Characterisation of anodic oxide films on zirconium formed in sulphuric acid: XPS and corrosion resistance investigations

Present work describes investigations of a two-step process consisting of galvanostatic anodising in a 1 M H₂SO₄ solution at 100 mA cm⁻² up to the limiting voltages of 20, 60, 80, 100 and 120 V, directly after which potentiostatic regime was employed and the current was allowed to drop. The total treatment time (5 min) was held constant for all samples. The treatment was carried out to improve the corrosion resistance of zirconium in physiological conditions, which was determined by electrochemical evaluation in Ringer’s solution. XPS studies revealed that after anodising sulphur was incorporated into the oxide film in the form of sulphated zirconia. The maximum content of sulphate in the oxide layer was observed after anodising at 80 V. Anodising at higher voltages resulted in formation of coatings with decreasing amount of sulphur. It was found that there is a strong correlation between the sulphur content in the oxide layers and the measured corrosion current density. On the other hand, the pitting corrosion resistance seemed to be unaffected by the presence of S and it was improving with the increasing limiting voltage of the treatment.

     

Electrical resistivity of microinhomogeneous PdMnxFe1−x alloys

The electronic band-structure calculations of the PdFe ferromagnet and the PdMn antiferromagnet performed in this work permit one to conclude that the specific features of the electrical resistivity observed in the ternary PdMn_xFe_1−x alloy system [the deviation from the Nordheim-Kurnakov rule ρ₀(x)∼x(1−x), which is accompanied by a high maximum of residual resistivity (not typical of metals) ρ ₀ ^m ∼220 μΩ cm at x _C∼0.8 and a negative temperature resistivity coefficient in the interval 0.5≤x≤1] are due to the microinhomogeneous (multiphase) state of the alloys and a variation in the band-gap parameter d spectrum caused by antiferromagnetic ordering of a PdMn-type phase. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 193–197. Original Russian Text Copyright ? 2002 by Kourov, Korotin, Volkova.     

Charge and orbital order in Fe3O4

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe3O4) is investigated using the local spin density approximation with Coulomb interaction correction method. While the difference between t(2g) minority occupancies of Fe(2+)(B) and Fe(3+)(B) cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.     

Coherent aperture synthesis by an incoherent signal

The procedure of aperture synthesis by an incoherent source during displacement of the receiving system in space is considered. The advantages of the method are the possibility of aperture synthesis with the help of antenna arrays consisting of two receivers and its applicability to both the source of a quasimonochromatic signal and that possessing a continuous spectrum. The method is applicable with the same limitations as the known methods using the procedures of signal phasing. The results of mathematical simulation of the synthesis upon location of two sources with close frequencies at different distances and in the presence of noise, as well as the results of the in situ experiment with a reverberation interference, are given. The algorithm for decreasing the time of aperture synthesis without resolution deterioration is proposed and tested in situ.     

Effect of doping by boron,carbon, and nitrogen atoms on the magnetic and photocatalytic properties of anatase

The effect of doping of titanium dioxide with the anatase structure by boron, carbon, and nitrogen atoms on the magnetic and optical properties and the electronic spectrum of this compound has been investigated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) band-structure method in the local spin density approximation explicitly including Coulomb correlations (LSDA + U) in combination with the semiempirical extended Hückel theory (EHT) method. The LSDA + U calculations of the electronic structure, the imaginary part of the dielectric function, the total magnetic moments, and the magnetic moments at the impurity atoms have been carried out. The diagrams of the molecular orbitals of the clusters Ti₃ X (X = B, C, N) have been calculated and the pseudo-space images of the molecular orbitals of the clusters have been constructed. The effect of doping on the nature and origin of photocatalytic activity in the visible spectral range and the specific features of the generation of ferromagnetic interactions in doped anatase have been discussed based on the analysis of the obtained data. It has been shown that, in the sequence TiO_{2 ? y} N _y → TiO_{2 ? y} C _y → TiO_{2 ? y} B _y (y = 1/16), the photocatalytic activity can increase with the generation of electronic excitations with the participation of impurity bands. The calculated magnetic moments for boron and nitrogen atoms are equal to 1 μ_B, whereas the impurity carbon atoms are nonmagnetic.     

Measurement of the parameters of the natural waveguide mode

The possibilities of using the decomposition of natural waveguide modes in a shallow-water sea in case there is a sound velocity gradient into sinusoidal modes of an ideal waveguide is grounded. The applicability range of such decomposition is shown. Dispersion in signals of the modes presented in such a way is determined by mathematical reversal without a test source. The structure of discrete modes in a natural waveguide is determined without utilizing the bottom parameters and sound velocity’s distribution over the waveguide depth. The coefficient of the mode signal’s correlation with the measured parameters of the mode signal and a real signal, introduced into it, is shown to be 0.973. The signals from a point emitter positioned at the depth of 50 m in the frequency range of 90–280 Hz in a shallow-water sea (the Barents Sea, a 120 m depth, a 7 km distance), received by a vertical antenna array comprising 32 receivers spaced equidistantly with a 3-m step are used in the experiment. A real signal has been successfully reversed using a mathematical model.     

The use of receiving arrays for measuring underwater noise levels of moving sources

A method of measuring underwater noise levels of moving ships with the use of a vertical receiving array is considered. An algorithm is proposed for synthesizing the weight vector that provides a given measurement accuracy with maximal noise suppression for the cases of an interference with a known structure and an a priori unknown interference (the adaptive synthesis). Results of both numerical simulation of the method and its experimental testing under actual sea conditions are presented.     

The influence of defects on magnetic properties of fcc-Pu

The influence of vacancies and interstitial atoms on magnetism in Pu is considered in the framework of the density functional theory. The crystal structure relaxation arising due to various types of defects is calculated using the molecular dynamics method with a modified embedded atom model. The local density approximation with explicit inclusion of Coulomb and spin-orbit interactions is applied in matrix invariant form to describe correlation effects in Pu with these types of defects. The calculations show that both vacancies and interstitials give rise to local moments in the f-shell of Pu in good agreement with experimental data for aged Pu. Magnetism appears due to the destruction of a delicate balance between spin-orbit and exchange interactions.     

Source detection in mechanoacoustic systems by the wave front inversion and time reversal methods

A combined experimental and theoretical method is proposed for finding the vibration sources in mechanoacoustic systems. The method is similar to the wave front inversion and time reversal focusing methods used in optics and acoustics. The method performs coherent measurements of the vibration field at a set of points and employs the reciprocity principle in its theoretical part. To enhance the stability of the method, modifications are proposed that perform nonlinear processing of vibration fields calculated by the finite element method. Possibilities of using the method in self-testing vibrating systems are discussed.     

Noise level decrease due to the matching of the receiving array with the medium

A stable (robust) decrease in noise level by 2–3 dB was observed when the receiving array was matched with the medium using the signal of a probing source. This effect is of the same physical nature as the reverberation level decrease due to the time reversal of waves, which was described in J. Acoust. Soc. Am. 115, 1525 (2004); J. Acoust. Soc. Am. 118, 1381 (2005). The noise level is found to decrease in the case of matching with the probing source positioned at different distances and even in the case of antimatching. The study of the effect shows that its origin lies in the decrease that occurs in the spatial correlation of noise received by the array because of the introduction of the matching factor. The experiments were carried out in the Barents Sea at distances of 7, 10.5, and 12 km with a sea depth of 120 m in the frequency range from 100 to 300 Hz.