INSPECTION OF FLUORESCENT LAMPTUBES
FLUORESCENCE OF GLASS WOLDEMAR A. WEYL Pennsylvania State College, State College, Penna.
A classification of fluorescent glasses is can be used to find out how N INCREASING numfast the glass melt moves from ber of physicists, chemsuggested which arranges them according the doghouse to the working ists, and electrical ento the role played by the vitreous phase in end of the tank. Without imgineers are now engaged in producing fluorescence : fluorescent enparting visible color, such a work on fluorescence phenomamels containing crystalline phosphors cerium-containing glass can be ena. The use of fluorescent easily recognized under ultra(Lenard type), fluorescent glasses containglass and of glass tubings conviolet light where it emits a taining coatings of crystalline ing crystalline residues on which the strong blue fluorescence. fluorescent materials is revoluactivator is adsorbed (Fischer type), fluoAnother application is based itionizing our modern system rescent glasses containing atoms or moleon the observation that uraof illumination. This rapid cules in an “energetically isolated” state, nium oxide by itself is nontechnical development makes and glasses containing cations or anionic fluorescent, but glasses containit worth while to present a ,picture of present knowledge of ing traces of uranium can be groups as fluorescence centers. the fluorescent properties of easily recognized by their charThe quenching of fluorescence in glass glass. Excellent monographs acteristic green fluorescence is discussed and may be due either to heat, when exposed to ultraviolet and handbooks are available concentration, or foreign atoms. light. Uranium, therefore, has ‘dealing with the fundamentals of fluorescence, and many treabeen used as an indicator to determine t h e temDerature tises have been written on a t which glass formation starts. fluorescence and phosphorescence of crystalline products. I n By introducing fluorescent ions such as uranium, UO2++, the field of glasses, however, literature has little t o offer. or europium, Eu+++, into different base glasses, the scientist Morey’s authoritative book ( l a )does not even mention the fluohas started to interpret their emission spectra and to draw conrescence of glass. There are numerous observations scattered i n scientific journals, but most of them are incidental and in clusions on the atomic structure and constitution of the glasses. Such an interpretation is usually based on previous experience many cases the composition of the glass has not been recorded. Fluorescence phenomena of glasses have been studied with with the fluorescence of the same ion when present in crystalline material of known s-etry and atomic arrangement. various applications in mind. Small additions of certain The most important practical application of fluorescent (elements producing a characteristic fluorescence may help to glass is, no doubt, its use in the so-called cold light sources. identify a glass and distinguish it from competitive products. Additions of fluorescent materials have been suggested in Here its function is to absorb cathode and ultraviolet radiorder to learn more about the currents in a glass tank. Withation produced in gas discharge tubes and transform their out affecting the production, a small amount of cerium oxide energy into visible radiation. i oI35
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Vol. 34, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n order to survey the wide field of fluorescent glass, some kind of classification has to be made. One method mould be to group the glasses according to the elements responsible for the fluorescence. Here another type of classification is sugges ted in which the glasses are arranged according to the role played by the vitreous phase in producing fluorescence. According to this classification we shall start with the fluorescent enamels, where the role of the glass is merely that of a bond and where fluorescence is completely due to crystalline materials embedded in a glassy matrix. There is no sharp limit between these enamels containing a crystalline phosphor and another group of glasses in which fluorescence is brought about by the interaction of a nonfluorescent or a weakly fluorescent glass and a crystalline phase which by itself is nonfluorescent too. The next group of fluorescent glasses comprises cases where again the fluorescence is not caused by the glam structure proper; here the glass provides a n inert medium which separates certain atoms or molecules so that they are energetically isolated and can display optical properties similar to those in their vapor state. The last group deals with fluorescent glasses in the true sense of the word; fluorescence is brought about by cations and anionic groups which are constituents of the glass.
Fluorescent Enamels Fluorescent enamels, from the viewpoint of the physicist interested in fluorescence phenomena, have nothing characteristic to offer. They contain crystalline phosphors like those of the Lenard type where a host lattice is activated by traces of heavy metals. Most of the crystalline phosphors, such as sulfides, silicates, and tungstates, are stable up to relatively high temperatures so that they can be introduced into enamels in the same way as the ordinary opacifying agents stannic oxide, titanium dioxide, or zirconium dioxide. This method was suggested by Sauvag6 and by Guntz (17). The composition of the frit or the glassy phase in these fluorescent enamels does not differ principally from that of other enamels except that they are high in zinc oxide in order to presrent the zinc sulfide and zinc orthosilicate from being dissolved during firing. The follov ing batch composition, in per cent, might serve as an example: 20 boric acid, 13 calcium oxide, 28 dehydrated borax, 21 zinc oxide, 18 sodium silicate. Enamels of this type have been applied to metal parts such as watch dials, pressure gages, or similar instruments. Instead of adding a crystalline phosphor to the enamel frit, its composition can be adjusted so that zinc sulfide or zinc orthosilicate crystallizes out. Glaze compositions from which zinc orthosilicate crystallizes on cooling have been developed for decorative purposes and form one group of the crystalline glazes. As the crystals of zinc sulfide and zinc orthosilicate require an activator such as manganese, a manganese compound has to be added to the melt. The manganese concentration is determined by the optimum concentration for the crystal and by the distribution of the manganese ion between the crystalline and the vitreous phases. Zinc and calcium borates have been repeatedly studied. They can be activated by manganese. When rapidly cooled, they form a glass of weak fluorescence. On heat treatment, crystallization takes place readily and the fluorescence increases up to thirty fold. The simple composition of a zinc borate glass containing 57 per cent zinc oxide and 43 per cent boric acid, as well as the ease with which this melt may be obtained in both vitreous and crystalline condition, makes it a suitable object for scientific investigations (%), Also, sodium silicates activated by copper have been used (15) to compare the fluorescence properties of a substance in the crystalline
and vitreous state, respectively. I n nearly all cases examined and for all sources of activation, crystalline materials exhibit stronger fluorescence than the corresponding glasses.
Fluorescent Glasses Containing Crystalline Residues I n the relatively high-melting glasses of the normal sodalime type, crystalline phosphors such as zinc sulfide or zinc orthosilicate are rapidly dissolved and their fluorescent properties destroyed. It is just as unfeasible to use a Lenard phosphor in ordinary glasses as it is t o use an enamel opacifier to produce a n opal glass. Fischer (5) d i o studied the interaction between crystalline phosphors and glasses, found an original method to overcome this difficulty and to produce a phosphor even in a glass of a composition suitable for the manufacture of tubes. His method consists of a synthesis of the phosphor in the melt. The activating ion-for instance, manganese, bismuth, lead, or an element of the rare earth group-is added to the glass batch so that it dissolves and distributes uniformly in the melt. A crystalline material which can act as a host lattice is then stirred into the completed melt before the glass is worked out. Aluminum oxide, zinc sulfide, zinc selenide, or a mixture of a sulfate n i t h a proper amount of metallic aluminum have been found suitable for this purpose. Heating magnesium sulfate and aluminum metal leads to a n intimate mixture of aluminum oxide and magnesium sulfide, both of which can be actirated: 3MgSOI
+ SA1
3MgS
+ 4A1103
The resulting glass is cloudy, for it contains the undissolved residues of these crystals. Some of the activating ions niay have a chance to diffuse into the crystal lattice; others are merely adsorbed at the glass-crystal interface. The effect is similar in both cases. Even the adsorption of a n ion or a molecule has proved sufficient to produce fluorescence. Many organic dyestuffs, wliicli in their aqueous solution possess no or only weak fluorescence, exhibit strong fluorescence when adsorbed on aluminum oxide, silica gel, cellophane, or organic fibers. The formation of the fluorescent center by a mere adsorption process can be seen from: 1. The shift of the fluorescence color towards shorter wave lengths. 2. The shift of the spectral range which causes excitation. 3. The fact that in many cases a ne\v emission band originates after the crystalline material has been stirred into the glass. 4. An increase in storage capacity. A glass of the Fischer type has a longer afterglow than the hoiiiogeneous glass which contains the activating ion only.
Figure 1 illustrates point 4. Curve I represents the decay of fluorescence intensity for a glass activated by manganese. Curve I1 is the same glass, but after completion of the melt some zinc oxide has been stirred in. Curve 111 represents a n addition of zinc sulfide, and curve IV, an addition of a mixture of magnesium sulfate and aluminum. This type of fluorescent glass assumes an intermediate position between the previously discussed enamels and the true fluorescent glasses which will be discussed later.
Glasses Containing Atoms or Molecules i n an “Energetically Isolated” State Studies on fluorescence phenomena brought out the fact that fluorescence occurs only when the atom or the molecule absorbing the exciting radiation is not too much disturbed by the neighboring atoms. This principle of energy isolation is to prevent energy dissipation in the form of additional thermal motion.
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
I n a gas the conditions are relatively simple. If the concentration of fluorescent molecules or atoms increases beyond a certain critical value, quenching takes place. Instead of being forced t o emit the absorbed energy in the form of fluorescent light, the molecules have an increased probability t o collide with others during the short lifetime of their excited state. Such a collision leads to energy transfer and dissipation.
>
rime in seconds FIGURE 1
I n solutions the fluorescent molecules-for instance, organic dyestuffs-are always surrounded by other moleculesnamely, those of the solvent. The solvation” (that is, the interaction between solute and solvent) varies and is a function of the dipole character of the solvent. The stronger the solvation, the greater are the possibilities for energy transfer; that means the less fluorescence intensity we have to expect. As a rule, the fluorescence color shifts to longer wave lengths. There is no better example to illustrate this relation than the fluorescence of dimethylnaphtheurhodine in different solvents which was studied by Kauffniann and Beisswenger (9) :
Solvent
Dielectric Constant
Color of Fluorescence
Ligroin Benzene (CnHe) Ether Chloroform Ethyl benzoate Ethyl oxalate Benzyl cyanide Ethyl alcohol Methyl alcohol
1.86 2.3 4.36 4.95 6.04 8.08 15.0 21.7 32.5
Green Greenish yellow Greenish yellow Yellow Yellow Yelloe Orange Orange Red-orange
The strong solvation of inorganic ions dissolved in water is responsible for their complete lack of fluorescence. Even the solutions of those salts which show a brilliant fluorescence in the crystallized state, l i e the uranyl compounds and the complex platinum cyanides, are nonfluorescent. I n order to develop glasses of high fluorescence efficiency, the interaction between the fluorescence center and the surrounding has to be kept to a minimum. There are different methods of accomplishing such an “energy isolation”. ORGANICMOLECULES IN GLASSES. The structure of inorganic glasses consists of a continuous network of anions and cations rather than of an accumulation or an aggregation of individual molecules. Most organic materials, on the other hand, consist of electrically neutral molecules which in their
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crystalline and vitreous state are held together by van der Waal forces. If an organic molecule like fluorescein or terephthalic acid is dissolved in a low-melting glass such as boric acid or phosphoric acid, the continuity of the glass network is interrupted, for the ionic forces acting between the glassforming ions cannot extend to the neutral organic molecule. The organic molecules are more or less isolated and when thpy absorb ultraviolet radiation they have to emit the absorbed energy in the form of light in order to change from their excited state back to the normal state. The molecules resemble a vapor which has been “frozen in” for they are separated by large distances, such as in the vapor state, and they lack the chance to collide with one another. Systems of this type belong to the most efficient fluorescent materials. Unfortunately the nature of the organic molecules prevents their use as activators in commercial glasses and restricts their application to chemically unstable borate and phosphate glasses of very low melting temperature. Tiede and his collaborators (20) made extensive studies on boric acid glasses activated by different organic molecules, and Chomse ( I ) investigated similar systems on a metaphosphate basis. The instability and water solubility of these systems, however, exclude their use for most technical purposes. COVALENT MOLECULES IN SILICATEGLASSES. I n normal silicate glasses organic substances would be destroyed by the high melting temperature. However, certain inorganic compounds resemble the organic molecules in the type of bonds Cadmium sulfide is a typical example. The cadmium and sulfide ions cannot be regarded here as separate individuals held together atic forces. The outer electronic shells are defor a way that a covalent bond results, just as in orga s. If cadmium sulfide is dissolved in a silicate glass, it can play the role of an activating molecule and exhibit fluorescence. Cadmium sulfide crystals do not fluoresce in the visible region. I n the glass, however, the cadmium sulfide molecules are isolated and therefore have to emit their energy. The same is true for cadmium sulfide which has been absorbed a t the surface of an inorganic or organic medium. If cadmium acetate is adsorbed on filter paper or on activated alumina, it can be converted into a molecular dispersion of cadmium sulfide by treating the dried material with gaseous hydrogen sulfide. i n case this treatment is applied in the dark under a fluorescence lamp, a brilliant yellow light emission can be observed as the reaction proceeds. Aggregation and recrystallization of the sulfide shift the fluorescence t o the red, and the crystallized mineral greenockite or the cadmium sulfide which has been precipitated from aqueous solutions emits only in the infrared region when excited by ultraviolet or blue rays. Introducing cadmium sulfide into a glass batch and cooling the melt rapidly results in a colorless glass which contains the cadmium and sulfur ions separated and statistically distributed in the network. Reheating such a glass up to the softening range causes “striking”, which means development of the yellow color typical for molecular undissociated cadmium sulfide due t o atomic rearrangement. The glass remains clear as long as the groups of cadmium sulfide molecules are too small to cause bending and reflection of the light. Excessive or prolonged heat treatment brings about crystallization and results in turbidity and finally in the formation of well crystallized hexagonal plates of cadmium sulfide. As a consequence of this aggregation the fluorescence of the glass shifts from greenish yellow to orange and red, and finally disappears in the infrared. Jaeckel (8) records that for some commercial yellow filter glasses the fluorescence is shifted with the absorption edge. All glasses have the same composition but they are heated for different lengths of time between 640’ and 720’ C.:
INDUSTRIAL AND ENGINEERING CHEMISTRY
1038 Absorption Edge
Color
425 mp 460 475 490
Whitish Yellow Orange Red
Fluorescence------Emission Range 670-450 mp 670-510 670-550 600-630
Cadmium sulfide glasses are commercially used as yellow filters for photographic purposes and in the form of tubings
FORMING TCRINGS FOR SEOK SIGNS
for neon signs. I n their manufacture, iron as an impurity has to be carefully avoided, for even traces of iron decrease the fluorescence. The solid solutions or mixed crystals between cadmium and iron sulfides do not possess the desirable sharp absorption edge of the pure cadmium sulfide glass. I n cadmium sulfide glasses the absence of iron can be easily determined, for only in the iron-free glasses is the color independent of thickness. The color of iron-containing glasses might still be a pure yellow in thin layers, but i t will look brownish when the disk is viewed edgewise. FLUORESCENCE CAUSEDBY METALVAPORS. Of all fluoresccnt systems the metal vapors are the most efficient. Most metal vapors exhibit resonance fluorescence; that is, under proper temperature and pressure conditions metal atoms are in a position to emit light quanta of exactly the same size as those they absorb. Metals are only slightly soluble in molten salts or glasses. When a glass melt containing metal atoms is cooled rapidly, the atoms are “frozen in” in this state of subdivision. They remain energetically isolated, embedded in a rigid glassy matrix. Obviously such an atomic dispersion represents a very unstable state, and the atoms possess a strong tendency to aggregate and form crystal nuclei. This aggregation to crystals of colloidal dimensions is the reason why the ruby color develops in a chilled copper or gold ruby glass on reheating. The metal atoms present in the chilled colorless glass exhibit the properties characteristic of their vapor state and not of their compact state. They lack the intense light
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absorption and metallic gloss of the compact state, but they do show fluorescence just as in the vapor state. As aggregation progresses, the properties gradually change from those of the vapor state to those of the metallic state. Color appears and fluorescence disappears. I n many glasses the origin of fluorescence is due to metal atoms. The best known example is that of atomic silver (24) but also zinc, cadmium, bismuth, thallium, and lead introduced into glasses seem to be able to form metal vapors under proper conditions. Chilled copper ruby glasses have an intense brown fluorescence. It is not possible, however, to decide whether this fluorescence i s caused partially by atomic copper or is due entirely to the cuprous ions present. Cuprous ions are known to exhibit fluorescence in crystalline cuprous chloride. This fluorescence is very similar to that of copper glasses, so that we are justified in assuming that most of the fluorescence is due to the monovalent copper ion and not to the atomic copper. When selenium compounds are introduced into a soda-lime glass and slightly oxidizing melting conditions are maintained, a pink color results. The selenium pink color is produced by the element selenium in atomic subdivision. Its concentration in glass is limited by its high vapor pressure. Such a glass can be excited to fluoresce when irradiated with green light, exactly like the selenium vapor. The red fluorescence can be obtained only when the iron content of the base glass is low and when it is free of lead oxide. Selenium amber glasses whose color is due t o compounds of selenium with lead or iron are nonfluorescent. The formation of selenium atoms in glass is a consequence of rather complicated chemical reactions and cannot be used as an example for general methods t o produce metal atoms in glasses. Silver in this respect provides a much better example. When silver compounds are introduced into a sodalime glass in amounts not exceeding 0.2 per cent and when oxidizing melting conditions are maintained, a clear colorless glass results. I n such a glass the silver ions take part in the glass formation and assume similar positions like the sodium ions. Silver ions are not very stable a t high temperature but have a tendency to form neutral atoms. The tendency of metal ions to decrease their valency with increasing temperature (the law of valency isobars) is characteristic for many glass-forming oxides. It is, however, not generally realized that the dissociation of many oxides in glass extends as far as the neutral atom. It is generally accepted that hexavalent chromium or trivalent iron which are stable in low-melting glazes change into trivalent chromium and divalent iron, respectively, if the glass is melted a t high temperature; but it is not so well understood that in glasses containing Pb++, Zn+r, Cd++, Bif++, and similar metal ions a change into the neutral atoms may take place. I n other words, the oxides PbO, ZnO, CdO, and RizOs dissociate to a certain extent into the elements a t the melting temperature of the glass. Gold and platinum compounds dissociate readily when introduced into a low-melting glass. Silver compounds dissociate partially but require a relatively high melting temperature or the help of a reducing agent. The fluorescence of these glasses is not very strong, for the concentration of metal atoms is small. The highest concentration of metal atoms
.. .
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obtainable by thermal dissociation depends upon the solubility and the vapor pressure of the metal. Exceeding the solubility leads to cloudy glasses or even to droplets of the metal a t the bottom of the crucible. The temperature required to produce metal atoms by mere thermal dissociation depends upon the electropositive character of the metal. I n silicate glasses melting temperatures of 1400' to 1450" C. are sufficient to form silver atoms but not tin atoms. If, however, tin oxide is introduced into pure silica glass, the high melting temperature leads to atomic tin. Such a glass (Stannosil) has been found to exhibit strong fluorescence (11). I n order to increase fluorescence intensity, methods have to he found to reduce metal ions in glass to the element a t temperatures sufficiently low to prevent their aggregation and crystal formation. One way to accomplish this is by bombardment with electrons. If a glass containing silver ion is exposed to cathode radiation, the electrons combine with the silver ions and neutralize their positive charges: Ag+
+ E = Ag
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light. The brownish tint they assume is caused by the formation of metallic arsenic. Especially if cerium compounds are introduced simultaneously with the arsenic (3) a rapid solarization can be observed according to the equation:
+
5As+++ = 3As+++++ 2As As+++ 3Ce+++ = 3Ce++++
+
+ As
The trivalent arsenic ions are reduced t o the metal a t the expense of certain ions which are oxidized into higher states of valency. Similar reactions are responsible for the darkening of oxides and pigments, especially when suspended in organic materials; titanium, thallium, and zinc compounds have been studied in this respect (24). They are reduced by organic material such as glycerol, citric acid, or resins. When zinc oxide is embedded in certain plastics, solarization leads to atomic zinc and fluorescence results. The formation of metal atoms in crystals has been known for some time. Sodium chloride and other halides discolor and become fluorescent when irradiated with radium. The color of the blue rock salt has been traced back to metallic sodium. Lithium fluoride assumes a red color (19) when irradiated with ultraviolet radiation. The puzzling observation that even a very pure zinc sulfide and zinc oxide, completely free from the usual activators, can exhibit fluorescence has found its explanation (18) from the partial dissociation of zinc sulfide a t high temperature and the formation of free zinc atoms distributed in the flaws of the crystal lattice. When certain salts such as sodium chloride are fused with small additions of silver salts, a colorless, nonfluorescent mixed crystal results. Treatment with hydrogen around 100" C . cause8 the formation of silver atoms and, therefore, fluorescence. These examples might be sufficient to illustrate that metal vapors are a possible source of fluorescence. The phenomenon is in no way characteristic for glasses, nor are the methods t o produce metal vapors in a solid phase limited to glasses. Further systematic investigation along these lines-for instance, the hydrogen treatment of crystals containing heavy metal ions in solid solution-will probably extend the field of phosphors.
Cathode rays provide a powerful tool for reducing ions in glass. Care has to be taken not to reduce the silica; othermise a brown discoloration results. Cathode rays have been found particularly useful when silver ions are to be reduced in phosphate glasses where they are much more stable than in silicate glasses. When a glass containing silver ions is treated with hydrogen in a temperature range between 100" and 200" C., reduction to elemental silver takes place, while the rigidity of the glass does not allow the atoms to aggregate readily. The result of such a treatment is a glass which contains neutral silver atoms instead of silver ions. The temperature for the hydrogen treatment has to be chosen so that the diffusion speed of the hydrogen is high enough to make the reduction possible but the diffusion speed of silver atoms is still too low to cause aggregation. I n silicate glasses containing about 0.125 per cent silver, the reduction can be completed at 150" C. within an hour when fine powder is used. At higher temperature, but still below the softening range the diffusion speed of the silver atoms becomes noticeable. Whenever silver atoms migrate through the glass network and collide, Cations or Anionic Groups as Fluorescence they stick together and gradually build up a crystal nucleus. , Centers With the formation of silver crystals the colorless glass becomes yellow, brown, and finally gray, and the fluorescence I n the previous section fluorescing systems were discussed decreases. Table I illustrates this change of a soda-lime glass in which the glass acts as an inert medium separating neutral containing 0.125 per cent silver. When cooled rapidly from atoms, such as silver and tin, or covalent compounds, such as fused conditions, such a glass is colorless with a weak, just cadmium sulfide, so that they can display their fluorescence noticeable fluorescence. properties. This section will deal with glasses whose fluorescence is caused hy glass-forming cations or anionic groups. The TABLEI. CHANQE I N SODA-LIMEGLASSCONTAININQ 0.125 absorption spectra of cations in glasses have a more dePERCENTSILVER tailed, fine structure than those in aqueous solutions (%). -FluorescenceThis indicates that the electronic transitions in the absorbing Treatment Color Intensity ion are less disturbed in the rigid glass than in liquid systems. Melted under oxidizin conditions, cooled rapid1 owfered Bluish white Very weak The same, even to a higher degree, applies to the fluorescence Treated with &%ogen 5 hr. at spectra. No ion of the transition elements can produce Yellowish white Very strong 1200 c. Reheated for 5 min. to fluorescence in aqueous solutions. The probability of'emitYellow Strong 175' C.. 2200 c. Yellow-brown Medium ting light can be considerably increased when the ion is freed Brown Weak 275' C . of its hydration shell and introduced into sulfuric acid. I n Brown Weak 325' C . Dark brown Verv weak 3750 c. concentrated sulfuric acid the uranyl ion is able to fluoresce. 4300 c. ...... No -fluorescence I n tetraphosphoric acid even the divalent manganese can be excited to fluoresce. I n silicate glasses as well as in borate glasses, the number of fluorescing ions is much greater. Another type of reaction leading to free metals in glass INFLUENCE OF VALENCY. With few exceptions the outer should be mentioned even if it has not yet been used to proelectronic orbits of an ion are the seat of light absorption and duce fluorescent systems. Glasses containing trivalent aremission. If the outer orbits are changed by splitting off senic or antimony are liable to discolor when exposed to sunelectrons, the optical properties of the element are changed
INDUSTRIAL AND ENGINEERING CHEMISTRY
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fundamentally too. KO simple relation between the properties of the different states of ionization of one element can be expected.. As in their chemical properties, each state of ionization has to be treated as an individual. Most of the coloring ions which can be introduced into glasses occur in different states of valency, and the equilibrium between them depends upon the oxidizing or reducing conditions, the me1t)iiig temperature, and the composition of the base glass. With the exception of the europium ion, which in the divalent state exhibits a n-eak green and in the trivalent a strong red fluorescence, no other elements are knov-n which fluoresce in glass in more than one state of valency. In the case of Ce+++,Cu+, or Mn++it is the lower state of oxidation, and in the case of V++++++and V+++++ only the highest state of oxidation ~vhichshows fluorescence. Oxidizing or reducing melting conditions, therefore, greatly affects the fluorescence of glaeses. The same is true for the melting ternperature. It has been mentioned before that high melting temperature favors lower valency. A phosphate glass containing uranium loses its fluorescence when heated too high. That has been attributed to the loss of vater ( I S ) . The reason is that hexavalent uranium (the only form which fluoresces) changes into the tetravalent state which is nonfluorescent. Heating such a glass with water and remelting it a t low temperature re-establishes fluorescence; this behavior seems to confirm the conclusion that water is an essential constituent of fluorescent uraniurri glasses. Such a treatment, however, allom the tetravalent uranium ion C++++to reoxidize and form the original hexavalent uranium, U 0 2 + ~ . ISFLCENCE OF COORDIKATIOI~ NUMBER.In most glasses the fluorescent ion is surrounded by oxygens, and this surrounding has to be taken into consideration if fluorescence is to be fully understood. I n the case of uranium glasses it can be seen how important the surrounding oxygens are, for only glasses which contain the C02++(uranyl) groups show fluorescence ($6). Without a change in the state of ionization (in both cases we have to deal with the hexavalent uranium) the fluorescence is destroyed when UO,-- groups (uranates) are formed. This is in accordance with the properties of crystalline uranium compounds where only the uranyl salt's fluoresce but not the uranates. Base glasses favoring the formation of uranvl mourn. such as borosilicates and Dhosahates. produce the strongest fluorescence. On the other hand, glasses rich in alkali, lead, or titanium favor the formation of uranates so that these additions hasre a quenching effect on fluorescence. In the case of ~anadiuiiithe situation is just the reverse. Only the anionic group \TOl--- is able to fluoresce. This again is in agreement with crystalline vanadium compounds. From all vanadium salts only some vanadates are able to fluoresce; zinc vanadate is outstanding with its stiong yellon, fluorescence. The decisive influence of the coordination number on the optical properties of an ion has been recognized by Hantzsch, n h o attributed the various colors of cobalt salts to the position of the cobalt ion in the crystal and to the number of atoms or anionic groups which surround each cobalt atom. He even v e n t so far as t o say that the number of surrounding atoms has a much greater influence on the light absorption than their nature. The color change from pink to blue which solutions of cobaltous chloride exhibit on heating is an indication that even in solutions the change of coordination is a chief factor in determining the absorption spectrum. Similar changes can be observed in glasses where the coordination numbers of the glass-forming ions and, therefore, the absorption spectrum are subject to change with temperature. Manganese, the most frequently used activator in silicates, varies in fluorescence color from green to yellow, orange, and Y
u
*
I
Y
Vol. 34, No. 9
red. Linwood and \Teyl (10) recently showed that two different, types of emission spectra have to be attributed to two different types of coordinations. In glasses, as well as in crystalline silicates, green fluorescence occurs when the )In++ ion is surrounded by four oxygens. I n a more spacious surrounding (that is, a t the center of an octahedron whose corners are oxygen atoms) the fluorescence color of the divalent manganese ion is orange or red. INFLUENCE OF BASE GLASS. The t w o most important factors determining the fluorescence of an element' in glassthe state of ionization and the coordination number-are both affected by the coniposit'ioii of the base glass. No simple relation between fluorescence and glass composition can therefore be expected. However, soiiie general rules can be applied to both fluorescent spectra and absorption spectra of glassforming ions. The fine structure of these spectra is greatly impaired by t'he alkali content of the glass. Comparing the different alkali ions, we find that their perturbing action decreases from lithium to sodium and potassium. Potassium glasses, as a rule, give the brightest colors, and their spectra may show fine structure even when the corresponding sodium glass does not. A similar but weaker influcnce is exerted by the divalent ions where, again, the large barium ion is more suitable for fluorescent glasses than the smaller calcium. The influence of these basic oxides on the fine structure was studied by Rosenhauer and Weidert (16) who used neodymium as a color indicator. Obviously, a relation exists between the field strengths of these ions and their perturbation effe As far as the anionic groups SO4----, BO3---, and Pod--are concerned, a similar influence exists, but in comparing silicate, borate, and phosphate glasses, other factors have to be considered as w l l . A theoretical interpretation of the emission spectrum of an ion as it is modified by different base glasses could be given oiily in the case of the trivalent europium ion. For this ion Gobrecht (7) was able to calculate and predict' splitting and shift of the spectral lines by electric fields of different symmetry and intensity. On this basis Tomascliek and Deutschbein (21) used europium as a fluorescence indicator to draiv certain conclusions on the symmetry of the glass structure.
.
Quenching Fluorescence in Glass TEXPERATURE. I n glasses. as in crvstals. fluorescence is greatly impaired when %lie temperature- is increased and vice \'etsa. Most of them show a more brilliant fluorefcence a t the temperature of liquid air. The reason for this temperature quenching can be easily understood, for the stronger the thermal motion of the surrounding atoms, the greater nil1 be the chance for collisions of the second-order and energy dissipation. The temperature coefficient of fluorescence is determined by the geometry and the nature of the neighboring atoms. The greater the polarizability of the surrounding atoms, the more pionounced is the influence of temperature. Oxygen or fluorine atoms n i t h their lorn polarizability are therefore much more suitable constituents for the desTelopment of phosphors, fluorescing a t room temperature, than bromine, chlorine, and iodine. Most chlorides and bromides hich are fluorescent a t the temperature of liquid air lose this property a t room temperature. For manganese ions as activators in glasses and crystals it can be shown that the geometry of the surroundings, especially the coordination number, affects their temperature quenching. The decrease of fluorescence of glasses with temperature is generally knonn. Gibbs ( 6 ) determined the changes of the emission spectrum when uranium glass \%as heated and cooled. Increasing temperature not only decreases the intensity but shifts the bands to longer nave lengths and causes the fine structure to disappear
September, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
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These reversible changes of fluorescence are different from quenching properties are much weaker. This is certainly one the irreversible changes observed in cadmium sulfide or silver of the rearons why phosphate glasses are suitable for fluoglasses of different heat treatment which were discussed above. rescent tubings. CONCENTRATION. Typical for most fluorescing systems When the manufacture of fluorescent glasses and crystals (crystal or solution) is the decrease of fluorescence when the was started on a commercial basis, their high prices as comconcentration of the activator exceeds a certain critical pared with the relatively low cost of the raw materials lured value. This concentration quenching can be observed in many companies into the field of producing fluorescent glasses too. The optimum concentration of an activator as a materials. Despite the apparent ease with which crystalline rule is higher in glasses than in crystals, but varies widely zinc orthosilicate or any fluorescent glass can be produced, with the nature of the ion and its role in glass structure. For many of the new enterprises were condemned to failure. The manganese glasses, concentration quenching increases with chief reason was the difficulty of obtaining phosphors of high the probability that two manganese ions may become tied up efficiency. Successful production of fluorescent materials has as a first requirement the elimination of certain impurities, to the same oxygen. For cerium 2-3 per cent presents the optimum concentration. especially iron, to an extent not known in other branches of FOREIGN ATOMS. I n aqueous solutions certain anions such induptry. as chlorine, bromine, or iodine are known as typical quenchers. The blue fluorescence of a diluted solution of quinine sulfatr iq Acknowledgment completely destroyed when chlorine ions are added. West and Jette (3%’)arranged ions according to their quenching The photographs are from A. J. Phillips’ book on “Glass: effect and found that quenching is proportional to the polariThe Miracle Maker”, published by the Pitman Publishing zability of the quenching anion. The softer or the more Corporation, through the courtesy of Ceramic Industry. deformable the outer electronic configuration of a n anion, the more easily it diverts the energy of the excited cation. Literature Cited As far as oxygen can be replaced in glass by these anions, the same trend applies (23). Addition of bromides or iodides to (1) Chomse, H., 2 . anorg. allgem. Chem., 233, 140-4 (1937). (2) Cohn, B. E , J . Am. Chem. Soc., 55, 953-7 (1933). uranium or cerium glass decreases fluorescence, but man(3) Eckert, F., and Schmidt, K., Glastech. Ber., 10,80-5 (1932). ganese glasses are much less affected. (4) Fischer, H., French Patent 767,436 (1934); U. S. Patent Under proper conditions nearly all elements which can be 2,049,765 (1936). introduced into a glass can be excited to fluoresce. The only ( 5 ) Fisoher, H., Glastech. Ber., 16, 162-3 (1938). (6) Gibbs, R. C., P h y s . Rev., 28,361-76 (1909); 30, 377-84 (1910); exception so far seems to be iron. Despite its characteristic 31,463-88 (1910). ultraviolet absorption iron is not only nonfluorescent but also (7) Gobrecht, H., Ann. P h y s i k , 28, 673-700 (1937). impairs the fluorescence of other ions to a remarkable degree. (8) Jaeckel, G., 2. tech. P h y s i k , 7,301-4 (1926). (9) Kauffmann, H., and Beisswenger, A , , 2. physik. Chem., 50, T o most fluorescent glasses even traces of iron are detri350-4 (1905). mental. On the other hand, if a glass can be melted reason(10) Lmwood, S. H., and Weyl, W. A . , J . Optical SOC.Am., 32, 443 ably free from iron, even small amounts of activators are (1942). sufficient to make it fluoresce. According to Fischer (4, (11) Maddock, A. J., J . SOC.Glass Tech., 23,372-7 (1939). glasses containing less than 0.005 per cent ferric oxide can be (12) Morey, G. W., “Properties of Glass”, 1938. (13) Nichols, E. L., and Slattery, M. K., J. Optical Soc. Am., 12, activated by as little as 0.001 per cent lead, 0.0005 per cent 449-66 (1926). uranium, or 0.0005 per cent samarium. The influence of iron (14) Renz, C., Helu. Chim. Acta, 2,704-17 (1919); 4, 950-68 (1921); on flporescence is a typical quenching effect. It cannot be Goodeve, C. F., T r a n s . Faraday Soc., 33, 343-7 (1937). explained by its ultraviolet absorption or by a permanent (15) Rexer, E., Glastech. Ber., 16, 90-1 (1938). (16) Rosenhauer, K., and Weidert, F., Ibid., 16, 51-7 (1938). chemical reaction with the fluorescence centers. The absorp(17) SauvagB, F., French Patent 579,284 (1923); Guntz, A , , I b i d . , tion spectrum of the fluorescent ion remains unchanged 582,407 (1923). whether iron is present or not, and that is the surest indiYlS) Schleede, A,, 2. angew. Chem., 50, 908 (1937). cation that no chemical reaction has taken place. It is (19) Schneider, E. G., J . Optical Soc. Am., 26, 305 (1936). (20) Tiede, E., and Wulff, P., Ber., 55, 588-97 (1922). difficult to understand the strong effect of iron, especially (21) Tomaschek, R., and Deutschbein, O., Glastech. Ber , 16, 155-63 since iron is effective even when separated from the fluo(1938). rescence center by several hundred atoms. There is no doubt (22) West, W., and Jette, E., Proc. R o y . Soc. (London), A121, 299that the quenching effect is connected with the ability of 307 (1928). (23) Weyl, W. A.,Glass I n d . , 23, 135-7 (1942). iron to form both Fe++ and Fe+++ ions. The equilibrium (24) Weyl, W. A., Sprechsaal, 70,578-80 (1937). between these two ions in glasses makes iron a willing donor (25) Weyl, W. A., VerOfentl. Kaiser Wilhelm-Institut Silikatforsch. and acceptor of electrons. I n those phosphate glasses where Berlin-Dahlem, 7, 167-203 (1935). iron is tied up as a complex anion rather than a cation, its (26) Weyl, W. A., and Thumen, E . , Sprechsaal, 67, 95-7 (1934).
