Iulica Zana

Design optimization of an 8 W, microscale, axial-flux, permanent-magnet generator

This paper presents the design optimization and characterization of a microscale, permanent-magnet (PM) generator, capable of supplying 8 W of dc power to a resistive load at a rotational speed of 305 000 rpm. The generator is a three-phase, axial-flux, PM machine, consisting of a stator with Cu surface windings and a multi-pole SmCo PM rotor. Optimization of the machine geometries has enabled a 30% improvement in power density (for the same rotational speed) over a previously reported machine. Furthermore, these design improvements, in combination with higher rotational speeds, have enabled a >7x improvement in total output power and a net power density of 59 W cm -3 .

Electrodeposition of Co-Pt films with high perpendicular anisotropy

Co 8 0 Pt 2 0 alloy films from 125 to 1000 nm thick have been grown by electrodeposition on Cu(111) seed layers under constant current. Their structure, morphology, and magnetic properties have been investigated. Growth conditions can be identified for which the resulting Co-Pt films consist of a hexagonal close-packed phase only, with the c axis perpendicular to the substrate. Grain size in the film plane remains almost constant with increasing thickness, X-ray, atomic force microscopy, and magnetic data pointing toward a columnar growth. The films exhibit hard magnetic properties in the out-of-plane direction with coercivity up to 6.1 kOe.

Microfabricated High-Speed Axial-Flux Multiwatt Permanent-Magnet Generators—Part II: Design, Fabrication, and Testing

This paper presents the design, fabrication, and characterization of permanent-magnet (PM) generators for use in microscale power generation systems. The generators are three-phase, axial-flux, synchronous machines, each consisting of an eight-pole surface-wound stator and PM rotor. The devices are fabricated using a combination of microfabrication and precision assembly. Characterization is achieved using a custom-built experimental test stand that incorporates an off-the-shelf gas-driven spindle to power the machines. At a rotational speed of 120 000 rpm, one generator demonstrates 2.5 W of mechanical-to-electrical power conversion and, coupled to a transformer and rectifier, delivers 1.1 W of dc electrical power to a resistive load. This watt-scale electrical power generation demonstrates the viability of scaled PM machines for practical power generation applications

Pinhole analysis in magnetic tunnel junctions

Pinholes in the insulating layer of magnetic tunnel junctions are local shortcuts and cause malfunction of such devices. The need for reduction of the tunnel resistance by reduction of the insulator thickness will make this problem even more severe. Therefore, the development of low-resistance magnetic tunnel junctions requires analyzing the pinhole density. We developed a method for pinhole imaging using electrodeposition of copper. Selective nucleation at pinholes produces characteristic structures that can be visualized by conventional microscopy techniques. The experimental conditions were carefully chosen in order to avoid uncontrolled damage of the insulator layer.

A low-power resonant micromachined compass

This paper describes a micromachined magnetic field sensor based on magnetic resonant structures. A micromechanical resonator fabricated using surface micromachining techniques is modified so as to incorporate a magnetic material. The shift of the fundamental mechanical resonant frequency of the device, caused by the interaction of the external magnetic field and the magnetic component of the resonant system, is used to determine the amplitude or the direction of the external field. We have designed, fabricated and tested two types of micromachined magnetic field sensors relying on the proposed principle of operation. The fabrication of the sensors follows CMOS-compatible and low temperature processes based on surface micromachining. Devices have been fabricated which exhibit a minimum resolution of 45° at 30 µT or less, at an excitation voltage of 10 V, demonstrating their utility as a magnetic compass. The power consumed to actuate the resonator is on the order of 20 nW. A theoretical model of the magnetic field sensor was developed using vibration analysis and nonlinear deflection theory. Good agreement was observed between the predicted and observed behavior of the compass.

Magnetic induction machines integrated into bulk-micromachined silicon

This paper presents the design, fabrication, and characterization of laminated, magnetic induction machines intended for high-speed, high-temperature, high-power-density, silicon-based microengine power generation systems. Innovative fabrication techniques were used to embed electroplated materials (Cu, Ni/sub 80/Fe/sub 20/, Co/sub 65/Fe/sub 18/Ni/sub 17/) within bulk-micromachined and fusion-bonded silicon to form the machine structures. The induction machines were characterized in motoring mode using tethered rotors, and exhibited a maximum measured torque of 2.5 /spl mu/N/spl middot/m.

Microfabricated High-Speed Axial-Flux Multiwatt Permanent-Magnet Generators—Part I: Modeling

This paper presents the modeling of permanent-magnet (PM) generators for use in microscale power generation systems. The generators are three-phase, axial-flux, synchronous machines, each consisting of a multipole, surface-wound stator and PM rotor. The machines are modeled by analytically solving two-dimensional (2-D) magneto-quasi-static Maxwells equations as a function of radius. The 2-D field solutions are then integrated over the radial span of the machine to determine circuit parameters such as open-circuit voltage and inductance as well as hysteresis loss in the stator core and eddy current losses in the stator core and windings. The model provides a computationally fast method to determine power and efficiency of an axial-flux PM machine as a function of geometry, speed, and material properties. The open-circuit voltage predictions are also shown to agree well with 3-D finite-element analysis simulation results. 1700

Electroplated metal microstructures embedded in fusion-bonded silicon: conductors and magnetic materials

Fabrication methods for integrating thick (tens or hundreds of micrometers) electroplated metallic microstructures inside fusion-bonded silicon wafers are proposed and validated. Cu and Ni/sub 80/Fe/sub 20/ (permalloy) test structures were embedded inside of cavities in silicon wafers, which were fusion-bonded at 500/spl deg/C for 4h with nearly 100% yield. Resistance tests validated the electrical integrity of the metals after annealing, and magnetic measurements indicated the Ni-Fe maintained its magnetic performance after annealing. Additional mechanical tests verified a strong, uniform bond, and that the presence of the metals does not degrade the bond strength. These results demonstrate the ability to integrate conductive and magnetic materials in wafer-bonded silicon, a method useful for a variety of multiwafer, MEMS devices.

Fabrication and characterization of Fe/sub 81/Ga/sub 19/ thin films

Summary form only given. Certain substitutional additions to Fe can increase both its resistivity and magnetostriction /spl lambda/. Recently Fe-Ga has been investigated as a bulk magnetostrictive material, with maximum /spl lambda/ localized along the . Further, it has been discovered that /spl lambda/ along increased from /spl sim/300 to /spl sim/350 ppm when the samples were quenched, rather than furnace-cooled. Fe-Ga, due to its low anisotropy and high magnetostriction is an attractive candidate as a high magnetostrictive susceptibility MEMS material. In order to investigate the thin film properties of Fe-Ga materials epitaxial fabrication is necessary, in order to locate the in specific orientations.

Vertically laminated magnetic cores by electroplating Ni-Fe into micromachined Si

The fabrication and characterization of vertically laminated, electrodeposited Ni-Fe magnetic cores in micromachined silicon for the low megahertz frequency range is presented. Laminated cores were fabricated by etching vertical trenches (60-180 /spl mu/m wide and 525 /spl mu/m deep) through silicon wafers and directly plating Ni-Fe onto the etched sidewalls. The Ni-Fe was plated over a range of thicknesses (3-53 /spl mu/m) in order to study the influence of eddy currents. The measured impedances in the range 10 kHz-40 MHz confirm the reduction of eddy current losses. This work demonstrates a simple fabrication method for achieving high-aspect-ratio, vertically laminated magnetic cores where the geometry of the structures can be easily tailored for various applications.