Porous-sphere combustion

FLAME EXTINCTION FROM THE UPSTREAM PORTION OF A DROP IN MOTION. In his studies of the influence of relative air velocity on the combustion of liquid fuel spheres, Spalding (51, 56) noted a critical velocity above which flame could not be supported at the upstream portion of the sphere. He observed that the flame blew off and resided solely in the sphere s wake. In tests with kerosine, the air (20° C.) velocity at extinction varied linearly with sphere diameter (range 0.7 to 2.6 cm.), and the ratio Ubi2n was about 100 seconds-1. A similar result is obtained from the data on flame extinction of burning camphor spheres (15, 59). The near proportionality between extinction velocity and diameter was taken as supporting evidence for a theory on flame extinction advanced by Spalding (59). More recent experimental work with porous spheres and n-butyl alcohol as fuel does not support this relationship (1), because it was found that the extinction velocity is proportional to the square root of the drop diameter. 

Kiley (35) attempted to burn liquid monopropellants in nitrogen by the method of suspended droplets and by the technique of a porous Alundum sphere (7 mm. in diameter). These experiments did not yield stable combustion in an inert gaseous atmosphere. For combustion in air he reports the evaporation constants for hydrazine and nitromethane (Table IV). 

Combustion Synthesis of AIN Porous-Shell Hollow Spheres... 

As a result, the inner A1 core was gradually evacuated, and finally the AIN porous-shell hollow spheres were obtained. Actually, many gray powders were observed in the combustion chamber after the reactions. XRD result (not shown here) indicated that these powders were hexagonal AlN phase, and carbon phase could not be detected, which might be due to the limitations of the measurement technique. 

In summary, ID AIN nanowhiskers, 3D flower-like AlN microstructure, and AlN porous-shell hollow spheres with uniform morphologies have been successfully fabricated by CS methods. The morphologies of products can be controlled by manipulating the combustion parameters. The present processes for synthesis of the AIN micro/nanostructures are facile, productive, reproducible, and energy saving. The as-synthesized AIN nanowhiskers have been applied to produce fillers as reinforcement for electrical packaging. However, in order to realize... 

Porous Si02 spheres were prepared successfully by means of fume pyrolysis using gel solutions derived from Si-alkoxides. Particle sizes in all samples were about 1.5 pm large but surface areas varied from 20 m2.g-1 to about 500 m2.g-1 with viscosities of source solution. Pores in Si02 spheres were formed with almost the same size and the mean pore sizes were controlled in the range of about 2.2 nm to 0 7 nm in radii. The pore size and surface area were closely connected with the preparation condition and it was proved that the properties of formed Si02 were associated with the gel structures in the fumed droplets and the combustion conditions under O2 stream. 

The formation of open and porous structures with extremely large surface area is of high technological significance, because this structure type is very suitable for electrodes in many electrochemical devices, such as fuel cells, batteries and sensors [1,2], and in catalysis applications [3]. The template-directed synthesis method is most commonly used for the preparation of such electrodes. This method is based on a deposition of desired materials in interstitial spaces of disposable hard template. When interstitial spaces of template are filled by deposited material, the template is removed by combustion or etching, and then the deposited material with the replica structure of the template is obtained [4, 5]. The most often used hard templates are porous polycarbonate membranes [6, 7], anodic alumina membrane [8-10], colloidal crystals [11, 12], echinoid skeletal stractures [13], and polystyrene spheres [14, 15]. 

---