Flash Sintering Process Research Articles

Flash sintering of HUST-1 lunar regolith simulant: Effects of processing temperature on microstructure characteristics and mechanical properties

In-situ construction method is expected to be considered for lunar base construction because of its reducing transportation cost. Among the many in-situ construction processes, the flash sintering process can significantly reduce the processing time and become a potentially effective in-situ resource utilization (ISRU) technology. In this study, the flash sintering process was investigated for the first time to realize the sample forming of HUST-1 lunar regolith simulant (LRS). The effects of sintering temperature on the micro- and macrostructural characteristics and mechanical properties of the samples were systematically studied. The results showed that the densities and mechanical properties of the sintered samples were improved with the increase of the sintering temperature (from 1050 °C to 1290 °C) in a low vacuum environment for 4 min. Joule heating effect was investigated to cause rapid high-temperature solid-phase reactions between the LRS particles. At 1290 °C, the compressive strength of the sintered sample reached maximum 59.47 MPa. This study provides an efficient forming method for lunar construction in the future.

Flash sintering of ZrO2–SnO2–ZrSnO4 composite nanofibers: Fabrication, properties, and biochemical effects in Drosophila melanogaster

ZrO2–SnO2–ZrSnO4 composite nanofibers (CNFs) were fabricated by an electrospinning technique in three different ratios by volume (%v/v). Changing the composition ratio of CNFs affected the temperature and sinter time variation of both the band gap and the flash sintering (FS) phenomenon. FS experiments were used at a current cut-off of 3.77 mA/mm2 under thermal (844–878 °C) and electric field (420 V/mm). Highly dense CNFs were obtained in less than 80 s with a maximum power absorption of 1.58 W/mm3. In addition to the FS process, CNFs were sintered with the conventional sintering (CS) method at 1200 °C for 1 h. As the amount of Sn in the composition (%v/v) increases, the density increases, but the hardness value decreases. The FS process was proven to produce faster, denser, and harder material than CS.Long-term and permanent results are obtained by designing biocompatible nanocomposite materials in health. However, determining the long-term effects of CNFs, which are considered non-toxic, is necessary. Drosophila melanogaster female food was covered with these different v/v% ratios CNFs (ZrSn, Zr2Sn; 2ZrSn) and contact by life span: The effects were evaluated on movement, weight, egg production, longevity, and antioxidant system (Superoxide dismutase-SOD, catalase-CAT, glutathione-GSH, and 2,2-diphenyl-1-picrylhydrazil-DPPH). It can be said that Zr2S CNFs were not toxic by not changing the life span, egg production, climbing, and biochemical parameters of the fly. However, it was determined that CNFs used in ZrSn and 2ZrSn ratios slightly reduced the behavioral (movement and longevity) and antioxidant activities (SOD and CAT) of the individual.

Effects of frequency on the AC flash sintering behavior and microstructure of Ga-LLZO

In this paper, the effects of frequency on the flash sintering behavior and microstructure of Li6.4Ga0.2La3Zr2O12(0.2Ga-LLZO) were investigated. The results showed that an increase in frequency had little effect on the onset temperature, but this increase reduced the duration of the weak current stage the incubation stage. Moreover, this phenomenon led to a decrease in the power loss during the constant current stage. Scanning Electron Microscope images showed that the microstructure was symmetrically distributed, and the uniformity increased with increasing frequency. Finite element simulations and an IR thermocamera were used to analyze the temperatures of the specimens. The symmetric distribution of temperature during the flash sintering process was the root cause. The results of AC impedance spectroscopy showed that when the frequency of flash sintering was ≥ 400 Hz, the ionic conductivity of Ga-LLZO was greater than that of conventional sintering. The results of this work demonstrated the important role of frequency in controlling the microstructures of ceramic materials in flash sintering.

Reusing blended leach residue by flash sintering method

AbstractIn this study, the effects of different sintering methodology on grain formation, density, and hardness of blended leach residue (B‐LR) pellets is investigated. For the first time, the flash sintering (FS) method, an environmentally friendly process, is used to make multiphase ceramic composites using B‐LR under 100 V/mm at 675C. The outcomes of this study revealed that the FS process is faster and more energy‐efficient than the conventional sintering (CS) methods performed at 800°C for 4 h. The formation of abnormal grain growth is prevented by FS in B‐LR material composition after it is exposed to a max power absorption 61.4 mW/mm3 and whole FS is completed less than 100 s. Besides, FS method does not lead to any melting on grain boundaries due to excessive joule heating, compared with the CS sample. It is noted that FS also provides better density and hardness values for this material system along with compositional integrity in this multiphase system. The significant outcome of this study is reducing lead volatility and emission by decrement on the sintering temperature. Ultimately, this study has planned the practicality of reusage and recycle of this material with green, safe, and eco‐friendly methods.

Reactive flash sintering of TiZrN and TiAlN ternary metal nitrides

This study demonstrated the reactive flash sintering (RFS) for two powder mixtures: TiN-ZrN (both conducting) and AlN-TiN (TiN conducting but AlN insulating), targeting ternary metal nitrides (TMN) of Ti0.5Zr0.5N and Ti0.5Al0.5N, respectively. A constant volage and pressure (e.g., 8 V DC, ∼15 MPa) at room temperature triggered the flash (current density up to 27 A/mm2) without pre-heating, and the entire RFS process finished in a few minutes. For TiZrN, the flash was instantaneous whereas for TiAlN, there was a long incubation before the flash followed by a quick and dramatic flash. Both conventional ex situ XRD and in situ synchrotron study had been carried out. They showed a uniform Ti0.57Zr0.43N solution formed in RFS and persisted upon cooling, while (Ti, Al) N solid solution formed at high temperature was not stable and likely went through a very quick phase separation in the cooling process. The final products from RFS had been characterized using SEM/EDS for microstructure. Both TiZrN and TiAlN were dense. Distribution of Ti, Zr, and N was uniform for TiZrN; for TiAlN, Ti and N distribution was uniform, while association of Al with oxygen was observed. TGA-DSC revealed the onset oxidation temperature for TiZrN was comparable to TiN and ZrN, while it was higher by ∼200 °C for TiAlN, likely due to the formation of Al2O3. In terms of mechanical properties such as hardness or fracture toughness, forming a single-phase solid solution (like TiZrN) does not offer obvious benefits. while large grain size from RFS seemed to be unfavorable. Future optimization of RFS condition and in-depth study by both experiments and simulation are needed to fully understand the composition-processing-structure-property relationships for such TMN from the reactive flash sintering process.

Dielectric barrier discharge based defect engineering method to assist flash sintering

Oxygen vacancy plays an important role in the flash sintering (FS) process. In defect engineering, the methods to create oxygen vacancy defect include doping, heating, and etching, and all of them often have complex process or equipment. In this study we used dielectric barrier discharge (DBD) as a new defect engineering technology to increase oxygen vacancy concentration of green billet with different ceramics (ZnO, TiO₂ and 3 mol% yttria-stabilized zirconia (3YSZ)). With a 10 kHz AC power supply, the low-temperature plasma was generated and the specimen could be treated in different atmosphere. The effect of DBD treatment was influenced by atmosphere, treatment time, and voltage amplitude of power supply. After DBD treatment, the oxygen vacancy defect concentration in the ZnO samples increased significantly, and the resistance test showed that the conductivity of the samples increased by 2–3 orders of magnitude.  Moreover, the onset electric field of ZnO FS decreased from 5.17 kV/cm to 0.86 kV/cm at room temperature, while in the whole FS the max Power dissipation decrease from 563.17 W to 27.94 W. The defect concentration and conductivity of the green billets for TiO₂ and 3YSZ also changed by DBD, and then the FS process was modified. It is a new technology to treat green billet of ceramic in very short time, applicable to other ceramics, and beneficial to regulate the FS process.

Role of surface carbon nanolayer on the activation of flash sintering in tungsten carbide

Flash sintering has been recently successfully activated also in conductive ceramics like tungsten carbide (WC). The present work aims at understanding how the WC particles surface chemistry can influence the electrical properties of the material and play a fundamental role in the flash sintering phenomenon. An electrical contact resistance (ECR) model was developed to understand the role of resistive surface layers on the electrical behaviour of WC green compacts under different applied pressures during the initial stages of the processes. It is established that the large resistivity measured on green compacts can be attributed to the sole presence of an ultrathin carbon layer on the particles' surface. A carbon nanolayer with a thickness of about 1–2 nm, as detected by XPS and TEM analyses, is found to be responsible for the high resistance reached at the particles' contact points while evolving during the flash event. Flash sintering conditions can be achieved during the electrical resistance flash sintering (ERFS) process in WC nanoparticles covered by such carbon layer and independently of the presence of W oxides.

Microstructural and electrical properties of flash sintered ZnO–Bi2O3–Sb2O3 based varistors: Effect of current density with a controlled current ramp

The ZnO–Bi2O3–Sb2O3-based varistors were produced by the flash sintering method. The effects of current density passed through the sample during the sintering process on the microstructure, and electrical properties of Varistors were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), impedance spectroscopy, and electrical measurements. The average grain size increased from 1.46 μm to 4.85 μm by increasing the current density from 80 to 200 mA/mm2, respectively. The XRD analysis showed that the pyrochlore phase was decomposed in the samples sintered by the current density of 200 mA/mm2. Increasing the current density passed through the sample during the flash sintering process resulted in the modified nonlinear characteristics of varistors. The highest relative density (94% of theoretical density) and the optimum electrical properties were obtained in varistors sintered by 200 mA/mm2, i.e., the nonlinear coefficient, leakage current density, and Schottky barrier height of 39.1, 12 μA/cm2, and 0.43 eV, respectively.

Microstructure and properties of ZnO-Bi2O3-based varistor ceramics via flash sintering

PurposeThis paper aims to study the effects of MnO2 on the ZnO–Bi2O3-based varistor prepared via flash sintering (FS)Design/methodology/approachMnO2-doped ZnO–Bi2O3-based varistors were successfully prepared by the FS with a step-wise increase of the .current in 60 s at the furnace temperature <750°C under the direct current electric field of 300 V cm−1. The FS process, microstructure and the electrical performance of ZnO–Bi2O3-based varistors were systematically investigated.FindingsThe doping of MnO2 significantly decreased the onset temperature of FS and improved the electrical performance of FS ZnO varistor ceramic. The sample with 0.5 mol% MnO2 doping shows the highest improvement, with the nonlinear coefficient of 18, the leakage current of 16.82 µA, the threshold voltage of 459 V/mm and the dielectric constant of 1,221 at 1 kHz.Originality/valueFS is a wonderful technology to enhance ZnO varistors for its low energy consumption, and a short sintering time can reduce grain growth and inhabit Bi2O3 volatilize, yet few research studies work on that. In this research, the authors analyzed the FS process and improved the electrical characteristics through MnO2 doping.

Gas-discharge induced flash sintering of YSZ ceramics at room temperature

This is the first study to conduct the flash sintering of 3 mol% yttria-stabilized zirconia (3YSZ) ceramics at room temperature (25 °C) under a strong electric field, larger than 1 kV/cm. At the standard atmospheric pressure (101 kPa), the probability of successful sintering is approximately half of that at low atmospheric pressure, lower than 80 kPa. The success of the proposed flash sintering process was determined based on the high electric arc performance at different atmospheric pressures ranging from 20 to 100 kPa. The 3YSZ samples achieved a maximum relative density of 99.5% with a grain size of ∼200 nm. The results showed that as the atmospheric pressure decreases, the onset electric field of flash sintering decreases, corresponding to the empirical formula of the flashover voltage. Moreover, flash sintering was found to be triggered by the surface flashover of ceramic samples, and the electric arc on the sample surfaces floated upward before complete flash sintering at overly high pressures, resulting in the failure of flash sintering. This study reveals a new method for the facile preparation of flash-sintered ceramics at room temperature, which will promote the application of flash sintering in the ceramic industry.

Rapid synthesis of Li4Ti5O12 as lithium‐ion battery anode by reactive flash sintering

AbstractIn this work spinel Li4Ti5O12 (LTO) was successfully synthesized via flash sintering technique at a furnace temperature of 580°C within only 5 min for the first time. Phase evolution, microstructures, and electrochemical properties were investigated and compared with conventionally synthesized LTO. The flash sintered LTO shows good crystallinity, high purity, and small particle size. More importantly, it suggests that the flash sintering process could help inhibit lithium volatilization and control the particle size, which is vital to the improvement of the lithium‐ion diffusion process and battery performance. Compared with the conventionally synthesized samples, the flash sintered LTO exhibits higher specific capacities, better cycling stabilities, and rate capabilities. Therefore, flash sintering shows potentials in improving the electrochemical performance of anode materials, and thus suggests an efficient strategy for rapid synthesis of high‐performance anode materials for lithium‐ion batteries.

A current-controlled flash sintering processing leading to dense and fine-grained typical multi-element ZnO varistor ceramics

Comparing to conventional sintering, the densification, microstructure and electric properties of typical multi-element ZnO-Bi2O3-based varistor ceramics on different constant furnace temperatures (850–1000 °C) were studied at electric field of 250 V/cm by flash sintering. The flash-sintered specimens of near full densification could be obtained within 35 s, which had finer grain size and more homogenous microstructure. The effect of different limited current on densification was investigated in flash sintering process. The results showed the relative densities of specimens from 89.9% to 92.0%, 95.5% while limited current increased from 1.0 A to 1.5 A, 2.0 A. The 950 °C flash-sintered specimen with the relative density of 98.0% had the highest threshold voltage of 656 V/mm with the nonlinear coefficient of 41.9. The dielectric loss of flash-sintered specimens had the value lower than 0.2. Thus, typical multi-element ZnO varistor ceramics could show uniform microstructure and excellent electric properties by constant furnace temperature flash sintering.

Ludwig–Soret effect formulated from the grain-boundary-phase model

The Ludwig–Soret effect is a phenomenon wherein thermal diffusion is induced by a temperature gradient. The governing differential equation to explain this phenomenon has been derived phenomenologically based on the Onsager theorem in non-equilibrium thermodynamics. In this study, we applied the grain-boundary-phase (GBP) model to the Ludwig–Soret effect. This model has been originally proposed for calculating the amount of grain boundary segregation in alloys. The flux equation for the thermal diffusion of vacancies was reasonably derived through parallel-tangent construction of the Gibbs energy curves utilized in the GBP model. Moreover, the thermal vacancy diffusion in a pure metal was simulated. The results illustrated that the excess vacancies in the pure metal preferentially moved to the high-temperature region. The direct application of the thermodynamic Gibbs energy parameters in the CALPHAD method is essential to analyze the thermal diffusion phenomenon. Furthermore, the oxygen vacancy diffusion in Zr(O,Va)2 under a considerably large temperature gradient was calculated, and a similar result was obtained, wherein the excess oxygen vacancies moved to the high-temperature region. This finding may explain the rapid atom diffusion observed during the flash sintering process.

Particle Characteristics' Influence on FLASH Sintering of Potassium Sodium Niobate: A Relationship with Conduction Mechanisms.

The considerable decrease in temperature and time makes FLASH sintering a more sustainable alternative for materials processing. FLASH also becomes relevant if volatile elements are part of the material to be processed, as in alkali-based piezoelectrics like the promising lead-free K0.5Na0.5NbO3 (KNN). Due to the volatile nature of K and Na, KNN is difficult to process by conventional sintering. Although some studies have been undertaken, much remains to be understood to properly engineer the FLASH sintering process of KNN. In this work, the effect of FLASH temperature, TF, is studied as a function of the particle size and impurity content of KNN powders. Differences are demonstrated: while the particle size and impurity degree markedly influence TF, they do not significantly affect the densification and grain growth processes. The conductivity of KNN FLASH-sintered ceramics and KNN single crystals (SCs) is compared to elucidate the role of particles’ surface conduction. When particles’ surfaces are not present, as in the case of SCs, the FLASH process requires higher temperatures and conductivity values. These results have implications in understanding FLASH sintering towards a more sustainable processing of lead-free piezoelectrics.

Evaluation of machine learning based models to predict the bulk density in the flash sintering process

Flash Sintering (FS) has still unexplained issues of the flash phenomenon that do not allow the construction of dynamic models to predict the ceramic microstructure. This work investigates computational models based on supervised machine learning techniques for modelling the FS process to predict the bulk density (BD) using the holding time (TH) and electric current density (J). Samples of 3YSZ (zirconia stabilised with 3 mol% of yttria) were prepared and shaped in cylindrical geometry (6 mm in diameter and 4 mm in height). The FS parameters were adjusted at a heating rate of 20 °C/min, maximum current density of 100 mA/mm² and an electric field in alternating current mode of 90 V/cm (1 kHz). FS processes were performed for different J values set to 80, 100 and 200 mA/mm² and different TH values set to 60 s, 120 s and 180 s. After preliminary regression analysis, four learning methods were applied to generate the models: Artificial Neural Network, K-Nearest Neighbours, Random Forest, and Support Vector Machine. The results show moderate correlations (>0.5) between the values measured and predicted by the different models in the prediction of BD through TH. The Support Vector Machine based method allowed the best BD prediction models to be built, with r equal to 0.62, MAE equal to 0.95 and RMSE equal to 1.37. These results indicate the potential of applying the methods and allow the definition of new hypotheses to improve the models.

One-pot synthesis of surface oxidation-suppressed multidimensional copper particles for photonically sintered, directly printed conductive features

Recently, an exploit of highly conductive printed features has been recognized as one of main technologies for facilitating multi-functional integrated microelectronics systems. Among various metallic substances, printable Cu materials with excellent cost-effectiveness and electrical properties have gained tremendous attention in a conjunction with the usage of the instantaneous photonic sintering process. In this study, we have suggested a facile methodology of synthesizing multidimensional Cu particles, a mixture of nano-sized spherical particles and micro-sized flakes, from a single one-batch chemical/mechanical reaction. The chemical factors involved in obtaining the sub-micron Cu particles, with multimodal size distribution, are elucidated with an investigation of the role of surface capping molecules. A subsequent mechanical milling procedure enables one-pot synthesis of multidimensional Cu particles through morphological transformation from multimodal sub-micron Cu particles. It is demonstrated that, by an instant flash sintering process for 1–2 msec, the multidimensional printed Cu features exhibit superior electrical properties with values of resistance of 2.0 and 2.8 Ω/cm on polyimide and polyethylene naphthalate substrates, respectively.

Flash sintering of hydroxyapatite ceramics

ABSTRACT The flash sintering process has been regarded as a noble method to densify ceramic materials. The distinctive feature of the flash sintering is the reduced sintering time which is caused by the electric field. In this study, flash sintering of the hydroxyapatite bioceramics was carried out at temperatures of 900–1200°C and electric fields of 500–1200 V/cm. Dense sintered sample was prepared in a short time within 10 seconds. Electric field required for flashing became reduced as the furnace temperature increased. Abrupt rise of the sample temperature of up to 120°C was observed when the flash occurred. The incubation time for flashing became shorter with increasing temperature and electric field. The flashing was found to occur at lower temperature and lower electric field in vacuum than in air. Microstructure of the flash sintered sample was not different from that of conventionally sintered sample.

Effect of external electric field on diffusivity and flash sintering of 8YSZ: A molecular dynamics study

The atomistic structural modification and ionic diffusivity in 8YSZ at elevated temperature with the presence of an external electric field (E-field) were investigated by molecular dynamics (MD) simulations. As an example, a sufficiently large system of atomic configuration for a bi-crystal ∑5(310)/[001] grain boundary (GB) model was studied. MD results show that the E-field promotes the formation of Frenkel pair defects. Notably, some Zr ions diffuse out of GB regions at E-field>∼500-1000V/cm, resulting in an “avalanche” of cation vacancies at GBs and reduction of GB space charge. Consequently, the diffusivities of cations and anions are enhanced in the 8YSZ system with the presence of E-field. The atomistic level understanding of E-field induced structural modifications and ionic diffusivities provide an in-depth insight to unravel the flash sintering mechanisms of ionic ceramics, especially the coupled thermal-field effect during the flash sintering process.

Temperature effect on mechanical response of flash-sintered ZnO by in-situ compression tests

ZnO has been widely used for various industrial applications. Compared to the extensive investigations on the functional properties of ZnO, the mechanical behavior of ZnO is less well understood. Here we investigated temperature-dependent deformation mechanisms of flash-sintered and conventionally sintered ZnO by in-situ microcompression test up to 600 °C. When tested at room temperature, prominent transgranular cracking occurred in the flash-sintered ZnO at a flow stress of 1.2 GPa, higher than that for the conventionally sintered ZnO (0.9 GPa). A high density of dislocations formed near grain boundaries and fracture surfaces led to the high strength and work-hardening capability in the flash-sintered ZnO. At 200 °C, the flow stress of the flash-sintered ZnO decreased to 0.7 GPa, and the fracture mode evolved to intergranular cracking. When compressed at 400 °C, severe deformation localized at the pillar top was dominated by grain boundary-mediated processes. At 600 °C, plastically deformed grains and intergranular cracks increased prominently, accompanied by a decrease in the density of geometrically necessary dislocation. This study shed lights on understanding the influence of the flash sintering process on the mechanical behaviors of ZnO.

Fabrication and electrical characteristics of flash-sintered SiO2-doped ZnO-Bi2O3-MnO2 varistors

The dense ZnO-Bi2O3-MnO2-xSiO2 (ZBMS) varistors for x = 0, 1, 2, 3 wt% were fabricated by flash sintering method under the low temperature of 850 °C within 2 min. The sample temperature was estimated by a black body radiation model in the flash sintering process. The crystalline phase assemblage, density, microstructure, and electrical characteristics of the flash-sintered ZBMS varistors with different SiO2-doped content were investigated. According to the XRD analysis, many secondary phases were detected due to the SiO2 doping. Meanwhile, the average grain size decrease with increasing SiO2-doped content. The improved nonlinear characteristics were obtained in SiO2-doped samples, which can be attributed to the ion migration and oxygen absorption induced by the doped SiO2. The flash-sintered ZBMS varistor ceramics for x = 2 wt% exhibited excellent comprehensive electrical properties, with the nonlinear coefficient of 24.5, the threshold voltage and leakage current of 385 V·mm−1 and 11.8 µA, respectively.