Chemical and Physical Stability of Silicate Phosphors

Abstract

The question is .raised whether phosphors, and in particular the silicate phosphors used in fluorescent or F lamps, are subject to deterioration either before application or during normal burning of the lamps. It is shown that Zn-Be silicate phosphors usually contain a small amount of free Mn20 8, lodged at the crystal surface, and responsible for slight discolorations and lower than "normal" fluorescent bright-heSS. This brown oxide forms when the phosphors are exposed to air or oxidizing atmospheres in a range of surface instability between roughly 350 ~ and 1,000 ~ C, either during cooling from higher temperatures or upon reheating. In reducing atmospheres or in vacuo this process of chemical deterioration is reversible. The phosphors may be stabilized by removal of superficial manganese. This is accomplished, for example, with an acid SO2 solution. In order to measure the brightness of phosphors taken from finished or burned F lamps, the powders must be freed of mercury and its compounds (oxide, nitride). This is best done by vacuum distillation at low temperatures. Phosphors removed from many lamps were demercurized and found to be about as bright as phosphors removed from unburned lamps of the same manufacture. Occasional gains as well as losses, both of the order of 1% to 3%, are accounted for. Recovery of flfll brightness is taken as one proof that the phosphors do not further deteriorate during lamp life. Other tests were made with jacketed lamps, such as a quartz lamp inside a glass envelope bearing the phosphor, and a quartz-vycor :~791 combination inner tube in a glass lamp coated with a special phosphor. In the former experiment, the phosphor was exposed to excited mercury and to short ultraviolet (uv) radiation for 600 hr. During this time the lamp output declined 25%, but the recovered phosphor was 103% of the original in brightness. The quartz-vycor combination served to demonstrate a strong afterglow which is excited by X 1,849 radiation in the mercury arc. Photographic and photoelectric measurements of this afterglow, made on lamps of standard design, showed that the /, 1,849-stimulated phosphorescence of this special phosphor depreciates at the same rate during lamp life as the full h 1,849 + 2,537stimulated fluorescence, as measured by the lumen output of the lamps. t Manulcript received February 15, 1945. $ Lamp Development Laboratory, General Electric Co., Cleveland, Ohio. 429 unless CC License in place (see abstract). ) ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-04-04 to IP

Phosphor Deterioration in Fluorescent Lamps

“…The equation implies three different deterioration mechanisms but does not permit identification. Also, since the accuracy with which experimental deterioration dependences can be measured is not too good, their comparisons with an equation containing six arbitrarily adjustable constants appears to be open to question, We observed a surprisingly good fit of experimental lamp phosphor degradations to a very much simpler formula B/Bo : exp (--~/t~) [1] where t is the time and B/Bo the brightness ratio with B0 as the zero-hour brightness. The only arbitrary parameter in this equation is the time constant, T, denoting the time necessary to let the brightness, B, de creas~ to exp (--1) ~ 37% of the original.…”

“…Several possible models have been proposed. (i) Absorption of mercury (or, possibly, mercury oxide) on the phosphor particle surfaces (1)(2)(3)(4)(5)(6). Involvement of mercury certainly does occur at least in some cases.…”

Relation of Some Surface Chemical Properties of Zinc Silicate Phosphor to its Behavior in Fluorescent Lamps

“…Froelich (18) concluded from his studies of Mnactivated silicate phosphors that oxidizing matter, presumably Mn ~+, is located at or near the surface of the phosphor crystals. Thus, he observed that Zn-Be silicate, which is isostructural with Zn~SiO~, activated by manganese acquired a tan coloration and high oxidizing potential when fired between about 350 ~ and 1000~ Experiments showed that the higher oxides of manganese did not develop during phosphor synthesis but only after the material was heated in the temperature range of instability previously mentioned.…”

“…Froelich (18) concluded from his studies of Mnactivated silicate phosphors that oxidizing matter, presumably Mn ~+, is located at or near the surface of the phosphor crystals. Thus, he observed that Zn-Be silicate, which is isostructural with Zn~SiO~, activated by manganese acquired a tan coloration and high oxidizing potential when fired between about 350 ~ and 1000~…”

Use of Radioactive Mercury to Study the Relation of Mercury to Depreciation of Fluorescent Lamps

“…It has been postulated (1,2) that mercury in one form or another is responsible for the darkening of fluorescent (F-) lamps. Some authors (3,4) have discussed the interaction between mercury and the fluorescent powder and proposed reaction mechanisms.…”