Summary of Device Performance

The thin-film metal (Au-Ni) TEG module explored in this dissertation (~47 mm3₎ showed power output of up to 0.8 µW (17µW/cm3) using a hot gas stream at 195 °C. Moreover, at this gas temperature, a temperature difference of 90 °C was sustained across the thermoelements, generating up to 80 mV. While the power output of the prototype device here was quite low, thermal and electrical models showed that by using p- and n-type semiconductors, much higher power of 1.3 mW (27 mW/cm3_{) may} be possible with a similar structure using hot gas streams at 400 °C. Additionally, stacking multiple modules, could potentially generate tens of milliwatts of power for in- line, heat-powered sources.

In practice, the voltage and power generated by the TEG may change due to thermal variations from the heat source, thus, a power management circuit or converter would be required to properly power a device. Typically, a power management system may consist of a charge pump, for instance, followed by a dc/dc converter with a variable conversion factor, and some feedback circuit [59] as shown in Figure 6-1. Simple step-up (boost) circuits are commonly employed, but alternative configurations for dc/dc conversions with switching capacitors have been published including the Fibonacci-type converter [60] and the Dickson charge pump [61].

For energy harvesting applications, a power management system converts the generated energy from the TEG and can then store it in a storage capacitor for later use or re-charge a microbattery. For instance, a TEG output voltage in the range of ~300 mV can activate the charge pump and dc/dc step-up circuit [62], which would then increase the voltage (~1 Volts range) to a suitable level for microelectronics. In ultra low-power wireless sensor applications, the required power is typically within a range of 50 μW – 100 μW [59]. One of the main concerns is the power consumption by the step- up control/converter system which can be up to 70 μW [63]. As a result, new power management circuits with integrated dc/dc converter have been implemented as more appropriate for micromachined TEG modules. For instance, a (Dickson) charge pump dc/dc converter implemented in CMOS technology [59], exhibited a total power (current) consumption of the control circuit as low as 2.1 μW (corresponding to a current

consumption of 1.4 μA). This type of converter demonstrated system efficiencies of up to ~58%, but can vary depending on converter components and operating conditions [59].

Furthermore, more active TEG modules can be stacked and connected in series for higher voltage (and power) from a hot gas stream. Thus, when the output voltage of the TEG modules rises above a certain set value, e.g. hundreds of milliVolts, then the dc-dc step-up system can be activated. The converter system would then step-up the voltage and an output current would be supplied to a suitable storage capacitor. The process of recovering waste heat energy from a hot gas stream, for instance, can proceed autonomously. Ultimately, sufficient electrical energy would be generated and become available for low-power wireless sensor applications.

Figure 6-1. Diagram of power converter system consisting of a dc/dc converter and feedback/control circuit. TEG energy at the input is converted at the output, which can then be stored in a capacitor or delivered to a load through a dc regulator.

Table 6-1 summarizes the device performance compared to other reported small- scale TEGs. One of the attractive features of the radial TEG module was the thermal isolation between the inner and outer silicon rings, discussed in Section 6.4.1. For the

Buffer or Storage Capacitor Iin Step Up Output Step Up Input VIn VOut IOut RTEG RL Voc ~ΔT

Charge Pump and dc/dc Converter

Feedback/ Control Circuit

TEG module, high module thermal resistance (~1250 K/W for single module) was

obtained by using thin supporting polyimide/oxide membranes extending 1 mm between concentric silicon rings. Other similar miniature TEG designs fabricated on silicon substrates showed much lower overall thermal resistances (~2 – 27 K/W). Thus, the radial design explored here is fundamentally expected to provide a larger ∆T and hence larger output power for a given hot side temperature.

Table 6-1. Comparison of the thermal isolation and electrical performance between the proposed radial TEG and a few of the other miniature TEG designs fabricated on silicon substrates.

Refs. Materials TE Element Fabrication Method Device Size (mm2₎ Thermal Resistance (K/W) DPF (µW/cm2_K2₎ Notes [30] p- and n-Bi-Sb-Te (300 µm) Micropacking alloy powders

100 for the device:

2 -- High density:10000 thermocouples [19] n-poly-Si (0.7 µm) Al (0.25 µm) CVD and

sputtered 100 for the device: 2 0.016, but with p/n- BiSbTe: 0.81 P=0.014µW, Max. ∆T=0.95K [35] n-doped-Si with Al (1 µm) CVD 16.5 supporting (10 µm) Si layer: 27 0.091 P=1.5µW Max. ∆T=10K This

work Au (0.5 µm) Ni (0.6 µm) Sputtered 132.7 for single device: ~1250 Supporting poly/ox membrane/air : ~4500 0.001 at mid radius, rmid P=0.8µW, V =80mV, Max. ∆T=90K at 195 °C gas

From the test results however, the device power factor (DPF)—a metric for normalizing the output power by area and ∆T—was fairly low, only 0.001 µW/cm2_·K2_. The primary reason this power metric was so low (10 – 100X smaller than other

leg pairs. As expected, other TE devices using hybrid thermoelement pairs of semiconductor and metal exhibited greater DPF values than an all-metal design. Consequently, combining the thermal isolation and direct gas coupling advantages of the radial design with higher-performing p- and n-type TE legs, can arguably improve the overall performance.