Preparation And Characterization Of Ta-Hf(Zr)-C Ternary Ceramics

Ta-Hf(Zr)-C as solid solution of Ta C and Hf(Zr)C,having ultra high temperature melting point and excellent resistance to oxidation and ablation,is an ideal material system applied in future extreme thermal environment.Search for appropriate methods to synthesize Ta-Hf(Zr)-C ceramics remains topical.The preparation of Ta-Hf(Zr)-C ceramics has been reported via two carbides directly solid solution strengthening sintering,which has some shortcomings,such as high sintering temperature,insufficient solid solution,coarse grain size.This paper prepares Ta-Hf(Zr)-C ceramics with ultrafine grained microstructure and single phase solid solution via synthesizing of single phase solid solution Ta-Hf(Zr)-C ceramic nano powder,followed by low temperature sintering.Details are as follows:Based on the thermodynamics calculations,MOx-C are certified to react,implying that the preparation of Ta-Hf(Zr)-C ceramics is theoretically feasible.The theoretically synthesis temperature showed a decrease when the content of Hf(Zr)C solution by the calculations.So the 1Ta C·1Hf(Zr)C are more difficult to synthesize within the Ta-Hf(Zr)-C ceramics,and the theoretically synthesis temperature of that are about 1600 K.The key technology of this paper is the synthesis of pure,highly disperied,and single phase solid solution Ta-Hf(Zr)-C nano powder via solvothermal treatment joint carbothermal reduction(1Ta C·1Hf(Zr)C,2Ta C·1Hf(Zr)C and 4Ta C·1Hf(Zr)C).Multi-layer core-shell structure MOx@C precursor was obtained via solvothermal treatment with chloride(Ta Cl5,Hf Cl4,and Zr Cl4)served as metal source and phenolic resin used as carbon source.The obtained MOx@C precursor was highly dispersed with ball shape,in which the mean diameter of 500 nm.What’s more,the centre aera of the MOx core was rich with Ta,and the outspace was rich with Hf(Zr).Besides,the shell was carbon.So the multi-layer core-shell structure solved the problems when carbon reducing with multi-oxide,including preferential precipitated phases,insufficient solid solution and residual carbon.Both solvothermal treatment parameters and carbothermal reduction process were studied and optimized.The preferential process parameters of Ta-Hf(Zr)-C nano powders were: the temperature and holding time of solvothermal treatment were 200 ℃ and 12 h,respectively.Besides,the filling factor was chosen to be 60 %.The temperature and holding time of carbothermal reduction were 1600 ℃ and 3h,respectively.Besides,the heating rate was selected to be 100 ℃/min.The following formation mechanism of the Ta-Hf(Zr)-C ceramics was proposed.The sol to be gel in the solvothermal environment accompanied with self-assembly and growth of core-shell.The multi-layer core-shell structure MOx@C precursor would change during the carbothermal reduction process.The innerspace of the MOx core were changed to be(Hf/Zr)O2 phase,the transition part were to be Ta Zr2.75O8 and Ta2Hf6O17 solid solution,the the outspace area were to be rich with Ta2O5 phase.Besides,the shell would be carbonization.Carbon diffusion from shell to core would produce Ta-Hf(Zr)-C、Ta C and Hf(Zr)C phases successively,and solid solution to be Ta-Hf(Zr)-C single phase.The chemical contents,microstructure and crystalline structure of Ta-Hf(Zr)-C powders were studied(1Ta C·1Hf(Zr)C,2Ta C·1Hf(Zr)C and 4Ta C·1Hf(Zr)C).The evolution of Ta-Hf(Zr)-C powders in high temperature with and without oxygen was invitigated.The prepared uniform Ta-Hf(Zr)-C powders with an average partical size(500～600 nm),was a typically solid solution single phase recognized in XRD patterns.Besides,the outerspace of the crystallite was residual amorphous carbon layer with thickness of 1~2nm.The content of impurity element was low to be 0.88wt%.The grain size of Ta-Hf(Zr)-C powders was about 20~30nm.With the contents of Hf(Zr)C solution decrasing,the lattice distortion and lattice parameter of Ta-Hf(Zr)-C powders showed a same decrease trend,and the grain sizes showed a opposite increase trend.Besides,the change would be more obvious when the solution phase was Zr C.The raman spectras of Ta-Hf(Zr)-C powders were characterized.The intensity of lower frequency acoustic branches was far larger than that of higher frequency optical branches,and the O2 optical branche would be red shifted with the increase of Hf(Zr)C solution.The surface property of Ta-Hf(Zr)-C powders was investigated.The adsorption isotherm of the Ta-Hf(Zr)-C ceramic exhibited the characteristics of a type-IV isotherm(according to IUPAC classification).The main pores are slit mesoporous,with a mean diameter of 4 nm.The surface area of Ta-Hf-C powders are 30~80 m2/g,and that of Ta-Zr-C powders are 20~30 m2/g.Besides,the high surface activity make these powders oxidated easily,and formed 3~6 at.% Hf(Zr)-O-C in the Ta-Hf(Zr)-C powders.The evolution of Ta-Hf(Zr)-C powders in high temperature was investigated.These Ta-Hf(Zr)-C powders were kept to be single phase solid solution without phase separation and desolvation,even treated at high temperature of 2000~2400 ℃.The particle size of these powders was not exceeding 700 nm,which showed excellent high temperature stability.However,because of high surface activity,these powders would be oxidated at 200 ℃ in the presence of oxygen,especially for Ta-Zr-C powders.Ta-Hf(Zr)-C ceramics with ultrafine grained microstructure and single phase solid solution were prepared with the self-synthesised Ta-Hf(Zr)-C powders.The preparation and characterization were studied,and the property including mechanical property,oxidation property,ablation property and physicochemical property were characterized.Ta-Hf(Zr)-C ceramics(1Ta C·1Hf(Zr)C,2Ta C·1Hf(Zr)C and 4Ta C·1Hf(Zr)C)were prepared by HP process with 2000 ℃.The structure of the obtained ceramics was well organizational without visible defects and cracks.The relative density of the ceramics was 94%~98%,with a particle size of 3~7 μm.The main sinering mechanisms were surface diffusion,lattice diffusion,and grain boundary migration.Ta-Hf(Zr)-C ceramics showed topical elasticoplatic deformation when tested by the nanoindentation.The hardness are 25~40 GPa,the elasticity modulus were 480~590 GPa,and the toughness were 2.4~3.64 MPa·m1/2.Besides,there would be a positive correlation between the hardness and the content of Hf(Zr)C solution.The elasticity modulus showed same trend like hardness with the change of content of Hf(Zr)C solution.However,the toughness of the ceramics showed an increase with a decrease of content of Hf(Zr)C solution.Ta-Hf(Zr)-C ceramics showed good oxidation resistance when tested below 1000 ℃ in air with an electric muffle furnace.The structure of the ceramics remained intact,and the oxidation increase was not exceeding 1%.At higher oxidation temperature environment,the Hf(Zr)O2 phase would inhibit the growth of t-Ta2O5 phase.The newly formed Hf6Ta2O17 and Ta Zr2.75O8 phase had a very low oxygen diffusion coefficient,which would make the ceramics have good higher temperature oxidation resistance.Besides,the presence of Hf6Ta2O17 and Ta Zr2.75O8 phase would reduce the loss of Ta2O5 phase,and enhanced the ablation resistance of Ta-Hf(Zr)-C ceramics.The mass ablation rate and linear ablation rate of 1Ta C·1Hf C were 0.73mg·s-1 and 0.0123 mm·s-1.The electrical property,thermal property and optical property of Ta-Hf(Zr)-C ceramics were investigated.The electric conductivity were 2.10×106 ~3.48×106(Ω·m)-1,the thermal conductivity were 26.24~62.21(W·m-1·K-1),and the electric conductivity and thermal conductivity both showed an increase with a decrease of the content of Hf(Zr)C solid solution.Besides,the thermal conductivity of Ta-Zr-C was higher than that of Ta-Hf-C ceramics.The absorptivity rate of Ta-Hf(Zr)-C ceramics reaches the maximum of 90% in visible bands.The corresponding band-gap energy of Ta-Hf(Zr)-C ceramics were in the range of 2.065~2.861 ev.