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Thermoelectrics is the simplest technology applicable for direct energy conversion between heat and electricity. After over 60 years of fruitful research efforts, recent boom in flexible electronics has promoted the rapid development of flexible thermoelectrics with rising performances, discovery of new materials and concepts, unconventional device configuration, and emerging applications not possible for traditional thermoelectric (TE) semiconductors. In this Perspective, we first overview representative flexible TE materials, then discuss recent breakthroughs for flexible TE devices assembled from various types of TE materials employing different technical routes. They exhibit promising power generation and sensing performances, and aim for applications in wearable electronics, such as the power supply harvesting heat from body for low-power electronics, temperature sensors for tactile e-skin, and newly emerged application as a thermo-haptic device in an extended reality system.

Polymer-based capacitors are very promising for high-power systems due to their high power density and ultrafast charge–discharge speed, yet reaching high dielectric constant and high breakdown strength simultaneously in dielectric polymers required by high-performance capacitors still remains a huge challenge. Herein, poly(vinylidene fluoride- co-trifluoroethylene) (PVDF-TrFE) and poly(vinylidene fluoride- co-hexafluoropropylene) (PVDF-HFP) were coaxial electrospun in core–shell structured fibers to create locally inhomogeneous microstructures deliberately. Through adjusting the functional group HFP/TrFE monomer ratio, P(VDF-HFP)/P(VDF-TrFE) hybrid polymer films with topological composition distribution have been elaborately designed, enabling gradient polarization distribution from core to shell. Compared with homogeneous hybrid films of the same composition, the core–shell structure significantly boosts breakdown strength, thus resulting in a significantly improved energy storage capacity. At an HFP/TrFE monomer ratio of 10:1, an optimal comprehensive energy storage performance has been achieved with Ue ∼ 20.7 J/cm3 and efficiency 67.8%; moreover, the film could maintain its energy storage performance after 106 charge/discharge cycles without reduction. Molecular dynamic simulation and finite element analysis have been employed in combination to reveal the dipole moments distribution at the molecular level and polarization distribution at the microscale, which further demonstrates that elaborate polarization distribution adjustment is an effective strategy toward high-performance electrostatic energy storage capacitors.

It is crucial to effectively control the heat flux in thermal functional devices and electronic systems. Here, we numerically simulate and experimentally design a simple but effective thermal manipulator based on transformation thermodynamics, which is capable of realizing an extreme heat flux concentration in a small area and guiding the heat flux to the region we desired. The experimental results show that the thermal manipulator has compressed the heat flux from the line-shaped thermal source with an extremely high concentration efficiency of 91.1% and a region concentration ratio of 39.7. The temperature gradient in the small area is 4.42 K/mm, which is about 115% of the applied temperature gradient, indicating a very strong heat concentration capability. Furthermore, the thermal manipulator can physically rotate the heat flux under a constant temperature gradient through a diagonal thermal conduction vector as well. By concentrating and deflecting the heat flux, the extreme concentration and tunable control of the heat flux have been demonstrated, providing an effective thermal manipulating method for the heat flux control in the small area.

Mn-doped BiFeO3 films with Mn contents of 5 and 10 mol. % were prepared via a chemical route. A carefully controlled amount of Bi deficiency was introduced to further tune the lattice structure and the functionality of multiferroic BiFeO3. The crystal structure of Bi1−δFe1−xMnxO3 films was investigated by X-ray diffraction and Raman spectra; a rhombohedral-to-orthorhombic phase transition was revealed. The observed double hysteresis loops and two capacitance maxima from polarization vs electric field and capacitance-voltage measurements indicate an antiferroelectric-like behavior. Additionally, the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases in Bi1−δFe1−xMnxO3 films was revealed from the domain structures obtained by piezoelectric force microscopy. The effects of Mn substitution in conjunction with Bi deficiency on the FE-AFE phase transition and electrical behavior of BiFeO3 films are discussed in detail. Meanwhile, magnetic and photoluminescence measurements on the films illustrate that Mn substitution gives rise to the net magnetic moment and the defects induced by both Bi deficiency and Mn substitution influence the electronic structure of BiFeO3 films. This study thus shows a simple and effective way to control the functionalities of BiFeO3 films.

A modified chemical route via the layer-by-layer alternative deposition of BaTiO3 and BiFeO3 two compositions followed by the solution processing has been developed to prepare Ba and Ti co-doped multiferroic BiFeO3 thin films. The Ba and Ti co-doped BiFeO3 films crystallize in single perovskite phase and are composed of uniformly distributed grains of tens of nanometers as evidenced by X-ray diffraction and scanning electron microscopy images, respectively. But the films show local composition fluctuation with further high-resolution transmission electron microscope analysis, which leads to increased inner strain and thus a phase transition from R3c to P4mm occurs as Ba and Ti contents reach 50 mol. %. Accompanying the structural phase transition, a saturated P-E hysteresis loop with Pr ∼ 27 μC/cm2 and Ps ∼ 87 μC/cm2 and, moreover, a transition from antiferromagnetic to ferromagnetic state with Mr ∼ 1.46 emu/cm3 and Hc ∼ 1000 Oe have been observed. A strain induced structural change is proposed to explain for the observed synchronous enhancement in ferroelectric and magnetic properties, and elastic energy calculation is carried out to verify the viewpoint on magnetic behavior. The modified multiferroic behaviors of Ba, Ti co-doped BiFeO3 film imply an improved magnetoelectric coupling, which makes the material good candidate as memory devices and sensors.

α-In2Se3 is of large bandgap (∼1.4 eV) semiconductor and its structure is based on two-layer hexagonally packed arrays of selenium atoms with 1/3 of the sites of indium atoms being empty. Here we report a bandgap Eg reduction due mainly to the formation of a Cu2Se slab in the host In2Se3, which is responsible for the remarkable improvement of thermoelectric performance of bulk polycrystalline In2−xCuxSe3 (x = 0.1–0.2). When x = 0.2 the dimensionless figure of merit ZT and power factor were increased by a factor of 2 and 3, respectively, at 846 K if compared to those of Cu-free In2Se3. Interestingly, an incorporation of Cu into the lattice of In2Se3 results in a change in morphology from amorphouslike structure represented by In2Se3 to a visible polycrystalline form attributed to partial crystallization of the structure. This change enhances lattice thermal conductivities κL over the very low values of In2Se3. However, the enhancement is only moderate because of the effective scattering of phonons in the p...

In this paper, the nucleation field HN, hysteresis loops, and coercivity mechanism for hard/soft multilayers have been investigated within a self-contained micromagnetic model, which change with the strength of the interface exchange coupling Ji as well as the thickness of the hard and soft layers Lh and Ls. Analyses show that the nucleation field goes up as the interface exchange coupling increases. However, Ji has a significant effect on HN only when Ls is about 10% of the domain wall width of the soft phase while the thickness of the hard layer is larger than its domain wall width. The hysteresis loops of the Nd2Fe14B/α-Fe/Nd2Fe14B trilayers for various Ji have been calculated. The calculated pinning field decreases when Ji increases. It is found that, as Ls increases, the coercivity mechanism changes from pinning to nucleation. The change of exchange coupling at the interface influences the soft layer thickness, at which the coercivity starts to deviate from pinning. The critical thickness of the soft...

The demagnetization process of a hard/soft/hard sandwich has been investigated systematically within a self-contained micromagnetic model, with particular attention on the cases with small hard layer thickness. The microscopic and macroscopic hysteresis loops, as well as the angular distributions of the magnetization between nucleation and pinning have been obtained numerically, with the formula for the nucleation field derived. It is found that both nucleation and pinning fields, as well as the gap in between decrease as the hard layer thickness reduces. The hard layer thickness has great effect on the hysteresis loops only when the thickness is very small, where the hysteresis loop is nearly square and the dominant coercivity mechanism is the nucleation. The thickness regions at which the theoretical and practical giant energy products can be achieved have been discussed. In most cases, the hard layer can be taken as sufficiently thick so that the magnetization at its surface obeys a simple coherent rotation model. In these cases, the calculation can be simplified significantly, with only the influence of the soft layer thickness accounted.

Nucleation and evolution of domain walls as well as the magnetic reversal process in perpendicularly oriented hard/soft trilayers have been investigated within a micromagnetic approach, concentrated on the situation with very thin hard layer. The formula for the nucleation field has been derived, which falls as the soft layer thickness rises. Negative nucleation field occurs at thick soft layer when the shape anisotropy surpasses the crystalline one, where the magnetization in the center of the soft layer obeys the coherent rotation model. The microscopic and macroscopic hysteresis loops have been calculated numerically, with the angular distribution of the magnetization between the nucleation and pinning fields obtained. The area of the hysteresis loop decreases while the coercivity mechanism changes from nucleation to pinning as the soft layer thickness increases. The effect of the hard layer thickness is contrary to that of the soft layer, however, the former affects the demagnetization process and hysteresis loops significantly only when the layers are thin enough.

The magnetic properties of as-grown single-crystal SrMn3O6−δ nanoribbons synthesized by a simple molten-salt method can be significantly improved by a heat-treatment procedure, which enhances the ferromagnetism and spin-glass behavior. These enhanced properties might originate from a spin structure modulated by the thermodynamical equilibrium nonuniform oxygen distribution. Measurements of ac susceptibility prove the existence of a spin-glass phase in the postgrowth annealing SrMn3O6−δ nanoribbons, which differs from the only antiferromagnetic phase in the corresponding bulk material. Asymmetric magnetization hysteresis loops observed in applied magnetic fields H≤5 T are attributed to an exchange coupling between the spin-glass shell and the antiferromagnetic core of postgrowth annealing SrMn3O6−δ nanoribbons. Results suggest that the intrinsic phase inhomogeneity due to the surface effects of the nanostructural manganites may induce exchange anisotropy, which is of special interests for potential applica...