This review article aims to provide an updated and comprehensive description of the development of the Electric Current Activated/assisted Sintering technique (ECAS) for the obtainment of dense materials including nanostructured ones. The use of ECAS for pure sintering purposes, when starting from already synthesized powders promoters, and to obtain the desired material by simultaneously performing synthesis and consolidation in one-step is reviewed. Specifically, more than a thousand papers published on this subject during the past decades are taken into account. The experimental procedures, formation mechanisms, characteristics, and functionality of a wide spectrum of dense materials fabricated by ECAS are presented. The influence of the most important operating parameters (i.e. current intensity, temperature, processing time, etc.) on product characteristics and process dynamics is reviewed for a large family of materials including ceramics, intermetallics, metal–ceramic and ceramic–ceramic composites. In this review, systems where synthesis and densification stages occur simultaneously, i.e. a fully dense product is formed immediately after reaction completion, as well as those ones for which a satisfactory densification degree is reached only by maintaining the application of the electric current once the full reaction conversion is obtained, are identified. In addition, emphasis is given to the obtainment of nanostructured dense materials due to their rapid progress and wide applications. Specifically, the effect of mechanical activation by ball milling of starting powders on ECAS process dynamics and product characteristics (i.e. density and microstructure) is analysed. The emerging theme from the large majority of the reviewed investigations is the comparison of ECAS over conventional methods including pressureless sintering, hot pressing, and others. Theoretical analysis pertaining to such technique is also proposed following the last results obtained on this topic.

Consolidation/synthesis of materials by electric current activated/assisted sintering

This review paper focuses on interdigital electrodes-a geometric structure encountered in a wide variety of sensor and transducer designs. Physical and chemical principles behind the operation of these devices vary so much across different fields of science and technology that the common features present in all devices are often overlooked. This paper attempts to bring under one umbrella capacitive, inductive, dielectric, piezoacoustic, chemical, biological, and microelectromechanical interdigital sensors and transducers. The paper also provides historical perspective, discusses fabrication techniques, modeling of sensor parameters, application examples, and directions of future research.

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Interdigital sensors and transducers

Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

The nano-sized coupled oxides ZnO/SnO 2 in a molar ratio of 2:1 (Z 2 S) and 1:1 (ZS) were prepared using the co-precipitation method and characterized with X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy and specific surface area (Brunauer–Emmett–Teller (BET)). Their photocatalytic activities were also evaluated using methyl orange (MO) as a model organic compound. The isothermal adsorption behavior of MO on Z 2 S and the factors affecting the photocatalytic activity, such as the heat-treating temperature for the photocatalyst, the pH value of the reaction suspension and the addition of NaCl, KNO 3 and K 2 SO 4 into the suspension, have been studied. It is found that the change in phase of Z 2 S occurs with the calcination temperature. The band gap energy value observed by UV–VIS diffuse reflectance spectroscopy is 3.15 eV for Z 2 S compared with those of 3.12 eV for ZS, 3.17 eV for ZnO and 2.53 eV for SnO 2 . The isothermal adsorption behavior is a two-stage process. The photocatalytic degradation rate of MO on Z 2 S is faster than that on ZS and ZnO by 40.2 and 66.1%, respectively. A schematic diagram of photocatalytic activity is also presented to explain the results.

Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO2 coupled photocatalysts

Publisher Summary Piezoelectric ceramics can sense and actuate by rapidly converting mechanical and thermal signals into electrical ones, the reverse also being true. The piezoelectric properties and quick response characteristics have made piezoelectric ceramics one of the most commonly used smart materials. The intrinsic brittleness of piezoelectric ceramics and damageability of the materials under electric field — making the materials prone to fracture — are of major concern for product reliability. The fracture of piezoelectric ceramics under combined electrical and mechanical loading has been among the most prevalent research topics. The chapter describes piezoelectricity, ferroelectrics, spontaneous polarization, and electric domains; and discusses the poling process, the hysteresis loop of polarization versus the electric field strength, and the butterfly loop of strain versus the electric field. The chapter focuses on the basic equations commonly used in the study of the fracture behavior of piezoelectric ceramics within the thermodynamics framework, the general solution based on Stroh's formalism (a powerful tool for solving two-dimensional electroelastic problems), analysis of Green's functions for insulating elliptical cavities and cracks, study of conductive elliptical cavities and cracks, study of piezoelectric interface cracks, and three-dimensional electroelastic problems. The four types of nonlinear approaches considered are: electrostriction, domain switching, domain wall kinetics, and polarization saturation at a crack tip. The polarization saturation model that treats piezoelectric ceramics as mechanically brittle and electrically ductile materials is also discussed. The chapter provides an overview of experimental observations whose results show that microstructure and temperature have a profound influence on the fracture behaviors of piezoelectric ceramics under purely mechanical loads, and also discusses the commonly used failure criteria, the electric saturation model, the stress intensity factor criterion and the stress criterion.

Fracture of piezoelectric ceramics

The successive phase transformations in MgO-doped BaTiO 3 were studied. Upon MgO doping, dielectric anomalies corresponding to lower phase transformations were broadened and depressed, while an anomaly for a cubic-tetragonal transformation remained and shifted to a lower temperature. XRD peak splitting upon tetragonality of BaTiO 3 was decreased, and the peaks exhibited abnormally broadened profiles which are different from the one for cubic BaTiO 3 above T c . Raman spectroscopy revealed the existence of orthorhombic phase at room temperature for the solid solution with 0.5 mol% or more MgO. The temperature dependence of the Raman spectrum showed that orthorhombic and rhombohedral phases in MgO-doped BaTiO 3 were stabilized at higher temperatures than pure BaTiO 3 .

Effect of MgO Doping on the Phase Transformations of BaTiO3

Lead zirconate titanate based composites containing silver particles were fabricated from commercially available PZT and Ag2O powder mixtures. Densification behavior and mechanical and dielectric properties were evaluated as a function of the volume fraction of silver. No unwanted reaction phases between the PZT matrix and silver could be detected in the X-ray diffraction analysis. The added silver particles were homogeneously dispersed in the matrix. The densification of PZT was substantially accelerated by the addition of silver particles. Mechanical properties, e.g., fracture toughness and fracture strength, were improved by the addition of silver particles. The relative dielectric constant also increased when the volume fraction of silver was increased. This observed enhancement in dielectric constant may be associated with the effective dielectric field developed by the existence of conducting silver particles.

Low‐Temperature Sintering and High‐Strength Pb(Zr,Ti)O3‐Matrix Composites Incorporating Silver Particles

Pb(Zr,Ti)O 3 (PZT) nanocomposites were prepared from high purity PZT powder and small amount (0.1-1.0 vol%) of oxides, i.e. Al 2 O 3 , MgO and ZrO 2 . Effects of the additives on mechanical and piezoelectric properties of the nanocomposites were investigated. The fracture strength and hardness of PZT nanocomposites with 0.5 vol% Al 2 O 3 or 0.1 vol% MgO additives were significantly improved. The reduced grain size of the nanocomposites is considered to be responsible for the excellent mechanical properties. Though the dielectric constants of the composites were decreased with an addition of Al 2 O 3 or MgO, the planar electromechanical coupling factor, K p , of MgO-added composites was higher than that of Al 2 O 3 -added. As a consequence, the PZT/0.1 vol% MgO nanocomposite showed good mechanical and suitable piezoelectric properties. Addition of ZrO 2 had little effect on mechanical and electrical properties of the composites.

PZT nanocomposites reinforced by small amount of oxides

The Curie temperatures of lead zirconate titanate-based (PZT-based) composites that were incorporated with silver particles were evaluated as a function of the silver content. Although the substitution of the silver ion has been considered to decrease the Curie temperature, PZT/silver composites exhibited higher Curie temperatures than those of monolithic PZT ceramics. It was found that the Curie temperature gradually increased as the silver content increased. It was suggested that the observed Curie temperature anomaly resulted from the relaxation of transformation-induced internal stress by intergranular silver particles.

Curie Temperature Anomaly in Lead Zirconate Titanate/Silver Composites

High-toughness and high-strength lead zirconate titanate (PZT) composites that contain fine silver particles were successfully fabricated at low sintering temperatures. Addition of silver to a PZT matrix did not result in unwanted reaction phases; however, some silver diffused toward the perovskite crystal structure. A small quantity of silver accelerated the sinterability of the PZT composites. The formation of oxygen vacancies due to the partial substitution of silver appeared to enhance the sinterability of the PZT. Fracture toughness depended on the size and degree of sphericity of the silver particles, and SEM observations on crack propagation suggested that the toughening mechanism in the PZT/Ag composites involves crack bridging resulting from the ductile behavior of silver particles. It is proposed that high fracture strength in PZT/1 to 5 vol% Ag composites results from the relaxation of transformation-induced internal stress by the silver particles.

Mutsuo Sando

Papers

Effect of MgO Doping on the Phase Transformations of BaTiO3

Low‐Temperature Sintering and High‐Strength Pb(Zr,Ti)O3‐Matrix Composites Incorporating Silver Particles

PZT nanocomposites reinforced by small amount of oxides

Curie Temperature Anomaly in Lead Zirconate Titanate/Silver Composites

Fabrication, Sinterability, and Mechanical Properties of Lead Zirconate Titanate/Silver Composites