Defect and Diffusion Forum Vol. 420

Abstract: Maraging steels are martensitic steels hardened by intermetallic compounds that precipitate during aging heat treatments. During aging of these steels complex phenomena involving nucleation and growth of several phases as well as changes in the precipitates, morphology and stoichiometry take place. The present work aims to study the kinetics of precipitation in a maraging 350 steel through the KJMA and Austin-Rickett (AR) equations. Analysed data were obtained from Vickers microhardness measurements carried out in samples heat-treated between 440 and 560 °C. Variation in the n-constant has been observed for both equations, indicating changes in the precipitation behavior. However, the n-constant values obtained from AR equation follow the microstructural changes observed in previous works on maraging steels. Interpretation of the n-constants using the AR equation was linked to the precipitation on dislocations at 440 °C, the growth of finite long cylinders in comparison to their separation at 480 °C, and general particle growth from small dimensions at 520, and 560 °C.

Abstract: Metallic glasses are materials that present absence of periodic atomic order of long scale and seem to be promising to application in the fields of science and engineering. They were first obtained in 1960 through the technique of rapid cooling with thinner thickness, but the development of new production techniques and the combination of elements allowed the production of amorphous condition in millimeter-thick alloys, which received the denomination of bulk metallic glasses. Two alloys of metallic glasses, with the Cu_47.5Zr_45.5Al₅Er₂ and Cu_47.75Zr_47.75Al_4.5 compositions, were here studied. Their production was made through arc furnace, and the characterization through X-ray diffraction (XRD), with Cu Kα radiation, Scanning Electron Microscope (SEM), ultra-microhardness tester, and Atomic Force Microscopy (AFM), from Nanoscope IIIA. The analysis of the atomic structure of the Cu_47.5Zr_45.5Al₅Er₂ alloy showed great structural disorder, and the Cu_47.75Zr_47.75Al_4.5 sample presented peaks arising from crystalline phases, like Zr₄Cu₂O (big cubic phase) formed due to the affinity between zirconium and oxygen, in the amorphous matrix. Using the AFM, the Cu_47.5Zr_45.5Al₅Er₂ sample presented the values of 2.15 nm from Ra (roughness average) and 3.14 nm from RMS (Root Mean Square). Another sample has shown better results of roughness, where Ra was 1.88 nm and the RMS was 2.53 nm. As it is known, roughness is an import tribological parameter in materials and lower values allow that less surface area is exposed to harmful atmospheric effects such as corrosion, as evidence of the advantage of the sample with erbium.

Abstract: Pseudoboehmite is an aluminium compound with a structure similar to that of boehmite. The unit cell of the pseudoboehmite is slightly larger than that of the boehmite because of the greater incorporation of water in the same structure [1]. It has particles of nanometric dimensions and can be synthesized with extremely high purity. Several papers published in the literature show the potential of using this material in the controlled release of drugs, including cancer treatment [2,3]. Using drugs in complexes incorporated into polymer matrices and ceramic gels can lead to adequate control of gastrointestinal absorption when administered orally. Consequently, there is the possibility of promoting a gradual action through the progressive release of the drug, thus increasing its efficiency and reducing dose and toxicity. Previous studies have shown that pseudoboehmite is non-toxic and can be used for drug delivery. During the tests to determine the toxicity of this material, it was observed that the blood glucose content was reduced in mice that took pseudoboehmite by gavage. Since pseudoboehmite is obtained via low-cost inorganic synthesis, different physicochemical characteristics can be incorporated into this high purity medium. The formed structure was evaluated as a drug delivery system to establish a profile of the influence of the physicochemical properties of a molecule in the process of interaction with pseudoboehmite, a drug with recognized pharmacological activity, simvastatin (IUPAC name (1S,3R,7S,8S,8aR)-8-{2-[(2R,4R)-4-hydroxy-6-oxooxan-2-yl]ethyl}-3,7-dimethyl-1,2,3,7,8,8a-hexahydronaphthalen-1-yl 2,2-dimethylbutanoate) was choose. It also emphasizes the importance of pseudoboehmite as a drug carrier, serving as a precursor to new therapeutic systems. The synthesis of pseudoboehmite from ammonium hydroxide and aluminium nitrate was optimized to release simvastatin in vitro. After absorption and hydrolysis in the liver to form the active β-hydroxy acid metabolite, simvastatin acts as a potent reversible, competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, an early and rate-limiting enzyme in the biosynthesis of cholesterol.

Abstract: The improved oil recovery (IOR) is a way of enhancing the reservoir properties with the use of nanomaterials to detach the oil molecule from the trapped zone. The polarization effect on reservoir sandstone under an electric double layer is one of the major research interests. The nanoparticles agglomeration such as graphene nanofluid due to poor dispersion in reservoir zone can be a major challenge that can lead to low reservoir permeability are well elucidated. This study investigated the influence of graphene nanofluid on the ionic polarization under an electric double layer in reservoir sandstone. Saturated Berea sandstone was used to investigate the interaction of ionic species on reservoir sandstone with the aid of Field-emission microscopy (FESEM), Energy dispersive X-ray mapping (EDX), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectral analysis, and Core flooding experiment. This research gives information on the adsorption of ions within an electric double layer and its polarization mechanisms. It was revealed from the experimental result that ionic polarization occurs at 9.97 GHz with a 5.8nm wavelength shift which improves the mobility of the reservoir and in turn increases oil recovery factors. Graphene nanoparticles show a positive effect on both reservoir oil viscosity and stabilization characteristics of drilling fluids, wettability alteration, interfacial tension, and improving the emulsion Keywords: Nanomaterials, Sandstone, Electric double layer, Graphene

Abstract: The behavior of hydrogen atom isotopes in a diamond-like lattice without defects is studied. The movement of the hydrogen atom along the interstices is considered. The collective nature of the movement of the hydrogen atom (and its isotopes) is taken into account, which consists in the fact that any movement of the hydrogen atom is accompanied by reversible displacements of the nearest lattice atoms. To take into account this nature of the movement, local chains are built. As a result, a new one-dimensional Lagrangian is derived, the equations of motion from which lead to a soliton-like solution in the form of a kink - Frenkel-Kontorova soliton. The study of the structure of such a kink and the distribution of displacement velocities in it reveals that the kink formed by tritium moves faster through the lattice than the kinks formed by deuterium and hydrogen.

Abstract: The stability of dispersed nanoparticles in the base fluid has always been one of the most important challenges in using nanofluids as a coolant in heat transfer applications in different industries such as modern electronic equipment, heat exchangers, solar technologies, etc. In the present study, the dynamic light scattering (DLS) method is used to obtain the particle size distribution of Al₂O₃-ZnO dispersed in DI water. After adjusting the optical arrangement and designing the DLS setup, the correlation curves are plotted by analyzing the detected signals of the experiments. Then, a decay rate is derived by fitting an exponential function to the correlation curve to get the particle size distribution by using the Stoke-Einstein equation. In order to investigate the stability of Al₂O₃-ZnO water-based nanofluid, the particle size distribution profiles are studied several times. In addition, the stability of Al₂O₃-ZnO-CNT hybrid nanofluid is followed by absorbance measurements using a UV-Vis spectrophotometer. Moreover, the thermal conductivity coefficient and electrical conductivity of the Al₂O₃-ZnO hybrid nanofluid with and without CNT particles are determined by utilizing KD2 Pro and PCT-407 devices, respectively. The results showed that the peak in the particle size distribution curve for Al₂O₃-ZnO hybrid nanofluid shifted from 476 nm to 128 nm after 5 days. Furthermore, the inclusion of carbon nanotubes increased the stability of zinc oxide particles in the nanofluid. In addition, by adding carbon nanotubes in a ratio of 1:1:0.5 to Al₂O₃-ZnO nanofluid and forming 0.05 wt.% hybrid nanofluid, the thermal conductivity coefficient was enhanced by 30% in comparison with deionized water, while a 0.05 wt.% hybrid nanofluid without CNT particles improved the thermal conductivity by 19%. Although the electrical conductivity increased by adding nanoparticles to the base fluid, it didn’t change significantly for nanofluids containing CNTs compared to nanofluids without CNT particles.

Abstract: In this paper, the diffusion isotope effect and the Manning factor are investigated by means of Molecular Dynamics simulations in liquid Cu-Ag alloys. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms that are moving cooperatively in a basic diffusion event as ‘seen’ by a given (tracer) atom. On average, in the considered alloys and considered temperatures, this is limited to between 5 and 15. This is consistent with results of Molecular Dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, a majority of the neighbouring atoms are involved in a basic diffusion event. Results for the Manning factor (MD simulation) for Cu-Ag liquid alloys are seemingly in agreement with the direct exchange mechanism where only two atoms are involved in the elementary diffusion event. This is not in apparent agreement with the isotope effect results. It was shown, however, that any ring mechanism, or, more complex, cage mechanism are, in fact, a combination of several simultaneously happening direct exchanges. Any other possible mechanisms for diffusion in liquids is most likely a combination of direct exchanges as well. It can be seen then that the collective nature of all considered mechanisms is very similar and follows the direct exchange signature properties.

Abstract: Stokes-Einstein relation is a convenient way to evaluate diffusion properties in liquids from viscosity results (and vice-versa). However, the accuracy of this relation in the case of atomic fluids is often debated as it was initially established in the case of a big Brownian particle immersed in a fluid. Especially, the question is raised to properly define the hydrodynamic radius entering the formula, as well as the constant depending on the boundary conditions at the surface of the particle. In this study, we use our results of viscosity and self-diffusion coefficient obtained by molecular dynamics simulations in the case of alkali metals and their alloys to evaluate the applicability of Stokes-Einstein relation in the case of these liquids. In the case of pure metals, its validity is discussed over a wide range of thermodynamic states, from ambient pressure up to several gigapascals. In the case of alloys, the evolution of its accuracy as a function of temperature and composition is considered. Both definitions of hydrodynamic radius and boundary conditions constant are examined.

Abstract: In this work, the comparison between the Newtonian and the Power Law models was pointed out to determine the flow field within a system that includes the bypass of a narrowing zone (Stenosis) of an ideal artery. The simulations were conducted in a transient regime considering the pulsed effect inside the veins. The results show that the Power Law model does not sufficiently coincide with the Newtonian model, especially for describing the wall stresses. More specifically, the Power Law model predicts a velocity profile that remains parabolic for the entire time range analyzed, while the Newtonian model captures vorticities.

Abstract: Despite many desirable properties, most phase change materials (PCMs) undergo timing issues during the phase change process due to a low thermal conductivity, which limits their application in heat storages. Thus, many techniques have been pointed out to overcome these disadvantages and improve heat transfer, such as coupling PCMs with metal inserts, like high porosity open-cell metal foams. Indeed, the presence of a metal foam increases the effective thermal conductivity of the composite medium and speeds up the charging and discharging phases. In the present paper, a numerical model developed in COMSOL ambient has been calibrated by comparison with experimental results on the melting of pure and metal-foam loaded PCMs, placed in a small case and heated from the top by an electric resistance. The numerical model considers the metal foam as a static solid, filled with a phase changing fluid and employs a literature correlation to evaluate the effective thermal conductivity of the composite medium. The performance of two different paraffinic PCMs (RT35 and RT35HC by Rubitherm GmbH, D), loaded either with a copper foam (20 PPI, 95% porosity, by Porometal, China) or with an aluminum one (10 PPI, 96% porosity, by Recemat, NL) has been investigated in terms of speed rate of the phase change front, time required to complete the melting process, temperature distribution and effect of foam porosity. The obtained results clearly evidence the significant heat transfer improvement yielded by metal foams, whose presence increases the effective thermal conductivity of the composite medium (from 0.2 to 7.03 W/mK for copper foam and to 3.52 W/mK for aluminum one), leading to a significant decrease of the charging time and to a lower temperature gradient within the PCM (from about 16 to 3 K).