J. C. Ho

Magnetic properties of partially-inverted zinc ferrite aerogel powders

Fine powders of ZnFe2O4 with an average particle size of 10 nm and inversion parameter of 0.21 were synthesized by the aerogel procedure. Portions of the powders were calcined in air at 500 and 800u2009°C and other portions were ball-milled for 10 h. The materials were characterized by x-ray diffractometry, vibrating sample, and SQUID magnetometry, Mossbauer spectrometry, and low temperature calorimetry. Upon calcination the powders underwent significant changes in grain size, inversion parameter, and hence magnetic properties. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. Upon ball-milling the grain size varied widely and the inversion parameter attained a value of 0.55. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe2O4 powders having comparable grain size and inversion parameters.

Low temperature heat capacities of Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2

For the binary Ti–Al system, an ordering transformation in Ti3Al has been shown to result in a significant lowering of the electronic heat-capacity coefficient, γ, by removing electrons from conducting states. When γ is normalized to a per Ti atom basis, the same tendency is found in low temperature calorimetric studies of the conducting ternary carbides Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2 reported herein. As a consequence of C- or N-induced covalent-like bond formation, the Debye temperatures in these ternaries are in excess of 700 K.

Titanium Aneurysm Clips: Part I-Mechanical, Radiological, and Biocompatibility Testing

Most aneurysm clips are made of cobalt-based alloys. Although these clips are nonferromagnetic, they still produce artifact that degrades the quality of magnetic resonance (MR) images. A new aneurysm clip of pure titanium was developed to minimize artifact on postoperative MR images. We evaluated these clips in a series of mechanical tests in vitro, biocompatibility tests in rabbits, and radiological tests in greyhound dogs. The clip sizes and shapes matched those of conventional aneurysm clips. The average closing forces ranged between 151.6 and 181.8 g and were not diminished by repeated sterilization or stress. After > 20 million cycles of high-pressure and high-frequency pulsations, the clips did not open and the closing forces were not reduced. Titanium aneurysm clips implanted in the subarachnoid space of 12 rabbits for 1 or 6 months produced mild gliosis identical to that produced by implantation of cobalt alloy clips in 12 control rabbits. Based on pre- and postoperative weights and electron microscopic scans, the titanium implants did not corrode. The artifact on computed tomographic and MR imaging produced by a titanium clip placed on the internal carotid artery of a greyhound was less than that produced by an identical cobalt-chrome alloy clip by a factor of two to three. This study demonstrated that titanium aneurysm clips are mechanically equivalent to conventional clips, biocompatible, and corrosion resistant. Furthermore, titanium clips have superior imaging characteristics, creating less computed tomographic and MR imaging artifact and permitting better resolution of anatomic structures than cobalt alloy clips.

Mössbauer studies of melt-spun Pr2Fe14B ribbons

Melt-spun ribbons of tetragonal Pr2Fe14B exhibit favorable hard magnet characteristics. Technically relevant materials based on this compound, however, generally contain a certain amount of soft magnetic α-Fe or Fe3−xBx for remanence enhancement through exchange coupling. The nominal off-stoichiometric compositions lead to metallurgical complications, which are not easily resolvable by standard phase identification techniques such as x-ray diffraction and thermal magnetic analysis. As a viable alternative, 57Fe-Mossbauer spectroscopy can be used to delineate individual Fe sites. To provide a basis for such an approach, this report gives Mossbauer parameters including hyperfine magnetic field, isomer shift, and quadrupole splitting as obtained from a single-phase melt-spun Pr2Fe14B ribbon.

Mössbauer evaluation of cobalt ferrite nanoparticles synthesized by forced hydrolysis

Spinel cobalt ferrite in the form of nearly spherical particles, having a narrow range of particle size around 7.6nm, was produced by forced hydrolysis. Between the magnetization blocking temperature of 180K and the Mossbauer blocking temperature of 270K, the magnetic anisotropy energy decreases rapidly with temperature. The occupancy of the octahedral sites by only half of the Co2+ cations may have weakened the crystalline anisotropy. Significant spin canting was also observed at these sites.

Low temperature heat capacity of Ti3SiC2

Calorimetric measurements between 2 and 10 K have been made on Ti3SiC2. The molar heat capacity, c, data can be fitted to the sum of an electronic and a lattice contribution: c=γT+βT3. The β value of 0.049 mJ/molu200aK4 yields a Debye temperature of 620 K, typical of high stiffness and hardness ceramic materials such as TiC. This value is significantly higher than the Debye temperature measured from elastic measurements. The γ value of 5.21 mJ/molu200aK2 is a measure of the density of states at the Fermi level. This value, when normalized to a per Ti atom basis, is higher than that of TiC0.97, but roughly half that of Ti metal, implying that covalent-type bonding induced electron localization increases in going from Ti to Ti3SiC2 to TiC. It also partially explains the excellent electrical and thermal conductivities of Ti3SiC2.

Low-temperature heat capacities of Ni3Al

Heat capacities of two single‐crystal Ni3Al samples have been measured between 2 and 14 K. The data can be well fitted to the sum of a lattice and an electronic term, without a magnetic cluster contribution as previously reported by de Dood and de Chatel. The resulting Debye temperature values are comparable to those derived from elastic constant measurements, but the calorimetrically determined density of states at the Fermi level differs appreciably from that based on band‐structure calculations.

Magnetic properties of Ni-Co-Cr-base Elgiloy

The Ni–Co–Cr-base Elgiloy is one of the commonly used engineering materials. Most applications rely on its high strength, ductility, corrosion resistance and excellent fatigue life over a wide temperature range. However, in the medical application of cerebral aneurysm clips, the alloy is often subjected to strong magnetic fields associated with magnetic resonance imaging (MRI). Its paramagnetic behavior meets MRI safety requirements, but is the source of relatively large artifacts and thus less MRI-compatible for MRI procedure involving the brain. This article reports superconducting quantum interference device (SQUID) measurements on the magnetic properties of a series of Elgiloy wires in either as-drawn or heat-treated conditions. Furthermore, low-temperature calorimetry was employed to reveal the existence of submicroscopic clusters containing ferromagnetic elements such as Ni or Co in the macroscopically paramagnetic matrix.

Low Temperature Heat Capacities of Several High Strength Materials

Abstract : Low temperature calorimetric measurements have been made on three metallurgically prepared high strength materials: NiAl, W-2 (a heavy tungsten alloy), and Al-Fe-Ce (a lightweight alloy with dispersion strengthening). To fit the heat capacity data between 3 and 10 K, not only the electronic and lattice contributions having T and T3 dependence, respectively, need to be considered, there is an additional temperature-independent term presumably associated with magnetic clusters. These results provide information on the fundamental characteristics, as well as the heterogeneous nature of the materials not always detectable by standard metallurgical techniques. Keywords: Low temperature, Heat capacity, Adiabatic calorimeter, Magnetic clusters, Nickel, Aluminum compounds, Iron, Cesium.

Low‐temperature heat capacities of silicon carbide

Temperature dependence of heat capacity data between 4 and 12 K can be represented by a single term, βT3, associated with lattice vibrations. The coefficient β corresponds to a large Debye temperature of 990 K consistent with the high melting point and hardness of this refractory ceramic material.