Origins of the Rio Capim kaolinites (northern Brazil) revealed by δ18O and δD analyses

Deuterium (δD) and oxygen (δ¹⁸O) isotope data from the Rio Capim kaolin, northern Brazil, were combined with optical studies in order to better understand the genesis and evolution of the kaolinites. The results show that δ¹⁸O values from a lower soft kaolin unit range from 6.0‰ to 19.2‰ for Ka (size ranging from 1 to 3 μm) and Kb (size ranging from 10 to 30 μm) kaolinites, and from 15.4‰ to 24.9‰ for Kc (size < 200 nm) kaolinites. The δD values range from − 63.1‰ to 79.5‰ for the Ka + Kb kaolinites, and from − 68.8‰ to − 244.35‰ for the Kc kaolinites. An upper semi flint kaolin unit, dominated by Kc kaolinites, displays δ¹⁸O and δD values ranging from 15.1‰ to 21.8‰, and − 71.3‰ to − 87.4‰, respectively. Based on these data, and on the δ¹⁸O and δD values obtained for the surface meteoric water and groundwater, it can be concluded that the kaolinites are not in equilibrium with the modern weathering environment, but they reflect isotopic compositions of the formation time, probably due to the interaction with fossil groundwater. However, mineralogical contaminations derived from replacements of framework grains also had great influence in the isotopic composition of these kaolinites. In addition, the isotope values of the Kc kaolinites from the semi-flint kaolin unit is variable, which is due to the presence of Kc kaolinites of different origins, including kaolinites derived from the underlying soft kaolin unit, kaolinites formed during different phases of paleoweathering, as well as later phases of coarse-grained kaolinites formed along fractures. Due to these complexities, binary diagrams contrasting δ¹⁸O and δD values, worldwide applied for distinguishing supergenic from hypogenic kaolinites, as well as those formed under weathering conditions, can not be applied to interpret the origin of the kaolinites in the Rio Capim Kaolin.     

Superconductivity in diamond

We survey methods of synthesis of boron doped diamond with high pressure-high temperature techniques. New route is proposed for synthesis of relatively large heavily boron doped diamond single crystals, which exhibit superconductivity. Superconducting boron-doped diamond samples were synthesized with isotopes of ¹⁰B, ¹¹B, ¹³C and ¹²C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the “diamond–carbon”-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm^? 1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron.     

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr_1−xCa_xFe_1−yAl_yO_3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO₂ gradient of 0.21/0.021 atm vary from 3.7×10⁻¹⁰ mol s⁻¹ cm⁻² to 1.5×10⁻⁷ mol s⁻¹ cm⁻² at 950 °C. The maximum solubility of Al³⁺ cations in the perovskite lattice of SrFe_1−yAl_yO_3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr_1−xCa_xFe_0.5Al_0.5O_3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO₃-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe_0.5Al_0.5O_2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe_0.7Al_0.3O_3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO₂ stability limits of Sr_1−xCa_xFe_1−yAl_yO_3−δ seem essentially independent of composition, varying between that of LaFeO_3−δ and the Fe/Fe_1−γO boundary. Thermal expansion coefficients of Sr_1−xCa_xFe_1−yAl_yO_3−δ ceramics in air are 9×10⁻⁶ K⁻¹ to 16×10⁻⁶ K⁻¹ at 100–650 °C and 12×10⁻⁶ K⁻¹ to 24×10⁻⁶ K⁻¹ at 650–950 °C. Doping of SrFe_1−yAl_yO_3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.     

Electronic conductivity,oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ is approximately 15%. The incorporation of Al³⁺ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr_0.7Ce_0.3Mn_0.9Al_0.1O_3−δ, measured in the oxygen partial pressure range from 10⁻¹⁸ to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O₂) below 10⁻⁴ to 10⁻² atm. The oxygen permeability of dense Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO₃-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ ceramics in air are (10.8–11.8) × 10⁻⁶ K⁻¹.     

Synthesis,structure and oxygen thermally programmed desorption spectra of La1−xCaxAlyFe1−yO3−δ perovskites

Perovskites La_1−xCa_xAl_yFe_1−yO_3−δ (x, y = 0 to 1) were prepared by high-temperature solid-state synthesis based on mixtures of oxides produced by colloidal milling. The XRD analysis showed that perovskites La_0.5Ca_0.5Al_yFe_1−yO_3−δ with a high Fe content (1 − y = 0.8–1.0) were of orthorhombic structure, perovskites with a medium Fe content (1 − y = 0.8–0.5) were of rhombohedral structure, and perovskite with the lowest Fe content (1 − y = 0.2) were of cubic structure. Thermally programmed desorption (TPD) of oxygen revealed that chemical desorption of oxygen in the temperature range from 200 to 1000 °C had proceeded in the two desorption peaks. The low-temperature α-peak (in the 200–550 °C temperature range) was brought about by oxygen liberated from oxygen vacancies; the high-temperature β-peak (in the 550–1000 °C temperature range) corresponded to the reduction of Fe⁴⁺ to Fe³⁺. The chemidesorption oxygen capacity increased with increasing Ca content and decreased with increasing Al content in the perovskites. The Al³⁺ ions restricted, probably for kinetic reasons, the reduction of Fe⁴⁺ and the high-temperature oxygen desorption associated with it.     

Crystal structure,thermal expansion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)

Ba_xSr_1−xCo_0.8Fe_0.2O_3−δ (0.3 ≤ x ≤ 0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x ≤ 0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10⁻⁶ °C⁻¹. The conductivity were larger than 30 S cm⁻¹ above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba_0.4Sr_0.6Co_0.8Fe_0‘2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ are potential cathode materials.     

Diffusion coefficient of carbon in Fe–Ni alloy during synthesis of diamond under high temperature and high pressure

Synthetic diamond particles were prepared under high temperature and high pressure using arrayed seeds. A dense Fe–Ni alloy shell covered each diamond seed during synthesis; the growth of diamond particles was controlled by the diffusion of carbon through the metallic shell. The diffusion coefficient of carbon through Fe–Ni melt at 1600 K and 5.5 GPa is about 5×10^?6 cm²/s, with an activation energy for diffusion of 336 kJ/mol.     

High resolution electron energy loss spectroscopy of hydrogenated polycrystalline diamond: Assignment of peaks through modifications induced by isotopic exchange

In this work we unambiguously determine the origin of the different peaks which appear in the High Resolution Electron Energy Loss Spectrum (HREELS) of hydrogenated polycrystalline diamond films for an incident electron energy of 5 eV and loss energies extending to 700 meV. High quality diamond films deposited by hot filament chemical vapor deposition from various isotopic gas mixtures: ¹²CH₄ + H₂, ¹²CD₄ + D₂, ¹²CH₄ + D₂, ¹²CD₄ + D₂, ¹³CH₄ + H₂ were characterized. The different vibrational modes, fundamentals and overtones, were directly identified through the modifications of the HREEL spectra induced by the isotopic exchange of H by D and ¹²C by ¹³C Three types of peaks were identified: (1) pure C–C related peaks (a diamond optical phonon at ∼ 155 meV and its overtones at 300, 450 and 600 meV), (2) pure C–H related peaks (C–H bend at ∼ 150 meV and C–H stretch of sp³ carbon at 360 meV), (3) coupling of C–H and C–C peaks (510 meV peak due to coupling of the C–H stretch at 360 meV with either the C–C stretch or the C–H bend at ∼ 155 meV). The overtones at 300, 450 and 600 meV (associated with electron scattering at diamond optical phonons) indicate a well defined hydrogenated diamond surface since they are absent in the HREEL spectrum of low energy ion beam damaged diamond surface.     

Internal friction investigation of phase transformation in nearly stoichiometric LaMnO3+δ

Rhombohedral LaMnO_3+δ powders, prepared by two different soft chemistry routes (co-precipitation and hydrothermal synthesis), are sintered at 1400 °C for 2 h in air. Measurements of internal friction Q⁻¹(T) and shear modulus G(T), at low frequencies from −180 to 700 °C under vacuum, evidence three structural transitions of nearly stoichiometric orthorhombic LaMnO_3+δ. The first one, at 250 or 290 °C, depending on the processing followed, is associated to either a Jahn–Teller structural transition or a phase transformation from orthorhombic to pseudo-cubic. The second one at 610 or 630 °C is related to a phase transformation from pseudo-cubic or orthorhombic to rhombohedral. Below the Neel temperature, around −170 °C, a relaxation peak could be associated, for samples prepared according to both processing routes, to the motion of Weiss domains.     

X-ray single phase LSGM at 1350 °C

Synthesis of X-ray-phase-pure (La_1−xSr_xGa_1−yMg_yO_3−δ, LSGM, where x = 0.1, y = 0.15 and 0.17) powders were achieved at temperatures as low as 1350 °C via organic precursor method using tartaric acid as the carrier material. LSGM materials were characterized for their phase purity, crystallization and electrical properties. Pellets sintered at 1350 °C for 6 h were single phase and dense (>99%). Electron microscopy analysis of X-ray single-phase pellets revealed MgO precipitates with sizes ranging from 50–300 nm. Phase formation and distribution in this complicated multi-cation-oxide system as a function of temperature were reported and discussed. Amorphous LSGM first crystallizes at 625 °C. However, elimination of undesired phases require higher temperatures. Impedance measurements as a function of temperature up to 545 °C revealed that the X-ray phase pure pellets may have extrapolated ionic conductivity values as high as 0.14–0.16 S/cm at 800 °C. Possible implications of limited MgO solubility on the ionic conductivity are presented.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

Characteristics of Cu and Mo-doped Ca3Co4O9−δ cathode materials for use in solid oxide fuel cells

In this study, Cu and Mo ions were doped in Ca₃Co₄O_9−δ to improve the electrical conductivity and electrochemical behavior of Ca₃Co₄O_9−δ ceramic and the performance of a solid oxide fuel cell (SOFC) single cell based on NiO-SDC/SDC/doped Ca₃Co₄O_9−δ-SDC were examined. Cu substitution in the monoclinic Ca₃Co₄O_9−δ ceramic effectively enhanced the densification, slightly increased the grain size, and triggered the formation of some Ca₃Co₂O₆; however, no second phase was found in porous Mo-doped Ca₃Co₄O_9−δ ceramics even when the sintering temperature reached 1050 °C. Substitution of Cu ions caused slight increase in the Co³⁺ and Co⁴⁺ contents and decrease in the Co²⁺ content; however, doping with Mo ions showed the opposite trend. Doping the Ca₃Co₄O_9−δ ceramic with a small amount of Cu or Mo increased its electrical conductivity. The maximum electrical conductivity measured was 218.8 S cm⁻¹ for the Ca₃Co_3.9Cu_0.1O_9−δ ceramic at 800 °C. The Ca₃Co_3.9Cu_0.1O_9−δ ceramic with a coefficient of thermal expansion coefficient of 12.1×10⁻⁶ K⁻¹ was chosen as the cathode to build SOFC single cells consisting of a 20 μm SDC electrolyte layer. Without optimizing the microstructure of the cathode or hermetically sealing the cell against the gas, a power density of 0.367 Wcm⁻² at 750 °C was achieved, demonstrating that Cu-doped Ca₃Co₄O_9−δ can be used as a potential cathode material for IT-SOFCs.     

Production and properties of substituted LaFeO3-perovskite tubular membranes for partial oxidation of methane to syngas

Tubular membranes of La_0.6Ca_0.4Fe_0.75Co_0.25O_3−δ and La_0.5Sr_0.5Fe_1−yTi_yO_3−δ (y = 0, 0.2) for the application of partial oxidation of methane to syngas were produced by thermoplastic extrusion and investigated by oxygen permeation measurements. The optimum ceramic content in the feedstock for extrusion was found to be 51 vol% as a result of rheology measurements. Tubes with an outer diameter of 4.8–5.5 mm and thickness of 0.25–0.47 mm were produced with densities higher than 95% of the theoretical density. The oxygen permeation flux of the tubular membranes was measured with air on one side and Ar or Ar + CH₄ mixture on the other side. The oxygen permeation rate decreased with Ti-substitution while it was considerably increased by introduction of 5% methane into the system. The normalized oxygen fluxes in air/Ar gradient at 900 °C were measured to be 0.06, 0.051, and 0.012 μmol cm⁻² s⁻¹ for LCFC, LSF, and LSFT2, respectively, and 0.18 μmol cm⁻² s⁻¹ for LSFT2 with 5% methane.     

Electrochemical performance of intermediate temperature micro-tubular solid oxide fuel cells using porous ceria barrier layers

We describe the manufacture and electrochemical characterization of micro-tubular anode supported solid oxide fuel cells (mT-SOFC) operating at intermediate temperatures (IT) using porous gadolinium-doped ceria (GDC: Ce_0.9Gd_0.1O_2−δ) barrier layers. Rheological studies were performed to determine the deposition conditions by dip coating of the GDC and cathode layers. Two cell configurations (anode/electrolyte/barrier layer/cathode): single-layer cathode (Ni–YSZ/YSZ/GDC/LSCF) and double-layer cathode (Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF) were fabricated (YSZ: Zr_0.92Y_0.16O_2.08; LSCF: La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ). Effect of sintering conditions and microstructure features for the GDC layer and cathode layer in cell performance was studied. Current density–voltage (j–V) curves and impedance spectroscopy measurements were performed between 650–800 °C, using wet H₂ as fuel and air as oxidant. The double-cathode cells using a GDC layer sintered at 1400 °C with porosity about 50% and pores and grain sizes about 1 μm, showed the best electrochemical response, achieving maximum power densities of up to 160 mW cm⁻² at 650 °C and about 700 mW cm⁻² at 800 °C. In this case GDC electrical bridges between cathode and electrolyte are preserved free of insulating phases. A preliminary test under operation at 800 °C shows no degradation at least during the first 100 h. These results demonstrated that these cells could compete with standard IT-SOFC, and the presented fabrication method is applicable for industrial-scale.     

Subsolidus solution and electrical properties of Sr-substituted bismuth magnesium niobate pyrochlores

Here we investigated the structural and dielectric properties of (Bi_3.36Mg_0.64-xSr_x)(Mg_1.28Nb_2.72)O_13.76 (0≤ x≤0.5) subsolidus solution. Sr-substituted bismuth magnesium niobate (BMSN) pyrochlores were prepared by solid-state reaction at 1025 °C over 1–2 days. X-ray diffraction (XRD) analysis confirmed that the BMSN pyrochlores crystallise in cubic symmetry, space group Fd3m with lattice parameters in the range 10.5968 (4)-10.5671 (17) Å. The surface morphologies of these samples, as confirmed by Scanning Electron Microscopy (SEM), were composed of irregular shaped grains. Both Scherrer and Williamson-Hall methods revealed that the crystallite sizes were in the range 46–75 nm. No thermal event was discernible over the temperature range 30–1000 °C, thus confirming the thermal stability of these materials. On the other hand, Arrhenius conductivity plots showed the BMSN pyrochlores to be highly insulating with activation energies of ~1.20–1.49 eV. At ~30 °C and 1 MHz, BMSN pyrochlores exhibited moderate high bulk dielectric constants, ɛ’, 90–186 and low dielectric losses, tan δ in the order of 10⁻²–10⁻¹, respectively. Both the ɛ’ and tan δ values of the BMSN pyrochlores showed a nearly two-fold decrease with increasing Sr concentration. Negative temperature coefficient of capacitances, TCC, −408 to −713 ppm/°C were recorded over ~30–300 °C at 1 MHz.     

Electro-spinning Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ nanofibers infiltrated with Gd0.2Ce0.8O1.9 nanoparticles as cathode for intermediate temperature solid oxide fuel cell

Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (PSCFN) nanofibers and their corresponding Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ–Gd_0.2Ce_0.8O_1.9 (PSCFN–GDC) composites have been synthesized and applied as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this paper, PSCFN nanofibers were obtained through electro-spinning and the following pyrolysis process. The resultant PSCFN nanofibers were infiltrated with GDC precursor to prepare nanofiber-structured PSCFN–GDC composite cathodes. The optimal PSCFN: GDC mass ratio of 1: 0.10 was identified to possess the lowest interfacial polarization resistances of 0.264, 0.155, 0.039 and 0.018 Ω cm² at 650, 700, 750 and 800 °C, respectively, lower than those of the PSCFN–GDC nanoparticle-structured composite cathode. The PSCFN–GDC (1: 0.10) shows an excellent stability of electrochemical activity under a current density of 200 mA cm⁻² for 100 h at 800 °C. All results proved that the nanofiber-structured PSCFN–GDC composite could act as a highly efficient cathode candidate for the IT-SOFCs.     

Ceramic materials for H2 transport membranes applicable for gas separation under coal-gasification-related conditions

This work focuses on the synthesis, characterization and testing of mixed protonic–electronic conducting membrane materials for H₂ separation from gas mixtures capable of operating in a membrane reactor at temperatures higher than 600 °C. La_5.5WO_12−δ and selected substituted barium zirconates with stoichiometries BaCe_0.5Zr_0.4Y_0.1O_3−δ and BaCe_0.2Zr_0.7Yb_0.08Ni_0.02O_3−δ were therefore characterized and tested under coal-gasification-related conditions at 600–900 °C. Sintered samples of the synthesized substituted barium zirconates were characterized by measuring the total conductivity and the thermal expansion coefficients. Also particle size distributions, BET surface-areas and elemental analysis of the starting powders, including commercial La_5.5WO_12−δ were specified. The compounds were exposed to syngas with steam, as well as to an atmosphere mainly consisting of CO₂. The microstructure and phase composition of the membrane materials were studied by SEM, EDX and XRD before and after exposure. BaCe_0.2Zr_0.7Yb_0.08Ni_0.02O_3−δ shows a very promising chemical stability from 600 °C to 900 °C and La_5.5WO_12−δ at 900 °C.     

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti⁴⁺ in the lattice of apatite-type La_9.83Si_6−xTi_xO_26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La_9.83Si_6−xTi_xO_26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O₂) range from 10⁻²⁰ to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H₂–90%N₂ gradients at 700–950 °C, suggesting the stabilization of Ti⁴⁺ in the apatite structure. Titanium addition in La_9.83Si_6−xTi_xO_26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O₂) = 10⁻²⁰ atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr⁴⁺ in the La_9.83Si_6−xZr_xO_26.75 system was found to be negligible, while the maximum concentration of Ce⁴⁺ in La_9.4−xCe_xSi₆O_27−δ is approximately 5% with respect to the number of lanthanum sites.     

The effect of Zn addition on the structure and transport properties of BaCe0.9−xZrxY0.1O3−δ

BaCe_0.9−xZr_xY_0.1O_3−δ (0.1 ≤ x ≤ 0.9) are ceramic proton conductors widely investigated for different electrochemical devices, such as Solid Oxide Fuel Cell (SOFC) electrolytes, however, their applications are limited by the high sintering temperatures necessary to achieve densification. Polycrystalline powders of BaCe_0.9−xZr_xY_0.1O_3−δ (BCZ) have been prepared by freeze-drying precursor method at 1000 °C. These powders were mixed with a zinc nitrate solution to decrease the sintering temperature to 1200–1300 °C. The addition of Zn has several effects on the structure and microstructure of BCZ: the densification and grain growth are enhanced at lower temperature and a reduction of the crystallographic symmetry is observed for samples with low Zr-content (x ≤ 0.3). Furthermore, the bulk and specific grain boundary conductivities are only slightly affected by the addition of Zn.     

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH₄-H₂ gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H₂ flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH₄. The highest growth rate, 5 μm h⁻¹, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm⁻¹ K⁻¹. Diamond disks with very low optical absorption (loss tangent tgδ<10⁻⁵) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.