Projects / Programmes
Micro-electromechanical and electrocaloric layer elements
| Code |
Science |
Field |
Subfield |
| 2.09.00 |
Engineering sciences and technologies |
Electronic components and technologies |
|
| Code |
Science |
Field |
| T170 |
Technological sciences |
Electronics |
| Code |
Science |
Field |
| 2.05 |
Engineering and Technology |
Materials engineering |
electrocalorics, micro-electromechanical systems, ceramics, microstructure
Researchers (21)
Organisations (4)
Abstract
Increasing needs of society for electricity, heating, or cooling have become a global priority, thus efficient ways of energy production, conversion, storage, and consumption are needed. A contribution to solutions to the described problems may be the inovative use of ferroelectric or relaxor ferroelectric ceramic materials in energy conversion applications, namely in dielectric cooling by exploiting the electrocaloric effect (ECE). The latter is defined as a reversible temperature change in a material upon application of an external electric field at adiabatic conditions. For an EC-cooling device according to our concept ceramic cooling elements, thin plates of the EC material with a large surface area, which allow the heat transfer to the surrounding liquid, are required.
The objectives of the project are:
-To synthesise advanced materials with a high EC temperature change. Until now lead-based relaxor ferroelectrics have exhibited the largest ECE, such as lead magnesium niobate-lead titanate, and therefore such materials will be investigated. Furthermore, due to environmental concerns lead-free relaxors will be also considered for EC applications. Chemical composition of EC materials will be adjusted to maximise the ECE close to room temperature.
-To establish the relation between the microstructure and the ECE for selected materials compositions which exhibit the largest EC temperature change close to room temperature.
-To enhance the dielectric breakdown strength of the EC material, both by modifying the chemical composition by doping, and by designing the microstructure (grain size and size distribution, porosity). Enhancing the dielectric strength will allow applying higher amplitudes of electric field to the material and thus achieving larger EC temperature changes.
-To fabricate self-standing EC layer-elements with optimised microstructure and thicknesses not exceeding a few 100 micrometers so that, according to the present knowledge, voltages not exceeding a few kV would suffice to obtain large enough EC temperature change in a demonstrator device.
-To demonstrate additional functionalities of such optimised layer elements, mainly in micro-electro-mechanical systems, for actuator or energy harvesting applications, and, further to consider possible multifunctionality of such elements.
Significance for science
Research on electrocaloric (EC) materials and their possible application in solid-state cooling technologies is a hot topic since the report of Mischenko and colleagues on a giant EC effect in lead-based ferroelectric (PZT) thin films (Science, 2006). In this exciting field with only good 10 years of history the project group has successfully merged the expertise and skills in the fields of materials, condensed matter physics and mechanical engineering. The paper on a conceptual cooling device is undoubtedly the synergetic result of the project (Applied Physics Letters, 2015, also among Editor's Picks for 2015). The demo device which is one of the first working prototypes, included cooling elements (plates) of relaxor-ferroelectric ceramic 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMN-10PT) and exhibited an efficient heat regeneration. The temperature span between the hot and the cold sides of the regenerator exceeded for a few times the temperature change achieved within a single element. The result was commented in the same year in a review paper (Moya et al., Nature Phys, 2015). The chemical composition of a relaxor ferroelectric determines the largest EC response (which should be close to room temperature from the application point of view). We showed that also the design of the microstructure plays a role in maximising the EC effect. The correlation between the grain size and EC effect reaches a peak at a certain grain size as a consequence of two main contributions. One is the maximum polarization that increases with increasing grain size to a certain peak value, and the other is the dielectric breakdown strength that is decreasing with increasing grain size. Thus, for the material with about 98 % relative density and 3.6 micrometre grains, the EC temperature change of 3.45 K is achieved at an electric field amplitude of 160 kV/cm, which is the highest reported value so far for lead-based perovskites. According to the EU directives WEE: Waste Electrical and Electronic Equipment Directive and ROHS: Restriction of Hazardous Substances Directive, environment-friendly lead-free alternatives should replace toxic, yet efficient lead-based materials in near future. The EC effect of a lead-free relaxor 0.85K0.5Na0.5NbO3-0.15SrTiO3 (KNN-STO) was thus probed. The material exhibits the EC temperature change of 1.6 K at 159 kV/cm, and a high value persists over a range of a few 10 K, which is good from the application point of view. EC temperature changes of at least 2 K are needed for practical application, and they are reached at high electric field amplitudes, usually exceeding 100 kV/cm, which is close to dielectric breakdown, Logically, lower applied voltages are obtained by reducing the dielectric thickness, but due to the inherent brittleness of ceramics, the thickness of bulk ceramic elements, like plates, is in the range of 100 – 200 micrometres. We were among the first to report the relaxor-ferroelectric multilayer elements with individual layer thickness of a few 10 micrometres and with the composition and microstructure optimised for a good EC response. The maximum measured EC temperature change was 2,26 K at 100 kV/cm and at 105 °C, which is in a good agreement with the values obtained on bulk ceramics of similar mass and dimensions, but at a few times lower applied voltage. We also studied the long-term stability of the EC effect in multilayer elements. The elements were exposed to 10exp6 unipolar cycles of electric field amplitude of 110 kV/cm. As confirmed by direct EC measurements the initial EC temperature change (1.45 K) reduced only for 0.01 K, confirming the fatigue-less effect. Furthermore, this study shows that EC multilayers are a viable solution for solid-state cooling.
Significance for the country
The EU patent on electrocaloric (EC) cooling (inventors: teams of B. Malič and Z, Kutnjak, Jožef Stefan Institute, and A. Kitanovski, Faculty of Mechanical Engineering, University of Ljubljana) has been sold to a house-appliance company Gorenje. The patent applications have been filed also in USA and in China. The principle of caloric cooling is interesting for the company, and the collaboration between the project partners and the company continues on this topic. The results of the project are useful for our co-funder, company KEKO-Equipment. Even after the end of the project they offer us support in fabrication of multilayer elements. Our plan is to further enhance the efficiency of heat transfer by an optimised layer design, supported by finite element modelling. The internal platinum electrodes are too expensive for upscaling, so we have developed an optimised process which includes much cheaper silver-palladium internal electrodes. We note that we have already received some inquiries for EC multilayer elements from USA. We are considering to establish a spin-off company. Project partners dr. Marko Vrabelj and PhD students Lovro Fulanović and Andraž Bradeško have developed a business idea: CeraSense – Smart Ceramic Solutions, and presented it at the 10th International Conference on Technology Transfer (Ljubljana, 2017) and their idea was one of the first three selected. They propose to build compact EC cooling systems integrated into protective clothing of professionals working at extreme conditions such as high temperatures. Such advanced protection could actively contribute to lowering of the temperature of the vital parts of the body and would contribute to less severe working conditions. In the frame of M I D E M 207: the 53rd International Conference on Microelectronics, Devices and Materials we organised the Workshop on Materials for Energy Conversion and their Applications: Electrocalorics and Thermoelectrics. This event was a good chance to bring to Slovenia the top experts in the field from Europe, and to attract researchers and postgraduate students from different fields: mechanical and electric engineering, materials, physics giving everyone the possibility to meet and discuss.
Most important scientific results
Annual report
2014,
2015,
final report
Most important socioeconomically and culturally relevant results
Annual report
2014,
2015,
final report
