| Due to the inherent anharmonic lattice vibrations,most solids expand upon heating,which is referred to as positive thermal expansion(PTE),leading to variations in dimension as temperature changes.In the fields of precision optical instruments and microelectronic devices,the thermal expansion of materials will severely affect the accuracy and normal function of the instrument.The difference in the coefficients of thermal expansion between different parts will cause catastrophic consequences such as interfacial stress,exfoliation or even fracture failure.In areas such as aerospace and cryogenic engineering,cyclical temperature fluctuations can cause stress concentration and fatigue failure,thereby shortening the service lifetime of materials or devices.Materials with negative thermal expansion(NTE)characteristics,which can be compounded with PTE materials and adjust the coefficient of thermal expansion(CTE)effectively,or materials with zero/low CTE characteristics have great potential for applying in many fields of contemporary industry.The iron-based Laves phase RFe2(R=Zr,Hf,Sc,Ti,etc.)alloys and their doping compounds exhibit an anomalous thermal expansion near their magnetic transition temperature,i.e.,the CTE of the magnetic ground state is lower than that of the paramagnetic(PM)state.Among them,Hf1-xTaxFe2 compounds have attracted much attention owing to their rich magnetic properties.The compounds with x～0.16-0.22 exhibit an abrupt volume shrinkage coupled with a magnetic transition from ferromagnetic(FM)to antiferromagnetic(AFM)state with increasing temperature,and the maximum magnitude of △V/V reaches 1%.However,the narrow temperature window(only a few K)of the phase transition,which occurs in low temperature,limits their application as NTE materials.In this paper,Hf1-xTaxFe2 compounds were systematically studied.By adjusting the Ta content,the abrupt volume shrinkage of Hf1-xTaxFe2 was successfully modified into a continuous one.A new material with a large negative CTE over a wide temperature range covering room temperature is obtained,and the mechanism responsible for the broadened volume shrinkage was discussed.On the basis of these preliminary results,a series of low-CTE composites with wide temperature range applicable to different working temperatures were obtained by means of introducing over-stoichiometric Fe,the substitution of Hf/Ta with excessive Cu,and the substitution of Fe by Ni.The main contents of the paper are listed as follows:1.By reducing the concentration of Ta,the sharp volume change in Hf1-xTaxFe2 was broadened to a continuous volume contraction and the phase transition is moved to around room temperature.Hence,the optimum NTE was obtained in x=0.13,which exhibits NTE over a wide temperature range from 222 to 327 K(△T=105 K)with a large NTE coefficient(αL=-16.3 ppm/K).Electron spin resonance(ESR)reveals that the NTE transition is closely related to the FM ordering process of Fe atoms at 2a and 6h sites,and the broadened temperature window of NTE is originated from the asynchronous FM ordering process at two Fe sites at the FM-PM transition.In addition,Hf1-xTaxFe2 compounds show attractive performance of good thermal conductivity and mechanical properties.2.The off-stoichiometric effect of Hf0.87Ta0.13Fe2+x on the thermal expansion and magnetic properties were studied.For x<0.4,the excess Fe resides on part of Hf/Ta sublattice by forming anti-site substitutional defects,giving rise to a slower FM ordering process of Fe atoms.The NTE of x=0 is gradually suppressed and finally changed to near-zero thermal expansion for x=0.4.When x>0.4,a secondary α-Fe phase precipitates in the matrix of C14 structure.α-Fe plays a role in bonding and strengthening the matrix,which greatly improves the mechanical properties and thermal conductivity of the material.As a result,x=1.0 exhibits a low CTE {αL=3.3 ppm/K)in the temperature range of 250-490 K(△T=240 K),concurrent with good thermal stability,high thermal conductivity(15.6 W·m-1·K-1)and improved compressive strength(1081 MPa).3.The effect of Cu addition on the thermal expansion and thermal conductance of Hf0.87Tao.i3Fe2Cux was studied.Hf/Ta atoms are partially substituted by Cu ones,causing a continuous suppression of NTE as well as a shift of the NTE window towards lower temperatures.Further increasing Cu content leads to the precipitation of Cu as a secondary phase.Finally,low CTE spanning from room to cryogenic temperatures was obtained,and the thermal conductivity and compressive strength of the material are markedly improved owing to the high thermal conductivity and strengthening effect of Cu.For example,x=1.25 has a CTE of only 1.7 ppm/K from 50 to 293 K,which is comparable to that the commercial Invar alloy.Moreover,Hf0.87Ta0.13Fe2Cux compound still maintain good thermal conductivity and mechanical properties in the temperature range of liquid nitrogen,suggesting the potential value for cryogenic application.4.The iron-based Laves phase has two crystal structures,i.e.,C14 and C15,which are close to each other in energy.Samples of Hf0.87Ta0.13Fe2-xNix series were prepared by arc melting.The compounds are a two-phase mixture of C14 and C15 for 0.1 ≤ x ≤ 0.4.The compounds of x≤ 0.1 crystallize in a single phase of C14 structure,and NTE characteristics similar to x=0 are retained in x≤0.1,though the NTE window is moved to lower temperatures.As for x≥0.15,the NTE of C14 phase at low temperatures is gradually weakened by the appearance of C15 phase.Due to the spontaneous volume magnetostriction effect,the C15 phase exhibits a lower CTE in its magnetic ground state(the transition temperature is higher than the NTE temperature of the C14 phase),causing a gradual decrease in CTE in the higher temperature region as the C15 phase content increases.Consequently,x=0.25 shows a low CTE characteristics in the whole temperature region investigated,i.e.,the linear CTE was 3.5 ppm/K in the temperature range of 50-360 K(△T=310 K). |
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