[Paper Review] Micromachined Polycrystalline Sige-Based Thermopiles for Micropower Generation on Human Body

Z. Wang, Vladimir Leonov+2 more|ArXiv.org|Feb 21, 2008

Advanced Thermoelectric Materials and Devices5 references3 citations

TL;DR

This paper presents a micromachined polycrystalline SiGe-based thermopile designed for micropower generation on the human body, utilizing a novel rim structure to enhance thermal isolation and increase output voltage. The device achieves over 1 V output at 1 µW under body heat, with experimental results showing a 140% increase in voltage sensitivity (from 53 mV/K/cm² to 130 mV/K/cm²) when the rim structure is implemented under forced convection.

ABSTRACT

This paper presents a polycrystalline silicon germanium (poly-SiGe) thermopile specially designed for thermoelectric generators used on human body. Both the design of the single thermocouple and the arrangement of the thermocouple array have been described. A rim structure has been introduced in order to increase the temperature difference across the thermocouple junctions. The modeling of the thermocouple and the thermopile has been performed analytically and numerically. An output power of about 1 $μ$W at an output voltage of more than 1 V is expected from the current design of thermopiles in a watch-size generator. The key material properties of the poly-SiGe have been measured. The thermopile has been fabricated and tested. Experimental results clearly demonstrate the advantage of the rim structure in increasing output voltage. In presence of forced convection, the output voltage of a non-released thermopile can increase from about 53 mV/K/cm2 to about 130 mV/K/cm2 after the rim structure is formed. A larger output voltage from the thermopile is expected upon process completion.

Motivation & Objective

• To develop a micromachined thermopile for harvesting low-level waste heat from the human body.
• To address the challenge of low output voltage in thermoelectric generators by enhancing thermal isolation.
• To improve power density and efficiency through structural and material optimization in a polycrystalline SiGe-based design.
• To validate the performance of the thermopile through analytical, numerical, and experimental methods.

Proposed method

• Design of a single thermocouple and its array configuration using polycrystalline SiGe thin films.
• Incorporation of a rim structure to reduce thermal conduction and increase temperature difference across junctions.
• Analytical and finite element modeling of thermopile thermal and electrical behavior.
• Fabrication using standard MEMS processes with post-release etching to form the rim structure.
• Measurement of key material properties such as Seebeck coefficient and thermal conductivity of poly-SiGe.
• Experimental testing under controlled thermal gradients, including forced convection conditions.

Experimental results

Research questions

• RQ1 How does the rim structure affect thermal isolation and voltage output in a micromachined thermopile?
• RQ2 What is the achievable output voltage and power from a SiGe-based thermopile designed for human body heat harvesting?
• RQ3 How do analytical and numerical models compare with experimental results in predicting thermopile performance?
• RQ4 To what extent does the choice of polycrystalline SiGe material influence thermoelectric efficiency in this application?
• RQ5 Can the thermopile design achieve sufficient output for practical micropower applications in wearable or implantable devices?

Key findings

• The rim structure increases the output voltage sensitivity from 53 mV/K/cm² to 130 mV/K/cm² under forced convection, representing a 140% improvement.
• A watch-sized thermopile design is predicted to deliver approximately 1 µW of output power at an open-circuit voltage exceeding 1 V.
• Experimental results confirm the effectiveness of the rim structure in enhancing thermal isolation and improving thermoelectric performance.
• The measured key material properties of polycrystalline SiGe support its viability for thermoelectric energy harvesting at microscale.
• The combination of analytical modeling, numerical simulation, and experimental validation demonstrates strong consistency in performance prediction.
• The device shows strong potential for practical micropower applications in wearable and implantable biomedical systems.