Yingbin Cao

Synthesis of submicrometer zirconium carbide formed from inorganic–organic hybrid precursor pyrolysis

Zirconium carbide (ZrC) was synthesized from inorganic–organic hybrid precursor’s pyrolysis by solution-based processing. Zirconium-containing complexes, which were obtained by chelation of oxide bidentate ligands to zirconium, were used to combine with phenolic resin to form precursors for ZrC. The precursors using specific ligands including acetylacetone, ethyleneglycol, and salicylic acid transformed into pure ZrC at a relatively low temperature (1,550xa0°C) in addition to that using lactic acid. As a comparison, synthesis of ZrC only using zirconium oxychloride octahydrate (ZrOCl2·8H2O) and phenolic resin was also conducted. The synthesized powders had a small average crystallite size (~300xa0nm), and a low oxygen content (~2.5xa0at.%). The conversions from as-synthesized preceramic precursors to ceramics were studied by means of FTIR, SEM, EDS, XRD, and XPS. The oxidation behavior of the synthesized ZrC in air was studied by DSC-TG analysis.

Fabrication of high density three-dimensional carbon fibre reinforced nitride composites by precursor infiltration and pyrolysis

Abstract Three-dimensional carbon fibre reinforced nitride composite materials were fabricated by precursor infiltration and pyrolysis using a hybrid precursor. The curing stage of precursor infiltration and pyrolysis was investigated and improved. The densification behaviour, mechanical properties and ablation properties of the composites were studied. The results show that high pressure curing method can be used to fabricate high density composites. With the increase of density from 1˙50 to 1˙65 g cm−3, the flexural strength increases from 156˙4 to 192˙3 MPa, and the elastic modulus increases from about 45˙9 to 56˙3 GPa, while the linear ablation rate decreases from 0˙069 to 0˙056 mm s−1. The load–displacement curves show that carbon fibres cause efficient reinforcement, and the composites have good toughness.

Preparation of Silicon Carbide Coatings from Liquid Carbosilanes by Chemical Vapor Deposition

Liquid carbosilanes which were synthesized from polydimethylsilane (PDMS) were analyzed by infrared spectra (IR) and H-nuclear magnetic resonance (NMR). Silicon carbide coatings were prepared by chemical vapor deposition from liquid carbosilanes onto several different substrates. The coatings were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and IR. The results show that liquid carbosilanes are mixtures of several oligomers which have Si-C backbone. The coatings prepared at 850xa0°C contain some organic segments, and those prepared at 900xa0°C are comparatively pure SiC which is partly crystallized. The coatings are glazed and very hard.

Lightweight C/SiC mirrors for space application

Challenges in high resolution space telescopes have led to the desire to create large primary mirror apertures. Ceramic mirrors and complex structures are becoming more important for high precision lightweight optical applications in adverse environments. Carbon-fiber reinforced silicon carbide (C/SiC) has shown great potential to be used as mirror substrate. This material has a high stiffness to weight ratio, dimensional stability from ambient to cryo temperatures, and thermal conductivity, low thermal expansion as well. These properties make C/SiC very attractive for a variety of applications in precision optical structures, especially when considering space-borne application. In this paper, lightweight C/SiC mirror prepared for a scan mirror of a high resolution camera is presented. The manufacturing of C/SiC mirror starts with a porous rigid felt made of short chopped carbon fibers. The fibers are molded with phenolic resin under pressure to form a carbon fiber reinforced plastic blank, followed by a pyrolization process by which the phenolic resin reacts to a carbon matrix. The C/C-felt can be machined by standard computer controlled milling techniques to any virtual shape. This is one of the most significant advantages of this material, as it drastically reduces the making costs and enables the manufacture of truly ultra-lightweight mirrors, reflectors and structures. Upon completion of milling, the C/C-felt preform is mounted in a high-temperature furnace together with silicon and heated under vacuum condition to 1500°C at which the silicon changes into liquid phase. Subsequently, the molten silicon is infiltrated into the porous preform under capillary forces to react with carbon matrix and the surfaces of the carbon fibers to form a density C/SiC substrate. The C/SiC material retains the preform shape to within a tight tolerance after sintering means the ceramization process is a nearly net shaping process. Reactive melt infiltrated C/SiC, followed by chemical vapor deposited silicon carbide (CVD SiC) cladding, is used to fabricate a 225-mm×165-mm ellipse mirror 18-mm thick and 0.41 kg weight. The mirrors backing structure contains hexagon pockets. The individual ribs are only 2-mm thick, each contains a large cutout for structural efficiency and improves the mirrors thermal properties. Open back honeycomb lightweight structure is produced to gain 14kg/m2 area density. CVD SiC has an excellent adherence to C/SiC. It also has an excellent thermal strain match. Approximately 150-μm of this material is used to clad the mirror substrate. The CVD SiC cladding is polished to be a super smooth surface with less than 0.372-nm RMS surface roughness. Experimental results indicate that reactive melt infiltrated C/SiC can be used as optical mirror substrates. Currently, experiments are under way to fabricate a large-scale lightweight C/SiC optical mirror in diameter larger than 600-mm.

Synthesis of zirconium carbide nanoparticles by polymerised complex route

A polymerised complex route, based on polyesterification between tartaric acid–zirconium complex and ethylene glycol (EG), has been demonstrated to synthesise ZrC powders at a relatively low temperature (1300°C). The Fourier transform infrared results confirmed the coordination of tartaric acid to the zirconium ion. Furthermore, this complex reacted with EG through polyesterification to immobilise the metal ion in the rigid polymer. The intimate mixing of the reaction components has thus been achieved in ZrO2/carbon ‘coke” due to the suppression of segregation of the reaction components during pyrolysis. Finally, well crystallised ZrC nanoparticles (50–150u200anm) were obtained through the carbothermal reduction of ZrO2/carbon ‘coke’, concerning the formation of the intermediate oxycarbide phase.

Mechanical behaviour and microstructure of Cf/ZrC, Cf/SiC and Cf/ZrC – SiC composites

Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites were successfully prepared by polymer infiltration and pyrolysis (PIP) using polycarbosilane and a liquid ZrC precursor. The densification process, mechanical properties and microstructures were studied in a view of comparison. After the same total 20 PIP cycles, the Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites had flexural strengths of 50.1±5.3, 285.7±22.6, 141.5±13.1u2005MPa respectively; elastic moduli of 7.8±0.9, 57.1±3.2 and 45.1±2.6u2005GPa respectively; and fracture toughness of 2.5±0.2, 10.4±0.9 and 10.9±1.1u2005MPa m1/2 respectively. With the introduction of high modulus SiC phase into the ZrC matrix, the densification and modulus of the matrix were improved; as a result, the Cf/ZrC–SiC composite showed higher mechanical properties compared to Cf/ZrC.

Preparation and characterization of diamond-silicon carbide-silicon composites by gaseous silicon vacuum infiltration process

Diamond-SiC-Si composites have been prepared using gaseous silicon vacuum infiltration. The evolution of the phases and microstructures of the composites have been analyzed using X-ray diffraction technique and scanning electron microscopy. It has been found that the diamond-SiC-Si composite is composed of β-SiC, diamond, and residual Si. The diamond particles were distributed homogeneously in the dense matrix of the composites. Besides, the effects of particle size and content of diamond on the properties of diamond-SiC-Si composites have been analyzed. The thermal conductivity of the composites increases with particle size and content of diamond. When the particle size and content of diamond are 300 μm and 80 wt %, respectively, the thermal conductivity of the composites approaches the value of 280 W·m−1·K−1.