Near complete densification of flash sintered 8YSZ: controlled shrinkage rate effects

Rate-controlled sintering of 8 mol%Y₂O₃-doped ZrO₂ (8YSZ) polycrystals was performed by rating the current limit of alternating current (AC) electric field during a flash event; this method promotes constant linear shrinkage of the green compact with respect to the sintering time. A power dissipation peak associated with the flash event was suppressed by setting the initial current limit at 100 mA; after AC current hits 100 mA, the current limit was rated to a maximum value of 1100?2000 mA to keep shrinkage rate constant. The power dissipation behavior during the current-rating process exhibited a characteristic inverted S-shaped curve as a function of time. 8YSZ polycrystal with a near-theoretical density of 5.97 g/cm³ and a grain size of approximately 3.4 μm was produced in AC electric field (150 V/cm at 1000 Hz) with a final maximum current limit of 1150 mA as a rectangular bulk style.     

Flash sintering produces eutectic microstructures in Al2O3–LaPO4 versus conventional microstructures in 8YSZ–LaPO4

While monazite (LaPO₄) does not flash sinter even at high fields of 1130 V/cm and temperatures of 1450°C, composite systems of 8YSZ–LaPO₄ and Al₂O₃–LaPO₄ have been found to more readily flash sinter. 8YSZ added to LaPO₄ greatly lowered the furnace temperature for flash to 1100°C using a field of only 250 V/cm. In these experiments, -Al₂O₃ alone also did not flash sinter at 1450°C even with high fields of 1130 V/cm, but composites of Al₂O₃–LaPO₄ powders flash sintered at 900-1080 V/cm at 1450°C. Alumina–monazite (Al₂O₃–LaPO₄) composites with compositions ranging from 25 vol% to 75 vol% Al₂O₃ were flash sintered with current limits from 2 to 25 mA/mm². Microstructures were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A eutectic microstructure was observed to form in all flash sintered Al₂O₃–LaPO₄ composites. With higher power (higher current limits), eutectic structures with regular lamellar regions were found to coexist in the channeled region (where both the current and the temperature were the highest) with large hexagonal-shaped -Al₂O₃ grains (up to 75 m) and large irregular LaPO₄ grains. With lower power (lower current limits), an irregular eutectic microstructure was dominant, and there was minimal abnormal grain growth. These results indicate that Al₂O₃–LaPO₄ is a eutectic-forming system and the eutectic temperature was reached locally during flash sintering in regions. These eutectic microstructures with lamellar dimensions on the scale of 100 nm offer potential for improved mechanical properties.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Intergranular amorphous films formed by DC electric field in pure zirconia

It is difficult to obtain pure ZrO₂ sintered compacts with a bulk style at room temperature because a large volumetric expansion from tetragonal to monoclinic phase (t/m) transformation occurs at around 1000°C, which is lower than the sintering temperature. In contrast, pure monoclinic ZrO₂ can be consolidated without shattering using flash‐sintering at 1350°C for 5 minutes under an applied DC electric field of 175 V/cm. High‐resolution transmission electron microscopy and electron energy loss spectroscopy have revealed that amorphous films are formed along grain boundaries after flash‐sintering at 1350°C for 5 minutes. Monoclinic ZrO₂ flash‐sintered compact including the amorphous films are able to survive without shattering through the t/m transformation, as the amorphous films partially absorb the large volumetric expansion arising from the t/m transformation. The formation of the amorphous films results from the severe reducing condition due to the applied DC electric fields during flash‐sintering.     

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.     

Excess oxygen-vacancy formed by FAST regime of direct-current electric field during flash sintering for 3 mol%–10 mol% Y2O3-doped ZrO2

The amount of excess oxygen vacancies that are generated during FAST regime of direct-current flash sintering was estimated semi-quantitatively for 3 mol%–10 mol% Y₂O₃-doped ZrO₂ (3–10YSZ) using the temperature dependence of the electrical conductivity. For the estimation, the electrical conductivity in the temperature range, where shrinkages are below a few percent, was used to avoid the effect of large microstructural changes on the electric conductivity. For all Y₂O₃ contents except 3YSZ, the amount of excess oxygen vacancies increased above a compact temperature of approximately 840 °C, which depended on Y₂O₃ content, and was highest in 8YSZ. Approximately 0.13% excess oxygen vacancy was generated by direct-current flashing in 8YSZ in the present electric field condition. In contrast, the increase in the excess oxygen vacancies is negligible in 3YSZ. The dependence of the amount of excess oxygen vacancies on the Y₂O₃ content was related possibly to the dependence of the electrode overvoltage on the Y₂O₃ content.     

Abnormal grain growth in DC flash sintered 3-mol% yttria-stabilized zirconia ceramics

The origin of nonuniform microstructure and abnormal grain growth (AGG) was investigated in flash sintered 3 mol% yttria-stabilized zirconia (3YSZ) ceramics. The microstructural homogeneity decreased with increasing direct current (DC) density and with dwell time in a flash state, eventually resulting in AGG in the specimen core, the first observation of AGG in 3YSZ. Abnormal grains up to 100 μm in size emerged when the DC density was ≥160 mA/mm², and the specimen's density exceeded 99% of theoretical, starting from the cathode and propagating toward the anode. The results are discussed by comparison with established mechanisms and previous experimental evidence concerning AGG in oxides, focusing on the possible effects of the electrochemical reduction at the cathode end of the specimen.     

Flash sintering of alumina/reduced graphene oxide composites

The flash sintering behavior of Al₂O₃/reduced graphene oxide (rGO) composites was investigated. rGO was used as a composite component and a conductive additive. Under the electric fields of 250–400 V cm⁻¹, the flash event occurred at extremely low temperatures of 236–249 °C. The current density limit played a significant role in the degree of densification. A larger current density resulted in a higher density of the sample. However, current densities larger than 33.33 A cm⁻² resulted in broken samples because of the localization of high current density coupled with the formation of hot spots. Flash sintering at a furnace temperature of 800 °C, electric field of 300 V cm⁻¹ and current density limit of 33.33 A cm⁻² produced nearly completely dense Al₂O₃/rGO composites. In addition to the current limit, the furnace temperature is also a key parameter that controls the degree of densification to achieve “safe” flash sintering.     

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.     

Flash self-joining of Y-TZP ceramics assisted with an AC electric field

In this study, flash joining experiments were conducted using an AC field on 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (Y-TZP) bodies. Furthermore, the necessary conditions to obtain an almost complete self-joining of Y-TZP bodies were clarified. The specimens were successfully joined by applying an AC field at 60 mA mm⁻² for 80 s at a furnace temperature of 1000°C, thus resulting in a successfully joined specimen with 92% of the flexural strength of the as-sintered Y-TZP body. Almost complete self-joining of Y-TZP was achieved at current densities above 30 mA mm⁻², and input energy densities of >24 J mm⁻³. Both the input energy density and electric current were critical factors for producing the reliable joining of ceramics.     

On the catalytic effect of zirconia on flash sintering of alumina

Flash sintering of alumina is more difficult than of yttria-stabilized zirconia (YSZ). Whereas (MgO doped) alumina requires fields greater than 1 kVcm^–1 and temperatures often significantly higher than 1000°C, YSZ can be flashed sintered at ∼100 Vcm^–1 at temperatures below 800°C. Mixed powders of such bi-phasic ceramics, on the other hand, can be flashed under conditions below those for alumina. This effect is usually subscribed to the influence of YSZ on the overall electrical conductivity of the composite. However, such rationalization leaves open the mechanism by which YSZ catalyzes the flash event in alumina. Here, we present results for the onset of flash in a layered structure of YSZ and alumina where both constituents extend from one electrode to the other. We find that the flash initiates, at first, exclusively in the YSZ layer, under conditions identical to those in usual voltage-to-current experiments in single phase YSZ, and then, after a brief incubation period, spreads transversely through the thickness of the alumina layer at a speed of ∼3.3 mm s^–1, while the power supply is held at constant current. This observation opens a new question as to how flash once initiated in an “easy” phase can migrate normal to itself into a second ceramic, which is nominally more-difficult-to flash. (In the present experiments, the alumina layer sintered to full density with all the shrinkage being accommodated in the thickness direction, consistent with an earlier study that articulated that flash obviates constrained sintering.) It is noteworthy that the catalytic effect depends not only on the volume fraction of YSZ, but also on the architecture of the green state (for example a two-phase powder mixture vs. layered structure), which may affect the initiation of the flash in YSZ but, likely, not its migration behavior into the second phase.     

Field assisted sintering of ceramic constituted by alumina and yttria stabilized zirconia

We show that a two-phase 50 vol% 3YSZ-alumina ceramic flash-sinters at a furnace temperature of 1060 °C under an electrical field of 150 V cm⁻¹. In contrast undoped, single-phase alumina remains immune to field assisted sintering at fields up to 1000 V cm⁻¹, although single-phase 3YSZ flash sinters at 750 °C (furnace temperature). The mechanisms of field assisted sintering are divided into two regimes. At low fields the sintering rate increases gradually (FAST), while at high fields sintering occurs abruptly (FLASH). Interestingly, alumina/zirconia composites show a hybrid behavior such that early sintering occurs in FAST mode, which is then followed by flash-sintering. The specimens held in the flashed state, after they had sintered to near full density, show much higher rate of grain growth than in conventional experiments. These results are in contrast to earlier work where the rate of grain growth had been shown to be slower under weak electrical fields.     

Shrinkage rate control during a flash state by current-ramping for 3 mol% Y2O3-doped ZrO2 polycrystals

3 mol% Y₂O₃-doped ZrO₂ green compacts with rectangular shapes were sintered by maintaining the shrinkage rates at constant values under alternating electric fields by ramping the electric current during flash states. Green compacts were furnace-heated under 40-100 V_rms/cm until current hits an initial current limit of 100 mA_rms. After then, current-ramping was started to keep the shrinkage rates constant by increasing the limit current value using a programmable power supply operating in a current control mode. Highly densified 3 mol% Y₂O₃-doped ZrO₂ polycrystals with a density of 6.05 g/cm³ as a bulk density and a grain size of about 0.4 μm were obtained at a furnace temperature of about 930℃, 50 V_rms/cm with 1000 Hz and shrinkage rate of about 120 μm/min (0.8%/min against initial lengths of green compacts). The Vickers hardness and indentation fracture toughness of the compact exhibit similar values to those obtained from thermally sintered compacts.     

Formation of Al2O3–GdAlO3 eutectic ceramics with a fine anisotropic structure in a flash event

An Al₂O₃–GdAlO₃ (GAP) composite with a fine eutectic microstructure was obtained through local melting and rapid solidification during a flash event. An AC field of 1 kV/cm at a frequency of 1 kHz was applied to a calcined Al₂O₃–GAP body at a furnace temperature of 1400°C to induce the flash event. The Al₂O₃–GAP body exhibited local melting through the current path. Solidification proceeded with the maximum cooling rate of about 140°C/s just after the power supply was turned off. Using this flash treatment, a refined eutectic structure with rodlike GAP phases in the Al₂O₃ matrix was obtained within 1 min. The interphase spacing of the obtained structure was approximately 170 nm, comparable with the finest Al₂O₃–GAP eutectic material obtained in previous studies. A flash event and subsequent rapid cooling can contribute to forming fine eutectic ceramics with a relatively low furnace temperature and short processing time.     

The influence of the processing parameters on the reactive flash sintering of ZrO2-CeO2

Reactive flash sintering (RFS) is a method that was recently developed to produce dense single-phase bulk ceramic parts through solid-state reactions in a single-step that only takes a few minutes. The influence of the RFS parameters on the phase purity of a simple mixed oxide, (Zr_0.8,Ce_0.2)O₂, was investigated. Parameters such as furnace temperature, furnace atmosphere, electric current density, and alternating current (AC) or direct current (DC) were examined. It was found that (Zr_0.8,Ce_0.2)O₂ pellets with high densities, above 90% of its theoretical density, can be produced by RFS in a few minutes when RFS occurs under oxidizing atmospheres, AC fields with current densities of 100 mA·mm⁻², and at a furnace temperature of 1200°C. Reducing conditions such as Ar-H₂ atmosphere and DC fields, low furnace temperatures, and low current densities resulted in phase impurities and poor reactions between the ZrO₂ and the CeO₂ powders. These results show that RFS is a useful method to produce mixed oxides, but it is very sensitive to the processing parameters. This is the first time that the influence of most of the RFS processing parameters has been studied systematically. Thus, the present work aims to provide guidelines on selecting the right processing parameters when exploring RFS.     

Suppressed atomic diffusion in flash sintering of bismuth telluride

Bi₂Te₃ materials were synthesised by the recently developed flash sintering (FS) method, and the rapid densification effect was studied. Whereas a fully densified sample can be obtained with a feeding current of 1.2 kA for 1 s, the limited heating effect reduces grain growth and atomic diffusion. Interestingly, the significant chemical reaction suppression with oxygen contaminants, which induces electron doping, has a meaningful electronic properties impact. The large negative Seebeck coefficient of ? 138.9 μV/K in the sample prepared by conventional current sintering of 773 K for 3 min, in which the oxygen diffuses into the Bi₂Te₃ phase, is significantly reduced to ? 8.5 μV/K in the FS sample, much closer to the intrinsic p-type conduction in Bi₂Te₃ raw powder. These results suggest that the limited FS heating effect may contribute to preserving the intrinsic raw powder properties in the sintered material by avoiding excess atomic diffusion and undesirable reactions.     

Liquid phase flash sintering in magnesia silicate glass-containing alumina

Magnesia silicate glass-containing alumina was flash sintered using an E-Field in the 500–1500 V/cm range. The addition of glass allows to reduce the current needed for densification and improves the shrinkage obtained during field-assisted sintering process. This behaviour is related to the different sintering mechanisms involved in the two materials, i.e. solid state sintering for pure alumina and liquid phase sintering for glass-containing alumina.The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355 °C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO₂-Al₂O₃. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Flash sintering of a three‐phase alumina,spinel, and yttria‐stabilized zirconia composite

Three‐phase ceramic composites constituted from equal volume fractions of α‐Al₂O₃, MgAl₂O₄ spinel, and cubic 8 mol% Y₂O₃‐stabilized ZrO₂ (8YSZ) were flash‐sintered under the influence of DC electric fields. The temperature for the onset of rapid densification (flash sintering) was measured using a constant heating rate at fields of 50‐500 V/cm. The experiments were carried out by heating the furnace at a constant rate. Flash sintering occurred at a furnace temperature of 1350°C at a field of 100 V/cm, which dropped to 1150°C at a field of 500 V/cm. The sintered densities ranged from 90% to 96%. Higher electric fields inhibited grain growth due to the lowering of the flash temperature and an accelerated sintering rate. During flash sintering, alumina reacted with the spinel phase to form a high‐alumina spinel solid solution, identified by electron dispersive spectroscopy and from a decrease in the spinel lattice parameter as measured by X‐ray diffraction. It is proposed that the solid solution reaction was promoted by a combination of electrical field and Joule heating.     

Multicomponent bulk metal nitride (Nb1/3Ta1/3Ti1/3)N1−δ synthesis via reaction flash sintering and characterizations

In this research, near fully dense single phase bulk multicomponent transition metal nitride (Nb_1/3Ta_1/3Ti_1/3)N_1−δ has been successfully synthesized from mixed commercial powders of NbN, TaN and TiN via reaction flash sintering technique. This was performed with an applied pressure of ~ 35 MPa at 25°C under a constant DC electric field (~24-32 V/cm). The flash event, which is the abrupt increase in current (up to ~ 25.2 A/mm²) and temperature, occurred without preheating. The threshold power dissipation on the sample right before the flash is ~ 0.7 W/mm³. The formation of single phase (Nb_1/3Ta_1/3Ti_1/3)N_1−δ random solid solution and its compositional uniformity were confirmed by XRD and EDS, respectively. The effects of ball milling duration and limiting current density on phase formation were studied. Simulation based on Joule heating provides an estimate of the ultimate sample temperature of ~ 1850°C. Vickers hardness of the obtained (Nb_1/3Ta_1/3Ti_1/3)N_1−δ is 17.6 ± 0.6 GPa, which is comparable to similarly flash sintered ingredient binary nitrides of TaN and NbN. TGA in air shows that the oxidation resistance of (Nb_1/3Ta_1/3Ti_1/3)N_1−δ is better than that of TaN and NbN but inferior to TiN. The study demonstrates that reaction flash sintering can be a highly efficient technique for synthesizing bulk multicomponent ceramics for both material fundamental investigations and application development.