Göran Grimvall

Cohesive properties and vibrational entropy of 3d-transition metal carbides

Abstract We make an extensive study of cohesive properties such as the enthalpy of formation Δ0H and the standard vibrational entropy 0S at 298.15 K of metal carbides MeC, Me3C2 Me2C, Me7C3, Me5C2, Me3C and Me23C6, with Me being a 3d-transition metal, Sc, Ti, V, Cr, Mn, Fe, Co and Ni. Direct and indirect experimental information is used. For systems where experimental data are lacking, we rely on interpolation and extrapolation procedures, assuming that the bonding properties vary smoothly as a function of the average number of valence electrons per atom in the compound, ne,. In 0S the atomic masses enter in such a way that they can be taken into consideration exactly, and only an appropriate averaged interatomic force constant ks needs to be estimated. The systematic variations with ne were previously noted by us in 3rd-transition metal carbides and nitrides (Fernandez Guillermet and Grimvall, Phys. Rev. B40, 10,582 (1989)), and can be theoretically understood through an electron-band filling argument. That approach is considerably extended in the present work. Many of the compounds studied are metastable, i.e. the phase does not appear in the stable metal-carbon phase diagram. This fact is used as important indirect information which, together with our estimated entropy, gives limits to Δ0H of metastable carbides through CALPHAD calculations of phase diagrams. Our results are summarized in tables of the room-temperature enthalpy of formation Δ0H, the entropy Debye temperature θs, the corresponding average interatomic force constant ks and the standard vibrational entropy 0S(298.15). Δ0H is in good general agreement with predictions from Miedemas formula, with some small but significant differences. The general plcture of the cohesion in the carbides suggests important p−d bonding for Sc, Ti and V compounds, with a maximum strength for TiC, and weaker and more metallic bonding in Cr, Mn, Fe, Co and Ni carbides.

Temperature dependent effective masses of conduction electrons

Abstract The temperature dependence of the effective electron mass which appears in the heat capacity, cyclotron resonance and amplitude of de Haas-van Alphen effect is discussed. An accurate calculation is reported for sodium at low temperatures. An Einstein model gives the temperature dependence of the thermal massas a universal function of T θ E . This function is evaluated and plotted.

Bonding properties and vibrational entropy of transition metal MeB2(AlB2) diborides

Abstract The transition metal diborides MeB2 were chosen as an example to illustrate an approach introduced by us, by which existing thermodynamic information can be systematized and new information generated. We consider trends in the melting temperature, the enthalpy of formation and lattice vibration properties as the metal Me varies along the 3d, 4d and 5d transition metal series. The trends show a consistent picture which can be explained from the electronic density of states by a rigid-band like argument. This fact gives us confidence to apply the regularities and obtain new information by interpolation and extrapolation procedures. In particular, we estimate the standard entropy S⊖ at 298.15 K for stable AlB2 structure compounds where there are no previous measurements and for some metastable compounds (Me ≡ Sc, V, Cr, Mn, Y, Mo, Tc, WandRe). The results are compared with estimates based on various empirical methods. In the Me-B systems, the MeB2 phase may be stable at all temperatures, or be a high-temperature phase or a metastable phase. We explain the observed trend. Through the use of thermodynamic phase diagram calculations (the so-called CALPHAD method), we study the thermodynamic properties of the metastable AlB2 structure phase FeB2.

Thermal conductivity of cast iron: Models and analysis of experiments

Cast iron can be viewed as a composite material. We use effective medium and other theories for the overall conductivity of a composite, expressed in the conductivities, the volume fractions, and the morphology of the constituent phases, to model the thermal conductivity of grey and white cast iron and some iron alloys. The electronic and the vibrational contributions to the conductivities of the microconstituents (alloyed ferrite, cementite, pearlite, graphite) are discussed, with consideration of the various scattering mechanisms. Our model gives a good account of measured thermal conductivities at 300 K. It is easily extended to describe the thermal conductivity of other materials characterized by having several constituent phases of varying chemical composition.

Electrical resistivity of steels and face-centered-cubic iron

Literature data for the electrical resistivity of austenitic and ferritic steels are analyzed in a model that takes into account their composition and microstructure. The resistivity in these syste ...

Phase diagram and lattice instability in tungsten–rhenium alloys

Abstract Tungsten has been suggested as a material in applications where it is irradiated by neutrons and undergoes transmutation to rhenium. Pure W has the bcc lattice structure. According to the equilibrium phase diagram about 30 at.% Re can go into a bcc W–Re solid solution. However, ab initio electronic structure calculations show that the bcc lattice becomes dynamically unstable at high Re concentrations. Through a detailed calculation of the phonon spectrum, we find that bcc W1−cRec does not become dynamically unstable until c≳0.7, i.e., well above the equilibrium solubility limit of Re in bcc W. Concentration fluctuations of Re in irradiated W will therefore not lead to any significant number of regions where the bcc lattice collapses due to a dynamical instability.

Heat capacity of actinide dioxides

Abstract We analyse available experimental information on the vibrational heat capacity of ThO 2 , with particular emphasis on anharmonic effects, and on the similarities in the vibrational properties of actinide dioxides of the CaF 2 type structure. The result is then used in a discussion of the heat capacity of UO 2 . The contribution from crystal-field split 5f-electron states is calculated from published spectral data, which allows remaining nonvibrational and controversial contributions to the heat capacity to be separated out. An important aim of the paper is also to demonstrate a method of analysing high-temperature thermodynamic data.

Bonding and Debye temperatures in alkali-earth-metal halides

Abstract We consider 20 compounds of the composition AB 2 , where A = Be, Mg, Ca, Sr or Ba, and B = F, Cl, Br or I. From experimental data on their entropy we derive the logarithmic average ω (0) of the phonon frequencies in each solid. In this particular average, the atomic masses separate from the interatomic forces. We define a quantity E s , that has the dimension of energy and is directly related to the force-constant part of ω 2 (0). E s allows us to describe ω (0), or the equivalent Debye temperature θ s , to better than ± 4% (r.m.s. deviation). Our semi-empirical method is useful to accurately estimate unknown θ s in a class of chemically similar compounds, where θ s of some compounds are known. The connection to the Lindemann melting rule is briefly discussed.

Phase stability properties of transition metal diborides

We systematize available thermodynamic information on transition metal diborides, with emphasis on the enthalpy of formation, the vibrational entropy and the melting temperature. When experimental information is lacking, e.g., for metastable compounds, we obtain estimates through interpolation and extrapolation procedures introduced by us. Our results are used, i.a., to account for the change from stable to metastable transition metal diborides as one proceeds along a transition‐metal line in the Periodic Table. Our method of analysis combines available experimental thermodynamic and lattice–parameter data with CALPHAD calculations of phase diagrams and knowledge from solid state theory on electron states and lattice vibrations. An important point in the analysis is that in the high‐temperature vibrational entropy, atomic masses are separated from the interatomic forces. That allows us to introduce a quantity with the dimension of energy, which depends only on the electronic structure and shows a regular ...

Transverse conductivity in non-ideal fiber composite geometries

Abstract An ideal fiber composite is defined as one with parallel circular fibers forming a regular array. We consider how the thermal conductivity transverse to the overall fiber direction is affected by departures from the ideal geometry, such as irregular stacking pattern, misalignment of fibers and unequal or non-circular cross sections. Illuminating examples are discussed, quantitatively and qualitatively. In particular, we derive an effective-medium result for slightly misaligned fibers. It is found that for fiber concentrations not close to a percolation threshold, the effect of non-ideal geometry is normally too small (