Electric Sintering Research Articles

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.

Densification, microstructure, and properties of W-Mo-Cu alloys prepared with nano-sized Cu powders via large electric current sintering

• The sintering densification of W-Mo-Cu alloy was achieved with a short time and relatively low temperature. • The densification of W-Mo-Cu alloy was significantly improved using nano-sized Cu powders. • W-Mo-Cu alloy contained three new phases aside from W, Mo, and Cu phases. This study rapidly fabricated a novel W-Mo-Cu alloy by large current electric field sintering at a relatively low temperature, and the effects of the powder size of Cu on the densification, microstructure, and properties were comprehensively investigated. The particle size of Cu did not influence the phase type but significantly affected the densification, microstructure, and properties. XRD and TEM results showed that the alloy contained three new phases aside from W, Mo, and Cu phases, i.e., Mo-W ordered phase, Mo-Cu solid solution, and Cu 0.4 W 0.6 intermetallic compound. Copper powders with smaller sizes were beneficial to improving the distributional homogeneity of elements and the sintering densification. Therefore, the alloy prepared with 100 nm Cu powders had a denser and more homogeneous microstructure and better comprehensive properties than that prepared with 5 μm Cu powders. Overall, the W-Mo-Cu alloy prepared with 100 nm Cu powders at 980 °C proposed the best comprehensive properties, and its relative density can reach 98% approximately.

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.

Detailed microstructural and mechanical evaluation of yttria-stabilized zirconia-Ti composite compacts developed using spark plasma and electric discharge sintering processes

In this work, electric discharge sintering (EDS) and spark plasma sintering (SPS) methods are used to fabricate different yttria-stabilized zirconia–titanium (YSZ-Ti) composite compacts. The compacts were prepared using the ball-milled precursors and by varying the YSZ content of the YSZ-Ti composite (0, 1, 3, 5, 10 wt%). Then, the microstructures and mechanical properties of these composite compacts are analyzed in detail and compared. The results indicated that the YSZ contents of the SPS produced composite compacts are agglomerated and dispersed mostly at the Ti particle boundaries, according to the microstructural analyses. In the EDS composite compacts, the YSZ component is uniformly distributed throughout the Ti matrix. As a result, the mechanical properties such as the hardness (from 88 HV to 611 HV), yield strength (366 MPa–922 MPa), and tensile strength (from 743 MPa to 1944 MPa) of the EDS composite compacts are improved significantly. In comparison, as the YSZ content of Ti increased, the mechanical properties of the SPS prepared composite compacts are also enhanced but not as much as those of the EDS processed composite compacts (hardness (from 219 HV to 382 HV), yield strength (321 MPa–502 MPa), and tensile strength (from 692 MPa to 1076 MPa). To summarize, a composite comprising a Ti matrix reinforced with YSZ was efficiently fabricated by EDS rather than by SPS using Ti powder with surface-embedded YSZ. The excellent mechanical properties of the Ti-YSZ composites fabricated by EDS can be ascribed to the uniform and homogeneous distribution of YSZ in the Ti matrix phase. EDS is thus a promising powder metallurgy technique for the production of low-cost isotropic Ti matrix composites reinforced with ceramic particles.

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.

Engineering Silver Microgrids with a Boron Nitride Nanotube Interlayer for Highly Conductive and Flexible Transparent Heaters

AbstractThe use of flexible transparent conductive electrodes (TCEs) as printed heaters offers unique advantages where transparency is a necessary design feature. Many existing TCE materials however suffer from poor flexibility and require complex fabrication processes and thus are not commercially viable for such applications. The design and process optimization of screen‐printable silver metal microgrids over a layer of boron nitride nanotubes (BNNT) to produce highly conductive and mechanically robust transparent heaters with high transparency and low power requirements is reported. Square and hexagonal geometries are investigated alongside varying line width and pitch combinations to produce 16 different grid designs with optical transparency ranging between 65% and 89% and resistance values as low as 2.90 Ω sq−1. The BNNT thin film coating on polyethylene terephthalate substrates provides mechanical stability to the heater architecture by reducing the effects of deformation by up to 400%. The BNNT interlayer also contributes to an increase in thermal performance, achieving temperatures as high as 74.0 °C by initiating electrical sintering of the microgrids during heater operation which increases the Joule heating capacity of the features. Overall, this physical and material optimization provides a low‐cost, printable microgrid architecture for high performing and stable applications as TCE‐based heaters.

Effects of electro-thermal cycles on the densification and properties of W–Mo–Cu alloys prepared via large current electric field sintering

In this study, we fabricated a novel ternary alloy of W–Mo–Cu by constant and cyclic large current electric field sintering at a relatively low temperature (950 °C) and within a short time (5 min). We subsequently investigated the effects of the electro-thermal cycles on the densification, microstructure, and properties of the sintered alloy. The electro-thermal cycles process generated greater amounts of Joule heating and provided more energy under the additional effect of field emission by repeating the heating and cooling treatments. W–Mo–Cu alloy was experimentally proven to densify significantly under the effect of electro-thermal cycles. Indeed, as compared with the microstructures of W–Mo–Cu alloy prepared via constant temperature sintering, the microstructures derived by cyclical sintering were far denser. It was also found that the relative density and hardness of cyclically sintered W–Mo–Cu alloy were higher, and its electrical resistivity was lower than those of constantly sintered alloy. In general, cyclically sintered W–Mo–Cu alloy, with 10 wt% Cu added, exhibited optimal comprehensive properties. It can be concluded that cyclic large current electric field sintering is a promising method for preparing W–Mo–Cu alloy with enhanced densification and properties.

Microstructure and phases investigations of W–Mo–Cu alloy prepared by large current electric field sintering

W–Mo–Cu alloy was fabricated by large current electric field sintering at a low temperature of 950 °C. The interrupted sintering at different sintering stages was performed to identify the phases and microstructure evolution during the densification process. The transmission electron microscope (TEM) images clearly show that the nanoscale phases form and distribute at the boundaries of W, Mo, and Cu phases. Moreover, the high-resolution TEM images incorporating with a selected area fast Fourier transform and inverse fast Fourier transform (IFFT) treatment present the structure of formed phases. Meanwhile, the lattice parameters calculated through X-ray diffraction patterns and IFFT images jointly evidence the existence of defects and atom diffusion during the sintering. The dislocations and lattice strain induced by the ultrahigh current and pressure provide the stored energy and nucleation sites, thus promoting the diffusion of elements and the formation of fine grains.

Effect of pressure on the electrical resistance flash sintering of tungsten carbide

The application of external pressure during the Electrical Resistance Flash Sintering of tungsten carbide (WC), a “metallic ceramic” with positive temperature coefficient (PTC) for resistivity, results in a limited densification of the powder. In the present work, the effect of pressure on the ultrafast sintering of WC is analysed in detail. In particular, the application of pressure at different stages of flash sintering is studied by varying independently the processing parameters i.e., voltage, time and pressure. Microstructural characterization performed by FE-SEM and XRD identifies biphasic structures of WC and W2C, which compositions result almost unchanged for different sintering parameters. Increasing the external load from 4 up to 100 MPa leads to microporous materials, limiting the attainable final density. Pressure is demonstrated to control the flash phenomenon directly by affecting the flash duration and intensity, thus controlling the electrical energy dissipated during the power surge with respect to the overall process.

Magnetic properties of iron powder sintered by medium-frequency electrical resistance sintering

Medium-frequency electrical resistance sintering (MF-ERS) is a technique that uses the application of pressure and heat, heat coming from the Joule effect, simultaneously to metallic powders. In this study, this technique consolidates commercial iron powders, and the porosity distribution and hysteresis curves of the compacts were analysed. Compact consolidated by conventional powder metallurgy (PM) was compared with the results obtained.

Thermodynamic Study of Phase Formation for W-Mo-Cu Composite Via Large Current Electric Field Sintering

Thermodynamic Study of Phase Formation for W-Mo-Cu Composite Via Large Current Electric Field Sintering

Mechanical alloying and amorphization of Ti75Cu25 alloy

Ti-Cu alloys are used in dental and medical applications, due to their good mechanical properties, corrosion resistance and antibacterial properties. These properties are sensitive to microstructure and can be improved by formation of intermetallic compounds or amorphization of crystalline Ti-Cu powder. This study focuses on the amorphization of Ti75Cu25 powders prepared by mechanical alloying (MA). Ball milling, up to 60 h, of Ti and Cu powders leads to an amorphous structure. Laser granulometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were carried out for different milling times. Future research on consolidation of amorphous powder via electrical resistance sintering will be performed to obtain bulk samples with amorphous phase and/or nanocrystalline phase with Ti3Cu and/or Ti2Cu intermetallic compounds.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ВНУТРЕННИХ НАПРЯЖЕНИЙ НА БАЛЛИСТИЧЕСКУЮ СТОЙКОСТЬ МЕЛКОЗЕРНИСТОГО ОКСИДА АЛЮМИНИЯ, ПОЛУЧЕННОГО МЕТОДОМ ЭЛЕКТРОИМПУЛЬСНОГО ПЛАЗМЕННОГО СПЕКАНИЯ

The results of tests for ballistic resistance of ceramic samples based on aluminum oxide (Al2O3 + 0.25MgO) obtained by high-speed electric pulse plasma sintering (EIPS) are presented. Samples 30 mm in diameter, 6.5 and 7 mm thick had a high relative density (more than 99%), a uniform fine-grained microstructure (average grain size ~1–1.5 mm), and increased hardness (more than 17 GPa). Using the finite element method, using the ANSYS Workbench package, it is shown that during rapid cooling at a rate of 50 °C/min, an uneven temperature field is formed in the samples and a difference in temperatures arises on two opposite sides of the ceramic samples. It has been established that non-uniform cooling during SPS leads to the formation of compressive internal stresses (up to –450 MPa), and there is a significant difference in the magnitude of internal stresses on opposite sides of ceramic samples. The differences in the parameters of the microstructure and microhardness on opposite sides of the samples are insignificant. Ballistic resistance tests were carried out at a speed of 700 m/s, using cylindrical-conical impactors made of hardened steel 52100. The ballistic resistance of the ceramics was evaluated by the anti-barrier action, i.e., the penetration depth of the striker into the aluminum witness sample located behind the tested ceramic plate. It is shown that the barrier action during testing of a ceramic plate with a large value of compressive internal stresses (from –390 to –450 MPa) is 1.5–2 times less than during ballistic testing of a ceramic plate, in the surface layer of which internal compressive stresses are small (from –60 to –90 MPa). The results obtained for the first time demonstrated the importance of the formation of compressive fields of internal stresses in ceramics, which makes it possible to further increase the ballistic resistance of protective ceramic plates.

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.

Ultrahigh Temperature Flash Sintering of Binder-Less Tungsten Carbide within 6 s.

We report on an ultrarapid (6 s) consolidation of binder-less WC using a novel Ultrahigh temperature Flash Sintering (UFS) approach. The UFS technique bridges the gap between electric resistance sintering (≪1 s) and flash spark plasma sintering (20–60 s). Compared to the well-established spark plasma sintering, the proposed approach results in improved energy efficiency with massive energy and time savings while maintaining a comparable relative density (94.6%) and Vickers hardness of 2124 HV. The novelty of this work relies on (i) multiple steps current discharge profile to suit the rapid change of electrical conductivity experienced by the sintering powder, (ii) upgraded low thermal inertia CFC dies and (iii) ultra-high consolidation temperature approaching 2750 °C. Compared to SPS process, the UFS process is highly energy efficient (≈200 times faster and it consumes ≈95% less energy) and it holds the promise of energy efficient and ultrafast consolidation of several conductive refractory compounds.

The Effect of Current Supply Duration during Stepwise Electrical Sintering of Silver Nanoparticles

We studied the effect of current supply duration at final-step currents during the stepwise electrical sintering of silver (Ag) nanoparticles (NPs). Ag NPs ink was inkjet-printed onto Eagle-XG glass substrates. Constant final-step currents of 0.4 and 0.5 A with various time intervals were applied to the printed samples. The final-step current of 0.5 A damaged the line at a comparatively shorter time duration. On the other hand, the lower final-step current of 0.4 A prevented the line damage at longer time durations while producing comparatively lower Ag NPs specific resistance. The minimum specific resistances of the printed samples sintered at 0.4 and 0.5 A were 3.59 μΩ∙cm and 3.79 μΩ∙cm, respectively. Furthermore, numerical temperature estimation and scanning electron microscope (SEM) analysis were conducted to elaborate on the results. The numerical temperature estimation results implied that the lower estimated peak temperature at the final-step current of 0.4 A helped prevent Ag NP line damage. The SEM micrographs suggested that a high surface porosity—caused by higher sintering peak temperatures—in the case of the 0.5 A final-step current resulted in a comparatively higher Ag NP line-specific resistance. This contribution is a step forward in the development of Ag NP sintering for printed electronics applications.

Obtaining a High-Entropy Fe–Cr–Co–Ni–Ti Alloy by Mechanical Alloying and Electric Spark Plasma Sintering of a Powder Mixture

Obtaining a High-Entropy Fe–Cr–Co–Ni–Ti Alloy by Mechanical Alloying and Electric Spark Plasma Sintering of a Powder Mixture

Revealing specific features of structure formation in composites based on nanopowders of synthesized zirconium dioxide

Peculiarities of formation of microstructure in composites based on chemically synthesized zirconium nanopowders obtained by the method of decomposition from fluoride salts were considered. Hydrofluoric acid, concentrated nitric acid, aqueous ammonia solution, metallic zirconium, and polyvinyl alcohol were used. It was established that the reduction of porosity in nanopowders in the sintering process is the main problem in the formation of high-density materials. Analysis of various initial nanopowders, their morphology, and features of sintering by the method of hot pressing with direct transmission of electric current was made. Peculiarities of obtaining the composites based on them with the addition of Al2O3 nanopowders applying the electric sintering method were considered. It was shown that the increase in the content of alumina nano additives leads to an increase in strength and crack resistance of the samples due to simultaneous inhibition of abnormal grain growth and formation of a finer structure with a high content of tetragonal phase. The influence of sintering modes on the formation of the microstructure of zirconium nanopowders has been studied for different contents of alumina additives. Electric current promotes the surface activity of nanopowders and its variable value promotes partial fragmentation of agglomerated grains thus affecting the composite structure. Physical-mechanical properties of the obtained samples, optimal compositions of mixtures, and possibilities of improving some parameters were determined. It was found that nanopowders of zirconium dioxide obtained by the method of decomposition from fluoride salts are quite suitable for the production of composite materials with high physical and mechanical properties. They can compete with imported analogs and enable obtaining of crack resistance of 7.8 MPa·m1/2 and strength of 820 MPa.