Huiling Gong

Core-satellite BaTiO3@SrTiO3 assemblies for a local compositionally graded relaxor ferroelectric capacitor with enhanced energy storage density and high energy efficiency

Dielectric capacitors with high energy density and low energy loss are of great importance in high power electric and electronic systems. Traditional BaTiO3 (BT) or its solid solutions have been widely explored as high energy density materials owing to their notably high dielectric constants. However, these materials often suffer from significant drawbacks of strong dielectric nonlinearity, low breakdown strength and high hysteresis loss, limiting the energy storage density and energy utilization efficiency. In this study, by using core-satellite structured nanocubic SrTiO3 (ST) decorated BT assemblies, a composite capacitor with enhanced breakdown strength and weaker dielectric nonlinearity was successfully fabricated in contrast with the pure ferroelectric BT ceramic, resulting in elevated energy storage density and high energy efficiency as extracted from the polarization-electric field loops. The mechanism behind the improved electric and dielectric performances was discovered to be the remarkable suppression of grain size owing to the existence of the ST nanocubes and also the ferroelectric relaxor behaviors arising from the local compositionally graded structure due to the controlled sintering and modulated diffusion of Sr. This work provided a new approach for fabrication of dielectric materials with promising high energy density and low loss.

Dielectric Enhancement in Graphene/Barium Titanate Nanocomposites

GN/BT nanocomposites were fabricated via colloidal processing methods, and ceramics were sintered through two-step sintering methods. The microstructure and morphology were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. XRD analysis shows that all samples are perovskite phases, and the lattice parameters a and c almost decrease linearly with the increase of graphene nanosheets. The dielectric properties were tested by using precision impedance. The maximum dielectric constant at the Curie temperature for the nanocomposites with graphene addition of 3 wt % is about 16,000, almost 2 times more than that of pure BaTiO3 ceramics. The relaxation, band structure, density of states, and charge density distribution of GN/BT superlattices were calculated using first-principles calculations for the first time, and results showed the strong hybrid interactions between C 2p states and O 2p and Ti 3d orbitals.

Electrical and reliability characteristics of Mn-doped nano BaTiO3-based ceramics for ultrathin multilayer ceramic capacitor application

Nano BaTiO3-based dielectric ceramics were prepared by chemical coating approach, which are promising for ultrathin multilayer ceramic capacitor (MLCC) applications. The doping effects of Mn element on the microstructures and dielectric properties of the ceramics were investigated. The degradation test and impedance spectroscopy were employed to study the resistance degradation and the conduction mechanism of Mn-doped nano-BaTiO3 ceramic samples. It has been found that the reliability characteristics greatly depended on the Mn-doped content. Moreover, the BaTiO3 ceramic with grain size in nanoscale is more sensitive to the Mn-doped content than that in sub-micron scale. The addition of 0.3 mol. % Mn is beneficial for improving the reliability of the nano BaTiO3-based ceramics, which is an important parameter for MLCC applications. However, further increasing the addition amount will deteriorate the performance of the ceramic samples.

Comparative study of metastable phase formation in the immiscible Cu–W system by ab initio calculation and n-body potential

The lattice constants and cohesive energies of some possible metastable Cu–W compounds are obtained by ab initio calculation and the formation of a metastable phase at Cu75W25 is predicted for the equilibrium immiscible Cu–W system. The prediction is in agreement with the fact that a metastable hcp phase was indeed observed in the Cu75W25 multilayer films upon ion beam mixing. Furthermore, some of the ab initio calculated properties are used in deriving an n-body Cu–W potential under the embedded atom method. The constructed Cu–W potential is then used to predict the phase stability of the metastable Cu–W phases over the entire composition and the prediction is also supported by some experimental observations.

Unusual alloying behavior at the equilibrium immiscible Cu-Nb interfaces

Assisted by ab initio calculation, an n-body potential is derived for the equilibrium immiscible Cu–Nb system. Based on the potential, molecular dynamics simulations reveal that among the nine Cu∕Nb interfaces stacked by possible combinations of (100), (110), and (111) atomic planes, only the (111)Cu∕(110)Nb and (100)Cu∕(110)Nb interfaces could remain stable up to a temperature of 873K, and that introducing a disordered interlayer at these two interfaces could trigger solid-state amorphization, suggesting that the Cu∕Nb close-packed plane does not serve as a growth barrier, which was frequently the case observed previously in many other systems.

Preparation and Characterization of X8R Fine-Grained Dielectric Ceramics

Fine-grained BaTiO3-based nonreducible ceramics were obtained by a conventional mixing method and the dielectric and electrical properties were characterized. The average grain size was less than 200 nm. The materials provided a dielectric constant of 1300 and satisfied the Electronic Industries Association (EIA) X8R specification. Fine-grained ceramics showed a better performance under a direct current (DC) field at high temperatures, compared with coarse-grained ceramics. Impedance analysis was conducted to determine the activation energy and to evaluate the ionic transference number. Moreover, capacitance variation under a DC field was also largely improved for fine-grained ceramics and relative mechanisms were examined.

Sintering behavior and reliability characteristics of BaTiO3-based ceramics prepared by different methods

Fine-grained BaTiO3-based ceramics with core–shell structures were prepared using the chemical coating method and the solid-state method. The sintering behavior and microstructure evolution were investigated for the samples prepared using different methods. The dielectric properties of modified BaTiO3 ceramics were also investigated, and the TEM–EDS results provided a detailed explanation for the different temperature coefficient of capacitance behaviors. In addition, the insulation resistivity for the ceramic samples under elevated temperature and high voltages was observed. The highly accelerated lifetime test results indicate that the samples prepared by the chemical coating method exhibited higher insulation resistance and a smaller degradation rate. Impedance analysis demonstrated that the grain boundary activation energy was much higher for the samples prepared by the chemical coating method.

Interfacial diffusion behavior in Ni-BaTiO 3 MLCCs with ultra-thin active layers

The interfacial structure and diffusion behavior between the dielectric layers (BaTiO3) and internal electrode layers (Ni) in X5R-type multilayer ceramic capacitors (MLCCs, from −55°C to 85°C, at a temperature capacitance coefficient within ±15%) with ultra-thin active layers (T = 1–3 µm) have been investigated by several microstructural techniques (SEM/TEM/HRTEM) with energy-dispersive x-ray spectroscopy (EDS). In the MLCC samples with different active layer thicknesses (1–3 µm), weak interfacial diffusion was observed between BaTiO3 and Ni. It was also found that the diffusion capability of Ni into the BaTiO3 layer was stronger than that of BaTiO3 to the Ni electrode, which indicated that the diffusion of Ni was the dominant factor for the interfacial diffusion behavior in the ultra-thin layered MLCCs. The mechanism of Ni diffusion is discussed in this study as well.