22 января, 2016 
Pokraskin 5.5.5.1
Application Areas and Phosphors Used
An overview of phosphors used in the most important application areas is given in Table 5.21. In many applications, the blue — and the red-emitting CRT phosphors are coated with a daylight absorbing pigment, to enhance contrast in daylight viewing conditions. The blue-emitting phosphor is coated with a particle coating of (blue) CoAl2O4, the red-emitting phosphor with a particle coating of (red) a-Fe2O3. Coating the green-emitting phosphor with a green-absorbing pigment is less useful in view of the high sensitivity of the human eye to green light. Work on CRT phosphors has almost stopped in view of the strong advances in liquid crystal displays (LCDs) and plasma display panels. However, even LCDs also require luminescent materials, as they need a backlight: either thin or flat fluorescent lamps or LEDs.
In plasma display panels, the most severe energy loss is encountered in the discharge. This is a consequence of the small dimensions of the picture elements (pixels). The phosphors used have been known for a long time. The operational lifetime of the blue emitting phosphor still poses a problem.
In fluorescent lamps, the tricolor mix results in white light generating lamps with higher brightness than lamps based on the halophosphate, because ofa better matching of the emission spectrum of the tricolor phosphor mixture to the eye sensitivity, while the ability of the lamps to reproduce all colors in a natural way (color rendering) is still very good. Fluorescent lamps contain mercury, which is an environmental disadvantage. One possibility is to reduce Hg consumption by fluorescent lamp constituents. Present fluorescent lamps contain much more Hg than neces-
sary for proper lamp operation. Alternatively, Hg-free fluorescent lamps, based on a Xe-discharge might be a solution. However, as the Xe-discharge generates radiation in the VUV, the energy efficiency of the phosphors will be lower, reducing the energy efficiency of the lighting device. However, on excitation with VUV radiation, more than one visible photon can be generated per absorbed VUV photon, principally enabling Hg-free fluorescent lamps to be as efficient as current fluorescent lamps. This will be dealt with in the Outlook.
Research on phosphors for X-ray detection mainly deals with applications in Computed Tomography (CT) and Position Emission Tomography (PET). The X-rays (CT) or y-rays (PET) are converted into visible light by these phosphors. The visible light detection is by photomultipliers or photodiodes. Apart from energy efficiency (energy efficient X-ray phosphors reduce the amount of harmful radiation to which a patient is exposed), the temporal behavior is also important. The emission decay time should be short (ns for PET, which is a consequence of the measuring principle: the two photons, which are generated in the positron annihilation process have to be detected in coincidence; ps for CT to enhance the scan speed). In addition, obtaining translucent or even transparent ceramic materials (scintillators) is an issue here. The thickness of these crystals is of the order of millimeters for CT and centimeters for PET scintillators. In general, the scintillators are made by single crystal growth or by hot pressing techniques.
X-ray phosphors are also used in X-ray intensifying screens. The X-rays are first converted into visible photons, which subsequently irradiate the film. In most cases, the photographic film is sandwiched between two phosphor sheets. Light moving into the direction away from the photographic film can nevertheless be used by application of TiO2 reflecting layers. The typical thickness of such a phosphor layer is of the order of a few hundred micrometers.
In LED lamps based on a blue-emitting LED, addition of a red-emitting phosphor (the emission of which is due to Eu2+) to the yellow-emitting (Y, Gd)3Al5O12:Ce enables the production of white-emitting LED lamps which generate light with a low color temperature. Phosphor converted LEDs, based on (Y, Gd)3Al5O12:Ce only emit a rather bluish white, i. e. the light has a rather high color temperature. Red — emitting materials, which can be excited effectively with blue or near-UV light require rather covalent hosts. Interestingly, a class of materials, which has been unknown to phosphor applications till now, looks very promising for this application: multinary nitrido silicates. Phosphors such as LaSi3N5:Eu2+,O2- or Ba2Si5N8:Eu2+ are likely the first representatives of a potentially large group [5.226, 5.227]. LED lamps can also be based on near-UV emitting LEDs. At least two phosphors are needed to generate white light in such a case (blue and yellow). Alternatively, three phosphors are used, emitting in the blue, green and red part of the visible spectrum. At present, there are no proven concepts here. In addition, there might be an issue with the UV stability of the LED encapsulation and finally UV-emitting LEDs might have an intrinsically lower efficiency. Charge-transporting layers, transporting electrons and holes to the emitting (In)GaN layer, generally consist of doped InGaN layers. For optimal light output, the charge-transporting layer should have a larger band gap than
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the emissive layer. This implies that the active layer should have a higher In content than the charge-transporting layers. This condition cannot be realized for (almost) pure GaN emissive layers unless efficient charge-transport layers with larger band gaps (implying a different chemical composition) are found.
5.5.5.2
Опубликовано в рубрике 