Field-assisted sintering technology has revolutionized material processing by integrating temperature, mechanical, electrical, and magnetic fields to achieve unprecedented densification efficiency and microstructural control. Recent advances in techniques such as hot oscillatory pressing, cold sintering, high/ultra-high pressure sintering, spark plasma sintering, ultrafast high-temperature sintering, and flash sintering have enabled the fabrication of previously unattainable materials, including ultrafine-grained ceramics, nanostructured composites, and functionally graded materials. These materials possess exceptional performances under extreme conditions, expanding applications in aerospace, electronics, energy, and biomedicine. However, the rapid development of these methods has exposed limitations in conventional sintering theory, particularly in describing mass transport and interface evolution under multi-physics coupling. This review systematically examines representative field-assisted sintering technologies and discusses their principles, equipment configurations, and application cases. By analyzing current challenges and opportunities, we aim to bridge fundamental understanding with industrial implementation, providing insights for the design and fabrication of next-generation high-performance materials.
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Open Access
Review
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High-entropy oxides (HEOs) have attracted much attention in the field of electrochemistry due to their distinctive structural characteristics and unique properties. The multiple-principal elements in HEOs offer the multiple redox pairs and multiple possible active sites, which can enhance the energy storage capacity and the electrocatalytic performance. Although the notable progress is achieved in the development of HEOs electrodes, their electrochemical properties should be further improved to meet the requirements of high-performance supercapacitors and OER electrocatalysts. The abundant active sites for the Faradic redox reactions and short pathways for charge transportation could be constructed through the design of novel HEOs with advanced microstructures, thus improving the electrochemical properties. As advanced microstructures, a hollow structure has a great promise for energy storage and conversion because it can provide more accessible storage sites, more catalytic centers and a larger electrode/electrolyte contact area. It is thus expected that the construction of hollow structure is an alternative route to significantly promote the electrochemical properties of HEOs electrode materials. However, it is difficult to prepare the HEOs with a hollow structure due to the complexity of the high-entropy system.
In this work, a hollow spherical high-entropy perovskite oxide of La(Cr0.2Mn0.2Fe0.2Ni0.2Cu0.2)O3(HS-HEPs) was prepared by microwave solvothermal process and subsequent calcination treatment. The as-prepared HS-HEPs exhibited the excellent electrochemical performance when used as an electrode material for supercapacitors and OER electrocatalysts due to the advantages resulted from the combination of high-entropy perovskite and special hollow structure.
HS-HEPs were prepared by microwave solvothermal process and subsequent calcination treatment. Typically, 0.134 mmol Cr(NO3)3·6H2O, 0.134 mmol Mn(NO3)2·4H2O, 0.134 mmol Fe(NO3)3·9H2O, 0.134 mmol Ni(NO3)2·6H2O, 0.134 mmol Cu(NO3)2·3H2O, and 0.5 mmol La(NO3)3·6H2O were dissolved in 30 mL ethanol under stirring for 1 h to obtain the homogeneous solution. Afterwards, 60 mg of carbon spheres were added in the solution under ultrasonic treatment for 30 min. The resulting mixture was transferred to a 50 mL microwave digestion vessel. The vessel was heated in a microwave oven at a power of 210 W for 10 min. Subsequently, the obtained mixture was centrifuged, washed with deionized water, and dried in a vacuum drying oven at 70 ℃ for 12 h. Finally, the obtained precursor powder was calcinated in a tube furnace with a heating rate of 3 ℃/min at 650 ℃ for 2 h to acquire HS-HEPs.
The crystalline structure of the sample was determined by X-ray diffraction (XRD, D8 Davinci, Bruker Co., Germany). The morphology and microstructure of sample were characterized by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi Co. Ltd., Japan) equipped with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, 2100F, JEOL Co., Japan). The X-ray photoelectron spectra were obtained by a X-ray photoelectron spectrometry (XPS, ESCALab 250, Thermo VG Co., USA). The supercapacitor and OER performance of the sample were measured on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China).
The as-prepared samples display a cubic perovskite crystalline structure and a hollow sphere morphology. According to the XPS analysis, the variable oxidation states of Cr, Fe and Mn present in the HS-HEPs, which benefits the Faradaic redox reactions and increases the capacitance. In addition, the existence of high concentration of oxygen vacancies in HS-HEPs is beneficial to enhancing the capacitance and OER activity. Based on the GCD curve, the specific capacitance of HS-HEPs is estimated to be 406 F/g at 1 A/g. After GCD cycles of 5000 at a current density of 5 A/g, 65% capacitance is retained, implying a good long-term electrochemical stability. An asymmetric supercapacitor device (HS-HEPs//AC) with a two electrode configuration is assembled. A maximum energy density of 39.4 W·h/kg is achieved at power density of 746 W/kg. The OER activity of HS-HEPs is evaluated by a linear sweep voltammetry (LSV) polarization curve in 1 mol/L KOH aqueous solution using a standard three-electrode system. The overpotential of HS-HEPs is identified as 347 mV versus RHE for achieving a current density of 10 mA/cm2, which is smaller than that of commercial IrO2 (372 mV). The HS-HEPs possess the excellent electrochemical performance, which can be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure.
High-entropy La(Cr0.2Mn0.2Fe0.2Ni0.2Cu0.2)O3 hollow spheres with a perovskite crystalline structure were prepared by microwave solvothermal process and subsequent calcination treatment. The HS-HEPs possessed the excellent electrochemical performance, which could be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure. Based on the electrochemical performance, HS-HEPs could be used as supercapacitor electrode material and OER electrocatalysts. This work could provide a strategy to design and prepare high-entropy oxides with a hollow sphere structure, having promising applications in energy storage and conversion.
Open Access
Rapid Communication
Issue
The introduction of oxygen vacancies into zirconia is an effective strategy for enhancing its light absorption ability and photocatalytic performance. However, the cost-efficient preparation of oxygen-deficient zirconia (ZrO2−x) remains challenging, which severely limits its broad application. In this study, flash sintering treatment was used to fabricate ZrO2−x bulk in very short time of 90 s. Oxygen vacancies were introduced into ZrO2 bulk through electrochemical reduction reactions. The as-prepared black ZrO2−x exhibited excellent optical absorption capability, a small band gap (2.09 eV for direct and 1.67 eV for indirect), and a reduced conduction band energy, which is ascribed to the generation of oxygen vacancies and reduction of Zr cations. The as-prepared ZrO2−x showed remarkable photocatalytic activity due to excellent solar light absorption and low recombination rate of electron‒hole pairs. Flash sintering treatment provides a cost-efficient approach for rapidly fabricating ZrO2−x bulk materials with high concentrations of oxygen vacancies, which can also be applied to other materials.
Open Access
Research Article
Issue
The challenge in synthesizing high-entropy ceramic (HEC) nanopowders is to suppress severe grain coarsening and particle agglomeration, which occur at elevated temperatures. This challenge could be addressed by the polyacrylamide gel method. In this work, single-phase high-entropy (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7 and (La0.2Nd0.2Y0.2Eu0.2Gd0.2)2Zr2O7 nanopowders without agglomeration were successfully synthesized using the polyacrylamide gel method for the first time. The results showed that phase composition, particle size, and agglomeration degree of the nanopowders were greatly influenced by the molar ratio of acrylamide (AM)/Zr and calcination temperature. These as-synthesized high-entropy zirconate (HEZ) nanopowders could be sintered into fully dense ceramics at 1500 ℃ for 2 h. These HEZ nanopowders showed a phase transformation from a defect-fluorite phase to a pyrochlore phase with the increase of sintering temperature. Additionally, two-step sintering of these nanopowders was conducted, and the HEZ ceramics with fine grains were prepared. The polyacrylamide gel method is simple and easily operated, which is a facile approach of producing the HEC nanopowders with excellent sinterability.
High-entropy oxides (HEO) are a novel material with the unique electrochemical properties, which are stabilized due to the high configurational entropy. In this paper, a high-entropy (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O nanopowder with the sizes of 40–65 nm and a rocksalt structure was synthesized by a polyacrylamide gel method and subsequent calcination at 900 ℃ for 2 h in the presence of a mole ratio of acrylamide/metal cations of 120:1. The results show that the temperature of forming a single-phase (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O nanopowder decreases with the increase of the mole ratio of acrylamide/metal cations. The HEO nanopowder has a specific capacitance of 402 F/g at a current density of 1 A/g. A rate capability of 62% appears at 20 A/g. The capacitance retention is 61% after 2000 cycles at a current density of 5 A/g. It is indicted that (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O nanopowder could be used as a prospective electrode material for supercapacitors.
Rapid preparation of dense BiFeO3–BaTiO3 lead-free piezoelectric ceramics at low temperatures is beneficial to preventing the Bi volatilization, reducing the energy consumption and developing novel materials with some unique properties. In this paper, dense BiFeO3–BaTiO3 ceramics with good ferroelectric and piezoelectric properties were prepared in an electric field of 200 V/cm with a current limit of 280 mA via reactive flash sintering of multiphase precursor powders (i.e., Bi2O3, Fe2O3, TiO2 and BaCO3) at 415 ℃ for 30 s. The solid-state reaction in sintering occurs during the flash event. The current limit has an effect on the phase transformation and densification. It is indicated that the reactive flash sintering technology could prepare the single-phase and dense ceramic material through flash sintering of starting precursors in one step, thus providing an effective approach for the rapid production of ceramic materials.
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