Chialun Tsai

Dropwise condensation on superhydrophobic surfaces with two-tier roughness

Dropwise condensation can enhance heat transfer by an order of magnitude compared to film condensation. Superhydrophobicity appears ideal to promote continued dropwise condensation which requires rapid removal of condensate drops; however, such promotion has not been reported on engineered surfaces. This letter reports continuous dropwise condensation on a superhydrophobic surface with short carbon nanotubes deposited on micromachined posts, a two-tier texture mimicking lotus leaves. On such micro-/nanostructured surfaces, the condensate drops prefer the Cassie state which is thermodynamically more stable than the Wenzel state. With a hexadecanethiol coating, superhydrophobicity is retained during and after condensation and rapid drop removal is enabled.

An isolated tunable capacitor with a linear capacitance-voltage behavior

A unique tunable capacitor has been designed that facilitates a completely isolated capacitance and a truly linear capacitance-voltage behavior. Using a low-temperature adhesive bonding process and device layer transfer techniques, a linear analog tunable capacitor has been fabricated and tested. The device shows a linear capacitance-voltage behavior using a +-10 V input voltage and tunes over a 1.78 to 3.88 pf range. The isolated design also allows greater flexibility in circuit design because the capacitor is not required to be a capacitor to ground.

A high Q, large tuning range, tunable capacitor for RF applications

Using a new, double-sided adhesive process, an analog tunable capacitor has been designed and fabricated with an extremely large tuning range and a high Q. New design components such as two-sided metal deposition, low resistivity silicon, thicker device layers, and double beam suspensions have improved RF performance drastically. In the 200-400 MHz range that this device is intended for, Q values are in excess of 100. In addition, an 8.4 to 1 tuning ratio has been achieved with continuous tuning over a 1.4 to 11.9 pF range. To further improve dynamic performance, devices were operated in a high viscosity gas environment and near critical damping was achieved.

Studies of material and process compatibility in developing compact silicon vapor chambers

The performance and long-term reliability of a silicon vapor chamber (SVC) developed for thermal management of high-power electronics critically depend on compatibility of the component materials. A hermetically sealed SVC presented in this paper is composed of bulk silicon, glass-frit as a bonding agent, lead/tin solder as an interface sealant and a copper charging tube. These materials, in the presence of a water/vapor environment, may chemically react and release noncondensable gas (NCG), which can weaken structural strength and degrade the heat transfer performance with time. The present work reports detailed studies on chemical compatibility of the components and potential solutions to avoid the resulting thermal performance degradation. Silicon surface oxidation and purification of operating liquid are necessary steps to reduce performance degradation in the transient period. A lead-based solder with its low reflow temperature is found to be electrochemically stable in water/vapor environment. High glazing temperature solidifies molecular bonding in glass-frit and mitigates PbO precipitation. Numerous liquid flushes guarantee removal of chemical residual after the charging tube is soldered to SVC. With these improvements on the SVC material and process compatibility, high effective thermal conductivity and steady heat transfer performance are obtained.

Investigations of High Heat Flux Cooling Using Carbon Nanotube Bi-Wick Structure and Integrated Platinum Thermometer/Heater

With the increase of power consumption in compact electronic devices, passive heat transfer cooling technologies with high heat flux characteristics are highly desired in microelectronics industries. Carbon nanotube (CNT) cluster/forest has high effective thermal conductivity, nano pore size and large porosity, which can be used as wick structure in a heat pipe heatspreader and provides high capillary force for high heat flux thermal management. In this research, investigations of high heat flux cooling of the CNT bi-wick structure are associated with the development of a reliable thermometer and high performance/interface free heater. A 100nm thick and 600μm wide Z-shaped platinum wire resistor is fabricated on the backside of a CNT sample substrate to heat a 2×2mm2 wick area. As a heater, it provides direct heating effect without thermal interface and is capable of over 800°C high temperature operation. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation with temperature after the annealing process is applied. The CNT sample substrate is silicon. The backside of the silicon substrate is thermally oxidized to create a 2μm thick and pinhole-free SiO2 layer so that the platinum heater and thermometer can survive from the server CNT growth environments and without any electrical leakage. For high heat flux cooling, the CNT bi-wick structure is composed of 250μm tall, 100μm wide stripe-like CNT clusters and 50μm empty space. Using 1×1cm2 CNT bi-wick samples, experiments are completed in both the open and saturated environments. Testing results of CNT bi-wick structure demonstrate 600W/cm2 heat transfer capacity and good thermal & mass transport characteristics in the nano level porous media.© 2009 ASME

Micromachined stack component for miniature thermoacoustic refrigerator

This paper reports a novel miniature MEMS based thermoacoustic refrigerator design for thermal management of electronic and optoelectronic devices. This technique utilizes high-frequency acoustic energy to provide the heat pumping effect. The goal of the Rockwell Scientific-led HERETIC (Heat Removal by Thermo-Integrated Circuits) project is the development and demonstration of a miniaturized refrigeration device based on the thermoacoustic refrigeration principle. Utilizing MEMS technology such as high aspect ratio through wafer etching, bonding and coating techniques, three to four mm thick bonded MEM-TAR (Thermoacoustic Refrigerator) stacks with only 10 to 15 micron wide fine patterns were demonstrated. With our current design, numerical models predict device capability of 1 W heat transport at 20/spl deg/C below ambient when pressurized with 10 atm of He/Ar gas mixture. Preliminary results using 1 atm air achieve as high as 10 degrees of stable cooling below ambient.

A miniaturized micro‐machined thermoacoustic cooler

A thermoacoustic cooler of miniature scale with a micro‐machined stack has been numerically modeled and experimentally realized. deltae and Fluent were used to perform the numerical modeling and simulation. The acoustic pressure, velocity, and temperature profiles in the stack and exchangers were simulated. The stack is manufactured using MEMS technology for precise dimension control and flexible geometry design. The MEMS stack as thick as 4.2 mm with high aspect ratio (1/50) thin rib structures, 10 to 15 μm, have been realized. Commercially available PZT drivers were initially used to deliver acoustic power into the resonator at around 4 kHz. A more powerful acoustic driver design using PLZT is under development. The simulation result, device performance, and system integration will be reported. [Work supported by DARPA.]

Fluid Mixing in Micro Scale Channel Patterned Hydrophobic/Hydrophilic Surface

Micro scale fluid control or mixing is critical for chemistry and life sciences. Successful performance of on-chip biochemical analysis processes, such as DNA hybridization and PCR amplification, highly depend on rapid mixing of multiple fluid species. In this paper, a set of initial designs is developed for flow mixing. In micro channels with 100 and 200μm width, alternating regions of hydrophobic/hydrophilic surface are created on silicon surfaces by photolithography and dry etch techniques. Experimental results show that in the micro channels with 20mm length, effective mixing is observed on the device patterned by incline hydrophobic/hydrophilic grilles in which eddy diffusion mixes two liquids. In contrasts, slight mixing is caused by the development of liquid instability induced by alternating hydrophobic/hydrophilic patterns orthogonal to the flow direction.© 2006 ASME