Distribution of intercalates in CrO3H2SO4-graphite and CrO3HClO4-graphite bi-intercalation compounds

Binary graphite intercalation compounds with sulfuric acid (H₂SO₄GICs), perchloric acid (HClO₄GICs) as well as chromium trioxide (CrO₃GICs) are known to be readily de-intercalated due to cathodic reduction and/or washing with aqueous solutions. The subsequent electrochemical intercalation of sulfuric and perchloric acid into the host CrO₃GICs has resulted in the ternary CrO₃H₂SO₄GICs and CrO₃HClO₄GICs, respectively. The products of the subsequent intercalation composed of the bi- and co-intercalation domains appeared to be very resistant to de-intercalation. The comparison of the results of the X-ray microprobe and X-ray diffraction analysis obtained for the original CrO₃GICs and the cathodically reduced ternary compounds have allowed insight into the structural and chemical changes developing during the subsequent intercalation/de-intercalation process. The knowledge of these properties is helpful for both the better understanding of the intercalation mechanism of the acids into the host CrO₃GIC and the elucidation of the extremely high stability of the ternary compounds.     

Behavior of unsaturated organic molecules in the nanospace of stage-2 cesium–graphite intercalation compounds

Intercalation reaction of benzene (C₆H₆) into CsC₂₄ and the characteristics of resulting ternary graphite intercalation compound (GIC) have been investigated, in comparison with the similar CsC₂₄–ternary GICs with ethylene (C₂H₄) and acrylonitrile (C₂H₃CN), and the behavior of these organic molecules within the interlayer nanospace has been discussed. The stability of these ternary GICs under exposure in air, in both stage structure and electrical conductivity was found to be in the order of C₂H₄–, C₆H₆– and C₂H₃CN–GICs. This result was reasonably explained as follows: the C₂H₄ molecules in the interlayer space oligomerize considerably and form network preventing diffusion of the Cs atoms in the interlayer, while the C₆H₆ molecules form only dimers or trimers and unable to form network, and the C₂H₃CN molecules, in spite of their strong tendency to polymerize, apparently do not form oligomers in the interlayer space, and its ternary GIC tends to degrade easily. By the electrical and galvanomagnetic measurements, it is revealed that the dominant carrier of these ternary GICs is electrons, even when the electrical conductivity decreases considerably after prolonged exposure in air, while the host graphite, Grafoil, shows two-carrier conduction. This fact indicates that the degradation of the ternary GICs occurs mainly at the periphery or surface layer of the crystallite, and at the inside of the crystallite, the ternary stage structure is retained unchanged.     

The stress corrosion cracking of high strength CrMnSiNi and CrMo steels

Practically all constructional steels are working under applied loads and environments. Below some stress levels the deterioriation of the material occurs by typical corrosion modes. Modern constructions are often loaded with enough high stresses to promote catastrophic failures due to stress corrosion crack propagation. Actually the whole range of metallic materials used in reliable constructions which are exposed to corrosive environments should be tested for their sensitivity to stress corrosion cracking. By measuring the material constant K_ISCC (critical stress intensity factor for stress corrosion cracking) it is possible to construct the reliable parts working in a safe range of stresses, which cannot be computed knowing yield strength of the material only. For measuring K_ISCC values of high strength CrMnSiNi and CrMo steels, original stands were built and long-term (> 10³h) tests applied by means of the cantilever beam method. Sensitivity of tested steels to stress corrosion cracking was expressed as a ratio K_ISCC:K_IC. Some other observations concerning kinetics of crack propagation and other properties of the materials have been carried out.     

The oxidation behaviour of austenitic FeCrNi alloys containing dispersed phases

The high temperature oxidation behaviour of FeCrNi austenitic alloys containing 1% Ti which, in some cases, had been converted into an oxide dispersion has been examined. The oxide dispersions were produced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. Generally, the effect of the dispersed oxide was much less pronounced than in corresponding nickel-free, ferritic alloys. Nevertheless, the time-to-breakaway of the protective Cr₃O₃ scale which developed on Fe18CrNi alloys was substantially increased, although the differences between the untreated and the internally oxidized alloys reduced with increasing nickel content. An Fe14Cr20Ni alloy did not show any improvement after internal oxidation. Unlike the ferritic alloys, no coarsening of the dispersoid phase was observed during exposure.     

Computer simulation of order-disorder phase transition in the intercalation compounds M1／2TiS2 （M = Fe, Co, Ni）

Based on cluster variation method (CVM) and natural iteration method (NIM), order-disorde phase transition inthe intercalation compounds M_(1/2)TiS_2 is simulated by computer. The favorble conditions, under which 3a_0~(1/2)×a_0 super-structure is formed, are given, and the results are in good agreement with the experiments and theoretical calculations. Therelationship between critical temperature and M-ion-vacancy interaction parameter is linear.     

The effect of oxygen on the open-circuit passivity of Fe18Cr

Electrochemical studies were conducted to determine the critical amount of oxygen necessary to prevent corrosion by maintaining the open-circuit passivity of Fe18Cr samples initially passivated at 0.6 V(NHE) in 1N H₂SO₄ solutions. Samples passivated at 0.6 V(NHE) and then released to open circuit in O₂-saturated (29.4 ppm O₂ dissolved) solution maintained a state of passivity. Samples passivated and released to open circuit in N₂-purged solution decayed to a state of active corrosion in 800–2000 min. A passive state, however, could be maintained if O₂ were added to the N₂ flow so that a minimum of 1.7 ppm O₂ was present in solution. Furthermore, this critical amount of O₂ had to be added before open-circuit decay reached 0.45 V(NHE). Auger electron spectroscopy measurements indicated that following open-circuit stabilization of passivity by O₂ the percentage chromium in the film increased with increasing open-circuit potential. X-ray photo-electron spectroscopy measurements indicated that the film thickness decreased with increasing open-circuit potential.     

The dissolution and passivation kinetics of stainless alloys containing molybdenum—1. Coulometric studies of FeCr and FeCrMo alloys

The evolution of active current density with time has been measured on new surfaces of FeCr and FeCr-1.5 at. % Mo alloys. The solutions used were 1M and 4M HCl. At several potentials, and at steady-state current densities as high as 150 mA/cm¹, the inhibiting effect of molybdenum was established after the passage of a charge density equivalent to less than three monolayers of alloy. The results suggest that molybdenum acts by enriching at surface kink or step sites in the active state.     

Crystal structures of PrAlxGe2−x compounds

The PrAl_xGe_2−x cross-section of the Pr–Al–Ge system at 1073 K was studied. The homogeneity ranges of two ternary praseodymium alumogermanides, PrAl_1.55–1.48Ge_0.45–0.52 and PrAl_1.42–0.98Ge_0.58–1.02, were determined and their crystal structures refined from X-ray single-crystal diffraction data. The former has a hexagonal structure of the AlB₂ type (hP3, P6/mmm, a=4.3223(3), c=4.2585(4) Å for x=1.476(2)) and the latter a tetragonal structure of the LaPtSi type (tI12, I4₁md, a=4.2534(5), c=14.641(2) Å for x=1). For both structures, the Al–Ge interatomic distances are close to the sum of the covalent radii, whereas the Pr–Al(Ge) distances agree well with the sum of the metallic radii. The solubility of Ge in the cubic Laves phase PrAl₂ was found to be less than 2 at.%, that of Al in off-stoichiometric PrGe_2−x less than 8 at.%. Al(Ge)-centered R₆ trigonal prisms constitute a common geometrical feature of the structures of the germanides RGe_2−x and alumogermanides RAl_xGe_2−x formed by the light rare-earth elements. Substitution of Ge for Al, or replacement of a large rare-earth element by a smaller one, leads to a reduction of the prism volume.     

Microwave synthesis of LiCoO2 cathode materials

1INTRODUCTION LiCoO2isacommerciallyusedcathodemateri alforlithiumionbattery.Comparedwithothercathodematerials,suchasLiNiO2,LiMn2O4and theirdopedcompounds,LiCoO2hastheadvanta gesofhighpotential,excellentreversibility,longcyclelifeandreliableperformance[14].Thereare mainlyseveralmethodsforsynthesisofLiCoO2,suchasconventionalceramicmethod[5,6],sol gel synthesis[7],andlow temperaturesolutioncom bustion[8].Amongthesemethods,theconvention alceramicmethodhasbeenappliedtoindustrial productio…     

Mechanism of influence of ferric ion on electrogenerative leaching of sulfide minerals with FeCl3

1 Introduction Electrogenerative leaching process is a newly- developed technique in hydrometallurgy. Although its principle has developed since the late 1960s[1], this technique has been overlooked in metallurgy until ZHANG et al[2] introduced it to the …     

Solid-state synthesis of Sr-and Co-doped LaMnO_3 perovskites

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The effect of phosphorus addition on the corrosion behavior of ARC-MELTED Ni10TaP alloys in 12 M HCl

The corrosion resistance of arc-melted Ni10TaP alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30 °C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni10Ta alloys containing 0 and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni10Ta20P alloy increases almost linearly in the initial 2 h. The Ni10Ta alloy consists of intermetallic Ni₈Ta and immersion in 12 M HCl results in faceting dissolution. Ni10Ta10P alloy is composed of major Ni₈Ta and Ni₃P phases and minor Ni₂Ta and Ni₂P phases. Immersion of Ni10Ta10P alloy leads to preferential dissolution of the Ni₈Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni 10Ta20P alloy consists of Ni₂Ta, Ni₃P, Ni₂P and NiP phases. Immersion of Ni10Ta20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni10Ta20P alloy than Ni10Ta10P alloy is attributable to the formation of the Ni₂Ta phase with a higher tantalum content than the Ni₈Ta phase which is the readily corroded major intermetallic phase in the Ni10Ta10P alloy.     

Non-equilibrium phase synthesis in Al2O3–Y2O3 by spray pyrolysis of nitrate precursors

The phase evolution in the Al₂O₃–Y₂O₃ system has been studied for 4, 10, 15 and 37.5% Y₂O₃ amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO₃ and γ-Al₂O₃ (spinel structure). Partitionless crystallisation, on the other hand, leads directly to a γ-Al₂O₃ solid solution in dilute alloys, and garnet at temperatures as low as 800°C in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated γ-Al₂O₃ leads to the precipitation of the orthorhombic YAlO₃ that is stable up to temperatures as high as 1600°C, while hexagonal YAlO₃ converts to garnet, Y₃Al₅O₁₂. A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy–composition curves for the various competing phases.     

Chemical synthesis of nanocrystalline ZrO_2-SnO_2 composite powders

ZrO_2-SnO_2 composite nanoparticles were prepared by heating the hydrate precursors synthesized by the chemicalco-precipitation reaction of ZrOCl_2 and SnCl_4. The precursors were examined by differential thermal analysis (DTA) andthermogravimetric analysis (TGA). The composite powder was characterized using X-ray diffraction (XRD), transmissionelectron microscopy (TEM) and desorption isotherm (Barrett-Joyner-Halenda method). The average crystal size of thenanoparticle ranges from 15 to 30 nm for the sample containing 5%-25% ZrO_2 (mass fraction). Most of the pores in theZrO_2-SnO_2 nanoparticles are about 10-20 nm in diameter. The composite powder is promising for chemical sensors.     

The effect of Cl− ions on the passivation of Fe26Cr alloy

Anodic oxide films formed on Fe26Cr in pH 2.0 H₂SO₄ solution in the presence and absence of Cl⁻ ions have been investigated using electrochemical techniques and Auger electron spectroscopy (AES) combined with ion sputtering. It is possible to incorporate Cl⁻ ions into passive oxide films formed over the entire passive potential range only when Cl⁻ is present in the solution from the very beginning of film formation. Cl⁻ ion incorporation does not cause any change in film thickness or Fe/Cr ratio, or any film thinning or film breakdown. A relatively short anodization in Cl⁻-free solution is sufficient to prevent any subsequent Cl⁻ ion incorporation. The susceptibility of the passive film to Cl⁻ attack appears to depend on the presence of small amounts of impurity in the alloy. A 99.97% pure alloy does not pit, whereas a 99.93% pure alloy, with larger concentrations of C, S, Mn, Co and Ni, does suffer intergranular attack in Cl⁻ solution.     

Combustion synthesis of FeAl−Al2O3 composites with TiB2 and TiC additions via metallothermic reduction of Fe2O3 and TiO2

Combustion synthesis involving metallothermic reduction of Fe₂O₃ and TiO₂ was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate FeAl-based composites with dual ceramic phases, TiB₂/Al₂O₃ and TiC/Al₂O₃. The reactant mixture included thermite reagents of 0.6Fe₂O₃+0.6TiO₂+2Al, and elemental Fe, Al, boron, and carbon powders. The formation of xFeAl−0.6TiB₂−Al₂O₃ composites with x=2.0−3.6 and yFeAl−0.6TiC−Al₂O₃ composites with y=1.8−2.75 was studied. The increase of FeAl causes a decrease in the reaction exothermicity, thus resulting in the existence of flammability limits of x=3.6 and y=2.75 for the SHS reactions. Based on combustion wave kinetics, the activation energies of E_a=97.1 and 101.1 kJ/mol are deduced for the metallothermic SHS reactions. XRD analyses confirm in situ formation of FeAl/TiB₂/Al₂O₃ and FeAl/TiC/Al₂O₃ composites. SEM micrographs exhibit that FeAl is formed with a dense polycrystalline structure, and the ceramic phases, TiB₂, TiC, and Al₂O₃, are micro-sized discrete particles. The synthesized FeAl−TiB₂−Al₂O₃ and FeAl−TiC−Al₂O₃ composites exhibit the hardness ranging from 12.8 to 16.6 GPa and fracture toughness from 7.93 to 9.84 MPa·m^1/2.     

Hydrothermal synthesis and characterization of Eu-doped GaOOH/α-Ga2O3/β-Ga2O3 nanoparticles

Eu-doped GaOOH nanoparticles with size of 5-8 nm were prepared by hydrothermal method using sodium dodecylbenzene sulfonate (SDBS) as surfactant. Eu-doped α-Ga₂O₃ and β-Ga₂O₃ were further fabricated by annealing GaOOH:Eu and then characterized by X-ray diffraction(XRD), transmission electron microscopy (TEM) and photoluminescence (PL). The TEM results show that monodisperse Eu³⁺-doped GaOOH nanoparticles form and then transform into Eu³⁺-doped α-Ga₂O₃ and β-Ga₂O₃ through annealing the GaOOH:Eu nanoparticles at 600 and 900 °C, respectively. PL studies indicate that GaOOH:Eu has the highest intensity at 618 nm. Luminescence quenching is observed at higher Eu³⁺ concentration in all samples.     

Self-propagating high temperature synthesis of MoSi2 matrix composites

MoSi2 is presently regarded as the most important material for electrical heating and as one with huge potential for high temperature structural uses. MoSi2 and MoSi2 matrix composites were prepared by self-propagating high temperature synthesis （SHS）. Pure MoSi2 was obtained and a compound of MoSi2 and WSi2was synthesized in the form of predominant solid solution （Mo,W）Si2. By adding aluminum of 5.5 at.% to Mo-Si, the crystal structure of MoSi2 changed into a mixture of tetragonal Cllb MoSi2and hexagonal C40 Mo（Si,Al）2. The （Mo,W）Si2-Mo（Si,Al）2-W（Si,Al）2 composite materials were synthesized by adding aluminum of 5.5 at.% to Mo-W-Si. However, if the amount of the added aluminum was not larger than 2.5 at.%, it did not have any significant effect. SHS is an effective technology for synthesis of MoSi2 and MoSi2 matrix composites.