We have investigated the temperature dependence of elastic modulus for various ferroelectric ceramics in the temperature range of 20–90°C.The Na0.5Bi0.5TiO3(NBT)ceramics has a phase transition at 200°C,thus exhibits minimal change in elastic modulus up to 90°C,while the elastic modulus of the BaZr0.07Ti0.93O3(BZT-7)shows 12.5%change at the phase transition temperature of70°C and that of the BaZr0.15Ti0.85O3(BZT-15)ceramics shows 34.6%change at the phase transition temperature of60°C.The variations of elastic modulus will affect the temperature stability of devices made by these lead-free ferroelectric ceramics.
Cadrniurn-doped zinc oxide nanocrystals in the quantum confinement region have been firstly synthesized by a fast and facile sonochernical method. The alloyed structure of the nanocrystals is confirmed by X-ray diflraction, transmission electron rnicroscopy, and infrared analysis. With the increase of cadmium to zinc molar ratio from 0 to 2.0, the crystallite sizes of the samples decrease from 5.1 nrn to 2.6 nrn, and the band gaps of the samples show a red shift then a blue shift, and a red shift again. The variations of band gaps of the samples can be interpreted by the crystallite size and the composition. It is found that both the non-therrnal equilibrium environrnent established in the sonochernical reaction and the coordination ability of triethylene glycol solvent play crucial roles in the current preparation.
The speed-of-sound variance will decrease the imaging quality of photoacoustic tomography in acoustically inhomo- geneous tissue. In this study, ultrasound computed tomography is combined with photoacoustic tomography to enhance the photoacoustic tomography in this situation. The speed-of-sound information is recovered by ultrasound computed to- mography. Then, an improved delay-and-sum method is used to reconstruct the image from the photoacoustic signals. The simulation results validate that the proposed method can obtain a better photoacoustic tomography than the conventional method when the speed-of-sound variance is increased. In addition, the influences of the speed-of-sound variance and the fan-angle on the image quality are quantitatively explored to optimize the image scheme. The proposed method has a good performance even when the speed-of-sound variance reaches 14.2%. Furthermore, an optimized fan angle is revealed, which can keep the good image quality with a low cost of hardware. This study has a potential value in extending the biomedical application of photoacoustic tomography.
We design a planar metasurface to modulate the wavefront of a water surface wave（WSW） on a deep sub-wavelength scale. The metasurface is composed of an array of coiling-up-space units with specially designed parameters, and can take on the work of steering the wavefront when it is pierced into water. Like their acoustic counterparts, the modulation of WSW is ascribed to the gradient phase shift of the coiling-up-space units, which can be perfectly tuned by changing the coiling plate length and channel number inside the units. According to the generalized Snell＇s law, negative refraction and ＇driven＇ surface mode of WSW are also demonstrated at certain incidences. Specially, the transmitted WSW could be efficiently guided out by linking a symmetrically-corrugated channel in ＇driven＇ surface mode. This work may have potential applications in water wave energy extraction and coastal protection.
We present a detailed theoretical description of wave propagation in an acoustic gradient-index system with cylindrical symmetry and demonstrate its potential to numerically control acoustic waves in different ways.The trajectory of an acoustic wave within the system is derived by employing the theory of geometric acoustics,and the validity of the theoretical descriptions is verified numerically by using the finite element method simulation.The results show that by tailoring the distribution function of the refractive index,the proposed system can yield a tunable manipulation of acoustic waves,such as acoustic bending,trapping,and absorbing.
Pulse decomposition has been proven to be efficient to analyze complicated signals and it is introduced into the photo-acoustic and thermo-acoustic tomography to eliminate reconstruction distortions caused by negative lobes.During image reconstruction,negative lobes bring errors in the estimation of acoustic pulse amplitude,which is closely related to the distribution of absorption coefficient.The negative lobe error degrades imaging quality seriously in limited-view conditions because it cannot be offset so well as in full-view conditions.Therefore,a pulse decomposition formula is provided with detailed deduction to eliminate the negative lobe error and is incorporated into the popular delay-and-sum method for better reconstructing the image without additional complicated computation.Numerical experiments show that the pulse decomposition improves the image quality obviously in the limited-view conditions,such as separating adjacent absorbers,discovering a small absorber despite disturbance from a big absorber nearby,etc.
The electric field enhancement properties of an active gold nanoshell with gain material inside have been investigated by using Mie theory. As the gain coefficient of the inner core increases to a critical value, a super-resonance appears in the active gold nanoshell, and enormous enhancements of the electric fields can be found near the surface of the particle. With increasing shell thickness, the critical value of the gain coefficient for the super-resonance of the active gold nanoshell first decreases and then increases, and the corresponding surface enhanced Raman scattering （SERS） enhancement factor （G factor） also first increases and then decreases. The optimized active gold nanoshell can be obtained with an extremely high SERS G factor of the order of 1019-1020. Such an optimized active gold nanoshell possesses a high-efficiency SERS effect and may be useful for single-molecule detection.
