There are many potential uses for microshells having an outside diameter considerably below one millimeter. As one example, the microshells can be used as a filler material in structural members such as reinforced beams. Inasmuch as the strength-to-weight ratio of a mass of such shells increases as the size of the pores (here the bubbles in the shells) decreases, shells of especially small size are desirable.
Shells of a size down to about one-half millimeter can be readily produced by a number of methods. In one method, the molten shell material is extruded through an outer nozzle, while a gas is extruded through a concentric inner nozzle, to form a hollow stream that breaks up into gas-filled shells. As the diameter of the nozzles decreases, it becomes difficult to machine them and to position the tiny inner nozzle, and there is more frequent blocking of the space between nozzles by unwanted solid particles. Shells can be formed by dry chemical-blowing processes, but the materials are limited by available compatible frit-combinations, and are usually useful only for low-melt materials such as polymers, plastics, and low-temperature glasses. A method for producing microshells of a wide variety of materials, including refractory metals and alloys, would be of considerable value.