Artificial Nucleation Sites Research Articles

Coarsening phenomenon of Si clusters on artificial nucleation sites

A coarsening phenomenon of Si clusters is investigated in the process of nucleation and growth. In the early stage of chemical vapor deposition, multiple fine clusters of Si are nucleated on the small portion of an artifical nucleation site. A large cluster emerges among the fine clusters and grows rapidly as the deposition proceeds. As a result, the site is occupied by only one large cluster, while the fine clusters seem to disappear. Such a phenomenon is responsible for the mechanism in which a single nucleus is selected to grow on the site.

Manipulation of nucleation sites and periods over amorphous substrates

A selective nucleation based crystal growth technique over amorphous substrates is originated. The method manipulates nucleation sites and periods, and hence, controls the grain boundary location by modifying the substrate surface. In Si, Si3 N4 provides artificial nucleation sites, 1–2 μm in diameter, 100 μ m in period, which is surrounded by SiO2 . One Si nucleus is formed exclusively in a small portion of Si3 N4 . The highly faceted and periodically located nuclei grow over SiO2 up to 100 μm in diameter before impingement. A field-effect transistor fabricated inside the island operates comparably to the bulk Si control.

Investigation of boiling heat transfer at a surface with a system of cylindrical cavities under conditions of free motion

Expressions are proposed for calculating the boiling curve for water, ethyl alcohol and R 113 on a surface with artificial nucleation sites in the form of cylindrical cavities.

Boiling Heat Transfer in a Shallow Fluidized Particulate Bed

Experimental data are presented which indicate the effects of a thin layer of unconfined particles on saturated pool boiling heat transfer from a horizontal surface. Results are presented for two different types of particles: (1) 0.275 and 0.475-mm-dia glass spheres which have low density and thermal conductivity, and (2) 0.100 and 0.200-mm-dia copper spheres which have high density and thermal conductivity. These two particle types are the extremes of particles found as corrosion products or contaminants in boiling systems. To ensure that the surface nucleation characteristics were well defined, polished chrome surfaces with a finite number of artificial nucleation sites were used. Experimental results are reported for heat fluxes between 20 kW/m2 and 100kW/m2 using water at 1 atm as a coolant. For both particle types, vapor was observed to move upward through chimneys in the particle layer, tending to fluidize the layer. Compared with ordinary pool boiling at the same surface heat flux level, the experiments indicate that addition of light, low-conductivity particles significantly increases the wall superheat, whereas addition of heavier, high-conductivity particles decreases wall superheat. Heat transfer coefficients measured in the experiments with a layer of copper particles were found to be as much as a factor of two larger than those measured for ordinary pool boiling at the same heat flux level. The results further indicate that at least for thin layers, the boiling curve is insensitive to layer thickness. These results are shown to be consistent with the expected effects of the particles on nucleation, fluid motion, and effective conductivity in the pool at or near the surface. The effect of surface nucleation site density on heat transfer with a particle layer present is also discussed.

Photographic Observations of Bubble Formation in Flashing Nozzle Flow

Visual observations have been made of bubble growth in the nucleation region of flashing flow of initially subcooled water in a converging-diverging nozzle. Experiments performed under various flow rates, saturation temperatures, turbulence levels, noncondensable gas content, and artificial nucleation sites failed to produce isolated spherical bubbles of the size or density predicted by common bubble nucleation and growth models. Heterogeneous nucleation in the bulk flow was never observed and it is concluded from bubble growth rates that the role of convection in the heat and mass transfer environment of the bubbles is an important consideration in the physics of flashing flows near the nucleation region.

Bubble nucleation in saturated and subcooled boiling

AbstractAn experimental investigation is reported for water boiling at atmospheric pressure on a copper surface in which the nucleation of bubbles was observed at an artificial nucleation site for five levels of heat flux between 11,700 Btu/hr ft3 and 19,600 Btu/hr ft2 and various levels of subcooling from 0°F to 32°F. Four mathematical models were evaluated. The experimental results were found to provide excellent agreement with the predictions of the Han and Griffith model for bubble nucleation.

Pool boiling from large arrays of artificial nucleation sites

Pool boiling heat transfer from large and controlled arrays of artificial nucleation sites was studied experimentally. Organic liquids were boiled from artificial sites of uniform size, shape and spacing, drilled in superfinished horizontal surfaces at site densities of 8 and 16 per cm 2. The results confirm the existence of two heat transfer regimes which are characteristic of boiling from a constant number of sites [2]. Appropriate correlations, based on a physical model, were developed for predicting the heat transfer rate in the two regimes over a wide range of site densities and Prandtl numbers. A heat transfer mechanism is proposed and discussed.