real world of indu~tridchemi~try
edited by
R. J. Araujo Sullivan Park, Corning Glass Works, Corning, NY 14831
A material whose absorption spectrum changes when it is irradiated with light and reverts to its initial state upon cessation of irradiation is said to he photochromic. The change of absorption with time during and after irradiation at two different intensities is shown schematicallv in Fieure 1. A small number of homogeneous glitssrs, inrluding ciidmium tx)rosilicates ( 1 I and stronrlv rrrlured alkali silicates ( 2 ) .exhibit such behavior. In coniiast to the homogeneous &ses, some glasses are ~hotochromichecause thev contain. in a passive matrix, a suspension of small particles that are sensitive to irradiation. Iftht! particles rre small enough, noapprecinl~lelight scatkring orrurs and tht!system is triul;ipareni. The physical properties can he varied indruendet~tlvof the optic& properiiesto a much larger extent in these glasses than they can he in the homogeneous glasses. E x a m ~ l e sof the heterogeneous glasses are r hose containing ropper-cadmium halides (31and thos*:containing silver h a l i h r 4 ) . The latter are the most thoroughly studied and the only ones that have achieved commerical importance and consequently will he the only ones included in the present discussion. Photochromic glasses are made by ordinary glass-forming techniques. Salts of silver and halogens are added to the other ingredients such as sand, boric oxide, and sodium carbonate, and the hatch is melted a t a ~ ~ r o x i m a t e l1400°C. v Glass objects produced by the standaid techniques of pressing or drawing are not ordinarily photochromic as made because the silver a i d halogen ions &ally stay in solution in any fahrication process involving moderately fast cooling. Precipitation of the photoactive silver halide phase is accomplished by maintaining the glass object at approximately 600°C for about a quarter of an hour. The temperature dependence of the silver halide solubility that is required for the precipitation process is not found in every glass. In general, only glasses containing large amounts
472
Journal of Chemical Education
Figure 1.
induced absaption versus time.
ASSUMED BORON SPECIES
Figure 2. Assumed boron species
W. C. FERNELIUS Kent State University Kent. OH 44242
HAROLD 303 Chemsoh Systems. W~TTCOFF Broadway Inc.. Tarrytown. NY 10591
active halogen atom formed upon the relinquishing of an electron reacts with the organic matrix. XO
+ matrix
-
modified matrix
(3)
In contrast to this process, the electron is made available in photochromic glasses by the net conversion of a cuprous ion to a cupric ion. Electrons can tunnel from the silver speck to a cupric ion as part of the process by which the glass reverts to its original state if the cupric ion is sufficiently close to the silver speck. The time required for the cupric ion to diffuse to a position in the close proximity of the silver speck is the factor limiting ( 6 ) .The com~licatedinthe sneed of the "clearine" .. orocess . teraction uf tunneling and diffusion is rellonsihle for the fact that the rate of rlearinr is inflwnced hv the intensity of the radiation used to darken the glass (see Fig. 1). The tinv" snecks . of silver formed during the excitation of photochromic glasses give rise to an absorption spectrum that is fairlv flat across the entire visible range because they varv in the& geometric anisotropy and each differently shaped p article absorbs in a sliehtlv different region of the spectrum i7). The wavelength c&esponding to &e maximum in the aborption spectrum is plotted as a function of the anisotropy in Figure 3. When the electric vector is perpendicular to the long axis of the particle the peak absorption wavelength moves toward smaller values but only slowly. In contrast, when the electric vector is parallel to the long axis the peak absorption moves to longer wavelengths very rapidly with increasing anisotropy. Thus a reasonably small variation in the shapes of the silver specks produces a very broad absorption band. Several interesting phenomena derive from this fact. Bleaching with light limited to a narrow range of wavelengths produces color. Bleaching with polarized light causes the nhotochromic elass to become a nolarizine elass. Of course, when the glass allowed to fade tdits clear state it is no longer colored or nolarizine. " For reasons that are not well understcod, the glass possesses memory of its optical bleaching and when the " elass is aeain darkened bv. exposure to anv exciting . - light source it darkens to its previous color or state of polarization. The speeds of darkening and especially of fading can be varied over considerable ranges. Glasses which show no measuruhlr fading over 3 periudof 10 years may IN: vrry useful for informatim storage. Glmses that fade essentially to clarity in a small number of minutes are widely used as automatic sunglasses. A remote computer terminal utilizing photochromic glass as the face plate of the CRT was introduced to the marketplace in the late 'sixties. Other proposals for the use of photochromic glasses have included photographic contrast enhancement and contrast reduction devices, protective goggles for nuclear flash, hosts for secret messages (based on ideas discussed in previous paragraph) and even toy dolls that would apparently "tan" when brought into the sun. The orocesses involved in the ~recioitation and ~hotolvsis . . of si1vt.r halide particles in photochromic glasses are coml~licared indeed. I n the hands oi industrial researchers, this complexity has provided a fertile basis for the development of a myriad of useful materials and devices. ~
C/ a Figure 3.
WavelengM of maximum abswption versudelongation ratio.
of boric oxide are suitable for the production of transparent photochromic glasses. I:seful &si compositions are further limited co certain ranges of alkali and alkaline earth concentrations because onl; in such composition ranges does the boron change coordination as a function of temperature (5). Figure 2 indicates the three possible honding schemes for boron in oxide glasses. The dashes through the oxygen atoms are meant to imulv that those narticular oxveen atoms are covalently bond;dwto other boron atoms or &silicon atoms. Thus these oxygen atoms are referred to as "bridging" oxygen atoms. The oxygen atom not cut by a dashed line is not covalentlv bonded to network formers and carries a negative charge. A positively charged cation such as an alkali is loosely associated with such a "non-bridging" oxygen atom. The structure indicated as B2 tends to be stable a t high temperatures whereas the one indicated as B4 tends to he stable at low temperatures. This phenomenon is the basis underlying . .. the high temperature dependence of the silver halide solut~ilitys;, necessary to the formntion uf photochn~micglass. A halogen awm can replace a non-hridg&g oxygen atom in the B2 structure but cannot replace any of the bridgingoxygen atoms in the B4 structure. Thus when the transformation to the low temperature structure occurs, halogen is ejected from the network and is made available for the precipitation of silver halide. The change in color of the glass upon irradiation is analogc~usto the formation of a print-out image in photographic emulsions. In hoth systems electrons excited by the irrad~ation neutralize silver ions which aggregate into specks large enough to absorbvisible light. These processes are represented by the following equations hv+X--XO+eAgo.-,
+ Agt + e-
-
~
.
Literature Cited
(1)
Ago.
(2)
where X is a halogen atom. Process (1)would he reversible if X underwent no further reactions. The process is irreversihle in photographic emulsions because in such a system the re-
(3) Arauj0.R. J.,U.S.Patent3,325.299,1967. (4) Armirtead, W . H., and Stmkey, S. D., Science, 144.150 (1964). ( 5 ) Alaula. R. J.,J . Non-Crystoil. Sol.,42.209 (19801. (6) Arauj0.R J.,Bormlfi,N.F.,andNalsn,D.A.,Philoa. Mog.B.44.453 (1981). (7) Araujo, R. J., "Treatise on Materials Science: Vol. 12, (Editors: Tomaesw. M. and Doremus, R. H., Academic Press. New York. 1977.p 91.
Volume 62
Number 6 June 1985
473
