GUAN Lili, ZHAO Jiaming, WANG Huitao, XU Fuxing

(School of Materials Science and Engineering, Inner Mongolia University of Science and Technology,

Baotou 014010, Inner Mongolia, China)

Extended abstract:

[Background and purposes] Dense lanthanum chromite-based ceramics exhibit promising properties, such as excellent electrical conductivity and high-temperature stability, rendering them attractive for various applications. However, achieving densification of lanthanum chromite is challenging, as conventional sintering requires high temperatures above 1600 ℃. To address this, Flash Sintering technology (FS) has been applied to the sintering of lanthanum chromite. This approach not only suppresses Cr volatilization and grain growth during ultra-rapid heating and holding stages, thereby promoting densification, but also has advantages of the high electrical conductivity of lanthanum chromite-based materials to initiate FS under relatively moderate conditions. This helps overcome the issues of poor densification and high sintering temperatures associated with the traditional methods. Furthermore, Sr doping can be used to modify material properties and further reduce both the FS environment temperature and the required electric field strength. In this study, DC Flash Sintering is employed to achieve rapid densification of Sr-doped lanthanum chromite at ambient temperatures below 60 ℃. The FS behavior and the influence of electric field parameters on the microstructure are studied, providing insights and references for the low-temperature rapid sintering of similar high-temperature ceramics.

[Methods] Sr-doped lanthanum chromite-based ceramic powders, La_0.95Sr_0.05CrO₃ and La_0.9Sr_0.1CrO₃ (abbreviated as LSC05 and LSC10, respectively), were synthesized via a sol-gel method. The as-prepared powders were calcined at 800 ℃ for 3 h to obtain the target phases. Subsequently, the powders were pressed into dog-bone-shaped green bodies with a central circular hole. Pt wire electrodes were attached, while a direct current (DC) electric field was applied. The variations in current and voltage during the process were monitored in real-time using two multimeters connected in series and parallel, respectively. Crystal structure of the synthesized materials was characterized by using X-ray diffraction (XRD) over 20°–90° at a scanning speed of 5 (°)·min^−1. Surface microstructure of the sintered ceramics was examined using a field-emission scanning electron microscope (FE-SEM). Additionally, elemental chemical states and oxygen species were analyzed by using X-ray photoelectron spectroscopy (XPS).

[Results] A DC electric field of 30 V·cm⁻¹ was applied to the LSC10 sample. As the ambient temperature increased, the power density rose, followed by an abrupt surge in power density at a specific environment temperature, which was identified as the Flash-Sintering point. At different limiting current densities, the Flash-Sintering point remained within the range of 27–30 ℃. After the power density peaked, the sample exhibited intense incandescence and rapid heat generation and the power supply mode switched to constant current output with a corresponding drop in voltage. After sintering for 60 s, a single-phase lanthanum chromite ceramic was obtained, with homogeneous elemental distribution. Furthermore, at 30 V·cm⁻¹, the required ambient temperature was only 29 ℃ for LSC10. As the doping level increased, the threshold electric field for FS decreased from 35 V·cm⁻¹ to 20 V·cm⁻¹, which is attributed to the doping-induced changes in material properties. Analysis of the Cr 2p and O 1s XPS spectra indicated that, with increased doping content of Sr, more Cr³⁺ was oxidized to Cr⁶⁺ (reaching 30.8%) to maintain charge neutrality. Concurrently, the ratio of adsorbed oxygen to lattice oxygen increased from 0.77 to 0.98, suggesting the formation of more oxygen vacancies on the carrier surface. These changes collectively enhanced electrical conductivity of the sample. Consequently, at an applied electric field, more carriers or defects migrate at an accelerated rate, thereby lowering the threshold conditions required for Flash Sintering. Taking LSC10 as an example, the actual temperature of the sample at different electric fields was significantly higher than the ambient temperature, with the peak temperature increasing as the electric field was strengthened. A characteristic inflection point in power dissipation occurred at about 7 W·cm⁻³, which falls within the reported range of 1–15 W·cm⁻³ for the onset of Flash Sintering. For LSC10 sintered at 30 V·cm⁻¹ for 60 s, the microstructure exhibited a characteristic trend. With increasing current density, both the grain growth and densification were improved. Full densification was achieved at 90 A·cm⁻², while signs of oversintering appeared at 100 A·cm⁻². At DC field, slight gradients were observed across different regions of the samples. The area near the positive electrode showed slightly larger grains and higher density, as compared with the center and the region near the negative electrode. This variation is likely ascribed to the hole conduction mechanism of p-type carriers and the specific migration behavior of oxygen vacancies.

[Conclusions] It is demonstrated that Sr doping effectively enables the low environment temperature sintering of lanthanum chromite. Through the application of a direct current (DC) electric field, the occurrence of Flash Sintering and densification were achieved at temperatures below 60 ℃. The sintered samples exhibited a single-phase lanthanum chromite without secondary phases and showed uniform elemental distribution. As the Sr doping level is increased from 0.05 to 0.10, the electric field strength required to initiate FS decreased from 35 V·cm⁻¹ to 20 V·cm⁻¹, while the corresponding FS onset temperature at the same field was also reduced. This reduction can be attributed to charge compensation mechanisms induced by the lower-valence ion doping. To maintain charge neutrality, a portion of Cr³⁺ was oxidized to higher-valence Cr⁶⁺ (reaching 30.8%), accompanied by the transformation of lattice oxygen into adsorbed oxygen and an increase in oxygen vacancy content. These changes collectively enhance the electrical conductivity of the material, thereby lowering the threshold conditions for Flash Sintering. Furthermore, at different electric field strengths, the characteristic inflection point in power density consistently occurred at about 7 W·cm⁻³. A fully dense ceramic was obtained after sintering for 60 s at a current density of 90 A·cm⁻². At DC field, the region near the positive electrode exhibited slightly larger grain size and higher density, as compared with the central and negative electrode areas, although the overall microstructural variation remained modest.

Key words: flash sintering; densification; Sr doped lanthanum chromate; high-valent Cr; oxygen vacancy