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

Electric 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 Gd_0.1Ce_0.9O_2-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.     

Polarity-induced grain growth of gadolinium-doped ceria under field-assisted sintering technology/spark plasma sintering (FAST/SPS) conditions

This study aims to understand the effect of the electrical field on microstructure evolution during field-assisted sintering or spark plasma sintering (FAST/SPS) of 10 mol% gadolinium-doped ceria (GDC) with experimental and numerical methods. The novelty of this study has been the observation of enhanced grain growth in the region closer to the anode, even under FAST/SPS conditions with electrical fields less than 5 V/cm. The grain growth kinetics, including determination of activation energy and grain-boundary mobility, were analyzed along the cross section of the samples for different temperatures and dwell periods. With an increase in distance from the anode, reduction in the activation energy for grain growth and grain-boundary mobility was observed. These observations attributed to the attraction of oxygen ions to the anode region under an electrical field with an increase in defects along the grain boundaries. Thereby an increase in the grain-boundary mobility and larger grains in that region were observed. A homogenous microstructure was observed in a case where the current did not flow through the sample. Furthermore, a numerical strategy has also been developed to simulate this behavior in addition to heat generation, heat transfer, and densification using Finite Element Methods (FEM) simulations. The simulation results provided an insight into the presence of a potential difference across the cross section of the samples. The simulation results were also in good agreement with the experimental observations.     

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.     

Ultrafast synthesis and densification of ZrO2 doped KNN ceramics by reactive flash sintering

Fabrication of dense KNN-based lead-free piezoelectric ceramics at low temperatures in short time through a cost-effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO₃-NaNbO₃ into (K,Na)NbO₃ and densification occur simultaneously during the flash event. Most importantly, ZrO₂ doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash-sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO₂ doped and undoped KNN ceramics show a comparable coercive field E_c, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid-state reaction and densification rates.     

Oxidation effects on spark plasma sintering of molybdenum nanopowders

Surface-oxidized molybdenum nanopowders are compacted by spark plasma sintering (SPS). The oxide impurity behavior is analyzed under various sintering temperatures. The densification mechanism of the nanopowders with a melted oxide phase is identified in situ by regression analysis of the experimental data on the temperature-dependent porosity change and on the SPS multistep pressure dilatometry. To increase the density of the compacted pellets, the nanopowders with the oxide phase are consolidated by SPS using the two in situ oxide removal methods: carbothermic reduction and particle surface cleaning by the electric current flow through the powders. The advantages and disadvantages of these methods in terms of the density, grain size, and mechanical properties of the final products are discussed.     

Densification behaviour and physico-mechanical properties of porcelains prepared using spark plasma sintering

Porcelain powder was consolidated using spark plasma sintering (SPS) at a constant heating rate of 100°C?min^?1 to peak temperatures ranging from 1000 to 1200°C and was observed to sinter at relatively low temperature ～920°C under the SPS conditions while conventional sintering requires ～1050°C. SPS produced densification rates about 10 times greater than conventional sintering. The dwelling step at the optimal peak temperature was negligible due to rapid flow of the molten glass assisted by applied pressure. SPSed samples exhibited denser microstructures, resulting in improved physico-mechanical properties compared with conventionally sintered samples such as apparent bulk density improved from 2.38 to 2.48?g?cm^?3, Vickers hardness improved from 3–5 to 6–7?GPa, and fracture toughness improved from 2–3 to 4–6?MPa?m^1/2.     

Microstructure and mechanical properties of TaC ceramics with 1–7.5 mol% Si as sintering aid

Metallic Si as sintering aid was effective in densifying tantalum carbide ceramic (TaC) by spark plasma sintering (SPS) at 1700°C. Full density was reached at 5.0 mol% Si addition (equivalent to 1.088% Si in weight) and above. Enhanced densification of TaC ceramic with Si was associated with decrease in oxygen content from ~0.24 wt% in TaC powder to ~0.03 wt% in consolidated specimen. Rest of the oxygen species was collected at multigrain conjunctions to form SiO₂‐based liquid at high temperatures. Upon cooling, Ta, Si, O, and C dissolving in the liquid precipitated minor phases of TaSi_x and SiC of low concentrations. Microstructure of TaC ceramics was refined by the Si addition, with average grain size decreasing from 11±8 μm at 1.0 mol% Si to 3±2 μm at 7.5 mol% Si addition. Ta solute in SiC and Si solute in TaC were evidenced. TaC ceramic containing 7.5 mol% Si had a relatively good flexural strength and fracture toughness of 646±51 MPa and 5.0 MPa·m^1/2, respectively.     

Pressure-assisted flash sintering of ZnO ceramics

Implementing pressure-assisted flash sintering of ZnO powder without pretreatment by a new experimental configuration is presented. Rapid and energy-concentrated heating of electrode-sample-electrode area by induction heating allows preheating and flash sintering of loose-pack powder in the die with pressure assistance. Using an insulated die enables the current to flow through the sample during flash sintering. ZnO ceramics with a relative density of 95.1% can be achieved in less than 3 min. The whole process includes 104 s of preheating by a low-power induction heating device and 30 s of flash sintering assisted by a pressure of 26 MPa using the pulsed direct current (DC). The process characteristics of pressure-assisted flash sintering using the pulsed DC are discussed. The effect of pressure on densification and grain size is analyzed in detail, and some potential mechanisms are provided.     

Study of the densification and grain growth mechanisms occurring during spark plasma sintering of different submicronic yttria-stabilized zirconia powders

Densification and grain growth mechanisms of Yttria-Stabilized Zirconia sintered by Spark Plasma Sintering are investigated. Sintering trajectories of four commercial submicronic powders with different average particle sizes and yttria amounts have been established and sintering regimes determined. Densification mechanisms are determined in the regime where densification is occurring without grain growth using a model derived from hot-pressing. Grain growth mechanisms are determined using the conventional power law in the regime where ceramics are fully densified. Densification occurs by grain boundary sliding accommodated by an in-series interface-reaction/lattice diffusion of cations or by an overlapping of surface diffusion and grain boundary sliding mechanisms for tetragonal stabilized zirconia and by dislocation climbing for fully stabilized zirconia. A normal grain growth occurs for each ceramic, all composed of a single phase, contrary to the two-phased ceramics obtained in literature where grain growth occurs by segregation at grain boundaries.     

Densification kinetics of boron carbide with medium entropy alloy as a sintering aid during spark plasma sintering

The high sintering temperature of pure B₄C considerably limits its widespread application, thus searching an effective sintering aid is critical. In this work, B₄C-based ceramic with 1 vol.% nonequiatomic Fe₅₀Mn₃₀Co₁₀Cr₁₀ medium entropy alloy as a sintering aid were fabricated at 1900-2000°C by spark plasma sintering (SPS) under applied pressure, and their mechanical properties were examined and compared with pure B₄C ceramic sintered at same condition. The maximal flexural strength of 255.59 MPa, microhardness of 2297.6 Hv_0.2 and fracture toughness of 3.62 MPa m^1/2 could be obtained at optimized SPS pressure of 50 MPa, which were all higher than those of pure B₄C ceramic. To better understand the densification kinetics mechanisms, the densification ratio as a function of SPS temperature and pressure was theoretically analyzed using steady creep model. It was found that densification controlled by grain-boundary sliding at lower pressure transferred to power law creep regime at higher pressure, which were proved by the dislocation net shown in transmission electron microscopy image.     

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics. Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C. Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering. By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N=3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering. The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process.     

Fabrication of translucent AlN ceramics with MgF2 additive by spark plasma sintering

Translucent AlN ceramics with 0‐2 wt.% MgF₂ additive were prepared by spark plasma sintering. AlN powder was heated temporarily up to 2000°C, before holding at 1850°C for 20 minutes in N₂ gas. The sintered ceramics consisted of a single phase of hexagonal AlN, and showed a transgranular fracture mode. The total transmittance was improved remarkably by the additive, to reach 74% at a wavelength of 800 nm for 1 wt.% MgF₂. For 2 wt.% MgF₂, the transmittance was slightly lower than that for 1 wt.% MgF₂, and an absorption band was observed apparently at around 400 nm. The addition of MgF₂ along with the temporary heating at higher temperatures than the sintering temperature contributed to improve the transmittance remarkably.     

Consolidation and high-temperature strength of monolithic lanthanum hexaboride

In 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 m^1/2, 300 ± 20 MPa). However, at 1600°C, these parameters would decrease to 1.3 ± 0.1 MPa m^1/2 and 120 ± 40 MPa, respectively.     

Effect of titanium diboride on the homogeneity of boron carbide ceramic by flash spark plasma sintering

A novel method, namely flash spark plasma sintering (FSPS), combining flash sintering and electric field assisted sintering, was utilized to densify boron carbide/titanium diboride (B₄C/TiB₂) composites. Further, sintering homogeneity of the composites with different contents of TiB₂ was systematically investigated and theoretical model was built. Results indicated that addition of 50?wt% TiB2 led to the densification of B₄C/TiB₂ composite by up to 97.7% with regional range 1.9% at 1872?°C under pressure of 4?MPa in 60?s. The preferential pathway of TiB₂ network proves to disperse the central current and distribute thermal flow throughout the specimen possibly via tunneling, electronic field emission effect at first stage and lower-resistance composite pathway latter, contributing to the increased homogeneity.     

Transparent Er2O3 ceramics fabricated by high-pressure spark plasma sintering

Fabrication of transparent Er₂O₃ ceramics was carried out by high-pressure spark plasma sintering (HP-SPS). The color and in-line transmittance of these ceramics was highly sensitive to the sintering parameters. Samples exhibited a strong pink or wine color after sintering at 1150 °C under 600 MPa or 1250 °C under 250 MPa, respectively. This was confirmed to be a result of oxygen vacancies created during the sintering process and high sensitivity of Er₂O₃ to the strong reducing atmosphere in the SPS apparatus. Post-sintering annealing in an air furnace led to elimination of oxygen vacancies and increased transparency. Additionally, the photoluminescence intensity and phosphorescence lifetime of annealed (pink) samples was higher and shorter, respectively, compared to that of the reduced (wine-colored) samples.     

Ultraviolet emission transparent Gd:YAG ceramics processed by solid-state reaction spark plasma sintering

Transparent 1% Gd-doped YAG and YAG ceramics were synthesized via solid-state reaction spark plasma sintering using commercially available powder and TESO as a sintering additive. The highest in-line transmission values achieved were 77.1% at 550 nm and 80.6% at 800 nm in the 1% (at.%) Gd-doped YAG transparent ceramic with 99.90% relative density. Ultraviolet emission at 312.5 nm was observed in 1% Gd-doped YAG ceramic via photoluminescence excitation, making it a promising material for applications in solid-state UV devices.     

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB₂) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO₂ and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000 °C, which was supposed to be the synthesis temperature of HfB₂ particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050 °C with a 30 min hold time under 60 MPa uniaxial pressure leads to obtain monolithic HfB₂ ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12 GPa) were achieved, which were directly attributed to the low bonding between HfB₂ grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12 MPa m^1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06 MPa m^1/2) and the fracture mode changed from transgranular to mixed mode.     

Low-angle twist grain boundary in SrTiO3 fabricated by spark plasma sintering techniques

A SrTiO₃ bicrystal with a low-angle twist grain boundary was fabricated using the spark plasma sintering (SPS) instrument. The atomic and electronic structure of the grain-boundary core was characterized using scanning transmission electron microscopy techniques. It was determined that the boundary is comprised of 2 types of defects with distinct electronic structures: screw dislocations and dislocations with an [001] edge component. The dislocations with an [001] edge component dissociated into 2 partial dislocations, separated by a stacking fault consisting of 2 Ti–O layers. The screw dislocations are attributed to the twist component of the grain boundary, while dislocations with an [001] edge component are attributed to surface steps on the original (100) SrTiO₃ surfaces prior to diffusion bonding. The observed repeat distances between the dislocations with edge components along the grain-boundary plane are smaller than those discovered during traditional diffusion bonding experiments. The higher planar defect density observed in this study results partly from higher heating rates, lower processing temperatures, and shorter holding times during SPS processing.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.