Defay, E

Hirose, S

Mathur, ND

Moya, X

MRS Bulletin

English

Xavier Moya

Emmanuel Defay

Neil D. Mathur

Sakyo Hirose

Crossref

Electrocaloric effects in multilayer capacitors for cooling applications

Apollo (Cambridge)

MRS Bulletin	Article Template	Author Name/Issue DateElectrocaloric effects in multilayer capacitors for cooling applicationsXavier Moya, Emmanuel Defay, Neil D. Mathur* and Sakyo Hirose*For more than a century, mankind has achieved refrigeration by exploiting volatile gases that harm the environment when released to the atmosphere. More recently, the observation of electrocaloric effects in commercial multilayer capacitors has inspired the possibility of environmentally friendly cooling. Here we describe electrocaloric effects and multilayer capacitors, compare the electrocaloric performance of existing multilayer capacitors, and discuss the improvements required for practical cooling devices.Keywords: electrocaloric effects, multilayer capacitors, ferroelectric materialsElectrocaloric effectsElectrocaloric (EC) effects1-8 are reversible thermal changes that arise in electrically polarizable materials due to changes of electric field ΔE. An example of how they can be exploited in cooling cycles for heat pumps is shown in Figure 1. EC effects are normally parameterised as adiabatic changes in temperature ΔT or isothermal changes in entropy ΔS, which we can be expressed1 using equilibrium thermodynamics as:     and     ,where P is electrical polarisation, c is specific heat capacity, Ei is the initial applied field, and Ef is the final applied field. EC effects are therefore expected to be large near ferroelectric phase transitions1, where the electrical polarisation varies strongly with temperature.For several decades, EC effects in ferroelectric oxides and polymers have been investigated1-8 using thinned bulk samples and films of various thicknesses. The EC effects tend to be small in the thinned bulk samples, as they are too thick to support very large electric fields without breakdown1,5. By contrast, thin films can support large electric fields and so they show larger EC effects. However, EC films can only pump a small amount of heat, and they are typically attached to substrates that constitute thermal anchors.Multilayer capacitors (MLCs) based on EC materials do not suffer from either of the above problems, because they comprise many electroded films11. Moreover, the metallic electrodes can be exploited to help transfer heat between the thermally insulating EC layers11 and the rest of the thermal pathway in a given cooling device. Here we describe MLCs, and discuss their use in EC cooling devices.Multilayer capacitorsA capacitor is an electronic device that stores electrical charge using two closely spaced metallic plates that are separated by an electrically polarizable medium (or free space). Over the last three decades, trends towards miniaturization, high performance, and low power consumption have dominated the ceramic capacitor industry, motivated to a large extent by the rise of mobile phones and compact computers. An MLC comprises many electrically polarizable layers that lie electrically in parallel due to interdigitated inner electrodes (Figure 2). The large resulting capacitance between the two terminals (outer electrodes) permits a large amount of charge to be stored in a small volume.MLCs based on micron-thick layers of high-permittivity ceramic oxides are mass‑produced worldwide in their billions via the following fabrication process12. (i) Powdered oxides, binders and solvents are mixed to form a slurry. (ii) The slurry is tape-cast to form green ceramic sheets. (iii) Metallic electrodes are screen-printed onto these green ceramic sheets. (iv) The electroded green ceramic sheets are stacked, laminated and cut in order to produce individual multilayer structures. (v) The multilayer structures are fired in a furnace. (vi) The two ends of the multilayer structures are dipped into a metallic ink based on Cu in order to metallise the two terminals, and then the structure is fired again. (vii) The two terminals are electroplated with a layer of Ni (for various reasons that include its role as a diffusion barrier), and then a layer of Sn to facilitate soldering.Commercial MLCs are broadly divided into two types, depending on the selection of the active material and the application. MLCs based on paraelectric materials such as TiO2 and CaZrO3 provide temperature-independent capacitances that are small13, whereas multilayer capacitors based on doped ferroelectric materials such as BaTiO3 provide large capacitances that vary relatively strongly with temperature and field13. The latter are necessarily not suitable for power electronics, but they are widely used as decoupling capacitors in microelectronics, and nowadays they also represent highly desirable EC modules as described below.Electrocaloric performance of multilayer capacitorsThe MLC geometry was used long ago in order to directly measure changes of temperature in thin EC layers14. MLCs were only later suggested as EC cooling devices in their own right15, leading to studies based on a very limited range of active materials, namely three ferroelectric oxides (based on BaTiO3, PbMg1/3Nb2/3O3‑PbTiO3 and PbMg1/3Nb2/3O3), and one ferroelectric polymer [poly(vinylidene fluoridetrifluoroethylene)16]. Oxide MLCs17-26 show adiabatic temperature changes of |ΔT| < 1 K near room temperature, and |ΔT| < 2.5 K at high temperature (Figure 3, Table 1). This has been sufficient for the development of five EC cooling devices27-31, some of which are described in Figure 4.OutlookFurther study of MLCs that show EC effects is now timely given the growing number of EC cooling devices that are being developed worldwide, and improvements in performance could lead to applications. The key goal is to increase the adiabatic temperature change that can be driven over a range of temperatures that lie near room temperature, most naturally by using better EC materials. Improvements in the fabrication process and device structure would also be important if they increase the breakdown field, permitting larger fields to drive larger EC effects. For example, device structure can be optimised via finite element analysis in order to maximise heat exchange via the terminals and also the faces. Once these improvements have been made, it will then be important to ensure resistance to fatigue during operation, e.g. by avoiding microcracks26 and occasionally reversing the polarity of the applied field32. MLCs for EC applications therefore represent a good challenge for materials scientists, and we look forward to the forthcoming developments as the area matures.AcknowledgmentsThis work was supported by the ERC Starting Grant No. 680032. X. M. is grateful for support from the Royal Society. E. D. thanks the Luxembourg National Research Fund (FNR) that supported this work through COFERMAT grant No. FNR/P12/4853155/Kreisel. References[1] X. Moya, S. Kar-Narayan, N. D. Mathur, Nature Mater. 13, 439 (2014).[2] Electrocaloric Materials, Editors T. Correia and Q. Zhang, Springer, 2014.[3] S. G. Lu and Q. Zhang, Adv. Mater. 21, 1983 (2009).[4] J. F. Scott, Annu. Rev. Mater. Res. 41, 229 (2011).[5] M. Valant, Prog. Mater. Sci. 57, 980-1009 (2012).[6] X. Li, S.-G. Lu, X.-Z. Chen, H. Gu, X.-S. Qian, and Q. M. Zhang, J. Mater. Chem. C 23, 1 (2013).[7] S. Alpay, J. Mantese, S. Trolier-McKinstry, Q. Zhang, and R. Whatmore, MRS Bulletin 39, 1099 (2014).[8] Electrocaloric Effect: Theory, Measurements, and Applications, Z. Kutnjak, B. Rožič, and R. Pirc, Wiley Encyclopedia of Electrical and Electronics Engineering, 2015.[9] G. V. Brown, J. Appl. Phys. 47, 3673 (1976).[10] S. Crossley, B. Nair, R. W. Whatmore, X. Moya, and N. D. Mathur, submitted.[11] S. Kar-Narayan and N. D. Mathur, Appl. Phys. Lett. 95, 242903 (2009).[12] www.murata.com/en-eu/products/emiconfun/capacitor/2011/06/28/en-20110628-p1.[13] H. Kishi, Y. Mizuno, and H. Chazono, Jpn. J. Appl. Phys. 42, 1 (2003).[14] W. N. Lawless and C. F. Clark, Phys. Rev. B 36, 459 (1987).[15] A. Mischenko and N. Mathur, Solid State Electrocaloric Cooling Device with Heat Switches and Method of Cooling, GB patent PCT/GB2005/050207 (2005).[16] H. Gu, X. Qian, X. Li, B. Craven, W. Zhu, A. Cheng, S. C. Yao, and Q. M. Zhang, Appl. Phys. Lett. 102, 122904 (2013).[17] S. Kar-Narayan and N. D. Mathur, J. Phys. D: Appl. Phys. 43, 032002 (2010).[18] Y. Bai, G. Zheng and S. Shi, Appl. Phys. Lett. 96, 192902 (2010).[19] S. Kar-Narayan, S. Crossley, X. Moya, V. Kovacova, J. Abergel, A. Bontempi, N. Baier, E. Defay, and N. D. Mathur, Appl. Phys. Lett. 102, 032903 (2013).[20] S. Hirose, T. Usui, S. Crossley, B. Nair, A. Ando, X. Moya, and N. D. Mathur, APL Materials 4, 064105 (2016).[21] Y. Liu, H. Strozyk, B. Dkhil, and E. Defay, Appl. Phys. Lett. 109, 212902 (2016).[22] C. Molin and S. Gebhardt, Ferroelectrics 498, 111 (2016).[23] T. Usui, S. Hirose, A. Ando, S. Crossley, B. Nair, X. Moya, and N. D. Mathur, J. Phys. D: Appl. Phys. 50, 424002 (2017).[24] R. Faye, H. Strozyk, B. Dkhil, and E. Defay, J. Phys. D: Appl. Phys. 50, 464002 (2017).[25] L. Fulanović, J. Koruzac, N. Novak, F. Weyland, B. Malič, and V. Bobnar, J. Eur. Ceram. Soc. 37, 5105 (2017).[26] L. Fulanović, S. Drnovšek, H. Uršič, M. Vrabelj, D. Kuščer, K. Makarovič, V. Bobnar, Z. Kutnjak, and B. Malič, J. Eur. Ceram. Soc. 37, 599 (2017).[27] Y. Jia and Y. S. Ju, Appl. Phys. Lett. 100, 242901 (2012).[28] P. Blumenthal, C. Molin, S. Gebhardt, and A. Raatz, Ferroelectrics 497, 1 (2016).[29] Y. D. Wang, S. J. Smullin, M. J. Sheridan, Q. Wang, C. Eldershaw, and D. E. Schwartz, Appl. Phys. Lett. 107, 134103 (2015).[30] D. Sette, A. Asseman, M. Gérard, H. Strozyk, R. Faye, and E. Defay, APL Materials 4, 091101 (2016).[31] T. Zhang, X.-S. Qian, H. Gu, Y. Hou, and Q. M. Zhang, Appl. Phys. Lett. 110, 243503 (2017).[32] F. Weyland, T. Eisele, S. Steiner, T. Frömling, G. A. Rossetti, Jr, J. Rödel, and N. Novak, J. Eur. Ceram. Soc. 38, 551 (2018).Figure 1. EC cooling cycle. (a) Schematic based on a single layer capacitor. (b) Plot of entropy S versus temperature T. (a,b) At starting temperature T1, the fast application of a voltage (corresponding to electric field E) adiabatically polarises the hitherto unpolarized active material, whose temperature is thus increased to T2. Heat is then dumped by relative motion along the length of a (fluid) regenerator9, such that the temperature of the material is reduced to T3. Fast removal of the electric field then adiabatically depolarises the active material, whose temperature is thus further reduced to T4. Finally, heat is absorbed from the regenerator by relative motion in the opposite direction along the regenerator, such that the temperature of the material is increased back to T1. This completes the cycle. We assume that the EC effect does not vary with starting temperature, such that no net heat is exchanged between the material and the regenerator (true regeneration10), as seen in (b). In (a), we also assume that the active material is a displacive ceramic ferroelectric above the Curie temperature, such that no dipoles are present at zero field.Figure 2. MLCs as existing electronic components. (a) Rear view of an Intel® Xeon® E3 processor, showing an assembly of MLCs that provide protection from current spikes by providing a charge reservoir. (b) Schematic of one such MLC with two outer electrodes (terminals). For the MLC highlighted in (a), there are 200 inner electrodes of 2 µm-thick Ni that address 2 µm‑thick layers of doped BaTiO3 (BTO). (c) Optical microscopy image of an MLC in cross-section.Figure 3. EC effects in MLCs. Peak values of EC temperature change |ΔT| arise due to the maximum changes of field |ΔE| at optimum starting temperature T0. The active layers comprise doped BaTiO3 (BTO), PbMg1/3Nb2/3O3-PbTiO3 (PMN-PT), and PbMg1/3Nb2/3O3 (PMN).Figure 4. Recent EC cooling devices based on MLCs of doped BTO. (a) Two silicon heat switches sandwich an EC module of 20 commercially manufactured MLCs. The temperature lift of the unloaded device was 0.3 K. (b) An EC module of 150 commercially manufactured MLCs was translated along a regenerative fluid column. The temperature lift of the unloaded device was 0.13 K, as measured between the top and bottom of the regenerator. (c) Two EC modules that each comprise a ring of 12 bespoke MLCs were rotated in opposite directions. The temperature lift of the unloaded device was 2 K, as measured between aluminium plates at the top and bottom (the two plates seen at the top are not labelled). Images reprinted from refs [29-31], with the permission of AIP Publishing.ECmaterial	EC layerthickness (µm)	Number ofEC layers	T0(K)	|ΔT|(K)	|ΔE|(kV cm-1)	Measurementmethod	ReferenceBTO	6.5	200	360	0.55	300	thermistor	17BTO	1.4	180	353	1.8	176	DSC	18BTO	6.5	200	293	0.5	308	IR/SThM	19BTO	11	150	298	0.6	182	IR	21BTO	9	137	298	0.86	222	IR	24BTO	12	30	296	0.9	165	thermocouple	310.9PMN-0.1PT	38	49	373	1.2	103	thermocouple	200.9PMN-0.1PT	35-39	19	380	2.7	288	thermocouple	230.9PMN-0.1PT	35-39	19	380	2.3	231	thermocouple/IR	230.92PMN-0.08PT	95	7	293	0.2	21	thermocouple/IR	280.9PMN-0.1PT	60	5	378	2.3	100	thermistor	25PMN	32	5	299	1.5	110	thermistor	26Table 1. Adiabatic EC temperature change |ΔT| due to the maximum changes of field |ΔE| at optimum starting temperature T0, for selected MLCs. The active layers comprise doped BaTiO3 (BTO), PbMg1/3Nb2/3O3-PbTiO3 (PMN‑PT), and PbMg1/3Nb2/3O3 (PMN). The abbreviated measurement techniques are differential scanning calorimetry (DSC), infrared thermometry (IR) and scanning thermal microscopy (SThM).Author biographiesXavier Moya is a Royal Society University Research Fellow at the Department of Materials Science, University of Cambridge. He received his BSc in Physics in 2003, and his PhD in Physics in 2008 from the University of Barcelona. He is interested in phase transitions in functional materials whose structural, magnetic, electrical and thermal properties display strong coupling. His research focuses on magnetocaloric materials, electrocaloric materials and mechanocaloric materials for cooling applications. He has been a Fellow of Churchill College since 2014. He was awarded the Ramon Margalef Prize 2009 for his PhD work, and the Spanish Royal Society of Physics Young Researcher in Experimental Physics Prize 2015 and the SRUK Emerging Talent Award 2017 for his continuous work on caloric materials.Emmanuel Defay is currently leading the Ferroic Materials for Transducers group at Luxembourg Institute of Science and Technology. His research interests are piezoelectrics and electrocalorics. He obtained his master in Engineering in 1994 from ENSMM Besançon (France), his PhD in Physics in 1999 from INSA Lyon (France) and his habilitation in 2007 from INPG Grenoble (France). Before LIST, he was with CEA LETI from 2000 to 2014, working on the integration of piezoelectric materials for microsystems. He also had a 2-year stay at the University of Cambridge to study electrocalorics (UK) from 2010 to 2012.Neil Mathur completed his first degree in Physics at Cambridge, and subsequently stayed there to do a PhD with Gil Lonzarich on magnetically mediated superconductivity. In 1996, he moved across Cambridge to work on manganites as a postdoc in the Department of Materials Science, and he has stayed at this address ever since. In 1999 he became a Research Fellow funded by The Royal Society; in 2005 he was hired to the faculty; and in 2013 he became Professor of Materials Physics. He currently studies the physics of materials and systems with notable electrical and magnetic properties, namely electrocaloric materials, magnetoelectric heterostructures, and spintronic devices. In 2012 he was elected a Fellow of the American Physical Society “for seminal contributions to the science and technology of magnetic and multiferroic oxides”. In 2009 he was awarded the Rosenhain Medal & Prize by The (UK) Institute of Materials, Minerals and Mining for “for his work on device materials and his contribution to the understanding of magnetic and electronic materials”.Sakyo Hirose is a Principal Researcher at Murata Manufacturing Co., Ltd. He received his B.E. and M.E. from Tokyo Institute of Technology, and joined Murata as a researcher in 2000. He received his PhD from Osaka University in 2012 as a result of his research into grain boundary engineering in electroceramics. His current research focussed mainly on electric-field-driven devices for future electronics, particularly for resistive-switching memories, magnetoelectric devices, and electrocaloric devices.PAGE  2

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Electrocaloric effects in multilayer capacitors for cooling applications

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