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Now showing items 1 - 16 of 51

  • Quasi-Ballistic Thermal Transport Across MoS2 Thin Films

    Sood, Aditya   Xiong, Feng   Chen, Shunda   Cheaito, Ramez   Lian, Feifei   Asheghi, Mehdi   Cui, Yi   Donadio, Davide   Goodson, Kenneth E.   Pop, Eric  

    Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (K-z) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure k(z) across MoS2 films of varying thickness (20-240 nm) and uncover evidence of very long c-axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that similar to 50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the k(z) of nanometer-thin films of MoS2 scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, similar to 10 m(2) K GW(-1). This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO2 substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS2 for optoelectronic and thermoelectric applications.
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  • Understanding the switching mechanism of interfacial phase change memory

    Okabe, Kye L.   Sood, Aditya   Yalon, Eilam   Neumann, Christopher M.   Asheghi, Mehdi   Pop, Eric   Goodson, Kenneth E.   Wong, H. -S. Philip  

    Phase Change Memory (PCM) is a leading candidate for next generation data storage, but it typically suffers from high switching (RESET) current density (20-30MA/cm(2)). Interfacial Phase Change Memory (IPCM) is a type of PCM using multilayers of Sb2Te3/GeTe, with up to 100x lower reported RESET current compared to the standard Ge2Sb2Te5-based PCM. Several hypotheses involving fundamentally new switching mechanisms have been proposed to explain the low switching current densities, but consensus is lacking. Here, we investigate IPCM switching by analyzing its thermal, electrical, and fabrication dependencies. First, we measure the effective thermal conductivity (approximate to 0.4Wm(-1)K(-1)) and thermal boundary resistance (approximate to 3.4m(2)KGW(-1)) of Sb2Te3/GeTe multilayers. Simulations show that IPCM thermal properties account only for an approximate to 13% reduction of current vs standard PCM and cannot explain previously reported results. Interestingly, electrical measurements reveal that our IPCM RESET indeed occurs by a melt-quench process, similar to PCM. Finally, we find that high deposition temperature causes defects including surface roughness and voids within the multilayer films. Thus, the substantial RESET current reduction of IPCM appears to be caused by voids within the multilayers, which migrate to the bottom electrode interface by thermophoresis, reducing the effective contact area. These results shed light on the IPCM switching mechanism, suggesting that an improved control of layer deposition is necessary to obtain reliable switching.
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  • Enhanced Heat Transfer Using Microporous Copper Inverse Opals

    Lee, Hyoungsoon   Maitra, Tanmoy   Palko, James   Kong, Daeyoung   Zhang, Chi   Barako, Michael T.   Won, Yoonjin   Asheghi, Mehdi   Goodson, Kenneth E.  

    Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of similar to 10 mu m and pore diameter of 5 lm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient (HTC) and critical heat flux (CHF) compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum CHF of 225 W/cm(2) (at 16.2 degrees C superheat) or about three times higher than that of smooth Si surface (80 W/cm(2) at 21.6 degrees C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency.
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  • Thermal Conduction across Metal-Dielectric Sidewall Interfaces

    Park, Woosung   Kodama, Takashi   Park, Joonsuk   Cho, Jungwan   Sood, Aditya   Barako, Michael T.   Asheghi, Mehdi   Goodson, Kenneth E.  

    The heat flow at the interfaces of complex nanostructures is three-dimensional in part due to the nonplanarity of interfaces. One example common in nano systems is the situation when a significant fraction of the interfacial area is composed of sidewalls that are perpendicular to the principal plane, for example, in metallization structures for complementary metal-oxide semiconductor transistors. It is often observed that such sidewall interfaces contain significantly higher levels of microstructural disorder, which impedes energy carrier transport and leads to effective increases in interfacial resistance. The impact of these sidewall interfaces needs to be explored in greater depth for practical device engineering, and a related problem is that appropriate characterization techniques are not available. Here, we develop a novel electrothermal method and an intricate microfabricated structure to extract the thermal resistance of a sidewall interface between aluminum and silicon dioxide using suspended nanograting structures. The thermal resistance of the sidewall interface is measured to be similar to 16 +/- 5 m(2) K GW(-1), which is twice as large as the equivalent horizontal planar interface comprising the same materials in the experimental sample. The rough sidewall interfaces are observed using transmission electron micrographs, which may be more extensive than at interfaces in the substrate plan in the same nanostructure. A model based on a two-dimensional sinusoidal surface estimates the impact of the roughness on thermal resistance to be similar to 2 m(2) K GW(-1). The large disparity between the model predictions and the experiments is attributed to the incomplete contact at the Al-SiO2 sidewall interfaces, inferred by observation of underetching of the silicon substrate below the sidewall opening. This study suggests that sidewall interfaces must be considered separately from planar interfaces in thermal analysis for nanostructured systems.
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  • Enhanced Thermal Conduction Through Nanostructured Interfaces

    Park, Woosung   Sood, Aditya   Park, Joonsuk   Asheghi, Mehdi   Sinclair, Robert   Goodson, Kenneth E.  

    Interfaces dominate heat conduction in nanostructured systems, and much work has focused on methods to enhance interfacial conduction. These approaches generally address planar interfaces, where the heat flux vector is everywhere normal to the interface. Here, we explore a nanostructured interface geometry that uses nonplanar features to enhance the effective interfacial conductance beyond what is possible with planar interfaces. This interface consists of interdigitating Al pillars embedded within SiO2 with characteristic feature size ranging from 100 to 800 nm. The total sidewall surface area is modulated to highlight the impact of this additional channel by changing the pillar-to-pillar pitch L-P between 1.6 mu m and 200 nm while maintaining the same Al:SiO2 fill fraction. Using optical pump-probe thermoreflectance measurements, we show that the effective conductance of an similar to 65 nm thick fin layer monotonically increases with decreasing L-P and that the conductance for L-P =3D 200 nm is more than twice the prediction for a layered stack with the same volume ratio and a planar interface. Through a combination of Boltzmann transport modeling and finite element calculations, we explore the impact of the pitch L-P and the pillar aspect ratio on effective thermal conductance. This analysis suggests that the concept of nanostructured interfaces can be extended to interfaces between diffusive and quasi-ballistic media in highly scaled devices. Our work proposes that the controlled texturing of interfaces can facilitate interfacial conduction beyond the planar interface regime, opening new avenues for thermal management at the nanoscale.
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  • Investigation of 3D manifold architecture heat sinks in air-cooled condensers

    Kharangate, Chirag R.   Libeer, Will   Palko, James   Lee, Hyoungsoon   Shi, Jessica   Asheghi, Mehdi   Goodson, Kenneth E.  

    Power plants account for a high rate of freshwater utilization in the United States. Use of air-cooled condensers (ACC) can significantly reduce or completely eliminate freshwater withdrawals for steam-electric plants but suffer from low heat transfer of single-phase air flow. In the current study, we experimentally and computationally investigate the thermal-hydraulic performance of the air-side of a traditional ACC heat sink (EVAPCO fins) and conduct an extensive comparative CFD study of a novel 3D manifolding architecture heat sink design. A parametric investigation was performed on the 3D manifold heat sinks with fin height ranging from 7.3 to 15.3 mm, three fin densities with fin pitch ranging from 1 to 3 mm, and fin angles between 0 degrees and 45 degrees. It is concluded that there is not a single optimal design over the range of flow rates/heat flux, and the heat sink performances are a strong function of the target operating heat flux. Overall, various manifold designs were able to offer improved COP over EVAPCO fins that covered a large range of the operating heat fluxes. Manifold designs also require less fin array material, making them a good alternative for EVAPCO ACC systems if it is desired to increase the heat flux by 3 times for the existing EVAPCO units.
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  • Phonon Conduction in Silicon Nanobeam Labyrinths

    Park, Woosung   Romano, Giuseppe   Ahn, Ethan C.   Kodama, Takashi   Park, Joonsuk   Barako, Michael T.   Sohn, Joon   Kim, Jin   Cho, Jungwan   Marconnet, Amy M.   Asheghi, Mehdi   Kolpak, Alexie M.   Goodson, Kenneth E.  

    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 =3D 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from similar to 47 W m(-1) K-1 for straight beam to similar 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 similar to 8%. The model prediction for the most tortuous path (s =3D 395 nm) is reduced by similar to 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.
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  • Enhanced Capillary-Fed Boiling in Copper Inverse Opals via Template Sintering

    Zhang, Chi   Palko, James W.   Barako, Michael T.   Asheghi, Mehdi   Santiago, Juan G.   Goodson, Kenneth E.  

    Capillary-fed boiling of water from microporous metal surfaces is promising for low thermal resistance vapor chamber heat spreaders for hot spot management. Vapor transport through the void spaces in porous metals enables high heat fluxes at low evaporator superheat. In this work, the critical heat fluxes of capillary-fed boiling in copper inverse opal (IO) wicks that consist of uniform pores with 3D periodicity is investigated. Template sintering is used to enlarge the necks, or hydraulic vias, that bridge adjacent IO pores of diameters from 0.6 to 2.1 mu m. The enhanced neck size increases the hydraulic permeability for vapor extraction by an order of magnitude, and subsequently the CHF from 100 to 1100 W cm(-2). Modeling of the boiling limit accounts for the vapor pressure drop through an IO wick using Darcy's law at a given bubble departure rate. This work links the effect of wick structure design on the boiling crises phenomenon in microporous surfaces and demonstrates material capabilities for ultrathin and low superheat thermal management solutions for high-power-density electronic devices.
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  • Impact of thermally dead volume on phonon conduction along silicon nanoladders

    Park, Woosung   Sohn, Joon   Romano, Giuseppe   Kodama, Takashi   Sood, Aditya   Katz, Joseph S.   Kim, Brian S. Y.   So, Hongyun   Ahn, Ethan C.   Asheghi, Mehdi   Kolpak, Alexie M.   Goodson, Kenneth E.  

    Thermal conduction in complex periodic nanostructures remains a key area of open questions and research, and a particularly provocative and challenging detail is the impact of nanoscale material volumes that do not lie along the optimal line of sight for conduction. Here, we experimentally study thermal transport in silicon nanoladders, which feature two orthogonal heat conduction paths: unobstructed line-of-sight channels in the axial direction and interconnecting bridges between them. The nanoladders feature an array of rectangular holes in a 10 m long straight beam with a 970 nm wide and 75 nm thick cross-section. We vary the pitch of these holes from 200 nm to 1100 nm to modulate the contribution of bridges to the net transport of heat in the axial direction. The effective thermal conductivity, corresponding to reduced heat flux, decreases from approximate to 45 W m(-1) K-1 to approximate to 31 W m(-1) K-1 with decreasing pitch. By solving the Boltzmann transport equation using phonon mean free paths taken from ab initio calculations, we model thermal transport in the nanoladders, and experimental results show excellent agreement with the predictions to within approximate to 11%. A combination of experiments and calculations shows that with decreasing pitch, thermal transport in nanoladders approaches the counterpart in a straight beam equivalent to the line-of-sight channels, indicating that the bridges constitute a thermally dead volume. This study suggests that ballistic effects are dictated by the line-of-sight channels, providing key insights into thermal conduction in nanostructured metamaterials.
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  • Power density optimization for micro thermoelectric generators

    Dunham, Marc T.   Barako, Michael T.   LeBlanc, Saniya   Asheghi, Mehdi   Chen, Baoxing   Goodson, Kenneth E.  

    Microfabricated thermoelectric generators (mu TEGs) can harvest modest temperature differences to provide reliable solid-state electricity for low-power electronics, sensors in distributed networks, and biomedical devices. While past work on mu TEGs has focused on fabrication and demonstration, here we derive and explore comprehensive design guidelines for optimizing power output. A new closed-form thermoelectric device model agrees well with the traditional iterative approach. When thermoelectric leg length is limited by thin-film fabrication techniques, a very low (<10%) active thermoelectric fill fraction is required to optimize device power output, requiring careful selection of filler material. Parasitic resistance due to electrical interconnects is significant when a small number of thermocouples is used, and this loss can be reduced by increasing the number of thermocouples while decreasing the cross-sectional area of the legs to maintain the same fill fraction. Finally, a discussion of the "incompleteness of Zr" shows that different combinations of thermal conductivity, electrical conductivity, and Seebeck coefficient resulting in the same ZT will result in different device performance and optimization decisions. For pTEGs, we show it is best to increase Seebeck coefficient, followed by decreasing thermal conductivity for short leg lengths and increasing electrical conductivity for long leg lengths. (C) 2015 Elsevier Ltd. All rights reserved.
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  • Tailoring Permeability of Microporous Copper Structures through Template Sintering

    Zhang, Chi   Palko, James W.   Rong, Guoguang   Pringle, Kenneth S.   Barako, Michael T.   Dusseault, Thomas J.   Asheghi, Mehdi   Santiago, Juan G.   Goodson, Kenneth E.  

    Microporous metals are used extensively for applications that combine convective and conductive transport and benefit from low resistance to both modes of transport. Conventional fabrication methods, such as direct sintering of metallic particles, however, often produce structures with limited fluid transport properties due to the lack of control over pore morphologies such as the pore size and porosity. Here, we demonstrate control and improvement of hydraulic permeability of microporous copper structures fabricated using template assisted electrodeposition. Template sintering is shown to modify the fluid transport network in a manner that increases permeability by nearly an order of magnitude with a less significant decrease (similar to 38%) in thermal conductivity. The measured permeabilities range from 4.8 X 10(-14) to 1.3 X 10(-12) m(2) with 5 mu m pores, with the peak value being roughly 5 times larger than the published values for sintered copper particles of comparable feature sizes. Analysis indicates that the enhancement of permeability is limited by constrictions, i.e., bottlenecks between connecting pores, whose dimensions are highly sensitive to the sintering conditions. We further show contrasting trends in permeability versus conductivity of the electrodeposited microporous copper and conventional sintered copper particles and suggest these differing trends to be the result of their inverse structural relationship.
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  • Thermal conduction in lattice-matched superlattices of InGaAs/InAlAs

    Sood, Aditya   Rowlette, Jeremy A.   Caneau, Catherine G.   Bozorg-Grayeli, Elah   Asheghi, Mehdi   Goodson, Kenneth E.  

    Understanding the relative importance of interface scattering and phonon-phonon interactions on thermal transport in superlattices (SLs) is essential for the simulation of practical devices, such as quantum cascade lasers (QCLs). While several studies have looked at the dependence of the thermal conductivity of SLs on period thickness, few have systematically examined the effect of varying material thickness ratio. Here, we study through-plane thermal conduction in lattice-matched In0.53Ga0.47As/In0.52Al0.48As SLs grown by metalorganic chemical vapor deposition as a function of SL period thickness (4.2 to 8.4 nm) and layer thickness ratio (1:3 to 3:1). Conductivities are measured using time-domain thermoreflectance and vary between 1.21 and 2.31 W m(-1) K-1. By studying the trends of the thermal conductivities for large SL periods, we estimate the bulk conductivities of In0.53Ga0.47As and In0.52Al0.48As to be approximately 5 W m(-1) K-1 and 1 W m(-1) K-1, respectively, the latter being an order of magnitude lower than theoretical estimates. Furthermore, we find that the Kapitza resistance between alloy layers has an upper bound of approximate to 0.1 m(2) KGW(-1), and is negligible compared to the intrinsic alloy resistances, even for 2 nm thick layers. A phonon Boltzmann transport model yields good agreement with the data when the alloy interfaces are modeled using a specular boundary condition, pointing towards the high-quality of interfaces. We discuss the potential impact of these results on the design and operation of high-power QCLs comprised of In1-xGaxAs/In1-yAlyAs SL cores. (C) 2014 AIP Publishing LLC.
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  • Thermal Conduction in Vertically Aligned Copper Nanowire Arrays and Composites

    Barako, Michael T.   Roy-Panzer, Shilpi   English, Timothy S.   Kodama, Takashi   Asheghi, Mehdi   Kenny, Thomas W.   Goodson, Kenneth E.  

    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 30) 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.
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  • Phonon Conduction in Periodically Porous Silicon Nanobridges

    Marconnet, Amy M.   Kodama, Takashi   Asheghi, Mehdi   Goodson, Kenneth E.  

    Thermal conduction in periodically porous nanostructures is strongly influenced by phonon boundary scattering, although the precise magnitude of this effect remains open to investigation. This work attempts to clarify the impact of phonon-boundary scattering at room temperature using electrothermal measurements and modeling. Silicon nanobeams, prepared using electron beam lithography, were coated with a thin palladium overlayer, which serves as both a heater and thermometer for the measurement. The thermal conductivity along the length of the silicon nanobeams was measured using a steady-state Joule heating technique. The thermal conductivities of the porous nanobeams were reduced to as low as 3% of the value for bulk silicon. A Callaway-Holland model for the thermal conductivity was adapted to investigate the relative impact of boundary scattering, pore scattering, and phonon bandgap effects. Both the experimental data and the modeling showed a reduction in thermal conductivity with increasing pore diameter, although the experimentally measured value was up to an order of magnitude lower than that predicted by the model.
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  • Reactive Metal Bonding of Carbon Nanotube Arrays for Thermal Interface Applications

    Barako, Michael T.   Gao, Yuan   Won, Yoonjin   Marconnet, Amy M.   Asheghi, Mehdi   Goodson, Kenneth E.  

    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 mm(2).K.W-1, which is more than an order of magnitude less than the nonbonded contact.
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  • Electrothermal Modeling and Design Strategies for Multibit Phase-Change Memory

    Li, Zijian   Jeyasingh, Rakesh Gnana David   Lee, Jaeho   Asheghi, Mehdi   Wong, H. -S. Philip   Goodson, Kenneth E.  

    Electrothermal transport and crystallization dynamics govern the speed and bit stability of multibit phase-change memory (PCM). This paper develops a transient simulation methodology incorporating electrical, thermal, and phase transition models to investigate multibit PCM cell structures and programming strategies. The simulations evaluate two standard PCM structures, namely, the mushroom cell and the confined pillar cell, with feature sizes smaller than 40 nm. The transient simulation captures the phase distribution and cell resistance profile, which are corroborated by transmission electron microscope imaging and the corresponding measured resistance values. This paper also explores a more compact architecture, i.e., the stacked vertical cell, with precise control of Joule heating and potentially more stable intermediate resistance levels. For an electrode area of 10 nm x 20 nm, a low programming current of 60-90 mu A generates sufficient heating power to amorphize the phase-change elements sequentially, resulting in four distinct resistance levels distributed over a two-order-of-magnitude resistance range with promise for multibit storage.
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