Demonstration of Low Frequency and High-Power Density Aln-Based Piezoelectric Vibration Energy Harvesters Using High Density Tungsten Proof Masses

In this study, we experimentally evaluate the performance of piezoelectric energy harvesters integrated with tungsten proof masses. A wafer level process was successfully developed to fabricate multiple devices. Tungsten proof masses were integrated in MEMS harvesters, which previously used silicon proof masses. This integration helped reduce the resonant frequency of the harvesters closer to ambient vibration and enhance device performance. The results demonstrate significant improvement in the Q factor for the tungsten-integrated devices, increasing from 250 to 2000 for one of the devices. Normalized power densities of $244.8 \mathrm{~mW} \cdot \mathrm{~g}^{-2} \cdot \mathrm{~cm}^{-3}$ at 50 Hz and $69.4 \mathrm{~mW} \cdot \mathrm{~g}^{-2} \cdot \mathrm{~cm}^{-3}$ at 553 Hz were achieved for different devices. These results demonstrated that we have achieved some of the highest performance levels among piezoelectric energy harvesters while operating at low frequencies typical of available ambient vibration in the environment of IoT devices, without the need for vacuum packaging.