| Nickel-iron alloy is an important transitional bimetallic material for its long history of application and wide application.In order to understand the important role played by nickel-iron alloys in various physicochemical processes,it is important to explore the structural characteristics and thermodynamic stability of nickel-iron alloys.Both the Curie temperature and the order-disorder transition temperature are important parameters for describing the structural characteristics and thermodynamic stability of nickel-iron alloys.The excellent properties of nanomaterials continue to attract the attention of researchers,as well as for nickel-iron alloys.It is well known that the various properties of nanomaterials will change as the scale decreases,but there are few studies on the scale effect of the Curie temperature and the order-disorder transition temperature of Ni-Fe nanoalloy systems.Complex magnetic-chemical effect makes it difficult to study the physical properties of nickel-iron alloys experimentally and theoretically.And there is no theoretical model for the effect of the Curie temperature and the order-disorder transition temperature of Ni-Fe nanoalloys.To solve the above problems,we have devised a method,based on the model of a regular solution and taking into account the interactions between the nickel and iron nano-atoms,for quantitatively describing the scale and composition of Ni-Fe nanoalloys for their Curie temperature and order-none The thermodynamic model of the influence of order transition temperature.According to the calculation results of the model,we found that the Curie temperature and the order-disorder transition temperature of Ni-Fe nanoalloy particles decrease as the scale decreases and the composition of nickel increases,and the magnitude of the order transition temperature decrease with the scale is less than the decay amplitude of the Curie temperature.In addition,the model calculation results also show that the non-magnetic surface layer is an intrinsic property of nickel-iron alloy,and its surface layer thickness also decreases with increasing scale,and tends to be a fixed value when the scale is greater than 40 nanometers,rather than other theoretical calculations of which is disappeared.At the same time,we also made electrodeposited Ni-Fe nanoalloy samples,and used a vibrating sample magnetometer(VSM)and differential scanning calorimetry(DSC)to measure the Curie temperature and the order-disorder transition temperature of the samples.The experimental results were compared with the results of our thermodynamic model calculations and found that the model was in good agreement with the experimental test results.The correctness of the model will provide an important guiding role for the rational design and application of Ni-Fe nanoalloy devices in the future.The main contents are summarized as follows:(1)Electrodeposition method was used to prepare the 4 Ni-Fe nanoalloy samples.The electrolyte was purified before the deposition while the substrate was deoiled.Electrochemical deposition using galvanostats and setting 3.8,5.0,6.8,and 7.8 A/dm2 at four different current densities to regulate the composition of the desired Ni-Fe nanoalloy,followed by measurement of four Ni-Fe nanoalloy samples.We used XRFS to measure the compositions,represented as Ni65Fe35,Ni62Fe35,Ni69Fe31 and Ni76Fe24,respectively.The four samples were also measured by transmission electron microscopy.The average particle size of the four Ni-Fe nanoalloy samples samples was approximately 28,28,23 and 20 nanometers,respectively,as measured by the cut-linemethod.(2)Using the vibrating sample magnetometer to measure the Curie temperatures of the four Ni-Fe nanoalloy samples according to the Curie-Weiss Law,and the values are 748,738,717 and 728 K,respectively,and the values are lower than the curie temperature of corresponding coarse crystals.Two differential absorption peaks(peak one and peak two)were found on the DSC curve by differential scanning calorimetry,and the corresponding values were Ni65Fe35:707 and 743 K;Ni62Fe3g:705 and 750,respectively.K;Ni69Fe3i:673 and 825 K;Ni76Fe24:680 and 741 K.Furthermore,the temperature increase rate in both the VSM and DSC tests is lOK/min.At the same time,the diffraction ring of the ordered superlattice structure of Ni-Fe nanoalloy was observed in selected area electron diffraction.(3)Based on the normal solution model,considered the interaction between the two component atoms at the same time,and considered that when the alloy scale is reduced,its structure remains unchanged.So we believed that this consideration can be extended to the nanoscale.And based on general quantum chemistry considerations,the bond order-length-strength correlation mechanism and the Ising model,the scale effect function of the interaction between alloying atoms and the scale effect function of cohesive energy are consistent,then the normalized Curie temperature is derived.The normalized Curie temperature is equal to the ratio of the Curie temperature of the bulk alloy and nanoalloy,which is weighted average of the two components.Many studies have pointed out that the order-disorder transition temperature is proportional to the Debye temperature,and therefore it is proportional to the square root of the binding energy Ec.Thus,we have obtained a thermodynamic model of the order-disorder transition temperature accordingly.Comparing the experimental test results with our thermodynamic model results to verify the correctness of our model.The thermodynamic models were then used to quantitatively analyze the variation of the Curie temperature and the order-disorder transition temperature of Ni-Fe nanoalloy with decreasing scale and increasing nickel composition.The intrinsic property of the non-magnetic layer on the surface of the Ni-Fe nanoalloy is calculated and decreases with increasing particle size,and tends to a fixed value when the scale is large enough. |
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