Particle melting

Allen G L ef a/1986 Small particle melting of pure metals Thin Solid Films 144 297... 

The reacting particles melt rapidly, and the droplets fall to the slag layer. The sulfide drops settle through it to form the matte phase. Any oxidi2ed copper is reduced to the matte by the following reaction ... 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

The combustion wave of an HMX composite propellant consists of successive re-achon zones the condensed-phase reachon zone, a first-stage reaction zone, a second-stage reaction zone, and the luminous flame zone. The combustion wave structure and temperature distribution for an HMX propellant are shown in Fig. 7.47. In the condensed-phase reaction zone, HMX particles melt together with the polymeric binder HTPE and form an energetic liquid mixture that covers the burning surface of the propellant. In the first-stage reaction zone, a rapid exother-... 

The Mg particles melt at the burning surface and are partially oxidized by the fluorine produced by thermal decomposition of the Tf particles. Meanwhile, the Tf particles decompose completely to produce fluorine and other gaseous fragments. During decomposition of the Tf particles, some of the Mg particles melt and form agglomerates on and above the burning surface, while others are ejected into the gas phase whereupon they are rapidly oxidized by the fluorine. The oxidation of each Mg particle occurs at the molten layer on its surface. 

The metal particles melt and bum as near spherical droplets as they are ejected at typical speeds of about 0.3 to 3 msAfter the titanium is consumed, rapidly cooling spheres and hollow spheres of oxide remain which either fall to the ground or drift away with the smoke, depending on their size. 

This applies, however, only within limits. Many solids are somewhat mobile and can flow very slowly. In that case methods and models of capillarity can be applied. One case where capillarity plays an important role is sintering. In sintering a powder is heated. At a temperature of roughly 2/3 of the melting point of the material the surface molecules become mobile and can diffuse laterally. Thereby the contact areas of neighbouring particles melt and menisci are formed. When cooling, the material solidifies in this new shape and forms a continuous solid. 

The metal product to be coated is preheated to a temperature which will melt the powder. Then the product is dipped into the fluid bed. The powder particles melt and flow onto the metal surface. Coatings up to 2.5 mm (0.1 in.) thick can be applied in a single dip. 

When the powder particle melts, it wets the substrate (Figure 10-12). The liquid is pulled over the surface by a line tension a. This depends on the interfacial tensions of liquid, solid and gas it is lower than the surface tension. This tension is counteracted by forces due to the viscosity r]. The flattening will also depend on the initial size Rq of the drop. Finally you would expect the drop to become flatter with increasing time t. 

The onset of particle mobility often coincides with particle melting and results in rapid sintering of supported metal catalysts. Thus, prediction of the melting temperature of supported metal clusters is of more than academic interest. 

It is well known that small particles melt at a lower temperature than the corresponding bulk solid. Buffat and Borel measured a monotonic lowering of melting temperature with decreasing diameter for gold clusters supported on carbon substrates (7). They accounted for their data by means of a thermodynamic model due to Pawlow (16), which states that the melting temperature of small particles is inversely proportional to the particle radius. 

The problem of glass transition, therefore, reduces to finding out the temperature dependence of cluster size, which in turn is related to the rate at which the surface particles melt into the tissue. Using plausibility arguments, an assumption has been made that the population of the vibrational ground state in the tissue remains constant till Tg. If fo represents the fraction of particles in the ground vibrational state of tissue region at a temperature T and if N, is the total number of particles in the... 

In some samples, a difference between the melt-onset temperatures collected from the DSC and the hot-stage accessory may be observed. These differences may be as small as tenths of °C, or as large as 10 to 20°C. In general, the finer particles melt initially, followed by the larger particles. 

Note that it has been reported that Mg/AI particles melt in two stages, first releasing Mg from the solid particle [Popov].)... 

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