K. W. Dennis

Metal-bonded Co-ferrite composites for magnetostrictive torque sensor applications

A new class of magnetomechanical sensor materials, Co-ferrite (CoO/spl middot/Fe/sub 2/O/sub 3/) and metal-bonded (Ag, Ni, Co) Co-ferrite composites, has been investigated. These materials exhibited magnetostriction in excess of 200 ppm and high d/sub 33/ (/spl part//spl lambda///spl part/H)/sub /spl sigma// coefficient, 1.3/spl times/10/sup -9/ A/sup -1/ m, at low applied field (<100 kA/m). Selected compositions were formed into test samples in the form of rings brazed to stainless steel through-shafts. Changes of surface axial magnetic field in response to applied torque as high as 64 AN/sup -1/ m/sup -2/ were observed for a demonstration sample of dimensions 25 mm OD, 12.5 mm ID, and 8 mm high. A hysteresis of /spl plusmn/0.5 N/spl middot/m was observed. These materials appear to be promising candidates for torque sensors, and other magnetostrictive sensor and actuator applications.

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites (CoMnxFe2?xO4 with 0 ? x ? 0.6) was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10–400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoMn0.2Fe1.8O4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases.

Origins of coercivity in the amorphous alloy Nd-Fe-Al

Results obtained from detailed studies of the microstructure and devitrification behavior, as revealed by advanced electron microscopy and synchrotron x-ray diffraction techniques, were compared with magnetic and thermal analysis characterization to obtain insight into the appreciable coercivity of nominally-amorphous melt-spun ribbon of Nd/sub 60-2/3x/Fe/sub 30-1/3x/Al/sub 10+x/(-2</spl times/<6.5). The as-spun microstructure consists of an amorphous Nd-rich, magnetically-soft matrix with clusters of approximate 1.2 nm diameter that apparently confer the hard magnetic properties to the alloy. The clusters are tentatively identified as sub-unit-cells of Nd/sub 6/Fe/sub 13-x/Al/sub 1+x/, by virtue of their unique and analogous temperature-dependent magnetic character as well as considerations of the ternary phase relationships during solidification.

Mechanically induced reactions in organic solids: liquid eutectics or solid-state processes?

The solvent-free reaction between o-vanillin and p-toluidine was investigated using NMR, DSC and XRD analyses. At room temperature, o-vanillin and p-toluidine react in a liquid eutectic formed upon grinding, while below 10 °C the same materials appear to react without the formation of a liquid phase, which most likely remains hidden behind solid reactants and reaction products.

Phase equilibria in the PrBaCuO system under varied oxygen partial pressures

Abstract Phase equilibria in the Prue5f8Baue5f8Cuue5f8O system under varied oxygen partial pressure (pO2) is investigated. Stoichiometric PrBa2Cu3O7−δ (Pr123) does not exist for pO2

Substitution for Ba by Pr, La, and Eu in Eu(Ba1−xRx)2Cu3O7+δ solid solutions

Abstract We did a detailed study on a series of samples of Eu(Ba 1− x R x ) 2 Cu 3 O 7+δ (where R=La, Pr and Eu) by X-ray diffraction, neutron diffraction, magnetic and thermopower measurements. All R 3+ substitutes went to the Ba 2+ site and suppress superconductivity. The suppression of superconductivity does not have a simple relation to the ionic size of the substitutes. On the other hand, the change in crystal structure from orthorhombic to tetragonal has a strong dependence on the ionic size of the substitutes. Pr on Ba site did not cause unexpected structural distortions yet suppress superconductivity at a rate faster than other rare earths. Data of thermopower measurements and neutron diffraction from our rare earth substituted Eu123 samples show that the suppression of superconductivity by Pr on Ba site is electronic, not magnetic in nature.

Novel pre-alloyed powder processing of modified alnico 8: Correlation of microstructure and magnetic properties

Progress is reviewed on development of an improved near-final bulk magnet fabrication process for alnico 8, as a non-rare earth permanent magnet with promise for sufficient energy density and coercivity for electric drive motors. This study showed that alnico bulk magnets in near-final shape can be made by simple compression molding from spherical high purity gas atomized pre-alloyed powder. Dwell time at peak sintering temperature (1250u2009°C) greatly affected grain size of the resulting magnet alloys. This microstructure transformation was demonstrated to be useful for gaining partially aligned magnetic properties and boosting energy product. While a route to increased coercivity was not identified by these experiments, manufacturability of bulk alnico magnet alloys in near-final shapes was demonstrated, permitting further processing and alloy modification experiments that can target higher coercivity and better control of grain anisotropy during grain growth.

Studies of sintered MRE-Fe-B magnets by DyF3 addition or diffusion treatment (MRE = Nd + Y + Dy)

Sintered MRE2(Fe, Co)14B magnets by DyF3 blending or diffusion treatment were investigated. “Base-line” magnets with a thickness of 1.5u2009mm were coated (painted) with DyF3 powder and heated to promote Dy diffusion at 800–900u2009°C, i.e., “Diffusion” magnets. For comparison, the magnet alloy powder for making Base-line magnets was blended with 3–5u2009wt.% DyF3 powder and then made into sintered magnets, i.e., “Blended” magnets. The coercivity and (BH)max of Base-line magnets were 9.7 kOe and 32.7 MGOe, respectively, while the coercivity of Diffusion magnets was increased to 15 kOe and the (BH)max was nearly unchanged at 31.4 MGOe. Blended magnets with 5u2009wt% DyF3 had a coercivity of 17.8 kOe, but the (BH)max was reduced to 25.4 MGOe, due to a considerable reduction of remanence. The total Dy concentration in the MRE2(Fe, Co)14B Diffusion magnets with a β value of −0.5%/°C was 5.3u2009wt%, while typical commercial Nd-based Nd2Fe14B magnets require at least 7.5 wt% Dy to achieve the same β. Therefore, the MRE2(Fe, Co)14...

Structural and magnetic properties of Ti4+/Co2+ co-substituted cobalt ferrite

The variations in the structural magnetic properties of cobalt ferrite due to Ti4+/Co2+ co-substitution for 2Fe3+ are presented. The non-linear relation in the variation of the lattice parameter agrees with a previous study on cation distribution, which showed that the rate of substitution of cations into the A-sites and B-sites varies with Ti-concentration. Such variation in the rate of substitution into the cation sites was also observed in the magnetization, coercive field, and susceptibility data. The coercive field and differential susceptibility are inversely related. Although the coercive field of the Ti-substituted cobalt ferrite generally decreased compared to the un-substituted cobalt ferrite, magnetic susceptibility was higher at higher Ti-concentrations.

Magnetic and Thermoelectric Properties of Cobalt Ferrite

Vacuum sintering has been found to result in large decrease in the electrical resistivity of cobalt ferrite samples which led to the enhancement of the thermoelectric properties compared to samples prepared by other methods. Magnetization of the sample was found to be affected by a (Co1-xFex)O phase developed due to the reducing nature of the vacuum chamber in which the samples were sintered. The Curie temperature obtained for the samples is similar.