In-situ Growth Of HfC Nanowires Andmodified C/C Composites

Carbon/carbon?C/C?composite has the characteristics of low density,high specific strength,excellent high temperature performance,etc.With the rapid development of aerospace vehicles,more stringent requirements have been put forward for the anti-oxidation and ablation ability of C/C composites under high temperature and high-speed airflow.Introducing HfC ceramics with ultra high melting point into C/C composite matrix is an effective way to improve its anti-oxidation and ablation performance.However,the introduction of HfC ceramic particles into the matrix of C/C composites will lead to the decrease of mechanical properties of the composites.HfC nanowires not only possess the high temperature resistance of HfC ceramic blocks,but also possess excellent mechanics due to their special one-dimensional structure and morphology.The C/C composite was introduced as the second reinforcing phase to achieve synchronous improvement of the oxidation ablation and mechanical properties of the C/C composite.However,there are few studies on the modification of C/C composites by HfC nanowires at present,and the key technical bottleneck lies in the inability to realize the in situ controllable growth of HfC nanowires in carbon fiber prefabricated bodies.In this paper,by optimizing process parameters of low-pressure catalytic chemical vapor deposition?CVD?,the in-situ controllable growth of HfC nanowires in the2.5D carbon fiber prefabricated body was realized.The morphology,structure,phase composition,chemical composition and distribution of HfC nanowires in the prefabricated body under different process conditions were investigated,and the growth mechanism was analyzed.On this basis,the effects of HfC nanowires on the deposition rate,texture and degree of graphitization of CVI pyrolysis carbon of C/C composites were analyzed,and the variation rules of microzone mechanical properties,thermal conductivity,oxidation resistance and ablation properties of HfC nanowires modified C/C composites(HfC_nw-C/C)were summarized.The main research contents and results are as follows:?1?in-situ growth of HfC nanowires.By low pressure CVD method with CH₄ as the carbon source,HfCl₄ as the source of hafnium,Ni?NO₃?₂ as catalyst,at 1075?synthetic HfC nanowires.The yield of HfC nanowires increased with the deposition time,and the synthesis rate of HfC nanowires was the fastest in the period from 60min to 120min,which was3.5mg/min.When the catalyst concentration was 0.05mol/L,it was conducive to the in-situ growth of HfC nanowires in the 2.5D carbon felt.When the solubility of the catalyst was0.05mol/L and 0.1mol/L,HfC nanoribbon was generated in the synthesized one-dimensional HfC nanoribbon.When QH₂/QCH₄ is 10/1 and 15/1,HfC nanowires will be affected by impurity oxygen with rough surface and low purity.With the increase of QH₂/QCH_4,the purity of HfC nanowires increases.The growth mechanisms of HfC nanowires and HfC nanoribbon accord with VLS mechanism.?2?Study on the influence of HfC nanowires on CVI pyrolysis carbon deposition rate and microstructure.Carbon fiber prefabricated HfC nanowires grown in situ were densified by atmospheric pressure CVI method with CH₄ as the gaseous precursor and N₂ as the protective and diluting gas.After the introduction of HfC nanowires into carbon fiber prefabricated body,the deposition rate of pyrolytic carbon was accelerated in the early stage.When the introduction amount of HfC nanowires was 3.25%,the micro-nano pore structure between HfC nanowires was blocked by pyrolytic carbon during the deposition process,reducing the deposition rate of CVI pyrolytic carbon in the later stage.HfC nanowires can restrain the interlayer of the pyrolytic carbon craze,HfC nanowires enrichment region pyrolytic carbon Ae=0°,pyrolytic carbon texture to the ISO;In the low HfC nanowire content region,the pyrolytic carbon texture was LT.Compared with the unmodified C/C composites,the grain size and degree of crystallization of the matrix microcrystalline carbon on different crystal surfaces were significantly different,in which the La?110?of the carbon atom plane layer on the direction of axis a decreased and the degree of crystallization decreased.However,the carbon atom planar stacking layer Lc?002?in the direction of the c axis increases and the degree of crystallization increases.?3?Study on the properties of HfC_nw-C/C composites.The thermal diffusivity and thermal conductivity of HfC_nw-C/C composites perpendicular to the direction without cloth are 1.5times and 2 times of that of the unmodified C/C composites,respectively.The results of constant heating rate oxidation test and isothermal oxidation test showed that the introduction of HfC nanowires into C/C composites could slow down the oxidation loss rate and reduce the final oxidation loss mass.Compared with the unmodified C/C composites,the line ablation rate and mass ablation rate of the HfC_nw-C/C composites were reduced by 22.7%and 9.0%respectively after the oxy-acetylene flame ablation 20s.