Processing,microstructure and piezoelectric properties of Li-doped BCZT ceramics

Li₂CO₃ is a promising additive to reduce the sintering temperature for (Ba_0.85Ca_0.15) (Zr_0.1Ti_0.9)O₃ (BCZT) ceramics, however, the solubility of Li₂CO₃ in water and the high volatility of Li₂O at elevated temperatures make the processing and densification of BCZT-Li₂CO₃ ceramics (known as BCZT-L) challenging. In our work, an optimized processing route was developed to obtain dense and flat BCZT-L ceramics made with 0–10 mol% of Li₂CO₃ and involving sintering at 1300 °C–1400 °C. The chemical and structural evolution of BCZT-L ceramics during sintering with and without a BCZT powder bed are comprehensively documented and the distribution of Li in the matrix has been observed through TOF-SIMS to explain the effects of Li doping on the piezoelectric properties. The d₃₃ and k_p of BCZT-L initially increased with Li content, but then decreased with excess Li. The decreased d₃₃ and k_p with excess Li is associated with Li aggregation in the BCZT matrix.     

Interaction between an electrically permeable crack and the imperfect interface in a functionally graded piezoelectric sensor

For a cracked piezoelectric sensor with an imperfect interface, the interaction between the crack and the imperfect interface is a problem of practical significance. Such a problem is investigated by the method of singular integral equation in the present work. The interface is assumed to be mechanically compliant and weakly conducting. Parametric studies on stress intensity factors (SIFs) indicate that when the crack is near to the interface SIFs increase as the interface change from perfection to imperfection, and the mechanical imperfection generally has more remarkable influence on SIFs than the dielectric imperfection does. For a crack in the piezoelectric layer and near to the interface, the SIF gets less sensitive to the variation of the substrate thickness if the interface becomes imperfect. The interfacial imperfection has less influence on the fracture behavior of a stiffer piezoelectric layer.     

Design and fabrication of medical micro-nebulizer

The application of a spray device to treat respiratory disease has long been known to be effective. Clinical studies have shown that the finer the droplet size, the deeper position of trachea that can be reached. With the aim of enhancing the curative effect, this study proposes a new design of micro-nebulizer, integrating a piezoelectric actuator, micro-nozzle plate, and the cavity of the micro pump to achieve a high-quality atomizing effect. In this research, we built a novel portable micro-nebulizer device with a changeable and cleanable medication chamber, which can generate a spray droplet of less than 4.04 μm in size (sauter mean diameters, SMD) and has a nebulizer rate of 0.5 ml/min. The device has an electrical requirement of drive voltage only 3 V, with a low power consumption of 1.2 W and an optimal operation frequency of 120 kHz. According to the simulations and experiments, it can be inferred that the droplet size distribution of the micro-nebulizer described in this research is finer and more uniform than that of the current conventional devices; furthermore, the system power consumption is lower than that of the ultrasonic-type nebulizer.     

New type of piezo-damping epoxy-matrix composites with multi-walled carbon nanotubes and lead zirconate titanate

A new type of rigid piezo-damping epoxy-matrix composites containing multi-walled carbon nanotubes (CNT) and piezoelectric lead zirconate titanate (PZT) was prepared, and the electrical and the damping properties were investigated. Dynamic mechanical thermal analysis reveals that the loss factors of the composites were improved by incorporation of PZT and CNT under the concentration above a critical electrical percolation. Based on this piezo-damping material, the PZT contributes to the transformation of mechanical noise and vibration energies into electric energy, while the CNT serve in the shorting of the generated electric current to the external circuit. An optimum formulation for the piezo-damping epoxy-based materials can be designed on the basis of the results of this study.     

Optimization of composite plates with piezoelectric stiffener-actuators under in-plane compressive loads

Stiffeners which are used to strengthen a plate can be constructed of piezoceramic materials and subsequently used as piezo actuators to improve the load carrying capacity of the plate. In the present study, a fibre composite plate with initial imperfections and under in-plane compressive loads is studied with a view towards minimizing its deflection using the piezo actuators and the fibre orientations. Piezoceramic stiffeners are bonded symmetrically on the top and bottom of the plate and deployed as actuators. Two cases of electric fields, namely, the in-phase and out-of-phase voltages are applied to the actuators. The presence of initial deflections leads to deformation under the in-plane compressive loads which should be less than the critical buckling load. Two cases of initial imperfections are considered, and the first one is the deterministic initial deflections which are known a priori and as such they are given as input parameters for the problem. In the second case the initial deflections are uncertain and they have to be obtained according to a given criterion. In the present study they are determined to produce the least favourable initial deflection (largest deflection) at a given point and the solution is obtained by convex modelling. The effect of the actuators, the ply angles and the voltage are studied and their effects on the transverse deflection are investigated. A performance index involving the L₂ norm of the deflections is minimized using the piezo effect and as well as the ply angles the optimal values of which are determined for various problem parameters.     

Cement-based piezoelectret

A cement-based piezoelectret is reported for the first time. Both poling during setting and sodium silicate liquid admixture strengthened the piezoelectret effect. The electret voltage increased upon compressive strain, with partial reversibility; the voltage change was up to 450 V per unit strain (i.e., piezoelectret coupling coefficient up to 4.2 × 10⁻¹⁵ m/V). The effect was relatively strong for a Na⁺ concentration of 0.5 M in the water, in combination with a poling electric field of 225 V/m. The effect increased with increasing magnitude of the constant compressive stress. The direct piezoelectric effect was observed as a minor effect, with the voltage decreasing upon compressive strain; the voltage change was up to 6.7 V per unit strain (i.e., piezoelectric coupling coefficient down to −2.3 × 10⁻¹⁶ m/V). An Na⁺ concentration of 0.5 M gave superior performance than 1.0 M. For 0.5 M, the compressive modulus and piezoelectret coupling coefficient were higher. The poling reduced the compressive modulus and caused pore formation in the vicinity of the electrodes, but it enhanced the piezoelectret effect.     

Interface crack between two interface deformable piezoelectric layers

Based on an interface deformable piezoelectric bi-layer beam model, a bonded piezoelectric bi-material beam with an interface crack perpendicular to the poling axis is analyzed within the framework of the theory of linear piezoelectricity. The layer-wise approximations of both the elastic displacements and electric potential are employed, and each sub-layer is modeled as a single linearly elastic Timoshenko beam perfectly bonded together through a deformable interface. Using the impermeable crack assumption, the closed form solutions for the energy release rate (ERR) and crack energy density (CED) are derived for the layered piezoelectric beam subjected to combined uniformly distributed electromechanical loading. Based on superposition principle, both the ERR and CED and their components are all reduced to the functions of the crack tip loading parameters. Loading dependence of the total CED with respect to the applied electric field is manifested with the analytical results, showing that there is a transformation from an even dependence to an odd dependence for the normalized CED when the applied mechanical loading increases. Compared with the commonly used equivalent single layer model, the proposed analysis augments the crack driving force by alleviating the stress concentration along the interface and thus increases the loading parameters at the crack tip. The proposed model provides improved solutions for fracture analysis of piezoelectric layered structures and sheds light on the loading dependence of the fracture parameters (i.e., the ERR and CED) with respect to the applied electromechanical loadings.     

声表面波器件的研究进展

声表面波器件作为一种新型的电子器件,近年来引起了人们极大的关注,在科学研究领域有了较大的进展,在现代无线通讯领域的应用范围日趋广泛.本文简述了目前国际上出现的声表面波器件的制备方法、性能研究及其应用,展望了其今后的发展趋势.     

Polarization effects in nitride semiconductors and device structures

Wide bandgap nitride semiconductors have recently attracted a great level of attention owing to their direct bandgaps in the visible to ultraviolet regions of the spectrum as emitters and detectors. Moreover, this material system with its favorable hetero-junctions and transport properties began to produce very respectable power levels in microwave amplifiers. If and when the breakdown fields achieved experimentally approach the predicted values, this material system would also be very attractive for power switching devices. In addition to the premature breakdown, high concentration of defects, and inhomogeneities, a number of scientific challenges remain including a clear experimental investigation of polarization effects. In this paper, following a succinct review of the progress that has been made, spontaneous and piezoelectric polarization effects and their impact on sample device-like hetero-structures will be treated. Received: 5 April 1999 / Reviewed and accepted: 19 April 1999