Low Temperature Synthesis of Single-crystal Alpha Alumina Platelets by Calcining Bayerite and Potassium Sulfate

Single-crystal alpha alumina （α-Al2O3） platelets were synthesized by calcining a powder mixture of bayerite （α-AI（OH）3） and potassium sulfate （K2SO4） at 900℃. The crystalline phase evolutions and morphologies of the samples were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The synthesized samples mainly consisted of single-crystal α-Al2O3 platelets with a diameter of 0.5-1.5 μm and a thickness of 50-150 nm. Moreover, with 3, 5, and 8 wt% （referred to the obtained alumina） α-Al2O3 seeds adding into the powder mixture of bayerite and potassium sulfate, the average diameter of α-Al2O3 platelets can be reduced to 450, 240, and 220 nm, respectively. It is found that the sequence of the phase transformation is the bayerite （α-Al（OH）3） → boehmite （γ-AIOOH） →γ-Al2O3 → α-Al2O3. Further analysis indicated that K2SO4 can promote the phase transformation from α-Al2O3 to α-Al2O3 and the formation of single-crystal α-Al2O3 platelets might be attributed to the liquid phase K3AI（SO4）3.

Facile synthesis of high-entropy zirconate nanopowders and their sintering behaviors

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(Lao.2Ndo.2Smo.2Gdo.2 Ybo.2)2Zr2O7 and(Lao.2Ndo.2Yo.2Euo.2Gdo.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 C 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.

Rapid fabrication of oxygen-deficient zirconia by flash sintering treatment

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(ZrO_(2−x))remains challenging,which severely limits its broad application.In this study,flash sintering treatment was used to fabricate ZrO_(2−x) bulk in very short time of 90 s.Oxygen vacancies were introduced into ZrO2 bulk through electrochemical reduction reactions.The as-prepared black ZrO_(2−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 ZrO_(2−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 ZrO_(2−x) bulk materials with high concentrations of oxygen vacancies,which can also be applied to other materials.