Michael T. Barako

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.

Thermal Conduction in Vertically Aligned Copper Nanowire Arrays and Composites

The ability to efficiently and reliably transfer heat between sources and sinks is often a bottleneck in the thermal management of modern energy conversion technologies ranging from microelectronics to thermoelectric power generation. These interfaces contribute parasitic thermal resistances that reduce device performance and are subjected to thermomechanical stresses that degrade device lifetime. Dense arrays of vertically aligned metal nanowires (NWs) offer the unique combination of thermal conductance from the constituent metal and mechanical compliance from the high aspect ratio geometry to increase interfacial heat transfer and device reliability. In the present work, we synthesize copper NW arrays directly onto substrates via templated electrodeposition and extend this technique through the use of a sacrificial overplating layer to achieve improved uniformity. Furthermore, we infiltrate the array with an organic phase change material and demonstrate the preservation of thermal properties. We use the 3ω method to measure the axial thermal conductivity of freestanding copper NW arrays to be as high as 70 W m(-1) K(-1), which is more than an order of magnitude larger than most commercial interface materials and enhanced-conductivity nanocomposites reported in the literature. These arrays are highly anisotropic, and the lateral thermal conductivity is found to be only 1-2 W m(-1) K(-1). We use these measured properties to elucidate the governing array-scale transport mechanisms, which include the effects of morphology and energy carrier scattering from size effects and grain boundaries.

Quasi-ballistic Electronic Thermal Conduction in Metal Inverse Opals.

Porous metals are used in interfacial transport applications that leverage the combination of electrical and/or thermal conductivity and the large available surface area. As nanomaterials push toward smaller pore sizes to increase the total surface area and reduce diffusion length scales, electron conduction within the metal scaffold becomes suppressed due to increased surface scattering. Here we observe the transition from diffusive to quasi-ballistic thermal conduction using metal inverse opals (IOs), which are metal films that contain a periodic arrangement of interconnected spherical pores. As the material dimensions are reduced from ∼230 nm to ∼23 nm, the thermal conductivity of copper IOs is reduced by more than 57% due to the increase in surface scattering. In contrast, nickel IOs exhibit diffusive-like conduction and have a constant thermal conductivity over this size regime. The quasi-ballistic nature of electron transport at these length scales is modeled considering the inverse opal geometry, surface scattering, and grain boundaries. Understanding the characteristics of electron conduction at the nanoscale is essential to minimizing the total resistance of porous metals for interfacial transport applications, such as the total electrical resistance of battery electrodes and the total thermal resistance of microscale heat exchangers.

Solder-bonded carbon nanotube thermal interface materials

Vertically-aligned carbon nanotube (CNT) films offer an attractive combination of properties for thermal interface applications, specifically high thermal conductance and mechanical compliance. In this work, we examine the use of a solder bonding layer to attach and transfer CNT films from the silicon growth substrate onto metalized surfaces. Indium foil is considered as a bonding layer for low-temperature (<;150°C) applications while a tin-plated aluminum/nickel foil is used for high temperature applications (<;1000°C). The intrinsic thermal conductivity of the CNT film and the thermal boundary resistances between the CNT film and the surrounding materials are measured with comparative infrared microscopy before and after solder bonding. The thermal properties are measured over a range of applied compressive stress. In general, compressive stress reduces the thermal boundary resistance and improves the thermal conductivity of the CNT films. Solder bonding of the exposed (non-growth) interface reduces the thermal boundary resistance by up to a factor of 30 over a dry unbonded contact.

Reactive Metal Bonding of Carbon Nanotube Arrays for Thermal Interface Applications

Vertically aligned carbon nanotube (CNT) arrays can offer an attractive combination of high thermal conductance and mechanical compliance for thermal interface applications. These arrays require a reliable, thermally conductive bonding technique to enable integration into devices. This paper examines the use of a reactive metal bonding layer to attach and transfer CNT arrays to metal-coated substrates, and the thermal performance is compared with CNT arrays bonded with indium solder. Infrared microscopy is used to simultaneously measure the intrinsic thermal conductivity of the CNT array and the thermal boundary resistance of both the bonded and growth CNT interfaces over a range of applied compressive stresses. A coarse-grained molecular simulation is used to model the effects of compressive pressure on the CNT array thermal conductivity. Reactive metal bonding reduces the thermal boundary resistance to as low as 27 mm2 · K · W-1, which is more than an order of magnitude less than the nonbonded contact.

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.

Inverse opals for fluid delivery in electronics cooling systems

We report the fabrication and fluid flow characterization of a class of open-cell copper foams known as copper inverse opals (CIOs). This material has finely controlled structure at the pore level, which may enable its use in microscale heat exchangers for microelectronics cooling. We fabricated CIOs by electrodepositing copper around a sacrificial template of packed polystyrene microspheres. We then removed the CIOs from their substrates and used electroetching to vary the pore structure and porosity. We characterized the geometry of the samples at various stages of fabrication with visual inspection and image analysis of scanning electron micrographs. We characterized the permeability with a through-plane flow rig and developed computational models for fluid flow in ideal face-centered cubic and hexagonally close-packed unit cells. Here we report the simulated and experimentally measured values of permeability. We also report experimental challenges that arise from the microscale dimensions of the samples.

Thermal conduction in nanoporous copper inverse opal films

Copper inverse opal films offer an attractive combination of conduction and convection transport properties that yield a low total thermal resistance for microfluidic heat exchanger applications. In this work, we present an integrated synthesis and characterization strategy to fabricate nanoporous copper inverse opal films and to measure the effective thermal conductivity. We synthesize inverse opal films with sub-micron pore diameters using a sacrificial packed multilayer nanosphere bed to mold the geometry of an electrodeposited copper film. We characterize the effective thermal conductivity using the 3ω method, where the nanoporous copper film is deposited directly above a microfabricated and electrically-passivated 3ω device. The effective thermal conductivity is measured to be as large as 170 W m-1 K-1. This experimental data is compared to finite element simulations as well as common conduction models for heterogeneous media, including Maxwells model and differential effective medium theory. This provides insight into the design of nanoengineered surfaces and two-phase vapor-venting microfluidic heat exchangers for ultrahigh heat flux cooling.

High heat flux two-phase cooling of electronics with integrated diamond/porous copper heat sinks and microfluidic coolant supply

We here present an approach to cooling of electronics requiring dissipation of extreme heat fluxes exceeding 1 kW/cm2 over ~1 cm2 areas. The approach applies a combination of heat spreading using laser micromachined diamond heat sinks; evaporation/boiling in fine featured (5 μm) conformal porous copper coatings; microfluidic liquid routing for uniform coolant supply over the surface of the heat sink; and phase separation to control distribution of liquid and vapor phases. We characterize the performance of these technologies independently and integrated into functional devices. We report two-phase heat transfer performance of diamond/porous copper heat sinks with microfluidic manifolding at full device scales (0.7 cm2) with heat fluxes exceeding 1300 W/cm2 using water working fluid. We further show application of hydrophobic phase separation membranes for phase management with heat dissipation exceeding 450 W/cm2 at the scale of a single extended surface (~300 μm).