Joshua U. Otaigbe

Modeling of magnetic properties of polymer bonded Nd-Fe-B magnets with surface modifications

Abstract The effects of surface modification on the magnetic properties of polymer-bonded Nd–Fe–B magnets have been studied. Two sets of Nd–Fe–B powders, coated and uncoated, were blended and compression molded with polyphenylene sulfide in isotropic form. Their magnetic properties were measured using a Helmholtz coil and a SQUID. The results showed that the effect of the coating significantly improved the irreversible loss in flux and energy product of the polymer-bonded magnets. The results have been interpreted using an isotropic model of hysteresis that takes into account energy losses. The modeling showed that the presence of soft magnetic materials in the Nd–Fe–B powders caused by oxidation reduces the interaction among magnetic particles, however, the coating treatment alters the magnetic properties by increasing the remanence of polymer-bonded magnets via increasing the interparticle coupling coefficient.

Polymer-bonded magnets: III. Effect of surface modification and particle size on the improved oxidation and corrosion resistance of magnetic rare earth fillers

Abstract We studied the effect of surface modification and particle size on the oxidation and corrosion behavior of neodymium–iron–boron (Nd–Fe–B) magnetic alloy powders by using thermal analysis to accelerate efforts to develop useful poly(phenylene sulfide) bonded Nd–Fe–B magnets. The results indicate that coating the Nd–Fe–B powders with a coupling agent provides the required oxidation and corrosion resistance for the targeted processing conditions. The coated rare earth powders are thermally and environmentally stable at elevated temperatures and in a 100% humid environment, where commercial rare earth alloy magnets are not usable. The coating treatment more significantly affects the small particle powders. A mechanism accounting for the improved thermal and environmental stability is presented and discussed. Magnetic measurements were performed to verify the thermal analysis results.

Melt crystallization of zinc alkali phosphate glasses

Abstract Melt crystallization of two zinc alkali phosphate glasses was studied with X-ray diffraction (XRD) experiments to accelerate efforts to melt process these glasses with organic polymers. The inorganic glasses differed markedly in chemical durability (water sensitivity) and crystallization rates. They were studied at room temperature prior to and after melt processing with XRD experiments and in situ at melt temperatures without flow in a novel differential scanning calorimeter/XRD apparatus. The glasses were found to be amorphous at room temperature and semi-crystalline above their glass-transition temperatures. Higher temperatures and shear (mixing) rates increased the crystallization rate of the glasses. The non-durable (water-sensitive) glass was observed to contain significant levels of crystalline matter after melt processing at 400°C. This process-induced crystallization of the glasses must be controlled, possibly during processing and/or glass formulation, otherwise it may lead to formation of unwanted phase-separated defects in the glass. If high levels of the crystalline matter are present during melt processing, they may lead to irreversible plugging of the processing equipment.

Generation, Characterization, and Modeling of Polymer Micro- and Nano-Particles

Polymer micro- and nano-particles are important in many technological applications, including polymer blends or alloys, biomaterials for drug delivery systems, electro-optic and luminescent devices, and polymer powder impregnation of inorganic fibers in composites. They are also critical in polymer-supported heterogeneous catalysis. This article reviews recent progress in experimental and simulation methods for generating, characterizing, and modeling polymer micro- and nano-particles in a number of polymer and polymer blend systems. A description of the use of gas atomization (of melts) and microdroplet (solution) approaches to generation and characterization of spherical polymer powders and microparticles represents their unique applications, giving the non-specialist reader a comprehensive overview. Using novel instrumentation developed for probing single fluorescent molecules in submicrometer droplets, it is demonstrated that polymer particles of nearly arbitrary size and composition can be made with uniform size dispersion. This interesting finding is ascribed to new dynamic behavior, which emerges when polymers are confined in a small droplet of solution the size of a molecule or molecular aggregates. Solvent evaporation takes place on a time scale short enough to frustrate phase separation, producing dry pure polymer or polymer blend microparticles that have tunable properties and that are homogeneous within molecular dimensions. In addition, it shows how a number of optical methodologies such as Fraunhofer diffraction can be used to probe polymer particles immobilized on two-dimensional substrates or levitated in space using a three-dimensional quadrupole (Paul) trap.

Linear rheology of multi-cation polyphosphate glasses

Abstract The viscoelasticity of the liquid state of two multi-cation polyphosphate glasses were studied under oscillatory shear flows in a parallel plate–plate rheometer to determine if organic polymers could be incorporated into the liquids and to provide insights into selection of optimum processing conditions. The viscosity was time-independent near the glass transition temperature and it increased monotonically with time at higher temperatures. The viscosity increase was exponential at times >84 min and the transition time to the exponential dependence was dependent on the shear strain. The results were consistent with the Hookean dumbbell model and an average molecular weight of the liquid structural species was estimated from a modified Rouse theory, making it possible to quantitatively assess melt processability of the multi-cation polyphosphate glasses.