Ferroelectric (FE) thin films have recently attracted renewed interest in research due to their great potential for designing novel tunable electromagnetic devices such as large intelligent surfaces (LISs). However, the mechanism of how a polar structure in the FE thin films contributes to desired tunable performance, especially within the microwave frequency range, which is the most widely used frequency range of electromagnetics, has not been illustrated clearly. In this paper, we described several straightforward and cost-effective methods to fabricate and characterize Ba_0.6Sr_0.4TiO₃ (BST) thin films at microwave frequencies. The prepared BST thin films here exhibit homogenous structures and great tunability ( $\eta$) in a wide frequency and temperature range when the applied field is in the out-of-plane direction. The high tunability can be attributed to high concentration of polar nanoclusters. Their response to the applied direct current (DC) field was directly visualized using a novel non-destructive near-field scanning microwave microscopy (NSMM) technique. Our results have provided some intriguing insights into the application of the FE thin films for future programmable high-frequency devices and systems.

The sodium (Na) and Ce co-doped calcium bismuth titanate (CBT; CaBi₄Ti₄O₁₅) Aurivillius ceramics in a Ca_1−x(Na_0.5Ce_0.5)_xBi₄Ti₄O₁₅ (CNCBT; doping content (x) = 0, 0.03, 0.05, 0.08 and 0.12) system were synthesized by the conventional solid-state sintering method. All compositions show a single-phase orthorhombic (space group: A2₁am) structure at room temperature. The shift of the Curie point (T_C) towards lower temperatures (T) on doping results from the increased tolerance factor (t). The substitution-enhanced ferroelectric performance with large maximum polarization (P_m) and facilitated domain switching is evidenced by the developed electrical polarization–electric field (P–E) and electrical current–electric field (I–E) hysteresis loops. The piezoelectric coefficient (d₃₃ = 20.5± 0.1 pC/N) of the x = 0.12 sample is about four times larger than that of pure CBT. The improved piezoelectric properties can be attributed to the high remanent polarization (P_r) and relatively high dielectric permittivity (ε′). In addition, multi-sized (micron and sub-micron) domain structures were observed in the CNCBT ceramics by the piezoresponse force microscope (PFM). The multiple-sized ferroelectric domain structure with smaller domains is beneficial to the easy domain switching, enhanced ferroelectric performance, and improved piezoelectric properties of the CNCBT ceramics. The designed Aurivillius-phase ferroelectric ceramics with the T_C around 765 ℃ and high piezoelectric coefficient (d₃₃) are suitable for high-temperature piezoelectric applications.

As a high-temperature thermoelectric (TE) material, ZnO offers advantages of non-toxicity, chemical stability, and oxidation resistance, and shows considerable promise as a true ready-to-use module under air conditions. However, poor electrical conductivity and high thermal conductivity severely hinder its application. Carbon nanotubes (CNTs) are often used as a reinforcing phase in composites, but it is difficult to achieve uniform dispersion of CNTs due to van der Waals forces. Herein, we developed an effective in-situ growth strategy of homogeneous CNTs on ZnO nanoparticles by exploiting the chemical vapor deposition (CVD) technology, in order to improve their electrical conductivity and mechanical properties, as well as reducing the thermal conductivity. Meanwhile, magnetic nickel (Ni) nanoparticles are introduced as catalysts for promoting the formation of CNTs, which can also enhance the electrical and thermal transportation of ZnO matrices. Notably, the electrical conductivity of ZnO is significantly boosted from 26 to 79 S·cm⁻¹ due to the formation of dense and uniform conductive CNT networks. The lattice thermal conductivity ( ${\kappa }_{\text{L}}$) is obviously declined by the intensification of phonon scattering, resulting from the abundant grain boundaries and interfaces in ZnO–CNT composites. Importantly, the maximum dimensionless figure of merit (zT) of 0.04 at 800 K is obtained in 2.0% Ni–CNTs/ZnO, which is three times larger than that of CNTs/ZnO prepared by traditional ultrasonic method. In addition, the mechanical properties of composites including Vickers hardness (HV) and fracture toughness $(K_{\text{IC}})$ are also reinforced. This work provides a valuable reference for dispersing nano-phases in TE materials to enhance both TE and mechanical properties.

Ferroelectric polymer poly (vinylidene fluoride) (PVDF) shows excellent electro-activity and is promising for flexible electronic devices. However, the processing of PVDF into the favourable ferroelectric structure (β-phase) presents difficulties, while its copolymer with trifluoroethylene (PVDF-TrFE) can directly crystallize into β-phase, but shows limited thermal stability and high-cost processing. As a result, an easily implementable method, pressing-and-folding (P&F), was used to produce highly compatible blended films of PVDF and PVDF-TrFE without using any hazardous solvent or complex polymer processing equipment. Hot-pressed PVDF (molecular weight: 530 kg/mol) and PVDF-TrFE (molar ratio: 51/49) films were firstly stacked before undergoing P&F treatment. Compared to extrusion-blended films before and after P&F, the P&F stacked films showed isotropic crystalline structure of β-phase, as confirmed using X-ray diffraction and infrared spectroscopy. The ferroelectric remnant polarization of the P&F stacked films is 0.068 C/m², surpassing pure PVDF-TrFE (0.062 C/m²) and the simulated value of remnant polarization of pure PVDF (~0.065 C/m²). The above findings promise to provide inspirations for new processing strategy on PVDF-based functional polymers.

Reversible luminescence modulation behavior upon the photochromic effect endows the photochromic ceramics with great potential in anti-counterfeiting and data storage applications. Here, Sm³⁺-doped KSr₂Nb₅O₁₅ photochromic ceramics exhibit superior anti-counterfeiting ability: good covertness and considerable modulation ratio of luminescent emission intensity after photochromic reaction. The results show that the photochromism originated from oxygen and cation vacancies, which were directly identified by electron paramagnetic resonance and positron annihilation lifetime spectra. Unexpectedly, oxygen vacancies work more effectively than cation vacancies during photochromic reactions. Moreover, the extraordinary anti-counterfeiting ability was attributed to the high energy transfer rate, which was particularly caused by the short mean distance below 1 nm between the Sm³⁺ and vacancies. The work here has provided atomic-scale structural evidence and made a progress in understanding the photochromic origins and mechanism in color-center theory.

Cu nanowires covered by Ag particles is studied for potential applications in the next-generation microelectronics. To date, the deformation mechanism in the Cu-Ag core-particle is not clear. Here, molecular dynamics simulation is used to describe the Cu-Ag core-particle system. The results show that the equilibrium structure of Ag particles is reconstructed, when the particle ≤1.0 nm. At low temperature (1 K) indicate that three different deformation processes take part in the core-particle structure, depending on the size of Ag particles. When the particle diameter ≤2.0 nm, the prevailing deformation mechanism is the emission of dislocations from the Cu surface. For the particle diameters ranging from 3.0 to 6.0 nm, the emission of misfit dislocations from the Ag-Cu interface is the dominant deformation mechanism. If the Ag particle ≥6.0 nm, the deformation mechanism can be characterized by the slip band, consisting of the dislocations and amorphous atoms. For elevated temperatures (2–400 K), the mechanical properties of the Ag-Cu core-shell system are nearly independent of temperature, whereas the structure with particles larger than 2.0 nm showed a strong dependence of its mechanical properties on temperature. Based on the results, the diameter-temperature plastic deformation map is proposed.