| A2B2O7 oxides are the potential candidate materials for thermal barrier coatings and nuclear wasteforms with the high melting point,low thermal conductivity,low oxygen permeability,anti-sintering and anti-irradiation.However,the coefficient of thermal expansion,fracture toughness and thermal cycle life of A2B2O7 oxides are relatively low,which limit its application in the thermal barrier coatings.In addition,A2B2O7 oxides have a strong selectivity to radionuclides when it is used as nuclear wasteforms.it can only selectively immobilising radioactive nuclides with appropriate valence states and ionic radius,and its compatibility with radionuclides needs to be further improved.A2B2O7 oxides can carry out various chemical substitution at the A and B sites,and the crystal structure also tolerates vacancies,which provides the possibility to regulate the properties by component design to solve the above problems.In recent years,the high-entropy ceramics provides a new window for the component design of A2B2O7 oxides.High-entropy ceramics have the characteristics of adjusting component,strong designability and lattice distortion.It is expected to show the four-effects of high-entropy materials.High-entropy ceramics usually have better thermal,mechanical,electrical and magnetic properties than the traditional single-component ceramics.Therefore,the properties of high-entropy ceramics are highly consistent with the requirements of thermal barrier coatings and radionuclide wasteforms,which provides a new opportunity for the practical application of A2B2O7 oxides.The multi-component A2B2O7 oxide ceramics with different cation sites(A and B sites)and components(rare earth elements and transition metal elements)were designed,and the phase,relative density,microstructure and element distribution of the oxide ceramics were analyzed.The relative density and grain size of the A-site and B-site oxide ceramics were compared.Based on this,A-site high-entropy ceramics were selected to explore the properties in the thermal barrier coatings,including Young's modulus,mechanical properties,coefficient of thermal expansion,thermal conductivity,oxidation resistance,sintering resistance and thermal shock resistance,and the mechanism of performance improvement was deeply analyzed.In addition,B-site high-entropy ceramics were selected to explore the properties in the radionuclide wasteform.The leaching resistance of B-site high-entropy ceramics was evaluated by static leaching experiment,and the mechanism of improving the leaching resistance was analyzed.The main research contents and results are as follows:(1)A2B2O7 oxide ceramics with different cationic sites(A and B)and principal elements were designed by pressureless sintering method.La2O3,Nd2O3,Sm2O3,Eu2O3,Gd2O3 and ZrO2 powders were used as raw materials.The ternary medium-entropy ceramics(La1/3Nd1/3Sm1/3)2Zr2O7,quaternary medium-entropy ceramics(La1/4Nd1/4Sm1/4Eu1/4)2Zr2O7 and quinary high-entropy ceramics(La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2Zr2O7 were prepared.La2O3,Ti O2,ZrO2,HfO2,Nb O2 and CeO2 powders were used as raw materials.The ternary medium-entropy ceramics La2(Zr1/3Hf1/3Ce1/3)2O7,quaternary medium-entropy ceramics La2(Ti1/4Zr1/4Hf1/4Ce1/4)2O7 and quinary high-entropy ceramics La2(Ti1/5Zr1/5Hf1/5Nb1/5Ce1/5)2O7 were prepared.The multi-component A-site high-entropy ceramics are single-phase pyrochlore structure,and the multi-component B-site high-entropy ceramics are dual-phase pyrochlore and monoclinic structure.The average grain sizes of ternary,quaternary and quinary A-site ceramics are 1.23?m,1.03?m and0.94?m,respectively.The relative densities of ternary,quaternary and quinary A-site ceramics are75.5%,78.1%and 77.7%,respectively.The average grain sizes of ternary,quaternary and quinary B-site ceramics are 0.64?m,1.10?m and 2.38?m,respectively.The relative densities of ternary,quaternary and quinary B-site ceramics are 66.0%?98.6%and 100.0%,respectively.The above research provides the basis for the application direction and performance analysis of multi-component A2B2O7 ceramics.(2)Due to the most serious lattice distortion,excellent sintering resistance and small grain size of quinary A-site high-entropy ceramics,the performance of it applied to the thermal barrier coatings was studied.La2O3,Nd2O3,Sm2O3,Eu2O3,Gd2O3 and ZrO2 powders were used as raw materials to prepared the high-entropy ceramic powders(La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2Zr2O7 by solid state reaction method at 1100? for 1 h.The synthesized powders have the pyrochlore structure with an average particle size of 0.76?m,and the elements are homogeneously distributed.High-entropy ceramic powders exhibit excellent thermal stability at high temperature in the range of room temperature to 1400?.Then the synthesized high-entropy ceramic powders were used as raw materials to prepared the high-entropy ceramic bulks at 1650? for 5 h by pressureless sintering method.The results show that:a)The high-entropy ceramic bulks have the pyrochlore structure with an average grain size of 1.27?m,and the elements are homogeneously distributed.The relative density of the high-entropy ceramic bulks is 98.6%.b)The Young's modulus,Vickers hardness,fracture toughness,coefficient of thermal expansion,thermal conductivity and electric conductivity of the high-entropy ceramic bulks are 220±9 GPa,10±0.2 GPa,1.0±0.4 MPa·m1/2,10.7×10-6 K-1(1100?),1.81 W·m-1·K-1(1100?)and 4.3×10-11 S·cm-1(750?),respectively.The synthesized high-entropy ceramic powders were used as raw materials to prepared the high-entropy ceramic coatings on the surface of nickel-base superalloy by atmospheric plasma spraying.The high-entropy ceramic coatings have the fluorite structure with a porosity of 9.17%,and the elements are homogeneously distributed.The thermal cycle life of the high-entropy ceramic coating is 53 times at 1100?.The high-entropy ceramics(La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2Zr2O7 have the excellent thermal stability,thermal properties,oxidation resistance,sintering resistance and thermal shock resistance,which are very promising candidate materials for thermal barrier coatings.(3)Due to the most serious lattice distortion,easy sintering and coarse grain size of quinary B-site high-entropy ceramics,the performance of it applied to the wasteforms was studied.Eu2O3,Gd2O3,Ti O2,ZrO2,HfO2,Nb2O5 and CeO2 were used as raw materials to prepared the dual-phase high-entropy ceramics Eu2(Ti1/5Zr1/5Hf1/5Nb1/5Ce1/5)2O7(HE-E),Eu Gd(Ti1/5Zr1/5Hf1/5Nb1/5Ce1/5)2O7(HE-EG)and Gd2(Ti1/5Zr1/5Hf1/5Nb1/5Ce1/5)2O7(HE-G)with uniform element distribution and coexistence of pyrochlore and fluorite structure by pressureless sintering method at 1550? for 2 h.The grain sizes of the HE-E,HE-EG and HE-G ceramics were 2.12?m,2.01?m and 1.93?m,respectively.The relative density of the HE-E,HE-EG and HE-G ceramics were 97.5%,97.0%and94.4%,respectively.The difficulty of preparing ceramic wasteforms with high relative density is significantly reduced by bring in Nb element with low melting point through composition design.After 42 days of static leaching test,the leaching rate of simulated radionuclides Ce and Gd is about10-8?10-6 g·m-2·d-1,which is 2 orders of magnitude lower than that of ternary A2B2O7 oxide ceramics.high-entropy ceramics(Eu1-xGdx)2(Ti1/5Zr1/5Hf1/5Nb1/5Ce1/5)2O7(x=0,0.5,1.0)have excellent resistance to leaching and relatively higher nuclide immobilising ability,which are the very promising candidate for wasteforms. |
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