Refrigeration based on plastic crystals

Plastic phase transitions in tris(hydroxymethyl)aminomethane perchlorate

Organic crystal materials with metal-free feature and intrinsically low molecular mass are highly desirable for applications in flexible smart devices. Here, we reported a plastic crystal, tris(hydroxymethyl)aminomethane perchlorate (Tris-HClO4), which crystallizes in the R3̄ space group at room temperature and undergoes plastic phase transition at 369 K, showing a large entropy gain of 70.5 J mol-1 K-1, much higher than its fusion entropy gain (12.9 J mol-1 K-1). PXRD measurement indicates that it has cubic lattice symmetry in the high-temperature phase. Moreover, it exhibits excellent dielectric permittivity switching properties and robust cyclic stability. This work could be the pathway for chemical designing multifunctional switchable materials with the motive of combining the idea of symmetry breaking and plastic phase transition.

Colossal room-temperature electrocaloric strength aided by hydrostatic pressure in lead-free multiferroic solid solutions

Solid-state cooling applications based on electrocaloric (EC) effects are particularly promising from a technological point of view due to their downsize scalability and natural implementation in circuitry. However, EC effects typically involve materials that contain toxic substances and require relatively large electric fields (∼100-1000 kV cm^-1) that cause fateful leakage current and dielectric loss problems. Here, we propose a possible solution to these practical issues that consists of concertedly applying hydrostatic pressure and electric fields on lead-free multiferroic materials. We theoretically demonstrate this strategy by performing first-principles simulations on supertetragonal BiFe_1-xCo_xO₃ solid solutions (BFCO). It is shown that hydrostatic pressure, besides adjusting the occurrence of EC effects to near room temperature, can reduce enormously the intensity of driving electric fields. For pressurized BFCO, we estimate a colossal room-temperature EC strength, defined as the ratio of the adiabatic EC temperature change by an applied electric field, of ∼1 K cm kV^-1, a value that is several orders of magnitude larger than those routinely measured in uncompressed ferroelectrics.

Rotational Dynamics of Discoid Colloidal Particles in Attractive Quasi-Two-Dimensional Plastic Crystals

Plastic crystals formed from anisotropic molecules or particles are an important state of matter characterized by the presence of long-range positional order and the lack of long-range orientational order. The rotational motion of molecules or particles in plastic crystals is the most attractive characteristic of the system. Here the rotational dynamics of the discoid particles in quasi-two-dimensional colloidal plastic crystals stabilized via depletion interactions are quantitatively studied using time-resolved confocal microscopy. The measured probability distribution of particle orientation reveals the existence of a strong coupling between the lattice symmetry and particle rotation, resulting in anisotropic rotational dynamics modes resembling the underlying hexagonal crystalline symmetry. Furthermore, the orientational distribution function provides information about the potential surface of rotational dynamics. The observed slow rotational diffusion can be attributed to the presence of orientational minima and potential barriers on the potential surface. Our findings with a real experimental system provide important insights into the role of attraction in the phase behaviors of plastic crystals.

Colossal barocaloric effects in the complex hydride Li[Formula: see text]B[Formula: see text]H[Formula: see text]

Traditional refrigeration technologies based on compression cycles of greenhouse gases pose serious threats to the environment and cannot be downscaled to electronic device dimensions. Solid-state cooling exploits the thermal response of caloric materials to changes in the applied external fields (i.e., magnetic, electric and/or mechanical stress) and represents a promising alternative to current refrigeration methods. However, most of the caloric materials known to date present relatively small adiabatic temperature changes ([Formula: see text] to 10 K) and/or limiting irreversibility issues resulting from significant phase-transition hysteresis. Here, we predict by using molecular dynamics simulations the existence of colossal barocaloric effects induced by pressure (isothermal entropy changes of [Formula: see text] J K[Formula: see text] kg[Formula: see text]) in the energy material Li[Formula: see text]B[Formula: see text]H[Formula: see text]. Specifically, we estimate [Formula: see text] J K[Formula: see text] kg[Formula: see text] and [Formula: see text] K for a small pressure shift of P = 0.1 GPa at [Formula: see text] K. The disclosed colossal barocaloric effects are originated by a fairly reversible order-disorder phase transformation involving coexistence of Li[Formula: see text] diffusion and (BH)[Formula: see text] reorientational motion at high temperatures.