Flash Spark Plasma Sintering Research Articles

Densification of zirconia-based ceramics by non-conventional sintering processes and chemical pathways

The present work reports an overview of the main results we have obtained from non-conventional sintering approaches towards the densification of 3mol.% yttria stabilized zirconia (3YSZ), the lessons learned from these results, and open questions that still need to be addressed. A wide range of techniques have been developed in the field of ceramic sintering, both in high temperature regime with extreme heating rates (>10 000 °C/min), as well as in very low temperatures (<400 °C) regime involving chemical reactivity to activate sintering mechanisms. In this context, we have recently explored several of these methods in order to understand and control the different sintering parameters and their impact on microstructure, crystallinity, grain boundaries and defect chemistry. Due to its complex chemistry, 3YSZ is a playground for the exploration of sintering capabilities and their influence on ceramic properties. Our recent studies in this field are presented in the present work, highlighting the effect of high heating rates through Flash Spark Plasma Sintering ( Flash-SPS) and Ultra High temperature Sintering (UHS) on sintering, but also the combination of low temperature Cold Sintering Process (CSP) and the associated precursors chemistry, with Spark Plasma Sintering (SPS). It also shows the possible densification by reactive hydrothermal sintering, using hydroxide precursors, and the effect of High Pressure SPS (HP-SPS) on 3YSZ nanopowders. All of these results touch upon current trends in the field of sintering, and highlight the possibilities in terms of chemical reactivity of precursors and physical parameter control. In particular, they show the importance of defect chemistry, related to the sintering atmospheres and their impact on both microstructures and properties.

Structural and compositional gradients in alternating current sintered aluminum-doped zinc oxide

Flash sintering revitalizes the exploration of electric field and current effects on the microstructure with novel phenomena including rapid densification, low-temperature sintering, and unique microstructures. The aim of this study is to investigate the influence of electric current on the microstructural development of Al-doped ZnO (nAl=4at.%). We apply two sintering techniques, namely spark plasma sintering (SPS) and flash spark plasma sintering (FSPS), both conducted under identical conditions (AC, f=50Hz, |E→0|=25V/cm, p=180MPa) for three different temperatures 600∘C, 800∘C and 1000∘C. The distinguishing factor between the two techniques lies in the passage of electric current flow through the specimens in the case of FSPS. The resulting microstructures of the sintered pellets are examined by XRD, SEM, EDX, TEM and SAED to identify the process-related differences. While SPS specimens reveal a homogeneous microstructure, FSPS specimens exhibit a complex microstructure which consists of two distinct regions. The major volume of the FSPS pellets exhibits a structural gradient with coarse ZnO grains and ZnAl2O4 spinel precipitates in the center to fine ZnO grains at the periphery. This microstructure results from Joule heating and geometry-dependent heat dissipation, creating a symmetric temperature profile with a hot center. A minor volume of the FSPS specimens shows a gradient in microstructure and composition. Here, ZnAl2O4 spinel precipitates accumulate in the center and the periphery reveals Al-depleted, dense and very coarse grains. Electromigration of Al-cations and the thermodynamically favored formation of ZnAl2O4 in the hot center are proposed as underlying causes for the observed microstructure. The electric current in FSPS significantly alters the microstructure and composition of sintered specimen where dopant electromigration is most significant under high current densities above |J→|=29(5)A/cm2.

Direct Recycling of Hot‐Deformed Nd–Fe–B Magnet Scrap by Field‐Assisted Sintering Technology

Recycling of Nd–Fe–B magnets is an ongoing challenge regarding circular economy. State‐of‐the‐art magnet production methods, such as hot deformation, have limitations with respect to direct recycling of magnet scrap particles that differ from pristine melt‐spun Nd–Fe–B powder. Recent work has shown that a combination of presintering by field‐assisted sintering technology/spark plasma sintering (FAST/SPS) and hot deformation by flash spark plasma sintering (flash SPS) has the potential to directly produce Nd–Fe–B magnets from 100% scrap material. Both processes have the capability to adjust and monitor process parameters closely, resulting in recycled magnets with properties similar to commercial magnets but made directly from crushed and recycled Nd–Fe–B powder that partially or completely replaces pristine melt‐spun Nd–Fe–B powder. Herein, a systematic study is done inserting recycled magnet particles into a flash SPS deformed magnet, considering the effects of different weight percentages of scrap material of varied particle size fractions. In some cases, coercivity HcJ of &gt;1400 kAm−1 and remanence Br of 1.1 T can be achieved with 20 wt% scrap material. The relationship between particle size fraction, oxygen uptake, and percentage of recyclate in a final magnet are all explored and discussed with respect to magnets made from pristine material.

Interfacial Evolution of WC-Co/AISI 304L Diffusion Bonded Joint Obtained by Flash SPS Technique

Interfacial Evolution of WC-Co/AISI 304L Diffusion Bonded Joint Obtained by Flash SPS Technique

Flash spark plasma sintering of 3YSZ: Introducing structural and microstructural heterogeneity by high heating rates

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS at various temperatures ranging from 700° to 2060°C using heating rates ⁓11,000 °C/min, and leading to densification in seconds. The influence of peak temperature on microstructure is then analyzed and displays unusual sintering trajectory. Densification shows a maximum of 91 % around 1600 °C and further decreases with increasing sintering temperature. The structural and microstructural homogeneities are assessed, showing the presence of remaining monoclinic phase from starting powder in the core of the pellets, even after sintering at 2060 °C, suggesting strong thermal gradient. This also results in grain size and morphology heterogeneity. This work aims at showing the importance of assessing homogeneity over the whole sample volume, especially for ultra-rapid sintering techniques.

Nanocrystalline Nd–Fe–B Anisotropic Magnets by Flash Spark Plasma Sintering

Flash spark plasma sintering (flash SPS) is an attractive method to obtain Nd–Fe–B magnets with anisotropic magnetic properties when starting from melt‐spun powders. Compared to the benchmark processing route via hot pressing with subsequent die upsetting, flash SPS promises electroplasticity as an additional deformation mechanism and reduced tool wear, while maximizing magnetic properties by tailoring the microstructure—fully dense and high texture. A detailed parameter study is conducted to understand the influence of Flash SPS parameters on the densification and magnetic properties of commercial MQU‐F powder. It is revealed that the presintering conditions and preheating temperature before applying the power pulse play a major role for tailoring grain size and texture in the case of hot deformation via Flash SPS. Detailed microstructure and magnetic domain evaluation disclose the texture enhancement with increasing flash SPS temperature at the expense of coercivity. The best compromise between remanence and coercivity (1.37 T and 1195 kA m−1, respectively) is achieved through a combination of presintering at 500 °C for 120 s and preheating temperature of 600 °C, resulting in a magnet with energy product (BH)max of 350 kJm−3. These findings show the potential of flash SPS to obtain fully dense anisotropic nanocrystalline magnets with high magnetic performance.

Electrical and Heat Distributions and Their Influence on the Mass Transfer during the Flash Spark Plasma Sintering of a Cu/Cr Nanocomposite: Experiments and Numerical Simulation.

The nanocomposite Cu–Cr powder was consolidated by flash spark plasma sintering (FSPS), which involves applying an extremely rapid change in the electrical power passing through the bulk of the sample. It was demonstrated that an essentially fully dense material could be obtained in 15 s. Such short-term treatment typically preserves the nanostructured features of the material. However, investigation revealed a nonuniformity in the microstructure of the alloys obtained under such extreme conditions. To better understand the observed effects, the FSPS process was simulated. It was observed that a rapid change in the applied electrical power resulted in nonuniform distributions of current density and temperature along the body of the consolidated material. Specifically, the current density was higher on the periphery of the sample, and the temperature was higher in the middle. These findings explain the observed structural transformation during FSPS and suggest an optimization strategy to avoid microstructural nonuniformity.

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.

Thermoelectric properties of higher manganese silicide consolidated by flash spark plasma sintering technique

We investigated thermoelectric properties of higher manganese silicide obtained by the simultaneous use of ball milling and flash spark plasma sintering (FSPS) technique, and the results were compared those of the samples prepared with conventional spark plasma sintering (SPS). Despite the very short sintering process less than 50 s, FSPS samples reproduced well the crystallographic and thermoelectric properties of SPS sample made with long sintering time with more than 3500 s. Notably, the grain growth was significantly suppressed in the FSPS samples, although it has had minor effects on the transport properties.

Consolidation and high‐temperature strength of monolithic lanthanum hexaboride

AbstractIn this study we explored the densification, microstructure evolution, and high‐temperature properties of bulk lanthanum hexaboride. LaB6 bulks were consolidated using spark‐plasma sintering only in the temperature range between 1400°C and 1700°C. We adopted flash spark plasma sintering (SPS) of LaB6 using a direct current heating without a graphite die. We observed a peculiar grain‐size gradient when coarse grains exceeding 300 μm were observed on the top side of the specimen, while the bottom side had a grain size of 15–20 μm. Such large grain was not observed using SPS at 2000°C, suggesting that these might originate from a local overheating.Based on the three‐point flexural tests, it was observed that the toughness and strength of the LaB6 were acceptable at room‐temperature (3.1 ± 0.2 MPa m1/2, 300 ± 20 MPa). However, at 1600°C, these parameters would decrease to 1.3 ± 0.1 MPa m1/2 and 120 ± 40 MPa, respectively.

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.

Flash Spark Plasma Sintering of SiC: Impact of Additives

Flash spark plasma sintering (FSPS) of SiC was carried out using additives (10 vol%) with different electrical conductivities: alumina (insulator), boron carbide (semiconductor), and titanium carbide metallic (conductor). The electrical conductivity of the additives plays a determinant role on the FSPS process, it controls the electrical SPS power dissipation, the development of gaseous/liquids/solid phases and time needed to complete densification. The microstructure of FSPSed SiC samples confirmed localized heating in the case of Al2O3 additive, resulting in undesired carbothermal reduction and formation of gaseous species. In the presence of TiC additive, solid state sintering led to SiC sublimation without achieving a complete densification. The presence of B4C as an additive led to a liquid phase formation, which promoted sintering contributing and the densification of the samples. The present study rationalizes the selection FSPS sintering additives based on their electrical and chemical compatibility with the SiC matrix.

Flash spark plasma sintering of pure TiB2

Flash Spark Plasma Sintering (FSPS) was used to rapidly sinter pure titanium diboride (TiB2). A pre-sintered sample (Ø ​= ​20 ​mm with relative density 60%) was crossed under a current of 2–2.5 ​kA flowing entirely across the sample. The samples were locally densified up to 98% of relative density in a very short time of 20 ​s. The rapid heating (≈6000 ​°C/min) prevented the complete evaporation of B2O3, leading to the formation of rarely seen segregation of boron at the grain boundaries. Compared to SPS or hot press, the rapid FSPS processing promoted the formation boron rich grain boundaries during sintering, thus enhancing consolidation. The FSPS approach might be suitable to consolidate other refractory borides.

Comparison of Conventional and Flash Spark Plasma Sintering of Cu–Cr Pseudo-Alloys: Kinetics, Structure, Properties

Spark plasma sintering (SPS) is widely used for the consolidation of different materials. Copper-based pseudo alloys have found a variety of applications including as electrodes in vacuum interrupters of high-voltage electric circuits. How does the kinetics of SPS consolidation for such alloys depend on the heating rate? Do SPS kinetics depend on the microstructure of the media to be sintered? These questions were addressed by the investigation of SPS kinetics in the heating rate range of 0.1 to 50 K/s. The latter conditions were achieved through flash spark plasma sintering (FSPS). We also compared the sintering kinetics for the conventional copper–chromium mixture and for the mechanically induced copper/chromium nanostructured particles. It was shown that, under FSPS conditions, the observed maximum consolidation rates were 20–30 times higher than that for conventional SPS with a heating rate of 100 K/min. Under the investigated conditions, the sintering rate for mechanically induced composite Cu/Cr particles was 2–4 times higher compared to the conventional Cu + Cr mixtures. The apparent sintering activation energy for the Cu/Cr powder was twice less than that for Cu–Cr mixture. It was concluded that the FSPS of nanostructured powders is an efficient approach for the fabrication of pseudo-alloys.

Power‐controlled flash spark plasma sintering of gadolinia‐doped ceria

AbstractElectric field‐assisted sintering (FAST) is a rapidly growing scientific and engineering domain. In the present paper, we describe the process of flash sintering (FS) in a configuration of classical spark plasma sintering (SPS) (graphite punch and boron nitride (BN) die), also called flash spark plasma sintering (FSPS). The densification process of Gd0.1Ce0.9O2‐x powder is studied in detail with a focus on the transition from FAST to FS. We discuss the electrical, geometrical, and thermal evolution of the process and the characteristics of the final compacts. Low electrical fields are sufficient for the onset of FS. Ceria is a material difficult to sinter by FAST techniques due to its known mechanochemical transformations. We observed the disintegration of pellets after experiments with well‐pronounced flash event.

Graphite creep negation during flash spark plasma sintering under temperatures close to 2000 °C

Graphite creep has high importance for applications using high pressures (100 MPa) and temperatures close to 2000 °C. In particular, the new flash spark plasma sintering process (FSPS) is highly sensitive to graphite creep when applied to ultra-high temperature materials such as silicon carbide. In this flash process taking only a few seconds, the graphite tooling reaches temperatures higher than 2000 °C resulting in its irreversible deformation. The graphite tooling creep prevents the flash spark plasma sintering process from progressing further. In this study, a finite element model is used to determine FSPS tooling temperatures. In this context, we explore the graphite creep onset for temperatures above 2000 °C and for high pressures. Knowing the graphite high temperature limit, we modify the FSPS process so that the sintering occurs outside the graphite creep range of temperatures/pressures. 95% dense silicon carbide compacts are obtained in about 30 s using the optimized FSPS.

Controlling current flow in sintering: A facile method coupling flash with spark plasma sintering.

A facile method is described to couple flash sintering (FS) and spark plasma sintering (SPS). Flash spark plasma sintering (FSPS) combines advantages of both techniques: the use of pellet-shaped samples under mechanical load with the controlled passage of electric current through the sample. FSPS is realized by partially replacing graphite pressing tools (two punches and one matrix) used in standard SPS. An insulating boron nitride matrix substitutes the conducting graphite matrix to force the electric current through the sample. Additionally, external heating of the boron nitride matrix is implemented. Microstructures of standard and flash-SPS are compared using aluminum doped zinc oxide as an example. Scanning electron microscopy reveals that different microstructures are generated for SPS and FSPS. The new setups provide novel processing routes for different current sintering methods of materials under mechanical load and assist in identifying the role of the electric current or field in the microstructure.

Interfacial reaction between ZrNbHfTa foil and graphite: Formation of high-entropy carbide and the effect of heating rate on its microstructure

The interfacial reaction between graphite and a ZrNbHfTa foil was investigated using a spark plasma sintering (SPS) machine. The interlayer was converted during the treatment into a high-entropy carbide by the reaction between the alloy and the graphite substrate. The degree of conversion depended on the processing time and temperature, but could be completed in only 16 s using the flash-spark plasma sintering process.The final microstructure was strongly influenced by the heating rate. Flash-SPS processing (heating rate about 120 °C/s) melted the alloy, which was then squeezed and deformed before its conversion to the high-entropy carbide (Zr,Nb,Hf,Ta)C. On the contrary, when conventional heating was used (i.e., 10 °C/min) the metal reacted with the graphite substrate before it melted. This concept appears to be generally applicable to different graphite/metal systems, with particular implications in the fields of graphite/metal joining.

Reactive, nonreactive, and flash spark plasma sintering of Al2O3/SiC composites—A comparative study

AbstractThe influence of different SPS‐based methods, that is, conventional spark plasma sintering (SPS), flash SPS (FSPS), and reactive SPS (RSPS) on the properties of Al2O3/SiC composite was investigated. It was shown that the application of preliminary high energy ball milling of the powders significantly enhances the sinterability of the ceramics. It was also demonstrated that FSPS provides unique conditions for rapid, that is, less than a minute, consolidation of refractory ceramics. The Al2O3‐20 wt% SiC composite produced by FSPS possesses the highest relative density (~99%), fracture toughness (7.5 MPa m1/2), hardness (20.3 GPa) and wear resistance among all ceramics produced by other SPS‐based approaches with dwelling time 10 minutes. The RSPS ceramics hold the highest Young's modulus (390 GPa). Substitution of micron‐sized Al2O3 particles by nano alumina does not lead to measurable enhancement of the mechanical properties.

Flash joining of conductive ceramics in a few seconds by flash spark plasma sintering

The feasibility of flash joining conductive ceramics using spark plasma sintering is demonstrated. It is shown that graphite disks can be joined in a few seconds (6–10 s electric discharge time) using a SiOC precursor (methyl silicone resin) as an interlayer. Differently from usual flash experiments, the process does not require any pre-heating, allowing a dramatic reduction of the processing time. XPS analysis of the joint revealed a clear evolution of the chemical environment of silicon with a progressive transition from SiOC to SiO2/SiC. Mechanical tests were performed to determine the fracture toughness (1.0 ± 0.2 MPa m0.5) and fracture energy (40.6 ± 9.8 J/m2) of the interfaceFlash joining can be applied beyond graphite joining, and opens a novel and flexible processing route for many conductive ceramics, as demonstrate by preliminary work on Kanthal® Super MoSi2 and Cf-reinforced SiC.