Surface morphology and dielectric properties of alkoxy-derived Sr2Ta2O7 and Sr2Ta, Nb2O7 thin films

Sr₂Ta₂O₇ and Sr₂(Ta, Nb)₂O₇ thin films were prepared from molecular structure-controled alkoxide solutions. The Sr₂Ta₂O₇ thin films initiated to crystallize at around 650 °C and showed random orientation. The surface topography developed was dependent on the solutions. In the thin films prepared from the ethanolic solution, grains formed at low temperatures. However, many small pores still remained and no grains were observed in the thin films prepared from the methoxyethanolic solution. In contrast, the 800 °C-annealed Sr₂(Ta, Nb)₂O₇ thin films showed the (0 k 0) orientation as in Sr₂Nb₂O₇ thin films. The dielectric constants and loss factors of the 750 °C-annealed Sr₂Ta₂O₇ and Sr₂(Ta_0.7Nb_0.3)₂O₇ thin films were around 90 and less than 0.05 at 100 kHz, respectively.     

Synthesis and Properties of (Sr1 – xMx)2Nb2O7 (M = Cu, Ni) Solid Solutions

(Sr_{1 – x} M _x )₂Nb₂O₇ (M = Cu, Ni) layered perovskite solid solutions were synthesized, and their stoichiometry ranges were determined. The electrical properties of the solid solutions were compared with those of Sr₂Nb₂O₇.     

Evolution of the microstructure of undoped and Nb-doped SrTiO3

Undoped and Nb-doped SrTiO₃ specimens with excess titania compositions were prepared by sintering in air at 1420 or 1480 °C. Large grains due to liquid-phase sintering were obtained for undoped specimens containing 0.6 mol % excess titania and fired at 1480 °C. On the other hand uniform fine grains were observed for samples fired at 1420 °C, resulting from grain-growth inhibition due to exsolved TiO₂ second phase. The solubility of excess titania seemed less than 0.2 mol% under our experimental conditions. The microstructural behaviour of Nb-doped SrTiO₃ could be explained well by the Sr-vacancy compensation model. According to this model, the solubility of excess titania in SrTiO₃ increased with Nb₂O₅ dopant concentration. Thus, for specimens which had high excess titania compositions and were sintered at 1480 °C, large grains were observed when the Nb content was low enough to retain sufficient excess titania-forming liquid phase. For specimens having the same compositions and fired at 1420 °C, uniform fine grains were obtained due to grain growth inhibition by the exsolved TiO₂ second phase, when the Nb content was low. If the excess titania was less than the solubility determined by the amount of Nb dopant, Ruddlesden-Popper-type phases were believed to be formed and resulted in poor densification. Although excess titania was the major factor in determining the grain size of the specimens, the niobium dopant enhanced grain growth.     

Effect of the level of addition of Nb2O5/Co2O3 and Ba(Nb2/3Co1/3)O3 on the structure, microstructure, and dielectric properties of BaTiO3

In this work attempt is made to study the effects of the level of the addition of two dopant systems, that is, Nb₂O₅/Co₂O₃ and Ba(Nb_2/3Co_1/3)O₃ on the structure, microstructure, and dielectric properties of BaTiO₃(BT) prepared by ceramic route. For the BT samples doped with 2 and 3 at % Nb₂O₅/Co₂O₃ a broad maxima for _r–T variations was observed. Higher levels of addition, that is, 6, 8, and 10 at % Nb₂O₅/Co₂O₃, however, caused a drastic reduction in the magnitude of _r and vanishing of maxima peaks observed. The reduction in the magnitudes of the _r for these samples is related to the observed reduction of the tetragonality of doped BT samples as well as the existence of some non-ferroelectric extra phase, evidenced by our XRD analysis. The existence of this extra phase is also believed to be mainly responsible for the observed increase of the dielectric loss. For the BT samples doped with Ba(Nb_2/3Co_1/3)O₃ perovskite solid solution, the _r maxima peak could only be detected for the samples doped with 2 at %. In the case of the samples doped with 4, 6, 8, and 10 at % perovskite solid solution, however, a very uniform distribution of _r–T was observed. The increase of the level of dopant for these series of samples was also seen to give rise in the reduction of the magnitude of _r. However, this reduction was not as strong as the cases observed for the samples doped with 8 and 10 at % Nb₂O₅/Co₂O₃. XRD patterns obtained for these series of samples also revealed the reduction of tetragonality of the BT samples when doped with Ba(Nb_2/3Co_1/3)O₃. Further, microstructural studies carried out by scanning electron microscopy (SEM) on both series of samples revealed a contrasting picture. While a uniform grain size distribution was observed for the whole series of the samples doped with Ba(Nb_2/3Co_1/3)O₃, a non-uniform size distribution of grain sizes was observed in the case of samples doped with Nb₂O₅/Co₂O₃. This is thought to be due to the possible formation and non-uniform distribution of a liquid phase due to the formation of Ti-rich region in the shell region of Nb₂O₅/Co₂O₃-doped samples. The existence of such liquid phase, in case of Nb₂O₅/Co₂O₃-doped samples, would have the consequence of an increased rate of diffusion of Nb/Co into the BT cores leaving narrow compositional fluctuations for the shell regions.     

Effect of dopants on the microstructure and lattice parameters of lead zirconate-titanate ceramics

The effect of dopants on the microstructure and lattice parameters of Pb_0.94Sr_0.06(Zr_0.53Ti_0.47)O₃ ceramics was studied. Small amounts (0 to 0.3 wt%) of Cr₂O₃, MnO₂, Co₂O₃ and U₃O₈, were used as dopants. In a few compositions two oxides were used, namely U₃O₈ with Cr₂O₃ as well as MnO₂ with Cr₂O₃ X-ray diffraction investigation of powdered samples showed that all the compositions (both modified and unmodified) were of perovskite type with tetragonal symmetry. The Curie temperature was decreased with the decrease of tetragonal distortion indicated by the axial ratio,c ₀/a ₀, in all cases. The average grain sizes for the doped and undoped compositions were determined from SEM photomicrographs of polished and acid-etched surfaces using the linear intercept technique. The average grain size for the undoped composition was found to be 10.7m. The addition of a small amount of dopants controlled the grain growth to give rise to the average grain size of 6.4 to 9.5m, which appeared to have some effect on the improvement of the piezoelectric properties of the ceramics.Communication No. 4123 from the National Chemical Laboratory, Poona, India.     

Synthesis of (BaxSr1−x)(Zn1/3Nb2/3)O3 ceramics by reaction-sintering process and microstructure

(Ba_xSr_1−x)(Zn_1/3Nb_2/3)O₃ complex perovskite ceramics produced by reaction-sintering process were investigated. Secondary phases ZnNb₂O₆ and (Cu₂Zn)Nb₂O₈ were found at the perovskite matrix grain boundaries. 98.5–99.8% of the theoretical density was obtained. In (Ba_0.5Sr_0.5)(Zn_1/3Nb_2/3)O₃ and (Ba_0.7Sr_0.3)(Zn_1/3Nb_2/3)O₃, the amount and volume of second phases decreased clearly at 1500 °C.     

Effect of Nb addition on magnetic,structural and superconducting properties of (Bi,Pb)-2223 superconductors

In this work, the effects of Nb₂O₅ addition with different ratios on the structural and magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y (x = 0.00–0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (J_c) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y superconductor. Critical onset (T _c ^on ) and loss peak temperatures (T_p) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108–98 K with increasing Nb addition (x = 0.00–0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (J_c) by means of the Bean’s model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca₂PbO₄ as the minor. It was also shown from XRD measurements that volume fraction of high-T_c phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.     

Grain orientation and electrical properties of Sr2Nb2O7 ceramics

Sr₂Nb₂O₇ ceramics sintered at 1753 K and the hot-pressed compacts annealed at 1073 K possess low density because of the formation of voids by large expansion. The expansion is attributed to cleavage and due to the preferential grain growth along the <010> direction. An increase in dielectric constant at temperatures over 973 K and a parabolic decrease of resistivity with increasing temperature are observed. The constant variation corresponds well to the decrease in thermal strain and the grain orientation factor along the <0K0> direction.     

Properties of sol-gel derived strontium barium niobate ceramics and the effect of V2O5 additive

Strontium barium niobate, Sr _x Ba_1–x Nb₂O₆, (SBN) ceramics with a range of Sr/Ba ratios were fabricated using a solution chemistry approach with alkoxides dissolved in acetic acid. Powders obtained from dried gels were calcined at 800°C for three hours, a heat treatment that produced fully crystalline powders. After preparing bulk ceramics from these powders by sintering at 1200°C for 1–24 hours, the tetragonal tungsten bronze (TTB) phase was present in all compositions between 0 x 1, although not predicted by the phase diagram for BaNb₂O₆and SrNb₂O₆. Vanadium pentoxide (V₂O₅) additions increased the densification, the amount of tungsten bronze phase, and the dielectric constant for all compositions except for Sr_0.65Ba_0.35Nb₂O₆(SBN65) ceramics.     

Molten salt synthesis of anisometric particles in the SrO-Nb2O5-BaO system

Acicular particles of KSr₂Nb₅O₁₅, Sr_0.5Ba_0.5Nb₂O₆, and SrNb₂O₆ were synthesized in the SrO-BaO-Nb₂O₅ system, using KCl or SrCl₂·6H₂O salts. In the SrCl₂·6H₂O + Nb₂O₅ system, acicular SrNb₂O₆ particles were formed by a solid state reaction between the salt and Nb₂O₅. Large, irregularly shaped (Sr-rich) Sr₂Nb₂O₇ particles formed with increasing reaction temperature and time. Small but finite solubility of SrO in the KCl + SrNb₂O₆ system favored the formation of acicular KSr₂Nb₅O₁₅ and blade-like Sr₂Nb₂O₇ particles (at higher temperatures). Uniformly sized, acicular KSr₂Nb₅O₁₅ particles were easier to reproduce compared to the formation of acicular Sr_0.5Ba_0.5Nb₂O₆ and SrNb₂O₆ particles.     

Optical and structural properties of TiO2 films as a function of Nb doping concentration

Ti_1?Cx Nb _x O₂ (x?=?0?C0.06) films were prepared on quartz substrates by sol?Cgel spin coating and characterized by a variety techniques. The effect of Nb doping on the structural and optical properties of Ti_1?Cx Nb _x O₂ films were mainly investigated by X-Ray diffraction (XRD), Raman spectroscopy, Field-emission scanning electron microscopy and UV?Cvis transmittance spectroscopy. XRD and Raman study showed that the Nb doping inhibited the grain growth. The grain size decreased from 39.4 to 23.4?nm as the doping concentration increased from 0 to 0.06 in atomic percent. The UV?Cvis transmittance spectroscopy analysis revealed that the films exhibited 55?C65% transmittance in the visible region and the band gap of films became wider with increasing Nb doping concentration.     

Formation of niobium-tantalum pentoxide orthorhombic solid solutions under chlorine-bearing atmospheres

This paper studies, by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), the structural changes suffered by amorphous hydrated Nb oxide and mixtures of amorphous and crystalline Nb and Ta oxides subjected to thermal treatments in air and chlorine atmospheres. The air heating of amorphous Nb₂O₅·nH₂O leads to different crystalline phases depending on the working temperature. The hexagonal phase of Nb₂O₅ is obtained at 773 K and the monoclinic phase -Nb₂O₅ is obtained at 1173 K. The thermal treatment of amorphous Nb oxide in chlorine atmosphere decreases the temperature at which phase -Nb₂O₅ appears and at 1173 K the stable monoclinic phase -Nb₂O₅ is obtained. Air calcination for 5 hours at temperatures between 973 and 1273 K of different amorphous Nb₂O₅·nH₂O - Ta₂O₅·nH₂O mixtures does not lead to solid solution of these oxides. Thermal treatment in chlorine atmosphere of amorphous Nb and Ta oxides leads to the formation of Nb₂O₅ and Ta₂O₅ orthorhombic solution in one hour at 973 K and in 24 hours at 1223 K, when starting from crystalline oxides. The effect of chlorine is due to the dissolution-recrystallization of the metallic chlorides and the oxygen formed, when the system evolves to chemical equilibrium between solid phases and gaseous chlorine.     

Optical Study of the Pseudogap State in Single Layer Bi2Sr2CuO6+δ

We present infrared reflectivity measurements of the ab-plane response of Pb- and La-doped Bi ₂ Sr ₂ CuO ₆₊ single crystals. A suppression of scattering rate is observed for La-doped samples below and above T _c , suggestive of a pseudogap. Compared with double-layer Bi ₂ Sr ₂ CaCu ₂ O ₈₊ , the suppression is weak and appears at lower frequencies. This may reflect a weak pairing interaction and/or smaller gap amplitude.     

Magnetic and Transport Properties of Bilayered Perovskite Sr3Ir2O7

We report magnetic and transport properties of polycrystalline samples of bilayered perovskite Sr₃Ir₂O₇, which is synthesized by using high pressure technique. Magnetic susceptibility shows weak ferromagnetism below 290K with small size of remanent moment (10^–3B/Ir). Resistivity shows insulating behavior below 290K. We discuss the magnetism and conductivity in Sr₃Ir₂O₇ comparing with those of Sr₃Ru₂O₇, which has a quite similar crystal structure to Sr₃Ir₂O₇, from the view point of rotation angle of MO₆ (M=Ir, Ru) octahedra.     

Oxides formed between ZrO2 and Nb2O5

High-purity monoclinic ZrO₂ and monoclinic Nb₂O₅ powder samples were mixed in varying ratios, and heated at 1300°C for 24 h before furnace cooling. X-ray diffraction analysis showed that two compounds, Nb₂O₅-6ZrO₂ and Nb₂O₅·8ZrO₂ (or (Zr, Nb)₈O₁₇ and (Zr, Nb)₁₀O₂₁), were formed. Some of the reflections from the two compounds were overlapped and interfered with those from the zirconium oxide, especially the tetragonal ZrO₂. A thermodynamic analysis was used to demonstrate that it is possible to have ZrO₂ as well as Nb₂O₅ formed on Zr-Nb alloys in an oxidizing environment. The relevance of these results (experimental and calculated) to the corrosion of a Zr-Nb alloy in high temperature aqueous environments is discussed. It is suggested that the incorporation of niobium into the oxide during the corrosion of Zr-Nb alloys could be by the formation of compounds such as (Zr, Nb)₈O₁₇ and (Zr, Nb)₁₀O₂₁. Also, the tetragonal ZrO₂, which has been found to be a barrier layer oxide, could, in fact, be either of the ternary compounds Nb₂O₅·6ZrO₂ and Nb₂O₅·8ZrO₂ both of which are orthorhombic with lattice parameters very close to those of the tetragonal ZrO₂.     

Effect of Zr+4 ion substitution on the structural, dielectric and electrical properties of Sr5LaTi3Nb7O30 ceramics

Polycrystalline Zr-modified Sr₅LaTi₃Nb₇O₃₀ compound was prepared by a high-temperature solid-state reaction technique. X-ray structural analysis confirmed the formation of compound in an orthorhombic system at room temperature. Detailed studies of the dielectric parameters (dielectric constant and tangent loss) as a function of frequency (1–100 kHz) at different temperature (150 to 650 K) were carried out. It was found that as Zr⁺⁴ concentration increases in Sr₅LaTi_{3 – x} ZrxNb₇O₃₀, the value of dielectric constant decreases. These compounds show ferroelectric-paraelectric phase transition of diffuse type at 283, 305 and 320 K for x = 0, 1 and 2 respectively. The transition temperature (T _c) shifts towards higher temperature and maximum dielectric constant value (_max) decreases with increasing Zr⁺⁴ concentration for x = 0 to x = 2. When Ti⁺⁴ cations were completely replaced by Zr⁺⁴ cations (i.e., Sr₅LaZr₃Nb₇O₃₀), the compound does not show any phase transition. Impedance-spectroscopic studies provided an insight into the electrical properties and understanding of relaxation behavior of the material. Measurement of electrical conductivity as a function of temperature suggests that the compounds have a negative temperature coefficient of resistivity (NTCR) with a typical semiconductor behavior.     

Formation of the La0.03Sr0.255Ba0.7Nb2–yTiyO6–y/2 ferroelectric ceramic system

The study of the formation of the Sr_0.255La_0.03Ba_0.7Nb_{2 – y} Ti _y O_{6 – y/2} ferroelectric ceramic system and the influence of the sintering conditions and titanium concentration on the densification process is reported. A single-phase compound is observed for low titanium content (y 0.1) in the XRD spectra, being isostructural with the tetragonal tungsten bronze SBN phase. For high titanium concentrations (y > 0.1) the XRD patterns show, besides the tetragonal phase reflections, several small peaks evidencing an additional phase in the compound. This second phase was identified as isostructural to hexagonal Ba₆Nb₉Ti₇O₄₂. The grain size shows a linear increase with titanium content in the region of monophasic compositions and a possible liquid phase aided sintering process is analyzed.     

Insulators for YBa2Cu3O7 Josephson Junction Technology

Because ideal PrBa₂Cu₃O₇ superconducts at 90 K, it should be replaced by RBa_2–u Sr _u Cu₂MO₈ (where R is a light-mass rare earth and M is Nb or, preferably, Ta) as the insulator of choice for YBa₂Cu₃O₇ Josephson junction or microstructure technology. Furthermore, (110)- or (100)-oriented substrates of RSr₂Cu₂MO₈ may provide an opportunity for optimal device fabrication.     

Effects of Nb doping on the microstructure,ferroelectric and piezoelectric properties of 0.7BiFeO<Subscript>3</Subscript>–0.3BaTiO<Subscript>3</Subscript> lead-free ceramics

Donor-doped lead-free Bi_0.7Ba_0.3(Fe_0.7Ti_0.3)_1?x Nb_0.66x O₃ + 1 mol% MnO₂ ceramics were prepared by a conventional oxide-mixed method and the effects of Nb-doping on microstructure, piezoelectric and ferroelectric properties of the ceramics were investigated. All the ceramics exhibit a pure perovskite structure with rhombohedral symmetry. The grain growth of the ceramics is inhibited after the addition of Nb doping. High electric insulation (R = 10⁹–10¹⁰ Ω?cm) and the poor piezoelectric performance and weak ferroelectricity are observed after the addition of Nb₂O₅ in the ceramics. Different from the donor effect of Pb-based perovskite ceramics, the introduction of Nb into 0.7BiFeO₃–0.3BaTiO₃ degrades the piezoelectricity and ferroelectricity of the ceramics. The Bi_0.7Ba_0.3(Fe_0.7Ti_0.3)_1?x Nb_0.66x O₃ + 1 mol% MnO₂ ceramic with x = 0 exhibits the optimum piezoelectric properties with d ₃₃ = 133 pN C^?1 and k _p = 0.29 and high Curie temperature (T_C = 603^°C).