Electrochemical characteristics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ–Sm0.2Ce0.8O1.9 composite materials for low-temperature solid oxide fuel cell cathodes

In this paper, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ–xSm_0.2Ce_0.8O_1.9 (BSCF–xSDC, x = 0–60 wt.%) composite cathodes were prepared by soft chemical methods, and then examined for potential applications in lower temperature solid oxide fuel cells. Both DC polarization and AC impedance spectroscopy measurements indicated that the addition of SDC electrolyte into BSCF remarkably improved the electrochemical properties. The optimum composition was found to be BSCF–30SDC, which exhibited 5.5 times higher polarization current density and 15.1% polarization resistance, compared with the pure-phase BSCF cathode at 550 °C.     

Role of La0.5Sr0.5CoO3 template layers on dielectric and electrical properties of pulsed-laser ablated Pb(Nb2/3Mg1/3)O3-PbTiO3 thin films

Relaxor ferroelectric thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) deposited on platinized silicon substrates with and without template layers were studied. Perovskite phase (80% by volume) was obtained through proper selection of the processing conditions on bare Pt/Ti/SiO₂/Si substrates. The films were initially grown at 300 °C using pulsed-laser ablation and subsequently annealed in a rapid thermal annealing furnace in the temperature range of 750-850 °C to induce crystallization. Comparison of microstructure of the films annealed at different temperatures showed change in perovskite phase formation and grain size etc. Results from compositional analysis of the films revealed that the films initially possessed high content of lead percentage, which subsequently decreased after annealing at temperature 750-850 °C. Films with highest perovskite content were found to form at 820-840 °C on Pt substrates where the Pb content was near stoichiometric. Further improvement in the formation of perovskite PMN-PT phase was obtained by using buffer layers of La_0.5Sr_0.5CoO₃ (LSCO) on the Pt substrate. This resulted 100% perovskite phase formation in the films deposited at 650 °C. Dielectric studies on the PMN-PT films with LSCO template layers showed high values of relative dielectric constant (3800) with a loss factor (tan δ) of 0.035 at a frequency of 1 kHz at room temperature.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Laser deposition of oriented BiMeVOx films

The oxygen-ion conducting ceramics Bi₂VO_5.5 and Bi₂Cu_0.1V_0.9O_5.35 were synthesized and deposited on several substrates by the pulsed laser deposition technique. Defect-free films of Bi₂Cu_0.1V_0.9O_5.35 were obtained on SrTiO₃ substrates, while films of Bi₂VO_5.5 exhibited severe cracking. Reasons for the different results are discussed.     

Crystal structure, thermal expansion and electrical conductivity of Nd0.7Sr0.3Fe1−xCoxO3 (0≤x≤0.8)

The crystal structure, thermal expansion and electrical conductivity of the solid solution Nd_0.7Sr_0.3Fe_1−xCo_xO₃ for 0≤x≤0.8 were investigated. All compositions had the GdFeO₃-type orthorhombic perovskite structure. The lattice parameters were determined at room temperature by X-ray powder diffraction (XRPD). The pseudo-cubic lattice constant decreased continuously with x. The average linear thermal expansion coefficient (TEC) in the temperature range from 573 to 973 K was found to increase with x. The thermal expansion curves for all values of x displayed rapid increase in slope at high temperatures. The electrical conductivity increased with x for the entire temperature range of measurement. The calculated activation energy values indicate that electrical conduction takes place primarily by the small polaron hopping mechanism. The charge compensation for the divalent ion on the A-site is provided by the formation of Fe⁴⁺ ions on the B-site (in preference to Co⁴⁺ ions) and vacancies on the oxygen sublattice for low values of x. The large increase in the conductivity with x in the range from 0.6 to 0.8 is attributed to the substitution of Fe⁴⁺ ions by Co⁴⁺ ions. The Fe site has a lower small polaron site energy than Co and hence behaves like a carrier trap, thereby drastically reducing the conductivity. The non-linear behaviour in the dependence of log σT with reciprocal temperature can be attributed to the generation of additional charge carriers with increasing temperature by the charge disproportionation of Co³⁺ ions.     

La(1−x)SrxCoO3 for thin film thermocouple applications

In this study, the LSCO (lanthanum strontium cobalt oxide) family has been investigated for thin film thermocouple applications. Thin films of La_(1−x)Sr_xCoO₃ (x=0.3,0.5,0.7) were prepared on sapphire substrates by pulsed laser deposition. The films were annealed at different temperatures in air and characterized for phase, composition and microstructure to determine their thermal stability. From the phase and composition analyses, it is clear that as the Sr content in LSCO increases, the thermal stability decreases. Among the three compositions studied, x=0.3 had the best phase and chemical stability, and microstructural properties. It was observed that La_0.7Sr_0.3CoO₃ possesses excellent phase, composition and microstructural stability up to 1273 K. Above 1273 K, however, LSCO decomposes resulting in the loss of cobalt and formation of individual oxide phases. Electrical resistivity and Seebeck coefficients were measured in situ as a function of temperature in air up to 1023 K. The electrical and Seebeck coefficient properties were found to be stable for all the three compositions up to 1023 K and studies indicated that electrical conduction occurs through a small polaron hopping mechanism. In conclusion, LSCO possessed good thermal stability in air up to 1273 K and exhibits excellent potential in thin film thermocouple applications.     

Growth of La2Mo2O9 films on porous Al2O3 substrates by radio frequency magnetron sputtering

Synthesis conditions of La₂Mo₂O₉ thin film by radio frequency (RF) sputtering technique on Al₂O₃ ceramic substrates are studied. It is found that the deposition temperature and oxygen partial pressure are the most important factors for obtaining pure La₂Mo₂O₉ films. Varying both parameters, Mo-rich, stoichiometric, and Mo-deficient films are obtained. With increasing the La:Mo ratio, films become denser. A crust layer is observed on top of the Mo-rich and the Mo-deficient films. The formation of the La₂Mo₂O₉ phase is discussed with respect to the sputtering mechanism.     

Improved dielectric properties of CaCu3Ti4O12 thin films on oxide bottom electrode of La0.5Sr0.5CoO3

We report on the effect of La_0.5Sr_0.5CoO₃ (LSCO) bottom electrode to the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown on Ir/Ti/SiO₂/Si substrates. Compared with the films grown directly on Ir/Ti/SiO₂/Si substrates, the dielectric constant has been increased greatly about 100%, and the dielectric loss decreased to lower than 0.2 in the frequency range of 1-100 kHz. The origin has been discussed in details based on the analysis of the X-ray diffraction and impedance spectra measurements. Results of the impedance spectra suggest that the absence of undesired interfacial layer between Ir/CCTO thin films might be one of the major reasons of the improvement of the dielectric properties when the LSCO was introduced as the bottom electrode.     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

Structural and electrochemical characterization of LiMn2−δVδOy spinels

The spinel phase compounds with the composition of LiMn_2−δV_δO_y were prepared by solid reaction of the mixture of LiNO₃·H₂O, MnCO₃ and NH₄VO₃ powders. Evolution of the crystalline phases of the samples versus the vanadium content was analyzed using X-ray diffraction (XRD) technique, EPR and FT-IR spectroscopes. Cubic spinel is the predominant phase in the powders under heat treatment at 550 °C for 5 h. The valence state of manganese ion changed from +4 to +3 with vanadium substitution for charge compensation. The vanadium substitution of manganese leads the decline in capacity and cyclic behavior of the powders. The electrochemical behaviors relating to the variation of structure corresponding to the vanadium substitution were discussed.     

Self-patterned Nano Structures in Structurally Gradient Epitaxial La0.5Ba0.5CoO3 Films

Highly epitaxial La_0.5Ba_0.5CoO₃ (LBCO) thin films with sharp interface and a thickness of 200 nm were epitaxially grown on (001) SrTiO₃ substrates using pulsed laser deposition. High-resolution transmission electron microscopy and electron diffraction analysis revealed that the films have a triple-layered structure. The first layer, close to the film/substrate interface, has a thickness of ~ 6 nm and is a defect free single crystal disordered cubic structure (a = 3.882 Å) which has a lattice mismatch of − 0.59% with respect to the substrate. The second layer which dominates the film structure has a single crystal disordered cubic structure (a = 3.854 Å) which has a lattice mismatch of − 1.31% with respect to the substrate. The third layer located on the top of the film has a thickness of several nanometers and consists of 112-type ordered tetragonal structure. The cubic structures in the first and second layer have an orientation relationship of (001)_LBCO//(001)_STO and < 100 > _LBCO//< 100 > _STO with respect to the substrate. Self-patterned 3-dimensional nano structures with a dimension range from 2 to 10 nm were formed in the second and third layers. These nano structures were formed by the enclosure of anti-phase boundary planes which are parallel to the {100} of the cubic structure. Epitaxial LBCO thin films with such nano structures are hard ferromagnetic with a large coercive field value and magnetoresistance effect value (~ 24%), and exhibit semiconductor behavior at temperatures < 300 K.     

Ultraviolet emitting (Y1−xGdx)2O3−δ thin films deposited by radio frequency magnetron sputtering: Combinatorial modeling, synthesis, and rapid characterization

Gadolinium-doped, yttrium oxide thin films have been deposited on silicon (001) substrates by radio-frequency (RF) magnetron reactive sputtering that exhibit cathodoluminescence (CL) at ultraviolet frequencies. The maximum CL brightness occurred at λ314–315 nm characteristic of the ⁶P_3 / 2 → ⁸S (λ = 314 nm) transition observed in Gd-doped, yttrium oxide powders. The radiative recombination takes place at the rare earth activator Gd³⁺ site embedded in the Y₂O_3−δ host; the optical transition resides within the band gap of the Y₂O_3−δ host and the transition observed is characteristic of atomic gadolinium. A combinatorial approach to sputtering was used to deposit a film of variable composition from 1 to 23 at.% Gd in Y₂O_3−δ in order to rapidly discern the composition node of optimal CL brightness. A simulation was created for the purpose of predicting the film combinatorial composition for binary and ternary alloys prior to sputtering experiments in order to facilitate our combinatorial thin film synthesis technique. The model prediction varied from the real experimental composition profile by only 2.2 at.% Gd ± 1.6 at.% proving the predictor as a useful aide to complement combinatorial thin film experiments. A film of composition Y_1.56Gd_0.44O_3.25 (8.3 at.% Gd) yielded the maximum CL brightness. CL brightness increased continuously up to the 8.3 at.% Gd composition due to the increased number of activators present in the host. Beyond this composition the brightness drastically decreased. The oxygen composition in the combinatorial film was strongly dependent on the Gd composition; films were sub-stoichiometric δ > 0 below 6 at.% Gd and was over-stoichiometric δ < 0 beyond this composition.     

A simple approach of fabricating thermoelectric γ-NaxCoO2 and superconductive Nax(H2O)yCoO2-δ films using the sol–gel spin-coating method

We report a simple approach of fabricating thermoelectric γ-Na_xCoO₂ film with the c-axis orientation using the sol–gel spin-coating method. The inferred sodium content is x = 0.65 according to the correlation between the c-axis lattice constant and x. Temperature dependence of both the resistivity and thermopower resembles that of the γ-Na_0.68CoO₂ film grown by the reactive solid-phase epitaxy. The fitted thermopower data show that the bandwidth of γ-Na_xCoO₂ is found to be ~ 101 meV, being close to the quasi-particle band (70–100 meV) derived from an angle-resolved photoemission study of γ-Na_0.7CoO₂. These results enable the possibility of low-cost fabrication of γ-Na_xCoO₂-based thermoelectric film devices. Furthermore, we have also topotactically transformed the of γ-Na_xCoO₂ film to a superconducting Na_x(H₂O)_yCoO_2-δ film with T_{c, onset} = 4.12 K.     

Combinatorial pulsed laser deposition and thermoelectricity of (La1−xCax)VO3 composition-spread films

La_1−xCa_xVO₃ composition-spread film library was fabricated by combinatorial pulsed laser deposition and their thermoelectric properties were evaluated paralelly by the multi-channel probes of Seebeck coefficient and electric conductivity. Concurrent X-ray analysis verified the formation of solid soluted films in the full composition range (0x1) as judged from the linear variation of the lattice constants. The Seebeck coefficients of La_1−xCa_xVO₃ changed from a large negative value to almost zero with the increase of x, due presumably to the variation of valence in vanadium ions.The power factor in this library was as high as 0.6 μW/cm K², which was obtained at x=0, i.e. pure LaVO₃ grown at 800 °C.     

High-temperature oxygen non-stoichiometry, conductivity and structure in strontium-rich nickelates La2−xSrxNiO4−δ (x = 1 and 1.4)

Oxygen non-stoichiometry, electrical conductivity and thermal expansion of La_2−xSr_xNiO_4−δ phases with high levels of strontium-substitution (1 ≤ x ≤ 1.4) have been investigated in air and oxygen atmosphere in the temperature range 20–1050 °C. These phases retain the K₂NiF₄-type structure of La₂NiO₄ (tetragonal, space group I4/mmm). The oxygen vacancy fraction was determined independently from thermogravimetric and neutron diffraction experiments, and is found to increase considerably on heating. The electrical resistivity, thermal expansion and cell parameters with temperature show peculiar variations with temperature, and differ notably from La₂NiO_4±δ in this respect. These variations are tentatively correlated with the evolution of nickel oxidation state, which crosses from a Ni³⁺/Ni⁴⁺ to a Ni²⁺/Ni³⁺ equilibrium on heating.     

Simple chemical solution deposition and thermoelectric properties of epitaxial La0.95Sr0.05CoO3 thin films

In this paper, simple chemical solution deposition method is used to prepare La_0.95Sr_0.05CoO₃ thin films on SrTiO₃ (001) substrates by acetate-based precursors. The derived film is characterized by x-ray diffraction, field-emission scanning electron microscopy and transmission electronic microscopy. The derived film is epitaxial growth with < 001>[100] La_0.95Sr_0.05CoO₃||<001>[100] SrTiO₃, indicating that the chemical solution deposition is an effective route to obtain the cobalt-based films. The resistivity, Seebeck coefficient and thermal power factor are 0.05Ω cm, 250 μV/K and 0.21 mWK^− 2m^− 1 at 300 K, respectively, which is higher than these of the ceramics, indicating epitaxial thin film is an effective route to enhance the thermoelectric properties of La_0.95Sr_0.05CoO₃.     

Phase relations, lattice thermal expansion in Ce1−xEuxO2−x/2 and Ce1−xSmxO2−x/2 systems and stabilization of cubic RE2O3 (RE: Eu, Sm)

The phase relations in CeO₂–Eu₂O₃ and CeO₂–Sm₂O₃ systems have been established under slow-cooled conditions from 1400 °C. The two-phase relations differ as the CeO₂–Eu₂O₃ system showed only two monophasic phase fields, namely F-type cubic and C-type cubic, whereas CeO₂–Sm₂O₃ system showed three phase fields namely F-type cubic, C-type cubic and a biphasic field comprising of C-type cubic and monoclinic phase. An interesting observation of this investigation is the stabilization of C-type rare-earth oxide after Ce⁴⁺ substitution, which is attributed to decrease in average cationic size on Ce⁴⁺ substitution at RE³⁺ site. The lattice thermal expansion behavior of F-type solid solution and C-type solid solution in CeO₂–Eu₂O₃ system was investigated by high-temperature XRD.     

Photoelectrochemical properties of sol–gel-derived Ti1−xVxO2 solid solution film photoelectrodes

Ti_1−xV_xO₂ solid solution film photoelectrodes were prepared by the dip-coating sol–gel method. X-ray diffraction and X-ray photoelectron spectroscopy were employed to ensure the formation of the solid solution and their composition. Obvious photoresponses were observed in the visible region for the solid solution film electrodes with x0.05 and the red shift of the photoresponse was enhanced with increasing x. Moreover, the solid solution film electrodes were found to be photoelectrochemically stable. However, the onset potential of photocurrent shifted positively with increasing x. Band model of the solid solution was suggested to explain the effects of the vanadium incorporation on the photoelectrochemical properties.     

Fabrication of Y2O3-doped BaZrO3 thin films by electrostatic spray deposition

Electrostatic spray deposition (ESD) technique was used to fabricate dense Y₂O₃-doped BaZrO₃ (BYZ) thin films, which have been extensively studied for the protonic ceramic fuel cell electrolyte. Effects of the ESD process parameters (i.e. substrate temperature, type of precursor, flow rate and applied voltage) on the microstructure of as-deposited films were studied. The uniform as-deposited films were obtained using a mixture of zirconium acetylacetonate, barium chloride dihydrate and yttrium chloride hexahydrate precursors in a solvent mixture of butyl carbitol and deionized water at a volume ratio of 50:50. The optimum deposition parameters were obtained at the substrate temperature of 250 °C with the applied voltage and flow rate in a range of 10-12 kV and 1.4-2.8 ml/h, respectively. The as-deposited films were subsequently annealed at 1350 °C for 10 h to ensure the complete chemical reactions of the precursors. X-ray diffraction patterns reveal the perovskite structures of the annealed BYZ films (deposited on yttria stabilized zirconia substrates) with only traces of Y₂O₃ phase, which could arise from the loss of BaO at high annealing temperatures.     

Structural and dielectric properties of (001) and (111)-oriented BaZr0.2Ti0.8O3 epitaxial thin films

We have grown and characterized BaZr_0.2Ti_0.8O₃ (BZT) epitaxial thin films deposited on (001) and (111)-oriented SrRuO₃-buffered SrTiO₃ substrates by pulsed laser deposition. Structural and morphological characterizations were performed using X-ray diffractometry and atomic force microscopy, respectively. A cube-on-cube epitaxial relationship was ascertained from the θ-2θ and φ diffractograms in both (001) and (111)-oriented films. The (001)-oriented films showed a smooth granular morphology, whereas the faceted pyramid-like crystallites of the (111)-oriented films led to a rough surface. The dielectric response of BZT at room temperature was measured along the growth direction. The films were found to be ferroelectric, although a well-saturated hysteresis loop was obtained only for the (001)-oriented films. High leakage currents were observed for the (111) orientation, likely associated to charge transport along the boundaries of its crystallites. The remanent polarization, coercive field, dielectric constant, and relative change of dielectric permittivity (tunability) of (111)-oriented BZT were higher than those of (001)-oriented BZT.