Low-frequency magnetoelectric interactions in single crystal and polycrystalline bilayers of lanthanum strontium manganite and lead zirconate titanate

The magnetoelectric (ME) characterization of bilayers of lead zirconate titanate (PZT) and single crystal or hot-pressed polycrystalline lanthanum strontium manganite (LSMO) are discussed. Data on ME voltage coefficient have been obtained as a function of strength and orientation of bias magnetic field H, temperature, and frequency. The bilayers exhibit superior ME coupling compared to thick film multilayer composites and the strongest ME interactions are measured for samples with single crystal LSMO. Bilayers with single crystal LSMO show strong dependence of ME coefficient on H orientation and temperature, with a maximum value of 190 mV/cm Oe at 86 K. The frequency dependence of ME coefficient reveals a resonance enhancement due to radial acoustic modes. There is excellent agreement between theory and data for the H variation of ME coefficients.     

Switching time of planar ferroelectric capacitors using strontium titanate and barium strontium titanate films

A microstripe cavity was used to make microwave (f∼10 GHz) measurements of the time of variation of the permittivity of SrTiO₃ and (Ba,Sr)TiO₃ films exposed to unipolar video-frequency voltage pulses. It was shown that the permittivity of SrTiO₃ films varies over times less than 30 ns. For (Ba,Sr)TiO₃ films two characteristic times of variation of the permittivity were identified: a fast variation over times less than 30 ns and a slower variation, on the order of 20 μs. Pis’ma Zh. Tekh. Fiz. 24, 19–25 (October 12, 1998)     

Sintering mechanisms in strontium doped lanthanum chromite

The sintering behavior of (La_0.7Sr_0.3)_xCrO₃ (0.95 x 1.05) is investigated to compare liquid phase sintering phenomena occuring in stoichiometric and non-stoichiometric compositions. Shrinkage analysis revealed marked contrast between the densification characteristics of the A-site enriched (x > 1.00) and A-site depleted (x < 1.00) materials. A-site depleted samples typically exhibited a single liquid phase sintering event at 1250 °C attributed to the melting of an exsoluted SrCrO₄ phase. A-site enriched samples indicated two rapid shrinkage events due to the melting of SrCrO₄, and a Sr_2.67(CrO₄)₂ phase with a melting temperature of 1450 °C. Sr_2.67(CrO₄)₂ was shown to evolve from a decomposition reaction between SrCrO₄ and La₂CrO₆, detected together in A-site enriched samples from 800–1000 °C. Maximum densities (93% theoretical density) were achieved for (La_0.7Sr_0.3)_xCrO₃ x = 1.00 after sintering at 1700 °C for two hours.     

Sandwich-type composite multilayer films of strontium titanate and barium strontium titanate and their controllable dielectric properties

It is a challenge to reduce the dielectric loss and increase the tunability of pure barium strontium titanate(BST)films for microwave tunable application because these two properties change simultaneously.Herein,a novel composite of strontium titanate(ST)and potassium-doped BST(KBST)has been designed as ST/KBST/ST sandwich-type film with various ST and KBST layers.X-ray diffraction patterns show that the film exhibits cubic perovskite polycrystalline structure composed of BST and ST phase,mainly grow along(110)crystal plane with average grain size of less than 20 nm and decreasing BST phase/ST phase ratio with increasing film thickness.Scanning electron microscope shows that no interfacial layer can be observed,indicating that ST and KBST are fully compounded.Low dielectric loss and high tunability at-10-10V and stable and excellent dielectric properties at 1 GHz are achieved,meeting the needs of microwave tunable application at high frequency.The surface structures are also studied by other analysis methods,and ST/MgBST/ST sandwich-type film is compared.     

Transitions of barium strontium titanate ferroelectric ceramics for different strontium content

Ceramic based barium strontium titanate (BST) solid solutions with the formula Ba_1 − xSr_xTiO₃ are very important candidates for a wide range of device applications. Several doped (Mn and Mg) and undoped samples were prepared by standard solid-state reaction. Special emphasis was put on compositions with x = 0.35 and 0.60, with high potential for applications. The samples were sintered at temperatures in the 1200 ÷ 1260 °C range. Structural X-ray diffraction analysis preformed confirms the perovskite structure. The dielectric parameters were investigated in a wide temperature range between − 150 and 150 °C. The temperature was cyclically changed in both directions, up and down, at a rate of less than 2 °C/min. Both permittivity and dielectric loss were measured at low frequencies, 1 kHz. The peak values of the permittivity are increasing from 2000 to 4000 with the sintering temperature increase. Moreover the dielectric parameters were measured at room temperature in microwave domain (1 ÷ 2 GHz). The Curie temperature of BST samples with x = 0.35 and x = 0.6 is in agreement with the Curie point dependence on Sr content, as we have previously reported.     

Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO₃ ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO₂ and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.     

Colloidal processing of lead lanthanum zirconate titanate ceramics

Well-dispersed aqueous slurries of fine ceramic powders with high solids loading are often required for various shape forming techniques such as slip and tape casting in order to fabricate advanced ceramics with a dense and uniform microstructure. Colloidal processing of lead lanthanum zirconate titanate (PLZT) powders was conducted at various pH using ammonium polymethacrylate as a dispersant. Suspensions were characterized by viscosity and zeta-potential measurements. The effect of pH on polymer adsorption and the rheological behavior of the slurries were investigated and stabilization mechanisms discussed. Through optimization of the dispersant concentration and pH, solids loadings of the suspensions up to 50 vol.% with a relatively low viscosity were obtained.     

Synthesis of barium strontium titanate nanorods in reverse microemulsion

Perovskite Ba_0.7Sr_0.3TiO₃ nanorods were synthesized in Triton X-100 reverse microemulsion at room temperature and characterized by XRD, ICP-AES, ED and TEM. The Ba_0.7Sr_0.3TiO₃ nanorods with a diameter of 60-100 nm and a length of 450-1200 nm showed a single-crystalline tetragonal structure. The size of nanorods was effectively tuned by changing w value (molar ratio of water to surfactant), aging time and reactant concentration.     

Lattice constant of strontium titanate at low temperatures

A double-crystal X-ray diffraction technique has been applied to the lattice constant measurement of SrTiO₃ in the temperature range 17 to 300 K. A relative accuracy of 10^?5 is attained. No anomaly on the lattice constant vs. temperature curve is detected except at the cubic-to-tetragonal transition at 106 K. The splitting of the h00 and 00l Bragg peaks are completely resolved below 85 K.     

Anomalous crystallization behavior of strontium titanate aluminosilicate glass-ceramics

The crystallization of glasses with a nominal composition consisting of 65 wt% SrTiO₃, 29 wt% SiO₂, and 6 wt% Al₂O₃, were investigated. Two glass batches of this composition, prepared from different chemicals and similar processing conditions, resulted in glass-ceramics with very different crystalline phases. One glass batch resulted in glass-ceramics with perovskite SrTiO₃ as the primary crystalline phase, while the second glass batch resulted in glass-ceramics with fresnoite Sr₂TiSi₂O₈ as the primary crystalline phase. The different crystallization sequences of glass-ceramics of the two glass batches were traced to differences between the first phases to crystalline from each glass. The crystallization behavior of these glass-ceramics was further complicated by the presence of several unidentified phases.     

Dielectric properties of lanthanum substituted barium titanate microwave ceramics

In this study, we report the high dielectric constant lanthanum substituted barium titanate ceramic material for its possible applications at microwave frequencies. The microwave dielectric characterization of Ba_6 − 3xLa_8 + 2xTi₁₈O₅₄ solid solutions with 0.0 ≤ x ≤ 0.7 prepared by conventional mixed oxide route method has been carried out. The lattice parameters were obtained from the X-ray diffraction patterns. It was observed that lattice parameters increased with respect to an increase in the ‘La’ content. The crystal symmetry investigated was orthorhombic with space group of Pbam. From the evaluation of microwave dielectric properties of lanthanum doped barium titanate ceramics, it was observed that a maximum value of dielectric constant (ε′) = 157 and a minimum tangent loss (tanδ) = 0.0572 was obtained. The minimum value of a.c. conductivity (σ_a.c.) was observed to be 1.76e − 07 S/m.     

Silica-coated lanthanum–strontium manganites for hyperthermia treatments

La_0.76Sr_0.24MnO_3 + δ particles, prepared by performing a traditional, solid-state method of synthesis, were coated by uniform layers of silica via initiating hydrolysis and condensation of TEOS in aqueous–alcoholic alkali environment. The eventually obtained samples exhibited Curie temperature at ∼40 °C, and comprised core-shell particles of ∼250 nm in diameter. By varying stoichiometric ratio of cations within manganite cores of the particles, Curie point of the resulting material can be varied too, thus opening a way for the simple design of biocompatible, temperature-self-regulating particles for application in hyperthermia treatments, with Curie point thereof adjusted to a destined biological context of application.     

Dielectric properties of lead zirconate titanate thin films seeded with barium strontium titanate nanoparticles

A low temperature synthetic method recently proposed by the authors was applied to the fabrication of lead zirconate titanate (PZT) thin films containing crystalline seeds of barium strontium titanate (BST) nanoparticles. PZT precursor and the BST particles were prepared with complex alkoxide methods. Precursor solution suspending the BST particles was spin-coated on Pt/Ti/SiO₂/Si substrate to film thickness of 500–800 nm at particle concentrations of 0–25.1 mol%, and annealed at various temperatures. Seeding of BST particles prevented the formation of pyrochlore phases, which appeared at temperatures above 400 °C in unseeded PZT films, and induced crystallization of PZT into perovskite structures at 420 °C, which was more than 100 °C below the crystallization temperature of the unseeded PZT films. Measurement of dielectric properties at 1 kHz showed that the 25.1 mol% BST-seeded PZT films annealed at 450 °C had a dielectric constant as high as 300 with a dissipation factor of 0.05. Leakage current density of the film was less than 1×10⁻⁶ A/cm² at applied electric field from 0 to 64 kV/cm.     

Effect of lanthanum replacement by strontium in lanthanum nickelate crystals synthetized using gelatin as organic precursor

La_1-xSr_xNiO₃ (0 ≤ x ≤ 0.8) nanopowders were successfully synthesized using gelatin as a polymerization agent. Powders were calcined for 4 h at 700 and 900 °C and characterized using thermal analysis, X-Ray diffraction and scanning electron microscopy. Perovskite phase formation was achieved at 700 °C, with an increase in crystallinity and secundary phases forming in powders calcined at 900 °C. The structure of the lanthanum nickelate was confirmed as rhombohedral and transition to tetragonal structure occurred by partially substituting lanthanum with strontium.     

Atomic layer deposition of ruthenium films on strontium titanate

Atomic layer deposition of ruthenium on SrTiO3 layers was investigated using (C2H5C5H4). (NC4H4)Ru and air as precursors. For comparison, the growth was studied also on ZrO2 films and SiO2/Si surfaces. Deposition temperature was 325 degrees C. Using rather short but intense air pulses, smooth and uniform Ru films were deposited on SrTiO3. The films were crystallized at early stages of the growth. The nucleation density and rate on SrTiO3 were notably lower compared to that on ZrO2 and SiO2, but the physical qualities including the film conductivity were considerably enhanced after reaching Ru film thickness around 10 nm.