High-temperature Sintering Technique Research Articles

Water-mediated production of K0.5Na0.5NbO3 piezoelectric ceramics with cold-sintering assisted route: Enhanced dielectric properties

Cold sintering is an attractive method to densify ceramics, providing the opportunity to produce ferroelectrics efficiently. This is particularly important for cases where ceramics like perovskites are prone to high A-site element volatilization at high temperatures. The resulting uncontrolled creation of defects is often detrimental to the ferroelectric properties. In this work, cold-sintering assisted production of KNN ceramics with water (no corrosive transient liquid) leads to a homogenous, high-density pure KNN with high dielectric permittivity and lower dielectric losses as compared to high-temperature sintering techniques. Our work could, therefore, open a new venue for further modification of defect chemistry in lead-free piezoelectrics.

Boron-incorporated IrO2-Ta2O5 coating as an efficient electrocatalyst for acidic oxygen evolution reaction

The Ir-based electrocatalysts for the acidic oxygen evolution reaction (OER) have demonstrated remarkable durability. Enhancing the Ir-based electrocatalytic activity still remains crucial owing to the scarcity of iridium. Here, a high-temperature sintering technique is employed to fabricate a boron (B)-incorporated IrO2-Ta2O5 coating with an almost perfect rutile-type crystal structure on a corrosion-resistant titanium substrate, ensuring exceptional stability for the acidic OER. The B-incorporated IrO2-Ta2O5 electrode fabricated in a mixed solution of 0.6 M H3BO3, exhibits an overpotential of 210 mV at a current density of 10 mA cm−2 and a lower Tafel slope of 34.2 mV dec−1 in a 0.5 M H2SO4 solution, which is far lower than the 272 mV overpotential and the 45.3 mV dec−1 of the IrO2-Ta2O5/Ti electrode. The electrode possesses a minimal potential increase even after undergoing continuous OER for 400 h at a high current density of 100 mA cm−2 in a 0.5 M H2SO4 solution. The incorporation of B species into IrO2-Ta2O5 effectively fine-tunes the electronic structure of Ir active sites, leading to a substantial enhancement of the intrinsic electrocatalytic activity. This study provides promising prospects for reducing the energy consumption of noble IrO2-based electrocatalysts in the practical application of electrochemical industry for the acidic OER.

Dielectric, Magnetic and Surface Analytical Study of Dy and Tb Doped (Bi, Pb)(Fe,Ti)O3 Multiferroic Ceramics

Polycrystalline samples of Bi0.8A0.1Pb0.1Fe0.9Ti0.1O3(where A = Dy and Tb) ceramics have been prepared through high temperature sintering technique. X-ray diffraction analysis showed that the ceramics possess perovskite phase with hexagonal crystal symmetry. Dielectric and electrical properties indicate that the Curie temperature shifted to higher temperature with Dy doping. AC conductivity of the sample increases with increase in temperature which confirms the negative temperature coefficient of resistance (NTCR) behaviour of material. Substitution of Dy and Tb ions at A-site suppress the spiral spin structure of BiFeO3 which is further responsible for the appearance of weak ferromagnetism atroom-temperature.Induced destruction of the spin cycloid due to the Dy and Tb substitution, these multiferroic properties are attributed because of the enhanced magneto-electric interaction.

Design, Fabrication, and Screening of Environmental‐Thermal Barrier Coatings Prepared by Ultrafast High‐Temperature Sintering

AbstractThe demand for more efficient gas turbines relies heavily on the development of new environmental‐thermal barrier coatings (ETBCs). However, there is still uncertainty about which alloys and composites will be used for the next generation of turbine blades, as well as the most promising coating materials. Herein, an ETBCs development strategy is presented by integrating the coating design, synthesis, and screening using an ultrafast high temperature sintering (UHS) technique to accelerate coating improvements. The initial basis for composition selection is their thermal expansion mismatch with the substrate alloys; for which a temperature‐dependent coefficient of thermal expansion database is created. By combining tape casting method with the UHS technique a high‐throughput synthesis of single and multi‐layer coatings are realized with different compositions, layer stacking sequences, and layer thicknesses. To evaluate the coatings, thermal cycling tests from room temperature to 1300 °C are conducted. The approach enabled coatings on objects with complex geometries, multi‐layer ETBCs, and porosity tailoring by using staged UHS that runs with different temperatures and durations. The fast iteration strategy is more cost‐effective for the screening of ETBCs compared to conventional methods and greater throughput which can be further extended for rapid optimization of other materials systems.

Ultra-fast high-temperature synthesis and densification of high-entropy diborides and diboride-carbide ceramics

The strong covalent bonds and low self-diffusion coefficients of high-entropy diboride (HEBs) and diboride-carbide (HEB-HEC) ceramics render their densification difficult, which limits their applications. Here, we used an ultra-fast high-temperature sintering technique to synthesize and densify several HEBs and HEB-HEC (containing 5–8 cations) ceramics within an isothermal duration of 1 min and a total sintering period of 6 min. All ceramics formed a single-phase or dual-phase high-entropy solid solution, and showed high density, high hardness and good fracture toughness. The rapid densification mechanisms were determined, and the preferential dissolution of metal cations in HEB and HEC phases was explained using a simplified ideal solution model. A good fracture toughness in the range of 3.6-5.4 MPa·m1/2 resulted from crack deflection or blunting and bridging of the rod-like diboride or boron-rich phase. This paper presents a simple, economical, and efficient method for the synthesis and densification of high-entropy ceramics and other advanced materials.

Enhancing densification rate and the unusual 4H-SiC polytype stabilization in ultrafast high-temperature sintering of α-SiC

Novel Powder medium-based Ultrafast High-temperature Sintering (P-UHS) technique brings unexplored opportunities for the ultrarapid consolidation of large-sized and difficult-to-sinter ceramics with a complex shape. Herein, the P-UHS was demonstrated to be very effective means for the densification of large-sized α-SiC bulks (∼16 cm3) with boron and carbon additions under heating rates exceeding 1500 °C/min. Bulk ceramics with ∼97% density and homogenous microstructure were obtained within only 6 min. The ultrafast heating led to two orders of magnitude increase in the densification rate contributing to an energy saving of 90% if compared to conventional sintering process. In addition, the preferential 4H-SiC polytype stabilization was observed only when ultrafast heating rate was employed. The non-equilibrium transient chemistry and the ultrarapid shrinkage were considered as the major drivers for this unusual phenomenon.

Rapid Li compensation toward highly conductive solid state electrolyte film

Garnet type solid-state electrolytes (SSEs) possess high stability against Li metal, intrinsic safety, and wide electrochemical window. To achieve the all-solid-state batteries (ASSBs) with high gravimetric/volumetric energy density and high power density, flat thin garnet SSEs with a high ionic conductivity of > 10−4 S/cm are required. However, the current fabrication processes are challengeable to maintain both the high ionic conductivity and the flat skeleton due to the severe Li loss during processing and the fragile nature of thin films. Herein, we introduce a rapid Li compensation (RLC) technique that combines the ultra-fast high-temperature sintering (UHS) technique with the tape casting process to produce highly ionic conductive, flat garnet SSE thin films. The freestanding Li6.4La3Zr1.4Ta0.6O12 (LLZTO) thin film with a thickness of 40 µm, fabricated using this technique, displays a surface roughness of 5 µm and a relative density of 95%. After a 10-seconds Li compensation, the compensated LLZTO film displays an ionic conductivity of 4.3 × 10−4 S/cm, 2 times higher than the non-compensated one. As a proof of concept, this RLC technique proposes a new strategy for the fabrication of high-performance ceramic thin films and also lays a solid foundation for the application of ASSBs.

Mechanism of cordierite formation obtained by high temperature sintering technique

Cordierite (2MgO·2Al2O3·5SiO2) is a kind of important ceramic with low thermal expansion coefficient and low dielectric constant. In this study, the influence of raw material compositions and various sintering temperatures on the synthesis mechanism and reaction path of cordierite were studied. The results reveal that the binary compounds forsterite and magnesia-alumina spinel are firstly generated in the low-temperature section while using pure single oxide materials. Then, forsterite and magnesia-alumina spinel are respectively combined with Al2O3 and SiO2 to form cordierite. The reaction of magnesia-alumina spinel with SiO2 to form cordierite requires a higher temperature. The addition of CaO and TiO2 tends to form low melting point substances, thus promoting the formation of cordierite phase. This study has a certain guiding significance for the cordierite synthesizing from natural minerals and solid wastes.

A TiO2 modified whisker mullite hollow fiber ceramic membrane for high-efficiency oil/water emulsions separation

Design and preparation of membranes with ultrahigh separation performance and antifouling property for oil-in-water (O/W) emulsions remains challenging. In this study, a high flux mullite/TiO2 ceramic composite membrane was prepared via multi-precipitation of TiO2 on a whisker mullite hollow fiber support synthesized by combining phase inversion and high-temperature sintering techniques. The results showed that the generated whisker mullite structure improved the permeation flux, and the micro-nano structured TiO2 functional layer endowed the membrane surface with superhydrophility and stability. The retention of the optimal composite membrane (M20T13) that was soaked in the titanium solution 20 times for 13 min each time for the O/W emulsions like n-hexane, toluene and engine oil maintained over 98 %, and the flux after 6 h filtration was 668.34 L·m−2·h−1, 487.25 L·m−2·h−1 and 258.66 L·m−2·h−1, respectively, much higher than that of the optimal substrate (F3A1, mass ratio of fly ash: Al2O3 = 3:1). Moreover, the flux recovery rate of M20T13 was much higher than that of F3A1 after chemical backwashing. This work manifests great potential in O/W treatment fields.

Hierarchically structured powder metallurgy austenitic stainless steel with exceptional strength and ductility

Powder metallurgy stainless steels have attracted much attention due to their wide applications. However, they often exhibit a low relative density (<96%) and a strength-ductility trade-off dilemma. We herein study the microstructure and mechanical property of a dense hierarchically structured stainless steel, which is synthesized by a high-pressure (4 GPa)/high-temperature (1200 °C) sintering technique and exhibits a relative density of approx. 99%. The hierarchically structured stainless steel is composed of 27.5% microcrystalline phase with an average grain size of 9 μm and 72.5% ultrafine-grained phase with an average grain size of 259 nm. These unique microstructures together with the high relative density enable us to achieve an exceptional combination of high yield strength of 1188 ± 59 MPa and a relatively high tensile elongation of 15 ± 2.6%, evading the strength-ductility dilemma observed in conventional powder metallurgy stainless steels synthesized by pressureless or low-pressure sintering. The high strength is mainly caused by the ultrafine-grained phase, which can be obtained even after sintering nanocrystalline stainless steel powders at a high temperature of 1200 °C (∼87% of melting temperature). The ultrafine-grained phase and the grain boundary strengthening factor in the ultrafine-grained phase contributes 91% and 48% to the total strength, respectively. Both experiments and calculations indicate that increasing the sintering pressure from 40 MPa to 4 GPa can significantly lower the grain growth rate by approx. 64%. The high ductility is mainly caused by the microcrystalline phase, which causes the formation of a large number of dislocations and the generation of nano-twins.

Realizing High Thermoelectric Performance in Bi0.4Sb1.6Te3 Nanosheets by Doping Sn Element

The bottom-up method has been universally used to synthesize low-dimensional nanomaterials because the existence of a large number of interfaces contributes to low thermal conductivity compared to the top-down method. In this work, SnxBi0.4-xSb1.6Te3 (abbreviated as SnxBST, where x = 0, 0.04, 0.06, or 0.08) ingots are synthesized by a one-step hydrothermal method, combining a fast annealing technique with a high-temperature sintering technique. The existence of defects contributes to the dramatic increase of carrier concentration and then optimizes the electrical conductivity. The maximum power factor (PF) value is 3.11 mW/m/K2 for the Sn0.08BST thermoelectric ingot, which is an increase of 1.59 times compared to that of the pure sample at room temperature. Moreover, the maximum weighted mobility (μw) value is 339.1 cm2/(V·s) at 303 K, which is 1.54 times that of the pure sample. By considering the anisotropy of the thermal conductivity, combining PF perpendicular to the pressure direction and thermal conductivity along the pressure direction, the over-estimated ZTmax and ZTave values are up to 1.34 at 393 K and 1.24 between 303 and 483 K, respectively, which are 1.43 and 1.52 times those of the pure sample, respectively. Finally, this work could be extended to apply to the synthesis of thermoelectric materials via solvothermal methods.

One-step synthesis of t-ZrO2 from Zircon using magnesium-calcium minerals as stabilizers

Stabling crystal structure at room temperature is a classic problem in the study of Zirconium dioxide (ZrO2). However, there are few investigations on making tetragonal zirconia (t-ZrO2) in one step at a low cost. In this research, t-ZrO2 is synthesized using a one-step high-temperature solid-state sintering technique with magnesite, dolomite, and limestone as stabilizers and zircon as the raw material. The most suitable stabilizers and reaction conditions are determined, and the mechanism of zirconia structure stabilization is explored. The findings suggest that magnesite has the lowest effect as a crystal structure stabilizer, whereas dolomite and limestone are pretty close, but dolomite introduces more impurities. The ideal reaction conditions were 60% mol limestone at 1500 °C. The stabilization mechanism is zirconia gap correction, according to XRD and EPR data. The characterization of the SEM demonstrates that the heat treatment temperature and stabilizer had little effect on the morphology of t-ZrO2. When limestone was introduced throughout the process, EDS data revealed that some amorphous silicon-calcium compounds occurred in the product. The focus of follow-up work will be on how to lessen the impact in this area. This research offers vital reference value for reducing the cost of the synthetic t-ZrO2 process.

Ultrafast high-temperature sintering of barium titanate ceramics with colossal dielectric constants

Application of Ultrafast High-temperature Sintering (UHS) technique to rapidly densify barium titanate ceramics has been explored for the first time. Bulk ceramic with ~94% density was obtained by UHS at ~1340 °C for 60 seconds. The densification process was accompanied with progressive sample discolouration from light to dark grey. Further analysis indicates that oxygen vacancy and its associated Ti-rich phase Ba4Ti12O27 are present in the ceramics. Their roles in ultrafast densification and sample discoloration are discussed. Due to the presence of oxygen vacancies, the UHSed ceramics generally exhibit a colossal dielectric constant of ~ 15–30k at 1 kHz, with dielectric loss of ~0.07–0.10, while the ceramics without oxygen vacancy retain a dielectric constant of ~3000–6000 and dielectric loss of ~ 0.06 at 1 kHz which are comparable to that of the conventionally sintered ceramics. Furthermore, the challenges in applying UHS to sinter thick BT ceramics are discussed, aided by thermal simulations.

Effect of Graphene Addition on the Thermal and Persistent Luminescence Properties of Gd2.994Ce0.006Ga3Al2O12 and Gd2.964Ce0.006Dy0.03Ga3Al2O12 Ceramics.

The gadolinium, gallium, aluminum garnet doped with cerium and co-doped with dysprosium ions were prepared using sol gel method. The SEM images show that after synthesis, the grains are below 100 nm. The powders were ultrasonically mixed with graphene nanoflakes and ceramics were prepared using the high pressure low temperature sintering technique. A series of the ceramics was prepared using different graphene content. The structure of the samples was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman techniques. The spectroscopic properties were checked using conventional and persistent luminescence spectra measurements. The thermoluminescence glow curves and fading time of persistent luminescence measurements were performed to check how the graphene presence affects the electron traps number and depth. It was found that the addition of graphene improved the thermal conductivity of co-doped samples. This resulted in faster release of deeper traps and an increase in fading of persistent luminescence. The possibility of releasing energy from deep traps without additional stimulation may allow the use in different applications, the matrices and luminescent ions, which so far did not show persistent luminescence at room temperature.

Rapid Pressureless Sintering of Glasses

Silica glasses have wide applications in industrial fields due to their extraordinary properties, such as high transparency, low thermal expansion coefficient, and high hardness. However, current methods of fabricating silica glass generally require long thermal treatment time (up to hours) and complex setups, leading to high cost and slow manufacturing speed. Herein, to obtain high-quality glasses using a facile and rapid method, an ultrafast high-temperature sintering (UHS) technique is reported that requires no additional pressure. Using UHS, silica precursors can be densified in seconds due to the large heating rate (up to 102 K s-1 ) of closely placed carbon heaters. The typical sintering time is as short as ≈10 s, ≈1-3 orders of magnitude faster than other methods. The sintered glasses exhibit relative densities of >98% and high visible transmittances of ≈90%. The powder-based sintering process also allows rapid doping of metal ions to fabricate colored glasses. The UHS is further extended to sinter other functional glasses such as indium tin oxide (ITO)-doped silica glass, and other transparent ceramics such as Gd-doped yttrium aluminum garnet. This study demonstrates an UHS proof-of-concept for the rapid fabrication of high-quality glass and opens an avenue toward rapid discovery of transparent materials.

Ultrafast high-temperature synthesis and densification of high-entropy carbides

Recently, high-entropy carbides have attracted great attention due to their remarkable component complexity and excellent properties. However, the high melting points and low self-diffusion coefficients of carbides lead to the difficulties in forming solid solution and sintering densification. In this work, six dense multicomponent carbides (containing 5–8 cations) were prepared by a novel ultrafast high-temperature sintering (UHS) technique within a full period of 6 min, and three of them formed a single-phase high-entropy solid solution. The solid solubility of the UHSed multicomponent carbides was highly sensitive to the compositional variation. The presence of Cr3C2 liquid had significant contributions to the formation of solid solution and the densification of multicomponent carbides. All UHSed multicomponent carbides exhibited high hardness, which, unexpectedly, did not simply increase with increasing number of the components. The highest nanohardness with a value of 36.6 ± 1.5 GPa was achieved in the (Ti1/5Cr1/5Nb1/5Ta1/5V1/5)Cx high-entropy carbide. This work is expected to expedite the development of high-entropy carbides and broaden the application of UHS in the synthesis and densification of advanced ceramics.

Achieving the synergy of low thermal conductivity and superior mechanical properties of Ce doping La2Zr2O7 ceramics for thermal protection coatings: Experiment and first-principles calculations

The bulk La2(Zr0.7Ce0.3)2O7 (LZ7C) ceramics were successfully fabricated by a novel high-temperature pressureless sintering technique using solid sintering synthesized La2(Zr0.7Ce0.3)2O7 powders. The synthesized ceramic powders presented a mixed pyrochlore and fluorite phase with a smooth average grain size of 410 nm, which was benefit to bulk ceramic densification. The relative density of LZ7C sample was enhanced with the increasing sintering temperature, leading to an enhancement in mechanical properties, especially in fracture toughness, from 0.28 ± 0.05 MPa·m1/2 under 1300 °C increased to 1.41 ± 0.20 MPa·m1/2 under 1500 °C. The calculated thermal conductivity based on the Slack's model was approached to 0.467 W/(m·K) at 300K, which was close to the experimental result (0.493 W/(m·K), RT) of LZ7C-1400 sample. The calculated average coefficient of thermal expansion using QHA method was approached to 6.45 × 10−6 K−1, lower than that of experimental result (8.75 × 10−6 K−1) of LZ7C-1400 sample, which was mainly attributed to the defects, pores and grain boundaries in experiment samples.

Ultra-fast High-temperature Sintering (UHS) of translucent alumina

Pressure-less consolidation of transparent alumina requires several hours dwelling at high temperature (1600 °C) under a strongly reducing atmosphere (hydrogen). Here, we report on the ultrafast consolidation of translucent alumina ceramics at 1360 °C. Transparency was promoted by synergistic effects of high initial green density (62.7%) and rapid sintering using Ultra-fast High-temperature Sintering (UHS) technique. The proposed approach, using a heating rate of 430 °C/min and dwelling time of 15 min, resulted in ultra-fine-grained translucent alumina ceramics with a grain size of 0.39 μm, and an in-line transmittance of 28.7% at a wavelength of 700 nm. For comparison, conventionally fired counterparts were opaque due to their incomplete densification and pore coalescence.

Effect of components on the microstructures and properties of rare-earth zirconate ceramics prepared by ultrafast high-throughput sintering

Rare-earth zirconate ceramics are conventionally sintered at high temperatures for long durations to achieve a full density. Herein, we synthesized and densified five lanthanide-group rare-earth zirconates (Ln2Zr2O7, Ln = La, Nd, Sm, Eu, Gd) containing different numbers of Ln cations in a high-throughput mode using a novel ultrafast high-temperature sintering technique within a cycle of 5 min, and investigated the effects of components on the structure and properties of the resultant ceramics. Under the high-throughput mode, the same sintering conditions make the analysis and comparison of the results more reasonable. The average grain size decreased while hardness and Young’s modulus increased with an increase in the number of the Ln components in the zirconate ceramics. The mechanisms were ascribed to the sluggish diffusion and lattice distortion effect caused by the increase in entropy.

Ultrafast high-temperature sintering of bulk oxides

Conventional ceramic sintering often requires high temperatures and hours of duration to achieve near full density. Herein, we adopted an innovative ultrafast high-temperature sintering (UHS) technique to densify several representative bulk oxides within an isothermal duration of 30 s and a total sintering period of several minutes, and revealed the relationship between ceramic structures and processing parameters. These achievements demonstrate the great potential of the UHS technique in the rapid densification of ceramics. Moreover, this flexible technique is expected to accelerate the material screening rate and facilitate the development of novel materials.