Field-assisted sintering technology has revolutionized material processing by integrating temperature, mechanical, electrical, and magnetic fields to achieve unprecedented densification efficiency and microstructural control. Recent advances in techniques such as hot oscillatory pressing, cold sintering, high/ultra-high pressure sintering, spark plasma sintering, ultrafast high-temperature sintering, and flash sintering have enabled the fabrication of previously unattainable materials, including ultrafine-grained ceramics, nanostructured composites, and functionally graded materials. These materials possess exceptional performances under extreme conditions, expanding applications in aerospace, electronics, energy, and biomedicine. However, the rapid development of these methods has exposed limitations in conventional sintering theory, particularly in describing mass transport and interface evolution under multi-physics coupling. This review systematically examines representative field-assisted sintering technologies and discusses their principles, equipment configurations, and application cases. By analyzing current challenges and opportunities, we aim to bridge fundamental understanding with industrial implementation, providing insights for the design and fabrication of next-generation high-performance materials.
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Open Access
Review
Issue
The accumulation of surface charges can easily lead to surface flashover accidents. Regarding the issue of charge accumulation on the surface of basin-type insulators for DC-GIS and DC-GIL under DC voltage, as well as post insulators for ultra-high voltage DC wall bushings, existing research has mainly focused on the gas-solid interface charge accumulation laws of polymers such as epoxy resin and polytetrafluoroethylene at DC voltages. There is relatively few research on the charge accumulation and dissipation characteristics of ceramics and a lack of comparative studies on different ceramic materials, which limits the application of ceramic insulation materials. To clarify the mechanism of charge accumulation and dissipation on ceramic surfaces and to establish a model for charge accumulation and dissipation of ceramic materials under direct current electric fields, charges were injected into the surface of the samples with a needle-plate electrode. The surface charge density, of three ceramic materials, Al2O3, AlN and Si3N4, was tested using the electrostatic probe method, while their trap distribution characteristics were analyzed using the isothermal surface potential decay method.
Before the experiments, the humidity within the sealed chamber was controlled and stabilized at ±3% of the target value. The samples were cleaned with an ultrasonic cleaning machine and then dried in an oven for 12 h to eliminate any pre-existing surface charges. Subsequently, the samples were placed under the electrostatic probe, where the measured potential value was close to 0, indicating that the original charges had been mostly eliminated. The samples were then moved beneath the needle electrode, aligning the needle electrode with the center of the sample. The grounding switch of the displacement platform was closed and the power supply was turned on to apply a +10 k V voltage between the needle electrode and the platform in a stepwise manner for 3 min, injecting charges onto the surface of the sample. Then, the power supply was turned off, while simultaneously disconnecting the grounding switch of the displacement platform to maintain the floating potential. The samples were then moved to a position 3 mm below the electrostatic probe to begin measuring the surface potential. When measuring the central charge density, the electrostatic probe was positioned above the center of the sample to record the central potential values over time. To measure the surface charge distribution, the displacement platform was controlled to move the probe in a square area with a side length of 60 mm centered on the sample. The scanning path started at the center of the area, moved to the lower-left corner and then scanned the entire area along an S-shaped path to the upper-right corner, finally returning to the center of the area. The scanning process was symmetric in time and space. The scanning speed was set at 20 mm·s−1 and the entire scanning process took approximately 60 s. Considering that the potential of the sample surface may decay during this time, the measured potential values were corrected.
At a relative humidity of 32%, the initial central potential after removing the voltage of the three samples was measured and the central charge density was calculated. Si3N4 had the highest charge density, followed by Al2O3, while AlN had the lowest value. All three samples accumulated positive charges of the same polarity as the applied voltage. The potential was highest at the center of charge injection and decreased radially. The area outside the sample on the displacement platform exhibited almost no charge distribution. At 60% and 32% humidities, the surface potential values of the three samples were measured over time after the voltage was removed. Subsequently, a double-exponential function was used to fit the changes in surface potential. Comparatively, at 32% humidity, the initial surface potential values of the samples were slightly higher than those at 60% humidity, while the potential decay rate was significantly lower than that at 60% humidity. In terms of initial charge density at 60% humidity, the order was: Al2O3＞Si3N4＞AlN, and at 32% humidity, it was Si3N4＞Al2O3＞AlN. Regarding the charge dissipation rates, regardless of low or high humidity, the order was AlN＞Si3N4＞Al2O3. At 32% humidity, using the moment of voltage removal as the initial time, the charge distribution morphology of the three samples was measured at the initial moment, 5 min, 15 min and 60 min. All three samples conformed to have the characteristic of "no significant change in charge distribution morphology", when bulk dissipation was the main dissipation pathway, while no pronounced volcanic-like distribution or expansion of charge distribution range was observed. In Al2O3 and Si3N4, a phenomenon was observed where the potential at a distance of 4 mm from the center first increased and then decreased after the voltage was removed, but remained lower than the potential at the center. This was ascribed to the charge received from the center direction, which exceeded the charge lost from bulk dissipation at that location, thereby resulting in an increase in charge density and a rise in potential. This phenomenon indicates that there exists a tangential electric field induced by surface charges or a charge dissipation process driven by charge density differences at different locations along the surface. In AlN, the aforementioned phenomenon was not observed, as its bulk dissipation rate was relatively fast, meaning that the charge lost from non-central areas due to bulk dissipation was always greater than the charge gained from high charge density areas due to surface dissipation, resulting in a continuous decrease in potential in non-central areas.
All three ceramic surfaces accumulated charges of the same polarity as the applied voltage, with charge density decreasing radially from the injection point. At low humidity, the central accumulated charge density was Si3N4＞Al2O3＞AlN. At high humidity, the order was Al2O3＞Si3N4＞AlN. The dissipation of surface charge after removing the applied voltage is the result of the combined action of surface dissipation and bulk dissipation, with dissipation rate of AlN＞Si3N4＞Al2O3. The surface conductivity of materials is higher at high humidity, leading to a faster dissipation of charges along the surface. According to the analysis of the isothermal surface potential decay model, shallow traps are the main types of traps in AlN, while deep traps are the main types of traps in Al2O3 and Si3N4.
Open Access
Research Article
Issue
Room-temperature flash sintering (FS) for ceramics is a highly efficient and energy-saving new ceramic sintering technique. Addressing the current challenges in room-temperature flash sintering research, such as small product sizes, shape limitations, and high power requirements, limits their real application in the FS industry. In particular, for dog bone shape and small size, which are usually smaller than 10 mm, no records of sizes larger than 20 mm have been reported. In this study, a novel flash sintering device based on a composite layered carbon electrode structure was developed to conduct large-diameter sample flash sintering at room temperature (RT) in an air atmosphere under a direct current (DC) voltage below 100 V. Specifically, room-temperature flash sintering was achieved for ZnO ceramic disks with diameters of 40.0 mm and thicknesses of 1.80 mm, achieving a maximum relative density of 96.02%. Furthermore, room-temperature flash sintering was achieved for ZnO varistor ceramic disks with a diameter of 40.0 mm and a thickness of 1.93 mm, reaching a maximum relative density of 99.27%, a maximum voltage gradient of 330.5 V·mm−1, and the highest nonlinearity coefficient (α) of 23.0. Room-temperature flash sintering was also achieved for 3 mol% yttrium-doped zirconia (3YSZ) ceramic disks, achieving a maximum relative density of 98.48%. The proposed flash sintering device and corresponding process demonstrate broad applicability for the ceramics industry.
Open Access
Rapid Communication
Issue
For the first time, the flash sintering (FS) of high-purity alumina at room temperature, which was previously considered unachievable due to its low electrical conductivity, was conducted herein. The electrical arc originating from surface flashover was harnessed to induce FS at room temperature and low air pressure. The successful FS of high-purity alumina was realized at 60 kPa under the arc constraint, resulting in a notable relative density of the alumina sample of 98.7%. The electric–thermal coupling between the arc and high-purity alumina sample during the arc-induced FS process was analyzed via the finite element simulation method. The results revealed the thermal and electrical effects of the arc on the sample, which ultimately enhance the electrical conductivity of the alumina sample. The formation of a conductive channel on the sample surface, a result of increased electrical conductivity, was the pivotal factor in achieving FS in high-purity alumina at room temperature. The arc constraint technique can be applied to numerous materials, such as ionic conductors, semiconductors, and even insulators, under room-temperature and low-air-pressure conditions.
Open Access
Research paper
Issue
BiSe with intrinsic low thermal conductivity has considered as a promising thermoelectric (TE) material at nearly room temperature. To improve its low thermoelectric figure of merit (zT), in this work, Sb and Te isovalent co-alloying was performed and significantly optimized its TE property with weakly anisotropic characteristic. After substituting Sb on Bi sites, the carrier concentration is suppressed by introduction of Sb- Se site defects, which contributes to the increased absolute value of Seebeck coefficient (|S|). Further co-alloying Te on Se of the optimized composition Bi0.7Sb0.3Se, the carrier concentration increased without affecting the |S| due to the enhanced effective mass, which leads to a highest power factor of 12.8 μW/(cm·K2) at 423 K. As a result, a maximum zT of ~0.54 is achieved for Bi0.7Sb0.3Se0.7Te0.3 along the pressing direction and the average zT (zTave) (from 300 K to 623 K) are drastically improved from 0.24 for pristine BiSe sample to 0.45. Moreover, an energy conversion efficiency ~4.0% is achieved for a single leg TE device of Bi0.7Sb0.3Se0.7Te0.3when applied the temperature difference of 339 K, indicating the potential TE application.
Open Access
Research Article
Issue
Oxygen vacancy OV plays an important role in a flash sintering (FS) process. In defect engineering, the methods of creating oxygen vacancy defects include doping, heating, and etching, and all of them often have complex processes or equipment. In this study, we used dielectric barrier discharge (DBD) as a new defect engineering technology to increase oxygen vacancy concentrations of green billets with different ceramics (ZnO, TiO2, and 3 mol% yttria-stabilized zirconia (3YSZ)). With an alternating current (AC) power supply of 10 kHz, low-temperature plasma was generated, and a specimen could be treated in different atmospheres. The effect of the DBD treatment was influenced by atmosphere, treatment time, and voltage amplitude of the power supply. After the DBD treatment, the oxygen vacancy defect concentration in ZnO samples increased significantly, and a resistance test showed that conductivity of the samples increased by 2–3 orders of magnitude. Moreover, the onset electric field (E) of ZnO FS decreased from 5.17 to 0.86 kV/cm at room temperature (RT); while in the whole FS, the max power dissipation decreased from 563.17 to 27.94 W. The defect concentration and conductivity of the green billets for TiO2 and 3YSZ were also changed by the DBD, and then the FS process was modified. It is a new technology to treat the green billet of ceramics in very short time, applicable to other ceramics, and beneficial to regulate the FS process.
Open Access
Rapid Communication
Issue
In this study, we reported that flash sintering (FS) could be efficiently triggered at room temperature (25 ℃) by manipulating the oxygen concentration within ZnO powders via a versatile defect engineering strategy, fully demonstrating a promising method for the repaid prototyping of ceramics. With a low concentration of oxygen defects, FS was only activated at a high onset electric field of ~2.7 kV/cm, while arcs appearing on the surfaces of samples. Strikingly, the onset electric field was decreased to < 0.51 kV/cm for the activation of FS initiated, which was associated with increased oxygen concentrations coupled with increased electrical conductivity. Thereby, a general room-temperature FS strategy by introducing intrinsic structural defect is suggested for a broad range of ceramics that are prone to form high concentration of point defects.
Open Access
Research Article
Issue
This is the first study to conduct the flash sintering of 3 mol% yttria-stabilized zirconia (3YSZ) ceramics at room temperature (25 ℃) under a strong electric field, larger than 1 kV/cm. At the standard atmospheric pressure (101 kPa), the probability of successful sintering is approximately half of that at low atmospheric pressure, lower than 80 kPa. The success of the proposed flash sintering process was determined based on the high electric arc performance at different atmospheric pressures ranging from 20 to 100 kPa. The 3YSZ samples achieved a maximum relative density of 99.5% with a grain size of ~200 nm. The results showed that as the atmospheric pressure decreases, the onset electric field of flash sintering decreases, corresponding to the empirical formula of the flashover voltage. Moreover, flash sintering was found to be triggered by the surface flashover of ceramic samples, and the electric arc on the sample surfaces floated upward before complete flash sintering at overly high pressures, resulting in the failure of flash sintering. This study reveals a new method for the facile preparation of flash-sintered ceramics at room temperature, which will promote the application of flash sintering in the ceramic industry.
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