Fabrication of columnar structured lanthanum zirconate films by laser CVD

Columnar lanthanum zirconate (LZ) films were deposited by laser chemical vapor deposition (LCVD). The effects of deposition conditions, preheat temperature and laser power, on crystal structure, morphology, and deposition rate were investigated. A large region in the terms of preheat temperature and laser power for the formation of cubic pyrochlore‐structured LZ films was mapped under a fixed total pressure of 950 Pa. The morphology of LZ films without laser irradiation showed a dense and glass‐like structure with a flat cauliflower surface. It was changed into a columnar structure with an unflat cauliflower surface by low‐power laser irradiation during deposition process. The transformation occurred once a critical laser power beyond 16 W/cm². Thus, the effect of laser on the deposition of LZ films was a result of double‐side action including both pyrolytic effect and photolytic effect. Plasma gas around the substrate resulted in significant increases in deposition temperature and deposition rate due to self‐propagating high‐temperature reactions between the precursors and O₂. The maximum deposition rate can reach 250 μm/h by LCVD.     

Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

High‐temperature time‐of‐flight neutron diffraction experiments were performed on cubic yttria‐stabilized zirconia (YSZ, 10 mol% YO_1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO₂ laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10^?6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10^?6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10^?6/K at 2350°C–2550°C with oxygen displacement parameters (U_iso) reaching 0.1 Å², similar to behavior observed in UO₂. Acquisition of powder‐like high‐temperature neutron diffraction data from solid‐levitated samples is feasible and possible improvements are outlined. This methodology should be applicable to a wide range of materials for high‐temperature applications.     

Ga-doped IZO films obtained by magnetron sputtering as transparent conductors for visible and solar applications

C-axis textured thin films of gallium-doped indium zinc oxide (GIZO) with a 2% ratio of Ga/Zn, were obtained via RF-magnetron sputtering with high transparency and electrical conductivity. A Box-Behnken response surface design was used to evaluate the effects of the deposition parameters (In₂O₃ target power, deposition time, and substrate temperature) on the chemical composition, optical, electrical, and structural properties of the GIZO films. The optical constants and the electrical properties were obtained using optical models. The GIZO stoichiometry, and therefore the In/Zn atomic ratio, affected the crystallinity, crystalline parameters, band gap, and charge carrier mobility of the GIZO films. The charge carrier density was related to the change in the crystalline parameters of the hexagonal structure and the In/Zn atomic ratio. The best electrical conductivity values (1.75?×?10³ Ω^?1 cm^?1) were obtained for GIZO films with In/Zn ratio ≥?1. Several figures of merit (FOM) defined for the visible and solar regions were comparatively used to select the optimal In/Zn atomic ratio that provided the best balance between the conductivity and the transparency. The optimal In/Zn ratio was in a range of 0.85–0.90 for the GIZO films.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

Coagulation of polymer–Cu2+ complexes in polymer films and its application for producing semiconducting CuI surface layers

In poly(vinyl alcohol) and polyacrylamide films containing the corresponding polymer–Cu²⁺ complexes, the reason why the films may gain surface electrical semiconductivity as high as 10^?3 Ω^?1 when treated with acetone solution of iodine was investigated. Optical and scanning electron microscope observations indicated that the coagulated polymer–Cu²⁺ complexes favor the appearance of the high conductivity and that the state of coagulation depends on the anions of the copper salts used as well as two parameters, F₁ ≡ [Cu²⁺]/[MU] and F₂ ≡ [OH^?]/[Cu²⁺], where [MU] is the molar concentration of monomeric units of the polymer and [OH^?] is that of hydroxide ions added. The effectiveness of the anions in causing coagulation decreases in the order of SO₄^2? > Cl^? > NO₃^? ≈ Br^?. The whitish substance that appears on the film surface after the iodine treatment gives x-ray Debye–Scherrer rings characteristic of γ-CuI. The γ-CuI surface layer adheres to the film rather firmly, at least in polyacrylamide, and is responsible for the conductivity. By controlling the state of coagulation of the complexes and hence the formation of the γ-CuI surface layer, we have produced films with anisotropic surface electrical semiconductivity, i.e., σ_∥ ≈ 10^?4 Ω^?1 and σ_∥/σ_⊥ = 1 ? 10³. Optical and ESR spectra are also obtained to understand the mechanism of γ-CuI formation and to clarify the optical properties of the films.     

Ionic conductivity of hybrid films based on polyacrylonitrile and their battery application

The alternate current (AC) and direct current (DC) ionic conductivity of hybrid films composed of polyacrylonitrile (PAN), lithium perchlorate (LiClO₄), and a plasticizer was studied. Three kinds of the plasticizer [ethylene carbonate (EC), propylene carbonate (PC), N,N-dimethylformamide (DMF)] were used. Suitability of these hybrid films for lithium battery was investigated. The AC conductivity, which represents bulk ionic conductivity, was dependent on the component and the composition of the hybrid films, ranging from 10^?4?10^?8 Scm^?1. The AC conductivity was mainly determined by the molar ratio of [plasticizer]/[LiClO₄] in the hybrid films and increased with the increase in this ratio. The effect of the plasticizer on the enhancement in the AC conductivity was in the following order. DMF>EC>PC. The hybrid films with both electrodes of lithium showed the stable DC conductivity of about 1/10 of the AC conductivity, except for the hybrid films containing DMF. The hybrid films were found to be effective as a lithium ionic conductor. The galvanic cell. Li/sample/MnO₂, at the discharge current density of 90 μA/cm² showed the stable electromotive force of about 3 V for 70 h.     

Optical and electrical properties of (111)-oriented epitaxial SrVO3 thin films

Perovskite oxide SrVO₃ (SVO) is a transparent conductor with excellent optical and electrical properties. Most of the previous works have focused on (001)-oriented SVO thin films. As an alternative to tin-doped indium oxide (ITO), the other orientations of SVO thin films are important to be considered as well. In the present work, the optical and electrical properties of (111)-oriented SVO epitaxial films have been investigated. Excellent electrical conductivity (2.92?×?10⁴?S?cm^?1) and optical transparency (56.6%) have been demonstrated, which are comparable to those of ITO and expand the applications of epitaxial SVO thin films in other orientations as transparent conducting oxide.     

Characterization of non-stoichiometric Ga2O3-x thin films grown by radio-frequency powder sputtering

We report the synthesis and characterization of non-stoichiometric Ga₂O_3-x thin films deposited on sapphire (0001) substrates by radio-frequency powder sputtering. The chemical and electronic states of the non-stoichiometric Ga₂O_3-x thin films were investigated. By sputtering in an Ar atmosphere, the as-grown thin films become non-stoichiometric Ga₂O_2.7, due to the difference in sputtering yield between Ga and O species of the Ga₂O₃ target. The electronic states of the thin films consist of ~85% Ga³⁺ and ~15% Ga¹⁺, corresponding to Ga₂O₃ and Ga₂O, respectively. The films have the electrical characteristics of a semiconductor, with electrical conductivity of approximately 5.0 × 10^-4 S cm^-1 and a carrier concentration of 4.5 × 10¹⁴ cm^-3 at 300 K.     

Fabrication and electrical properties of Bi2-xSbxTe3 ternary nanopillars array films

Highly oriented Bi_2-xSb_xTe₃ (x?=?0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi_2-xSb_xTe₃ films have pure rhombohedral phase with well-ordered nanopillars array. Bi, Sb and Te atoms uniformly distributed throughtout films with no precipitation. Electrical conductivity of Bi_2-xSb_xTe₃ films transforms from n-type to p-type when x?>?1.1. Metal-insulator transition was observed due to the incorporation of Sb in Bi₂Te₃. Bi_2-xSb_xTe₃ film with x?=?1.5 exhibits optimized electrical properties with maximum electrical conductivity σ of 2.95?×?10⁵ S?m^?1 at T?=?300?K, which is approximately ten times higher than that of the undoped Bi₂Te₃ film, and three times higher than previous report for Bi_0.5Sb_1.5Te₃ films and bulk materials. The maximum power factor PF of Bi_0.5Sb_1.5Te₃ nanopillars array film is about 3.83?μW?cm^?1 K^?2 at T?=?475?K. Highly oriented (Bi,Sb)₂Te₃ nanocrystalline films with tuned electronic transport properties have potentials in thermoelectric devices.     

Thermal conductivity of air plasma sprayed yttrium heavily-doped lanthanum zirconate thermal barrier coatings

The yttrium heavily doped La₂Zr₂O₇ solid solutions coatings, with a Y to La molar ratio of 1:1, have been successfully prepared by air plasma spraying technique. The evolution of phase composition, phase structure and thermal conductivity of such coatings with annealing at 1300?°C has been investigated. The results show that, a single pyrochlore structure can be retained for coating after annealing up to 48?h, beyond which the fluorite phase begins to precipitate out. By comparing thermal conductivities to those undoped counterparts at a similar porosity level, we find a considerably flat thermal conductivity versus temperature (k-T) curve, suggesting the existence of a strong phonon scattering source, which is inferred as rattlers. In addition, after the segmentation of the fluorite phase, the thermal conductivity of corresponding coatings rises considerably, indicating that the fluorite phase has a higher thermal conductivity than that of pyrochlore phase. Moreover, while the as-sprayed coatings show a clear indication of radiative thermal conduction beyond 1000?°C, the thermal conductivity of annealed coatings do not show such an uprising trend after 1000?°C, suggesting that the radiative thermal conduction has been greatly suppressed. The reason is proposed as the formation of local dipoles due to local enrichment of certain elements influences the propagation of electromagnetic waves and thus suppresses the radiative thermal conduction.     

Distribution of relaxation time in solution-processed polycrystalline CZTS thin films: Study of impedance spectroscopy

Here we report the complex impedance spectroscopic analysis of polycrystalline CZTS thin films synthesized by sol-gel spin coating technique without any post deposition sulphurization. The films are characterized by microstructural, compositional, optical and electrical studies to confirm the formation of kesterite phase of CZTS comprises of well distributed compact grains with the optical band gap 1.44?eV. Room temperature electrical characterizations of the CZTS thin films by four-probe and Hall effect technique revealed the p-type conductivity of the films with resistivity ~ 1.45?×?10^?2 Ω?cm, mobility ~ 3.7?×?10³ cm² V^?1 s^?1 and carrier concentration ~ 1.82?×?10¹⁷ cm^?3. The distribution of relaxation time (DRT) function with improved frequency resolution is reconstructed from the impedance spectra of CZTS film recorded in the frequency range 50?Hz to 5?MHz at room temperature to identify the number of electrical processes in the polycrystalline film. The Nyquist plot is fitted into electrical model consist of three parallel combinations of resistor (R) and capacitor (C) in series as three major peaks in DRT function indicates the presence of different relaxation processes with major contributions from core grains along with smaller contributions from grain boundary and interfaces. The room temperature frequency dependence of dielectric constant, loss tangent and ac conductivity is also studied for the CZTS films.     

Densification behavior and properties of hot-pressed ZrC ceramics with Zr and graphite additives

Densifications of hot-pressed ZrC ceramics with Zr and graphite additives were studied at 1800-2000 °C. ZrC with 8.94 wt% Zr additive (named ZC10) sintered at 1900-2000 °C achieved higher relative densities (>98.4%) than that of additive-free ZrC (<83%). The densification improvement was attributed to the formation of non-stoichiometric ZrC_0.9, whereas there had rapid grain growth with grain size about 50-100 μm in ZC10. By adding co-doped additive of Zr plus C and adjusting the molar ratio of Zr/C, ZrC with co-doped additives with Zr/C molar ratio at 1:2 (named ZC12), ZrC ceramics with both high relative density (98.4%) and fine microstructures (grain size about 5-10 μm) were obtained at 1900-2000 °C. Effect of formation of non-stoichiometric ZrC_1−x on densification of ZrC was discussed. The Vickers hardness and indentation toughness of ZC10 and ZC12 samples sintered at 1900 °C were 17.8 GPa and 3.0 MPa m^1/2, 16.2 GPa and 4.7 MPa m^1/2, respectively.     

Electrochemical synthesis of sulfonated polypyrrole in FSO3H/acetonitrile solution

The incorporation of SO₃^? groups into a polypyrrole backbone was successfully carried out using FSO₃H/acetonitrile medium. The degree of sulfonation (sulfonation or sulfur‐to‐nitrogen ratio) of polymer was controlled by varying FSO₃H concentration while the film was being electrodeposited. The cyclic voltammograms of the electrodeposited films were taken between ?1.00 and +2.00 V (versus Ag/AgCl) in blank solution. A substantial effect on the electrical conductivity was observed upon the incorporation of SO₃^? groups in the polypyrrole polymeric chain. The elemental analysis results of the freshly prepared and reduced films give independent evidence that the SO₃^? groups are covalently bound to the structure. FTIR, UV‐Vis, spectroscopy and SEM techniques were used to characterize the electrosynthesized polypyrrole films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 526–533, 2004     

Realizing tremendous electrical transport properties of polycrystalline SnSe2 by Cl-doped and anisotropy

SnSe₂ is regarded as an attractive thermoelectric material for its structural and chemical analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe₂ polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purification, diffusion doping, and spark plasma sintering. The phase structure, lamellar morphology and crystallite orientation were studied for the synthesized SnSe₂ powder and the sintered pellets. The structural evolution was traced from the SnSe₂ hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe₂ was verified by phase composition, lattice parameter, element distribution, and chemical valance. The doped Cl increased both the carrier concentration and the mobility. The anisotropic thermoelectric properties of SnSe₂ bulk materials were investigated as functions of temperature from 50?°C to 300?°C and the doping amount, respectively. The Seebeck coefficient was less anisotropic than the electrical and thermal conduction. The grain orientation influenced the anisotropy of the electrical and thermal conductivity at a similar ratio. The power factors were less dependent on temperature with an optimum in-plane 1.06?mW?m^?1 K^?2 and out-of-plane 0.41?mW?m^?1 K^?2. The highest ZTs of 0.3 were attained at 300?°C in both directions.     

Consequences of lead incorporation in fluorite structured thoria

The outcome of incorporation of lead in fluorite structured thoria in terms of structure and electrical properties have been discussed. Samples with stoichiometries Th_1-xPb_xO_2-δ (x?=?0.00 to 0.70) have been synthesized by a solution combustion route and characterized extensively. The maximum solubility limit of lead in fluorite structured thoria was deduced to be 50?mol % from powder X-ray diffraction and Raman spectral measurements. The micro strain of the fluorite lattice increased with the introduction of lead. X-ray photoelectron spectroscopy analysis of Th_0·50Pb_0·50O_2-δ showed the existence of lead in both +2 and + 4 oxidation states. The ratio between the intensities of peaks located at 548 cm^?1 and T_2g mode (I₅₄₈/I_T2g) was found to increase with increase in lead content in the samples and quantified utilizing grain size and half width half maxima. The defect concentrations in these samples have been estimated in the range 10¹⁷-10²⁰?cm^?3. The direct band gap values shifted from the insulating regime (for thoria) to semiconducting regime (3.4–2?eV) for lead substituted samples. From the frequency dependent conductivity plots of Th_1-xPb_xO_2-δ samples over temperature range (323–773?K), highest conductivity of 1.62?×?10^?6 Scm^?1 was obtained for Th_0·50Pb_0·50O_2-δ at 773?K. From the impedance measurements of lead substituted thoria under flowing oxygen (1?atm), thermal motion of oxygen ion vacancies was found responsible for the observed conductivity.     

Sol-gel derived low temperature HfO2-GPTMS hybrid gate dielectric for a-IGZO thin-film transistors (TFTs)

In this study, we prepared inorganic-organic HfO₂-GPTMS hybrid films by a simple sol-gel method at low temperature for high-k dielectric gate applications. The hybrid films were deposited by spin coating process, followed by annealing at 150?°C. The hybrid dielectric material was characterized by Spectroscopic ellipsometry (SE), AFM, FESEM, FTIR, TGA, and XPS techniques. The resulting hybrid films exhibit homogeneous and smooth surface with high optical transparency. Their dielectric properties were analysed by measuring leakage current and capacitance versus voltage of metal-insulator-metal (MIM) capacitor structures. From this analysis, the leakage current density at ??5?V, capacitance and dielectric constant at 1?MHz measured on the hybrid films were 10^?7 A/cm², 51.3?nF/cm² and of 11.4 respectively. Finally, to investigate the electrical performance of the hybrid thin films as a dielectric gate in thin film transistors (TFTs), bottom-gate TFTs were fabricated by depositing the HfO₂-GPTMS dielectric gate layer on ITO-coated glass substrate and subsequently a sputtered a-IGZO thin film as the channel layer. The electrical response of the resulting TFTs demonstrated good saturation mobility of 4.74?cm² V^?1 s^?1, very low threshold voltage of 0.3?V and I_on/I_off current ratio of 10⁴, with low operating voltage under 8?V.     

Highly ordered columnar ITO thin film with enhanced thermoelectric and mechanical performance over wide temperature range

Indium tin oxide (ITO) based thin films offer the possibility to improve the performance of high-temperature thermocouples by providing good sensitivity and reliability over a wide temperature range. In this study, a thin but robust ITO-based thin-film thermocouple, with a low-crystallised highly ordered columnar structure, was fabricated. The electrical conductivity exhibited a high temperature-dependent sensitivity owing to the increasing density with increasing temperature. The nano-hardness and interfacial robustness were evaluated and found to exhibit excellent service reliability at high temperatures because of the low thermal stress. Furthermore, the similar mechanical and electrical performances of the thin films, after annealing at 600 and 800 °C, demonstrated that the enhanced performance was mainly determined by the orientation of the ITO thin films. An enhanced Seebeck coefficient (～100 μV K^?1) was obtained for the ITO thin film after annealing at 1000 °C, resulting in a special structure with profuse nanoholes. These results highlight the good mechanical performance and stability of the thermoelectric properties of highly ordered columnar thin films over a wide temperature range, and can serve as a guide for the preparation of thin but robust functional ceramic-based materials.     

Carbon nanotubes branch on cobalt oxide nanowires core as enhanced high-rate cathodes of alkaline batteries

Directional synthesis of carbon/metal oxide core-branch arrays is of great importance for development of advanced high-rate alkaline batteries. In this work, we report a facile hydrothermal-chemical vapor deposition (CVD) method for controllable fabrication of Co₃O₄ @CNTs core-branch arrays. Interestingly, free-standing Co₃O₄ core nanowires are intimately decorated by cocoon-like branch CNTs with diameters of 20–30?nm, which act as a highly conductive network and structure stabilizer. The electrochemical performance of the designed Co₃O₄ @CNTs core-branch arrays are tested as cathodes of alkaline batteries. Arising from enhanced electrical conductivity, larger surface area and improved structural stability, the Co₃O₄ @CNTs arrays show superior high-rate electrochemical performance with a higher capacity (116 mAh g^?1 at 2.5?A?g^?1), lower polarization and better cycling stability than the pure Co₃O₄ nanowires arrays (76 mAh g^?1 at 2.5?A?g^?1). Our directional composite strategy can be extended to preparation of other carbon-based core-branch arrays for applications in electrochemical batteries and catalysis.     

Theoretical investigation of intrinsic point defects and the oxygen migration behavior in rare earth hafnates

In this work, the composition-dependent point defect types and formation energies of RE₂Hf₂O₇ (RE = La, Ce, Pr, Nd, Pm, Sm, Eu and Gd) as well as the oxygen diffusion behavior are systematically investigated by first-principles calculations. The possible defect reactions and dominant defect complexes under stoichiometric and non-stoichiometric conditions are revealed. It is found that O Frenkel pairs are the predominant defect in stoichiometric pyrochlore hafnates. Hf-RE cation anti-site defects, accompanied by RE vacancies and/or oxygen interstitials, are stable in the non-stoichiometric case of HfO₂ excess. On the other hand, RE-Hf anti-site defects together with oxygen vacancies and/or RE interstitials are preferable in the case of RE₂O₃ excess. The energy barriers for the migration along the V_O48f - V_O48f pathway of pyrochlore hafnates were calculated to be between 0.81 eV and 0.89 eV. Based on these results, a defect engineering strategy is proposed and the pyrochlore hafnates investigated here are predicted to exhibit potential oxygen ionic conductivity.     

Enhancing the electrical conductivity of vanadate glass system (Fe2O3, B2O3, V2O5) via doping with sodium or strontium cations

Novel glass-ceramics of the nominal molar compositions 20Fe₂O₃·20B₂O₃·(60-x)V₂O₅· (xNa₂O or xSrO) (where x?=?0 or 10) were prepared by traditional melt technique. Differential thermal analysis (DTA) was implemented to study the thermal behavior of the prepared glasses. Vanadium pentoxide (V₂O₅), iron vanadate (FeVO₄), sodium vanadate (Na₃VO₄) and strontium vanadate (with different formulae) were crystallized and identified by X-ray diffraction (XRD) analysis under certain conditions of heat-treatment. Further characterization of glass and glass ceramics samples were performed using scanning electron microscope (SEM), density, electrical and dielectric measurements. In conclusion, our study elucidated that the substitution of vanadium by Na⁺ and Sr²⁺ ions enhanced the conductivity at 180?°C from 5.11?×?10^?4 for unmodified glass to 2.93?×?10^?3 and 1.03?×?10^?2?S?cm^?1 for Na- and Sr-modified glasses.