Flash Sintering Research Articles

Role of oxygen vacancy on rapid densification and giant dielectric properties of flash sintered Ca2+ and Ta5+ co-doped TiO2 ceramics

(Ca, Ta) co-doped TiO2 ceramics were successfully prepared using flash sintering technology, and the effects of oxygen vacancies on the rapid densification and giant dielectric properties were studied. The results suggest that the maximum volume fraction of oxygen vacancies in flash-sintered samples is approximately 10%. Oxygen vacancies decrease the barrier energy level of atomic diffusion and provide convenient diffusion channels, resulting in rapid densification of the sample within 24 min at 1100 °C. All flash-sintered samples exhibit high relative density, exceeding 95%. Oxygen vacancies promote the electron-pinning defect dipole effect at low temperatures. As the temperature increases, carriers such as oxygen vacancies move to the grain boundary, forming an internal barrier layer capacitance effect. The optical dielectric constant reaches 7.25 × 104, and the dielectric loss drops to 0.09. Oxygen vacancy play an important role in both flash sintering technology and the giant dielectric properties of co-doped TiO2 ceramics.

Electrochemical grand potential-based phase-field simulation of electric field-assisted sintering

An electrochemical grand potential functional was proposed to describe the sintering of an ionic ceramic green body. The resultant phase-field description enables simulation of the consolidation of an arbitrary number of granular particles and their interactions with the surrounding void phase. The model includes the effects of charged vacancies and the associated interactions between internal and applied electric fields. Defect segregation to grain boundaries is also accounted for, as well as enhanced interfacial defect mobilities. The model was parameterized for Y2O3. Simulations of two-particle systems showed that the applied electric field had an increasingly important impact on neck growth as particle size increased. A sudden rapid increase in temperature occurred for larger field strengths, which has been reported to be correlated to the onset of a flash event in flash sintering. Simulations of many particles showed that internal heat generation by Joule heating was localized at particle–particle contacts (grain boundaries), even though their conductivities were lower than nearby internal particle-void interfaces. A percolative path for ionic charge across the green body and the ceramic sintered solid was thus defined, accelerating the Joule heating process as the porosity of the green body is removed.

Ultrafast fabrication of W/Cu composites using flash sintering

When the proportion of reinforcing phases is high and the wettability between the secondary phase and the metal is poor, preparing fully dense particle-reinforced metal matrix composites becomes challenging. This study employs the flash sintering method to prepare fully dense WCu20 composite materials within a few tens of seconds. It is found that during the “thermal runaway' process of flash sintering, the Cu phase undergoes rapid melting and solidification, and the core mechanism of material densification is that pressure causes Cu liquid to quickly fill the gaps between tungsten particles. Pressure not only promotes the contraction of the framework formed by tungsten particles but also facilitates the complete filling of gaps within the tungsten frameworks by the Cu liquid and promotes the rapid expulsion of bubbles from the Cu liquid, thereby achieving rapid densification of WCu20 composite materials. Moreover, an increase in the temperature enhances the fluidity of the Cu liquid, which aids in filling the gaps between tungsten particles and promotes the discharge of bubbles from the Cu liquid. Flash sintering is a common technique for the sintering and densification of metal powders, suitable for the rapid preparation of various metal matrix composites.

In situ investigation on the evolution mechanism of heterogeneous micro-scale structure in potassium-niobate microwave flash sintering

Heterogeneity in the final product of flash sintering is a significant limitation for massive development. Synchrotron radiation computed tomography (SR-CT) was initially used to investigate the in-situ experiment of potassium-niobate (KNbO3) prepared by microwave flash sintering, revealing the temporal evolution of KNbO3 microstructure. The study has revealed the critical role of spatial arrangement between incompact particles and sintered agglomerates in achieving homogeneity in flash-sintered samples. Particles located the angle formed by the sintered agglomerate are more likely to experience electric field concentration, resulting in densification, and the densification rate decreases as the increases of angle size formed by the dense sintered agglomerates. This letter highlights that the presence of particles between sintered agglomerates is a significant factor contributing to heterogeneous sintering. The phenomenon is explained through a combination of experiments and simulation, which holds promise for enhancing the homogeneity of the sintering process.

Flash luminescence, resistive switching and metal-insulator transitions in ceramic oxides

A combination of voltage-induced, reversible OFF-ON resistive switching and electroluminescence is reported in two materials that are both mixed conductors of oxide ions and electron holes. This occurs at high temperatures, 450–700 °C in cubic, fluorite-structured hafnia, YxHf1-xO2-x/2 and at more modest temperatures, 200–300 °C in Ca-doped BiFeO3 with a cubic perovskite structure. In both cases, a small voltage in the range 2–40 V is applied across mm-thick ceramic samples. Both effects are influenced greatly by oxygen exchange with the surrounding atmosphere and involve a combination of electron injection at the cathode, hole injection at the anode and generation of internal pn junctions. There are strong similarities between these effects and flash sintering but without the significant sintering, thermoluminescence and Joule heating that may be observed with flash sintering.

Innovations in Electric Current-Assisted Sintering for SOFC: A Review of Advances in Flash Sintering and Ultrafast High-Temperature Sintering

This review discusses the groundbreaking advancements in electric current-assisted sintering techniques, specifically Flash Sintering (FS) and Ultrafast High-Temperature Sintering (UHS), for their application in Solid Oxide Fuel Cells (SOFCs). These innovative sintering methods have demonstrated remarkable potential in enhancing the efficiency and quality of SOFC manufacturing by significantly lowering sintering temperatures and durations, thereby mitigating energy consumption and cost. By providing a detailed overview of the mechanisms, process parameters, and material characteristics associated with FS and UHS, this paper sheds light on their pivotal role in the fabrication of SOFC components such as electrolytes, electrodes, multilayered materials, and interconnect coatings. The advantages, challenges, and prospective opportunities of these sintering technologies in propelling SOFC advancements are thoroughly assessed, underlining their transformative impact on the future of clean and efficient energy production technologies.

Flash sintering of metal-like ceramics: A short overview

Flash sintering was officially discovered about fifteen years ago as a ground-breaking method to consolidate ceramics in very short time at furnace temperature much lower than that used for conventional sintering. Since then, it has been applied to several ceramics and composites with the common feature of being characterized by a negative temperature coefficient for resistivity while few attempts have been made on materials with metal-like electronic conduction. In the present overview the results of the investigation carried out in the last years aiming at consolidating tungsten carbide and zirconium diboride by flash sintering are reported and the conditions for inducing a thermal runaway phenomenon like in other ceramics and understand the physical mechanisms behind the flash effect are analysed. The investigation allows to point out the importance of the fundamental processing variables like the applied pressure and voltage for obtain high density material. The key role of the starting particles’ surface chemistry is identified to generate specific microstructures and determine peculiar physical and mechanical properties in the flash sintered ceramic.

Flash sintering of ZrO2–SnO2–ZrSnO4 composite nanofibers: Fabrication, properties, and biochemical effects in Drosophila melanogaster

ZrO2–SnO2–ZrSnO4 composite nanofibers (CNFs) were fabricated by an electrospinning technique in three different ratios by volume (%v/v). Changing the composition ratio of CNFs affected the temperature and sinter time variation of both the band gap and the flash sintering (FS) phenomenon. FS experiments were used at a current cut-off of 3.77 mA/mm2 under thermal (844–878 °C) and electric field (420 V/mm). Highly dense CNFs were obtained in less than 80 s with a maximum power absorption of 1.58 W/mm3. In addition to the FS process, CNFs were sintered with the conventional sintering (CS) method at 1200 °C for 1 h. As the amount of Sn in the composition (%v/v) increases, the density increases, but the hardness value decreases. The FS process was proven to produce faster, denser, and harder material than CS.Long-term and permanent results are obtained by designing biocompatible nanocomposite materials in health. However, determining the long-term effects of CNFs, which are considered non-toxic, is necessary. Drosophila melanogaster female food was covered with these different v/v% ratios CNFs (ZrSn, Zr2Sn; 2ZrSn) and contact by life span: The effects were evaluated on movement, weight, egg production, longevity, and antioxidant system (Superoxide dismutase-SOD, catalase-CAT, glutathione-GSH, and 2,2-diphenyl-1-picrylhydrazil-DPPH). It can be said that Zr2S CNFs were not toxic by not changing the life span, egg production, climbing, and biochemical parameters of the fly. However, it was determined that CNFs used in ZrSn and 2ZrSn ratios slightly reduced the behavioral (movement and longevity) and antioxidant activities (SOD and CAT) of the individual.

Processing map for touch‐free flash sintering of a whiteware ceramic

AbstractFlash sintering has evolved into touch‐free sintering, where free‐standing workpieces can be sintered without attaching electrodes. Instead, the flash is transmitted from the surface of a reactor into the workpiece with superimposition of a magnetic field. Thus, sintering now depends on two independent parameters: the current used to sustain the flash in the reactor and the current flowing through the induction coil. We present a first report on the influence of these two parameters on the quality of the sintered workpiece. The specimens were made from whiteware, consisting of aggregates of ceramic particles interspersed with particles of a glass phase. The results are presented in a map with the reactor current and the induction current as the control variables. Three regimes are identified: insufficient sintering, good sintering, and the formation of defects. The reactor current emerges as an important variable: densification is poor if it is too low, and defects form if it is too high, with high density achieved in the intermediate regime. High induction currents are needed to achieve good sintering. Touch‐free flash sintering has also been shown to sinter and at the same time transform powders of elemental oxides into a single‐phase multicomponent ceramic.

Ultra-fast immobilization of simulated TRPO waste into as-prepared Gd2Zr2O7 by flash sintering

Flash sintering can prepare ceramics in extremely short time, which can significantly improve fabrication efficiency and is promising for processing nuclear waste. Herein, simulated TRPO waste (16 components containing low-melting components) was directly added into as-prepared Gd2Zr2O7, and a series of waste forms were successfully prepared by flash sintering at 950 °C for only 3 min. It was demonstrated that samples prepared by flash sintering showed almost no loss of simulated low-melting-point radionuclide, while samples prepared by pressureless sintering have certain losses of simulated low-melting-point radionuclide. The 42 d leaching rates of Mn, Ce, Nd, and Gd of the flash sintered samples were 8.21 × 10−5, 5.31 × 10−7, 4.04 × 10−6, and 1.04 × 10−6 g m−2 d−1, respectively, showing excellent chemical durability. This work proves that flash sintering technology can effectively suppress nuclear leaks during immobilization, along with excellent chemical durability.

Plasma formation during flash sintering of boron carbide – Part I: Plasma characteristics

In so-called Field Assisted Sintering Techniques such as Spark Plasma Sintering (SPS) or Flash Sintering (FS), ceramic powder compacts are densified at high sample temperature in the presence of an electrical field. The formation of a plasma is often discussed without actual evidence for its appearance. In this work, which is the first of two parts, the mechanisms of plasma formation as well as its properties under FS conditions were investigated. Field induced breakdown of various atmospheres was conducted by a DC-field (up to 350 V/cm) at temperatures between 1000 and 1500 °C. Experiments were conducted both with and without a sample mounted between the electrodes. Alumina, boron carbide and silicon carbide in green or sintered form were used as the samples. The electrical and optical properties of the plasma were characterised, and the early stages were visualised by high-speed camera recordings. The results showed that the breakdown is caused by a flashover mechanism and the conductance of the plasma under the present conditions was of the same order as the conductivity of a boron or silicon carbide green body. Plasma formation did not lead to extensive sintering but the plasma was able to infiltrate the green body and cause transfer of matter from the interior to the surface and beyond. Based on the present results, it seems unlikely that a gas plasma is formed under the normal conditions of SPS or FS.

Flash viscous flow sintering of nanostructured amorphous ZrO2–Y2O3 composite ceramics starting from a partially amorphous precursor powder

Nanostructured amorphous composites are novel materials comprising nano-sized crystal phases embedded in an amorphous matrix, but often necessitating ultra-high-pressure molding for preparation. In this study, nanostructured amorphous ZrO2–Y2O3 composite ceramics were rapidly prepared for the first time using flash viscous flow sintering without the application of pressure. The relative density of the ZY ceramics increased with increasing current density, and ZY composite ceramics with relative density of 86%–97 % were prepared by FS in a short time of 60 s under 700 V/cm with 0.16–0.56 A/cm2. Furthermore, the sintering activities in FS were compared between partially amorphous powder and crystalline powder. The ceramics prepared by flash sintering of partially amorphous powder show a much higher relative density than that from crystalline powder, suggesting that partially amorphous powder achieves easier densification than crystalline powder in flash sintering under given current density and power dissipation. This phenomenon was attributable to the differing sintering mechanisms between the two materials, i.e., solid-state sintering for crystalline powder and viscous flow sintering for partially amorphous powder.

Electrothermal Instabilities in Barium-Titanate-Based Ceramics

An electrothermal analysis for barium-titanate-based ceramics is presented, combining the Heywang–Jonker model for the electric resistivity with a heat dissipation mechanism based on natural convection and radiation in a one-dimensional model on the device level with voltage as the control parameter. Both positive-temperature-coefficient (PTC) and negative temperature coefficient (NTC) effects are accounted for through the double Schottky barriers at the grain boundaries of the material. The problem formulated in this way admits uniform and non-uniform multiple-steady-state solutions that do not depend on the external circuit. The numerical bifurcation analysis reveals that the PTC effect gives rise to several multiplicites above the Curie point, whereas the NTC effect is responsible for the thermal runaway (temperature blowup). The thermal runaway phenomenon as a potential thermal shock could be among the possible reasons for the observed thermomechanical failures (delamination fracture). The theoretical results for the NTC regime and the thermal runaway are in agreement with the experimental flash sintering results obtained for barium titanate, and 3% and 8% yttria-stabilized zirconia.

Effect of alumina morphology on flash sintering and mechanical properties of 3YSZ/alumina composites: Whiskers vs powders

In this paper, 3YSZ/alumina composites containing different content of alumina with different forms (powders and whiskers, respectively) were fabricated by flash sintering. The flash sintering behavior and mechanical properties of the composites were characterized and were compared with those prepared by conventional sintering. The onset temperature of the 3YSZ/alumina whiskers composites was lower than that of the 3YSZ/alumina powders composites with identical alumina content. The reason can be attributed to electrical conductivity; the conductivity of the former was lower than that of the latter. For the 3YSZ/alumina powders composites which can be densified by conventional sintering method, their relative density, Vickers hardness and fracture toughness were comparable to those prepared by flash sintering. While for the 3YSZ/alumina whiskers composites where constrained sintering existed, the flash sintering method revealed obvious advantage over the conventional sintering. A relative density of 94.5 %, hardness of 13.77 GPa and fracture toughness of 5.36 MPa m1/2 were obtained for the flash sintered 3YSZ/30 vol% alumina whisker composites. In contrast, the relative density, hardness, and fracture toughness of the conventional sintered ones were 86.4 %, 11.60 GPa and 4.82 MPa m1/2, respectively.

Expanding the scope of multiphase-flash sintering: Multi-dogbone configurations and reactive processes

In this work, we have expanded the possibilities of the multiphase-flash sintering (MPFS) technique by investigating several configurations that involve multiple dogbone specimens (ranging from 1 to 3) and multiple phases (also ranging from 1 to 3). Unlike the traditional MPFS approach using complex 3D or cylindrical samples, this new method allows for a direct comparison with the established conventional flash sintering technique. Our experimental results with dense 8-mol% Yttria-stabilized zirconia demonstrate a significant reduction in the onset temperature as the number of phases and dogbones increases. Building on these findings, we achieved the preparation of pure bulk specimens of SrFe12O19 for the first time through reactive multiphase-flash sintering.

Flash sintering can induce an anisotropic microstructure in ceramics. A phase field modeling insight

Field-assisted sintering techniques are at the forefront of advanced energy-saving procedures to create fully dense ceramic materials. At present, there is an extraordinary interest in understanding the sintering process and effectively controlling the final specimen's microstructure. This latter aspect is critical, but unfortunately, the short sintering times and intense electric fields under non-equilibrium conditions prevent a rigorous prediction in most cases. This paper is a theoretical contribution to gain insight into an outstanding effect of applying intense electric fields: the formation of non-spherical grain structures. A phase field simulation approach has been carried out to reproduce and determine the origin of this effect. The final simulated microstructures are proved to be textured. These results are explained in the framework of a recent model and compared with experimental evidence reported in the literature.

Residual chlorine prevents full densification of flash sintered yttria-stabilized zirconia ceramics

The densification of yttria-stabilized zirconia (YSZ) during flash sintering is strongly dependent on the amount of residual chlorine in the sintered bodies. The residual chlorine, present as a remnant from powder synthesis, hinders densification after it is trapped and vaporized in the rapidly closing pores, limiting the final density at around 87 – 93% of theoretical. We show that this phenomenon is likely to affect all YSZ powders synthesized from chloride precursors unless the residual chlorine is reduced to ≤ 0.03 wt% before the onset of the flash sintering. This can be universally achieved by thermal purification of green bodies at a high temperature or by chemical purification in ammonium hydroxide. Furthermore, added binder in commercially available Tosoh zirconia TZ-powders (TZ-3YB, TZ-3YB-E) helps to significantly reduce chlorine content during binder burnout which in turn allows flash sintering into a high density even without green body purification.

A numerical model for investigating the effect of material and process parameters on the running of flash sintering

Flash sintering of ceramic powders is a complex process that involves electric, thermal and mechanical phenomena. Modelling should help to better understand and control it, with a view to producing safe and homogeneous parts. A finite element model that couples all relevant phenomena, including the densification and shrinkage of the sintering material, has been worked out for this purpose. This model is used here to investigate the flash sintering of disk-shaped parts. Particular attention is paid to the appearance and growth of heterogeneities within the part, known as hot spots, which are highly dependent on material and process parameters. Leads are provided for limiting the development of such heterogeneities.

Effect of limited current on ultrarapid densification and giant dielectric properties of flash sintering (La, Ta) co-doped TiO2 ceramics

Effect of limited current on ultrarapid densification and giant dielectric properties of flash sintering (La, Ta) co-doped TiO2 ceramics

Flash‐sintering behavior of the composites of 3YSZ and graphene oxide

AbstractFlash sintering is a novel technique that involves the sintering of green bodies in few seconds at relatively very low furnace temperature as compared to what is required to sinter them conventionally. In the present work, we flash‐sintered composites of 3 mol.% yttria‐stabilized zirconia (3YSZ) and graphene oxide (GO). The main aim of this work was to see if the decomposition or oxidation of carbonaceous materials can be prevented by the high heating rates during flash sintering and if composites of 3YSZ and GO can be processed in air. 3YSZ with 5, 10, and 20 wt.% of GO were prepared and subjected to constant heating rate flash‐sintering experiments where electric field was applied across the sample right from room temperature. A DC electric field of 100 V/cm was applied and the current density was set to 100 mA/mm2. It was observed that GO decomposed from the surface of the samples; however, in the core of samples, GO was retained but was reduced to graphite. Grain size of the 3YSZ phase increased with increase in the GO content, whereas the hardness decreased. It was observed that the composites containing carbonaceous reinforcements need to be flash sintered in inert atmosphere to realize their full potential.