Flash Sintering Research Articles

The impact of flash sintering on densification and plasticity of strontium titanate: High heating rates, dislocation nucleation and plastic flow

Flash sintering involves very rapid densification of ceramic powder compacts during a thermal runaway induced by an applied voltage and current. The mechanisms of fast densification are still not well-understood. The present study investigates the impact of high heating rates during flash sintering on densification, dislocation density and plasticity of SrTiO3. After flash sintering, a high dislocation density of almost 1014 m−2 was observed by TEM. Uniaxial compression at 1150 °C revealed very high deformation rates. It is argued that for SrTiO3, dislocations are generated and migrate during flash sintering. This becomes possible by the very high heating rates, which conserve high driving forces for sintering up to high temperatures. High driving forces of several 10 MPa are preserved up to high temperatures. Thus, the sintering stress can be above the flow stress of SrTiO3 (5 MPa), and the nucleation of dislocations occurs, paving the path for plastic flow.

Preparation of Gd2Zr2O7 nanoceramics by flash sintering and two-step flash sintering

Preparation of Gd2Zr2O7 nanoceramics using a simple and low energy-consuming approach is significant for their wide range of applications. In this work, flash sintering (FS) and two-step flash sintering (TSFS) were first utilized to prepare Gd2Zr2O7 nanoceramics. The effects of electric field intensity and current limit on the densification and grain growth were investigated. The grain size was decreased with the increase of electric field intensity. Meanwhile, both grain size and density were increased with the increase of current limit. The Gd2Zr2O7 nanoceramics with an average grain size of 98 nm and a relative density of 92.8% were prepared by FS. By contrast, the Gd2Zr2O7 nanoceramics with the higher relative densities of 96.3–97.5% and the smaller average grain sizes of 85–90 nm were prepared by TSFS. Compared to FS, TSFS can achieve suppressed grain growth and fast densification. The as-prepared Gd2Zr2O7 nanoceramics showed excellent hardness and fracture toughness. The sample breakage caused by current localization can be avoided by controlling the current ramp after the occurrence of the flash event.

A Viewpoint on Hot Spots in Microwave Sintering and Flash Sintering

Microwave sintering and flash sintering are field‐assisted ceramic processing techniques in which heating is totally or partly obtained inside the material itself by a coupling between the ceramic material and the electromagnetic or electrical field. They frequently give rise to the observation of localized temperature excess known as hot spots that can lead to dramatic effects on the sintered parts. This article explores and discusses the common origin of hot spots in both techniques, their development, and the possible ways for limiting their effects.

Flash Sintering Preparation and Characterizations of (M0.5Ta0.5)0.05Ti0.95O2 (M = Al, Ga, In) Colossal Dielectric Ceramics

In this paper, (M0.5Ta0.5)0.05Ti0.95O2 (M = Al, Ga, In) ceramic materials were successfully prepared by Flash Sintering (FS) under an electric field of 550 V cm−1 at 1200 °C for 18 min. The influences of different ionic radius on flash sintering behavior, crystal structure, microstructure and dielectric properties of TiO2-based colossal permittivity (CP) ceramics were systematically investigated. FS can reduce the sintering temperature by 200 °C and shorten the sintering time by more than 12 times. The results show that all flash sintering samples were pure rutile TiO2 structure. The grain size decreased with increasing ionic radius. ATTO had the lowest dielectric loss of 0.2 and ITTO had a maximum dielectric constant of 8100 at 1 kHz. XPS analysis confirms that Ti3+ content increased gradually with increasing ion radius. Impedance spectroscopy analysis shows that all flash sintering ceramics were composed of semi-conductive grains and insulating grain boundaries. The CP behavior was explained using the internal barrier layer capacitance effect. This work provides a very meaningful method for the preparation of co-doped TiO2 CP ceramic materials.

Chemical bonding and crystal structure in flash sintered Y2O3 under DC or AC field

Undoped Y2O3 polycrystals were densified by flash sintering under DC or AC electric field with varying holding times up to 60 min, and their chemical bonding state and crystal structure were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Only DC flash sintering with longer holding times (≥ 30 min) resulted in blackening in cathodic regions, indicating the electrode-dependent introduction of neutral oxygen vacancies. The Y 3d3/2 and Y 3d5/2 binding energies decreased with increasing holding time in both DC- and AC-flash-sintered Y2O3 specimens, regardless of polarity, indicating that positively charged oxygen vacancies were introduced by the flash sintering in an electrode-independent manner, even under DC electric field. In addition, the XRD analysis revealed that flash-sintered specimens with longer holding times exhibited short-range structural fluctuations. These electrode-dependent and independent point defect generations under electric fields may be related to the enhanced atomic diffusion during flash sintering.

Dielectric barrier discharge based defect engineering method to assist flash sintering

Oxygen vacancy plays an important role in the flash sintering (FS) process. In defect engineering, the methods to create oxygen vacancy defect include doping, heating, and etching, and all of them often have complex process or equipment. In this study we used dielectric barrier discharge (DBD) as a new defect engineering technology to increase oxygen vacancy concentration of green billet with different ceramics (ZnO, TiO₂ and 3 mol% yttria-stabilized zirconia (3YSZ)). With a 10 kHz AC power supply, the low-temperature plasma was generated and the specimen could be treated in different atmosphere. The effect of DBD treatment was influenced by atmosphere, treatment time, and voltage amplitude of power supply. After DBD treatment, the oxygen vacancy defect concentration in the ZnO samples increased significantly, and the resistance test showed that the conductivity of the samples increased by 2–3 orders of magnitude.  Moreover, the onset electric field of ZnO FS decreased from 5.17 kV/cm to 0.86 kV/cm at room temperature, while in the whole FS the max Power dissipation decrease from 563.17 W to 27.94 W. The defect concentration and conductivity of the green billets for TiO₂ and 3YSZ also changed by DBD, and then the FS process was modified. It is a new technology to treat green billet of ceramic in very short time, applicable to other ceramics, and beneficial to regulate the FS process.

Structural, Vibrational, and Magnetic Characterization of Orthoferrite LaFeO3 Ceramic Prepared by Reaction Flash Sintering.

LaFeO3 perovskite ceramics have been prepared via reaction flash technique using Fe2O3 and La2O3 as precursors. The obtained pellets have been investigated using several techniques. The formation of LaFeO3 has been clearly confirmed by X-ray diffraction. The scanning electron microscopy micrographs have shown the microporous character of the obtained pellets due to the low temperature and dwell time used in the synthesis process (10 min at 1173 K). The orthorhombic-rhombohedral phase transition has been observed at approximately 1273 K in differential thermal analysis measurements, which also allows us to determine the Néel temperature at 742 K. The fitted Mössbauer spectra exposed the presence of a single sextet ascribed to the Fe+3 ions in the tetrahedral site. Finally, magnetic measurements at room temperature indicate the antiferromagnetic character of the sample.

Improvements in Flash Sintering for Practical Application

Improvements in Flash Sintering for Practical Application

Micromechanical properties and microstructures of AC and DC flash-sintered alumina

Alumina (α-Al2O3), one of the most widely used structural ceramics for industrial applications, requires high temperature and long sintering time to achieve a fully dense structure through conventional pressureless consolidation and sintering methods. Recently, several attempts have been made to reduce sintering time and temperature of Al2O3 through the application of electric field with limited success. In this study, alternating current (AC) and direct current (DC) flash sintering techniques were employed to obtain high-density Al2O3 within 10 min. The density of flash-sintered (FS) specimens was greater than the specimens prepared by the pressureless sintering technique (hereinafter designated as conventional sintering) at the same temperature for 10 h. Furthermore, AC technique can reduce the electric field necessary for flash sintering substantially comparing to the DC flash sintering. The microindentation measurements reveal that the AC-FS Al2O3 provides the highest fracture toughness and a high hardness among all sintered specimens. The improved mechanical properties of AC-FS Al2O3 are attributed to defects, such as dislocations as well as stacking faults observed in the FS specimens. This study provides a fresh perspective on the low-energy manufacturing of structural ceramics with improved properties.

Reduced interfacial stresses in laminated ceramics during flash sintering enabled by an extremely low uniaxial viscosity

Constrained sintering is a crucial issue for the development of laminated or composite ceramics. It had been suggested that flash sintering (FS) may eliminate the defects caused by constrained sintering. In this work, the interfacial tensile stresses of laminated ceramics during conventional sintering (CS), field assisted sintering technique (FAST) and FS were estimated and compared by combining a set of free sintering, bending creep and constrained sintering experiments with theoretical analysis. The interfacial tensile stress in constrained FS is significantly lower than those of both CS and FAST, and can only be explained by an extremely low uniaxial viscosity of the materials achieved during FS. The possible effects of temperature, density and microstructure on the uniaxial viscosity reduction have been carefully assessed to show that the electrical field itself is the key factor for the extremely low viscosity. Plausible mechanisms for the reduction in the uniaxial viscosity under different levels of electrical fields are also discussed.

In situ studies on defect formation dynamics in flash-sintered TiO2.

Flash-sintered (FS) ceramics have shown promising mechanical deformability at room temperature compared to conventional sintered ceramics. One major contributing factor to plasticity is high-density defects, such as dislocations, stacking faults and point defects, resulted presumably from the high electrical field during flash sintering. However, such direct experiemtnal evidence for defect formation and evolution under the electric field remains lacking. Here we performed in situ biasing experiments in FS and conventionally sintered (CS) polycrystalline TiO2 in a transmission electron microscope (TEM) to compare the defect evolution dynamics. In situ TEM studies revealed the coalescence of point defects under the electrical field in both FS and CS TiO2 and the subsequent formation of stacking faults, which are often referred to as Wadsley defects. Surprisingly, under the electrical field, the average fault growth rate in the FS samples is 10 times as much as that in the CS TiO2. Furthermore, the Magnéli phase, a 3D oxygen-deficient phase formed by the aggregation of Wadsley defects, is observed in the FS samples, but not in the CS samples. The present study provides new insights into defect dynamics in FS ceramics.

Particle size effect on the microstructure and the aging process of flash-sintered barium titanate from micro and nanopowders

Flash sintering experiments on different-sized BTO powders are performed, and the influence of the conditions over the achieved microstructures and functional properties is shown. For smaller powder sizes a time instability of properties is revealed.

A novel two-step field-assisted sintering method to prepare (MgCoNiCuZn)O high entropy oxide

A two-step sintering experiment of (MgCoNiCuZn)O high entropy oxide was performed to lower the flash onset temperature and shorten the process time. The flash onset temperature was efficiently minimized by regulating the first step heating process due to the variation of the conductivity and density of the compacts. The lowest onset temperature was only 307 °C at a 300 °C first step sintering. Subsequently, the as-sintered material with mainly rock-salt structure can be obtained in just 20 s soaking time after the field-assisted flash sintering procedure. The unusual densification was mainly caused by the higher configuration entropy and significant temperature gradients resulted from the preferential Joule heating of grain boundaries. This novel two-step sintering method would promote the rapid synthesize of high entropy oxides.

Low Temperature Magnetic Transition of BiFeO3 Ceramics Sintered by Electric Field-Assisted Methods: Flash and Spark Plasma Sintering

Low temperature magnetic properties of BiFeO3 powders sintered by flash and spark plasma sintering were studied. An anomaly observed in the magnetic measurements at 250 K proves the clear existence of a phase transition. This transformation, which becomes less well-defined as the grain sizes are reduced to nanometer scale, was described with regard to a magneto-elastic coupling. Furthermore, the samples exhibited enhanced ferromagnetic properties as compared with those of a pellet prepared by the conventional solid-state technique, with both a higher coercivity field and remnant magnetization, reaching a maximum value of 1.17 kOe and 8.5 10-3 emu/g, respectively, for the specimen sintered by flash sintering, which possesses the smallest grains. The specimens also show more significant exchange bias, from 22 to 177 Oe for the specimen prepared by the solid-state method and flash sintering technique, respectively. The observed increase in this parameter is explained in terms of a stronger exchange interaction between ferromagnetic and antiferromagnetic grains in the case of the pellet sintered by flash sintering.

Role of surface carbon nanolayer on the activation of flash sintering in tungsten carbide

Flash sintering has been recently successfully activated also in conductive ceramics like tungsten carbide (WC). The present work aims at understanding how the WC particles surface chemistry can influence the electrical properties of the material and play a fundamental role in the flash sintering phenomenon. An electrical contact resistance (ECR) model was developed to understand the role of resistive surface layers on the electrical behaviour of WC green compacts under different applied pressures during the initial stages of the processes. It is established that the large resistivity measured on green compacts can be attributed to the sole presence of an ultrathin carbon layer on the particles' surface. A carbon nanolayer with a thickness of about 1–2 nm, as detected by XPS and TEM analyses, is found to be responsible for the high resistance reached at the particles' contact points while evolving during the flash event. Flash sintering conditions can be achieved during the electrical resistance flash sintering (ERFS) process in WC nanoparticles covered by such carbon layer and independently of the presence of W oxides.

Role of microstructure reactivity and surface diffusion in explaining flash (ultra-rapid) sintering kinetics

The competition between sintering and coarsening is cited by numerous authors as one of the potential factors for explaining the ultra-rapid sintering kinetics of flash sintering. In particular, surface diffusion is a mechanism decreasing the driving force of sintering by changing the initial highly reactive microstructures (particle contact) into poorly reactive porous skeleton structures (spherical porosity). We show by finite element simulations that flash SPS experiments high specimen temperatures close to 2000 °C. These high temperatures are not sufficient to explain the ultra-rapid sintering kinetics if typical spherical pore theoretical moduli are employed. On the contrary, reactive experimentally determined moduli succeed in explaining the ultra-rapid sintering kinetics. Mesoscale simulations evidenced that the origin of such reactive experimental moduli is a porous skeleton geometry with a significant delay in surface diffusion and particle rearrangement. This highlights the important role of the surface diffusion negation (favoring higher stress intensification factor) in flash sintering.

Effects of doping cations on the densification and microstructure of flash-sintered α-Al2O3

The effects of dopants with different valences on the flash sintering behavior of α-Al2O3 are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al2O3. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al2O3 because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different.

Microstructural and electrical properties of flash sintered ZnO–Bi2O3–Sb2O3 based varistors: Effect of current density with a controlled current ramp

The ZnO–Bi2O3–Sb2O3-based varistors were produced by the flash sintering method. The effects of current density passed through the sample during the sintering process on the microstructure, and electrical properties of Varistors were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), impedance spectroscopy, and electrical measurements. The average grain size increased from 1.46 μm to 4.85 μm by increasing the current density from 80 to 200 mA/mm2, respectively. The XRD analysis showed that the pyrochlore phase was decomposed in the samples sintered by the current density of 200 mA/mm2. Increasing the current density passed through the sample during the flash sintering process resulted in the modified nonlinear characteristics of varistors. The highest relative density (94% of theoretical density) and the optimum electrical properties were obtained in varistors sintered by 200 mA/mm2, i.e., the nonlinear coefficient, leakage current density, and Schottky barrier height of 39.1, 12 μA/cm2, and 0.43 eV, respectively.

Facile formation of porous, multilayer reduced graphene oxide electrodes using electrophoretic deposition and flash sintering

A reduced graphene oxide (rGO) film is mechanically fragile; therefore, transferring it to another substrate without damage is crucial. Hence, a technique that facilitates the formation of an integrated rGO film that holds adhesion with the substrate is desired. Moreover, a rGO film on metal is promising as an electrode for energy storage devices. This work presents the formation of rGO film on copper using electrophoretic deposition (EPD) and flash sintering. Briefly, a stable aqueous dispersion of graphene oxide (GO) layers was used to form a uniform GO film on copper at a fixed deposition voltage and time duration. To reduce the GO film without detaching it from copper, pulse energy (1–10 J cm−2) was varied for a fixed exposure time (10 ms) and a single micropulse. At pulse energy of 6 J cm−2, the structural studies indicated turbostratic stacking (decreased interlayer spacing) and restored crystallinity (decreased peak intensity ratio of D and G bands). The morphology showed voids between stacked rGO layers and high roughness in the film. The reduction resulted in mesoporous and high surface area (373 m2 g−1) multilayer rGO film. Due to restored electrical conductivity, the rGO film showed a specific capacitance of 475.8 F g−1 at a scan rate of 2 mV s−1. An increase in contact angle from ∼66° to ∼81° also validated the reduction. The combination of EPD and flash sintering offers rapid and eco-friendly formation of binder-free rGO film on copper, promising as an electrode for supercapacitors.

Influence of geometry on thermal gradients and hotspot formation during flash sintering

Hotspot or preferential current path formation is a major problem halting the advance of flash sintering as an industry applicable densification method. In-situ infrared thermal imaging shows significant dependence of the surface temperature distribution on the sample geometry, causing significant changes in the onset parameters, the course of the flash event and sample quality. Current-ramped flash sintering is compared to conventional flash sintering experiments and reveals an effective reduction in temperature differences. The role of the setup design on the thermal losses and the gradients is highlighted.