Insulator phosphors

Molecular, Insulator, and Semiconductor Phosphors. Luminescent materials can be divided into three broad categories molecular phosphors, insulator phosphors, and semiconductor phosphors (6). 

Most minerals fall into the class of insulator phosphors. The characteristics of the luminescence are usually defined by the electronic structure of an activator ion as modified by the crystal field of the host crystal structure. Although some energy transfer takes place between nearby ions, appearing as the phenomena of co-activation, luminescence poisons, and activator pair interactions, the overall luminescence process is localized in a "luminescent center" which is typically 2 to 3 nm in radius. From a perspective of band theory, luminescent centers behave as localized states within the forbidden energy gap. 

Activators. It is convenient to classify activator ions for insulator phosphors in terms of their electronic configuration. 

The standard EL device (Fig. 1 b) employs a transparent substrate, typically glass, coated with a transparent conducting layer, which serves as the bottom electrode. The bottom insulator, phosphor, and top insulator layers reside between the bottom transparent conductor and a top opaque conducting layer. This layer serves both as an electrical contact and as a reflector to direct light generated in the phosphor layer... 

Fig. 1. A schematic diagram of the foibidden gap (Eq) of an insulating phosphor. Various defect levels are shown. The arrows show possible transitions during the excitation, trapping and emission (see text).

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

Muller (1951, 1956) developed this instrument, which for the first time enabled extensive details of the atomic structure of a solid surface to be seen directly. Figure 1.1 illustrates schematically the basic construction of a FIM. The specimen is prepared in the form of a fine wire or needle, which has been chemically or electrochemically polished to a sharp point with an end radius typically 50-100 nm. It is mounted along the axis of a vacuum chamber, about 50 mm from a phosphor screen (perhaps 75 mm in diameter). The specimen is mounted on an electrical insulator within a cryostat, and it can be raised to a high positive potential (3-30 kV) by means of the leads attached. 

Phenolic resins also find use in varnishes, electrical insulation, and in other protective coatings. Heat-settings adhesives which are based on phenolics find use in producing plywood. These also find use in the production of ionexchange resins having amine, sulphonic acid, hydroxy or phosphoric acid functional groups. 

Several classes of potentially useful fluorescent materials exist. Most lamp phosphors and many solid state materials are insulating compounds containing ionic activators. The spectra of the rare-earth activators resemble, to a first approximation,... 

The choice of sensor material determines range, sensitivity, and stability. By considering the latter factors, it is found that inorganic insulating compounds, such as most lamp phosphors and many solid state laser materials, are the most suitable materials for thermometric applications. Indeed, these materials are most commonly used in the existing commercial fluorescence thermometer schemes. 

Early work in the 1960s on thin-hlm EL (TEEL) involved the excitation of Al/ZnS (activated by Mn or Cu) deposited on glass, but the key step forward was taken when the double-insulator structure was developed as shown in Figure 3.30. As can be seen the ZnS film is sandwiched between two insulating layers, limiting the amount of charge transferred to the device and also totally encapsulating the phosphor film. In spite of this the EL device still suffered from lifetime problems and it was not until 1978, when Sharp demonstrated a 240 x 320 display, that this problem was overcome. 

When a ferrous alloy is immersed in phosphoric acid, il initially forms a soluble phosphate. As the pH rises at the mclal/solutiun interface, the phosphate becomes insoluble and crystallizes epitaxially on Ihe substrate metal. The phosphate coating thus produced consists of a nonconduciivc layer nf crysinlx that insulates the metal from any subsequently applied film and provides a topography with enhanced tooth" for increased adhesion. The cry stals insulate microanode and microcathode centers caused by stress or imperfections in the metal surface. This greatly reduces Ihe severity of electrochemical corrosion. 

The Srs(P04)3Cl crystals are hexagonal needles with lattice parameters ah = 9.953 A. and ch = 7.194 A. The needle axis corresponds to the crystallographic c axis. The europium(II) doped sample is a phosphor, readily excitable with electrons, x-rays, and both short and long ultraviolet light. It emits in the blue with a peak at 445 nm. Crystals of strontium chloride vanadate(V) are orthorhombic platelets with lattice constants a = 7.43 A., b = 11.36 A., and c = 6.54 A., with the b axis corresponding to the thin dimension of the flakes. Strontium chloride vanadate(V) is a self-activated phosphor giving broadband emission with a peak at 423 nm. when excited with 2537-A. radiation. All compounds are insulators, with resistivities >1012 ft-cm. 

In semiconductor phosphors the energy band structure of the host crystal plays a central role. Some semiconductor luminescence arises from decay of exciton states, other emission arises from decay of donor states generated by impurity or defect centers. It is not the magnitude of the band gap itself that separates insulator from semiconductor phosphors it is a question of whether the spectrum is characteristic of impurity energy levels as perturbed by the local crystal structure or whether the spectrum is characteristic of the band structure as modified by impurities. 

Crystal Structure and Luminescence of Insulator Mineral Phosphors... 

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