Research On CVD Synthesis And Field Emission Properties Of One-dimensional HfC And Modified C/C Composites By One-dimensional HfC

One-dimensional (1D) hafnium carbide (HfC) materials realize the combination of goodintrinsic properties of HfC with the unique geometry of1D nanostructures. Thus,1D HfCmaterials have excellent properties based on their high melting point, good mechanicalproperties, high thermal and chemical stability, low work function and high aspect ratio. Dueto these properties,1D HfC sturctures acting as field emission (FE) cathode materials andmodification materials of carbon/carbon (C/C) composites have wide application prospects invacuum microelectronics, aviation and aerospace.In this thesis, aims of the research are to synthesize1D HfC by CVD and extend itsapplications as FE cathode materials and additives of C/C composites. Micrometer-scale HfCwhiskers were synthesized using Ni-catalytic low pressure chemical vapor deposition (CVD).Based on the method of the synthesis of HfC whiskers, HfC nanowires were synthesized byCVD below the C–Hf–Ni eutectic temperature (TE). The effect of process parameters on thesynthesis was systemically studied. Morphologies and microstructures of1D HfCnanomaterials were characterized by XRD, FESEM, TEM, HRTEM, SAED, and EDX.One-dimensional HfC nanomaterials with various morphologies and structures wereassembled into field emitters, and their FE properties were examined by employing a platformfor FE testing. HfC nanowires modified C/C composites were fabricated. The thermalconductivity properties, thermal expansion properties, ablation resistance and mechanicalproperties were examined. The major research contents and results of the thesis are listed asfollowing:HfC whiskers were prepared by Ni-catalytic low pressure CVD method. The effect of Ocontent of the product and deposition temperature on phases and structures of HfC whiskerswas studied. The growth mechanism of HfC whiskers was discussed. The formationmechanism of HfO2in the product was also analyzed by thermodynamic calculation. Theresults show the quantity of HfO2decreases and the crystallinity of HfC gradually improveswith the decrease of O content within the product; the morphologies of the product changefrom the porous coating to rod-like whiskers with rough side faces with the decrease of the Ocontent; with the increase of deposition temperature, the yield, crystallinity and diameter ofHfC whiskers increase. The growth of HfC whiskers occurred by VLS mechanism. Theresults of the thermodynamic analysis confirm that HfO2can be generated and stably exist in the high-temperature reducing atmosphere; this implies that we should minimize the impactfrom oxygen impurities on1D HfC materials to prepare high-quality and high-purity1D HfCmaterials by CVD.On the base of the CVD method for the synthesis of HfC whiskers, HfC nanowires with adiameter of~50nm and a length of tens of microns was prepared at1025℃, which is belowthe eutectic temperature (TE) of C–Hf–Ni alloys. HfC nanowire growth is dominated by VLSmechanism. The effect of deposition pressure and flow ration of H2and CH4on the synthesisof HfC nanowires is systemically investigated. Under negative pressure, the diameterincreases with the decrease of the deposition pressure. On the one hand, excess H2isconducive to the formation of HfC nanowires, and growth of straight or zigzag HfCnanowires is controllable by adjusting H2flow rate. On the other hand, excess CH4inhibits theanisotropic growth of the nanowires.Four kinds of1D HfC nanostructures, including HfC nanowires, HfC/HfO2core/shellnanoneedles, HfC nanocrystal chains, and mixtures of HfC nanobelts and nanowires, areprepared under various processing conditions. Their growth process follows VLS mechanism.The results of FE testing show1D HfC nanomaterials exhibit good FE properties with lowturn-on field and high field enhancement factor. For example, HfC nanowires have a turn-onfield of1.6–1.7V μm-1, a field enhancement factor of above4869.HfC nanowires modified C/C composites (HfCnw-C/C) was successfully fabricated bylow-pressure CVD and isothermal CVI. The effects of HfC nanowires on the thermalconductivity properties, thermal expansion properties, ablation resistance and mechanicalproperties of C/C composites were investigated. The results show the above-mentionedproperties of C/C composites are improved by introducing HfC nanowires into the matrixalthough HfC nanowires lead to formation of the low-textrured carbon in the matrix of C/Ccomposites. Compared with unmodified C/C composites, HfCnw-C/C have a higher thermalconductivity within a temperature range of100–2500℃along vertical direction of carboncloth, which is5.5–17.5W m-1K-1. Because introducing HfC nanowires with high thermalconductivity leads to formation of low-textured pyrolytic carbon with high thermalconductivity along the vertical direction, the thermal conductivity of HfCnw-C/C wasimproved. At2500℃, the thermal conductivity of HfCnw-C/C increases by about four fifths.Coefficient of thermal expansion along parallel direction of carbon cloth is3.2–5.8×10-6K-1 within a temperature range of900–2500℃. Compared with unmodified C/C composites,coefficient of thermal expansion of HfCnw-C/C increases within the range of900–2060℃andreduces within the range of2060–2500℃. Low-textured pyrolytic carbon with highcoefficient of thermal expansion along the parallel direction is formed by introducing HfCnanwires with high coefficient of thermal expansion, which results in the increase ofcoefficient of thermal expansion of C/C composite. At2500℃, coefficient of thermalexpansion reduces by about one fourth. Ablation resistance of HfCnw-C/C is significantlyimproved. During the ablation process, on the one hand, oxidation of HfC nanowires canconsume oxidizing gases and heat, on the other hand, networks of HfC or HfO2can fasten thepyrolytic carbon. They result in improvement of ablation resistance of C/C composite. After20s ablation, the linear ablation rate and the mass ablation rate is lower by about one thirdand two fifths than that of unmodified C/C composites, respectively. Because HfC nanowiresincrease bonding strength of interface between matrix and carbon fibers, the shear andflexural failure of modified C/C composites present the brittle damage code. In addition, theshear strength of HfCnw-C/C is improved due to increase of the bonding strength, which ishigher by about three fifths than unmodified C/C; the flexural strength and modulus have noobvious improvement.