Ca Nb Research Articles

Investigation of hydrogen embrittlement sensitivity of X65 pipeline steel with different compositions employing thermal simulation

The microstructure and hydrogen embrittlement sensitivity of two X65 pipeline steels at different t8/5 (the time required to cool from 800 °C to 500 °C) were investigated employing thermal simulation. With the increase of t8/5 for both steels, the martensite transferred to acicular ferrite (AF) and bainite, and further converts to bainite and martensite/austenite (M/A) constituents. Elongated polygonal Mn–Nb–S inclusions appear in steel A, which are easy to fall off and the hydrogen embrittlement sensitivity is thus increased. However, spherical Al–Ca–Nb–N–O–S inclusions appear in steel B, which can promote nucleation of AF, thus obtaining lower hydrogen embrittlement sensitivity under lower t8/5.

Effect of calcium on niobium solubility in alkaline solutions

Abstract The solubility of niobium-94 (94Nb) in calcium alkaline solutions is one of the important parameters in safety assessment of intermediate-depth disposal which are assumed to use cementitious materials. Nb solubility and solubility-limiting solid phases of Nb in these systems remain unclear. The oversaturation solubility experiments were performed systematically in the 0.001–0.1 mol dm−3 (M) CaCl2 solutions under alkali conditions, and the characterization of precipitated solid phase controlling Nb solubility was conducted. The negative dependence of Nb solubilities on pH and calcium (Ca) concentration was observed in solubility experiments, and the Ca/Nb molar ratio of precipitated solid phase was 0.66. The pH and Ca dependence of Nb solubilities was reproduced by the reaction with Nb aqueous species Nb(OH)6 − and Ca–Nb oxide with the Ca/Nb ratio of 0.66, e.g., Ca4Nb6O19 (am). With increasing pH, Nb concentrations in the 0.001–0.1 M CaCl2 solutions were significantly lower than those calculated from thermodynamic data without Ca–Nb solid. This work provides systematic evidence that the presence of Ca clearly affects Nb solubility. Since calcium is a major component of groundwater and cement pore water, the Ca–Nb solid phase should be considered in the Nb solubility assessment.

Consideration on modeling of Nb sorption onto clay minerals

Abstract Sorption distribution coefficient (Kd) of niobium-94 on minerals are an important parameter in safety assessment of intermediate-depth disposal of waste from core internals etc. The Kd of Nb on clay minerals in Ca(ClO4)2 solutions were, however, not successfully modeled in a previous study. The high distribution coefficients of Nb on illite in Ca(ClO4)2 solutions were successfully reproduced by taking Ca–Nb–OH surface species into account. Solubility of Nb was studied in Ca(ClO4)2 solutions and the results were reproduced by taking an aqueous Ca–Nb–OH complex species, CaNb(OH)6 +, into account in addition to previously reported Nb(OH)6 − and Nb(OH)7 2−. Based on this aqueous speciation model, the Ca–Nb–OH surface species responsible for the sorption of Nb on illite in Ca(ClO4)2 solutions was presumed to be X_OCaNb(OH)6. Although uncertainties exist in the speciation of aqueous Ca–Nb–OH species, the result of this study proposed a possible mechanism for high distribution coefficient of Nb on illite in Ca(ClO4)2 solutions. The mechanism includes Ca–Nb–OH complex formation in aqueous, solid and surface phases.

Factors Influencing the Direct Electrochemical Reduction of Nb2O5 Pellets to Nb Metal in Molten Chloride Salts

Powder compacted and sintered Nb2O5 pellets were cathodically polarised against graphite anode in calcium chloride melt at 1173 K to study the influence of various factors on the electrochemical reduction of the oxide. The parameters were; duration and temperature of electrolysis, open porosity of pellets, nature of anode, mode of electrolysis and configuration of the oxide cathode. The experiments were also conducted in KCl, KCl-25 mol% CaCl2 and NaCl melts to understand the effect of melt composition on the electroreduction. Different Ca–Nb–O and Nb–O intermediates were found in the pellets electrolysed for different durations of time in CaCl2 melt which eventually reduced to Nb. The current efficiency of the process decreased with increasing duration of electrolysis. Decrease in electrolysis temperature from 1173 to 1073 K led to the decrease in the rate of reduction of the oxide pellets. Pellets with high open porosity reduced faster. Carbon contamination of the melt was relatively less when pyrolytic graphite was used as anode. Of all the melts studied, the reduction was found to be better in calcium chloride melt, that too when alumina crucible was used as container of the melt.

1,3-Dipolar Cycloaddition of Nitrones to Gold(III)-Bound Isocyanides

Treatment of gold(III)-isocyanides [AuCl3(CNR1)] (R1 = Xyl 1, Cy 2, But 3) with an equimolar amount of 5,5-dimethyl-1-pyrroline-N-oxide (4) in CH2Cl2 at −74 °C leads to the generation of the heterocyclic aminocarbene species [AuCl3{C(ONaCMe2CH2CH2CbH)═NeR1}(Na–Cb)(Cb–Ne)] 8 (for R1 = But) or gold(III) complexes cis-[AuCl2{Na(CMe2CH2CH2CbNeR1)Cd═O}(Na═Cb)(Ne–Cd)] 9 and 10 (for R1 = Xyl and Cy) in good isolated yields (75–87%). DFT calculations show that deprotonation of the endocyclic CH group in the carbene ligand leads to spontaneous N–O bond cleavage, and acidity of this group is a factor controlling the different chemical behavior of 1–3 depending on the nature of substituent R1. The reaction of equimolar amounts of the aldonitrone p-TolCH═N+(Me)O– (5) or the ketonitrones Ph2C═N+(R2)O– (R2 = Ph 6, CH2Ph 7) with 1–3 in CD2Cl2 at −70 °C in air (or under N2) revealed the formation of the carbene complexes [AuCl3{C(ONMeCaH-p-Tol)═NbR1}(Ca–Nb)] (R1 = Cy 11, Xyl 12, But 13), [AuCl3{C(ONPhCaPh2)═NbR1}(Ca–Nb)]...

Growth of Shape-Controlled Ca3NbGa3Si2O14 and Sr3NbGa3Si2O14 Single Crystals by Micro-Pulling-Down Method and Their Physical Properties

Shape-controlled langasite-type piezoelectric single crystals, Ca3NbGa3Si2O14 (CNGS) and Sr3NbGa3Si2O14 (SNGS), were grown by the micro-pulling-down (µ-PD) method and their physical properties were investigated. Columnar-shaped CNGS and SNGS crystals with a- and c-axes in the growth direction were grown and the diameters of the grown columnar-shaped crystals were controlled to be approximately 3 mm from the initial to the later part. The grown crystals without cracks indicated relatively high crystallinities and a single phase of the langasite-type structure in the powder X-ray diffraction patterns. Meanwhile, in the outside area of the crystals, parts of second phases were detected and energy dispersive X-ray spectroscopy measurements indicated that the second phases of the CNGS and SNGS crystals were the Ca–Nb–O and Sr–Nb–O systems, respectively. The d11 of the CNGS crystal indicated 3.98 pC/N, which was coincident with those of previous reports.

Growth of Shape-Controlled Ca3NbGa3Si2O14and Sr3NbGa3Si2O14Single Crystals by Micro-Pulling-Down Method and Their Physical Properties

Shape-controlled langasite-type piezoelectric single crystals, Ca3NbGa3Si2O14 (CNGS) and Sr3NbGa3Si2O14 (SNGS), were grown by the micro-pulling-down (µ-PD) method and their physical properties were investigated. Columnar-shaped CNGS and SNGS crystals with a- and c-axes in the growth direction were grown and the diameters of the grown columnar-shaped crystals were controlled to be approximately 3 mm from the initial to the later part. The grown crystals without cracks indicated relatively high crystallinities and a single phase of the langasite-type structure in the powder X-ray diffraction patterns. Meanwhile, in the outside area of the crystals, parts of second phases were detected and energy dispersive X-ray spectroscopy measurements indicated that the second phases of the CNGS and SNGS crystals were the Ca–Nb–O and Sr–Nb–O systems, respectively. The d11 of the CNGS crystal indicated 3.98 pC/N, which was coincident with those of previous reports.

Li2Ca1.5Nb3O10from X-ray powder data

Lithium calcium niobium oxide (2/1.5/3/10), Li2Ca1.5Nb3O10, has been synthesized by conventional solid-state reaction. Its structure consists of triple-layer perovskite slabs of corner-sharing NbO6 octa­hedra inter­leaved with lithium ions; Ca cations partially occupy the perovskite A sites at 75% occupancy probability. All eight atoms in the asymmetric unit are on special positions: one Nb atom has site symmetry 4/mmm; the second Nb, both K, the Sr and two O atoms have site symmetry 4mm; the remaining two O atoms have site symmetries 2mm. and mmm., respectively.

Engineered Interfaces of Artificial Perovskite Oxide Superlattices via Nanosheet Deposition Process

Combining different materials into desired superlattice structures can produce new electronic states at the interface and the opportunity to create artificial materials with novel properties. Here we introduce a new, rather unexpected, and yet simple way to such a superlattice assembly of perovskite oxides: in the Dion-Jacobson phase, a model system of layered perovskites, high-quality bicolor perovskite superlattices (LaNb(2)O(7))(nL)(Ca(2)Nb(3)O(10))(nC) are successfully fabricated by a layer-by-layer assembly using two different perovskite nanosheets (LaNb(2)O(7) and Ca(2)Nb(3)O(10)) as a building block. The artificially fabricated (LaNb(2)O(7)/Ca(2)Nb(3)O(10)) superlattices are structurally unique, which is not feasible to create in the bulk form. By such an artificial structuring, we found that (LaNb(2)O(7)/Ca(2)Nb(3)O(10)) superlattices possess a new form of interface coupling, which gives rise to ferroelectricity.

N Doping of Oxide Nanosheets

Photo-N doping for Nb-O system oxide nanosheets such as Nb(6)O(17), TINbO(5), and Ca(2)Nb(3)O(10) was succeeded in by treatment under mild conditions, where their H-restacked forms containing tetrabuthylammonium (TBA) ion were illuminated by UV light at room temperature. A relatively strong absorption in visible light region (400 approximately 600 nm) was observed for the N-doped samples because of formation of p-band or valence band hybridization of N in the Nb-O bandgap. The N doping proceeded only under TBA presence in water without O(2), and Pt loading promoted this doping reaction. It is proposed as a mechanism from the results of the doping for the Ti-O and Nb-O nanosheet mixed samples, that the Nb-O nanosheet acts as a photocatalyst for the N-doping reaction. This is the first report of N doping to oxide nanosheets.

Photocatalyst of Lamellar Aggregates of RuOx‐Loaded Perovskite Nanosheets for Overall Water Splitting.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A synchrotron X-ray study of triclinic LiCa2Nb3O10 with perovskite-type slabs.

The triclinic superstructure of a small crystal of LiCa(2)Nb(3)O(10), lithium dicalcium triniobium decaoxide, has been investigated by synchrotron X-ray diffraction. The unit cell is an almost rectangular parallelepiped, although there is a 0.245 degrees offset from orthogonality for beta. The structure essentially belongs to a homologous series of Li[Na(n-3)Ca(2)Nb(n)O(3n+1)] with n = 3, where the moiety in square brackets has a perovskite-type slab structure. The superstructure has a doubled unit-cell volume with respect to the tetragonal aristotype. The NbO(6) octahedra are rotated about axes parallel to [110] by approximately 10 degrees. Adjacent slabs are connected by Li atoms and are geometrically related by 4(2) pseudosymmetry lying parallel to c. There are twice as many sites as Li atoms, providing a variation of population at these Li sites.

Transmission Electron Microscopy Study of α-Decay Damage in Aeschynite and Britholite

ABSTRACTThe aeschynite structure-type (Ce,Nd,La,Th,U,Ca)(Nb,Ti)2O6, and the rare-earth silicate apatite structure-type with the formula (Ce,La,Nd,Ca,Th)10(SiO4,PO4)6(O,F,OH)2 are important rare-earth and actinide host phases for high-level nuclear waste. Natural phases of these structure-types have calculated alpha-decay doses up to ∼1017 α-events/mg which have accumulated over hundreds of millions of years. Transmission electron microscopy has been used to study the microstructure of α-decay damage in aeschynite and britholite. Electron diffraction analysis of natural aeschynite revealed that minerals originally crystalline gradually lost their crystallinity with increasing alpha-decay doses. Helium bubbles were found in the aeschynite which have accumulated up to ∼2×1016 α-events/mg. These bubbles may nucleate within collision cascades during a-decay damage. Electron irradiation has an enhanced rare-gas migration and the formation of larger bubbles. High-resolution electron microscopy (HRTEM) revealed that amorphization during accumulation of a-decay damage was from alpha-recoil nuclei collision cascades, in both the aeschynite and britholite.

Structure analysis of a Ca [sbnd]Nb [sbnd]Ga garnet

Neutron powder-diffraction intensities were measured in the range from room temperature to 800°C for a Ca Nb Ga garnet (CNGG) grown from a non-stoichiometric melt. A single crystal of CNGG was also studied at room temperature by means of X-ray diffraction. The space group I2 13 (No. 199) was tentatively assigned to CNGG based both on Rietveld analysis and on X-ray single-crystal structure analysis. There are some vacancies at one of the Ca 2+ sites and partial substitutions of Ca 2+ and Ga 3+ for Nb 5+. CNGG exhibited appreciably different thermal-expansion coefficients at room temperature and above 400°C. The dielectric constant measured parallel to the [001] direction for CNGG starts to increase non-linearly at 400°C, but exhibits no peak up to 600°C.

Oxydes de type perovskite du systeme CaNbO

Three perovskite-like phases of the CaNbO system have been isolated, Ca x NbO 3 (0.9 ⩽ x ⩽ 1), Ca(Ca x Nb 1−x) O 3 (0 ⩽ x ⩽ 1 3 ) , and Ca 3Nb 2O 8− x . X-ray powder diffraction and electron microscopy and diffraction were used to obtain crystallographic data of these phases. The high temperature form of Ca(Ca x Nb 1− x )O 3 perovskites, where calcium ions occupy partially the octahedral sites, presents an orthorhombic symmetry and quasi-periodic twin bands like Ca x NbO 3 compounds. Two monoclinic forms (for x = 1 3 and x = 1 4 ) were found by heating Ca(Ca x Nb 1− x )O 3 compounds at lower temperature for long periods; ordering of calcium and niobium ions in octahedral sites is proposed for that structure. The Ca 3Nb 2O 8− x compound presents a quadratic cell: ordering of the cations derived from (NH 4) 3FeF 6 structure is proposed in agreement with the observed space group P4 nnc . A new multiple-phase family, with general formula Ca n Nb n O 3 n+2 , was obtained for n = 4.25, 4.5, and 5; electron microscopy shows intergrowth phenomena.