Volume 45, Number 910, 2000
General Physics  Gravitation
Motion of the Mass Centers in a Scalar Theory of Gravitation. I. Definition of Mass Centers and General Form of the Equations of Motion Mayeul Arminjon Romanian Journal of Physics 45,
645658 (2000)
A scalar theory of gravitation with a preferred reference frame is considered, that accounts for special relativity and reduces to it if the gravitational field cancels. The gravitating system consists of a finite number of perfectfluid bodies. An «asymptotic» postNewtonian (PN) approximation scheme is used, allowing an explicit weakfield limit with all fields expanded. Exact mass centers are defined. The PN equations of motion of these mass centers are derived: the zeroorder equations are those of Newtonian gravity, and the equations for the firstorder (PN) corrections depend linearly on the PN fields. In a forthcoming paper, the latter equations are simplified for the case of wellseparated bodies with rigid motion.
Motion of the Mass Centers in a Scalar Theory of Gravitation. II. Explicit Equations of Motion in the Relevant Approximation Mayeul Arminjon Romanian Journal of Physics 45,
659678 (2000)
In a foregoing paper, equations for postNewtonian (PN) corrections to the motion of the mass centers have been derived in a scalar theory of gravitation with a preferred reference frame (PRF). One will assume here «verywellseparated» rigidly moving bodies with spherical selffields of the zeroorder approximation. This is appropriate for PN corrections to the motion of the mass centers in typical systems, especially in the solar system. The PN corrections reduce then to a time integration and include spin effects, which might be significant. It is shown that the Newtonian masses are not correct zeroorder masses for the PN calculations. An algorithm is proposed, in order to minimize the residual and to assess the velocity in the PRF.
Nuclear Physics
Investigation of Raw Materials and Related Final Products Involved in the Ironmaking Process by Thermal Neutron Activation Antoaneta Ene, C. Besliu, Agata Olariu, T. Badica, I. V. Popescu, Al. Ivănescu, Dana Jianu Romanian Journal of Physics 45,
679684 (2000)
Neutron activation analysis (NAA) was applied to investigate some samples of raw and auxiliary materials (sinter, pellets, coke) and related final products (cast iron, slag, blastfurnace ﬂue dust) involved in the ironmaking process in the SIDEX Iron and Steel Works at Galati (Romania). A series of elements were determined in the analyzed samples: Al, As, Cu, Fe, K, Mn, Na, V and W. We sought for the transfer of As, Cu, K, Na, V and W from auxiliary materials to cast iron during the blast furnace process.
Condensed Matter
Study of Finite Differences Simulations of the Ultrasound Propagation in Christensen's Media Dan Iordache, Marco Scalerandi, Viorica Iordache Romanian Journal of Physics 45,
685704 (2000)
Starting from the constitutive equation of Christensen's media, we have found that the equation corresponding to harmonic waves is equivalent to a Zener's equation with specific effective parameters. The obtained expressions of the effective relaxation times under constant stress and strain, resp. and of the effective relaxed modulus as functions of frequency and of the characteristic times of each relaxation are rather intricate, but they allow us to use the procedure of FD simulations of the ultrasound propagation in Zener's media also for Christensen's media. The convergence and stability features of the FD schemes used to describe the Christensen's media are the same as the ones for the FD schemes used for Zener media characterized by the frequency dependent effective parameters of the studied Christensen's media. The obtained results concerning the FD description of the propagation of harmonic waves and ultrasound pulses through different types of attenuative multilayers are also reported.
The WeakField Magnetoresistance Skewness Effect and the Related Galvanomagnetic Measurements in Cubic or NonCubic Crystals D. S. Kyriakos Romanian Journal of Physics 45,
705721 (2000)
The action of a magnetic field of arbitrary direction on any crystalline material is to induce an anisotropy in the electric resistivity, even in the case of isotropic materials. Generally. the magnetoresistance skewness effect appears, that is the change of the principal directions of electrical resistivity under the inﬂuence of the magnetic field. This effect can be used for the experimental determination of the weakfield magnetoresistance coefficients. For the measurements the Van der Pauw and Wasscher methods are used, for the isotropic and anisotropic case respectively. Finally a study of flat circular samples, especially for isotropic materials, is presented.
Synthesis and Properties of YBCO Type Superconductor Thin Films Mihaela Stegarescu, Lucian Trupina, Elena Dimitriu, [fbox]constantin Constantin Romanian Journal of Physics 45,
723731 (2000)
YBCO high temperature superconducting films were prepared by two methods: Magnetron RFsputtering (MRFS), and Chemical Solution Deposition (CSD) onto polycrystalline alumina substrate with CeO_{2} buffer. Typical $J_c$ values of about 10^{5} A/cm^{2} and 10^{6} A/cm^{2} at 77K for MRFS and CSD, respectively have been obtained. The measured transition temperature was about 90K. The superconducting phases have been identified by Xray diffraction technique.
A Way to Estimate the Lindemann Number V. Sandu Romanian Journal of Physics 45,
733736 (2000)
Using the fishtail effect in high temperature superconductors and assuming this effect as a fingerprint of Bragg glass to a disordered glass transition in the flux lines system we obtained the value of the Lindemann number $c_L = 0.18$. We also have built a model for the dependence of peak field on temperature.
H_{2}OInduced Enhancement of Conductance in AlGeAu Surface Barrier Devices A. A. Oberafo, A. I. Mukolu Romanian Journal of Physics 45,
736749 (2000)
The enhancement of conductance as a result of adsorbed water vapour interaction with the surface of planar structures of AluminiumGermaniumGold (AlGeAu) surface barrier devices has been investigated. Germanium films of thickness 1000 Å were deposited by filament evaporation in vacuum of about 10^{5} Torr. Some of the Ge samples were annealed in vacuum at 400°C for 30 minutes. Al and Au films were contacted to the ends of the Ge layer in a twopoint probe configuration. All the probe electrodes were 500 Å thick. Current ($I$) — voltage ($V$) measurements show that all the devices were softly rectifying. Specifically, that at relatively low positive bias (< 20 V) on the Al electrode, the Al/Ge junction presented the major resistance to current flow, while for V > 20 V, as well as for all negative bias on the Al electrode, the evaporated Ge film presented the major resistance to current flow. The results also show that in water vapour ambient the specific contact conductance ($\sigma_c$) of Al/Ge junction is increased relative to the vacuum value by factor ranging from 0.3 to 6.0, while the specific surface conductance ($\sigma_s$) of the aGe film is increased by factor ranging from 1 to 4 as the relative humidity (RH) of the ambient varied from 20 to 80%. The barrier height (BH) of Al/Ge junction was found to be 0.48 eV.
Inﬂuence of the Initial Forming Pressure and Sample Geometry on the Final Density of PZT Type Sintered Ceramics Samples C. F. Miclea, C. Tănăsoiu, C. Miclea, R. Iorgulescu Romanian Journal of Physics 45,
751758 (2000)
The density of sintered ceramic samples of PZT type materials, were determined as a function of initial forming pressure (between 600 and 1800 atm) and sample geometry (cylinders with diameter to thickness ratios from 2 to 16). The samples were pressed in steel dies from PZT type powders with mean grain size of 0.11 μm. The sintered samples densities do not practically depend on the initial pressure, but they show an optimum for $D/h$ ratio around 10 and an extremely small difference (within 0.7 %) for high and low compacting pressures. These results are important from practical point of view, since for different applications there must be chosen the tight shapes sample in order to obtain the optimum characteristics of the PZT elements.
Effect of Different Dopants on the Structure and Properties of Modified Lead Titanate Ceramics L. Amarande, C. Miclea, C. Tănăsoiu, C. F. Miclea Romanian Journal of Physics 45,
759768 (2000)
The main effects of dopants and processing conditions on the structure and properties of modified lead ntanate (PT) ceramics were reviewed. Alkaline (Ca^{2+}, Sr^{2+}, Ba^{2+}) and rare earth (La^{3+}, Nd^{3+}, Sm^{3+}, Gd^{3+}) substitutions stabilize lead titanat ceramics against cracking by lowering their lattice anisotropy. The inﬂuence of additives on electrical, mechanical and dielectric properties of these materials was analysed. Thus, the dielectric constant at room temperature, the electrical resistivity and the Poisson's ratio increase, while the spontaneous polarization, the transition temperature and the corresponding maximum of the dielectric constant decrease with increasing amount of both types of dopants. Very large piezoelectric anisotropy, along with a large thickness coupling factor ($k_t = 0.5$) were attained in modified lead titanate ceramics, at room temperature, under certain processing conditions (sintering temperature of 1250°C, polydomain grains and poling field higher than 7 kV/mm), if proper types and amounts of additives (such as 24 mol% Ca or 10 mol% Sm) were chosen. These results were explained by assuming that piezoelectric constant $d_{31}$ may reach values around zero, as a result of averaging the piezoelectric constants of singledomain crystals over all possible orientations of grains and domains, thus being possible that planar coupling factor becomes extremely small. Effects of poling conditions (field and temperature) and poling reversals on modified PT ceramics, as well as piezoelectric relaxation in these materials were also discussed.
Inﬂuence of Me^{2+}O (Me = Cu, Cd, Co, Ca, Mn, Mg) on the Structure and Properties of a NiZn Ferrite E. Rezlescu, N. Rezlescu, P. D. Popa, L. Sachelarie Romanian Journal of Physics 45,
769773 (2000)
The effect of Cu, Cd, Co, Ca, Mn and Mg ions on the physical properties of Ni_{0.5}Zn_{0.5}Fe_{2}O_{4} ferrite was investigated. It was found that the magnetic and electrical properties of Ni_{0.25}Me_{0.25}Zn_{0.5}Fe_{2}O_{4} ferrites considerably changed with the substitution species. Cu ions play an important role in lowering the sintering temperature and increasmg density. Mn and Co ions increased electrical resistivity while the Ca and Mn ions improved thermal stability of permeability.
Electronic Properties of RbK, CsK and CsRb Alloys P. N. Gajiar, B. Y. Thakore, A. R. Jani Romanian Journal of Physics 45,
775784 (2000)
The perturbative treatment incorporated in the pseudopotential formalism has been used to study the electronic properties of alloys composed of alkali metals. A simple one parametric local pseudopotential is proposed to determine pseudoalloyatom (PAA) for the Rb$_{1x}$K$_x$, Cs$_{1x}$K$_x$ and Cs$_{1x}$Rb$_x$ solid solutions. The heat of solutions, $\Delta E$, and interaction parameter, $\Delta E/[x(1  x)]$, are calculated as a function of the atomic fraction $x$. In the present study we have incorporated three different localfield correction functions viz.; Hartree, Taylor and IchimaruUtsumi. Present findings of $\Delta E$ and $\Delta E/[x(1  x)]$ are in good agreement with other such available data. Then, the same PAA model is applied to investigate pressurevolume relation for the representative Rb_{0.5}K_{0.5}, Cs_{0.5}K_{0.5} and Cs_{0.5}Rb_{0.5}. It is found that the CsRb is more compressible than CsK and RbK equiatomic solid solutions. In order to obtain clearly the alloying effect, we have shown the pressure $P(x,\Omega/\Omega_0 = 0.6)$ at the representative compressed volume $\Omega/\Omega_0 = 0.6$ versus atomic fraction $x$. It is also seen that $P(x, \Omega/\Omega_0 = 0.6)$ of the solid solutions are monotonous functions of atomic fraction, but deviated from the linear interpolation between pure alkali metals.
Geophysics
Predicting Peak Ground Horizontal Acceleration for Vrancea Large Earthquakes Using Attenuation Relations for Moderate Shocks IrenAdelina Moldovan, Bogdan Dumitru Enescu, Constantin Ionescu Romanian Journal of Physics 45,
785800 (2000)
The estimation of strong ground motion dependent parameters is an important step in attenuation studies and implicitly in hazard assessment. The dependent parameter selected in this work is the peak ground horizontal acceleration and the used database consists in 84 horizontal peak accelerations from 5 earthquakes (including the April 28, 1999 earthquake, M = 5.5) recorded by the K2 strong motion array in Romania, array centered on Vrancea seismic region. The obtained attenuation relationship for moderate Vrancea earthquakes (4 < M < 5.5), following a two step regression model, was used for predicting the peak ground horizontal acceleration for large earthquakes. These accelerations were compared with those obtained from other attenuation laws [1]. Also, comparisons were made between our obtained attenuation curves and those published by Mârza and Pântea [2]. The Vrancea source induces a high seismic risk in the densely built zones of the SouthEast of Romania because the fundamental periods of the peak accelerations recorded at the large intermediatedepth shocks (11.5 s) are close to the natural periods of modern tall structures, contributing to severe damages of such structures. A number of 32 reinforced concrete multistorey buildings completely collapsed during the shock that hit Bucharest on March 4, 1977. That is why, predicting the ground motion accelerations during a major earthquake is of crucial importance for assessing more accurately the seismic hazard and for proposing hazard mitigation actions.
[1] T. Lungu, T. Cornea, C. Nedelcu, Vrancea earthquakes: Tectonics, Hazard and Risk Mitigation, Kluwer Academic Publishers, 251267 (1999). [2] V. Mârza, A. Pântea, Proc. XXIV ESC General Assembly (1994).
