Effect of MnCO3 doping on the dielectric and tunable properties of BSTO/MgO composite for phased array antennas

Undoped and MnCO₃ doped (0.3, 0.5, 1, 2mol%) Ba_0.55Sr_0.45TiO₃/MgO composites were prepared by conventional ceramic processing method and their structural, surface morphological, tunable properties and their dielectric properties at low and microwave frequencies were extensively investigated. The results indicate that the addition of MnCO₃ slightly decreases the average grain size. The proper doping of MnCO₃ can dramatically improve the properties of Ba_0.55Sr_0.45TiO₃/MgO composite with lower dielectric loss and higher tunability. For 0.5mol% MnCO₃ doped samples, the microwave loss at 4.19 GHz is 2.91 × 10⁻³ and the value of tunability is 7.4% with the external DC field 2 kV mm⁻¹, which presented the best properties and can basically meet the requirements of phased array applications.     

Effect of ZrO2 doping on the tunable and dielectric properties of Ba0.55Sr0.45TiO3/MgO composites for microwave tunable application

Undoped and ZrO₂ doped (0.5, 1.0, 2.0, 3.0 wt.%) Ba_0.55Sr_0.45TiO₃/MgO composites were prepared by traditional ceramic processing. The ZrO₂ doping effect on the structural, surface morphological, tunability and dielectric properties were systemically investigated. The result shows that the tunability of Ba_0.55Sr_0.45TiO₃/MgO composites was improved by using ZrO₂ dopant. The Ba_0.55Sr_0.45TiO₃/MgO composite with 1.0 wt.% ZrO₂ dopant exhibits high tunability (17% at 2.5 kV/mm), low dielectric constant (200) and a low microwave loss (0.005, 2.7 GHz), which are suitable for microwave tunable application.     

Microstructure and dielectric properties of BST/MZO ceramic composites for tunable microwave applications

Ba_0.6Sr_0.4TiO₃/Mg_0.9Zn_0.1O (BST/MZO) ceramic composites with different MZO contents were prepared by traditional ceramic process. The crystal structure, fracture surface morphology, and dielectric properties were systematically investigated. The results show that the BST/MZO ceramic composites possess diphase structure, dense, and uniform morphology. The composites have relatively low dielectric loss (in the order of 10⁻³) at microwave frequency. The ceramics all retain substantial tunability (more than 20% at 8 kV/mm DC field) and excellent dielectric strength (more than 19.5 kV/mm). The BST-50 wt% MZO sample has the optimal FOM value (about 256) and should be a better candidate for tunable microwave applications.     

Microstructures and dielectric tunable properties of Ba0.5Sr0.5TiO3–MgO–Mg2TiO4 composite ceramics

Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composite ceramics were prepared by a solid-state reaction method, and the dielectric tunable properties were investigated. It is observed that the addition of MgO–Mg₂TiO₄ into the Ba_0.5Sr_0.5TiO₃ forms ferroelectric (Ba_0.5Sr_0.5TiO₃)–dielectric (Mg₂TiO₄–MgO) composites. Increasing Mg₂TiO₄ content causes an increase of Curie temperature T_c towards room temperature and a decrease of dielectric constant peak ε_max. The dielectric constant and loss tangent of Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composites have been reduced and the overall tunability is maintained at a sufficiently high level. With the increase of Mg₂TiO₄ content and the decrease of MgO content, the dielectric constant and tunability of Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composite ceramics increase and the Q × f values decrease. Ba_0.5Sr_0.5TiO₃–Mg₂TiO₄–MgO composites have dielectric constant of 123.0–156.5 and tunability of 14.4–28.5 % at 10 kHz under 3.9 kV/mm, indicating that they are promising candidate materials for tunable microwave applications requiring a low dielectric constant.     

Compositionally graded ferroelectric multilayers for frequency agile tunable devices

Recently, there has been significant interest toward the development of tunable dielectric materials for voltage-controlled, frequency-agile phase shifters and filters operating in the microwave regime. The fundamental challenge in designing materials systems for such tunable devices is the simultaneous requirement of high dielectric tunability (>40%) over a large temperature interval (−10 °C to +90 °C) coupled with low dielectric losses (between 3.0 dB and 4.0 dB in operational bandwidths ranging from several hundred MHz up to 30 or more GHz). We show that a high- and temperature-insensitive tunability can be realized in compositionally graded ferroelectrics and provide a brief review of the results of experimental and theoretical studies on the dielectric properties of Barium Strontium Titanate (Ba_1−x Sr _x TiO₃ or BST) multilayer heterostructures. Theoretically, we discuss the role of thermal stresses on the dielectric properties using a non-linear thermodynamic model coupled with basic electrostatic considerations to describe the interlayer interactions between the ferroelectric layers. We show that the thermal strains arising from the thermal expansion coefficient mismatch between the multilayered film and the substrate may have a significant effect on the dielectric permittivity and tunability of BST multilayers. Experimentally, compositionally graded BST multilayers (5 mol% MgO doped and undoped) were grown via metallo-organic solution deposition (MOSD) on Pt–Si substrates and electrically characterized. Optimum conditions were found to exist in BST multilayers consisting of three distinct layers of ~220 nm nominal thickness with compositions corresponding to Ba_0.60Sr_0.40TiO₃ (BST 60/40), BST 75/25, and BST 90/10. At room temperature, the BST heterostructure has a small-signal dielectric permittivity of 360 with a dissipation factor of 0.012 and a dielectric tunability of 65% at 444 kV/cm. These properties exhibit minimal dispersion as a function of temperature ranging from 90 °C to −10 °C. Our results also show that MgO doping improves dielectric loss (tan δ = 0.008), but results in a moderate dielectric tunability of 29% at 444 kV/cm. Electrical measurements at microwave frequencies display a decrease in the dielectric permittivity and tunability for both undoped and MgO-doped BST multilayers. At 10 GHz, the dielectric response, tunability, and the loss characteristics for graded undoped BST are 261, 25% (at 1,778 kV/cm), and 0.078, respectively, and 189 and 15% (at 1,778 kV/cm), and 0.039, respectively, for the MgO-doped graded BST.     

Structure and dielectric properties of Ba1−xSrxCuSi2O6-Ba0.55Sr0.45TiO3 ferroelectric-dielectric composite ceramics

yBa_1?xSr_xCuSi₂O₆-(1?y) Ba_0.55Sr_0.45TiO₃ (x?=?0, 0.2 and y?=?10 wt%, 30 wt%) ferroelectric-dielectric composite ceramics were prepared by the solid-state sintering method. The relationship among phase composition, microstructures and dielectric properties has been investigated. By adjusting the content of the Ba_1?xSr_xCuSi₂O₆ and the content of Sr²⁺ in the Ba_1?xSr_xCuSi₂O₆, the effect of the dielectric material on BST was studied. With the increase of Ba_1?xSr_xCuSi₂O₆ content and the increase of the Sr²⁺ content, the dielectric peak of the composite material is gradually suppressed and broadened, and the Curie temperature (Tc) moves to the low temperature. The introduction of Ba_1?xSr_xCuSi₂O₆ reduced the permittivity of the composite material to a certain extent, at the same time, the dielectric tunability and quality factor (Q) value are maintained at a high level. The dielectric tunability of the four composite ceramics obtained is still higher than 10% under the applied electric field (at 30 kV/cm). The 30 wt% BCSO–70 wt% BST55 system and the 30 wt% BSCSO–70 wt% BST55 system has lower dielectric permittivity (830, 577), higher dielectric tunability (17.3%, 10.2% at 30 kV/cm) and a decent Q value (149 at 1.25 GHz, 168 at 1.31 GHz).

     

Dielectric, ferromagnetic and ferroelectric properties of the (1 ? x)Ba0.8Sr0.2TiO3–xCoFe2O4 multiferroic particulate ceramic composites

The (1 − x)Ba_0.8Sr_0.2TiO₃–xCoFe₂O₄ ceramic composites (x = 0–1) were prepared by standard solid state reaction method. X-ray diffraction and SEM indicate the Ba_0.8Sr_0.2TiO₃ (BST) phase and CoFe₂O₄ (CFO) phase coexist in the composites. The dielectric constant and dielectric loss for the composites were studied as a function of frequency (40 Hz–1 MHz) and temperature (30–600 °C). Magnetic and ferroelectric tests show that the ceramic composites display ferromagnetic and ferroelectric properties simultaneously. The saturated polarization of the composites decrease with ferrite concentration increasing, while the remnant polarization of the composites increase with increasing ferrite concentration. The enhanced ferroelectricity of composites may be attributed to space charge contribution in the composites.     

Effect of Mn doping on the dielectric tunable properties of Ba0.5Sr0.5TiO3–MgO–Mg2SiO4 composite ceramics

Mn-doped Ba_0.5Sr_0.5TiO₃–MgO–Mg₂SiO₄ composite ceramics were prepared by a solid-state reaction method, and the dielectric tunable properties were investigated. It was observed that the composite ceramics show three crystalline phases: Ba_0.5Sr_0.5TiO₃, MgO and Mg₂SiO₄ phases. Mn doping significantly improves the dielectric tunable properties of Ba_0.5Sr_0.5TiO₃–MgO–Mg₂SiO₄ composite ceramics. Mn-doped Ba_0.5Sr_0.5TiO₃–MgO–Mg₂SiO₄ composites have dielectric constant of 114.2–127.0, and tunability of 11.3–13.0 % at 10 kHz under 3 kV/mm, indicating that they are promising candidate materials for tunable microwave applications requiring a low dielectric constant.     

Dielectric properties of planar structures based on ferroelectric Ba0.5Sr0.5TiO3 films

An experimental investigation has been made of the dielectric properties of planar Cu-Cr/Ba_0.5Sr_0.5TiO₃ and YBa₂Cu₃O_7−δ /Ba_0.5Sr_0.5TiO₃ structures in the temperature range 78–300 K. It is shown that the use of YBa₂Cu₃O_7−δ electrodes in Ba_0.5Sr_0.5TiO₃ film structures ensures that there is no dielectric hysteresis in the paraelectric phase. At the same time, the dielectric nonlinearity is preserved and the dielectric losses are reduced. Pis’ma Zh. Tekh. Fiz. 23, 46–52 (August 12, 1997)     

Compositionally graded multilayer Ba<Subscript>x</Subscript>Sr<Subscript>0.95−x</Subscript>Ca<Subscript>0.05</Subscript>TiO<Subscript>3</Subscript> ceramics prepared by tape casting for use as dielectric materials

Compositionally graded multilayer Ba_xSr_0.95−xCa_0.05TiO₃ (BSCT) ceramics were prepared via tape casting method using nanometer powders from co-precipitation. Microstructures and dielectric properties of the BSCT system were investigated. The powders were characterized by using transmission electron microscope and BET surface area measurement. Surface morphologies of the sintered samples and multilayer structure were examined by scanning electron microscopy. BSCT particles were of spherical shape with diameters in the range of 73–93 nm. Their specific surface areas were in the range of 11.7–14.6 m²/g. The graded BSCT ceramics with nine layers laminated in vertical way exhibited a higher sintered density, with an average grain size of 0.4 μm, after sintered at 1,200 °C. Dielectric constant, dielectric loss and tunability of the graded ceramics were 2223.94, 1.5 × 10⁻³ at 2 MHz and 42.9% at 3.0 kV/mm, with good dielectric temperature and frequency stability, which made it a promising candidate used for tunable ceramic capacitors and phase shifters.     

Dielectric dispersion and tunability of sol-gel derived Ba x Sr1−x TiO3 thin films

Ba_0.5Sr_0.5TiO₃, Ba_0.6Sr_0.4TiO₃, Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃ thin films were fabricated by a modified sol-gel technique on Pt(111)/Ti/SiO₂/Si(100) substrates. All Ba _x Sr_1–x TiO₃ films crystallized in the perovskite structure with a crack free microstructure and clear grain boundaries. Highest relative permittivity and dielectric tunability was observed in the Ba_0.7Sr_0.3TiO₃ thin film. Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃ compositions demonstrated ferroelectric hysteresis loops indicating the presence of ferroelectricity at room temperature. The paraelectric compositions of Ba_0.5Sr_0.5TiO₃ and Ba_0.6Sr_0.4TiO₃ showed significant tunability with negligible loss tangent. The tunability of Ba_0.5Sr_0.5TiO₃ thin film decreased with the increase of frequency from 100 kHz to 100 MHz. As the frequency increases, especially above 10 MHz, the relative permittivity decreases while the loss tangent increases. Since Ba_0.5Sr_0.5TiO₃ thin film is paraelectric at room temperature, relaxation due to ferroelectric domains cannot occur. Therefore this behaviour has originated from the contact resistance and finite sheet resistance of both the bottom and top electrodes. To analyse the thin film capacitor, the parallel plate capacitor structure can be modeled based on an equivalent circuit, which contain electrode and contact resistance.     

Electrical behavior of Y-doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> thin films

Ba_0.6Sr_0.4TiO₃ (BST) and 1.5 at% Y-doped Ba_0.6Sr_0.4TiO₃ (Y-BST) thin films have been deposited on single-crystal (100) oriented LaAlO₃ substrates using pulsed-laser deposition technique (PLD), respectively. X-ray diffraction (XRD) scanning revealed that the two kinds of films could be epitaxially grown in pure single-oriented perovskite phases, but Y-BST thin films showed an enhanced crystallization effect. The dielectric properties of the pure and Y-BST thin films were measured at 10 kHz and 300 K with a parallel-plate capacitor configuration. The results revealed that the addition of Y as an acceptor doping is very effective to increase dielectric tunability, and to reduce leakage current of BST thin films. The figure-of-merit (FOM) factor value increases from 17.32 for BST to 25.84 for Y-BST under an applied electric field of 300 kV/cm. The leakage current density of the BST thin films at a negative bias field of 300 kV/cm decreases from 2.45 × 10⁻⁴ A/cm² to 1.55 × 10⁻⁶ A/cm² by Y doping. The obtained results indicated that the Y-doped BST thin film is a promising candidate material for tunable microwave devices.     

Effect of lead on formation and dielectric tunability of (Pbx,Sr1? x )0.85Bi0.1TiO3 thin films

(Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films with the perovskite phase structure were prepared on an ITO glass substrate by sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and an impedance analyzer were respectively used in order to characterize the phase status, morphology and dielectric properties of the thin films. The results show that during the formation process of (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin films, the nucleus of the perovskite phase are initially formed and then congregated. These aggregated nucleus are then transformed as the perovskite-phase crystalline in the thin film. Finally, the crystalline phase grows and separates gradually to form the perfect crystalline-phase structure. The content of the perovskite phase formed in the thin film under rapid thermal process (RTP) is more than that formed under traditional heat treatment with kinetic equilibrium. This is due to the high active decomposed ions that form the perovskite phase directly when heat-treated by RTP. The formation of the perovskite phase therefore overcomes a much lower barrier under RTP than that under traditional calcinations. The structure of the perovskite phase has a close relation to the ratio of Pb/Sr in the system because of the radius difference between Pb²⁺ and Sr²⁺. The transformation temperature between the cubic and the tetragonal structures of the perovskite phase increases with increasing Pb²⁺ content because the radius of Pb²⁺ is larger than that of Sr²⁺. It appears at room temperature when the content of Pb²⁺/Sr²⁺ is about 40/60 in the thin film. Meanwhile, the tetragonality of the perovskite phase is increased when Pb²⁺ ions increase due to its high polarization. The higher tunability of the (Pb_x,Sr_1−x )_0.85Bi_0.1TiO₃ thin film is exhibited when the film composition is close to the transformation point between the paraelectric and ferroelectric phases. Pb²⁺ ions show a dominant factor to affect the Curie point of the system and then changing tunability. Translated from Journal of Inorganic Materials, 2006, 21(2): 466–472 [译自: 无机材料学报]     

Two-step sol–gel method for preparing MgO–MnO2–B2O3–Li2O co-added core-shell structural Ba0.55Sr0.40Ca0.05TiO3 powders and the dielectric properties of the composite ceramics

The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite powders possessing core-shell structure, small particle size and high specific surface area were prepared by the novel two-step sol–gel method which was based on the citric acid (CA)–ethylene glycol (EG) system. In this experiment, the Ba_0.55Sr_0.40Ca_0.05TiO₃ (BSCT) powders were firstly synthesized by chemical co-precipitation method. MgO [(MgO/BSCT)_mass = 1] and MnO₂ [(MnO₂/BSCT)_mole = 0.01] were added into the BSCT powders in the first sol–gel step. 1.5 wt% B₂O₃–Li₂O as sintering aids was added into the composite powders during the second sol–gel step. The BSCT–MgO composite powders were detected to have a perfect core-shell structure which was detected by the transmission electron microscope. Perovskite BSCT and periclase MgO were confirmed to exist in the BSCT–MgO composite powders according to the X-ray diffraction patterns. The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite ceramics that synthesized with the core-shell powders were sintered at 1,000 °C for 2 h. According to the scanning electron microscope images, the grain sizes of BSCT and MgO in the BSCT–MgO composite ceramics were in the range of 0.5–2.0 μm and 1.0–2.5 μm, respectively. The dielectric constant of the BSCT–MgO composite ceramics was 210 and the dielectric loss was 0.0012 when tested at 1 MHz, room temperature. The BSCT–MgO composite ceramics were expected to be a promising candidate for applying as phase shifters or tunable components, etc, in the microwave field.     

Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders

Composite ceramics of Ba_0.6Sr_0.4TiO₃ + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg²⁺ into the lattice of the Ba_0.6Sr_0.4TiO₃ phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm.     

Studies on magnetic and magnetoelectric properties of the NiFe2O4–Ba0.7Sr0.3TiO3 composites

The magnetic and magnetoelectric properties of magnetoelectric (ME) composites consisting of with nickel ferrite (NiFe₂O₄) and barium strontium titanate (Ba_0.7Sr_0.3TiO₃) were investigated. The composites were prepared by standard double sintering ceramic method. The X-ray diffraction analysis was carried out to confirm the phases formed during sintering and also to calculate the lattice parameters. The hysteresis measurements were done to determine saturation magnetization (M_s), remenance magnetization (M_r) and coercivity (H_c) of the samples. The magnetoelectric voltage coefficient (dE/dH)_H was studied as a function of intensity of the magnetic field. The measured magnetoelectric (ME) response demonstrated strong dependence on the volume fraction of NiFe₂O₄ and the applied magnetic field. A large ME voltage coefficient of about 560 μVcm⁻¹Oe⁻¹ was observed for 15% NiFe₂O₄ + 85% Ba_0.7Sr_0.3TiO₃ composite.     

Influence of bismuth on properties and microstructures of Sr0.5Ba0.5−xBixTiO3 thin films

The influence of bismuth (Bi) on the dielectric and ferroelectric properties of Sr_0.5Ba_0.5−xBi_xTiO₃ (BST, 0 ≤ x ≤ 0.030 mol) thin films was studied. The results showed that the dielectric constant (ε_r) and dielectric loss (tan δ) decreased, and temperature, T_m, for maximum and ε_r (Curie temperature), moved to lower temperature with increasing Bi content. The P_r, P_s and E_c were 0.22 μC/cm², 0.32 μC/cm² and 60 kV/cm, respectively for Sr_0.5Ba_0.485Bi_0.015TiO₃ thin films measured at 100 Hz, 20 V. The microstructure of BST thinfilms was studied by XRD and TEM. Tetragonal perovskite grains existed in BST thin films, but the grain size decreased with increasing doping ratio in BST. The characteristic absorption band for octahedron [TiO₂] (471.65 cm⁻¹) was shifted to lower wave number.     

Effect of La2O3 doping on the tunable and dielectric properties of BST/MgO composite for microwave tunable application

Undoped and La₂O₃doped (0.5, 1.0, 2.0, 3.0 wt.%) Ba_0.55Sr_0.45TiO₃/MgO composites were prepared by traditional ceramic processing and their structural, surface morphological, tunable properties and their dielectric properties at low frequency and microwave frequency were systemically examined. The result shows that La₂O₃ dopant has a strong effect on the average grain size. The La₂O₃ doped samples have lower temperature coefficient of capacitance than the undoped. The 0.5 wt.% La₂O₃ doped sample has a little higher tunability than the undoped and the tunability of other doping concentration samples is lower as compared to the undoped. The addition of La₂O₃ decreases the dielectric constant and increases quality factor (Q × f) at microwave frequency. The 0.5 wt.% La₂O₃ doped samples have the best properties among these samples and have a higher tunability, lower dielectric constant and lower dielectric loss tangent at microwave frequency and these properties are very beneficial to the development of the tunable devices application.     

Enhanced microwave dielectric properties of Ba<Subscript>0.40</Subscript>Sr<Subscript>0.60</Subscript>TiO<Subscript>3</Subscript>–Zr<Subscript>0.80</Subscript>Sn<Subscript>0.20</Subscript>TiO<Subscript>4</Subscript> composite ceramics

The (1−x) Ba_0.40Sr_0.60TiO₃ (BST)−xZr_0.80Sn_0.20TiO₄ (ZST) composite ceramics with x = 10, 20, 30, and 40 wt% were fabricated by conventional solid-state reaction method. With increasing of ZST content, the dielectric constant of composite ceramics was decreased and dielectric loss increases. The effect of ZnO addition to 70 wt% BST–30 wt% ZST composition on the microstructure and dielectric properties was investigated. The improvements in dielectric constant, dielectric loss, and microwave dielectric properties of composite ceramics can be achieved by ZnO addition. The sample with 98 wt% (70 wt% BST–30 wt% ZST)–2 wt%ZnO composition exhibits promising dielectric properties, with dielectric constant, loss tangent and tunability at 4 kV/mm, of 125, 0.0016 and 12%, at 10 kHz and room temperature. At ~2 GHz, it possesses a dielectric constant of 101 and a Q factor of 187, which makes it a good candidate for tunable microwave device applications.     

Dielectric and tunable properties of Pb0.25BaxSr0.75−xTiO3 thin films fabricated by a modified sol-gel method

A modified sol-gel method was used to fabricate (Pb_0.25Ba_x Sr_0.75−x)TiO₃ (PBST) thin films with x = 0.05,0.1,0.15 and 0.2 on Pt/TiO₂/SiO₂/Si substrate. The structure, surface morphology, dielectric and tunable properties of PBST thin films were investigated as a function of barium content (x). X-ray diffraction and scanning electron microscopy analysis showed that we could get pure PBST perovskite phase and relative fine density thin films with smooth surface. It was found that the crystal lattice constant, grain size, room temperature dielectric constant, dielectric loss and tunability of Ba solutionizing PST thin films increased with the increase in Ba content. For (Pb_0.25Ba_0.2Sr_0.55)TiO₃ thin film, it had the highest dielectric constant of 1390 and the largest tunability of 80.6%. The figure of merit parameter reached a maximal value of 28.9 corresponding to the (Pb_0.25Ba_0.05 Sr_0.7)TiO₃ thin film, whose dielectric constant, dielectric loss and tunability measured at 1 MHz were 627, 0.024 and 69.4%, respectively.