Pressure Enhanced Cvd Synthesis, Characterization And Performance Of Sialon Nanobelts

This paper introduced a synthesis process of SiONCH-Al precursor by molecule design. High containing hydrogen polysiloxane and diaminoethane, which are free from hydroxyl and organic group, soluble in organic solvent and able to polymerize, were used to produce SiONCH sols. SiONCH-Al precursors were synthesized by ball mill SiONCH sols with Al powders. Then SiONCH-Al precursors were pyrolyzed at N₂ gas to form SiONCH-Al powders. The invented technology achieved some goals, such as molecule design of precursor, control of the process and purity of the products. Then the as-synthsized powders were annealed to fabricate quasi-one-dimensional (1D) Sialon nanobelts N₂ atmosphere. The influences of the raw materials, annealing temperature and gas pressure on the growth of Sialon nanobelts were systematically investigated, and the optical and electrical properties of the obtained 1D nanomaterials were also studied.The precursor powders contain amorphous substances with high content, such as Si-N groups, Si-O-N and Al-O-N groups. There was also Si, AlN, SiC, Al₃O₃N microcrystallite to be found. For adding Al to precursor, the materials show tendency to crystallite. In the pyrolysis of SiONCH-Al precursor, Si·, C·and O·ions were produced at a relative low temperature. At a relative high temperature, recomposition reaction took place between Si-O/Si-O, Si-O/Si-C, Si-H/Si-O, Si-O/Si-N, Si-N/Si-C and AlSi-O to form Si-Si, Si-O and Al-N groups. The particle sizes of precursor powders was about 50-150nm with high surface area. On during precursor powders were suitable to synthesize nanobelts.Thermodynamics calculation indicated higher N₂ pressure tended to make Si₃N₄ stable. For example, Si₃N₄ could be stable at 1464°C under 1MPa N₂ pressure. Beyond this temperature, SiC tended to form. Phase equilibrium of Si₂N₂O andβ-Si₃N₄ was affected by oxygen pressure. A high oxygen partial pressure made Si₂N₂O stable. For example, the pressure under which Si₂N₂O can not form at 1464°C is 10^-8.6atm. Field-emission scanning electron microscopy observations reveal the nanobelts with the length of 0.2-12 mm (more photos see see ?ESI), width of 0.2-0.8μm and thickness of 10-40 nm (Figure 2b, 2c and 2d). The width and thickness of a single nanobelt are quite uniform along its entire length. The high-resolution transmission electron microscopy images of the Sialon nanobelt (Figure 2e) show a core-shell structure, a crystal Sialon core and an amorphous carbon shell. The nanobelt axes lies along the [210] direction of the Sialon.The yield of Sialon nanobelts was 16Wt% at 1450°C with holding time 2 hours. The yield decreased with increasing temperature. The SiC with bead structures were observed at 1600°C. At much high temperature, hollow-belt and micro-wire could product. The study of influence factor to synthesize products indicates precursor composition has a relative small affect to the final products when the ratio of Si and Al is more than 10. But as this ratio is less than 5, the yield of the product decreases sharply. The gas pressure is a key factor to the growth of Sialon nanobelt. For example, the final products contain O-Sialon orβ-Si₃N₄ when the nitrogen pressure belows 0.8Mpa or beyonds 1.8Mpa respectively. A proper nitrogen pressure is benefit to increase yield and purity of the products. Large scale synthesis of Sialon nanobelt was investigated by use overlap graphite bracket. This methor results in the formation of Sialon nano cloth with tickness of 1-3mm and area of 200.96 cm2.The growth mechanism of Sialon nanobelt was pointed to VS mechanism because no droplets were found on the tip of nanobelt. The growth ofβ-Sialon is based onβ-Si₃N₄ where Al-O pairs replaced Si-N pairs with the lattice parameter increased. For the crystal grown from the vapor phase, it is generally believed that the growth form is closely related to the supersaturation ratio. While a certain level of supersaturation is necessary for crystal growth, a relatively low supersaturation ratio is considered asfavorable for the formation of whiskers, whereas a higher supersaturation ratio would encourage the formation of powders. Based on the supersaturation ratio theory, a Sialon knuckles structure can be proposed by alternating change of supersaturation ratio. Further, the mechanism of Sialon nanobelt and its twin structure were proposed also.High pure Sialon nanobelts were fabricated. The samples display type-II adsorption isotherm typical of mesoporous materials by the N₂ adsorption–desorption isotherm. The BET measurements indicate the speci?c surface area was 214.6 m²g^-1 and average pore diamete was about 2.2nm. Emission spectra ofβ-Si₅AlON₇ nanobelts, showing two broad bands centred at 354 and 652 nm. The emission spectrum was measured under the 411 and 445 nm excitation. The intensity of emission peak at 652 nm and 362nm increase with the growth temperature rising. The electric properties of Sialon nanobelt were investigated. The results showed different conductance have different properties. The HCHO and Oxygen sensor based on Sialon nanobelt were also studied. It is believe Sialon nanobelt can be potential materials for semiconductor applications.