| As an abnormal physical phenomenon,negative thermal expansion(NTE)has attracted more and more attention not only because of its complex physical mechanism of coupling among spin,electron,phonon and lattice,but also its unique value in practical application.Compared with other NTE materials,metal-based NTE materials have the advantages of good mechanical properties,high thermal conductivity and electrical conductivity.Therefore,based on the NTE materials,this work reveals the NTE mechanism from the aspects of crystal structure,magnetic structure and electronic structure,and actively and effectively regulates the thermal expansion,so as to meet different practical application requirements.At the same time,explore the universal method to further improve thermal conductivity while ensuring zero thermal expansion(ZTE).The Er2(Fe1-xCox)14B solid solutions are formed by introducing Co into Er2Fe14B.The results of synchrotron X-ray powder diffraction show that the thermal expansion can be continuously adjustable(-3.7 × 10-6 K-1??V?7.5 × 10-6 K-1),especially when x=0.05,it shows ZTE in wide temperature range(?V=1.5 ×10-6 K-1,120?475 K).It should be noted that the ZTE at x=0.05 has the widest temperature range(?T=355 K)compared with the currently known ZTE alloys.It is found that the temperature dependence of magnetic moment plays a decisive role in the NTE.With the substitution of Co,the exchange interaction between Fe and Fe is enhanced,which makes it more difficult to break the coupling between magnetic atoms,and higher temperature is needed to achieve it.Therefore,the rate of magnetic moment changing with temperature becomes slower.As a result,the thermal expansion is regulated from NTE to ZTE and positive thermal expansion(PTE).Both HfFe2 and TiFe2 exhibit PTE characteristics,but peculiar NTE performance occurred after forming(Hf1-xTix)Fe2 solid solutions.With the increase of x,the NTE temperature region moves to lower temperature,and the widest NTE temperature range has achieved at x=0.4.When x=0.6,it shows ZTE at low temperature.(Hf0.6Ti0.4)Fe2 has been studied in detail by means of synchronous X-ray powder diffraction,macroscopic magnetic measurement and neutron powder diffraction.It is found that the large volume ferromagnetic phase and small volume antiferromagnetic phase coexist in the NTE temperature region.Although both phases show PTE performance,the decrease of ferromagnetic phase and the increase of antiferromagnetic phase with the increase of temperature resulted in the NTE of the whole volume.The strong NTE of YbMn2Ge2(?V=-32.9×10-6 K-1,400?575 K,?T=175 K)is investigated by neutron powder diffraction and X-ray absorption near edge structure.It is found that the strong NTE is not caused by a single mechanism,but by the magneto volume effect of Mn magnetic moment and the valence transition of Yb.This dual effect mechanism is not only limited to the magnetovolume effect and valence transition,but also applicable to the rigid unit model and ferroelectric effect.Therefore,it is a universal method for finding strong NTE materials.Combined with the advantages of NTE material and high thermal conductivity of copper,the core-shell structure of NTE material coated with copper is constructed by electroless plating,and composites with continuously adjustable thermal expansion(-9.6 × 10-6 K-1??V?17 × 10-6 K-1)are obtained.In particular,a ultra-low thermal expansion over a wide temperature range(?1=2.14×10-6 K-1,300?700 K,?T=400 K)has been obtained when the volume fraction of copper is 35%.At the same time,3D thermal conductivity network copper structure is obtained,which is more conducive to play the advantages of high thermal conductivity of copper,and high thermal conductivity(60 W·m-1·K-1)is achieved,which is higher than that of other ultra-low thermal expansion materials.The electroless plating process in this work has universality and can be applied to metal-based or non-metallic NTE materials,and can improve the current situation of low thermal conductivity of ultra-low thermal expansion materials. |
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