Архивы рубрики ‘Industrial Inorganic Pigments’

Long-Afterglow Phosphors

In long-afterglow phosphors, optical excitation energy is stored in the lattice by trap­ping photoexcited charge carriers. The most prominent example is SrAl2O4:Eu, Dy: after optical excitation of Eu2+, Eu2+ is oxidized to Eu3+ and Dy3+ is reduced to Dy2+. Thermal excitation of Dy2+ to Dy3+, followed by capture of the electron by Eu3+ and subsequent […]

Donor-Acceptor Pair Luminescence

This luminescence mechanism is found in some semi-conducting materials that are doped with both donors and acceptors. The mechanism is depicted in Figure 5.50, in which step IV results in luminescence. Ionized donors, the resulting holes in the valence band caused by ionized acceptors, capture electrons that are excited into the conduction band. The emission […]

Charge Transfer Luminescence

In the case of charge transfer, the optical transition takes place between different kinds of orbitals or between electronic states of different ions. In these cases, too, the width and position of the emission bands depend on the chemical environment. A very well known example is CaWO4, used for decades for the detection of X-rays, […]

Center Luminescence

In the case of center luminescence, the emission is generated on an optical center, in contrast to e. g. emission which results from optical transitions between host lattice band states. Such a center can be an ion or a molecular ion complex. One speaks of characteristic luminescence when, in principle, the emission could also occur […]

Luminescence Mechanisms

Luminescent materials, also called phosphors, are mostly solid inorganic materials consisting of a host lattice, usually intentionally doped with impurities (Figure 5.43). The impurity concentrations generally are low, in view of the fact that at higher con­centrations the efficiency of the luminescence process generally decreases (concentra­tion quenching). Recently organic luminescent materials have gained considerable interest, […]

Historical Overview [5.198, 5.199]

Contrary to common belief, luminescent materials have been known and used for about 2000 years. Already in ancient times, the Romans used luminescent materials for hair decoration [5.199]; the material used is thought to be CaS [5.200]. Also other luminescent materials were found in nature (e. g. naturally doped willemite, scheelite or calcite crystals). Around […]

Luminescent pigments

5.5.1 Introduction Luminescent materials (or phosphors) are generally characterized by the emission of light with energy beyond thermal equilibrium, therefore the nature of luminescence is different from black body radiation. As a consequence, external energy has to be applied to luminescent materials to enable them to generate light. Luminescence can occur as a result of […]

Transparent Zinc Oxide

The most significant property of transparent zinc oxide is again the absorption of UV radiation. The main application of zinc oxide is in sunscreens. The usual industrial manufacturing process is either direct combustion of high-purity zinc metal in a plasma with oxygen [5.196] according to the reaction: 2 Zn + O2 ^ 2 ZnO resulting […]

Transparent Titanium Dioxide

Transparent titanium dioxide exists in both rutile and anatase forms. The average primary particle size (uncoated) ranges from 8 to 25 nm. The specific surface area has values between 80 and 200 m2 g-1. Typical applications are: — UV absorbers — Catalysts (DENOX) — Effect pigments in coatings — Gas purification by absorption — Heat […]

Transparent Functional Pigments

Transparent functional pigments do not have any “color properties” in the visible light range. Their main field of application is based on their ability to absorb UV radiation. They can be used to protect organic materials such as plastics and coating resins as well as human skin (in the form of sun-creams). However, care must […]