Study On The Negative/Zero Thermal Expansion Effect In La?Fe,Si?₁₃ And ?Hf,Ta?Fe₂ Alloys

Negative thermal expansion materials have developed rapidly in recent years,which have important applications in electronics,medical,communications,aerospace and other advanced fields.Negative thermal expansion?NTE?alloys with magnetic phase transformation have attracted wide attention due to their large isotropic negative thermal expansion effect and good thermal and electrical conductivities.However,due to the rapid phase transformation and large lattice constant changes,these alloys usually have poor mechanical properties and relatively narrow NTE operation-temperature window,which hinders further research and application.In this paper,we chose La?Fe,Si?₁₃ magnetic alloys which have giant NTE effect and prepared La?Fe,Si?₁₃/?-Fe composite material by adding excessive Fe elements into the system.The NTE of matrix 1:13 phase is compensated by?-Fe phase.An extremely low coefficient of thermal expansion?CTE?of-6.5×10^-7 K^-1 between 261K and 282 K is obtained when the fraction of?-Fe phase reaches 31.08%,leading to the establishment of ZTE.Additionally,as a toughening phase,?-Fe phase is helpful to block crack propagation,thus enhancing the mechanical integrity and yield compressive stress.At the same time,the thermal conductivity and electrical conductivity of the composites have also been improved.In addition,we chose Hf_1-xTa_xFe₂ magnetic alloys as the research object and controlled the CTE of the alloy by adjusting the amount of Ta doping in the system.It is found that the content of Ta not only affects the NTE of the system,but also has a significant effect on the thermal expansion in the low temperature range.At x=0.13,an extremely low CTE of 2.7×10^-7 K^-1 between 124 and 230 K is obtained.Further study shows that the zero thermal expansion effect in the system is caused by the asynchronous FM orderings of Fe moments at 6h and 2a Fe sites and the slow and continuous FM orderings of Fe atoms at 2a.