Phosphors for CRTs

Phosphors for CRTs are predominantly based on zinc and cadmium sulfides. Selected CRT phosphors are listed in Table 1 which also includes their precursors, their P-numbers , and their uses. (The system used to classify CRT phosphors came from JEDEC, the former Joint Electronic Device Engineering Council, now JT- Committee on Phosphor and Optical Characteristics within the Electronic Industries Association. JEDEC established the original code for these phosphors in terms of P-numbers .8 Many manufacturers of CRTs use these when specifying phosphor screen characteristics.) Although the precursors are tabulated, the mole ratios used in the syntheses are not. 

Different routes have to be chosen in cases where the concentration of dopants is very low, e.g. in the ZnS phosphors for CRT application. Here, the dopant concentrations are of the order of 100 ppm (by mole). Minute amounts of these activator ions can be precipitated onto ZnS grains, e.g. by preparing a suspension of ZnS in... 

Optical phenomena based on electron transition Optical adsorption (electrochromism, photochromism) Luminescence (fluorescence, electroluminescence) EC display Chromatic glasses Optical switch Phosphors for CRT, lump, etc. Solid state laser EL display... 

P-1 Zn2Si04 Mn2+(Pb2+) ZnO, Si02, MnC03, (PbF2) Green-emitting (525 nm) phosphor for display (CRT)... 

Rare earth elements have earned an important role in the CRT (cathode ray tube) phosphor industry - a highly significant one for both the color TV and the rare earth businesses. This paper will review how rare earths earned this role, and will attempt to build a framework for discussion of future use of rare earths in CRT phosphors. Color TV phosphors will be emphasized since color TV is the largest market for CRT phosphors both generally, and in particular, those which contain rare earth elements. 

An already substantial and rapidly growing market for CRT phosphors and tubes is in the area of data displays, both alphanumeric and graphic, e.g., computer terminals and word processors. In spite of dramatic advances in other technologies, CRT s are still the most cost effective way to present information, and very likely always will be. Most data display tubes do not use rare earth phosphors because of their high cost, rare earth phosphors find use only when there is a compelling need for their special properties. At the present time, this is limited to the use of Eu3+ reds in tricolor tubes that use the same shadow mask principle as conventional color TV. 

Several areas of recent research activity that have an impact on rare earth usage in CRT phosphors have already been mentioned—pigmented phosphors for color TV, and voltage penetration and current saturable phosphors for two-color displays. 

Eu O consumed, totaling 6-7 tons in the U.S., is used for phos-phor manufacture—80% to 90% of this for CRT s, the remainder for lighting. About 2/3 of the Y O consumed ( v 100 tons U.S.) is used for phosphors. This fraction might decrease if some other envisioned uses for yttria-based ceramics, e.g., automotive emission sensors, reach fruition. 

The green phosphors include GdaOaSiTb and Y2SiOg Tb. Because stability, brightness output with current density and body color changes are a major problem in projection CRTs, phosphors for this application have been evaluated in terms of ... 

Uses. The main appHcation for strontium is in the form of strontium compounds. The carbonate, used in cathode ray tubes (CRTs) for color televisions and color computer monitors, is used both in the manufacturing of the glass envelope of the CRT and in the phosphors which give the color. 

For the central measurement, the field of view is rectangular (e.g., 0.3 by 1.25°) and for other eccentricities, the field is an annular arc similar to that provided in minimum motion photometry. The use of a CRT monitor introduces the same problem as the use of LEDs in HFP, namely, the broadband nature of the screen phosphors, and a correction must be made before reporting the peak MP optical density. A system that could employ lamps and filters instead of a CRT monitor would be difficult to design because of the complexity of the visual stimulus. 

It is worth summarizing at this point the different excitation methods used for phosphors that will be referred to throughout this chapter. There are three types photoluminescence (PL) which is based on initial excitation by absorption of light, cathodoluminescence (CL) which is based on bombardment with a beam of electrons, as in a cathode ray tube (CRT) and electroluminescence (EL) which is based on application of an electric field (either a.c. or d.c.) across the phosphor. 

The entire CRT industry is based on the quality and reproducibility of the phosphors used. As stated earlier the phosphors used for the different kinds of CRT displays are constantly being researched and improved. For high-definition television screens the phosphors must be prepared with great control over powder morphology and particle size (see Section 9.15.4.2.1). 

P-15 ZnO Zn ZnO (reducing atmosphere) Used originally in a flying spot scanner is applicable for any CRT requiring a highly visible screen and a fast decay. Green-emitting phosphor (515 nm). 

VFD tubes operate by the same principle as CRTs but use low-energy electrons (10-100 V) to excite the phosphor. The electrons are emitted from a cathode (wire) and are accelerated and controlled so that they bombard a phosphor-coated anode, causing the phosphor to emit light. ZnO Zn (green-emitting) phosphors were used in early VFD devices, but as the application range expanded, demand developed for multicolored displays.29... 

Precipitation may also be employed in solvents other than water. For example, nanocrystals of ZnS Tb could be prepared by a coprecipitation was effected by addition of aqueous sodium sulfide to Tb(N03)3 and zinc acetate dissolved in methanol.114 ZnS TbF3 and ZnS Eu were prepared similarly. The photoluminescent intensities of the nanocrystals of ZnS Tb and ZnS Eu prepared in this way were 2.5 and 2.8 times stronger than those of bulk (conventionally prepared) phosphors, and these nanocrystals have been proposed for FED, EL, PDP, and CRT applications.114... 

There are not many uses for dysprosium. Scientists continue to experiment with it as a possible alloy metal (it has a high melting point) to be mixed with steel to make control rods that absorb neutrons in nuclear reactors. There are only a few commercial uses for dysprosium, such as a laser material and as a fluorescence activator for the phosphors used to produce the colors in the older TV and computer cathode ray tubes (CRTs). When combined with steel or nickel as an alloy, it makes strong magnets. 

The manufactnring process for the important zinc and cadmium sulfide phosphors involves precipitation of the sulfide from purified salt solutions, e.g. the snlfate, with hydrogen snlfide. For ZnS Cu, the copper activator is added, as a readily decomposed derivative, to the sulfides and after grinding the components are fired in fnmaces at temperatures in the range 800-1200 °C. ZnS Ag, the bine phosphor nsed in CRTs, is prepared by firing zinc sulfide with silver nitrate at 1000 °C, nsnally in the presence of sodinm chloride to give the co-activating chloride ions. 

The standard color TV blue, ZnS Ag,Al, has a broad emission spectrum, but i s peak at 445 nm matches the z curve well. The emission of Eu in Sr Cl(PO,) lEu " " (] ) while broad, is narrower than that of the sulfidef so the match is even better and z is larger. Unfortunately, the CRT energy efficiency (unlike that for uv excitation, which is exceptionally good) is so low that the spectral virtues of this phosphor cannot be used to advantage. 

SrGa S Ce C 14). The motivation for choosing sulfides for de-velopment is, in part, simply that the most efficient families of CRT phosphors are sulfides ZnS, CdS, and the alkaline earth sulfides. However, rare earth based sulfides have not achieved the CR efficiency of the conventional sulfides. 

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