Neodymium silicates

The third oxide used for physical decolorizing is neodymium oxide. Its absorption curve closely compliments an average mixture of ferrous and ferric oxides especially with the strong absorption band at 589 nm. Neodymium oxide is also stable against any state of oxidation change in the furnace. Neodymium is exceptionally good as a decolorizer for potassium silicate and lead glasses. If the redox balance is not quite correct for the... 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

His experiments were primarily upon a single soda-lime silicate, which was chosen because it had been studied extensively in the literature and can be made in small amounts with fair optical quality. The decay for this glass with 0.8 per cent Nd203 added is shown in Fig. 33. It is quite clearly not a single exponential. One finds that the slope changes by a factor of three over the range examined and that the curvature increases with neodymium concentration. The measurements made at long times were done with a mechanical shutter that shielded the photomultiplier tube from the initial intense fluorescence. When this was not done, hysteresis in the photomultiplier tube led to spurious results. 

Hirayama and Lewis (131) studied the fluorescent decay of the 3/2 state of neodymium in a very large number of alkali silicate and alkali germanate glasses. Data were taken by exciting a flat sample with a xenon... 

Fluorescent and Physical Properties of Neodymium-Doped Alkali Silicate and Germanate Glasses... 

Gandy and co-workers (134) have also observed energy transfer from cerium to neodymium in a silicate glass. The evidence is that this is again an example of nonradiative-energy transfer. Intensity measurements indicate... 

In traditional areas such as ceramic tiles, praseodymium oxide in a zirconium silicate matrix is used as a yellow stain. Cerium oxide is used as an opacifier or to give esthetic effects in the glaze. Other pigment colours, such as orange with yttrium oxide and light purple with neodymium oxide can be prepared. The compositions of these pigments are given in Table 12.19. 

Silicate is depleted from surface waters by biological processes and remineralized in the deep water. Moreover, silicate tends to accumulate in water masses as they age (cf. Broecker and Peng, 1982). These processes account for both the increasing concentration of silicate with depth, and its increasing concentrations from the North Atlantic to the circum-Antarctic to the Pacific. With a few exceptions (notably the North Atlantic), neodymium abundances show a smooth increase with depth, and in deep water they are highest in the Pacific, lowest in the Atlantic, and intermediate in the Indian (Figure 6). 

Considered separately, the neodymium-isotope ratios and the neodymium concentrations have very different implications, (i) Neodymium-isotope ratios are variable in the different oceans and within an ocean they fingerprint the advective paths of water masses. This requires that the residence time of neodymium is shorter than the ocean mixing time of 10 yr (e.g., Broecker and Peng, 1982). (ii) Neodymium concentrations appear to mimic silicate, implying orders of magnitude longer residence times of 10" yr... 

Northern and Southern end-members using salinity or silicate as conservative water-mass mixing proxies (Figure 10). Comparing neodymium-isotope ratios and concentrations, the compositions of many samples are also consistent with North-South water-mass mixing (Figure 14(a)). However, a substantial portion of the data is outside any... 

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