Reactions at the interface La0.5Ca0.5MnO3-YSZ/Al2O3 under anodic current

The reaction at the interface of a solid electrolyte cell between air electrode (La_0.5Ca_0.5MnO₃) and YSZ-electrolyte with different Al₂O₃-contents was investigated by electron microscopy (SEM) and x-ray diffraction (XRD). Formation of MnAl₂O₄ was detected in a 5 μm diffusion zone within the electrolyte. MnAl₂O₄ formation can be explained by diffusion of Mn-ions into the electrolyte and subsequent reaction with the α-Al₂O₃ grains during sintering. Cell performance and long-term stability in SOFC operation are not negatively affected by MnAl₂O₄ formation. However a rise in electrode resistance and slow delamination of perovskite oxide electrode were observed after some hours of electrolysis. This reaction is the consequence of oxygen gas pressure at the electrolyte in the MnAl₂O₄ diffusion zone. It is caused by local increase of electronic conductivity by MnAl₂O₄ formation. Long-term stability also for electrolysis conditions has been achieved by an additional intermediate YSZ-layer between air electrode and electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)₂), iron trihydroxide (Fe(OH)₃), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from M?ssbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the M?ssbauer subspectral area and recoil-free fraction of each phase. The different M?ssbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 μm. This revised version was published online in August 2006 with corrections to the Cover Date.     

LiMn2O4 films grown by pulsed-laser deposition

LiMn₂O₄ films have been deposited onto silicon wafer by pulsed-laser deposition (PLD) technique in order to test their reliability as cathode materials in rechargeable lithium microbatteries. The film formation has been studied as a function of the preparation conditions, i.e., composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Depending on the conditions of deposition, Mn₂O₃ was present as an impurity phase. When deposited onto silicon substrate maintained at 300 °C in an oxygen pressure of 100 mTorr from the target LiMn₂O₄+15 % Li₂O, the PLD films are well-textured with crystallite size of 300 nm. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by x-ray diffraction and Raman scattering measurements. Surface morphologies of layers were investigated by SEM. The cells Li//LiMn₂O₄ have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the range 3.0–4.2 V. The voltage profiles show the two expected steps for Li_xMn₂O₄ with a specific capacity as high as 120 mC/cm² μm. The chemical diffusion coefficients for the Li_xMn₂O₄ thin films appear to be in the range of 10⁻¹¹-10⁻¹² cm²/s. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

An anomaly in the magnetic susceptibility of the Mg1−x CuxO solid solutions with 0.01≤x≤0.20

The problem of localized superconductivity has motivated the preparation of Mg_1−x Cu_xO solid solutions with NaCl structure and 0.01≤x≤0.20, as well as a study of the magnetization and magnetic susceptibility χ in the 2–400 K temperature range and in magnetic fields of up to 5 T. The temperature dependence of χ is described for all compositions by the Curie-Weiss law, χ = C/(T − θ), where the constant C is close to the value calculated for each composition for μ_eff = 1.7–1.9μ_B, and θ is close to zero. For T < 30 K, χ(T) deviates for all compositions toward lower χ, which can be attributed to magnetic ordering of exchange-coupled clusters in the solid solution. At T∼320–330 K, an anomaly of a diamagnetic type, i.e., a decrease of χ by 6–30% of its paramagnetic value, has been observed for all compositions against the background of the generally paramagnetic χ(T). A discussion is presented of alternative reasons for this anomaly and of its possible connection with localized superconductivity. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 4, 2000, pp. 701–703. Original Russian Text Copyright ? 2000 by Samokhvalov, Arbuzova, Viglin, Naumov, Smolyak, Korolev, Lobachevskaya.     

Preparation techniques and materials for long term stable SOFC - Single cell membranes

Chemical reactions and thermal expansion mismatch between electrodes and electrolyte may reduce the long term stability of SOFC-single cells and can cause undesirable thermomechanical stresses. In solid electrolyte cells the formation of MnAl₂O₄ was detected between the air electrode (La_0.5Ca_0.5MnO₃) and the electrolyte (YSZ/Al₂O₃) in a 5 μm diffusion zone within the electrolyte. The electronically conducting spinel MnAl₂O₄ is thought to be the main factor for delamination of the air electrode under anodic current (electrolysis). The performance and long term stability of the air electrode/electrolyte interface can be improved for electrolysis conditions by an additional intermediate YSZ-layer made by sol-gel technique. The mismatch in thermal expansion between the electrode materials and the electrolyte have been eliminated via optimized doping and by adding small amounts of a silicate-based substituent with a very low thermal expansion co-efficient for the cathode and anode, respectively. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Determination of amorphous layers on thick film Nasicon in dependence on different sintering processes

In thick film gas sensors Nasicon is used as a solid electrolyte with high Na⁺ ionic conductivity. Sensors like CO₂, O₂, Pt ▮ Na₂CO₃, BaCO₃ ∥ NASICON [Pt]_glass, O₂, CO₂ are suitable to measure the CO₂-concentration over 5 orders of magnitude. To characterize the screen printed Nasicon as a main component of such sensors grazing incidence diffractometry (GID), SEM, impedance spectroscopy and dc polarization measurements are performed in order to improve the long-term stability. The sintering process of the thick film influences the chemical surface composition of Nasicon and as a consequence the response of the sensor. Nasicon films sintered at temperatures between 1070 and 1210 °C show an amorphous layer increasing up to 1.1 μm thickness on the surface. Impedance measurements show, that cells using in such a way prearated Nasicon are responsible for water vapour. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Investigations of a number of alternative negative electrode materials for use in lithium cells

There is a considerable interest in the replacement of graphite as the negative electrode reactant in rechargeable lithium batteries by composite electrodes containing alloys or convertible oxides. Some such materials can have much higher theoretical specific capacities than graphite, more than a factor of ten in some cases. In addition it would be desirable to eliminate the irreversible loss of capacity during the first charging cycle that is characterisitic of graphite electrodes, as well to raise the operating potential somewhat in order to reduce the danger of the formation of elemental lithium during recharging. The several strategies that have been followed in the search for attractive alternatives will be briefly described. It has been found to be difficult to obtain the desired combination of high capacity, low first cycle loss and capacity retention upon cycling. Investigations of the electrochemical behaviour of elemental boron and borides (B₄C, CaB₆, LaB₆, AlB₂, SiB₃), elemental silicon and silicides (Mg₂Si, FeSi₂, CoSi₂, NiSi₂, TiSi₂, VSi₂) and of siliconmonoxide, SiO, will be reported. The galvanostatic cycling method was used, with thick layer electrodes (30 μm) deposited upon copper foil in coffee bag-type cells with a liquid electrolyte. Lithium foil was used for the counter and reference electrodes. The results of the investigation of the morphological changes upon cycling, as observed by the use of SEM, will also be presented. Paper presented at the 7th Euroconference on Solid State Ionic, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Semiconducting properties of CoO thin films

This paper reports electrical properties of CoO thin films of different thickness in the range 0.375 – 7.95 μm. Both electrical conductivity and thermopower were measured at elevated temperatures (1223 – 1423 K) and under controlled oxygen partial pressure (5 − 2.1x10⁴ Pa). It was found that at low p(O₂) the electrical conductivity decreases with film thickness. The activation energy of the electrical conductivity (E_a) in air decreases with the oxide thickness from 0.56 eV at 0.375 μm to 0.52 eV for massive CoO while at low p(O₂)=5 Pa the E_a is independent of the thickness (E_a = 0.46 eV). The reciprocal of the p(O₂) exponent of the electrical conductivity (n_δ) in the range 1223 K – 1373 K is close to four for the 7,95 μm film and is about 3.5–3.7 for the 0.375 μm film. The electrical properties of the CoO thin films are considered assuming different defect structures in the bulk phase and the surface layer.     

Electrical resistivity of microinhomogeneous PdMnxFe1−x alloys

The electronic band-structure calculations of the PdFe ferromagnet and the PdMn antiferromagnet performed in this work permit one to conclude that the specific features of the electrical resistivity observed in the ternary PdMn_xFe_1−x alloy system [the deviation from the Nordheim-Kurnakov rule ρ₀(x)∼x(1−x), which is accompanied by a high maximum of residual resistivity (not typical of metals) ρ ₀ ^m ∼220 μΩ cm at x _C∼0.8 and a negative temperature resistivity coefficient in the interval 0.5≤x≤1] are due to the microinhomogeneous (multiphase) state of the alloys and a variation in the band-gap parameter d spectrum caused by antiferromagnetic ordering of a PdMn-type phase. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 193–197. Original Russian Text Copyright ? 2002 by Kourov, Korotin, Volkova.     

Manganese location and magnetic structure determination of SrMn2—W hexagonal ferrite by means of neutron diffraction

The Mn cation distribution in the seven sublattices of theW-type structure has been determined from high temperature neutron diffraction data of a SrMn₂Fe₁₆O₂₇ powder sample. The Mn²⁺ ions enter the tetrahedral sites of blockS with a preference for site 4e. Mn³⁺ ions in small amounts are found in the octahedral sites, substituting for Fe³⁺, while 0.3 Fe²⁺ has been found in site 6g. The sample has the formula SrMn_2.4Fe_15.6O₂₇. The crystal structure has been described in the non-centrosymmetric Pˉ62c space group, instead of the usual P6₃/mmc one. The magnetic structure, obtained from low temperature (4.2 K) data, is collinear, with the spins parallel to thec axis, according to the Gorter model. The magnetic moments of the sites give an experimental magnetization valueM _s=28.4(1.5) μ_B/f.u., in good agreement with magnetic measurements and the theoretical value.     

Measurement of molecular hydrogen densities by resonance fluorescence

Using a pulsed capilary discharge through helium as a light source fluorescence within the Lyman bands (transitionX ¹ ∑ _g ⁺ →B ¹ ∑ _u ⁺ of molecular hydrogen and deuterium has been investigated. Selecting a narrow spectral range (width ≈ 8 ?) from the continuum radiation with a spectrometer, lines in the wavelength range from 1060 ? to 1110 ? have been excited having a vibrational quantum number υ″=0 in the lower state, and ≦υ′≦4 in the upper state. The fluorescence intensity has been measured as a function of υ′ and of the hydrogen density. Agreement with calculations has been found to be within ±30%. The method at present allows the determination of densities between 10¹⁰ and 10¹⁵ molecules per cm³ with a temporal resolution of 1μs and with a spacial resolution of 0.1 cm³     

Method for measuring integrated sensitivity of solar cells and multielement photoconverters using an X-Y scanner

We describe a method for automated measurement of the integrated sensitivity of solar cells (SCs) and multielement photoconverters (MPCs) using an experimental apparatus including a Pentium III personal computer (PC), an HP-34401A digital multimeter (DM), a stabilized radiation source (SRS), a controllable focusing system, an X-Y positioning device based on CD-RW optical disk storage devices. The method provides high accuracy in measuring the size of photosensitive areas of the solar cells and multielement photoconverters and inhomogeneities in their active regions, which makes it possible to correct the production process in the development stage and during fabrication of test prototypes for the solar cells and multielement photoconverters. The radiation power from the stabilized radiation source was ≤1 W; the ranges of the scanning steps along the X, Y coordinates were 10–100 μm, the range of the transverse cross sectional diameters of the focused radiation beam was 10–100 μm, the measurable photocurrents were 10⁻⁹ A to 2 A; scanning rate along the X, Y coordinates, ≤100 mm/sec; relative mean-square error (RMSE) for measurement of the integrated sensitivity of the solar cells, 0.2 ≤ γS _int ≤ 0.9% in the ranges of measurable photocurrents 1 mA ≤ I_ph ≤ 750 mA and areas 0.1 ≤ A ≤ 25 cm² for number of measurements equal to ≤ 2· 10⁵; instability of the radiation power (luminosity) ≤ 0.08% for 1 h or ≤ 0.4% for 8 h continuous operation; stabilized power range for the stabilized radiation source, 10⁻²–10² W. The software was written in Delphi 7.0. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 670–675, September–October, 2006.     

Tailoring ceramics for specific applications: A case study of the development of all-solid-state lithium batteries

Metal oxides with the nominal chemical compositions Li₅La₃M₂O₁₂ (M=Nb, Ta), possessing a garnet-like structure, exhibit ionic bulk conductivities of the order of magnitude of ∼10⁻⁶ S/cm at 25 °C. Partial substitution of La by alkaline earth elements (Ca, Sr, Ba) in Li₅La₃M₂O₁₂ yields new members of compounds with garnet-like structure with the composition Li₆ALa₂M₂O₁₂ (A=Ca, Sr, Ba). Among the investigated compounds, so far, the Ba-compound Li₆BaLa₂Ta₂O₁₂ exhibits the highest bulk conductivity of 4.0×10⁻⁵ S/cm at 22 °C with an activation energy of 0.40 eV. All Ta-members were found to be stable against chemical reaction with molten elemental lithium. Li₆ALa₂Ta₂O₁₂ (A=Sr, Ba) exhibit also high electrochemical stability of ∼6 V vs. lithium and chemical stability against reaction with LiCoO₂ cathode material. A novel high voltage thin-film battery was constructed using spinel-type Li₂MMn₃O₈ (M=Co, Fe) as positive electrode, LiPON as electrolyte and Al as negative electrode material. Li₂MMn₃O₈ (M=Fe, Co) electrodes show two reversible plateaus during the charging and discharging cycle at ∼4 and ∼5 V vs. Li. The former plateau is due to the valence change of Mn³⁺ to Mn⁴⁺ and the latter one is due to the oxidation of M³⁺ to M⁴⁺. The chemical diffusion coefficient ( ) was found to be in the range 10⁻¹³–10⁻¹² cm²/sec for any composition x of Li_2−xMMn₃O₈ (M=Fe, Co) in the range from 0.1 to 1.6 by employing the galvanostatic intermittent titration technique (GITT). AC impedance studies revealed an electrolyte-electrode charge transfer resistance of 260–290 Θ and an electrode double layer capacity of ∼45–70 μF for an electrode area of 6.7 cm² at room temperature. The chemical diffusion coefficient of the Al,LiAl negative electrode is about three orders of magnitude higher than that of the positive electrode materials. Accordingly, we believe that the diffusion of Li into and out of the cathode material is the rate determining process. Paper presented at the Patras Conference on Solid State Ionics - Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Tailoring the microwave permittivity and permeability of composite materials

The microwave permittivity (ɛ_r) and permeability (μ_r) of composite materials are tailored by adding various loading agents to a host plastic and are subsequently modeled using the Maxwell Garnett theory and second order polynomials. With the addition of manganese zinc ferrite, strontium ferrite, nickel zinc ferrite, barium tetratitanate and graphite powders, materials with values of ɛ′, e″, μ′, μ″ as high as 22, 5, 2.5 and 1.7 have been obtained. Permittivity and permeability data are calculated at 2.0245 GHz from reflection and transmission measurements performed in a 7 mm coaxial test line. The Maxwell Garnett (MG) theory successfully models ɛ_r if the filling factor is less than 0.30 and ratio |ɛ₁| (host)/ |ɛ₂| (powder) is greater than 0.04. As this ratio decreases, the MG theory is shown to be independent of ɛ₂ and second order polynomials are used to effectively model the dielectric constant. Polynomials are also used for the ferrite composites because it was determined that the MG theory was unable to model μ_r. This deficiency is attributed to the difference of domain structures that exist in powdered and sintered ferrites.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

Lithium ion conductivity of the B-site substitution in Li3xLa0.67−xRE y III Ti1−2yPyO3 system (REIII=Sc3+, Y3+, Nd3+, Sm3+, Eu3+, Yb3+)

The B-site substituted perovskite solid solution systems Li_3xLa_0.67−xRE_yTi_1−2yP_yO₃ (RE=Sc, Y, Nd, Sm, Eu, Yb) have been investigated. Perovskite solid solutions formed in the range of x=0.10, y<0.10 for RE=Sc³⁺, Y³⁺, Yb³⁺, x=0.10, y≤0.05 for RE=Nd³⁺, Sm³⁺, Eu³⁺. Li_0.3La_0.57Nd_0.05Ti_0.9P_0.05O₃ has the highest bulk conductivity of 4.31×10⁻⁴ S·cm⁻¹ and the highest total conductivity of 2.52×10⁻⁴ S·cm⁻¹ at room temperature in all prepared compositions. The compositions have low activation energies of about 24–30 kJ/mol in the temperature ranges of 298–523 K. SEM studies showed that the sample made by solid-state reaction has a sphere-like morphology and a rough particle with particle size of about 50 μm. The research results also indicated that the reaction temperature decreases and the electrochemical stabilities of the titanate-based perovskite-type solid solutions are improved by using RE³⁺ and P⁵⁺ replaced Ti⁴⁺ on B-site in the Li_3xLa_0.67−xTiO₃ parent.     

利用原位傅里叶变换红外光谱诊断催化剂的特性和催化活性

The present work establishes a systematic approach based on the application of in-situ Fourier transform infrared spectroscopy (FTIR) for the investigation of the crystal structure, thermal stability, redox behavior (temperature-programmed reduction/temperatureprogrammed re-oxidation) as well as the catalytic properties of Co₃O₄ thin films. The syntheses of Co₃O₄ were achieved by chemical vapor deposition in the temperature range of 400-500℃. The structure analysis of the as-prepared material revealed the presence of two prominent IR bands peaking at 544 cm^-1 (υ₁) and 650 cm^-1 (υ₂) respectively, which originate from the stretching vibrations of the Co-O bond, characteristic of the Co₃O₄ spinel. The lattice stability limit of Co₃O₄ was estimated to be above 650℃. The redox properties of the spinel structure were determined by integrating the area under the emission bands υ₁ and υ₂ as a function of the temperature. Moreover, Co₃O₄ has been successfully tested as a catalyst towards complete oxidation of dimethyl ether below 340 ℃. The exhaust gas analysis during the catalytic process by in situ absorption FTIR revealed that only CO₂ and H₂O were detected as the final products in the catalytic reaction. The redox behavior suggests that the oxidation of dimethyl ether over Co₃O₄ follows a Mars-van Krevelen type mechanism. The comprehensive application of in situ FTIR provides a novel diagnostic tool in characterization and performance test of catalysts.     

Measurement of atomic number and mass attenuation coefficient in magnesium ferrite

Pure magnesium ferrite sample was prepared by standard ceramic technique and characterized by X-ray diffraction method. XRD pattern revealed that the sample possess single-phase cubic spinel structure. The linear attenuation coefficient (μ), mass attenuation coefficient (μ/ρ), total atomic cross-section (σ _tot), total electronic cross-section (σ _ele) and the effective atomic number (Z _eff) were calculated for pure magnesium ferrite (MgFe₂O₄). The values of γ-ray mass attenuation coefficient were obtained using a NaI energy selective scintillation counter with radioactive γ-ray sources having energy 0.36, 0.511, 0.662, 1.17 and 1.28 MeV. The experimentally obtained values of μ/ρ and Z _eff agreed fairly well with those obtained theoretically.      

Fabrication of anode supported thick film ceria electrolytes for IT-SOFCs

Anode supported thick film ceria electrolyte unit cells were fabricated using a colloidal dip coating method for IT-SOFCs. Pre-sintering temperature of the anode substrate and the final sintering temperature were found to be the primary parameters determining the density of the film. With Ni-Ce_0.89Gd_0.11 O_2–δ cermet anode, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ cathode and 15 μm Ce_0.89Gd_0.11 O_2–δ electrolyte, the cells were tested in a fuel cell configuration with air at the cathode and moist H₂ at the anode. At 650 °C, the cell indicated a maximum power density of ∼0.27 W/cm² at a current density of 0.62 A/cm². Cell performance was compared with oxygen at the cathode and the cell indicated a maximum power density of ∼0.50 W/cm² at 1.14 A/cm², 650 °C. Activation energy for the area specific resistance (ASR) of the cell suggests that with air at cathode, the cell performance was limited by gaseous diffusion at cathode and with oxygen at cathode, by oxygen ion transport across the electrolyte.