Initial stage sintering mechanism of NaNbO3 and implications regarding the densification of alkaline niobates

The behavior of submicron- and nano-sized NaNbO₃ powder compacts during conventional sintering was studied using optical dilatometry and microstructure analysis. Microstructure-development trajectories revealed the dominance of grain growth during the initial sintering stage, while densification occurred only during later stages. Surface diffusion with low activation energy in the range of 50–60 kJ/mol was found to be the dominant material-transport mechanism during the initial sintering stage. The early activation of surface diffusion reduced the sintering driving force, decreased the rate of the densifying mechanisms and was thus identified as the main cause for poor densification of NaNbO₃. Same explanation could be valid also for other alkaline niobate based lead-free piezoelectric ceramics. Finally, alternative sintering methods are discussed and the efficiency of the pressure-assisted sintering was demonstrated in successful production of highly-dense fine-grained NaNbO₃ ceramics, with relative density and grain size of 98% and 700 nm, respectively.     

A comparative study of direct and indirect evaluation of piezoelectric properties of electrophoreticaly deposited (Ba,Ca) (Zr,Ti)O3 lead-free piezoceramics

The development and optimisation of piezoceramics are targeted usually to enhance their piezoelectric properties evaluated by both the direct or indirect measurement methods. The presented work aims to elaborate on the correlation of one direct (Berlincourt) and two indirect (convert and field-dependent) piezoelectric measurement methods on various material states. The role of the ceramic powder treatment by ball milling and electrophoretic deposition (EPD) technique on the determined electric properties as well as basic physical and mechanical properties of (Ba_0.85Ca_0.15) (Zr_0.1Ti_0.9)O₃ ceramics (BCZT) was investigated. It was found that the EPD technologically supported by milling allows obtaining thick and dense deposits (>2 mm). After sintering, the BCZT ceramics with a relative density of >95%, hardness in the range of 2.3–2.9 GPa and piezoelectric coefficients of d₃₃* = 940 pm/V, d_33(E=0) = 427 pm/V and d₃₃ = 364 pC/N can be achieved. Reported results also suggest that indirect (field-dependent) and direct (Berlincourt) measurements of the piezoelectric coefficients can be comparable at optimal poling conditions.     

Bi-templated grain growth maximizing the effects of texture on piezoelectricity

Templated grain growth is beneficial for piezoelectric materials, the properties of which become the best in their single crystalline form. Nevertheless, a textured ceramic prepared by a templated grain growth technique often fails in exhibiting as good properties as expected in single crystals even with a high degree of orientation factor. Here, we propose a new strategy for maximizing texturing effect by suppressing the growth of untextured matrix grains. The textured ceramics made by our method, so-called bi-templated grain growth, are featured by ultrahigh piezoelectric properties (d₃₃ = ～1,031 pC/N, d?g = ～59,000, k_p = ～0.76). A special emphasis is on the achieved electric-field-induced strain of 0.13 % at 1 kV/mm, which is as high as that of single crystals. This work demonstrates that not only the degree of texture but also the coarsening of untextured matrix grains should be well-controlled to best exploit the templated grain growth technique.     

对运用压电陶瓷智能材料土木工程结构的振动控制回顾和总结

对土木工程中结构的振动控制进行了总结,对具有压电控制驱动的智能结构给予重点总结。过去的十年中,全世界范围内智能结构在螺旋形地发展着。这些智能结构方面的成就主要依靠材料来实现,如压电陶瓷、形状记忆合金、可控流体如磁流变液、电流变液、光纤维传感器和多种其他材料。压电陶瓷是一种低成本、轻质、便于成型的材料,可应用于结构振动的主动控制。压电陶瓷有多种形式如刚性片、柔性片、叠层式压电陶瓷堆、粗纤维复合驱动器和压电陶瓷摩擦阻尼器。只需要对原始结构做出轻微的改变,就可以将压电陶瓷驱动器粘结在结构高应变区域的表面,或者将其植入结构中形成组合结构。另一方面,在对原结构做出轻微改进之后,叠放型驱动器可以与这种需要高控制力和微小位移的结构结合成一体。首先介绍了压电陶瓷材料、各种驱动控制方法和压电陶瓷驱动器的类型,然后回顾了压电陶瓷驱动器在各种不同类型的土木工程结构中如梁、桁架、钢框架和悬索桥等的应用研究。     

Analysis of indentation size effect (ISE) in nanoindentation hardness in polycrystalline PMN-PT piezoceramics with different domain configurations

Piezoelectric materials contain microstructural features (e.g., domain walls, interdomain spacing, and grain size) that span across several length scales, i.e., few nm in the case of interdomain wall spacing to several μm in case grain sizes. Recent experimental findings indicated that the domain configurations have more influence on the hardness of these materials than the grain size. In this study, nanoindentation experiments are conducted on polycrystalline PMN-PT (a relaxor ferroelectric material) with a focus to investigate the influence of domain configurations on the indentation size effect (ISE) in hardness, H. Different domain configurations are achieved by selectively annealing the as poled samples above and below the Curie temperature. Nanoindentation hardness is obtained in the load range of 1–5 mN with the maximum penetration depth well below the grain size of the samples. The experimental results reveal that all the samples, albeit to a different order, exhibit strong Reverse Indentation Size Effect (RISE) and normal ISE in H. The observed ISE is then analyzed using classical Meyer's law, the proportional specimen resistance (PSR) model and modified PSR (mPSR) model. The critical analysis of nanoindentation data reveals that the PSR model provides a satisfactory understanding of the genesis of RISE and ISE considering the elastic resistance of test material and frictional resistance at indenter facet/test material.     

Characteristics of piezoceramic and 3–3 piezocomposite hydrophones evaluated by finite element modelling

The transducer characteristics of hydrophones manufactured from porous 3–3 piezocomposites are compared with dense piezoceramic disc hydrophones using finite element modelling. Due to the complex porous structure of the 3–3 piezocomposites, a real-size 3-dimensional model was developed while a 2-dimensional axisymmetric model was constructed for the simple dense disc hydrophone. The electrical impedance and receiving sensitivity of the hydrophones in water were evaluated in the frequency range 10–100 kHz. The model results were compared with the experimental results. The sharp resonance peaks observed for the dense piezoceramic hydrophone were broadened to a large extent for porous piezocomposite hydrophones due to weaker coupling of the structure. The receiving sensitivity of piezocomposite hydrophones is found to be constant over the frequency range studied. The flat frequency response suggests that the 3–3 piezocomposites are useful for wide-band hydrophone applications.     

Microstructure and electrical properties of 3-0 type composite of Na0.5Bi2.5Nb2O9-based bismuth-layered piezoceramics

The microstructure and electrical properties of 3-0 type composite of Na_0.5Bi_2.5Nb₂O₉-based bismuth layered piezoceramics modified by Al₂O₃ addition are investigated. The darker and plate-like grains, locating at the grain boundaries, are confirmed to be pure α-Al₂O₃ by high resolution transmission electron microscope, not a Bi₂AlNbO₇ pyrochlore phase. This 3-0 type Na_0.5Bi_2.5Nb₂O₉-Al₂O₃ composite piezoceramics have a large piezoelectric constant d₃₃ of 15.2pC/N with good temperature stability up to 600 °C, and good ferroelectric properties with a relatively large remnant polarization of ~11.6 μC/cm². These demonstrate that designing a 3-0 type composite structure would be an effective approach to tailor the microstructure and improve the electrical properties of bismuth layered piezoceremics for their potential applications at temperature up to 600 °C.     

Enhancement of piezoelectric properties stability of submicron-structured piezoceramics obtained by spark plasma sintering

Grain-size effect on functional properties of piezoceramics is nowadays an important topic due to the potential applications of these materials at the submicron scale. In this context, we explore the effect of the grain size reduction on the functional properties stability of piezoceramics. Submicron- and micro-structured Pb(Zr,Ti)O₃-based materials are taken as model samples to carry out this study. Rayleigh analysis is used to quantitatively evaluate the domain wall contribution. The results show that the stability of properties depends strongly on grain size. Specifically, the nonlinear response decreases significantly when the grain size is reduced to the submicron scale. This behavior can be explained by taking into account that a grain size reduction involves a grain boundary increment, which may clamp domain walls. The results suggest that grain size engineering may be an effective alternative to the widely used compositional engineering in order to control the undesirable nonlinear behavior in piezoceramics.     

压电陶瓷式电子提花机的研究与设计

为了减少电子提花机的能耗,提高其稳定性,介绍了使用压电陶瓷片代替电磁铁作为电子提花机组件内核进行选针控制的工作原理,并详细论述了步进电机驱动方式和控制系统的组成和原理.测试数据表明该装置达到了预期效果,在节能、可靠性等方面是一种有发展潜力的提花装置.