Flash Sintering Research Articles

Short Review of Flash Sintering: Mechanisms, Microstructures, and Mechanical Properties

This review article highlights the potential of flash sintering as a novel densification technology for advanced ceramics. Conventional ceramic sintering methods involve heating a powder compact at high temperatures for several hours to trigger the solid-state diffusion of atoms. In contrast, flash sintering takes advantage of electric field and current to drastically lower processing time and temperature, providing a promising solution to reduce the economic, energetic, and environmental costs associated with traditional ceramic sintering methods. The effects of electric field and current during flash sintering result in unique non-equilibrium microstructures that enhance the mechanical properties of advanced ceramics through defect-mediated inelastic deformation mechanisms. This article provides an overview of the flash sintering mechanisms, the unique microstructural features observed in flash-sintered ceramics, and their impacts on mechanical properties.

A Review on Flash Sintering of Graphene Oxide Films: Mechanism and Recent Advances in the Field.

This review highlights the significance of flash sintering, a photothermal route, in reducing graphene oxide (GO) films. Essentially, extensive efforts are devoted to form graphene electrodes due to its distinctive properties, such as high surface area, excellent electrical conductivity, and optical transparency, owing to which it finds widespread use in energy storage devices, wearable electronics, sensors, and optoelectronics. Thus, rapidly rising market demands for these applications necessitate the need of a technique offering ease of manufacturability and scalability for production of graphene electrodes. The solution-processed graphene electrodes (SPGEs) are promising to fulfil these requirements. Particularly, SPGEs are fabricated by reducing GO film to graphene/reduced graphene oxide (rGO) by utilizing any reduction method, such as chemical, solvothermal, electrochemical, etc. Lately, flash sintering has garnered substantial attention as a promising reduction route for rapid, clean, and green production of graphene electrodes. This review briefly describes the underlying principle, mechanism, and parameters of flash sintering to develop an insight and advantages of this method over extensively used reduction methods. The review is a systematic summarization of the electrical, optical, and microstructural properties of rGO films/electrodes fabricated using this technique.

Low-temperature preparation of La0.9Ca0.1CrO3 semiconductor ceramics by flash sintering

Flash sintering of La0.9Ca0.1CrO3 based ceramics can be triggered at very low furnace temperature compare with pure LaCrO3. Due to Ca2+ doped LaCrO3 could lead to the formation of oxygen vacancy, which increases the conductivity of the sample, and reduces the one set temperature of FS. La0.9Ca0.1CrO3 as a p-type semiconductor may be sintered to a dense state at a furnace temperature of 311 °C–140 °C under 15–35 V/cm. EDS results show that the volatilization of Cr elements is well controlled. The current density increases non-linearly to the set limit value in the voltage control stage under an initial electric field. ln(E2/Ton4) and 1/Ton have a good linear relationship, according to this relationship, the onset temperature (Ton) under electric field conditions of 25 and 22.5 V/cm is estimated to be completely consistent with the experimental value. Furthermore, flash sintering resulted in a higher density and smaller particle size within the sample compared to conventional sintering.

Sintering and deformation properties of forsterite + diopside aggregates in an electrical field

A new technique called “flash sintering” has been proposed in the field of materials science. When a compact powder of oxide material is sintered in an electrical field, densification accelerates at a certain temperature, and the sintering is completed in a shorter time at a lower temperature as compared with conventional sintering experiments. In addition, tensile deformation experiments conducted on a dense oxide body in an electrical field have shown that plastic deformation can be enhanced at lower temperatures with lower stresses, as compared with conventional deformation experiments. Fluctuations in the magnetic and electrical fields derived from outside Earth, such as solar activity, induce an electrical field inside Earth. The strength of the electrical field in Earth's mantle is still debated, but it could be up to 10−6 V/cm. Although electrical fields inside Earth may affect the deformation behavior of rocks, no experimental studies have been conducted on geomaterials to assess this effect. Therefore, based on these recent findings in materials science, we conducted sintering and deformation experiments to examine the effects of an electrical field on high-temperature mass transport. We used forsterite + diopside crystal aggregates, which are good analogs for Earth's mantle. The sintering experiments demonstrated that the relative density increases with an increasing electrical field at a constant temperature. In the deformation experiment with a constant displacement rate, the samples in an electrical field of 1000 V/cm were deformed by 4.4× lower stress than for samples not in an electrical field. These features suggest that diffusional mass transport is enhanced in an electrical field. We propose that the presence of an electrical field within Earth may accelerate the deformation of mantle materials.

Reactive flash sintering of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7: Microstructural evolution and aqueous durability

Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the microstructural evolution for the first time. It was demonstrated that the densification rate of (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10−6∼10−7 g m−2 d−1 after 42 days, which were much lower than most reported pyrochlore materials.

Rapid and controllable synthesis of LiCoO2 cathode materials by reactive flash sintering

AbstractLiCoO2 (LCO) has been widely adopted as the electrode for lithium‐ion batteries (LIBs) which provide a solution for settling problems referring to energy and environment. The demand for rapid and controllable synthesis of LCO increases dramatically. In this work, reactive flash sintering (RFS), for the first time, was introduced to synthesize LCO rapidly and controllably based on simple oxide precursors. The as‐prepared LCO, investigated by physicochemical property characterization, showed well‐constructed and high crystallinity and controllable morphologies, in which the grain size varied with the magnitude of the applied limited current value, allowing for grain size modulation. Furthermore, the as‐prepared LCO exhibited excellent performance in cyclic stability and rate capacity by contrast with that synthesized by the conventional solid‐phase sintering method. This is admittedly attributed to better crystallinity and less lithium loss. Therefore, RFS provides a new approach for the manufacture of LCO electrode and has the potential to be applied to other electrode materials for LIBs.

Issue Information

Journal of the American Ceramic SocietyVolume 106, Issue 7 p. 3979-3982 ISSUE INFORMATIONFree Access Issue Information First published: 05 May 2023 https://doi.org/10.1111/jace.18564AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract Cover Photograph: Ultrafast high-temperature sintering (UHS) and flash sintering are novel methods for rapid sintering of ceramics, often completed in just a few seconds. The authors of https://ceramics.onlinelibrary.wiley.com/doi/full/10.1111/jace.19070 this issue's feature article show that both also share two additional features: an abrupt rise in electrical conductivity, which is electronic, and electroluminescence, through simple yet elegant experiments on a commercial gas-fired lantern mantle. Read the article https://doi.org/10.1111/jace.19070 for more information. Volume106, Issue7July 2023Pages 3979-3982 RelatedInformation

Flash and Breakdown: Thermal Runaway and Dielectric Breakdown as Competing Mechanisms During Flash Sintering of Barium Titanate

Herein, hot‐spot generation in bulk barium titanate samples is investigated during controlled current ramping at various rates using in situ dilatometry. The incubation of the flash event is separated from dielectric breakdown at high current densities, which has previously been attributed to cause flash incubation in barium titanate. The lower boundary of the onset temperature of the flash event in barium titanate is investigated through conventional flash experiments at high electric fields. Despite incubating through thermal runaway, the lower boundary does not coincide with the Debye temperature of barium titanate.

Mechanisms responsible for the onset of selective laser flash sintering of 8‐YSZ

AbstractSelective laser flash sintering (SLFS) is a variation of flash sintering where the only external heat source is a scanning laser. The scanning laser locally heats a region of the sample between two electrodes, initiating measured current flow at a threshold electric field and laser energy density. This study focuses on understanding how charge transport occurs during stage I SLFS in 8 mol% yttria stabilized zirconia. Two potential charge transport mechanisms are considered: (1) a continuous current moving along a hot line between electrodes and (2) charge transported in a discrete bundle within a hot spot, localized to the area under the laser beam. Two laser scan patterns are employed to experimentally determine how charge is transported during stage I SLFS. Numerical modeling is used to estimate the temperatures at which SLFS initiates, which allows the calculation of charge carrier densities and mobilities relevant for the onset of SLFS. Results demonstrate that both a discrete bundle of charges and continuous flow of charge carriers contribute to the current at the onset of SLFS.

Reactive flash synthesis and phase compositions of Lix(CoCrFeMnNi)Oy (x = 0–1) oxides

In this paper, the reactive flash sintering method was adopted to synthesize a series of Lix(CoCrFeMnNi)Oy oxides with x in the range of 0–1. The phase compositions and electrical conductivities of the oxides at different Li incorporation level were characterized. A single phase of spinel structure (Fd-3m) was obtained for the sample (CoCrFeMnNi)3O4. With incorporation of Li, the phase transformed into Rock-salt structure (Fm-3m) and then layered structure (R-3m). The ionic and electronic conductivities of the samples were characterized by impedance spectrometry. The ionic and electronic conductivities of the samples decreased with increasing Li incorporation level at first along with the spinel to Rock-salt structure transformation, and then increased with further increasing Li incorporation level along with the Rock-salt to layered structure transformation.

Electric Current‐Assisted Sintering of 8YSZ: A Comparative Study of Ultrafast High‐Temperature Sintering and Flash Sintering

Electric current‐assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short‐term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra‐high temperatures, and electric field. However, the key consolidation‐enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high‐temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ.

A Perspective on Emerging and Future Sintering Technologies of Ceramic Materials

Novel sintering methods have emerged in the recent past years, which have raised great interest in the scientific community. Relying on electric field effects, high heating rates, the use of mechanical pressure, or hydrothermal conditions, they offer fundamental advantages compared to conventional sintering routes like minimizing the energy consumption and enhancing the process efficiency. This perspective aims at explaining these effects in a general way and presenting the status quo of using them for the processing of high‐performing ceramic materials. In detail, this work focuses on flash sintering, ultrafast high‐temperature sintering, spark plasma sintering, cold sintering, and photonic sintering methods based on different light sources. The specificities, potentials, and limitations of each method are compared, especially in the light of a possible industrialization.

Sintering behavior of 8YSZ‐Ni cermet: Comparison between conventional, FST/SPS, and flash sintering

AbstractCermets are ceramic metal composites. The metallic phase in the cermet typically undergoes oxidation during sintering in air. Electric field‐assisted sintering processes such as field‐assisted sintering technology/spark plasma sintering (FAST/SPS) and flash involves very high heating rates, short processing time and low processing temperature. The main aim of this work was to see if field‐assisted sintering techniques can prevent the oxidation of the metallic phase in the cermet. Sintering behavior of 8YSZ‐5 wt.% Ni cermet was studied by three different techniques namely; conventional sintering, FAST/SPS and flash sintering. Phases and microstructure were analyzed through X‐ray diffraction and scanning electron microscopy, respectively. Temperature and time required for sintering the samples via FAST/SPS and flash sintering was significantly lower than that during conventional sintering. In addition, we found limited grain growth during FAST/SPS and flash sintering. During conventional sintering in reducing atmosphere (Ar and vacuum), Ni particles retained their elemental state, however the extent of densification was poor in the cermet. FAST/SPS in argon and vacuum resulted in almost complete densification (relative density > 97%) and Ni particles were retained in their elemental state in the cermet. During flash sintering in air, the samples sintered to a high densification (relative density ∼98%), however, Ni particles were completely oxidized.

In‐operando synchrotron experiments of flash sintering carried out in current rate mode

AbstractWe report first‐time results for in‐operando flash sintering synchrotron experiments carried out in current rate mode where the specimen, held at a constant temperature, is fed current that is increased at a constant rate. These experiments are unique because the time dependence of the sintering behavior can be stretched out over a longer period (by changing the current rate) than in voltage‐to‐current experiments in which sintering occurs in a burst at the onset of the flash. Two results are presented: (i) A comparison of temperatures measured with the platinum standard to those predicted by the black body radiation model leading to estimates of the emissivity as a function of porosity whereby emissivity increases from 0.65 to 0.9 as the sample sinters from its green state to full density, and (ii) measurements of the excess lattice expansion as a function of density as the sample sinters continuously while the current is increased. The present work highlights the promise of current rate experiments to obtain results while the sample sinters gradually from its green density to full density (somewhat akin to conventional sintering) for gaining further insights into the mechanisms of flash sintering.

On the confluence of ultrafast high‐temperature sintering and flash sintering phenomena

AbstractUltrafast high‐temperature sintering (UHS) and flash sintering are novel methods for rapid sintering of ceramics, often completed in just a few seconds. Here, we show that both also share two additional features: an abrupt rise in electrical conductivity, which is electronic, and electroluminescence. More fundamentally, both are related to phonon physics where MD calculations have shown that proliferation of phonons at the edge of the Brillouin zone can induce Frenkel pairs without the application of electrical fields. Here, we show that, indeed, heating without the application of electric field, can also induce flash: Rapid heating processes of thin films of an oxide‐salt deposited on silk fibers, with a propane torch, are shown to induce electronic conductivity, electroluminescence, and rapid sintering of the oxide. The discussion in this article harkens back to two inventions, more than a century ago, which can now be related to flash and UHS: (i) the Nernst glow lamp circa 1900, made from zirconia, and (ii) the Welsbach mantle, constituted from ceria doped thorium oxide, in the late nineteenth century. Thus, the confluence between high heating rate and electric field induced flash phenomena links the past to the new. The emerging question is how injection of phonons that has been shown to create Frenkels can further induce high electronic conductivity and electroluminescence in oxides. Both electronic conductivity and luminescence are likely related to the generation of electron–hole pairs.

Flash sintering in metallic ceramics: finite element analysis of thermal runaway in tungsten carbide green bodies

Flash sintering is a powerful tool for the ultrarapid consolidation of green ceramic compacts, although its activation mechanisms in electrically conductive PTC (Positive Temperature Coefficient for resistivity) materials' is poorly understood. It was argued that a flash event could be initiated and sustained for a transitory period in certain PTC ceramics because of an initial negative dependence of the green material resistivity with temperature. The thermal runaway phenomenon and its activation conditions on binderless tungsten carbide (WC) green bodies are investigated in the present work by numerical simulations using finite element methods (FEM). The flash event is recreated and studied within the COMSOL Multiphysics software at the macroscale, i.e., considering the flash as an electrical power surge driven by an increasing sample's conductivity. During the flash, very high temperatures in the range of 1800–2000 °C can be reached in the WC green sample in a few seconds. The accurate numerical simulation of such event results in heating rates exceeding 1000 °C/s, a condition that theoretically brings a powder compact at temperatures high enough to accelerate and prioritize sintering densifying mechanisms over non-densifying ones. Therefore, the sample's regions where the maximum sintering temperature is reached more slowly because of thermal contacts with the electrodes remain highly porous at the end of the process.

Experimental and Numerical Studies of Densification and Grain Growth of 8YSZ during Flash Sintering

As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra‐high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering ( =424 kJ mol−1) compared to the conventional sintering ( = 691 kJ mol−1). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data.

A comparative study of microstructure and electrical properties of (Al, Nb) co-doped TiO2 ceramics by different sintering methods

(Al, Nb) co-doped TiO2 colossal dielectric ceramics were effectively made using conventional sintering (CS), flash sintering (FS) and two-step flash sintering (TSFS), respectively. The structure and electrical characteristics of ceramic samples were thoroughly investigated. FS and TSFS can decrease the processing temperature by 20% and shorten the sintering time by 30 times compared with CS. All sintered samples were pure rutile TiO2 structure. The TSFS samples with finer grain size had higher permittivity than the CS samples. With increasing sintering temperature, the permittivity of the co-doped TiO2 ceramic increased first and then decreased. When the furnace temperature was 1080 °C, the TSFS ceramic sample showed the best overall performance: the dielectric constant (ε’) of 1.4 × 105, the dielectric loss (tanδ) of 0.57 at 1 kHz, and the nonlinear coefficient of 5.8. XPS and IS spectra indicated that the excellent dielectric properties are attributed to the internal barrier layer capacitance model. Therefore, the TSFS method provides an ideal way to prepare co-doped TiO2 colossal dielectric ceramics.

Ultra-fast synthesis of Bi2Te3−xSex based compounds with micro-nano composite structure at room temperature by reactive flash sintering

The synthesis of Bi2Te3−xSex (x = 0 and 0.3) compounds in a few seconds at room temperature was realized by reactive flash sintering method using Bi, Te and Se raw powders. The phase transition and microstructure evolution during flash sintering were systematically studied relating to the duration time, power density and sample temperature. The synthesized Bi2Te3−xSex compounds have micro-nano composite structure, which are composed of 5–10 μm lamellar crystal and 10–20 nm nanograin. It is suggested that the instantaneously generated local Joule heat throughout the sample and the resulted ultra-high self-heating rate approaching tens of thousands K/min are responsible for the synthesis of highly crystalline Bi2Te3−xSex. The synthesis mechanism is proposed, including Joule heating, melting, chemical reaction, nucleation and grain growth. This highly efficient and economic synthesis method of Bi2Te3−xSex compounds with micro-nano composite structure has great potential for application in the field of thermoelectric industry.

Effect of Surface Dispersion of Fe Nanoparticles on the Room-Temperature Flash Sintering Behavior of 3YSZ

Arc floating in surface flashover can be controlled by reducing the interfacial charge-transfer resistance of ceramics. However, thus far, only a few studies have been conducted on methods of treating ceramic surfaces directly to reduce the interfacial charge-transfer resistance. Herein, we explore the flash sintering behavior of a ceramic surface (3 mol% yttria-stabilized zirconia (3YSZ)) onto which loose metal (iron) powder was spread prior to flash sintering at room temperature (25 °C). The iron powder acts as a conductive phase that accelerates the start of flash sintering while also doping the ceramic phase during the sintering process. Notably, the iron powder substantially reduces the transition time from the arc stage to the flash stage from 13.50 to 8.22 s. The surface temperature (~1600 °C) of the ceramic substrate is sufficiently high to melt the iron powder. The molten metal then reacts with the ceramic surface, causing iron ions to substitute Zr4+ ions and promoting rapid densification. The YSZ grains in the metal-infiltrated area grow exceptionally fast. The results demonstrate that spreading metal powder onto a ceramic surface prior to flash sintering can enable the metal to enter the ceramic pores, which will be of significance in developing and enhancing ceramic-metal powder processing techniques.