Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics.These unique properties are strongly related to their layered crystal structures and microstructures. The co...Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics.These unique properties are strongly related to their layered crystal structures and microstructures. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy （STEM） and transmission electron microscopy （TEM）, selected area electron diffraction （SAED）, convergent beam electron diffraction （CBED） represents a powerful method to link microstructures of materials to macroscopic properties, allowing layered ternary ceramics to be investigated in an unprecedented detail. Vicrostructural information obtained using TEM is useful in understanding the formation mechanism, layered stacking characteristics, and defect structures for layered ternary ceramics down to atomic-scale level; and thus provides insight into understanding the ＂Processing-Structure-Property＂ relationship of layered ternary ceramics. Transmission electron microscopic characterizations of layered ternary ceramics in Ti-Si-C, Ti-Al-C, Cr-Al-C, Zr-Al-C, Ta-Al-C and Ti-Al-N systems are reviewed.展开更多
M_(3)A_(2)X phases,named 321 phases,are an atypical series of MAX phases featuring in the MA-triangular-prism bilayers,with the A=As/P,exhibiting excellent elastic properties.This work systematically studies the therm...M_(3)A_(2)X phases,named 321 phases,are an atypical series of MAX phases featuring in the MA-triangular-prism bilayers,with the A=As/P,exhibiting excellent elastic properties.This work systematically studies the thermal expansion properties of 321 phases.We found their average linear thermal expansion coefficients(TECs),α_(L)=5-6μK^(–1),are the lowest among the reported values of MAX phases.The lowest average TEC was found in Nb_(3)As_(2)C(αa=4.46(4)μK^(–1),αc=5.09(4)μK–1,αL=5.09(4)μK–1).The average TEC and anisotropy factor(αc/αa)of Nb_(3)As_(2)C and Nb_(3)P_(2)C were lower than the ones of the corresponding 211 phases.The best isotropy performance was found in Nb_(3)P_(2)C (αc/αa=1.11).Moreover,our first-principles calculations demonstrate that the weaker chemical bonding between Nb-As/P than Nb-C induces thermal expansion in M_(3)A_(2)X phases.Furthermore,a relatively weaker anharmonic effect in 321 phases than in the 211 phases was revealed by the as-calculated average Grüneisen parameters,which account for the lower TECs in 321 phases.The low TECs and enhanced thermal isotropy make 321 phases outstanding among MAX phases,which could be sound candidates for varying-temperature structural-functional components.展开更多
文摘Layered ternary ceramics represent a new class of solids that combine the merits of both metals and ceramics.These unique properties are strongly related to their layered crystal structures and microstructures. The combination of atomic-resolution Z-contrast scanning transmission electron microscopy （STEM） and transmission electron microscopy （TEM）, selected area electron diffraction （SAED）, convergent beam electron diffraction （CBED） represents a powerful method to link microstructures of materials to macroscopic properties, allowing layered ternary ceramics to be investigated in an unprecedented detail. Vicrostructural information obtained using TEM is useful in understanding the formation mechanism, layered stacking characteristics, and defect structures for layered ternary ceramics down to atomic-scale level; and thus provides insight into understanding the ＂Processing-Structure-Property＂ relationship of layered ternary ceramics. Transmission electron microscopic characterizations of layered ternary ceramics in Ti-Si-C, Ti-Al-C, Cr-Al-C, Zr-Al-C, Ta-Al-C and Ti-Al-N systems are reviewed.
基金financially supported by the National Science Foundation for Young Scientists of China(No.51902055)the Natural Science Foundation of Fujian Province(No.2021J011077)the Fuzhou Science and Technology Plan Project(No.2021-P-049).
文摘M_(3)A_(2)X phases,named 321 phases,are an atypical series of MAX phases featuring in the MA-triangular-prism bilayers,with the A=As/P,exhibiting excellent elastic properties.This work systematically studies the thermal expansion properties of 321 phases.We found their average linear thermal expansion coefficients(TECs),α_(L)=5-6μK^(–1),are the lowest among the reported values of MAX phases.The lowest average TEC was found in Nb_(3)As_(2)C(αa=4.46(4)μK^(–1),αc=5.09(4)μK–1,αL=5.09(4)μK–1).The average TEC and anisotropy factor(αc/αa)of Nb_(3)As_(2)C and Nb_(3)P_(2)C were lower than the ones of the corresponding 211 phases.The best isotropy performance was found in Nb_(3)P_(2)C (αc/αa=1.11).Moreover,our first-principles calculations demonstrate that the weaker chemical bonding between Nb-As/P than Nb-C induces thermal expansion in M_(3)A_(2)X phases.Furthermore,a relatively weaker anharmonic effect in 321 phases than in the 211 phases was revealed by the as-calculated average Grüneisen parameters,which account for the lower TECs in 321 phases.The low TECs and enhanced thermal isotropy make 321 phases outstanding among MAX phases,which could be sound candidates for varying-temperature structural-functional components.
