After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

Lead zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity of lead and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union in this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their preparation, structure–property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. From the above study, it is concluded that some lead-free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large number of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of the others.

/pdf/review-environmental-friendly-lead-free-piezoelectric-1rosofvdqd.pdf

Review: environmental friendly lead-free piezoelectric materials

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

The last decade has witnessed significant advances in energy harvesting technologies as a possible alternative to provide a continuous power supply for small, low-power devices in applications, such as wireless sensing, data transmission, actuation, and medical implants. Piezoelectric energy harvesting (PEH) has been a salient topic in the literature and has attracted widespread attention from researchers due to its advantages of simple architecture, high power density, and good scalability. This paper presents a comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Various key aspects to improve the overall performance of a PEH device are discussed, including basic fundamentals and configurations, materials and fabrication, performance enhancement mechanisms, applications, and future outlooks.

A comprehensive review on piezoelectric energy harvesting technology: Materials, mechanisms, and applications

Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion. In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.

Oxide materials for high temperature thermoelectric energy conversion

Improvement of thermoelectric properties with the addition of Sb to ZnO

Formation process and microwave dielectric properties of the R2V2O7 (R=Ba, Sr, and Ca) ceramics were investigated. Transient RV2O6 phases, which were formed at 300°∼500°C by the reaction of RCO3 and V2O5, interacted with remnant RCO3 to form the homogeneous triclinic R2V2O7 phases at 600°–700°C. The Sr2V2O7 and Ca2V2O7 ceramics sintered at 1000° and 950°C, respectively, for 10 h showed a dense microstructure with the microwave dielectric properties of ɛr=10.4–12.1, Q×f=15 200–19 520 GHz, and τf=−30.9–34.8 ppm/°C. However, the Ba2V2O7 ceramic was well sintered at 900°C for 10 h and it showed the good microwave dielectric properties of ɛr=10.1, Q×f=51 630 GHz, and τf=−26.5 ppm/°C.

Formation Process and Microwave Dielectric Properties of the R2V2O7 (R=Ba, Sr, and Ca) Ceramics

The effect of Zn on the microstructure and electrical properties of Mn1.17−xNi0.93Co0.9ZnxO4 (0 ≤ x ≤ 0.075) NTC thermistors

The formation process and microwave dielectric properties of the Mg2V2O7 ceramics were investigated. The MgV2O6 phase that was formed at around 450°C interacted with remnant MgO above 590°C to form a homogeneous monoclinic Mg2V2O7 phase. Finally, this monoclinic Mg2V2O7 phase was changed to a triclinic Mg2V2O7 phase for the specimen fired at 800°C. Sintering at 950°C for more than 5 h produced high-density triclinic Mg2V2O7 ceramics. In particular, the Mg2V2O7 ceramics sintered at 950°C for 10 h exhibited the good microwave dielectric properties of ɛr=10.5, Q×f=58 275 GHz, and τf=−26.9 ppm/°C.

Formation and Microwave Dielectric Properties of the Mg2V2O7 Ceramics

(Na0.5K0.5)NbO3 (NKN) ceramic with 1.5 mol% CuO added (NKNC) was well sintered even at a low temperature of 900°C with the addition of ZnO. Most of the ZnO reacted with the CuO and formed the liquid phase that assisted the densification of the specimens at 900°C. A few Zn2+ ions entered the matrix of the specimens and increased the coercive field (Ec) and Qm values of the specimens. High-piezoelectric properties of kp=0.37, Qm=755, and ɛ3T/ɛ0=327 were obtained from the NKNC ceramics containing 1.0 mol% ZnO sintered at 900°C for 2 h.

Low-Temperature Sintering and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

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