Synthesis, characterization, and electrical conductivity of new Aurivillius-type oxide-ion conductor

A new oxide-ion conductor of Aurivillius family with a general formula Bi₂Al _x V_{1 − x} O_{5.5 − x − δ}; 0 ≤ x ≤ 0.20 (BIALVOX) was synthesized by the sol-gel citrate route. Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analyses confirmed that the calcination of BIALVOX xerogels is fully completed at around 500°C after three hours of thermal treatment. It has been found that the β-orthorhombic phase is stabilized with compositions x ≤ 0.07, whereas the stabilization of the γ′-phase takes place for x ≥ 0.10. AC impedance spectroscopic investigation suggested that the charge accumulation at grain boundaries is thermally activated process. However, the maximum electrical conductivity (7.73 × 10⁻⁵ S cm⁻¹) noticed for BIALVOX.13 at 300°C was attributed to the maximum vacancy concentration in the equatorial planes, responsible for the ion diffusion through the structure. This has been further evidenced by the temperature dependence of dielectric permittivity.     

Thermal Expansion in the Zr and 1-, 2-valent Complex Phosphates of NaZr2(PO4)3 (NZP) Structure

A_5–4xZr_xZr(PO₄)₃ (A=Na, K;0≤x≤1.25), Na_1-xCd_0.5xZr₂(PO₄)₃ (0≤x≤1), Na_5–xCd_0.5xZr(PO₄)₃ (0≤x≤4) compositions which belong to the NZP structural family were synthesized using the sol-gel method. The lattice thermal expansion of members of these rows were determined up to 600°C by high-temperature X-ray diffractometry. The axial thermal expansion coefficients change from -5.8·10^-6to 7.5·10^-6 °C^-1 (α_a) and from 2.6·10^–6 to 22·10^–6 °C^-1 (α_c). These results, in addition to those for other NZP compounds allow us to explain their low thermal expansion. The mechanism can be attributed to strongly bonded three-dimensional network structure, the existence of structural holes capable to damp some of the thermal vibrations and anisotropyin the thermal expansion of the lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Crystallization in the SiO2-Al2O3 System from Amorphous Powders

The crystallization of amorphous chemically homogeneous powders in the SiO₂.Al₂O₃ system has been studied by differential scanning calorimetry and X-ray diffraction. Up to 1300°C only one exotherm has been observed. Only mullite crystallizes for compositions ≤69 mol% Al₂O₃ and spinel for those ≤80%. The crystallizations into mullite and spinel are sharp and exothermic, with an enthalpy of 250–300 J/g. The chemical composition of the crystallized mullite regularly increases from 68 to 76 mol% Al₂O₃ with increasing bulk composition from 60 to 75 mol% Al₂O₃.     

Kinetics of cordierite crystallization from diphasic gels

Diphasic cordierite gels were prepared from colloidal silica, aluminum and magnesium nitrates and citric acid. The mechanism of xerogel decomposition was studied by infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The thermal decomposition of the xerogel forms a solid mixture of MgO, Al₂O₃ and SiO₂ at around 250 °C. Cordierite crystallization was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA). Xerogels were initially thermally treated, and this sample crystallized to μ-cordierite at 850 °C, at 900 °C α-cordierite crystallizes and at 1150 °C α-cordierite is the major phase and μ-cordierite is totally consumed. The apparent activation energy for cordierite crystallization process was determined based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, Ligero methods and the Arrhenius law for dependence of activation energy with temperature. The apparent activation energy was (466.8 ± 34.3) kJ/mol, the exponent of Avrami was (1.9 ± 0.2) and the frequency factor was (1.55 × 10²⁰) s⁻¹. The Avrami value indicates a nucleation controlled process, which can be a consequence of the high xerogel homogeneity, a consequence of the early and simultaneous formation of the MgO, Al₂O₃ and SiO₂ mixture.     

Potassium-ionic conduction in the gallate-ferrite solid electrolytes

New potassium-conducting solid electrolytes in the mixed gallate-ferrite systems (1 − x)Ga₂O₃ · xFe₂O₃ · 0.25TiO₂ · K₂O and 1.5[(1 − x)Ga₂O₃ · xFe₂O₃] · TiO₂ · 2K₂O are synthesized and studied. The electrolytes exhibit high ionic conductivity in the test temperature range of 300 to 750°C (above 10⁻² S/cm at 300°C and above 10⁻¹ S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. Possible reasons for the effect of Ga/Fe ratio in the structure of solid electrolytes on their transport properties are discussed.     

FT-EPR Spectroscopic Study of Silicophosphate Glasses Derived from Gels

(100-x) mole% SiO₂-x mole% P₂O₅-glasses withx=1–9 have been prepared by the sol-gel process using tetraethylorthosilicate and triethylphosphate as precursors. The gels were fired at various temperatures up to 950°C and then exposed to γ-ray irradiation to induce paramagnetic centers. CW-EPR and FT-EPR Spectroscopies were employed at temperatures between 4 and 300 K in order to determine the resulting structures. The dried gels exhibited four types of O ₂ ⁻ -ions trapped in pores of different sizes. The gels fired atT=670°C exhibited theE’ ₁-center and non-bridging oxygen as a results of the fracture of the Si-O-Si bonds. At higher temperatures, the spectra of the POHC, POHC ^b ,E’, and CH ₃ ⁻ -centers have been detected that are a function ofx andT.     

Thermal properties of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–PbO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glass system

Thermal behavior of xGa₂O₃–(50 − x)PbO–50P₂O₅ (x = 0, 10, 20, and 30 mol.% Ga₂O₃) and xGa₂O₃–(70 − x)PbO–30P₂O₅ (x = 0, 10, 20, 30, and 40 mol.% Ga₂O₃) glassy materials were studied by thermo-mechanical analysis (TMA) and differential thermal analysis (DTA). Replacement of PbO for Ga₂O₃ is accompanied by increasing glass-transition temperature (263 ≤ T _g/°C ≤ 535), deformation temperature (363 ≤ T _d/°C ≤ 672), crystallization temperature (396 ≤ T _c/°C ≤ 640) and decreasing of coefficient of thermal expansion (5.1 ≤ CTE/ppm K⁻¹ ≤ 16.7). Values of Hruby parameter were determined (0.1 ≤ K _H ≤ 1.3). The thermal stability of prepared glasses increases with increasing of concentration of Ga₂O₃.     

Hydrothermal alteration of BaxMIVxCe2?2 x (PO4)2 [MIV=Zr, Hf] as hosts for minor actinides

Ba_xM^IV _xCe_2−2x (PO₄)₂ [M^IV=Zr, Hf] monazite-like compounds were succesfully synthesized by solid state reaction for x≤0.2 (M^IV=Zr) and x≤0.1 (M^IV=Hf). The low miscibility of BaM^IV(PO₄)₂ (M^IV=Zr, Hf) compounds in CePO₄ was explained on the basis of the monoclinic-to-trigonal phase transition that occurs at 733 K in BaZr(PO₄)₂ and at 798 K in BaHf(PO₄)₂. The hydrothermal alteration of these compounds was tested using a modified MCC-1 static leaching test in acid (1 mol·dm⁻³ HCl) and basic (1 mol dm⁻³ KOH) solutions at 373 K, 473 K and 573 K; both the experimental fluids and the reacted solid specimens were analyzed by different analytical techniques and the reaction mechanisms were elucidated. All the tested compounds are stable in 1 mol·dm⁻³ HCl until 573 K. The stability of the monazites in 1 mol·dm⁻³ KOH is a function of the temperature.     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Composition and formation kinetics of strontium polyvanadates in vanadium(IV,V) solutions

The phase and chemical compositions of the precipitates forming in the Sr(VO₃)₂-VOCl₂-H₂O system in the V⁴⁺/V⁵⁺ = 0.11–9 range at 80–90°C are reported. At pH 1–3 and V⁴⁺/V⁵⁺ = 0.25−9, the general formula of the precipitated compounds is Sr _x V _y ⁴⁺ V_12−y ⁵⁺O_31−δ·nH₂)(0.37 ≤ x ≤ 1.0, 1.7 ≤ y ≤ 3.0, 0.95 ≤ δ ≤ 2.1). Polyvanadates containing the largest amount of vanadium(IV) are obtained at an initial V⁴⁺/V⁵⁺ ratio of 9 and pH 1.9. Precipitation from solutions at pH 3 takes place only in the presence of the VO²⁺ ion, and the highest precipitation rate is observed at V⁴⁺/V⁵⁺ = 0.11. The process is controlled by a second-order reaction on the polyvanadate surface. Under hydrothermal conditions at 180°C, Sr_0.25V₂O₅·1.5H₂O nanorods are obtained from solutions with a V⁴⁺/V⁵⁺ molar ratio of 0.1 at pH 3. The nanorods, 30–100 nm in diameter and up to 2–3 μm in length, have a layered structure with an interlayer spacing of 10.53 ± 0.08 ?.     

Sintering, microstructures and dielectric properties of Ba1−xPbx(Ti1−xLix)O3−3xF3x ferroelectric ceramics

Various compositions (1−x)BaTiO₃ + xPbF2 + xLiF were prepared, shaped to pellets then sintered at 900°C for 2 h in gold sealed tubes. The purity and the symmetry of the obtained samples were checked by X-ray diffraction. A new solid solution with Ba_1−x Pb _x (Ti_1−x Li _x ) O_3−3x F_3x formula occurs in the composition range 0 ≤ x ≤ 0.20. SEM observations were performed on polished and fractured ceramics. The complex permittivity was measured as a function of temperature (−120°C ≤ T ≤ 250°C) and frequency (50 Hz ≤ f ≤ 4 × 10⁷ Hz). The dielectric performances are the best for ceramic Ba_0.97Pb_0.03(Ti_0.97Li_0.03)O_2.91F_0.09. The real component ε′, exhibits a maximum of approximately 7500 at the ferroelectric Curie temperature T _C ≈ -18°C, the dielectric losses tan δ value being 0.012. At room temperature, the relaxation frequency f _r is around 40 MHz for this ceramic. This novel ferroelectric oxifluoride is a promising material for applications, in particular in the field of Z5U multilayer capacitors.      

Phase equilibria of the Dy–Al–Si system at 500 °C

The isothermal section at 500 °C of the Dy–Al–Si system was studied in the whole concentration range. The alloys were characterized by X-ray powder diffraction, scanning electron microscopy and electron micro-probe analysis. A few samples were analysed by differential thermal analysis. The following intermetallic compounds, some of them showing variable composition, were found: DyAl₂Si₂ (τ₁), hP5-CaAl₂O₂ structure type, Dy₂Al₃Si₂ (τ₂) mS14-Y₂Al₃Si₂ structure type, Dy₂Al_1+x Si_2−x (τ₃), 0 ≤ x ≤ 0.25, oI10-W₂CoB₂ structure type and Dy₆Al₃Si (τ₄), tI80-Tb₆Al₃Si structure type. A number of binary phases dissolve the third element forming ternary solid solutions: Dy(Al_1−x Si _x )₃, 0 ≤ x ≤ 0.5, hP16-Ni₃Ti structure type, Dy(Al _x Si_1−x )₂, 0 ≤ x ≤ 0.1, oI12-GdSi₂ structure type, Dy(Al _x Si_1−x )_1.67, 0 ≤ x ≤ 0.2, oI12-GdSi₂ structure type, DyAl _x Si_1−x , 0 ≤ x ≤ 0.2, oC8-CrB, and Dy₅(Al _x Si_1−x )₃, 0 ≤ x ≤0.3, hP16-Mn₅Si₃ structure type. The melting point of Dy₆Al₃Si was determined.     

Untersuchung der thermischen Zersetzung von Aluminium-dodekawolframatosilicat

Investigation on the Thermal Degradation of Aluminium-12-Tungstosilicate The dehydration of aluminium-12-tungstosilicate AlH[SiO₄W₁₂O₃₆] · 29 H₂O gives the anhydrous salt at 440°C. By means of X-ray heating patterns, thermal analysis, and i.r. spectroscopy the formation of the new phase 1/2 Al₂O₃ · SiO₂ · 12 WO₃ (I) at 500°C is observed, stable at room temperature. Above 800°C from I tetragonal W₃, Al₂(WO₄)₃, and amorphous SiO₂ are formed. Amorphous SiO₂ crystallizes to high-temperature cristobalite at 1000°C. High-resolution ²⁷Al NMR (MAS-technique) is used to determine the coordination number of aluminium in the different phases.     

Sol-Gel Processing of Sm2+-Doped Glass and Its Spectral Hole Burning at Room Temperature

The sol-gel process was applied to the preparation of Sm²⁺ ion-doped silicate glasses, which show persistent spectral hole burning at room temperature. The gels synthesized by the hydrolysis of metal alkoxides and SmCl₃·6H₂O were heated in air at 500°C, were then reacted with H₂ gas to form the Sm²⁺ ion. The Al₂O₃−SiO₂2 glasses are appropriate to reduce the Sm³⁺ ion with H₂ gas and show intense photoluminescence of Sm²⁺ ion. Persistent spectra hole burning was observed in the excitation spectrum for the⁷F₀→⁵D₀ transition of the Sm²⁺ ion by the irradiation of DCM dye laser. The hole width and depth were ∼16 cm⁻¹ and ∼10% of the total intensity, respectively, at 20°C.     

Rubidium-cationic conductance in the systems Rb2 ? 2xAl2 ? xAxO4 (A = P, V)

RbAlO₂-based solid solutions are synthesized in the systems Rb_{2 − x} Al_{2 − x} A _x O₄ (A = P, V). Temperature and concentration dependences of their conductance are studied; the rubidium-cationic character of the conductance is corroborated. The high ionic conductivity of the synthesized phases (∼5 × 10⁻³ S cm⁻¹ at 300°C, ∼2 × 10⁻² S cm⁻¹ at 700°C) is caused by (a) formation of rubidium vacancies as a result of the replacing of Al³⁺ ions by fivefold-charged cations of phosphorus or vanadium and (b) disordered crystal structure of γ-KAlO₂ type. The obtained data are compared with the results of the studying of similar systems. Original Russian Text ? G.Sh. Shekhtman, E.I. Volegova, E.I. Burmakin, 2009, published in Elektrokhimiya, 2009, Vol. 45, No. 4, pp. 495–499.     

Kinetic studies of the crystallization process of glass-ceramics based on basalt

The crystallization kinetic of the basalt glass ceramic of the oxide composition, (%): SiO₂ − 50.82; Al₂O₃ − 12.05; Fe₂O₃ − 9.28; CaO − 15.48; MgO − 11.08; Na₂O+K₂O − 1.14; TiO₂ − 0.15, with addition of 10% TiO₂ as nucleating agent has been studied using thermal analysis under non-isothermal conditions. In this order, the non-isothermal DTA curves were obtained at different heating rates between 4 and 20°C min⁻¹ in the temperature range of 25–1000°C using a Derivatograph-C (MOM, Hungary). The kinetic parameters of the crystallization process were calculated on the basis of Ozawa-Flynn-Wall, Friedman, Budrugeac-Segal and non-parametric kinetic methods.     

Electrochemical synthesis of ammonia using a cell with a Nafion membrane and SmFe0.7Cu0.3?xNixO3 (x = 0?0.3) cathode at atmospheric pressure and lower temperature

Samaria-doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) and SmFe_0.7Cu_0.3−x Ni _x O₃ have been synthesized by the sol-gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The electrochemical synthesis of ammonia was investigated at atmospheric pressure and low temperature, using the SFCN materials as the cathode, a Nafion membrane as the electrolyte, nickel-doped SDC (Ni-SDC) as the anode and silver-platinum paste as the current collector. Ammonia was synthesized from 25 to 100°C when the SFCN materials were used as cathode, with SmFe_0.7Cu_0.1Ni_0.2O₃ giving the highest rates of ammonia formation. The maximum rate of evolution of ammonia was 1.13 × 10⁻⁸ mol·cm⁻²·s⁻¹ at 80°C, and the current efficiency reached as high as 90.4%. Supported by the National Natural Science Foundation of China (Grant No. 20863007)     

Experimental investigation of the Nd–Al–Si system

The isothermal section of the Nd–Al–Si ternary system at 500 °C has been investigated using differential thermal analysis, X-ray diffraction analysis, scanning electron microscopy and electron micro-probe analysis. Four ternary intermetallic compounds were confirmed: NdAl₂Si₂ (τ₁), hP5-CaLa₂O₂ structure type, Nd₂Al₃Si (τ₂), hP3-AlB₂ structure type, NdAl_1−x Si_1+x , 0.25 ≤ x ≤ 0.3 (τ₃), tI12-αThSi₂ structure type and Nd₂Al_1−x Si_1+x , 0 ≤ x ≤ 0.2, (τ₅), oS8-CrB structure type. A new ternary intermetallic phase (τ₄) was found: Nd₄Al₃Si₃, orthorhombic oS20, isotypic with Pr₄Al₃Ge₃.     

Structural features of the formation of La1?xCaxFeO3?δ (0 ≤x ≤ 0.7) hetero valent solid solutions

A synthesis method with the use of polymer-salt compositions (calcination temperature 800°C) provides the preparation of various solid solutions of a La_1−x Ca _x FeO_3−δ series in the 0≤ x≤ 0.7 range, which belong to the perovskite structure type. A morphotropic phase transition occurs from the orthorhombic perovskite modification (0≤ x ≤ 0.4) to the cubic one (0.5 ≤ x≤ 0.7). A growing number of microdistortions in the perovskite structure and the formation of a microblock structure in the morphotropic phase transition region are observed with increasing degree of calcium substitution for lanthanum. Calcination of solid solutions with x = 0.6 and 0.7 at temperatures above 1000°C in the air or under conditions of reduced oxygen partial pressure (laboratory vacuum of 10⁻³ Torr) results in the formation of a nanostructured state with coherently grown blocks of perovskite and Grenier phase, which is due to irreversible oxygen loss.     

Ionic conduction of lithium for Perovskite-type compounds, Li x La(1− x )/3NbO3 and (Li0.25La0.25)1− x Sr0.5 x NbO3

Perovskite-type compounds, Li _x La_{(1− x )/3}NbO₃ and (Li_0.25La_0.25)_{1− x} Sr_{0.5 x} NbO₃ as lithium ionic conductors, were synthesized by a solid-state reaction. From powder X-ray diffraction, the solid solution ranges of the two compounds were determined to be 0≤x≤0.25 and 0≤x≤0.125, respectively. In the Li _x La_{(1− x )/3}NbO₃ system, the ionic conductivity of lithium at room temperature, σ₂₅, exhibited a maximum value of 4.7 × 10⁻⁵ S · cm⁻¹ at x = 0.10. However, because of the decrease in the lattice parameters with increasing Li concentration x˙, σ₂₅ of the samples decreased with increasing x from 0.10 to 0.25. Also, in the (Li_0.25La_0.25)_{1− x} Sr_{0.5 x} NbO₃ system, the lattice parameter increased with the increase of Sr concentration and the σ₂₅ achieved a maximum (7.3 × 10⁻⁵ S · cm⁻¹ at 25 °C) at x = 0.125. Received: 12 September 1997 / Accepted: 15 November 1997