Лако-красочные материалы — производство

Phosphor research is not only triggered by new application areas, the reverse situation can also occur. Quantum dots are a very good example of this [5.233-5.235]. Due to size quantization effects, which occur in semi-conductors with radii typically less than 10 nm, the spectral position of the emission can be shifted over a large spectral […]

This class of luminescent materials with more than one absorbed or emitted photon is gaining interest. Fundamentally, a well-selected combination of energy levels, normally f-levels of lanthanide ions, is required to establish a cascade transition. With CaWO4:Yb3+,Er3+ as a first example, absorption of two photons with lower energy (e. g. deep red or IR), followed […]

About 100 years of research on luminescent materials has resulted in phosphors with impressive maturity. In almost all applications, phosphors perform at their physical limits. We will close this section by considering two research areas, which may lead to important new application fields: cascade phosphors and quantum dots. 5.5.7.1

Traditionally, luminescent materials are prepared by mix and fire techniques: small grains of reactants (generally with diameters in the pm range or smaller) are thor­oughly mixed (either dry or in a suspension) and heated. The heating temperatures typically exceed 1000 °C. To facilitate the reaction and to obtain luminescent materials of sufficient crystallinity (which generally […]

In general, the luminescent materials applied operate at physical limits, in terms of absorption of the exciting radiation and the quantum efficiency (number of visible photons generated divided by the number of photons absorbed) with which lumi­nescence is generated. In cathode-ray tubes, the energy efficiency of the phosphors used is of the order of 20% […]

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 […]

After absorption of electrons or high-energy photons (e. g. X-ray quanta) impinging on the phosphor material, secondary charge carriers, i. e. electron-hole pairs, are generated in the lattice. The electron-hole pairs thermalize, eventually leading to band gap excitations. After thermalization, the excitation is transferred to an activator (or sensitizer), resulting in emission. For each absorbed […]

The advent of the blue light emitting diode (LED) and of organic electroluminescent structures has strongly revitalized interest in this luminescence mechanism. For the first time, efficient light-emitting structures can be realized which do not require either high or low pressure. In addition: hitherto luminescent devices always used a cascade: in fluorescent lamps, first a […]

High Voltage Electroluminescence High voltage electroluminescence relies on an electrical breakthrough in a semicon­ducting material, which rationalizes the necessity to use rather high voltages. The voltage applied is typically of the order of 100 V. Charge carriers are generated and accelerated in the host lattice. In a next step, they can excite an activator ion. […]

5.5.4.1 Optical Excitation of Luminescence and Energy Transfer When absorption of UV or even visible light leads to emission, one speaks of optical excitation of luminescence. This process takes place in e. g. fluorescent lamps and phosphor-converted LEDs in which phosphors are used to at least partly change the wavelength of the radiation emitted by […]