Resilient energy harvesting systems

Abstract

Developing resilient sensor systems for deployment in extreme environments is a challenge which silicon carbide, along with other wide band gap materials, stands to play a major role in. However, any system developed will be hindered in its usefulness unless the problem of providing a power supply in these extreme conditions is addressed. This work addresses this need; a wireless sensor node conceived of standard o the shelf components was first developed and used as the basis for the design considerations required for a silicon carbide sensor node. The silicon system developed uses a piezoelectric energy harvester for the power supply and exhibits favourable operating characteristics for low vibration environments. It is capable of continuous operation at 120 mg (1.177 ms􀀀2) and at 40 mg operates with a system duty cycle of 0.05. PZT, a standard piezoelectric energy harvesting material, was characterised to 300 C to test its resilience to the conditions found in hostile environments. The material degrades considerably with temperature, with a decrease in Youngs modulus from 66 GPa at room temperature to 8.16 GPa at 300 C. The room temperature value is repeatable once cooled with an observed hysteresis in the upper temperature range. The peak output voltage at resonance also varies with temperature, resulting in an 11.6% decrease in room temperature voltage once the device is heated to 300 C. The output voltage at 300 C is found to be 2.05 V, a considerable decrease from the initial 11.1 V output at room temperature. The decrease in voltage with temperature is not monotonic as maybe expected, the data showing that at 473 K there is an increase in output voltage which is caused by a decrease in mechanical damping. SiC pin diodes were fabricated with wide drift regions to promote a large depletion width, in order to maximise the capture cross section of incident light on the devices. The large drift region produces a high series resistance. However, ll factors above 0.7 show that the device is not signi cantly a ected. SiC is shown to be an e ective UV harvester with an observed increase in output power from 0.17 mWcm􀀀2 at room temperature to 0.32 mWcm􀀀2 at 600 i K. Fill factor also remains stable with temperature, indicating that the device is not a ected by variation in parameters such as shunt and series resistances or the ideality factor. There are current technological di culties which preclude the manufacture of large area silicon carbide solar cells and as such, an alternative networking solution is presented as a way to increase the output power of the devices. Given that these devices would be subject to long term high temperature exposure, a 700 hour thermal stress test is carried out at 450 C to explore the failure mechanism of the devices. There is an observed decrease in device ll factor which indicates that the device su ers increasing degradation. The data shows that this is caused by increasing series resistance, which reduces the devices ability to output power. SEM imaging and SIMS analysis show this is likely caused by signifcant metal diusion in the contact stack which could potentially be overcome by the addition ofan alternative di usion barrier. Once energy is generated by an energy harvester is must be stored so that it can be used when required. To this end both substrate and on chip storage technologies are discussed in the forms of AlN and HfO2 metal insulator metal (MIM) capacitors. To test the feasibility of both solutions, AlN and HfO2 MIM capacitors were characterised to 300 C. The HfO2 device leakage has a strong temperature dependence as observed in the IV characteristics and the capacitance density does not scale according to parallel plate theory. However, the devices can be e ectively networked and their leakage reduced with series connection. The internal voltage decay of the device is reduced with series connection, due to the di er-ing work functions of the metal-insulator contacts. The alternative AlN solution exhibits substantially weaker temperature dependance and signi cantly improved lm quality. The data shows no existence of a barrier at the insulator - metal interface, as observed in the HfO2 device IV characteristics. The extracted activation energy is stable with temperature at 1.26 +/- 0.15 eV indicating a trap assisted leakage mechanism. This method is more suitable to fabrication of large area storage as it can be fabricated o chip on a less expensive substrate and the devices fabricated exhibit a higher yield than the HfO2 devices.