Defect and Diffusion Forum Vols. 273-276

Abstract: We report a model for the explanation of the single molecule magnet behavior of the [CuIILTbIII(hfac)2]2 cluster. The model takes into account the crystal field acting on the TbIII –ion and the exchange interaction between the TbIII and CuII ions. The energies of the low-lying levels are shown to increase with the decrease of the mean value of the z-projection of the total angular momentum of the cluster, thus forming a barrier for magnetization reversal that is in accordance with the experimental evidence.

Abstract: The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.

Abstract: Interstitial solutes in body-centered cubic metals, such as oxygen in tantalum, produce ideally Snoek effects when they are in solutions enough diluted. However, for higher concentration of these solutes, more complex relaxation process can occur, as interaction between interstitial solutes and dislocations. Anelastic relaxation measurements were carried out in polycrystalline tantalum samples, using torsion pendulum inverted, operating between 300 K and 680 K and oscillation frequencies in the hertz bandwidth, for three different experimental sample conditions: as received sample, annealed and annealed followed by a treatment in an oxygen atmosphere. These measurements have revealed the following behavior: the intensity of the internal friction peak associated to matrix-interstitial interaction Ta-O decreased between the first run and the next runs, and this phenomenon did not occur for the others conditions. The variation of relaxation strength of Ta-O peak, with number of runs is due to a decrease of an amount of oxygen in solid solution, which can be associated with the precipitation of new phases in Ta sample and with the trapping of oxygen atoms by dislocations.

Abstract: The nitrided layer formed on Type 304 steel after gas nitriding was studied using TEM and so on. (1) Dependency of the nitriding temperature and time on the depth of the nitrided layer was studied. The following equation was obtained. L2=8.7*108*EXP(-146,000/RT)*t (2) Nitrided layer formed at both 570°C for 3 hrs and 410°C for 48 hrs had high density of dislocation, stacking fault and lattice strain. (3) Nitrided layer formed at 570°C was mainly composed of є-Fe3N, and had much Fe4N and CrN. (4) On the other hand, the nitrided layer formed at 410°C was mainly composed of S-phase, є-Fe3N was not detected and Fe4N and CrN were very little. N content in the nitrided layer formed at 410°C was about 7-8 mass%. (5) Nitrided layer formed at 410°C showed good corrosion resistance.

Abstract: Intermittent drying is an alternative technology to continuous drying that improves the product quality as result of diminishing the average material temperature and the heating time. In this study, the application of the intermittent drying technology to mango drying (Mangifera indica L.) was analyzed and optimized. To formulate the optimization problem, an objective function was proposed in order to improve the product quality and decrease the energy consumption. To quantify the objective function, a mathematical model for a cubic shape was formulated and validated, considering diffusional mass and heat transfer with non-negligible external resistances.

Abstract: Metals that present bcc crystalline structure, when receiving addition of interstitial atoms as oxygen, nitrogen, hydrogen and carbon, undergo significant changes in their physical properties, being able to dissolve great amounts of those interstitial elements, thus forming solid solutions. Niobium and most of its alloys possess bcc crystalline structure and, as Brazil is the largest world exporter of this metal, it is fundamental to understand the interaction mechanisms between interstitial elements and niobium or its alloys. In this paper, mechanical spectroscopy (internal friction) measurements were performed in Nb-2.0wt%Ti alloys containing nitrogen in solid solution. The experimental results presented complex internal friction spectra and with the addition of substitutional solute, it was observed interactions between the two types of solutes (substitutional and interstitial), considering that the random distribution of the interstitial atoms was affected by the presence of substitutional atoms. Interstitial diffusion coefficients, pre-exponential factors and activation energies were calculated for nitrogen in the Nb-2.0wt%Ti alloys.

Abstract: The scientific and technological development in the area of new materials contributed to several applications of niobium and its alloys in nuclear power plants as well as in aerospace, aeronautics, automobile and naval industries. This paper presents the interstitial diffusion coefficients of nitrogen in solid solution in the Nb-1.0wt%Zr alloy using internal friction measurements obtained by mechanical spectroscopy, which uses a torsion pendulum operating at an oscillation frequency between 1.0 Hz and 10.0 Hz. The temperature range varies from 300K to 700K, at a heating rate of 1 K/min and vacuum better than 2 x 10-6 Torr. The results showed an increase of the interstitial diffusion coefficient of nitrogen that was correlated with configurational considerations for the octahedral interstitials.

Abstract: Since the discovery of high-temperature superconductivity of cuprate oxides, it has been clear that it is strongly affected by the oxygen content, which is also a crucial factor to determine other physical properties of high Tc superconductors. Non-stoichiometric (interstitial) oxygen strongly influences the physical properties of various superconducting oxides, in particular by creating conducting holes. It is now ascertained that the amount of holes injected depends not only on the content of interstitial oxygen, but also on its ordering. Rearrangement of the oxygen ordering may occur even below room temperature due to the unusual high mobility of these atoms. This way, mechanical spectroscopy is one of the most adequate techniques for the study of the mobility (diffusion) of oxygen atoms. This technique allows the determination of the jump frequency of an atomic species precisely, regardless of the model or the different possible types of jumps. In order to evaluate the mobility and the effect of oxygen content on these oxides, ceramic samples we prepared and submitted to several oxygen removal cycles alternately with mechanical relaxation measurements. As for SBCO, it was assumed that the peak was due to O(1)-O(5) jumps of oxygen atoms at the chain terminals or in chain fragments in the orthorhombic phase. In the case of BSCCO, the results showed complex anelastic relaxation structures, which were attributed to interstitial oxygen atom jumps between two adjacent CuO planes.

Abstract: We study thermodynamically the behaviour of PdSe2 while subjected to high pressure under isothermal conditions. The present paper discusses the volumetric-level calculations and results. Experiments under two certain temperature levels are performed: 20oC and 300oC. Calculations and analyses are done according to the method for thermodynamical analysis developed by us in [1]. We detected the order of phase transition from PdS2 structure type to pyrite one to be first order notwithstanding the temperature level. Values of transition pressure were found to be 12.24 GPa and 9.785 GPa at 20oC and 300oC, respectively. Adjusted entropy generation during compression was calculated aiming to study stability of treated compound. Influence of compression temperature level was analysed, as well as duration of pressure plateaux.

Abstract: We study thermodynamically the behaviour of PdSe2 while subjected to high pressure under isothermal conditions. The present paper continues the study started in [1]. Here we present the results of the axial calculations and analyses. Specific lengths, linear adjusted Gibbs free energy changes and linear adjusted entropy generations were studied along each spatial axis separately. We found that the first-order transition from PdS2 structure type to pyrite one at 20oC is accompanied by saltatory contraction of a and b specific lengths and respective saltatory expansion of c specific length. Under 300oC all specific lengths contract saltatory. In the transition point under 20oC PdSe2 gains saltatory stability along a and b axis and looses along c one, respectively. Besides, the loose along c axis is bigger than the gains along a and b ones. Under 300oC the transition is accompanied by slight gain of stability along all three spatial axes. Plateaux duration affects the stability of PdSe2 strongly under higher temperature.