Frank P. Missell

Effect of Plastic Deformation on the Excess Loss of Electrical Steel

The interpretation of the effect of plastic deformation on the calculated excess loss component (anomalous-loss) supports the concept of loss separation. Magnetic losses and Barkhausen noise of nonoriented electrical steel sheets were measured on Epstein strips taken from a single coil of 0.8% Si nonoriented electrical steel. Sheets were extracted in the annealed condition, without any skin pass and with a grain size of 18 μm. This material was cold rolled in order to obtain sets of samples with true strain from 2% up to 29%. X-ray diffraction was used to estimate the dislocation density. The analysis of magnetic properties was performed by Barkhausen noise measurements and also by analyzing the hysteresis loops obtained from Epstein frame measurements for different inductions and different frequencies (including the quasi-static regime for hysteresis loss measurements). These data allowed us to observe that most of the well known total loss increase with plastic deformation is due to an increase in the hysteresis loss component, while excess loss decreases to become negligible. This behavior can be explained if it is assumed that the plastic deformation lead to an increase in the number of domain walls per unit volume, thereby decreasing the excess loss. Barkhausen peak area increases with plastic deformation, reproducing results taken from samples of different silicon content.

Effect of surface roughness on performance of magnetoelastic biosensors for the detection of Escherichia coli.

Escherichia coli are bacteria that must be controlled in the food industry and the hospital sector. Magnetoelastic biosensors offer the promise of rapid identification of these and other harmful antigens. In this work, strips of amorphous Metglas 2826MB3 were cut to size (5 mm × 1 mm) with a microdicing saw and were then coated with thin layers of Cr and Au, as verified by Rutherford backscattering spectroscopy (RBS). Several sensor surfaces were studied: 1) as-cast strip, wheel side; 2) as-cast strip, free surface; and 3) thinned and polished surface. A layer of cystamine was applied to the Au-covered magnetoelastic substrate, forming a self-assembledmonolayer (SAM), followed by antibodies, using a modified Hermanson protocol. The cystamine layer growth was verified by Fourier transform infrared spectroscopy (FTIR) and scanning electronmicroscopy (SEM). The biosensors were exposed to solutions of bacteria and the resonant frequency of the sensors was measured with an impedance analyzer for times up to 100 min. Reductions in the resonant frequency, corresponding to bacteria capture, were measured after optimizing the signal amplitude. For times up to 40 min, high capture rates were observed and thereafter saturation occurred. Saturation values of the frequency shifts were compared with the number of bacteria observed on the sensor using fluorescence microscopy. Parameters associated with capture kinetics were studied for different sensor surfaces. The rough surfaces were found to show a faster response, while the thinned and polished sensors showed the largest frequency shift.

Barkhausen Noise and Magnetic Properties of Plastically Deformed Silicon Steels

We present Barkhausen noise and magnetic measurements on two fully processed, nonoriented electrical steels which had been cold-rolled to thickness reductions of up to 60%. Both coercive field Hc and hysteresis loss Wh show an almost linear increase with thickness reduction up to the highest deformations. These changes are almost fully reversed after vacuum annealing for 2 h at 760°C. The hysteresis loss can be conveniently subdivided into high and low induction components as suggested by recent modelling. Electron back-scatter diffraction (EBSD) shows no texture change during initial phases of cold-rolling. Barkhausen noise measurements were obtained on both cold-rolled and annealed samples. The undeformed material shows a Barkhausen signature consisting of two small peaks which coalesce into one peak upon plastic deformation and thereafter grow steadily. Annealing the material brings back the two-peaked signature. These results are discussed and hypotheses are presented for the behavior of the Barkhausen noise.

Electron backscattered diffraction texture analysis of SmCo5 magnets

The remanence of sintered SmCo5 magnets is a direct function of their crystallographic texture. The Jr∕Js ratio was determined by electron backscattered diffraction (EBSD) for SmCo5 magnets which had been oriented in a pulsed field of 6T, compressed isostatically, and then sintered at 1150–1170°C for 1h. The experimentally determined EBSD distribution does not follow a Gaussian or a function of the form f(θ=cosnθ). EBSD measurements provided the same Jr∕Js value (0.91) as a microstructural model plus remanence measurements. X-ray diffraction Schulz pole figures resulted in a slightly higher Jr∕Js value (0.92), probably due to neglecting misaligned grains.

Estimate of the anisotropy field in isotropic SmCo 2:17 magnets with the Stoner-Wohlfarth CLC model

The Callen-Liu-Cullen (CLC) modification of the Stoner-Wohlfarth model was found able to describe properly the hysteresis curves of isotropic Sm(CoFeCuZr)z magnets. The SW-CLC model uses three parameters, and all of them have physical meaning. One of the parameters is related to the saturation magnetization, another to the anisotropy field, and another is 1/d, which evaluates the interaction between grains or particles. The model was applied for several magnets, indicating an anisotropy field of 6-7 T, which is compatible with other methods for anisotropy field determination. The model also gives insight into the abnormal temperature dependence of the coercivity found in SmCo 2:17 magnets. For compositions with a low z, the parameter 1/d is significant. These compositions with a low z are those showing the most abnormal coercivity behavior with temperature.

Case depth in SAE 1020 steel using barkhausen noise

The most widely used thermochemical process for surface hardening of steels is case hardening. Using several different heat treatments, martensitic surface layers were formed on SAE 1020 steel into which carbon had been diffused. Case depths were measured by traditional destructive techniques. Barkhausen noise measurements were made and both the RMS Barkhausen pulse envelope and the fast Fourier transform (FFT) were obtained from numerical calculation. The FFT amplitudes, functions of frequency, were associated with distance from the sample surface using the skin depth equation δ = 1/ (πfσµ)½ , where f is the frequency of the electromagnetic wave, s is the electrical conductivity, and µ is the magnetic permeability. We define a normalized power index (NPI) which can be used to estimate case depths. The NPI is discussed in relation to the sample microstructure and it is shown that the case depth is most easily determined when the magnetic properties of the surface layer and core are substantially different.

Influence of microstructural constituents on the hysteresis curves in 0.2%C and 0.45%C steels

Steels with 0.2% and 0.45%C were submitted to different types of heat treatment, leading to different microstructures, with different amounts of the microstructural constituents: ferrite, pearlite and martensite. Measurements of magnetic hysteresis and Barkhausen noise were performed at different frequencies. A relationship was found between coercive force and the volume fraction of the microstructural constituents. The influence of the microstructural constituents on the shape of hysteresis curve is also discussed. The presence of martensite leads to more magnetization reversal by domain rotation, reducing the relevance of domain wall movement as a reversal mechanism. When domain wall movement predominates, permeability increases and coercivity decreases. As a consequence, when the martensite volume fraction increases, coercivity also increases. The results help clarify the relation between microstructure of steels and Barkhausen noise measurements in non-destructive testing.

Effect of Ti-C and Cr Additions on Magnetic Properties of Nanocrystalline (Pr,Nd)-Fe-B Alloys

The addition of both Ti-C and Cr as grain refiners in Nd-Fe-B nanocomposites substantially increases the coercive field Hc. This motived our investigation of the effect of Ti-C and Cr on Pr-Fe-B nanocomposites. Melt-spun ribbons of composition (Pr_9.5 Fe_84.5 B₆ )_0.97 _- _x Cr_x (TiC)_0.03(x = 0; 0.25; 0.5; 0.75; 1) and (Nd _9.5Fe _84.5B ₆) _0.97-x Cr_x(TiC) _0.03( x = 0.5 and 1) were produced for study. For a Pr nanocomposite with 1% Cr, Hc = 12.5 kOe. However, the energy product was limited to 13.6 MGOe by the remanence value. Rietveld analysis of X-ray spectra showed the ribbons to consist of predominantly hard ( ~ 70 wt%) R ₂Fe ₁₄B, the soft phase being ( ~ 30 wt%) alpha-Fe. Mossbauer measurements at 300 K are consistent with a reduced hyperfine field for the hard magnetic phase due to the Cr addition. Analysis of transmission electron microscopy images showed the Pr nanocomposite with 1% Cr to have an increased average grain size.

Magnetic characterization of the (Zr,Sm)Co3 phase in Sm(CoFeCuZr)z magnets

Magnets with composition Sm(CobalFe0.2Cu0.1Zrx)8 (x=0;0.02;0.04;0.06,0.08) were studied using x-ray diffraction Rietveld analysis. In addition to the majority 2:17 and 1:5 phases, the magnets present a rhombohedral (SmZr)1(CoFeCu)3 (PuNi3 structure) phase, whose volume fraction increases with Zr addition. Higher Zr contents decrease coercivity and lead to the appearance of impurity phases such as 6:23 (Th6Mn23 structure), 2:7R (Gd2Co7 structure), and 5:19R (Ce5Co19 structure) in the magnets. The 1:3R phase was reproduced in a Fe57-doped sample of Sm0.33Zr0.67Co3 and its Mossbauer spectra, at several temperatures, were adjusted using four sextets and a nonmagnetic doublet. The beneficial effect of small additions of Zr on the coercivity of the magnets is discussed.

Influence of antibody immobilization strategies on the analytical performance of a magneto-elastic immunosensor for Staphylococcus aureus detection

Magneto-elastic (ME) sensors have a great advantage in microbiology due to their ability to be queried wirelessly. Staphylococcus aureus is one of the most common bacteria widespread in the environment and a major human pathogen related to numerous illnesses. Immunosensors are affinity-based assays where the analyte is highly selective. The immobilization of antibodies (Ab) is an important step in the development of such devices. This study compared the effects of two antibody immobilization strategies on the analytical performance of a magneto-elastic immunosensor: (1) random antibody covalent immobilization (CysAb) and (2) specific-oriented antibody covalent immobilization (PrGAb). Immunosensors were exposed to solutions containing S. aureus at different concentrations (104 to 108CFU/ml) and sensor resonant frequencies were measured. In order to confirm that the frequency shifts were mainly caused by the binding of S. aureus to the sensors surface, scanning electron microscope (SEM) and indirect immunofluorescence (IIF) images were taken after bacteria exposure at 108CFU/ml. Sensor surfaces were further monitored by non-contact topographic atomic force microscopy (AFM) images. In the covalent-oriented strategy, PrG was first bound covalently to the surface, which in turn, then binds the anti-S. aureus antibody in an oriented manner. Topographic AFM images showed different surface patterns between the two antibody immobilization strategies. Specific-oriented antibody covalent immobilization (PrGAb) strategy gave the highest anti-S. aureus antibody immobilization density. Therefore, the covalent-oriented strategy presented the best performance for S. aureus capture, detecting 104CFU/ml.