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Now showing items 1 - 16 of 32

  • Fundamental Cooling Limits for High Power Density Gallium Nitride Electronics

    Won, Yoonjin   Cho, Jungwan   Agonafer, Damena   Asheghi, Mehdi   Goodson, Kenneth E.  

    The peak power density of GaN high-electron-mobility transistor technology is limited by a hierarchy of thermal resistances from the junction to the ambient. Here, we explore the ultimate or fundamental cooling limits for junction-to fluid cooling, which are enabled by advanced thermal management technologies, including GaN-diamond composites and nanoengineered heat sinks. Through continued attention to near-junction resistances and extreme flux convection heat sinks, heat fluxes beyond 300 kW/cm2 from individual 2-μm gates and 10 kW/cm2 from the transistor footprint will be feasible. The cooling technologies under discussion here are also applicable to thermal management of 2.5-D and 3-D logic circuits at lower heat fluxes.
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  • Nonhomogeneous morphology and the elastic modulus of aligned carbon nanotube films

    Won, Yoonjin   Gao, Yuan   Guzman de Villoria, Roberto   Wardle, Brian L   Xiang, Rong   Maruyama, Shigeo   Kenny, Thomas W   Goodson, Kenneth E  

    Carbon nanotube (CNT) arrays offer the potential to develop nanostructured materials that leverage their outstanding physical properties. Vertically aligned carbon nanotubes (VACNTs), also named CNT forests, CNT arrays, or CNT turfs, can provide high heat conductivity and sufficient mechanical compliance to accommodate thermal expansion mismatch for use as thermal interface materials (TIMs). This paper reports measurements of the in-plane moduli of vertically aligned, single-walled CNT (SWCNT) and multi-walled CNT (MWCNT) films. The mechanical response of these films is related to the nonhomogeneous morphology of the grown nanotubes, such as entangled nanotubes of a top crust layer, aligned CNTs in the middle region, and CNTs in the bottom layer. To investigate how the entanglements govern the overall mechanical moduli of CNT films, we remove the crust layer consisting of CNT entanglements by etching the CNT films from the top. A microfabricated cantilever technique shows that crust removal reduces the resulting moduli of the etched SWCNT films by as much as 40%, whereas the moduli of the etched MWCNT films do not change significantly, suggesting a minimal crust effect on the film modulus for thick MWCNT films (>90 μm). This improved understanding will allow us to engineer the mechanical moduli of CNT films for TIMs or packaging applications. In electronic components, it is crucial to minimize the thermal resistances between the heat sink and heat spreader by minimizing air gaps. The surface roughness of planar surfaces limits the actual contact area between the two solids to values as low as 2% for lightly loaded interfaces [1]. These components also experience repeated temperature cycles that cause mechanical stresses at the interface between the two different materials. Therefore, mechanically compliant thermal interface materials (TIMs) are used to join surfaces to improve heat transfer across the interface and minimize the effects of thermomechanical mismatch. Here, vertically aligned carbon nanotube (VACNT) films have been proposed as ideal TIMs due to their ability to combine the mechanical compliance of a polymer with the high thermal conductivity of a metal [2–7]. While the out-of-plane elastic modulus has been studied using nanoindentation [1, 8], there is limited data for the in-plane elastic modulus (perpendicular to the primary direction of the carbon nanotubes (CNTs). Past nanoindentation experiments have produced estimates of the out-of-plane (CNT axis direction) modulus ranging from 1 to 300 MPa as a result of nanotube morphology variations [2–11]. Our past results using nanoindentation measurements show that out-of-plane modulus of multi-walled CNT (MWCNT) films ranging from 0.2 to 2.8 MPa [8]. These results suggest that the overall density, roughness, array height, and other physical characteristics may strongly influence the mechanical properties of VACNT films. Previous measurements of the in-plane modulus of VACNT films also suggested that the wide distribution of modulus values stem from the variation of nanostructural features [12, 13]. The nonhomogeneous characteristics of VACNT films have been confirmed by several studies using various types of measurements, such as scanning electron microscopy (SEM), resonant Raman spectroscopy, and angle-resolved x-ray absorption [14–18]. Therefore, it is critical to relate the mechanical properties of VACNT films to variations (effectively this is spatial heterogeneity) in the morphology of the nanotube films. In recent work, we have investigated the mechanical compliance of single-walled CNT (SWCNT) films, and the mechanical responses are linked to their morphology and microscopic motions, including zipping, unzipping, and entanglements [19]. Performance of TIM materials as well as other VACNT-comprised structures [20, 21], particularly those used in microfabrication, will be directly affected by such structure-property heterogeneity, e.g. bioparticle separation elements and 3D structures formed via capillary infiltration of fluids into patterned arrays are likely very sensitive to such spatial heterogeneities [22–24]. In this paper, a resonator technique is used to infer the in-plane modulus of vertically aligned SWCNT and MWCNT films using a laser Doppler vibrometer (LDV) to ascertain resonance shifts. We additionally investigate the modulus after removing the crust from the VACNT films using plasma etching to understand those local morphological effects on the overall mechanical responses. Figure 1 shows the suggested mechanical characterization method before and after the etching step using a Si-CNT composite beam [12]. The fabrication process of the silicon (Si) cantilevers includes two deep reactive-ion etching (DRIE) steps in serial order to define cantilever outlines at the front and substrates open at the back. This is followed by removal of the oxide layer, which is used as an etch stop layer for the DRIE step. The processed wafer is cleaved into several pieces so different thicknesses of VACNT films through exposure are grown on samples. Vertically aligned SWCNT films are grown by using an alcohol catalytic chemical vapor deposition process, and detailed in [25]. A 50 nm SiO2 layer is first grown on the substrates and the catalyst films (10 nm-thick Al and 0.2 nm-thick Co) are deposited. SWCNT films are then synthesized using ethanol (1.3 kPa) as the carbon source at 800 °C for different time frames. Analysis of the SWCNTs reveals that the growth provides almost 100% of the SWCNTs with the number density of 1012 cm−2 [26, 27]. Vertically aligned MWCNT films are grown using a chemical vapor deposition process, as described in [28–30]. Prior to CVD, Al2O3/Fe layers are deposited, and all pre-heater gas lines (He, H2, and C2H4) are flushed for 10 min followed by 10 additional minutes of He to displace trapped air from the system at 750 °C. A flow of He (73 sccm) and H2 (400 sccm) is then introduced over the target substrate for 5 min. A mixture of He/H2/C2H4 (73/400/200 sccm) is introduced for reference substrates during which the CNT array emerges. This process provides a relatively thick film with the number density of ~1010 cm−2. Figure 1. (a) Schematic of the mechanical characterization process of in-plane modulus using a Si-CNT composite beam before and after the etching. Beam dimensions are scaled to achieve a range of resonant frequencies. VACNT film thicknesses are also varied to characterize the mechanical properties of different layer thicknesses. The fabrication process of the microcantilevers is detailed elsewhere [12]. (b) SEM image of a Si-MWCNT composite beam. Download figure: Standard High-resolution Export PowerPoint slide Scanning electron microscope (SEM) images reveal that the actual arrangements are substantially complicated by showing many crossings and connections between CNTs as shown in figure 1(b). We assume that a VACNT film behaves as a solid and continuous material structure where all the tube crossings are linked. Based on this continuum assumption, a resonator measurement technique is used to extract the effective in-plane moduli of CNT films. The resonator technique infers the mechanical properties of the tested film, such as effective in-plane modulus. An LDV characterizes the mechanical properties of the nanostructured film by capturing the resonant frequency of the cantilevers before and after the film growth, and after the etching. Using a Si-CNT composite beam, the effective modulus of CNT film can be extracted as [12]. We use the effective and , where E, I, ρ, and A, are respectively the modulus, the second moment of area, density, and cross-sectional area of the beam [12, 31, 32]. The subscripts, Si0, Si, and CNT denote the silicon layer of a silicon-only cantilever, the silicon, and the VACNT layer of a Si-CNT composite cantilever, respectively, and is the ratio of the resonant frequency shift with the Si-CNT beam to the original resonant frequency of the silicon beam. We use the transformed section method that converts the cross-section of a composite beam into an equivalent cross-section of an imaginary beam, which is then modeled as one material. The distances y to the centroid, values of , and from the measurement are used as input parameters to equation (1). Because of the modulus weighted centroid calculation, we iterate to find the correct ECNT and ICNT using Matlab code. The density (ρCNT), another input parameter to equation (1), is estimated based on the thickness and mass measurement of CNT layer using a microbalance (SE2 Ultra Micro Balance, Sartorius) with a 0.1 μg resolution and cross-sectional SEM. The details of the measurement process are explained elsewhere [12]. We validate this methodology with a finite element method (FEM) model, by using a film with same modulus and thickness to compare the resonant frequency shift [13]. A good agreement between the measurements and simulation has been observed for thin films. However, the modulus is particularly underestimated for thick films, which introduces a modification factor using FEM models from a previous study. The resulting modulus increases by about 20%. This might be because of transverse shear strain or damping effects, which are usually more significant for thick films. We recently reported that nanotube characteristics such as alignment, density, and height impact the film's mechanical properties, including elastic modulus [19]. The nonhomogeneous regions of VACNT films exist because of their growth process which results in a denser and entangled crust layer above layers which can decrease in alignment toward the substrate [33]. Bedewy et al [18, 34] identified variations in morphology by investigating the growth height and density over time and correlating observed morphology changes to the growth process dynamics including catalyst diffusion and coarsening, and CNT population dynamics such as crowding, self-organization, and termination. These studies have revealed four different stages of the growth process. The first stage is the interweaving of a thin layer of entangled and randomly oriented nanotubes, which forms a crust on the top of the VACNT film. The crust extent varies for a given process depending on growth conditions. The crust is observed to be 1 μm thick for the SWCNTs and 0.4 μm thick for the MWCNTs in this work. The first stage begins the self-organization that is manifested in the second stage morphology. The second stage is the vertically aligned growth beneath the top crust. As a next stage, the CNTs may have density decay region in the bottom. The CNTs grow at the same speed as the previous stage, but the mass density per unit area can slowly decrease. This results in less aligned and less dense morphology in the bottom region of the film. The final stage is the termination step of the nanotube growth. Note that the third and fourth stages are dependent on the film height, i.e. if film height is controlled by quenching the growth process (removing carbon feedstock and/or temperature) to achieve a desired height that is less than the termination height allowed by catalyst evolution (typically in the range of 1 mm for MWCNT arrays), then the third and fourth stages are less important. Detailed micrographs of a crust with a larger magnification are shown in figures 2(a) and (c). The mechanical moduli of ~10 µm-thick SWCNT films and ~90 µm-thick MWCNT films are shown in figures 3(a) and (b). The SWCNT films have an in-plane modulus ranging from 40 to 80 MPa. The MWCNT films have low modulus around 1 MPa. There is no thickness dependency since the crust (0.5–1 µm-thick in this work) is  <0.5% of the total thickness for the MWCNT films. Figure 2. SEM images of MWCNT and SWCNT films. (a) SEM image of a crust layer and (b) middle layer of a MWCNT film. (c) SEM image of a crust layer and (d) middle layer of a SWCNT film. SEM images (a), (c) show the top portion of CNTs indicate more entanglements and connections between CNTs. SEM images (b), (d) show the middle and bottom portion of CNTs are vertically aligned. Download figure: Standard High-resolution Export PowerPoint slide Figure 3. Effective modulus of (a) SWCNT films and (b) MWCNT films after the surface etching. Solid squares and crosses represent the moduli before and after the etching, respectively. The thickness nonuniformities are indicated by the horizontal error bars. The nonuniformity effects on the modulus are represented by the vertical error bars. Although the thick MWCNT films exhibit relatively large thickness nonuniformities, the effects on the modulus are reasonably small. Download figure: Standard High-resolution Export PowerPoint slide Past work shows a clear trend of decreasing in-plane modulus with increases in film thickness ranging from 0.5–200 µm [12, 13]. This is attributed to the presence of a thin and entangled crust layer as described above. The crust layer is measured to be denser than the middle layer. The higher density in combination with the horizontal orientation of the tubes suggests that the crust has higher in-plane stiffness than the middle layer [19]. In many cases, CNT films are metallized, which then allows a metal layer to form a solder bond between a CNT film and target substrates [35]. The metallization is only about 200 nm thick. The solder penetrates the crust layer of the CNT film, which might decrease the crust effect on the mechanical properties of these films when used as TIMs. The modulus measurements are performed before and after the surface etching of the VACNT films to isolate and distinguish the effect of the crust on the properties of the film. To understand the role of the crust layer, and to characterize films without the crust layer, we developed an etching technique capable of removing the crust layer. An O2 plasma etch process is used to remove the crust and top layer of the VACNT films resulting in a reduction of the VACNT film thicknesses [36]. We have used RF power (45 W) or O2 flow rate (80 sccm), resulting in an etching rate of 1–2 μm min−1. Figures 2 and 4 show SEM micrographs of SWCNTs and MWCNTs before and after O2 plasma etching. In figures 2(a) and (c), the tips of the CNTs are entangled, forming a wavy crust layer. Figure 4 shows that the top crust layer of the CNTs is removed by the O2 plasma etching. The transformed morphology is clearly illustrated in a close-up view in figures 4(a) and (b). Some tips of the CNTs attach and fix themselves to each other, forming needle shapes instead of displaying entanglements. Additionally, the CNTs appear more vertically oriented after the etching than they were before the etching. The enhancement in vertical orientation may be associated with the effect of a strong vertical electric field during the plasma etch process, which may induce vertical alignment of the CNTs [37]. Thus, the vertical alignment can contribute to a lower in-plane mechanical modulus. In addition, damage or nonuniformity after the surface etching is another potential factor affecting the overall mechanical response of the film. Therefore, this factor should be carefully investigated in future work. Figure 4. SEM images of MWCNT and SWCNT films. (a) MWCNTs and (b) SWCNTs after the surface etching. The surface etching removes the entanglements and connections between CNTs. Download figure: Standard High-resolution Export PowerPoint slide The SWCNT films received a 1 min surface etching. This etching reduces the film thickness from 11–14 μm to 9–12 μm and removes the crust layer with a height of ~1 μm. Since the portion of the crust is relatively large (7–11% of the film thickness), the crust can significantly affect the overall density and mechanical properties of the film. In figure 3(a), the moduli before the etching are shown with black squares, and the moduli after the etching are represented with gray crosses. Figure 5 shows that the modulus of as-grown films increases as the film thickness decreases due to the impact of the crust on the modulus. Figure 3(a) shows that the etched films have a lower modulus after the plasma etch. This reduction of modulus for thinned films is the opposite of the previous trend for the dependence of modulus on film thickness because of the removal of the crust layer in this experiment. The remaining film comprises the middle and base layers, which have a lower density and in-plane modulus than the crust. Therefore, the density and moduli of the entire films without the top crust layer will have decreased; the moduli of the etched films are smaller than the grown films with same heights. Figure 5. Modulus of SWCNT and MWCNT films [31] before and after the surface etching. Data sets show that the modulus decreases as the film thickness increases. The plot indicates the fitting line from the multi-layer model (MLD) [12] using the upper and lower limit to represent the crust and middle layer. The plot includes data on samples from a previous study that were not etched. The etched samples are boxed. (Inset) Schematics of vertically aligned SWCNT and MWCNT films before and after the etching. The SWCNT films have a crust and middle layer, and the MWCNT (>90 μm) films have a crust, middle, and bottom layer. Download figure: Standard High-resolution Export PowerPoint slide The MWCNT films have lower density, alignment, and more significant density decay in the bottom part than the SWCNT films. These MWCNT films received a 5 min surface etching. This etching removes the top crust and reduces the film thickness from 95–115 μm to 90–105 μm. In figure 3(b), the moduli before and after etching of the MWCNT films are represented with blue squares and crosses, respectively. There is no significant difference in modulus between the etched and as-grown films. For thick films (>90 μm), the crust of (~0.4 μm) occupies a smaller portion of the entire film (<0.5%); the crust effect on the overall density and extracted modulus is relatively small. In addition to the crust effect, the low modulus of MWCNT films may be attributed to the bottom region of the thick films resulting from the density decay stage [18, 34]. This bottom region of sparse or unaligned nanotubes might reduce the mechanical modulus of an entire film. In this paper, the microfabricated resonator technique is used to measure the in-plane moduli of ~10 µm-thick vertically aligned SWCNT films and ~90 µm-thick vertically aligned MWCNT films to be in the range of 80 MPa and 1 MPa, respectively. The mechanical data sets show the strong dependence of the SWCNT film modulus on film thickness due to morphology variations (crust, middle, and bottom regions) through the film thickness while the morphological effect is not obvious for the MWCNT films in the thickness ranges considered (~90 µm thick films). Removal of the unaligned crust layer reduces the in-plane modulus, consistent with micromechanics of aligned and unaligned fibrous structures. To use these VACNTs as TIMs, the crust region and bottom region of the VACNT film will be embedded in thin metal layers for use in a solder bonding process. Therefore, it is particularly important to isolate and understand the effect of the crust on the metal deposition and the resulting mechanical properties of VACNT films. The surface etching removes the crust of entangled nanotubes from the VACNT film and changes the film thickness, as revealed in SEM images. The moduli of the films before and after etching are measured using SWCNT and MWCNT films [31], and the images of the SWCNT and MWCNT films before and after the etching are investigated. Because of the large effect of the crust on the measured modulus values for SWCNT films, the etching step improves the mechanical compliance of the SWCNT films (~10 μm), which is suitable for use as an interface material. Regardless of the presence of the crust, the range of the low mechanical moduli of MWCNT films is promising for a variety of applications including thermal interfaces. Future work will address potential changes in morphology associated with surface etching. The new morphology may induce differences in nonuniformity and density, which can affect the mechanical behaviors of the entire films. To account for those factors, the etched films should be characterized using other techniques, such as Raman spectroscopy or x-ray absorption. Our plans for future work also include studies of bonding materials and integration of the CNT TIM between two surfaces that show different thermal expansion coefficients. This work was sponsored by the Office of Naval Research, SRC, the MARCO IFC, the Air Force Office of Scientific Research (AFOSR) through FA9550-12-1-0195, and the National Science Foundation. At the University of Tokyo, this work was supported by Grant-in-Aid for Scientific Research (22226006 and 25630063), Indium replacement by single-walled carbon nanotube thin films (IRENA) Project by Japan Science and Technology Agency and Strategic International Collaborative Research Program. At MIT, this work was supported by Boeing, EADS, Embraer, Lockheed Martin, Saab AB, Composite Systems Technology, Hexcel, and TohoTenax through MIT's Nano-Engineered Composite aerospace STructures (NECST) Consortium. We performed this work in part at the Stanford Nanofabrication Facility (a member of the National Nanotechnology Infrastructure Network), which is supported by the National Science Foundation under Grant ECS-Integrated Systems. At MIT, this work was carried out in part through the use of MIT's Microsystems Technology Laboratories and made use of the MIT MRSEC Shared Experimental Facilities supported by the National Science Foundation under award number DMR-0819762.
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  • The Control of Colloidal Grain Boundaries through Evaporative Vertical Self-Assembly

    Suh, Youngjoon   Pham, Quang   Shao, Bowen   Won, Yoonjin  

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  • Droplet Jumping on Superhydrophobic Copper Oxide Nanostructured Surfaces

    Lee, Jonggyu   Shao, Bowen   Won, Yoonjin  

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  • Capillary Wicking in Hierarchically Textured Copper Nanowire Arrays

    Lee, Jonggyu   Suh, Youngjoon   Dubey, Pranav P.   Barako, Michael   Won, Yoonjin  

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  • Phase purity and the thermoelectric properties of Ge2Sb2Te5 films down to 25 nm thickness

    Lee, Jaeho   Kodama, Takashi   Won, Yoonjin   Asheghi, Mehdi   Goodson, Kenneth E.  

    Thermoelectric phenomena strongly influence the behavior of chalcogenide materials in nanoelectronic devices including phase-change memory cells. This work uses a novel silicon-on-insulator experimental structure to measure the phase and temperature-dependent Seebeck and Thomson coefficients of Ge2Sb2Te5 films including the first data for films of thickness down to 25 nm. The Ge2Sb2Te5 films annealed at different temperatures contain varying fractions of the amorphous and crystalline phases which strongly influence the thermoelectric properties. The Seebeck coefficient reduces from 371 mu V/K to 206 mu V/K as the crystalline fraction increases by a factor of four as quantified using x-ray diffraction. The data are consistent with modeling based on effective medium theory and suggest that careful consideration of phase purity is needed to account for thermoelectric transport in phase change memory. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731252]
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  • Self‐Assembly: The Control of Colloidal Grain Boundaries through Evaporative Vertical Self‐Assembly (Small 12/2019)

    Suh, Youngjoon   Pham, Quang   Shao, Bowen   Won, Yoonjin  

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  • [IEEE 2013 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - Monterey, CA, USA (2013.10.13-2013.10.16)] 2013 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - Cooling Limits for GaN HEMT Technology

    Won, Yoonjin   Cho, Jungwan   Agonafer, Damena   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • [IEEE 2014 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - La Jolla, CA, USA (2014.10.19-2014.10.22)] 2014 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS) - Microfluidic Heat Exchangers for High Power Density GaN on SiC

    Won, Yoonjin   Houshmand, Farzad   Agonafer, Damena   Asheghi, Mehdi   Goodson, Kenneth E.  

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  • Evaporative Wicking Phenomena on Nanotextured Surfaces

    Duong Vy Le   Pham, Quang N.   Lee, Jonggyu   Zhang, Shiwei   Won, Yoonjin  

    As modern electronics become miniaturized with high power, thermal management for electronics devices has become significant. This motivates the implementation of new cooling solutions to dissipate high-heat levels from high-performance electronics. Evaporative cooling is one of the most promising approaches for meeting these future thermal demands. Thin-film evaporation promotes heat dissipation through the phase change process with minimal conduction resistance. In this process, it is important to design surface structures and corresponding surface properties that can minimize meniscus thickness, increase liquid-vapor interfacial area, and enhance evaporation performances. In this study, we investigate thin-film evaporation by employing nanotextured copper substrates for varying thermal conditions. The liquid spreading on the nanotextured surfaces is visualized using a high-speed imaging technique to quantify evaporative heat transfer for various surfaces. The permeability is calculated using an enhanced wicking model to estimate the evaporation effect combined with the mass measurements. Then, infrared (IR) thermography is employed to examine two-dimensional temporal temperature profiles of the samples during the evaporative wicking with a given heat flux. The combination of optical time-lapse images, evaporation rate measurements, and temperature profiles will provide a comprehensive understanding of evaporation performances using textured surfaces.
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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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  • Controlled Wetting Properties through Heterogeneous Surfaces Containing Two-level Nanofeatures.

    Dubey, Pranav P   Pham, Quang N   Cho, Hyunjin   Kim, Yongsung   Won, Yoonjin  

    Addressing the direct control of surface wettability has been a significant challenge for a variety of applications from self-cleaning surfaces to phase-change applications. Surface wettability has been traditionally modulated by installing surface nanostructures or changing their chemistry. Among numerous nanofabrication efforts, the chemical oxidation method is considered a promising approach because it allows cost-effective, quick, and direct control of the morphologies and chemical compositions of the grown nanofeatures. Despite the wide applicability of the surface oxidation method, the precise control of wetting behaviors through the growth of nanostructures has yet to be addressed. Here, we investigate the wetting characteristics of heterogeneous surfaces that contain two-level features (i.e., nanograsses and nanoflowers) with different petal shapes and structural chemistry. The difference in growth rates between nanograsses and nanoflowers creates a time-evolving morphology that can be classified by grass-dominated or flower-dominated regimes, which induces a wide range of water contact angles from 120 to 20=C2=B0. The following study systematically quantifies the structural details and chemistry of nanostructures associated with their wetting characteristics. This investigation of heterogeneous surfaces will pave the way for selective growth of copper nanostructures and thus a direct control of surface wetting properties for use in future copper-based thermal applications.=20
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  • Impact of nanotube density and alignment on the elastic modulus near the top and base surfaces of aligned multi-walled carbon nanotube films

    Gao, Yuan   Kodama, Takashi   Won, Yoonjin   Dogbe, Senyo   Pan, Lawrence   Goodson, Kenneth E.  

    The mechanical compliance of vertically aligned carbon nanotube (VACNT) films renders them promising as interface materials that can accommodate thermal expansion mismatch. Here we study the relationship between the detailed morphology and elastic modulus of multi-walled VACNT films with thicknesses ranging from 98 to 1300 am. A systematic analysis of scanning electron micrographs reveals variations in nanotube alignment and density among samples and within different regions of a given film. Nanoindentation of both top and bottom film surfaces using an atomic force microscope with spherical indenters with radii between 15 and 25 gm provides evidence of the modulus differences. The top surface is shown to have a higher modulus than the base, with out-of-plane modulus values of 1.0-2.8 MPa (top) and 0.2-1.4 MPa (base). The indentation data and microstructural information obtained from electron microscopy are interpreted together using an open cell foam model to account for differences in nanotube alignment and density, which are generally lower at the base and yield predictions that are consistent with the modulus data trends. This work shows that microstructure analysis complements property measurements to improve our understanding of nanostructured materials. (C) 2012 Elsevier Ltd. All rights reserved.
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  • Layered manganese metal-organic framework with high specific and areal capacitance for hybrid supercapacitors

    Shinde, Pragati A.   Seo, Youngho   Lee, Suchan   Kim, Hansung   Pham, Quang N.   Won, Yoonjin   Jun, Seong Chan  

    High capacitance, long cycling life, superior energy density, and ultrafast charge-discharge rates are some of the important characteristics for energy storage systems to meet the energy demands of modern electronics. The development of new emerging class of materials is necessary to rally these key requirements. Metal organic frameworks (MOFs) are generated tremendous interest as a new class of electrode materials for applications in energy storage owing to their large specific surface area, excellent porosity, composition and functionality. Herein, layered manganese-1, 4 benzenedicarboxylic acid-based MOFs [Mn(BDC).nDMF]n (Mn-MOFs) are fabricated using hydrothermal technique for supercapacitors application. The as-obtained Mn-MOF exhibits exceptionally high specific capacity (areal capacitance) of 567.5 mA h g(-1) (10.25 F cm(-2)) at a current density of 1 A g(-1). The hybrid supercapacitor fabricated with Mn-MOFs as a cathode and reduced graphene oxide (rGO) as an anode demonstrates specific and volumetric capacitances of 211.37 F g(-1) and 3.32 F cm(-3), respectively, specific energy of 66 Wh kg(-1) at a specific power of 441 W kg(-1), and capacity retention of 81.18% over 10,000 cycles. These excellent electrochemical results illustrate potential of utilizing MOF-based materials for supercapacitor application and provide innovative direction for the development of future high-performance energy storage systems.
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  • Boiling Heat Transfer with a Well-Ordered Microporous Architecture

    Pham, Quang N.   Zhang, Shiwei   Hao, Shuai   Montazeri, Kimia   Lin, Cheng-Hui   Lee, Jonggyu   Mohraz, Ali   Won, Yoonjin  

    Boiling heat transfer through a porous medium offers an attractive combination of enormous liquid-vapor interfacial area and high bubble nucleation site density. In this work, we characterize the boiling performances of porous media by employing the well-ordered and highly interconnected architecture of inverse opals (IOs). The boiling characterization identifies hydrodynamic mechanisms through which structural characteristics affect the boiling performance of metallic microporous architecture by validating empirical measurements. The boiling performances can be optimized through the rational design of both the structural thicknesses and pore diameters of IOs, which demonstrate up to 336% enhancement in boiling heat-transfer coefficient (HTC) over smooth surfaces. The optimal HTC and critical heat flux occur at approximately 3-4 mu m in porous structure thickness, which is manifested through the balance of liquid-vapor occupation within the spatial confinement of the IO structure. The optimization of boiling performances with varying pore diameters (0.3-1.0 mu m) can be attributed to the hydraulic competitions between permeability and viscous resistance to liquid-vapor transport. This study unveils thermophysical understandings to enhance multiphase heat transfer in microporous media for ultrahigh heat flux thermal management.
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  • Phase and thickness dependent modulus of Ge2Sb2Te5 films down to 25?nm thickness

    Won, Yoonjin   Lee, Jaeho   Asheghi, Mehdi   Kenny, Thomas W.   Goodson, Kenneth E.  

    The mechanical properties of phase-change materials including Ge2Sb2Te5 (GST) are strongly influenced by the complex interaction of phase and imperfection distributions, especially at film thicknesses relevant for phase-change memory devices. This work uses a micromechanical resonator as a substrate to study the phase dependent modulus of GST films with thicknesses from 25 nm to 350 nm. The moduli of amorphous GST and crystalline GST films increase with decreasing thickness to 10 GPa and up to 60 GPa, respectively. The phase purity is studied using X-ray diffraction and energy dissipation data, which provide qualitative information about inelastic absorption. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699227]
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