Effect of CeO2 and Nd2O3 on the Microstructure and Microwave Dielectric Properties of (Zr0.8,Sn0.2)TiO4 Ceramics

(Zr_0.8,Sn_0.2)TiO₄ (ZST) ceramics were prepared by solid-state reaction method with 1 wt% ZnO and 0.5–1.5 wt% CeO₂ or Nd₂O₃ as sintering aids. The effect of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of ZST ceramics were investigated. The dielectric constant (?_r) and temperature coefficient of the resonance frequency (τ_f) were not significantly affected by the addition of these additives. The unloaded quality factors (Q_u) were effectively promoted by CeO₂ and Nd₂O₃ additions. ?_r values of 40 and 38.3, Qxf_o values of 57,500 and 59,300 were obtained for the samples sintered with 1.5 and 0.5 wt% of CeO₂ and Nd₂O₃ respectively. The improvement in Qxf_o value is primarily attributed to the increase in uniform grain size and density.     

Effects of dysprosium oxide addition in bismuth sodium titanate ceramics

This research focuses on the preparation and characterization of dysprosium-doped bismuth sodium titanate (BNT) ceramics. The compounds were prepared using the conventional mixed-oxide method. The amount of dysprosium oxide used was varied from 0 to 2 at.%. The mixed powders were calcined at 800 °C and checked for phase purity using X-ray diffraction technique. The powders were then cold-pressed into small pellets which were subsequently sintered at 1050 °C for 2 h. The results from density measurement and SEM micrographs showed that highly dense and high-purity ceramics were obtained. The grain size of Dy-doped samples was found to decrease with increasing Dy content. Compared to pure BNT, the addition of Dy₂O₃ in BNT ceramics slightly increased the dielectric constant values near room temperature. In addition, the Dy doping resulted in a more diffused transition temperature, less frequency dependence of the dielectric constant and very low values of the dielectric loss.     

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.     

Mixed Electronic-Ionic Conductivity of Cobalt Doped Cerium Gadolinium Oxide

The effect of small amounts (<5 mol %) of cobalt oxide on the electrical properties of cerium oxide solid solutions has been evaluated. Ce_0.8Gd_0.2O_2-x (CGO) powder with an average crystallite size of 20 nm served as a model substance for the electrolyte material with a high oxygen ion conductivity and low electronic conductivity in its densified state. Doping the CGO powder by transition metal oxides (MeO) with concentrations below 2 mol % did not change the ionic conductivity nor the electrolytic domain boundary. After long sintering times (2 h) at temperatures above 900°C, MeO and CeO₂ form solid solutions. However, short sintering times or high dopant concentrations lead to an electronic conducting grain boundary phase short circuiting the ionic conductivity of the CGO grains. Choosing proper doping levels, sintering time and temperature allows one to tailor mixed conducting oxides based on CGO. These materials have potential use as electrolytes and/or anodes in solid oxide fuel cells and ion separation membranes.     

Enhancement of dielectric properties by optimization of sintering condition in tungsten–bronze structured Ba5SmTi3Nb7O30 ferroelectric ceramics

In this work, polycrystalline Ba₅SmTi₃Nb₇O₃₀ tungsten–bronze structured ferroelectric ceramics were synthesized by solid-state reaction technique at different sintering temperatures and durations. The X-ray diffractograms reveal the formation of the compounds in orthorhombic crystal system. The density of the compound is observed to increase with increase in sintering temperature and duration. Scanning electron microscopy (SEM) has been used for the microstructural investigation. Detailed dielectric properties of the compounds have been studied as a function of frequency and temperature. The variations of dielectric constant $\left( {\varepsilon \prime _r } \right)$ with temperature show that the compounds undergo a diffuse type ferro-paraelectric phase transition. The dielectric constant is found to increase with the increasing sintering temperature and duration. In all the samples, the variation of dielectric loss (tan δ) with temperature is observed to be almost constant initially but it increases as temperature is increased and a peak is observed only when the material is sintered at higher temperature for longer duration. The frequency dependence of dielectric constant and loss shows a decreasing trend up to nearly 10 kHz and beyond this frequency there is almost no variation. Also, the diffusivities of the samples have been calculated and it is found to increase with increasing sintering temperature and duration.     

Characterization of conduction in PZT thin films produced by laser ablation deposition

Abstract

Both direct current (d.c.) and alternating current (a.c.) conductivity measurements were undertaken on lead zirconate titanate (PZT) films synthesized by laser ablation deposition. Direct current (I) displayed an initial time dependence of the form I ∝ t^?γ (γ ∝ 0.5–1.0). The possible reasons for this time dependence are discussed. At lower temperatures, the a.c. electrical conductivity shows a frequency dependence of the form σ ∝ ω′ which is explained as electrical charge hopping. At higher temperatures, the d.c. component of electrical conductivity becomes dominant, and is accompanied by a strong low frequency dispersion of the dielectric constant. The results are compared to published data on conductivity in SrTiO₃, and discussed in terms of the latest theories for dielectric response of materials.     

Microstructures and electric characteristics of SrNdCoO4 ceramics with K2NiF4 structure

SrNdCoO₄ ceramics with tetragonal K₂NiF₄ structure were prepared by a solid-state reaction process. The frequency and temperature dependence of dielectric and conductive properties were studied. A peak was found at ～325 °C on both dielectric constant (?) and conductivity (σ) plots with temperature. After annealing in O₂, the peak of ? disappeared while that of σ just shifted a little toward high temperature side. So, some oxide vacancies existed in the polycrystalline SrNdCoO₄, which contributed much to ? but influenced σ only slightly in the measured temperature and frequency range. A lower resistivity of 3.7 Ωcm was obtained in SrNdCoO₄ ceramics sintered at 1150 °C.     

Dielectric Properties of Niobium and Lanthanum Doped Lead Barium Zirconate Titanate Relaxor Ferroelectrics

The lead barium zirconate titanate (PBZT) relaxor ferroelectrics are ideal for high voltage capacitor applications due to their high dielectric constant, stability under DC bias, and temperature stability. In this study the composition (Pb_0.65Ba_0.35)(Zr_0.70Ti_0.30)O₃ was selected as the base composition. It exhibited typical relaxor characteristics such as frequency dispersion and diffuse phase transition. The dielectric constant is 6000 at room temperature and remains almost constant under electric field as high as 20 kV/cm. To further enhance the dielectric properties, various amounts of niobium oxide and lanthanum oxide dopants were added to the base PBZT to alter the defect structure and hence the dielectric properties. It was found that the dielectric constant of 1% Nb-doped samples was increased by 20–25% while maintaining similar voltage stability. This increase was attributed to the abnormal grain growth in the Nb-doped sample, and the correlation between microstructure and dielectric constant was drawn through a grain size study. The La addition only caused a monotonic decrease of dielectric constant and slightly improved voltage stability.     

Influence of microstructure on oxide ionic conductivity in doped CeO2 electrolytes

Doped ceria (CeO₂) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for `low (500^∘–650^∘C)’ temperature operation of solid oxide fuel cells (SOFCs). In this study, some rare earth (eg. Gd, Sm, and Dy) doped CeO₂ nano-powders were synthesized via a carbonate co-precipitation method. Fluorite-type solid solution were able to be formed at low temperature, such as 400^∘C and dense sintered bodies were subsequently fabricated in the temperature ranging from 1000^∘ to 1450^∘C by conventional sintering (CS) method. To develop high quality solid electrolytes, the microstructure at the atomic level of these doped CeO₂ solid electrolytes were examined using transmission electron microscopy (TEM). The specimens obtained by CS had continuous and large micro-domains with a distorted pyrochlore structure or related structure, within each grain. We conclude that the conducting properties in these doped CeO₂ systems are strongly influenced by the micro-domain size in the grain. To minimize the micro-domain size, spark plasma sintering (SPS) was examined. SPS has not been used to fabricate dense sintered bodies of doped CeO₂ electrolytes, previously; carbon from the graphite dies penetrates the specimens and inhibits densification. To overcome this challenge, and to be able to produce dense sintered bodies of doped CeO₂ of a grain size that minimizes the microdomain growth, a combination of SPS and CS methods were examined. Using this combined method we report that we were able to produce fully dense specimens with improved conductivity. This is correlated with a reduction in the size of the micro-domains. Consequently we conclude that the control of micro-domain size within the grain structure is a key component in the successful design of electrolyte materials with improved conductivity.     

Fabrication and dielectric properties of Na0.5Bi0.5Cu3Ti4O12 from co-precipitation method

High dielectric Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were firstly prepared by co-precipitation method at low temperature. X-ray diffraction results revealed that pure phase of NBCTO was achieved by calcination at 950 °C for 2 h. Thermo-gravimetric analysis on a dried NBCTO precursor was carried out to study the thermal decomposition process. The microstructure and dielectric properties of NBCTO ceramics sintered at different temperatures were investigated. The results indicate that the sintering temperature has a sensitive influence on the microstructure and dielectric properties. Higher sintering temperature gave rise to increased dielectric constant and dielectric loss of NBCTO samples, and the sample sintered at 975 °C for 8 h exhibits high dielectric constant of 8.3?×?10³ and low dielectric loss of 0.069 at 10 kHz. The dielectric properties were further discussed by the impedance spectroscopy.     

Conductivity and space charge in LDPE containing nano- and micro-sized ZnO particles

DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm^-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm.     

Preparation and dielectric characterization of BaLaAlO4 ceramics

In the present work, BaLaAlO₄ ceramics with orthorhombic structure similar to K₂SO₄ in space group P2₁2₁2₁ were prepared by a solid state sintering process. The dense BaLaAlO₄ ceramics with minor amount of secondary phase have a low dielectric loss and a temperature stable dielectric constant with obvious frequency dependence. A dielectric constant around 15 was obtained at 12 GHz in the present ceramics together with a Qf value over 5,000 GHz.     

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.     

Structural and electrical studies of Ba5LaTi3V7O30 compound

The ferroelectric ceramic Ba₅LaTi₃V₇O₃₀ has been synthesized by solid-state reaction technique. Preliminary X-ray structural analysis confirmed a single-phase formation (orthorhombic crystal system) of the compound. Surface morphology of the compound was studied by scanning electron microscopy (SEM). Detailed studies of electrical properties (i.e., dielectric constant, loss tangent, ac and dc conductivity) as a function of temperature (RT - 773?K) at four different frequencies, 1?kHz, 10?kHz, 100?kHz and 1?MHz show ferroelectric—paraelectric phase transition of the compound. The impedance spectra show two distinctly separated regions in wide temperature range corresponding to grain boundary and grain interior contributions. The activation energy has been evaluated from ac conductivity and dc conductivity following Arrhenius equation is 0.15?eV at 1?MHz and 0.28?eV, respectively. The temperature dependence of electrical conductivity shows that the conductivity increases with increase in temperature suggesting that the compounds have a negative temperature coefficient of resistance (NTCR) behaviour. The conductivity pattern shows that it is strongly frequency dependent and obeys Jonscher’s power relation.     

The High Frequency Properties and Crystallization of PbTiO3 Glass-Ceramics by Sol-Gel Process

PbTiO₃ glass-ceramics were produced by a sol-gel process. DTA and X-ray diffraction analysis provided information about the structural transformations and crystallization temperature. The range of crystallization temperature of PbTiO₃ is larger than that of PbTiO₃ glass-ceramics. By increasing the glass composition, the crystallization temperature was increased. The phase transformation of PbTiO₃ from the cubic to the tetragonal phase depends on the crystal size caused by the different heat treatment temperatures. The dielectric measurements in high frequency range (> 10⁷Hz) exhibited evidence of a relaxation phenomena near 850 MHz. With increasing glass composition, the position of the relaxation frequency was increased. The temperature dependence of the dielectric constant of PbTiO₃ and PbTiO₃ glass-ceramics has an obvious discrepancy. The dielectric constant of PbTiO₃ glass-ceramics shows the characteristic dispersion at the Curie point. It indicated that the glass ceramic sample has a stronger tendency for a diffuse phase transition.     

High dielectric constant in (1 − x)SrTiO3/xCuO composite ceramics

Semiconductive secondary phase CuO was introduced into SrTiO₃ ceramic matrix to yield dielectric composite ceramics with high dielectric constant. The dense composite ceramics could be obtained by sintering at 1050°C in air for 3 h, in which the co-presence of SrTiO₃ and CuO/Cu₂O was confirmed. The dielectric constant of the present composite ceramics increased firstly and then decreased with increasing the content of CuO, and the highest dielectric constant was obtained at x?=?0.4. There were steps and peaks on the curves of dielectric constant vs temperature and dielectric loss vs temperature, respectively, and the peak temperatures of dielectric loss indicated the Debye-type relaxation.     

The electrical properties of Ba1-x SrxTiO3 thin films grown by sputter deposition

Abstract

Ba_1-x Sr_xTiO₃ (BST) thin films were deposited by reactive rf-magnetron sputtering onto Si substrates. The influence of the deposition parameters such as temperature and oxygen ambient on the dielectric constant of the films is presented. BST films deposited at 450°C and optimum conditions exhibited a dielectric constant of approximately 200 at frequencies as high as 1GHz. In addition, the films were found to have leakage current densities of <0.1μAmp/cm² at fields of 5×10⁵V/cm. An extrapolated lifetime greater than 10 years was obtained from stress tests at elevated temperatures and fields. These films compared favorably with published data.     

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.     

Structural, dielectric and conductivity studies of Na2Pb2La2W2Ti4Nb4O30 ferroelectric ceramic

A new single phase orthorhombic ferroelectric ceramic Na₂Pb₂La₂W₂Ti₄Nb₄O₃₀ (NPLWTN) was prepared via high-temperature solid-state reaction method. The grain morphology of the compound was analyzed by scanning electron microscopy (SEM). Studies of dielectric properties (??_r and tan??) of the compound at different frequencies (10²?C10⁶?Hz) in a wide temperature range (300?C700?K) showed multiple phase transitions in it. First phase transition observed at 335?K related to structural type (ferroelectric-ferroelectric) and the second one observed at 536?K is related to the ferroelectric to paraelectric. The ferroelectric property of the compound at room temperature was confirmed by polarization (hysteresis) study. Broadened dielectric peaks at low frequencies were observed above ferroelectric to paraelectric phase transition temperature (Tc). The values of exponent n(T) and pre-factor A(T) at and around Tc were obtained by the fitting ac conductivity data with Jonscher??s universal power law. From the variation of n(T) and A(T) with temperature, the strength of interaction among the charge carriers with the crystal lattice and the strength of polarisability at phase transition are observed. The activation energy of the compound in low and high temperature range suggests the conduction mechanism in the material.     

Characterization of MOCVD Pt electrode for ferroelectric thin films

Abstract

Platinum thin films were deposited by low pressure chemical vapor deposition (LPMOCVD) on SiO₂/Si and (Ba, Sr)TiO₃/Pt/SiO₂/Si substrates using Pt-hexafluoroacetylacetonate at various deposition temperatures. The shiny mirror-like Pt thin films of a high electrical conductivity were obtained, when the deposition temperature is between 325°C and 350°C, whereas above 375°C Pt thin films showed rough surface as well as poor adhesion property to oxide substrate. Pt thin films had a good step coverage of 90%. The results indicate that LPMOCVD Pt thin films can be applied for the top electrode of high dielectric thin film, which is thought to be one of the best candidate materials for a capacitor of ULSI DRAM.