The diversity of applications for spherical materials, often referred to as hollow or solid microspheres, microballoons, shells and microcapsules, span a wide range of technologies and attracts the talents of individuals from a broad set of disciplines. Consequently, this knowledge and capability is widely dispersed. A prime objective of this symposium was to bring that diverse community together to share, for mutual benefit, that knowledge and capability. This article highlights some of these diverse technologies and applications and identifies some of the many important literature sources encountered.

Sol-gel derived ceramic bubbles include shaped and fired, porous or impermeable nonvitreous ceramic microcapsules of metal oxide and non-oxide, are made by non-melt process involving the equilibrium boiling point method with controlled thermal gelation. Discrete, free-flowing, ceramic microbubbles consist of a non-oxide component and/or an oxide component having diameters in the range of I to 300 micrometers and each having a wall thickness of less than 10 percent of the diameter of the bubble. In this process, a sol precursor and a bloating agent, when added to a bubble promoting medium under proper conditions, provide green gelled microbubbles which after firing are strong, well formed, ceramic microbubbles. The microbubbles are non-vitreous, sol-gel derived, fine microstructured, uniform, hollow, smooth, and are essentially all oxide or non-oxide or combinations of both. The microbubbles can be used as fillers for metal, glass, ceramic, and polymer or filled with a selected solid, liquid or gas for purpose of storing, transporting, or facilitating the use of the same.

Two novel methods particularly suitable for continuously fabricating uniform glass spheres are described, one suitable for spheres micrometers to millimeters in diameter and the other for spheres of diameters in the submicron range, down to tens of nanometers. Both methods start from liquid precursors and combine droplet generation which is effected by a nozzle with sol-gel processing. One difference is that in the latter method, which is termed the charged liquid cluster beam (CLCB) technique, one employs flow-limited field-injection electrostatic spraying to further reduce the size of the drops to the nanometer range in the droplet generation process. Using the two methods, uniform hollow silica aerogel spheres, micrometer to millimeter in diameter, of controlled porosity and silica spheres with diameters in the nanometer range were produced. By utilizing the appropriate liquid precursors, the present methods can fabricate spheres composed of a variety of materials, including glass. Due to its ability to fabricate nanodrops, the CLCB technique is suited to fabricating nanoparticles as well as thin films of controlled stoichiometry and chemical composition.

Metastable aqueous borosilicate solutions may be processed to hollow microspheres by a combination of liquid feed atomization and thermal exposure. Techniques for feed preparation and the relationships between feed composition, stability and processibility are discussed. Optimization of feed atomization, hollow sphere formation and finishing operations are discussed relative to density, size, strength, and chemical durability properties of finished products. The connection between finished properties and product utility is presented for several applications including lightweight fillers for thermoset resins and vinyl plastisols, and physical sensitizers for emulsion explosives.

Pulsed electric fields have been used to enhance the dispersion of aqueous metal (Zr and Al) salt solutions from a nozzle and into a nonconducting liquid continuous phase that is immiscible with the aqueous phase. The diameter of the resulting microdroplets ranged in size from approximately 0.1 to 10 μm. Precipitation of hydrous metal oxides occurred as ammonia, which was dissolved in varying amounts in the continuous phase, diffused into the aqueous microdroplets. Spherical shells were formed at higher ammonia concentrations and microspheres were produced at lower ammonia concentrations. Upon drying, dimples appeared in the particles that were synthesized at higher ammonia concentrations. The latter result accords with the well known fact that under certain conditions spherical shells collapse when a fluid is extracted from the core of the particle. No dimples were observed in the microspheres that were produced at lower ammonia concentrations. Analog X-ray dot maps for aluminum and zirconium were done to determine the spatial distribution of each metal in the particles.

Spray pyrolysis offers a number of opportunities for the synthesis of either solid or hollow spherical particles. As predicted by the evaporation model, particles with different shell thicknesses can be obtained depending on the concentration gradient at the onset of precipitation. If the precipitate shell has a sufficiently high permeability for removal of the remaining solvent, then the shell can be preserved and a hollow particle obtained. By drying at a lower temperature solid particles are readily formed from precursor salts but hollow particle formation by SP at these low temperatures imposes demanding heating rate requirements.

Fabrication of three dimensional micro structures in silicon and silicon related materials is becoming increasingly important for the realisation of micro systems comprising of sensors, actuators, transducers and analytical assemblies. Fabrication of such devices so far has been mostly in form of structures defined by the crystal planes of silicon, or has involved sophisticated technologies such as ion beam machining, replication using LIGA, or micromachining techniques involving a sequence of alignment and etch stages using binary masks. Structures with circular symmetry are of great interest as micro optical components amongst others, and these are not easily amenable to microfabrication techniques commonly employed.

Many properties of small particle powders are significantly improved when the particle shape is spherical and the size distribution is narrow or monodisperse. By using a properly designed atomizer based on a rotating disc, perfectly spherical polymer particle with a narrow size distribution in the 20 μm to 300 μm range has been prepared directly from a polymer melt or solution. Furthermore, by using a large number of parallel discs in the same equipment, a capacity suitable for industrial scale production is possible. The polymer melt or solution is added to the rotating disk to form a thin film which is transported to the disc edge where it leaves as particles. The powder particles are formed when sufficient energy and mass is given for its formation. When the energy given by the rotation equals the energy required to create the new surface of the particle it will separate from the film and leave the disc edge. Examples such as powder coatings, chromatography media and biodegradable spheres will be shown.

Pure and manganese doped zinc hydroxicarbonate are precursors for materials based on zinc oxide with electrical and luminescent properties. This paper reports on investigation of the effect of chloride and sulfate anions in the morphology of pure and doped precursors obtained by aging diluted zinc salt solutions in presence of urea, and their thermally decomposed products Characterization was carried out by XRI), IR and SEM techniques. Hydrozincite Zn5(OH)6(CO3)2 and zincite ZnO with spherical shaped particles of 5–10 ptm were obtained independent of anion used Moreover, in a chloride medium the spheres are organized in an arrangement of acicularly crystallized units as spherulitic aggregates, and in sulfate medium porous spherical particles were observed In addition, there is a good topotaxic relationship between the precursor and oxide

Metal powders with a diameter ranging from 20 to 50 micrometers are currently produced by electrolysis at high current density in aqueous electrolytes. A lot of research is then devoted to produce by electrolysis and with a high yield, ultrafine metal powders. Two main techniques are actually developed: strong electrolyte stirring and pulsed currents in order to obtain high current densities and accordingly, high nucleation rates and small nuclei diameters. By combining pulsed current and pulsed ultrasound on the same electrode surface, it has been possible to produce by electrolysis, with a yield ranging from 80 to 95%, crystalline metal powders with a sharp diameter distribution around 100nm. Possible explanations of these results are presented. This new technique, called sonoelectrochemistry, is illustrated by metal powders analysed by HIRTEM, X-ray diffraction and laser diffusion. The perspectives of sonoelectrochemistry for the production of metal or ceramic nanopowders are studied.

Medium sized spherical particles (with diameters below 100 μm) of ceramic materials (Al203 and Al203 +8%TiO2) were produced in the following steps: (1) preparation of concentrated hydroxide sols of cations by extraction of anions using Primene JMT, (2) formation of sol emulsion in 2-ethylhexanol-1 containing various amounts of surfactants (EH), (3) gelation of emulsion drops by extraction of water with partially dehydrated EH, (4) non-destructive thermal treatment. Principal properties of the sol depend on the temperature of the extraction process. For preparation of lowanionic sols a two step extraction process, separated by digestion of the aqueous phase at boiling point, was necessary. It was found that the parameters of emulsion formation as well as the properties of the sols significantly influence the particle size distribution. It was concluded that lowering of the sol- EH interracial tension by the surfactant does not influence the process. On the contrary the “anti-clustering” effect of surfactants is essential for preparation of perfect spherical powders. The thermal decomposition of gels to oxides is determinant for densities, specific surface area and carbon content. Medium sized spherical powders are free flowing. The compactibility and sinterability of those powders are poor. However drastic reduction of particle size by milling (connected with their transformation to irregular shape) significantly improves the sinterability.

Using high-intensity ultrasound, we have developed a method for the synthesis of airfilled hemoglobin (Hb) microbubbles (≈2.5 μm in diameter). Transmission electron, scanning electron, and optical microscopy show spherical particles with a shell thickness of approximately 35 nm, or roughly six protein molecules thick. The mechanism of microbubbles formation has been determined to involve both the dispersion of gas into micron-sized bubbles and the chemical cross-linking of cysteine residues between protein molecules. The primary oxidizing agent is superoxide (HO2), which is sonochemically produced from oxygen and water during acoustic cavitation. The Hb microbubbles possess many of the desired characteristics of a blood substitute. The microbubbles are smaller than red blood cells and will not block capillaries. The microbubbles are air-filled and provide a large O2 carrying capacity. The hemoglobins of the microbubbles retain their ability to bind oxygen reversibly. In addition, the oxygen affinities are similar to those of native Hb. Even more surprisingly, microbubbles show extensive cooperativity, as indicated by Hill coefficients as high as 18, which means that in the microbubble shell, there is communication between several of the crosslinked Hb tetramers upon binding oxygen. The Hb microbubbles show minimal degradation (>25%) after storage for six months at 4 °C.

The association of lipid or surfactant molecules into spherical vesicles in solution constitutes a primary self-assembly process, although typical vesicles are not the equilibrium form of aggregation for most lipids. Such meta-stable vesicles can undergo a secondary self-assembly into higher order structures in a controlled and reversible manner by means of site-specific ligandreceptor coupling. Cryo-electron microscopy shows these structures to be composed of tethered vesicles in their original, unstressed state. In contrast, vesicles aggregated by non-specific forces are deformed. In this work, we show that equilibrium vesicles can also undergo a secondary selfassembly via ligand-receptor interactions, as evidenced by freeze-fracture electron microscopy. Such site-specific vesicle aggregation provides a practical mechanism for the production of stable, yet controllable, microstructured materials.

The targets used in inertial confinement fusion (ICF) experiments at the Lawrence Livermore National Laboratory are plastic capsules roughly 0.5 mm in diameter. This paper reviews the fabrication of these capsules, focusing on the production of the thinwalled polystyrene microshell mandrel around which the capsule is built. The relationship between the capsule characteristics, especially surface finish, and capsule performance is discussed, as are the methods of surface characterization and modification necessary for experiments designed to study the effects of surface roughness on implosion dynamics. Targets for the next generation of ICF facilities using more powerful laser drivers will have to be larger while meeting the same or even more stringent symmetry and surface finish requirements. Some of the technologies for meeting these needs are discussed briefly.

Methods for control of the shell diameter, wall thickness and wall nonuniformity and also methods for selection of shells in fluid have been developed and investigated. The methods allow us to separate shells according to the diameter with 0.25 μ;m accuracy, to the wall nonuniformity with 5–7% accuracy, to make measurements of the diameter with 0.35 μ;m accuracy and wall thickness with 3–10%.

These methods allowed us to develop the automated process cycle of control and selection of shells for laser fusion targets. A model automated line has been developed and joint operation of units has been investigated. The calculated automated line throughput during the shell analysis is 50–300 shells per hour, the control and selection accuracy for the diameter, thickness, and wall nonuniformity is 1%, 3–10% and 1–3%, respectively.

Thus the foundations have been laid for an automated, high-quality target production technology for power stations based on laser fusion.

We present a summary of the techniques used to produce hollow plastic microshells from solid polymer granules using drop tower techniques. Unlike solution drop tower techniques, the use of these granules allows one to produce good quality shells with diameters of 1 to 1.5 mm. The reasons for hollow plastic shell diameter limitations are discussed, and based on these studies we present a new drop tower furnace concept we call a ballistic furnace that has been designed to produce large (2 to 3 mm) polystyrene shells. The characteristics of the individual components such as the pellet injector and the fast cooling system are given.

A new technique for producing hollow shell laser fusion fuel capsules was developed that starts with a depolymerizable mandrel. In this technique we use poly(alpha-methylstyrene) (PAMS) beads or shells as mandrels which are overcoated with plasma polymer. The PAMS mandrel is thermally depolymerized to gas phase monomer, which diffuses through the permeable and thermally more stable plasma polymer coating, leaving a hollow shell. We have developed methods for controlling the size of the PAMS mandrel by either grinding to make smaller sizes or melt sintering to form larger mandrels. Sphericity and surface finish are improved by heating the PAMS mandrels in hot water using a surfactant to prevent aggregation. Using this technique we have made shells from 200 μm to 4 mm diameter with 15 to 100 μm wall thickness having sphericity better than 2 μm and local surface finish better than 10 nm RMS.

Lasting storage of hydrogen, its isotopes and other gases under high (1–2 thousand atm) pressure inside hollow microspheres as well as production of low-density syntaxis poroplasts to withstand high external loads - all these need hollow microspheres of high mechanical strength. By modern commercial technology the microspheres are mainly obtained from water- soluble alkali-silicate or alkali-boro-silicate glasses. These microspheres are not strong, their strength being 1000–1500 MPa.. Besides the low resistivity to hydrolysis and chemicals also restrict their applications. To gain strength 1.5–2 times higher the additional modification of microspheres is done either by leaching, or by annealing, or by ion exchange. Microspheres of stronger glasses being usually multi-component require a technology fitted to produce highly uniform glass [1,2].

A new fabrication method is presented to produce hollow microspheres of various glasses. They include simple alkali and alkali-boro-silicate glasses as well as multi-component glasses of commercially available compositions, alkali-free, refractory, and high-strength glasses.

Coating of small particles is often employed to impart special properties to the particles. One process that has been used to accomplish this is chemical vapor deposition while the particles are fluidized. Because the depositing solids can plug small orifices, the gas distributors used for chemical vapor deposition in a fluidized bed of particles are different from those used for most other fluidized bed processes. The turbulent mixing of the gases by the particle bed and the high collection efficiency of depositing species by the large surface area of the particle bed can produce unique coating microstructures. Examples of these unique microstructures are those of isotropic pyrolytic carbon, fine-grained silicon carbide and carbon-silicon two-phase mixtures.

Synthesis of Pb(Ti,Zr)O3 (PZT) coatings onto glass microspheres was demonstrated using solgel techniques. Characterization of the coatings was performed using XRD and SEM to demonstrate that piezoelectric PZT coating are possible. This study represents an initial feasibility study into the manufacture of so-called “smart” material fabrication utilizing different geometries.