JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 2673 – 2678 A flexible route to high strength α-alumina and aluminate spheres KIMBERLY A. DEFRIEND, ANDREW R. BARRON ∗ Department of Chemistry, Department of Mechanical Engineering and Materials Science, and Center for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, USA E-mail: arb@rice.edu The formation of hollow alumina spheres is accomplished by coating polystyrene beads of 3 μm and 50–80 μm diameter with carboxylic acid functionalized alumina nanoparticles (alumoxanes) from aqueous solution 2–8 wt%. The resulting coated beads were heated to 220 ◦ C to calcine the alumoxane to porous amorphous alumina before washing with toluene to remove the polystyrene from inside the ceramic coating. The resulting hollow spheres were sintered at 1000 ◦ C to form α-alumina. The α-alumina spheres have been characterized, by SEM (scanning electron microscopy), BET, and hardness measurements, that show the hardness of the hollow alumina sphere (1900 ± 100 Kg.mm -2 ) approaches that of corundum (ca. 2000 Kg.mm -2 ). Multilayered bi-phasic spheres may be prepared by subsequent coating the α-alumina spheres with a solution of a metal-doped alumoxane. After calcining, the mixed metal oxide phase (CaAl 12 O 19 , Er 6 Al 10 O 24 , MgAl 2 O 4 , Al 2 TiO 5 , and Y 3 Al 5 O 12 ) forms outside of the alumina sphere resulting in a composite like ceramic bi-layer sphere. Pre-formed hollow alumina spheres were incorporated into a resin and ceramic thin film formed from a 1 wt% A-alumoxane aqueous solution. The hardness of the composites is compared to the matrix materials themselves. C  2003 Kluwer Academic Publishers 1. Introduction Recognition that the macroscopic properties of materi- als depend not only on their chemical composition, but also on the size, shape and structure, has spawned inves- tigations into the control of these parameters for various materials [1]. In this regard, the fabrication of uniform hollow spheres has recently gained much interest. Hol- low capsules with nanometer and micrometer dimen- sions offer a diverse range of potential applications, including: utilization as encapsulants for the controlled release of a variety of substances, such as drugs, dyes, proteins, and cosmetics [2, 3]. When used as fillers for coatings, composites, insulating materials or pigments, hollow spheres provide advantages over the traditional solid particles because of their associated low densities [4]. A spherical morphology also allows for applica- tions in optical devices [5–8]. The geometry of the spheres has been shown to in- crease the strength of composite materials. Incorpo- rating hollow spheres into composite materials, im- prove the strength and the fracture strength of the ma- terial [9]. Typically, materials (organic or inorganic) are reinforced with fibers that retard the propagation of stress cracks. When hollow particles are incorporated into the fiber-reinforced composite, the crack growth is further stopped by the neighboring particles [10], for ∗ Author to whom all correspondence should be addressed. example, incorporation of glass beads into an epoxy resin [9]. Hollow particles have been fabricated from a variety of materials, such as polymers, metal, ceramics, and glass [3], however, a great deal of research has focused on various metal oxides, due to their chemical, ther- mal, and oxidative resistance, and because they have low dielectric constants and are optically transparent [11, 12]. Conventional methods to produce hollow ce- ramic spheres are vapor deposition, sputtering, molec- ular beam deposition and electrolytic deposition, how- ever, these processes do not always provide a uniform coating of individual particles [13]. Ceramic spheres exhibiting a uniform coating and thickness have been achieved with the sol-gel route [3, 4, 14, 15]. Typi- cally the spheres are formed by templating with either polystyrene spheres or silica spheres [2–4, 6, 16, 17]. The polystyrene or silica spheres are coated with the sol-gel, after which the core is etched away, and cali- cination results in a ceramic hollow sphere. Titanium dioxide, barium titanate, alumina, and aluminosilicate spheres have been fabricated using the sol-gel templat- ing technique [6, 12, 13, 18, 19]. We have previously shown that for alumina films and bodies, a low cost, flexible, alternative to sol-gels are chemically functionalized alumina 0022–2461 C  2003 Kluwer Academic Publishers 2673