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Organic-inorganic hybrid materials have attracted a lot of attention recently due to their fantastic multifunctional properties, namely their magnetic, multiferroic, optoelectronic and photovoltaic properties.
A team of researchers from Spain have now shown that a certain hybrid perovskite compound has great potential in solid-state cooling applications, due to exhibiting giant barocaloric effects near room temperature and under low pressures.
Current cooling technology in refrigeration and air conditioning units’ accounts for more than 20% of the world’s energy consumption, and the demand is expected to rise in the coming years. However, despite the increased need, current technology in commercially available products generally possess low efficiencies and can contain hazardous greenhouse gases, of which are banned in the UK and are set to be banned across the rest of Europe in the next few years.
There has now been a surge to find environmentally friendly alternatives for when these pollutants become banned across all of Europe. One approach that is promising is using solid-state materials that exhibit a large caloric effect.
This is when the refrigeration capacity of the material is associated with a large isothermal energy change, or when there is a large adiabatic temperature change induced by an external stimulus.
The Spanish researchers have now identified a hybrid perovskite material, [TPrA][Mn(dca)3], which exhibits giant barocaloric effects at room temperature, with minimal applied pressure. Other materials are known to exhibit high caloric effects at room temperature, but many of them require high pressures and are not feasible for commercial applications.
The material belongs to a class of compounds that is quite new, but is growing significantly. As such, new properties within this class of materials are currently being discovered. Within this class of hybrid materials, a wide variety of materials can be produced, which is owed the multiple combinations of organic and inorganic moieties, and a variety of structural and chemical properties that can be exploited.
The perovskite material was synthesized through standard wet chemical techniques. To quantify and characterize the material’s caloric effects, the researchers used a combination of X-ray diffraction (XRD, Siemens D-5000 diffractomer), differential scanning calorimetry (DSC, TA Instruments Q2000) and high-pressure DSC (Setaram mDSC7 EVO). They also used a synchrotron PXRD to obtain a Rietveld analysis and calculate the entropy change.
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The researchers carried out quasi-direct methods in the form of isobaric calorimetry measurements at both high and low pressures. They also used direct entropic methods through cyclic isothermal calorimetric measurements and compared them to indirect Clausius–Clapeyron estimates.
The researchers found a phase transition around 330 K, which involved a complex structural transition with a large response towards pressure and temperature. The material exhibits a perovskite-type structure above and below the transition temperature and the Mn2+ ions form an octahedral array.
The ions are bridged by the dicyanamide (dca) ligands to form a 3D network and the tetrapropylammonium (TPrA) cations occupy the pseudo-cuboctahedra holes in the lattice.
The researchers found that even at low pressure, less than 0.007 GPa, and in room temperature environments the material surpasses most of the best caloric materials available today and possess a large caloric effect of 37.0 J kg-1 K-1.
It was found that the entropy change of the hybrid material is determined by the entropic changes of the atomic arrangements that become partly ordered/disordered. As such, the researchers believe there is more room to obtain even greater values by improving the configurational, rotational and vibrational entropies.
The researchers expect follow on research to include doping, tuning and tailoring of the building blocks, for both this and similar hybrid materials, to achieve an optimised caloric performance.
The researchers reckon that this material will not be the only organic-inorganic hybrid material to exhibit such high caloric effects, as many organic-inorganic materials (from MOFs to coordination polymers) have the basic ingredients to produce large caloric effects. As such, this research could open the door to a whole new class of materials to solve a continent-wide coolant problem.
Sources and Further Reading
- “Giant barocaloric effect in the ferroic organicinorganic hybrid [TPrA][Mn(dca)3] perovskite under easily accessible pressures”- Bermudez-Garcia J. M., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15715