| Porous material is a new type material that has received much attention in the field of materials science. Porous materials have excellent properties such as good heat-resistance, thermal shock resistance, high strength, high chemical stability and high specific surface area. Therefore, these materials have important applications in filters, heat exchangers, catalyst supports, photoelectric devices and sensors etc. Using the conversion of precursor polymer to prepare porous ceramics has advantages over the traditional methods of ceramic preparation such as the low temperature processing, versatile shaping, designable molecular structure and hierarchical porous structure. The porosity and pore size distribution of the porous ceramics can be designed using the template method, and the preparation technology is very simple.In this work, the design, preparation and properties of the porous ceramics were as the research targets, porous SiOC ceramics and porous SiOC/BN composites were successfully prepared by changing the preceramic precursors and templates, or adding the inert filler. These porous ceramics displayed the controlled open porosity and pore size distribution. The effects of the technological parameters on the morphologies, microstructures and properties of the SiOC ceramics and their composites were observed. Furthermore, the growth mechanism of the nanowires in the pores was investigated. The main results were as follows:Porous SiOC ceramics were prepared by sintering after impregnation, using the polyurethane sponge as the template and silicone resin as the preceramic precursor. These SiOC ceramics displayed the honeycombed porous structure, and the open porosity of the samples was up to85%. With the increase of the sintering temperature, the SiOC glassses of the samples showed the phase separation, and were converted into SiO2, SiC nanocrystalline and SiC nanowires. When the sintering temperature was higher or the holding time was longer, the porous SiOC ceramics showed good N2absorption property, semiconductor property and thermal stability.Laminated porous SiOC ceramics were prepared by sintering after hot-pressing, using the filter paper as the bio-template and silicone resin as the preceramic precursor. These SiOC ceramics retained the fiber morphology of the filter paper, and the open porosity of the samples was up to55%. With the increase of the sintering temperature, the SiOC glassses of the samples were converted into SiC. Laminated porous ceramics showed the hierarchical porous structure with macro-and mesoporous. The increase of the temperature resulted in the increase of the specific surface area and pore volume, and the pore size distribution was converted from unimodal (3-11nm) into bimodal pattern (3-4nm and4-23nm). Furthermore, with increasing the temperature, the electrical conductivity and thermal stability were improved.Porous SiOC ceramics were obtained through impregnating, hot-pressing and sintering, using the wood powders as the natural bio-template and silicone resin as the preceramic precursor. The open porosity of the samples was up to54%. With increasing the temperature, the specific surface area of the samples increased first and then decreased, and the pore volume increased. The pore size distribution was converted from unimodal (3-4.3nm) into bimodal pattern (3-4.5nm and4.5-60nm). When the temperature was1300℃, with the increase of the amount of the wood powders, the specific surface area and pore volume of the samples decreased. With the increase of the temperature or amount of the wood powders, the electrical conductivity and thermal stability were improved.Solid polysiloxanes were synthetized from poly (methylhydrosiloxane) as the precursor, divinylbenzene as the crosslinking agent and chloroplatinic acid (H2PtCl6) as the catalyst. Then the porous SiOC ceramics were prepared by sintering the composites of the wood powders and solid polysiloxanes The open porosity of the porous SiOC ceramics was up to59%. With the increase of the sintering temperature, the β-SiC and a large amount of SiO2in the porous ceramics were observed. The specific surface area of the samples was from6.1m2/g up to180.5m2/g, and the pore volume increased gradually. And the pore size distribution was converted from unimodal (3-5nm) into multimodal distribution (3-4nm,7-9.5nm and10-12nm). The tribological performances of the porous composite ceramics were closely related to the porosity and microstructures of the material itself. The tribological performances of the porous ceramics obtained at a lower temperature were better due to the formation of the lubricating film during the friction process.Porous SiOC/BN composite ceramics were prepared by sintering the mixtures of the wood powders, solid polysiloxanes and BN as the inter filler, through hot pressing process. The open porosity of these ceramics was up to52%. With the increase of the temperature, the amount of SiC of the samples increased. The specific surface area and pore volume of the samples both increased, and the specific surface area was up to81.2m2/g. And the pore size distribution was converted from unimodal (3-4.5nm) into bimodal pattern (2.6-4nm and10.5-25nm). BN as a good lubricant could accelerate the formation of the lubrication film on the surface of the samples, which made the porous ceramics show better tribological properties. The wear mechanism of these porous SiOC and SiOC/BN ceramics was abrasive wear mechanism.The nanowires can be formed in the porous SiOC ceramics using different types of templates or polyceramic precursors under proper technological conditions. The bamboo-like and chain-like nanowires were formed in the porous structure of the ceramics using the sponge as the template. SiC/SiO2nanowires and SiC nanowires were formed in the interfacial channels and pores of the porous ceramics using the filter paper as the template. The straight and bead-like SiC nanowires were formed in the porous ceramics prepared from wood powders and silicone resin. Meanwhile, the straight and curve nanowires could be formed in the porous structure of the SiOC ceramics and SiOC/BN ceramics from wood powders and solid polysiloxanes. The growth mechanism of these nanowires was vapor-liquid-solid (VLS) and vapor-solid (VS) growth model. The generation of these nanowires gave the porous ceramics hierarchical porous structure and more excellent performances (such as high specific surface area).Porous ceramics prepared by different types of templates or ceramic precursors showed obvious differences in microstructures and properties. The porous SiOC ceramics prepared from sponge can obtain larger macropores (200-300μm) and open porosity. And the porous SiOC ceramics prepared from filter paper have laminated porous structure and the size of the macropores is up to100μm. The highest specific surface area of the porous SiOC ceramic obtained from wood powders as the template and silicone resin as the preceramic precursor is up to245m2/g. Furthermore, the porous SiOC ceramics show the good tribological performances. The porous SiOC/BN ceramics have the better tribological performances than that of SiOC ceramics (in the same process). This work provides the theoretical basis and application foundation for the development of the porous ceramics and the application of the related fields. |
PDF Full Text Request