905 research outputs found
Debye temperature of disordered bcc-Fe-Cr alloys
Debye temperature, TD, of Fe100-xCrx disordered alloys with 0<x<99.9 was
determined from the temperature dependence of the centre shift of 57Fe
Mossbauer spectra recorded in the temperature range of 80-300K. Its
compositional dependence shows an interesting non-monotonous behaviour. For
0<x<~45 as well as for ~75<x<~95 the Debye temperature is enhanced relative to
its value of a metallic iron, and at x=~3 there is a local maximum having a
relative height of ~12% compared to a pure iron. For ~45~95
the Debye temperature is smaller than the one for the metallic iron, with a
local minimum at x=~55 at which the relative decrease of TD amounts to ~12%.
The first maximum coincides quite well with that found for the spin-waves
stiffness coefficient, D0, while the pretty steep decrease observed for x>~95
which is indicative of a decoupling of the probe Fe atoms from the underlying
chromium matrix is likely related to the spin-density waves which constitute
the magnetic structure of chromium in that interval of composition. The
harmonic force constant calculated from the Debye temperature of the least
Fe-concentrated alloy (x>99.9) amounts to only 23% of the one characteristic of
a pure chromium.Comment: 15 pages, 7 figures, 26 reference
Magnetic ordering above room temperature in the sigma-phase of Fe66V34
Magnetic properties of four sigma-phase Fe_(100-x)V_x samples with
34.4<x<55.1 were investigated by Mossbauer spectroscopy and magnetic
measurements in the temperature interval 5-300 K. Four magnetic quantities viz.
hyperfine field, Curie temperature, magnetic moment and susceptibility were
determined. The sample containing 34.4 at% V was revealed to exhibit the
largest values found up to now for the sigma-phase for average hyperfine field,
B = 12.1 T, average magnetic moment per Fe atom, m = 0.89 mB, and Curie
temperature, TC = 315.5 K. The quantities were shown to be strongly correlated
with each other. In particular, TC is linearly correlated with m with a slope
of 406.5 K/mB, as well as B is so correlated with m yielding 14.3 T/mB for the
hyperfine coupling constant.Comment: 15 pages, 11 figures, 1 tabl
Antiferromagnetic correlations in Fe-Cu granular alloys: the role of the surface structure
Fe precipitates in a Cufcc matrix, prepared using the Bridgeman method and with an average composition of Cu97Fe3, displayed the coexistence of ferromagnetism ~FM!, spin glass-like ~SGL! behavior and antiferromagnetic ~AFM! correlations. The two former contributions may be attributed, respectively, to the segregation of FM, a-Febcc precipitates and to the few Fe spins distributed in the matrix. The annealing procedures increased the FM contribution and, as particle growth and phase segregation took place, the SGL behavior progressively disappeared. Results from high resolution transmission electron microscopy ~HRTEM!, x-ray photoelectron spectroscopy ~XPS!, and electron energy-loss spectroscopy ~EELS! suggest that the AFM correlations are due to the a-Fe particles that show a surface layer of a few nanometers in thickness, of either FeO and/or g-Fefcc . XPS and EELS measurements confirm the presence of FeO; however, the latter is only tentatively suggested by the HRTEM analysis of the particle/matrix interfaces
Modeling the series of (n x 2) Si-rich reconstructions of beta-SiC(001): a prospective atomic wire?
We perform ab initio plane wave supercell density functional calculations on
three candidate models of the (3 x 2) reconstruction of the beta-SiC(001)
surface. We find that the two-adlayer asymmetric-dimer model (TAADM) is
unambiguously favored for all reasonable values of Si chemical potential. We
then use structures derived from the TAADM parent to model the silicon lines
that are observed when the (3 x 2) reconstruction is annealed (the (n x 2)
series of reconstructions), using a tight-binding method. We find that as we
increase n, and so separate the lines, a structural transition occurs in which
the top addimer of the line flattens. We also find that associated with the
separation of the lines is a large decrease in the HOMO-LUMO gap, and that the
HOMO state becomes quasi-one-dimensional. These properties are qualititatively
and quantitatively different from the electronic properties of the original (3
x 2) reconstruction.Comment: 22 pages, including 6 EPS figure
Assessment techniques, database design and software facilities for thermodynamics and diffusion
The purpose of this article is to give a set of recommendations to producers of assessed thermodynamic data, who may be involved in either the critical evaluation of limited chemical systems or the creation and dissemination of larger thermodynamic databases. Also, it is hoped that reviewers and editors of scientific publications in this field will find some of the information useful. Good practice in the assessment process is essential, particularly as datasets from many different sources may be combined together into a single database. With this in mind, we highlight some problems that can arise during the assessment process and we propose a quality assurance procedure. It is worth mentioning at this point, that the provision of reliable assessed thermodynamic data relies heavily on the availability of high quality experimental information. The different software packages for thermodynamics and diffusion are described here only briefly
New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al and HTSC oxides
A new DTA (Differential Thermal Analysis) device was designed and installed
in a Hot Isostatic Pressure (HIP) furnace in order to perform high-pressure
thermodynamic investigations up to 2 kbar and 1200C. Thermal analysis can be
carried out in inert or oxidising atmosphere up to p(O2) = 400 bar. The
calibration of the DTA apparatus under pressure was successfully performed
using the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard
calibration references. The thermal properties of these metals have been
studied under pressure. The values of DV (volume variation between liquid and
solid at Tm), ROsm (density of the solid at Tm) and ALPHAm (linear thermal
expansion coefficient at Tm) have been extracted. A very good agreement was
found with the existing literature and new data were added. This HP-DTA
apparatus is very useful for studying the thermodynamics of those systems where
one or more volatile elements are present, such as high TC superconducting
oxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 kbar
under reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading
to the formation of the 2223 phase was found to occur at higher temperatures
when applying pressure: the reaction DTA peak shifted by 49C at 2 kbar compared
to the reaction at 1 bar. This temperature shift is due to the higher stability
of the Pb-rich precursor phases under pressure, as the high isostatic pressure
prevents Pb from evaporating.Comment: 6 figures, 3 tables, Thermodynamics, Thermal property, Bi-2223,
fundamental valu
Surface induced disorder in body-centered cubic alloys
We present Monte Carlo simulations of surface induced disordering in a model
of a binary alloy on a bcc lattice which undergoes a first order bulk
transition from the ordered DO3 phase to the disordered A2 phase. The data are
analyzed in terms of an effective interface Hamiltonian for a system with
several order parameters in the framework of the linear renormalization
approach due to Brezin, Halperin and Leibler. We show that the model provides a
good description of the system in the vicinity of the interface. In particular,
we recover the logarithmic divergence of the thickness of the disordered layer
as the bulk transition is approached, we calculate the critical behavior of the
maxima of the layer susceptibilities, and demonstrate that it is in reasonable
agreement with the simulation data. Directly at the (110) surface, the theory
predicts that all order parameters vanish continuously at the surface with a
nonuniversal, but common critical exponent. However, we find different
exponents for the order parameter of the DO3 phase and the order parameter of
the B2 phase. Using the effective interface model, we derive the finite size
scaling function for the surface order parameter and show that the theory
accounts well for the finite size behavior of the DO3 ordering but not for that
of B2 ordering. The situation is even more complicated in the neighborhood of
the (100) surface, due to the presence of an ordering field which couples to
the B2 order.Comment: To appear in Physical Review
Long-term behaviour of Nb and Cr nitrides nanostructured coatings under steam at 650°C. Mechanistic considerations.
There is an increasing demand for steam power plants to operate in super-critical conditions i.e. temperatures in excess of 600°C. Under these conditions creep resistant ferritic steels oxidize and therefore require coatings in order to last. Physical vapor deposition and especially High Power Impulse Magnetron Sputtering deposited CrN/NbN nano-scale multilayer coatings with a 2.45 Cr/Nb ratio showed excellent performance when exposed to 650 °C in pure steam environment up to 2,000 h. However the role of Nb in offering protection is unclear. In order to study the long term behaviour of this type of coatings as well as to determine the influence of Nb on their oxidation resistance, a CrN/NbN coating with a 1.16 Cr/Nb ratio was studied for 12,650 h. The coating is hard, well adhered and resistant to environmental corrosion, which are properties required in particular for coatings to be applied on turbine blades. The coating also protects P92 from steam oxidation at 650º C, however coating growth defects influence significantly the oxidation resistance. The long-time exposure allowed to study the protection/ degradation mechanisms provided by this type of ceramic coatings. It was found that oxide nodules grow due to the presence of coating defect originated from substrate defects. Moreover, the higher Nb CrN/NbN coating slowly oxidizes, consuming the coating to a large extent after 12,650 h. As a result, protective oxides containing Cr and Nb are developed, remaining well attached to the substrate for at least the test duration, and preventing further substrate oxidation by steam. Interestingly, thin voids present in the as deposited coating self-heal by forming Cr rich oxides, which block steam to reach the substrate
Sigma-phase in Fe-Cr and Fe-V alloy systems and its physical properties
A review is presented on physical properties of the sigma-phase in Fe-Cr and
Fe-V alloy systems as revealed both with experimental -- mostly with the
Mossbauer spectroscopy -- and theoretical methods. In particular, the following
questions relevant to the issue have been addressed: identification of sigma
and determination of its structural properties, kinetics of alpha-to-sigma and
sigma-to-alpha phase transformations, Debye temperature and Fe-partial phonon
density of states, Curie temperature and magnetization, hyperfine fields,
isomer shifts and electric field gradients.Comment: 26 pages, 23 figures and 83 reference
Thermal Diffusion and Quench Propagation in YBCO Pancake Coils Wound with ZnO-and Mylar Insulations
The thermal diffusion properties of several different kinds of YBCO
insulations and the quench properties of pancake coils made using these
insulations were studied. Insulations investigated include Nomex, Kapton, and
Mylar, as well as insulations based on ZnO, Zn2GeO4, and ZnO-Cu. Initially,
short stacks of YBCO conductors with interlayer insulation, epoxy, and a
central heater strip were made and later measured for thermal conductivity in
liquid nitrogen. Subsequently, three different pancake coils were made. The
first two were smaller, each using one meter total of YBCO tape present as four
turns around a G-10 former. One of these smaller coils used Mylar insulation
co-wound with the YBCO tape, the other used YBCO tape onto which ZnO based
insulation had been deposited. One larger coil was made which used 12 total
meters of ZnO-insulated tape and had 45 turns. The results for all short sample
and coil thermal conductivities were ~1-3 Wm-1K-1. Finally, quench propagation
velocity measurements were performed on the coils (77 K, self field) by
applying a DC current and then using a heater pulse to initiate a quench.
Normal zone propagation velocity (NZP) values were obtained for the coils both
in the radial direction and in the azimuthal direction. Radial NZP values
(0.05-0.7 mm/s) were two orders of magnitude lower than axial values (~14-17
mm/s). Nevertheless, the quenches were generally seen to propagate radially
within the coils, in the sense that any given layer in the coil is driven
normal by the layer underneath it.Comment: 58 pages, 5 tables, 16 fig
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