•Two honeycomb structures of BSi with two-atom thickness are predicted.•Phonon spectra show that both structures are thermodynamically stable.•C2-type BSi is a very promising candidate for photoelectric applications.•C2-type BSi has a good absorption of photons in the range of 1.5–2.5 eV.•Thermal conductivity of P1-type BSi is around 6 times higher than that of C2-type.Using first-principles calculations and evolutionary algorithm, we predict two honeycomb (P1- and C2-type) structures of BSi with two-atom thickness. Phonon spectrum calculations show that both structures are thermodynamically stable. A weak covalent bonding interaction between B atoms along [0 0 1] direction plays a very important role in structure stability of P1- and C2-type BSi. P1-type BSi is semi-metallic, while C2-type exhibits a semiconductor band structure and has a good absorption of photons in the range of 1.5–2.5 eV that is very promising for photoelectric applications. An e22 piezoelectric coefficient of 1.65 × 10−10 C m−1 is predicted for C2-type BSi, which makes it a potential candidate for piezoelectric applications at nano-scale. Moreover, three-phonon interactions are adopted to evaluate phonon lifetimes, phonon linewidths and lattice thermal conductivity. Although both P1- and C2-type BSi have a similar honeycomb structure and same elements, the maximum lattice thermal conductivity of P1-type BSi is around 6 times higher than that of C2-type BSi. Additionally, P1-type BSi exhibits remarkably higher phonon lifetimes than C2-type BSi.Download high-res image (170KB)Download full-size image

Backing materials play important role in enhancing the acoustic performance of an ultrasonic transducer. Most backing materials prepared by conventional methods failed to show both high acoustic impedance and attenuation, which however determine the bandwidth and axial resolution of acoustic transducer, respectively. In the present work, taking advantage of the structural feature of 3D graphene foam as a confined space for dense packing of tungsten spheres with the assistance of centrifugal force, the desired structural requirement for high impedance is obtained. Meanwhile, superior thermal conductivity of graphene contributes to the acoustic attenuation via the conversion of acoustic waves to thermal energy. The tight contact between tungstate spheres, epoxy matrix, or graphene makes the acoustic wave depleted easily for the absence of air barrier. The as-prepared 3DG/W80 wt %/epoxy film in 1 mm, prepared using ∼41 μm W spheres in diameter, not only displays acoustic impedance of 13.05 ± 0.11 MRayl but also illustrates acoustic attenuation of 110.15 ± 1.23 dB/cm MHz. Additionally, the composite film exhibits a high acoustic absorption coefficient, which is 94.4% at 1 MHz and 100% at 3 MHz, respectively. Present composite film outperforms most of the reported backing materials consisting of metal fillers/polymer blending in terms of the acoustic impedance and attenuation.

A tungsten/epoxy composite film integrated with graphene oxide (GO) is prepared via a layer-by-layer assembly method, which shows enhanced acoustic attenuating performance compared with a tungsten/epoxy composite film in the absence of GO. The basic structure consists of tungsten/epoxy/GO/epoxy (W/E/GO/E), in which the inner wrapped epoxy acts as a buffer layer to anchor GO nanosheets on W spheres, and the outer wrapped epoxy layer is designed to prevent GO nanosheets peeling off from W spheres. The design was beneficial in guaranteeing the independence and integrity of the W/E/GO/E structure. The structure of the core–shell composites was characterized with Fourier-transform infrared spectra, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The acoustic properties of the films were evaluated by a conventional pulse-echo overlap technique at the frequency of 9 MHz. It was found that the acoustic attenuation of the optimal W/E/GO/E composite films was much higher than those of traditional W films at a band frequency range from 5 MHz to 12 MHz, and 36.58 ± 0.2 dB cm−1 MHz−1 was obtained at 9 MHz. The wrapping of GO on the surface of W/E will create a crumpled surface for better mixing with the epoxy matrix due to hydrogen bonding and chemical bonding, leading to the preparation of a high quality composite film with minimal structural defects including bubbles and cracks. The enhanced acoustic absorption property of the composite films was attributed to a synergistic effect among its multicomponents, as well as the contribution of GO's thermoacoustic effect. W/E/GO/E composite films with such excellent attenuation loss properties have promise to be the backing material for ultrasonic transducers.

•Optical properties for KNN-based single crystal was studied for the first time.•Temperature dependence of band gap energies and phonon energy was calculated.•Structure of KNNT single crystal was characterized by Raman spectroscopy.The optical properties of KNN-based single crystals were investigated firstly in this work. The optical transmittance of (K0.56Na0.44)(Nb0.77Ta0.23)O3 (KNNT) single crystal was measured at different temperatures. The band gap energies of direct transition Egd, indirect transition Egi, and the phonon energies Ep involved in the indirect transition were calculated based on the optical transmittance spectra. It was found that Egd and Egi show abrupt change at orthorhombic–tetragonal phase transition temperature (TO–T) at 119 °C due to the change of crystal lattice, while Ep varies rapidly at around 60 °C. In order to further understand the phase transition behavior and the assistant phonons in the indirect transition, the KNNT single crystal was characterized by using Raman spectroscopy. It is clarified that the υ1 vibration mode is involved in the indirect transition below 60 °C and υ5 mode is involved above 60 °C. The Raman scattering of KNNT crystal shows distinct behavior in different phases. The phase transitions at TO–T = 119°C and TC = 291 °C can be confirmed respectively as orthorhombic–tetragonal and tetragonal–cubic by the discontinuities of the Raman peaks’ intensities and positions.

A high quality orthorhombic (K, Na, Li)(Nb, Ta)O3 single crystal with a large size of 9 × 9 × 11 mm3 was successfully grown using the top-seeded solution growth method. Its large size allowed us to perform comprehensive investigation on this crystal. A complete set of elastic, dielectric and piezoelectric coefficients was determined using combined resonance and ultrasonic methods. The longitudinal piezoelectric coefficient d33 and the transverse piezoelectric coefficient d31 of this lead-free piezoelectric single crystal are as high as 354 pC N−1 and −163 pC N−1, respectively, which are two times higher than those of KNN-TL ceramics and are comparable with those of PZT5. The electromechanical coupling factor k33 is as high as 82%, much higher than those of KNN-TL ceramics and PZT5. The coefficient d33,strain was found to be as high as 672 pm v−1 and the extrinsic contribution (~22%) was investigated by Rayleigh analysis. These high piezoelectric properties make this single crystal a promising candidate for replacing lead-based piezoelectric materials.

•Temperature behavior of piezoelectric properties in Mn-doped and undoped PIN–PMN–PT.•Higher curie temperature, coercive field and mechanical quality from Mn substitution.•Better temperature stability of k33, d33 and Qm in Mn-doped PIN–PMN–PT.The temperature dependence of dielectric, piezoelectric, and electromechanical coupling properties of 0.5 wt.% manganese-doped and undoped [0 0 1]c-poled 0.24Pb(In1/2Nb1/2)O3–0.47Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 (0.24PIN–0.47PMN–0.29PT) single crystals has been investigated. Compared to the undoped single crystal, the Mn-doped 0.24PIN–0.47PMN–0.29PT demonstrated higher level of coercive field (9.8 kV/cm), increased Curie temperature (187 °C), improved mechanical quality factor Qm (196), decreased piezoelectric constant d33 and comparable electromechanical coupling factor k33, indicating hardening effects caused by the manganese ion substitution. More importantly, it was found that the Mn substitution significantly enhanced temperature stabilities of k33, d33 and Qm, leading to 30–40 °C improvement of the usage temperature range. These results show the application potential of Mn-doped ternary PIN–PMN–PT single crystals for the high-temperature and high-power electromechanical devices.

The influence of temperature on electric-field-induced domain switching of [0 0 1]c oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–0.3PT) single crystal has been studied. The piezoelectric properties of PMN–0.3PT single crystal change drastically at one critical field at 30 °C and two critical fields at 90 °C corresponding to electric-field-induced domain switching. The domain structures were studied by polarizing light microscopy on the [1 0 0]c surface under the electric field applied along [0 0 1]c direction. The PMN–0.3PT single crystal exhibits a rapid increase in piezoresponse at 100 V/mm, which is related to R–MA phase transformation. At 90 °C, the M and T0 0 1 phases coexist at 100 V/mm, while T0 0 1 mono-domain appears at 300 V/mm. The domain switching process here can be identified as (T1 0 0 or T0 1 0) → M → T0 0 1. The experimental results show that the phase state and domain structures of the crystal are closely related to the piezoelectric behaviors.Highlights► The rapid increase in piezoresponse (at 100 V/mm) is related to the polarization rotation through R–MA phase transformation at 30 °C. ► The irregular shaped micro-domains are observed without drastic changes in piezoresponse at 30 °C. ► Two critical fields are observed at 90 °C, the domain switching process can be identified as (T1 0 0 or T0 1 0) → M → T0 0 1. ► The mono-domain state is induced beyond 300 V/mm at 90 °C.

The orientation dependence of electromechanical properties of relaxor based ferroelectric single crystals Pb(Zn1/3Nb2/3)O3–(6–7)%PbTiO3 and Pb(Mg1/3Nb2/3)O3–33%PbTiO3 has been calculated by coordinate transformation. Different from previous studies, the optimum cutting orientations have been predicted in terms of their piezoelectric responses in the corresponding crystal planes. The calculation results indicated that the anisotropic piezoelectric effects of [001]c and [011]c poled multi-domain crystals mainly come from the intrinsic contribution. However, the strong dielectric anisotropy of [001]c poled multi-domain crystals mainly comes from extrinsic domain and domain wall contributions. For [011]c poled multi-domain crystals, the intrinsic orientation effect enhances the dielectric anisotropy.

A tungsten/epoxy composite film integrated with graphene oxide (GO) is prepared via a layer-by-layer assembly method, which shows enhanced acoustic attenuating performance compared with a tungsten/epoxy composite film in the absence of GO. The basic structure consists of tungsten/epoxy/GO/epoxy (W/E/GO/E), in which the inner wrapped epoxy acts as a buffer layer to anchor GO nanosheets on W spheres, and the outer wrapped epoxy layer is designed to prevent GO nanosheets peeling off from W spheres. The design was beneficial in guaranteeing the independence and integrity of the W/E/GO/E structure. The structure of the core–shell composites was characterized with Fourier-transform infrared spectra, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The acoustic properties of the films were evaluated by a conventional pulse-echo overlap technique at the frequency of 9 MHz. It was found that the acoustic attenuation of the optimal W/E/GO/E composite films was much higher than those of traditional W films at a band frequency range from 5 MHz to 12 MHz, and 36.58 ± 0.2 dB cm−1 MHz−1 was obtained at 9 MHz. The wrapping of GO on the surface of W/E will create a crumpled surface for better mixing with the epoxy matrix due to hydrogen bonding and chemical bonding, leading to the preparation of a high quality composite film with minimal structural defects including bubbles and cracks. The enhanced acoustic absorption property of the composite films was attributed to a synergistic effect among its multicomponents, as well as the contribution of GO's thermoacoustic effect. W/E/GO/E composite films with such excellent attenuation loss properties have promise to be the backing material for ultrasonic transducers.