Densification behaviour and three-dimensional printing of Y2O3 ceramic powder by selective laser sintering

The selective laser sintering (SLS) of an yttria (Y₂O₃) ceramic powder was studied to understand both the effects of i) the initial yttria particle characteristics on the powder bed behaviour and ii) the process conditions (laser power, scanning speed, hatching space) on the sintering/melting of three-dimensionally printed objects. The roughness of the powder bed, a sensitive indicator of the layer bed quality, was determined through three-dimensional optical profilometry and the powder bed packing density was modelled using the discrete-element method. Complex shaped objects including spheres and open rings were successfully fabricated by the SLS three-dimensional printing. In addition, SLS cube-shaped samples were characterized by X-ray diffraction and scanning electron microscopy. The open pore volume fraction significantly decreased from 41% without a post-SLS heat treatment to 31% with a post-SLS heat treatment at 1750 °C for 20 h under secondary vacuum. Finally, an anisotropy in elastic properties has been highlighted, Young's modulus reaches 11 GPa in the stiffest direction.     

Effect of atmospheric conditions on the onset electric field of ZnO and Y2O3 ceramics flash sintering at room temperature

Flash sintering of zinc oxide (ZnO) ceramics can be induced at room temperature (25 °C) by electrical breakdown at high electric field strength. However, a strong discharge may degrade the sample. This study investigated the effects of atmospheric pressure and composition on the onset electric field for the flash sintering of ZnO. The experimental results show that flash sintering of ZnO under a low electric field at room temperature can be achieved by adjusting the atmospheric conditions. Compared with the onset electric field strength under normal atmospheric conditions, the value for flash sintering of ZnO at 20 kPa in a mixture of 20% air +80% argon (Ar) can be reduced by 82% to approximately 700 V/cm. This method also applies to yttrium oxide (Y₂O₃) for a low electric field in flash sintering at room temperature, and is the first report on flash sintering of Y₂O₃ at room temperature.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

A thermal perspective of flash sintering: The effect of AC current ramp rate on microstructure evolution

In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary.     

Electric field-assisted flash sintering of Bi2/3Cu3Ti4O12 starting from a multi-phase precursor powder

We show that powders of bismuth titanate, copper oxide, and titanium oxide can be transformed into a single Bi_2/3Cu₃Ti₄O₁₂ phase and subsequently flash sintered at furnace temperatures of 855−910 °C in a single running experiment, with an electric field ranging from 5 to 15 V cm^–1 and a current limit of 53 mA mm^–2. The (di)electrical properties of the flashed materials were assessed by impedance spectroscopy. These ceramics show a giant dielectric constant (ε′ > 10⁴), and an increasing resistivity with raising the field used in the flash, reaching a value as high as 6.7 MΩ cm for 15 V cm^–1. Thus, the one-step experiment process described here greatly simplifies the fabrication of high-density single-phase ceramics of this material while at the same time improving its properties for low leakage high dielectric applications.     

Rapid synthesis of Gd2Zr2O7 ceramics by flash sintering and its aqueous durability

The high radiation resistance and long time stability of Gd₂Zr₂O₇ ceramics make it a promising candidate for high level waste (HLW) immobilization materials. In this study, single phase nanocrystalline Gd₂Zr₂O₇ was successfully synthesized and consolidated at temperatures around 1050 °C for only 1 min by flash sintering for the first time. The phase evolution and microstructural development during flash sintering were systematically studied and compared with the conventionally sintered samples. The flash sintered Gd₂Zr₂O₇ exhibit defect fluorite structure, and a following heat treatment at 1400 °C could transform the Gd₂Zr₂O₇ ceramics from defect fluorite phase into pyrochlore phase. The MCC-1 leaching test shows that the flash sintered Gd₂Zr₂O₇ samples exhibit good aqueous durability.     

Processing and characterizations of flash sintered ZnO-Bi2O3-MnO2 varistor ceramics under different electric fields

The dense ZnO-Bi₂O₃-MnO₂ (ZBM) varistors were prepared by flash sintering under electric fields ranging from 200 V/cm to 400 V/cm at constant heating rate (CHR) and constant furnace temperature (CFT), respectively. The structure and electrical properties of the ZBM varistors were studied via the XRD, SEM and a DC parameter instrument. The onset temperature and incubation time of flash sintering decrease with increasing electric field. The effects of the maximum limiting current on the density of the samples were also investigated. The results showed that the density of samples increase with the increasing current values. The improved electrical characteristics were obtained during constant furnace temperature flash sintering. The ZBM varistor ceramics exhibited superior comprehensive electrical characteristics under a field of 250 V/cm, in which the nonlinear coefficient is 26.4, the threshold voltage and the leakage current is 466 V/mm and 12.32 μA, respectively.     

Phenomenological understanding of flash sintering in MnCo2O4

The dual role of electric field in the flash sintering process of conducting MnCo₂O₄ is demonstrated. The flash and conventionally sintered MnCo₂O₄ samples produced at different temperatures are characterized using energy dispersive X-ray and micro-Raman spectroscopy to elucidate the micro-level spatial distribution of evolved phases. Raman signal mapping over the two ways sintered samples exposes differently grown areas of cobalt oxide based secondary phase. Electrical conductivity of conventionally sintered sample is recorded as a function of temperature and E-field and is utilized to discover the charge carrier activation mechanism during the flash effect. The conductivity before the flash-onset is shown to be comparable to that occurs by Poole-Frenkel effect and Phonon-assisted tunneling i.e. by the mechanism that occurs before the dielectric breakdown of semiconductors and insulators. The observed results, finally, confirm that catalyst like drift action of E-field on cobalt oxide formation is responsible for enhancement in the flash-sintering.     

Microstructure and defect gradients in DC and AC flash sintered ZnO

In this study, the microstructure and defect characteristics of flash sintering under direct current (DC) and alternating current (AC) were investigated and compared for the ZnO system. DC flash sintering resulted in grain size and porosity gradients within the sample, whereas AC flash sintering produced a sample with homogeneous and fine grain size. Raman spectroscopy revealed asymmetric peaks correlated to the lattice disorder. Using the Breit-Wigner-Fano model, the asymmetric peak fitting revealed redistribution of defects within the DC flash sintered ZnO, while the AC flash sintered ZnO had a comparable defect concentration to conventionally sintered ZnO.     

Suppressed atomic diffusion in flash sintering of bismuth telluride

Bi₂Te₃ materials were synthesised by the recently developed flash sintering (FS) method, and the rapid densification effect was studied. Whereas a fully densified sample can be obtained with a feeding current of 1.2 kA for 1 s, the limited heating effect reduces grain growth and atomic diffusion. Interestingly, the significant chemical reaction suppression with oxygen contaminants, which induces electron doping, has a meaningful electronic properties impact. The large negative Seebeck coefficient of ? 138.9 μV/K in the sample prepared by conventional current sintering of 773 K for 3 min, in which the oxygen diffuses into the Bi₂Te₃ phase, is significantly reduced to ? 8.5 μV/K in the FS sample, much closer to the intrinsic p-type conduction in Bi₂Te₃ raw powder. These results suggest that the limited FS heating effect may contribute to preserving the intrinsic raw powder properties in the sintered material by avoiding excess atomic diffusion and undesirable reactions.     

Electric field-assisted flash sintering of tin dioxide

SnO₂ green pellets were submitted to ac electric fields at temperatures below 1350 °C. Electric current pulses occurred and a substantial modification was found in the microstructure of the pellets after application of 80 V cm⁻¹ at 900, 1100 and 1300 °C. Similar experiments were carried out in SnO₂ mixed to 2 wt.% MnO₂. The linear shrinkage of the pellets was monitored with a dilatometer during the application of the electric field. Scanning electron microscopy micrographs of the pellets show the grain structure evolution after the electric current pulses. The larger is the electric current flow through the SnO₂ pellet, the larger are the shrinkage and the average grain size. Even though sintering occurs without significant densification in SnO₂, the welding of the grains is evident. The apparent density of green pellets of SnO₂ with MnO₂ addition sintered at 1100 °C increased 110% with the application of 80 V cm⁻¹, 5 A.     

Tailoring the microstructure by a proper electric current control in flash sintering: The case of barium titanate

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO₃ ceramics. The results reveal that the most suitable way to control the sintering process is by using non-linear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO₃, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials.     

Field-assisted sintering of undoped BaTiO3: Microstructure evolution and dielectric permittivity

We report, for first time, how electric fields influence the sintering of undoped BaTiO₃, a ferroelectric material, and how this process affects the microstructure and the dielectric properties. Flash sintering is achieved at a furnace temperature of 688 °C under a field of 500 V cm⁻¹, producing specimens that are 94% dense. As a consequence, the grain size is much finer than in conventional sintering, which is shown to influence the Curie temperature and dielectric permittivity. Data obtained at different strengths of the electrical field, and current limits imposed on the specimen are presented in the form of a “processing map” that separates the safe region, where sintering is uniform, from the fail region, where the current flow in the sample becomes localized. The map illustrates that ceramics can respond by different mechanisms, with the dominant mechanism changing with the strength of the electrical parameters.     

Promoting core/surface homogeneity during flash sintering of 3YSZ ceramic by current path management: experimental and modelling studies

During flash sintering (FS) of ceramics, the heat loss by surface radiation is the main cause of temperature gradient between core and surface, which induces inhomogeneity in microstructure. To solve this problem, the judicious designing of sample geometry and electrodes configuration is proposed. Experimental and simulation results show that the application of dogbone shape, forked electrodes, and lower cross-section aspect ratio effectively shifts the current path in 3YSZ samples from core to near-surface during FS, compared to bar-shape samples with a single electrode at each end. Consequently, the temperature distribution becomes more uniform throughout the 3YSZ sample, resulting in increase in relative density from 92.7 % to 99.7 % and improved core/surface homogeneity in microstructure. These optimizations enable 3YSZ ceramics to obtain significant increase in flexural strength from 1203 ± 17 MPa to 1501 ± 15 MPa. A multiphysics model is implemented and compared with experimental results, which reveals the underlying mechanisms of improved sample homogeneity.     

Liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing

The effects were investigated of the starting particle size (i.e., nanometer or submicrometer powders), content of Y₃Al₅O₁₂ additives (YAG; in the range 5–20 wt.%), and difference of size scales between the two particle types on the liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing. It was found that flash sintering benefits from the refinement of the particles size, the increase in additive content, and the smaller size scale of the particulate additive. It was also found that under the present flash sintering conditions (i.e., 900 °C furnace temperature, 13 A current, and 50 s in flash state) the resulting ceramics are, despite the formation of liquid phase, porous to a greater or lesser extent, and exhibit decreasing porosity gradients from their surface to the centre. These observations are rationalized to extract guidelines for powder batch design contributing to the pressureless ultrafast sintering of non-oxide advanced ceramics.     

Preliminary investigation of hydroxyapatite microstructures prepared by flash sintering

Flash sintering is a novel and emerging route for sintering ceramics within a few seconds, even under pressure-less conditions. In the current study, hydroxyapatite (HA) was fully densified by flash sintering at a furnace temperature of 1020°C. Flash sintering with constant electric fields of 750 and 1000?V?cm^?1 reduced the grain growth rate significantly compared to that sintered in the absence of an electric field at 1400°C. The microstructure of HA consolidated by flash sintering was compared with that of the without electric field sintered samples. The flash-sintered samples showed smaller grains (160?～?320?nm) than the without electric field sintered samples (～15?µm). The samples with a higher applied electric field showed slightly better densification than those with the lower field by flash sintering. Overall, the electric flash reduces the sintering temperature effectively and decreases the holding time to densify highly insulating ceramics, such as HA.     

Fabrication of ZrO2/rGO nanocomposites by flash sintering under AC electric fields

Ceramic matrix nanocomposites containing graphene possess superior mechanical properties. However, these nanocomposites are very difficult to be prepared using the conventional methods due to severe grain growth and simultaneous degradation of the graphene at high sintering temperatures and long dwell time. Herein, the dense ZrO₂/rGO (reduced graphene oxide) nanocomposites are successfully fabricated by flash sintering of the green compacts consisting of ZrO₂ nanoparticles and graphene oxide (GO) at 893–951℃ in merely 5 seconds under the alternating current (AC) electric fields of 130–150 V cm⁻¹. The GO can be in situ thermal reduced during the flash sintering. The as-prepared ZrO₂/rGO nanocomposites exhibit excellent mechanical properties. This study presents a green and simple approach to fabricate the dense ceramic matrix nanocomposites reinforced with graphene at low temperatures in a short time.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.