Lurui Zhao

Ferrofluid liquid spring for vibration energy harvesting

This paper reports ferrofluid liquid spring used to suspend a magnet array for harvesting vibration energy. A new idea of ferrofluid-based suspension is used for a low resonant frequency for a microfabricated vibration-energy harvester as well as high reliability. The ferrofluid liquid spring has reduced the resonant frequency of a microfabricated electromagnetic energy harvester to around 340 Hz, at which 36 nW is delivered into a matched load of 2.3 Ω from 7 g acceleration.

Vibration energy harvester with low resonant frequency based on flexible coil and liquid spring

This paper reports an electromagnetic vibration-energy harvester with low resonant frequency based on liquid spring composed of ferrofluid. Cylinder magnet array formed by four disc NdFeB magnets is suspended by ferrofluid in a laser-machined acrylic tube which is wrapped by flexible planar coil fabricated with microfabrication process. The magnet array and coil are aligned automatically by the ferrofluid. Restoring force when the magnet array is deviated from the balance position is proportional to the deviated distance, which makes the ferrofluid work as a liquid spring obeying Hooks law. Experimental results show that the electromagnetic energy harvester occupying 1.8 cc and weighing 5 g has a resonant frequency of 16 Hz and generates an induced electromotive force of Vrms = 2.58 mV (delivering 79 nW power into matched load of 21 Ω) from 3 g acceleration at 16 Hz.

Integration of microfabricated low resistance and thousand-turn coils for vibration energy harvesting

This paper presents two microfabrication approaches for multi-layer coils for vibration-energy harvesters. A magnet array is arranged with alternating north- and south-orientation to provide a rapidly changing magnetic field for high electromagnetic energy conversion. Multi-turn spiral coils on silicon wafer are aligned to the magnet array for maximum magnetic flux change. One type of coil is made out of 300 μm-thick copper that is electroplated with silicon mold, and the other is built on 25 μm-thick copper electroplated with photoresist mold. The low resistive coils fabricated by the first approach are integrated in a microfabricated energy harvester of 17 × 7 × 1.7 mm3 (=0.2 cm3) weighing 0.8 g, which generates 14.3 μW power output (into 0.7 Ω load) from vibration amplitude of 6 μm at 250 Hz. The latter approach is used to make a 1080-turn coil for a microfabricated electromagnetic energy harvester with magnet array and plastic spring. Though the size and weight of the harvester are only 44 × 20 × 6 mm3 (=5.3 cm3) and 12 g, respectively, it generates 1.04 mW power output (into 190 Ω load) when it is vibrated at 75 Hz with vibration amplitude of 220 μm.

Microfabricated thousand-turn coils for mW power generation from sub-mm vibrations

This paper presents two microfabrication approaches for 3D multiple-layer coils for vibration-energy harvesters. One type of coils is made out of 300 μm thick copper that is electroplated with silicon mold, and the other is built on 25 μm thick copper electroplated with photoresist mold. The latter approach is used to obtain 1,080-turn coil for microfabricated electromagnetic energy harvester with magnet array and plastic spring. Though the size and weight of the harvester are only 26×13×6 mm3 and 12 gram, respectively, it generates 1.04 mW power output (into 190 Ω load) when it is vibrated at 75 Hz with vibration amplitude of 220 μm.

Stackable dual-layer coil based on wafer-level transfer technique for electromagnetic energy harvester

This paper reports a stackable dual layer coil based on ZnO sacrificial layer and thermal-release-tape transfer technique for electromagnetic energy harvester. The power density of electromagnetic energy harvester can be improved significantly using multilayer coil with high coil turns. Five of microfabricated dual-layer coil plates are stacked for 450 turns for an electromagnetic vibration-energy harvester, which produces 116 μW from 3.6 g vibration at resonant frequency of 160 Hz.

Non-resonant, broad-band vibration-energy harvester based on self-assembled liquid bearing

This paper reports a non-resonant, broad-band electromagnetic vibration-energy harvester based on self-assembled liquid bearing made of ferrofluid. The liquid bearing suspends magnet array over a microfabricated multi-layer coil plate such that the energy harvester does not rely on any mechanical resonance, and can harvest vibration energy over a broad frequency range. The non-resonant energy harvester (20.7 × 12 × 4.5 mm3) produces μW level of power from 1 g acceleration even at 2 - 4 Hz, a frequency range that would be too low for a resonant energy harvester.

Micromachined piezoelectric ultrasonic transducer based on dome-shaped diaphragm supported by flat square diaphragm

This paper reports a microfabricated ultrasonic transducer based on a piezoelectrically actuated dome-shaped diaphragm supported at the center of a flat square diaphragm. The dome-shaped diaphragm is fabricated by silicon isotropic wet etching while the whole diaphragm including the flat square diaphragm is released by KOH etching with a novel dicing-based front-to-back alignment method. Shadow masks are used for patterned film depositions on 3D dome diaphragm. With 30 Vpp drive voltage, the transducer produces sound pressure level (SPL) as high as 88.21 dB between 10 and 40 kHz when measured at 5 mm away in an open field. The linearity of the sound output as a function of input voltage is measured to be very good. In addition, post-process laser cutting was utilized to form a cantilever-like diaphragm structure, which boosted the maximum SPL to 95.12 dB without degrading the linearity.