Diffusion effects measurement

There is an international standard, ISO 691455 which covers both the continuous and intermittent procedures plus the simplified intermittent method. Strip test pieces are used, 1 mm thick to minimise oxygen diffusion effects. Measurement at a series of temperatures is recommended and results are presented in graphical form but no consideration is given to interpretation. British Standards did not accept this revision of ISO 6914 and BS 903 Part A5256 is identical to the 1985 ISO method. The revision was not accepted in the UK because mistakes in handling comments resulted in inconsistencies. As an example, the title is now stress relaxation but a note says that this term is avoided ... 

The study found that the slot-type inlet at the bottom of the cabinet door resulted in higher pressure losses (lower CJ than the diffuser or perforated plate inlet. The exhaust configuration had little effect on Q or tracer gas clearance time. The study also concluded that an exhaust rate 0.118 m s for a two-cylinder cabinet was sufficient as little improvement was seen with an increase to 0.165 m s F" The slotted inlet took longer to clear a leak than either the perforated plate or diffuser inlet. Measured coefficients of entry for a two-cylinder gas storage cabinet are shown in Table 10.9. 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

Since it was proposed in the early 1980s [6, 7], spin-relaxation has been extensively used to determine the surface-to-volume ratio of porous materials [8-10]. Pore structure has been probed by the effect on the diffusion coefficient [11, 12] and the diffusion propagator [13,14], Self-diffusion coefficient measurements as a function of diffusion time provide surface-to-volume ratio information for the early times, and tortuosity for the long times. Recent techniques of two-dimensional NMR of relaxation and diffusion [15-21] have proven particularly interesting for several applications. The development of portable NMR sensors (e.g., NMR logging devices [22] and NMR-MOUSE [23]) and novel concepts for ex situ NMR [24, 25] demonstrate the potential to extend the NMR technology to a broad application of field material testing. 

The likelihood that Fick s laws will be obeyed in a crystal containing dislocations is dependent upon the spacing between the defects. Provided that this spacing is much greater than the diffusion length (Dt)1/2, where D is the bulk diffusion coefficient, Fick s laws are obeyed, with an effective (measured) diffusion coefficient, Deff, given by... 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

Double-sided electrolytic contacts are favorable for this method of diffusion length measurement because they are transparent and the required SCRs are easily induced by application of a reverse bias. Therefore homogeneously doped wafers need no additional preparation, such as evaporation of metal contacts or diffusion doping, to produce a p-n junction. Furthermore, a record low value of surface recombination velocity has been measured for silicon surfaces in contact with an HF electrolyte at OCP [Yal], Note that this OCP value cannot be further decreased by a forward bias at the frontside, because any potential other than OCP has been found to increase the surface recombination velocity, as shown in Fig. 3.2. Note that contaminations in the HF electrolyte, such as Cu, may significantly increase the surface recombination velocity. This effect has been used to detect trace levels (20 ppt) of Cu in HF [Re5j. 

For small chains in solution the translational diffusion significantly contributes to the overall decay of Schain(Q>0- Therefore precise knowledge of the centre of mass diffusion is essential. Combing dynamic light scattering (DLS) and NSE revealed effective collective diffusion coefficients. Measurements at different concentrations showed that up to a polymer volume fraction of 10% no concentration dependence could be detected. All data are well below the overlap volume fraction of (p =0.23. Since no -dependence was seen, the data may be directly compared with the Zimm prediction [6] for dilute solutions ... 

A further benefit of the low absorbtivity of most samples in the NIR is that measurements involving scattering effects (both diffuse transmission and diffuse reflectance) are possible. The penetration depth of the sampling beam in diffuse reflectance measurements of powders in the NIR can be on the scale of millimeters, despite the numerous refractive index interfaces and extended optical path involved. Thus relatively large volumes of material can be interrogated, avoiding problems of surface contamination and sample nonhomogeneity. 

F. Girard, P. Poulet, I.J. Namer, J. Steibel, J. Chambron, Localized T-2 measurements using an osiris-CPMG method—application to measurements of blood oxygenation and transverse relaxation free of diffusion effect, NMR Biomed. 7 (1994) 343-348. 

Use of the lower time would give a big advantage in respect to the upper limits of current density at which an electrode kinetic measurement can be made See of diffusion control. At 0.1 ms, the current density will be free of diffusion effects because it is 100 times higher than that at 1 s, when diffusion will in any case affect the measurement (Fig. 8.4). 

In the rest of this section, the various ways transients can be used to reduce diffusion effects and the effects of contamination of the electrode surface by adsorption from solution will be discussed, as well as any counter-limitations that some types of transient measurements may involve. 

Gravitational sedimentation causes a change in the particle size distribution anywhere in and below the cloud compared with the size distribution at stabilization time. Thus, to reconstruct the size distribution at stabilization time, corrections must be applied to the size distributions measured in the samples. These corrections were calculated by assuming Stokesian settling modified by a drag slip correction. It was assumed further that at stabilization time the cloud was axially symmetric and consisted of spherical particles. Wind and diffusion effects were neglected. 

The primary difference between D and D is a thermodynamic factor involving the concentration dependence of the activity coefficient of component 1. The thermodynamic factor arises because mass diffusion has a chemical potential gradient as a driving force, but the diffusivity is measured proportional to a concentration gradient and is thus influenced by the nonideality of the solution. This effect is absent in self-diffusion. 

The pure compound rate constants were measured with 20-28 mesh catalyst particles and reflect intrinsic rates (—i.e., rates free from diffusion effects). Estimated pore diffusion thresholds are shown for 1/8-inch and 1/16-inch catalyst sizes. These curves show the approximate reaction rate constants above which pore diffusion effects may be observed for these two catalyst sizes. These thresholds were calculated using pore diffusion theory for first-order reactions (18). Effective diffusivities were estimated using the Wilke-Chang correlation (19) and applying a tortuosity of 4.0. The pure compound data were obtained by G. E. Langlois and co-workers in our laboratories. Product yields and suggested reaction mechanisms for hydrocracking many of these compounds have been published elsewhere (20-25). 

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