Nanocrystalline Wires Research Articles

Fatigue-driven acceleration of abnormal grain growth in nanocrystalline wires

Molecular dynamics simulations were employed to simulate the mechanical response and grain evolution in a Ni nanowire for both static and cyclic loading conditions at both 300 and 500 K for periods of 40 ns. The loading conditions included thermal annealing with no deformation, constant 1% extension (creep loading) and cyclic loading with strain amplitudes of 0.5% and 1% for 200 cycles. Under cyclic loading, the stress–strain response showed permanent deformation and cyclic hardening behavior. At 300 K, modest grain evolution was observed at all conditions within the 40 ns simulations. At 500 K, substantial grain growth is observed in all cases, but is most pronounced under cyclic loading. This may result mechanistically from a net motion of the boundaries associated with boundary ratcheting. There is a striking qualitative consistency between the present simulation results and the experimental observation of abnormal grain growth in nanocrystalline metals as a precursor to fatigue crack initiation.

Comprehensive Evaluation of the Properties of Ultrafine to Nanocrystalline Grade 2 Titanium Wires.

This paper describes the mechanical properties and microstructure of commercially pure titanium (Grade 2) processed with Conform severe plastic deformation (SPD) and rotary swaging techniques. This technology enables ultrafine-grained to nanocrystalline wires to be produced in a continuous process. A comprehensive description is given of those properties which should enable straightforward implementation of the material in medical applications. Conform SPD processing has led to a dramatic refinement of the initial microstructure, producing equiaxed grains already in the first pass. The mean grain size in the transverse direction was 320 nm. Further passes did not lead to any additional appreciable grain refinement. The subsequent rotary swaging caused fine grains to become elongated. A single Conform SPD pass and subsequent rotary swaging resulted in an ultimate strength of 1060 MPa and elongation of 12%. The achieved fatigue limit was 396 MPa. This paper describes the production possibilities of ultrafine to nanocrystalline wires made of pure titanium and points out the possibility of serial production, particularly in medical implants.

Energy Losses Analysis of Fe-based and CoFe-based Soft Ferromagnetic Wires

The CoFe-based amorphous and Fe-based nanocrystalline alloys possess excellent soft magnetic properties due to their nearly zero-magnetostriction and very low magnetic anisotropy. So they are good candidate materials for power electronics and giant magneto-impedance (GMI) sensor applications. As an indication of their AC performance, energy losses were determined by the measurements of impedance for Co68.2Fe4.3Si12.5B15 amorphous wire and Fe73.5Cu1Nb3Si13.5B9 nanocrystalline wires with different domain structure. The measurements were carried out at a frequency range of 10 – 104 Hz and induction amplitudes of 0.1 – 1.2 T. Although the measured results show good agreement with the scale theory model of energy loss, different origins of energy loss in the investigated samples have been concluded by comparing the experimental data with the Bertotti’s statistical model. The differences of energy loss in these samples could be explained by considering the magnetic structure and circular technical magnetization processes.

Effect of low-temperature aging treatment on thermally- and stress-induced phase transformations of nanocrystalline and coarse-grained NiTi wires

The phase transformation behaviors of nanocrystalline NiTi alloys coupling with grain size poses a challenge in functional property configuration. To realize this configuration and simultaneously avoid undesirable grain growth, the low-temperature aging (LTA) treatment at 573K for 2h was applied to both the nanocrystalline and coarse-grained NiTi wires in this study and the effect of LTA on both the thermally- and stress-induced phase transformations was respectively investigated. The results show that, after LTA, B2↔R transformation temperature of nanograins was elevated when R→B19′ transformation was maintained suppressed. The stress hysteresis and residual strain of nanograins were increased while those of coarse grains were decreased. Nanograins required higher stress to activate stress-induced R-phase transformation than coarse grains. Aged NiTi coarse grains presented larger thermal hysteresis but smaller stress hysteresis compared with non-aged ones. To have an in-depth understanding of these differences, the microstructures and microhardness were further studied. It turns out that the nanoprecipitation, lattice recovery, as well as the preservation of the preformed grain size are responsible for the differences. This study thus suggests the potential of configuring the functional properties while simultaneously maintaining the constant grain size via LTA treatment, which may facilitate the application of NiTi nanocrystalline.

Elastocaloric effect induced by the rubber-like behavior of nanocrystalline wires of a Ti-50.8Ni (at.%) alloy

Elastocaloric effects in shape memory alloys are usually caused by latent heat associated with the stress-induced martensitic transformation. We report here that the rubber-like behavior of the R-phase in nanocrystalline Ti-50.8Ni (at.%) wires, realized by an aging heat treatment under tensile stress, also contributes a significant elastocaloric effect. The efficiency of the elastocaloric effect caused by the rubber-like behavior is higher than that caused by the stress-induced B2↔B19′ transformations because of the small energy dissipation. The usage of the rubber-like behavior is a method to extend the temperature window of the elastocaloric effect in shape memory alloys.

A model of axial heterostructure formation in III–V semiconductor nanowires

A kinetic model of the formation of axial heterostructures in nanocrystalline wires (nanowires, NWs) of III–V semiconductor compounds growing according to the vapor–liquid–solid (VLS) mechanism is proposed. A general system of nonstationary equations for effective fluxes of two elements of the same group (e.g., group III) is formulated that allows the composition profile of a heterostructure to be calculated as a function of the coordinate and epitaxial growth conditions, including the flux of a group V element. Characteristic times of the composition relaxation, which determine the sharpness of the heteroboundary (heterointerface), are determined in the linear approximation. A temporal interruption (arrest) of fluxes during the switching of elements for a period exceeding these relaxation times must increase sharpness of the heteroboundary. Model calculations of the composition profile in a double GaAs/InAs/GaAs axial heterostructure have been performed for various NW radii.

Stochastic quantum confinement in nanocrystalline silicon layers: The role of quantum dots, quantum wires and localized states

Nanocrystallites of Silicon have been produced by electrochemical etching of crystal wafers. The obtained samples show photoluminescence in the red band of the visible spectrum when illuminated by ultraviolet light. The photoluminescence spectra can be deconvolved into three components according to a stochastic quantum confinement model: one band coming from Nanocrystalline dots, or quantum dots, one from Nanocrystalline wires, or quantum wires, and one from the presence of localized surface states related to silicon oxide. The results fit well within other published models.

Influence of cold drawing on the magnetic properties and giant magneto-impedance response of FINEMET nanocrystalline wires

The effect of annealing and applied stresses on the magnetic properties and giant magneto-impedance effect in as-cast and cold drawn FINEMET wires has been studied. The results show major changes in the evolution of the magnetic permeability, coercivity, and magneto-impedance response with the annealing and cold drawing stages. These changes have been explained considering the structural transformations, intrinsic, applied, and drawing-induced stresses, as well as their effect on the global magnetostriction of the wires. The observed sensitivity to applied stresses recommends this class of cold drawn nanocrystalline wires for applications in miniaturized magnetic sensors with enhanced sensitivity.

Multifunctional Sensor Based on a Hybrid Ferromagnetic/Sol–Gel TiO2 Coating Nanostructure

Development of a multifunction sensor for detection of environmental parameters such as temperature and relative humidity is described in this work. The multifunctional sensor is based on the combination in the same sensor unit of a magnetoinductive temperature sensor (soft magnetic FeCrSiBCuNb nanocrystalline wire) and a capacitive ceramic element (TiO2 microscale coating) for humidity detection. The sensing principle is based on the changes of electrical impedance of both the inner ferromagnetic wire (self-inductance, L) and the ceramic TiO2 coating (capacitance, C). In the magnetoinductive temperature element, the sensor span can be suitably selected through control of the Curie temperature of the residual amorphous phase (i.e., Cr content of the residual amorphous phase). With respect to the TiO2 coating, its doping with alkali elements (K) improves the humidity sensor sensibility. The designed cylindrical metal/TiO2/metal capacitive structures display resistive switching phenomena (memresistive behav...

Analysis of heating effects (magnetic hyperthermia) in FeCrSiBCuNb amorphous and nanocrystalline wires

The induction heating effects in amorphous and nanocrystalline wires, Fe73.5-xCrxSi13.5Cu1B9Nb3 (x = 3, 7, and 10), are analyzed in this work. In these alloys, the Curie temperature of the amorphous phase, TC, can be tailored through the Cr content of the alloy or the volume crystalline fraction after nanocrystallization. Four samples were selected; amorphous with x = 0 and 10 and nanocrystalline x = 7 with different crystalline fractions. The Curie temperature of the residual amorphous phase, TCa, was experimentally determined by the temperature dependence of the self-inductance of the samples. The analysis of the frequency dependence of the complex magnetic susceptibility enabled the estimation of the magnetic power losses in the samples. The heating effects on the wires were analyzed under the application of an ac magnetic field employing a home-made hyperthermia set-up. A single piece of a wire was immersed in a water bath (initial temperature from 291 K to 325 K) and subjected to the ac magnetic field. The specific absorption rate (SAR) was estimated through the initial slope of the temperature increase as a function of time. Maximum SAR values were obtained in the amorphous sample (x = 3) with the highest TC and enhanced magnetic power losses. In the nanocrystalline samples (x = 7), the detected heating effects above TCa are interpreted as a consequence of the magnetization process of the ferromagnetic grains. However, in spite of the low SAR displayed by the amorphous wire with TC ≈ 300 K (x = 10), interesting self-regulated characteristics are observed in this sample.

Torsion-Impedance in Soft Magnetic Amorphous and Nanocrystalline Wires

The measurements of magneto-impedance,, have been performed, at the frequency f = 1 MHz, on amorphous and nanocrystalline wires under the application of torsion stress ( = 0-125 rad/m). At , exhibit single-peak due to the axial easy direction of the wires. The torsion-impedance,, is defined as the impedance change with at the condition of the axial field H = 0. We found that at the low torsion , shows a maximum at = 0.87 and 5.23 rad/m for the amorphous and the nanocrystalline samples, respectively; with increasing ,of the amorphous wire decreases monotonously and tends to the saturation, but of the nanocrystalline sample shows two more peaks at = 70.65 and 104.67rad/m. These results have been discussed considering the modification of domain structure by the applied torsion.

Magnetic induction heating of FeCr nanocrystalline alloys

In this work the thermal effects of magnetic induction heating in (FeCr)73.5Si13.5Cu1B9Nb3 amorphous and nanocrystalline wires were analyzed. A single piece of wire was immersed in a glass capillary filled with water and subjected to an ac magnetic field (frequency, 320kHz). The initial temperature rise enabled the determination of the effective Specific Absorption Rate (SAR). Maximum SAR values are achieved for those samples displaying high magnetic susceptibility, where the eddy current losses dominate the induction heating behavior. Moreover, the amorphous sample with Curie temperature around room temperature displays characteristic features of self-regulated hyperthermia.

Improvement of Giant Magneto-Impedance Effect in Fe-Based Nanocrystalline Wires

In order to obtain materials with more ideal GMI effect, (DZ/Z）max, the substitution effects of R (R = Co, Ni and Cr) for Fe in Fe63.5R10Cu1Nb3Si13.5B9 soft magnetic wires have been studied. We found that although the Finemet composition shown excellent GMI effect, the substitution of Co can raise notably the field sensitivity, Q, Ni makes a little improvements of (DZ/Z）max, and Q, and Cr diminishes the undesirable hysteretic effect. In additions, the influence of annealing conditions upon the GMI effect of the best composition Fe63.5Cr10Cu1Nb3Si13.5B9 was also investigated. It indicated that the sample obtained by the ways of Joule heating under the application of suitable tensile stress exhibits 200% and 30%¤Oe for (DZ/Z）max and Q, respectively. This sample can serve as a proper choice for magnetic sensors.

Influence of AC current on the profile of GMI effect in Fe-based nanocrystalline wire

The influence of AC current (with amplitud of Iac ＝0.2—20 mA and frequency of f＝1 kHz—1 MHz) upon the profile of gian magnetoimpedance (GMI) effect of a Fe-based nanocrystalline wire with transverse domain structure have been investigated. The experiment indicates that the GMI effect of the sample exhibits a two-peak feature. With the increasing of f, the peak field Hm increases, while Hm decreases even to H=0 when Iac increases. According to the current theoretical models, the peak field Hm should conrrespond to the transverse anisotropy field Hk, Hm=Hk. However, we found that this conclusion is correct only for the case of very small AC current. So a relationship Hm=Hk cos3θ, where θ is the angle between saturation magnetisation and the axis of wire, has been deduced by minimizing the energy in the process of magnetisation rotation. The experiments can be explained by this result.

Influences of axial field upon circular magnetization and eddy‐current anomaly factor in Fe‐based nanocrystalline wire

AbstractFe‐based soft magnetic wire with transverse magnetic anisotropy was obtained by stress current annealing. Its circular permeability, µ = µ′ – jµ″, has been determined from the measurements of impedance, Z = R + jωL, as a function of frequency (f = 10 – 105 Hz), AC current amplitude (I = 0.1 – 100 mA) and DC bias field (H = 0 – 5.4 kA/m). An experimental study of the influence of H upon the circular magnetization and the f‐dependent µ = µ′ – jµ″ has been performed. It is concluded that the axial field, H, evolution of the circular permeability, µ = µ′ – jµ″, the relaxation frequency, fτ, and the eddy‐current anomaly factor, η, can be explained in terms of the modification of domain structure by the applied axial field H.

Prediction of Energy Loss of Ni0.58Zn0.42Fe2O4 Nanocrystalline and Fe3O4 Nanowire Arrays

The hysteretic differential susceptibility in both cases magnetization and demagnetization processes was modeled for Ni0.58Zn0.42Fe2O4 nanocrystalline and Fe3O4 nano wire arrays. It was formulated hysteretic loop of nanoscale magnetic materials in both magnetization and demagnetization processes. Energy loss was predicted in an expression as a result of this model. Equations of hysteretic loop of magnetic nanoscale materials were achieved for Ni0.58Zn0.42Fe2O4 nanocrystalline and Fe3O4 nanowire. A predicted energy loss per unit mass to heat per cycle of both Ni0.58Zn0.42Fe2O4 nanocrystalline and Fe3O4 nanowire array was estimated.

Eddy-current anomaly in Fe-based nanocrystalline wires

The circular permeability, μ= μ′− jμ″, of two Fe-based nanocrystalline wires obtained by furnace annealing (S1) and by current annealing (S2) were determined from the measurements of the impedance, Z= R+ jωL, as a function of frequency ( f=0.1–100 kHz) and DC bias field ( H=0–89 kA/m). So the H-dependent low- f eddy-current anomaly factor, η( H), has been investigated. The experimental results indicate that the sample S2 by current annealing has a larger η=4 than the sample S1 by furnace annealing with η=2.8 at H=0. With increasing H, the η( H) of two samples decreases initially until about H=450 A/m, and then increases very quickly. These results have been analyzed by the helical magnetization model considering the contribution of domain wall displacements (DWD) and domain magnetization rotation (DMR).

Magnetic spectra of Fe-based nanocrystalline wires with axial or transverse domains

The circular permeability μ′= μ′− jμ″ of two Fe-based soft magnetic wires with axial and transverse domains, has been determined from the measurements of impedance Z= R+ jX as functions of frequency ( f=10–10 5 Hz) and AC current amplitude ( I=0.1–100 mA). From the magnetic spectra of μ′− f and μ″− f for a few circular fields ( H φ =0.4, 1.2, 4, 12, 40 A/m), we found that the sample with axial domain structure exhibits a relaxational feature, while for the one with transverse domain resonance-like spectra were observed when the circular field H φ ≥4 A/m. These results have been discussed in terms of domain structure and circular magnetization processes.

Temperature Dependence of Magnetoimpedance and Anisotropy in Nanocrystalline Finemet Wire

Here we present the unusual dependence of magnetoimpedance effect on applied axial magnetic field for Finemet-type nanocrystalline wires in the low-temperature range between - 195degC to 200degC. The dependence exhibits two maxima, with a stronger maximum at the magnetic field value of the order of anisotropy, when applied in the direction of previous saturation. Such behavior is treated in terms of strong induced transversal magnetoelastic anisotropy due to the thermal expansion of the wire. The experiment is confirmed by a simple model which describes also the temperature dependence of the transversal magnetic anisotropy.

Frequency- and axial-field-dependent circular magnetization reversal in a stress-annealed Fe-based nanocrystalline wire

Absract After suitable annealing under a tensile stress, Fe73.5Cu1Nb3Si13.5B9 amorphous wire becomes the nano-structured material together with a transverse anisotropy field Hk=−3.2 kA/m. Its circular permeability, μ=μ′−jμ″, was determined from the measurements of the impedance, as a function of the frequency (f=333–33,333 Hz) and amplitude (IAC=0.1–100 mA) of AC current and the axially applied DC field (H=0–89 kA/m). We found that the circular technical magnetization of this sample carried out by the spontaneous domain nucleation process. The influences of the AC current frequency and the axial DC field on the circular magnetization have been studied.