoxide the initial stage of decay was completely absent, and the decay had the characteristics of the slow second stage from the instant the exciting radiation was extinguished. The intensity of fluorescence was reduced as more arsenic was added, an effect that became more noticeable above 0.05 per cent arsenic pentoxide, as is shown graphically in figure 2. This means that full advantage cannot be taken TABLE 1 Characteristics of second stage of decay w m ox DMESION
PEOY rmsr STAGE
TREAIXFXZ OF PXOSWOE
ceut
Normal.. ....................... Quenched. ..................... Normal. ........................ Normal. ........................ Normal.. ....................... Normal. ........................ Normal.. .......................
millirecads
0 0 0.002 0.005
44
4
25 15 10
16 34
0.01
7
0.05 0.5
4
O
I
49 59 74 100
Percentage As, 0,
FIQ.2. Fluorescence intensity of zinc silicate phosphors containing arsenic
of the favorable effect on decay without at least somewhat of a sacrifice in initial intensity. To elucidate these relationships, the data of figure 1 are replotted in figure 3 in terms of the intensity of the arsenic-free phosphor. I€ is interesting to note that the intensity levels of the second stage of decay rise with increase in arsenic content to a maximum that occurs a t about 0.05 per cent arsenic pentoxide and thereafter decreases. With 5.0per cent arsenic pentoxide the phosphorescence lay below what the phosphoroscope could measure.
EXAGGERATED PHOSPHORESCENCE OF ZINC SILICATE
PHOSPHORS
881
The arsenic associates itself chemically with the zinc oxide in the phosphor. When a mixture of these two oxides by themselves in the proportion to form Znl(AsO& was fired a t 720"C., a product was obtained which had lost only 6 per cent in weight and which lost nothing more in weight during 33 hr. a t 1OOO"C. This product w&s non-fluorescent under 2537 d.,with a low pale green fluorescence under cathode rays. When prepared with 0.5 per cent manganese, it formed a phosphor emitting a moderate rose fluorescence under 2537 d. and a deep brilliant red under cathode rays; there was no appreciable phosphorescence
2-
\
\
\B
I
in either case. When arsenic oxide was fired a t 750°C. in a mixture with silica, the former volatilized out completely, denoting that no stable arsenic silicate can be formed. When 0.02 per cent of zinc arsenate was added to the mixture for a zinc silicate phosphor and fired, the resulting phosphor had the same long, persistent phosphorescence as when prepared by the introduction of arsenic oxide. This demonstrates that the cause of the enhanced phosphorescence due to the use of arsenic pentoxide does not lie in the volatility of the oxide or in an evolution of oxygen from it, but simply in the presence of zinc arsenate itself.
882
HERW4N C. FROELICH AND GORTON R. FONDA
From a practical viewpoint it appears rather unfortunate that the arsenicstimulated phosphorescence occurs in the second stage of decay rather than in the first. These silicate phosphors are widely used in fluorescent lamps where they are subjected to an intermittent excitation depending upon the frequency of the supply current. In average installations with 60-cycle current this means sixty on-off flickers per second, allowing 16.7 millisec. for a complete cycle of excitation and extinction of the phosphor. This results in a very slight unsteadiness of the light which, however, becomes more noticeable in installations with only 25-cycle current. According to figure 1 the drop in intensity of phosphorescent zinc silicate is around 30 per cent during the first few milliseconds after excitation has ceased. Since the intensity level is high initially, the afterglow is of some benefit in reducing the flicker in green lamps. However, the intensity of phosphorescence in zinc beryllium silicate is much lower and, while the same rate of decay may prevail, it is therefore much less helpful in reducing the flicker of the practically more important white lamps. Other silicates show exaggerated phosphorescence as well. Cadmium silicate, activated with manganese and 0.01 to 0.05 per cent arsenic, shows an orange or pink afterglow .depending upon the amount of manganese present; zinc beryllium silicates, depending upon their composition, likewise give yellow to pink phosphorescence. In either of these phosphors the intensity of afterglow, however, is not as great as in zinc silicate. The effect of arsenic has not been studied yet in yellow and red zinc silicates. We find that arsenic is effective under cathode and x-ray excitation as well as under ultraviolet, giving essentially the same phosphorescent light in all cases. It becomes less effective in zinc silicates with a high concentration of manganese, such as 3 per cent or more. The effect can be killed completely by small amounts of lithium chloride added as a flux in the phosphor preparation. Lithium chloride appears to be the strongest agent in suppressing afterglow in silicates. The arsenic need not be added as the pentoxide; the trioxide, or any oxideyielding material, is equally effective. In fact, most qualitative experiments were carried out with arsenic trioxide which was added from a dilute aqueous solution. Nearly all arsenic added can be recovered quantitatively in the finished product, indicating its presence as some temperature-stable compound such as zinc arsenate. Among other elements tried to produce afterglow only antimony gave a somewhat similar effect in zinc silicate, but not by far as strong an effect as arsenic. The effect of other elements in the fifth or neighboring groups of the Periodic Table was either negative or insignificant. So far as our present knowledge goes, the effect of arsenic is confined to the silicates mentioned. There are, for instance, other silicate phosphors which likewise are activated by manganese, such &s calcium silicate and magnesium silicate. Their luminescence is excited by cathode rays. The introduction to them of 0.05 per cent arsenic pentoxide was found to increase the phosphorescence of the magnesium silicate only by an inconsequential amount and actually
EXAGGERATED PHOSPHORESCENCE OF ZINC SILICATE PHOSPHORS
883
to decrease the phosphorescence of the calcium silicate. On the other hand, there is a cadmium silicate phosphor activated by lead. Under 2537 A. i t emits a yellowish white fluorescence and under cathode rays a greenish blue, with a pronounced phosphorescence in both cases. The introduction to it of 0.05 per cent arsenic pentoxide served only to poison its emission under both
TREATMENT 01 PEOSPEOP
Am01
i
UTE
K
a m
psr cent
Piormal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quenched. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 0 0.002 0.005
72 48 34
0.010 0.050
20 20
29
forms of excitation, reducing the intensity of fluorescence and shortening the phosphorescence. The introduction of arsenic oxide had no effect on the phosphorescence of the zinc sulfide phosphors, the decay of which follows a different type from that of the silicates. It had no effect, furthermore, in stimulating phosphorescence in calcium and magnesium tungstates, which in their normal forms are virtually devoid of phosphorescence, or in borates, phosphates, chlorophosphates, or similar inorganic phosphors.
884
HERMAN C. FROELICH AND QORTON R. FONDA RISE IN FLUORESCENCE DURING INDUCTION PERIOD
The same factors that produce phosphorescence also give rise to an induction period in the emission of fluorescence. It has been shown (1) that when a phosphorescent phosphor is subjected to exciting radiation, the intensity of fluorescence emitted by it does not rise instantaneously to a stable equilibrium value but rather increases a t a measurable rate. In the case of zinc silicate, the introduction of excess silica was found to lower this rate. It has now been found that the rate of increase is slowed up still more markedly by these new factors,-quenching and the introduction of arsenic. The same procedure was used as before. The rise was found again to be exponential, following the monomolecular equation F - Ft = Fe-k', where F represents the stable intensity of fluorescence emitted when an equilibrium condition has been reached and F1 is the fluorescence at the instant t . The results are given in table 2 and are shown in the curves of figure 4. D1SCUSSION
The occurring of a second stage of phosphorescence as well as the existence of an induction period have generally been ascribed to the existence within the body of the phosphor of trapping states capable of retaining electrons sufficiently to retard their return to an excited manganese atom (3). In this respect a distinction has been made between the mechanism of the first and second stages of decay, in that the monomolecular character of the first was ascribed to the return of these excited electrons which of necessity remained free because of the saturated condition of the trapping states (1). Light sum is the name given to the total amount of luminescence that is emitted during the second stage of decay of phosphorescence. As the source of this emission lies in the electrons which are temporarily trapped, so the magnitude of the emission must be dependent upon the number of these electrons. The especially high value for the light sum that characterizes the phosphors containing arsenic as well as the long duration of their induction period signify an abnormally large number of trapped electrons. A simple experiment demonstrated that this in turn signifies that the presence of arsenic has increased the number of trapping states rather than increased the energy available for holding electrons in the normal number of such states. Two zinc silicate phosphors, with and without arsenic, were cooled in liquid air and excited by radiation with 2537 A. When they were allowed to warm up slowly, side by side, both began emitting phosphorescence simultaneously. The phosphorescence from the arsenic-containing phosphor, however, was more intense and more persistent as the heating was continued. Both quenching and the introduction of arsenic produce effects in exaggerated form that had already been observed, though to a much less extent, by the introduction of other foreign bodies, such as silica, boric acid, and ground porcelain: namely, an increase in the light sum of phosphorescence and an increase in duration of the induction period. The basis for these effects has been taken to lie in an increase in the number of available trapping states. It is presumable
TEERMAL DATA.
XV
885
that this is brought about in all these cases by the same physical agent. Inasmuch as quenching and the introduction of neutral material can better be regarded as producing the same physical effect,-namely, a disruption in the regularity of the crystalline lattice and consequently an increase in the occurrence of lattice defects,-not only does the conclusion previously arrived a t seem reasonable, that such defects serve physically as the location of trapping states for excited electrons, but it also seems reasonable to conclude that the introduction of arsenic or, more specifically, of zinc arsenate produces the same effect. The fact that zinc arsenate can itself be converted to a phosphor by activation with manganese is probably only incidental. As already noted, the presence of arsenic serves actually to reduce the luminescence of zinc silicate activated by manganese. SUMhfARY
Measurements have been made on zinc silicate phosphors containing traces of arsenic, with the aim of showing the character of their decay as well as of their initial rise in fluorescence during the induction period. The results are compared with other, though less effective, means for increasing phosphorescence, such as quenching and the introduction of inert material. It is concluded that the action of arsenic is the same as that of the other agents and consists in the formation of additional lattice defects which thereby increase the number of trapping stam. REFERENCES (1) FONDA,G. R . : J. Applied Phys. 10, 408 (1939). (2) FROELICA, H.C . : U. S. patent 2,206,280 (July 2, 1940); British patent 536,305 (May 9, 1941). (3) JOHNSON, R. P.: J. Optical SOC. Am. 29, 387 (1939).
THERMAL DATA. XV THE
HEATSOF COMBUSTION AND FREEENERGIES OF SOMECOMPOUNDS CONTAININQ THE PEPTIDE BOND HUGH M. HUFFMAN
William G. Kerckhof Laboratories of the Biological Sciences, California Inshtute of Technology, Pasadena, California Received May 20, 1948
In the preceding paper in this series (8) the heat-capacity data and entropy values for five compounds that contained the peptide bond were presented. In this paper are given the experimental results of the heats of combustion of four of these compounds. These results are then utilized, in conjunction with other data, to calculate the standard free energies of formation of dl-alanylglycine, glycylglycine, hippuric acid, hippurylglycine, and dl-leucylglycine.
