Millimeter Scale MEMS Air Turbine Generator by Winding Wire and Multilayer Magnetic Ceramic Circuit ()
1. Introduction
Strong demand for miniaturized power sources becomes remarkable with progressive miniaturization of electronic equipment. Both small size and high power density are required for various applications. Although lithium secondary batteries have been used as the small and high power density sources, the power density is approaching the theoretical limit. Instead of the batteries, miniaturized generators have been studied as one of the candidate for exceeding the limit. Since UMGT (Ultra Micro Gas turbine) was presented by MIT group [1,2], a lot of MEMS (Micro Electro Mechanical Systems) micro generators have been studied widely.
Although a lot of studies of MEMS micro generator have been focused electrostatic types, the electromagnetic induction type shows low output impedance. Therefore, the electromagnetic induction type is more desirable than electrostatic type even in the miniaturized generator. Some outstanding studies were reported [3-12]. Among these studies, milli-watt to watt class generators were achieved. Also the complex three phase windings of copper conductor were reported.
In the commercial size generator, three dimensional wire windings such as helical structure and magnetic materials such as coil cores are used widely. However, in MEMS micro generator, the windings are usually designed in the planer structure such as spiral, meander or equivalent. Moreover the magnetic materials such coil cores have not been reported.
In the electromagnetic induction, the wide area and high magnetic flux density result large effect. Therefore, the combination of the helical structure that produces wide cross section and the magnetic material that concentrates magnetic flux is advantageous. Moreover, since the magnetic flux is focused in magnetic material of the small area, it is possible to shorten the conductor length. The shorter length results the less conductor loss.
The authors considered the three dimensional helical structure of the conductor and also introducing magnetic materials to the MEMS micro generator. Multilayer magnetic ceramic technology was used to realize helical windings and the magnetic material.
In this present study, the author fabricated a millimeter size generator with a MEMS micro air turbine and the multilayer magnetic ceramic circuit of the helical winding structure. As a fundamental benchmark study, we also prepared a simple winding wire circuit and compared output power, DC resistance of the conductor and so on. Although the obtained output power was micro watt level, some features were extracted. The obtained generator was monolithic structure in which high permeability magnetic material and helical winding conductor was formed. The output power density of the multilayer type achieved almost same in smaller volume with the less turn number of the coil than that of the winding wire. The DC resistance of the multilayer circuit was quarter of the simple winding wire type.
2. MEMS Air Turbine
The schematic illustration of the air turbine is shown in Figure 1. The air turbine part was made of 7 silicon layers. The upper layers were assigned for the air passage to the stator. The lower layers were formed for the passage to the fluid dynamic bearing system. The dimensions of the air turbine were 3.0, 3.0, 3.0 mm length, width and height, respectively.
The ring shape magnet was attached to the rotor and placed inside the hole of the stator. The shape of the rotor blade was referred to quasi-ultrasonic wings. The number of rotor blades were 20, the height was 100 μm.
The rotor and each layer were fabricated by a photolithography process. Aluminum etched pattern was used as resist films for high aspect ratio photolithograph, and resin photo resist was used for the low aspect ratio lithography. Each part layer was assembled by making use of the alignment pin and the hole, which reduced the dimensional error.
Figure 2 shows the component fabricated by photolithographic process. Also, Figure 3 shows the diameters of the rotor and the stator hole measured by a confocal microscope. The designed diameters of the rotor and stator were the 1590 μm and 1600 μm, respectively. And then the machined diameters were 1578.27 μm and 1603.32 μm, respectively. It is found that the error was quite small. In addition, Figure 4 shows the etched rotor structure observed by Scanning Electron Microscope (SEM). It is observed that the walls of the blade were formed perpendicular to the substrate.