This paper describes the design, fabrication, and characterization of tightly focused (ƒ-number close to 1) high frequency needle-type transducers based on lead-free Na_0.5Bi_4.5Ti_3.975Co_0.025O₁₅ (NBT-Co) piezo-ceramics. The NBT-Co ceramics, are fabricated through solid-state reactions, have a piezoelectric coefficient d₃₃ of 32 pC/N, and an electromechanical coupling factor k_t of 35.3%. The high Curie temperature (670 °C) indicates a wide working temperature range. Characterization results show a center frequency of 70.4 MHz and a −6 dB bandwidth of 52.7%. Lateral resolution of 29.8 μm was achieved by scanning a 10 μm tungsten wire target, and axial resolution of 20.8 μm was calculated from the full width at half maximum (FWHM) of the pulse length of the echo. This lead-free ultrasonic transducer has potential applications in high resolution biological imaging.

Currently, severe electromagnetic circumstances pose a serious threat to electronic systems. In this paper, the damage effects of a high-power electromagnetic pulse (EMP) on the GaN high-electron-mobility transistor (HEMT) were investigated in detail. The mechanism is presented by analyzing the variation in the internal distribution of multiple physical quantities in the device. The results reveal that the device damage was dominated by different thermal accumulation effects such as self-heating, avalanche breakdown and hot carrier emission during the action of the high-power EMP. Furthermore, a multi-scale protection design for the GaN HEMT against high-power electromagnetic interference (EMI) is presented and verified by a simulation study. The device structure optimization results demonstrate that the symmetrical structure, with the same distance from the gate to drain (Lgd) and gate to source (Lgs), possesses a higher damage threshold compared to the asymmetrical structure, and that a proper passivation layer, which enhances the breakdown characteristics, can improve the anti-EMI capability. The circuit optimization results present the influences of external components on the damage progress. The findings show that the resistive components which are in series at the source and gate will strengthen the capability of the device to withstand high-power EMP damage. All of the above conclusions are important for device reliability design using gallium nitride materials, especially when the device operates under severe electromagnetic circumstances.

In order to meet the urgent need for high temperature piezoelectric materials with a Curie temperature over 400 °C, the Mn/Nb co-doped strategy has been proposed to improve the weak piezoelectric performance of the Aurivillius-type Na_0.5Bi_4.5Ti₄O₁₅ (NBT) high temperature piezoelectric ceramics. In this paper, the crystal structure, electrical properties, and thermal stability of the B-site Mn/Nb co-doped Na_0.5Bi_4.5Ti_4-x(Mn_1/3Nb_2/3)_xO₁₅ (NBT-100x) ceramics were systematically investigated by the conventional solid-state reaction method. The crystal structural analysis results indicate that the NBT-100x ceramics have typical bismuth oxide layer type phase structure and high anisotropic plate-like morphology. The lattice parameters and the grain sizes increase with the B-site Mn/Nb co-doped content. The electrical properties were significantly improved by Mn/Nb co-doped modifications. The maximum of the piezoelectric coefficient d₃₃ was found to be 29 pC/N for the NBT-2 ceramics, nearly twice that of the unmodified NBT ceramics. The highest values of the planar electromechanical coupling factor k_p and thickness electromechanical coupling factor k_t were also obtained for the NBT-2 ceramics, at 5.4% and 31.2%, respectively. The dielectric spectroscopy showed that the Curie temperature Tc of the Mn/Nb co-doped NBT-100x ceramics is slightly higher than that of unmodified NBT ceramics (646 °C). The DC resistivity of the NBT-2 ceramics is higher than 10⁶ Ω∙cm at 500 °C. All the results together with the good thermal stability demonstrated the Mn/Nb co-doped ceramics as an effective method to improve the NBT based piezoelectric ceramics and the potential candidates of the Mn/Nb co-doped NBT-100x ceramics for high temperature piezoelectric applications.