Yu Hui Huang

From core–shell Ba0.4Sr0.6TiO3@SiO2 particles to dense ceramics with high energy storage performance by spark plasma sintering

Electrostatic capacitors with high charge/discharge speed and long cycle lifetime play an essential role in advanced electronic and electrical power systems. However, their low energy density (usually less than 1 J cm−3) has limited their development. In this study, core–shell structured Ba0.4Sr0.6TiO3@SiO2 nanoparticles were synthesized by a wet-chemical method, and dense ceramics with enhanced energy storage density were fabricated by spark plasma sintering (SPS). In situ TEM and DSC techniques were used to investigate the interface reaction between Ba0.4Sr0.6TiO3 and SiO2. The results revealed that the secondary phase ((Ba,Sr)2TiSi2O8) was unavoidable, but it could be suppressed due to the low sintering temperature and short sintering period used in the SPS technique. With the increasing SiO2 coating amount, the polarization decreased monotonously, whereas the dielectric breakdown strength increased to a maximum of 400 kV cm−1 and then decreased slightly. The enhancement in the dielectric breakdown strength was ascribed to the formation of nanosized (Ba,Sr)2TiSi2O8 coating on Ba0.4Sr0.6TiO3 grains, whereas the subsequent degradation of performance might have been caused by the sub-micrometric secondary phase precipitated at the grain boundaries. The effect of microstructure on the breakdown strength was further confirmed by numerical simulation using COMSOL. As a result, the Ba0.4Sr0.6TiO3 ceramics with 8 mol% SiO2 showed a maximum energy storage density of 1.60 J cm−3 at 400 kV cm−1 with an ultrahigh energy efficiency of 90.9%. This study opens an effective way for the design of high-performance dielectric ceramics.

The effects of melamine on the formation of carbon xerogel derived from resorcinol and formaldehyde and its performance for supercapacitor

Carbon aerogel/xerogel can be easily tuned to have hierarchical pores ranging from micropores to macropores. Nitrogen doping is considered to enhance the wettability and conductivity of the carbon electrode, hence improve the electrochemical performance. To prepare N-doped carbon xerogel and study the effects on the structure and the electrochemical performance of resorcinol and formaldehyde derived carbon xerogel, a series of Resorcinol-Melamine-Formaldehyde derived N-doped carbon xerogel were prepared by a facile sol-gel process and ambient drying method. With the increasing amount of melamine, the inside channels become larger, which contributes to faster ion transport and smaller charge transfer resistance (Rct). The Nitrogen content in carbon xerogel is also increased, enhancing the capacitance of carbon electrode by pseudocapacity effect, while damaging the rate performance by introducing more defects and larger degree of disorder. As a result, the best electrochemical performance is achieved in the RM-6-4 sample (resorcinol:melamine = 6:4), showing the largest capacitance of 139 F g-1.

Oxygen-vacancy-induced reversible control of ferroelectric polarization in Ba4Eu2Fe2Nb8O30 ceramics

Ba4Eu2Fe2Nb8O30 ceramics with a tetragonal tungsten bronze structure were synthesized under different sintering and annealing conditions. The maximum polarization of air-sintered samples decreased from 4.0 μC/cm2 to 3.6 μC/cm2 after annealing in an oxygen atmosphere. In contrast, the maximum polarization of oxygen-sintered samples improved from 3.5 μC/cm2 to 7.0 μC/cm2 after annealing in a nitrogen atmosphere. A reversible enhancement of remanent polarization (0.2 μC/cm2) induced by oxygen vacancies was also detected by the positive up negative down method. This reversible control of ferroelectric polarization was assumed to be associated with the structure modulation induced by oxygen vacancies. To prove this, selected area electron diffraction patterns were studied. The results showed that the oxygen vacancies would influence the direction of O-octahedral tilting and promote the commensurate modulation in Ba4Eu2Fe2Nb8O30, which contributed to the enhanced ferroelectric properties.Ba4Eu2Fe2Nb8O30 ceramics with a tetragonal tungsten bronze structure were synthesized under different sintering and annealing conditions. The maximum polarization of air-sintered samples decreased from 4.0 μC/cm2 to 3.6 μC/cm2 after annealing in an oxygen atmosphere. In contrast, the maximum polarization of oxygen-sintered samples improved from 3.5 μC/cm2 to 7.0 μC/cm2 after annealing in a nitrogen atmosphere. A reversible enhancement of remanent polarization (0.2 μC/cm2) induced by oxygen vacancies was also detected by the positive up negative down method. This reversible control of ferroelectric polarization was assumed to be associated with the structure modulation induced by oxygen vacancies. To prove this, selected area electron diffraction patterns were studied. The results showed that the oxygen vacancies would influence the direction of O-octahedral tilting and promote the commensurate modulation in Ba4Eu2Fe2Nb8O30, which contributed to the enhanced ferroelectric properties.