Environment-friendly lead-free piezoelectric ceramics have been studied extensively in the past decade with great progress particularly in systems based on a niobate perovskite compound formulated as (K, Na)NbO3 (abbreviated as KNN). A comprehensive review on the latest development of KNN-based piezoelectric ceramics is presented in this article, including the phase structure, property enhancement approaches, and sintering processes as well as the status of some promising applications. The phase structure of KNN was reexamined and associated with the effect of chemical modification on its tetragonal-to-orthorhombic transition. Then, a special focus is placed on the temperature dependence of piezoelectric properties of KNN-based ceramics, followed by reviewing the recent approaches devoted to the temperature-stability enhancement. The processing fundamentals related to the sintering of KNN-based ceramics are also presented with an emphasis on compositional and microstructural control. Finally, this review introduces several industrial attempts of traditional piezoceramic products using KNN-based ceramics and the studies on some promising application in authors' laboratory.

(K, Na)NbO3-Based Lead-Free Piezoceramics: Fundamental Aspects, Processing Technologies, and Remaining Challenges

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1–100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and ...

/pdf/continuous-hydrothermal-synthesis-of-inorganic-nanoparticles-4pr8muq3n8.pdf

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanorods, nanowires, and nanotubes of ferroelectric perovskites have recently been studied with increasing intensity due to their potential use in non-volatile ferroelectric random access memory, nano-electromechanical systems, energy-harvesting devices, advanced sensors, and in photocatalysis. This Review summarizes the current status of these 1D nanostructures and gives a critical overview of synthesis routes with emphasis on chemical methods. The ferroelectric and piezoelectric properties of the 1D nanostructures are discussed and possible applications are highlighted. Finally, prospects for future research within this field are outlined.

One-dimensional nanostructures of ferroelectric perovskites.

Single-crystalline Bi4Ti3O12 nanosheets with rectangular shape and exposed {0 0 1} facets were successfully synthesized via a sol–gel hydrothermal process for the first time. Their average side lengths were around 100 and 150 nm, respectively, and thickness ∼20 nm. As a result, the Bi4Ti3O12 nanosheets reached as high as 79.2% of photodegradation efficiency of Rhodamine B (6 ppm of initial contentration) under 90 min of sunlight irradiation, which was ∼3 times that of conventionally calcined Bi4Ti3O12. The synthesized nanosheet catalyst also exhibited high reusability for the photodegradation reaction due to the chemical stability of its single crystalline structure. Moreover, photodegradation efficiency of the Bi4Ti3O12 nanosheets increased as decreasing Rhodamine B concentrations in the range of 6–20 ppm or pH values in the range of 3.1–11.2. Addition of 0.1 M SO42− to reaction solution increased almost 6% of photodegradation efficiency of Bi4Ti3O12 nanosheets under 90 min of illumination. These results demonstrate the superiority of the novel synthetsis technique for preparing bismuth titanate photocatalyst. The much-enhanced photocatalytic performance of Bi4Ti3O12 nanosheets is attributed to their increased active surface sites, efficient separation and transfer of photogenerated charge carriers and greatly suppressed recombination rate of e−/h+ pairs, arising from their large specific surface area, small particle size, single crystallinity, and specific exposed facets.

Enhanced photocatalytic performance over Bi4Ti3O12 nanosheets with controllable size and exposed {0 0 1} facets for Rhodamine B degradation

Nanostructured materials are central to the evolution of future electronics and information technologies. Ferroelectrics have already been established as a dominant branch in the electronics sector because of their diverse application range such as ferroelectric memories, ferroelectric tunnel junctions, etc. The on-going dimensional downscaling of materials to allow packing of increased numbers of components onto integrated circuits provides the momentum for the evolution of nanostructured ferroelectric materials and devices. Nanoscaling of ferroelectric materials can result in a modification of their functionality, such as phase transition temperature or Curie temperature (TC), domain dynamics, dielectric constant, coercive field, spontaneous polarisation and piezoelectric response. Furthermore, nanoscaling can be used to form high density arrays of monodomain ferroelectric nanostructures, which is desirable for the miniaturisation of memory devices. This review article highlights some research breakthroughs in the fabrication, characterisation and applications of nanoscale ferroelectric materials over the last decade, with priority given to novel synthetic strategies.

Ferroelectric nanoparticles, wires and tubes: synthesis, characterisation and applications

Formation and transformation of ZnTiO3 prepared by sol–gel process

Synthesis and characterization of lead-free K0.5Bi0.5TiO3 ferroelectrics by sol–gel technique

Sol-gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

The sol-gel-hydrothermal processing of (Na 0.8 K 0.2 ) 0.5 Bi 0.5 TiO 3 (NKBT) nanowires as well as their densification behavior were investigated. The morphology and structure analyses indicated that the sol-gel-hydrothermal route led to the formation of phase-pure perovskite NKBT nanowires with diameters of 50-80 nm and lengths of 1.5-2 μm, and the processing temperature was as low as 160°C, but the conventional sol-gel route tended to lead to the formation of NKBT agglomerated porous structured nanopowders, and the processing temperature was higher than 650°C. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires at a low temperature. Owing to the better packing efficiency and therefore a good sinterability of the freestanding nanowhiskers, the pressed pellets made by NKBT nanowires showed >98% theoretical density at 1100°C for 2 h. The sol-gel-hydrothermal-derived ceramics have typical characteristics of relaxor ferroelectrics, and the piezoelectric properties were better than the ceramics prepared by the conventional sol-gel and solid-state reaction.

Lei Hou

Papers

Formation and transformation of ZnTiO3 prepared by sol–gel process

Synthesis and characterization of lead-free K0.5Bi0.5TiO3 ferroelectrics by sol–gel technique

Sol-gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

(Na0.8K0.2)0.5Bi0.5TiO3 Nanowires: Low-Temperature Sol–Gel–Hydrothermal Synthesis and Densification

Comparative study of K0.5Bi0.5TiO3 nanoparticles derived from sol-gel-hydrothermal and sol-gel routes