Temperature-activation process

In contrast to the influence of velocity, whose primary effect is to increase the corrosion rates of electrode processes that are controlled by the diffusion of reactants, temperature changes have the greatest effect when the rate determining step is the activation process. In general, if diffusion rates are doubled for a certain increase in temperature, activation processes may be increased by 10-100 times, depending on the magnitude of the activation energy. 

As stated in the introduction to this chapter, CVD can be classified by the method used to apply the energy necessary to activate the CVD reaction, i.e., temperature, photon, or plasma. This section is a review of temperature-activation process commonly known as thermal CVD. 

The drawing of fibrils and their breakage have been described by means of a classical stress and temperature activated process, with activation energies and... 

Table 10.4 also shows the values obtained by Kondo et al. [137], with varying ethanol concentrahons at different temperamres. As the concentration of water in the feed increases, the separation factor increases. The water adsorbs in the inter- and intracrystalline pores of the zeolite A, hindering the permeahon of ethanol, which typically occurs up to a value around 10 wt% of H2O where there is no more increase, as it was observed by several authors [115,141]. The water flux increases as the water concentration increases, because the driving force for pervaporation increases. The temperature does not affect signihcantly the separation factor however, the flux rises markedly since the mobility and diffusion of the molecules increases with temperamre, due to surface diffusion is a temperature-activated process. 

Creep and yielding are stress- and temperature-activated processes, which in many mateiMs, including pdymers, follow the Eyring rate equation ... 

Because surface smoothing, coarsening, grain growth, and sintering all generally involve solid-state mass transport, they tend to be temperature-activated processes that obey an exponential Arrhenius-type rate law. Thus, the kinetic constants appearing in the equations for these processes (e.g., B , Kj),K, C)... 

Furthermore, an interesting aspect of the Ru effect relates to the effect of temperature and the optimum Pt Ru ratio. Gasteiger et al. showed that dissociative methanol adsorption can occur on Ru sites as well, but it is a temperature-activated process [94]. Therefore, at low temperatures (e.g., 298 K) a higher Pt Ru atomic ratio (above 1 1) is required to facilitate the dissociative adsorption and dehydrogenation of methanol preferentially on Pt, whilst at high temperatures (e.g., 333 K and above) a surface richer in Ru is beneficial (e.g., 1 1 at. ratio) since Ru becomes active for chemisorption and the rate determining step switches to flie reaction between COad and OHad [94]. 

It has been shown that there exists a correlation between the properties of samples obtained via an abrasion technique, to the position within the active carbon particles. Anisotropy of porous structure is due to burn -off of the carbonaceous substance which reduces radially from the outer surface of the particle to its inner core. The drop of burn-off, in turn, is the result of mass transfer resistance caused by the high temperature activation process. 

Micropore Diffusion. In very small pores in which the pore diameter is not much greater than the molecular diameter the diffusing molecule never escapes from the force field of the pore wall. Under these conditions steric effects and the effects of nonuniformity in the potential field become dominant and the Knudsen mechanism no longer appHes. Diffusion occurs by an activated process involving jumps from site to site, just as in surface diffusion, and the diffusivity becomes strongly dependent on both temperature and concentration. 

Catalyst CAS Registry Number Activator Processing temperature, °C... 

Electrical conduction ia glasses is mainly attributed to the migration of mobile ions such as LE, Na", K", and OH under the influence of an appHed field. At higher temperatures, >250° C, divalent ions, eg, Ca " and Mg ", contribute to conduction, although their mobiUty is much less (14). Conduction ia glass is an activated process and thus the number of conducting ions iacreases with both temperature and field. The temperature—resistivity dependence is given... 

The conductivity of solid dielectrics is roughly independent of temperature below about 20°C but increases according to an Arrhenius function at higher temperatures as processes with different activation energies dominate [ 133 ]. In the case of liquids, the conductivity continues to fall at temperatures less than 20°C and at low ambient temperatures the conductivity is only a fraction of the value measured in the laboratory (3-5.5). The conductivity of liquids can decrease by orders of magnitude if they solidify (5-2.5.5). 

In chemical activation processes, the precursor is first treated with a chemical activation agent, often phosphoric acid, and then heated to a temperature of 450 -700 °C in an activation kiln. The char is then washed with water to remove the acid from the carbon. The filtrate is passed to a chemical recovery unit for recycling. The carbon is dried, and the product is often screened to obtain a specific particle size range. A diagram of a process for the chemical activation of a wood precursor is shown in Fig. 3. 

---