Single-phase Ceramic Materials Research Articles

Synergistic effect of dielectric resonance and plasma oscillation on negative permittivity behavior in La1-xSrxMnO3 single-phase ceramic

Ceramic-based composites with negative permittivity property can be realized by dielectric resonance or plasma oscillation, but it is still a challenge to integrate two kinds of negative permittivity into a single-phase material. In this study, the La1-xSrxMnO3 (LSMO) single-phase ceramic (x = 0.3, 0.4, 0.5, 0.6 and 0.7) was successfully fabricated by a sol-gel auto-combustion and pressureless sintering method. Just by adjusting the content of Sr2+, the Lorentz-like and Drude-like negative permittivity were simultaneously observed in LSMO ceramic under the synergistic effect of dielectric resonance and plasma oscillation. The amplitude of dielectric resonance was lessened because of the increase of damping factor, and the resonance frequency gradually moved to a lower frequency region with the increase of Sr2+. When the content of Sr2+ reached 0.7, the Drude-like negative permittivity played an overwhelming role due to an enhanced double-exchange effect in La1-xSrxMnO3. Meanwhile, the tanδ of LSMO was as low as 0.085, which was scarcely observed in previous investigations. The generation and regulation mechanism of negative permittivity was further clarified. This work opened up a way to adjust negative permittivity behavior in single-phase ceramic material which has great value in high-power microwave filters and wireless ring resonators.

Conductivity of perovskites La0.9Sr0.1Sc1–x Fe x O3–α (x = 0.003–0.47) in oxidizing and reducing atmospheres

The conductivity of single-phase ceramic materials based on proton-conducting perovskite La0.9Sr0.1ScO3–α containing from 0.3 to 47 at % Fe in the scandium sublattice has been studied. Synthesis has been performed by burning with ethylene glycol. Measurements have been carried out by the four-probe (500–900°C) and impedance (100–500°C) methods in oxidizing and reducing atmospheres, as well as at different pressures \({p_{{O_2}}}\) (2.1 × 104–10‒15 Pa) and \({p_{{H_2}O}}\) (0.04–2.5 kPa). Substitution of scandium with iron significantly decreases the proton conductivity.

Novel SnO2 ceramic surge absorbers for low voltage applications

To make SnO2–CoO–Nb2O5–Cr2O3–Y2O3–Bi2O3 ceramics suitable for low voltage varistor applications, the amount of Bi2O3 addition to a well-known SnO2-based composition for high voltage varistors was optimized. The addition of 0.15mol% Bi2O3 has resulted in a single phase ceramic material with the average grain size of 30μm and electrical properties useful for low voltage varistors. The obtained SnO2 low voltage varistors exhibit excellent electrical properties as surge absorbers.

Microstructure evolution of ceramic materials during solidification from melts in high-gravity combustion synthesis

A variety of bulk ceramic materials were prepared via melt-casting by high-gravity combustion synthesis, and the microstructure evolution during solidification was investigated. Most single-phase ceramics like Al 2O 3, YAG, and MgAl 2O 4 showed a faceted crystal shape because they have higher melting entropies. The cooling condition had an evident influence on the solidification process, resulting in different microstructures. As an example, a strongly textured structure consisting of well-arranged faceted crystals was observed in the surface layer of Al 2O 3 samples. Compared with single-phase ceramic materials, multiphase eutectic ceramics have lower melting temperatures and can provide more opportunity for tailoring microstructures. From the experimental results, the microstructure of eutectic ceramics was related with the volume fraction of each component, and various eutectic structures with submicron interphase spacings were produced such as lamellae, fibers, and three-dimensional interpenetrating frameworks. No cracks were found at the interface between different phases in the eutectic ceramics, indicating an excellent interfacial bonding strength.

Synthesis of cordierite by crystallization from MgO–Al2O3–SiO2 tempered glass

Physicochemical research is conducted to examine the phase formation under dynamic and static annealing of MgO–Al 2 O 3 –SiO 2 tempered glass (composition range adjacent to stoichiometric cordierite). Thirty glass compositions are examined in this system and the range is determined over which single-phase ceramic materials with the lowest possible thermal expansion coefficient are synthesized under specific crystallization conditions.

Conductivity of the nanostructured ceramic material Zr0.88Sc0.1Ce0.01Y0.01O1.955 prepared from mechanically activated powders

The structure of the Zr0.88 Sc0.1Ce0.01Y0.01O1.955 solid solution, a candidate for the use as a solid electrolyte in fuel cells with a low temperature, has been investigated using x-ray powder diffraction and Raman spectroscopy. Single-phase ceramic materials have been produced from powders prepared by the mechanochemical synthesis from ZrO2 nanoprecursors purified of the impurities introduced during grinding of commercial zirconia. The solid solution has a rhombohedral structure at room temperature owing to the partial ordering of oxygen vacancies. The electrical conductivity of the ceramic materials sintered at temperatures below 1570 K exhibits a hysteresis due to the delay of the martensitic transition from the cubic phase to the rhombohedral phase upon cooling of the sample. The nanostructured ceramic materials are characterized by a high mechanical strength and unusually close values of the activation energies for bulk and grain-boundary electrical conduction.

The heat pulse transport in single phase ceramics

The phonon transport through the single phase ceramic materials at low temperatures is studied. Simple transport equation is derived under the assumption that phonons move ballistically within the grains. The phonon effective diffusion coefficient is shown to depend crucially upon how the phonons pass the contact between neighbouring grains. The heat pulse signal maximum arrival time can either rise of fall as a function of the generator's temperature provided the excitation level is low. The low temperature part of heat conductivity should deviate in such ceramics from T 3 law. The experimental results on heat pulse propagation prove both the aforementioned possibilities.

On subcritical crack growth in ceramics as influenced by grain size and energy-dissipative mechanisms

A model is presented concerning the possibility that an unstable crack becomes stabilized by the initiation of some energy-dissipative mechanism if the stress intensity reaches a threshold value KLD. Depending on the toughness and the grain size distribution of the material, different crack propagation modes may occur. For sintered alumina, measurements of the fracture induced photon emission can be explained in terms of this model, substantiating in this way the assumption that microcracking is the dominating form of energy dissipation in this single phase ceramic material.