Carrier velocity

The Johnsonfigure of merit, based on saturated carrier velocity and dielectric strength (product of power x frequency squared x impedance), predicts the suitability of a material for high power applications. It is normalized with the value of one given to silicon. As shown in Table 13.2 below, diamond is clearly the preferred material on this basis. 

The smaller the particle size, the faster the rate of generating theoretical plate (HETP) per unit of time. Figure 14.8 shows a plot of HETP versus linear carrier velocity u for small particles. It indicates that the smaller the particle size, the lower the HETP. It is also important to note that small particles provide nearly the same HETP over a wider range of flow rate.14... 

As a rule of thumb the optimum depletion layer thickness is that defined by half the period of the required modulation response, i.e., at 2 GHz bandwidth and a carrier velocity of 100,000 m/sec the optimum depletion layer thickness is ca. 50... 

Finally, the liquid carrier velocity should be considered. The flow velocity is strictly related to the operating mode. In normal elution mode, both analysis time and resolution decrease as flow velocity decreases. In general, in normal mode, flow rates in the 0.1-0.5mL min range are employed. Steric and hyperlayer modes operate at flow rate values higher than those typical of normal mode. 

In addition to the number of electrons, the other factor in Eq. (6.9) that affects conductivity is the electron mobility, pe. The mobility is the average charge carrier velocity, or drift velocity, v, divided by the electric field strength, ... 

Figure 7.33 Variation of tape thickness with carrier velocity and slurry viscosity. Adapted from J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

From a column performance point of view a gas having a small diffusion coefficient is desirable (high molecular weight, e.g., n2 co2 Ar) or l°w carrier velocities while large diffusion coefficients (low molecular weight, e.g.,, He) are best at high... 

This equation shows that R is given by the ratio of particle displacement velocity V to average carrier velocity , or equivalently to the ratio of channel void volume V° to the retention volume V. Thus R is experimentally accessible because V is the measured volume required to displace a given particle size" through the channel. However, the equation also shows that R is related to X, which is related to d through the previous equation. Thus a linkage is formed between particle diameter d and experimental retention volume V. It has been estimated that particle diameters accurate to 1-3% can be obtained by using this approach (2). 

By refining column design, the Instrumental band broadening H, has been reduced to a level insignificant in comparison with the other terms of Equation 7. The nonequilibrium term, linear in carrier velocity , also depends on channel thickness w and inversely on particle diffusion coefficient D. The dependence of coefficient x on X is well known, although the function is complex (14). Its limiting form is described by... 

The high-field saturation of the carrier velocity can have various origins, e.g. a finite bandwidth of a non-parabolic transporting (here valence) bands, or the emission of optical phonons. It is believed that the high-field saturation of the drift carrier velocity in the crystal directions where the band model concept can be applied is due to the first one. Then [420],... 

A further experimental problem is caused by the action of hydrodynamic lift forces in A-Fl-FFF. As the mean carrier fluid velocity varies along the channel length in the rectangular channel geometries, the equilibrium positions of the particles also vary. Hence conditions may be encountered where the carrier velocity close to the outlet of a rectangular A-Fl-FFF channel falls to such a low level that lift forces are unable to counter the drag of the flow through the membrane. These particles then make contact with the membrane and do not elute [250]. 

Of the three common mobilities used for TTFT assessment, the effective mobility is probably the best channel mobility figure-of merit since (i) it is evaluated in the linear regime of operation where the channel is most uniform and the concept of mobility as a proportionality constant betw een the electric field and carrier velocity is most appropriate, and (ii) the term... 

The solute capacity factors measured at the optimum linear carrier velocity increase monotonously as the inverse of the column diameter, as shown in Figure 2. This result is to be expected, since the phase ratio decreases in the same fashion. However, since the mobile-phase volume decreases geometrically x on reducing the column radius, raw analysis times for a givmi linear carrier velocity, while longer with the columns of smaller ID, are not inordinately so. The values of tjjm (min m ) for columns 1 to 6 respectively were 2.38, 2.56, 2.62, 2.6 3.03, 3.63, while column 7 gave 3.14 (see below). 

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