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

Field-assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non-equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

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Field-Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

https://dr.ntu.edu.sg/bitstream/10356/93903/2/125.%20Progress%20in%20Synthesis%20of%20Ferroelectric%20Ceramic%20Materials%20via%20High-Energy%20Mechanochemical%20Technique.pdf

Progress in synthesis of ferroelectric ceramic materials via high-energy mechanochemical technique

A novel finger-sensing nanocomposite with remarkable and reversible piezoresistivity is successfully fabricated by dispersing homogeneously conductive graphite nanosheets (GNs) in a silicone rubber (SR) matrix. Because of the high aspect ratio of the graphite nanosheets, the nanocomposite displays a very low percolation threshold. The SR/GN nanocomposite with a volume fraction of conductive nanosheets closest to that for the percolation threshold presents a sharp positive-pressure coefficient effect of the resistivity under very low pressure, namely, in the finger-pressure range (0.3–0.7 MPa), whereby the abrupt transition could be attributed to compressive-stress-induced deformation of the conducting network. The super-sensitive piezoresistive behavior of the nanocomposite is accounted for by an extension of the tunneling conduction theory which provides a good approximation to the piezoresistive effect.

Piezoresistive behavior study on finger-sensing silicone rubber/graphite nanosheet nanocomposites

Research on sintering of dense ceramic materials has been very active in the past decades and still keeps gaining in popularity. Although a number of new techniques have been developed, the sintering process is still performed at high temperatures. Very recently we established a novel protocol, the “Cold Sintering Process (CSP)”, to achieve dense ceramic solids at extraordinarily low temperatures of <300°C. A wide variety of chemistries and composites were successfully densified using this technique. In this article, a comprehensive CSP tutorial will be delivered by employing three classic ferroelectric materials (KH2PO4, NaNO2, and BaTiO3) as examples. Together with detailed experimental demonstrations, fundamental mechanisms, as well as the underlying physics from a thermodynamics perspective, are collaboratively outlined. Such an impactful technique opens up a new way for cost-effective and energy-saving ceramic processing. We hope that this article will provide a promising route to guide future studies on ultralow temperature ceramic sintering or ceramic materials related integration.

Cold Sintering Process: A Novel Technique for Low‐Temperature Ceramic Processing of Ferroelectrics

The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B2O3–Bi2O3—CdO. It exhibited the following dielectric and piezoelectric properties: Kp= 0.52 to 0.58, Qm= 1000, eT33/e0= 800 to 1000, tan δ= 50 × 10−4, ρ= 7.56 to 7.64 g/cm3. Ceramics with the aid of suitable dopants (CdO, SiO2, and excess PbO) in the Pb-(Ni1/3Nb2/3)O3—PZT family could be sintered at 860° to 900°C. For these materials, Kp= 0.56 to 0.61, Qm= 1000, eT33/e0= 1500 to 2000, tan δ≤ 50 × 10−4, ρ= 7.80 to 8.03 g/cm3. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

Low temperature sintering of quarternary system lead magnesium nickel niobate zirconate titanate (PMN-PNN-PZT) piezoelectric ceramics has been investigated. This ceramic with proper additives could be sintered at about 900°C and exhibited excellent piezoelectric properties: Kp = 0.61–0.65, Qm ≥ 1000, ϵT/33/ϵ0 = 2300–2800, d 33 = 430–465 × 10−12 C/N, tgδ ≤ 50 × 10−4, ρ = 7.9–8.01 g/cm3. The microstructure and sintering mechanism of the materials has been studied by means of SEM and TEM.

Low temperature sintering of lead magnesium nickel niobate zirconate titanate (PMN-PNN-PZT) piezoelectric ceramic, with high performances

The binary system lead zirconate titanate (PZT) ceramics and monolithic piezoelectric transformer (MPT) have been investigated. The sintering temperature of PZT ceramic could be reduced from about 1250°C to about 960°C by the addition of a small amount of lower melting frit which is composed of B2O3-Bi2O3-CdO. The properties of the material are: Kp = 0.52 − 0.58. Qm = 1000, ϵ T/33/ϵ0 = 800 − 1100, tgδ = 50 × 10−4 ρ = 7.56 − 7.64 g/cm3. The Ag/Pd = 80/20 internal electrodes of MPT were cofired with ceramic green sheets at a temperature of about 960°C. The experimental results show that MPT has very high voltage step up ratio, lower driving voltage. The alternating current no-load voltage step up ratio of 9000 has been obtained. It is 30–40 times higher than that of the conventional single layer piezoelectric transformer.

Lead zirconate titanate ceramics and monolithic piezoelectric transformer of low firing temperature

Low temperature sintering of PZT piezoelectric ceramics has been studied. The sintering temperature of PZT ceramics could be reduced from about 1250°C to about 960°C by the addition of small amounts of lower-melting frit (B2O3-Bi2O3-CdO). The electrical properties obtained were: Kp = 0.52–0.58, Qm ≥ 1000, ϵ T/33/ϵ0 = 800–1000, tg δ ≤ 50 × 10−4. The microstructure, sintering mechanism and the effects of additions have been analyzed by means of SEM, EPMA, XPS, etc. A monolithic piezoelectric transformer using this material has been made.

Low temperature sintering of PZT ceramics

Textured bismuth titanate ceramics were successfully produced using spark plasma sintering and platelike bismuth titanate particles prepared using a molten-salt method. The microstructure and dielectric properties of the samples were investigated. The results showed that the dielectric property of the textured bismuth titanate ceramics was anisotropic in various directions and that spark plasma sintering was an effective sintering technology to obtain textured, dense bismuth titanate ceramics at a low temperature.

Li Longtu

Papers

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

Low temperature sintering of lead magnesium nickel niobate zirconate titanate (PMN-PNN-PZT) piezoelectric ceramic, with high performances

Lead zirconate titanate ceramics and monolithic piezoelectric transformer of low firing temperature

Low temperature sintering of PZT ceramics

Preparation of Textured Bismuth Titanate Ceramics Using Spark Plasma Sintering