Possibilities of NOx and CHx determination using galvanic cells with perovskite-electrodes on YSZ

Perovskite-type mixed oxides with the formula La_1−xA_xMe_1−yB_yO_3±δ (x=0.01...0.2; y=0.01...0.5; A=Ca, Sr; Me=Cr, Mn, Fe, Co; B=Mg, Ga) were investigated with the aim to use the oxides as electrode materials for galvanic cells with Y₂O₃-stabilized ZrO₂ solid electrolytes (YSZ). The catalytic activity of the oxides for the oxidation, reduction and decomposition of gas components was varied by changing the transition metal and by partial substitution of lanthanum and the metal. The behaviour of different substituted lanthanum chromite/YSZ and manganite/YSZ electrodes in the presence of nitric oxides (NO_x) and combustible components (CH_x) in gases containing oxygen was investigated. Various electrode combinations were tested in gas-symmetrical cells. For the NO determination cells with a catalytically active Pt electrode and a La_0.8Sr_0.2MnO₃ electrode with a low catalytic activity for the decomposition of NO were used. The results show the possible quantitative measurement at low and constant p(O₂) on the basis of a calibration. For the determination of C₃H₆ a CH_x-sensitive Au and an O₂-sensitive La_0.99Sr_0.01CrO₃ electrode were combined in a potentiometric thick film sensor. In gases containing oxygen with small amounts of C₃H₆ the sensor provided a utilizable voltage in dependence of the C₃H₆ concentration. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Reference electrodes for low-temperature gas sensors

The behavior of sodium oxide bronzes in contact with sodium-conducting solid electrolytes in atmospheres of oxygen, carbon dioxide, hydrogen sulfide, and humid air was investigated. The exchange currents at the Na_xCoO_y/NASICON interface was determined. It is shown that among the sodium-cobalt oxide bronzes the β-phase of the composition Na_0.6CoO₂ is the most reversible one (exchange current 236 μA/cm²). Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept. 1994     

Platinum–aluminum alloy electrode for retention improvement of gadolinium oxide resistive switching memory

Platinum–aluminum (Pt–Al) alloy top electrode on the retention improvement of gadolinium oxide (Gd _x O _y ) resistive switching memory was investigated. The aluminum oxide (Al _x O _y ) formation at the Pt–Al alloy top electrode and Gd _x O _y interface will lead to the high Schottky barrier height. Further, the more aluminum incorporation can suppress the crystallization of platinum electrode after the post-metallization annealing. Both the crystallization suppression of Pt top electrode and the interfacial aluminum oxide formation will prevent the oxygen ions from out-diffusion through Pt grain boundaries, responsible for the retention enhancement of the Gd _x O _y resistive switching memory.     

Evaluation of the electrical capacitance of M,O2/O2− interfaces (M=Pt, Au, Pd, In2O3; O2− = zirconia based electrolyte) exhibiting constant phase element behavior

The behavior of the electrode systems M,O₂/O² (M = porous Pd, Pt, A and dense In₂O₃; O²⁻ = ZrO₂-based single-crystal solid electrolyte) was studied by means of impedance measurements. The examination of the Pt,O₂/O²⁻ electrode system showed that the constant phase element (CPE) can be attributed to a nonuniform distribution of current at the electrode surface. It was observed that the CPE parameters n and B in the expression Y_CPE = B (jω)ⁿ may be related by B=(C_dl)ⁿ (R_Ω)_n-1, where C_dl is the double layer capacitance and R_Ω the resistance of the electrolyte in the cell. Then, C_dl of the electrode - electrolyte interface could be determined. The specific C_dl of the oxidized noble metals and india electrodes is nearly one order of magnitude lower than C_dl of the electrodes in the metallic state. The C_dl value of all the electrodes studied depends little or is independent of temperature and oxygen pressure. It is concluded that the Helmholtz model of double layer structure does not contradict the C_dl behavior.     

Phase stratification in the Nd1 − x BaxCoO3 − δ (0.3 ≤ x ≤ 0.54) system

Slowly cooled Nd_{1 ? x} Ba_xCoO_{3 ? δ} samples were two-phase in the concentration interval 0.3 ≤ x ≤ 0.46. One of the phases had O-orthorhombic lattice distortions (Pbnm) characteristic of ferromagnetic samples with x ≤ 0.3, and the other phase had tetragonal distortions (P4/mmm) characteristic of samples with x ≥ 0.46. Tetragonal distortions were caused by ordering of Nd³⁺ and Ba²⁺ ions. Samples with ordered neodymium and barium ions (Nd_{1 ? y} Ba_{1 + y} Co₂O_{6 ? γ} at ?0.08 ≤ y ≤ 0.08) experienced metal-dielectric and orientation magnetic phase transitions.     

Concentration-phase transitions in solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 and their manifestations in Raman spectra

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃, Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ (x = 0?0.16, y = 0–1). It has been revealed that, for the solid solutions Li _x Na_{1 ? x} Ta_0.1Nb_0.9O₃ and Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860–873 and 900–905 cm^?1 upon variation of the concentration of tantalum for solid solutions Li_0.12Na_0.88Ta _y Nb_{1 ? y} O₃, and NaTa _y Nb_{1 ? y} O₃ is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.     

Mechanoelectric effect in O<Superscript>2−</Superscript> conducting cell: Relations to phase transformations and other lattice changes in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + <Emphasis Type="Italic">x</Emphasis></Subscript>-like oxides

The origin of mechanoelectric voltage arising in an ^2?-conducting solid-state cell O₂,Pt|ZrO₂|Pt, O₂, YBCO due to the pressure applied on the YBCO sample is discussed. It is demonstrated that the mechanoelectric voltage is the sum of the electronic and ionic mechanoelectric contact potentials. The mechanoelectric contact potentials are determined by the work done by the load during an elementary ordering act or another elementary process taking place when the sample absorbs one oxygen atom. Effects related to oxygen atoms being ordered under the load, such as pressure-induced oxygen absorption, enhanced deformation, and change in the order parameter, are discovered.     

Synthesis of monolayers and bilayers of cobalt oxyhydrates (Na,K) x (H2O) y CoO2− δ

Potassium sodium cobalt oxyhydrates (Na,K)_x(H₂O)_yCoO_{2? δ} were synthesized from γ-Na_0.7CoO₂ by using aqueous KMnO₄ solution in a one-pot process. Chemical and structural analyses revealed that a partial or even almost complete replacement of K⁺ for Na⁺ in the alkaline layers occurs. Direct formation of the c ≈ 13.9 Å phase is apparently associated with the larger size of K⁺ (～1.4 Å) as compared to Na⁺ (～1.0 Å). Formation of (Na,K) _x (H₂O) _y CoO_{2? δ} not only involves de-intercalation, oxidation and hydration processes, but also an ion exchange reaction. Based on a systematic study, the phase formation of (Na,K) _x (H₂O) _y CoO_{2? δ} with c ≈ 19.6 Å is a slow process, particularly when using aqueous KMnO₄ solution with low molar ratio of KMnO₄/Na. When comparing the Co K-edge X-ray absorption spectra of (Na,K) _x (H₂O) _y CoO_{2? δ} with those of Na _x (H₂O) _y CoO₂ obtained from Br₂/CH₃CN solution, the edge energy of the main peak of the bilayered hydrate is found to be 3.5 eV higher than that of the monolayered hydrate for (Na,K) _x (H₂O) _y CoO_{2? δ}. In contrast, the edge energy of the main peak of the bilayered hydrate is 0.4 eV lower than that of the monolayered hydrate for Na _x (H₂O) _y CoO₂. In addition, the hydration behavior of monolayered of (Na,K) _x (H₂O) _y CoO_{2? δ} is different from that of Na _x (H₂O) _y CoO₂. These results seem to suggest that they are two different systems.     

Lithium ion conductive glass and its application to solid state batteries

Three kinds of coin-type battery, In-Li_x / Li_1−xCoO₂, Li_4/3+xTi_5/3O₄ / Li_1−xCoO₂, and Li_2+xFeS₂ / Li_1−xCoO₂, were fabricated with a Li⁺ ion conductive glass as an electrolyte, and their properties were investigated. They show excellent performance thanks to the solid electrolyte. Iron sulfide is found to be an excellent electrode material in solid state rechargeable batteries. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Surface properties of palladium catalysts supported on ternary ZrO2–Al2O3–WOx oxides prepared by the sol–gel method: Study of the chemical state of the support

The surface properties of Pd and Pd–Pt catalysts supported on binary ZrO₂–WOx and ternary ZrO₂–Al₂O₃–WOx oxides prepared by the sol–gel method were studied. Special attention was paid to the study of the texture of the catalysts as well as the chemical state of tungstated zirconia and tungstated zirconia promoted with alumina in the palladium catalysts. The catalysts were tested in the isomerization of n-hexane and were characterized by N₂ physisorption, XRD, TPR, Raman spectroscopy, XPS and FT-IR of adsorbed pyridine. The catalysts had bimodal pore size distributions with mesopores in the range 55–70 Å and macropores of 1000 Å in diameter. The catalysts had a surface WOx coverage (4.4–6.0 W nm^?2) lower than that of the theoretical monolayer (7.0 W nm^?2). A lower acidity of the ternary ZrO₂–Al₂O₃–WOx oxide as compared to the binary ZrO₂–WOx oxide was found. Higher activity in the isomerisation of n-hexane was obtained in the Pd–Pt catalysts supported on ternary ZrAlW oxides prepared by sol–gel that is correlated with the coexistence on the surface of W⁴⁺ (WO₂) or W⁰ and W⁶⁺ (Al₂(WO₄)₃) species, ZrO₂ in the tetragonal phase and a high amount of ZrOx suboxides species in a low oxidation state (Zr³⁺ and Zr²⁺).     

Electronic structure, x-ray spectra, and magnetic properties of the LiCoO2−δ and NaxCoO2 nonstoichiometric oxides

This paper reports on a study of the magnetic susceptibility, x-ray photoelectron, and x-ray emission spectra of the LiCoO_2?δ and Na_xCoO₂ nonstoichiometric oxides. The valence-band structure of LiCoO₂ was analyzed. The hole concentration in the oxygen 2p band of LiNiO₂ and LiCoO₂ was derived from measurements of the O Kα emission spectra. Measurements of Co 2p and Co 3s photoelectron spectra showed that the Co³⁺ ions reside in the low-spin state with S=0. The deficiency of oxygen in the LiCoO_2?δ reduced oxides gives rise to the formation of divalent cobalt ions. The deficiency of the alkali metal in Na_xCoO₂ initiates the formation of holes in the oxygen 2p band while not changing the electronic configuration d ⁶ of the cobalt-ion ground state.     

Control of luminescence and conductivity of delafossite-type CuYO2 by substitution of rare earth cation (Eu, Tb) and/or Ca cation for Y cation

Delafossite-type oxides of CuTb_yY_1−yO₂, CuEu_yY_1−yO₂, CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ have been prepared by solid state reactions. The lattice-parameter dependence on the composition implies substitution of the Tb³⁺, Eu³⁺ and Ca²⁺ cations for the Y³⁺ site. Noticeable sharp emission lines due to the f-f transitions (⁵D₄→⁷F_J, J=3-6) of Tb³⁺ or due to the f-f transitions (⁵D₀→⁷F_J, J=0-4) of Eu³⁺ are observed at room temperature. Electrical conductivities of CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ are larger than those of CuTb_yY_1−yO₂ and CuEu_yY_1−yO₂, indicating the increase of the hole concentration caused by the substitution of Ca²⁺ for the Y³⁺ site. These results indicate the controllability of the luminescence and conductivity in CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ delafossite-type oxides by simultaneous substitution of the rare earth Tb³⁺ or Eu³⁺ cation and the Ca²⁺ cation for the Y³⁺ site.     

Permeation barrier properties of an Al2O3/ZrO2 multilayer deposited by remote plasma atomic layer deposition

We report the permeation barrier properties of Al₂O₃/ZrO₂ multi-layers deposited by remote plasma atomic layer deposition. Electrical Ca degradation tests were performed to derive the water vapor transmission rate (WVTR) of Al₂O₃, ZrO₂ and Al₂O₃/ZrO₂ multi-layers at 50 °C and 50% relative humidity (RH). Al₂O₃/ZrO₂ multi-layers exhibit better barrier properties than Al₂O₃ and ZrO₂ layers, and when more individual layers were deposited in the same total thickness, the WVTR value was reduced further, indicating a better barrier property. The WVTR of the Al₂O₃ and ZrO₂ layers were 9.5 × 10⁻³ and 1.6 × 10⁻² g/m² day, respectively, but when deposited alternatively with 1 cycle of each layer, the WVTR decreased to 9.9 × 10⁻⁴ g/m² day. X-ray diffraction results indicated that ZrO₂ has a monoclinic structure but Al₂O₃ and Al₂O₃/ZrO₂ multi-layers show an amorphous structure. Cross sectional Al₂O₃/ZrO₂ multi-layer structures and the formation of a ZrAl_xO_y phase are observed by transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) results indicate that Al₂O₃ and ZrO₂ contain 33.7% and 37.8%, respectively, Al–OH and Zr–OH bonding. However, the ZrAl_xO_y phase contained 30.5% Al–OH and Zr–OH bonding. The results of transmittance measurement indicate that overall, Al₂O₃, ZrO₂ and Al₂O₃/ZrO₂ multi-layers show high transmittance greater than 80% in the visible region.     

Thermodynamic properties of titanium dioxide,niobium dioxide and their solid solutions at high temperature

By means of thermogravimetric measurements, it has been possible to obtain information on the nature of the intrinsic and extrinsic defects of TiO₂, NbO₂ and solid solutions Nb_yTi_1?yO_2±x Pure TiO₂ is an oxygen-deficient oxide The main defects are oxygen vacancies, doubly ionized V^.._o or singly ionized V^._o, and interstitial titanium Ti³_i NbO₂ is a metal-deficient oxide The main defects are neutral niobium vacancies. The solid solutions Nb_yTi_1?yO_2±x may be divided into two groups If y > 0 04, the behavior is analogous to that of NbO₂; with the same defects, but the width of the homogeneity range decreases with the titanium content and Nb_{0 04}Ti_{0 96}O₂ is a stoichiometnc oxide If y < 004, the oxides are both metal deficient and oxygen deficient according to the oxygen partial pressure. We have particularly studied the solution Ti_{0 995}Nb_{0 005}O₂ In the oxygen-deficient domain, the main defects are assumed to be neutral or singly ionized oxygen vacancies In the metal-deficient domain, the main defects are metal vacancies V⁴_Ti From these results we have deduced the nature of intrinsic defects in TiO₂ to be Schottky defects: 2V^.._O + V⁴_T1.     

Some ABO3 oxides with defect pyrochlore structure

Studies on the formation, structures and properties of the ternary oxides TlNbO₃, TlTaO₃ and BiVO_3+x occurring in A₂B₂O_7-y defect pyrochlore structure are reported. The reaction of Tl₂ CO₃ with Nb₂O₅/Ta₂O₅ initially leads to the formation of TlNbO_3+x [x ～ 0.3] and TlTaO_3+x [x ～ 0.4]. On further heating, the phases lose oxygen forming TlTaO₃ and TlNbO₃. The occurrence of ABO₃ oxides having Tl⁺, Pb²⁺ or Bi³⁺ as the A cation in defect pyrochlore structure in preference to the perovskite structure is attributed to the formation of a “trap-mediated” A-A bond through 6s² electrons.     

Effect of allied and alien ions on the EPR spectrum of Mn-containing lithium-manganese spinel oxides

Electron paramagnetic resonance spectroscopy was used for studying the effect of allied and alien ions on the EPR spectrum of Mn⁴⁺-containing lithium-manganese spinel oxides. Manganese spinel oxides with paramagnetic Mn⁴⁺ and diamagnetic substituents in the 16d spinel sites were studied: Li[Mg_0.5Mn_1.5]O₄, Li[Mg_0.5−xCo_2xMn_1.5−x]O₄, 0<x≤0.5, and Li[Li_1/3Mn_5/3]O₄. Ni²⁺-ions with integer-spin-ground state (S=1) were selected as alien ions: Li[Mg_0.5−xNi_xMn_1.5]O₄ (0≤x≤0.5), Li[Li_(1−2x)/3Ni_xMn_(5−x)/3]O₄ (0≤x≤0.5), and Li[Ni_0.5Mn_1.5−yTi_y]O₄ (0≤y≤1.0). It was shown that in Ni-substituted oxides the low temperature EPR response comes from magnetically correlated Ni-Mn spins, while at high registration temperature Mn⁴⁺ ions give rise to the EPR profile. Analysis of the EPR line width allows differentiating between the contributions of the density of paramagnetic species and the strength of the exchange interactions in magnetically concentrated systems. The density of allied and alien paramagnetic species has no effect on the EPR line width in cases when the strengths of antiferro- and ferromagnetic interactions on an atomic site are close. On the contrary, when antiferro- or ferromagnetic interactions on an atomic site are dominant, the EPR line width increases with the density of paramagnetic species.     

Effect of different doped elements on visible light absorption and ultraviolet light emission on Er3+:Y3Al5O12

In this progress report, seven kinds of novel carefully designed and fabricated up-conversion luminescence agents, Er³⁺:Y₃Al₅O₁₂, Er³⁺:Yb_nY_3?nAl₅O₁₂, Er³⁺:Y₃B_aAl_5?aO₁₂, Er³⁺:Y₃Ga_bAl_5?bO₁₂, Er³⁺:Y₃Al₅N_xO_12?x, Er³⁺:Y₃Al₅F_yO_12?y and Er³⁺:Yb_nY_3?nB_aGa_bAl_5?a?bN_xF_yO_12?x?y, are successfully synthesized using sol–gel methods. After that, their corresponding photocatalysts, Er³⁺:Y₃Al₅O₁₂/TiO₂, Er³⁺:Yb_nY_3?nAl₅O₁₂/TiO₂, Er³⁺:Y₃B_aAl_5?aO₁₂/TiO₂, Er³⁺:Y₃Ga_bAl_5?bO₁₂/TiO₂, Er³⁺:Y₃Al₅N_xO_12?x/TiO₂, Er³⁺:Y₃Al₅F_yO_12?y/TiO₂ and Er³⁺:Yb_nY_3?nB_aGa_bAl_5?a?bN_xF_yO_12?x?y/TiO₂, are also prepared by sol–gel coating process. The obtained up-conversion luminescence agents and photocatalysts were characterized by using XRD, XPS, SEM, UV–vis and fluorescence spectrophotometer. Synchronously, several kinds of organic dyes are used to test their photocatalytic degradation using prepared photocatalysts. It indicates that the up-conversion luminescence ability of Er³⁺:Y₃Al₅O₁₂ can be improved obviously through doping of some elements. And then, the photocatalytic activity of TiO₂ is markedly enhanced by modified up-conversion luminescence agents which can transform much visible light into ultraviolet light.     

X-ray spectra and specific features of the structure of lithium-sodium cobaltite Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Na<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>CoO<Subscript>2</Subscript>

The electronic structure and specific features of the structure of nonstoichiometric cobaltite Li _x Na _y CoO₂ (x = 0.42, y = 0.36) have been studied comprehensively. The calculated multiplet for the lowspin state of the Co³⁺ ion agrees with the experimental spectra. It has been established using X-ray absorption spectra measured in the total photoelectric effect yield and total fluorescence yield modes that the Li _x Na _y CoO₂ cobaltite is stoichiometric with respect to the alkali metal near its surface and is defective inside. It has been demonstrated that the charge compensation in the case of an alkali metal deficit in LixNayCoO₂ is due to holes in O 2p states.     

Electrode reaction at Pt,O2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

This study aims to make clear the reaction kinetics at Pt, O₂(g)/zirconia electrodes in the oxygen partial pressure, P_O2, of ∼ 10⁻⁴ − 1 atm at ∼ 400–∼800°C. By a critical review on the preceding studies, problems were pointed out in the application of the Langmuir adsorption isotherm to the P_O2 dependence of electrode conductance, in the assumption of electric double layer at the electrode interface, and of the inconsistency between the recent reaction model of surface diffusion controlled kinetics and the absolute rate theory. It was shown that the charge transfer kinetics cannot be the rate determining step (RDS) of the electrode reaction. The possible RDS was concluded to be either (i) dissociative adsorption of oxygen molecules on the Pt surface or (ii) surface diffusion of O_ad atoms on the Pt surface to the Pt/zirconia contact. The diffusion of O_ad atoms on the Pt surface was considered to be proportional to θ(1−θ)(∂μ_O/∂x), where θ is the occupancy of O_ad atoms on Pt and μ_O is the oxygen chemical potential on the Pt surface. The rate equation, current-potential relationship, and the electrode conductivity, σ_E, were calculated for the cases the RDS is (i) and (ii), respectively. By comparing the calculated σ_E versus log P_O2 relationship with the reported ones, it was shown that the RDS is (i) for T ≲ 500°C and is (ii) for T ≳ 600°C. In the former case, σ_E is essentially constant irrespective to P_O2, and in the latter case, σ_E maximum appears on the σ_E versus log P_O2 relations.     

Spectroscopy of mixed molybdenum-vanadium oxides and catalytic oxidation of toluene

The EPR, FTIR, and XRD techniques are used to examine individual and mixed vanadiummolybdenum oxides of composition (1 ? x)V₂O₅: xMoO₃ prepared by coprecipitation. The phase and structural heterogeneity of these oxides is characterized. The oxidation of toluene with air oxygen to maleic anhydride on V₂O₅ and mixed oxides involves singlet oxygen ¹O₂. A correlation between the content of V(IV) ions in the mixed catalysts and the amount desorbed ¹O₂ is observed. It is demonstrated that the oxidation of toluene causes changes in the spatial distribution of the V(IV) ions in the matrix and the phase composition of the samples.