Controlling Negative Thermal Expansion In GaXMn₃?X=N,C?Compounds Via Magnetic Element Doping And Magnetic Field

Due to the non-harmonic vibrations of the atoms,most materials expand upon heating while shrink upon cooling.The effect tends to produce stress concentration in the defective parts such as the grain boundary and vacancy of the material,which leads to thermal fatigue or mechanical fatigue,resulting in a decrease in structural stability,reduced safety and reliability,and a shortened service life of components.In addition,aerospace,precision instruments,optical devices,low temperature engineering and other areas of the material requirements of the high precision require the size of the workpiece is not sensitive to temperature.Negative thermal expansion materials that expand upon cooling while shrink upon heating(which can compensate for the positive expansion and decrease of the expansion coefficient of the general material)and near-zero expansion materials have a great potential value in above fields.The anti-perovskite structure manganese-based compound AXMn3(A is the main group or part of the transitional element;X is N or C)is a kind of material system with abnormal thermal expansion that are interesting for fundermental research and practical application.Such materials exhibit abundant lattice orders and magnetic orders,and usually the crystal structure excibits a sudden shrinkage when the magnetic phase transition occurs.As reported previously,negative expansion effect can be realized by broadening the magnetic/lattice phase transition zone by doping non-magnetic element at A-site(introducing local lattice distortion)or reducing the grain size(introducing disorder).At present,the negative thermal expansion reported mainly occurs at room temperature or above,while there are few studies on low-temperature negative thermal expansion.In addition,new methods other than mentioneds above for controlling negative expansion and new effects based on anomalous thermal expansion need to be explored.In this paper,we reported new materials showing negative thermal expansion in different temperature regions obtained by the magnetic element doping in antiperovskite compounds GaXMn3(X = N,C);The preliminary study on the preparation of low-temperature near-zero-expansion composites was carried out;The effects of magnetic field on the anomalous thermal expansion were studied,and the giant magnetostrictive effect with switching characteristics was found.The main contents are as follows:1.The effect of Mn doping on the negative thermal expansion of GaN0.8Mn3 was studied.The polycrystalline samples Ga1-xMnxN0.8Mn3(0.1 ? x ? 0.3)were prepared by solid-state reaction.As the Mn doping increases,the negative thermal expansion window expands and moves toward low temperatures.For example,Ga0.75Mn0.25N0.8Mn3 shows large negative thermal expansion in the wide temperature region near room temperature[linear expansion coefficient is-42 ppm/K(10-6/K)between 255-309 K].The magnetization measurements show that the ferromagnetic order coexists with the antiferromagnetic order associated with negative thermal expansion in the negative expansion zone.The presence of the ferromagnetic order disturbs the antiferromagnetic ordering,resulting in negative thermal expansion.This is different from the previously reported negative thermal expansion caused by local lattice distortion.Indeed,the result of atomic pair distribution function shows that the local lattice of Gao.75Mno.25No.8Mn3 does not distort when the nagetive thermal expansion occured.2.The effect of Cr doping on the thermal expansion of GaN0.83Mn3 was studied.The polycrystalline samples Ga1-xCxN0.83Mn3(0 ? x ? 0.3)were prepared and their thermal expansion,magnetism,electrical transport,specific heat were studied.The results of magnetic studies show that Cr doping leads to the enhancement of ferromagnetism,which inhibits the antiferromagnetic ordering,which expands the negative thermal expansion zone and moves it towards low temperature.Specific heat studies have shown that enhanced ferromagnetism from Cr doping can be attributed to an increase in the electron energy density at the Fermi surface.In addition,when the negative thermal expansion occurs,the magnetic and resistivity also change accordingly,indicating a correlation between the lattice,spin and charge in the vicinity of the phase transition of Ga1-xCrxN0.83Mn3.3.The low-temperature negative expansion behavior of(Ga0.7Cu0.3)1-xMnxNMn3 was studied,and the low temperature expansion coefficient of epoxy resin was tuned by using this material.(Ga0.7Cu0.3)NMn3 with a large lattice shirnk near 200 K was selected as the parent compound.When(Ga0.7Cu0.3)was partially replaced by Mn,negative thermal expansion at low temperatures were obtained,and the coefficient of thermal expansion of(Ga0.7Cu0.3)0.7Mn0.3NMn3 is-22.8 ppm/K below 120 K.Compared with the existing low temperature negative thermal expansion material,the current material has advantage in terms of the larger negative expansion coefficient and wider temperature zone.In addition,(Ga0.7Cu0.3)0.7Mn0.3NMn3 powder were mixed with epoxy resin,and thus a composite material with near zero thermal expansion at liquid nitrogen temperatures was prepared(1.1 ppm/K below 120 K).The thermal conductivity of the composite material is much higher than that of pure epoxy resin.Such low-temperature negative temperature expansion materials and zero thermal expansion composite materials have potential application in the crogenic engineerings.4.The influence of magnetic field on the abnormal thermal expansion of GaCMn3 lattice was studied.GaCMn3 shows three phase transitions,namely,the antiferromagnetic-intermediate magnetic state,followed by the intermediate magnetic-ferromagnetic state,and finally the ferromagnetic-paramagnetic phase.The lattice sudden shrink at the anti ferromagnetic-intermediate magnetic phase transition at?150K.This phase transition is very sensitive to the magnetic field.For example,when placed in a magnetic field of 50 kOe,the lattice mutation moves to a low temperature by 30 K and remains steep.Furthermore,GaCMn3 exhibits an isotropic magnetostriction(-1700 ppm)below the antiferromagnetic transition temperature,whose magnitude order is comparable to that of the commercial giant magnetostrictive material Terfenol-D.The magnetostrictive strain of GaCMn3 is derived from its steep field-induced magnetic phase transition.As long as the critical field is reached,magnetostrictive occurs suddenly,showing a switching effect.Also,the magnetostrain has good reversibility and stability.Compared with conventional magnetostrictive materials,the switching-magnetostrictive effect has a significant advantage in terms of magnetic energy(electrical energy)/mechanical energy conversion.