Radio age research, manufacturing, communications, broadcasting, television (1941)

» Crystallization, the final step in the synthesis of a phosphor, is rela- tively brief but spectacular. A quartz or platinum crucible, filled with the precisely compounded mix- ture of pure ingredients, is placed in an electric furnace operating at 2000-3000° F. At the elevated fur- nace temperature, the haphazard agglomeration of the various chem- icals becomes mobile and, naturally, the different varieties of atoms commingle to a homogeneous mass on a molecular scale. After some minutes, the white hot crucible con- tents are withdrawn from the fur- nace and allowed to cool. As the cooling proceeds, the slithering atoms move more and more slug- glishly until, at the crystallization temperature, the entire assemblage locks into perfect three-dimensional rows which are characteristic of a true crystal. The crystallization process is analogous to the manner in which trained soldiers snap out of the sprawling confusion of "at rest" into neat marching array. An accurately organized crystal- line condition is essential to good phosphors. The luminescent mech- anism usually involves electron transfer through extensive regions within the crystal. Manifestly, elec- tron transfer would be seriously impeded if the mass were a disor- dered jumble such as exists in glass, amorphous solids, and other non- crystalline materials. Incongruously, it is necessary to have carefully devised imperfec- tions scattered sparsely throughout the otherwise faultless phosphor crystal. Such scientifically produced faults are essential for persistent luminescence. The desired imper- fections provide temporary halting places or traps for the electrons en- gaged in the luminescent processes. Minute concentrations, viz., one part in a million, of cei'tain "for- eign" elements such as copper are suitable for this purpose. A very diminutive Alice in Won- derland who could act as an observ- er when a swift cathode ray or quantum of ultraviolet struck a phosphor crystal, might report the following happenings. One of the "bombed" atoms in the crystal would be stripped of its least tena- cious electron, which electron would wander through the crystal until it was trapped in one of the few im- perfections. Some time later, latent heat energy would again liberate the electron so that it could once more wander about until it chanced near its own "home" or another vacant site. On close approach, the electron would dive into the parent haven, which would acknowledge re- ceipt by emitting a momentary scin- tillation of light. Multiply that sim- plified act by "skintillions" and you have an inkling of the process whereby electric energy is con- verted into a television image or into a flood of light from a fluores- cent lamp. The electron guns of television Kinescopes or the in- visible radiations from agitated mercury atoms in fluorescent lamps furnish the cathode ray or ultra- violet quanta "bombs" which excite light from phosphors. Testing of the completed phos- phors is an art unto itself. It is necessary to provide and measure both visible and invisible radiations with accurate determinations of in- tensities ranging from those of starlight to that of sunlight. In- genious high - voltage electronic equipment is used to measure atom- ic spacings in crystals, wavelengths and energies of luminescent light, surface potentials of phosphors, TWO QUARTZ AND FIVE PYREX ELECTRON- ICALLY-CONTROLLED STILLS FURNISH QUADRUPLY DISTILLED WATER AND ACIDS —THE "LIFEBLOOD" OF LABORATORY OPERATIONS. and many other pertinent data. One rather impressive apparatus, the electronic phosphoroscope, makes measurements of light intensity within time intervals as short as millionths of a second or as long as days after excitation of the phos- phor has ceased. Our present knowledge of phos- phors is far from being exact or complete. Despite the great for- ward steps of the past decade, re- search on luminescent materials still relies considerably on scien- tific intuition for its direction. This state of affairs reflects the defiance of the subatomic domain with re- spect to science's attempts at de- tailed investigation. Although it is not possible to guarantee the predicted properties of untried phosphors, many lumi- nescent materials have been cus- tom-made to fulfill special require- DR. R. E. SHRADER, RESEARCH PHYSICIST, TESTS COMPLETED BUT UNTRIED SYN- THETIC PHOSPHORS WITH A RECORDING SPECTROKADIOMETER. [RADIO AGE 9]