Dongzhi Guo

Electrocaloric characterization of a poly(vinylidene fluoride- trifluoroethylene-chlorofluoroethylene) terpolymer by infrared imaging

The electrocaloric effect in thin films of a poly(vinylidene fluoride-trifluoroethylene chlorofluoroethylene) terpolymer (62.6/29.4/8 mol. %, 11–12 μm thick) is directly measured by infrared imaging at ambient conditions. The adiabatic temperature change is estimated to be 5.2 K for an applied electric field of 90 V/μm. The temperature change is independent of the operating frequency in the range of 0.03–0.3 Hz and is stable over a testing period of 30 min. Application of this terpolymer is promising for micro-scale refrigeration.

Material Characterization and Transfer of Large-Area Ultra-Thin Polydimethylsiloxane Membranes

Fabrication of ultra-thin (1-20-μm thickness) polydimethylsiloxane (PDMS) films is enabled with a hexane dilution process and an underlying gelatin release layer. The release and transfer of these films over large areas (>5 cm) allows measurement of the thickness-dependent and process-dependent mechanical properties of ultra-thin PDMS membranes, reported for the first time. The effective Youngs modulus of 1-μm-thick PDMS, measured by bulge testing, is approximately ten times larger than that of 0.5-mm-thick material, following a continuous power-law relationship over the entire thickness range. Mesh-patterned metal electrodes of 2-μm minimum feature size are embedded in selected membranes. Metal evaporation and subsequent reactive ion etch patterning on PDMS increases its Youngs modulus due to the increase in cross-link formation and hardening of the surface. The results are meaningful in design and fabrication of soft electronics, microsensors, microvalves, and micropumps.

Design and Evaluation of a MEMS-Based Stirling Microcooler

A new Stirling microrefrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated. The cooling elements are to be fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Air at a pressure of 2 bar is the working fluid and is sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. Parametric study of the design shows the effects of phase lag between the hot space and cold space, swept volume ratio between the hot space and cold space, and dead volume ratio on the cooling power. Losses due to regenerator nonidealities are estimated and the effects of the operating frequency and the regenerator porosity on the cooler performance are explored. The optimal porosity for the best system coefficient of performance (COP) is identified.

Release and transfer of large-area ultra-thin PDMS

This paper reports on the fabrication of ultra-thin (~10 μm) polydimethylsiloxane (PDMS) films with embedded metal electrodes of 2 μm minimum feature size, as well as the release and transfer of large area films (>5 cm). The initial motivation for this work is the development of a miniature pump actuator for moving working fluid in an electrocaloric microcooler. PDMS diaphragms with electrodes are released and transferred onto contoured silicon chambers formed by gray-scale lithography and deep-reactive ion etching.

Large stroke electrostatic actuated PDMS-on-silicon micro-pump

We introduce a large stroke electrostatic micro-pump made with a thin PDMS diaphragm embedded with thin-film metal electrodes and bonded over a smoothly shaped Si substrate that acts as the counter electrode. The thin PDMS layer creates a highly compliant diaphragm. The spline-shaped substrate height significantly reduces electrostatic actuation voltage, and is refined in its smoothness by a modification to conventional grayscale lithography for Si MEMS. Combining the effects of the compliant diaphragm and the “zipper” electrode gap, a diaphragm displacement of 100 μm is achieved, giving a displacement volume of 1 μL/stroke and a pumping rate of 60 μL/min at 1 Hz.

Design of a Fluid-Based Micro-Scale Electrocaloric Refrigeration System

The electrocaloric effect (ECE) is a phenomenon in which reversible temperature and entropy changes of a material due to polarization appear under the application and removal of an electric field. Materials with a giant ECE have recently been reported, suggesting practical application in cooling devices. In this paper, a refrigeration system composed of silicon MEMS cooling elements is designed based on the ECE in a terpolymer. Finite element simulations are performed to explore the system performance. The effect of the form of the applied electric field is studied. The time lag between the electric field and the diaphragm motion is found to affect the cooling power significantly. A parametric study of the operating frequency is also conducted. The results indicate that when the system is operated at a temperature difference of 5 K, a cooling power density of 2 W/cm2 is achieved for one element.Copyright

Numerical Modeling of a Micro-Scale Stirling Cooler

Micro-scale coolers have a wide range of potential application areas, such as cooling for chip- and board-level electronics, sensors and radio frequency systems. Miniature devices operating on the Stirling cycle are an attractive potential choice due to the high efficiencies realized for macroscale Stirling machines. A new micro-scale Stirling cooler system composed of arrays of silicon MEMS cooling elements has been designed. In this paper, we use computational tools to analyze the porosity-dependence of the pressure and heat transfer performance in the regenerator. For laminar flow in the micro-scale regenerator, the optimal porosity is in a range of 0.85∼0.9 based on maximizing the system coefficient of performance (COP). The system’s thermal performance was then predicted considering compressible flow and heat transfer with a large deformed mesh in COMSOL. The Arbitrary Lagrangian-Eulerian (ALE) technique was used to handle the deformed geometry and the moving boundary. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model was used to replace the pillars in the model, allowing for numerical predictions of full-element geometry. Parametric studies of the design demonstrate the effect of the operating frequency on the cooling capacity and the COP of the system.Copyright

Thermal-Aware Micro-Channel Cooling of Multicore Processors: A Three-Stage Design Approach

This study goes beyond the common micro-channel cooling system composed of uniform parallel straight micro-channels. Due to the highly non-uniform power dissipation on a multicore processor, the micro-channel cooling system is designed to fit with the heat load on the multicore processor. By applying effective strategies and arranging key design parameters, stronger cooling is provided under the high power core area, and less cooling is provided under the low power cache area to save the precious pumping power. The well designed thermal-aware micro-channel cooling systems could effectively lower the hot spot temperature and temperature gradients on chip.A three-stage approach to design thermal-aware micro-channel cooling system for multicore processor is developed. Two micro-channel cooling systems are specifically designed for a 2 core 150W Intel Tulsa processor and an 8 core 260W (doubled power) Intel Nehalem processor, to illustrate the design approach. The working fluid is single phase HFE7100. For the Tulsa processor, a strategy named strip-and-zone approach is used. The final design leads to 30kPa pressure drop and 0.094W pumping power while maintains the hot spot temperature to be 75 °C. For the Nehalem processor, a split flow micro-channel system and a widen-inlet strategy are applied. The final design takes 15kPa pressure drop and 0.0845W pumping power while maintains the hot spot temperature to be 82.8 °C. The design approach in this study provides the basic guide for the industrial applications of effective multicore processor cooling using micro-channels.© 2013 ASME

Design and Evaluation of MEMS-Based Stirling Cycle Micro-Refrigeration System

A Stirling cycle micro-refrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated thermodynamically. The cooling elements are each 5 mm-long, 2.25 mm-wide, have a thickness of 300 μm, and are fabricated in a stacked array on a silicon wafer. A 0.5 mm-long regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms. The diaphragms are 2.25 mm circles driven electrostatically. Helium is the working fluid, pressurized at 2 bar and sealed in the system. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally 90° out of phase such that heat is extracted to the expansion space and released from the compression space. The bulk silicon substrate on which the device is grown is etched with “zipping” shaped chambers under the diaphragms. The silicon enables efficient heat transfer between the gas and heat source/sink as well as reduces the dead volume of the system, thus enhancing the cooling capacity. In addition, the “zipping” shaped substrates reduce the voltage required to actuate the diaphragms. An array of vertical silicon pillars in the regenerator serves as a thermal capacitor transferring heat to and from the working gas during a cycle. In operation, the push-pull motion of the diaphragm makes a 300 μm stroke and actuates at a frequency of 2 kHz. Parametric study of the design shows the effects of phase lag, swept volume ratio between the hot space and cold space, and dead volume ratio on cooling capacity. At TH = 313.15 K and TC = 288.15 K and assuming a perfect regenerator, the thermodynamic optimization analysis gives a heat extraction rate of 0.22 W per element and cooling capacity of 30 W/cm2 for the stacked system. Evaluation of the stacked system shows that the COP will reach 6.3 if the expansion work from the cold side is recovered electrostatically and used to drive the hot side diaphragm.Copyright