Phosphors for Lighting

This chapter covers both traditional and more recent synthetic methods of preparing phosphors. It is important to note that as phosphors are used in all types of display, and are, therefore, an important part of a multibillion-dollar industry research and development is always ongoing to improve light output and, therefore, ease of viewing. The same is also true in the use of phosphors for lighting. 

Yeh CW, Liu YP, Xiao ZR, Wang YK, Hu SF, Liu RS (2012) Luminescence and density functional theory (DFT) calculation of undoped nitridosilicate phosphors for light-emitting diodes. J Mater Chem 22 5828... 

Liu TC, Cheng BM, Hu SF, Liu RS (2011) Highly stable red oxynitride p-SiA10N Pr phosphor for light-emitting diodes. Chem Mater 23 3698... 

Kim JS, Jeon PE, Choi JC, Park HL (2005) Emission color variation of M2Si04 Eu (M = Ba, Sr, Ca) phosphors for light-emitting diode. Solid State Commun 133 187-190... 

Various phosphors have been developed for different application systems, wherein the oxide phosphors are particularly noticed and mosfly widely used because of their excellent properties such as high luminescence efficiency and thermal stability as well as the superiority of simple preparation methods and inexpensive raw materials. In this chapter, oxide phosphors for lighting and display will be introduced, and more attention will be focused on the performance improvement of practical oxide phosphors for three-band fluorescent lamps, plasma displays, and white light-emitting diodes. Due to the space limitation, the aim of this chapter is not to discuss all of the works involved in the progress of oxide phosphors but to introduce the efforts made by our research group in the General Research Institute for Nonferrous Metals, China. 

Lin CC, liu RS (2011) Advances in phosphors for light-emitting diodes. J Phys Chem Lett 2 1268... 

Molycorp, a wholly owned subsidiary of Unocal Corp., was the only company to mine rare earth minerals in the United States in 2002. The rare-earth separation plant operations stopped in 2003. Molycorp mined bastnasite, a rare earth fluorocarbonate mineral, as a primary product at Mountain Pass, California. The value of domestic ore production was estimated at 31 million in 2002 the estimated value of refined rare earth minerals was more than 1 billion. The end uses for rare earth products in 2000 were as follows automotive catalytic, 22 percent glass polishing and ceramics, 39 percent permanent magnets, 16 percent petroleum refining catalysts, 12 percent metallurgical additives and alloys, 9 percent rare earth phosphors for lighting, televisions, computer monitors, radar, and x-ray intensifying film, 1 percent, and miscellaneous,... 

Nonradiative Decay. To have technical importance, a luminescent material should have a high efficiency for conversion of the excitation to visible light. Photoluminescent phosphors for use in fluorescent lamps usually have a quantum efficiency of greater than 0.75. AH the exciting quanta would be reemitted as visible light if there were no nonradiative losses. 

The Calcium Halophosphate Phosphors. Early fluorescent lamps used various combinations of naturally occurring fluorescent minerals. The development of the calcium halophosphate phosphor, Ca (P0 2(Cl, F) Sb ", Mn, in the 1940s was a significant breakthrough in fluorescent lighting (7). As is often the case in new phosphor discoveries, this phosphor was found accidentally while searching for phosphors for radar screens. 

The phosphors described in this chapter are usually based on metal oxide, metal oxysulfide, and metal sulfide lattices, which act as hosts for light-emitting centers that are doped into them. This is denoted by, for example, ZnS Pb (meaning a trace of Pb doped into a ZnS host lattice) or LiGdF4 Er3+,Tb3+ (meaning a mixture of Er3+ and Tb3+ ions doped into a LiGdF4 host lattice). 

Figure 1.3 Left. Detailed view of the Nb K-edge XANES data of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo fVJpry) as a function of temperature [31]. A change in niobium oxidation state, from Nb5+ to Nb4+, is identified between 350 and 420°C by a relative increase in absorption about 19.002 keV, and can be connected with the activation of the catalyst for light alkane oxidation. Right. Radial Fourier-transform EXAFS function for the NbPMo (V)pyr sample heated to 420°C [31 ]. The two peaks correspond to the Nb-O (1.5 A) and Nb-Mo (3 A) distances in the heteropolymolybdate fragments presumed to be the active phase for alkane oxidation. (Reproduced with permission from Elsevier.)...

In the previous chapter we have introduced the physical basis of the interpretation of optical spectra of centers in crystals. The main effect of these centers is to introduce new energy levels within the energy gap of the crystal, so that the transitions among these levels produce new optical bands that are not present in the perfect crystal. Due to these absorption and emission bands, centers in crystals are relevant for a variety of applications, such as solid state lasers, amplifiers and phosphors for fluorescent lighting and cathode ray tubes. In this chapter, we will describe the main characteristics of the relevant centers for these applications. 

Uses. Yttrium is mixed with rare earths as phosphors for color television receivers oxide for mantles in gas and acetylene lights in ceramics in superconductors... 

Zinc fluoride is used in the manufacture of phosphors for fluorescent lights. It also is used in electroplating baths, in preservation of wood, in glazes and enamels for ceramics, and in fluorination reactions of organics. 

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. 

Screen-Film Systems. According to Ludwig (5), crucial to the application of x-ray phosphors for intensifying screens is the efficiency with which the incident x-ray energy is converted to useful light energy as given by the expression ... 

Very broadly speaking, two situations have to be considered in explaining devices such as those we have mentioned. In the first, which is relevant to the ruby laser and to phosphors for fluorescent lights, the light is emitted by an impurity ion in a host lattice. We are concerned here with what is essentially an atomic spectrum modified by the lattice. In the second case, which applies to LEDs and the gallium arsenide laser, the optical properties of the delocalised electrons in the bulk solid are important. 

In color picture tubes, the final color is formed by additive mixing of red, green, and blue light. Each of the three phosphors (Table 58) is excited by its own modulated electron beam and emits light of intensity corresponding to the particular degree of excitation. The phosphors for color picture tubes have an average particle size of 8-10 pm. 

The majority of phosphors for cathode-ray tubes are coated with an oxide, silicate, or phosphate before use to improve their processing properties and stability to bum-in. Pigmenting of Y202S Eu3+ with finely divided Fe203 and of ZnS Ag+ with ultramarine or cobalt aluminate is also known. The pigment, which has the same body-color as the emission color of the phosphor, absorbs incident ambient light, effecting an increase in contrast [5.430]. 

Phosphors for cathode-ray tubes, television screens, monitor screens, radar screens, and oscilloscopes are tested under electron excitation. Electron energy and density should be similar to the conditions of the tube in which the screen will be used. The phosphors are sedimented or brushed onto light-permeable screens and coated with an evaporated aluminum coating to dissipate charge. The luminescence brightness and color of the emitted light are measured with optical instruments such as photomultipliers or spectrophotometers. 

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