Redox behaviour of cerium oxychloride in molten MgCl2-NaCl-KCl eutectic

The electrochemical behaviour of cerium oxychloride in MgCl₂-NaCl-KCl ternary eutectic was investigated by cyclic voltammetry at 823 K. The cyclic voltammogram of UO₂Cl₂-CeOCl in MgCl₂-NaCl-KCl eutectic shows two peaks during the cathodic sweep as well as anodic sweep. The reduction of UO₂²⁺ is by a single step two-electron transfer and that of CeO⁺ is by a single step one-electron transfer. The reduction of CeO⁺ was found to be quasi-reversible.The reduction potentials of UO₂²⁺/UO₂ and CeO⁺/CeO versus Ag(I)/Ag reference electrode at 823 K are 0.103 and −0.299 V, respectively. The diffusion coefficient of CeO⁺ at 823 K is in the range of (1.7-1.9) × 10⁻⁵ cm² s⁻¹. The cyclic voltammogram for 0.015 mol% CeOCl shows an additional peak during the anodic sweep at −0.056 V, which is being attributed to monolayer dissolution of CeO at the glassy carbon working electrode. Electrochemical impedance data of 0.015 mol% CeOCl in MgCl₂-NaCl-KCl eutectic at the open circuit potential was fitted to a Randles cell from which the heterogeneous rate constant was estimated. X-ray photoelectron spectroscopy was used to confirm that the oxidation state of cerium in the eutectic is +3.     

Influences of carbon structure on the reactivities of lignite char reacting with CO2 and NO

Raw and demineralized lignite samples were pyrolyzed from 773 to 1673 K to generate chars. The chars were characterized with Raman spectroscopy for the structure evolution. The reactivities of the chars reacting with CO₂ and NO were measured with thermogravimetric analysis. The derived reactivity indexes were correlated with the treatment temperature and the Raman structural parameters to demonstrate the applicability of Raman spectroscopy for evaluation of the reactivities of char CO₂ gasification and char-NO reaction. It was found that char microstructure evolution with the treatment temperature could be represented by Raman band area ratios. I_D1/I_G and I_G/I_ALL represented the evolution of the ordered carbon structure while the combination of I_D3/(I_G + I_D2 + I_D3) reflected the evolution of the amorphous carbon structure of the lignite chars with increasing the treatment temperature from 773 to 1673 K. Reactivity indexes of the demineralized chars reacting with both CO₂ and NO were found to increase with increasing the treatment temperature, implying that the structure ordering did result in the losses of the reactivities. Higher reactivities of the non-demineralized chars indicated the catalytic role of inorganic matter in the reactions with both gases. I_D1/I_G and I_G/I_ALL had good linear correlations with the reactivities particularly of the demineralized chars if considering the structure evolution behaviors at lower and higher temperatures, respectively. I_D3/(I_G + I_D2 + I_D3) was found to have fairly good linear correlations with the reactivity indexes of the lignite chars generated over the whole temperature range.     

Phase transformation in tungsten carbide–cobalt composite during high temperature treatment in microwave hydrogen plasma

To study phase formation up to 2300 K, tungsten carbide–cobalt (WC–Co) samples were exposed to concentrated solar radiation in hydrogen plasma. The reducing atmosphere was a non-equilibrium hydrogen plasma created in a microwave cavity at the nominal power of 1000 W. The dissociation fraction of hydrogen molecules in plasma was of the order of 10% assuring for almost optimal reduction of any oxide that could be formed on the surface of samples due to the residual atmosphere. Micro-structural characterization was performed by XRD and AES depth profiling. The results showed the appearance of Co₆W₆C phase at the temperature of 1050±50 K. This phase almost vanished at the temperature of 1300±90 K and was replaced by the Co₃W₃C phase. This phase vanished at 1690±150 K where only WC and Co peaks were detected by XRD. The AES depth profiles showed enrichment of the surface film with Co at elevated temperatures. At extremely high temperature of 2300 K, the Co vanished from the surface layer but remained in crystalline form in the bulk material. SEM imaging showed an evolution of the material crystallinity up to perfectly recognizable crystals of the dimension of approx. 1 μm at the maximum temperature. The results are explained by mobility of Co atoms, surface segregation and sublimation at very high temperature.     

Densification, microstructure and mechanical properties of SiO2-cBN composites by spark plasma sintering

SiO₂-cBN composites were consolidated by spark plasma sintering at 1473-1973 K. The effects of cBN content and sintering temperature on the relative density, phase transformation, microstructure and mechanical properties of the SiO₂-cBN composites were investigated. The relative density of the SiO₂-cBN composites increased with increasing SiO₂ content. The phase transformation of cBN to hBN in SiO₂-cBN composites was identified at 1973 K, showing the highest transformation temperature in cBN-containing composites. The SiO₂-20 vol% cBN composites sintered at 1673 K showed the highest hardness and fracture toughness of 12.5 GPa and 1.5 MPa m^1/2, respectively.     

The influence of pentavalent Nb substitution for Zr on electrical property of oxide-ion conductor Gd2Zr2O7

Gd₂(Zr_1−xNb_x)₂O_7+x (0 ≤ x ≤ 0.2) ceramics are prepared via the solid state reaction process at 1973 K for 10 h in air. Gd₂(Zr_1−xNb_x)₂O_7+x (x = 0.1, 0.2) ceramics exhibit an ordered pyrochlore-type structure, whereas Gd₂Zr₂O₇ has a defective fluorite-type structure. The electrical property of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics is investigated by electrochemical impedance spectroscopy over a frequency range of 10 Hz to 8 MHz from 623 to 923 K. The electrical conductivity obeys the Arrhenius equation. The grain conductivity of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics varies with doping different Nb contents, and exhibits a maximum at the Nb content of x = 0.1 in the temperature range of 623-923 K. The conductivity in hydrogen atmosphere is a little bit higher than in air in the temperature range of 723-923 K, which indicates that the doping of Zr⁴⁺ by Nb⁵⁺ can increase the proton-type conduction and reduce the oxide-ionic conduction. The conduction of Gd₂(Zr_1−xNb_x)₂O_7+x is not a pure oxide-ionic conductor.     

Effects of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels in acidic chloride solutions

The role of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels was investigated in 0.5 M H₂SO₄, 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques. The passive film stability was enhanced in 0.5 M H₂SO₄ and the pitting resistance was improved in 3.5% NaCl solution by more nitrogen addition. The influence of nitrogen extended the whole anodic polarization region in 0.5 M H₂SO₄ + 0.5 M NaCl solution, as demonstrated by the enhanced dissolution resistance, promoted adsorption and passivation process, improved film protection and pitting resistance with increasing nitrogen content. Possible mechanisms relating to the role of nitrogen in different potential regions were discussed.     

A TDS study of D adsorption on terraces and terrace edges of graphite (0 0 0 1) surfaces

Adsorption of thermal (2000 K) D atoms on (0 0 0 1) surfaces of various highly oriented pyrolytic graphite (HOPG) and natural graphite substrates was studied under ultra high vacuum (UHV) conditions with thermal desorption spectroscopy (TDS). D chemisorption on terrace and terrace edge sites of graphite (0 0 0 1) surfaces was identified. Recombinative desorption of D adsorbed on terraces was observed between 400 and 600 K. The analysis of TD spectra from various graphite surfaces reveals the existence of three desorption states intrinsic to graphite (0 0 0 1), and proposed as being due to adsorbate structures composed of one (monomer) and two neighbouring (dimer) chemisorbed D atoms, and aggregates thereof (mixed). The dimer structure is supposed to exhibit higher stability than the monomer. Reaction of D with terrace edges leads to the formation of CD, CD₂ and CD₃-groups at edge C atoms. These groups decompose during heating between 790 and 1300 K via release of gaseous D₂ and CD_x, C₂D_x and C₃D_x-hydrocarbons.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Synthesis, characterization and electrochemical properties of a novel triphosphate LiFe2P3O10

A novel lithium iron phosphate has been synthesized by a solution route at moderate temperature. The structure was determined from powder by XRD, HRTEM and SAED experiments. LiFe₂P₃O₁₀ tripolyphosphate crystallizes in the monoclinic system, space group P2₁/m, with lattice constants a = 4.597(7) Å, b = 8.566(4) Å, c = 9.051(4) Å, β = 97.47° and Z = 2. Internal and external vibrational modes (Raman and FTIR) show that the dominant spectral features come from the (P₃O₁₀)⁵⁻ oxo-anions displaying internal and external modes along with the P-O-P bridging modes. Magnetic measurements are consistent with the high-spin configuration of Fe²⁺ cation with an effective magnetic moment 5.51 μ_B. A weak antiferromagnetic ordering is observed below the Néel temperature at T_N = 19 K. Electron paramagnetic resonance spectroscopy confirms this electronic configuration and provides evidence of the presence of a carbonaceous layer onto the particle surface. Electrochemical measurements were carried out in lithium cells with LiPF₆-EC-DEC electrolyte at 25 °C. The material delivered a capacity 70 mAh/g in the voltage range 2.7-3.9 V, close to the theoretical value (72 mAh/g). The resulting cyclic voltammogram indicates a stable structure with a good reversibility with the redox peaks at 3.26 and 3.13 V vs. Li⁰/Li⁺.     

Modification of titanium oxide layer by calcium and phosphorus

The aim of the study was to investigate the possibility of calcium and phosphorus ion implantation into an oxide film applied onto titanium during anodic passivation. The corrosion resistance of modified titanium in Tyrode's physiological solution has been identified. Anodic oxidation was carried out in two solutions. The first contained 20 g dm⁻³ NaH₂PO₂ in 4.3 M H₃PO₄ (K1), whereas the other, 20 g dm⁻³ Ca(H₂PO₂)₂ in 4.3 M H₃PO₄ (K2). Voltage of 100 and 150 V was applied. It has been found out that it is possible to incorporate Ca and P into the emerging passive layer. The application of the voltage of 150 V makes it very porous. It has been also demonstrated that titanium so modified presents higher resistance to corrosion in the investigated environment than titanium not modified in Tyrode's solution.     

Transparent yttria produced by spark plasma sintering at moderate temperature and pressure profiles

Transparent yttria (Y₂O₃) bodies were fabricated by spark plasma sintering, and the effects of the sintering temperature on relative density, microstructure, and the optical and mechanical properties of Y₂O₃ bodies were investigated. Fully dense Y₂O₃ bodies were obtained at sintering temperatures 1473-1873 K. The average grain size was 0.24-0.32 μm at 1473-1573 K, and steadily increased to 1.97 μm with an increase in temperature to 1823 K. The highest transmittance was obtained in the Y₂O₃ body sintered at 1573 K and annealed at 1323 K, showing 81.7% (99% of the theoretical value) at a wavelength of 2000 nm.     

Synthesis, characterization, and hydrotreating activity of carbon-supported transition metal phosphides

A series of nickel, molybdenum, and tungsten metal phosphides deposited on a carbon black support (Ni₂P/C, MoP/C, and WP/C) were synthesized by means of temperature-programmed reduction. The samples were characterized by BET surface area, CO uptake, X-ray diffraction (XRD), elemental analysis, and extended X-ray absorption fine structure (EXAFS) measurements. The activity of these catalysts was measured at 613 K and 3.1 MPa in a three-phase, packed-bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) with a model liquid feed containing 500 ppm sulfur as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 3000 ppm sulfur as dimethyl disulfide, and 200 ppm nitrogen as quinoline. The Ni₂P/C catalyst was found to exhibit the best hydroprocessing performance based on equal CO chemisorption sites (70 μmol) loaded in the reactor. An optimum Ni loading for HDS and HDN activity was found as 1.656 mmol g⁻¹ (11.0 wt.% Ni₂P) which gave an HDS conversion of 99% and an HDN conversion of 100% at a molar space velocity of 0.88 h⁻¹. These were much higher than those of a commercial Ni-Mo-S/γ-Al₂O₃ catalyst which gave an HDS conversion of 68% and an HDN conversion of 94%, and a previously reported best Ni₂P/SiO₂ catalyst which gave an HDS conversion of 76% and an HDN conversion of 92%. The use of carbon instead of silica as a support gave rise to other differences, which included smaller particle size, higher CO uptake, lessened retention of P on the support, and reduced sulfur deposition. The stability of the 11.0 wt.% Ni₂P/C catalyst was also excellent with no deactivation observed over 110 h of time on stream. The activity and stability of the Ni₂P/C catalyst were affected by the phosphorous content, both reaching a maximum with an initial Ni/P ratio of 1/2. EXAFS and elemental analysis of the spent samples indicated the formation of a surface phosphosulfide phase on the Ni₂P, which was beneficial for hydrotreating activity, while the bulk structure of the phosphides was maintained during the course of reaction as revealed from the XRD patterns.     

New hybrid inorganic-organic complexes based on poly(3-butylthiophene) and titanium tetrachloride: Synthesis, structure and conductivity

This report describes the preparation of two new hybrid inorganic-organic polymer materials to use as electro-active layers in micro-electronic devices. By reacting poly(3-butylthiophene) (PBT) with titanium tetrachloride in a nitrogen atmosphere the hybrid inorganic-organic [(Bu-Th)₂TiCl₂]_n (A) complex (Bu-Th = 3-butylthiophene) was prepared. {[(Bu-Th)₂Ti(OH)₂(H₂O)₂]·1.5(H₂O)}_n (B) was obtained by exposing A to the air. The materials’ morphology was examined by scanning electron microscopy (SEM). FT-IR studies provided information about: (a) the structural characteristics of the products; (b) the degree of polymerization; (c) the interactions occurring between the aromatic rings of PBT macromolecular ligand and the titanium metal centers; (d) the effects of the hydration process on coordination geometry of Ti complexes and on the three-dimensional structure of the resulting hybrid inorganic-organic polymer. The thermal behaviour of materials was investigated by thermogravimetry (TGA) and differential scanning calorimetry (DSC). The electrical properties of pristine PBT, A and B materials were studied by broadband dielectric spectroscopy (BDS) in the 40 Hz to 10 MHz range, and in the range of temperatures from 173 K to about 423 K. The conductivity mechanism was interpreted in terms of charge carrier hoppings between different sites followed by polymer network reorganizations both in bulk grains, and inter-grain interfaces.     

Effect of synthesis temperature on hardness of carbon phases prepared from C60 and nanosized diamonds under pressure

Vicker’s hardness and Raman scattering spectra have been studied for carbon phases prepared from C₆₀ fullerene and nanosized diamonds at high temperatures and a pressure of 6 GPa. It was found that the hardness dependence on annealing temperature has a maximum near ∼1100 K for both fullerene and nanosized diamonds as initial materials. This temperature is only slightly higher than the temperature at which the C₆₀ cage collapses, and appears to correspond to the termination of intercluster bonding in the case of nanosized diamonds. The hardness maximum is interpreted as a result of competition between an increase in intercluster/intercage bonding and local instability for graphitic-like ordering.     

Dielectric spectroscopy of poly(butylene succinate) films

Dielectric properties of poly(butylene succinate) crystallized under different conditions have been reported in the temperature range of 163-383 K and in the frequency range of 0.01-10⁵ Hz. Both the dipolar α and β processes have been identified at low temperatures: the α process is associated with the amorphous fraction while the β with the relaxations in both the amorphous and crystalline fractions. The space charge effect dominates the high temperature dielectric spectra. These spectra have been analyzed in the light of an equivalent circuit model. The Maxwell-Wagner-Sillars polarization, electrode polarization and free charge motion are well resolved. At 383 K, near the melting temperature (387 K), massive melting and subsequent recrystallization have been observed. The peculiar evolution of the spectra is also analyzed using the same equivalent circuit model. The relationship between the fitting parameters and the evolved microstructures is discussed.     

Electronic structure modifications and HER of annealed electrodeposited Ni overlayers on Mo polycrystalline surface

The nature, the chemical state and electrocatalytic activity of Ni overlayers electrochemically prepared on a polycrystalline Mo surface were studied by means of X-ray and UV photoelectron spectroscopies (XPS, UPS) and steady state potentiostatic polarization measurements for the H₂ evolution reaction in 1 M NaOH solution. Upon cathodic polarization, a Ni(OH)₂-α phase adlayer is formed by precipitation which is gradually reduced by annealing under UHV (ultra-high vacuum) conditions forming a new Ni-Mo intermetallic phase at temperatures as low as 670 K. This was confirmed both by following the Ni 2p core-level shift and the appearance of new features in the UV spectrum thus denoting the modification of the electronic structure of Ni due to the formation of a common strongly hybridised Ni-Mo electronic state. The steady state polarization measurements revealed that sub-monolayer deposition of Ni on the Mo surface (0.8 ML) with subsequent heating at 870 K results in the threefold enhancement of its electrocatalytic activity for the hydrogen evolution reaction (HER) with respect to the Mo polycrystalline foil and the corresponding increase in its desorption kinetic constants in comparison to both Ni and Mo polycrystalline surfaces. As has been recorded for the first time by UPS, in the case of Ni-Mo, this can be attributed to the downshift of the d-band center of the Ni-Mo intermetallic phase, thus resulting in a weaker bonding strength of H_ad with respect to its adsorption on the Ni foil.     

Characterization of active sites for methane decomposition on carbon black through acetylene chemisorption

The use of carbon black as a stable catalyst for methane decomposition for the production of hydrogen without CO_x emissions has recently been reported. In order to characterize the catalytically active sites on carbon black, acetylene adsorption was examined at 773 and 873 K by using a pulse technique. As the injection was repeated at 773 K, the adsorbed amount gradually decreased and eventually the adsorption ceased. At 873 K a constant amount of C₂H₂ was consumed repeatedly after several injections. Good relationships were obtained between the methane decomposition rate at 1123 or 1173 K and the cumulative acetylene adsorption at 773 K or the constant acetylene consumption at 873 K. Acetylene adsorption at 773 K can be used for quantitative determination of the number of active sites on the carbon black and the constant consumption at 873 K can be utilized as a probe reaction for comparison of activities of different carbon catalysts. Different physicochemical processes taking place at 773 and 873 K were discussed and a model for these adsorption processes was proposed.     

Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures

Hydrogen adsorption measurements have been carried out at different temperatures (298 K and 77 K) and high pressure on a series of chemically activated carbons with a wide range of porosities and also on other types of carbon materials, such as activated carbon fibers, carbon nanotubes and carbon nanofibers. This paper provides a useful interpretation of hydrogen adsorption data according to the porosity of the materials and to the adsorption conditions, using the fundamentals of adsorption. At 298 K, the hydrogen adsorption capacity depends on both the micropore volume and the micropore size distribution. Values of hydrogen adsorption capacities at 298 K of 1.2 wt.% and 2.7 wt.% have been obtained at 20 MPa and 50 MPa, respectively, for a chemically activated carbon. At 77 K, hydrogen adsorption depends on the surface area and the total micropore volume of the activated carbon. Hydrogen adsorption capacity of 5.6 wt.% at 4 MPa and 77 K have been reached by a chemically activated carbon. The total hydrogen storage on the best activated carbon at 298 K is 16.7 g H₂/l and 37.2 g H₂/l at 20 MPa and 50 MPa, respectively (which correspond to 3.2 wt.% and 6.8 wt.%, excluding the tank weight) and 38.8 g H₂/l at 77 K and 4 MPa (8 wt.% excluding the tank weight).     

An experimental study of sulphate transformation during pyrolysis of an Australian lignite

The transformation of sulphate minerals during pyrolysis of an Australian lignite has been studied using pure sulphates (CaSO₄, FeSO₄ and Fe₂(SO₄)₃), a high mineral (HM) lignite sample and a low mineral (LM) lignite sample collected from different locations of the same deposit, and samples of acid-washed LM doped with sulphates (CaSO₄+ LM and FeSO₄+ LM), respectively. Thermogravimetric analysis and fixed-bed reactor techniques were used for the pyrolysis experimentation and the lignite samples and their chars were analysed using FTIR and XRD. The TGA experiments showed that CaSO₄ decomposes between 1400 and 1700 K in nitrogen and a 50/50 N₂/CO₂ mixture, while in air CaSO₄ decomposes between 1500 and 1700 K. Using a TGA-MS it was found that only a small fraction of CaSO₄ in CaSO₄+ LM decomposed at 653 K, releasing SO₂. CaSO₄ was still observed in the char recovered at 1073 K as confirmed by the FTIR and XRD analysis. FeSO₄·7H₂O released the bound water below 543 K and the remaining FeSO₄ decomposed between 813 and 953 K. FeSO₄ in FeSO₄+ LM decomposed at 500 K to release SO₂. The inherent sulphates in HM were dominated by iron sulphates which started to decompose and release SO₂ at around 500 K and all sulphate had been decomposed at 1073 K. It was observed that during the fixed-bed pyrolysis at 1073 K in nitrogen, approximately 36% of the total sulphur in the CaSO₄+ LM decomposed, 88% of the total sulphur in the FeSO₄+ LM decomposed and around 76% of the total sulphur in HM decomposed. It was also confirmed that FeSO₄+ LM produced more volatile sulphur than CaSO₄+ LM during pyrolysis.     

Processing and performance of solid oxide fuel cell with Gd-doped ceria electrolyte

Fuel Cell performance was measured at 792-1095 K for Ni-GDC (Gd-doped ceria) anode-supported GDC film (60 μm thickness) with a (La_0.8Sr_0.2)(Co_0.8Fe_0.2)O₃ cathode using H₂ fuel containing 3 vol% H₂O. A maximum power density, 436 mW/cm², was obtained at 1095 K. The electrical conductivity of GDC electrolyte in N₂ atmosphere of 10⁻¹⁵-10⁰ Pa oxygen partial pressures (Po₂) at 773-1073 K was independent of Po₂, which indicated the diffusion of oxide ions. The conductivity of GDC in H₂O/H₂ atmosphere increased because of the further formation of electrons due to the dissociation of hydrogen in GDC (H₂ → 2H⁺ + 2e⁻). The hole conductivity was observed at 873 K in Po₂ = 10⁰-10⁴ Pa. The key factors in increasing power density are the increase of open circuit voltage and the suppression of H₂ fuel dissolution in GDC electrolyte. These are controlled by the cathode material and Gd-dopant composition.