The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

Enhanced magnetic properties of Nd–Fe–B thin films crystallized by heat treatment

Nd₂Fe₁₄B Φ phase crystallites were formed in Nd_16.7Fe_65.5B_17.8 thin films prepared by RF sputtering with subsequent heat treatment. The 2 μm-thick films were deposited onto 0.1 mm Mo sheets at an average substrate temperature (T_s) of 365°C. The enhanced magnetic properties of the magnetically anisotropic thin films were investigated using different heating rates (h_r) of 10°C, 20°C, 50°C and 100°C/min in an annealing experiment. Transformation from the amorphous phase to the crystalline phase is clearly manifested by the formation of fine crystallites embedded as a columnar matrix of Nd₂Fe₁₄B phase. High-resolution scanning electron microscope data of the cross-section of the annealed films show columnar stacking of Nd₂Fe₁₄B crystallites with sizes <500 nm. Transmission electron microscope observations revealed that the microstructure of these films having out-of-plane magnetization consists of uniformly distributed Φ phase with grain size around 400 nm together with small Nd rich particles. This grain size of Φ phase is comparable to the single domain particle diameter of Nd₂Fe₁₄B. Significant change in _iH_c, 4πM_r and 4πM_s with h_r was confirmed. Annealing conditions with a heating rate of 50°C/min to an annealing temperature (T_a) of 650°C for 30 min was consequently found to give optimum properties for the NdFeB thin films. The resulting magnetic properties, considered to be the effect of varying h_r were _iH_c= 1307–1357 kA/m, 4πM_r=0.78–1.06 T and 4πM_s=0.81–1.07 T.     

Ln1−xSrxCoO3(Ln = Sm, Dy) for the electrode of solid oxide fuel cells

The perovskite-type oxides were synthesized in the series of Ln_1−xSr_xCoO₃(Ln = Sm, Dy). The formation of solid solutions in Dy_{1 − x}Sr_xCoO₃ was limited, compared with that in Sm_{1 − x}Sr_xCoO₃. The electrical conductivities of the sintered samples were measured as a function of x in the temperature range 30 to 1000 °C. The highest conductivity of around 500 S/cm at 1000 °C was found in Sm_0.7Sr_0.3CoO₃. The reactivity of all the samples with YSZ was examined at 800–1000 °C for 96 h. The Sr-doped perovskite oxides were more reactive with YSZ and produced SrZrO₃ at 900 °C after 96 h. However, no reaction product between SmCoO₃ and YSZ was observed at 1000 °C for 96 h. The cathodic polarization of the oxide electrodes, sputtered on yttria stabilized zirconia (YSZ), was studied at 800–1000 °C in air. SmCoO₃ shows no degradation of the electrode performance at higher temperatures. The thermal expansion measurements on the sintered samples were carried out from room temperature to 1000 °C. Large thermal expansion coefficients were found in these samples.     

The effect of heat treatment on explosively compacted (Y---Ba---K---Cu---O) superconductive powders

A ceramic powder mixture corresponding to the stoichiometric ratio of YBa_1.95K_0.05Cu₃O_y superconducting ceramic with 10% w/w Ag was heat treated at 950°C for 3 h in flowing oxygen. The resultant superconducting powder was consolidated in a silver tube by the explosive compaction technique (pre-heat treated ceramic). The same fabrication technique, i.e. explosive compaction, was employed to the initial stoichiometric powder mixture, without prior heat treatment and the green product was subsequently subjected to a series of heat-treating cycles (920°C, 70 h), in order to obtain the required superconducting properties (post-heat treated ceramic). The microstructure and physical properties of the pre- and post-heat treated ceramics were analyzed and compared using various characterization techniques. X-ray diffraction performed on the pre-heat and post-heat treated ceramics indicated that maximum 123-phase content retained in the former case (pre-heat treated ceramic), whilst DC magnetic susceptibility measurements have shown the characteristic Meissner shielding effect at 99 and 92 K, respectively. Furthermore, the pre-heat treated compacted billet was characterized by a critical current density, J_c equal to 2800 A/cm² at 77 K after suitable annealing at 850°C for 10 h in oxygen stream.     

Phase transition and dielectric characteristics of nano-grained BaTiO3 ceramics synthesized from surface-coated nano-powders

Nano-grained BaTiO₃ (BTO) ceramics were synthesized by pressureless sintering BTO nano-powders coated with Mn materials. The BTO ceramics, which were prepared at 800 °C from nano-powders of 40 nm size, are of a cubic structure with a small fraction of tetragonal phase, similar to that of the starting BTO nano-powders of the same size. On the other hand, the BTO ceramics with about grains of 1300 nm size, which were prepared at sintering temperature ranging from 1100 to 1200 °C, have a mainly tetragonal phase. By increasing the sintering temperature and/or the amount of the coating materials, the crystallinity of the tetragonal phase was increased. The relation of the dielectric features of the nano-grained BTO ceramics with the structural variation is discussed in terms of process parameters.     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

Low temperature fabrication of vanadium oxide films for uncooled bolometric detectors

Vanadium oxide films with temperature coefficient of resistant of −2.6% K⁻¹ have been fabricated on Si₃N₄-film-coated Si substrates by ion beam sputtering in a controlled Ar/O₂ atmosphere, at a relatively low growth temperature of 200 °C. The as-deposited films show no semiconductor-to-metal phase transitions even heated up to 150 °C. X-ray diffractometry shows that the main compound of the VO_x film is a metastable phase of vanadium dioxide (VO₂(B)) and the VO₂(B) film can be transformed into VO₂ film by post-growth annealing at 450 °C in flowing Ar atmosphere.     

The temperature dependence of the short wavelength transmittance limit of vacuum ultraviolet window materials—I. Experiment

The short wavelength transmittance limit or cut-off wavelength, λ_co, of LiF, MgF₂, CaF₂, LaF₃, BaF₂, sapphire, synthetic crystal quartz and fused quartz has been measured from about 100°C to about 10°K. λ_co is not a well denned quantity, so for the purpose of this experiment it has been arbitrarily taken as the wavelength where transmittance could just be measured, usually 0.1-0.5 per cent. With one exception λ_co shifted to shorter wavelengths as the sample was cooled; the shift varied from about 40 to 80 Å over the temperature range from 100°C to 10°K, depending on the material, with the largest shift occurring in BaF₂;. The exception was LaF₃ which showed no measurable change in λ_co wth temperature. Over the temperature range from 20° to 100°C the slope of λ_co, with temperature for all materials was fairly constant, but below 20°C it decreased, approaching zero as the temperature approached 20°-10°K. In the case of synthetic crystal quartz, for example, the slope changed from about 0.28 Å/°K at room temperature to about 0.055 Å/°K at 80°K.     

Origin of cathodic degradation and new phase formation at the La0.9Sr0.1MnO3/YSZ interface

New phase formation at the La_0.9Sr_0.1MnO₃/YSZ interface and its effects on the cathodic performances were studied at 900 °C in air. The resistance caused by the interfacial product layer kept increasing with time to reach up to 40% of the total resistance after 500 h. The interfacial product was identified as La₂Zr₂O₇ by XRD measurement. The electrical conductivity of La₂Zr₂O₇ (2.4 × 10⁻⁵ S cm⁻¹ at 1000 °C), measured by AC impedance and current interruption methods, was 4 to 7 orders of magnitude smaller than those of La_0.9Sr_0.1MnO₃ electrode or YSZ electrolyte. Either the electronic conductivity or the electrochemical O₂ reduction activity of La₂Zr₂O₇ was negligible. Combining these results, a conclusion was made that the cathodic degradation comes mainly from the growth of interfacial product layer and its contribution to the cell resistance increment is ohmic in nature.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

Densification and conductivity enhancement of Na4Zr2Si3O12-based solid electrolytes using TiO2 as a sintering aid

Densification of Na₄Zr₂Si₃O₁₂ (NZS) solid electrolytes was performed by dispersing TiO₂ (0.8–5.9 wt. %, corresponding to 5–30 mol %) in NZS powders prior to sintering at 1200°C. Increases in pellet density, from ca. 65 to 94% of the theoretical (X-ray density) value, and in electrical conductivity from 10⁻⁷ to 10⁻⁶ S/cm at 50°C were observed for small additions of TiO₂, which acts as a sintering aid. AC impedance spectroscopy reveals that the enhancement is not a bulk effect but instead is associated with a reduction in inter-granular constriction resistances within porous NZS ceramics. The presence of adsorbed water species in NZS powders prepared via a sol-gel route is found to have a dramatic effect on the conductivity enhancement.     

Phase evolution during the melting and recrystallization of ceramic core in the (Bi,Pb)-2223 tape

The phase evolution during melting and recrystallization of (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ ((Bi,Pb)-2223) core in a Ag-sheathed monofilamentary tape has been investigated. The tape was fabricated by PIT process with powders containing nearly pure (Bi,Pb)-2223 phase. Short samples were melted at 805 °C, 808 °C, 812 °C, 816 °C, 831 °C, slowly cooled at 1.5 °C/h under flowing 1.6% O₂ balanced with argon and quenched in air at room temperature. X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS) were applied for the phase identification. The results show that (Bi,Pb)-2223 core is partially melted into a liquid and alkaline earth cuprates (AECs), mainly 2:1-AEC, at 805 °C, 808 °C, 812 °C, and well reforms directly from the melt during the slow cooling. More (Bi,Pb)-2223 phase is decomposed at temperatures higher than 816 °C, but cannot recrystallize, indicating that a partial melting at some temperatures around a given temperature range is essential to (Bi,Pb)-2223 phase reformation. The melt composition moves from that between “2223” and “2212” stoichiometries towards 2212-like stoichiometry with increasing temperature. This seems to lead to the conclusion that (Bi,Pb)-2223 phase decomposes incongruently into a 2212-like liquid and (Ca,Sr)-cuprates. 2:1-AEC plays the most important role in (Bi,Pb)-2223 melt-recrystallization process. Our results also reveal that plate-like shape (Bi,Pb)-2223 grains can be obtained via melting and recrystallization if the optimum processing conditions are used.     

Structural-phase transition and electrical conductivity in tin-modified zirconium titanate

The structural phase transition in tin-modified zirconium titanate was investigated using high-temperature X-ray, DTA, DSC and electrical conductivity. In a dilute solid solution of Sn (x 0.2) in Zr_1−xSn_xTiO₄, we ascribed to the successive phase transition from normal to incommensurate the break of electrical conductivity, thermal anomalies, and specific heat anomalies occurring at temperatures of 1121 °C, 1124 °C and 1125 °C on the heating run, and at temperatures of 1121 °C, 1116 °C and 1117 °C on the cooling run, respectively. Furthermore, the super-lattice reflection intensity was inversely proportional to the Sn content, cooling rate and electrical activation energy. With a composition of Zr_0.8Sn_0.2TiO₄, the phase transition from normal to incommensurate was completely inhibited.     

The phase transition and optimal synthesis temperature of LiNiO2

The phase transition of LiNiO₂ has been studied by using impedance spectroscopy, DTA, and X-ray diffraction methods. At 720°C the hexagonal phase changes in air to the cubic phase. The phase transition process is rather slow and partially reversible. A peculiar conductivity behavior was observed. From 324°C the conductivity deviates from Arrhenius behavior and is temperature-invariant until 613°C. Subsequently, it decreases rapidly with increase of the temperature. This phenomenon is related to the occupation of Ni²⁺ in Li⁺ positions. The optimal sintering temperature for LiNiO₂ is suggested to be 700°C.     

Structural evolution of Sc-containing zirconia electrolytes

A powder of nominal composition (ZrO₂)_0.886(Sc₂O₃)_0.104(Al₂O₃)_0.01 was synthesized by spray drying with the purpose of testing the performance of solid oxide fuel cells containing scandia-stabilized zirconia (ScSZ) as electrolyte. The phases resulting from calcination and sintering at different temperatures were investigated by XRD, impedance and Raman spectroscopy. At sintering temperatures of 1200–1400 °C nearly equal amounts of cubic and rhombohedral ScSZ were detected, whereas heat treatment higher than 1500 °C led to a cubic single-phase material. A preliminary reaction mechanism of phase formation is proposed with respect to the various results depending on heat treatment.     

Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

Heat-treatment studies of molybdenum oxide-monohydrate

The dehydration of molybdic acid, MoO₃---H₂O, was studied by thermal analysis, X-ray diffractometry and FTIR spectroscopy. The results show that an intermediate phase, MoO₃-2/3H₂O is formed at 216 °C and the monoclinic form of MoO₃ is grown above 350 °C. The mechanism of dehydration and structural rearrangement were confirmed by the features of the infrared spectra showing formation of corner-shared MoO₆ octahedra.     

Chemical stability at noble metal M/YBa2Cu3O6.8 interfaces (M = Pt, Ag, Au)

The chemical compatibility between YBa₂Cu₃O_6.8 (Y123) and Pt, Ag or Au was studied using quasi-infinite diffusion couples which were encapsulated and annealed at 650 to 800°C for 5 to 80 h. The phase formation at the interface was analyzed in cross sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. In addition, bulk powder mixtures of Y123 with Pt, Ag or Au were annealed at 800°C for 100 h and phase analysis was performed using X-ray diffraction.

At the Pt/Y123 interface a reaction zone grows slowly but decisively at 800°C. Its microstructure is multiphase with Y---Cu-oxides and a fine-grained dispersion of Y₂BaCuO₅ (Y211) with Ba---Cu-oxides. Additional Ba---Pt oxides and other phases are seen in powder mixtures annealed for longer times. Barium is suspected to diffuse out from the superconductor along grain boundaries from as deep as 1 mm, causing the decomposition of Y123 into Y---Cu-oxides in the depleted regions. Ag and Au form a stable contact at the interface to Y123 with no reaction zone or new phases. Interdiffusion at the Ag/Y123 interface at 800°C was too low to be clearly detected. In contrast, Au diffuses very fast into Y123 and at 800°C the solubility is 4.2 mass% Au. Yttrium and barium diffuse much slower into the (Au) phase, Cu diffusion was not detected. Weak traces of decomposition products, mostly Y211 and Ba---Cu-oxides, were observed in bulk powder mixtures of Y123 with Ag or Au annealed at 800°C for 100 h in closed capsules. These decomposition products are considered to be due to the high oxygen pressure in the closed capsule, exceeding the stability limit of Y123, and not due to the reduction of Y123. Both Ag and Au are virtually non-reactive with Y123.     

Superconducting oxide formation in the Yb---Ba---Cu---O system

Conditions for forming YbBa₂Cu₄O₈, Yb₂Ba₂Cu₃O_7−δ and YbBa₂Cu₃O_7−δ were determined in oxygen at 1.0 atm pressure by experiments with oxidized Yb---Ba---Cu---Ag alloys. YbBa₂Cu₃O_7−δ formed in less than 10 s when oxidized alloy ribbons were baked in the 805°C–890°C range. Large amounts of Ba₂Cu₃O₅ formed below 870°C. Yb₂Ba₄Cu₇O_15−δ and YbBa₂Cu₄O ₈ then formed in the 10²s–10³s range at temperatures in the 840°C–870°C and <840°C ranges, respectively. Ba₂Cu₃O₅ decomposition supplied Cu²⁺ and O²⁻ for the rapid transformation of YbBa₂Cu₃O_7−δ grains into Yb₂Ba₄Cu₇O_15−δ grains by intercalation. Intercalation was much slower in the absence of Ba₂Cu₃O₅. All of these transformation products underwent coarsening with increasing bake time. Rapid YbBa₂Cu₄O₈ and Yb₂Ba₄Cu₇O_15−δ formation, compared to the slow formation of these phase types when Yb was substituted by Y, Is due to the differing effects of Y and Yb on formation kinetics. Superconducting oxide-silver composite ribbons containing mainly YbBa₂Cu₃O_7−δ, Yb₂Ba₄Cu₇O_15−δ or YbBa₂Cu₄O₈ can be prepared from Yb---Ba ---Ba---Cu---Ag alloys.     

Synthesis and physical behaviour of In2S3 films

In₂S₃ layers have been grown by close-spaced evaporation of pre-synthesized In₂S₃ powder from its constituent elements. The layers were deposited on glass substrates at temperatures in the range, 200–350 °C. The effect of substrate temperature on composition, structure, morphology, electrical and optical properties of the as-grown indium sulfide films has been studied. The synthesized powder exhibited cubic structure with a grain size of 63.92 nm and S/In ratio of 1.01. The films grown at 200 °C were amorphous in nature while its crystallinity increased with the increase of substrate temperature to 300 °C. The films exhibited pure tetragonal β-In₂S₃ phase at the substrate temperature of 350 °C. The surface morphological analysis revealed that the films grown at 300 °C had an average roughness of 1.43 nm. These films showed a S/In ratio of 0.98 and a lower electrical resistivity of 1.28 × 10³ Ω cm. The optical band gap was found to be direct and the layers grown at 300 °C showed a higher optical transmittance of 78% and an energy band gap of 2.49 eV.