Reduction-diffusion process

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. 

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 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 ... 

Adam G and Delbrtick M 1968 Reduction of dimensionality in biological diffusion processes Structural Chemistry and Molecular Biology ed A Rich and N Davidson (San Francisco Freeman)... 

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. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Here F is the Faraday constant C = concentration of dissolved O2, in air-saturated water C = 2.7 x 10-7 mol cm 3 (C will be appreciably less in relatively concentrated heated solutions) the diffusion coefficient D = 2 x 10-5 cm2/s t is the time (s) r is the radius (cm). Figure 16 shows various plots of zm(02) vs. log t for various values of the microdisk electrode radius r. For large values of r, the transport of O2 to the surface follows a linear type of profile for finite times in the absence of stirring. In the case of small values of r, however, steady-state type diffusion conditions apply at shorter times due to the nonplanar nature of the diffusion process involved. Thus, the partial current density for O2 reduction in electroless deposition will tend to be more governed by kinetic factors at small features, while it will tend to be determined by the diffusion layer thickness in the case of large features. 

It is state of the art to deposit the platinum catalyst on the membrane surface by a diffusion process in which platinum salt solutions (from the cathode side of the membrane) and a reductant, like hydrazine, are counter-diffusing causing reductive precipitation of dispersed platinum close to and on the surface of the membrane. The BBC Membrel cell is reported to contain a catalyst load of only 0.2 mg Pt per square centimeter (97). The cathode is reported to exhibit only from 50 to 70 mV overpotential at current densities of 1 A/cm2 and 80°C. 

Because of the slow diffusion process of chlorofluoromethanes through the tropopause into the stratosphere and a slow photodissociation process of chlorolluoromethanes by sunlight in the stratosphere the maximum reduction of O, is estimated to occur 10 years after the release of chlorofluoromethanes at ground level [Rowland and Molina (843)]. 

Thus the rate of electrolysis of O is determined by the magnitude of k in concert with the diffusion process. Metal complexes such as cadmium nitrilotriacetic acid Cd(NTA), in which the metal complex first dissociates to form a hydrated metal ion that then undergoes reduction, fall into this category [6] ... 

Murray J. D. (1977) Reduction of dimensionality in diffusion processes antenna receptors of moths. In Lectures on Nonlinear-Differential-Equation Models in Biology, pp. 83-127. Oxford University Press, Oxford. 

Sm2Fei7Nx powders are produced commercially since 1999 by the reduction and diffusion process [136]. The currently mass-produced powder has a BHimx of 290 kJ/m3 with /ioMt = 1.35 T and fi(iHc = 1.1 T. Milling and surface treatment have been optimized to improve the squareness of the demagnetization curve and resistance to corrosion [137]. 

Corrosion current density — Anodic metal dissolution is compensated electronically by a cathodic process, like cathodic hydrogen evolution or oxygen reduction. These processes follow the exponential current density-potential relationship of the - Butler-Volmer equation in case of their charge transfer control or they may be transport controlled (- diffusion or - migration). At the -> rest potential Er both - current densities have the same value with opposite sign and compensate each other with a zero current density in the outer electronic circuit. In this case the rest potential is a -> mixed potential. This metal dissolution is related to the corro-... 

---