Field Assisted Sintering Technique Research Articles

Channelling electric current during the field-assisted sintering technique (FAST) to control microstructural evolution in Ti-6Al-4V

Perhaps the most defining feature of field-assisted sintering technology (FAST) is the application of an electric current, in addition to the uniaxial pressure, to create resistive heating in and around the sample region. However, with a few exceptions, most research takes this as an unchangeable part of the process. Here, this current flow has been directed to specific regions within the toolset, using boron nitride as electrically insulating material. This caused the heating to occur in differing regions within the Ti-6Al-4V sample and mould over four insulating configurations, with the shift in current density resulting in an extreme disparity in the final microstructures. The samples were imaged and analysed with deep learning in MIPAR, alongside comparisons with finite element analysis (FEA) models for 20 s and 5 min dwell times, to provide the technique with predictive capabilities for grain size and microstructure. The results gathered imply significant potential for this concept to improve the flexibility of FAST, and reduce negative effects such as undesirable temperature profiles in size scaling sintering for industry.

Development of sub-miniaturised testing methodology for W/Cu joints extracted from the ITER-specification monoblock

A testing methodology and a new “anchor” geometry for tensile samples were developed for evaluation of the W/Cu joint strength between the W monoblock and the CuCrZr cooling pipe joined via the Cu interlayer in the ITER-specification monoblock. The proposed samples can be machined from the ITER-specification monoblock. These monoblock tensile samples are designed for uniaxial tests allowing their remote manipulation for testing in hot cells after neutron irradiation, which is crucial for the assessment of the effect of this irradiation. For validation purposes, tensile tests with standard miniaturised dog-bone samples were performed using prototype block-to-block joints (W/CuCrZr) produced with the field-assisted sintering technique. The results are obtained from the conventional and the new tensile samples. Conventional samples were machined from the W/CuCrZr block-to-block joints, while the new “anchor” tensile samples were machined from the W/CuCrZr block-to-block joints and ITER monoblock containing the W block and Cu interlayer. A good agreement in the evaluation of the joint strength was obtained. The fracture surfaces and the joint interface were investigated with scanning electron microscopy. Micro-hardness was characterised across the joint interface.

Insights into microstructural evolution of tungsten-tungsten carbide plasma facing composite materials prepared by field assisted sintering technique

Combining a stable, resilient W-W2C or WC as plasma-facing material, with thermally more conductive and ductile W as a structural material, may provide a solution for the demanding environment in the fusion reactor chamber. In order to ensure the structural stability of such layered composite, their phase composition and thermal stability at elevated temperatures and cyclic heat loads needs to be evaluated.In situ densification and joining of W and W-xWC materials have been performed by field-assisted sintering technique. Microstructural analysis revealed that the interface layer formed between W and pure WC consists primarily of large W2C grains. Initially, equiaxed W2C grains are formed following the decarburisation of WC due to the large C concentration gradient. Further C diffusion results in the formation of larger anisotropic grains growing preferentially in the 〈0001〉 direction. A similar microstructure was observed when joining WC-reinforced W (W-xWC, x = 7, 17, 28, 33 at. % C) with WC. On the other hand, the joining of W with WC-reinforced W resulted in a sound joint with no observable grain growth at the interface, even at large C concentrations (e.g. 28 at. %). As formed, microstructures were stable after thermal ageing at 1600 °C for 24 h with no observable change in major microstructural features.The combination of WC-reinforced W as high strength plasma facing armour material and W-structural material offers a promising composite solution for improved thermal stability under high cyclic heat loads expected in future fusion power plants.

Mechanical Properties of Ti–5Al–5V–5Mo–3Cr Alloy Sintered From Blended Elemental Powders

The field-assisted sintering technique (FAST) was successfully used to sinter, homogenize, and age the Ti–5Al–5V–5Mo–3Cr (Ti-5553) aerospace alloy from blended elemental powders in a single processing run. Sintering at a comparatively high temperature of 1500 °C assured chemical homogeneity of the material. Subsequently, without cooling to the room temperature, the temperature was lowered to 600 °C for 30 min to provide annealing treatment and to produce a desired lamellar α + β microstructure. Using an in-house designed assembly, a fully dense rod of 15 mm in diameter and 70 mm in length was successfully sintered. Microstructural and mechanical properties of the alloy were investigated and compared with the conventionally processed Ti-5553 alloy. A good combination of strength (1183 MPa) and ductility (6 pct) was achieved; these values are fully comparable to the conventional cast, forged, and aged Ti-5553 alloy. It was shown that homogeneous Ti-5553 alloy can be produced by FAST, FAST can instantly provide necessary annealing steps, and the FAST process can be upscaled to produce homogeneous rods.

Powder Metallurgy Route to Ultrafine-Grained Refractory Metals.

Ultrafine-grained (UFG) refractory metals are promising materials for applications in aerospace, microelectronics, nuclear energy, and many others under extreme environments. Powder metallurgy (PM) allows to produce such materials with well-controlled chemistry and microstructure at multiple length scales and near-net shape manufacturing. However, sintering refractory metals to full density while maintaining a fine microstructure is still challenging due to the high sintering temperature and the difficulty to separate the kinetics of densification versus grain growth. Here an overview of the sintering issues, microstructural design rules, and PM practices towards UFG and nanocrystalline refractory metals are sought to be provided. The previous efforts shall be reviewed to address the processing challenges, including the use of fine/nanopowders, second-phase grain growth inhibitors, and field-assisted sintering techniques. Recently, pressureless two-step sintering has been successfully demonstrated in producing dense UFG refractory metals down to ≈300nm average grain size with a uniform microstructure and this technological breakthrough shall be reviewed. PM progresses in specific materials systems shall be next reviewed, including elementary metals (W and Mo), refractory alloys (W-Re), refractory high-entropy alloys, and their composites. Last, future developments and the endeavor towards UFG and nanocrystalline refractory metals with exceptionally uniform microstructure and improved properties are outlined.

Investigation of the Tribological Behaviour of Various AMC Surfaces against Brake Lining Material.

AlSi7Mg/SiCp aluminium matrix composites (AMCs) with a high ceramic content (35 vol.%) that were produced by using the field-assisted sintering technique (FAST) were subjected to tribological preconditioning and evaluated as a potential lightweight material to substitute grey cast iron brake discs. However, since an uncontrolled running-in process of the AMC surface can lead to severe wear and thus to failure of the friction system, AMC surfaces cannot be used directly after finishing and have to be preconditioned. A defined generation of a tribologically conditioned surface (tribosurface) is necessary, as was the aim in this study. To simulate tribological conditions in automotive brake systems, the prepared AMC samples were tested in a pin-on-disc configuration against conventional brake lining material under dry sliding conditions. The influence of the surface topography generated by face turning using different indexable inserts and feeds or an additional plasma electrolytic treatment was investigated at varied test pressures and sliding distances. The results showed that the coefficient of friction remained nearly constant when the set pressure was reached, whereas the initial topography of the samples studied by SEM varied substantially. A novel approach based on analysing the material ratio determined by 3D surface measurement was developed in order to obtain quantitative findings for industrial application.

Recent Developments of High-Pressure Spark Plasma Sintering: An Overview of Current Applications, Challenges and Future Directions.

Spark plasma sintering (SPS), also called pulsed electric current sintering (PECS) or field-assisted sintering technique (FAST) is a technique for sintering powder under moderate uniaxial pressure (max. 0.15 GPa) and high temperature (up to 2500 °C). It has been widely used over the last few years as it can achieve full densification of ceramic or metal powders with lower sintering temperature and shorter processing time compared to conventional processes, opening up new possibilities for nanomaterials densification. More recently, new frontiers of opportunities are emerging by coupling SPS with high pressure (up to ~10 GPa). A vast exciting field of academic research is now using high-pressure SPS (HP-SPS) in order to play with various parameters of sintering, like grain growth, structural stability and chemical reactivity, allowing the full densification of metastable or hard-to-sinter materials. This review summarizes the various benefits of HP-SPS for the sintering of many classes of advanced functional materials. It presents the latest research findings on various HP-SPS technologies with particular emphasis on their associated metrologies and their main outstanding results obtained. Finally, in the last section, this review lists some perspectives regarding the current challenges and future directions in which the HP-SPS field may have great breakthroughs in the coming years.

Reduced interfacial stresses in laminated ceramics during flash sintering enabled by an extremely low uniaxial viscosity

Constrained sintering is a crucial issue for the development of laminated or composite ceramics. It had been suggested that flash sintering (FS) may eliminate the defects caused by constrained sintering. In this work, the interfacial tensile stresses of laminated ceramics during conventional sintering (CS), field assisted sintering technique (FAST) and FS were estimated and compared by combining a set of free sintering, bending creep and constrained sintering experiments with theoretical analysis. The interfacial tensile stress in constrained FS is significantly lower than those of both CS and FAST, and can only be explained by an extremely low uniaxial viscosity of the materials achieved during FS. The possible effects of temperature, density and microstructure on the uniaxial viscosity reduction have been carefully assessed to show that the electrical field itself is the key factor for the extremely low viscosity. Plausible mechanisms for the reduction in the uniaxial viscosity under different levels of electrical fields are also discussed.

Field assisted sintering of Ta–Al2O3 composite materials and investigation of electrical conductivity

Ta–Al2O3 composite samples with different compositions are prepared using Field Assisted Sintering Technique (FAST). Two different alumina powders are used to investigate the influence of the starting powders particle size on the microstructural features and the resulting electrical conductivity of the prepared composite materials. Percolation threshold of the two material systems is influenced by the metal fraction, as well as the alumina particle size of the starting powder. The percolation threshold for the fine- and the coarse-grained alumina is found to be at 15 vol.-% Ta and 7.5 vol.-% Ta, respectively. Microstructural investigations show significant differences in terms of particle shape of both, Ta and Al2O3 after sintering, most likely being the reason for the different percolation thresholds of the investigated materials. Anisotropy effects resulting from the processing using FAST and the influence on electrical properties are also shown.

Rapid Sinterability of Mechanically Alloyed MgAl/MgO Composite with the Field-Assisted Sintering Technique

Rapid Sinterability of Mechanically Alloyed MgAl/MgO Composite with the Field-Assisted Sintering Technique

A universal all-solid synthesis for high throughput production of halide perovskite

Halide perovskites show ubiquitous presences in growing fields at both fundamental and applied levels. Discovery, investigation, and application of innovative perovskites are heavily dependent on the synthetic methodology in terms of time-/yield-/effort-/energy- efficiency. Conventional wet chemistry method provides the easiness for growing thin film samples, but represents as an inefficient way for bulk crystal synthesis. To overcome these, here we report a universal solid state-based route for synthesizing high-quality perovskites, by means of simultaneously applying both electric and mechanical stress fields during the synthesis, i.e., the electrical and mechanical field-assisted sintering technique. We employ various perovskite compositions and arbitrary geometric designs for demonstration in this report, and establish such synthetic route with uniqueness of ultrahigh yield, fast processing and solvent-free nature, along with bulk products of exceptional quality approaching to single crystals. We exemplify the applications of the as-synthesized perovskites in photodetection and thermoelectric as well as other potentials to open extra chapters for future technical development.

High-throughput characterization of elastic moduli of Ti-Nb-Zr-O biomedical alloys fabricated by field-assisted sintering technique

Compositionally graded samples of Ti-xNb-7Zr-yO alloys (11 ≤ x ≤ 35, 0.49 ≤ y ≤1.01 wt%) were prepared from elemental powders and TiO2 and compacted by the field-assisted sintering technique. By this approach, four layered samples covering together twenty four different chemical compositions were prepared. It allowed efficient sampling of alloys for potential biomedical use. Elastic constants of each layer (the shear modulus G and the Young’s modulus E) were determined using resonant ultrasound spectroscopy. The results show that in the considered compositional space, elastic constants exhibit a dependence on the volume fraction of β-stabilizing niobium and interstitial oxygen. Minimum elastic modulus was achieved for the compositions close to the stability limit between parent β matrix and martensitic α″ phase, while maximum elastic constants were caused by the presence of isothermal ω particles.

Assessing the effect of compaction pressure on the mechanical properties of polytetrafluoroethylene elaborated by field assisted sintering technique

We study the effect of compaction pressure on the mechanical properties of polytetrafluoroethylene (PTFE) synthesized by the field-assisted sintering technique (FAST). PTFE is a commonly polymer use material with various technological applications in the automotive, aerospace, and chemical industries. While the manufacture of PTFE is problematic due to its high molecular weight and hence a very high melt viscosity, FAST is an adequate process which can fix this problem by offering rapid densification (a few minutes). In this context, we explore the mechanical behavior of sintered PTFE produced with different compaction pressures, all other operating parameters being held constant. The stress-strain curves of PTFE investigated by uniaxial tension tests show a significant dependence of the compaction pressure on the elastic and plastic mechanical characteristics. An attempt is made to correlate the applied pressure during the sintering process to microstructural changes according to the de Gennes creep model. For plastic deformation, the stress-strain curves are compared to the predictive Hollomon's and Haward and Thackray's models and discussed in physical terms. We conclude that a better mechanical behavior of sintered PTFE materials can be achieved by using an optimum value of compaction pressure which is not necessary found above the yield flow. • PTFE samples produced by the field-assisted sintering technique are characterized. • Stress-strain curves show significant dependence on compaction pressure. • Hollomon's and HT analysis allow us to characterize the strain hardening behavior. • Optimal properties correspond to sintered samples at pressure less than 5 MPa.

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.

Ti-Mo-xTiC composites manufactured by U-FAST reactive sintering

The paper presents the characteristics of Ti-Mo-xTiC composites manufactured under experimentally selected conditions using the upgraded field-assisted sintering technique (U-FAST). Mixtures of microstructural titanium powders and nc-Ti0.9Mo0.1C/C carbide powders protected from oxidation by a carbon shell were subjected to sintering. The powders with nc-Ti0.9Mo0.1C/C contents of 10 and 20 wt% were used. The content of carbon forming the carbon shell was approximately 3 or 40 wt%. Composites with near full density were reinforced with titanium carbides in a Ti-Mo matrix. The composites with the highest content of reinforcing phase are characterized by the highest values of hardness, Young's modulus and wear resistance. Although the hardness of these composites is similar to that of ceramics, their nature is not brittle. Despite the high value of the Young's modulus, the addition of hard particles of the reinforcing phase to the titanium matrix significantly increases the values of the H/E ratio of the composites compared to the reference samples of cp-Ti and Ti6Al4V. A comparison of the research results for the composites with the highest share of titanium carbides showed that lowering the sintering temperature from 1300 to 1150 °C resulted in the inhibition of grain growth, a reduction in composite heterogeneity, composite roughness and hardness as well as a rise in the Young's modulus. An increase in the sintering temperature from 1150 to 1300 °C contributed to the higher high angle grain boundaries (HAGB) content.

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.

Garnet-Based Composite Cathodes for All-Solid-State Lithium Batteries

Garnet-based all-solid-state lithium batteries (ASSLB) provide high intrinsic safety, extended operational temperature range, and high energy density. As the first two are intrinsic to the materials system, one prerequisite to obtain high energy densities with such ASSLBs is the manufacturing of composite cathodes. Preferably, cathode active material (CAM) and electrolyte should form an intertwined 3D-network with an intimate extended contact between two phases. The interface between the CAM and the electrolyte should feature high total surface area with a low impedance to enable an efficient charge transfer and transport and minimize the resistance losses in a battery.Composite solid cathode microstructures can be formed via a co-sintering of solid electrolyte and CAM powders. However, many CAMs such as Li[Ni1-x-yCoxMny]O2 (NMC), Li[Ni1-x-yCoxAly]O2 (NCA) or Li2NiMn3O8 (LMO) cannot withstand sintering temperatures required, for oxide class solid electrolytes as Li7La3Zr2O12 (LLZ) or Li1+xAlxTi2-x(PO4)3, to obtain sintered composite cathodes. Furthermore, the thermodynamic stability of CAMs significantly decreases in the presence of solid electrolytes. Still, several groups have demonstrated that thermodynamic limitations can be significantly lessened via a kinetic control of the process such as reduction of reaction times and a careful control of powder morphology during ceramic sintering process.Following this work, we demonstrate that Field Assisted Sintering Technique / Spark Plasma Sintering (FAST/SPS) enables fabrication of different functional solid-state battery components within minutes such as dense LLZ separator layers with a high ionic conductivity, thick LiCoCO2 (LCO)/LLZ composite cathodes without any interfacial reactions, and integrated cathode half-cells consisting of a composite cathode and a LLZ separator layer at temperatures as low as 675 °C. . The analysis of these composite cathodes revealed well sintered pure phases and a homogenous distribution of cubic LLZ and cathode active material. Electrochemical tests showed promising electronic and ionic conductivity (0.4 mS cm-1) values and an increased capacity (4 mA cm-2) compared to cathodes with pure active material.The structural analysis after the electrochemical characterization shows the mechanical integrity of the LLZ/LCO interface, which means that a mechanically stable rigid LLZ/LCO interface can be obtained.Such a mechanically stable LLZ/LCO cathode allows to analyze possible electrochemistry induced degradation, which is suggested by theoretical work but lacks experimental validation. A detailed analysis of the LLZ/LCO interface by SEM, TEM, Raman, and XRD provide insights into the processes occurring during electrochemical cycling. The findings can pave the way to increase the cycling stability of ceramic all-solid-state batteries via interface optimization Figure 1

Polymer-Garnet-Based Composite Cathodes for Solid-State Li Batteries

All-solid-state lithium batteries (ASSLBs) can potentially outperform conventional Li-ion batteries with liquid or polymer electrolyte. One example for solid electrolytes is the garnet-type oxide Li7La3Zr2O12 (LLZO). LLZO has a wide electrochemical window, stability vs. lithium, and good ionic conductivity at room temperature. The cathode in ASSLBs is manufactured from a cathode active material (CAM), such as LiCoO2 (LCO). The efficiency of Li-ion storage can be improved by the use of a composite cathode consisting from a CAM and an ion-conducting ceramic, e.g. LCO/LLZO. In such a composite cathode, LLZO delivers Li-ions through the whole bulk enhancing the volumetric loading of LCO. In this work the addition of polymer electrolyte into LCO/LLZO composite cathode was proposed, aiming at further increase of cell performance due to facilitation of CAM usage, similar to the approach of manufacturing of polymer-ceramic electrolytes. They are fabricated mostly by tape casting of slurry with polymer matrix, ceramic filler and a solvent. An alternative technology includes free sintering of tape-casted LCO/LLZO porous network and subsequent infiltration by liquid or polymer electrolyte. Free sintering of LCO/LLZO composite requires relatively high temperature and/or long sintering time. This results in loss of volatile Li with decrease in electrochemical performance.In the present work the LCO/LLZO composite cathode was manufactured in a powder-based process by Field-Assisted Sintering Technique also known as Spark Plasma Sintering (FAST/SPS). Fast heating (100°C/min and higher) and application of mechanical pressure during FAST/SPS enable reduction of sintering temperature and processing time needed for fabrication of nearly-fully-dense composite.[1] Thereby, Li evaporation and grain growth can be significantly reduced. This technology was used in our previous work for fabrication of half-cells with dense LLZO electrolyte and dense LCO/LLZO composite cathode. However, the appearance of side phase after sintering at low pressure and a residual porosity was observed. The reason for that was partial reduction of oxides by carbon originated from graphite foil in FAST/SPS setup. In the presented work, the graphite foil was replaced by carbon-free mica foil. This measure enabled FAST/SPS sintering of porous LCO/LLZO network without side phase formation. The obtained porous skeleton was infiltrated with polymer electrolyte to fabricate a polymer-ceramic composite cathode. The cathode was assembled with an anodic half-cell consisting of dense FAST/SPS-sintered LLZO electrolyte and attached indium (In) foil used as anode. The ASSLB with polymer-ceramic composite cathode showed significantly lower interfacial impedance and remarkably higher area-specific storage capacity as compared to the similar ASSLBs with pure ceramic (porous or dense) composite cathodes. Thus, the functionality and the advanced storage capacity of the proposed polymer-ceramic cathode and related ASSLB architecture were demonstrated.[2]

Development and comparison of field assisted sintering techniques to densify CeO2 ceramics

Cerium dioxide (CeO2) was densified by conventional and field assisted sintering techniques in order to examine the effect of a range of sintering parameters on the resultant pellet microstructure, namely: temperature, hold time, atmosphere, electric field strength and polarity. CS at 1400 °C for 2 h in static air atmospheres provided the highest densities and grain sizes compared with an argon atmosphere, due to the retention of near stoichiometry maintaining sintering kinetics. SPS produced dense ceramics at similar sintering temperatures (1300 °C), but with greatly reduced sintering hold (5 min) and cycle times. However, microstructural inhomogeneity arose from the direct current polarity which led to oxygen ion diffusion toward the positive electrode. FS was performed with an alternating current electric field and produces samples of comparable density at sintering temperatures of ~1100 °C for a hold time of around 1 h with no inhomogeneity due to the alternating current employed.