Insulating Boundary Layer Research Articles

Ferroelectric tungsten bronze-based ceramics with high-energy storage performance via weakly coupled relaxor design and grain boundary optimization

A multiscale regulation strategy has been demonstrated for synthetic energy storage enhancement in a tetragonal tungsten bronze structure ferroelectric. Grain refining and second-phase precipitation (perovskite phase) are introduced in the BaSrTiNb2-xTaxO9 ceramics by regulating the composition and sintering process. Disordered polarization and distribution, chemical inhomogeneity, and insulating boundary layers are achieved to provide the fundamental structural origin of the relaxation characteristic, high breakdown strength, and superior energy storage performance. Thus, an ultrahigh energy storage density of 12.2 J cm−3 with an low energy consumption was achieved at an electric field of 950 kV cm−1. This is the highest known energy storage performance in tetragonal tungsten bronze-based ferroelectric. Notably, this ceramic shows remarkable stability over frequency, temperature, and cycling electric fields. This work brings new material candidates and structure design for developing of energy storage capacitors apart from the predominant perovskite ferroelectric ceramics.

Electromagnetic field effects in the microwave sintering of electrically conductive powders

Rapid sintering of compacted electrically conductive powders under heating by intense microwave radiation is investigated within a one-dimensional simulation model. The model accounts for the effective dielectric and magnetic properties of the powder material, electromagnetic field distribution, heat losses by thermal radiation, and densification. It is demonstrated that metal powders with insulating boundary layers on particles have low dielectric and magnetic losses, which results in a resonant character of microwave absorption. The influence of the interparticle contacts is investigated by introducing an effective electrical conductivity of the boundary layers, which can depend on the density. It is demonstrated that the losses in the boundary layers can stabilize the microwave heating process, facilitating rapid densification.

Inter-grain tunnelling in the half-metallic double-perovskites Sr2BB'O6(BB'= FeMo, FeRe, CrMo, CrW, CrRe)

The zero-field conductivities ($\sigma$) of the polycrystaline title materials, are governed by inter-grain transport. In the majority of cases their $\sigma$(T) can be described by the "fluctuation induced tunneling" model. Analysis of the results in terms of this model reveals two remarkable features: 1. For \emph{all} Sr$_2$FeMoO$_6$ samples of various microstructures, the tunneling constant (barrier width $\times$ inverse decay-length of the wave-function) is $\sim$ 2, indicating the existence of an intrinsic insulating boundary layer with a well defined electronic (and magnetic) structure. 2. The tunneling constant for \emph{all} cold-pressed samples decreases linearly with increasing magnetic-moment/formula-unit.

Three-dimensional magnetohydrodynamic simulations of the Helicity Injected Torus with Steady Inductive drive

The Helicity Injected Torus with Steady Inductive drive (HIT-SI) [P. E. Sieck, W. T. Hamp, V. A. Izzo, T. R. Jarboe, B. A. Nelson, R. G. O’Neill, A. J. Redd, and R. J. Smith, IEEE Conference Record—Abstracts. 31st IEEE International Conference On Plasma Science (IEEE Catalog No. 04CH37537), 2004, p. 160] is a spheromak driven by steady inductive helicity injection (SIHI) and consists of the toroidally symmetric spheromak confinement region and two nonsymmetric helicity injectors. The three-dimensional (3D) magnetohydrodynamic code NIMROD [A. H. Glasser, C. R. Sovinec, R. A. Nebel, T. A. Gianakon, S. J. Plimpton, M. S. Chu, and D. D. Schnack, Plasma Phys. Controlled Fusion, 41, A747 (1999)] is used to simulate HIT-SI operation, but the code’s toroidally symmetric boundary requires a creative treatment of the injectors. Sustained HIT-SI operation is simulated with nonaxisymmetric boundary conditions. In driven simulations at low Lundquist number S no n=0 fields are generated as a result of relaxation of the predominantly n=1 injector fields until the injectors are quickly shut off. At S=500, an n=0 component arises due to relaxation during sustainment. As S is increased further, the ratio of n=0 (equilibrium) fields to n=1 (injector) fields increases. The effects of a thin insulating boundary layer on the plasma decay time are also discussed.

Mechanism for Developing the Boundary Barrier Layers of CaCu3Ti4O12

CaCu3Ti4O12 (CCTO) was prepared by solid‐state reaction and identified by X‐ray diffractometry. The evolution of the microstructure was observed by scanning electron microscopy (SEM). It was found that discontinuous grain growth developed during sintering, and large abnormal grains played a very significant role in the dielectric behavior. Cu ions segregated to the boundaries and CuO located at the triple‐point sites of the abnormal large grains were observed by electron energy loss and energy‐dispersive X‐ray spectroscopy, respectively. Moreover, two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under the thermal etching conditions at 960°C for 72 min, which solves the contradiction related to the interpretation of the giant dielectric response between ceramic and single‐crystal CCTO. Complex impedance spectroscopy was used to analyze the conductivity of ceramic CCTO, which suggests that it consists of conducting domains with two kinds of insulating barrier layers of domain and grain boundaries. The insulating domain and grain boundaries were attributed to orderly arranged dislocations and segregation of Cu ions, respectively. The conduction of CCTO was found to be related to the porosity, the grain size, and the thickness of the insulating boundary layers. For a sample sintered at 1065°C for 3 h, the anomalously low resistivity and temperature‐dependent dielectric constant were due to the fact that domain boundaries were not substantially formed. The possible reasons for the development of barrier layers and the variation of the dielectric constant with the sintering time are discussed. A barrier‐layer model with dielectric response based on the Maxwell–Wagner type of relaxation for ceramic CCTO is proposed. Two kinds of dielectric responses occur, depending on the microstructure: they are dominated by the domain and domain boundary and by the grain and grain boundary for large grains and fine grains, respectively.

On the escape of cosmic rays from radio galaxy cocoons

The escape rate of cosmic ray (CR) particles from radio galaxy cocoons is a problem of high astrophysical relevance: e.g. if CR electrons are stored for long times in the dilute relativistic medium filling the radio cocoons ( radio plasma in the following) they are protected against Coulomb losses and thus are able to produce a significant non-thermal Comptonisation signature on the CMB. On the other hand, CR protons and positrons which leak out of radio plasma can interact with the ambient medium, leading to characteristic gamma ray radiation by pion decay and pair annihilation. In order better understand such problems a model for the escape of CR particles from radio galaxy cocoons is presented here. It is assumed that the radio cocoon is poorly magnetically connected to the environment. An extreme case of this kind is an insulating boundary layer of magnetic fields, which can efficiently suppress particle escape. More likely, magnetic field lines are less organised and allow the transport of CR particles from the source interior to the surface region. For such a scenario two transport regimes are analysed: diffusion of particles along inter-phase magnetic flux tubes (leaving the cocoon) and cross field transport of particles in flux tubes touching the cocoon surface. The cross field diffusion is likely the dominate escape path, unless a significant fraction of the surface is magnetically connected to the environment. Major cluster merger should strongly enhance the particle escape by two complementary mechanisms. i) The merger shock waves shred radio cocoons into filamentary structures, allowing the CRs to easily reach the radio plasma boundary due to the changed morphology. ii) Also efficient particle losses can be expected for radio cocoons not compressed in shock waves. There, for a short period after the sudden injection of large scale turbulence, the (anomalous) cross field diffusion can be enhanced by several orders of magnitude. This lasts until the turbulent energy cascade has reached the microscopic scales, which determine the value of the microscopic diffusion coefficients.

Analysis of impingement freezers performance

In recent years, several manufacturers have developed mechanical freezing tunnels incorporating impingement technology. This technology consists in directing high velocity air jets against the food surface to break the insulating boundary layer that surrounds the product. This operation helps to reduce processing times considerably, giving freezing times similar to those provided by cryogenic equipment. In addition, operating costs are similar to those of traditional mechanical equipment. The objective of the present work is to study heat and mass transfer that occur in impingement freezing equipment. An extensive bibliographic search was carried out to find correlations that adequately predict heat transfer coefficients. A previously developed numerical model was used to analyze the simultaneous heat and mass transfer during freezing in such equipments. The predicted values satisfactorily fitted experimental data from the literature.

An Equivalent Cicuit for CuO Modified Surface Barrier Layer Capacitors

Nb2O5-doped (Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 (BSTZ) is calcined at 1170°C for the complete formation of ABO3 phases. After calcination, CuO is added to BSTZ as a liquid phase promoter and an insulating boundary layer material. After sintering in a reducing atmosphere, the fired ceramics are cooled to an annealing temperature, then the cermics are annealed in air. The loss tangents of the fabricated ceramics increase with amount of CuO addition, but the maximum grain growth and maximum dielectric constant will reveal in 1 wt% CuO-added BSTZ. For BSTZ with different amount of CuO addition, the effective dielectric constant increases with the sintering temperature and decreases critically with annealing temperature and annealing time. When 60 min is used as annealing time, lower the annealing temperature is, more broad dielectric constant-temperature curves are. It is believed that the semiconductive BSTZ, which is residual in the center of the reoxidized BSTZ, will dominate the result.

Characterization of Insulating Boundary Layers of BL-Capacitors

A microstructure of a barrier-layer capacitor with Bi2O3 diffused SrTiO3 has been studied by means of SEM, TEM, and XPS. Samples for SEM observations were prepared by electrolytic etching, which allowed a visual characterization of insulating boundary layers. They were O2-diffused layers wider than Bi-diffused layers, which observed as lattice-disorder region by TEM. Metal Bi was observed in addition to Bi2O3 by XPS. Barrier-layer model unique in case of Bi2O3 was investigated for oxygen supplyment from Bi. And we revealed it's propriety using standard free energy of metal oxide.

Influence of sintering and annealing temperature on grain boundary barrier layer capacitors in a modified reduction-reoxidation method

To produce grain boundary barrier layer capacitors with a simpler fabrication process and more-stable dielectric characteristics, a modified reduction-reoxidation method was developed. Nb2O5-doped (Ba0.8 Sr0.2)(Ti0.9 Zr0.1)O3 (BSTZ) was calcined at 1170 °C for complete formation of ABO3 phases. After calcination CuO was added to BSTZ as a liquid-phase promoter and insulating boundary layer material. The ceramics were sintered in a reducing atmosphere, and then the fired samples were annealed in air to reoxidize the reduced CuO to form insulating layers. The dielectric constant of the fabricated capacitors was decreased for higher annealing temperature, longer annealing time and smaller grain size. The loss tangent of the fabricated capacitors was increased for BSTZ with more CuO added, and was almost unchanged with annealing temperature, annealing time and grain size because of the existence of an insulating boundary layer material (CuO).

Distribution of Electric Resistance of Dielectric Semiconductive Ceramics

The distribution of electric resistance in dielectric semiconductive ceramic can be observed using optical microscope and scanning electron microscopes by electrolytic etching in dilute acid. Using this method, insulating layers of BaTiO3 surface semiconductive ceramic capacitor and SrTiO3 boundary layer capacitors can be successfully observed. It is considered that the latter insulating boundary layers are different from Bi diffused layers.