Thermal diffusion process

Carburization by Thermal Diffusion. Carburization of chemically processed metal or metal-compound powders is carried out through sohd-state, thermal diffusion processes, either in protective gas or vacuum. Carbide soHd solutions are prepared by the same methods. Most carbides are made by these processes, using loose or compacted mixtures of carbon and metal or metal-oxide powders. HaUdes of Group 5 (VB) metals recovered from ores by chlorination are similarly carburized. 

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

There exist a number of other methods for the separation of diamondoids from petroleum fluids or natural gas streams (1) a gradient thermal diffusion process [54] is proposed for separation of diamondoids (2) a number of extraction and absorption methods [53,83] have been recommended for removing diamondoid compounds from natural gas streams and (3) separation of certain diamondoids from petroleum fluids has been achieved using zeolites [56, 84] and a number of other solid adsorbents. 

In a study that addressed the effect of doping on quantum dots, the donor and acceptor levels were found to be practically independent of particle size [De3]. In other words, shallow impurities become deep ones if the dot size is reduced. Experimental observations show that the luminescence is not affected by doping if a thermal diffusion process, for example using a POCl3 source, is used [Ell]. Implantation, in contrast, is observed to effectively quench the PL [Tal4]. If the pores are filled with a medium of a large low-frequency dielectric constant, such as water or any other polar solvent, it is found that deep impurity states still exist,... 

The ThFFF separation system is made up of a flat ribbon-like channel obtained by placing a trimming-spacer between two flat bars kept at different temperatures (at the upper wall) and (at the lower wall), with AT = Tg- The thickness of the spacer defines the channel thickness w. In the channel cross section, the thermal diffusion process pushes the analyte toward the so-called accumulation wall, usually the cold wall (thermophobic substances) the combination of the flow profile and the thermal diffusion produces the fractionation. 

One of the unique characteristics of Th-FFF is that retention depends not only on the molar mass but also on the chemical composition of the polymer. This chemical differentiation is due to the dependence of the underlining thermal diffusion process on polymer (and solvent) composition [84]. This effect can likely be used to determine compositional distributions in copolymers and blends [111]. Figure 10 compares the resolving power of Th-FFF and SEC on two polymers of similar molecular weight but varying chemical composition. The polymers coelute in SEC because their sizes are similar whereas Th-FFF resolves the polymers because they differ in chemical composition. 

In contrast, due to the typical temperature effect on the lattice-controlled process of a four-center photopolymerization, in the case of a few diolefin crystals such as m-PDA Me (m.p. 138 °C), only the amorphous oligomer is produced at all the temperature ranges attempted. In the polymerization of m-PDA Me higher temperatures favor chain growth. This behavior is reasonably well explained by lattice-controlled dimerization followed by random cyclobutane formation yielding the oligomer through the thermal diffusion process (Sect. IV.b.)22. ... 

The thermal diffusion factor a is proportional to the mass difference, (mi — mo)/(mi + m2). The thermal diffusion process depends on the transport of momentum in collisions between unlike molecules. The momentum transport vanishes for Maxwellian molecules, particles which repel one another with a force which falls off as the inverse fifth power of the distance between them. If the repulsive force between the molecules falls off more rapidly than the fifth power of the distance, then the light molecule will concentrate in the high temperature region of the space, while the heavy molecule concentrates in the cold temperature region. When the force law falls off less rapidly than the fifth power of the distance, then the thermal diffusion separation occurs in the opposite sense. The theory of the thermal diffusion factor a is as yet incomplete even for classical molecules. A summary of the theory has been given by Jones and Furry 15) and by Hirschfelder, Curtiss, and Bird 14), Since the thermal diffusion factor a for isotope mixtures is small, of the order of 10", it remained for Clusius and Dickel (8) to develop an elegant countercurrent system which could multiply the elementary effect. 

The thermal diffusion process, however, is not affected by the shear and so the same equations as before apply. Thus, the thickness of the thermal layer becomes closer to that of the viscous layer. 

The first term is connected with isobaric entropy fluctuation, which gives a diffusive component, and the second term is connected with an adiabatic pressure fluctuation, which gives rise to a high-frequency acoustic wave. The pressure wave is an acoustic standing wave oscillating with a period of Tac = A/v. This component decays by a mechanical acoustic damping or run out effect of the wave if the number of the fringes is limited. After the complete decay of this wave, the isobaric wave appears. This wave just stays where it is and decays by the thermal diffusion process as described in Section I1.B.2. This equation may be further expanded as... 

Thermal diffusion of UF . The thermal diffusion process makes use of the small difference in 23su/Js u ratio that is established when heat flows through a mixture of UFj and UF. The principle of the process is described in Chap. 14. The process was used [Al] in 1945 in the United States by the Manhattan Prqect to enrich uranium to 0.86 percent U. This slightly enriched material was used as feed for an electromagnetic separation plant. Although the process could be put into production quickly because of the simplicity of the equipment, it... 

Abstract Shales present particular problems to drilling activities. Swelling, shrinkage and strength degradation may occur from pressure, chemical and thermal diffusion processes, leading to hole control problems that may lead to excessive times, hole loss, even blowouts. The article will describe qualitatively the major physical processes, field conditions, and explain how various mud systems address these issues. 

The dependence of retention on Dt has been used to develop a method for determining this constant, which is difficult to do using other methods. This has resulted in an expansion in the number of polymer types that now have tabulated Dy values that may contribute to the development of theories for the thermal diffusion process. 

The Bquid thermal diffusion process had been evaluated in 1940 by the Uranium Committee, when Abelson was at the National Bureau of Standards. 

The systems have been realized by embossing using polymer materials such as PMMA and PC The sealing of different layers is achieved by a thermal diffusion process In this process the layers are oriented in respect to each other, heated up to a temperature above the glass transition temperature of the material employed and welded together under a slight positive pressure... 

Meanwhile, other Manhattan Project scientists were enriching uranium through the thermal-diffusion process. By heating a thin, vertically held film of uranium on one side and cooling it on the other, they were able to draw the U-235 molecules to the top of the film. 

Uranium hexafluoride has a vapor pressure at 56°C equal to 765 torr, so UEs is a gas at 60°C and 1.000 atm. Various diffusion and thermal diffusion processes were used in the Manhattan Project of the United States in World War II to separate gaseous UEs molecules from UFg molecules. 

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