Diffusion Foundations Vol. 22

Abstract: Solidification and cooling processes proceeding in the metal domain can be described in different ways. One of them consists in the application of the Fourier-type equation in which the additional term (source function) controlling the solidification process is introduced. In this paper this type of energy equation is used, but for the phase change modeling the equation discussed is in some way transformed. Such a modification is possible if one considers the pure metal for which the solidification point is a constant value. The numerical model used at the stage of computations is based on the Control Volume Method. In the final part of the paper, examples of computations are shown.

Abstract: This study investigated moisture diffusion in natural rubber (NR) hybrid composites filled with silica and bentonite clay. Natural bentonite (BNT) was treated with tetradecyldimethylammonium chloride and coco diethanolamide to produce modified bentonite (M-BNT). Varied proportions of silica, M-BNT, and BNT fillers were added to raw NR according to a third-degree simplex lattice mixture design of experiment. The addition of fillers affects the vulcanization characteristics, mechanical properties, and hardness of NR hybrid composites. Moisture diffusion behavior was studied by monitoring the water uptake of NR composites during immersion in deionized water at 80^°C. Data from sorption experiments were fitted on the classical Fickian and Langmuir-type diffusion models. The Fickian model overestimates and underestimates the water uptake of NR composites in the early and later stages of moisture diffusion, respectively. On the other hand, the Langmuir-type model adequately captures the anomalous diffusion behavior of moisture in NR composites. Parameters of the Langmuir model (equilibrium water uptake and diffusion coefficient) vary with the composition of hybrid fillers. Optimum proportions of silica, M-BNT, and BNT in rubber composites were obtained by considering the effect of fillers on mechanical properties and moisture diffusion characteristics of NR.

Abstract: The phenomena of transport in porous media arises in many diverse fields of science and engineering, ranging from agricultural, biomedical, building, ceramic, chemical, and petroleum engineering to food and soil science. Several authors provide an extensive description of the problems involving porous media. For building engineering, obtaining a good understanding of moisture transport in building envelopes is becoming one of the most important tasks. In the last few decades, many studies investigating moisture transport in building envelopes have been published, which have helped to improve overall building envelope design. This work presents a brief review of these studies.

Abstract: The influence of annealing at 1200 °C for 2, 4 and 8 h on the carbide microstructure of AISI M2 type high-speed steel obtained by both the foundry and conventional metallurgy technologies has been studied. The primary focus was on the kinetics of eutectic carbide decomposition and dissolution in both the cast and wrought M2 high-speed steels under the effect of high temperature.

Abstract: The structure of MgH₂ samples has been investigated by the neutron diffraction method at room temperature and 5 K. Samples of MgH₂ have been prepared with vacuum extraction technique at high temperature. Obtained neutron data demonstrated that samples contain coexisting Mg and MgH₂ phases in different rate. The distribution of hydrogen atoms in the structure of the samples is comparable at both temperatures. Collected neutron data and results of X-ray analysis show that microstructure of the samples is different at room and at low temperature. Non-stability of partly desorbed MgH₂ samples after low temperature treatment has been discussed on basis of different diffusion of hydrogen atoms in varied microstructure.

Abstract: This study reports on the elaboration and characterization of bulk nanocomposites samples obtained by dispersion of metallic powders at the nanoscale as reinforcements in a polymer matrix. Elemental Fe powders were successfully nanostructured via high-energy ball milling. Structural characterization of the produced powders was conducted using X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM). The Halder-Wagner approach was adopted to determine the powder’s average grain size, internal strain, lattice parameters and the mixing factors. Structural parameters evolution and morphological changes according to milling progression are discussed. Bulk nanocomposites samples were shaped in a home moulder by dispersion of coarse Fe and nanostructured Fe powders in a continuous matrix of commercial epoxy resin. The obtained bulk samples match the metallic X-band wave-guide WR-90 dimensions used for electromagnetic characterization. The two-port S_ij scattering parameters were measured via an Agilent 8791 ES network analyzer. The measured scattering parameters served to calculate the loss factor of samples and to extract the dielectric permittivity via the Nicholson-Ross-Weir conversion. Spectra evolution of the scattering parameters, the loss factor and the dielectric constant for epoxy resin with coarse Fe and nanostructured Fe reinforcements are commented.

Abstract: The electrochemical current noise signal of a high-alloy cast steel was investigated in a 0.1 M sulfuric acid solution and in a 5 wt.% sodium chloride solution. In the sulfuric acid solution, the current time signal reveals characteristic spikes of high amplitudes. In the chloride containing solution, spontaneous power drops with a subsequent recovery of comparatively low intensity characterize the noise signal. Both noise records were analyzed by the discrete wavelet transform. For the noise signal in the sulfuric acid solution, the received wavelet coefficients exhibit the highest values in the fine scale, which signal the dominance of short-time corrosion events that were attributed to the observed hydrogen bubble evolution. In the chloride containing medium, the signal decomposition by the wavelet analysis reveals the highest coefficients predominantly in the coarse scale, indicating a preferred initiation of corrosion processes of high duration. The subsequent observations by scanning electron microscopy, reveal an attack by micro pitting, which is associated with the noise events.

Abstract: The effect of cold-rolling on mechanical, thermal, and electrical properties as well as microstructure behaviour of the Al-2.93wt.%Mg-0.34wt.%Mn-0.33wt.%Si-0.22wt.%Fe-0.19wt.%Cr-0.24wt.%Sc-0.06wt.%Zr was studied. The material was investigated during step-by-step isochronal annealing in a temperature range from room temperature up to 540 °C and during isothermal annealing at 200, 450 and 550 °C. Precipitation reactions were studied by electrical resistometry, conductivity, (micro) hardness measurements and differential scanning calorimetry. The hardening effect appears due to the additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles. The distinct changes in residual resistivity ratio above ~ 330 °C are probably caused by precipitation of the Mn (,Fe,Cr)-containing particles. This precipitation process is highly influenced by cold rolling but it has a negligible effect on hardness. The apparent activation energy values for additional formation of the Al₃Sc and/or Al₃(Sc,Zr) particles were determined. The kinetics of the Al₃(Sc,Zr)-phase precipitation seems to be independent of Mn-and Mg-addition in the studied alloys. A partial recrystallization of the cold-rolled alloy was registered by electron backscatter diffraction after annealing at 550 °C. The initial difference in microhardness introduced by cold rolling is almost removed after annealing at 550 °C/30 min.

Abstract: Cuprous oxide (Cu₂O) has a high optical absorption coefficient and favourable electrical properties, which make Cu₂O thin films attractive for photovoltaic applications. Using reactive radio-frequency magnetron sputtering, high quality Cu₂O thin films with good carrier transport properties were prepared. This paper presents the characteristics of Cu₂O thin films that were sputter deposited on quartz substrates and subjected to post-deposition rapid thermal annealing. The thickness of the thin films and the optical constants were determined by ellipsometry spectroscopy (SE). The optical transmittance increased in lower wavelength region after annealing at 900 ̊C in rapid thermal annealing (RTA). The structural and morphological properties of the Cu₂O thin films were investigated by electronic scanning microscopy (SEM) and atomic force microscopy (AFM), whereas elemental analysis was performed by X-ray fluorescence spectroscopy (XRF). The carrier mobility, carrier density and film resistivity were changed after post-deposition rapid thermal annealing from respectively ~14 cm2/Vs, ~2.3 x 10¹⁵ cm^-3 and ~193 Ωcm for the as-deposited Cu₂O film to ~49 cm2/Vs, ~5.0 x 10¹⁴ cm^-3 and ~218 Ωcm for the annealed Cu₂O film. The investigation suggests that the sputter-deposited Cu₂O thin films have good potential for application as absorber layers in solar cells.