Structural transformation and magnetic properties of CaMn1 − xFexO3 − δ (0.0 ≤ x ≤ 0.5)

We have investigated the structural and magnetic properties of CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). Solid state method is used for the synthesis of these samples. Sintering of these compositions at 1300 °C stabilizes higher ionic radii Fe^+ 3 (0.645 Å) at Mn^+ 4 (0.53 Å) site in CaMn_1 − xFe_xO_3 − δ. Structural transformation from orthorhombic to tetragonal to pseudo cubic crystal system and the increase in lattice parameters have been observed with the substitution of Fe at Mn site in CaMn_1 − xFe_xO_3 − δ (0.0 ≤ x ≤ 0.5). The magnetization data show the transformation of G type of antiferromagnetic arrangement of Mn^+ 4 electrons spins in CaMnO₃ into paramagnetic spin type arrangement with the substitution of Fe at Mn site. The compositions x = 0.05, x = 0.1and x = 0.2 show a small kink ~ 100 K in the magnetization data, which resulted due to the competition between antiferromagnetic and paramagnetic states with the Fe substitution.     

Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Highly (100)-oriented Ce1 − xFexO2 − δ solid solution films prepared by laser chemical vapor deposition

Ce_1 − xFe_xO_2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce_1 − xFe_xO_2 − δ phase at x ≤ 0.15, while CeO₂ and Fe₂O₃ phases were found for higher Fe content. Highly (100)-oriented Ce_1 − xFe_xO_2 − δ (x = 0.02) films were obtained at laser power, P_L = 50-200 W and deposition temperature, T_dep = 800-1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P_L = 50-150 W. X-ray photoelectron spectroscopy revealed the presence of Fe³⁺, Ce⁴⁺ and Ce³⁺ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO₂.     

Microstructural characterization of Ce1−xTbxO2−δ (0.60 ≤ x ≤ 0.90) sintered samples

To understand the secondary phase formation in ceria based oxide, the microstructure need to be studied in a wide compositional range. However, in most previous studies, the doping concentration is lower than 50 at.%. In this work, the microstructure of Ce_1−xTb_xO_2−δ sintered samples with Tb concentrations of 0.60 ≤ x ≤ 0.90 were investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Besides the fluorite-structured matrix, a secondary phase with a superstructure formed from a structural modulation of the fluorite structure along [1 1 0] and [0 0 1] directions was observed, whose amount and size reached a maximum at x = 0.90. It has a cubic structure with a lattice constant twice as large as that of the fluorite-structured matrix.     

Fabrication of LiNi0.5 + δMn0.5 − δO2 nanofibers by electrospinning

Polycrystalline tetranary LiNi_0.5 + δMn_0.5 − δO₂ nanofibers have been successfully fabricated by a sol-gel assisted electrospinning technique. The structures and properties of fabricated nanofibers were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). After heat treatment of the electrospun fibers at a temperature of 800 °C, the LiNi_0.5 + δMn_0.5 − δO₂ phase was found without other trace phases. Multilayered nanoparticles with a grain size of 50 nm or less within a single fiber are notable from TEM. In this study, it was shown that the sol-gel assisted electrospun LiNi_0.5 + δMn_0.5 − δO₂ fibers could be formed with the α-NaFeO₂ type crystal structure at a temperature lower than that in a typical solid-state or sole sol-gel process and possess good thermal stability as high as 800 °C.     

Study on the chemical stability of Y-doped BaCeO3 − δ and BaZrO3 − δ films deposited by aerosol deposition

Barium cerate (BaCeO₃) has high proton conductivity but rather poor chemical stability in CO₂-containing atmospheres. Barium zirconate (BaZrO₃), in contrast, is a rather stable material, but exhibits poor sinterability. In the present work, powders of Y-doped BaCeO₃ and BaZrO₃ were synthesized via the solid solution reaction method, and dense ceramic membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ were prepared by the aerosol deposition method at room temperature. Aerosol deposition method is a technique that enables the fabrication of ceramic films at room temperature with a high deposition rate as well as strong adhesion to the substrate. The powders and aerosol-deposited membranes were characterized by X-ray diffraction, particle size analysis, scanning electron microscopy, and X-ray elemental mapping. The chemical stability of powders and aerosol-deposited membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ against water and carbon dioxide has been investigated, and it was found that BaZr_0.85Y_0.15O₃ materials showed a better chemical compatibility.     

Thin film growth of boron nitride on α-Al2O3 (0 0 1) substrates by reactive sputtering

Boron nitride thin films were grown on α-Al₂O₃ (0 0 1) substrates by reactive magnetron sputtering. Infrared attenuated total reflection (ATR) spectra of the films gave an intense signal associated with in-plane B-N stretching TO mode of short range ordered structure of BN hexagonal sheets. X-ray diffraction for the film prepared at a low working pressure (ca. 1 × 10⁻³ Torr) gave a diffraction peak at slightly lower angle than that corresponding to crystal plane h-BN (0 0 2). It is notable that crystal thickness calculated from X-ray peak linewidth (45 nm) was close to film thickness (53 nm), revealing well developed sheet stacking along the direction perpendicular to the substrate surface. When the substrates of MgO (0 0 1) and Si (0 0 1) were used, the short-range ordered structure of h-BN sheet was formed but the films gave no X-ray diffraction. The film showed optical band gap of 5.9 eV, being close to that for bulk crystalline h-BN.     

The 27 Ω rhodium-iron ceramic sensor

The 27 Ω rhodium-iron ceramic sensor is suitable for the measurement of temperatures from 1.2 to 300 K, and can be used up to 800 K. The sensor has been available for over 30 years, and around 12,000 units have been produced in this period. A new manufacturer of the rhodium-iron wire has resulted in different resistance-temperature behaviour with each wire batch. This has required a re-analysis of the calibration data. We discuss two types of calibration data: multi-point across the temperature range, and three-point at temperatures close to 4.2 K, 77 K, and 273 K. In both types it is shown that a reference function is of help with the data analysis. We propose a method for improving the interpolation accuracy for a three-point calibration when using a reference function.     

Phase transitions in Sr1 + xCo0.8Fe0.2O3 − δ oxides

The phase transitions that take place in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (− 0.2 ≤ x ≤ 0.1) oxides are reported here. Thermogravimetric analysis (TGA) showed that the oxides with − 0.2 ≤ x ≤ 0 were prone to undergo oxygen-vacancy disorder-order phase transitions, while others with x = 0.05, 0.1 had more stable crystal structures during oxygen-desorption processes in nitrogen. These results were further confirmed by high-temperature in-situ X-ray techniques. The changes in activation energies of three typical oxides, Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0, 0.1), used as oxygen-permeable membranes were investigated. The phase transitions in Sr_1 + xCo_0.8Fe_0.2O_3 − δ (x = − 0.2, 0) have also been detected in differential scanning calorimetry (DSC) profiles.     

Crystallization of amorphous silicon thin-film on glass substrate preheated at 650 °C using Xe arc flash of 400 μs

Experimental and theoretical investigations on flash lamp annealing (FLA) of amorphous silicon (a-Si) film on glass were carried out with a view to practical applications in large-window display industries. A Xe arc flash lamp of 950 mm in length and 22 mm in bore diameter was applied with nominal input voltage of 7 kV and flash duration of 400 μs. Prior to the annealing process, the specimen for FLA was preheated at 650 °C, which was very close to the service temperature of the glass specimen used in this study. By employing a focusing elliptic reflector, maximum light energy density of up to 8.4 J/cm² could be attained with an active exposure width of 2 cm. Crystallization of a-Si could be achieved in solid-phase by applying a flash beam with light density of at least 5 J/cm², and its phase-transition characteristics that varied with energy densities could be explained by theoretically estimated temperature fields. Electron microscopy observations confirmed that solid-phase crystallization preceded melting of a-Si due to relatively long flashing (heating) duration of 400 μs, which was comparable to solid-phase crystal-growth times at elevated temperatures.     

Dielectric properties of Ba3−xKxCaNb2O9−δ (0.5 < x < 1.25) (KBCN) double perovskites

We report synthesis, structure and dielectric properties of double perovskite-type Ba_3−xK_xCaNb₂O_9−δ (x = 0.5, 0.75, 1, 1.25) (KBCN). Powder X-ray diffraction (XRD) confirmed the formation of double perovskite-type structure and lattice constant decreases with increasing K in KBCN. AC impedance study showed a single semicircle over the investigated temperatures and frequencies in dry H₂, H₂ + 3% H₂O, 3% H₂O + N₂, while two semicircles were observed at low temperatures in air, which could be attributed to bulk and grain-boundary contributions. Unlike un-doped BCN, KBCN exhibits negligible grain-boundary and electrode effects to the total electrical properties and is consistent with perovskite-type K-doped BaZrO₃. The bulk dielectric constant and dielectric loss were found to increase with increasing K content in KBCN and also found to change with sintering temperature. Among the samples investigated, Ba_1.75K_1.25CaNb₂O_9−δ sintered at 1100 °C showed the highest dielectric constant of 65 at 10⁶ Hz and dielectric loss of 0.14 at 400 °C in air. Isothermal dielectric constant and electrical conductivity at 1 MHz were found to be independent at elevated temperatures, while vary at low-frequency and low temperatures. Below 700 °C, dielectric constant and dielectric loss decreases with increasing frequency, whereas an opposite trend was observed for the electrical conductivity.     

Oxygen permeation and electrical transport of Gd1−xPrxBaCo2O5 + δ dense membranes

Double-perovskite Gd_1−xPr_xBaCo₂O_5 + δ membranes showed appreciable oxygen permeability at moderate temperatures. The overall oxygen permeation process of GdBaCo₂O_5 + δ was found to be controlled mainly by the bulk diffusion step with the membrane thickness larger than 0.8 mm, and the limitation by oxygen surface exchange came into play at reduced thickness of 0.8 mm. The electrical conductivity measurement showed Gd_1 - xPr_xBaCo₂O_5 + δ samples possessed a semiconducting behavior at a wide temperature range below 300 °C and a metallic behavior at 300-850 °C with a high conductivity of nearly 10³ S cm^− 1.     

Properties of superconducting, polycrystalline dysprosium-doped Bi1.6Pb0.5Sr2−xDyxCa1.1Cu2.1O8+δ (0 ≤ x ≤ 0.5)

The structural and superconducting properties of dysprosium (Dy) doped (Bi,Pb)-2212 superconductor have been studied. Dy concentration is varied from x = 0.0 to 0.5 in a general stoichiometry of Bi_1.6Pb_0.5Sr_2−xDy_xCa_1.1Cu_2.1O_8+δ. It is found that the Dy atoms enter into the crystal structure by replacing Sr atoms and induce significant change in lattice parameter, microstructure, hole-concentration and normal state conductivity of the system. The critical temperature (T_C) and critical current density (J_C) at self-field of the Dy-doped samples enhance considerably at optimum doping levels. Maximum T_C of 92.3 K (for x = 0.4) and J_C of 1390 A/cm² at 64 K (for x = 0.2) are observed for doped samples as against 79.4 K and 127 A/cm², respectively, for the pure sample. The results are discussed on the basis of the change in hole-concentration due to Dy-doping at Sr-site of (Bi,Pb)-2212 superconductor.     

Dielectric properties,thermal expansion and heat capacity of (1 − x)Na0.5Bi0.5TiO3–xBaTiO3 single crystals (x = 0, 0.02, 0.025, 0.0325 and 0.05)

High-quality and large-size lead-free (1 − x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ single crystals (x = 0, 0.025, 0.0325 and 0.05) were grown using Czochralski method. The obtained samples were of pure perovskite structure with rhombohedral symmetry at room temperature. Thermal expansion, heat capacity, ferroelectric and dielectric properties were measured in a wide temperature range. The broad anomalies observed in thermal expansion and heat capacity were corresponded to structural, ferroelectric and dielectric anomalies, related to temperature features of polar regions and formation of a long-range order ferroelectric phase. The Burns temperature was found to increase with increasing BaTiO₃ content. At low-frequency (100 Hz–100 kHz) the samples showed diffuse phase transitions. The obtained results were discussed in terms of local electric and strain fields caused by a difference in ionic radii between (Na,Bi) and Ba ions.     

Significant improvement of the oxygen permeation flux of tubular Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes covered by a thin La0.6Sr0.4Ti0.3Fe0.7O3 − δ layer

We present a new method to improve the oxygen flux properties and stability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ tube membrane using a thin layer of La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ as protective coatings. The first relevant result is that the La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ protective layer had an extraordinary positive effect on improving the oxygen permeation flux of the tubular Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ membranes. La_0.6Sr_0.4Ti_0.3Fe_0.7O_3 − δ-coated Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ tubular membrane showed the highest oxygen permeability with the flux reaching ~ 3 ml cm⁻² min⁻¹ (oxygen purity > 99%) at 950 °C in static atmospheric pressure through a 1.0 mm thick membrane.     

The corrosion and wear resistances of magnesium alloy (LZ91) electroplated with copper and followed by 1 μm-thick chromium deposits

The Mg-Li-Zn alloy, LZ91, is a potential material for industrial application owing to its high specific strength. However, the LZ91 has so high chemical reactivity that it is easily corroded in air and an aqueous environment. In this study, an eco-friendly electrodeposition method was proposed to obtain a protective Cr/Cu coating on the LZ91 substrate. That is, the LZ91 surface electroplated a Cu undercoat in an alkaline Cu-plating bath and followed by decorative 1 μm-thick Cr electroplating in a plating bath with trivalent chromium ions. After electroplating, some of the Cr/Cu-deposited specimens were heated with a reduction flame for 0.5 s to increase the hardness of Cr-deposit. The wear resistance of as-plated and flame-hardened Cr/Cu-coated LZ91 specimens was estimated with ball-on-plate wear tester by using a steel-ball counterpart. Whereas, their corrosion resistance was evaluated in a 0.1 M H₂SO₄ solution before and after wear test. Experimental results show that a Cu deposit could be uniformly electroplated on the LZ91 surface by using our proposed electroplating method. The corrosion resistance of LZ91 was markedly improved after decorative 1 μm-thick Cr electroplating. After wear test, the steel counterpart was ground obviously with either as-plated or flame-hardened Cr/Cu-coated LZ91 specimen. According to the results of electrochemical corrosion test, some parts of the Cr deposit were peeled off from worn flame-hardened Cr/Cu LZ91 surface. This reduces its corrosion resistance significantly. On the other hand, the corrosion resistance of as-plated Cr/Cu LZ91 was not changed after wear test. That is, an as-plated Cr/Cu-coated LZ91 specimen could have good corrosion and wear resistance.     

Improved critical current densities in thick YBa2Cu3O7 − δ multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

We have fabricated and studied quasi-multilayered thick YBa₂Cu₃O_7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa₂Cu₃O_x nanodots, grown by Pulsed Laser Deposition on SrTiO₃ (100) substrates. Magnetization J_c(B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa₂Cu₃O_7 − δ films of same or even smaller thickness. A high J_c(B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I_c − w. Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa₂Cu₃O_x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I_c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.     

Crossover of magnetoresistance from positive to negative in La0.5Sr0.5CoO3 − σ/Si heterostructure

The magnetoresistance (MR) properties of a heterostructure fabricated by depositing a La_0.5Sr_0.5CoO_3 − σ film on an n-type Si substrate have been studied. The heterostructure exhibits a good rectifying behavior. A negative MR at T = 210 K and a positive MR at T = 300 K are observed for all bias currents whereas; for temperatures ranging from 240 to 280 K the MR changes from being positive to negative with the increase of the bias current. The observed behavior of the MR effect is discussed in terms of current-induced ferromagnetic spin order.     

Direct observation of oxygen vacancy and its effect on the microstructure, electronic and transport properties of sputtered LaNiO3 − δ films on Si substrates

Highly (100)-oriented LaNiO₃ films with different oxygen content were deposited on Si substrates by radio frequency sputtering and post-annealed in oxygen and vacuum conditions. The formation of oxygen vacancies is directly observed by a decrease of lattice oxygen ratio in O 1s core-level photoelectron spectroscopy. X-ray diffraction measurement indicates that low oxygen pressure during the deposition or annealing process has a significant influence on the lattice constant of LaNiO₃ films. Further valence-band spectra and transport measurements demonstrate that the oxygen vacancies also have a significant influence on the electronic structure and transport behaviors of final LaNiO₃ films.