Gunn Hwang

Experimental study on the thermal performance of micro-heat pipe with cross-section of polygon

Abstract The electronic industries have been tried to develop the electronic packaging technology for high-density, light weight, and miniaturization of mobile personal information terminals such as notebook PC, personal digital assistant (PDA), or handheld PC as the performance and the portability of the system have been become important more and more. The generated heat from the CPU of the electronic system should be dissipated effectively for the stability in operation and the long life-time of the system. In the present study, micro-heat pipes (MHPs) with cross-section of polygon have been manufactured and tested for operating characteristics and heat transfer limit. An additional process for installing a wick that is an inevitable component of heat pipes is not required for the MHP. For a small-sized heat pipe in the present study, a high precision technology is needed in manufacturing process. The MHPs tested in this paper have a triangular cross-section with curved sides and a rectangular cross-section with curved sides. The material of the MHP is copper and the working fluid of it is pure water. The test was performed in a vacuum chamber to minimize heat loss. The operating temperatures of the MHP were considered from 60 to 90 °C. From the test results, the MHP with triangular cross-section can dissipate up to a thermal load of 7 W.

Improving thermal performance of miniature heat pipe for notebook PC cooling

Abstract A miniature heat pipe (MHP) with woven wire wick was used for cooling a notebook PC. The cross-sectional area of the pipe is reduced by about 30% of the original, when the diameter of the MHP is pressed from 4 to 2 mm for packaging in a notebook PC. In the present study, a test of the MHP has been performed in order to review the thermal performance by varying pressed thickness, total length of MHP, wall thickness, heat flux and inclination angle. New wick types were considered for overcoming low heat transfer limits, which occur when the MHP is pressed to a thin plate. Through a performance test, the limiting thickness of pressing is shown to be within the range of 2–2.5 mm. When the wall thickness of 0.4 mm is reduced to 0.25 mm for minimizing conductive thermal resistance through the wall of heat pipe, the heat transfer limit and thermal resistance of the MHP were improved about 10%. While the thermal resistance of the MHP with central wick type is lower than that of the MHP with circular oven wire wick, the thermal resistance of the MHP with composite wick of woven/straight wire is higher than that of the MHP with circular woven wire wick. From the performance test conducted on the MHP cooling modules with woven wicks, it is seen that the T jc (junction temperature of the processor) satisfies a demand condition of being between 0 and 100 ° C. This shows the stability of the MHP as a cooling system of notebook PCs.

Fabrication of morphological defect-free vertical electrodes using a (1 1 0) silicon-on-patterned-insulator process for micromachined capacitive inclinometers

This paper presents a novel fabrication method of scalloping-free and footing-free vertical electrodes for micromachined capacitive inclinometers with a high sensing resolution. The proposed fabrication method is based on additional crystalline wet etching of a (1 1 0) silicon that is bonded to a silicon substrate with a patterned insulator layer. The sensing electrodes, which are aligned to the (1 1 1) plane, have very smooth sidewalls because the morphological defects formed by the silicon deep reactive ion etching (DRIE) process are drastically reduced in the crystalline wet etching. The fabricated capacitive inclinometer with smooth sensing electrodes was evaluated in terms of capacitance change and resolution. The capacitance of the fabricated inclinometer is changed from −0.246 to 0.258 pF for the inclination angle (−90° to 90°). The temporal deviation of the capacitance is as small as 0.2 fF, which leads to a high resolution of 0.1° or less for ±45°.

Experimental study on the performance of miniature heat pipes with woven-wire wick

The thermal density of electronic system has been increased continuously because high speed and high density are required for them. The heat dissipation of CPU for a notebook PC has been recently increased to be more than 10 W, but, on the other hand, the available packaging space has been decreased. Therefore, it has become inevitable to perform cooling by using miniature heat pipes (MHPs). In the present study, a new woven-wire-type wick for the MHP is developed, which has a large capillary limit and a high productivity. These MHPs with diameters of 3 mm or 4 mm are applicable to small-sized electronic parts such as CPU of a notebook PC. Because the operational characteristics of MHPs with the diameters of 3 mm or 4 mm are different from those of general medium-size heat pipes, performance tests have been conducted in order to review heat-transfer characteristics and effects of various factors on the performance of MHPs. The design factors under consideration are fill ratio of working fluid, length of heat pipe, length of evaporator and condenser, inclination angle of installation, number of wick strand and thermal load.

Biocompatible wireless power transferring based on ultrasonic resonance devices

To increase application area of implantable devices for medical treatment including implantable cardiac defibrillator or deep brain stimulator, wireless power transfer is highly required. Previous technologies such as magnetic resonance and induction coupling have limited applications because of short transfer distance compared to device size and magnetic field intensity limitation for the safety of body exposure. The biocompatible wireless power transferring technology is proposed using ultrasonic resonance method. In power transfer through tissue, since the distance between transmitter and receiver is not adjustable, the optimum power transfer condition is changed. For efficient power transferring, operating frequency should be adjusted based on acoustic radiation and spatial pressure distribution. The ultrasonic resonance transmitter and receiver are manufactured with 50mm diameter and 250 kHz resonance frequency. In case of water medium, since it is easy to adjust frequency at fixed distance, it bring...

Development of the micro capillary pumped loop for electronic cooling

Electronic devices have been minimized but the performance of those is becoming better and better. Therefore it is needed to develop new cooling methods suitable for a thin packaging structure with high thermal density. The thin flat plate type micro CPL(capillary pumped loop) with the thickness less than 2 mm was developed in this study. The proposed micro CPL has two staged grooves in evaporator instead of poles for preventing backflows of the vapor bubble and the simpler structure than that of a micro CPL with the poles. Also a large vapor space from the evaporator to the condenser was constructed in the middle plate therefore flow resistance of the vapor could be reduced. The micro CPL was fabricated using MEMS technology. The micro CPL was composed of lower, middle and upper substrates. The lower substrate was made of silicon and the middle and upper substrates are made of Pyrex glass for visualization. Through a preliminary test it was checked that there was no leakage at the adhesion interface between lower and middle or upper substrates and at the bonding interface between lower substrate and fill tube. Although the experimental studies for the micro CPL have been poor till now, we have obtained the reasonable experimental results in this study. The performance test result has showed 8.5 W of the heat transfer rate for the micro CPL and we could observe the operating characteristics of circulating or evaporating and condensing by visualization. Pure distilled water was used as the working fluid.

Resonant ultrasonic wireless power transmission for bio-implants

In this paper, we present the ultrasonic wireless power transmission system as part of a brain-machine interface (BMI) system in development to supply the required electric power. Making a small-size implantable BMI, it is essential to design a low power unit with a rechargeable battery. The ultrasonic power transmission system has two piezoelectric transducers, facing each other between skin tissues converting electrical energy to mechanical vibrational energy or vice versa. Ultrasound is free from the electromagnetic coupling effect and medical frequency band limitations which making it a promising candidate for implantable purposes. In this paper, we present the design of piezoelectric composite transducer, the rectifier circuit, and rechargeable battery that all packaged in biocompatible titanium can. An initial prototype device was built for demonstration purpose. The early experimental results demonstrate the prototype device can reach 50% of energy transmission efficiency in a water medium at 20mm distance and 18% in animal skin tissue at 18mm distance, respectively.

Modeling and Thermal Optimization of a Micro Heat Pipe With Curved Triangular Grooves

Heat transfer and fluid flow characteristics in a micro heat pipe with curved triangular grooves are investigated using numerical and experimental methods. In the numerical part, a one-dimensional mathematical model for micro heat pipe with curved triangular grooves is developed and solved to obtain the maximum heat transport rate, the capillary radius distribution, the liquid and the vapor pressure distributions along the axial direction of the micro heat pipe under the steady-state condition. In particular, the modified Shah method is applied to calculate the pressure drop induced by the liquid-vapor interfacial shear stress. Experiments are conducted to validate the numerical model. In the experiments, the micro heat pipe with 0.56 mm in hydraulic diameter and 50 mm in length is tested. The experimental results for the maximum heat transport rate agree well with those of the numerical investigations. Finally, thermal optimization of the micro heat pipe with curved triangular grooves is performed using the numerical model.Copyright

Novel Micro Capacitive Inclinometer with Oblique Comb Electrode and Suspension Spring Aligned Parallel to {111} Vertical Planes of (110) Silicon

A novel high resolution micro capacitive inclinometer has been developed using (110) silicon. KOH crystalline wet etching was employed after silicon deep reactive ion etching (DRIE) to reduce morphologic defects on the sidewalls of oblique comb electrodes aligned parallel to vertical {111} plane. Suspension springs are also parallel to other vertical {111} plane to secure the width during KOH wet etching. The sensitivity (pF/°) was increased because the oblique comb electrodes change both the overlapped area and gap during operation. The capacitance changed from -0.8 to 0.8 pF for -90° -90° and resolution was estimated at 0.18° or less for ±80°.

Design, Fabrication, and Characterization of a Readout Integrated Circuit (ROIC) for Capacitive MEMS Sensors

A novel readout integrated circuit (ROIC) with tunable sensitivity, variable resolution, and especially multiple and selectable input ranges is designed, fabricated, and characterized for various capacitive MEMS sensors. The ROIC was fabricated by using 0.35 mum 3.3 V CMOS process and consisted of a capacitance-to-time (C-T) converter, a high speed signal counter, a phase-locked loop (PLL), control logics and synchronous peripheral interfaces (SPI). By using SPI, the clock frequency (fCLK) of the ROIC can be changed from 10 MHz to 160 MHz, which results in resolutions up to 14 bit. The measured minimum detectable capacitance of the ROIC were 2.86 fF for fCLK = 10 MHz and 180 aF for fCLK = 160 MHz, which were equivalent to 9 bit and 14 bit resolutions, respectively.