Sol–gel formation,Mössbauer studies,optical absorption and impedance characteristics of Ba0.5Sr0.5Zn0.2Fe0.8O3−ξ powder

ABO_3−ξ-type oxides have gained prominence because of their usefulness in gas separation, solid oxide fuel cell, gas sensor, etc. Of particular interest is barium and zinc substituted strontium ferrite – an alternative cathode material due to its high structural stability, accommodation of considerable anion deficiency, and good oxygen permeability. An attempt has therefore been made to synthesize Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−ξ powder by a novel oxalate sol–gel route to investigate its formation, nature of iron species, optical absorption, and impedance behaviour. The synthesis process involves gel formation, digestion for 6 h, drying at 150 °C for 24 h, and decomposition of oxalate at 950 °C for 15 h in air. The product is shown to exhibit (i) a perovskite-type cubic phase with a = 3.975 ± 0.002 Å, Z = 1, and space group Pm3m, (ii) Fe³⁺_0.5 and Fe⁴⁺_0.3 ions, (iii) oxygen deficiency parameter ξ ∼ 0.45, and (iv) optical absorption at ∼370 nm (∼3.4 eV) and ∼797 nm (∼1.56 eV) – arising due to charge transfer transition from O²⁻(2p) to Fe³⁺(3d) and octahedral crystal field splitting of iron t_2g and e_g orbitals, respectively. Moreover, the high impedance values observed below 10 kHz over a temperature range of 303–413 K have been attributed to space charge polarization; activation energy of the relaxation process being 0.2 eV. The motion of induced polarons is possibly responsible for the decrease of impedance with increase of temperature in the range 303–413 K.     

Cadmium oxide, indium oxide and cadmium indate thin films obtained by the sol-gel technique

Thin films of the mixed CdO-In₂O₃ system were deposited on glass substrates by the sol-gel technique. The precursor solution was obtained starting from the mixture of two precursor solutions of CdO and In₂O₃ prepared separately at room temperature. The In atomic concentration percentages (X) in the precursor solution with respect to Cd (1 − X), were: 0, 16, 33, 50, 67, 84 and 100. The films were sintered at two different sintering temperatures (Ts) 450 and 550 °C, and after that, annealed in a 96:4 N₂/H₂ gas mixture at 350 °C. X-ray diffraction patterns showed three types of films, excluding those constituted only of CdO and In₂O₃ crystals: i) For X ≤ 50 at.%, the films were constituted of CdO + CdIn₂O₄ crystals, ii) For X = 67 at.%, the films were only formed of CdIn₂O₄ crystals and iii) For X = 84 at.% the films were constituted of In₂O₃ + CdIn₂O₄ crystals. In all films in the 0 < X < 100 range, the formation CdIn₂O₄ crystals of this material was prioritized with respect to the formation of CdO and In₂O₃ materials. All films showed high optical transmission and an increase of the direct band gap value from 2.4 (for CdO) to 3.6 eV (for In₂O₃), as the X value increases. The resistivity values obtained were in the interval of 8 × 10^− 4 Ω cm to 10⁶ Ω cm. The CdIn₂O₄ films had a resistivity value of 8 × 10^− 3 Ω cm and a band gap value of 3.3 eV.     

Study on ZnGa2O4:Cr a.c. powder electroluminescent device

An a.c. powder electroluminescent (EL) device using ZnGa₂O₄:Cr³⁺ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr³⁺ centers via self-activated Ga-O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L₀exp(− bV ^− 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr³⁺ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer-Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m² and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa₂O₄:Cr³⁺ shows excellent stability in comparison with the conventional sulfide powder EL device.     

Spray-pyrolysis Cd2SnO4 films for electrochemical applications

Conductive cadmium stannate (Cd₂SnO₄,) films were grown by a simple spray-pyrolysis technique using aerosols ultrasonically generated from solutions containing Cd(thd)₂(TMEDA) and ⁿBu₂Sn(AcAc)₂, and monoglyme as solvent (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate, TMEDA = N,N,N′,N′-tetramethylethylenediamine, AcAc = acethylacetonate). The overall film growing procedure was carried out at or below 400 °C thus allowing low-melting temperature materials like glass to be used as film substrates. Typical resistivity values of Cd₂SnO₄ films were found to be ∼ 2 · 10 ⁻³ Ωcm. The films exhibit excellent electrochemical activity with comparable or higher electron transfer rates than cadmium stannate films obtained via sol-gel methods at high annealing temperature.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Epitaxial growth and properties of cubic Zn0.7Mg0.3O films on TiN-buffered Si(100) substrates by in situ pulsed laser deposition

A novel cubic Zn_0.7Mg_0.3O film on silicon substrate is conducted by KrF excimer pulsed-laser ablation system. By introducing a thin TiN buffer, layer-by-layer growth of cubic Zn_0.7Mg_0.3O film epilayer has been realized. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the Zn_0.7Mg_0.3O films were strongly affected by the TiN buffer layer. The Zn_0.7Mg_0.3O film obtained at an optimal buffer layer exhibited high quality and good surface. For the metal-insulator-metal (MIM) structure of Pt/Zn_0.7Mg_0.3O (200 nm)/TiN (20 nm)/Si (400 μm) prepared at the optimal growth conditions achieved a very low leak current density of ∼10⁻⁶ A cm⁻² at an electric field of 9 × 10⁵ V cm⁻¹ and the permittivity (?_r) of about 8.1, agreed well with that of acquired MgO film and MgO single crystal.     

Enhanced thermoelectric properties of NaCo2O4 by adding ZnO

The primary phase present in the as-sintered Na(Co_1 − xZn_x)₂O₄ (0 ≤ x ≤ 0.1) bodies was the solid solution of the constituent oxides with a bronze-type layered structure. The electrical conductivity of the Na(Co_1 − xZn_x)₂O₄ samples significantly increased with an increase in ZnO content. The sign of the Seebeck coefficient for all samples was positive over the whole temperature range (723-1073 K), i.e., p-type conduction. The power factor of Na(Co_0.95Zn_0.05)₂O₄ showed an outstanding power factor (1.7 × 10^− 3Wm^− 1 K^− 2) at 1073 K. The power factor was above four times superior to that of ZnO-free NaCo₂O₄ (0.4 × 10^− 3Wm^− 1 K^− 2). This originates from an unusually large Seebeck coefficient (415 μVK^− 1) accompanied with high conductivity (127Ω^− 1 cm^− 1) at 1073 K.     

Microwave direct synthesis of MgB2 superconductor

Intermetallic compound superconductor MgB₂ was synthesized from spherical magnesium powder and lower purity amorphous boron powder by microwave direct heating. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is homogeneous and the particle size is about several hundreds of nanometers. The onset superconducting transition temperature of the MgB₂ sample measured by the temperature dependence of magnetization measurement is about 37.6 K. The critical current density J_c calculated according to the Bean model are about 2.0 × 10⁵ A/cm² at 20 K in self-field and 1.0 × 10⁵ A/cm² at 20 K in 1 T applied field.     

Synthesis of LiNi0.9Co0.1O2 cathode material for lithium secondary battery by a novel route

The cathode material, LiNi_0.9Co_0.1O₂ was prepared using a rheological phase reaction method with LiOH·H₂O, home-made Ni(OH)₂, and Co₂O₃ as starting materials. At first, the mixture of reactants and a proper amount of water reacted to form a rheological precursor. Then the dried precursor was heated at 730 °C in one step to yield the product. The effects of calcination time (between 0.5 and 10 h) on the structural, morphological and electrochemical properties were investigated. All obtained powders show a single phase with α-NaFeO₂ structure (R-3m space group). The sample prepared in 2.5 h delivers the largest initial discharge capacity of 218 mA h g^− 1 (3.0-4.35 V, 25 mA g^− 1) and still remains 192 mA h g^− 1 after 15 cycles. The method is simple, economical and effective and is promising for practical application.     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Multiferroic properties of BiFeO3/Bi4Ti3O12 double-layered thin films fabricated by chemical solution deposition

Multiferroic BiFeO₃/Bi₄Ti₃O₁₂ (BFO/BTO) double-layered film was fabricated on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The effect of an interfacial BTO layer on electrical and magnetic properties of BFO was investigated by comparing those of pure BFO and BTO films prepared by the same condition. The X-ray diffraction result showed that no additional phase was formed in the double-layered film, except BFO and BTO phases. The remnant polarization (2P_r) of the double-layered film capacitor was 100 μC/cm² at 250 kV/cm, which is much larger than that of the pure BFO film capacitor. The magnetization-magnetic field hysteresis loop revealed weak ferromagnetic response with remnant magnetization (2M_r) of 0.4 kA/m. The values of dielectric constant and dielectric loss of the double-layered film capacitor were 240 and 0.03 at 100 kHz, respectively. Leakage current density measured from the double-layered film capacitor was 6.1 × 10^− 7 A/cm² at 50 kV/cm, which is lower than the pure BFO and BTO film capacitors.     

Local fluctuations of absorber properties of Cu(In,Ga)Se2 by sub-micron resolved PL towards “real life” conditions

We analyze Cu(In,Ga)Se₂ absorber layers for solar cells in a confocal microscope setup by photoluminescence (PL) experiments. We present results on lateral inhomogeneities of absorbers in terms of local fluctuations of the splitting of quasi-Fermi levels (E_Fn − E_Fp), which determines the local open circuit voltage (V_oc) of the polycrystalline cell. These results can be extracted from spectrally resolved PL scans across several tens of microns. Excitation fluxes amount to 10² − 5 × 10⁴ suns equivalent at 83-300 K. We analyze the statistical distribution of the occurring fluctuations of (E_Fn − E_Fp) which we plot in histograms, seemingly showing Gaussian-like shapes. The width of these — showing substantial dependence on excitation flux and temperature — has been extrapolated towards 1 sun equivalent light fluxes. Furthermore, we use these results to correct the absolute values (E_Fn − E_Fp) which can be derived from non-laterally resolved, calibrated PL-studies at 300 K and 1 sun equivalent on comparatively large areas (1 mm²). The latter ones provide access to the spatially averaged PL-yields (∑Y_PL,xi) and their respective quasi-Fermi level splitting ((E_Fn − E_Fp)~ ln(∑Y_PL,xi)), while the average of the (E_Fn − E_Fp) from laterally resolved measurements reads (∑ln(Y_PL,xi)). We show a comparison of the two magnitudes and thus strongly appeal for sufficient high spatial resolution for a consistent quantitative interpretation of luminescence experiments.     

Dependence of Zr content on electrical properties of Bi3.15Nd0.85Ti3-xZrxO12 thin films synthesized by chemical solution deposition (CSD)

The Bi_3.15Nd_0.85Ti_3-xZr_xO₁₂ (BNTZ) thin films with Zr content (x = 0, 0.05, 0. 1, 0.15, and 0.2) were prepared on Pt/Ti/SiO₂/Si (100) substrates by chemical solution deposition (CSD) technique. The crystal structures of BNTZ films were analyzed by X-ray diffraction (XRD). The effects of Zr contents on the ferroelectric, dielectric properties, and leakage current of BNTZ films were thoroughly investigated. The XRD results demonstrated that all the films possessed a single phase bismuth-layered structure and exhibited the highly preferred (117) orientation. Among these films, the film with Zr content x = 0.1 held the maximum remanent polarization (2P_r) of 50.21 μC/cm² and a low coercive field (2E_c) of 210 kV/cm.     

High frequency characteristics of tin oxide thin films on Si

High frequency characteristics of tin oxide (SnO₂) thin films were studied. SnO₂ thin films have been successfully grown on n-type Si (111) substrates by using a spray deposition technique. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of the metal-oxide-semiconductor (Au/SnO₂/n-Si) Schottky diodes were investigated in the high frequency range from 300 kHz to 5 MHz. It has been shown that the interface state density, D_it, ranges from 2.44 × 10¹³ cm⁻² eV⁻¹ at 300 kHz to 0.57 × 10¹³ cm⁻² eV⁻¹ at 5 MHz and exponentially decreases with increasing frequency. The C-V and G/ω-V characteristics confirm that the interface state density and series resistance of the diode are important parameters that strongly influence the electrical parameters exhibited by the metal-oxide-semiconductor structure.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Sr3.5Mg0.5Si3O8Cl4: Eu bluish-green-emitting phosphor for NUV-based LED

Sr₄Si₃O₈Cl₄:Eu²⁺ and Sr_3.5Mg_0.5Si₃O₈Cl₄:Eu²⁺ phosphors were prepared by a conventional solid state reaction (SS). Excited by 370 nm near-ultraviolet light, the phosphors show an efficient bluish-green wide-band emission centering at 484 nm, which originates from the 4f5d¹ → 4f⁷ transition of Eu²⁺ ion. The excitation spectra of the phosphors are a broad band extending from 250 nm to 400 nm. Mg²⁺-codoping greatly enhances the bluish-green emission of the phosphors. An LED was fabricated by coating the Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ phosphor onto an ~ 370 nm-emitting InGaN chip. The LED exhibits bright bluish-green emission under a forward bias of 20 mA. The results indicate that Sr_3.5Mg_0.5Si₃O₈Cl₄:0.08Eu²⁺ is a candidate as a bluish-green component for fabrication of NUV-based white LEDs.     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.     

Chemical diffusion coefficient of lithium ion in Li3V2(PO4)3 cathode material

The kinetic properties of monoclinic lithium vanadium phosphate were investigated by potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS) method. The PSCA results show that there exists a linear relationship between the current and the square root of the time. The D?_Li values of lithium ion in Li_3-xV₂(PO₄)₃ under various initial potentials of 3.41, 3.67, 3.91 and 4.07 V (vs Li/Li⁺) obtained from PSCA are 1.26 × 10^− 9, 2.38 × 10^− 9, 2.27 × 10^− 9 and 2.22 × 10^− 9 cm²·s^− 1, respectively. Over the measuring temperature range 15-65 °C, the diffusion coefficient increased from 2.67 × 10^− 8 cm²·s^− 1 (at 15 °C) to 1.80 × 10^− 7 cm²·s^− 1 (at 65 °C) as the measuring temperature increased.