Defect and Diffusion Forum Vols. 326-328

Abstract: The temperature dependence of mechanical properties of aluminum foams has been investigated. Youngs modulus and Poissons ratio have been determined from simulations of a unidirectional tensile test using a finite element model of aluminum foam networks. The Youngs modulus of the network structure for different relative densities and foam cell regularity factors have been plotted over temperature. It has been found that the Youngs modulus decreases with increasing temperature, whereas the Poissons ratio remains constant. This trend has been observed for various foam structures of different sizes, relative densities and network irregularities.

Abstract: Semiconductors have been suggested as ideal candidates for many electrical and optical applications and several groups have reported their successful synthesis in recent years. In particular, ZnS that is a direct wide band gap (3.91 eV) semiconductor and an important phosphor host lattice material, can be used in electroluminescent devices (ELD), due to its large band gap, which enables emission of visible light without absorption and efficient electron transport. In this work, we report synthesis and structural characteristics of gold covered core-shell nanometer size of ZnS/SiO₂ thin films as a patterning materials for use as a protective layer in optical phase change discs prepared through a simple sol-gel process. The results obtained by transmission electron microscopy (TEM) images showed that, as prepared core-shell materials were well-aligned nanoprticles grew in the same direction and through X-ray diffraction (XRD) analysis, we found that ZnS/SiO₂ core shell material was composed of crystalline ZnS core covered by an amorphous SiO₂ shell. Photoluminescence (PL) measurements performed on core-shell samples before and after gold coverig samples have provided information regarding stoichiometric vacancies or interstitial impurities and enhenced intensity of the green emission of ZnS nanowires at room temperature due to sheelding with SiO₂.

Abstract: The action flux of ions of inert gas on the substratum Si (100) leads to porosity into the crystal lattice and self-organization of these defects. The kinetic stochastic model of the phase transition at the initial stage is applied to find distributions of defects in sizes and on their coordinates in the layers. The accumulation of stress is determined by computer simulation. Layers of pores and cracks precede to solid state epitaxy of silicon carbide.

Abstract: In the present study, the thermal diffusivity of four sand casting magnesium alloys: Mg-9Al-1Zn, Mg-6Al-2Sr, Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr were studied. Sand casting was performed at 730-780°C temperatures. Thermal diffusivity was measured by a LFA 427 Netzsch apparatus. The thermal diffusivity of the investigated alloys was chemical composition and temperature dependent and increased with increasing temperature. The thermal diffusivity of Mg-Al-Ca-Sr alloys was higher than that of Mg-Al alloy, because the total volume fraction of intermetallic phases in alloys containing calcium and strontium is larger than that in Mg-Al alloy. The formation of intermetallic phases caused the consumption of the solute element in the α-Mg matrix, and improved the thermal diffusivity of the Mg-Al-Ca-Sr magnesium alloy.

Abstract: In this paper, the corrosion resistance of two sand-casting creep resistant magnesium alloys Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr in the salt environment has been investigated. Specimens of each alloy has been immersed in 3.5% NaCl solution at room temperature and successively taken out after 1, 2, 4, 5 and 9 days. After immersion test, the microstructure and the appearances of the corroded structure were examined. The corrosion rates of both investigated alloys increased lineally with increasing the exposure time in both solutions. Mg-9Al-1.5Ca-0.3Sr alloy exhibits the higher corrosion rate during the immersion test than Mg-9Al-2.2Ca-0.8Sr. The corrosion layer of both alloys consists of MgO, MgOH and phases containing Cl, Na, Al and Ca. The increase of Ca content in the Mg-9Al-2.2Ca-0.8Sr alloy improved the corrosion resistance due to the formation of the reticular (Mg,Al)₂Ca phase, which acted as an effective barrier against corrosion.

Abstract: In this work we discuss the problem of coupled saturated and unsaturated flow in porous media under centrifugation. Our mathematical model is described by Darcy's equation in saturated region and Richard's nonlinear and degenerate equation in unsaturated region. We use Van Genuchten model with soil parameters and express the Richard's equation in both head and saturation. Our numerical method is based on space discretization. We reduce the problem to an ODE system with an algebraic condition to keep the water mass balance (DAE system). In our mathematical model we consider two interfaces separating fully saturated, partially saturated and dry subregions. Our goal is the determination of soil parameters. We control the input/output of infiltrated water in the sample. The only measurements concerning the sample require the measuring of the time moments in which the moving water achieves prescribed values of momentum. These data are sufficient to determine the soil parameters.

Abstract: The purpose of this paper is to present an experimental study of brick drying. For the drying experiments, industrial brick (clay) was dried in an oven under controlled conditions of air velocity, air temperature and air relative humidity. The continuous drying experiments ended when the mass reached constant weight. In order, to obtain the balanced moisture content, each sample was kept under the same drying air temperature for 48 hours inside the oven. The tests were performed under atmospheric pressure. Results of the drying and heating kinetics and volume variations during the process are shown and analyzed. It was verified that air temperature has big influence in the drying rate during process. It was verified that the largest temperature, moisture content and stress gradients are located in the vertexes of the brick. The drying process happens in the falling drying rate period.

Abstract: This paper presents the results of research on aluminide protective coatings manufactured on hightemperature creep resistant cast steel. The main purpose of these coatings is protection against the high temperature corrosion, at carburizing and oxidizing potential atmosphere. Coatings were obtained on cast steel type GXNiCrSi 3018 by slurry cementation in air atmosphere. The tests of carburizing and oxidizing were carried out. The structure of the coatings before and after carburizing and oxidizing is described in the present paper. The chemical composition, thickness and microstructure of coatings were determined. These coatings could protect equipment against hot corrosion at carburizing and oxidizing atmosphere and have thermal shocks resistance.

Abstract: The evaluation of the dispersion in vegetated beds may allow indentifying mechanisms that affect the transport and reaction of solutes, namely organic and nitrogen compounds. A set of non-reactive tracer experiments (slag injection) was performed in a vegetated bed (a mesocosm with a LECA-based substratum and colonized with Phragmites australis) used for the removal of organic and nitrogen pollutant loads. Loads of approximately 300 mg COD/L and 30 mg NH₄-N/L and a hydraulic loading rate of 3.5 cm/d were used. The results showed a delay in all the residence time distribution (RTD) curves and a variation in the dimensionless residence time (μ_(m,θ)) of the E(θ) curves, which means that the mass centre of the impulse was late relatively to the expected one. A strong dispersion and tracer retention (due to the presence of stagnated areas and internal recirculation) was observed, especially in the first 33 cm of the bed, which seems to have been related to the presence of complex clusters of roots, solid material, biofilm and LECA particles. An analytical solution of the Multiple-Tanks-in-Series (MTS) model well represents the RTD curves obtained in the tracer experiments. The detected dispersion and dead volume ratios (7% to 12%) did not affect the performance of the bed, which presented mean removal efficiencies of 85% and 60.4% for COD and NH₄-N, respectively.

Abstract: C-B-N diffusion layers on the samples of wrought and as-cast high-speed steels of AISI M2 grade have been developed thermo-chemically. Thermo-chemical treatment was carried out after full heat treatment and included holding for 1 and 3 h at 550°C in order to produce different C-B-N diffusion layers. The microstructure and component depth profiles in the C-B-N diffusion layers were studied by scanning electron microscopy and energy dispersive X-ray spectrometry. It was shown that well adhered (diffusion controlled) carbon-boron-nitrogen layers with nanosized microstructure can be thermo-chemically developed at 550°C for 1 h on the wrought and as-cast M2 high-speed steel substrates. In the samples made of the wrought M2 high-speed steel and held at 550°C for 3 h the C-B-N diffusion layer develops in the fiber-like morphology.