Showing papers in "Journal of The Less Common Metals in 1983"
TL;DR: In this paper, the three Judd-Ofelt intensity parameters for erbium(III) are given in 17 different environments, and the effect of covalent chemical bonding on the intensity is discussed.
Abstract: The three Judd-Ofelt intensity parameters for erbium(III) are given in 17 different environments. The Ω 2 parameter is strongly affected by covalent chemical bonding. In contrast, the Ω 6 parameter is related to the rigidity of the medium in which the ions are situated.
624 citations
TL;DR: In this paper, a model was developed for predicting which sites and how many of them could be occupied by hydrogen atoms in each of the intermetallic compounds listed, and the resulting stoichiometries were determined by experimentally filling the interstices of model lattices constructed of Friauf polyhedra without violating either of the adopted geometric criteria.
Abstract: Hole sizes and intersite distances were calculated at various hydrogen concentrations for the interstices in hydrides of TiCr 1.8 , ZrTi 2 , ZrV 2 , ZrCr 2 , ZrFe 2 , HfV 2 and TaV 2 with the cubic MgCu 2 C15 structure and of ZrMn 2 and ErMn 2 with the hexagonal MgZn 2 C14 structure. From a survey of other metallic hydrides, empirical criteria were chosen for the minimum hole size (0.40 A) and the minimum H-H distance (2.1 A) for hydrogen atom occupancy in stable hydrides. With these criteria, a model was developed for predicting which sites and how many of them could be occupied by hydrogen atoms in each of the intermetallic compounds listed. The resulting stoichiometries were determined by experimentally filling the interstices of model lattices constructed of Friauf polyhedra without violating either of the adopted geometric criteria. For each of the compounds rationales are presented to explain the experimental observations reported by other researchers.
236 citations
TL;DR: In this paper, the first few hydriding-dehydriding cycles, referred to as activation, are discussed and new H 2 -D 2 isotope scrambling results are presented which indicate that metallic iron does not play a catalytic role.
Abstract: The processes involved in the first few hydriding-dehydriding cycles, referred to as activation, are discussed. The controversial activation of TiFe is reviewed and new H 2 -D 2 isotope scrambling results are presented which indicate that metallic iron does not play a catalytic role. The hydriding-dehydriding kinetics are resolved into three interacting processes: intrinsic kinetics, heat flow and mass flow. The intrinsic kinetics are further resolved into three processes occurring simultaneously and in series: a surface process, diffusion and phase transformation. The problem of determining the rate-limiting intrinsic process from the time-temperature-pressure ( t T P ) dependences of the different processes is discussed. The employment of surface modification techniques to complement the t T P analysis is emphasized. Most kinetics analyses that have identified an intrinsic process as rate limiting are criticized as being invalid because 1. (1) the rate theory used has been inappropriate. 2. (2) heat flow has not been adequately taken into account. The viewpoint adopted is that only nucleation and growth (NG) theory is applicable. The NG formalism is developed and is employed to treat the interplay between the simultaneous intrinsic processes which is generally ignored. NG theory is also employed to model kinetics behavior in order to discuss heat flow. The control of heat flow by the previously overlooked, but very useful, control of heat capacity is discussed.
185 citations
TL;DR: The absorption of hydrogen in magnesium was studied in the temperature range 260-425 °C and at pressures from the equilibrium value to 2 MPa above equilibrium as discussed by the authors, at constant temperature the absorption rate depends on the pressure whereas total absorption is attained in approximately the same time regardless of the pressure.
Abstract: The absorption of hydrogen in magnesium (purity, 99.8%–99.94%) was studied in the temperature range 260–425 °C and at pressures from the equilibrium value to 2 MPa above equilibrium. At constant temperature the absorption rate depends on the pressure whereas total absorption is attained in approximately the same time regardless of the pressure. The final composition is very close to stoichiometric MgH2 except at the lowest temperatures and highest pressures investigated when the reaction becomes extremely slow or ceases completely at 80%–90% of the stoichiometric composition. Accurate determination of the relative molar enthalpy gives a value of −70 kJ (mol H2)−1. No hysteresis in the ordinary sense was observed, but although no change in the plateau pressure occurred desorption of from 5%–15% of the remaining hydrogen required almost zero pressure. The actual value appears to be dependent on the material.
167 citations
TL;DR: The chemical behavior of the actinides and lanthanide and actinide elements in aqueous media is compared for similarities and differences as mentioned in this paper, and the stability of oxidation states and the thermodynamic parameters of complexation are evaluated to reflect how the decreased shielding of the 5f orbitals relative to that of the 4f orbits causes the actinsides to have somewhat different properties from those of the lanthanides.
Abstract: The chemical behavior of the lanthanide and actinide elements in aqueous media is compared for similarities and differences. The stability of oxidation states and the thermodynamic parameters of complexation are evaluated to reflect how the decreased shielding of the 5f orbitals relative to that of the 4f orbitals causes the actinides to have somewhat different properties from those of the lanthanides. Although the interaction of the cations of both series with complexing ligands can be described satisfactorily by an ionic model, the freeenergy and enthalpy values of complexation of the trivalent actinides indicate a slightly greater covalency in bonding than that of the comparable lanthanide complexes. This is seen particularly in the metal-nitrogen-bonded systems.
155 citations
TL;DR: In this paper, the authors used a systematic elimination procedure to find three atomic property expressions which could be used to distinguish the crystal structures of AB intermetallic compounds and used them to predict the structure type of unknown AB phases with high accuracy.
Abstract: We used a systematic elimination procedure to find three atomic property expressions which could be used to distinguish the crystal structures of AB intermetallic compounds We started from 182 sets of tabulated physical properties and calculated atomic properties and combined them for the binary phases of A and B elements according to the modulus sums, differences, products and ratios The three expressions giving the best separation are the magnitude Δ(r s + r p ) Z AB = ¦(r s + r p ) Z A −(r s + r p ) Z B ¦ of the difference of Zunger's pseudopotential radii sums, the magnitude Δ MB AB = ¦X MB A −X MB B ¦ of the Martynov-Batsanov electronegativity difference and the sum ∑ZVEAB of the number of valence electrons We used these three axes in a three-dimensional structural stability diagram to separate 20 AB structure types representing 998 compounds into distinct regions with only 22 violations However, the diagram allows, as it must, an overlap of two (three) different structure types in regions that contain phases which have two (three) polymorphic structural forms The use of this diagram in conjunction with Miedema's model (for positive or negative ΔHfo) is a powerful tool for predicting the structure type of unknown AB phases with high accuracy
150 citations
TL;DR: In this article, a table of transition metal elements is constructed to show their dominant electronic features, and the actinide series is central to this new view, which is the most common transition metal element in the periodic table.
Abstract: A table of the transition metal elements is constructed to show their dominant electronic features. The strictly periodic table of the elements is misleading for the study of many useful metallic materials. The actinide series is central to this new view.
148 citations
TL;DR: In this paper, a review of qualitative and quantitative models that have been used to rationalize observed hydrogen atom site occupancies is critically reviewed, emphasizing the need for theoretical treatment leading to fundamental understanding of such systems.
Abstract: Numerous attempts have been made to explain the observed stabilities, stoichiometries and preferred hydrogen atom sites in hydrides of intermetallic compounds. The stabilities of some of these hydrides have been shown to correlate with cell volume or hole size or elastic properties or heat of formation for the intermetallic compound itself. It appears that each of these factors may influence the stability, but none plays the dominant role for all systems. The development of qualitative and quantitative models (based on the concept of imaginary binary hydrides) that have been used to rationalize observed hydrogen atom site occupancies is critically reviewed. The basis of these models is empirical and thus the successes reported for them may possibly be fortuitous. The example of LaNi5H6.5 is used to demonstrate a geometric model that uses only a minimum hole radius (0.40 A) and a minimum H-H distance (2.10 A) in the development of a rationale for the observed stoichiometries in hydrides of intermetallic compounds. This review emphasizes the need for theoretical treatment leading to fundamental understanding of such systems.
148 citations
TL;DR: In this paper, the intrinsic reaction rates for the hydriding and dehydriding of LaNi 5 were determined over a wide range of pressures in the temperature range 60-65 °C.
Abstract: The intrinsic reaction rates for the hydriding and dehydriding of LaNi 5 were determined over a wide range of pressures in the temperature range 60–65 °C. Approximately isothermal conditions were maintained by a new thermal ballast technique. The reaction rates measured are faster than those determined previously because previous measurements were heat flow limited. The data analysis takes into account the simultaneous operation in series of surface, diffusion and α-β interface intrinsic processes. It is concluded that the hydriding rate is under the mixed control of a surface process with an activation energy Q S / R of 1100 K and a bulk process with an activation energy Q B / R of 5480 K, a value similar to that for diffusion. The bulk process tends to become rate limiting at low temperatures and at high pressures. Dehydriding appears to be rate limited by a surface process with Q S / R = 0 K.
134 citations
TL;DR: In this paper, a model for calculating the coefficients of performance (COPs) is presented for a chemical heat pump which contains two metal hydrides and which consists of four reactors.
Abstract: A model for calculating the coefficients of performance (COPs) is presented for a chemical heat pump which contains two metal hydrides and which consists of four reactors Two reactors contain the first metal hydride MH 1 and the other two contain the other metal hydride MH 2 A concept of the sensible heat exchange between the reactors containing the same hydride is introduced into the model as one of the means of improving the COP The dependence of the COP on the various design parameters is discussed The results of the numerical calculation show that a sufficiently flat plateau, low reactor heat capacities and a highly efficient sensible heat exchange are essential to the achievement of a high COP
125 citations
TL;DR: The stoichiometric boron-to-carbon ratios of various carbon-rich Boron carbide phases were investigated in this paper, and the standard deviation of the results was evaluated.
Abstract: The stoichiometric boron-to-carbon ratios of various carbon-rich boron carbide phases were investigated. Many powders or sintered pellets were analysed and the total boron, total carbon, free-boron and free-carbon concentrations were measured for each specimen before calculating the stoichiometric ratio. The methods described were applied to a large number of analyses and the standard deviation of the results was evaluated. The stoichiometric boron-to-carbon ratio for carbon-rich boron carbide phases was found to depend on the synthesis conditions. Boron dissolves up to 21.6 at.% C at low temperatures and up to 24.3 at.% C near the melting point. Free carbon is deposited as the result of a solid state phase transformation. The stoichiometry of boron carbide prepared by the magnesothermal reaction is B 3.63 C (approximately B 11 C 3 ). Boron carbide produced by arc melting boron oxide and carbon is a solid solution. This single carbide melts congruently at a temperature of approximately 2450°C with a carbon content of 18.4 at.% (B 4.45 C).
TL;DR: In this paper, the results of investigations of the formation-decomposition kinetics and morphology of CeMg12 and Mg-10wt.%Ni alloys performed at pressures and temperatures where only the formation of the MgH2 hydride is reversible support the hypothesis that the high reactivity of such alloys with hydrogen is due, at least partially, to the effects of morphological parameters such as mechanical stresses and strains and incoherent grain boundaries which increase hydrogen diffusion and hydrides nucleation.
Abstract: The proposition that the apparent differences between the reactivities of magnesium and hydrogen are partially due to a lack of appreciation of the effects of various parameters on the kinetics is developed. The results of investigations of the formation-decomposition kinetics and morphology of CeMg12 and Mg-10wt.%Ni alloys performed at pressures and temperatures where only the formation of the MgH2 hydride is reversible support the hypothesis that the high reactivity of such alloys with hydrogen is due, at least partially, to the effects of morphological parameters such as mechanical stresses and strains and incoherent grain boundaries which increase hydrogen diffusion and hydride nucleation and which are produced by the formation of finely divided precipitates of CeH3 or Mg2Ni. The decrease in the hydrogen capacity on cycling is interpreted as a consequence of the removal of defects.
TL;DR: In this paper, the CrTe phase diagram was investigated in the composition range 30-100 at using differential thermal analysis and powder diffraction methods and six NiAs-related phases were characterized between 52.4 and 62 at.% Te.
Abstract: The CrTe phase diagram was investigated in the composition range 30–100 at.% Te using differential thermal analysis and powder diffraction methods. Six NiAs-related phases were characterized between 52.4 and 62 at.% Te. Two of them, hexagonal Cr 1− x Te (52.4–53.5 at.% Te) and monoclinic Cr 3 Te 4 -h (53.5–59.3 at.% Te), are high temperature phases. At lower temperatures monoclinic Cr 3 Te 4 -l (56.4–59.2 at.% Te) and trigonal Cr 2 Te 3 (59.5–60.0 at.% Te) are stable. Two phases, monoclinic Cr 5 Te 8 -m (59.6–61.5 at.% Te) and trigonal Cr 5 Te 8 -tr (61.5–62.0 at.% Te), were found on the tellurium-rich side of this phase system. At 75 at.% Te monoclinic CrTe 3 , a novel polytelluride, is formed in a peritectic reaction at 753 K. There are also indications of the formation of another tellurium-rich phase at about 70 at.% Te and 734 K.
TL;DR: In this article, an orthorhombic CeNi compound was shown to behave as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K.
Abstract: Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.
TL;DR: In this article, the interstitial hole radii, trigonal saddle point radii and intersite distances were calculated for the compounds LaNi 5 D 6.5, LaNi 4 AlD 4.8 and La Ni 4 MnD 5.9.
Abstract: Interstitial hole radii, trigonal saddle point radii and intersite distances were calculated for the compounds LaNi 5 D 6.5 , LaNi 4 AlD 4.8 and LaNi 4 MnD 5.9 in space group P 6/ mmm and for LaNi 5 D 6 in space group P 31 m . Two criteria, a minimum hole size of 0.4 A and a minimum H-H distance of 2.1 A, were adopted to allow rationalization of the observed interstitial occupancies and stoichiometries of each of these deuterides. The rationale includes consideration of diffusion mechanisms and their ramifications for site occupancy.
TL;DR: In this paper, the initial stages of the oxidation of polycrystalline tantalum and niobium in an oxygen atmosphere at low pressures (10−6 Pa ⩽ po2⩽ 10−3 Pa) and room temperature are characterized.
Abstract: Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to characterize the initial stages of the oxidation of polycrystalline tantalum and niobium in an oxygen atmosphere at low pressures (10−6 Pa ⩽ po2 ⩽ 10−3 Pa) androomtemperature. Theoxidationofbothmetalsproceedsinasimilarway: thegrowthoftheoxidelayersiscomparableandfollowsthesamekinetics. Twodistinctstagesareobserved. Atfirstrapidoxygenuptaketakesplaceuptoanoxygenexposureofabout 1.5L (1L = 10[su−6 Torr s = 1.32 × 10−4 Pa s) followed by a continuous decrease in the oxygen sorption rate until an oxygen exposure of 7–10 L is reached. This stage is characterized by the formation of an oxide film 2–3 monolayers thick (TaO and TaO2 on tantalum and NbO and NbO2 on niobium). At oxygen exposures above 10 L a second stage with a low reaction rate is observed which is characterized by the formation of Ta2O5 (Nb2O5) according to a logarithmic growth law until an oxygen exposure of 104 L is attained.
TL;DR: In this paper, the effects of pressure-induced absorption-desorption cycling on the degradation properties and the hydrogen purification ability of TiMn alloy hydrides were studied.
Abstract: The effects of pressure-induced absorption-desorption cycling on the degradation properties and the hydrogen purification ability of TiMn alloy hydrides were studied. The lines in the characteristic X-ray diffraction pattern were much broader and weaker in the cycled alloys but the C14 hexagonal structure was maintained and no second phase was observed. The hydrogen capacity was reduced by 30% for TiMn binary alloy and by 20% for TiMn multicomponent alloys after about 10 000 cycles in contrast with the marked reduction observed for LaNi5. In some cases slight recovery could be produced by heating at 500 °C for 1 h in a vacuum after cycling.
Gas chromatography measurements on hydrogen released from TiMn1.5 hydride showed that the hydrogen purity was better than 99.9999% (except for an H2O impurity) after a purge release of only a few per cent of hydrogen when the purity of the commercial grade hydrogen absorbed by the hydride was 99.99%. It can be concluded from the results of these measurements that hydrogen purification using TiMn alloy is related to two factors. The removal of N2, CO and CO2 proceeds by surface poisoning (i.e. oxidation, physisorption or chemisorption), whereas the removal of CH4, O2, H2O and Ar is controlled by surface poisoning and concentration. The hydrogen purification effect of TiMn alloy hydrides showed little degradation even after a large number of absorption-desorption cycles.
TL;DR: In this article, the structure and properties of amorphous metal hydrides, including hydrogen absorption-desorption characteristics, hydrogen-induced surface segregation, the static and dynamic environments around hydrogen atoms, the electronic density of states, positron annihilation and normal and superconducting properties, are reviewed.
Abstract: Current progress in investigations of the structure and properties of amorphous metal hydrides, including hydrogen absorption-desorption characteristics, hydrogen-induced surface segregation, the static and dynamic environments around hydrogen atoms, the electronic density of states, positron annihilation and normal and superconducting properties, is reviewed.
TL;DR: In this paper, the activation of FeTi was studied using transmission electron microscopy and it was concluded that the surface oxidation process proceeds mainly according to 2 FeTi + O 2 → Fe 2 Ti + TiO 2 + 2 Fe 2 O 2 + O O 2 ) → 2 Fe+TiO 2 or Fe+O 2 → 1 Fe + Ti O 2.
Abstract: The activation of FeTi was studied using transmission electron microscopy. Samples were annealed at different p ( H 2 ) and p ( O 2 ) levels including conditions typical of an activation treatment. Electron diffraction revealed the following surface phases: rutile TiO 2 , an ordered suboxide FeTiO x and Fe 2 Ti. No elemental iron was detected on the activated surfaces. It was concluded that the surface oxidation of FeTi proceeds mainly according to 2 FeTi + O 2 → Fe 2 Ti + TiO 2 . The elemental iron observed in magnetic measurements is presumably a product of rather severe oxidation and may be formed in side reactions such as Fe 2 Ti + O 2 → 2 Fe + TiO 2 or FeTi + O 2 → Fe + TiO 2 . A model with the following features is proposed for the activation of FeTi: 1. (1) catalytically active bare FeTiO x surfaces are formed by annealing FeTi in a vacuum or in hydrogen; 2. (2) additional catalytic activity is provided by oxide precipitates (TiO 2 , Ti n O 2 n −1 and Fe 7 Ti 1 O 3 ); 3. (3) a new catalytic surface area is repeatedly created by cracking. This model does not depend on the existence of catalytic iron clusters.
TL;DR: In this paper, the authors studied the hydrogen absorption-desorption characteristics of multicomponent alloys such as MmNi5−xAl(Mn)y−zMz and MnNi5 −xMnyM2.
Abstract: In order to reduce the large hysteresis effect in MmNi hydrides (Mm misch metal) we studied the hydrogen absorption-desorption characteristics of multicomponent alloys such as MmNi5−xAl(Mn)y−zMz and MmNi5−xAl(Mn)yM2 (M = Co, Cr, Cu, Nb, Ti, V, Zr; x = 0.3 − 0.5; y = 0.3 − 0.5; z = 0.05 − 0.1). The substitution or addition of the element M eliminates the large hysteresis effect that occurs during an absorption-desorption cycle. The hysteresis factor (equal to In( P a P d )) for MmNi4.7Al0.3M0.1 hydrides decreased for additives M in the following order: Zr > Co > Cr > Ti,V > Cu. Investigations of the effects of cycling showed that spalling occurred more rapidly for MmNi4.5Mn0.5 than for MmNi4.5Mn0.5Zr0.005 and MmNi4.5Mn0.5Zr0.1.
TL;DR: In this article, the authors investigated the kinetics of well-activated LaNi5 using a mass flowmeter under constant hydrogen pressure and showed that the overall rate is controlled by the rate of reaction at the interface which can be expressed as ======k ln(PPet=1 − (1−X)1/3
Abstract: The kinetics of well-activated LaNi5 were investigated using a mass flowmeter under constant hydrogen pressure. The overall rate is controlled by the rate of reaction at the interface which can be expressed as
k ln(PPet=1 − (1−X)1/3
where P is the hydrogen pressure and Pe is the equilibrium plateau pressure.
TL;DR: ReSi2 was found not to be isotypic with MoSi2 but to crystallize in space group Immm (no. 71) with the orthorhombic cell a = 3.128.
Abstract: ReSi2 was found not to be isotypic with MoSi2 but to crystallize in space group Immm (no. 71) with the orthorhombic cell a = 3.128(1) A, b = 3.144(1) A, c = 7.677 (2) A and Z = 2. ReSi2 is isopuntal (identical space group and site sets but different atomic coordination) with MoPt2 although the distortions of the MoSi2 type are very weak. Measurements of the electrical resistance and the optical reflectivity characterize ReSi2 as a semiconductor with an energy gap of about 0.2 eV. This property appears to require localized Re-Re bonds which, however, were not found.
TL;DR: In this article, the effects of impurities on the permeation characteristics of multicomponent palladium alloy membranes were investigated at comparatively high pressures and temperatures, and the effect of chemical reactions in the palladium diffuser and thermal cycling on permeability were also examined.
Abstract: The palladium alloy membrane method has been proposed as a means of purifying fusion reactor fuel. The applicability of this method was studied by investigating the effects of impurities on the permeation characteristics of multicomponent palladium alloy membranes. The experiments were performed at comparatively high pressures and temperatures.
The hydrogen permeability of commercial Pd-25wt.%(Ag-Au-Ru) was not affected by impurities such as NH3, CH4, CO, CO2 and N2 which are present in hydrogen at low concentrations (10–10000 ppm). However, the permeability deteriorated on contamination with oil vapour but could easily be recovered by baking in air followed by hydrogen reduction. Chemical reactions in the palladium diffuser and the effects of thermal cycling on permeability were also examined.
TL;DR: In this article, an experimental investigation aimed at improving the effective thermal conductivity of activated metal hydride is reported, where a three-dimensional structure of a porous aluminium composite has been used to enhance heat transmission.
Abstract: An experimental investigation aimed at improving the effective thermal conductivity of activated metal hydride is reported. Of the many material configurations which have been used to enhance heat transmission through metal hydride beds, a three-dimensional structure of a porous aluminium composite has shown the best results giving transmissions a factor of 9 or 10 greater than those of the unmodified hydride bed. The experimental results were used to derive equations for estimating the effective thermal conductivity as a function of the system pressure, the temperature, the hydride composition and the void fraction of the materials used.
TL;DR: The structure of MgAlB14 (space group, Imam; a = 5.848(1) A; b = 8.112(1), c = 10.312(1); Z = 4) was reinvestigated using single-crystal X-ray diffractometry as discussed by the authors.
Abstract: The structure of MgAlB14 (space group, Imam; a = 5.848(1) A; b = 8.112(1) A; c = 10.312(1) A; Z = 4) was reinvestigated using single-crystal X-ray diffractometry. The crystal was grown from a high temperature AlB melt containing a small quantity of magnesium. A total of 1277 unique reflections (Mo Kα radiation; 2θ < 100°) was collected and used in a block diagonal leastsquares refinement to a final R value of 0.023. The boron framework in the crystal is the same as that reported by Matkovich and Economy. However, the distribution of the metal atoms is significantly different, i.e. no aluminium atoms are present at the magnesium site which is split into two positions separated by 0.39 A. The occupancies of the metals are 78% and 75% for the magnesium and aluminium sites respectively. The chemical composition determined by the structure analysis is Mg0.78Al0.75B14. Re-examination of the structure using the previously published intensity data indicates that the reported crystal probably had the same chemical composition and metal distributions as the present crystal.
TL;DR: In this article, the crystal structures of 19 ternary compounds were investigated by X-ray powder diffraction and the alloys were of (RE, Y)NiSn and REPdSn (RE rare earth) equiatomic composition.
Abstract: The crystal structures of 19 ternary compounds were investigated by X-ray powder diffraction. The alloys were of (RE, Y)NiSn and REPdSn (RE rare earth) equiatomic composition. The TiNiSi-type structure (oP12) was found.
TL;DR: In this article, the chemical vapour deposition of boron nitride (BN) from BCl3-NH3-H2, BCl 3-NH 3-H 2 and BF 3 -NH 3 -H 2 mixtures was studied from a thermodynamic point of view.
Abstract: The chemical vapour deposition of boron nitride (BN) from BCl3-NH3-H2, BCl3-NH3 and BF3-NH3 mixtures was studied from a thermodynamic point of view. In the BCl3-NH3-H2 system, the deposit is either pure BN or a mixture of hexagonal BN and elemental boron depending on the [NH 3 ] [BCl 3 ] ratio and the amount of hydrogen used. When the initial composition does not contain free hydrogen (BC13-NH3 system), elemental boron is not present in the deposit. In the BF3-NH3 system, the deposit contains only pure BN but no deposit is formed at high temperatures. A substantial amount of BF3 remains unreacted at equilibrium, resulting in a low BN yield. When a carbon substrate is used, a chemical reaction may occur between the vapour phase and the carbon at high temperatures with the evolution of HCN, CH4 or C2H2. The most appropriate system for the chemical vapour infiltration (CVI) of hexagonal BN within the pores of a carbon composite preform appears to be BF3-NH3. Various types of C-C/BN composite materials were obtained by CVI at low temperatures and low pressures and were characterized.
TL;DR: In this article, the selective absorption of hydrogen by LaNi 5 and FeTi from gas mixtures containing nitrogen, CH 4, CO 2, CO or H 2 S impurities was investigated by cyclic absorption-desorption tests.
Abstract: The selective absorption of hydrogen by LaNi 5 and FeTi from gas mixtures containing nitrogen, CH 4 , CO 2 , CO or H 2 S impurities was investigated by cyclic absorption-desorption tests. It was found that CH 4 and nitrogen do not affect the rate of hydrogen absorption significantly, even in concentrations of up to 20 vol.%. CO 2 passivates LaNi 5 in concentrations up to 20 vol.%. However, it deactivates FeTi and heat treatment is necessary to reactivate the sample. CO, even in concentrations of less than 1 vol.%, deactivates both alloys completely. H 2 S causes strong passivation effects in LaNi 5 , although reactivation by heating to 700 K is possible, and poisons FeTi.
TL;DR: The new compounds APd2P2 (A Ca, Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) and Ce6Pd6P17 were prepared by reacting the constituent elements in silica tubes as mentioned in this paper.
Abstract: The new compounds APd2P2 (A Ca, Sr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb), La6Pd6P17 and Ce6Pd6P17 were prepared by reacting the constituent elements in silica tubes. Their Guinier powder patterns show that they crystallize with ThCr2Si2- and La6Ni6P17-type structures. Their lattice constants are reported. A comparison of the cell volumes suggests that europium and ytterbium have intermediate valence in their respective compounds.
TL;DR: In this article, the standard free energy of formation of PdO(s) was determined in the temperature range 699-1060 K using the galvanic cell Pt, Pd, pdO¦15 wt.
Abstract: The standard free energy of formation of PdO(s) was determined in the temperature range 699–1060 K using the galvanic cell Pt, Pd, PdO¦15 wt.% YSZ¦air ( P o 2 = 0.21 atm), Pt (YSZ yttria-stabilized zirconia). After correcting for the standard oxygen content in the air reference electrode, the standard free energy of formation of PdO(s) was calculated to be ΔG f °( PdO ( s )) = −112789.8 + 99.81 T ± 274 J mol −1 . This value is consistent with experimental values of the dissociation temperatures of PdO(s) in air and in pure oxygen determined from thermogravimetry measurements. Thus the validity of the expression obtained here can be extended to 1130 K which is the thermodynamic dissociation temperature of PdO(s) in pure oxygen. This free-energy expression is compared with other measurements reported in the literature. Combining the data for PdO(s) with the values for S 298 ° and C p of PdO(s) reported in the literature gives a value of −116.248 ± 0.41 kJ mol −1 for ΔH f °( PdO ( s ), 298.15K).