Thermal Transport in Micro- and Nanoscale Systems

Small-scale (micro-/nanoscale) heat transfer has broad and exciting range of applications. Heat transfer at small scale quite naturally is influenced – sometimes dramatically – with high surface area-to-volume ratios. This in effect means that heat transfer in small-scale devices and systems is influenced by surface treatment and surface morphology. Importantly, interfacial dynamic effects are at least non-negligible, and there is a strong potential to engineer the performance of such devices using the progress in micro- and nanomanufacturing technologies. With this motivation, the emphasis here is on heat conduction and convection. The chapter starts with a broad introduction to Boltzmann transport equation which captures the physics of small-scale heat transport, while also outlining the differences between small-scale transport and classical macroscale heat transport. Among applications, examples are thermoelectric and thermal interface materials where micro- and nanofabrication have led to impressive figure of merits and thermal management performance. Basic of phonon transport and its manipulation through nanostructuring materials are discussed in detail. Small-scale single-phase convection and the crucial role it has played in developing the thermal management solutions for the next generation of electronics and energy-harvesting devices are discussed as the next topic. Features of microcooling platforms and physics of optimized thermal transport using microchannel manifold heat sinks are discussed in detail along with a discussion of how such systems also facilitate use of low-grade, waste heat from data centers and photovoltaic modules. Phase change process and their control using surface micro-/nanostructure are discussed next. Among the feature considered, the first are microscale heat pipes where capillary effects play an important role. Next the role of nanostructures in controlling nucleation and mobility of the discrete phase in two-phase processes, such as boiling, condensation, and icing is explained in great detail. Special emphasis is placed on the limitations of current surface and device manufacture technologies while also outlining the potential ways to overcome them. Lastly, the chapter is concluded with a summary and perspective on future trends and, more importantly, the opportunities for new research and applications in this exciting field

Master of Science

thesisAn experimental study was performed to explore the possibility of constructing a spiral microchannel heat sink using a laser-based xurographic technology with double-sided adhesive Kapton® tape, which has a low thermal conductivity. Xurography is a rapid prototyping micromanufacturing technology that, in contrast to expensive and time-consuming traditional microfabrication technologies, enables the fabrication of inexpensive microfluidics devices in a short time frame. A set of three xurographic spiral microchannel heat sinks with different channel length, width, hydraulic diameter, aspect ratio, and number of spirals were fabricated, tested and analyzed. For all test sections, channel depth was determined by the Kapton® tape film thickness, which is approximately 105µm. The heat sinks were experimentally tested at different flow rates and heat fluxes. Four sets of tests were performed on each heat sink. Distilled water, used as the working fluid, entered the test sections at room temperature (~22°C). The supplied heat ranged from 25 to 200 W, and the Reynolds number ranged from 200 to 1800. Results showed that the device with the widest channel (3 mm) extracts more heat than those with smaller widths and requires the least amount of driving pressure. The maximum heat dissipation rate for the devices was approximately 140 W, corresponding to a heat flux of approximately 10 W/cm². The maximum convection coefficient was on the order of 6500 W/m²/K and was achieved in the widest channel device with a corresponding Nusselt number of up to 2.2. Results indicate a thermal performance that is less than desirable. Performance was adversely affected by the low thermal conductivity Kapton® tape and no enhancement due to secondary flows in the spiral geometry was observed. Comparison with a well-known macroscale curved duct correlation revealed a significant difference between the experimental results and predications

A Permeable-Membrane Microchannel Heat Sink Made by Additive Manufacturing

Microchannel heat sinks are capable of removing dense heat loads from high-power electronic devices with low thermal resistance, but suffer from high pressure drops due to the small channel dimensions. Features that reduce the pressure drop, such as manifolds, increase fabrication complexity and are constrained by traditional subtractive manufacturing approaches. Additive manufacturing technologies offer improved design freedom and reduced geometric restrictions, expanding the types of features that can be produced and integrated into a heat sink. In this work, a novel permeable membrane microchannel (PMM) heat sink geometry is proposed and fabricated using direct metal laser sintering (DMLS) of an aluminum alloy (AlSi10Mg). In this PMM design, the cooling fluid is forced through thin, porous walls that act as both conducting fins and membranes that allow flow through their fine internal flow features for efficient heat exchange. The design leverages the ability of this fabrication process to incorporate complex, arbitrarily curved structures having internal porosity to enhance heat transfer and reduce pressure drop across the heat sink. The PMM heat sink geometry is benchmarked against a low-pressure-drop manifold microchannel (MMC) heat sink. A reduced-order model is used to explore the relative performance trends between the designs. Both heat sinks are experimentally characterized at flow rates of 50–500 mL/min using deionized water as the working fluid. At a constant pumping power of 0.018 W, the permeable membrane microchannel design offers both lower thermal resistance (17% reduction) and lower pressure drop (28% reduction) compared to the manifold microchannel heat sink

Microheated substrates for patterning cells and controlling development

Here, we seek to control cellular development by devising a means through which cells can be subjected to a microheated environment in standard culture conditions. Numerous techniques have been devised for controlling cellular function and development via manipulation of surface environmental cues at the micro- and nanoscale. It is well understood that temperature plays a significant role in the rate of cellular activities, migratory behavior (thermotaxis), and in some cases, protein expression. Yet, the effects and possible utilization of micrometer-scale temperature fields in cell cultures have not been explored. Toward this end, two types of thermally isolated microheated substrates were designed and fabricated, one with standard backside etching beneath a dielectric film and another with a combination of surface and bulk micromachining and backside etching. The substrates were characterized with infrared microscopy, finite element modeling, scanning electron microscopy, stylus profilometry, and electrothermal calibrations. Neuron culture studies were conducted on these substrates to 1) examine the feasibility of using a microheated environment to achieve patterned cell growth and 2) selectively accelerate neural development on regions less than 100$mu m$wide. Results show that attached neurons, grown on microheated regions set at 37$~^circ C$, extended processes substantially faster than those incubated at 25$~^circ C$on the same substrate. Further, unattached neurons were positioned precisely along the length of the heater filament (operating at 45$~^circ C$) using free convection currents. These preliminary findings indicate that microheated substrates may be used to direct cellular development spatially in a practical manner.$hfillhbox[1414]

A numerical study on the design trade-offs of a thin-film thermoelectric generator for large-area applications

Thin-film thermoelectric generators with a novel folding scheme are proposed for large-area, low energy-density applications. Both the electrical current and heat transfer are in the plane of the thermoelectric thin-film, yet the heat transfer is across the plane of the module - similar to conventional bulk thermoelectric modules. With such designs, the heat leakage through the module itself can be minimized and the available temperature gradient maximized. Different from the previously reported corrugated thermoelectric generators, the proposed folding scheme enables high packing densities without compromising the thermal contact area to the heat source and sink. The significance of various thermal transport, or leakage, mechanisms in relation to power production is demonstrated for different packing densities and thicknesses of the module under heat sink-limited conditions. It is shown that the power factor is more important than ZT for predicting the power output of such thin-film devices. As very thin thermoelectric films are employed with modest temperature gradients, high aspect-ratio elements are needed to meet the - usually ignored - requirements of practical applications for the current. With the design trade-offs considered, the proposed devices may enable the exploitation of thermoelectric energy harvesting in new - large-area - applications at reasonable cost.Comment: 26 pages,5 figures, post-peer-review, pre-copyedit version of an article published in Renewable Energ

Gas Flows in Microsystems

International audienc

Perspective Chapter: Smart Liquid Cooling Solutions for Advanced Microelectronic Systems

Thermal management is today a primary focus in the electronics industry due to the continuous increase of power density in chips increasingly smaller in size, which has become a critical issue in fast-growing industries such as data centers. As air-cooling fails to meet the high heat extraction demands of this sector, liquid cooling emerges as a promising alternative. Nevertheless, advanced microelectronic components require a cooling system that not only reduces the energetic consumption but also enhances the thermal performance by minimizing the thermal resistance and ensuring high-temperature uniformities, especially under variable heat load scenarios with high heat dissipating hotspot regions, where conventional liquid cooling solutions prove inefficient. This chapter provides an overview of different passive heat transfer enhancement techniques of micro heat sinks from the literature, focusing on intelligent and adaptive solutions designed to optimize the cooling performance based on local and instantaneous cooling requirements for non-uniform and time-dependent power distribution maps

Overview of Recent Trends in Microchannels for Heat Transfer and Thermal Management Applications

© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license. https://creativecommons.org/licenses/by/4.0/Distinctive recent research and experimental trends in microchannels for heat transfer and thermal management applications are investigated via a novel framework. The qualitative literature analysis was performed from four perspectives: materials, enhanced flow control, design, and sustainability (MEDS). The findings revealed that enhanced microchannel (MC) heat transfer performance (HTP) could be achieved by adding asymmetrical barriers, pin-fins, non-conventional geometries, mixed-wettability/biphilic surfaces, hybrid/silver nanofluids, and adopting innovative experimental and analysis methods. Additionally, researchers urged to focus on new microchannel designs and flow boiling/phase change-based experiments to understand the physics and different effects caused by various parameters. Furthermore, the qualitative analyses were transformed into quantitative results from the evaluated described methods and datasets, followed by a critical discussion of the findings. Finally, this article points out a set of promising future investigations and draws conclusions about current state-of-the-art. It is observed that, despite the decent progress made so far, microchannel-based applications still rely on traditional rectangular shapes, water-based working fluids, and numerical methods. Therefore, the role and focus on Industry 4.0 technologies to drive further innovations and sustainability in microchannel technologies are still in the early stages of adoption; this arguably acts as a barrier that prevents meeting current thermal and heat transfer needs.Peer reviewe

On-chip micro-evaporation: Experimental evaluation of liquid pumping and vapor compression cooling systems

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Thermal designers of data centers and server manufacturers are showing a great concern regarding the cooling of new generation data centers, which are more compact and dissipate more power than is currently possible to cool by conventional air conditioning systems. With very large data centers exceeding 100 000 servers, some consume more than 50 MW [1] of electrical energy to operate, energy which is directly converted to heat and then simply wasted as it is dissipated into the atmosphere. A potentially significantly better solution would be to make use of on-chip two-phase cooling [2], which, besides improving the cooling performance at the chip level, also adds the capability to reuse the waste heat in a convenient manner, since higher evaporating and condensing temperatures of the two-phase cooling system (from 60-95°C) are possible with such a new green cooling technology. In the present project, two such two-phase cooling cycles using micro-evaporation technology were experimentally evaluated with specific attention being paid to energy consumption, overall exergetic efficiency and controllability. The main difference between the two cooling cycles is the driver, where both a mini-compressor and a gear pump were considered. The former has the advantage due to its appeal of energy recovery since its exergy potential is higher and the waste heat is exported at a higher temperature for reuse.This study is supported by: the Swiss Commission for Technology and Innovation (CTI) contract number 6862.2; the LTCM laboratory; IBM Zürich Research Laboratory (Switzerland) and Embraco (Brazil)