Cellular Ceramics / p2

2.8 Hollow Spheres 187

hollow glass spheres for fusion targets needs special mention as it is capable of making spheres with smooth surfaces and uniform wall thickness. A great deal of effort has gone into making these hollow spheres meet stringent product specifications set by the fusion targets. In a coaxial-nozzle technique, similar to the liquid-droplet generation technique, ceramic hollow spheres are generated by passing a slurry of ceramic particles through an outer nozzle while a gas flows through the inner nozzle, and as a result spherical hollow (occupied by a gas; see Fig. 9) drops are formed due to surface tension and hydrodynamic forces. The size of the hollow sphere and wall thickness are determined by the diameter of the outer nozzle, the spacing between the inner and outer nozzles, and the slurry concentration. This process is capable of producing monosized hollow spheres with uniform wall thickness. Spheres with tight tolerances have been produced due to the uniformity of processing (variations in diameter of less than – 5 %, spherodicity of – 4 %, and bulk density variations of – 6 %) [15]. The capabilities of different processing techniques and aspect ratios of hollow spheres produced by various techniques are given in Tab. 2.

Fig. 9 Schematic of generation of hollow droplets by the coax- ial-nozzle technique.

Table 2 Size and aspect ratio of ceramic hollow spheres produced by different techniques.

188Part 2 Manufacturing

2.8.2.5

Reaction-Based and Other Methods

The interest in ceramic hollow spheres has increased tremendously in the last few years, as is evident from the host of new techniques developed for producing them [76–83]: radio-frequency (RF) plasma [76], assembly during reaction [77], gelation and hydrothermal treatment [78], chemical conversion of hollow templates (carbon nanotubes) [79], self-assembly [80], solid-state reaction [81], oxidation [82], and solution chemistry [83]. The mechanism of formation of hollow spheres from hollow templates is straightforward, whereas that of hollow-sphere formation in reactionbased methods remains to be investigated.

2.8.3

Cellular Ceramics from Hollow Spheres (Syntactic Foams)

Cellular ceramics have been made by sintering ceramic hollow spheres [84–86]. Sintered hollow glass spheres have been fabricated by sintering hollow glass spheres between 500 and 550 C for various times [84]. The relative density (density of foam/- density of the hollow-sphere wall material) of the resultant foams was in the range of 9–24 %. It was observed that the sintering of hollow spheres is a complex process, as there was no systematic correlation between the wall thickness of the spheres before and after sintering (thickness decreased in some cases and increased in others). Al2O3 foams with 10 % relative density have been produced by a two-step process [86]: sacrificial cores were coated with Al2O3 powder slurry followed by a second coating which connected the coated particles. The compact was dried, calcined at 600 C, and sintered in air at 1600 C for 1 h. Cylindrical foams with diameters in the range 2–4 mm have been made by this technique. The compressive strength of the individual hollow spheres, made from different cores, was in the range of 25–31 MPa. The compressive strength of foams made of hollow spheres was in the range of 1–4 MPa.

2.8.4

Properties

Very limited information is available on the properties of ceramic hollow spheres. Green [84] and Green and Hoagland [85] studied the mechanical properties of lightweight ceramics made by sintering hollow glass spheres. In addition, they proposed a micromechanical model, based on shell theory, to predict the elastic modulus and fracture toughness. The data predicted by the model were in reasonable agreement with the experimental data, especially in the nonlinear variation of elastic modulus and fracture toughness as a function of density. Chung et al. studied the compressive mechanical behavior of hollow spheres made of alumina by both experiments and finite-element modeling [87]. The sphere strength (the load per maximum

2.8 Hollow Spheres 189

cross-sectional area of the sphere at fracture) was shown to be proportional to the square of the relative density (density of sphere/density of sphere wall material), as given in Eq. (1)

where rs is sphere strength, C a constant depending on the type of loading, ro the tensile strength of the wall material, s the density of the hollow sphere, and o the density of the sphere wall material.

The value of C was dictated by the stress analysis and fracture criterion used for different types of loading, and it is an order of magnitude larger for contact loading (0.774) than for concentrated loading (0.0275). The value of the constant determined from the experimental results was lower by a factor of three, and the discrepancy has been attributed to the imperfections in the sphere such as nonuniformity of wall thickness, imperfect sphere geometry, flaws, tails, and so on. The sphere strength increased from about 3 MPa to about 10 MPa when the relative density was increased from 0.05 to 0.14.

The optical properties of titania (TiO2) hollow spheres used to make photonic crystals have been studied [88]. In a photonic band gap crystal, an electromagnetic wave cannot pass though it in any direction in a particular frequency range. The band gap of the photonic crystals can be controlled by controlling the wall thickness as well as the connectivity between the hollow spheres. Liu and Wilcox, Sr., modeled the dielectric behavior of cordierite (2 MgO·2Al2O3·5 SiO2) reinforced with silica–a- lumina hollow spheres over a frequency range of 1 kHz to 1 MHz [89]. The dielectric constant of cordierite decreased from 5.7 to 3.6 when the hollow sphere loading was increased from 0 to 48 vol % (equal to 37 vol% porosity). The Bruggeman effective medium theory was used to predict the dielectric constant of the composite, and the model predictions were in good agreement with the experimental results.

2.8.5

Applications

Cochran described potential applications of ceramic hollow spheres in different areas [90]. These range from individual spheres used as under water hydrophones, ultrasonic imaging, drug-delivery capsules, artificial cell carriers, and targets for inertial-confinement fusion to collections of hollow spheres in the form of syntactic foams, fillers in lightweight composites, photonic crystals, refractories, kiln furniture, energy-absorbing structures, and catalyst supports. Newnham et al. have extensively investigated the possibility of using ceramic hollow spheres, mainly lead zirconate titanate (PZT) and lead titanate, as single-element transducers and an array of transducers for exposimetry and tissue ablation, medical imaging, hydrophones, and underwater flat-panel arrays [15, 91–96]. The smaller the transducer the better would be the resolution. Ceramic hollow spheres have been investigated for reducing the thermal conductivity of materials used in radiant burners [14]. Cenospheres,