Free volume conduction and magnetic solitons in polystyrene composites containing transition metals

Films (200 to 600m thick) of polystyrene composites, containing nickel and cobalt metallic powders, were prepared by a casting method. The micrographical analysis revealed a morphological deformation in the polymeric matrix due to the metallic intrusions. No change in the main transition temperatures was observed by differential scanning calorimetry. The d.c. electrical resistivity and the magnetic susceptibility were measured in the temperature range 300 to 470 K. The electrical resistivity results were interpreted on the basis of the free volume model with protonic charge carriers. The magnetic results revealed a magnetic role of the polymeric matrix due to the collective states of the -electrons (magnetic solitons) in the presence of the ferromagnetic metallic clusters. An s-d type magnetic interaction was suggested between the ferromagnetic particles and the solitons in the close vicinity of the particle surface.     

A new spd tight-binding model of magnetism in transition metals

We present the extension of an efficient spd tight-binding model to the case of spin polarized materials by including electron–electron interactions, in an extended Hubbard–Hartree–Fock formalism. It is applied with success to rhodium and palladium clusters and slabs.     

Some topochemical reactions based on the mox lamellar oxychlorides of transition metals

Intercalation of molecules and alkali metals are possible in lamellar transition metal oxyhalides. Intercalations in such lattice depend on different factors, which are the rigidity of the lattice and the stability of the +3 oxidation state of the metal. This structure leads also to irreversible topochemical reactions involving the substitution of the chlorine atoms by organic molecules possessing one or two nucleophilic groups. In the first case simple grafting reactions occur giving new layers compounds as FeOOCH₃ or FeOOCOCH₃ by example. In the second one, if we choose rigid organic molecules we realize double grafting or pillaring reactions KRK + 2FeOCl → 2KCl + FeOROFe. A new and interesting type is based on the following process 2FeOCl + K₂Fe(CO)₄ → 2KCl + (FeO)₂Fe(CO)₄. Chlorine layers are removed as potassium halide and simultaneously tetrahedral anionic groups are added.     

Optical properties of oxide glasses containing transition metals: Case of titanium- and chromium-containing glasses

One of the major driving forces for the development of new glasses is the demand for high optical non-linearity with reduced cost and a higher damage resistance. Oxide glasses with large non-linear refractive index and non-linear absorption coefficient are promising materials for fiber telecommunication and for non-linear optical devices such as ultrafast optical switches, power limiters, real time holography, self-focusing, white-light continuum generation and photonic applications. To get insight into the optical absorption in amorphous materials, studies are still needed for revealing the nature of photoelectronic excitations in these materials by comparison with that in crystals which have been understood firmly based on band theory. Although the IR absorption loss in oxide glasses is larger than of fluorides, low light scattering loss is expected in these oxide glasses because they have lower glass transition temperature. In addition, small concentration of dopant such as alkaline metal and alkaline earth metal elements gives rise to the structural relaxation of the frozen-in density fluctuations even below glass transition temperature T_g, adding to the reduction of T_g as well. A review of the fundamentals and recent research advances in optical properties of oxide glasses containing chromium or titanium is presented.     

Hydrogen absorption by transition metals

Hydrogen absorption by metals is analyzed. An equation is derived according to which the characteristics playing a key role in determining the nature and amount of gas absorption are the negative enthalpy and the Fermi-level density of states in the electronic spectrum of the metal.     

Properties of interfaces between iron-group metals (Fe, Co, Ni) and HfC via first-principles modeling

Metal carbide ceramics offer potential as protective coatings for iron-group metals M (Fe, Co, and Ni) because of its favorable physical, chemical, and mechanical properties. We present a computational study to evaluate the adhesion of hafnium carbide coatings on iron-group metals substrates based on density functional theory. We consider the Fe(110)/HfC(100), Co(111)/HfC(100), and Ni(111)/HfC(100) interfaces and focus on the magnetism, adhesion, and bonding of C-site and Hf site interfaces. The C-site interfaces have the largest adhesion, forming polar covalent/ionic mixed bonds. The calculations reveal that the work of adhesion increases on passing from iron to nickel (Fe < Co < Ni). Our results suggest that the presence of a metallic acceptor and a carbide donor insures good adhesion between the two components.     

Formation of new carbides with diamond structure from carbon film containing transition metal

New carbide crystallites, which have a solid-solution phase with diamond structure, were formed from amorphous carbon film containing a transition metal such as Fe, Co, Mo or Ti by vacuum heating at 500-800 °C. The lattice constants for each solid-solution phase have been determined from electron diffraction patterns and high-resolution transmission electron microscope images. The formation of carbon polymorphs has been summarized as being dependent on the heat treatment temperature.     

Carbides of group VA transition metals

Abstract

Estimates are made of the cohesive energies of the group VA transition metal carbides for various C contents and crystal structures. In the cubic structure MC_x, several factors help to provide optimal cohesion at x < 1, even in the presence of excess C. Notable among these is a drop in the Fermi level when vacancies appear in the C sublattice. A strong C–C repulsion between very close neighbours prevents the hexagonal structure becoming stable when x is large, even though the underlying metal sub lattice is expected to prefer hcp to fcc at these electron concentrations. In M₂ C the C–C repulsive contribution is sufficiently reduced to allow the hexagonal structure to appear. The opportunity for afurther reduction in this repulsive contribution, together with a change in electrostatic energy, provides a strong tendency for ordering in the C sub lattice of M₂ C, leading to high ordering temperatures.

MST/3031     

Atomic layer deposition of transition metals

Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapours of two chemical reactants. ALD processes have been successfully demonstrated for many metal compounds, but for only very few pure metals. Here we demonstrate processes for the ALD of transition metals including copper, cobalt, iron and nickel. Homoleptic N,N'-dialkylacetamidinato metal compounds and molecular hydrogen gas were used as the reactants. Their surface reactions were found to be complementary and self-limiting, thus providing highly uniform thicknesses and conformal coating of long, narrow holes. We propose that these ALD layers grow by a hydrogenation mechanism that should also operate during the ALD of many other metals. The use of water vapour in place of hydrogen gas gives highly uniform, conformal films of metal oxides, including lanthanum oxide. These processes should permit the improved production of many devices for which the ALD process has previously not been applicable.     

Some kinetic laws and the mechanism of carbonization in the high-temperature synthesis of carbides of transition metals

Results of investigations of the kinetic laws and the mechanism of carbonizing Ti, Zr, Nb, Ta, Mo, and W over a wide temperature range and for different schemes of metal-carbon interaction (Me+carbon-containing gas, Me+graphite) are given. Much attention has also been given to establishing the interrelation between the microstructure of carbide phases and the kinetics and mechanism of their formation.Institute of Chemical Physics, Academy of Sciences of the Republic of Armenia, Erevan. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 423–427, October, 1993.     

Chemical interaction of fluoropolymers with transition metals

Chemical interaction of transition metals (Mo, W, Ta, Nb, and Ti) with a tetrafluoroethylene-vinylidene fluoride (TFE-VDF) copolymer (21 mol % TFE + 79 mol % VDF) has been studied by differential scanning calorimetry (DSC) and mass spectrometry. The DSC curves of mixtures of the fluoropolymer with Ta, Nb, and Ti showed exothermic peaks, and those of composites with W and Mo showed endothermic peaks. Mass spectrometric analysis indicated that the fluoropolymer reacted with the transition metals to form the higher fluorides TaF₅, NbF₅, TiF₄, WF₆, and MoF₆. In addition, WOF₄ and MoOF₄ molecules were detected in the case of tungsten and molybdenum. At temperatures above 700 K, the mass spectra of all the systems showed ions corresponding to low-molecular hydrocarbon molecules.     

A critical transition state in liquid metals

Temperature dependence of viscosities of liquid Sn, Cu, Pb_38.1Sn_61.9, Pb_44.8Bi_55.2, and Cu_0.7Sn_99.3 has been studied by a torsional oscillation viscometer. During the measurement process, a deviation from the Arrhenius behavior is found when the temperature is lowered to about 60 K higher than the melting point. The abnormal increasing of the viscosity and the activation energy indicates the changes of the elementary melt structure during cooling towards the melting point. The deviation region from Arrhenius viscosity is defined as the transition state. The differential scanning calorimeter experiments also show the existence of the critical transition state in liquid metals near the melting point. The drastic increase of the viscosity in the transition state can be regarded as continuum transition and be presented well by the amended Arrhenius equation which is based on the critical phenomenon.     

Local modes of interstitial atoms in transition metals

Local modes of the C, N, O and H interstitial atoms were measured in many transition metal solid solutions by inelastic neutron scattering. The metal-interstitial force constants were estimated from the experimental data and are graphically presented as functions of metal-interstitial distances. A correlation of these functions with the data, predicted by the embedded atom theory, is revealed and discussed.     

Interactions between transition metals and defective carbon nanotubes

Interactions between 3d transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) and (5,5) carbon nanotube (CNT) with a vacancy defect are quantitatively characterized using first-principles calculations. The binding energies between CNT and transition metals are found to be significantly enhanced when vacancy defects are introduced into the CNT. For the defective CNTs doped with Sc, Cr and Zn atoms, the structures of defective CNTs are found to be intact. The doping of Ti, Mn, Cu, Fe, Ni and Co alternates the structures of defective CNTs. Among all 3d transition metals, only the ferromagnetic metal atoms Fe, Co and Ni form bonds with carbon atoms of CNT, suggesting the important role of magnetic exchange interaction in the p–d hybridisation between carbons and transition-metal atoms. The results also indicate that the 3d transition-metal atoms acting as substitutional defects can substantially modify the electronic structure of CNT. It is suggested that these stable CNT-metal systems could become promising engineering materials in many fields such as CNT devices for various spintronics applications and CNT metal–matrix composites.     

Stability of hydrogen and nitrogen in group VIa metals (Cr,Mo, W) and iron-group metals (Fe,Co, Ni) evaluated by statistical thermodynamics

Solubility data of hydrogen and nitrogen in group VIa metals (Cr, Mo, W) and iron-group metals (Fe, Co, Ni) were analysed on the basis of statistical thermodynamics. The stabilities of hydrogen in iron-group metals were estimated to be comparable to one another and the same appeared to be true for nitrogen solutions. On the other hand, stabilities of nitrogen in group VIa metals worsened appreciably as the number of the period increased. The same trend seemed to be valid for hydrogen solution in group VIa metals but with a smaller extent of worsening stability.