J. Ferenc

The Nanocrystalline FeSiBCuNb Finemet Absorption Properties at Microwaves

The reflection and absorption properties of powdered nanocrystalline FeSiBCuNb (“Finemet”) alloy at microwaves have been presented. To visualize the shielding ability of this material, the complex permittivity and permeability of pulverized Finemet in the frequency range from 0.2 to 10 GHz have been studied. To measure the permittivity and permeability of powdered Finemet, the modification of the Nicolson, Ross, and Weir method was presented. The modification of this method was mainly based on the unwrapping technique of the measured S-parameter phases. The investigation of permittivity and permeability was realized for pure powders. Four groups of powders with different particle sizes below 200 μm have been investigated. To assess the shielding effectiveness of the nanocrystalline Finemet, two attempts were analyzed. The investigation of the reflection loss of material layer with metal sheet attached to the backside as a reflector was investigated. On the other hand, the reflection and absorption factors of 3-mm-thick layer in free space were additionally analyzed. The obtained results show that the pulverized nanocrystalline Finemet alloy with particle size below 25 μm possesses good absorption properties and, for such particles, the absorption factor is significantly higher than the reflection factor.

Correlation between microstructure and temperature dependence of magnetic properties in Fe60Co18(Nb,Zr)6B15Cu1 alloy series

Temperature dependence of magnetic properties of nanocrystalline Fe60Co18Cu1B15Nb6−xZrx (x=0, 3, 6) alloys has been studied at different stages of devitrification. Transmission electron microscopy shows nanocrystals of the size ∼5 nm, which remains almost constant along the nanocrystallization process. Curie temperature of the residual amorphous phase decreases as nanocrystallization progresses for all the studied alloys. Thermal dependence of the exchange stiffness constant is obtained from the measurement of specific magnetization and coercivity as a function of crystalline fraction and temperature for the three studied alloys.

Magnetostrictive Iron-Based Bulk Metallic Glasses for Force Sensors

Iron-based bulk metallic glasses have been known for several attractive properties, such as high mechanical strength and magnetic softness. They also exhibit sensitivity of magnetic properties to stress, which is the result of magnetoelastic coupling. In this paper, these combined features were exploited to build a simple, but effective magnetoelastic force sensor. It was found that when rods made of glassy (Fe,Co,Ni)-Nb-Si-B alloys, used as cores in a transformer, are subjected to compressive stress, voltage on secondary winding changes with the application of force. When proper measurement conditions are met, the dependence of output voltage versus force is nearly linear. Furthermore, rods with 2 mm in diameter are able to withstand forces reaching 8 kN, which gives an opportunity to design small-sized, high-load magnetoelastic force sensors.

Magnetic Properties and Stability of Magnetically Soft Nanomaterials for High-Temperature Applications

Two-phase iron-based nanocrystalline alloys are the newest generation of magnetically soft materials. FINEMET and NANOPERM are the well known nanocrystalline materials obtained by partial crystallization of metallic glasses, with magnetic properties better than those found for the amorphous counterparts. The two-phase amorphous-nanocrystalline alloys exhibit very soft magnetic behavior but only up to temperatures close to the Curie point of the amorphous matrix. Thus, their application is limited only up to about 200 ̊C. In order to increase the Curie point of the amorphous matrix and to elevate the application temperature of these alloys, iron is partially replaced by cobalt. Nanomaterials for high-temperature applications must fulfill two basic requirements: (i) very soft magnetic behavior at elevated temperatures, and (ii) stable performance at elevated temperatures for long time (application time). The second requirement is related to thermal stability of nanocrystalline structure and magnetic properties of these materials. This paper summarizes the current status of research in the field of magnetically soft nanocrystalline materials for high-temperature applications, especially highlighting the influence of alloy composition on structure and magnetic properties as well as their stability at elevated temperatures during very long time annealing. The original studies have been oriented on tailoring appropriate alloys for different application temperatures and studies of their stability during annealing for thousands of hours at temperatures up to 500 ̊C. FINEMET and NANOPERM alloys have been modified by alloying elements like Co, Si, Cu, Hf, Zr, Nb and B. The structure was studied using Differential Scanning Calorimetry, X-ray Diffraction and Transmission Electron Microscopy. Hysteresis loop and VSM measurements were used for magnetic properties characterization at room and elevated temperatures.

Magnetically Soft Nanocrystalline Materials Obtained by Devitrification of Metallic Glasses

This paper presents the main features of magnetically soft metallic glasses and nanocrystalline materials obtained by controlled crystallization of metallic glasses, a brief description of the principal methods of nanocrystallization as well as the recent developments in nanocrystalline materials for high-temperature applications. Two groups of alloys were investigated: (Fe, Co)-Si-Nb-Cu-B (FINE-MET-type) and (Fe, Co)-(Zr, Nb, Hf)-Cu-B (HITPERM-type). For FINEMET-type alloys it was found that the optimum combination of magnetic properties coercivity, Curie temperature, magnetostriction) is obtained when Fe:Co ratio is about 1:1. For HITPERM-type alloys, the best performance and stability are observed when alloys contain Hf, and the worst in the case of Nb. Optimum Hf content is 7 at.%, and 6 at.% B. The HITPERM-type alloys exhibit good stability of properties at 500°C for at least 700 hours.

Magnetically Soft Nanomaterials Obtained by Devitrification of Metallic Glasses

Magnetically soft nanomaterials obtained by controlled crystallisation of metallic glasses are the newest group of materials for inductive components. In particular, research is carried out in the field of alloys for high temperature applications. This kind of materials must meet two basic requirements: good magnetic properties and stability of properties and structure. In the present work the magnetic properties and structure of Fe-Co-Hf-Zr-Cu-B (HITPERM-type) alloys were investigated, as well as their stability. Differential thermal analysis, (DTA), X-ray diffractometry (XRD), transmission electron microscopy (TEM), magnetometry (VSM) and quasistatic hysteresis loop recording were used to characterise structure and properties of the alloys investigated. Optimisation against properties and their stability was performed, resulting in formulation of chemical composition of the optimum alloy, as well as its heat treatment.

Magnetically Soft Fe-Co-Based Nanocrystalline Alloys

In the present study, Fe 45Co43B3.6Cu1Zr7.4-xAM x (where x = 3.7 or 7.4, and alloying metals, AM, are Hf or Nb) alloys have been investigated in order t o assess the structure and soft magnetic properties, as well as their thermal stability. The amorphous alloys were first nanocrystallised at 500 and 600°C, and subsequently annealed at 500°C for up to 700 hours . X-ray diffractometry, differential scanning calorimetry, transmission electron microscopy and magnetic hysteresis loop measurement were used as investigation techniques. T he crystallisation temperature of the alloys is between 455 and 525°C. Substituting Zr with Nb decreases the first stage crystallisation temperature and increases the second stage cry stallisation temperature, while Hf acts in the opposite manner, but its influence is much weaker. The coercive f ield (Hc) of the nanocrystallised alloys increases slightly during the first 50 hour s f annealing, and then remains almost stable up to 700 h. The lowest values of H c are observed for alloys containing Hf, being between 35 and 50 A/m. This may be attributed to the grain size of -FeCo, which is expected to be the smallest for the alloys containing Hf. Further TEM investiga tions will be carried out to verify this assumption. Introduction The new magnetically soft nanocrystalline HITPERM alloys (Fe -Co-Zr-Cu-B) are the result of developments of NANOPERM alloys leading to their application at el evated temperatures [1,3]. The addition of Co to the original alloy increases the Curie temper atur of both constituent phases, i.e. -FeCo and the amorphous matrix, which improves magnetic properties at elevated temperatures, as well as increasing the crystallisation tem perature of the amorphous precursor. Consequently, HITPERM alloys maintain good soft magnetic properties (high magnetisation, relatively high permeability, low coercivity, and low core losses ), at temperatures of 500-600°C. These alloys are a group of attractive materials suitable for levated temperatures applications, such as rotors of aircraft generators, transformers and magnetic bea rings. Good magnetic and structural stability of the alloys in the service conditions is important in a chieving optimum service life of the final products. The aim of the present study is to investigate the i mpact of chemical composition of Fe-Co-AM-Cu-B alloys on the thermal stability of their magnetic properties . Experimental Amorphous ribbons of alloys with chemical compositions of Fe 45Co43Cu1B3.6Zr7.4-xAM x (at. %), where x = 3.7 and 7.4, and alloying metals, AM, are Hf and Nb, were prepa red in air using the meltspinning technique. Partial crystallisation was performed by isother mal annealing for 1 hour, in vacuum, at temperatures ranging from 400 to 700°C. Alloys, whose stability was to be examined by prolonged annealing, were initially nanocrystallised in vacuum at 500 and 600°C for 1 hour. Long term annealing was also performed in vacuum, at 500°C, for up to 700 hours. The tructure and properties of the partially crystallised alloys was investiga ted using x-ray diffractometry (XRD), differential scanning calorimetry (DSC), and quasi-static magne tic hysteresis loop measurement at room temperature [4]. Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 67-70 doi:10.4028/www.scientific.net/SSP.94.67

Mössbauer and magnetic studies of FeCoNiCuNbSiB nanocrystalline alloys

Abstract Nanocrystalline Fe80-x-yCoxNiyCu1Nb3Si4B12 alloys were prepared by the annealing of amorphous ribbons. Primary crystallization of the alloys annealed at temperatures of between 500 and 550°C was studied by X-ray diffraction and Mössbauer spectroscopy. Magnetic properties of the alloys were investigated using a hysteresis loop tracer and vibrating sample magnetometer. The annealed ribbons are composed of a two-phase nanostructure consisting of bcc Fe-based grains embedded in an amorphous matrix. Conversion electron Mössbauer spectroscopy (CEMS) measurements reveal a more advanced crystallization process in the surface layers when compared with the volume of the ribbons. The degree of saturation magnetization of the nanocrystalline alloys is of about 1.5 T. The coercive field varies from 1.0 to 6.5 A/m and peaks at an annealing temperature of 525°C. Magnetic softening of the nanocrystalline alloys observed after annealing at 550°C is correlated with a volume fraction of the nanocrystalline bcc phase.

Magnetic Anisotropy of Nanocrystalline HITPERM-Type Alloys and its Correlation with Application

Structure as well as magnetic and magnetoelastic properties of nanocrystalline (Fe,Co)-(Hf,Zr)-Cu-B alloys (HITPERM-type) were investigated in order to find out which factors are responsible for the magnetic hardening of these magnetically soft materials. Magnetoelastic anisotropy, caused by the presence of cobalt, was found to play the predominant role in the observed increase of coercive field. On this basis, guidelines on chemical composition and crystallisation process selection were suggested for two fields of application: soft magnetic cores and sensors or actuators cores.