Designing a piezoelectric energy harvester (PEH) with high power density and high fatigue resistance is essential for the successful replacement of the currently using batteries in structural health monitoring (SHM) systems. Among the various designs, the PEH comprising of a cantilever structure as a passive layer and piezoelectric single crystal-based fiber composites (SFC) as an active layer showed excellent performance due to its high electromechanical properties and dynamic flexibilities that are suitable for low frequency vibrations. In the present study, an effort was made to investigate the reliable performance of hard and soft SFC based PEHs. The base acceleration of both PEHs is held at 7 m/s² and the frequency of excitation is tuned to their resonant frequency (f_r) and then the output power (P_rms) is monitored for 10⁷ fatigue cycles. The effect of fatigue cycles on the output voltage, vibration displacement, dielectric, and ferroelectric properties of PEHs was analyzed. It was noticed that fatigue-induced performance degradation is more prominent in soft SFC-based PEH (SS-PEH) than in hard SFC-based PEH (HS-PEH). The HS-PEH showed a slight degradation in the output power due to a shift in f_r, however, no degradation in the maximum power was noticed, in fact, dielectric and ferroelectric properties were improved even after 10⁷ vibration cycles. In this context, the present study provides a pathway to consider the fatigue life of piezoelectric material for the designing of PEH to be used at resonant conditions for long-term operation.

The magnetic properties of ThMn₁₂-type Fe-rich compounds were investigated by producing Sm(Fe_0.8Co_0.2)₁₀Si₂ ribbons. The produced ribbons, with different conditions by varying the melt-spinning conditions, were characterized to investigate physical and magnetic properties. Weight fraction of ThMn₁₂-type phase decreased from 94.5 to 57.1 wt. % as the melt-spinning wheel speed increased from 6.5 to 39 m/s, and corresponding magnetizations and coercivities were substantially varied; the coercivity increased up to 0.175 T from 0.058 T by increasing the wheel speed from 6.5 to 26 m/s, and their magnetization also increased from 89.81 Am²/kg to 105.58 Am²/kg, even though the content of ThMn₁₂-type phase decreased. Morphologies of the ribbons were also observed to verify the melt-spinning effects on the surface conditions and grain sizes. It was found that the particle and grain sizes in the ribbons became smaller and striped patterns appeared as the wheel speed increased. The grain size decreased from about 1 μm to 250 nm by increasing the wheel speed from 6.5 to 39 m/s.

Rare-earth-free permanent magnets have attracted considerable attention due to their favorable properties and applicability for cost-effective, high-efficiency, and sustainable energy devices. However, the magnetic field annealing process, which enhances the performance of permanent magnets, needs to be optimized for different magnetic fields and phases. Therefore, we investigated the effect of composition on the crystallization of amorphous MnBi to the ferromagnetic low-temperature phase (LTP). The optimal compositions and conditions were applied to magnetic field annealing under 2.5 T for elemental Mg- and Sb/Mg pair-substituted MnBi. The optimum MnBi composition for the highest purity LTP was determined to be Mn₅₆Bi₄₄, and its maximum energy product, (BH)_max, was 5.62 MGOe. The Mg-substituted MnBi exhibited enhanced squareness (M_r/M_s), coercivity (H_c), and (BH)_max values up to 0.8, 9659 Oe, and 5.64 MGOe, respectively, whereas the same values for the Sb/Mg pair-substituted MnBi were 0.76, 7038 Oe, and 5.60 MGOe, respectively. The substitution effects were also investigated using first-principles calculations. The density of states and total magnetic moments of Mn₁₆Bi₁₅Mg and Mn₁₆Bi₁₅Sb were similar to those of pure Mn₁₆Bi₁₆. Conversely, the Sb-substituted MnBi resulted in a dramatic enhancement in the anisotropy constant (K) from a small negative value (−0.85 MJ/m³) to a large positive value (6.042 MJ/m³).

The physical properties of the lead-free piezoelectric material BaTiO₃ (BT) with complex perovskite phase additives, i.e., A(Cu_1/3Nb_2/3)O₃ (A = Ba, Sr, Ca), were investigated. The dense and crack-free thick films with thickness of > 5 µm were grown on platinized Si(100) wafers by using the aerosol deposition method (AD) at room temperature. Compared to pure BT grown by using AD, complex perovskite-substituted BT thick films showed improved physical properties and film quality even after post annealing. The optimal amount of the additives and the optimal post annealing temperature were determined in accordance with the dielectric and the ferroelectric properties.