Synthesis Of Si-Al-Zr-O And Ti-Si-O-C Advanced Com-posite Ceramics Via Ceramic-precursor Transformation Route

Advanced ceramic materials were synthesized through a controlled transformation of pre-ceramic precursors. Two major parts of work were included:I, Synthesizing Si-Al-Zr-O amorphous pre-ceramic bulk by deep cooling the melted oxides, then heat-treat as-made precursor to ob-tain crystalline ceramics; Ⅱ, Fabricating Ti-Si-O-C monolithic gel pre-cursor through controlled sol-gel routes and transform it into mesoporous TiO2/SiO2/C and C-TiC/SiC nanocomoposites. The unique nano struc-tures and outstanding properties were achieved for both ceramic systems. And the detail research results are shown as following.(1) Dense zirconia-mullite ceramics with uniform dispersed nano grains can be synthesized by a controlled crystallization of Si-Al-Zr-O amorphous bulk. Teragonal ZrO2was first crystallized at～1000℃and mullite was formed at relatively higher temperature,1150℃. The me-chanical properties were adjustable by chemical composition and heat treatment temperatures. The sample with15wt%zirconia (Z15) and heat treated at1150℃showed the highest flexural strength and fracture toughness,520MPa and5.13MPa-m1/2, respectively.(2) Z15sample heated at1150℃showed two kinds of zirconia grains, rod like and spherical nano grains. The size of rod like zirconia grains reached over100nm, the other type of zirconia grains which were embedded in mullite grains showed the size within60nnm. The ZrO2grains maintain as spherical shapes when it is smaller then55nm. The growth rate of ZrO2grains along<001> direction is higher than that along <100> direction, which result in rod-like ZrO2with time. Cordierite crystallized after1200℃, and higher temperature promoted the grain growth of ZrO2and mullite, and the amount of amorphous phase reduced drmatically and only existed between grains boundary.(3) Comparing to amorphous bulk precursor, the sintering of starting oxides powder requires higher heating temperature to form zirco-nia-mullite phase. Adding amorphous Si-Al-Zr-O precursor powder to starting oxides can promote the mechanical properties and the density of sintered ceramics. The most effective amount of amorphous precursor added for sintering is10w%.(4) The extent of the ploy(furfuryl alcohol) PFA phase separation can be controlled by adjusting the ratio of silica and titania precursors (TEOS and TTIP), the amounts of FA and surfactant P123. The BET surface area (23-397m2/g), pore size (2.6-7nm), pore volume and the me-so/micropore ratio of the composites are all tunable in varied degrees by changing the chemical composition. Increase of silica to titania ratio gives higher porosity of the composite but results in an intense separation of carbon rich domains from the matrix and a high microporosity. Calci-nation of the as-synthesized monolithic gel at550℃in air could produce mesoporous titania/silica composites containing different amounts of ma-cropores. The demonstrated research here shows a facile approach to synthesize macro/meso/microporous titania/silica/carbon and titania/silica composites. Moreover, it also provides a feasible route to control the phase separation of PFA through composition changes, which can be widely used as hybrid precursors for synthesis of porous carbon, oxides, oxide/carbon and carbide/carbon nanocomposites.(5) The BET surface area (1m2/g-288m2/g) and separated carbon size (sphere size0μm-10μm) on silica/titania/carbon nanocomposite can be tunled through controlment of the ageing time, drying process and ultrasonic treatment in the sol-gel process. Increasing the ageing time will promote polymerization of furfuryl alcohol, causing more carbon phase separation. High drying rate will decrease the agglomerate rate of PFA which also control the phase separation. The pore structure is determined by the dry process, and ageing time has no obvious effect on it. But the residual carbon content in the silica/titania/carbon nanocomposite de-crease with ageing time.(6) We have demonstrated the synthesis of nanostructured (pore size3-4nm) bulk mesoporous C-TiC/SiC ceramics with high crystallinity by direct carbothermal reduction of pre-fabricated Ti-Si-O-C precursors. Benefiting from the homogenously dispersed components and intercon-nected channels, the carbothermal reduction takes place very efficiently at a relatively low temperature (starting at1100℃and completing at1450℃). The high mesoporosity (272m2/g) is obtained by in situ reaction of metal oxide with the surrounding carbon. The small pores (～3nm) were generated by removing of the surfactant (P123) during gel calcination, and they partially remained in the residual carbon for TSC-1450. Large pores (～10nm) were formed by the packing of nanoparticles. The high homogeneity is benefitial to the synthesis of highly mesoporous carbide ceramics.(7) The nano sized carbide grains are bonded into bulk materials by ploy(furfuryl alcohol)(PFA) derived nanocrystalline carbon framework because of its "bridge","entanglement" and "adhesive" effects. The gra-phenes are found to bond on the (111) plane of TiC grains, while no simi-lar coating is found for SiC grains. The bonding of graphitic carbon lay-ers on carbide grains support the nanostructure and also result in the de-sired combination of functional and mechanical properties. The average values of reduced elastic modulus, Er, and hardness, H, of the mesopor-ous carbides sample (TSC-1450) obtained from the nanoindentation tests were13.78±0.71GPa (Er) and0.85±0.072GPa (H), respectively. The coating and bridging of graphene sheets on TiC grains maintained the nanostructure and also resulted in a desired bulk material. Moreover, the process described here also provides an alternative concept for fabrication of bulk carbide composites through bonding of a carbon framework, avoiding granular sintering.(8) We demnostrate a feasible route to bulk synthesis mesoporous TiC microspheres (BET267m2/g) through in situ carbothermal reduction of precursor beads derived from a modified spray drying process. The uniform beads can be obtained from the sol aged for3.5days and spray dried at150℃. The average diameter of the as-made precursor and car-bide microsphere are75μm and45μm, respectively. No template is ap-plied to build the pore structure of the microspheres and the diameter and uniformity of spheres can be tuned by changing the ageing time and dry-ing temperature. The nano sized TiC crystal (～50nm) are connected by nanocrytalline carbon network by the mechanism discussed above and reasonable strength is obtained to maintain the spherical morphology, which could provide potential application in energy related areas.