A thermal rectifier that utilizes thermal expansion to directionally control interfacial conductance between two contacting surfaces is presented. The device consists of two thermal reservoirs contacting a beam with one rough and one smooth end. When the temperature of reservoir in contact with the smooth surface is raised, a similar temperature rise will occur in the beam, causing it to expand, thus increasing the contact pressure at the rough interface and reducing the interfacial contact resistance. However, if the temperature of the reservoir in contact with the rough interface is raised, the large contact resistance will prevent a similar temperature rise in the beam. As a result, the contact pressure will be marginally affected and the contact resistance will not change appreciably. Owing to the decreased contact resistance of the first scenario compared to the second, thermal rectification occurs. A parametric analysis is used to determine optimal device parameters including surface roughness, contact pressure, and device length. Modeling predicts that rectification factors greater than 2 are possible at thermal biases as small as 3 K. Additionally, thin surface coatings are discussed as a method to control the temperature bias at which maximum rectification occurs.

Functional magnetic resonance imaging (fMRI) studies reveal altered food cue processing in brain reward centers [e.g. insula (INS), amygdala (AMY), orbitofrontal cortex (OFC), caudate (CAU), putamen (PUT)] of obese vs. normal weight individuals. We assessed the effects of acute aerobic exercise (Ex) and protein intake [normal (NP) or high (HP)] on appetite and the neural response to visual food cues (NRVFC) in 7 women (BMI 35.5 ± 1.0 kg/m2, 29 ± 3 y). All meals for 24h were provided, dinner was NP or HP (15 vs. 30% of energy), and subjects rested (R) or Ex for 30min before dinner, resulting in 4 testing days (NPR, NPEx, HPR, HPEx) with 4 fMRI per day (pre, 1, 2.5, and 4h after dinner). Regions of interest were defined by activation clusters (threshold p< 0.001) from the first fMRI scan and where activation peaks fell within left/right INS, AMY, OFC, CAU, and PUT. Appetite was assessed before and after fMRI, Ex/R, and dinner. Hunger and desire to eat decreased and fullness increased at all postprandial time points vs. pre-dinner with no difference among testing days. NRVFC was not different among time points or testing days. Data from this pilot study do not support an effect of acute exercise and protein intake on neural food cue processing in obese women, but interpretations should be made with caution due to the small sample size. Support: NIH UL1 TR001108, USDA 2011-38420-20038

Thermal interface materials (TIMs) serve a critical role in the thermal management of electronic systems by enhancing the flow of heat from source to sink. Nanostructured materials, such as arrays of carbon nanotubes (CNTs) have been shown to outperform many commercially available TIMs due to their low intrinsic resistance and large compliance that enables them to conform to rough surfaces. These characteristics, combined with their low density and ability to withstand vacuum environments and extreme temperatures, make CNT-based TIMs very suitable for space applications. In space, materials are exposed to high doses of gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years or even more, total radiation exposure can be significant and can affect the structure and performance of the TIM. In this work, the potentially adverse effects on the thermal performance of CNT TIMs of gamma-ray irradiation is reported. CNT TIMs were irradiated in a gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad. The thermal interface resistance was measured before and after gamma-ray irradiation using a transient photoacoustic (PA) method at room temperature and a contact pressure of 134 kPa and indicated no adverse effects of gamma-ray exposure on thermal performance.

Thermal contact resistance (TCR) is most commonly measured using one-dimensional steady-state calorimetric techniques. In these experimental methods, a temperature gradient is applied across two contacting beams and the temperature drop at the interface is inferred from the temperature profiles of the rods that are measured at discrete points. During data analysis, thermal conductivity of the beams is typically taken to be an average value over the temperature range imposed during the experiment. Here, a generalized theory is presented that accounts for temperature-dependent changes in thermal conductivity. The procedure presented enables accurate measurement of TCR for contacting materials whose thermal conductivity is any arbitrary function of temperature. For example, it is shown that the standard technique yields TCR values that are about 15% below the actual value for two specific examples of copper and silicon contacts. On the other hand, the generalized technique predicts TCR values that are within 1% of the actual value. The method is exact when thermal conductivity is known exactly and no other errors are introduced to the system.

Thermal interface materials (TIMs) serve a critical role in thermal management by enhancing heat transfer across contact interfaces. Specifically, they are most commonly used in electronics to enhance the flow of heat from source to sink by decreasing the overall thermal resistance of the system. In space, these materials are exposed to high doses of Gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years, total radiation exposure can be significant and can adversely affect the thermal contact resistance (TCR) of the TIM. In this manuscript, we report the effect of radiation-aging on the TCC of several commercially available electrically insulating, thermally conductive interface materials that are commonly used in satellite systems. Although radiation dose levels can vary significantly during the course of a space mission, a dosing of 10 Mrad per year for TIMs is a reasonable estimate. The TIMs were aged in a Gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad to simulate mission lengths of 5 and 10 years, respectively. The TCR of each radiation-aged sample, as well as un-aged samples, were measured under vacuum (less than 3 × 10−4 Pa). Radiation-aging of the TIMs led to a significant increase in the TCR of the tested samples. For example, the pressure-dependent TCR was shown to increase 20–150% for Cho-Therm 1671 and 50–250% for ThermaCool R10404 samples subjected to 50 Mrad of gamma-ray irradiation. These results show that radiation-aging of TIMs cannot be ignored in the design and simulation of space systems.

Removal of waste heat generated via Joule heating during the operation of electronic devices is critical to overall system performance and reliability. A significant fraction of the overall thermal budget is consumed by heat transfer across the interface of contacting materials. To enhance the flow of heat from source to sink, thermal interface materials (TIMs) are used to reduce thermal contact resistance (TCR) by increasing real contact area at the interface. In space systems, TIMs are exposed to high doses of gamma radiation not encountered in typical terrestrial applications. With typical design lifetimes of 5 years or more, total radiation exposure can be significant and can affect the structure and performance of the TIM. Here, we report measurements of the pressure-dependent TCR of metallic foils and carbon nanotube TIMs (CNT-TIMs) in both vacuum and ambient air environments. The TIMs were irradiated in a gamma cell at a rate of 200 rad/s to a total dose of 50 Mrad. TCR was measured before and afte...

Thermal rectification occurs when a device permits heat to flow preferentially in one direction direction while restricting it in the opposite direction. Thermal rectification can occur whenever an asymmetry is present in a device, and has been demonstrated to arise in bulk materials that have asymmetric geometry, in the contact of two materials with different thermal properties and in nanomaterials. Herein, a thermal diode that utilizes thermal expansion to directionally control interfacial conductance between two contacting surfaces is presented. Essentially, the device consists of two thermal reservoirs contacting a beam with one rough and one smooth end. When the temperature of reservoir in contact with the smooth surface is raised, a similar temperature rise will occur in the beam, causing it to expand, thus increasing the contact pressure at the rough interface and reducing the interfacial contact resistance. However, if the temperature of the reservoir in contact with the rough interface is raised, the large contact resistance will prevent a similar temperature rise in the beam. As a result, the contact pressure will be marginally affected and the contact resistance will not change appreciably. Owing to the decreased contact resistance of the first scenario compared to the second, thermal rectification occurs. A parametric analysis is used to determine optimal device parameters including surface roughness, contact pressure and device length. Modeling predicts rectification factors greater than 2 are possible at thermal biases as small as 3 K.

We report measurements of temperature- and length-dependent 1/f noise in vertical single-walled carbon nanotube (SWCNT) arrays. Carbon nanotubes are synthesized in a porous anodic alumina template with sub-micrometer channel lengths ranging from 100 to 700 nm. A significant difference is observed in the 1/f noise magnitude of quasi-ballistic and diffusive SWCNT devices, with quasi-ballistic devices exhibiting 1/f noise levels that are one to two orders of magnitude less than diffusively conducting devices. Furthermore, 1/f noise was measured from 90 to 400 K, and the noise prefactor decreased significantly at temperatures below 250 K.

DC electrical self-heating (Joule heating) is exploited to characterize the thermal behavior of Ni microbridges. The temperature rise of the devices due to self-heating is monitored using an infrared microscope for current densities up to 10 5 A/cm 2 . The obtained temperature profiles reveal significant heating at the bases of the microbridges. Simulations are performed in order to extract the thermal conductivity of the electroplated Ni thin film from the experimental data. The thermal conductivity is found to be 78.8 W/m · K or 13% less than that of bulk Ni. As current flows through the microbridges, they deflect upward, significantly changing the system response and pull-in voltage required for actuation. Additionally, the electrical resistivity and specific electrical contact resistances between the microbridges and the anchor points are reported. The electroplated Ni is found to have an electrical resistivity of 9.7 μΩ · cm which agrees with other values in the literature for thin-film Ni. By combining the electrical and thermal measurements, it is possible to determine the phonon and electron contributions to thermal conductivity. Although demonstrated on Ni films, this technique can be applied to any metallic film without modification. Such characterization of transport properties of constituent materials is important in the modeling of microelectromechanical systems and enables device performance to be predicted with improved accuracy.

Coalescence between two drops on a substrate is one of the important factors that can affect print quality in inkjet applications. Two stochastic models (constant contant angle mode and constant contact area mode) that consider drop placement error, drop impact, and drop evaporation are proposed for determining the probability of coalescence between adjacently printed drops on nonporous substrates. Experiments are conducted to measure the probability of coalescence with respect to deposition time difference between adjacently printed drops and compared to the predictions of the models. The measured coalescence follows the constant contact angle mode evaporation model during the initial phase of the life of the first drop, which is followed by a mix between the constant contact angle mode and the constant contact area mode models for the remainder of the life of the first drop. This study shows that for probabilities of coalescence between 10% and 80% the constant contact angle mode model can be used to determine deposition time difference threshold values for adjacent drops in applications promoting drop coalescence while the constant contact area mode model can be used for applications avoiding drop coalescence. Further efforts are needed to capture the dynamics of the mixed-model evaporation and to more accurately predict larger (greater than 80%) and smaller (less than 10%) occurrences of coalescence.

Accurate knowledge of thermophysical properties is needed to predict and optimize the thermal performance of microsystems. Thermal conductivity is experimentally determined by measuring quantities such as voltage or temperature and then inferring a thermal conductivity from a thermal model. Thermal models used for data analysis contain inherent assumptions, and the resultant thermal conductivity value is sensitive to how well the actual experimental conditions match the model assumptions. In this paper, a modified data analysis procedure for the steady state Joule heating technique is presented that accounts for bond pad effects including thermal resistance, electrical resistance, and Joule heating. This new data analysis method is used to determine the thermal conductivity of polycrystalline silicon (polysilicon) microbridges fabricated using the Sandia National Laboratories SUMMiT V™ micromachining process over the temperature range of 77-350 K, with the value at 300 K being 71.7 ± 1.5 W/(m K). It is shown that making measurements on beams of multiple lengths is useful, if not essential, for inferring the correct thermal conductivity from steady state Joule heating measurements.

Electrical noise is inherent in all conductors; the magnitude of this noise is proportional to device temperature in some frequency bands. Shot noise thermometry is a self-calibrating measurement technique that relates statistical fluctuations in DC current across a device to temperature. Historically, low frequency bands that contain 1/f noise have been filtered out in noise thermometry analysis. We report a noise analysis that accounts for 1/f noise in temperature measurements, thus reducing the required bandwidth of the measurement system. Numerical simulation is used to show the efficacy of this approach. Computer-generated noise signals containing Johnson, shot and 1/f noise are randomly generated. Data processing is used to determine device temperature and Fano factor. The impact of important factors such as data averaging, 1/f noise magnitude, bias range, ambient temperature and Fano factor on the accuracy and precision of fit values is investigated. This technique is then applied to experimental measurements on a vertical single-walled carbon nanotube array.

Several creatures, including insects, spiders, and lizards, have developed a unique clinging ability that utilizes dry adhesion. Geckos, in particular, have developed the most complex adhesive structures capable of smart adhesion the ability to cling to different smooth and rough surfaces and detach at will. These animals make use of about three million microscale hairs (setae) (about 14000/mm(2)) that branch off into hundreds of nanoscale spatulae (about a billion spatulae). This hierarchical surface construction gives the gecko the adaptability to create a large real area of contact with surfaces. Modeling of the gecko attachment system as a hierarchical spring model has provided insight into adhesion enhancement generated by this system. van der Waals forces are the primary mechanism utilized to adhere to surfaces, and capillary forces are a secondary effect that can further increase adhesion force. Preload applied to the setae increases adhesion force. Although a gecko is capable of producing on the order of 20 N of adhesion force, it retains the ability to remove its feet from an attachment surface at will. The adhesion strength of gecko setae is dependent on the orientation; maximum adhesion occurs at 30 degrees. During walking a gecko is able to peel its foot from surfaces by changing the angle at which its setae contact a surface. A man-made fibrillar structure capable of replicating gecko adhesion has the potential for use in dry, superadhesive tapes that would be of use in a wide range of applications. These adhesives could be created using microfabrication/nanofabrication techniques or self-assembly.

A carbon nanotube (CNT) thermometer that operates on the principles of electrical shot noise is reported. Shot noise thermometry is a self-calibrating measurement technique that relates statistical fluctuations in dc current across a device to temperature. A structure consisting of vertical, top, and bottom-contacted single-walled carbon nanotubes in a porous anodic alumina template was fabricated and used to measure shot noise. Frequencies between 60 and 100 kHz were observed to preclude significant influence from Vf noise, which does not contain thermally relevant information. Because isothermal models do not accurately reproduce the observed noise trends, a self-heating shot noise model has been developed and applied to experimental data to determine the thermal resistance of a CNT device consisting of an array of vertical single-walled CNTs supported in a porous anodic alumina template. The thermal surface resistance at the nanotube-dielectric interface is found to be 1.5 × 10 8 K/W, which is consistent with measurements by other techniques.

The one-dimensional, cylindrical nature of single-walled carbon nanotubes (SWCNTs) suggests that the ideal gating geometry for nanotube field-effect transistors (FETs) is a surround gate (SG). Using vertical SWCNTs templated in porous anodic alumina, SGs are formed using top-down processes for the dielectric/metal depositions and definition of the channel length. Surround gates allow aggressive scaling of the channel to 25% of the length attainable with a bottom-gate geometry without incurring short-channel effects. The process demonstrated here for forming SGs on vertical SWCNTs is amenable for large-scale fabrication of multinanotube FETs.

Vertical single-walled carbon nanotubes (v-SWCNTs) are synthesized within highly ordered porous anodic alumina (PAA) templates supported on Si substrates. A process for obtaining thin-film PAA with long-range ordered nanopores is presented in this paper. Each nanopore contains at most one v-SWCNT that is supported by a dielectric and addressed by electrochemically formed Pd nanowire source contacts and evaporated Pd drain contacts. Characteristics of these completely vertical, two-terminal nanotube devices are presented. Control of the v-SWCNT length is demonstrated using a straightforward etching process with lengths of less than 100 nm achieved without the need for complex/expensive lithography. This effective nanoscale length control of highly ordered v-SWCNTs provides a practical basis for the realization of CNT-based nanoelectronics.