Preparation And Properties Of B Or N Modified Sioc Base Ceramic

Polymer-derived SiOC ceramics has many advantages including low cost, simple synthetic process, excellent processability, superior thermal and mechanical properties compared to vitreous silica, and have been applied in both civilian and military fields without critical temperature demand. However, most of SiOC ceramics tend to decompose around 1300°C due to carbothermal reactions. Thus the applications of SiOC ceramics are limited. In order to improve the thermal stability of the SiOC ceramics, B and N elements are incorporated into the SiOC network structure and SiOCB and SiOCN ceramics have been successfully prepared in this work. The effects of B or N elements on the molecular structural evolution, the mechanism of the pyrolysis, crystallization behavior of the ceramics have been studied. It is demonstrated that the thermal stability of SiOCB and SiOCN ceramics are superior to that of the SiOC ceramics. The SiOCB and SiOCN precursors are also used to prepare SiOCB ceramic fibers, UD-C_f/SiOCB ceramic matrix composites and lotus-type SiOCN porous ceramics. The main research results are as follows:SiOCB precursor has been prepared using polymethylhydrogensiloxane (PHMS), ethanol and boric acid as raw materials by alcoholysis and sol-gel methods. B elements are incorporated into the network of the SiOC sol-gel via≡Si-O-B bridges. The transformation of SiOCB polymer into an amorphous SiOCB glass is achieved at 1000°C. The amorphous ceramic network is composed of mixed SiC_xO_4-x, BC_yO_3-y and B(OSi)₃ units. In the pyrolysis process, analytion, redistribution and mineralization occur. The ceramic yield of the SiOCB precursor is as high as 86.0 wt.%. The effect of B content on the ceramic yield is significant. SiOCB ceramics are thermal stability up to 1500°C. Phase separation of the SiOCB amorphous ceramics occurs between 1000-1500°C. SiC_xO_4-x and BC_yO_3-y units evloved into SiC₄ and B(OSi)₄ units in the phase separtion. The tendency toward crystallization of SiC and graphitization of free carbon is strengthened with the increase of boron content and pyrolysis temperature. SiOCB ceramics annealed at 1550°C have decomposed due to the carbothermal reactions.SiOCB sol exhibits excellent spinnability, which is attributed to the plenty of Si-O-Si chains existing in the sols. The rheological experiment indicates that the SiOCB sols with spinnability show shear thinning behavior. Circular SiOCB ceramic fibers with 10mm in diameter are fabricated by pyrolysis at 1000℃, which have smooth and defect-free surface and dense textural feature. Through the investigation of the effects of the heat treatment temperature on the fiber morphology, SiOCB ceramic fiber with B/Si=0.14 has most excellent temperature stability. No evident morphology changes could be observed up to 1500°C.SiOCB precursor has also been demonstrated to be a good precursor for composite materials. By combining fiber winding precursor dipping pyrolysis (PIP) fabrication techniques, UD-C_f/SiOCB ceramic composites have been fabricated. Because C fibers have not interface protection, the interface reaction between fiber and matrix is intense and brittle crack is encountered after 2 PIP cycles for the samples prepared at 1500°C. However, the interface reaction is weak for the samples prepared at 1000 and 1300°C. PIP technology can effectively improve the densification of composite materials and influence the mechanical properties of composite significantly. The mechanical properties of composites reach maximum after four PIP cycles for the samples prepared at 1000°C. The mechanical properties of composites achieve maximum after three PIP cycles for the samples prepared at 1300°C. In the optimized conditions, the UD-C_f/SiOCB composite have weak interface connection and fiber uproot and debonding and interface disintegrate can be observed, which exhibit ductile fracture characteristics. With the further increase of PIP cycle, the interface between fiber and matrix is enhanced and the mechanical properties of composites decrease.SiOCN precursor polymer has been prepared through hydrosilation and dehydrogenation using tetramethycyclosiloxane and allylamine as raw material. SiOCN precursor can be converted into SiOCN amorphous ceramics by pyrolysis at 1000°C after redistribution and mineralization. The ceramic yield is as high as 83.0 wt.%. The effect of N content on the ceramic yield is significant. SiOCN amorphous ceramics is composed of SiC₂O₂, SiCO₃, SiNO₃, SiO₄, C-N and free carbon. SiOCN ceramics has excellent temperature stability. The ceramic could keep amorphous up to1400°C and no evidence weight loss could be observed even at 1600°C. Phase separation occurs in the SiOCN amorphous ceramics between 1000-1600°C. SiC_yO_4-y uites are converted into SiC₄ and B(OSi)₃ units by redistribution reactions and SiN_xO_4-x is converted into SiN₃O units. In this process, Si₂N₂O and SiC crystallization and the ordering of the turbostratic graphite could be observed. SiC crystallization is weakened with the increase of the content of N elements and free carbon. Evident weight loss could be observed annealed at 1700°C, which is due to the decomposition of Si₂N₂O and carbothermal reactions.A unidirectional solidification-prolysis technique is developed for the fabrication of lotus-type porous ceramics. Lotus-type porous SiOCN ceramic has been fabricated from a polymethylhydrogensiloxane-allylamine mixture using a new process based on unidirectional solidification and pyrolysis. The porosity of porous ceramics is 65.5%, and the pore diameters are mainly distributed in the range of 30-110μm. The hardness of lotus-type porous SiOCN ceramics is 11±2GPa, elastic modulus is 83±2 GPa. Compression strength parallel to the hole is 15±4 MPa, and that perpendicular to the hole is 15±4 MPa, which is better than that of the traditional SiOCN foam ceramic. The lotus-type SiOCN porous ceramics has high temperature stability. No evident weight loss could be oberved up to 1500°C under air or Ar atmosphere.