Synthesizing of Stabilized Nano-Tetragonal-zirconia in C/C Composites and Its Ablation Resistence Behavior
Supervisor:zhao dong lin
C/C composites are irreplaceable materials in aviation,aerospace and military fields.To meet the special needs of the application fields,the improve of ablation resistace ability of C/C composites is necessary.To improve the ablation resistance ability of C/C composites,zirconia was designed to be synthesized in the carbon matrix to prepare modified anti-ablation C/C composites.The crystal form and structure of zirconia in carbon matrix were fine controlled to improve the ablation resistance ability of the material.The matrix modified C/C composites with certain ablation resistace properties was obtainedIn this paper,firstly,the stabilizing method of zirconia in zirconia/carbon system was studied.In physical mixed zirconia/carbon system,zirconia could be partly stabilized into tetragonal phase.The element of carbon shows a lag in the heating process.The lag could restrain the transformation from monoclinic phase to tetragonal or cubic phase of zirconia in the heating progress.In the cooling process,the transformation from monoclinic phase to tetragonal or cubic phase was triggered and promoted and the reverse transformation was restrained.So that the crystal structure of zirconia was stabilized.In molecular mixed zirconia/carbon system,zirconia could be fully stabilized into tetragonal phase.In the system of adding yttria,zirconia could be stabilized into cubic phase.Secondly,the preparation method of nano-particles in liquid phase was studied."Molecular-cage" effect was proposed."Molecular-cage" effect was simulated and studied in liquid phase system with synthesizing hydroxyapatite and zirconium hydroxide.The size of nano-particles is about 5-30nm.In liquid phase system,multi-functional additives and solvents could form the molecular-cage structure.The intermolecular forces and hydrogen bond between multi-functional compounds could bound the reactions in separate tiny regions.Thus,the nano-particles formed.The size of the particles decreased with the concentration of cage molecules increased within limits.Molecular-cage method was used to prepare nano zirconia particles in carbon matrix.Nano zirconia particles was prepared in carbon matrix after heating process.The particle size was about 20-30nm.The largest particle was less than 50nm.The above two methods were used to prepare modified carbon matrix.Zirconium oxychloride octahydrate and oxidized carbon nanotube were reacted to synthesize zirconium precursor.Use it as the zirconium source,mid-temperature coal-tar pitch was modified into zirconium-containing pitch.In the impregnation process,nano-scaled and homodispersed zirbonium source was carried out by the mid-temperature coal-tar pitch’s molecular-cage structure.Carbon matrix with homodispersed and close packed nano-tetragonal-zirconia particles was prepared after carbonization.Boron was added in the carbon matrix with boron carbide as the additive.Boron nitride and zirconium boride were formed after carbonization process.C/C composite was prepared with mid-temperature pitch,and modified C/C composites were prepared with boron-zirconium-containing pitch.Densification process of the composites was optimized.The compete between the decrease of pitch’s liquidity and the spill-out of small-molecule gases was detected as the main factor of densification.Sample CCO and CC1-CC4 were prepared.CCO was the normal C/C composite,and CC1-CC4 were modified C/C composites.The density of CCO and CC1-CC4 were 1.76 g/cm3.1.23 g/cm3、1.55 g/cm3、1.75 g/cm3 and 1.78 g/cm3.The mass fraction of zirconium in CC1-CC4 were 1.2%、2.1%、2.6%and 10.0%.The carry over effect of small-molecule gases and the closure effect of ceramic phase was the main reasons of the low density and zirconium content of sample CC1 and CC2.Sample CC3 and CC4 were impregnanted to the same times as sample CCO.The density of sample CC3 and CC4 gained to 1.81 g/cm3 and g/cm3.The mechanical properties of the samples were tested.Tetragonal zirconia in the carbon matrix could improve the mechanical properties of the materials in the condition of same density.Samples were tested in static oxidation condition and dynamic ablation condition.The weight loss ratio of sample CCO and CC1-CC4 with single impregnation and carbonization in 800℃ TG test was decreased rapidly.In static oxidation test of 800℃ and 1600℃,the percentages of weight loss ratio of CC4 to CC0 were 3.2%and 51.28%.The percentages of line-ablation resistance ratio and mass-ablation ratio of CC4 to CCO were 40.59%and 17.96%.The ablation resistance ability of CC4 were obviously improved than CCO.In static oxidation tests,the homodispersed nano-zirconia could decrease the diffusion rate of oxygen.Boric oxide was generated in the oxidation of boron compounds and liquid phase was formed.The oxidation rate of the materials was decreased.In the dynamic ablation tests,boric oxide assisted the sintering of zirconia into ceramic layer and decreased the seepage rate of oxygen.It improved the ablation resistance ability of the material.The inner close packed nano-zirconia layer could slow down the diffusion rate of oxygen and decrease the oxidation rate of the material.Besides,molten zirconia layer,liquid boic oxide layer and zirconium carbide layer could also impove the ablation resistance ability of the modified C/C composites.