Electric Field-assisted Sintering Research Articles

Rapid low-temperature densification of Ag2Se bulk material: mass transfer through the dissociative adsorption reaction of Ag protrusions and Se saturated vapor

In materials science, the impact of density on a material’s capabilities is profound. Conventional sintering requires high temperatures and is energy-demanding, propelling the pursuit of less intensive, low-temperature densification methods. Electric field-assisted sintering has recently gained attention for its simplicity and effectiveness, offering a new frontier in low-temperature densification. In this study, dense bulk materials were produced by subjecting monophasic Ag2Se powders to electric field-assisted sintering, where a direct current with an average value of 4 A was applied, achieving a peak temperature of 344 K. The novel low-temperature densification mechanism unfolds thus: nanoscale silver protrusions, stimulated by electrical current, engage in a dissociative adsorption reaction with the ambient saturated selenium vapor. This process swiftly engenders the formation of fresh silver selenide (Ag2Se) compounds, initiating nucleation and subsequent growth. Consecutively, these compounds seamlessly occupy and expand, perpetually bridging the interstices amidst the powders. In a scant 8 s, the density swiftly surpassed 99%, yielding a bulk material that exhibited a ZT value of 1.07 at 390 K. This investigation not only attains an unparalleled density at low temperatures but also charts a pioneering course for material densification in such conditions.

Electrochemical grand potential-based phase-field simulation of electric field-assisted sintering

An electrochemical grand potential functional was proposed to describe the sintering of an ionic ceramic green body. The resultant phase-field description enables simulation of the consolidation of an arbitrary number of granular particles and their interactions with the surrounding void phase. The model includes the effects of charged vacancies and the associated interactions between internal and applied electric fields. Defect segregation to grain boundaries is also accounted for, as well as enhanced interfacial defect mobilities. The model was parameterized for Y2O3. Simulations of two-particle systems showed that the applied electric field had an increasingly important impact on neck growth as particle size increased. A sudden rapid increase in temperature occurred for larger field strengths, which has been reported to be correlated to the onset of a flash event in flash sintering. Simulations of many particles showed that internal heat generation by Joule heating was localized at particle–particle contacts (grain boundaries), even though their conductivities were lower than nearby internal particle-void interfaces. A percolative path for ionic charge across the green body and the ceramic sintered solid was thus defined, accelerating the Joule heating process as the porosity of the green body is removed.

Microstructure characterization of electric field assisted sintering (EFAS) sintered metallic and ceramic materials using local thermal diffusivity measurement

Electric Field Assisted Sintering (EFAS, also referred to as spark plasma sintering) is a powerful technology for the consolidation of powder materials. The high heating rate during the sintering process is critical for minimizing energy consumption, but it can also cause microstructure heterogeneities in sintered parts, such as spatially varied porosity. The examination of localized porosity usually requires the use of a scanning electron microscope with a carefully prepared surface. In this paper, photothermal radiometry is used to measure local thermal diffusivity and extract localized porosity of EFAS-sintered parts by using a percolation-threshold model. Applying this approach, we identified the radial position-dependent porosity variation in EFAS parts, which is likely formed due to the large temperature gradient during the sintering process. This approach has a unique advantage because it can measure samples with minimal or no surface preparation, enabling the possibility of in situ characterization in EFAS with proper system modification. Necessary modifications on the measurement approach for EFAS deployment and in situ characterization are also discussed.

Enhanced diffusion bonding of alloy 617 using electric field-assisted sintering

The development of compact heat exchangers (CHXs) has gained increasing interest in many industries owing to their high thermal efficiency and reduced size. Diffusion bonding (DB) is an advantageous technique for fabricating CHXs. Alloy 617 is a candidate for manufacturing CHXs for high-temperature advanced nuclear reactors due to its elevated-temperature properties. Previous endeavors in DB of Alloy 617 were conducted by hot pressing (HP), which reported precipitates at the diffusion-bond interface, limited grain boundary (GB) migration, and significantly reduced high-temperature mechanical properties. To overcome these challenges, this study investigated DB of Alloy 617 using electric field-assisted sintering (EFAS). Stacks composed of three sheets were bonded with EFAS using different temperatures, pressures, and hold times. DB using HP as the zero-current analog of EFAS was also performed for comparison. The result shows that Cr- and Mo-rich precipitates were formed at the interface of the hot-pressed samples. The electric current and temperature in EFAS play a significant role in precipitation and GB migration. The electric current coupled with correct temperatures can effectively prevent precipitate formation at the interface and achieve excellent GB migration. Nanoscale Al-rich oxide was formed at the interface of the samples made by both HP and EFAS, but grain boundaries can ignore the nanoscale Al-oxide and migrate across the interface. The temperature, pressure, and hold time also affected diffusion. The temperature is a prerequisite for a successful GB migration, and GB migration can be enhanced by increasing pressure and hold time.

Brazing of Ti-coated SiC using a CoFeCrNiCu high entropy alloy filler via electric field-assisted sintering

Electric field-assisted sintering technology (FAST) was used to join SiC ceramic by using a CoFeCrNiCu high-entropy alloy filler. A thin Ti film was deposited on SiC surface by physical vapor deposition to reduce the residual thermal stress in the joint. The microstructure, bending strength, and bonding mechanism were systematically investigated. It was found that a discontinuous TiC layer could be formed at the interface and the interfacial reaction products changed with different brazing temperatures. A joint with a bending strength of 72 MPa was achieved at 1200 °C, which is 73% higher than that of the SiC brazed without Ti coating. This work provides a new insight in the interfacial design of ceramic brazing with high mechanical properties.

Research on densification mechanism of Inconel 718 alloy miniature gear by electric field-assisted sintering

The densification mechanism of the electric field-assisted sintering (E-FAST) for Inconel 718 alloy miniature gears was studied using the Bernard-Granger model. Under the conditions of sintering temperature of 950 °C, 1000 °C and 1050 °C and dwelling time of 240 s, 300 s and 360 s, the Inconel 718 powders were subjected to E-FAST using the Gleeble thermal simulator, and the densification rate was up to 10−3 s−1. With the increase of the sintering temperature, the peak of the densification rate of the sintered sample was enlarged, and the time for the peak to appear became shorter. The densification of Inconel 718 alloy miniature gears was mainly concentrated in the dwelling stage. When the dwelling time reached 300 s, the densification rates of the samples under different temperature conditions were similar, indicating that the gear samples were close to full density at this time. Simulation and sintering test results show that the densification stages of the gear samples can be divided into low sintering stress stage with stress index n = 0.4 and high sintering stress stage with stress index n = 3.5. The apparent activation energy of the densification changes with the variation of the sintering stress stage.

Electric field-assisted in situ fabrication of carbon/zirconia nanocomposites with tunable conductivity for electromagnetic interference shielding applications

Carbon/ceramic nanocomposites have been considered as ideal candidates for electromagnetic interference shielding (EMI) applications in harsh environments. However, the conventional fabrication methods of carbon/ceramic composites are often complicated, costly and time-consuming. Herein, by applying a DC electric field during hot pressing sintering, we realized the direct growth of graphene-like carbon nanosheets (GCNs) in zirconia ceramics during sintering, achieving in situ fabrication of GCNs/zirconia nanocomposites with both high EMI shielding effectiveness (>40 dB) and mechanical strength (>1000 MPa) in one step. In particular, we found that applying a DC electric field could significantly enhance the carburization during sintering, and GCNs are in situ formed at zirconia grain boundaries. When the initial electric field is constant, the amount of in situ formed GCNs could be controlled by simply adjusting the current density, resulting in the tunable electrical conductivity. These findings will provide new opportunities for both experimental and theoretical studies on carbon/ceramic EMI shielding materials and the DC electric field-assisted sintering technique.

Ultrafast low-temperature near-seamless joining of Cf/SiC using a sacrificial Pr3Si2C2 filler via electric current field-assisted sintering technique

A novel layered structure material, Pr3Si2C2, was synthesized at a low temperature of 850 °C using a molten salt approach for the first time, and subsequently used as the joining filler for carbon fibers reinforced SiC composites (Cf/SiC). A robust near-seamless Cf/SiC joint was successfully obtained at 1509 °C (Ti) for 30 s, while an ultrafast heating rate of 6000 °C/min was applied via electric field-assisted sintering technology. The near-seamless joining process was attributed to the newly precipitated SiC grains, which were densified well with the Cf/SiC matrix by liquid-assisted sintering. The liquid phase was in-situ formed by the eutectic reaction between Pr3Si2C2 and SiC. The shear strength of the near-seamless joint obtained at 1509 °C for 30 s was 17.6 ± 3.0 MPa. The failure occurred in the Cf/SiC matrix. The formation of near-seamless Cf/SiC joints dismisses the issues related to thermal mismatch between Cf/SiC matrices and traditional joining fillers.

Near-seamless joining of Cf/SiC composites using Y3Si2C2 via electric field-assisted sintering technique

A novel Y3Si2C2 material was synthesized at a relatively low temperature (900 °C) using a molten salt method for the first time, and subsequently used as the joining material for carbon fiber reinforced SiC (Cf/SiC) composites. The sound near-seamless joints with no obvious remaining interlayer were obtained at 1600 °C using an electric field-assisted sintering technique (FAST). During joining, a liquid phase was formed by the eutectic reaction among Y3Si2C2, γ(Y—C) phase, and SiC, followed by the precipitation of SiC particles. The presence of the liquid promoted the sintering of newly formed SiC particles, leading to their complete consolidation with the Cf/SiC matrix. On the other hand, the excess of the liquid was pushed away from the joining area under the effect of a uniaxial pressure of 30 MPa, leading to the formation of the near-seamless joints. The highest shear strength (τ) of 17.2±2.9 MPa was obtained after being joined at 1600 °C for 10 min. The failure of the joints occurred in the Cf/SiC matrix, indicating that the interface was stronger than that of the Cf/SiC matrix. The formation of a near-seamless joint minimizes the mismatch of thermal expansion coefficients and also irradiation-induced swelling, suggesting that the proposed joining strategy can be potentially applied to SiC-based ceramic matrix composites (CMCs) for extreme environmental applications.

Improved additive manufacturing of silicon carbide parts via pressureless electric field‐assisted sintering

High solids loading silicon carbide (SiC)-based aqueous slurries containing only .5 wt. % organic additives were utilized to create specimens of various geometries via an extrusion-based additive manufacturing (AM) technique. Pressureless electric field-assisted sintering was performed to densify each specimen without deformation. The combination of these techniques produced parts with >98% relative density despite containing only 5 wt.% oxide sintering additives. After sintering, specimens contained only the α-SiC and yttrium aluminum perovskite phases. This suggests the evolution of a nonequilibrium yttrium aluminate phase, as well as transformation from β-SiC to α-SiC. The fabrication method presented in this work has advantages over other AM techniques commonly used with SiC, because it does not require significant organic additives nor additional postprocessing steps such as chemical vapor infiltration or polymer impregnation and pyrolysis.

Accessing the role of Joule heating on densification during flash sintering of YSZ

Flash sintering is a novel electric field and current assisted sintering technique which densify ceramics at moderate furnace temperature and in short time (few seconds) as compared to the conventional sintering techniques. The onset of flash encompasses three unique characteristics, (i) non-linear rise in conductivity, (ii) luminescence and (iii) rapid densification. the mechanism behind the flash phenomena is not fully understood yet. The present work experimentally demonstrated that the densification in flash sintering is primarily dominated by Joule heating. Herein, we distinguished the onset of flash and the degree of densification of cubic zirconia in sintering atmospheres with different oxygen partial pressures. In this study, we demonstrate that the flash onset temperature and densification are independently influenced by the sintering atmosphere during flash sintering. Furthermore, the extent of densification is not directly influenced by the current density rather, by the power dissipated in the sample.

Joining of SiC using CoFeCrNiCuTi high entropy alloy filler by electric current field assisted sintering

SiC ceramics were brazed by electric field-assisted sintering technology using CoFeCrNiCuTi high-entropy alloy as joining filler. The effect on the interface microstructure and bend strength of brazed joints at different brazing temperature was systemically studied. The interfacial reaction was controlled by adjusting the brazing temperature. The main components in the brazing seam are high-entropy alloys FCC (HEAF), C, TiC, CrC, and Cr23C6 phase. Furthermore, the maximum bending strength of 54 MPa was found when brazed at a lower temperature of 1125℃. In addition, due to the electric field-assisted sintering technology and the high-entropy effect of the CoFeCrNiCuTi filler, the diffusion of elements and the formation of solid solution were accelerated. This suggests that the current field was beneficial to improve the inter-diffusion between the CoFeCrNiCuTi filler and SiC ceramics. Consequently, the low-temperature rapid brazing of SiC ceramics was realized, and this technology provides a new filler system for ceramic brazing.

The Effect of Reactive Electric Field-Assisted Sintering of MoS2/Bi2Te3 Heterostructure on the Phase Integrity of Bi2Te3 Matrix and the Thermoelectric Properties.

In this work, a series of Bi2Te3/X mol% MoS2 (X = 0, 25, 50, 75) bulk nanocomposites were prepared by hydrothermal reaction followed by reactive spark plasma sintering (SPS). X-ray diffraction analysis (XRD) indicates that the native nanopowders, comprising of Bi2Te3/MoS2 heterostructure, are highly reactive during the electric field-assisted sintering by SPS. The nano-sized MoS2 particles react with the Bi2Te3 plates matrix forming a mixed-anion compound, Bi2Te2S, at the interface between the nanoplates. The transport properties characterizations revealed a significant influence of the nanocomposite structure formation on the native electrical conductivity, Seebeck coefficient, and thermal conductivity of the initial Bi2Te3 matrix. As a result, enhanced ZT values have been obtained in Bi2Te3/25 mol% MoS2 over the temperature range of 300–475 K induced mainly by a significant increase in the electrical conductivity.

Low temperature seamless joining of SiC using a Ytterbium film

Monolithic SiC, for the first time, was seamless joined at a low temperature of 1200 °C using electric field-assisted sintering technology. A 300 nm Yb coating on SiC was used as the joining filler to form Yb3Si2C2 via an in-situ reaction with the SiC. A liquid phase was formed by an eutectic reaction between Yb3Si2C2 and SiC. Almost completely seamless joints were formed by the precipitated SiC grains, which were fully consolidated with the SiC matrix with the help of in-situ formed liquid phase, followed by its elimination under the uniaxial pressure. The bending strength of the seamless joint joined at 1500 °C for 15 min was as high as 257.2 ± 31.1 MPa, which was comparable to the strength of the SiC matrix. As a result, the failure occurred in the matrix indicated a sound joint was obtained. The proposed low temperature seamless joining could potentially be used for joining of SiC-based composite.

Effect of Pressure and Temperature on Densification in Electric Field-Assisted Sintering of Inconel 718 Superalloy.

Electric field-assisted sintering has ubiquitous merits over conventional sintering technology for the fabrication of difficult-to-deform materials. To investigate the effect of sintering pressure and temperature on the densification of Inconel 718 superalloy, a numerical simulation model was established based on the Fleck-Kuhn-McMeeking (FKM) and Gurson-Tvergaard-Needleman (GTN) models, which covers a wide range of porosity. At a sintering pressure below 50 MPa or a sintering temperature below 950 °C, the average porosity of the sintered superalloy is over 0.17 with low densification. Under a pressure above 110 MPa and a temperature above 1250 °C, the sintered superalloy quickly completes densification and enters the plastic yield stage, making it difficult to control the sintering process. When the pressure is above 70 MPa while the temperature exceeds 1150 °C, the average porosity is 0.11, with little fall when the pressure or temperature rises. The experimental results indicated that the relative density of the sintered superalloy under 70 MPa and 1150 °C is 94.46%, and the proportion of the grain size below 10 μm is 73%. In addition, the yield strength of the sintered sample is 512 MPa, the compressive strength comes to 1260 MPa when the strain is over 0.8, and the microhardness is 395 Hv, demonstrating a better mechanical property than the conventional superalloy.

Joining of Ti-coated monolithic SiC using a SiCw/Ti3SiC2 filler by electric field-assisted sintering

Monolithic SiC, for the first time, was successfully joined using a SiC whisker-reinforced Ti3SiC2 composite (SiCw/Ti3SiC2) filler via electric field-assisted sintering technique. A thin Ti coating layer was formed on the SiC surface to minimize the residual stress at the joint interface by transforming it into a TiC gradient layer. After optimizing process parameters, a joint strength higher than 250 MPa was obtained, which is higher than the other values reported in the literature. Failure occurred at the SiC base rather than the joining interface because of the improved joint strength by the incorporation of SiCw. The addition up to 15 wt. % SiCw in the filler layer improved the joint strength by various strengthening mechanisms. On the other hand, the joint strength was lower with 20 wt. % SiCw addition, indicating the importance of thermal expansion mismatch between SiCw and Ti3SiC2 to obtain a sound SiC joint.

Electron microscopy observation of electric field-assisted sintering of stainless steel nanoparticles

The intrinsic role of electrical current on the electric field-assisted sintering (EFAS) process of stainless steel 316L nanoparticles has been revealed by both ex situ and in situ experiments. A novel device on the Si chip has been designed and fabricated to fit into the sample holder of a transmission electron microscope for these experiments. The evolution of nanoparticle morphology and microstructures during the EFAS process has been studied using scanning electron microscopy and transmission electron microscopy, which has been combined with the simultaneous measurement of the electric voltage and current changes. A preliminary four-stage mechanism for the EFAS process of stainless steel 316L nanoparticles has been proposed based on these experimental investigations.

High-strength SiC joints with a novel in-situ formed SiC/Al4SiC4 joining filler

The in-situ formed SiC/Al4SiC4 joining layer was used to join monolithic SiC using an electric field-assisted sintering technique. A multiphase powder of Al4C3/SiC/Al4SiC4 was used as the initial joining material to obtain the in-situ reaction layer of SiC/Al4SiC4 via the appropriate interface reactions. The bending strength as high as 240.5 ± 6.6 MPa was obtained for the sample joined at 1800 °C, which was higher than the strength of the un-joined SiC matrix. Sound joints were obtained when Al4C3 was completely transformed to Al4SiC4, and a fully dense SiC/Al4SiC4 joining layer was consolidated. The integration of the joining layer with the SiC matrix was improved by a high amount of liquid phase formed at the interface. The proposed SiC/Al4SiC4 joining layer, with good thermal matching with SiC matrix, shows a great potential to be applied as a joining material for SiC-based ceramic matrix composites.

Liquid-phase flash sintering 8YSZ with alkali halide sintering aids

The properties of ZrO2: 8 mol% Y2O3 (8YSZ) ceramics with LiF and KCl sintering aids for liquid phase formation during electric field-assisted sintering were studied. Sintering experiments were carried out at 650 °C under 200 V cm−1 AC electric field by varying current density, current application time, as well as LiF and KCl contents. Pellets sintered with KCl addition had cavities, cracks and fractures. Pellets sintered with 15 wt.% LiF, on the other hand, were homogeneous after thermal removal of LiF upon Joule heating. Low electric current densities coupled with longer application times produced homogeneous specimens. Three different stages were identified during sintering experiments: (i) LiF melting with the electric field applied at furnace temperatures lower than its melting point, (ii) shrinkage due to liquid phase formation and LiF removal, (iii) final densification due to grain growth and pore elimination. The electrical behavior and an estimate of the porosity were carried out by electrochemical impedance spectroscopy measurements.

Electric field-assisted sintering (gadolinia-doped ceria/alkali salts) composite membranes

Composite ceramic membranes were prepared according to two routes: (i) vacuum impregnation of molten eutectic sodium-lithium carbonates (NLC) into porous ceria-20 mol% gadolinia (20GDC) solid electrolytes; (ii) electric field-assisted sintering of a 25 wt% NLC/75 wt% 20GDC mixture. Porous 20GDC ceramics were obtained by controlled thermal removal of 40 vol% KCl added as pore former. Electric field-assisted (flash) sintering was carried out monitoring thickness during application of 200 V cm−1 to the specimen positioned in a sample chamber inserted in a vertical dilatometer. The surfaces of the sintered membranes were observed in a scanning electron microscope. Electrochemical impedance spectroscopy measurements were performed in the 5 Hz to 13 MHz frequency range in the 280–580°C range. Arrhenius plots showed the transition from oxide ion conduction (due to the solid electrolyte) to carbonate ion conduction (due to the molten NLC). Membranes flash sintered at 420°C in 2 min showed electrical conductivity similar to membranes conventionally sintered at 690°C for 2 h.