Electrocaloric Devices Research Articles

Self-oscillating polymeric refrigerator with high energy efficiency.

Electrocaloric1,2 and electrostrictive3,4 effects concurrently exist in dielectric materials. Combining these two effects could achieve the lightweight, compact localized thermal management that is promised by electrocaloric refrigeration5. Despite a handful of numerical models and schematic presentations6,7, current electrocaloric refrigerators still rely on external accessories to drive the working bodies8-10 and hence result in a low device-level cooling power density and coefficient of performance (COP). Here we report an electrocaloric thin-film device that uses the electro-thermomechanical synergy provided by polymeric ferroelectrics. Under one-time a.c. electric stimulation, the device is thermally and mechanically cycled by the working body itself, resulting in an external-driver-free, self-cycling, soft refrigerator. The prototype offers a directly measured cooling power density of 6.5 W g-1 and a peak COP exceeding 58 under a zero temperature span. Being merely a 30-µm-thick polymer film, the device achieved a COP close to 24 under a 4 K temperature span in an open ambient environment (32% thermodynamic efficiency). Compared with passive cooling, the thin-film refrigerator could immediately induce an additional 17.5 K temperature drop against an electronic chip. The soft, polymeric refrigerator can sense, actuate and pump heat to provide automatic localized thermal management.

Simulation study on the effect of operating and materials parameters on electrocaloric device for battery cooling

The present simulation work discusses the effect of different electric pulses, operating temperatures, electrodes and substrate materials on electrocaloric(EC) performance. The electrocaloric effect (ECE) is analyzed using poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) P(VDF-TrFE-CFE) polymer with electrode. The P(VDF-TrFE-CFE) film (100 μm) shows an effective adiabatic temperature change (ΔT ad)effective (considering losses) of 4.01 K at 100 MV m−1 when a square electric field (E) pulse is used. An effect of electrode and substrate layer thickness is also analyzed; the electrocaloric performance decreases with an increase in the thickness of the substrate layer. A smartphone battery’s surface (7 cm × 3 cm) is considered for cooling by the proposed design. The simulation study shows battery’s surface/source temperature dropped from 313.15 K to ∼298 K, ∼297.5 K, and ∼293.5 K when air cooling, the EC device in contact with the battery (no ECE, E = 0) and EC device in contact with battery (with ECE, E = 100 MV m−1), respectively are used in ∼6.5 s. The proposed EC device design can provide new insight for mobile or other electronic cooling applications.

P(VDF-TrFE-CFE) polymer based electrocaloric device design for battery cooling

Electrocaloric (EC) cooling has the potential to be an alternative to traditional cooling methods. The polyvinylidene fluoride-trifluoro ethylene-chlorofluoroethylene terpolymer (P(VDF-TrFE-CFE) is used as electrocaloric material. A 3D equation-based modeling and finite element simulations are performed for EC cooling, where the battery's heat generation is considered a heat source for analysis. The electric field waveform has been varied to obtain the maximum possible ECE. Electric field cycles with 0.3 sec holding provide the maximum EC temperature change. The magnitude of the electric field is also varied and a maximum adiabatic temperature change of 5.41 K is achieved under 100 MV/m. The EC effect shows that the heat transfer from the source (battery surface) to the sink (environment) is faster than without EC.

Self-sustaining personal all-day thermoregulatory clothing using only sunlight

The human body must stay within a certain temperature range for comfort and safety. However, challenges for thermoregulatory clothing exist for harsh application scenarios, such as full day/night cycles, frigid polar regions, and space travel. We developed a flexible and sustainable personal thermoregulatory clothing system by integrating a flexible organic photovoltaic (OPV) module to directly acquire energy from sunlight and bidirectional electrocaloric (EC) devices. The flexible OPV-EC thermoregulatory clothing (OETC) can extend the human thermal comfort zone from 22°–28°C to 12.5°–37.6°C with a fast thermoregulation rate. The low energy consumption and high efficiency of the EC device allows for 24 hours of controllable and dual-mode thermoregulation with 12 hours of sunlight energy input. This self-powered wearable thermoregulatory platform has a simple structure, compact design, high efficiency, and strong self-adaptability with sunlight as the sole energy source.

A highly efficient solid-state heat pump.

The high efficiency of a newly developed electrocaloric device brings theory closer to reality.

Development of electrocaloric effect in BT-based lead-free ceramic via density adjustment strategy

Electrocaloric (EC) refrigeration materials have seen significant progress recently, with efficient Barium titanate-based (BT-based) lead-free materials particularly promising for microelectronics applications. However, their performance still lags behind their lead-based counterparts, highlighting the need for further improvements. This study employs a novel density adjustment strategy (DAS) to enhance the EC performance of (0.5-x)BCT-BTZ-xBST ceramics by carefully balancing the relationships among density, dielectric breakdown strength (BDS), and ΔT. Although ΔT intuitively decreases with increasing density, enhanced BDS can partially compensate for this decrease. An excellent EC performance with ΔT of 2.21 K and ΔT/ΔE of 0.471 K mm kV−1 is obtained in x = 0.5, together with a broader temperature span of 50.3 °C near room temperature, which surpasses or is comparable to other lead-free or lead-based ones. The results suggest that the DAS provides a promising approach for developing efficient EC, making speediness of application in EC devices.

Enhanced Electrocaloric Response in Electrostatically Frustrated Ferroelectrics

The electrocaloric effect (i.e., the temperature change of a material upon the adiabatic application of an electric field) can be large in ferroelectric compounds, to the extent of offering an eco‐friendly alternative to current, polluting and noisy, refrigeration technologies. So far, ferroelectric‐based electrocaloric devices employ materials (from perovskite oxides to polymers) that have been known for decades. At the same time, recent studies have shown that the functional response of a ferroelectric can be drastically enhanced by controlling its mechanical and electric boundary conditions. Here, we use atomistic second‐principles simulations to quantify how much the electrocaloric effect benefits from such an enhancement. Using ferroelectric/dielectric superlattices as representative model systems, we predict that temperature changes can indeed be increased significantly (more than two times at room temperature), suggesting electrostatic engineering—to induce very reactive frustrated ferroelectric states—as a promising route for electrocaloric optimization.

Highly efficient electrocaloric device based on composite materials with excellent heat transfer performance

Electrocaloric (EC) cooling technology is known for its efficiency and simplicity of the electricity-driving method. However, there still are many challenges and problems, i.e., the poor heat transfer of EC materials (polymers), and the limited electrocaloric temperature change, which result in the limited cooling efficiency and/or cooling power of the EC cooling prototypes. The lack of EC device with high cooling performance poses challenging obstacles for the industrial deployment of the EC cooling technology. In this work, we demonstrated that the fabrication of P(VDF-TrFE-CFE) nanocomposites with BaZr0.2Ti0.8O3 (BZT) particles is an efficient way to improve the heat transfer of EC materials without compromising EC capability. In order to valid the positive effect of developing the EC device rather than provide a pure material study, we developed a solid-fluid coupling EC cooling machine numerically. The introduction of BZT composites allows EC refrigeration devices to exhibit a good COP while maintaining a relatively high total cooling power (>150 W) when the temperature span is 10 K. Therefore, the composites exhibit great potential of ECE for future cooling applications.

A micro solid-state refrigeration prototype device based on the electrocaloric effect

The increase in chip power consumption and heat generation has created the need for new cooling technologies, and the electrocaloric effect (ECE) has become a promising micro cooling technology due to its high efficiency and environmentally friendly features. In this work, a solid-state refrigeration prototype device based on ECE were designed. We first developed a refrigeration process based on the Brayton cycle. After the structural design, electrocaloric material selection, device fabrication, and assembly, we obtained a miniature solid-state refrigeration prototype device. With a 250-V applied voltage and a 3-s cycle period, this prototype refrigeration device achieved a temperature span (Tspan, difference in temperature between the hot end and the cold end) of 0.46 K between the hot end and cold end at room temperature, providing a new design concept for the ECE in micro cooling technology.

An Active Pixel-Matrix Electrocaloric Device for Targeted and Differential Thermal Management.

More than 55% of electronic failures are caused by damage from localized overheating. Up to now, there is still no efficient method for targeted temperature control against localized overheating. Although some existing thermal management devices handle this issue by full coverage cooling, it generates a lot of useless energy consumption. Here, a highly efficient pixel-matrix electrocaloric (EC) cooling device is reported, which can realize a targeted and differential thermal management. The modified poly(vinylidene fluoride-tertrifluoroethylene-chlorofluoroethylene) reaches a large adiabatic temperature change of 7.8 K and is more suitable for thermal transfer and electrostatic actuation at high frequencies. All active pixels in the EC cooling device exhibit a stable temperature span of 4.6 K and a heat flux of 62mWcm-2 , which is more than twice that of the one-layer EC device. Each refrigeration pixel can be independently controlled and effectively cooled down the localized overheating site(s) in situ. The surface temperature of the simulated central processing unit decreases by 33.2 K at 120 s after applying this EC device. Such a compact, embeddable, low cost, and active solid-state pixel-matrix cooling device has great potential for localized overheating protection in microelectronics.

Transformation Toughening in Solid Dielectrics

High breakdown-resistant solid dielectrics are necessary in almost all kinds of dielectric systems, such as dielectric energy-storage devices, piezoelectric actuators, electrocaloric devices, and dielectric capacitors. However, the low breakdown strength of solid dielectrics, especially ceramic dielectrics, is the main obstacle limiting their practical applications. Inspired by the transformation toughening in brittle materials, an approach to enhance the breakdown strength of solid dielectrics is proposed in this work, and the internal mechanism is studied via a phase-field breakdown model. Simulation results indicate that the phase transformation has a significant effect on the dielectric breakdown strength, especially when the critical electric fields for the phase transformation and breakdown are comparable. Compared to dielectrics without phase transformation, the dielectric breakdown strength is nearly doubled when the phase transformation occurs during the breakdown process. Therefore, a reasonable design of phase-transformed dielectrics is expected to be an effective way to achieve the high breakdown resistance.

Role of Density and Grain Size on the Electrocaloric Effect in Ba0.90Ca0.10TiO3 Ceramics.

Pure perovskite Ba0.90Ca0.10TiO3 ceramics, with a relative density of between 79 and 98% and grain sizes larger than 1 µm, were prepared by solid-state reaction. The dielectric and electrocaloric properties were investigated and discussed considering the density and grain size of the samples. Room temperature impedance measurements show good dielectric properties for all ceramics with relative permittivity between 800 and 1100 and losses of <5%. Polarization vs. E loops indicates regular variation with increasing sintering temperature (grain size and density), an increase in loop area, and remanent and saturation polarization (from Psat = 7.2 µC/cm2 to Psat = 16 µC/cm2). The largest electrocaloric effect was 1.67 K for ceramic with GS = 3 µm at 363 K and electrocaloric responsivity (ζ) was 0.56 K mm/kV. These values are larger than in the case of other similar materials; thus, Ba0.90Ca0.10TiO3 ceramics with a density larger than 90% and grain sizes of a few µms are suitable materials for electrocaloric devices.

High-performance cooling and heat pumping based on fatigue-resistant elastocaloric effect in compression.

SummaryIn recent years, elastocaloric cooling has shown great potential as an alternative to vapor-compression refrigeration. However, there is still no existing elastocaloric device that offers fatigue-resistant operation and yet high cooling/heat-pumping performance. Here, we introduce a new design of an elastocaloric regenerator based on compression-loaded Ni–Ti tubes, referred to as a shell-and-tube-like elastocaloric regenerator. Our regenerator design, which can operate in both cooling and heat-pumping modes, enables durable operation and record performance with a maximum temperature span of 31.3 K in heat-pumping mode or maximum heating/cooling powers of more than 60 W, equivalent to 4,400 W/kg of the elastocaloric material (at temperature span of 10 K). In terms of both maximum performance metrics, these results surpass all previously developed caloric (magnetocaloric, electrocaloric, and elastocaloric) devices and demonstrate the enormous potential of compression-loaded elastocaloric regenerators to be used in elastocaloric devices for a wide range of cooling and heat-pumping applications.

Giant room-temperature electrocaloric effect within wide temperature span in Sn-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 lead-free thin films

The electrocaloric (EC) effect cooling technique of environmentally friendly lead-free thin film materials driven by electric fields has recently gained tremendous attention due to the urgent demand for microelectronic and integrated circuit refrigeration devices. However, the widespread use of lead-free materials in EC devices is seriously hindered due to the small electrocaloric temperature change (ΔT) within a narrow operation temperature span (Tspan) near room temperature. Here, lead-free Ba0.85Ca0.15Zr0.1Ti0.9-xSnxO3 (BCZT-xSn, 0 ≤ x ≤ 0.03) thin films were prepared on substrates (Pt/Ti/SiO2/Si) via a sol-gel route. The BCZT-0.02Sn thin film presents an excellent EC effect (ΔT = 32.74 K, ΔS = 37.18 J kg−1 K−1) and large EC strength (ΔT/ΔE = 0.033 K cm kV−1, ΔS/ΔE = 0.037 J cm K−1 kg−1 kV−1) over a wide Tspan (∼26 K) under 1000 kV cm−1 near room temperature. The giant ΔT is mainly attributed to the emergence of an intermediate O phase and the formation of a multiphase (R, O and T phases) coexistence structure at room temperature, while the diffuse phase transition behavior is responsible for the wide Tspan. Our study provides a new idea for developing environment-friendly EC materials with an excellent room-temperature ΔT over a broad operational temperature region.

Ferroelectric Smectic Liquid Crystals as Electrocaloric Materials

The 1980s saw the development of ferroelectric chiral smectic C (SmC*) liquid crystals (FLCs) with a clear focus on their application in fast electro-optic devices. However, as the only known fluid ferroelectric materials, they also have potential in other applications, one of which is in heat-exchange devices based on the electrocaloric effect. In particular, ferroelectric liquid crystals can be both the electrocaloric material and the heat exchanging fluid in an electrocaloric device, significantly simplifying some of the design constraints associated with solid dielectrics. In this paper, we consider the electrocaloric potential of three SmC* ferroelectric liquid crystal systems, two of which are pure materials that exhibit ferroelectric, antiferroelectric, and intermediate phases and one that was developed as a room-temperature SmC* material for electro-optic applications. We report the field-induced temperature changes of these selected materials, measured indirectly using the Maxwell method. The maximum induced temperature change determined, 0.37 K, is currently record-breaking for an FLC and is sufficiently large to make these materials interesting candidates for the development for electrocaloric applications. Using the electrocaloric temperature change normalised as a function of electric field strength, as a function of merit, the performances of FLCs are compared with ferroelectric ceramics and polymers.

Enhanced near-ambient temperature energy storage and electrocaloric effect in the lead-free BaTi0.89Sn0.11O3 ceramic synthesized by sol–gel method

Lead-free perovskite materials with high performance have high potential in clean energy storage applications and developments of electrocaloric devices. This work reports structural, dielectric, ferroelectric, energy storage, and electrocaloric properties near the ambient temperature in barium stannate titanate (BaTi0.89Sn0.11O3, BTS11) ceramic prepared by a sol–gel method. A single perovskite structure formation was confirmed using the X-ray diffraction analysis. Average grain size of 18.5 µm was found by the mean of the SEM micrograph with a density of 5.91 g/cm3. The multiphase coexistence at very near ambient temperature was proved using temperature-dependent micro-Raman measurements and differential scanning calorimetry. The BTS11 ceramic exhibits a high dielectric constant of 15,460 and a low dielectric loss (< 0.055) in a broad temperature range. Moreover, a high energy storage density of 122 mJ/cm3 was shown with an efficiency of 79%, a maximum value of electrocaloric temperature change (ΔT) of 0.86 K, and finally, an electrocaloric responsivity (ΔT/ΔE) of 0.24 K.mm/kV at the external electric field change of 35 kV/cm near ambient temperature. The enhanced dielectric, ferroelectric, and electrocaloric properties make BTS11 ceramics a great candidate for solid-state cooling technology and high energy storage applications near ambient temperature.

Analytical Solution of Heat Exchange in Typical Electrocaloric Devices

AbstractTo date, most of electrocaloric devices reported can be simplified as a multilayer structure in which thermal source and sink are different materials at two ends. The thermal conduction in the multilayer structure is the key for the performance of the devices. In this paper, the analytical solutions for the thermal conduction in a multilayer structure with four layers are introduced. The middle two layers are electrocaloric materials. The analytical solutions are also simplified for a hot/cold plate with two sides being different media—a typical case for thermal treatment of materials. The analytical solutions include series with infinite terms. It is proved that these series are convergent so the sum of a series can be calculated using the first N terms. The equation for calculating the N is introduced. Based on the case study, it is found that the N is usually a small number, mostly less than 40 and rarely more than 100. The issues related to the application of the analytical solutions for the simulation of real electrocaloric devices are discussed, which includes the usage of multilayer ceramic capacitor, influence of electrodes, and characterization of thin film.

Flexible microfluidic electrocaloric cooling capillary tube with giant specific device cooling power density

Inorganic fillers improve the mechanical strength of the entire composite material EC capillary tube realizes a maximum temperature change of 3.53 K A record specific device cooling power density is 702.1 mW cm−3 The sub-ambient silicone oil can be continuously obtained The microfluidic cooling system is one of the most promising micro-cooling solutions for chip-scale thermal management, in which the regeneration of flowing cryogenic liquid is a key limitation. Thanks to its high efficiency and ease of minimization, electrocaloric (EC) cooling is regarded as a feasible solution. However, there is no existing EC cooling device that offers a continuous sub-ambient liquid media. Here, we fabricated a self-standing flexible EC capillary tube with a maximum temperature drop of 3.53 K by coaxial wet-spinning technology. The self-encapsulating property of the EC tube reduces the volume of inactive substances. Thus, the highest specific device cooling power density of 702.1 mW cm−3 among all the reported EC devices was realized in the EC capillary tube. The EC cooling capillary tube opens up a unique way to apply EC technology to the thermal management of the micro-electromechanical system, such as the cooling of the lab-on-chip systems. The microfluidic cooling system is one of the most promising micro-cooling solutions for chip-scale thermal management, in which the regeneration of flowing cryogenic liquid is a key limitation. Thanks to its high efficiency and ease of minimization, electrocaloric (EC) cooling is regarded as a feasible solution. However, there is no existing EC cooling device that offers a continuous sub-ambient liquid media. Here, we fabricated a self-standing flexible EC capillary tube with a maximum temperature drop of 3.53 K by coaxial wet-spinning technology. The self-encapsulating property of the EC tube reduces the volume of inactive substances. Thus, the highest specific device cooling power density of 702.1 mW cm−3 among all the reported EC devices was realized in the EC capillary tube. The EC cooling capillary tube opens up a unique way to apply EC technology to the thermal management of the micro-electromechanical system, such as the cooling of the lab-on-chip systems.

Electrocaloric Effect of Structural Configurated Ferroelectric Polymer Nanocomposites for Solid-State Refrigeration.

To successfully complete the design of high-performance electrocaloric devices for advanced flexible cooling systems, it is necessary to comprehensively consider the optimization of composite materials, structural design of nanocomposites, and device integration. The cooling power density and energy storage density of various structural configurated poly(vinylidene fluoride) (PVDF)-based polymer nanocomposites are investigated using a phase-field model through the general formulation of a partial differential equation of COMSOL Multiphysics and finite element analysis through Maxwell's equation of conservation of charge. It is revealed that ferroelectric polymer nanocomposites composed of boron nitrate fibers (BNf) + BCZT@BaTiO3(f) + PVDF possess the optimal result regarding their cooling power as well as the energy storage density. The cooling power density of the core-shell-structured BNf + BCZT@BaTiO3(f) + PVDF nanocomposites is evaluated as a function of the volume content, frequency, and electric field, where a remarkable cooling power density of 162.2 W/cm3 is achieved at 4 Hz with energy storage density of 33.4 J/cm3 under a 500 MV/m field. Therefore, by performing the systematic study of the electrocaloric effect in structural configurated ferroelectric polymer nanocomposites for solid-state refrigeration, this opens an avenue for developing remarkably improved power density with reduced weight in aerospace energy storage technology.

Thermal Management: Electrostatic Actuating Double‐Unit Electrocaloric Cooling Device with High Efficiency (Adv. Energy Mater. 13/2021)

In article number 2003771, Rujun Ma and co-workers report a highly efficient double-unit solid-state refrigeration device based on two-layer modified electrocaloric polymer stacks actuated by electrostatic forces. The availability and the high-efficiency of the device for cooling the central processing unit provides a unique option for thermal management of microcircuits in the future.