Showing papers in "Journal of The Less Common Metals in 1981"

The influence of carbon on the microstructure and mechanical properties of sintered boron carbide

Abstract: It was found that boron carbide can be sintered pressureless to ultrafine-grained bodies with densities of more than 95% of the theoretical density of B 4 C, if small additions of carbon are made to boron carbide powders of submicron particle size and if the compacts are heated in inert atmospheres to temperatures near 2150 °C. In particular the carbon acts as an inhibitor of surface-to-surface matter transport so that densification via grain boundary and/or lattice diffusion is enhanced at lower sintering temperatures. The mechanical properties of this new sintered material are summarized and compared with those of the commercial hot-pressed product.

The enthalpy of formation of solid borides, carbides, nitrides, silicides and phosphides of transition and noble metals

Abstract: Enthalpies of formation for transition and noble metal borides, carbides, nitrides, suicides and phosphides are calculated according to the semiempirical model proposed by Miedema and coworkers and are compared with the available experimental data. On the whole there is fair agreement. A few compounds are exceptional since the structure-dependent contribution to the enthalpy of formation is relatively important here.

Analytical investigations in the BC system

Abstract: In this paper we study the phase composition of hot-pressed boron carbide specimens containing 0–60 at.% C with the help of various analytical techniques, namely chemical analysis of boron and carbon, chemical analysis combined with lattice parameter measurements, addition method adapted for quantitative X-ray phase analysis and electron microprobe analysis. The homogeneity range of rhombohedral boron carbide was determined. It was found that the composition range extends from B10.4C (8.8 at.% C) on the boron-rich side to stoichiometric B4.0C (20.0 at.% C) on the carbon-rich side.

The properties and structure of the boron carbide phase

Abstract: The specimens examined were completely characterized within the homogeneity range of the carbide phase (8.8–20.0 at.% C, i.e. B10.4CB4C). An X-ray study of the lattice dimensions together with accurate measurements of the density of the compounds examined allowed us to determine the number n of atoms contained in the unit cell. The value of n varies linearly with the carbon content between B12(CBC)0.666B1.333 (“B10.4C”; n = 15.33 atoms per unit cell) and B12−xCxC¦BxC1−xC) (“B10.4C”; 0

Kinetics and mechanism of the U-H reaction

Abstract: The kinetics of the hydrogenation reaction of uranium were studied over a wide range of pressures (10 2 –10 7 Pa) and temperatures (0–500 °C). The results are discussed in terms of two possible models: model 1, hydrogen diffusion through a protective product hydride layer; model 2, hydride growth at the hydride-metal interface. A two-dimensional ( i.e. pressure-temperature) grid of the experimental data was successfully fitted to the pressure-temperature rate relations derived from model 2. This model relates the microscopic distribution of hydrogen atoms in the metal to the phase transformation probability, from which the pressure-temperature dependence of the hydride growth rate is obtained. The model also qualitatively accounts for the negative apparent activation energy reported in the literature for the hydrogenation of the powdered metal.

The surface properties of TiC(001) and TiC(111) surfaces

Abstract: The surface properties of the TiC(001) and TiC(111) surfaces such as atomic structure, electronic structure and chemisorption were investigated using low energy electron diffraction, angle-resolved X-ray photoelectron spectroscopy, angle-resolved UV photoelectron spectroscopy and ionscattering spectroscopy. The TiC(001) surface has a 1 × 1 structure with a nearly stoichiometric chemical composition, and the chemisorption is very similar to that of platinum. In contrast, the topmost layer of the TiC(111) surface with a 1 × 1 structure consists of titanium atoms, and its chemisorption resembles that of titanium.

Thermal stability and magnetic properties of amorphous Zr1−xFex alloys

Abstract: Amorphous Zr 1− x Fe x alloys for concentrations in the range 0.2⩽ x ⩽ 0.9 were prepared by melt spinning. The crystallization behaviour of these alloys was studied using differential scanning calorimetry and X-ray diffraction. It is shown that the overall trend in the crystallization temperature ( T x increases with increasing x ) behaves in accordance with the hole model proposed earlier. It is shown that differences in nucleation and/or growth at different compositions mainly account for the deviations from a continuous increase in T x with increasing x . The crystallization of a number of alloys was studied using a variable heating rate and the results were analysed by Kissinger's method. In several iron-rich alloys we determined the Curie temperature and the saturation magnetization. The results were compared with literature data and with data obtained previously on vapour-quenched Zr-Fe alloys. From these results it is concluded that there are marked differences between the magnetic properties of liquid-quenched alloys and those of vapour-quenched alloys.

Relationships between intermetallic compound structure and hydride formation

Abstract: The structures of four families of intermetallic compounds are analyzed in terms of the number, sizes, positions and symmetry properties of tetrahedral interstitial holes. The structure types considered are hexagonal AB5 (D2d), cubic AB2 (C15), hexagonal AB2 (C14) and orthorhombic AB (Bf). Holes and clusters of holes are considered in terms of their suitability for hydrogen occupancy in the formation of intermetallic compound hydrides. In the cases of the hydrides LaNi5H6 and LaCo5H4, the parent hexagonal structure P6/mmm is changed on hydriding to trigonal P31m in the former case and to orthorhombic Cmmm in the latter case. The symmetry changes that correspond to these transformations are shown to be directly related to the amount of hydrogen absorbed; i.e. some potential sites in the parent compound are retained and others are not, depending on the structural change. In those cases where structural data for the hydride phases are not available or where the parent structure does not change on hydriding, it is shown that reasonable estimates of the extent of hydrogen absorption can be made based on the number and the sizes of available holes or hole clusters in the parent compound.

The Zr-Fe phase diagram in the range 20–40 at.% Fe and the crystalline structure of the intermetallic compound Zr3Fe

Abstract: Part of the Zr-Fe phase diagram between the compounds Zr3Fe and Zr2Fe was determined using X-ray, microstructural and thermal analyses The eutectoid equilibrium Zr2Fe Zr3Fe + ZrFe2 was found at about 775 ° C and a peritectoid equilibrium β + Zr2Fe Zr3Fe was determined at about 885 °C Differential thermal analysis was used to determine the temperatures 1 (1) of the peritectic equilibrium L + ZrFe2 Zr2Fe (974 °C) 2 (2) of the eutectoid equilibrium β α + Zr3Fe (830 °C) The crystallographic characteristics of the Zr3Fe compound were determined The crystal system is orthorhombic with unit cell parameters a = 8819 ± 0001 A , b = 3329 ± 0004 A and c = 10952 ± 0002 A , and the number of formula units in the unit cell is 4 The space group is A2122 and the powder pattern was indexed

Determination of the boron-rich side of the BSi phase diagram☆

Abstract: Part of the BSi phase diagram was determined on the boron-rich side. Results from thermal analysis and vaporization studies on alloys solidified under equilibrium or non-equilibrium conditions are presented. The variation in lattice parameters with composition (determined from X-ray powder diffraction patterns) indicates the existence of a solid solution of silicon in β boron and an SiBn phase. The presence of the SiBn phase is supported by measurements of thermodynamic properties of the alloys. The activity of silicon gives the range of n as 14–40 at 1700 K and 15–40 at 1800 K.

Zusammenhang zwischen strukturellen und thermo-dynamischen eigenschaften bei phasen der kupfer-familie in T10-B4-systemen

Abstract: Zusammenfassung Integrale Atomvolumina nickel-, palladium- und platinreicher Phasen mit Elementen der vierten Untergruppe wurden rontgenographisch ermittelt. Die davon hergeleiteten Atomvolumina der metallischen B 4 -Elemente fur grosse Verdunnung in fester T 10 -Matrix sind vom benutzten Losungsmittel unabhangig. Die partiellen Atomvolumina der halbmetallischen Elemente Silicium und Germanium sind jedoch vom Losungsmittel abhangig. Zwischen der Differenz der partiellen molaren Atomvolumina der T 10 - und B 4 -Elemente und der partiellen molaren Mischungsenthalpie fur grosse Verdunnung existieren zwei unterschiedliche Korrelationen, die der Aufteilung der B 4 -Elemente in Metalle und Halbmetalle entsprechen. Diese Korrelationen werden analysiert.

RGa6 (R≡ rare earth atom), a common intermetallic compound of the R-Ga systems

Abstract: A number of new RGa 6 compounds (where R is a rare earth atom) with the tetragonal PuGa 6 structure are reported. The lattice parameters of these compounds decrease on replacing the rare earth atoms in the order of increasing atomic number. The RGa 6 compounds are formed by a peritectic reaction at temperatures which decrease in the order mentioned. The thermal instability of these compounds as expressed by the decrease in peritectic temperature is discussed. Also the failure to detect these intermetallic compounds by other investigators is explained.

Phase relations and diffusion paths in the Ti-Ni-Co system at 900 °C

Abstract: We investigated the 900 °C cross section through the ternary phase diagram Ti-Ni-Fe by means of the diffusion couple technique and by the investigation of equilibrated alloys The course of a number of diffusion paths was established Use was made of optical, microprobe and X-ray analyses Some hitherto unaccountable results mentioned in the literature can now be explained

Electrical conductivity of Manganese-Magnesium ferrites

Abstract: The electrical conductivity of Mn-Mg ferrites of various compositions was studied as a function of composition and of temperature. Based on the Seebeck coefficients observed the mixed ferrites were divided into two groups. The mixed ferrites of group I have a slight excess of cations and show n-type conduction, whilst those of group II have a cation deficiency and show p-type conduction. The mechanism of the electrical conduction is explained in terms of the oxidation of Fe2+ by Mn3+ on octahedral sites. Plots of conductivity (log (σT)) versus temperature ( 10 3 T ) are almost linear; they show a transition near the Curie temperature and a change in slope of the line with further increase in temperature.

The thermodynamics of the LaNi5-H2 system by differential heat flow calorimetry I: Techniques; the α + β two-phase region

Abstract: The use of a heat flow calorimeter with the differential twin cell configuration to study intermetallic compound-hydrogen reactions is described. The techniques of calibration and operation of both the gas titration system and the calorimeter, which are required to obtain high precision enthalpies, are described and discussed. Enthalpies for the reaction in the two-phase region of the LaNi5-H2 system are independent of the overall composition but exhibit a small hysteresis with ΔHabs = −32.30 ± 0.07 kJ (mol H2)−1ΔHdes = 31.83 ± 0.09 kJ (mol H2)−1 where ΔHabs is the enthalpy of absorption, ΔHdes is the enthalpy of desorption and the errors are the standard deviations. The value of ΔHdes is the best value for the enthalpy of reaction between equilibrium phases with a total probable error of ±0.5 kJ (mol H2)−1.

Electronic structure and chemical bonding in nonstoichiometric compounds of refractory transition metals of the IVa and Va subgroups

Abstract: A study of the electronic structure and chemical bonding in a variety of MeXx non-stoichiometric compounds (Me ≡ Ti, V, Zr, Nb; X ≡ C, N, O) as a function of the concentration of metalloid vacancies is reported in this work. Semiempirical molecular orbital linear combination of atomic orbitals and X-ray emission spectroscopy methods were used. The results obtained are compared with electron spectroscopy for chemical analysis measurements of the energy of inner levels, band calculations and a number of physicochemical properties of MeXx.

Crystal structure of magnesium heptaboride Mg2B14

Abstract: The crystal structure of a magnesium heptaboride, whose structural formula is Mg2B14, was determined from X-ray powder patterns according to a profile refinement technique and on the basis of isotypism with MgAlB14. The symmetry is orthorhombic and the lattice parameters are a = 5.970 A , b = 8.125 A and c = 10.480 A . The refinement is based on the space group Imam. The unit cell contains five non-equivalent boron and two nonequivalent magnesium atoms. The structure consists of chains of B12 icosahedra extending in the direction of the c axis. The chains are linked laterally via inter-icosahedral bonds or BB bonds involving nonicosahedral atoms, thus forming a three-dimensional boron network. The magnesium atoms occupy two types of holes in this network, corresponding to coordination numbers 12 and 16.

Neue ternäre erdalkali-übergangselement-pnictide

Abstract: Zusammenfassung Die neu dargestellten Verbindungen BaZnSb2, BaCdSb2, BaZnBi2, SrCdBi2 und BaCdBi2 kristallisieren tetragonal in der SrZnBi2-Struktur (Raumgruppe, I4/mmm) mit folgenden Gitterkonstanten: BaZnSb2a = 458,4 ± 0,2 pm, c = 2305 ± 2 pm, c a = 5,03 BaCdSb2a = 455,8 ± 0,5 pm, c = 2416 ± 3 pm, c a = 5,30 BaZnBi2a = 484,6 ± 0,1 pm, c = 2198 ± 1 pm, c a = 4,54 SrCdBi2a = 463,5 ± 0,5 pm, c = 2288 ± 3 pm, c a = 4,94 BaCdBi2a = 476,8 ± 0,2 pm, c = 2360 ± 2 pm, c a = 4,95 CaMnSb2 und SrMnSb2 hingegen kristallisieren orthorhombisch (Raumgruppe Pnma) in der SrZnSb2-Struktur mit folgenden Gitterkonstanten: CaMnSb2a = 2211 ± 1 pm, b = 431,2 ± 0,1 pm, c = 434,4 ± 0,1 pm SrMnSb2a = 2319 ± 3 pm, b = 442 ± 1 pm, c = 446 ± 1 pm

Crystal structure and physical properties of new ternary rare earth borides

Abstract: Two ternary boride systems with a new type of structure are reported for MRh 3 B 2 (M ≡ Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) and MIr 3 B 2 (M ≡ Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y). These materials were found to crystallize in a base-centered monoclinic structure with space group C 2/ m and two formula units per unit cell. The proposed atomic positions were determined by a structural study of the ErIr 3 B 2 compound. The magnetic and superconducting properties are discussed by comparisons with those of other related binary and ternary compounds.

Das Zustandsdiagramm Sr-Sn und die verbindung Sr3SnO

Abstract: Zusammenfassung Das Sr-Sn-Zustandsdiagramm wurde vervollstandigt und die kongruent schmelzenden Verbindungen Sr 2 Sn, Sr 5 Sn 3 , SrSn und SrSn 3 dabei bestatigt. Die bisher unbekannte Verbindung Sr 3 SnO kristallisiert kubisch mit a = 512 ± 1 pm in der Perowskitstruktur.

A contribution to the crystal chemistry of ternary 1:1:1 alloys: RAgSn and RCuTl compounds (R ≡ rare earth)

Abstract: The ternary alloys of the rare earths with tin and silver and of the rare earths with copper and thallium were studied for the stoichiometric ratio 1:1:1. The structures of these alloys are of the hP6 CaIn 2 type. The trend in the average atomic volumes of these phases is compared with the trend in other series of isostructural compounds, and the general alloying behaviour of the rare earths in the ternary compounds is briefly summarized.

Electrical conductivity of lithium-nickel ferrites

Abstract: The electrical conductivity of Li-Ni ferrites was studied as a function of composition and of temperature. X-ray analysis and Seebeck coefficient studies were also carried out so that the conductivity of these mixed ferrites could be discussed. The dependence of the electrical conductivity of Li-Ni ferrites on composition is explained on the basis of the cation distribution. All the ferrites exhibit a transition near the Curie temperature with a change in slope of the conductivity (log(σT)) versus temperature (103T) curve. It is found that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region for all the ferrites.

On n-type β rhombohedral boron

Abstract: By doping β rhombohedral boron with iron the interstitial solid solution FeB 29.5 was obtained. Investigations of the electronic transport properties between 100 and 700 K have for the first time proved that n-type β rhombohedral boron can be obtained by defined doping. Thus it was shown that a deliberate reversal of the conductivity character of β rhombohedral boron can be produced; this is a decisive prerequisite for applications within semiconductor technology.

Neutron diffraction studies of TbB4 and ErB4

Abstract: Neutron diffraction data are presented on the rare earth tetraborides TbB 4 and ErB 4 which crystallize with the tetragonal space group P4 mbm . Positional and thermal parameters were refined by full matrix least-squares analysis from single-crystal measurements to agreement values R of 0.018 and 0.030 respectively. Collinear antiferromagnetic ordering was derived from the low temperature powder data ( T N = 43 K for TbB 4 and T N = 13 K for ErB 4 ). The magnetic structures differ in their absolute moment orientations, which are perpendicular to the tetragonal c axis for TbB 4 and along the c axis for ErB 4 .

Preparation and characterization of thin protective films in silica tubes by thermal decomposition of hexachloroborazine

Abstract: The thermal decomposition of molecules containing boron and nitrogen gives coatings with a composition close to BN and chemically inert towards halogens and halides. The decomposition at 900–950 °C of the cyclic hexachloroborazine (approximate formula B 3 N 3 Cl 6 ) was studied. The coatings were characterized using X-ray diffraction, mass and IR spectroscopy, scanning electron microscopy, transmission electron microscopy, secondary ion mass spectrometry and electron microprobe analysis (energydispersive X-ray analysis). The use of coated silica tubes for chemical vapour transport of TiB 2 was examined.

High pressure X-ray diffraction of thorium to 30 GPa

Abstract: The lattice parameter variation of thorium metal was measured up to 30 GPa in a diamond anvil cell with an energy-dispersive X-ray diffraction method. A new experimental set-up containing a conical slit for the diffracted radiation was used. The pressure was determined from the lattice parameter of NaCl using Decker's equation of state. Thorium conserved its f.c.c. structure up to 30 GPa. Fitting the results to a second-order Birch equation yielded a bulk modulus B 0 of 58 GPa and a pressure derivative B ' 0 of 4.2.

Lithium hydroxide halides: phase equilibria and ionic conductivities

Abstract: The phase equilibria of the quasi-binary systems LiOH-LiI and LiOH-LiBr were determined. The intermediate phases are Li2(OH)I, Li5(OH)4I, Li2(OH)Br and Li4(OH)3Br. All these compounds melt peritectically, at 179 °C, 303 °C, 245 °C and 291 °C respectively. The crystal structure of Li2(OH)Br is primitive cubic ( a = 4.056 A ) of the anti-perovskite type (space group Pm3m) with structural disorder; one-third of the lithium sublattice sites are vacant. The ionic conductivities σ of Li2(OH)I and Li5(OH)4I are 3 × 10 −6 Ω −1 cm −1 and 2 × 10 −5 Ω −1 cm −1 at 150 °C with activation enthalpies for σT of 1.24 eV and 0.52 eV respectively. The conductivities of Li2(OH)Cl and Li5(OH)3Cl2 are 3 × 10 −5 Ω −1 cm −1 and 7.5 × 10 −4 Ω −1 cm −1 respectively at 200 °C with the same activation enthalpy of 0.97 eV. The ionic conductivities of the lithium hydroxide bromides are about 5 × 10 −5 Ω −1 at 200 °C. The partial electronic conductivities are at least two orders of magnitude lower for all the lithium hydroxide halides.