Inorganic Functional Materials

M.Sc. Patricia Pop-Ghe


Kaiserstraße 2, R. A-207
Phone: +49 431 880-6208
Telefax: +49 431 880-6203



Ferroelectric, functional materials: caloric effects, waste heat and more

The world energy consumption statistics are readily made use of to motivate diverse research topics related to energy in a recurring manner. According to the IEA fuel report - October 2019 the biggest fraction in energy end-use is heat, which accounts for approximately 50 % of the world energy consumption[1]. However, the need for cooling is an increasingly growing demand, as not only the public desire (for air conditioning) increases constantly but also the industrial need. The DatacenterDynamics Global Census has shown that the global datacentre power demand increased by 19% in 2012 and 7% in 2013[2]. To this date, this demand is almost entirely satisfied by the century-old vapour-compression refrigeration technique, a technique, which is based upon highly toxic fluids – usually hydrofluorocarbons - that serve as a medium for heat absorption and release.

An alternate approach is the exploitation of the so-called caloric effects, which describe the reversible, thermal response of a material to an applied driving signal or driving force in general. This driving signal might be a magnetic (magnetocaloric) or electric field (electrocaloric), as well as isotropic (barocaloric) or uniaxial stress (elastocaloric). Among those, the electrocaloric (EC) effect is of special importance, as it combines solid-state cooling without the necessity for a liquid with the same driving signal that is used in most of modern electronic devices – a voltage and a resulting electric field respectively. Essentially, solid-state cooling is regarded as an environmentally friendly cooling technique since it obviates the need for toxic fluids.

However, lead plays an essential role with regard to candidate materials in electrocaloric solid-state cooling, as lead derivatives represent the state-of-the-art concerning achieved effect sizes of the EC effect. For the toxicity of lead for the environment and living beings, it is of special interest to research electrocaloric, lead-free alternatives, wherein ferroelectric potassium sodium niobate- (K0.5Na0.5NbO3) or barium titanate-based (BaTiO3) materials are regarded as most promising. This need to find lead-free alternatives has been recognised by the EU by the Directive 2011/65/EU[3].

Ferroelectric relation

Figure 1 Schematic correlation between ferroelectricity, pyroelectricity, piezoelectricity and dielectrics in general.

One of the most interesting aspects of the mentioned ferroelectrics is that their characteristic to show ferroelectric properties includes other properties like pyroelectricity (cf. Figure 1). Therefore, not only cooling properties can be investigated in those materials, but also the possibility to design energy conversion devices for the productive use of e.g. waste heat, as waste heat occurs on all levels of an industrial process. Still, a major drawback of lead-free electrocaloric materials is their persisting inability to compete with the state-of-the-art materials in terms of functional characteristics, despite a strong scientific effort to achieve comparable performances.

Mathematical approaches: theory of crystallographic compatibility

As for the lack of lead-free electrocaloric materials with adequate functional properties, a mathematical approach can be used to identify promising material compositions. The theory of crystallographic compatibility[4],[5] is a geometric set of conditions, which, if fulfilled, ensures the elimination of stressed interfaces between participating phases during a phase transition, therefore reducing fatigue and improving material properties towards application dramatically. The combination of innovative fabrication with these mathematical conditions is able to reveal fatigue-free, high performance materials[6].

Thin films and nanotechnology

When discussing solid-state cooling, it is especially important to have a look at the very same previously mentioned materials as thin films instead of bulk materials. Bulk materials might offer an alternative in large-scale refrigeration but thin films are needed if the effect is to be exploited in modern electronics’ fabrication lines e.g. in electronic devices or possibly even micro-electromechanical systems (MEMSs). One of the most prominent techniques to produce thin films is the magnetron sputtering technique (cf. Research: Superelastic Shape Memory Films and Applications), which is a highly reproducible and reliable physical vapour deposition (PVD) technique to produce thin films of micrometre and nanometre thicknesses.


[1] accessed on 20.02.2020
[2] accessed on 20.02.2020
[3] Directive 2011/65/EU of the European Parliament and of the Council on the restriction of the use of certain hazardous substances in electrical and electronic equipment. Official Journal of the European Union, 54, 88–110 (2011).
[4] Chen, X., Srivastava, V., Dabade, V. & James, R. D. Study of the cofactor conditions: Conditions of supercompatibility between phases. Journal of the Mechanics and Physics of Solids 61, 2566–2587 (2013).
[5] Song, Y., Chen, X., Dabade, V., Shield, T. W. & James, R. D. Enhanced reversibility and unusual microstructure of a phase-transforming material. Nature 502, 85–88 (2013).
[6] Gu, H., Bumke, L., Chluba, C., Quandt, E. & James, R. D. Phase engineering and supercompatibility of shape memory alloys. Materials Today 21, 265–277 (2018).
[7] Ma, R. et al. Highly efficient electrocaloric cooling with electrostatic actuation. Science 357, 1130–1134 (2017).


  • Pop-Ghe, P.; Stock, N.; Quandt, E. (2019). Suppression of abnormal grain growth in K0.5Na0.5NbO3: phase transitions and compatibility. Scientific Reports 9, 19775.
  • Pop-Ghe, P.; Stock, N.; Quandt, E. Fabrication and structural analysis of bulk and thin film lead-free ferroelectric ceramics within the potassium sodium niobate system in 4th Euro Intelligent Materials 2019, Kiel, Germany, June 17th - 19th, 2019.
  • Pop-Ghe, P.; Krückemeier, L.; Wöhrl, N.; Buck, V. Plasma-assisted CVD graphene synthesis and characterisation on nickel substrates in European Graphene Forum, Paris, France, June 1st - 3rd, 2016.
  • Pop-Ghe, P.; Krückemeier, L.; Wöhrl, N.; Buck, V. Plasma-assisted CVD graphene synthesis and characterisation on nickel substrates in 80. Jahrestagung der DPG und DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), Regensburg, Germany, March 6th – 11th, 2016.