Preparation Of RF/SiO₂ Hybrid Aerogels And The Formation Of Silicon Carbide And Silicon Nitride Nanowires Via The In-Situ Carbothermal Reaction

As novel, low density, nano-porous, non-crystalline, monolithic, nanometer materials, aerogels possess continuous network structures which can be controlled and tailored at a nano-scale. Because of their special network structures, aerogels show some excellent properties, such as the larger specific surface area and the alterable density. As a result, using aerogels in many fields is extremely attractive and promising. Nowadays, the research of aerogels is focused on the developing new style of aerogels, fabricating various multicomponent aerogels and exploring the new applications of aerogels.In this paper, in order to enrich the basic theory and widen the applications of aerogels, a novel binary-component hybrid aerogel, resorcinol-formaldehyde/SiO2 (RF/SiO2) hybrid aerogel, is fabricated by sol-gel polycondensation process. Based on the synthesis of RF/SiO2 hybrid aerogels, the transformations of RF/SiO2 into silicon carbide (SiC) and silicon nitride (Si3N4) nanowires were investigated in detail. The morphologies and structures of these materials were characterized by XRD, SEM, TEM, HRTEM, TG/DSC and FT-IR measurements, the formation mechanisms were elucidated and the research results were obtained.1. Preparation and characterization of RF/SiO2 hybrid aerogelsBased on the synthesis technique of two most typical aerogels, resorcinol-formaldehyde and SiO2 aerogels, a novel RF/SiO2 hybrid aerogel was fabricated by sol-gel process and supercritical fluid drying in petroleum ether. The influences of synthesis parameters on the apparent density of RF/SiO2 hybrid aerogels were investigated. The results show that the apparent density of RF/SiO2 hybrid aerogels has a positive corelation with the content of ethanol, and a negative corelation with the contents of water, RF component and heating rate of supercritical fluid drying. The as-prepared RF/SiO2 hybrid aerogels has an open-cell three-dimension network which is made up of interconnected skeletons and interconnected textural pores. The average diameter of the nanoparticles constructing the intercrossing network is around 10nm. By the contrastive analysis of the 900℃-carbonized RF/SiO2 hybrid aerogels and two kinds of derivative pristine aerogels, it was proved that there is a large contact area between the carbon and silica parts, indicating that the three-dimensional, inter-crossing network formed between the two components was very homogeneous. No chemical bonds were formed between the two components. The main effect between them is the Van der Waals attraction.2. Synthesis of SiC nanowires via gradual heating method from RF/SiO2 hybrid aerogelsIn this study, the RF/SiO2 hybrid aerogels were chosen to synthesize the cubic silicon carbide (β-SiC) nanostructures via a carbothermal reduction route without any catalyst at Ar atmosphere. The influence of C/Si atom ratio and heating treatment parameters on the products was investigated. It is found that under the condition of heating treatment at 1500℃for 2h, when the C/Si atom ratio was 3, the yield of SiC synthesized from RF/SiO2 hybrid aerogels was the best,80.0%. The heating treatment time had no obvious effect on the crystalinity of the SiC products. It was shown thatβ-SiC nanowires with diameters of 50 to 150 nm and high crystallinity were obtained at the temperatures from 1300 to 1500℃.By comparing the SiC products synthesized at different heating treatment temperatures, we proposed the V-S growth mechanism of SiC from the RF/SiO2 hybrid aerogels. By calculating the stacking fault densities, it was found that the heating treatment temperature could increase the stacking fault content. When the heating treatment temperature was raised from 1300℃to 1500℃, the stacking fault density was raised from lower than 1 to more than 4, indicating that the raise of temperature could increase the growth energy of (3-SiC, resulting in more stacking faults to decrease the growth energy. But the heating treatment time had no obvious effect on the stacking fault density.3. Synthesis of SiC nanowires via rapid heating method from RF/SiO2 hybrid aerogelsA large quantity ofβ-SiC nanowires were fabricated by the carbothermal reduction of the RF/SiO2 hybrid aerogel using a rapid heating method, that is, putting the precursor into the pre-heated furnace directly to reach the target temperature in several seconds. SiC nanowires were grown at 1500℃for 2 h in an argon atmosphere without any catalyst. The results showed that the nanowires are single crystalline p-SiC phase with diameters of about 20-80 nm and lengths of about several tens of micrometers, growing along the [111] growth direction with a fringe spacing of 0.25 nm. The stacking fault density of theβ-SiC nanowires synthesized via rapid heating method was lower than that via gradual heating method, indicating that the rapid heating method could decease the growth energy. The role of the interpenetrating network of RF/SiO2 hybrid aerogel in the carbothermal reduction was very important. So the partial pressure of gaseous SiO could be supersaturated in a moment to facilitate the formation of SiC nuclei and nanowires. The SiC nanowires were formed via V-S mechanism.4. Synthesis of SiC nanowires via gradual heating method from RF/SiO2 hybrid aerogels with Fe catalystIn order to synthesize SiC at lower temperatures, the ferric nitrate (Fe(NO3)3) was introduced as a catalyst. First, we prepared Fe(NO3)3-doped RF/SiO2 hybrid aerogels as the precursors. The precursors were heated at 1300℃for 1h, a large quantity of SiC nanowires with random coil shape were formed through the carbothermal reduction reaction. By the results of the characterization of the obtained SiC samples, it was found that the sample was made up of many curved SiC wires entangled with others with an average diameter of about 100 nm. The SiC random-coil-shaped nanostructures were formed via vapor-liquid-solid (VLS) formation mechanism. The stacking fault density in this study was as low as 0.1, indicating that the growth energy could be greatly deceased with the existence of Fe catalyst.5. Synthesis of Si3N4 nanowires from RF/SiO2 hybrid aerogelsRegularly-arrangedβ-Si3N4 nanowires with a diameter of about 80～300 nm and a length of several hundredμm can be formed via heating RF/SiO2 hybrid aerogels at 1500℃for 1h at a nitrogen atmosphere. The heating treatment time had no obvious effect on the crystalinity of the Si3N4 products at 1500℃. Theβ-Si3N4 could also be formed by heating the precursor at 1400℃for 5h, but the products contained some impurities, such asα-Si3N4 and Si2N2O, and showed irregular morphology.