Size-dependent artifacts

One may also wonder about the effect of impurities such as surface oxidation on the measurement. Although XRD and XPS revealed no impurities in the Ni samples, for instance, one cannot exclude the existence of trace impurities. However, if all the samples were prepared and measured under the same conditions and we use the relative change of the quantities, artifacts caused by impurities should be minimized, and the results are purely size dependent... 

In many molecular dynamics simulations, equilibration is a separate step that precedes data collection. Equilibration is generally necessary to avoid introducing artifacts during the heating step and to ensure that the trajectory is actually simulating equilibrium properties. The period required for equilibration depends on the property of interest and the molecular system. It may take about 100 ps for the system to approach equilibrium, but some properties are fairly stable after 10-20 ps. Suggested times range from 5 ps to nearly 100 ps for medium-sized proteins. 

The carbon raw material in the form of coke, coal or natural or synthetic graphite is ground and sieved (following calcination at 700-1300°C to control volatiles, if necessary) to give a desired particle size distribution. The distribution depends upon the size of the artifact to be formed and the method of forming. 

Graphitization is accomplished by passing an electrical current through cither bed. Considerable resistive heating occurs where temperatures exceeding 3000 °C are possible. Normal process parameters utilize heating rates between 30 to 70°C per hour to 2500°C. Total time at temperature depends on the size of the artifact. Several more days are needed to cool the furnace before unpacking. 

The preceding analysis shows that there is a distinct lower bound for the size of the simulation cell. Below this bound, finite size artifacts heavily bias the thermodynamics and kinetics of ion accumulation. Therefore, it seems unadvisable to design simulations using neutralizing counterions alone, especially if the objective is to understand ion-mediated conformational transitions or to understand how ions interact with a macroion. The minimal number of excess ions needed to simulate 800 mm excess monovalent salt was nearly identical 800 pairs) for all of the systems studied (32 nt A-RNA, B-DNA, and the Tar—Tar RNA kissing-loop complex), indicating a simple dependence on net macroion charge. For other macromolecular solutes or other concentrations of excess salt, a box-size... 

Data Transformations. The deconvolution of correlation functions yields the intensity-weighted diffusion coefficient distribution. Transformation to a size distribution requires division by the diameter raised to a high power and by an angular dependent function which oscillates over several orders of magnitude for particles larger than a half a micron. Given the artifacts from the primary deconvolution, extreme caution is advised when transforming data. 

An experimental test to verify the absence of significant concentration gradients inside the catalyst pellet is based on the inverse proportional relation between the effectiveness factor and the pellet diameter for strong internal diffusion limitations. Hence, a measured rate which is independent of the pellet size indicates that internal diffusion limitations can be neglected. Care should be taken to avoid artifacts. External heat transfer effects also depend on pellet size and for exothermic reactions might compensate the internal diffusion limitations. If the catalyst pellet consists of a support with an non-uniformly distributed active phase, crushing and sieving to obtain smaller pellets is hazardous. 

For now, the bimodal distribution may be an artifact. The two lifetimes can be considered as lower and upper bounds of the pore size distribution. This technique is the only one available that can provide non-destructive depth profiles without sample preparations other than mounting them in the vacuum system. Depth profiled lifetime data are currently being collected. This is practical due to the high data acquisition rate of 3TO3 to 104 lifetime events per second, depending on the implantation depth. 

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