Ferroelectric Properties of the La0.03Sr0.225Ba0.7Nb2–yTiyO6–y/2 Ceramic System

The study of the dielectric properties of the stoichiometric Titanium doped Sr_0.255La_0.03Ba_0.7Nb₂O₆ ceramics prepared according to the formula Sr_0.225La_0.03Ba_0.7Nb_2–yTi_yO_6–y/2 is reported. A single phase compound is observed for low Ti content (y<0.1) in the XRD spectra, being isostructural with the SBN phase. For high Ti concentrations (y<0.1), the XRD patterns show, besides the SBN phase reflections, several small peaks associated with an additional phase present in the ceramics. A possible liquid phase sintering is analyzed. Strongly broadened dielectric curves are obtained in dielectric measurements, where the transition temperature decreases with the Titanium content. The diffuse phase transition coefficients are calculated, corroborating that Titanium increases the diffuse character of the transition in the monophasic region.     

Dielectric Properties of La3+ Doped Sr0.3–3y/2Lay Ba0.7Nb2O6 Ceramics Prepared under Different Sintering Conditions*

A study of the dielectric properties of the Lanthanum doped Sr_0.3Ba_0.7Nb₂O₆ (SBN30) ceramic according to the stoichiometric formulation Sr_0.3–3y/2La_yBa_0.7Nb₂O₆ with y = 0.01, 0.03 and 0.05, and the influence of the sintering conditions is reported. The XRD shows single phase compounds for Sr_0.285La_0.01Ba_0.7Nb₂O₆ (LSBN1) and Sr_0.225La_0.03Ba_0.7Nb₂O₆ (LSBN3) ceramics, both samples having similar microstructure, densification and dielectric properties. The density increases linearly with ln t, where t is the sintering time, and the values of the maximum ferroelectric peaks of the permittivity increase steadily with t. Using the Bruggeman model to estimate the theoretical permittivity, it is concluded that the magnitude of the experimental permittivity peaks are mainly affected by the volume fraction of porosity of the samples. In this study we also establish that pore diffusion mechanisms behave according to the Ginstling-Brownshtein equation. For the Sr_0.225La_0.05Ba_0.7Nb₂O₆ (LSBN5) sample, XRD analysis reveals the presence of isostructural compounds of the intermediate phases BaNb₂O₆ and SrNb₂O₆, and the dielectric properties start to deteriorate. This fact indicates the existence of a solubility limit of Lanthanum ions in the SBN solid solution.     

Diffuse Phase Transition of the Sr0.3−3y/2LayBa0.7Nb2O6 Ceramic System

The effect of Lanthanum cation doping in the diffuse phase transition of the Sr_0.3–3y/2La_yBa_0.7Nb₂O₆ ceramic system is studied for y=0.01, 0.03 and 0.05. For these compositions, the transition temperature T _c shifts towards higher values with increasing frequency and the temperature dependence of the electrical permittivity presents strongly broadened curves which suggest a non Curie-Weiss behavior near the transition temperature for temperatures far from T _c. The diffuse phase transition coefficient () was also determined and its values lead us to conclude that the degree of disorder in the system increases with the presence of the Lanthanum cation. This result is corroborated by calorimetric measurements, where an increase in entropy with the Lanthanum concentration is also found.     

Effect of Nb Doping on (Sr,Ba)TiO3 (BST) Ceramic Samples

The effect of doping the Sr_0.3Ba_0.7Ti_(1–5y/4)Nb _y O₃ ceramic with different concentration of Nb is studied by scanning electron microscopy (SEM), X-ray diffraction and thermoelectric analysis. It is observed that the grain size decreases as the Nb concentration increases. The critical temperature T _c has a linear decrease at a rate of 19°C/mol% of Nb. The temperature dependence of the dielectric permittivity presents strongly broadened curves, which suggest a non Curie-Weiss behavior near the transition temperature. The diffuse phase transition coefficient () was also determined and its value leads to the conclusion that the degree of disorder in the system increases with the presence of the Nb cation.     

Dielectric and ferroelectric properties of Nb-doped Ba0.8Sr0.2TiO3 ceramics

Ferroelectric and dielectric properties were investigated for Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ ceramics with different Nb₂O₅ concentrations. The relations between the ceramic structures and those properties were discussed. The Ba_0.8Sr_0.2TiO₃ doping with 0.01mol% Nb₂O₅ appears to have a strong ferroelectric effect and better dielectric properties. The max permittivity (? _max) is up to 7,521.3 and Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ ceramics has higher permittivity even at room temperature. The permittivity presents broadened curves at large temperature ranges, which suggests a non Curie–Weiss behavior near the transition temperature. The diffuse phase transition coefficient (δ) for Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ doping with 0.01mol% Nb₂O₅ reaches 0.098, and its P–E loop expresses a diffusing curve. The remanent polarization (2P _r) and coercive field are 31.3 μC/cm² and 10 kV/cm, respectively. The P–E loop presents a diffusing curve, which is relative to the relaxor characteristic.     

Composition dependent electrocaloric behavior of (SrxBa1-x)Nb2O6 ceramics

This article presents electrocaloric effect in (Sr_xBa_1-x)Nb₂O₆ ceramics (where x = 0.25, 0.50 and 0.75) using an indirect approach based on Maxwell's relations. Here, we have calculated various parameters of electrocaloric effect like temperature change (ΔT), entropy change (ΔS) and heat carrying capacity (ΔQ) of material due to the change in polarization under two different electric fields of 30 kV/cm and 20 kV/cm. (Sr_xBa_1-x)Nb₂O₆ ceramics is well known pyroelectric material, where by increasing Sr/Ba ratio the ferroelectric behavior turns towards relaxor behavior. While in terms of electrocaloric effect performance (temperature change ΔT) is increase as the Sr/Ba ratio increases. Additionally, maximum ΔT (0.30 K) was found for (Sr_xBa_1-x)Nb₂O₆ ceramic having x=0.75 molar concentration under the electric fields of 30 kV/cm.     

Electrical and memory window properties of Sr0.8-xBaxBi2.2Ta2-yZryO9 ferroelectric gate in metal-ferroelectric-insulator-semiconductor structure

Electrical characteristics of Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ ferroelectric films grown on HfO₂/Si wafers by sol–gel spin coating technique were investigated from the viewpoint of application as ferroelectric gates in metal-ferroelectric-insulator-semiconductor (MFIS) stacks. It was observed that the leakage current density level was 10^-8 A/cm² under 14?V for moderate doping ratio. Determined memory windows from C-V characteristics of Sr_0.8Bi_2.2Ta₂O₉ (SBT) and Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ (x?=?0.04, 0.08, 0.12 and y?=?0.1, 0.2, 0.3) are 0.59, 0.65, 0.75, and 0.86?V at gate sweeping bias of 5?V, respectively. Some part of electronic properties of Sr_0.8-xBa_xBi_2.2Ta_2-yZr_yO₉ with the objective to enhance memory window up to 45?% were discussed. It was interpreted that defects which are formed in Ba and Zr modified SBT affected the electronic processes like leakage current, memory window and charge trapping.     

Ceramics Materials Based on (Ba, Sr)TiO3 Solid Solutions for Tunable Microwave Devices

Structure and electrical properties at radio frequencies as well as within the 3.5–35 GHz frequency range have been investigated for ceramic samples of the (1–y)(Ba_xSr_{1 – x})TiO₃ · yMgO (BSM) system where x = 0.4–0.6; y = 0.15–0.30. For the compositions studied the bulk ferroelectrics were synthesized with the dielectric constant of 400–600 and high tunability coefficient. We indicated that the quality factor of the samples was in the range of 100–1000 within the frequency band of 3.5–35 GHz. The phase correlations and unit cell constants of the perovskite phase of the BSM samples were studied. The low loss factor and high tunability of the bulk material allowed us using the BSM ferroelectric ceramic layer for tunable accelerating structures of the Argonne Dielectric Wakefield Accelerator and for high power switches design and development for the future linear colliders.     

Millimeter-Wave Loaded Line Ferroelectric Phase Shifters

Ferroelectric loaded line phase shifters operating at millimeter waves for phased array antenna applications are presented. Phase shifters were manufactured on using Ba_0.3Sr_0.7TiO₃ thin films. The magnetron sputtering process was used to fabricate these Ba_0.3Sr_0.7TiO₃ ferroelectric films with a thickness ～1 μm. The phase shifter operating at V-band (60 GHz) demonstrated continuous phase shift up to 220 deg and figure of merit (FOM) 22 deg/dB. The phase shifter operating at Ka-band (30 GHz) showed phase shift up to 360 deg and FOM 40 deg/dB.     

Microstructures and Microwave Dielectric Characteristics of Ba6 − 3x (Sm1 − y La y )8 + 2x Ti18O54 Solid Solutions (x = 2/3 and 0.75)

Ba_{6 – 3x} (Sm_{1 – y} La _y )_{8 + 2x} Ti₁₈O₅₄ solid solutions were prepared and characterized. For x = 2/3, tungsten bronze type solid solutions were observed in the entire range of y = 0–1. While, La substitution for Sm will change the phase constitution in Ba_{6 – 3x} Sm_{8 + 2x} Ti₁₈O₅₄ ceramics with x = 0.75. Though the single phase solid solutions were observed for the compositions at the vicinity of the end-members, La₄Ti₉O₂₄ secondary phase was detected for the compositions of y = 0.3–0.8. For x = 2/3, the dielectric constant increased continuously with increasing y, and the Q · f value increased slightly at first then decreased. The dielectric constant had more complex change with increasing y for the situation of x = 0.75 where Q · f value decreased continuously. In both cases, the temperature coefficient of resonant frequency varied from negative to positive with increasing y.     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Effect of Temperature on the Elastic and Anelastic Behaviour of Magneto-Ferroelectric Composites Ba0.8Pb0.2TiO3 + Ni0.93Co0.02Mn0.05Fe1.95O4-δ in the Ferroelectric Rich Region

Composites with composition xBa_0.8Pb_0.2TiO₃+ (1 –x) Ni_0.93Co_0.02Mn_0.05Fe_1.95O_4- in which x varies as 1.0, 0.9, 0.7 and 0.5 in molar percent have been prepared by the conventional ceramic double sintering process. The presence of the two phases has been confirmed by X-ray diffraction. These composites were prepared for their use as magnetoferrolectric devices. Variation of longitudinal modulus (L) and internal friction loss (Q ^–1) of these samples with temperature at 142 kHz has been studied in the wide temperature range 300 to 630 K. The elastic behaviour (L) showed a break at the ferroelectric Curie temperature (498 K) in the case of pure ferroelectric material (Ba_0.8Pb_0.2TiO₃). This break shifted to lower temperature side as the ferrite component increases in the composite. The temperature variation of internal friction loss (Q ^–1) showed a corresponding stress induced relaxation peak at the ferroelectric-non-ferroelectric phase transition. This behaviour is explained in the light of structural phase transition.     

Low dielectric dissipation and enhanced tunability of Ba0.6Sr0.4TiO3 thin films by the modified composition and multilayer structure

Ba_0.6Sr_0.4Ti_1+yO₃ (BST, y?=?0.1, 0.15, 0.2, 0.25, 0.3) thin films were fabricated on Pt-coated silicon substrates by modified sol-gel techniques. It was found that the tunability of BST thin films and dissipation factor decreased with the increase of Ti content. The multilayer structure of Ba_0.6Sr_0.4Ti_1+yO₃(200 nm)/Ba_0.6Sr_0.4TiO₃(100 nm)/Ba_0.6Sr_0.4Ti_1+yO₃ (200 nm; y?=?0.1, 0.2, 0.25) was designed to enhance the tunability. Our results indicated that the modified composition and multilayer structure were beneficial to lowering the dielectric dissipation and enhancing the tunability simultaneously. The tunability of 26.7% and dielectric dissipation of 0.013 were achieved for modified BST thin films.     

Preparation and properties of strontium barium niobate based glass-ceramics for energy storage capacitors

Na₂O–BaO–SrO–Nb₂O₅–B₂O₃–SiO₂ glass-ceramics were prepared by melt-casting followed by controlled crystallization. X-ray diffraction results show that Ba_0.27Sr_0.75Nb₂O_5.78 with tungsten bronze structure formed as the dielectric phases from the glass matrix at 800°C. However, a secondary phase NaSr_1.2Ba_0.8Nb₅O₁₅ occurs when crystallization temperature exceeds 850°C. The glass-ceramics exhibit excellent stability in permittivity values from room temperature to 200°C and low dielectric losses below 0.05. Electrical testing demonstrates that the breakdown strength increases with crystallization temperature. The P–E characteristics at room temperature do not show any clear ferroelectric behavior. The glass-ceramic material heated at 800°C/3 h + 950°C/3 h shows a breakdown strength of 1400 kV/cm and its energy storage density can reach up to 4.0 J/cm³, which may be a strong candidate for high energy density storage capacitors for portable or pulsed power applications.     

Substrate effect on in-plane ferroelectric and dielectric properties of Ba0.7Sr0.3TiO3 thin films

Heteroepitaxial Ba_0.7Sr_0.3TiO₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) (LSAT) and SrTiO₃ (001) (STO) single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure for films grown on both LSAT and STO substrates. The in-plane ferroelectric and dielectric properties of the films were studied using interdigital electrodes (IDE). The film grown on LSAT substrate exhibited an enhanced in-plane ferroelectricity, including a well-defined P-E hysteresis loop with the remnant polarization P _r = 10.5 μC/cm² and a butterfly-shaped C-V curve. Nevertheless, only a slim hysteresis loop was observed in the film grown on STO substrate. Curie temperature T _c of the film grown on LSAT substrate was found to be ∼105^∘C, which is nearly 70^∘C higher than that of the bulk Ba_0.7Sr_0.3TiO₃ ceramics. T _c of the film grown on STO substrate has almost no change compared to the bulk Ba_0.7Sr_0.3TiO₃ ceramics. The dielectric tunabilities were found to be 64% and 52% at 1 MHz for the films grown on LSAT and STO substrates, respectively.     

Dielectric and Mechanical Losses in (Ba,Sr)TiO3 Systems

In the application of tuneable microwave devices of ferroelectric (BaSr)TiO₃ systems the two critical parameters needed for optimal device performance are high tunability and low dielectric loss. The dielectric loss of the materials is strongly dependent on microstructure. This paper is concerned with an investigation of the variation in the dielectric and mechanical losses in Ba _x Sr_{1 – x} TiO₃ systems (x = 0.5, 0.6, 0.7 and 1.0) with microstructure (grain sizes from 1 m to 50 m). The magnitude of the loss peak and sharpness of the anomaly in the dielectric constant/elastic modulus observed for the phase transitions in Ba _x Sr_{1 – x} TiO₃, depend not only on the composition and but also on the grain size. A relaxation peak has been observed in large grain material, which is indication of interactions between different configurations of domain walls and the diffusion of oxygen vacancies in the domains.     

Dielectric and piezoelectric properties of La2O3 doped (Bi0.5Na0.5)0.92(Ba0.8Sr0.2)0.08 TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.92(Ba_0.8Sr_0.2)_0.08 TiO₃+x mol% La₂O₃(x = 0, 0.1, 0.3, 0.5, 0.8) were synthesized by conventional solid state reaction. The crystal structure of all compositions is mono-perovskite ascertained by XRD. The grain size decreased and diffuse phase transition behavior was more evident with the increasing amount of La₂O₃. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p showed the maximum value of 165 pC/N and 0.322 at 0.3% and 0.1% La₂O₃ addition, respectively, and rapidly decreased when La₂O₃ addition over 0.5%. The loss tangent tanδ linearly increased and the mechanical quality factor Q_m linearly decreased with the increasing amount of La₂O₃.     

Defect CHEMISTRY and Charge TRANSPORT in SrBi2Nb2O9

Equilibrium dc conductivity and thermopower measurements at 650–800°C on undoped and 1% acceptor-doped SrBi₂Nb₂O₉, SBN, indicate that the n-type conductivity is similar to that of a simple transition metal oxide that contains 1–2% donor excess. The donor content is attributed to the presence of Bi⁺³ on Sr⁺² sites in the perovskite-like layers of the structure. These centers arise from cation place exchange between these ions in the alternating layers of the crystal. This exchange is apparently not completely self-compensating, and there is local charge compensation in each layer. While the equilibrium conductivity of SrBi₂Ta₂O₉, SBT, is dominated by ionic conduction in the Bi layers, in SBN conduction by electrons in the perovskite-like layers prevails. The difference in behavior is attributed to the expected smaller band gap of the niobate. The electron mobility in SBN is extremely small, of the order of 10^–5 cm²/v · sec at 750°C, and is highly activated with an activation energy of about 1.6 eV. The resulting low mobility at ambient temperatures is proposed as the basis for the observed resistance to ferroelectric fatigue. Reports of metallic Bi on the surface of SBT and SBN by XPS analysis are shown to result from the highly reducing atmosphere of the XPS apparatus.     

Chemical solution processing and properties of tungsten bronze (ba,la)nb2o6 thin films

Abstract

Highly oriented (Ba,La)Nb₂O₆ thin films have been synthesized by a chemical solution deposition method. A homogeneous and stable (Ba_0.75La_0.167)Nb₂O₆ (BLN) precursor solution was prepared by controlling the reaction of metal alkoxides. BLN precursor films crystallized in the tetragonal tungsten bronze phase at 700°C. BLN thin films on MgO(100) and Pt(100)/MgO(100) substrates showed the prominent c-axis preferred orientation. BLN thin films on Pt(100)/MgO(100) exhibited the diffuse phase transition depending upon the frequency.     

Ionic Doping Effects in SrBi2Nb2O9 Ferroelectric Ceramics

Ionic doping effects of various ions in Bi-layered ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics were studied. Un-doped and doped SBN ceramics with Ba²⁺, Pb²⁺, Ca²⁺, Bi³⁺, La³⁺, Ti⁴⁺, Mo⁶⁺, and W⁶⁺ ions were made with solid state reactions. Temperature dependent dielectric constants were measured. Ferroelectric transition temperature (T_C) decreased with Ba²⁺ and Pb²⁺ ions but increased with Ca²⁺ ion which substitutes the 12-coordinated Sr²⁺ site. T_C increased with Ti⁴⁺, Mo⁶⁺, and W⁶⁺ ions which substitute the 6-coordinated Nb⁵⁺ sites. With trivalent Bi³⁺ and La³⁺ ions, T_C increased with Bi³⁺ ion but much decreased with La³⁺ ion. These results showed that the ion size plays an important role in ferroelectricity of SBN ceramics.