Diffusion-zone method

CoAd Blood Testing oa Sickle Cell Anemia and Related Vl6-oAdeAS by AlienachAomatogAaphlc Methods. The CM-Sephadex procedure easily detects Hb-S and Hb-C at birth but the diffuse zone of Hb-A has on occasion been difficult to see. Substitution of CM-Cellulose for CM-Sephadex has yielded a superior mlcrochromatographlc method, and the compact, well-defined zones of the CM-Cellulose column facilitate the Interpretation of the results even though the amount of sample Is only 20% as great. The CM-Cellulose method Is as simple and rapid as the original CM-Sephadex procedure (27. 28). 

Subject to the restrictions of no scatter and diffuse surface emission and reflection, the above equations are the most general matrix statement possible for the zone method. When P = 1, the directed exchange areas all reduce to the total exchange areas for a single gray gas. If, in addition, K = 0, the much simpler case of radiative transfer in a transparent medium results. If, in addition, all surface zones are black, the direct, total, and directed exchange areas are all identical. 

Figure l-(a) shows the schematic diagram of the method to make the compositional ly graded diffusion couple. The couple of Cu/Cu-10 massXSn has the diffusion zone of 2 mm in length after heating. 

In all cases, it is essential to solve the model equations efficiently and accurately. Some techniques are discussed in this book and in the appendices, for the solution of the highly non-linear algebraic, differential and integral equations arising in the modelling of fixed bed catalytic reactors. The most difficult equations to solve are usually the equations for diffusion and reaction in the porous catalyst pellets, especially when diffusional limitations are severe. The orthogonal collocation technique has proved to be very efficient in the solution of this problem in most cases. In cases of extremely steep concentration and temperature profiles inside the pellet, the effective reaction zone method and its more advanced generalization, the spline collocation technique, prove to be very efficient. 

In the "diffusion flame" method developed by von Hartel and Polanji (Z. physikal. Chem. B 1930,11, 97) sodium vapour is introduced throu a nozzle into an excess of organic halide and the extent of penetration before it is completely consumed by reaction is measured. In an experiment made by Cvetanovic and Le Roy (J. Chem. Phys. 1952, 20, 1016) at 532.7 K, a stream of sodium vapour at a partial pressure = 8.3 x 10" mm Hg in nitrogen carrier gas was passed through a nozzle of radius r = 0.125 cm into a stream of nitrogen and ethyl chloride vapour at a partial pressure p = 2.5 x 10" mm Hg. The radius R of the spherical zone of reaction made visible by illumination with sodium D-line resonance radiation was 1.55 cm. The partial pressure P of sodium at the visibility limit was estimated to be 7 X 10" mm Hg. The diffusion coef5cient D of sodium in the reaction mixture is 130 cm s. ... 

A maximum of the diffuse background was foimd in the spectrum of sample 7 (Fig. 2) on the left of the (200) selective reflection. This maximum was attributed to the presence of a second phase since it was not observed in the samples prepared by ihe floating molten zone method or by the Bridgman method (Fig. 3). 

While investigating reactive diffusion in a thin film of Al-Co by using the high-resolution method of atomic tomography it was established that nucleation of the first phase AI9C02 is possible only in the case of initial components interdiffusion to a depth of not less than 4 nm [6]. A numerical analysis of the nucleation possibility at the initial stage of reaction between Al and Co was carried out. Microscopic nucleation mechanisms in the diffusion zone were considered and it was realized... 

One of the modern hi -accuracy experimental methods is tomography atom probe (TAP). It enables the investigation of reactions in thin metallic diffusion couples by means of spatially resolved chemical analysis with a local accuracy up to interatomic distance [11, 12]. This technique was also applied to reactive diffusion in the thin film system Al-Co within the temperature range of 200-400 °C [6]. In accordance with the previous results obtained by the authors [13, 14] by DSC, the first phase to nucleate and grow in the temperature interval from 200 to 400 °C is AI9C02. The authors of [6] have established that in the diffusion zone between A1 and Co formation and growth of a solid solution layer occurs first, and nucleation of AI9C02 particles is never observed before the diffusion zone reaches about 3-4 nm thickness. Thus, the appearance of the product phase is controlled by the width... 

The sandwich-hke sample Al-Cd-Mg was treated at 575 K by the method of diffusion zones superposition [11]. A1 and Mg interacted through the Cd layer (0.2 xm). After isothermal aimealing for t = 10, 100, 200, 500, and 900 h and measuring the concentration distributions, the diffusion paths were built and thus the system s phase diagram was constructed (Figure 9.1b). In that, the first phase. 

In [37, 38], a phenomenological scheme for interdiffusion in the two-phase zone at a quasiequihbrium process, taking into account diffusion fluxes in both phases of the diffusion zone, was proposed. The analysis was performed for a model pseudobinary system (Figure 10.7), which, however, does not impose any limitations with regard to the generahty of the developed phenomenological method. 

A modification of the diffusion flame method is the life-time method [99, 510]. It implies that the rate constant (for instance of reaction Na + RX NaX + R) is expressed as k = Nq/Nc, where Nq stands for the number of sodium atoms entering into the reactor per second, N for the number of sodium atoms in the reactor, and c for RX concentration. The ratio N/Nq represents the mean lifetime of an atom in the reaction zone (hence the name of the method). 

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