Reduction-diffusion

Calciothermic reduction of samarium oxide, in the presence of cobalt powder, yields samarium-cobalt alloys in the powder form. The process is popularly known as reduction diffusion. Samarium oxide, mixed with cobalt powder and calcium hydride powder or calcium particles, is heated at 1200 °C under 1 atm hydrogen pressure to produce the alloys. Cobalt oxide sometimes partly replaces the cobalt metal in the charge for alloy preparation. This presents no difficulty because calcium can easily reduce cobalt oxide. A pelletized mixture of oxides of samarium and cobalt, cobalt and calcium, with the components taken in stoichiometric quantities, is heated at 1100-1200 °C in vacuum for 2 to 3 h. This process is called coreduction. In reduction diffusion as well as in coreduction, the metals samarium and/or cobalt form by reduction rather quickly but they need time to form the alloy by diffusion, which warrants holding the charge at the reaction temperature for 4 to 5 h. The yield of alloy in these processes ranges from 97 to 99%. Reduction diffusion is the method by which most of the 500 to 600 t of the magnetic samarium-cobalt alloy (SmCOs) are produced every year. 

The reduction diffusion process has also been used for the production of powders of the magnetic neodymium-iron-boron alloy (Nd15Fe77B8). The reaction involves use of a powder mix of neodymium oxide, iron, ferroboron and calcium. The reaction is conducted by heating the powder charge mixture at 1200 °C for 4 h under vacuum. Neodymium-iron-boron alloys are much more prone to oxidation than samarium-cobalt alloys and a proprietary leaching procedure is used for the separation of the alloy and calcium oxide. 

Table III. Comparison of Rates of Sulfate Reduction, Diffusion, and Retention...

Thus sorption, followed by intra-particle diffusion of the dyes, causes rapid initial loss followed by slow long term loss. Transformation pathways probably involve azo reduction. Diffusion may limit the dyes transformation rates because of the large size of the molecules. 

R. A. Sulfate reduction, diffusion, and bioturbation in Long Island Sound sediments Report of the FOAM group. 

FOAm (.Friend s of Anoxic Mud) Goldhaber, M.B., Allen, R.C., Cochran, J.K., Rosenfeld, J.K., Martens, C.S., and Berner, R.A. Sulfate reduction, diffusion, and bioturbation in Long Island sound sediments Report of the Foam Group. Amer. 

The second method involves the so-called calciothermic reduction of rare earth oxides by means of calcium in the presence of 3d metals under a protective atmosphere (Cech 1974, Herget and Domazer 1975, Sun Guangfei et al. 1985, Herget 1987, Zhou et al. 1987). This process is commonly referred to as the reduction-diffusion (R-D) process. Basically, it involves two steps. In the first step, performed at 900-1100 °C, the actual reduction of Nd203 takes place according to the reactions ... 

Fig. 26. Process outline for the production of rare earth-transition metal magnet alloy powders by calciothermic reduction and solid-state diffusion. R/D is reduction-diffusion, and KOR is Koreduktion...

In the reduction/diffusion version of the method, first developed by General Electric in the USA (Cech 1974), all the cobalt and any other TM is introduced as a fine metallic powder, blended with calcium granules, and the mixture reacted under a hydrogen atmosphere. The reaction according to the equation... 

Chemical changes of groundwaters are the result of complex processes taking place in the water-rock-atmosphere system. These are physical, chemical and biochemical processes taking place simultaneously or in a close sequence. Dissolution, hydrolysis, adsorption, ion exchange, oxidation and reduction, diffusion and osmosis are of decisive importance in the formation of a given chemical composition. 

Carbonyl sulfide makes up approximately 80% of the total sulfur content of the atmosphere and is the major source of stratospheric aerosols. Carbonyl sulfide is produced within surface waters by photolysis of dissolved organosulfur compounds. Therefore, surface water OCS levels within estuaries exhibit a strong diel trend. Carbonyl sulfide is also added to the water column by diffusion from anoxic sediments, where its production appears to be coupled to microbial sulfate reduction. Diffusion of OCS from the sediment to the water column accounts for 75% of the OCS supplied to the water column and is responsible for the higher OCS concentrations in estuaries relative to the open ocean. While supersaturations of OCS are observed throughout... 

The aerobic soil layer functions as an effectiv e sink for reductants diffusing from the underlying anaerobic layer. Both chemical and biological processes regulate the consumption of oxygen in the aerobic layer. Oxygen consumption by anaerobic soils is best described by a two-phase hrst-order reaction (Figure 6.24 Reddy et al., 1980) ... 

Chemical-deposition Electrode. Chemical techniques can also be employed in the fabrication of catalyst layers by platinizing the surface of a Nafion membrane. One approach [65, 66] is to expose one side of the Nafion membrane to a reductant solution (e.g., hydrazine solution) and the other side to a platinic acid solution. The reductant diffuses across the membrane to react with platinic acid and form a Pt catalyst layer. In another impregnation-reduction method [3, 67, 68] a cationic salt, such as Pt(NH3)4Cl2, is first impregnated into the Nafion membrane, followed by exposing this membrane to a reductant, such as NaBUj. As shown in Figure 19.13, a dense and porous platinum film can be formed using this chemical deposition technique. 

The water vapor formed during reduction diffuses from the interior to exterior of particle, and then flows through a-Fe produced by reduction. This process is one-way and inevitable. It will repeatedly oxidize and reduce the a-Fe to cause the growth of crystallite of a-Fe, and the activity of the catalyst is decreased as well. Therefore, the water vapor concentration should be strictly controlled during the whole reduction process, generally lower than 0.7g/m l.Og/m . 

The R-Fe-B alloys may be prepared by reducing with calcium, at high temperature, the respective rare-earth oxides, in the presence of a mixture of transition metals in powder form plus some transition metal oxides (Herget 1985a). Two versions of calciothermic processes for making rare-earth alloys were proposed the reduction-diffusion (R-D) process (Cech 1974, McFarland 1973) and the co-reduction process (Herget and Domazer 1975, Domazer and Strnat 1976). The last one consists of a simultaneous reduction of rare-earth and transition metal oxides in the presence of transition metal powder, followed by diffusional alloys formation. The process may be divided into two steps ... 

Since the R-Co alloys have become technically significant, many efforts have been made to lower the cost of the R-Co alloys which are the starting materials for the magnet production. In the last three years new methods have been developed, which avoid the preparation of pure R metals which are later melted with Co and other transition metals, but start with rare earth oxides (R20a) and Co oxide. These processes are known under the names reduction-diffusion process and co-reduction process . 

The reduction-diffusion (R-D) process is based on developments by General Electric guided by Cech (1974). The main reaction can be written as follows... 

The co-reduction process has been developed by H.G. Domazer and coworkers at Goldschmidt in Essen, Germany. Similar to the reduction-diffusion process they started with a rare earth oxide and not with a rare earth metal. In contrast to the modified (technological) reduction-diffusion process (see McFarland, 1973) C03O4 is used deliberately in order to achieve a strongly exothermic reaction. Therefore the rare earth oxide and the Co oxide are reduced simultaneously. The summary equation of the process is given below ... 

The reduction-diffusion method is another method to prepare fine powders (around 20-50 pm) of alloys with low lattice strain because the alloy is prepared by heating the mixture of mischmetal oxides, the reductant (Ca) and the metal powder, such as Ni, Co, Mn, Al, at 900-1000 C (Takeya et al. 1993). Almost the same electrode performances as those of the samples prepared by the melting method were obtained for the Mn-containing alloys. 

Unlike other global methods for spectral dimensionality reduction. Diffusion Maps does not compute a -nearest neighbour graph. Rather, the affinities are computed for each pairing of data points, that is, the affinities (Eq.2.6) are calculated for all points such that the affinity matrix W is dense. Since the affinities are calculated for all n points, the cost of computing the affinity matrix is 0 rp-). This computational... 

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