A. de Risi

Wearable and flexible thermoelectric generator with enhanced package

Present work shows recent progresses in thin film-based flexible and wearable thermoelectric generator (TEG), finalized to support energy scavenging and local storage for low consumption electronics in Ambient Assisted Living (AAL) applications and buildings integration. The proposed TEG is able to recover energy from heat dispersed into the environment converting a thermal gradient to an effective electrical energy available to power ultra-low consumption devices. A low cost fabrication process based on planar thin-film technology was optimized to scale down the TEG dimensions to micrometer range. The prototype integrates 2778 thermocouples of sputtered Sb2Te3 and Bi2Te3 thin films (1 μm thick) on an area of 25 cm2. The electrical properties of thermoelectric materials were investigated by Van der Pauw measurements. Transfer Length Method (TLM) analysis was performed on three different multi-layer contact schemes in order to select the best solution to use for the definition of the contact pads realized on each section of the thermoelectric array configuration to allow electrical testing of single production areas. Kapton polyimide film was used as flexible substrate in order to add comfortable lightweight and better wearability to the device. The realized TEG is able to autonomously recover the thermal gradient useful to thermoelectric generation thanks to an appropriate package designed and optimized by a thermal analysis based on finite element method (FEM). The proposed package solution consists in coupling the module realized onto Kapton foil to a PDMS layer opportunely molded to thermally insulate TEG cold junctions and enhance the thermal gradient useful for the energy scavenging. Simulations results were compared to experimental tests performed by a thermal infrared camera, in order to evaluate the real performance of the designed package. First tests conducted on the realized TEG indicate that the prototype is able to recover about 5°C between hot and cold thermocouples junctions with a thermal difference of 17°C initially available between body skin and environment, generating about 2 V of open circuit output voltage.

Carbon-monoxide (CO) Real Time Monitoring in Combustion Engines by an Optical Detection System

The experimental characterization of an innovative optical system for detection of carbon monoxide (CO) is reported. In this system a photodetector based on gallium nitride (GaN) and an UV light source are integrated. The gas flows between the light source and the GaN photodetector. The UV light source consists of a spark produced by an arc discharge which induced transitions in the gas, causing a modification of the light intensity as a function of gas composition. These transitions modify the fraction of light in the UV spectral region which is detected by the GaN-photodetector, as a function of the species concentration. By virtue of its structural properties, gallium nitride (GaN) allows to operate at high temperature and high speed and to work in-situ in the exhaust manifold of combustion engines at temperatures as high as 600degC, at which the deposited organic residuals on the detector can be oxidized. This assures the clear surface needed for a real time optical measurement of the species concentration to be used for a closed loop control of the fuel injection process. The system was applied to the detection of CO with concentration between 0-2.4% in a buffer of pure nitrogen gas, showing an increase in the measured photocurrent as a function of the above gases

GaN optical system for CO and NO gas detection in the exhaust manifold of combustion engines

We report on an optical system for the detection of carbon monoxide (CO) and nitride oxide (NO) concentrations in the exhaust manifolds of internal combustion engines. In this system a detector based on gallium nitride (GaN) and an UV arc discharge lamp are integrated. The arc discharge lamp induces electronic transitions in the gas molecules flowing between the light source and the GaN photodetector. These transitions modify the fraction of light in the UV spectral region which is detected by the GaN photodetector, as a function of the exhaust gas concentration. The system is designed to operate at temperatures as high as 600??C, so that deposited organic residuals on the detector can be oxidized, ensuring the clear surface needed for an optical measurement. The system was tested by measuring the concentrations of CO and NO diluted in pure N2, showing a strong increase in the measured photocurrent as a function of the above gases.

Structural reliability and thermal insulation performance of flexible thermoelectric generator for wearable sensors

Present work highlights the progress in the field of polymeric package reliability engineering for a flexible thermoelectric generator realized by thin film semiconductor technology on Kapton®. Together with mechanical enhancement, the thermal insulation performance of the realized 3D custom package was tested. The effect of different plasma treatments on the mechanical performance and interface of a Polydimethylsiloxane (PDMS)/Kapton® assembly were investigated; in order to increase the package mechanical stability of the realized wearable power source, Kapton® surface wettability was investigated by static contact angle measurements using deionized water and PDMS as liquid test. In fact, well known weak adhesion between PDMS and Kapton® leads to delamination of the package with unrecoverable damage of the generator. Plasma effect on adhesion performances was evaluated by scratch test method. By a numerical thermal analysis, the device packaging was optimized by coupling the module realized onto Kapton foil (by thin film PVD technology) to a PDMS layer opportunely molded to thermally insulate TEG cold junctions and enhance the thermal gradient useful for thermocouples operation. Fabrication process with optical lithography steps allows high resolution definition of thermoelectric semiconductors alloys. The main advances in wearable generator packaging technology is represented by increased structural robustness of PDMS/Kapton® assembly in terms of delamination and fatigue resistance.