Journal of Advanced Dielectrics

Electric-induced resistive switching effects have attracted wide attention for future nonvolatile memory applications known as resistive random access memory (RRAM). RRAM is one of the promising candidates because of its excellent properties including simple device structure, high operation speed, low power consumption and high density integration. The RRAM devices primarily utilize different resistance values to store the digital data and can keep the resistance state without any power. Recent advances in the understanding of the resistive switching mechanism are described by a thermal or electrochemical redox reaction near the interface between the oxide and the active metal electrode. This paper reviews the ongoing research and development activities on the interface engineering of the RRAM devices. The possible switching mechanisms for the bistable resistive switching are described. The effects of formation, composition and thickness of the interface layer on the resistive switching characteristics and consequently the memory performance are also discussed.

In this article, two concepts of temperature, i.e., Burns temperature for relaxor ferroelectrics and quantum temperature scale for quantum paraelectrics, are reviewed briefly. Since both temperatures describe the deviation of the dielectric constant from Curie–Weiss law, their relationship is discussed. Finally the concept of quantum temperature scale is extended to demonstrate the evolution process of quantum paraelectric behavior to relaxor ferroelectric behavior.

Specific features of the dielectric response of colloid systems with electrically interacting components (polar liquid matrix and solid spherical inclusions) have been revealed. A theory was developed to describe dielectric dispersion and anisotropy of the liquid phase caused by the electric field of surface charges on the solid particles. It was found that the local dielectric permittivity in different directions can vary over a wide range and strongly depends on the strength of the inner field. The existence of the interphase interaction leads to the formation of thin structured liquid layers in the contact area of two phases and, therefore, to a decrease in the average dielectric permittivity of the considered system. Nonuniform structure of the polar liquid was taken into account while the derivation of a mixing formula which clearly demonstrates the contribution of the interphase interaction to the dielectric response of the investigated colloid system.

Based on density-functional calculations, we have studied possible ferroelectric switching path in monodomain single crystal of rhombohedral BiFeO₃, a prototypical multiferroic compound. By carefully studying the behaviors of FeO₆ corner-sharing double-tetrahedrons, we find abrupt changes in total energy and oxygen atomic positions, and therefore polarizations, occur in the ferroelectric switching path of rhombohedral BiFeO₃. Detailed analyses suggest that such behavior might be caused by the frustrated magnetic ordering in the paraelectric phase of rhombohedral BiFeO₃, where three O atoms and the Bi atom are in the same plane perpendicular to the polarization direction. This is supported by the fact that the ferroelectric switching for paramagnetic BiFeO₃ is smooth and has a much lower energy barrier than that of antiferromagnetic BiFeO₃.

The physical properties of the new TeO₂ – B₂O₃ – PbO₂ (TBP)-based glass system, obtained from the usual melt-quench method, have been investigated taking into account the influence of the Pb(Zr_yTi_{1 - y})O₃ (PZT) ferroelectric system. A crystalline-like structure, which characterizes a glass-ceramic-like state, promoted by the ferroelectric phase, has been revealed from the X-ray diffraction results for the sample with higher PZT concentration. On the other hand, results on the dielectric response revealed noticeable dielectric anomalies, which until now have not been reported. In fact, the dielectric and ferroelectric properties suggest the existence of a structural phase transition induced by the PZT crystalline phase, which indeed have been related to the ferroelectric polar-phase regions embedded into the glass matrix.

The CaF₂–4LiF additive was added into SrTiO₃ ceramics in order to decrease the sintering temperature for compact pulse power application. The crystalline structure, microstructure and energy storage performance of sintered ceramics were studied. Incorporating CaF₂–4LiF additive to SrTiO₃ ceramics contributes to a notably enhancement of the energy storage density. The great enhancement in energy storage density occurred due to the notable increase in breakdown strength and the refinement of microstructure. With 2 at% additive, the samples exhibited an average breakdown strength of 31.8 kV/mm, and an energy storage density of 1.212 J/cm³ which is about 1.4 times higher than pure SrTiO₃.

Spontaneous polarization was found in Ta₂O₅ dielectric ceramics prepared by laser sintering technique. After heat corrosion, 90° and 180° domains in the ceramics was observed by field emission scanning electron micrographs. The crystal structure of the laser-sintered ceramics at RT was determined as monoclinic phase, which indicates the lattice distortion existed in the ceramics. The variation of O-Ta-O bending vibration mode was deduced from the variation of phonon mode at the low wave-number (57 cm^-1 and 250 cm^-1). The enhancement of effective electric field for tantalum atoms due to the lattice distortion was determined by calculation of charge distribution coefficients. The distortion of crystal structure due to the phase transformation from higher symmetric orthorhombic phase to lower symmetric monoclinic phase explained spontaneous polarization in the laser-sintered Ta₂O₅ ceramics.

High dielectric-permittivity (Ca_0.5Cu_0.5)TiO₃-based ceramics have been prepared by a sol-gel method combined with a solid state sintering process. The results indicate that the additives of H₃BO₃ have a remarkable effect on the sintering temperature, microstructure and dielectric properties. High density (Ca_0.5Cu_0.5)TiO₃ bulk ceramics can be obtained after sintering at 900°C. The as-sintered ceramics show high dielectric constants (~1000), and low losses (~0.05). The dielectric properties are nearly independent of frequency and temperature in a wide range. The activation energy is calculated as about 0.50 eV by impedance spectrum method.

The comparison of the low temperature sintering and the microwave dielectric properties of Li₂MgSiO₄ (LMS) ceramics prepared by citrate gel (CG) route and solid state (SS) ceramic route are discussed in this paper. The LMS prepared using CG route sintered at 1175°C/2 h has ε_r = 5.3 and tan δ = 1 × 10⁻³ at 9 GHz. The sintering temperature of LMS is lowered to 950°C with the addition of 5 wt% lithium magnesium zinc borosilicate glass and has ε_r = 5.6 and tan δ = 2 × 10^-3 at 9 GHz. The amount of glass required to lower the sintering temperature of ceramics prepared using CG are slightly higher than that of SS ceramic route. The LMS ceramics prepared using SS ceramic route shows excellent microwave dielectric properties with low sintering temperature compared to CG route.

In this paper, Eu³⁺-doped CaTiO₃ (CTO) nanoparticles were prepared by a sol-gel method. Three different charge compensation mechanisms were realized by substituting Eu ion at different sites. (i) Ca site substitution with Ca vacancy compensation (CTO-A); (ii) Ti site substitution with O vacancy compensation (CTO-B); (iii) simultaneous substitution at both Ca and Ti sites with self-compensation (CTO-AB). Strongly related to the site substitution, the photoluminescent (PL) intensities were increasing in the sequence of CTO-B > CTO-AB > CTO-A. The absorbance spectra analysis of pure CTO nanoparticles showed absorption in the UV light region, while the absorption of Eu-doped CTO samples exhibited the blue shift owing to the presence of Eu³⁺. Moreover, the photocatalytic activities of samples for decomposing dyes were reverse to the sequence of PL intensities. The PL properties and photocatalytic activity of the Eu³⁺-doped CTO nanoparticles were thus discussed in detail from the viewpoints of the microstructures and charge transfer-related processes.

The electrostrictive properties above T_{ε max} represented by the field-related and polarization-related M and Q coefficients have been measured for PbZr_{1 - x}Sn_xO₃ single crystals with aniferroelectric phase transitions. The investigations presented in this paper have proved that the M₁₁(T) and Q₁₁(T) runs bring direct information of the pre-transitional effects connected with the co-existence of local polar objects with nonpolar matrix. Due to this co-existence, nonlinear Q₁₁(T) dependence in the neighborhood of T_C has been detected. Observed even in a wide temperature above T_C, this co-existence is a natural feature of the material with ABO₃ perovskite structure that undergoes structural transition to polar phase.

Effects of the Co₃O₄ nanoparticle on the phase formation, microstructure and dielectric properties of barium titanate ceramics are investigated in this study. Co₃O₄-doped BaTiO₃ ceramics were prepared by the conventional mixed oxide method followed by normal sintering in air. Systematic studies of X-ray diffraction (XRD), scanning electron microscopy and dielectric spectroscopy with varying doping levels were performed. The cell parameters, tetragonality and crystallite size of doped ceramics also were calculated from XRD data. Results showed that the single phase of BaTiO₃ gave no evidence of unwanted peak forms in any of the samples. Cubic phase occurred after adding Co₃O₄ at 0.25 mol%. Various grain sizes and shapes were found in this system. Finally, adding Co₃O₄ in BaTiO₃ ceramics resulted in shifting of the Curie point to a lower temperature. The composition, x = 0.25, showed different dielectric behavior, which related to appearance of the cubic phase and fine grain microstructure.

The 0.96Na_0.5Bi_0.5TiO₃–0.04K_0.5Bi_0.5TiO₃ (NKBT96/4) lead-free single crystal was successfully grown by top-seeded solution growth method. The effective segregation coefficient for K was calculated to be 0.20. X-ray powder diffraction measurement showed that the as-grown crystal possesses rhombohedral perovskite structure at room temperature. X-ray rock curve measure indicated that the as-grown single crystal owned relatively higher quality. The NKBT96/4 single crystal exhibits apparent diffused dielectric behavior, where the diffusiveness index α is 1.79 for the [001]-oriented crystal planes. A notable thermal hysteresis, ΔT_h, of 25°C was disclosed for the tetragonal antiferroelectric-tetragonal paraelectric phase transition in the dielectric property studies between heating and cooling. The highest remnant polarization, P_r, 16.8 μC/cm², was obtained at 100°C. The electromechanical coupling coefficient k_t of the [001]-oriented crystal planes at room temperature was determined to be 38% using resonance–antiresonance technique. The room temperature piezoelectric constant d₃₃ reaches 120 pC/N.

In order to develop new lead-free pieozoelectric/ferroelectric materials, the y(K_0.5Na_0.5)NbO₃ – (1 - y)[(1 - x)Bi_0.5K_0.5TiO₃ – xBaTiO₃] solid solution system is synthesized and the partial ternary phase diagram in the KNN-rich region with compositions of y = 0.98 to y = 0.96 is established. All the compositions studied show a gradual phase change from a tetragonal to an orthorhombic structure as the BT component increases. A mixture of tetragonal and orthorhombic phases is found in the composition range of x = 0.25 to 0.60 and y = 0.97, and x = 0.50 to 0.75 and y = 0.96, indicating a ternary morphotropic phase boundary region. The piezoelectric and ferroelectric properties of the 0.97(KNN)–0.03[0.60BKT–0.40BT] ceramics are characterized. The increase in remnant polarization as compared with pure KNN shows improved ferroelectricity by forming the ternary solid solution.

New lead-free compositions of AMO₃ perovskite type were synthesized in the BaTi_1-x(Nb_1/2Sc_1/2)_xO₃(0 ≤ x ≤ 1) system, with regards to the comparable electronic configurations of the various 6-coordinated cations. Special solid state reaction was used. The effect of the replacement of titanium by scandium and niobium in the B-site on structural and dielectric properties was investigated. For this, we have used X-ray diffraction and dielectric characterizations. Dielectric measurements performed on ceramics with 0 ≤ x ≤ 0.25 composition revealed that the small change in composition from BaTiO₃ has a strong influence on the dielectric properties: fast decreasing of the ferroelectric transition temperature for low values of x and evolution from classical ferroelectric for 0 ≤ x ≤ 0.075 to relaxor behavior for 0.075 < x ≤ 0.25.

Ternary solid solutions of BiFeO₃, GdFeO₃ and BaTiO₃ have been prepared by solid-state reaction method. Dielectric properties of these ceramics have been characterized in the temperature range between room temperature and 673 K and magnetic properties between 5 K and 300 K. For the understanding of the multiferroic property, the relation between the crystal structures, magnetic transition and ferroelectric transitions with increasing temperature have been analyzed. All ceramic samples show single perovskite phase. When Ba content exceeds 0.2 wt%, the sintering ability is weakened and the phase structure of Bi_1-x-yGd_xBa_yFe_1-yTi_yO₃ (with x = 0.2 and y = 0.1, 0.2, 0.3, 0.4, 0.5) solid solution changes from rhombohedral to tetragonal phase. The maximum ferroelectric transition temperature (T_e of this system was in the range 154–165°C with the dielectric constant peak of 6000 for y = 0.5 at 100 kHz. Well-saturated piezoresponse hysteresis loops were observed for all compositions indicating room temperature ferroelectricity. With increasing Ba content (up to 0.3 wt%) the remanent magnetization M_r increased and the coercive magnetic field decreased.