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The Intrachip Enhanced Cooling Fundamentals (ICECool Fun) effort was launched by the Defense Advanced Research Projects Agency (DARPA) under the leadership of Dr. Avram Bar-Cohen during 2012–2015 to target an order of magnitude improvement in chip level and hot spot heat fluxes, compared to the then state-of-the-art (SOA). Evaporative cooling technologies to achieve potential targets of 1 kW/cm2 at the chip level and 5 kW/cm2 at the hot spot level were targeted. A key goal was to improve fundamental understanding of the evaporative cooling physics at the relevant scales, and a numerical modeling capability to enable the co-design of such solutions in emerging computing and communications systems. A summary of the five projects pursued under this effort is provided, including the key accomplishments and developed capabilities.

Mean radical lifetimes and fluctuations in lifetime are shown to decrease with conversion and polydispersity index for atom transfer radical polymerization (ATRP) of both methyl methacrylate and st...

The extended meniscus and the intermolecular and capillary forces that govern its behavior and connection to change-of-phase heat transfer have been the subject of an increasing body of research over the past 50 years. We have been fortunate to be at the forefront of this effort starting from the development of a capillary feeder, in Earth's gravity, to stabilize film boiling to running a series of transparent heat pipe experiments aboard the International space station hoping to better understand the role of intermolecular forces in microgravity. The use of ellipsometry and interferometry to highlight the location and state of the vapor–liquid interface have been key to these studies and have helped to uncover many new, interesting, and sometimes unexpected phenomena associated with fluid flow and change-of-phase heat transfer.

The Microfluidic, Extreme heat flux, CMOS compatible, Heat-eXchanger (MECH-X) is an embeddable silicon-based reactor-style heat sink which has been experimentally studied by the authors. The 800 µm thick MEMS heat sink discretizes the working fluid into stacked primary and secondary chambers to enhance phase change heat transfer. Piranha Pin-Fin (PPF) microstructures—reported previously by the authors—have been employed in the primary reaction chamber. The PPF structures vent higher-enthalpy fluid into a secondary, or booster, chamber for additional heat transfer. The MECH-X system has been shown to dissipate heat loads exceeding 10,000,000 W/m2 with dielectric fluid HFE7000 while maintaining surface temperatures below 95 °C. The present work reports on flow boiling experiments performed to characterize system-level performance of the MECH-X heat sink at mass fluxes of 680, 1440, and 3350 kg/s/m2. HFE7000 was used as a coolant at ambient lab temperature (∼23 °C) and a system pressure of 377 kPa. A 3.95 mm2 heating element, which simulated a heat generating component, was maintained below 95 °C with heats loads exceeding 1 kW/cm2. Experiments were terminated at this heat flux due to saturating a 50 VDC laboratory power supply. Results are also presented at a system pressure of 239 kPa, and results are compared to first generation PPF heat sinks.

Electrospinning is a simple method for producing nanoscale or microscale fibers from a wide variety of materials. Intrinsically conductive polymers (ICPs), such as polyaniline (PANI), show higher conductivities with the use of secondary dopants like m-cresol. However, due to the low volatility of most secondary dopants, it has not been possible to electrospin secondary doped ICP fibers. In this work, the concept of secondary doping has been applied for the first time to electrospun fibers. Using a novel design for rotating drum electrospinning, fibers were efficiently and reliably produced from a mixture of low- and high-volatility solvents. The conductivity of electrospun PANI-poly(ethylene oxide) (PEO) fibers prepared was 1.73 S/cm, two orders of magnitude higher than the average value reported in the literature. These conductive fibers were tested as electrodes for supercapacitors and were shown to have a specific capacitance as high as 3121 F/g at 0.1 A/g, the highest value reported, thus far, for PANI-PEO electrospun fibers.

An experimental heat transfer study using triethylamine (TEA)-water solution as a coolant in a Piranha Pin Fin (PPF) structure is reported here. The triethylamine-water solution undergoes phase separation when heated to a temperature over 18.2 °C at a TEA mass fraction of 32.1% (i.e., critical composition). This separation process was proven to enhance heat transfer in the plain channel geometry. With the aid of the PPF structure, the TEA-water solution was able to dissipate a heat flux of up to 500 W/cm2 at a mass flux of 600 kg/m2s and to keep the surface temperature below 80 °C. The phase separation of a TEA-water mixture in the PPF structure yields about 1.8 times greater heat transfer coefficient compared to a corresponding homogeneous mixture flow. Flow boiling was achieved during experiments when the heat flux exceeded 600 W/cm2. The effects of different configurations of PPF were also studied, and it is shown that the PPF configuration with fewer and smaller fins provides the best performance enhancement over water flow at the same mass flux. Two flow conditions, channel flow and extraction flow, were examined. The TEA-rich phase was extracted during the extraction flow experiment and the remaining water-rich phase results in favorable thermal transport performance. A string-shaped flow pattern is observed due to both shear force and the flow disturbance provided by the PPFs. The system exhibits reduced pressure drop as the fluid viscosities decrease after phase separation.

Achieving effective cooling within limited space is one of the key challenges for miniaturized product design. State-of-the-art micro-scale cooling enhancement techniques incorporate flow disturbances and boiling to reach high performance. However, these methods face the inherent issues of extra pressure drop, flow instability and dry-out that limits heat flux. Here we demonstrate that substantial cooling capability enhancement, up to 2.5 times, is realized by introducing the phase separation of a triethylamine (TEA)/water mixture at the micro-scale. Our experiments show that the enhancement behavior is closely related to the system’s initial composition, temperature, and flow conditions. Moreover, the mixture system exhibits reduced pressure drop after separation, which makes it more promising in serving practical applications. The results reveal new possibilities for liquid coolant selection and provide the experimental foundation for further research in this area.

Understanding the fluid dynamics and the phase change heat transfer process within a thin liquid film is important to improve the performance of many industrial processes like coating or distillation. Studies by our group and other research teams showed that thin liquid films begin to oscillate spontaneously as the heat flux increases. We also found that the oscillation amplitude and frequency increase with increasing heat input. This implies that there is a heat transfer advantage to an oscillating thin film. We developed a numerical model to try and understand if there is an advantage to oscillation and under what conditions that advantage occurs. We found that oscillation can enhance net evaporative heat transfer but only if a short period of condensation exists within each oscillation cycle. Such condensation can be driven by intermolecular forces, capillary forces, Marangoni forces, or combinations of all three as we concluded from recent heat pipe experiments. Condensation increases the liquid film thickness at the contact line, and therefore decreases the disjoining pressure impediment to evaporation. These short condensation periods followed by fast evaporation appear as “spikes” in the liquid film thickness over time. These “spikes” were observed experimentally and mimicked by the simulations. Our calculations also show that the heat transfer efficiency increases with increasing oscillation frequency and amplitude in qualitative agreement with experiments.

A composite film made of a stable gold nanoparticle (NP) array with well-controlled separation and size atop a TiO2 nanorod film was fabricated via the oblique angle deposition (OAD) technique. The fabrication of the NP array is based on controlled, Rayleigh-instability-induced, solid-state dewetting of as-deposited gold aggregates on the TiO2 nanorods. It was found that the initial spacing between as-deposited gold aggregates along the vapor flux direction should be greater than the TiO2 interrod spacing created by 80° OAD to control dewetting and produce NP arrays. A numerical investigation of the process was conducted using a phase-field modeling approach. Simulation results showed that coalescence between neighboring gold aggregates is likely to have caused the uncontrolled dewetting in the 80° deposition, and this could be circumvented if the initial spacing between gold aggregates is larger than a critical value smin. We also found that TiO2 nanorod tips affect dewetting dynamics differently than pl...

The Constrained Vapor Bubble (CVB), a simple, wickless, heat pipe design that depends on interfacial forces to drive corner flow in a square cuvette, was studied in the microgravity environment aboard the International Space Station (ISS). In this paper, we consider the effects of different condenser temperatures on the heat transfer and fluid flow behavior using pentane as the working fluid. As the condenser temperature was decreased, the performance of the system decreased. This performance decrease using the pure working fluid was opposite to the behavior observed when using a mixture of 94 vol% pentane and 6 vol% isohexane. The mechanism for the decline in performance as the condenser temperature was lowered was a stronger than expected increase in the apparent strength of Marangoni flows at the heater end of the system. A simple mathematical model was fit to the experimental data and used to extract an evaporator heat transfer coefficient for experiments where we held the condenser temperature constant while increasing the heater power and where we held the heater power constant while decreasing the condenser temperature. All the results could be collapsed onto a single Nusselt number vs. Marangoni number curve. In this formulation, the Nusselt number was found to decrease with increasing Marangoni number to the 1/3 power.

Understanding the dynamics of phase change heat and mass transfer in the three-phase contact line region is a critical step toward improving the efficiency of phase change processes. Phase change becomes especially complicated when a fluid mixture is used. In this paper, a wickless heat pipe was operated on the International Space Station (ISS) to study the contact line dynamics of a pentane/isohexane mixture. Different interfacial regions were identified, compared, and studied. Using high resolution (50×), interference images, we calculated the curvature gradient of the liquid-vapor interface at the contact line region along the edges of the heat pipe. We found that the curvature gradient in the evaporation region increases with increasing heat flux magnitude and decreasing pentane concentration. The curvature gradient for the mixture case is larger than for the pure pentane case. The difference between the two cases increases as pentane concentration decreases. Our data showed that the curvature gradient profile within the evaporation section is separated into two regions with the boundary between the two corresponding to the location of a thick, liquid, “central drop” region at the point of maximum internal local heat flux. We found that the curvature gradients at the central drop and on the flat surfaces where condensation begins are one order of magnitude smaller than the gradients in the corner meniscus indicating the driving forces for fluid flow are much larger in the corners.

Low- $\kappa $ SiCOH reliability is a growing concern for integrated circuit reliability. An important consideration for product qualification involves the accurate extrapolation to the low percentile failures based on the results from a group of samples. A method is presented to determine the root cause of failure distributions amongst a group of dielectric samples using voltage ramp data. Samples’ leakage current traces and breakdown voltages are compared with each other. Using this method, it was determined that the dielectric spacing variation dominates across-wafer failure, while variation of local breakdown strength affects in-chip failure.

The effect of cooling temperature on heat pipe performance has generally received little consideration. In this paper, we studied the performance of a Constrained Vapor Bubble (CVB) heat pipe using a liquid mixture of 94 vol%-pentane and 6 vol%-isohexane at different cooling temperatures in the microgravity environment of the International Space Station (ISS). Using a one-dimensional (1-D) heat transfer model developed in our laboratory, the heat transfer coefficient of the evaporator section was calculated and shown to decrease with increasing cooler temperature. Interestingly, the decreasing trend was not the same across the cooler settings studied in the paper. This trend corresponded with the change in the temperature profile along the cuvette. When the cooling temperature went from 0 to 20 °C, the temperature of the cuvette decreased monotonically from the heater end to the cooler end and the heat transfer coefficient decreased slowly from 456 to 401 (W m −2 K −1 ) (at a rate of 2.75 W m −2 K −2 ). However, when the cooling temperature increased from 25 to 35 °C, a minimum point formed in the temperature profile, and the heat transfer coefficient dramatically decreased from 355 to 236 (W m −2 K −1 ) (at a rate of 11.9 W m −2 K −2 ). A similar change in decreasing trend was observed in the pressure gradient and liquid velocity profile. The reduced heat pipe performance at high cooling temperatures was consistent with the reduced evaporation which was indicated by the decreasing internal heat transfer and the increasing liquid film thickness along the cuvette as seen in the surveillance images. The result obtained is important for future heat pipe design because we now have a better understanding of the working temperature ranges of these devices.

The Constrained Vapor Bubble (CVB) is a transparent, wickless heat pipe experiment carried out in the US Labs of the International Space Station (ISS). Experiments were carried out using the 40 mm CVB, 3 mm× 3 mm in cross-section, pentane as the working fluid, with the power inputs of up to 3 W. Due to the low Bond number (Bo) in microgravity and materials of construction, the CVB system was ideally suited to determine the contribution of the Marangoni forces toward the limiting heat pipe performance, and the transparent quartz shows exactly how that limitation occurs. Previous literature models and experimental temperature and pressure measurements suggested that at high enough temperature gradients, the working fluid should be subjected to enough Marangoni force to force it away from the heater and ultimately, dry out the hot end. The CVB experiment shows that high temperature gradients lead to a totally opposite behavior, i.e., ‘flooding’ of the heated end. Flooding of the heater end is attributed to a competition between Marangoni-induced flow due to high temperature gradients at the heater end and capillary return flow from the cooler. This creates a thick liquid layer in the corner of the cuvette at the heater end. At the point of flow balance, a thick layer of liquid is observed on the flat surface of the quartz cuvette. This is defined as the central drop. The region from the top of the heater end to the central drop is referred to as the interfacial flow region. The interfacial flow region develops at a power input of around 0.7 W, and increases in length to the power input of 2 W. At 2 W, the strength of the Marangoni forces saturate. As a result, the forces in the flooded interfacial region are not able to push the liquid further into the capillary region and a further penetration of liquid down the axis of the heat pipe is arrested. As the power input is increased to nearly 3 W, an increase in the vapor space is observed near the heater end at 3 W. This behavior suggests that the flooding might just be an intermediate stage in reaching the dry-out limitation. The flat quartz surface at the hot end is covered by a wavy thin liquid film due to the interfacial forces. The hot end region closest to the heater is a superheated vapor region that leads to the condensation. This additional observation is discussed in Appendix.

Previous studies on wickless heat pipes showed that a temperature induced “Marangoni flow” prevents liquid from recirculating to the heater end, and therefore reduces the effectiveness of the heat pipe. Recently, several research groups used a water and alcohol mixture, with a low concentration of alcohol, resulting in better performance of the heat pipe. The alcohol/water combinations were peculiar in that for a certain composition range, the surface tension increases with increasing temperature thereby driving liquid toward the hotter end. It was believed that changing the direction of the Marangoni stress or reducing its magnitude by differential evaporation of an ideal binary mixture would also improve the performance of the heat pipe. For the first time, an ideal fluid mixture of 94 vol%-pentane and 6 vol%-isohexane was used as the working fluid in the Constrained Vapor Bubble (CVB) heat pipe experiment on the International Space Station (ISS). Using a simple heat transfer model developed in our laboratory, an internal heat transfer coefficient in the evaporator section was determined and shown to be almost twice that of the case where pure pentane was used under the same conditions. The Marangoni stress in the mixture was five times lower. Interestingly, reducing the Marangoni stress led to less liquid accumulation near the heater end and surveillance images of the device, taken at the steady state, showed that the bubble gets much closer to the heater end in the mixture case instead of being isolated from the heater by a thick liquid pool as in the pure pentane case. The proximity of the bubble to the heater wall led to more evaporation at the heater end in the mixture case, and therefore a higher heat transfer coefficient. The pressure profile calculated from the Young–Laplace equation supports the observations made from the surveillance images.

In recent years, electrospun polymer fibers have gained attention for various antibacterial applications. In this work, the effect of positively charged polymer fiber mats as antibacterial gauze is studied using electrospun poly(caprolactone) and polyaniline nanofibers. Chloroxylenol, an established anti-microbial agent is used for the first time as a secondary dopant to polyaniline during the electrospinning process to make the surface of the polyaniline fiber positively charged. Both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli are used to investigate the antibacterial activity of the positively charged and uncharged polymer surfaces. The results surprisingly show that the polyaniline surface can inhibit the growth of both bacteria even when chloroxylenol is used below its minimum inhibitory concentration. This study provides new insights allowing the better understanding of dopant-based, intrinsically conducting polymer surfaces for use as antibacterial fiber mats.

A charge transport model was previously developed in our group to predict intrinsic dielectric failure as a function of voltage for low-k SiCOH and high-k SiN, two materials commonly used in integrated circuits[1], [2]. The model incorporates a set of fundamental mechanisms, including electronical conduction and defect generation, resulting in breakdown when a critical defect density is reached. It replicated electrical conduction through dielectric materials and so can describe the entire history of current flow through the dielectric. Furthermore, a revised version of this model was recently proposed, and it overcame two limitations of the original model: the lack of thickness and temperature dependence. One issue recently investigated was the assumption that the effective velocity of tunneling electrons was the same as mobile electrons. These velocities are used to calculate the electron flux/current. New models separating the two velocities were developed and to fit the experimental data. These newer models offered slightly worse reliability predictions, and so the initial assumption remains not only simpler, but also more accurate so far. This model will be applied to predict the filament formation in resistive switching memory devices.

Flow boiling of HFE7000 in a new class of structured microchannel, termed Piranha Pin Fin ( PPF ), is studied. The novel heat sink consists of an array of 150 micron diameter microstructures entrenched in a 2.4 mm wide silicon channel, with overall device dimensions of 12 by 28 mm. Mass fluxes ranging from 1200 to 7000 kg/s/m 2 are used to dissipate base heat loads up to 700 W/cm 2 , provided over a 6 mm 2 heating element located beneath the PPF array. Experimental single phase and flow boiling heat transfer data is presented for two flow configurations within the microdevice — open flow and extraction flow . The effect of system pressure on phase change heat transfer is experimentally examined at operating pressures of 1.4 atm and 2.8 atm. Single and two phase heat transfer coefficients, Nusselt numbers, exit quality, and pressure drop data is presented to develop a comprehensive system-level analysis of the unique heat sink.

Low temperature sintering of metallic nanoparticles can be used to enhance the thermal properties of composite, thermal interface materials. Here, we present an approach to achieve the coalescence and sintering of a gold nanorod (AuNR) assembly in a block copolymer pattern by thermal annealing at low temperatures. Prior to thermal annealing in a conventional furnace or a rapid thermal processing unit, self-assembly of AuNRs is directed by the guidance of nanochannels in the block copolymer thin films and the removal of surfactants by a brief plasma treatment. Both furnace thermal annealing and rapid thermal annealing are employed to study the sintering behavior of the AuNR assembly. It is found that the sintering process initially takes place locally, resulting in small AuNR aggregates. Eventually the aggregates grow into a globally continuous, percolating network structure. The condensation heat transfer coefficient was measured in an environmental scanning electron microscope by following droplet growth over time and it is shown to be 3.7 times higher for AuNR composites.