Cathode-Ray Phosphors

Figure 5. Spectral energy distributions of red-emitting cathode ray phosphors (8 ...

These orange-emitting phosphors have a long afterglow time and are therefore still used in special radar tubes and oscilloscopes [5.296], [5.307], [5.418], despite their low stability towards burn-in compared with other cathode-ray phosphors. 

Traditionally, yttrium has had many of the same uses as the rare earth elements. For example, it has been used in phosphors. A phosphor is a material that gives off light when struck by electrons. The color of the light produced depends on the elements of which the phosphor is made. Yttrium phosphors have long been used in color television sets and in computer monitors. They have also been used in specialized fluorescent lights and newer plat-panel displays. In 2007, approximately 89 percent of all the yttrium consumed in the United States was used for the manufacture of lamp and cathode ray phosphors. Another 10 percent was used in ceramics. 

However, there is a very usefiil reason for determining the type of luminescence decay curve present. Confirming the presence of an exponential decay curve means that only one type of emitter is jn sent. If a logarithmic decay process is found, it usually means that more than one type of emitting center is present, or that two or more decay processes are operative. While this does not occur very often, it is useful to know if such is present. This phenomenon occurs more in cathode-ray phosphors than in lamp phosphors, i.e.- sulfides vs oxide- hosts. 

From this discussion, it should be obvious that the two most important properties of cathode-ray phosphors are the response to electron-beaun excitation (brightness) and the decay time. We require a long-decay phosphor for radar applications and a short-decay phosphor for television usage. Nearly all the cathode-ray phosphors are based on the zinc and cadmium sulfides because they exhibit the highest efficiency to cathode-ray excitation. ZnS forms a series of solid solutions with CdS whose emission band can be shifted from the blue (ZnS Ag) to the red phosphor. 

The various cathode-ray phosphors have been classified over the years... 

Just as in the field of lamp phosphors and (partly) cathode-ray phosphors, CaW04 lost its leading position to rare-earth activated X-ray phosphors (see chapter 8). As a salute to this old champion, but also for your information, we give in Fig. 1.6 the crystal structure of CaW04 which illustrates the build-up of the lattice from Ca ions and luminescent WO4 groups, and in Fig. 1.7. an electron micrograph of a commercial CaW04 powder. 

An example of the former class is ZnS, the host lattice for cathode-ray phosphors. This compound is a semiconductor. Optical absorption occurs for energies laiger than Eg, the width of the forbidden gap. This absorption cremes an electron in the conduction band and a hole in the valence band. Since the lop of the valence band consists of levels with predominant sulfur character and the bottom of the conduction band of levels with a considerable amount of zinc character, the optical transition is of the charge transfer type. Its position can be shifted by replacing Zn and/or S in ZnS by other elements (see Table 2..3). 

It is in principle very simple to change the emission wavelength of the efficient blue cathode-ray phosphor ZnS Ag, vix. by replacing part of the zinc by cadmium. 

As a consequence the band gap decreases, so that the emission wavelength shifts to the ted. Actually Zno.eBCdo.32S Ag is a gieen-. and Zno.i3Cdoa7S Ag+ a red-emitting cathode-ray phosphor. The emission color is not determined by the nature of the luminescent center, but by the value of the band gap. Figure 7.2 shows the chromaticity diagram with three phosphors from the (Zn,Cd)S Ag+ family. 

Fig. 7.S. Emission specirum of the blue-emitting cathode-ray phosphor ZnS Ag...

Table 7.1. Luminescent properties of rcd-cmitiing cathode-ray phosphors (after Ref. [1)).

As argued in Sect. 7.1 one of the problems with cathode-ray phosphors in projection television is the saturation of their light output under high excitation density. Related to this is the temperature increase of the phosphor under these circumstances the screen temperature can rise to 100 C [8]. 

Fig. 7.9. Thermal quenching of ihe green emission of some cathode-ray phosphors... 

Fig. 7.10. Non-linearity of the red cathode-ray phosphors. Figs. 7.8-7.10 are reproduced with permission from Ref. [14]...

In the literature many other cathode-ray phosphors are known for several purposes. Two of these we will mention here, partly because they are generally known as luminescent materials, partly because their properties are also interesting from a fundamental point of view. They are Zn2Si0.i Mn, also known by the mineral name willemite, and the family of Ce -activated phosphors. Still more cathode-ray phosphors can be found in Ref. [I]. 

Willemite is used as a cathode-ray phosphor in terminal displays and oscilloscope tubes. The decay time is very long, viz. 25 ms [I]. This is mainly due to the spin- and parity- forbidden nature of the Ti -> Ai emission transition in the configuration of the Mn ion (Sect. 3.3.4.C), but there is also a contribution of afterglow. An even longer persislance is observed for samples to which As has been added. As a re.sult of the As addition, electron traps are formed ich trap the electrons for a certain time, so that the emission is delayed (Sect. 3.4). 

Cathode-ray phosphors with such a long persistence are suitable to avoid or minimize flicker in the display. This is especially of importance when high-definition figures need to be displayed. For application in television tubes (moving pictures) or high-frequency oscilloscopes, such a long persistence is of course fatal. 

The Ce -activated cathode-ray phosphors are used in applications where a very short decay time is a requirement [13). Since the emi.ssion is a completely allowed transition (5t/-> 4/, Sect. 3.3.3Ji), the decay time of Ce varies between about 15 and 70 ns, depending on the emission wavelength. One application is in the beam-index tubes which generate color images by means of one electron gun [1,13]. This system could, however, never compete with the above-mentioned shadow-mask tube. The beam-index phosphor indicates the location of the electron beam. Therefore, the emission should have a very short decay time and, in order not to disturbe the image, should be situated in the ultraviolet. A good choice is Y2Si2C>7 rCe with a radiant... 

The field of cathode-ray phosphors is relatively old and has reached a high level of maturity. Ilic quality of todays color television is a token of the success of these materials. For direct-view television there is not much more to desire as far as the luminescent materials are concerned. The combination ZnSrAg, ZnSiCu. Ap, and 2028 Eu is worldwide considered to be the most suitable combination. 

The terms X-ray phosphors and scintillators are often used in an interchangeable way. Some authors use the term X-ray phosphors when the application requires a powder screen, and the term scintillator when a single crystal is required. The physical processes in the luminescence of these two types of materials is, however, in principle the same and comparable to that in cathode ray phosphors (Chapter 7). 

Much of whai has been said before about the preparation of lamp phosphors (Sect. 6.3) and cathode-ray phosphors (Sect. 7.2), is also true for X-ray phosphors from which screens arc produced. Brixner [I] has described several preparation procedures. Here we give some examples. 

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