Showing papers in "Microscale Thermophysical Engineering in 1999"
TL;DR: In this paper, a general boundary condition that accounts for the reduced momentum and heat exchange with wall surfaces is proposed and its validity is investigated and it is shown that it is applicable in the entire Knudsen range and is second-order accurate in Kn in the slip flow regime.
Abstract: Rarefied gas flows in channels, pipes, and ducts with smooth surfaces are studied in a wide range of Knudsen number (Kn) at low Mach number (M) with the objective of developing simple, physics-based models. Such flows are encountered in microelectromechanical systems (MEMS), in nanotechnology applications, and in low-pressure environments. A new general boundary condition that accounts for the reduced momentum and heat exchange with wall surfaces is proposed and its validity is investigated. It is shown that it is applicable in the entire Knudsen range and is second-order accurate in Kn in the slip flow regime. Based on this boundary condition, a universal scaling for the velocity profile is obtained, which is used to develop a unified model predicting mass flow rate and pressure distribution with reasonable accuracy for channel, pipe, and duct flows in the regime (0 Kn). A rarefaction coefficient is introduced into this two-parameter model to account for the increasingly reduced intermolecular collisions...
1,106Â citations
TL;DR: In this paper, it was shown that the thermoelectric performance of any 3D material should show an enhanced performance when prepared as a 2D multi-quantum well superlattice, utilizing the enhanced density of states at the onset of each electronic subband, and the increased scattering of vibrational waves at the boundary between the quantum well and the adjacent barrier of the superlatinice.
Abstract: The study of thermoelectric materials has recently been revived as an active research field, in part due to the recent demonstration of enhancement in the thermoelectric figure of merit of a two-dimensional (2D) PbTe quantum well system, relative to its three-dimensional (3D) bulk counterpart. Calculations suggest that the thermoelectric performance of any 3D material should show an enhanced thermoelectric figure of merit, when prepared as a 2D multi-quantum well superlattice, utilizing the enhanced density of states at the onset of each electronic subband, and the increased scattering of vibrational waves at the boundary between the quantum well and the adjacent barrier of the superlattice. In principle, low dimensionality also allows certain materials such as bismuth, which are poor thermoelectrics in 3D, to become good thermoelectrics. Thus, the successful fabrication of 1D bismuth nanowires offers new possibilities for the study of 1D systems for possible thermoelectric applications.
110Â citations
TL;DR: In this paper, a simple model for the thermal runaway effect in doped silicon cantilevers was developed, which relates the electrical conductivity in the cantilever to the temperature-dependent carrier concentrations in silicon and is consistent with the available experimental data.
Abstract: A variety of micromachined sensors and actuators use coupled electrical and thermal transport in doped silicon bridges and cantilevers. One example is thermomechanical data storage cantilevers, in which Joule heating and atomic-scale forces yield indentations in an organic substrate. The thermal isolation of these structures augments the temperature rise during Joule heating, which can generate more intrinsic carriers and lead to thermal runaway in the presence of a constant bias voltage. This article develops a simple model for the thermal runaway effect in doped silicon cantilevers. The model relates the electrical conductivity in the cantilever to the temperature-dependent carrier concentrations in silicon and is consistent with the available experimental data.
73Â citations
TL;DR: In this paper, thermal energy transport in a micro counterflow heat exchanger with a numerical model that includes axial conduction is examined, and an analysis that permits end-wall temperature gradients to be determined, thus allowing the conduction heat transfer from the heat exchangers to be assessed.
Abstract: This article examines thermal energy transport in a micro counterflow heat exchanger with a numerical model that includes axial conduction. The unique aspect of this work is an analysis that permits end-wall temperature gradients to be determined, thus allowing the conduction heat transfer from the heat exchanger to be assessed. Since there are two unknown initial conditions out of four needed in the set of equations governing the temperature distributions, a two-value shooting approach is needed to solve the problem. Conduction losses are combined with nonunity effectiveness losses to obtain a total normalized heat loss for the system. The results of the study demonstrate the need for using very low thermal conductivity material in the construction of micro counterflow heat exchangers in order to achieve reasonable performance in small devices.
62Â citations
TL;DR: In this paper, the authors explored the application of the Marangoni effect in micro systems involving small gas or vapor bubbles in a liquid environment subjected to a temperature gradient and found that the optimum bubble size for these applications is expected to be of the order of 10 mu m.
Abstract: This work explores the application of the Marangoni effect in micro systems involving small gas or vapor bubbles in a liquid environment subjected to a temperature gradient. The Marangoni effect characterizes the variation of surface tension along the bubble surface resulting from the temperature gradient around the bubble, thus driving the bubble toward the higher temperature region. This phenomenon is more pronounced as the bubble becomes smaller and the temperature gradient becomes steeper, both of which can be achieved in microbubble systems. Potential applications based on the Marangoni effect include linear bubble actuators, dynamic microvalves, and hot-spot locators. The optimum bubble size for these applications is expected to be of the order of 10 mu m. A smaller bubble may be difficult to introduce into the working system and maintain its size. Presented for illustration is a feasibility analysis for both a noncondensable gas bubble and a vapor bubble situated above a microheater. The analysis y...
42Â citations
TL;DR: The recent progress in microscale thermophysical engineering as seen in fast-developing areas such as thermal transport phenomena, experimental and computational techniques, thermal microdevices and laser applications is reviewed in this article.
Abstract: The past decade has registered a rapid expansion of research effort focused on microscale thermophysical engineering. The increased technological demand for the miniaturization of devices requires a comprehensive understanding of the fundamental phenomena that govern thermal transport at short length and time scales. A substantial amount of microscale research has been devoted to microelectromechanical systems (MEMS) and ensuing design challenges. Improved fabrication, processing, and measurement and analysis tools have been critical to new technological achievements. This paper reviews the recent progress in microscale thermophysical engineering as seen in fast-developing areas such as thermal transport phenomena, experimental and computational techniques, thermal microdevices and laser applications. Promising research directions in these areas are also highlighted.
40Â citations
TL;DR: In this paper, a study of microfabricated enhanced structures for enhancing the thermal performance of a two-phase thermosyphon loop is presented. And the results show that the heat dissipation with these structures is at least three times better than that of a plain polished silicon surface, at a wall superheat of 30 degrees C.
Abstract: This report presents a study of microfabricated enhanced structures for enhancing the thermal performance of a two-phase thermosyphon loop. The structures used in this study have an array of channels and pores that make it highly porous. The structures were fabricated in silicon using wafer dicing, wet-chemical etching, and laser milling, and details are included in this report. The structures were employed inside the evaporator section of a thermosyphon loop for enhancing the boiling performance. Preliminary results show that the heat dissipation with these structures is at least three times better than that of a plain polished silicon surface, at a wall superheat of 30 degrees C. The results also indicate that increasing the channel width resulted in better thermal performance.
30Â citations
TL;DR: In this article, the longitudinal acoustic velocity in silica aerogel is presented as a function of the interstitial gas type and pressure, which is measured using air-coupled ultrasonic transducers configured for differential pulse transit time measurements.
Abstract: The longitudinal acoustic velocity in silica aerogel is presented as a function of the interstitial gas type and pressure. This was measured using air-coupled ultrasonic transducers configured for differential pulse transit time measurements. The results are interpreted in terms of the thermal relaxation of the acoustic pulse. The microscale temperature oscillations of the gas and solid phases of the aerogel due to the acoustic pulse are not identical if the rate of heat transfer between the two phases is slow compared to the period of the acoustic oscillation. The energy transferred from the gas to the solid phase is lost to the acoustic propagation and, thus, reduces the amplitude and velocity of the acoustic wave. The gas type and pressure may provide independent variables for probing these effects in aerogel.
24Â citations
TL;DR: In this paper, the concept, design, and experimental characterization of a novel micropump based on the temperature dependence of liquid viscosity is described, where the difference in flow resistance generated by local heating of the liquid in narrow channels is used for rectification of the flow.
Abstract: The concept, design, and experimental characterization of a novel micropump based on the temperature dependence of liquid viscosity is described. Since the difference in flow resistance generated by local heating of the liquid in narrow channels is used for rectification of the flow, the pump is free from mechanically movable valves while attaining high controllability. Small size is important in making this principle work effectively, as well as reducing the complexity of thermal and fluidic design. An experiment using a prototype device fabricated by silicon-based micromachining demonstrated the linear relationship between flow rate and the duration of heating, and the capability of bi-directional pumping.
16Â citations
TL;DR: In this article, the atomic force microscope (AFM) was used to obtain molecular-level images of the structure and dynamics of ice crystals formed from vapor phase on a mica surface.
Abstract: The atomic force microscope (AFM) was used to obtain molecular-level images of the structure and dynamics of ice crystals formed from vapor phase on a mica surface. The images show epitaxial growth of ice to form monolayers. Under contact force from the AFM tip, motion of these monolayers was observed. One of the interesting observations made in this study is that the adsorbed water layer freezes to form a polycrystalline structure consisting of a single monolayer. Detailed imaging of the grain boundaries shows highly disordered percolation structure with short-range alignment along crystallographic directions. It is proposed that the percolation grain boundaries are formed because of phase segregation during freezing of an ionic solution in the water monolayers.
13Â citations
TL;DR: In this article, the authors presented the theory and experimental results of an optical imaging technique that simultaneously measured the deflections of a focal plane array of bimaterial microcantilevers that are used as thermomechanical infrared sensors.
Abstract: This article presents the theory and experimental results of an optical imaging technique that simultaneously measures the deflections of a focal plane array of bimaterial microcantilevers that are used as thermomechanical infrared sensors. Based on Fourier optics, this technique is used for infrared vision of room-temperature objects with a noise-equivalent temperature difference (NETD) in the range of 2-5 K. Efforts are currently underway to improve the NETD to the range of 30-50 mK.
TL;DR: In this article, a new evanescent microwave method that is capable of mapping temperature distributions with ~ 1-micrometre spatial resolution was presented, and the temperature sensitivity was better than 0.19 V/degree C with a minimum detectable signal of 0.01 degree C and response time of faster than 1 microsecond.
Abstract: Very high spatial-resolution thermography is of great importance in electronics, biology, and in many other situations where local variations in temperature are needed to study heat dissipation or to monitor metabolism rate, which can be related directly to heat production. Infrared imaging techniques probably are the best way to obtain thermal maps of large structures. However, the spatial resolution of infrared imaging techniques is limited to a few 100 micrometres, and their temperature resolution is usually around 1 degree C. Here we report on a new evanescent microwave method that is capable of mapping temperature distributions with ~ 1-micrometre spatial resolution. The temperature sensitivity of this probe was better than 0.19 V/degree C with a minimum detectable signal of 0.01 degree C and a response time of faster than 1 microsecond.
TL;DR: In this paper, molecular dynamics simulations were carried out under nonequilibrium conditions to investigate the dynamic process of the absorption of water vapor into aqueous electrolyte solution, and the absorption rate was calculated and some molecular pictures were reported.
Abstract: In order to investigate the dynamic process of the absorption of water vapor into aqueous electrolyte solution, molecular dynamics simulations were carried out under nonequilibrium conditions. The two boxes at liquid-vapor equilibrium at different saturation pressure are coupled. One box contains water and the other box contains aqueous electrolyte solution. The variables are the temperature of the water, the temperature and concentration of the aqueous electrolyte solution, and the pressure difference between the two boxes. The absorption rate was calculated and some molecular pictures are reported. On the molecular scale, ions are negatively adsorbed at the surface, particularly in weak aqueous electrolyte solution, and highly concentrated aqueous electrolyte solution is effectively ''frozen'' and cannot rapidly adjust to the incident water vapor molecules. These structural and dynamical properties affect the absorption rate.
TL;DR: In this article, the absorptance of high-temperature superconducting YBa2Cu3O7-delta (YBCO) films, deposited on Si substrates, in the far infrared from 15 to 95 cm - 1 (wavelength from 667 to 105 mu m) at temperatures of 100, 200, and 300 K (i.e., in the normal state).
Abstract: This work presents the absorptance of high-temperature superconducting YBa2Cu3O7-delta (YBCO) films, deposited on Si substrates, in the far infrared from 15 to 95 cm - 1 (wavelength from 667 to 105 mu m) at temperatures of 100, 200, and 300 K (i.e., in the normal state). Our experiments show a significant difference in the absorptance for radiation incident on the film side as compared to radiation incident on the substrate side. Interference fringes associated with the Si substrate are observed from the measurement and used to analyze the interaction of radiation with the film - substrate composite at the interface. The film thickness is found to have a strong effect on the absorptance of the film - substrate composite, especially for radiation incident on the substrate side.
TL;DR: In this paper, a dynamic model for conjugate heat and mass transfer in a microchannel adsorption reactor is developed based on transient, two-dimensional, and compressible Navier-Stokes equations of motion as the governing conservation equations.
Abstract: A dynamic model for conjugate heat and mass transfer in a microchannel adsorption reactor is developed. The model is based on transient, two-dimensional, and compressible Navier-Stokes equations of motion as the governing conservation equations. Appropriate boundary conditions for the momentum, heat, and mass transfer at the channel wall in the presence of adsorption for the no-slip and slip flows are formulated and incorporated into the generalized single-equation-based framework for solving conjugate problems. The 500- mu m-long parallel-plate channel with spacing between walls 10 mu m and a wall thickness 2 mu m is considered as a prototype of the unit cell of the adsorption microreactor. Air is taken a carrier gas and water vapor as an adsorbable species. The flow conditions are characterized by the Reynolds and Knudsen numbers equal to 130x10-2 and 6.5x10-3, respectively. The Freundlich adsorption isotherm is utilized to specify the adsorption desorption equilibrium. The theoretical model developed i...
TL;DR: In this article, a polysilicon surface-micromachined thermal actuator-based vibromotor has been designed and fabricated, and a traveling speed of 3 mm s has been demonstrated with an AC input voltage of 2.0 V at 5 kHz plus a DC bias offset.
Abstract: A compact polysilicon surface-micromachined thermal actuator-based vibromotor has been designed and fabricated. Mechanical power transmission occurs during the impact of the thermal actuators on the sides of a movable guided element (slider.) Bi-directional operation is made possible through the impact head design. The thermal actuators have been driven at frequencies up to 10 kHz. A traveling speed of 3 mm s has been demonstrated with an AC input voltage of 2.0 V at 5 kHz plus a 10.0-V DC bias offset. A polysilicon hinged vertical micromirror has been moved for a full range of 350 mu m.
TL;DR: In this article, the time required to evaporate liquid out of a feature of given depth and relatively high aspect ratio is described in a non-dimensional form, provided that the temporal variation of the vapor pressure at the top of the feature is known.
Abstract: Liquid-phase processing is commonplace in semiconductor manufacturing. The present investigation focuses on evaporative drying of features such as trenches and vias. The time required to evaporate liquid out of a feature of given depth and relatively high aspect ratio is described in a nondimensional form. Provided that the temporal variation of the vapor pressure at the top of the feature is known, this description yields an estimate of the time required to evaporate liquid out of a feature of a given depth. The method developed is used to estimate the evaporation time for two realistic situations.
TL;DR: In this paper, a titanium target was ablated by a KrF excimer laser with fluences varying from 4 to 8 J/cm2 in an argon-filled environment with pressures ranging from vacuum to 1 torr.
Abstract: A titanium target was ablated by a KrF excimer laser with fluences varying from 4 to 8 J/cm2 in an argon-filled environment with pressures ranging from vacuum to 1 torr. The effects of laser fluence and background gas pressure on the kinetic energies of the ablated species were investigated by temporally and spatially resolved emission spectroscopy. The maximum surface temperatures were calculated by an one-dimensional conduction model. Experimentally obtained surface temperatures from the kinetic energy of the ejected plume were one order of magnitude higher than the calculated temperatures. This discrepancy is most likely due to the absorption of laser energy by the plasma that is formed early in the pulse. Temporally resolved imaging with 10-ns gate width was also employed to reveal the evolution of the ablated plume against the background gas. Separation of slower and faster components were observed for pressures above 50 mtorr, and angular concentration of titanium in the plume was determined.