Spark Plasma Sintering of double perovskite Ba2MgWO6 doped with Ce3+: Part I - Structural and microstructural characterizations

Ba2MgWO6 (BMW) translucent ceramics undoped or doped with Ce³⁺ were prepared by using Spark Plasma Sintering (SPS) method. Powders with submicrometric particles diameter and composed of pure BMW cubic phase were fabricated by simple solid-state reaction from primary powders. This work highlights that thermomechanical schedule applied during sintering must be carefully chosen as BMW tends to decompose for temperatures above 1400 °C. As a result, dense and translucent undoped BMW and BMW:Ce³⁺ ceramics were obtained at only 1350 °C for 5 min under 50 MPa of uniaxial pressure. Such lanthanide-free ceramics with high density (i.e. 6.8–7.2 g/cm⁻³) could be envisaged as new and more environmentally-sustainable scintillators for medical imaging.     

Phase evolution and sinterability of lanthanum phosphate – Towards a below 600 °C Spark Plasma Sintering

Lanthanum phosphate, due to its interesting thermal and mechanical properties is a widely studied material for refractory applications. Sintering processes have already been proposed to densify this material and drive its microstructure. Inspired by recent progress on low temperature sintering, we investigate a low temperature Spark Plasma Sintering (LowT-SPS) using hydrated precursor. First, lanthanum phosphate thermal behaviour was studied using TGA/DTA and XRD analysis on various heat-treated powders. Their SPS behaviour were explored by in situ dilatometry measurements. As hydrated precursor showed a low temperature densification, samples were sintered at temperatures from 160 °C to 350 °C under 400 MPa. Even if dense and nano-scaled microstructures were obtained, a residual hydration was observed. Finally, a well densified and fine-grained monazite type lanthanum phosphate was obtained at 550 °C and under 200 MPa. Its mechanical properties are then compared to conventional and Spark Plasma Sintered materials.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Densification of MnSO4 ceramics by Cool-SPS: Evidences for a complex sintering mechanism and magnetoelectric coupling

Dense ceramics of MnSO₄ composition have been successfully densified at 400?°C in only 5?min under a uniaxial pressure of 400?MPa, using Spark Plasma Sintering technique. Since the stable form of MnSO₄ in ambient atmosphere is its hydrate MnSO₄·H₂O, crystallizing in a different space group, dehydration is required to reach a purely anhydrous phase. In situ dehydration during Spark Plasma Sintering allows to lower both sintering temperature and time. Applied pressure strongly influences dehydration step and therefore is a key parameter to tune densification, so far as to obtain a dense MnSO₄·H₂O ceramic. The presence of a reversible phase transition to a β-MnSO₄ high temperature form seems to influence the dehydration temperature under pressure, and likely drives the sintering mechanisms. The high densification obtained, beyond 95% of theoretical density, added to the preservation of the structural and physical properties of MnSO₄ after sintering allowed to perform reliable and reproducible measurements showing a dielectric anomaly associated to the magnetic transition, and the hysteretic behaviour of capacitance versus magnetic field, which is a clue for an intrinsic magnetoelectric coupling in MnSO₄.     

Ti3SiC2-Cf composites by spark plasma sintering: Processing,microstructure and thermo-mechanical properties

MAX phases, and particularly Ti₃SiC₂, are interesting for high temperature applications. The addition of carbon fibers can be used to reduce the density and to modify the properties of the matrix. This work presents the densification and characterization of Ti₃SiC₂ based composites with short carbon fibers using a fast and simple fabrication approach: dry mixing and densification by Spark Plasma Sintering. Good densification level was obtained below 1400 °C even with a high amount of fibers. The reaction of the fibers with the matrix is limited thanks to the fast processing time and depends on the amount of fibers in the composite. Bending strength at room temperature, between 437 and 120 MPa, is in the range of conventional CMCs with short fibers and according to the resistance of the matrix and the presence of residual porosity. Thermo-mechanical properties of the composites up to 1500 °C are also presented.     

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.     

Sintering of a sodium-based NASICON electrolyte: A comparative study between cold,field assisted and conventional sintering methods

Scandium-substituted NASICON (Na_3.4Sc_0.4Zr_1.6Si₂PO₁₂) is a promising electrolyte material for sodium-ion solid state batteries, with the highest ionic conductivity reported to date for a NASICON material. Low-temperature densification and control of microstructure are important factors to enable the low-cost manufacturing of such new battery type. Non-conventional sintering techniques such as Field Assisted Sintering Technology / Spark Plasma Sintering (FAST/SPS) and Cold Sintering are therefore investigated and compared to conventional free sintering. FAST/SPS enables to get rapidly dense samples (99% TD) at lower temperatures than the ones required by conventional sintering routes and with similar electrical properties. Cold sintering experiments, involving the addition of aqueous solutions as sintering aids and high mechanical pressure, enable a moderate densification, but at temperatures as low as 250 °C. Further heat treatments still below the conventional sintering temperature increased the achieved density and ionic conductivity.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

The effect of SPS parameters on the production of yttria transparent ceramic

In this work, the spark plasma sintering (SPS) of commercial yttria nanopowder is investigated. The SPS parameters such as sintering temperature, applied pressure, and dwell time are varied. Densification without grain growth occurring at occurred up to a sintering temperature of 1400°C and grain growth without further densification taking place at the higher temperature. The optimum sample was obtained at a temperature of 1400°C with a pressure of 70 MPa and dwelling time of 15 minutes. The highest relative density of 99.8% and the average grain size of 1.26 μm were obtained at 1400°C. The yttria ceramic annealed at 1200°C had the in-line transmission of 5%-70% and 70%-82% in the visible and infrared wavelength region, respectively. The measurements of hardness and fracture toughness characteristics of the transparent yttria ceramic showed 9.2 GPa and 2.24 MPa.m^1/2, respectively.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Reactive Hydrothermal Liquid‐Phase Densification (rHLPD) of Ceramics – A Study of the BaTiO3[TiO2] Composite System

A densification process called reactive hydrothermal liquid‐phase densification (rHLPD), based on principles of hydrothermal reaction, infiltration, reactive crystallization, and liquid‐phase sintering, is presented. rHLPD can be used to form monolithic ceramic components at low temperatures. The densification of barium titanate–titania composite monoliths was studied to demonstrate proof of concept for this densification model. Permeable, green titania (anatase) compacts were infiltrated with aqueous barium hydroxide solutions and reacted under hydrothermal conditions in the temperature range 90°C–240°C. The effects of reaction time and temperature on the conversion of titania (anatase) into barium titanate were studied. Utilizing a 72 h reaction at 240°C between l.0 M Ba(OH)₂, an anatase (TiO₂) powder compact, and a corresponding Ba/Ti ratio of 1.5, it was possible to crystallize a composite 95 wt% (88 mol%) BaTiO₃ and 5 wt% (12 mol%) TiO₂. The composite had a relative density of ~90% with a compressive strength of 172 ± 21 MPa and a flexural strength of 49 ± 4 MPa.     

In situ X-ray diffraction study of a TiO2 nanopowder Spark Plasma Sintering under very high pressure

We investigated the effect of very high pressure on the sintering temperature, phase transition and the grain growth during Spark Plasma Sintering (SPS) of a 15 nm TiO₂ nanopowder. Using in situ synchrotron X-ray diffraction during sintering at 1.5 and 3.5 GPa, we followed the evolution of the crystalline phases and the crystallite size as a function of temperature. In comparison, in the laboratory, SPS experiments were performed on two original facilities: A Paris-Edinburgh press and a high-pressure module adapted to standard SPS equipment. We studied the effect of the pressure on the sintering in the range 76 MPa to 3.5 GPa. We have shown that highly dense nanostructured ceramics can be prepared under very high pressure at low sintering temperatures. At 1 GPa, we limited the grain growth to an average size of 233 nm by heating at only 560 °C, and achieved a relative density of 98 %.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

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.     

Isothermal sintering kinetic of 3YTZ and 8YSZ: Cation diffusion

We have studied the sintering kinetic of 3 and 8 mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450 °C. The sintering kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731 kJ mol^?1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400 °C in 4.05×10^?14 and 6.00×10^?11 cm² s^?1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Reactive FAST/SPS sintering of strontium titanate as a tool for grain boundary engineering

A high-pressure FAST/Spark Plasma Sintering method was used to produce dense SrTiO₃ ceramics at temperatures of 1050 °C, more than 250 °C below typical sintering temperatures. Combining SPS with solid-state reactive sintering further improves densification. The process resulted in fine-grained microstructures with grain sizes of ∼300 nm. STEM-EDS was utilized for analyzing cationic segregation at grain boundaries, revealing no cationic segregation at the GBs after SPS. Electrochemical impedance spectroscopy indicates the presence of a space charge layer. Space charge thicknesses were calculated according to the plate capacitor equation and the Mott-Schottky model. They fit the expected size range, yet the corresponding space charge potentials are lower than typical values of conventionally processed SrTiO₃. The low space charge potential was associated to low cationic GB segregation after SPS and likely results in better grain boundary conductivity. The findings offer a path to tailor grain boundary segregation and conductivity in perovskite ceramics.     

Densification behavior and properties of alumina–chrome ceramics: Effect of TiO2

Highly dense alumina–chrome bodies with low porosity are usually used as corrosion and thermal resistant refractories. Alumina–chrome refractory with molar ratio 1:1 was developed using chemical grade hydrated alumina and chromium (III) oxide by conventional sintering route. Batch materials were attrition milled, isostatically pressed and sintered in the temperature range from 1000 °C to 1700 °C with 2 h soaking at peak temperature. Phase development of the sintered materials with temperature was studied by X-ray diffraction. Sintering temperature, sintering condition and addition of sintering aid (TiO₂) have immense effect on the densification of the alumina–chrome refractory. Highly dense alumina–chrome refractory with almost nil apparent porosity was developed at 1500 °C in reducing atmosphere. Flexural strength of the sintered materials at room temperature and at 1200 °C was also measured. 1 wt% TiO₂ gives the optimum result with respect to densification and flexural strength.