Ferroelectric Barium Strontium Titanate Research Articles

Transparent Barium Strontium Titanate Ceramics Prepared by Spark Plasma Sintering

Transparent ferroelectric ceramics are valuable for many applications in electro-optical devices. In this paper, transparent barium strontium titanate (BST) ceramics were prepared by spark plasma sintering and thermal treatment. The microstructures and optical properties of the transparent BST samples were investigated. The densities of the BST samples were nearly theoretical and the samples were optically transparent. Both the transmittances at 633 nm of the Ba0.3Sr0.7TiO3 and Ba0.4Sr0.6TiO3 ceramics with a thickness of 0.3 mm are over 74%. The lead-free optical transparent ferroelectric BST ceramics are promising candidates for electro-optical applications.

ZrO2 Layer Thickness Dependent Electrical and Dielectric Properties of BST/ZrO2/BST Multilayer Thin Films

ABSTRACTRecently, high K materials play an important role in microelectronic devices such as capacitors, memory devices, and microwave devices. Now a days ferroelectric barium strontium titanate [BaxSr1-xTiO3, (BST)] thin film is being actively investigated for applications in dynamic random access memories (DRAM), field effect transistor (FET), and tunable devices because of its properties such as high dielectric constant, low leakage current, low dielectric loss, and high dielectric breakdown strength. Several approaches have been used to optimize the dielectric and electrical properties of BST thin films such as doping, graded compositions, and multilayer structures. We have found that inserting a ZrO2 layer in between two BST layers results in a significant reduction in dielectric constant, loss tangent, and leakage current in the multilayer thin films. Also it is shown that the properties of multilayer structure are found to depend strongly on the sublayer thicknesses. In this work the effect of ZrO2 layer thickness on the dielectric, ferroelectric as well as electrical properties of BST/ZrO2/BST multilayer structure is studied. The multilayer Ba0.8Sr0.2TiO3/ZrO2/Ba0.8Sr0.2TiO3 film is deposited by a sol-gel process on the platinized Si substrate. The thickness of the middle ZrO2 layer is varied while keeping the top and bottom BST layer thickness as fixed. It is observed that the dielectric constant, dielectric loss tangent, and leakage current of the multilayer films reduce with the increase of ZrO2 layer thickness and hence suitable for memory device applications. The ferroelectric properties of the multilayer film also decrease with the ZrO2 layer thickness.

Dual-tunable hybrid wave hexaferrite–ferroelectric millimetre-wave phase shifter

A novel electrically and magnetically tunable microwave phase shifter based on forward volume spin wave propagation in a layered structure containing films of oriented barium ferrite (BaM) and polycrystalline ferroelectric barium strontium titanate (BSTO) is proposed. Operating characteristics of the phase shifter are theoretically investigated. It is shown that the hexaferrite–ferroelectric waveguiding element allows the construction of compact devices with high electrical tunability and operation frequency up to ∼0.1 THz at reasonable bias magnetic fields.

Temperature-Dependent Electrical Characterization of Multiferroic BiFeO3 Thin Films

The polarization hysteresis and current leakage characteristics of bismuth ferrite, BiFeO3 (BFO) thin films deposited by pulsed laser deposition was measured while varying the temperature from 80 - 300 K in increments of 10 K, to determine the feasibility of BFO for capacitive applications in memory storage devices. Data is compared to the performance of prototypic ferroelectric barium strontium titanate, BaxSr1-xTiO3 (BST) under similar conditions. Finding contacts on the BFO samples that exhibited acceptable dielectric properties was challenging; and once identified, the polarization characteristics between them varied greatly. However, the non-uniformity among the contact points within each sample suggests that either the samples were defective (by contamination or growth process), or that the deposition process of the contacts may have undermined the functionality of the devices. Subjected to increasing temperatures, BFO's polarization improved, and though its polarizability was shown to be inferior to BST, the dielectric loss was less.

Electric-field control of ferromagnetic resonance in monolithic BaFe12O19–Ba0.5Sr0.5TiO3 heterostructures

This paper demonstrates an electric-field tuning of the ferromagnetic resonance (FMR) responses at millimeter wave frequencies for a monolithic magneto-electric heterostructure. The layered stack is comprised of c-axis oriented and low loss barium hexaferrite (BaM) and (111) oriented ferroelectric barium strontium titanate (BSTO) layers along with embedded platinum electrode layers, all fabricated by pulsed laser deposition technique. A tunability of the FMR frequency as large as 3.5 MHz/V has been observed at 60 GHz due to application of bias voltages in the range of several volts. The realization of such a large tunability relies on the quasi-lattice-to-lattice contact between the BaM and BSTO layers as well as the high quality of those layers.

A Low Loss X-Band Quasi-Elliptic Ferroelectric Tunable Filter

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A compact high quality factor four-pole X-band tunable quasi-elliptic bandpass filter is presented in this letter. The filter is enabled by high-Q ferroelectric barium strontium titanate capacitors and open-loop resonators. The central frequency of 8.35 GHz and the frequency tuning range of 500 MHz (6%) are achieved with a dc bias voltage of 30 V. The selective filter has a fractional 1 dB bandwidth ranging from 5.5% up to 7.3%. The measured insertion loss and return loss are 5.7–3.5 dB and 10.2–7.9 dB, respectively, with a dc bias range of 0 to 30 V. To the best of our knowledge, this is the best analog tunable performance and selectivity in X-band. </para>

Co-sintering of barium strontium titanate (BST) thick films inside a LTCC substrate with pressure-assisted sintering

Screen-printed ferroelectric BST thick films were co-fired inside a LTCC substrate by employing pressure-assisted sintering (PAS). Sintering pressure and time were varied and their effect on the microstructure and dielectric properties of the film was studied. Sintering conditions for the fabrication of buried BST thick films were experimentally defined and parallel plate capacitors were successfully fabricated inside a LTCC substrate. The relative permittivity ( ɛ r) of the films at 10 kHz varied from 188 to 220 depending on the sintering conditions, and the maximum tunability of the films was 44% (5.7 V/μm). The measured loss tangent (tan δ) at zero bias was 0.005.

Influence of crystallization on the spectral features of nano-sized ferroelectric barium strontium titanate (Ba 0.7Sr 0.3Tio 3) thin films

Ferroelectric barium strontium titanate (Ba 0.7Sr 0.3TiO 3)(BST) thin films have been prepared from barium 2-ethylhexanoate [Ba[CH 3(CH 2) 3CH(C 2H 5)CO 2] 2] , strontium 2-ethylhexanoate [Sr[CH 3(CH 2) 3CH(C 2H 5)CO 2] 2] and titanium(IV) isopropoxide [TiOCH(CH 3) 2] 4 precursors using a modified sol–gel technique. The precursor except [TiOCH(CH 3) 2] 4 were synthesized in the laboratory. Transparent and crack-free films were fabricated on pre-cleaned quartz substrates by spin coating. The structural and optical properties of films annealed at different temperatures have been investigated. The as-fired films were found to be amorphous that crystallized to the tetragonal phase after annealing at 550 °C for 1 h in air. The lattice constants “ a” and “ c” were found to be 3.974 A and 3.990 A, respectively. The grain sizes of the films annealed at 450, 500 and 550 °C were found to be 30.8, 36.0 and 39.8 nm respectively. The amorphous film showed very high transparency (∼95%), which decreases slightly after crystallization (∼90%). The band gap and refractive index of the amorphous and crystalline films were estimated. The optical dispersion data are also analyzed in the light of the single oscillator model and are discussed.

Dual tunable thin-film ferrite-ferroelectric slotline resonator

A novel microwave resonator of yttrium iron garnet and ferroelectric barium strontium titanate films in a slotline structure is proposed. The resonator is electric field tunable, by 75 MHz for E 5 V/ m applied to the slotline electrodes, and has several advantages: (i) a low cutoff frequency of a few megahertz compared with several gigahertz for open-dielectric waveguides, and (ii) a high degree of hybridisation of magnetostatic surface spin waves and electromagnetic modes that results in excellent tunability.

Method for manufacturing embedded variable capacitors in low-temperature cofired ceramic substrate

A method for manufacturing embedded variable capacitors (varactors) inside a low-temperature cofired ceramic substrate is presented. A varactor is fabricated by filling a via hole with ferroelectric barium strontium titanate paste. The measured capacitance of the varactor changed from 1.47 to 0.97 pF and the Q factor from 18.2 to 31.5 at 1 GHz when the bias voltage was increased from 0 to 200 V.

Multifunctional dual-tunable low loss ferrite-ferroelctric heterostructures for microwave devices

Oriented barium ferrite (BaM) and polycrystalline ferroelectric barium strontium titanate (BSTO) layered structures have been fabricated by pulsed laser deposition. The 0.5μm thick BaM layer has a saturation induction of 4kG, a uniaxial effective anisotropy field of 16kOe, and a relatively low ferromagnetic resonance linewidth of about 25Oe, values that are indicative of a high quality film. The dielectric constant of the 0.9μm thick BSTO layer drops by a factor of 2 for an applied voltage of 3V.

Microstructure and dielectric properties of ferroelectric barium strontium titanate ceramics prepared by hydrothermal method

Ba x Sr1-x TiO3, nanoparticles with different Ba compositions were synthesized by a hydrothermal method. The mechanism of hydrothermal reactions was discussed based on DTA/TG, XRD and TEM characterizations. The result showed that perovskite structure was developed through the mutual diffusion between the intermediate phases and TiO2 phase. The grain size of the Ba0.77Sr0.23TiO3 (BST77) powders was about 20–40 nm. BST ceramics were made from the hydrothermal-derived BST powders and the dielectric properties of the BST ceramics were measured. Due to the small grain size and active surface energy of the BST powders prepared by hydrothermal method, the BST ceramics showed low sintering temperature. It was found that the BST77 ceramics sintered at 1280 °C showed dielectric constant peak dispersion which was believed to be caused by dimension domino effect.

Multilayers and superlattices of ferroelectric barium strontium titanate

Epitaxial multilayers and superlattices of Ba0.8Sr0.2TiO3 and Ba0.4Sr0.6TiO3 were grown by pulsed laser deposition on MgO substrates using a La0.5Sr0.5CoO3 bottom electrode. The epitaxial relations, crystal perfection, and strains were studied by x-ray diffraction at room temperature. Pseudomorphic growth of superlattices was demonstrated for the period as large as 62unit cells. The obtained in-plane strain was consistent with that theoretically expected. Low-frequency functionality was found to depend on strain, period, and amount and quality of interfaces.

Synthesis and structural characterization of Ba0.6Sr0.4TiO3 nanotubes

Abstract Ferroelectric barium strontium titanate, Ba0.6Sr0.4TiO3, (BST40) nanotubes were fabricated by means of sol–gel method utilizing nanochannel porous anodic aluminum oxide (AAO) templates. XRD study confirmed the pure phase perovskite structure of the BST40 nanotubes. TEM analysis confirmed that the nanotubes are composed of nanoparticles in the range of 4–8 nm and the wall thickness of around 12 nm. Y-junctions and multi-branches were also observed in some of the BST40 nanotubes.

Structural and Optical Properties of Barium Strontium Titanate (Ba0.5Sr0.5TiO3) Thin Films

Ferroelectric Barium Strontium Titanate (Ba0.5Sr0.5TiO3) or BST thin films on quartz substrates have been prepared by using a modified sol gel processing technique. The starting materials are Barium 2-ethylhexanoate Ba[CH3(CH2)3CH(C2H5)CO2]2, Strontium 2-ethylhexanoate Sr[CH3(CH2)3CH(C2H5)CO2]2 and Titanium(IV) isopropoxide [TiOCH(CH3)2]4. The precursors except [TiOCH(CH3)2]4 were synthesized in the laboratory. Transparent and crack-free films were fabricated on quartz substrates by spin coating. The as-fired films were found to be amorphous, which crystallized to cubic phase after annealing at 550°C in air for 1 hr. In this paper we report the structural and optical properties of BST films prepared by the modified sol-gel process. Communicated by Prof. E. C. Subbarao

Ferroelectric properties of BaxSr1−xTiO3 solid solutions obtained by molecular dynamics simulation

Classical shell-model potentials for describing the complex ferroelectric behaviourof barium titanate and strontium titanate are developed and used to simulateBaxSr1−xTiO3 solid solutions. The temperature versus composition phase diagram is verywell described and the local behaviour of the structure and polarization isanalysed. It is shown that the ferroelectric properties of the solid solution canbe understood in terms of the effects of average density and the local chemicalenvironment. The experimentally observed static dielectric peak broadening aroundTc atlow x is reproduced in the simulation and seems to be related to the average volume rather thanto the local chemical environment.

An S-Band Reflection-Type Phase Shifter - A Design Example Using Ferroelectrics

AbstractOne of the challenges faced in using ferroelectrics in high frequency devices is how to effectively use the material in a circuit design. A compact reflection-type phase shifter fabricated on sapphire substrates coated with ferroelectric barium strontium titanate (BST) thin-films has been built which shows the promise of using BST thin films in the design of tunable microwave devices. The phase shifter, fabricated as one monolithic assembly, consists of a 3dB coupler, meandered line inductors and tunable interdigital capacitors. A continuously variable phase shift range of more than 100° using the branch-line coupler was obtained at a center frequency of 2.95 GHz, and more than 90° phase shift over 200 MHz bandwidth with a bias voltage range from 0 V to 175 V. The phase shifter using the Lange coupler has over 700 MHz bandwidth centered at 2.2 GHz with a phase shift of more than 90° and an insertion loss less than 2 dB and return loss of greater than 14 dB, over a bias voltage range from 0 V to 160 V. The loss of the BST phase shifter presented in this work is on the order of other commercially available RF front-end components, such as bandpass filters and RF switches. This holds promise for the practical realization of smart antenna systems in cellular handsets and wireless LAN cards.

Preparation of Ba xSr 1− xTiO 3 thin films with seeding layer by a sol–gel method

Ferroelectric barium strontium titanate (Ba x Sr 1− x TiO 3, BST) thin films have been prepared by depositing a very thin layer of BST on the substrate as a seeding layer, which was expected to enhance crystal growth during heat treatment. SEM analysis data indicate that bigger grains were obtained for films coated with a seeding layer compared with films without a seeding layer. The polarization at E=0 and the field at P=0 of the sample prepared with a seeding layer are 2.6 μC/cm 2 and 20 kV/cm, respectively. The leakage current density of the thin film prepared with a seeding layer at ±40 kV/cm (±1 V) is about 9×10 −8 A/cm 2.

Miniature frequency-output temperature transmitter based on a ceramic capacitive sensor

The challenge of developing cost-effective systems for the measurement of temperature in digital form is widely recognized. This paper describes a miniature temperature transmitter, which indluces a hybridized RC-oscillator based on a new ceramic capacitive sensor. The sensor material developed for this purpose is a ferroelectric barium strontium titanate compound. The characteristics of the transmitter and the capactive sensor are given. The main advantages of the temperature transmitter are good sensitivity (1.3%/°C), small non-lineairty (±0.4 °C) over the temperature rangte −25°C +85°C and small size. In addition, the transmitters are made interchangeable (±1.0°C) by calibrating them at one temperature.

Optical and Microstructural Variations with Electric Field in Barium‐Strontium Titanate Ceramics

Quasi‐thin sections of large‐grained ferroelectric barium‐strontium titanate ceramic compositions were observed under crossed polars. Dielectric constant and light transmission were measured as a function of temperature and applied dc voltages. Microscopic observations, such as changes in structure and in optical anisotropy with temperature and simultaneously applied electric fields, are described. Optical data indicate that static electric fields readily induce anisotropy in the isotropic grains at the ferroelectric to paraelectric transition of the ceramic. Below the transition temperature, the effect of the field on the optical and dielectric properties diminishes. Contrary to single‐crystal barium titanate, a field‐produced reorientation of domains in the ceramic at temperatures below the transition proceeds with difficulty, and is sometimes achieved only after long exposures to high‐biasing fields.