Nucleus supercritical

Nickel oligomers prepared in the presence of PA (Amax = 540 nm) (Section 20.4.2) may also act as catalysts for the reduction of Ni by hypophosphite ions. This requires, as shown by pulse radiolysis, a critical nuclearity, while free Ni cannot be reduced directly by H2PO2. Very low radiation dose conditions, just initiating the formation of a few supercritical nuclei, will lead to large particles of nickel [96]. 

Thus, consecutive addition (and detachment) of atoms leads eventually to the formation of supercritical nuclei that grow irreversibly and are not further considered in the steady state equations. Becker and... 

The non-steady-state time lag may approach zero when supercritical nuclei are already present. These nuclei are athermal. In this case, subsequent crystallization is determined by the crystal growth rate of the athermal crystals. 

A different kinetic realization of dynamic roughening is as follows If the rate of step-spreading is much greater than the nucleation rate, then a single layer can cover the facet before the subsequent nucleation event occurs. As the driving force is increased, this condition is more difficult to satisfy, and the facet becomes decorated with multiple supercritical nuclei simultaneously. Over a time period xs the nuclei... 

In the case of the Fe(III)-H20 system, the initial precipitation product is an amorphous ferric hydroxide, Fe(OH)3(s). It has been postulated by van der Woude and de Bruyn [35] that the formation of the corresponding supercritical nucleus may be viewed in terms of the reaction... 

Full shape adaptation may be limited by kinetics and geometry of the process. The most probable is the case when the shape optimization is restricted to the following the linear size of the supercritical nucleus must not be less than the size of a single unit cell (0.7 nm). In this case, the critical width of the diffusion zone which is close to the experimental result is obtained at a = 1.5 x 10 J/atom. When the diflFusion zone is 3 nm wide, the barrier amounts to lOOke T (nucleation is prohibited), and at 4 nm to 30k T (nucleation is possible). 

Nucleation is the process by which atoms (or ions) that are free in solution come together to produce a thermodynamically stable cluster. The cluster must exceed the critical nucleus size, n, and then it becomes a supercritical nucleus capable of further growth. If the nucleus is smaller than the critical size, spontaneous dissolution can occur. In the simulation, if the number of products inside the same droplet is smaller than the n parameter, they are considered free inside the droplet. When the number of products is equal or greater than n, they come together forming a stable nucleus. This nucleus has to be exchanged as a whole. 

In Figure 6 the sequence of events in a typical nucleation run is shown in the form of snapshots of the two-dimensional mixture. Beginning as before with a pure-A phase" (frame 1) the system evolves first to the metastable state (frame 2), where it remains for over 80,000 collisions. By collision 86,274 (frame 3), a supercritical nucleus has appeared. Thereafter this nucleus grows rapidly as the system evolves to the stable steady state (frame 4). This example was selected for display because it exhibits a particularly "long-lived" metastable state and, together with Figure 8, illustrates the wide range of induction times observed in the formation of (super-) critical nuclei. 

Neutrons produced in a chain reaction are moving very fast, and most escape into the surroundings without colliding with another fissionable nucleus. However, if a large enough number of uranium nuclei are present in the sample, enough neutrons can be captured to sustain the chain reaction. In that case, there is a critical mass, a mass of fissionable material above which so few neutrons escape from the sample that the fission chain reaction is sustained. If a sample is supercritical,... 

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

The rate, Rn, of random nucleation is therefore obtained from Eqns. (6.4) and (6.5) by recognizing that the addition of one more particle i to the critical nucleus makes it supercritical, which means that it will grow further. A simple way to represent the transfer frequency s of i across the surface of a critical nucleus is as follows... 

Many substances that are vapors at room temperature and atmospheric pressure may be used as NMR solvents in sealed tubes or at reduced temperature. For example, S02 has a vapor pressure of about 3 atm at room temperature and can be easily contained in sealed thin-walled, 5 mm diameter NMR sample tubes. Supercritical fluids are also used as NMR solvents in specialized sample tubes. For NMR studies of nuclei other than hydrogen and carbon, suitable solvents that do not contain the nucleus being studied are usually readily available. Frequently, the use of two or more solvents can provide valuable information on molecular structure, as indicated in Chapter 4. 

Using this technique, the spectra of thiophene (b.p. 84 °C) dissolved in supercritical ethylene and carbon disulfide was obtained. Because of the intrinsic insensitivity of the sulfur nucleus, 2 x 10 transients were acquired for these spectra, which resulted in signal-to-noise ratios of 4 1. The line widths obtained at 60°C and 200bar were approximately 200 Hz for thiophene in ethylene which correspond to narrowing factors of 7, when compared to the literature values <1985J(F2)63> for the line widths of the neat liquids at ambient conditions. 

In addition to the product nuclides, neutrons are produced in the fission reactions of This makes it possible to produce a self-sustaining fission process—a chain reaction (see Fig. 21.12). For the fission process to be self-sustaining, at least one neutron from each fission event must go on to split another nucleus. If, on the average, less than one neutron causes another fission event, the process dies out the reaction is said to be subcritical. If exactly one neutron from each fission event causes another fission event, the process sustains itself at the same level and is said to be critical. If more than one neutron from each fission event causes another fission event, the process rapidly escalates and the heat buildup causes a violent explosion. This situation is described as supercritical. 

Electron Transfer Mechanism. Thus, the first oxidation step of the hydroquinone occurs, provided the nuclearity is supercritical, n> nc, which means when the potential °(Ag, + i-Ag +i) becomes higher than the threshold °(Q "-QH2). Then the supercritical cluster acts as a nucleus for its own growth through an autocatalytic electron transfer (reactions 5,6) according to the mechanism that was summarized by reaction 12 ... 

---