Flash‐sintering of magnesium aluminate spinel (MgAl2O4) ceramics

The sintering behavior of commercially available MgAl₂O₄ spinel was investigated under DC electric field in a range of 0 and 1000 V/cm. Flash‐sintering results in densification close to theoretical density at 1410°C under the DC field of 1000 V/cm, in comparison to the higher sintering temperature of 1650°C in case of conventional sintering. It was observed that the fields less than 750 V/cm had no significant effect on the densification behavior. An abrupt increase in power dissipation was observed corresponding to the occurrence of the flash event. A significant enhancement in grain size was observed in case of flash‐sintered dense spinel samples. The gradual increase in the specimen conductivity observed in the electric field‐assisted sintering (FAST) regime led to Joule heating within the specimen. The increased specimen temperature triggered further increment of current and Joule heating, resulting in the immediate densification.     

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ～90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.     

Flash Sintering of Nanocrystalline Zinc Oxide and its Influence on Microstructure and Defect Formation

We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real‐time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.     

Understanding the Flash Sintering of Rare‐Earth‐Doped Ceria for Solid Oxide Fuel Cell

A novel electrical current applied technique known as flash sintering has been applied to rapidly (within 10 min) densify electrolytes including Ce_0.8Gd_0.2O_1.9 (GDC20), Ce_0.9Gd_0.1O_1.95 (GDC10), and Ce_0.8Sm_0.2O_1.9 (SDC20) for application in Solid Oxide Fuel Cells (SOFCs). The densification temperature for the three electrolytes was 554°C, 635°C, and 667°C, respectively, which is far below conventional sintering temperatures. All specimens after flash sintering maintained the pure fluorite structure and exhibited a well‐densified microstructure. To investigate the flash‐sintering mechanism, we have applied Joule heating effect with blackbody radiation theory, and found that this theory could reasonably interpret the flash‐sintering phenomenon by matching theoretically calculated temperature with the real temperature. More importantly, one of the materials inherent properties, the electronic conductivity, has been found correlated with the onset of flash sintering, which indicates that the electrons and holes are the primary current carriers during the start of flash‐sintering process. As a result, potential densification mechanisms have been discussed in terms of spark plasma discharge.     

A thermal perspective of flash sintering: The effect of AC current ramp rate on microstructure evolution

In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary.     

Consolidation of undoped,monoclinic zirconia polycrystals by flash sintering

Consolidated, monoclinic ZrO₂ polycrystal was produced from undoped ZrO₂ powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO₂ was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal‐to‐monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO₂ body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO₂ without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain‐boundary second phase in the flash‐sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash‐sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO₂ grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash‐sintered specimen for 5 minutes. The alignment of ZrO₂ grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO₃ ceramics. The results reveal that the most suitable way to control the sintering process is by using non-linear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO₃, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials.     

Effect of Li2O content and sintering temperature on the grain growth and electrical properties of Gd-doped CeO2 ceramics

The sintering behavior and electrical properties of Gd-doped CeO₂ (GDC), with and without Li₂O as a sintering additive, were examined. With increasing Li₂O content, the grain growth of GDC was enhanced because of an increase in the duration for which a Li-rich liquid phase existed during sintering. The migration of Li ions led to an increase in the electrical conductivity of the Li-doped GDC. When the Li-doped GDC was sintered at a high temperature of 1400 °C, the Li evaporated, resulting in a decrease in the electrical conductivity.     

Microstructural evolution of 3YSZ flash-sintered with current ramp control

The effect of a controlled current ramp during flash sintering (FS) on the densification and microstructural evolution of 3 mol% yttria-stabilized zirconia was investigated. The samples were flash sintered using a current ramp control with six different current ramp rates and compared with samples sintered by FS without current ramp control. In both cases, maximum electric current densities of 100 and 200 mA mm⁻² were used. The microstructure of cylindrical samples was observed, showing grain size heterogeneity between the curved surface and the core for the flash-sintered (FSed) samples regardless of the maximum current density used. By contrast, the current ramp FSed samples exhibited a homogeneous grain size when the electric current density of 100 mA mm⁻² was applied. Thus, controlling a current ramp during FS can be an alternative for avoiding grain size heterogeneity on ceramics sintered by this technique.     

Influence of electric field strength on structure,microstructure, and electrical properties of flash sintered 8% mol Yttria-stabilized zirconia as a solid oxide fuel cell electrolyte

To study the effect of electric field on the characteristics of flash sintered materials, 8% mol. Yttria-stabilized zirconia (8YSZ) was isothermally flash sintered under various electric field strengths as a solid oxide fuel cell (SOFC) electrolyte. Structural, microstructural, and electrical characteristics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Electrochemical impedance spectroscopy (EIS), respectively. Results show that the electric field did not affect the relative density of flash sintered 8YSZ. Electric fields stronger than 300 V cm^?1, however, transformed the cubic structure to tetragonal. Microstructural studies show that the average grain size of samples is independent of the applied electric field strength. Electrochemical impedance spectroscopy showed changes in the grain boundary characteristics upon using the electric field for flash sintering. Oxygen vacancy concentration in the grain boundary of flash sintered samples was more than ten times higher than conventionally sintered ones, which improved the conductivity in flash sintered samples.     

Influence of the electric field on flash-sintered (Zr + Ta) co-doped TiO2 colossal permittivity ceramics

In the preparation process for advanced ceramics, how to reduce the sintering temperature, shorten the processing time and refine grains is the key to obtaining high-performance ceramic materials. The flash sintering (FS) provides an effective method to solve this issue. Here, (Zr + Ta) co-doped TiO₂ colossal permittivity ceramics were successfully fabricated by conventional sintering (CS) and flash sintering under electric fields from 500 V/cm to 800 V/cm. The flash behavior, sintered crystal structure and microstructure, dielectric properties, and varistor characteristics were systematically investigated. The effects of the applied electric fields on the above behaviors were discussed. The results show that flash sintering can reduce the sintering temperature by 200 °C, decrease the processing time by 10 times and reduce grain sizes in TiO₂ ceramics. All sintered samples were single rutile structures. Flash sintering led to similar electrical properties to conventional sintering. In the flash-sintered samples, with increasing the electric field, the permittivity of co-doped TiO₂ ceramics increased at a frequency of 10³–10⁴ Hz. The flash-sintered sample under an electric field of 800 V/cm possessed the best comprehensive properties, a dielectric permittivity of >10⁵, a dielectric loss of ～0.77 at 10³ Hz, and a nonlinear coefficient of 5.2.     

Preliminary investigation of hydroxyapatite microstructures prepared by flash sintering

Flash sintering is a novel and emerging route for sintering ceramics within a few seconds, even under pressure-less conditions. In the current study, hydroxyapatite (HA) was fully densified by flash sintering at a furnace temperature of 1020°C. Flash sintering with constant electric fields of 750 and 1000?V?cm^?1 reduced the grain growth rate significantly compared to that sintered in the absence of an electric field at 1400°C. The microstructure of HA consolidated by flash sintering was compared with that of the without electric field sintered samples. The flash-sintered samples showed smaller grains (160?～?320?nm) than the without electric field sintered samples (～15?µm). The samples with a higher applied electric field showed slightly better densification than those with the lower field by flash sintering. Overall, the electric flash reduces the sintering temperature effectively and decreases the holding time to densify highly insulating ceramics, such as HA.     

DC electric field-assisted hot pressing of zirconia: Methodology,phenomenology, and sintering mechanism

Flash sintering (FS) is an important technique in the field of ceramic sintering. Nevertheless, conventional FS is less attractive for practical applications because of the complex shapes and small sizes of the specimens. In this study, using the novel electric field-assisted hot pressing (FAHP) technique, we successfully achieved FS during the net-shape hot pressing (HP) process for the first time. It was found that the 3 mol% yttria-stabilized zirconia (3YSZ) can be flash sintered at 909°C using a fairly low DC field of 33 V/cm under 30 MPa pressure. The grain sizes of the FAHP-sintered samples were 20% smaller than that of the HP-sintered sample. When the current density limit is ≥240 mA/mm², 3YSZ can be fully densified during the flash events. Careful analysis of the sintering curves suggests that although the carrier type or concentration is changed during flash events, it cannot explain the ultrafast densification. Additionally, we devised a qualitative method to analyze the densification mechanism. The results indicated that the ultrafast densification observed during flash events resulted from the synergistic effects of the rapid heating rate and peak sample temperature. Finally, the atomic force microscopy confirmed the lower grain boundary energy for the FAHP-sintered samples, which accounts for the smaller grain sizes than the HP-sintered sample. We believe that the FAHP technique could create new possibilities for theoretical and applied research on field-assisted sintering techniques.     

Flash sintering of lead zirconate titanate (PZT) ceramics: Influence of electrical field and current limit on densification and grain growth

Flash sintering of lead zirconate titanate ceramics were investigated under DC electric fields ranging from 300 to 600?V/cm. The onset temperature for flash sintering significantly decreased with the electrical field to a lower limit of furnace temperature of 538?°C at 600?V/cm. The retardation of grain growth was observed, and the grain size decreased with increasing the electrical field. The current limit had a great influence on the density and grain size of specimen. During the flash sintering process, power dissipation first rose abruptly to a maximum value, then declined sharply to a steady state. Meanwhile, optical glow of specimen was observed. Using black body radiation model, the actual specimen temperature was estimated, which was too low to obtain the full dense ceramics in 30?s. It was suggested that Joule heating, ultra-high heating rate and high concentrations of defects were responsible for flash sintering of PZT ceramics.     

A benzoate coprecipitation route for synthesizing nanocrystalline GDC powder with lowered sintering temperature

Electrolyte powders with low sintering temperature and high-ionic conductivity can considerably facilitate the fabrication and performance of solid oxide fuel cells (SOFCs). Gadolinia-doped ceria (GDC) is a promising electrolyte for developing intermediate- and low-temperature (IT and LT) SOFCs. However, the conventional sintering temperature for GDC is usually above 1200 °C unless additives are used. In this work, a nanocrystalline powder of GDC, (10 mol% Gd dopant, Gd_0.1Ce_0.9O_1.95) with low-sintering temperature has been synthesized using ammonium benzoate as a novel, environmentally friendly and cost-effective precursor/precipitant. The synthesized benzoate powders (termed washed- and non-washed samples) were calcined at a relatively low temperature of 500 °C for 6 h. Physicochemical characteristics were determined using thermal analysis (TG/DTA), Raman spectroscopy, FT-IR, SEM/EDX, XRD, nitrogen absorptiometry, and dilatometry. Dilatometry showed that the newly synthesized GDC samples (washed and non-washed routes) start to shrink at temperatures of 500 and 600 °C (respectively), reaching their maximum sintering rate at 650 and 750 °C. Sintering of pelletized electrolyte substrates at the sintering onset temperature for commercial GDC powder (950 °C) for 6 h, showed densification of washed- and non-washed samples, obtaining 97.48 and 98.43% respectively, relative to theoretical density. The electrochemical impedance spectroscopy (EIS) analysis for the electrolyte pellets sintered at 950 °C showed a total electrical conductivity of 3.83 × 10^?2 and 5.90 × 10^?2 S cm^?1 (under air atmosphere at 750 °C) for washed- and non-washed samples, respectively. This is the first report of a GDC synthesis, where a considerable improvement in sinterability and electrical conductivity of the product GDC is observed at 950 °C without additives addition.     

Electromechanical properties of flash sintered BNT-based piezoelectric ceramic

Lead-free, (BiNa_0.88K_0.08Li_0.04)_0.5Ti_0.995 Mn_0.015 O₃ piezoceramic has been successfully densified by a novel electrical current applied technique known as flash sintering (FS) at 880 °C. The effect of alternating and direct current, current density limit and holding time on the densification, crystal structure, electromechanical and electrical properties have been investigated. The optimum flash condition was obtained with a 1 KHz alternating current, 100 V·cm^?1 initial electric field and preset maximum current limit of 1.5 A·cm^-2. The flash sintered specimen is characterized with finer grain size (10–15 μm), slightly higher electromechanical properties and higher symmetry butterfly shape strain hysteresis loop compared to conventional sintering. Under both sintering conditions uniform distribution of elements and pure rhombohedral structure were observed. Flash sintering also results in lower resistivity and more significant grain boundaries contributions in the conduction mechanism.     

Field-assisted sintering of undoped BaTiO3: Microstructure evolution and dielectric permittivity

We report, for first time, how electric fields influence the sintering of undoped BaTiO₃, a ferroelectric material, and how this process affects the microstructure and the dielectric properties. Flash sintering is achieved at a furnace temperature of 688 °C under a field of 500 V cm⁻¹, producing specimens that are 94% dense. As a consequence, the grain size is much finer than in conventional sintering, which is shown to influence the Curie temperature and dielectric permittivity. Data obtained at different strengths of the electrical field, and current limits imposed on the specimen are presented in the form of a “processing map” that separates the safe region, where sintering is uniform, from the fail region, where the current flow in the sample becomes localized. The map illustrates that ceramics can respond by different mechanisms, with the dominant mechanism changing with the strength of the electrical parameters.     

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

Cermets 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.     

Densification behavior of flash sintered boron suboxide

A study to quantify the flash sintering kinetics of boron suboxide (B₆O) under various electric field strengths and cut‐off amperages is presented. B₆O is conventionally sintered at a prolonged temperature above 1800°C, near its thermal decomposition temperature, with an overpressure >3 atm. By applying a direct current (DC) electric field across a green powder compact, B₆O can be sintered at 1000°C at atmospheric pressure. During the flash sintering process, an intensive radiation was emitted (electroluminescence), which is distinct from the thermal radiation (thermoluminescence) that is expected in conventional sintering. It was observed that the degree of sintering of the large B₆O specimen was heterogeneous due to apparent localization of electrically conducting paths. The material near the surface was sintered, but the core of the specimen was not. It was found that the flash event occurred at a critical temperature, which was obtained by combining external heating via ambient furnace conditions and internal Joule heating. The progressive densification behaviors of the B₆O are also presented.