Calcium tungstates

Calcium hexacyanoferrate (II) is produced by mixing liquid hydrogen cyanide with an aqueous solution of ferrous chloride and milk of lime. The resulting solution is filtered and evaporated to crystallise the hydrated calcium salt [31.9]. 

Calcium hexacyanoferrate is usually converted into either the sodium and potassium salts, or into the hexacyanoferrates (III) (otherwise known as ferricyanides, [Fe(CN)6] ). It is used in the production of blue pigments and salt (as an anti-caking agent). 

Calcium silicon (CaSi2) is produced by reacting lime, quartz and coal at above 1200 °C in an electric arc furnace. The raw materials are heated to the required temperature and the carbon reduces the lime and silica (31.11) [31.10], 

Calcium silicon is used in the iron and steel industry as a de-oxidizer and desul-furizer and for the modification of non-metallic inclusions. A high calcium quicklime is required (e.g. 94 to 97 % CaO) with controlled levels of combined CO2 and water. 

Calcium dichromate (CaCr207) is made industrially by oxidative roasting of chromium-containing ores with quicklime (or limestone). The dichromate is extracted by leaching with sulfuric acid [31.11]. 

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Sal soda, see Sodium carbonate 10-water Saltpeter, see Potassium nitrate Scacchite, see Manganese chloride Scheelite, see Calcium tungstate(VI)(2—)... 

Phosphors usually contain activator ions in addition to the host material. These ions are dehberately added in the proper proportion during the synthesis. The activators and their surrounding ions form the active optical centers. Table 1 Hsts some commonly used activator ions. Some soflds, made up of complexes such as calcium tungstate [7790-75-2] CaWO, are self-activated. Also in many photolurninescence phosphors, the primary activator does not efficiently absorb the exciting radiation and a second impurity ion is introduced known as the sensitizer. The sensitizer, which is an activator ion itself, absorbs the exciting radiation and transfers this energy to the primary activator. 

Zinc and calcium siUcates, and calcium tungstate doped with activators such an Mn, Pb, Sn, and Eu are just a few examples of a large number of known ionicaHy bonded luminescent pigments. HaUde phosphates of the general formula 3Ca(P0 2 CaX2, where X = F or Cl, and Sb " and are... 

Resin cement materials are provided as a two-part powder—Hquid product. The powder consists largely of poly(methyl methacrylate) to which various fillers (qv) maybe added. These include calcium carbonate [471-34-1], siHca [7631-86-9], barium carbonate [513-77-9], and calcium tungstate [7790-75-2]. An organic peroxide, eg, benzoyl peroxide, capable of generating free radicals is also present (see Initiators Peroxides, organic). 

All commercial materials are based on calcium hydroxide and liquid alkyl salicylates (Prosser, Grolfman Wilson, 1982) and are supplied as a two-paste pack. Zinc oxide is sometimes added to the calcium hydroxide, as are neutral fillers. A paste is formed from this powder by the addition of a plasticizer examples include A-ethyl toluenesulphonamide (o- orp-) and paraffin oil, with sometimes minor additions of polypropylene glycol. The other paste is based on an alkyl salicylate as the active constituent containing an inorganic filler such as titanium dioxide, calcium sulphate, calcium tungstate or barium sulphate. Alkyl salicylates used include methyl salicylate, isobutyl salicylate, and 1-methyl trimethylene disalicylate. An example of one commercial material, Dycal, is given in Table 9.7, but its composition has been subjected to change over the years. 

Solid-state lasers using substitutional neodymium (Nd3+ ions) as the active defects are widely available. Practical lasers contain about 1% Nd3+ dopant. The most common host materials are glass, yttrium aluminum garnet (YAG), Y3A15012, and calcium tungstate, CaW04. In the crystalline host structures, the defects responsible for amplification are NdY and Ndca-... 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is... 

Zarowin (121) measured the fluorescent lifetime of neodymium in calcium tungstate at room temperature using a multiple-sampling technique. He found different values for the 4F3/2->479/2 and the 4F3/2 4/n/2 transitions, the results being 160 16 and 230 15 /xsec, respectively. As he points out, these results are indeed very surprising. It was not clear to Zarowin whether the experimental data are suspect or whether the lifetime is indeed different. 

Peterson and Bridenbaugh (122) made measurements on calcium tungstate Nd and charge compensated with sodium. The crystals were provided by Nassau and were of laser quality. Using a stroboscopic-measuring technique at room temperature, they found exactly the same lifetime for both the 4 3/2 >4 9/2 an[Pg.258]

Yariv and colleagues (74) studied the fluorescence and laser characteristics of praseodymium in calcium tungstate. For the G4 metastable state with a doping level of 0.5 per cent, they report a fluorescent decay time of 50 /nsec. This data was taken at 77°K and was obtained using an analog technique (74). 

The quantum yields of fluorescence of the different systems have also been determined relative to a single crystal of neodymium-doped YAG for which a quantum yield of unity has been assumed (Heller, 1968a). The quantum yields obtained, even if they are accurate only within a factor of two, follow the same trend as for the lifetimes, with the highest values for the acidic solutions 0.70 and >0.75 in presence of S11CI4 and SbCls, respectively. Neutral and basic solutions are less luminescent and have quantum yields of 0.5 and 0.4, respectively. Identical measurements performed on a sodium-compensated neodymium-doped calcium tungstate crystal lead to a value of 0.5. The high quantum efficiency and the low threshold (between 2 and 40 J) of these Nd3+ SeOCl2 systems clearly demonstrate that liquids are not inherently inferior to solids as laser materials. 

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