Understanding secondary phase inclusion and composition variations in the microstructure design of n-type Bi_(2)Te_(3) alloys via selective dissolution of KCl

This study investigated the effects of KCl treatment on microstructure and thermoelectric properties of n-type Bi_(2)Te_(2.7)Se_(0.3)(BTS)thermoelectric materials.The innovative KCl treatment was originally intended to introduce nanopores through selective dissolution of KCl from a mixture of thermoelectric materials and KCl.However,it unexpectedly induced substantial variations in material composition and microstructure during the subsequent spark plasma sintering(SPS)process.Hydroxyl groups adsorbed on the powder surface during the dissolution resulted in the emergence of a Bi_(2)TeO_(5) secondary phase within the BTS matrix after the SPS process at 450℃.The concentration of Bi_(2)TeO_(5) increased with an increase in the KCl content.Furthermore,a remarkable grain growth occurred at low KCl concentrations,likely due to the liquid-phase formation in a Te-rich composition during SPS.However,excessive Bi_(2)TeO_(5) at higher KCl concentrations hindered grain growth.These variations in the microstructure had complex effects on electrical properties:The TeBi antisite defects increased the electron concentration,and Bi_(2)TeO_(5) reduced electron mobility.Additionally,the lattice thermal conductivity decreased due to the presence of Bi_(2)TeO_(5),from 0.8 W·m^(−1)·K^(−1) at 298 K for the pristine BTS to 0.6 W·m^(−1)·K^(−1) at 298 K for BTS treated with 1 wt%KCl.These insights allowed precise adjustments of the electrical and thermal conductivities,leading to an enhancement in the maximum value of figure-of-merit(ZT)from 0.76 to 0.96 through the selective dissolution of KCl approach.We believe that our observations potentially enable advances in thermoelectric materials by engineering microstructures.

Dual-frequency piezoelectric micromachined ultrasound transducer based on polarization switching in ferroelectric thin films

Due to its additional frequency response,dual-frequency ultrasound has advantages over conventional ultrasound,which operates at a specific frequency band.Moreover,a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution.This facilitates the availability of more advanced techniques that simultaneously require low-and high-frequency ultrasounds,such as harmonic imaging and image-guided therapy.In this study,we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer(PMUT).Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging.To switch the ultrasound frequency,the only requirement is to tune a DC bias to control the polarization state of the ferroelectric film.Flextensional vibration of the PMUT membrane strongly depends on the polarization state,producing low-and high-frequency ultrasounds from a single excitation frequency.This strategy for dual-frequency ultrasounds meets the requirement for either multielectrode configurations or heterodesigned elements,which are integrated into an array.Consequently,this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.