Optimization Of Thermal And Mechanical Properties And Solid Solution Mechanism Of Samarium Tantalate Ceramics With Low Thermal Conductivity

Thermal barrier coatings(TBCs)are one of the most important materials in gas turbine to protect the high temperature components.They have excellent resistance to corrosion,increase working temperature,reduce fuel consumption,improve the service lifetime and show great potential for use as next-generation gas turbine.As the TBCs not only have high melting point,high-temperature phase stability,low thermal conductivity and high-temperature toughness,but also have good compatibility with matrix materials.However,the most widely used yttria stabilized zirconia(YSZ)ceramics is restricted by the problems of phase transformation,accelerated sintering,pronounced radiation heat transfer and molten salt corrosion at high temperature.Thus,it is integral to identify novel materials with better combination property for the next generation thermal barrier coating applications.Due to the high melting point(?1600?),low thermal conductivity,high thermal expansion coefficients and so on,rare earth tantalates ceramics are potential TBC materials.It is selected as the investigated object that the low thermal conductivity samarium tantalate ceramics as potential TBCs materials in this thesis.The thermal-mechanical properties are conducted by means of doping,alloying,nonstoichiometry and composition design,which is crucial to the materials selection and properties tailoring.The main contents contain:Firstly,SmTaO₄,Sm₃TaO₇ and Sm Ta₃O₉ ceramics were prepared via a high temperature solid-state reaction.The crystal structure was carried out to tailor the force-thermophysical properties of samarium tantalate ceramics.The results show that the minimum mean free path of SmTaO₄,Sm₃TaO₇ and Sm Ta₃O₉ ceramics is higher than the atomic distance.It indicates that thermal conductivity of samarium tantalate ceramics has room to decrease,and thermal conductivity is further decreased with the content increase of ions.Compared with that of 7-8 YSZ,Sm₃TaO₇ has a low thermal conductivity(1.3-1.7 W·m^-1·K^-1 at 100-900°C),the highest TECs at approximately9.82×10^-6 K^-1 at 1200°C and excellent Young's modulus(183 GPa)and hardness(10.1 GPa).The thermal conductivity of Sm Ta₃O₉ gradually transform from k?T^-1 to glass-like with a change of temperature.The high concentration of vacancies can produce a low energy“rattling”vibrational mode,leading to glassy thermal conductivities in a crystalline material.Secondly,SmTaO₄ and Sm₃TaO₇ ceramics doped with the different contents of CeO₂,ZrO₂ and HfO₂ by high temperature solid-state reaction.The solid solution mechanism in the high-temperature sintering process is revealed in detail.The effect of tetravalent ions with different ionic radius and atomic weight were investigated.The results show that CeO₂,ZrO₂ and HfO₂ doping was inordinately increased the thermal conductivity and thermal expansion coefficient of ceramics.The SmTaO₄ceramics doped with 2%of ZrO₂ had lower thermal conductivity(0.97 W·m^-1·K^-1,900°C),the Sm₃TaO₇ ceramics doped with 8%of ZrO₂ had higher thermal expansion coefficients(10.9×10^-6 K^-1,1200°C).At last,we choose Sm₃TaO₇ as the second phase to composite with 7YSZ in this work.It is shown that effective scattering of radiation heat transfer was achieved by controlling both the concent and the grain size of second phase Sm₃TaO₇,and thermal conductivity was significantly reduced.The results show that 7YSZ/Sm₃TaO₇composites have lower thermal conductivity(0.81 W·m^-1·K^-1,900°C)than 7–8-YSZ,with a minimum nearly 50%lower than YSZ.The highest thermal expansion coefficient is 5wt.%7YSZ/Sm₃TaO₇ composites(12.1×10^-6 K^-1),which is much higher than that of YSZ(10.0×10^-6 K^-1,1200°C).The 7YSZ/Sm₃TaO₇ composites are prepared on the bonding layer Co Ni Cr Al Y by atmospheric plasma spraying techniques.At 1100?,the longest thermal cycle life is 5wt.%7YSZ/Sm₃TaO₇composites coating,which is much longer than that of YSZ coating.The main reason is that 5wt.%7YSZ/Sm₃TaO₇ composites had a higher thermal expansion coefficient,which can reduce the mismatch of the thermal expansion coefficient between the TBC material and adhesive layer,and effectively alleviate the thermal stress concentration of the coating.