Showing papers in "Journal of The Less Common Metals in 1977"

Hydrogen absorption and desorption properties of AB2 laves-phase pseudobinary compounds

Abstract: The formation and properties of the hydrides of the pseudobinary intermetallic compounds Zr(CoxM1−x)2 and Zr(FexM1−x)2 (M = V, Cr, Mn; 0

A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds

Abstract: A correlation has been established between the stabilities of metal hydrides (measured in terms of the free energies of formation of the hydride phase per mole H 2 ) and the radii of tetrahedral holes in both hexagonal AB 5 (D2 d ) and cubic AB (B 2 ) intermetallic compounds. The correlation demonstrates that as the tetrahedral hole size increases, the stability increases. The results of this correlation show conclusively that hole size can be employed effectively in determining the stabilities of intermetallic compound-hydrogen phases. A model, which depends solely upon lattice parameters, was developed to compute the radii of tetrahedral holes in hexagonal AB 5 intermetallic compounds. A similar model, which requires a ratio of metallic radii in addition to the lattice parameter, has been employed to compute the radii of tetrahedral holes in the cubic AB intermetallic compounds. The thermodynamic and structural properties of hexagonal AB 5 -hydrogen systems and cubic AB-hydrogen systems have been compiled and are presented. The quantitative correlations are excellent. A change in valence of the A atom in the hexagonal AB 5 intermetallic compounds may have a significant effect on the stabilities of the hydride phases formed. In those cases where the B atoms were partially substituted in the hexagonal AB 6 and the cubic AB intermetallic compounds, a good correlation of stability with hole size was found. Gross deviations of the Ce-base AB 5 hydride data from the correlation line established by all the other AB 5 hydrides were observed. These were rationalized on the basis of well-documented stress-induced lattice contractions of Ce intermetallic compounds. The resulting contracted hole sizes of the CeNi 5 and CeCo 5 compounds modified the stability of their hydrides. These compound hydrides demonstrate a displaced but similar correlation to the other AB 5 hydrides, i.e . the smaller the hole size, the less stable is the system.

Étude thermodynamique du système binaire cuivre-indium I. Étude calorimétrique

Abstract: The integral enthalpy of formation hf of Cu-In liquid alloys has been measured by direct reaction calorimetry for 0 < xIn < 1 and 903 < T (K) < 1348. The excess heat capacity CEP which has been deduced is temperature independent. The hf minimum and the CEP maximum, at xIn ⋍ 0.25, are interpreted in terms of a local order in the melts. The integral enthalpy of formation of the β, γ and η solid phases has also been measured by dissolution calorimetry at 904 and 941 K. The entropy of melting of the γ phase has been determined. It is pointed out that many systems do not have zero excess heat capacity; thus the mean values of hf as calculated from measurements at different temperatures are incorrect.

The constitution of copper-hardened samarium-cobalt permanent magnets

Abstract: A study has been made of the constitution of copper-hardened samarium-cobalt alloys lying between SmCo5 and Sm2Co17 and containing up to 20 at.% copper. It is found that copper exerts a strong influence; the 2:17 phase is repressed in favour of the 1:5 phase with the peritectic line changing to a eutectic trough. The solvus line of the 1:5 phase is displaced towards the 2:17 phase in the composition range studied. SmCu5 and the 1:5 phase are mutually soluble above the latter's eutectoid transformation temperature, which falls as the copper level increases. Finally, the cobalt solid solution comes into direct equilibrium with a wide range of composition of the 1:5 phase in the ternary alloys (in the region of 800°C).

Étude hydrothermale du système VO2-VO2,5-H2O

Abstract: The system VO2-VO2,5-H2O has been studied at temperatures varying between 20 °C and 400 °C and at pressures close to the saturation vapour pressure of water or at supercritical pressures. Under these conditions, the following known oxides were observed: VO2 (with the rutile structure) [1–3], V6O13 [14 – 17], V3O7 [8 – 15] and V2O5. Other compounds have also been found, namely two additional oxides, VO2(A) and VO2(B), and seven hydrates including four VO2, xH2O hydrates, two V2O 4,8, nH2O hydrates and one V3O7,H2O hydrate.

Étude cinétique, diffusionnelle et morphologique de la nitruration du titane par l'azote à haute température: Propriétés mécaniques et structurales des solutions solides Tiα-azote

Abstract: Resume L'etude cinetique de la nitruration du titane a ete realisee entre 1230 et 1500 °C. Dans ce domaine de temperature, nous avons calcule a l'aide d'un modele simple de diffusion les coefficients de diffusion D α N = 0,144 × exp ( −53400 RT ) et D N β = 0,121 exp ( −53300 RT ) de l'azote dans les phases α et β du titane. Par des nitrurations a tres hautes temperatures, nous avons mis en evidence la phase ϵ jusqu'a 2000 °C, ainsi qu'une forme dendritique du nitrure de titane TiN, de parametre a = 4,2398 ± 0,0005 A . Des solutions solides homogenes titane α-azote ont ete elaborees jusqu'a des teneurs de 9 at.%. L'etude des alliages met en evidence l'influence de l'azote sur les proprietes structurales et mecaniques du titane.

Atomic radii and lattice parameters of the A15 crystal structure

Abstract: Some properties of the lattice parameters a of the A15 (A 3 B) crystal structure are reinvestigated on the basis of the previously reported experimental data for about seventy compounds. The relation between these parameters and the atomic radii of A and B atoms ( r A and r B ) in the A15 structure is discussed. In f.c.c. transition elements, r B is equivalent to the atomic radius of the element. The value of r B for non-transition elements varies according to whether the A site element is in Group IVA, Group VA or Group VIA of the periodic table. There is a linear relationship between a and r B . Taking account of such relationships, a unique set of A15 atomic radii is determined. Lattice parameters of the A15 compounds are calculated using these atomic radii.

The effect of the α-β phase change on the α phase solubility of hydrogen in palladium

Abstract: Following the α-β phase change in palladium, there is an enhanced solubility of hydrogen at a given equilibrium pressure within the low hydrogen content α phase. There are marked similarities between the absorption isotherms for hydrogen following the phase change and those exhibited by heavily cold-worked palladium. Partial thermodynamic parameters for hydrogen absorption are reported for palladium plastically deformed by the phase change.

Catalytic activity of the basal and edge planes of molybdenum disulphide

Abstract: CoMo—Al2O3 catalysts are used for hydrotreating and desulphurisation in petroleum processing. The catalyst prepared in the oxide form is inactive and is presulphided before use. ESCA analysis has shown that, during this activation, a state similar to Mo4+ sulphide in MoS2 is formed on the surface. To gain more insight into the properties of the sulphide formed by presulphiding, we have examined the properties of MoS2 and report findings with two samples, one prepared with a high basal plane area and the other with a high edge plane area. We used four experimental tests: reaction with carbon disulphide and with thiophene and adsorption of hydrogen and thiophene below reaction temperatures. ESCA was used to follow changes in the surface composition of the sulphides and the results were compared with those found with an active commercial CoMo catalyst. The MoS2 sample having a higher edge plane area adsorbed hydrogen and thiophene more strongly, gave bigger yields of methane from carbon disulphide and gave a higher proportion of butane relative to butenes in the hydrogenolysis of thiophene. However, the sample having mainly basal plane MoS2 showed the higher conversion of thiophene. Our results suggest that the basal plane of MoS2 catalyses hydrogenolysis whereas the edge plane promotes a variety of other hydrogenation reactions. Comparison with the performance of a sulphided CoMo—Al2O3 catalyst indicates that the dominant reaction site resembles the basal plane of MoS2. However, the commercial catalyst shows distinctive patterns of reaction with carbon disulphide during presulphiding and may also show differences in hydrogen adsorption, particularly when the surface contains excess sulphur.

Interfacial tensions in Zn, Zn-Sn and Zn-Sn-Pb systems

Abstract: Intergranular and solid-liquid interfacial tensions have been determined for Zn-Sn and Zn-Sn-Pb systems. The temperature coefficient of the intergranular tension of solid zinc is negative. Solid-liquid interfacial tension for the Zn-Sn system increases as the tin content increases; the σsl values fit an equation derived from statistical thermodynamics which allows us to calculate the solid-liquid interfacial tension of pure Zn. The isotherm at 330 °C of liquid-solid interfacial tension in the ternary Zn-Sn-Pb system shows a decrease in σsl as the Zn content in the liquid phase increases, which is in qualitative agreement with theoretical predictions.

Some phases in ternary alloys of thorium and uranium with the Al4Ba-ThCu2SI2-type structure

Abstract: The geometrical characteristics of Al4Ba-ThCu2Si2-type phases are discussed briefly in terms of reduced cell parameters (aRA). Special reference is made to those ternary compounds of Th and U which have this type of structure; the data already known from the literature and those now obtained for 21 new compounds are considered.

Nitriding of titanium in a radio frequency discharge

Abstract: The nitriding of titanium in a nitrogen or nitrogen-hydrogen plasma prepared with a radio frequency discharge was studied. Power was supplied from an inductively coupled 13.56 MHz 500 W generator. The reaction kinetics obeyed a parabolic relationship and the reaction constants were 10−12 - 10−10 g2 cm−4s−1 under various conditions. The surface of the nitrided metal was hardened by the formation of TiN. The rate-determining steps may be the absorption of ions and the diffusion of nitrogen in the metal.

High pressure syntheses of SmB2 and GdB12

Abstract: Borides with a composition LnB2 and LnB12 have been reported [1,2] for the smaller members of the lanthanide series. Diborides have been prepared [3–10] for Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc and dodecaborides are known [5,6,11–16] for Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc. The diborides are isomorphous, with an AlB2-type (C32) structure. This structure is hexagonal (P6/mmm) and consists of alternating layers of metal atoms and boron atoms. The metal atoms form close-packed (36) layers and the boron atoms, hexagonal (63) layers. The layers are arranged such that boron atoms are positioned over the faces of the triangles formed by the metal atoms. The dodecaborides (except SCB12) each assume a UB12-type(D2f) structure. This structure is cubic (Fm3m) with metal atoms at fcc positions and each metal atom surrounded by a B24 cubo-octahedron. The tetragonal structure of ScB12 is described by Matkovich, et al. [16].

Superconductivity of lanthanum pnictides

Abstract: On the basis of a discussion of their chemical bonding, the lanthanum pnictides are divided into a nonmetallic and a metallic group, with the semi-metallic monopnictides lying in between. With the exception of LaBi2, all the metallic phases were tested for superconductivity. They all show a transition to the superconducting state below 1 K. The anti-Th3P4-type compounds, of which La4As3 was synthesized for the first time, were studied in more detail. A drastic increase in Tc on reduction of the electron concentration was observed in the series La4Bi3−xPbx. Partial substitution of La by Y leads to a decrease in Tc.

High resolution measurements of proton NMR in VHx, NbHx and TaHx: Proton knight shift and magnetic susceptibility

Abstract: The Knight shift of protons (KH) has been measured in Group Va transition metal-hydrogen alloys VHx, NbHx and TaHx at temperatures between 34 and 200°C. The hydrogen concentration x is between 0.096 and 0.736 for VHx, 0.311 and 0.879 for NbHx. and 0.092 and 0.567 for TaHx. The measurements were made using a high resolution spectrometer with a resolution of ± 1 ppm; the effect of the demagnetizing field due to the bulk magnetic susceptibility was isolated by using a single sheet of foil as a specimen. This made it possible to determine the bulk magnetic susceptibility and KH simultaneously. KH obtained with respect to bare protons was found to be negative in all cases. The result is interpreted in terms of a direct contact interaction with the negative spin polarization in the interstitial region and some additional contribution from H-induced bonding states.

The chromium-sulphur system between 873 K and 1364 K

Abstract: The chromium-sulphur system was studied by direct and indirect (H 2 S/H 2 ) sulphur pressure measurements as a function of the composition in the range 50–60 at.% S and 873–1364 K. The boundary towards metallic Cr and three small two-phase regions were detected, separating four high temperature phases from each other. Phase relations are discussed in the light of results reported by different authors. CrS seems to be Cr deficient while Cr 2 S 3 is deficient in sulphur. It is suggested that Cr 3 S 4 exhibits a change in modification at about 1170 K.

Hydrogen absorption by rare earth-transition metal alloys

Abstract: The absorption of hydrogen by RTx alloys (R = Ce, La, Nd, Pr and MM (mischmetal); T = transition metals, chiefly Co, Ni and Fe) has been studied at the Federal Bureau of Mines with the object of developing a suitable alloy absorbent for hydrogen storage at 25 °C and a pressure range of 1 – 10 atm. The ability to absorb and desorb hydrogen reversibly under these conditions (i.e. the working capacity) reached a maximum with the composition RT5. LaNi5 exhibited the highest capacity. Studies with MMCo5 showed that variations in individual rare earth contents in mischmetal had little effect upon the absorption properties of the alloys, but partial substitution of cobalt by other transition metals had a substantial effect. Interstitial oxygen and carbon in MMCo5 considerably reduced both the hydrogen absorption and the working capacity. The optimum composition for mischmetal-cobalt alloys with partial substitution of cobalt by other transition metals was found to be between MMCo4.25Ni0.75 and MMCo3.5Ni1.5. Up to 2.6 g atom H mol−1 of alloy could be reversibly stored between pressure limits of 1 atm and 10 atm at room temperature. Absorption of hydrogen by MMCo4.25Ni0.75 was largely unaffected by He, N2 and CH4 impurities in the gas, but was severely inhibited by O2, H2O, CO2 and CO impurities. Cyclic studies of morphology indicated that alloy decrepitation occurred more rapidly for MMCo3.5Ni1.5 than for MMCo4.25Ni0.75 during repeated absorption and desorption.

Structure and growth of anodic oxide films on titanium and TA6V alloy

Abstract: The crystallographic structure of anodic oxides grown on TA6V or pure titanium is determined for various electrolytes (IN H 2 SO 4 or H 3 PO 4 ). In each case, a change from microcrystalline aggregates to crystalline TiO 2 is found when the voltage increases. The thickness d of the films, as a function of the anodic voltage V , is determined in the various electrolytes. In the range of growth of microcrystalline aggregates, a linear function d = α V is found. The parameter α is slightly smaller for TA6V than for titanium. The crystallization of anatase is easier in the sulfuric electrolyte.

Diffuse reflectance spectra of molybdenum ions supported by magnesia, γ-alumina or silica

Abstract: Supported molybdenum oxides, used as model catalysts for the oxidation of propene, have been examined by diffuse reflectance spectroscopy. Changes in the valence state and coordination number of molybdenum are functions of the support (MgO, SiO 2 or γ-Al 2 O 3 ), the preparation procedure, the oxidising pretreatment and the conditions of reduction by the olefin. Two principal types of preparation have been used: impregnation by an aqueous solution of (NH 4 ) 6 (Mo 7 O 24 ) and reaction of the support with MoCl 5 . It appears that this latter procedure is suitable for preparing molybdenum oxides of low Mo content in which the Mo 6+ and Mo 5+ coordination can be predominantly tetrahedral. The selectivity for acrolein formation during the oxidation of propene can thus be related to the presence of Mo ions in tetrahedral configuration.

Defect structure, thermal and electrical properties of Ti nitride and V nitride powders

Abstract: Nitride powders of titanium and vanadium prepared by vapour phase reactions have been investigated with regard to their defect structure, thermal stability, sinterability and electrical conductivity. Ti nitride and V nitride containing excess of the non-metallic element and with large numbers of vacancies on the metal and non-metal sublattices liberated nitrogen, with the disappearance of vacancies, when held above 600°C under vacuum. The fractional occupancies of both sublattices were near unity at an atomic ratio (N + O) M of 1.00. However, they decreased again when the (N + O) M ratio fell below 1.00. The dissociation pressures of Ti and V nitrides at 1100°C increased sharply when the (N + O) M ratio was close to or above 1.00. The fine powders of Ti and V nitrides (0.03–0.05μm particle size) showed very high sinterabilities. The sintering proceeded rapidly at temperatures of 1000–1300 °C for titanium nitride and 900–1100°C for vanadium nitride without noticeable grain growth. The resistivities of fine powder compacts of Ti and V nitrides were 10−2–10−1 Ω cm and about 10−3 Ω cm, respectively, in the range -173 –400 °C and the temperature dependences were slightly negative.

Crystal chemical considerations on rare earth-transition metal compounds RxT with x ≥ 1 (T = Co, Rh, Ni, Pd)

Abstract: A survey of the crystal structures of the compounds x T is given. These structures may be grouped into those having exclusively trigonal prisms as coordination polyhedra and those with other polyhedra. For the latter group a crystal chemical interpretation seems difficult. However, the first group (those with only trigonal prisms) may be considered from four different crystal chemical aspects 1. (a) the relationship between the trigonal prism linkage coefficient and the stoichiometry 2. (b) the formation of structural series 3. (c) the concept of structure block stacking 4. (d) the interpretation of structure types based on unit cell twinning of close-packed structures.

Corrosion inhibition by sodium molybdate

Abstract: A laboratory study was conducted on the corrosion inhibition of low carbon steel by sodium molybdate in simulated cooling waters. The inhibiting efficacy of sodium molybdate is shown to be influenced by water composition and by the presence of other water treatment chemicals. More molybdate is required to protect steel in softer waters and in those containing high chloride and sulphate concentrations. Some common water treatment chemicals, notably an alkyl phosphonate, sodium nitrite, zinc salt and sodium gluconate, synergistically improve molybdate inhibition. Recent data which show the insensitivity of fish and fish food to sodium molybdate are presented. Reference to the nontoxicity of molybdenum compounds is also included, as is a brief description of the inhibiting mechanism of molybdate.

Étude esca de l'interaction oxygène-titane

Abstract: Resume L'interaction entre oxygene et titane massique polycristallin est etudiee a temperature ambiante et pour des pressions d'oxygene variant entre 2 × 10 −8 et 40 Torr, par spectroscopie d'electrons ESCA. Les spectres de photoelectrons montrent qu'a une phase initiale rapide d'adsorption d'oxygene succede un debut d'oxydation du titane. II est trouve qu'a chaque pression d'oxydation correspond une couche d'oxyde d'epaisseur limite. Ces couches sont constituees essentiellement de TiO 2 , mais des sous-oxydes de titane sont aussi presents. Une estimation de l'epaisseur des couches est donnee. Le mode de preparation de la surface metallique influence l'epaisseur des couches d'oxydes formees: un essai d'interpretation se base sur la formation de couches d'oxydes de structures differentes.

Sur l'oxydation du titane α en atmosphère d'oxygène: rôle de la couche oxydée et mécanisme d'oxydation☆

Abstract: Resume De nombreux travaux ont ete consacres a l'etude de l'oxydation du titane a haute temperature. Neanmoins, il existe encore des lacunes significatives dans la connaissance et la comprehension du phenomene, en particulier en ce qui concerne la contribution de la couche d'oxyde aux processus d'oxydation et de diffusion. Nous presentons, dans cet article, les aspects cinetiques, morphologiques et diffusionnels de l'oxydation du titane entre 650 et 875 °C. Nous proposons une interpretation basee sur la diffusion de l'oxygene a travers la couche d'oxyde. Nous en deduisons les valeurs suivantes pour les coefficients de diffusion de l'oxygene dans le titane ( D 2 ) et dans la couche d'oxyde ( D 1 ): D 2 = 0,408 exp (−47 040/ RT ) cm 2 s −1 D 1 = 1,42 × 10 −2 exp (− 45080/ RT ) cm 2 s −1

Chemical, structural and catalytic modifications of pure and doped iron(III) molybdate

Abstract: In this paper various factors influencing the precipitation of Fe-Mo-O systems have been investigated. Stoichiometric Fe(III) molybdate, Mo-rich Fe(III) molybdate promoted with bismuth and an Fe-Mo-O system with excess of Mo obtained by precipitation at room temperature were studied and the changes in the catalyst properties with respect to the temperature of calcination were examined. The following techniques were employed: chemical analysis, X-ray diffraction, scanning electron microscopy, surface area measurements, differential thermal analysis and an abrasion resistance test. The oxidation of methanol to formaldehyde was also investigated using a fixed bed microreactor. Stoichiometric Fe(III) molybdate was confirmed as the active component. Excess of Mo had no influence on the specific activity of the investigated catalysts, but it was found to be responsible for the high coherence of the sample. The addition of Bi to this system improved mechanical properties by preserving the coherence at temperatures above 450 °C where a collapse of the structure was obtained in the absence of bismuth. Bi-promoted catalyst showed the formation of Bi molybdate 23 phase. Stoichiometric Fe(III) molybdate gave the highest conversions of methanol after prolonged treatment at high temperature.

Solubility and activity of oxygen in liquid gallium and gallium-copper alloys

Abstract: The solubility of oxygen in liquid gallium in the temperature range 775 –1125 °C and in liquid gallium-copper alloys at 1100 °C, in equilibrium with β-Ga2O3, has been measured by an isopiestic equilibrium technique. The solubility of oxygen in pure gallium is given by the equation log (at.% O) = −7380/T + 4.264 (±0.03) Using recently measured values for the standard free energy of formation of β-Ga2O3 and assuming that oxygen obeys Sievert's law up to the saturation limit, the standard free energy of solution of oxygen in liquid gallium may be calculated : View the MathML sourceΔ°298 = −52 680 + 6.53T (±200) cal where the standard state for dissolved oxygen is an infinitely dilute solution in which the activity is equal to atomic per cent. The effect of copper on the activity of oxygen dissolved in liquid gallium is found to be in good agreement with that predicted by a recent quasichemical model in which it was assumed that each oxygen is interstitially coordinated to four metal atoms and that the nearest neighbour metal atoms lose approximately half their metallic cohesive energies.