Smart devices, nowadays, are inspiring the infinite vitality and possibilities of intelligent life, such as self-power electromagnetic (EM) nanogenerator and microsensor, smart window, thermally-driven EM absorber, interstellar energy deliverer, and so on. Herein, the latest and most impressive works of 3D nano-micro architectures and their smart EM devices are highly focused on. The most key information, including assembly strategy and mechanism, EM response, and approach-structure-function relationship, is extracted and well-organized with profundity and easy-to-understand approach. The merit and demerit are revealed by comparison. What's more, the brightest and most cutting-edge smart EM devices constructed by 3D nano-micro architectures are reported as highlights, and the device principles are deeply dissected. Finally, a profound and top comment on the fast-growing field as well as challenges are proposed, and the future directions are predicted intelligently.

Assembling Nano–Microarchitecture for Electromagnetic Absorbers and Smart Devices

Developing eco-friendly high-performance piezoceramics without lead has become one of the most advanced frontiers in interdisciplinary research. Although potassium sodium-niobate {(K,Na)NbO3, KNN} based ceramics are believed to be one of the most promising lead-free candidates, the relatively inferior piezoelectric properties and strong temperature dependency have hindered their development for more than 50 years since being discovered in the 1950s. It was not until 2014 that our group initially proposed a new phase boundary (NPB) that simultaneously improved the piezoelectric properties and temperature stability of non-textured KNN-based ceramics to the level of partly lead-based ceramics. The NPB has been then proved by some researchers and believed to pave the way for "lead-free at last" proposed by E. Cross (Nature, 2004, 432, 24). However, the understanding of the NPB is still in its infancy, leaving many controversies, including the phase structure and physical mechanisms at the NPB as well as the essential difference when compared with other phase boundaries. In this context, we systematically summarized the origin and development of the NPB, focusing on the construction, structure and intrinsic trait of the NPB, the effects of the NPB on the performance, and the validity and related incipient devices of the NPB. Particularly, we concluded the phase structure and domain structure locating at the NPB, analyzed the physical mechanisms in depth, proposed the possible methods to further improve the performance at the NPB, and demonstrated the validity and scope of the NPB as well as the device application. Finally, we gave out our perspective on the challenges and future research of KNN-based ceramics with NPB. Therefore, we believe that this review could promote the understanding of the NPB and guide the future work of KNN-based ceramics.

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Emerging new phase boundary in potassium sodium-niobate based ceramics

Additive Manufacturing of Piezoelectric Materials

Reducing or even prohibiting the use of toxic lead in electronic devices has become one of the most cutting-edge topics in various disciplines. The recently proposed phase boundary engineering endows lead-free piezoceramics with comparable performances to that of some lead-based piezoceramics. However, the enhancement in performance hinges on the coexistence of multi-phases and complex domain structures, particularly the occurrence of nano-domains and polar nanoregions (PNRs). Although nano-domains have been significantly studied in lead-based piezoceramics, understanding the nano-domains and PNRs in lead-free piezoceramics is in its infancy and needs a systematic summary and in-depth analysis. Herein, we summarize the nano-domains and PNRs in three representative lead-free piezoceramics (i.e., potassium sodium niobate, barium titanate, and sodium bismuth titanate), focusing on their effects on macro performance. First, we introduce the foundation and tools for the observation of the domains. Then, we summarize the variations in the nano-domains with phase structure, electric field, and temperature, and their effects on performance including piezoelectricity, strain, temperature stability, aging, and fatigue. Finally, we present our perspectives on the future of nano-domains, concentrating on nano-domain engineering. Therefore, this review can help better understand the nano-domains and PNRs in lead-free piezoceramics, and be used for the further development of high-performance lead-free piezoceramics.

/pdf/nano-domains-in-lead-free-piezoceramics-a-review-4ho23rual5.pdf

Nano-domains in lead-free piezoceramics: a review

Progress and perspective of high strain NBT-based lead-free piezoceramics and multilayer actuators

High-performance potassium sodium niobate piezoceramics for ultrasonic transducer

In this paper, lead-free 0.965(K0.45Na0.55) (Nb0.96Sb0.04)O3-0.0375Bi0.5Na0.5Zr0.85Hf0.15O3 (KNNS-BNZH)/epoxy 1–3 composite was designed and fabricated with the dice-and-fill method. The composite material exhibited a high thickness electromechanical coupling coefficient ( $k_{t} = 0.7$ ), high piezoelectric constant ( $d_{33} = 350$ pC N−1), relatively low mechanical quality factor ( $Q_{m} = 5$ ), and relatively low acoustic impedance. An ultrasonic transducer with a center frequency of 5 MHz was produced based on the 1–3 KNNS-BNZH/epoxy composite, showing a broad bandwidth of 80% (−6 dB) and two-way insertion loss of −30 dB. Ultrasonic and photoacoustic images were further demonstrated. The outstanding performance of the 1–3 KNNS-BNZH/epoxy composite transducer competitive to Pb(Zr1– x Ti x )O3 (PZT)-based transducers suggests that the lead-free material can serve as a promising alternative to Pb-based piezoelectric materials for ultrasonic applications.

KNNS-BNZH Lead-Free 1–3 Piezoelectric Composite for Ultrasonic and Photoacoustic Imaging

Increasing array transducer bandwidth (BW) and signal-to-noise ratio (SNR) is a critical issue for producing a high-quality medical ultrasound image. However, array elements with small size tend to have poor sensitivity due to a much higher impedance compared with the electrical impedance of the transmitter and receiver circuit. Implementation of multilayer ceramic (MLC) is an effective way of reducing impedance, and thus, with a potential for improving SNR for an ultrasonic probe. In this work, we fabricated multilayer piezoelectric ceramic with a composition of 0.1Pb(Ni1/3Nb2/3)O3-0.35Pb(Zn1/3Nb2/3)O3-0.15Pb(Mg1/3Nb2/3)O3-0.1PbZrO3-0.3PbTiO3-4mol% excess NiO (PNN–PZN–PMN–PZ–PT), by a roll to roll tape casting process and co-fired with 90Ag/10Pd electrode at a low temperature of 950 °C. Using five-layer MLC (5L-MLC) as obtained, we designed and demonstrated a 5 MHz 32-element array transducer for ultrasonic and photoacoustic imaging. The five-layer transducer element exhibited a BW of 87% at −6 dB, substantially higher than 62% for single-layer ceramic (SLC) element. In addition, the insertion loss was improved by 16.2 dB over the SLC element with an external impedance of $50~\Omega $ . Both the experimental results and theoretical analysis showed that our array transducer made of the PNN–PZN–PMN–PZ–PT MLC is promising for acquiring high-quality ultrasonic and photoacoustic images.

Broadband Ultrasonic Array Transducer From Multilayer Piezoelectric Ceramic With Lowered Co-Firing Temperature

Emerging ultrasound imaging modality based on optical-generated acoustic waves, such as photoacoustic (PA) imaging, has enabled novel functional imaging on biological samples. The performance of the ultrasonic transducer plays a critical role in producing higher quality PA images. However, the high electrical impedance of the small piezoelectric elements in the transducer array causes an electrical mismatch with external circuitry and results in degraded sensitivity. One effective method for reducing the electrical impedance is to implement a piezoelectric multilayer configuration instead of the conventional single layer for the transducer. In this work, we introduced an ultrasonic transducer comprising a piezoelectric polymer multilayer structure produced by an innovative multicycle powder-based electrophoretic deposition, using a suspension of polymer nanoparticles. The multicycle electrophoretic deposition overcomes the redissolution issue in solution-based methods. The ultrasonic transducer comprising the piezoelectric polymer multilayer exhibits significantly enhanced receiving sensitivity as compared to the ultrasonic transducer using a single layer. Ultrasonic transducer with multielement array configuration is obtained using the piezoelectric polymer multilayer, and PA imaging with improved resolution is demonstrated. Theoretical analysis shows that the enhanced transducer performance is mainly attributed to the improved electrical impedance match between the piezoelectric polymer element in the transducer and external receiving circuit.

Ultrasonic Transducer From Piezoelectric Polymer Multilayer Through Electrophoretic Deposition for Photoacoustic Imaging

Implementation of piezoelectric multilayer ceramic (MLC) is an effective way to reduce impedance and improve the performance of linear-array transducer for ultrasonic system applications. However, the ultrasonic image derived from a planar linear-array transducer generally suffers from degradation of lateral resolution and contrast. In this article, we designed and fabricated a focused 5-MHz 128-element linear-array ultrasonic transducer with concave structure using five-layered 0.1Pb (Ni1/3Nb2/3)O3 -0.35Pb(Zn1/3Nb2/3)O3 -0.15Pb(Mg1/3Nb2/3)O3-0.1PbZrO3-0.3PbTiO3 (PNN-PZN-PMN-PZ-PT) piezo- electric ceramic. The transducer showed a bandwidth of 63% at −6 dB and the lateral resolution up to 0.33 mm. An improved transmission signal of 90% higher than a commercial single-layer ceramic transducer was also achieved. We further demonstrated high-resolution photoacoustic imaging with the obtained concave linear-array transducer.

Qingqing Ke

Papers

High-performance potassium sodium niobate piezoceramics for ultrasonic transducer

KNNS-BNZH Lead-Free 1–3 Piezoelectric Composite for Ultrasonic and Photoacoustic Imaging

Broadband Ultrasonic Array Transducer From Multilayer Piezoelectric Ceramic With Lowered Co-Firing Temperature

Ultrasonic Transducer From Piezoelectric Polymer Multilayer Through Electrophoretic Deposition for Photoacoustic Imaging

Concave Array Ultrasonic Transducer From Multilayer Piezoelectric Ceramic for Photoacoustic Imaging