Nature of low frequency dielectric permittivity behaviour in weakly and moderate concentrated KTaO3:Li

Abstract

The low frequency complex dielectric permittivity (′’ and ″) have been studied in the frequency-temperature region 100 Hz - 1 MHz and 5 K - 300 K in weakly (x <0.01) and moderatly (x < 0.1) concentrated K_1-xLi_x TaO₃ (KLT). The ″'(T) dependencies reveal one or two well-pronounced temperature maxima depending on the Li concentration. Only one maximum in the ″'(T) and ″ (T) behaviour is present in the weakly doped KLT, whose magnitude strongly depends on frequency. For the moderatly Li concentration an additional relaxation temperature maximum emerges with a larger potential barrier of the dipolar reorientation. In both cases the magnitudes of the permittivity maxima with temperature appear to be very large, which can not be explained only by the Li⁺ off-centre reorientations themselves. We present a theory, which considers two types of the Li⁺ related relaxors coupled to the soft TO mode that describes quantitatively the phenomena observed.     

Electrical conduction and dielectric relaxation in polyethylene terephthalate succinate

Electrical conduction and complex permittivity are examined in polyethylene terephthalate succinate, focusing on their relations to dielectric relaxation processes. Both the real and imaginary parts of the complex permittivity, namely, the dielectric constant ε′_r and the dielectric loss factor ε′′_r, increase with a decrease in frequency, especially at high temperatures. They are both ascribed to the transport of ionic mobile carriers. Namely, the carrier transport forms a conduction current that should contribute to ε′′_r. On this occasion, if charge exchange does not occur at the two electrodes, heterocharge layers should be formed before the electrodes. This should increase the charge density on the electrodes, thus contributing to ε′_r. In addition to the increase in ε′_r and ε′′_r due to mobile ions, two relaxation processes, one due to micro‐Brownian motion of dipoles and the other due to orientation and magnitude change of the dipole moment induced by two end groups in the polymer main chain, are observed. Corresponding to these two relaxation processes, two thermally stimulated discharge current (TSDC) peaks appear. The two TSDC peaks as well as the increments in ε′_r and ε′′_r become larger when the crystallinity of the sample decreases. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 1–8, 2010; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20956     

THE ELECTRICAL AND STRUCTURAL PROPERTIES OF Li2CO3 DOPED 0.7(Ba,Sr)TiO3-0.3MgO CERAMICS

ABSTRACT

We have studied the formation and characterization of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃-0.3MgO ceramics for the low temperature sintering and microwave applications. In this study 1 ～ 5 wt% of Li₂CO₃ was added to the 0.7(Ba,Sr)TiO₃-0.3MgO ceramic materials to reduce the sintering temperature. The MgO contents, which added in this experiment, play a role of improving dielectric permittivity such as low frequency dispersion and low loss tangent.

In this paper, we will discuss the crystalline properties, dielectric properties, and the microstructures of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃- 0.3MgO ceramics. No pyro phase was observed in the X-ray diffraction method. Very weak frequency dispersion of dielectric permittivity was observed from the 1 kHz to 1 MHz range. Different grain sizes of Li₂CO₃ doped 0.7(Ba_0.5Sr_0.5)TiO₃-0.3MgO ceramics were observed through the SEM methods.     

Dielectric behavior of Bi2/3Cu3Ti4O12 ceramic and thick films

The paper reports on synthesis, sintering and microstructure of Bi_2/3Cu₃Ti₄O₁₂, a lead-free, high-permittivity material with internal barrier layer capacitor behavior. Complex impedance and capacitance of the ceramic and thick films were studied as a function of frequency (10 Hz–2 MHz) and temperature (−170 to 400°C). Dc electrical conductivity of the samples was measured in the temperature range 20–400°C. Broad and high maxima of dielectric permittivity versus temperature plots were observed reaching 60,000 for ceramic and 5,000 for thick films. The maxima decrease and shift to higher temperatures with increasing frequency. Two arcs ascribed to grains and grain boundaries were found in the plots of imaginary part versus real part of impedance. Analysis of the impedance spectra indicates that Bi_2/3Cu₃Ti₄O₁₂ ceramic could be regarded as electrically heterogeneous system composed of semiconducting grains and less conducting grain boundaries. The developed thick film capacitors with dielectric layers based on Bi_2/3Cu₃Ti₄O₁₂ exhibit dense microstructure, good cooperation with Ag electrodes, high permittivity up to 5,000 and relatively low temperature coefficient of capacitance in the temperature range 100–300°C. Broad maxima in the dielectric permittivity versus temperature curves may be attributed to Maxwell–Wagner relaxation.     

Effect of cobalt doping on the dielectric response of Ba0.95Pb0.05TiO3 ceramics

Dielectric response of Ba_0.95Pb_0.05TiO₃ ceramics doped with 0.1 and 1 wt.% of Co₂O₃, synthesized by conventional high-temperature method, was studied in wide temperature and frequency range. The temperature dependences of the real and the imaginary parts of dielectric permittivity of the ceramics were compared with those of BaTiO₃ and Ba_0.95Pb_0.05TiO_3. The addition of Co³⁺ ions results in a broadening of dielectric anomalies related to the transition to paraelectric cubic phase, and the structural transition between the tetragonal and the orthorhombic phases. At low temperatures (125–200 K) the dielectric absorption of Co-doped Ba_0.95Pb_0.05TiO₃ ceramics was found to exhibit relaxor-like properties. The dielectric response has been found to contain the contributions characteristic of fluctuations of the polar nanoregion boundaries and reorientations of the dipole moments between allowed directions in the nanoregions in the rhombohedral and the orthorhombic phases. The behavior speaks in favor of ordering of polar defects in the host lattice of Ba_0.95Pb_0.05TiO₃ in a form polar nanoregions_.     

Dielectric Properties of Yttrium Vanadate Crystals from 15 K to 295 K

Yttrium Vanadate (YVO₄) is a birefringent crystal, which has similar dielectric constant as that of Sapphire. In this paper we have reported the measurement of the real part of permittivity and loss tangent of YVO₄ crystal in the temperature range 15–295 K at a frequency of 16.3 GHz. We have used the dielectric post resonator technique for the microwave characterisation of the YVO₄ dielectric rod. The multifrequency Transmission Mode Q-Factor (TMQF) technique has been used for data processing and hence precise values of permittivity and loss tangent are achieved. Easily machineable YVO₄ is characterized by low losses at microwave frequencies. At temperature of 15 K and frequency of 16.3 GHz the permittivity was 9.23 and loss tangent was 2 × 10^{− 5}. YVO₄ is identified as a potential candidate to replace expensive Sapphire in many microwave applications.     

Relaxation Motion and Possible Memory of Domain Structures in Barium Titanate Ceramics Studied by Mechanical and Dielectric Losses

The relaxation motion and memory effect of domain structures have been investigated using mechanical and dielectric loss measurements in BaTiO₃ ceramics with grains sizes varied from 1 m to 50 m. The measurements of mechanical loss, elastic modulus, dielectric loss and permittivity show that each phase transition induces a loss peak and an anomaly in the dielectric constants and elastic modulus, furthermore, a number of relaxation loss peaks due to ferroelectric domains in the samples with large grain have been observed. All the relaxation peaks can be analysed by Arrhenius relationship for a wide range of frequency from 10^–2 to 10⁶ Hz. The activation energies of relaxation peaks have been determined as 0.92 eV, 0.68 eV, 0.47 eV, and 0.29 eV for the peaks located in the tetragonal, orthorhombic, and rhombohedral phase, with Arrhenius perfactor in the order of 10^–13 s. Moreover, one relaxation process is insensitive to ferroelectric phase transitions, and it can exist in all the ferroelectric phases. This implies a possible memory effect of ferroelectric domain structures. Such a motion of domain wall is limited in fine-grained materials. Effect of vacuum annealing on the relaxation peak in the tetragonal phase is also studied to clarify the mechanisms of the peak. These relaxation peaks could be explained by the interaction between different domain walls and the diffusion of oxygen vacancy in the domains.     

Effects of Processing Conditions on the Dielectric Properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

There have been a number of recent reports of anomalously large permittivities (ε _r ≈ 10⁴) in the material CaCu₃Ti₄O₁₂. The dielectric spectra is characterized by a large, relatively temperature independent permittivity near room temperature which exhibits a dielectric relaxation above 100 K. The crystal structure of CaCu₃Ti₄O₁₂ can be described as pseudo-perovskite with a cubic unit cell with a lattice constant of 7.391 Å. The ubiquitous occurrence of this dielectric behavior in ceramics, single crystals, and thin films suggests that the polarization is not related to a simple conducting grain/insulating grain boundary-type system. While the precise origin of the dielectric response is not entirely clear, in this work it is shown that processing conditions have a significant influence on the room temperature dielectric properties. Specifically, the permittivity and loss exhibit a strong dependence on the oxygen partial pressure and sintering time. In fact, studies of the effects of sintering time and supporting evidence from capacitance-voltage measurements conclusively show that there is no direct relationship between the permittivity and grain size, as is the case in classical boundary layer systems. Lastly, with aliovalent doping the room temperature dielectric properties can be optimized to provide a high permittivity (ε _r ～ 8,000) dielectric with relatively low loss (tan δ < 0.05 at 1 kHz).     

Simultaneous preparation and densification of lead magnesium niobate-based ferroelectrics

Abstract

Ferroelectric lead titanate-doped lead magnesium niobate ceramics have been prepared and densified simultaneously by a new reaction sintering process. At low temperatures, the pyrochlore phase is firstly formed, which induces the expansion of the specimens. When the perovskite phase begins to produce at elevated temperatures, the specimens rapidly shrink, indicating the occurrence of reaction sintering. After sintering at 850°C, monophasic Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ solid solutions with high density are obtained. With an increase in the doping amount of PbTiO₃, the maximum dielectric permittivity and the Curie temperature increase; however, the relaxor characteristics of Pb(Mg_1/3Nb_2/3)O₃ become weakened. With the addition of 15 mol% PbTiO₃, the maximum dielectric permittivity is over 25000 at 1 kHz. It is concluded that monophasic and densified Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ with high dielectric permittivity are successfully prepared by the newly developed reaction sintering process.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

High-k Ceramic Materials Based on Nonferroelectric AFe1/2B1/2O3 (A-Ba,Sr,Ca; B-Nb,Ta,Sb) Perovskites

Very high values (10³–10⁴) of the dielectric permittivity showing a weak temperature dependence and a Debye-like frequency dependence were observed in ceramics of AFe_1/2B_1/2O₃ (A-Ba,Sr,Ca; B-Nb,Ta,Sb) perovskites. Such behavior is inherent to so-called high-k materials. The analysis of the experimental data have shown that a Maxwell-Wagner relaxation is likely to be a main source for the phenomenon observed.     

Barium strontium titanate and non-ferroelectric oxide ceramic composites for use in phased array antennas

Abstract

A ceramic ferroelectric phase shifting device has been demonstrated using Ba_1?xSr_xTiO₃ (BSTO) ceramics. ¹ As part of an effort to optimize the device performance, various composites of BSTO and other nonelectrically active oxide ceramics have been formulated. In general the composites have reduced dielectric constants, ε′, where ε = ε′-iε″ and reduced loss tangents, tan δ, compared to BSTO. The low dielectric constant and low loss tangent reduce the overall impedance mismatch and insertion loss of the device. In addition, the tunability (change in the dielectric constant with applied voltage) is maintained at a relatively high level (15% with an applied electric field of 1.5 V/μm) for dielectric constants of 200. The combination of electronic properties of these materials offer substantially higher operating frequencies, 10 GHz and above. The microstructures including grain size and phase analysis have been examined using SEM and EDX. X-ray diffraction has been used to identify the presence of any secondary phases formed in the composites. The analysis of the phase formation and compositional variations will be related to the electronic properties of the materials.     

The effect of Sb on the electrical and magnetic properties of Ni-Zn ferrites prepared by sol–gel autocombustion method

Polycrystalline Ni-Zn ferrites with a well-defined composition of Ni_0.4Zn_0.6Fe_2-xSb_xO₄ synthesized using sol–gel method. Morphological characterizations on the prepared samples were performed by high resolution transmission electron and field emission scanning electron microscopy. The powders were densified using microwave sintering method. The room temperature complex permittivity (ε′ and ε″) and permeability (μ′ and μ″) were measured over a wide frequency range from 1 MHz–1.8 GHz. The real part of permittivity varies as ‘x’ concentration increases and the resonance frequency was observed at much higher frequencies and there is a significant decrease in the loss factor (tanδ). The electrical resistivity and permeability of NiZn ferrites increased with an increase of Sb content. As the concentration of ‘x’ increases from 0 to 0.08 the saturation magnetisation decreases. The saturation magnetization (M_s)?≈?52.211 A.m²/Kg for x?=?0 at room temperature. The room temperature electro paramagnetic resonance (EPR) were studied.     

Dielectric and nonlinear electrical behaviors of Ce-doped CaCu3Ti4O12 ceramics

We present a remarkable effect of Ce doping on the dielectric and nonlinear electric properties of CaCu₃Ti₄O₁₂ ceramics. Microstructure analysis indicates that Ce-doped CCTO ceramics are composed of large and small grains. Our results indicate that only 2.5 at. % Ce doping can suppress the dielectric permittivity in CaCu₃Ti₄O₁₂ by up to two orders of magnitude (from 10⁴ to 10² ), and the nonlinear varistor characteristics disappear completely, which may be due mainly to the Cu⁺ ions?? disappearing caused by the Ce doping.     

Dielectric properties and scaling behavior of lithium tungsten phosphate glasses

In the present study a series of ternary (30 Li₂O, (70-x) P₂O₅, xWO3) glasses were prepared and their dielectric properties and ac conductivity were investigated. The measurements have been taken in the frequency range from 100?Hz to 100?kHz and over the temperature range from 296?K to 578?K. The temperature dependence of ac conductivity can be adequately explained by considering the contributions from mixed ionic and electronic mechanisms. In the studied glasses it is found that the ac conductivity increases with increasing frequency. By investigating the relation between temperature and the frequency exponent “s” of the power law σ_ac?=?Aω^s, it is found that the Correlated Barrier Hopping model (CBH) is appropriate for describing the conduction mechanism in the samples. In an attempt to investigate the universality of ac conductivity in these glasses, it is found that the data obtained follow Rolling scaling model. When considering the dielectric properties, it is found that the M″vs. M′ plots give master Cole-Cole curves at all temperatures. These results can be considered as an indication of the presence of space charge or accumulation of charges in some regions inside the samples. The relation between M″/ M_max″ and f/f _p represent a master plot at different temperatures. These scaling suggest the existence of a distribution of potential wells, in which the carriers are trapped.     

Temperature Dependence of Microwave and THz Dielectric Response in Srn + 1TinO3n + 1 (n = 1–4)

The microwave, near-millimetre and infrared (IR) dielectric response of Sr_{n + 1}Ti_nO_{3n + 1} (n = 1–4) Ruddlesden-Popper homologous series was studied in the temperature range 10 to 300 K. Remarkable softening of the polar optical mode was observed in Sr₄Ti₃O₁₀ and Sr₅Ti₄O₁₃ which explains the increase in microwave permittivity and dielectric loss upon cooling. However, both samples have a distinct content of SrTiO₃ dispersed between SrO layers. It is proposed therefore that the observed soft mode originates from the SrTiO₃ microscopic inclusions.     

Improved ferroelectricity of strained SrTiO3 thin films on sapphire

Magnetron sputtered and laser deposited SrTiO₃ thin films are deposited on CeO₂ buffered sapphire substrates. Their structural properties are investigated and correlated to the dielectric properties of the SrTiO₃ films. It is shown, that the biaxial compressive strain imposed by the substrate on the ferroelectric films leads to a considerable increase of the permittivity and tunability of SrTiO₃ thin films in technically relevant temperature regimes. Generally, the permittivity and tunability decreases with increasing strain. However, the ferroelectric phase transition of the SrTiO₃ films is shifted to higher temperatures compared to that of single crystalline SrTiO₃. As a consequence, the permittivity of the films is larger than that of undistorted SrTiO₃ single crystals for small strain (Δa/a < 0.005) and temperatures above the Curie temperature. Furthermore, a linear dependence of the loss tangent and the tunability on the permittivity is observed, which indicates, that all three properties are affected by the same mechanism that itself is affected by the lattice strain.     

Dielectric Properties and Sintering Characteristics of CaTiO<Subscript>3</Subscript>-(Li<Subscript>1/2</Subscript>Nd<Subscript>1/2</Subscript>)TiO<Subscript>3</Subscript> Ceramics

The dielectric properties and the sintering effect upon microstructure of (1–x) CaTiO₃-x(Li_1/2Nd_1/2)-TiO₃ Ceramics are investigated in this paper. Nd³⁺ and Mg^{2 +} ions co-substitution for Ca^{2 +} on A site improves the sintering characteristic of CaTiO₃ ceramics with forming orthorhombic perovskite structure. The structure of (1 – x) CaTiO₃-x(Li_1/2Nd_1/2)TiO₃ changes from orthorhombic to tetragonal as (Li_1/2Nd_1/2)TiO₃ addition increasing. Limited solubility of (Li_1/2Nd_1/2)TiO₃ in CaTiO₃ forming a part solid solution compound achieves the adjustment of for CaTiO₃ at low sintering temperature. The proper dielectric properties with = 78, tan = 0.0006, = +7 ppm/C are obtained for 0.8Ca_0.67(Nd,Mg)_0.22TiO₃-0.2(Li_1/2Nd_1/2)TiO₃ ceramics.     

Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

Several SrTiO₃ (STO) thin films without electrodes processed by pulsed laser deposition, of thicknesses down to 40 nm, were studied using infrared transmission and reflection spectroscopy. The complex dielectric responses of polar phonon modes, particularly ferroelectric soft mode, in the films were determined quantitatively. The compressed epitaxial STO films on (100) La_0.18Sr_0.82Al_0.59Ta_0.41O₃ substrates (strain 0.9%) show strongly stiffened phonon responses, whereas the soft mode in polycrystalline film on (0001) sapphire substrate shows a strong broadening due to grain boundaries and/or other inhomogeneities and defects. The stiffened soft mode is responsible for a much lower static permittivity in the plane of the compressed film than in the bulk samples.     

Comparative Characteristics of Na0.5K0.5NbO3 Films on Pt by Pulsed Laser Deposition and Magnetron Sputtering

Ferroelectric Na_0.5K_0.5NbO₃ (NKN) thin films were grown on the Pt₈₀Ir₂₀ polycrystalline substrates by pulsed laser deposition (PLD) and radio frequency-magnetron sputtering (RF) technique using the same stoichiometric Na_0.5K_0.5NbO₃ ceramic target. X-ray diffraction proved both PLD- and RF-made Na_0.5K_0.5NbO₃/Pt₈₀Ir₂₀ films are single phase and have preferential c-axis orientation. Temperature dependence of dielectric permittivity reveals the presence of two phase transitions around 210 and 410°C. Capacitance vs. applied voltage C-V @ 100 kHz, I-V, and P-E hysteresis characteristics recorded for the vertical capacitive structures yielded loss tanδ = 0.026 and 0.016, tunability about 44.5 and 30% @ 100 kV/cm, Ohmic resistivity 6.7 × 10¹² Ω·cm and 0.2 × 10¹² Ω·cm, remnant polarization 11.7 and 9.7 μC/cm², coercive field 28.0 and 94.6 kV/cm for PLD- and RF-films, respectively. Piezoelectric test carried out in hydrostatic conditions showed piezoelectric coefficient d _H = 21 for PLD-NKN and 15 pC/N for RF-NKN film.