| Owing to their superb features, such as low density, high specific surface area, controllable size, good dispersibility and novel multifunctional properties arising by combining different materials in the the hollow internal space, hollow spheres have been used in battery materials, catalysts and their carriers, nano/micro-scale chemical reactors, drug delivery carriers, buoyancy materials and many other fields. Various processing techniques for making hollow spheres have been developed, such as template method, spray drying, spray pyrolysis, Ostwald ripening, Kirkendall effect, etc. However, the shell of the hollow spheres obtained by these methods is either porous or very thin, and the yield is relatively unsatisfactory, thus hampers some applications of hollow spheres. As far as we know, thermal plasma provides unique advantages of high temperature, fast heat conduction, rapid quenching rate, controllable atmosphere and high yield rate. These exclusive features of thermal plasma inspire us to do the work presented here, and the main results are summarized as follows:(1) Hollow silica spheres (HSSs) are prepared via a two-step method:producing micrometer-sized agglomerated granules with nano-silica by spray drying then spheroidizing and hollowing the spray-dried granules by DC thermal plasma sintering. Firstly, the composition of two kinds of silica sources is investigated. Then, it is found out that the morphology, particle size distribution and porosity of spray-dried granules are affected by the silica concentration of the slurry. The shell structure and shell thickness of HSSs can be tuned by plasma sintering different kinds of spray-dried granules. The shell thickness of HSSs ranges from 0.9 to 7μm, the average true density ranges from 0.28 to 0.90 g/cm3, and the corresponding collapse strength ranges from 40 to 95 MPa. The cavity formation mechanisms of two different types of HSSs are further investigated:the heating way of spray-dried granules are determined by their characteristics:gradient heating leads to single large void; and gradientless (uniform) heating results in multiple small voids. The DC thermal plasma sintering method provides a facile way to produce HSSs with low density and high crush strength which have promising application in many fields.(2) The spray drying-thermal plasma sintering method is applied into alumina, which has sophisticated phase transition. Hollow alumina spheres are successfully prepared with nano-alumina and micro-alumina as raw material respectively. It is firstly observed that the internal hollow structure is constructed by polycrystal grains of alumina. The differences of cavity formation between hollow alumina spheres and hollow silica spheres are investigated. The reasons for the appearance of δ metastable phase and the hollow polycrystal grains structure are explained.(3) The spray drying-thermal plasma sintering method is further applied into Al2O3-SiO2 binary system to produce hollow mullite spheres. The silica sources are silica sol and nano-silica respectively and the alumina source is nano α-Al2O3. Different silica sources affect the morphology of spray dried granules and the mullite content of the hollow spheres. The RF thermal plasma sintered hollow spheres have 35～39% mullite component and relatively small hollow inner structure. We assume that the large temperature span of liquid phase and the short residence time in thermal plasma are the main reasons for the low mullite content and the very small inner void.(4) The spray drying-thermal plasma sintering method is finally applied into a sophisticated ceramic system with multiple components. We successfully use silicon nitride as major material, alumina and yttria as sintering additives to prepare silicon nitride hollow quasi-spheres (SNHQSs). Nine kinds of slurry with different content of sintering additives are prepared for spray drying, and the attained nine kinds of spray dried granules have similar morphology and properties. However, different spray dried granules generate diverse hollow structures via plasma sintering. The cavity formation mechanism of the SNHQSs is deeply investigated and this time the mechanism is quite different from the fonner ones. In addition, the obtained SNHQSs possess low apparent density, high compressive strength (up to 50 MPa) and good thermal stability (up to 1600℃). Finally, SNHQSs are used as in-situ pore-forming agent and in-situ sintering additives to make porous ceramics. Two kinds of porous composite ceramics are investigated. One is quartz-SNHQSs, the other is Si3N4-SNHQSs. The results show that they both possess low density, low dielectric constant and certain flexural strength. |
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