A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

Dielectric polymers with high dipole density have the potential to achieve very high energy density, which is required in many modern electronics and electric systems. We demonstrate that a very high energy density with fast discharge speed and low loss can be obtained in defect-modified poly(vinylidene fluoride) polymers. This is achieved by combining nonpolar and polar molecular structural changes of the polymer with the proper dielectric constants, to avoid the electric displacement saturation at electric fields well below the breakdown field. The results indicate that a very high dielectric constant may not be desirable to reach a very high energy density.

http://science.sciencemag.org/content/sci/313/5785/334.full.pdf

A Dielectric Polymer with High Electric Energy Density and Fast Discharge Speed

Fundamentals, processes and applications of high-permittivity polymer–matrix composites

The surface charge on some commercially available zinc oxide powders in aqueous solution has been studied. Aqueous suspensions of zinc oxide could not be stabilized electrostatically because of the decreasing pH of suspensions in the region between 7.2 and 12 as a result the transformation of colloidal Zn(OH)2(S) particles to Zn(OH)2(aq) ions. The major cationic impurity in the zinc oxide powders is lead, whereas in the aged suspensions of the same powders, zinc and sulphate ions dominate. These ions have the the most significant effect in creating an electrical double layer shell around the zinc oxide particles. The effect of these ions on the surface potential of zinc oxide was studied and the point of zero charge and charge reversal were found to be a direct function of the zinc and sulphate ions' concentration in the supernatant.

Effect of pH and impurities on the surface charge of zinc oxide in aqueous solution

We have studied the polarization-electric-field hysteresis, the dielectric permittivity dispersion, the piezoelectric properties, the electric-field-induced strain, and the interrelations between these properties for bismuth ferrite (BiFeO3) ceramics. The results indicate that the domain-wall movement in BiFeO3 is strongly inhibited by charged defects, most probably acceptor-oxygen-vacancy defect pairs. The domain-wall mobility can be considerably increased by preventing the defects from migrating into their stable configuration; this can be achieved by thermal quenching from above the Curie temperature, which freezes the disordered defect state. Similarly, Bi2O3 loss during annealing at high temperatures contributes to depinning of the domain walls and an increase in the remanent polarization. The possible defects causing the pinning effect are analyzed and discussed. A weakening of the contacts between the grains in the ceramics and crack propagation were observed during poling with constant field at 100 kV/cm. This is probably caused by an electrically induced strain associated with ferroelastic domain reversal. A relatively large piezoelectric d33 constant of 44 pC/N was obtained by “cyclic poling,” in which the electric field was released after each applied cycle with the purpose to relax the mechanical stresses and minimize the problem of cracking.

Strong ferroelectric domain-wall pinning in BiFeO3 ceramics

In this contribution the effect of alkaline-earth (AE) dopants Mg2+, Ca2+, Sr2+ and Ba2+ on sinterability and functional response of KNN is studied. Ceramic powders with the general formula (K0.5Na0.5)1−2yAEyNbO3, y = 0.005 were prepared by classical ceramic processing. The values of 94–95% of theoretical density (TD) are obtained for stoichiometric KNN after sintering at 1115 °C for 2 h. Calcium and strontium doping promotes densification while magnesium doping inhibits it. In the case of Ba-doping a secondary phase forms. The dielectric constant and piezo d33 coefficient determined by Berlincourt piezo d33-meter of Ca and Sr-doped KNN ceramics are 500 and 95 pC/N, respectively, while the values for undoped KNN are slightly lower.

http://web1.see.asso.fr/electroceramics/html/cdrom/pdf/Papers/C07-03-Ora.pdf

Alkaline-earth doping in (K,Na)NbO3 based piezoceramics

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

Marija Kosec

Papers

Effect of pH and impurities on the surface charge of zinc oxide in aqueous solution

Strong ferroelectric domain-wall pinning in BiFeO3 ceramics

Alkaline-earth doping in (K,Na)NbO3 based piezoceramics

Influence of the critical point on the electrocaloric response of relaxor ferroelectrics

Crystal structure and phase transitions of sodium potassium niobate perovskites