Electric Current Assisted Sintering Research Articles

Innovations in Electric Current-Assisted Sintering for SOFC: A Review of Advances in Flash Sintering and Ultrafast High-Temperature Sintering

This review discusses the groundbreaking advancements in electric current-assisted sintering techniques, specifically Flash Sintering (FS) and Ultrafast High-Temperature Sintering (UHS), for their application in Solid Oxide Fuel Cells (SOFCs). These innovative sintering methods have demonstrated remarkable potential in enhancing the efficiency and quality of SOFC manufacturing by significantly lowering sintering temperatures and durations, thereby mitigating energy consumption and cost. By providing a detailed overview of the mechanisms, process parameters, and material characteristics associated with FS and UHS, this paper sheds light on their pivotal role in the fabrication of SOFC components such as electrolytes, electrodes, multilayered materials, and interconnect coatings. The advantages, challenges, and prospective opportunities of these sintering technologies in propelling SOFC advancements are thoroughly assessed, underlining their transformative impact on the future of clean and efficient energy production technologies.

A comparative study of electric current-assisted and conventional sintering of Inconel 718 superalloy

In this study, In718 powder mixtures prepared from elemental powders following stoichiometric composition and supplied as a commercial product were produced by conventional powder metallurgy (1300 °C/4 h) and electric current-assisted sintering (ECAS)(1700-2300A/10 min) methods and followed by a double-aging heat treatment. The characterization studies determined that the best results were the sample produced by the ECAS method using the elemental powder mixture. The targeted precipitate γ′, γ″, and δ phases were obtained after the heat treatment, and a hardness value of 344 ± 41 HV0.5 and a relative density of 98.96% were achieved. The sample preserved its surface integrity by exhibiting high corrosion resistance from hot corrosion studies in the temperature range of 650–850 °C and NaSO4 + 60%V2O5 environment. The corrosion rate of the sample, which was subjected to electrochemical corrosion tests in 3.5% NaCl and 10% NaNO3 solutions, was determined as 45.59 mpy and 10.56 mpy, respectively.

Does flash sintering alter the deformation mechanisms of tungsten carbide?

The study aims at unravelling the effect of fast and ultrafast electric current-assisted sintering on the plasticity of ultrahard binderless tungsten carbide. The small-scale deformation of polycrystalline micropillars obtained from samples consolidated by Spark Plasma Sintering (SPS) and Electrical Resistance Flash Sintering (ERFS) processes was studied. Micropillars, 3 µm in diameter, were prepared by focused ion beam and compressed ex and in-situ at room and high temperatures (700 °C). Electron-transparent lamellas were milled out from the pillars plastically deformed at different strain levels to carry out Transmission Kikuchi diffraction (TKD) and HRTEM analyses. At room temperature, the micropillars show similar mechanical responses under compression, reaching outstanding yield strengths (8–11 GPa) with plastic strain not exceeding 3–5% because of a dislocation-assisted toughening mechanism. At 700 °C, the pillar's yield strength drops to around 1.5–2 GPa accompanied by the relevant temperature-activated plasticity. However, only the pillars prepared from flash-sintered material can be homogeneously deformed up to ≈50%; conversely, those derived from SPS ceramics fail macroscopically at strains of ≈20–25% strain upon the localisation of plastic strain at shear bands.

DEVELOPMENT of Cu MATRIX COMPOSITE CONTACTORS REINFORCED by NICKEL COATED SiC

In this study, Cu-Ni coated SiC composite samples were produced by electric current assisted sintering (ECAS) method by adding electroless nickel coated SiC powders to copper powders produced by cementation method at the ratios of 0.5, 1 and 1.5 wt%. The relative densities of the produced samples were measured by Archimedes’ principle, their microstructures were examined by SEM-EDS, dominant phases were determined by XRD technique; microhardness and electrical conductivity measurements were made. The relative density of undoped copper was determined as 99.42% and the relative density value of Cu-Ni SiC composite samples decreased to 98.35% at most with increasing SiC ratio. The hardness values of Cu-Ni SiC composite samples increased from 120HV to 145HV with the addition of SiC; electrical conductivity values decreased from 90.41 IACS (International annealed copper standard) to 58.56 IACS.

Electric Current‐Assisted Sintering of 8YSZ: A Comparative Study of Ultrafast High‐Temperature Sintering and Flash Sintering

Electric current‐assisted sintering (ECAS) is a promising powder consolidation technique that can achieve short‐term sintering with high heating rates. Currently, main methods of performing ECAS are indirect heating of the powder compact in a conductive tool or direct heating with current flowing through the powder compact. Various influencing factors have been identified to explain the rapid densification during ECAS, such as ultrahigh heating rates, extra‐high temperatures, and electric field. However, the key consolidation‐enhancing factor is still under debate. This study aims at understanding the role of heating rate on the enhanced densification during ECAS of 8 mol% Y2O3‐stabilized ZrO2 (8YSZ) by experimental and numerical methods. Two different heating modes, ultrafast high‐temperature sintering (UHS, indirect heating) and flash sintering (FS, direct heating), are studied. The novel UHS technique is successfully applied to consolidate the 8YSZ samples. Additionally, finite element methods (FEM) combined with a constitutive model is adopted to predict the densification and grain growth. Furthermore, a comparison of UHS and FS is performed to investigate the thermal effect (heating rate) and athermal effect (electric field) individually. The results indicate that the high heating rate is the key factor of the rapid densification during UHS and FS of 8YSZ.

Enhanced Room-Temperature Thermoelectric Performance of 2D-SnSe Alloys via Electric-Current-Assisted Sintering.

Single-crystalline tin-selenide (SnSe) has emerged as a high-performance and eco-friendly alternative to the lead-chalcogens often used in mid-temperature thermoelectric (TE) generators. At high temperature >800 K, the phase transition from Pnma to Cmcm causes a significant rise in the TE figure-of-merit (zT) curve. Conversely, the SnSe TE requires a booster at low temperatures, which allows broader applicability from a device perspective. Herein, a synergy of Cu alloy and Ag-coating is realized through a sequential multi-step synthesis, designed to combine different metal deposition effects. Single-crystalline (Cu2Se)x(SnSe)1-x alloys grown by the Bridgman method were then coated with a thin Ag layer by radio frequency (RF) sputtering, and the interlayer epitaxial film was observed via electric-current assisted sintering (ECAS). Consequently, the thin Ag-coating improves the electrical conductivity (σ) and reduces the thermal conductivity (κ) for (Cu2Se)0.005(SnSe)0.995+Ag alloy, increasing the zT curve at close to room temperature (373 K). The incorporation of multistep addition by ECAS enables tuning of the overall solubility of the alloy, which opens a new avenue to optimize TE performance in anisotropic 2D materials.

Tailoring oxidation resistance of Fe2CoCrNi0.5 based high entropy alloys by addition of alloying elements (Si, Cu and Si-Cu co-added)

Investigations about how to improve the properties of high entropy alloys (HEAs) play a key role in growing their potential for use in future industrial applications. Accordingly, this study aims to shed light on the effects of alloying elements (Si, Cu and Si-Cu co-added) on the microstructure and oxidation performance of the Fe 2 CoCrNi 0.5 based high entropy alloys. To this end, a series of high entropy alloys containing Fe 2 CoCrNi 0.5 , Fe 2 CoCrNi 0.5 Si 0.25 , Fe 2 CoCrNi 0.5 Cu 0.25 and Fe 2 CoCrNi 0.5 Si 0.25 Cu 0.25 were produced by electric current assisted sintering (ECAS) method, which is founded on the principle of simultaneous application of high-density electric current flowing through the powder mixture and uniaxial pressure within a short span of time. The oxidation test was carried out in air at 1123 K for 1200 h under cyclic conditions. The X-ray diffraction results showed that all HEAs have simple FCC solid solution phases. Among all HEAs, the oxidation resistance of the alloy exhibited an increasing trend with the Si addition and the best oxidation behavior at 1123 K for 1200 h was noted with Fe 2 CoCrNi 0.5 Si 0.25 , which experienced the least mass change of 4.72 mg cm −2 . Cu addition caused the highest mass change of 15.92 mg cm −2 and the thickest oxide scale of 38.2 µm after oxidation for 1200 h. Si addition is observed to enhance the amount of Cr-rich oxide in the scale. The oxide scale formed on HEA with Cu-added and HEA with Si-Cu co-added contained two oxide layers, including an outer layer of Fe-rich oxide and an inner layer of Cr-rich oxide, which showed continuous structure. However, the alloying elements did not play a favorable role in relieving the formation of internal oxidation. Additionally, it has been reported that Fe 2 CoCrNi 0.5 Si 0.25 HEA can exhibit high resistance to cyclic oxidation, which can be reasoned as an optimum material for structural and high-temperature applications. • Alloying element behavior has been shown to have a significant influence on the investigated properties of HEAs. • Si has proved to be effective in enhancing oxidation resistance. • Cu addition produced the thickest oxide scale. • One of the encouraging results of this investigation was the synergic effect of Si-Cu co-added.

NdFeB Magnets with Well‐Pronounced Anisotropic Magnetic Properties Made by Electric Current‐Assisted Sintering

Electric current‐assisted sintering (ECAS) technologies are highly promising for processing of NdFeB magnets. Due to the combination of direct Joule heating and application of external load, even powders, whose particle size distribution and morphology are not optimum for conventional powder processing like melt‐spun powders or magnet scrap, can be easily sintered to high densities. A systematic study is done to demonstrate the potential of field‐assisted sintering technique/spark plasma sintering (FAST/SPS) and flash spark plasma sintering (flash SPS) for sintering of NdFeB powders. Melt‐spun, commercial NdFeB powder (Magnequench MQU‐F) is used as starting material. Its platelet‐like shape makes this powder extremely difficult to sinter by conventional methods. This study clearly reveals that especially in the case of flash SPS application of external pressure in combination with short cycle times enables to achieve well‐pronounced anisotropic magnetic properties without the need of subsequent upset forging. Optimized flash SPS parameters are applied to NdFeB magnet scrap with broad particle size distribution, demonstrating the general potential of ECAS technologies for recycling of waste magnet materials. Finally, the results are benchmarked with respect to established NdFeB processing technologies and electrodischarge sintering (EDS), another promising ECAS technology with very short cycling time.

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.

A Novel Hermetic Sealing Method for the Metal Package Based on Electric-Current-Assisted Sintering of Silver Paste

Based on electric-current-assisted sintering (ECAS) of silver paste, we propose a novel hermetic sealing method for the metal package. The monitoring of the temperature field during the bonding process shows that hermetic packaging based on ECAS of silver paste is a local heating method which has little impact on the component. During the process of ECAS, the heating rate of sintered joint dropped a lot with the removal of organics, and the reliable sintered joint is gradually formed after the removal of organics. Increasing the current gets higher peak temperature of the sintered joint, which is beneficial to form a denser and robust joint. The hermeticity of the sealed specimen and the shear strength of the sintered joint are improved with the increase of peak temperature. When the peak-temperature reaches above 400 °C, the joint will be dense enough to ensure the leakage rate of specimen below the reject limit.

High Temperature Corrosion Behavior of Eutectic Structured NiAl-34Cr and NiAl-28Cr-6Mo Alloys Produced by Electric Current Activated Sintering

In this study, NiAl-34Cr and NiAl-28Cr-6Mo eutectic alloys were produced by electric current assisted sintering (ECAS) method in a 3500-4200 A current range with a waiting time of 47 minutes. Phase examinations of the obtained samples were carried out with the help of X-ray diffraction analysis (XRD). While determining NiAl and Cr phases in NiAl-34Cr alloy from XRD patterns; It was determined that NiAl-28Cr-6Mo alloy consists of two phases, together with Mo residues, NiAl and CrMo. According to the Archimed principle, the relative density of NiAl-34Cr and NiAl-28Cr-6Mo alloys was determined as 96.2%, 97.9% respectively. The hardness values of NiAl-34Cr and NiAl-28Cr-6Mo samples were approximately 275 ± 13 HB and 255 ± 20 HB detected. In addition, the corrosion properties of the samples were examined by hot corrosion tests at 800, 900 and 1000°C for 165 hours (15 cycles) in 25% wt. K2SO4 + 75% wt. Na2SO4 salt medium. Weight changes, microstructure (SEM-EDS) and phase analysis of the samples after corrosion were carried out, and the corrosion properties of NiAl-34Cr alloy were found to be better compared to the Mo-added alloy.

Microstructure and magnetic properties of Ni0·75Zn0·25Fe2O4 ferrite prepared using an electric current-assisted sintering method

Using a Ni0·75Zn0·25Fe2O4 nanopowder synthesized by means of a hydrothermal method as a raw material, polycrystalline nickel zinc (NiZn) ferrite ceramics composed of sub-micron grains were successfully prepared via an electric current-assisted sintering method. Temperatures ranging from 800 °C to 950 °C and a dwell time of 20 min were employed. The phase composition and microstructure of the samples were characterized via X-ray diffraction and scanning electron microscopy, respectively. Moreover, the magnetic properties of the samples were investigated using a vibrating sample magnetometer and a ferromagnetic resonance system. The results revealed that each sintered sample was mainly composed of a spinel phase. With increasing sintering temperature, the specific saturation magnetization increased from 71.85 emu/g to 74.58 emu/g, owing mainly to the increase in the relative density and the average grain size of the NiZn ferrites. The coercivity and ferromagnetic resonance linewidth of the ferrite ceramics decreased monotonically with increasing sintering temperature, owing mainly to the magnetostriction coefficient, saturation magnetization, and porosity of the sintered ferrites.

Laser re-melting influence on isothermal oxidation behavior of electric current assisted sintered CoCrFeNi, CoCrFeNiAl0.5 and CoCrFeNiTi0.5Al0.5 high entropy alloys

In this study, it was aimed to determine the effect of laser re-melting on the oxidation behavior of high entropy alloys produced with electric current assisted sintering (ECAS). CoCrFeNi, CoCrFeNiAl0.5 and CoCrFeNiAl0.5Ti0.5 high entropy alloys (HEAs) were produced using ECAS. After the production of HEAs, the laser re-melting (LR) process was applied to the surface of sintered samples. Then, isothermal oxidation tests were carried out to HEAs at 1000 °C for 5, 25 and 75 h. Before and after the oxidation tests, the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) mapping analysis. The obtained results show that the LR process significantly improved the microstructural properties of ECAS-HEAs. More homogeneous microstructure, lower porosity and single-phase formations were observed in HEAs after LR. This has enabled a more stable microstructure compared to ECAS samples. After the oxidation tests, lower oxide layer thickness, lower oxide growth rates and lower inner oxide formation were obtained in laser re-melted HEAs. The presence of Al-rich phases enables the formation of alumina layer on the surface of HEAs. The best oxidation performance was obtained with laser re-melted CoCrFeNiAl0.5.

Magnetic Field Generated during Electric Current-Assisted Sintering: From Health and Safety Issues to Lorentz Force Effects

In the past decade, a renewed interest on electromagnetic processing of materials has motivated several investigations on the interaction between matter, electric and magnetic fields. These effects are primarily reconducted to the Joule heating and very little attention has been dedicated to the magnetic field contributions. The magnetic field generated during electric current-assisted sintering has not been widely investigated. Magnetism could have significant effects on sintering as it generates significant magnetic forces, resulting in inductive electrical loads and preferential heating induced by overlapping magnetic fields (i.e., proximity effect). This work summarizes the magnetic field effects in electric current-assisted processing; it focuses on health and safety issues associated with large currents (up to 0.4 MA); using FEM simulations, it computes the self-generated magnetic field during spark plasma sintering (SPS) to consolidate materials with variable magnetic permeability; and it quantifies the Lorentz force acting at interparticle contact points. The results encourage one to pay more attention to magnetic field-related effects in order to engineer and exploit their potentials.

Ceramic Material Processing Towards Future Space Habitat: Electric Current-Assisted Sintering of Lunar Regolith Simulant.

In situ utilization of available resources in space is necessary for future space habitation. However, direct sintering of the lunar regolith on the Moon as structural and functional components is considered to be challenging due to the sintering conditions. To address this issue, we demonstrate the use of electric current-assisted sintering (ECAS) as a single-step method of compacting and densifying lunar regolith simulant JSC-1A. The sintering temperature and pressure required to achieve a relative density of 97% and microhardness of 6 GPa are 700 °C and 50 MPa, which are significantly lower than for the conventional sintering technique. The sintered samples also demonstrated ferroelectric and ferromagnetic behavior at room temperature. This study presents the feasibility of using ECAS to sinter lunar regolith for future space resource utilization and habitation.

Application of Electric Current‐Assisted Sintering Techniques for the Processing of Advanced Materials

Highly efficient energy conversion and storage technologies such as high‐temperature solid oxide fuel and electrolysis cells, all‐solid‐state batteries, gas separation membranes, and thermal barrier coatings for advanced turbine systems depend on advanced materials. In all cases, processing of ceramics and metals starting from powders plays a key role and is often a challenging task. Depending on their composition, such powder materials often require high sintering temperatures and show an inherent risk of abnormal grain growth, evaporation, chemical reaction, or decomposition, especially in the case of long dwelling times. Electric current‐assisted sintering (ECAS) techniques are promising to overcome these restrictions, but a lot of fundamental and practical challenges must be solved properly to take full advantage of these techniques. A broad and long‐term expertise in the field of ECAS techniques and comprehensive facilities including conventional field‐assisted sintering technology/spark plasma sintering (FAST/SPS), hybrid FAST/SPS (with additional heater), sinter forging, and flash sintering (FS) devices are available at the Institute of Energy and Climate Research: Materials Synthesis and Processing (IEK‐1). Herein, main advantages and challenges of these techniques are discussed and the concept to overcome current limitations is introduced on selected examples.

Investigation of Cyclic Oxidation Behavior of (Cr, Mo, Si)-Containing (α2 + γ) TiAl-Based Alloys Synthesized by ECAS Sintering

Ti–48Al–2Cr–2Mo–(0 and 0.5 at %) Si intermetallic alloys were synthesized by electric current- assisted sintering (ECAS). Microstructures of the alloys were characterized using scanning electron microscopy (SEM), electron diffraction spectroscopy (EDS), and X-ray diffraction (XRD) methods of analysis. It was found that the microstructures of the produced alloys consist of α2-Ti3Al and γ-TiAl phases. The cycle oxidation behaviors of the alloys were carried out between 700 and 900°C temperatures for 180 h in air. The oxide scale has comprised the TiO2 and Al2O3 mixture layer, intermediate Al2O3 layer, and outermost TiO2 layer. The oxidation rate constants and activation energies of the alloys were calculated.

Nb and B effect on mechanical properties of Ti–Al based intermetallic materials

Ti–TiAl3 in-situ composites containing different percentages of Nb and B were effectively produced from Ti, Al, Nb and B powders by electric current assisted sintering (ECAS) technique which is a powder metallurgy processing method. Samples are sintered for 90 s with 2000 A current. The effect of B and Nb on the hardness, fracture toughness and wear resistance of samples were studied. The microstructure properties of the sintered samples were analysed with scanning electron microscopes (SEM), the phases in the samples were determined with XRD and their hardness and fracture toughness values were measured with a Vickers hardness tester with a load of 0.98 N and 98 N respectively. The highest fracture toughness value has been obtained with wt %10 Nb addition as 5.23 MPa m1/2, whereas the highest hardness was determined as 965 HV for wt%5 B reinforced in situ-Ti-TiAl3 composite. Best wear resistance was obtained in the 47.5Ti-47.5Al–5B sample. While Nb additive had a negatory effect on wear resistance, additive B had a positive effect on wear resistance.

Reduction of temperature gradient and carbon contamination in electric current assisted sintering (ECAS/SPS) using a “saw-tooth” heating schedule

A novel approach for the reduction of temperature gradients and carbon contamination of transparent ceramics prepared via electric current assisted sintering (ECAS) is proposed and successfully demonstrated in the case of transparent spinel ceramics. A so-called “saw-tooth” heating schedule was applied to attain more homogeneous densification, hereby lowering the level of carbon contamination. An important feature of this saw-tooth heating schedule is that the average heating rate can be retained. The approach is a general one that can be used for other ceramics as well.

Process investigation and mechanical properties of electro sinter forged (ESF) titanium discs

Classified as an electric current–assisted sintering (ECAS) process, electro sinter forging (ESF) represents a sintering process following the resistance heating approach. The powder is simultaneously compacted and heated in a closed-die setup. The heating is generated by the Joule effect from the electrical current. Near net shape components of conductive materials are made in the closed-die setup within a short process time (100–400 ms). The final relative density is an important quality measure for the sintered parts. In the present work, samples of commercially pure titanium are produced with up to 98% relative density by optimisation of the main process parameters, namely electrical current density, compaction pressure and sintering time. Metallographic observations revealed that porosities were mostly found at the perimeter of the sintered samples. Mechanical testing by μ-Vickers hardness test, uniaxial compression and indirect tensile tests showed improved properties of the material with increasing density. The achieved mechanical properties were compatible with the theoretical values for bulk titanium.