Corrected diffusivity, temperature

The temperature dependence of the corrected diffusivity follows the usual Eyring expression... 

The substrate temperature is a very important deposition parameter, as it directly affects the kinetics of ad- and desorption of growth precursors, surface diffusion, and incorporation. Actual substrate temperatures may differ from substrate heater setpoints. Calibration of temperature readings is needed, so as to report the correct substrate temperature. 

Corrected Diffusion Coefficients (D x 109 cm2/s) of H-ZSM-11 and H-SSZ-24 Zeolites at Different Temperatures... 

Arrhenius plot showing temperature dependence of corrected diffusivity for N2 in various different samples of 4A crystals. 

Optional At 273 K, 7 23 (for He-Ar) is 0.653 cm s . From Du, Di, and D23 at room temperature, calculate c/12, c/13, and c/23. (To correct diffusion constants from one temperature to another, assume a dependence if the temperature change is small. This is only approximate, since the ds may vaiy in some degree with the temperature.) Then obtain d-[, di, and c/3 and use these to calculate D, Di, and D, from Eq. (V-35). Determine the ratios of the self-diffusion constants to their respective viscosities. How do these compare with the theoretical ratios discussed in the introductory section of Chapter V ... 

Fig. 5 Loading dependence of corrected diffusivities (Do) for ethylbenzene in H-ZSM-5 at various temperatures, measured by piezometric technique. From Schumacher et al. [20] with permission...

Fig. 8 Effect of temperature on the amounts (a, upper part) adsorbed and (b, lower part) desorbed of ethylbenzene in ZSM-5 for different pressure jumps and three final partial pressures (115, 230, and 460 Pa) and effect on the (corrected) diffusivities (vide infra). The uptake curves 1, 2, 3, and 4 were determined at 355, 375, 395, and 415 K, respectively...

Dq pressure- and temperature-corrected diffusion coefficient in free air. 

Figure 8 shows the self-diffusion coefficients, Db, of methane, ethane and propane as a function of soibate loading at various temperatures. These corrected diffusion coefficients were calculated from Equation 12 using diffusion coefficients obtained fi m both sorption and desorption half-cycles. These diffusion coefficients were identical. The corrected, Db, diffusion coefficients in Figure 8 are <5 smaller than the intracrystalline self-diffusion coefficients measured directly by NMR which are also included in this figure and are in close agreement with the corrected diffusion coefficients obriuned by the full FR method. 

Using the temperature dependence form for the Henry constant and the corrected diffusivity... 

Djo DOmuO corrected diffusivity at reference temperature Tq mVsec... 

The relationship between sorbate activity and concentration for zeolitic systems is highly nonlinear so, except at very low concentrations approaching the Henry s law region, the thermodynamic correlation factor d np/d nc [Eq. (5.6)] is large, approaching infinity in the saturation region of the isotherm. In analyzing the dependence of diffusivity on concentration and temperature, it is therefore important to consider the corrected diffusivity or the self-diffusivity rather than the transport diffusivity. 

The temperature dependence of the corrected diffusivity is illustrated in Figure 5.23, and the correlation of diffusional activation energy with van der Waals radius is indicated in Figure 5.12. It is evident that the trend is very similar to that observed with 4A zeolite although the magnitude is somewhat larger. 

The elevation of the buffer temperature and the induced temperature gradient would in turn affect the temperature-dependent properties of the buffer and sample, including the viscosity, dielectric constant, electrical conductivity, diffusivity, electromobilities, and pH value. Therefore, various cooling methods, such as the use of a liquid coolant in a well-thermostatted system and the use of air by natural or forced convection in a nonthermostatted system, have been developed to maintain the correct capillary temperature and thus minimize Joule heating effects. On the other hand, a potential use of Joule heating effects to precisely control the temperature in microfiuidic devices was recently reported. Analyses based on simplified the-... 

Figure 12. Variation of diffusional time constant (Do/R ), dimensionless Henry constant (K) and the product KDo with temperature. (From data of Chen et al [64]). The value of Do/R derived from the reaction rate measurements ( ) is also shown. Corrected diffusivities are derived from the reported integral diffusivities according to the analysis of Garg and Ruthven [69].

Do thermodynamically corrected Tabsolute temperature diffusivity (see Eq. 7) mutual diffusivity... 

Another special application instrument known as the laser pyrometer incorporates a built-in laser projection system and uses the reflected energy of the active laser to measure target reflectance. A built-in microcomputer calculates target emissivity based on the measured reflectance and uses this to provide a corrected true temperature reading. The laser pyrometer is useful for high-temperature diffuse target surfaces. 

Physical-chemical data representative of the site appear in Table II. A particular PCB congener, 2, 4, 2 , 4 -tetrachlorobiphehyl was used to specify a molecular diffusivity, and value of the biodiffusion coefficient representative of Lake Huron sediments was used in the absence of one for the Fox River. The latter was corrected for temperature a factor of 2 increase for each 10 C increase was assumed (14). A correlation developed for stream-beds was used to estimate p as a function of flow rate (9). The results of these computations appear in Table II. 

Diffusion coefficient of hydrogen in water. At 298 K, the diffusion constant of hydrogen in water is 5.85 x 10" cmVs Perry and Chilton 1973). Correcting for temperature using >h2 Pw/7 = constant Logan 1999) where p , is the dynamic viscosity of water at temperature T (K), we calculate at 303 K the diffusion coefficient is... 

X 10 m /s. Diffusion coefficients may be corrected for other conditions by assuming them proportional to Schmidt numbers depend only weaMy on temperature (113). 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

Class B direct dyes have poor leveling power and exhaustion must be brought about by controlled salt addition. If these dyes are not taken up uniformly in the initial stages it is extremely difficult to correct the urdevelness. They are dyes that have medium—high affinity and poor diffusion. In their apphcation the cellulose is entered into a dyebath containing ordy dye. The salt is added gradually and portionwise as the temperature is increased and possibly the final additions made after the dyebath has come to the bod. 

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