Nucleation mode

Avrami exponent Crystal geometry Nucleation mode Rate determination Equation ... 

Some examples, which illustrate these different nucleation modes, and the conditions under which they apply, are given by the ubiquitous ice-water system ... 

By using the classical theory of ion induced nucleation to describe the growth of radon daughters from the free activity mode to the nucleation mode, we loose information about the size of the subcritical clusters. These clusters are all lumped together between the size of a pure H2O ion cluster at 75% r.h. and the size of the critical H2O-H2SO4 cluster. The model only does keep track of the growth by condensation of the radon daughters once they arrived in the nucleation mode. 

The environmental conditions for each of the cases considered below are summarized in Table III all these parameters are constant in time. The build up of the nucleation mode of the stable particles and the build up of both the nucleation and accumulation modes of the radon decay products is calculated, and the results are given after a process time of one hour. Figures 1 to 5 show the size distributions of stable and radioactive particles, and Table IV gives the disequilibrium, the equilibrium factor F, the "unattached fraction" f and the plate-out rates for the different daughters. 

Avrami coefficient, n Nucleation mode Growth dimensionality... 

Number concentrations are dominated by submicron particles, whereas the mass concentrations are strongly influenced by particle concentrations in 0.1-10 pm diameter range [13]. Similarly, the variability of the number-based measurements is strongly dominated by variability in smaller diameter ranges, whereas the variability of mass-based properties, such as PM10, are dominated by variability in the accumulation mode (usually around 500 nm of mass mean diameter) and in the coarse mode. This means the variabilities of these properties are not necessarily similar in shorter timescales, due to sensitivity of variance from very different air masses and thus aerosol types. This is demonstrated in Fig. lb, where the variance of the each size class of particle number concentrations between 3 and 1,000 nm is shown for SMEAR II station in Hyytiala, Finland. The variance has similarities to the particle number size distribution (Fig. la), but there are also significant differences, especially on smaller particles sizes. Even though in the median particle number size distribution the nucleation mode is visible only weakly, it is a major contributor to submicron particle number concentration variability. 

Particles in nucleation mode are generally formed due to condensation of the vapour present in the exhaust gases and nucleation (gas-to-particle conversion) in the atmosphere after rapid cooling and dilution of exhaust emissions [31,32]. These particles originate mainly from unbumed fuel and lubricating oil consisting of sulphates, nitrates and organic compounds [33]. 

The nucleation mode (ii ie < 0.1 pm) accounts for the majority of particles by number but because of their small size, these particles rarely account for more than a few percent of the total mass of atmospheric particles. These particles originate from condensation of supersaturated vapors from combustion processes and from the nucleation of atmospheric particles to form fresh particles (Seinfeld and Pandis, 1998 Horvath, 2000). 

The accumulation mode (0.1 < d ic < 1pm) particles included in this mode originate from coagulation of particles in the nucleation mode and from condensation of vapors onto existing particles. These particles usually accounts for a substantial part of the aerosol mass and for most of the aerosol surface area (Seinfeld and Pandis, 1998). 

Recently, a fourth mode has been introduced into this nomenclature It appears that particles with sizes less than 0.1 pm consist of two modes, the nucleation mode, which includes particles with dae between 0.01 and 0.03 pm representing quite recently formed particles, and the Aitken mode containing particles between 0.03 and 0.1pm (Horvath, 2000). 

Figure 8. Nucleation mode in a small Z,l0 particle. Due to reduced anisotropy at the surface, the reversal starts at the surface [89],...

Figure 10. Nucleation modes in homogeneous magnets (a) coherent rotation in a sphere, (b) curling in a sphere, and (c) curling in a cylinder. The arrows show the local magnetization M = Mz ez + m, where ez is parallel to the axis of revolution of the ellipsoid (cylinder).

In addition to the three modes described above, recent measurements have shown that there is often a distinct particle mode under 10-nm diameter (Fig. 2). There is no current agreement for the name of particles in this mode, which are interchangeably called ultrafme particles, nanoparticles, or nucleation mode particles. There are also alternative definitions for these terms, which can be a source of confusion. For example, the term ultrafme particles is sometimes employed to refer solely to particles with Dp = 3 -10 nm (e.g., in nucleation studies) or to all particles with Dp < 100 nm (e.g., in health and emission studies). Similarly, the term nanoparticles is sometimes employed as a description for all particles of Dp < 50 nm (regardless of mode), sometimes for particles of 10-nm diameter or less, and occasionally for any particle with Dp < 1 pm. In this review we use the common current definitions of ultrafme particles as those with Dp < 100 nm and nanoparticles as those with Dp < 50 nm. 

Kulmala M, Toivonen A, Ma kela JM, Laaksonen A (1998b) Analysis of the growth of nucleation mode particles observed in boreal forest. Tellus B 50 449-462 Kulrnala M, Piijola U, Ma kela JM (2000) Stable srrlpbadusters as a source of new atmospheric particles. Nature 404 66-69... 

It has therefore been concluded that (i) the most common process of zeolite nucleation relies on a primary nucleation mechanism and (ii) the most probable primary nucleation mode is heterogeneous and centred upon the amorphous phase of the reaction mixture (which for most clear solution syntheses is colloidal in nature). 

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