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The electrocaloric effect in thin films of a poly(vinylidene fluoride-trifluoroethylene chlorofluoroethylene) terpolymer (62.6/29.4/8 mol. %, 11-12 lm 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/lm. 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. V C 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4890676] Micro-scale refrigeration systems are widely used for the cooling of integrated circuits, microelectromechanical sensors, and biomedical devices.1 Environment-friendly cooling technologies with a high efficiency are attractive due to growing energy demands and stringent environmental requirements. 2Although thermoelectric cooling is commonly applied and has been scaled to the micro-domain, 3-6 the low efficiency and challenges in material fabrication suggest that alternatives are needed. 6 While refrigeration based on the magnetocaloric effect 7 can be employed to achieve extremely low temperatures, miniaturization of devices is challenging while maintaining a high cooling performance due to the difficulty of realizing the large magnetic fields required. 2The electrocaloric (EC) effect is a phenomenon in which reversible, polarization-related temperature and entropy changes occur when an electric field is applied to certain materials. EC cooling, which operates on a refrigeration cycle analogous to magnetocaloric cooling, is an emerging technology. 8 The highest reported adiabatic temperature change in a bulk EC material is 2.5 K at an electric field of 3 V/lm and a temperature of 434 K 11 The P(VDF-TrFE-CFE) terpolymer demonstrates an adiabatic temperature change of 16 K at an electric field of 150 V/lm near room temperature 12 and is easily and economically fabricated, making it favorable for mass production.13 These findings point to the potential of applying EC cooling in micro-devices using polymer thin films.Direct and indirect techniques can be applied to measure the EC effect. In the indirect measurement, a differential scanning calorimeter is used to measure the heat flow under a high electric field and isothermal conditions. 14,15 This technique is best suited to bulk materials as the output heat flow signal for a thin film sample is very small. Jia and Ju 16 reported an approach for characterizing the EC effect in a thin film sitting on an insulating substrate. In this approach, the temperature response of a resistance thermometer deposited on the bottom of the EC film is monitored as an electric field is turned on and off. In the reported measurements, the temperature change is less than 10% of that expected because the heat loss from the EC film to the substrate is large. Lu et al.17 employed a specially designed calorimeter to measure the EC effect in a thin film. In this approach, the heat generated in the...

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A new Stirling microrefrigeration system composed of arrays of silicon MEMS cooling elements has been designed and evaluated. The cooling elements are to he 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 peiformance are explored. The optimal porosity for the best system coefficient of performance (COP) is identified.

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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.

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.