Woosung Park

Thermal Cycling, Mechanical Degradation, and the Effective Figure of Merit of a Thermoelectric Module

Thermoelectric modules experience performance reduction and mechanical failure due to thermomechanical stresses induced by thermal cycling. The present study subjects a thermoelectric module to thermal cycling and evaluates the evolution of its thermoelectric performance through measurements of the thermoelectric figure of merit, ZT, and its individual components. The Seebeck coefficient and thermal conductivity are measured using steady-state infrared microscopy, and the electrical conductivity and ZT are evaluated using the Harman technique. These properties are tracked over many cycles until device failure after 45,000 thermal cycles. The mechanical failure of the TE module is analyzed using high-resolution infrared microscopy and scanning electron microscopy. A reduction in electrical conductivity is the primary mechanism of performance reduction and is likely associated with defects observed during cycling. The effective figure of merit is reduced by 20% through 40,000 cycles and drops by 97% at 45,000 cycles. These results quantify the effect of thermal cycling on a commercial TE module and provide insight into the packaging of a complete TE module for reliable operation.

Investigation of energy loss mechanisms in micromechanical resonators

Micromechanical resonators with resonant frequencies from 500 kHz to 10 MHz were built and examined for several energy loss mechanisms. Thermoelastic damping, clamping loss and air damping were considered. The devices were shown to be limited by thermoelastic damping, providing experimental verification of this phenomenon at the microscale. Resonators with scaled dimensions also matched well with scaling theory of damping at a given pressure. An energy loss mechanism other than thermoelastic dissipation, most likely clamping loss, was shown to be dominant for resonators whose ratio of length to width was less than 10:1. The devices were fabricated using a single-wafer encapsulation process.

Using MEMS to Build the Device and the Package

MEMS devices must be packaged to be used. Unfortunately, MEMS packages are challenging to develop, and the packaging of MEMS devices often dominates the cost of the product. In recent years, our group has worked with a team from Bosch to develop and demonstrate a novel wafer-scale encapsulation approach for MEMS. This process uses MEMS fabrication steps to build the device and the package at the same time. The main advantage of this approach is that the wafers emerge from the fabrication facility with all the fragile MEMS structures completely buried within the wafer, allowing all existing standard handling and packaging approaches, such as wafer-dicing, pick/place, and injection mold packaging to be used. This encapsulation process enables CMOS integration, embedding, and extreme miniaturization of complete systems. In this paper, we describe some advantages for performance, size and cost that can come from this approach.

Modulation of thermal and thermoelectric transport in individual carbon nanotubes by fullerene encapsulation

The potential impact of encapsulated molecules on the thermal properties of individual carbon nanotubes (CNTs) has been an important open question since the first reports of the strong modulation of electrical properties in 2002. However, thermal property modulation has not been demonstrated experimentally because of the difficulty of realizing CNT-encapsulated molecules as part of thermal transport microstructures. Here we develop a nanofabrication strategy that enables measurement of the impact of encapsulation on the thermal conductivity (κ) and thermopower (S) of single CNT bundles that encapsulate C 60, Gd@C 82 and Er 2@C 82. Encapsulation causes 35-55% suppression in κ and approximately 40% enhancement in S compared with the properties of hollow CNTs at room temperature. Measurements of temperature dependence from 40 to 320 K demonstrate a shift of the peak in the κ to lower temperature. The data are consistent with simulations accounting for the interaction between CNTs and encapsulated fullerenes.

A reliability study with infrared imaging of thermoelectric modules under thermal cycling

Thermoelectric (TE) modules undergo performance degradation and mechanical failure due to thermal cycling. In the present study, TE modules are subjected to thermal cycling, and the thermoelectric performance is evaluated at periodic intervals. Both the thermoelectric figure of merit, ZT, and the individual components of ZT are measured at each interval. The thermopower and thermal conductivity are measured using steady state infrared microscopy, and the electrical conductivity and ZT are evaluated using a variation of the Harman technique. These properties are tracked over many cycles until device failure. Critical failure occurred after 45,000 thermal cycles, and the mechanical failure of the TE module is analyzed using high-resolution infrared microscopy and scanning electron microscopy. These results quantify the effect of thermal cycling on a commercial TE module performance and provide insight into the packaging of a complete TE module for reliable operation.

Effect of thermal cycling on commercial thermoelectric modules

The large temperature gradients experienced by thermoelectric modules induce significant thermal stresses which eventually lead to device failure. The impact of thermal cycling on a commercial thermoelectric module is investigated through characterization of the electrical properties. In this work, we measure the evolution of the thermoelectric and electrical properties with thermal cycling. One side of the thermoelectric module is cycled between 30°C and 160°C every 3 minutes while the other side is held at ~20°C. The thermoelectric figure of merit, ZTET̅, and electrical resistivity are measured after every 1000 cycles. The measured ZTET̅ value is compared using both a modified Harman method and an electrical measurement technique analyzed with an electrical circuit model. In addition, the change in output power and resistivity with cycling are reported. This study provides insight into characterization methods for thermoelectric modules and quantifies reliability characteristics of thermoelectric modules.

On-chip ovenization of encapsulated Disk Resonator Gyroscope (drg)

This paper reports, for the first time, a single-chip ovenization of a fully-encapsulated MEMS gyroscope to improve the stability of the scale factor and bias. We use the frequency output of the gyroscope as a thermometer, and, in turn heat the device through an on-chip silicon heater defined in the encapsulation layer. During temperature-controlled operation, the scale factor holds constant and the bias remains less than 1°/s, even as external temperature changed from 0-80°C.

In-plane thermal conductivity measurement on nanoscale conductive materials with on-substrate device configuration

In this study, we measure the in-plane thermal conductivity of palladium (Pd) nanowire with varying length (3-50 μm) and width (100-250 nm). The bridges are fabricated by electron beam lithography with an on-substrate measurement configuration. The measurements are performed on substrates with 190 nm and 2.9 μm thick thermal oxide using a 4-probe steady-state DC Joule heating method, and several suspended structure are also prepared to investigate the accuracy of the on-substrate results. For the on-substrate measurements, the thermal conductivity is estimated for short nanowires assuming the magnitude of the heat loss to the substrate from measurements of longer nanowires. As a result, the measured thermal conductivity is 30 ± 5 W/mK for suspended short nanowires at room temperature, and the estimated thermal conductivity for the on-substrate samples are consistent with this value. The measurements on the substrate with 2.9 μm oxide result in small variations between samples (± 5 W/mK), while the results on 190 nm thick oxide has a larger variation and uncertainty (>; ± 20 W/mK) due to the uncertainty in the magnitude of the heat loss to the substrate. Sufficient measurement accuracy is only achieved if the heat loss to the substrate can be estimated or measured with high accuracy.

Phonon Conduction in Silicon Nanobeam Labyrinths.

Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed “line-of-sight” (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s = 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from ~47 W m−1 K−1 for straight beam to ~31 W m−1 K−1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within ~8%. The model prediction for the most tortuous path (s = 395 nm) is reduced by ~14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.

Phonon conduction in silicon nanobeams

Despite extensive studies on thermal transport in thin silicon films, there has been little work studying the thermal conductivity of single-crystal rectangular, cross-sectional nanobeams that are commonly used in many applications such as nanoelectronics (FinFETs), nano-electromechanical systems, and nanophotonics. Here, we report experimental data on the thermal conductivity of silicon nanobeams of a thickness of ∼78 nm and widths of ∼65 nm, 170 nm, 270 nm, 470 nm, and 970 nm. The experimental data agree well (within ∼9%) with the predictions of a thermal conductivity model that uses a combination of bulk mean free paths obtained from ab initio calculations and a suppression function derived from the kinetic theory. This work quantifies the impact of nanobeam aspect ratios on thermal transport and establishes a criterion to differentiate between thin films and beams in studying thermal transport. The thermal conductivity of a 78 nm × 65 nm nanobeam is ∼32 W m−1 K−1, which is roughly a factor of two smal...