Negative Thermal Expansion Research Of MnNiGe Compound With Ni₂In-type Structure

Most materials expand on heating and contract on cooling,showing positive thermal expansion(PTE).As a result of PTE,materials or devices are prone to produce thermal stress,which leads to the failure of materials in microstructure or even in macroscale.Besides,when the temperature changes abruptly,large difference in coefficients of thermal expansion(CTE)between different materials could also lead to accuracy decline or performance failure due to thermal stress.Controlling the coefficients of PTE materials then to improve the dimensional stability of materials has become one of the most important topics in the design and manufacture of modern materials.Negative thermal expansion(NTE)materials,which exhibit opposite expansion behaviors with that of PTE,can act as the inhibitor of the PTE one to reduce the CTE of materials and solve the related problems resulted by the PTE.Therefore,it is very important to research and develop the NTE materials with excellent expansion characteristics(such as large coefficient of NTE,wide NTE temperature range covering the room temperature,etc.)The Ni2In-type structure compound MnCoGe reported in recent years is a new type of NTE material,which shows the volume contraction of about-3.9%upon the martensitic(MA)transformation.Previous researchers indicated that the MA transformation of the MnCoGe compound can be tuned to near room temperature by chemical doping and the NTE temperature range of MnCoGe-based compounds can be broadened by introducing internal stress.Accordingly,the giant NTE spanning a wide temperature range across room temperature was obtained.However,the MnCoGe-based compounds show strong ferromagnetism after MA transformation,which is not suitable for their application in magnetic environment.In this paper,we report the NTE behavior of antiferromagnetic(AFM)(TN=346 K)MnNiGe compound.The AFM NTE across the room temperature or lower temperature was obtained by alloying isomorphic compounds or substituting Ge with large size atoms in MnNiGe,which in turn expanded the application field of this kind of compound as NTE materials.The main contents are as follows:1.The AFM NTE covering room temperature or liquid nitrogen temperature was obtained by doping Ni2In structure FeCoSn into MnNiGe to disturb the magnetic order of(MnNiGe)1-x(FeCoSn)x compounds.The average coefficient of linear expansion reaches-35 ppm/K(175 K-330 K)for x=0.02 and-52 ppm/K(74 K-144 K)for x=0.04,respectively.The magnetic testing and the related researches show that the Co and Sn can stabilize the AFM state of MnNiGe within a certain doping component,while the Fe is not conducive to the stability of the AFM state due to its strong ferromagnetism.2.The NTE and magnetic properties of(MnNiGe)1-x(MnCoSn)x compounds are studied.The results show that the NTE coefficient of(MnNiGe)1-x(MnCoSn)x(0.02 ?x?0.3)compounds increases first then decreases with increasing x,and the maximum NTE reaches-96 ppm/K(174 K-280 K)for x=0.04.The magnetic test results show that the NTE of x?0.1 compounds presents the AFM and its magnetization under the same magnetic field decreases with increasing x,which indicates that the material system has changed from helical AFM to collinear one.All of these are related with the combined action of the Co substitutiing for Ni and Sn substituting for Ge in MnNiGe,and the Co promotes the transition from helical AFM to the collinear AFM.3.The influence of Pb doping on the NTE and magnetic properties of MnNiGe was studied.The results show that the NTE of MnNiGe1-xPbx compounds continuously moves from high temperature to low temperature due to the doping of Pb on Ge site,and the AFM NTE over a wide temperature range is realized within the content of 0.02? x ? 0.2.The NTE coefficient of MnNiGe1-xPbx compounds was improved first then degraded with increasing Pb cotent,and the maximum coefficient of linear expansion reaches-97 ppm/K(150 K-250 K).The magnetic testing shows that the antiferromagnetism of MnNiGe1-xPbx compounds gradually weakens with increasing Pb concentration.Just like the case of Sn doping in MnNiGe1-xSnx compounds,the increase of Mn-Mn coupling distance due to the substitution of large size atoms for Ge promotes the transition from the helical AFM to the fully collinear AFM.