Defect and Diffusion Forum Vol. 353

Abstract: AZ91 Mg alloy was micro arc oxidized under constant electrical parameters in silicate based and phosphate based electrolytes with and without addition of organic chemicals, namely Hexamethylenetetramine (HMTA), TRIS (hydroxymethyl) aminomethane (THAM) and Glycerol in two different concentrations. Following oxidation, samples were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), coating thickness measurements, hardness measurements and electrochemical corrosion tests. Results showed that coating layers mainly consisted of MgO, Mg₂SiO₄ and MgF₂ for silicate based electrolytes, and MgO for phosphate based electrolytes. Incorporation of organic chemicals into electrolyte composition did not change the type of the phases in the coating. However, when they are added in silicate based electrolytes, pore density and coating thickness are reduced and pore size is increased. On the other hand, there is no significant change in surface morphology when organic chemicals are added in phosphate based electrolyte. In the view point of corrosion resistance, organic chemicals did not enhance corrosion resistance of the samples oxidized in silicate based electrolytes, but exhibited some increment in corrosion resistance of the samples oxidized in phosphate based electrolytes.

Abstract: The interaction among heavy interstitial atoms present in metals with bcc structure is studied using anelastic spectroscopy. This technique makes it possible to obtain information on interstitial concentration, precipitation, solubility limit, and diffusion. The diffusion coefficients of nitrogen in niobium were obtained using the relaxation parameters obtained from anelastic spectroscopy measurements for different oscillation frequencies of the system. The results showed the interstitial diffusion of nitrogen present in solid solution in niobium when submitted to different charges of nitrogen at a temperature of 1373 K and a partial pressure in the order of 10^-4 Torr. The exponential variation of the pressure experimentally in function of the time was thus obtained.

Abstract: The microstructure of sodium or potassium added Mg-23.5 mass%Ni eutectic alloy was observed and the effect of sodium and potassium addition on the eutectic structure was investigated. All alloys showed the eutectic structure which composed of Mg phase and Mg₂Ni phase, and no evidence of the sodium or potassium precipitation occurred. For the sodium added and furnace cooled alloy, the lamellar spacing in the eutectic structure became a little narrow and the Mg₂Ni phase tends to become fragmentary as the amount of sodium increases. The sodium addition has a little effect for the refinement of the eutectic structure. For the potassium added and furnace cooled alloy, the lamellar spacing of the eutectic structure became clearly narrow even by the 0.1 mass% potassium additions. The morphology drastically varied in more than 0.5 mass% potassium added specimens, that is, the refinement and fragmentation of the Mg₂Ni phase occurred. The potassium addition has a large effect for the refinement and the fragmentation of the Mg₂Ni phase in the eutectic structure. For the water quenched specimens, the eutectic structure was extremely fine and globular shape with and without the additive element. The refinement effect by the water quenching is remarkably high even as the non-added specimen. The effect of sodium and potassium addition on the refinement of eutectic structure is not clear in the case of the rapid cooling speed during solidification.

Abstract: A high purity of strontium-doped lanthanum cobaltite with formula of La_0.6Sr_0.4CoO_3-δ (LSCO) was synthesized via a combined citrate-EDTA route. LSCO slurry was prepared by mixing LSCO and polyvinyl pyrrolidone (PVP) in ethanol solution. This slurry was manually painted onto both surfaces of yttrium-doped cerate-zirconate, BaCe_0.54Zr_0.36Y_0.1O_2.95 (BCZY) electrolyte to fabricate a symmetrical cell of LSCO|BCZY|LSCO. The scanning electron microscopy (SEM) analysis result revealed that the LSCO was well adhered onto the BCZY electrolyte with no formation of crack or air gap/hole at the LSCO|BCZY interface. Elemental composition of LSCO cathode and BCZY electrolyte elements such as lanthanum (La), barium (Ba) and cerium (Ce) at the interface region was confirmed by electron dispersive spectroscopy (EDS) analysis. The electrochemical performance of the fired cell was analyzed in air by an electrochemical impedance spectroscopy (EIS) as a function of temperature ranging from 500 – 800°C. It is found that the fabricated cell exhibits low polarization resistance (R_p) at the operating temperatures and the values are comparable with those reported in literature. This significant result indicates that LSCO is a promising candidate to be used as a cathode material for BCZY electrolyte at intermediate temperatures.

Abstract: β-SiC particles were synthesized using silicon powder and carbon black at 1300°C. The effect of grinding the silicon powder raw material into smaller particles on the synthesis and crystallinity of the SiC particles was investigated. The size of the silicon particles decreased and the surface of the silicon particles increasingly oxidized into silicon oxide with increasing grinding time, thus decreasing the yield of SiC because of the higher SiO₂ content in the ground silicon powder, since SiO₂ transforms into SiC at a higher temperature than does silicon. Although the sizes of the silicon particles in the raw materials were different, all the synthesized SiC crystallites showed similar size. Transmission electron microscopy-energy dispersive X-ray spectroscopy analysis showed that although most of the synthesized SiC particles were smaller than 100 nm, the primary SiC particles had aggregated, and some unreacted silicon was observed inside the large aggregated SiC particles.

Abstract: Since mesoporous silica such as MCM-41 and SBA-15 was developed, the study of the properties of high-surface area materials was accelerated. Moreover, the mesoporous silica is used as a template to produce high-surface materials by nanocasting technology. The purpose of this paper is the synthesis of a high surface silicon carbide sphere by the nanocasting technology. In this study, KCC-1 silica sphere was used as a template, and polycarbosilane and poly (phenyl carbosilane) were selected for precursor of silicon carbide. Carbosilane polymer gives advantage of synthesis silicon carbide under low temperature, and hollow spheres were produced. In this study, the polycarbosilane was more effective for the synthesis of SiC hollow spheres by inversion of template structure showing a fibrous morphology on the sphere wall. And it was confirmed that the sphere was composed of nanosized SiC crystals, and has high surface area using TEM, XRD and BET analysis.

Abstract: The characterization and fracture toughness with hard coatings formed at the surface of gray cast irons class 30 is evaluated in the present study. The formation of hard coatings was obtained out means of the pack boriding process; the treatment was carried out at temperatures of 1173 and 1223 K during 6 hours. The layers were evaluated by the techniques of X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and microindentation across the thickness of the iron boride layer. Three-point bending tests are carried out to examine the fracture toughness of gray cast irons boriding according to the ASTM 399 standard. Consequently, the stress intensity factor was evaluated by means of the finite element method (FEM) using the package ANSYS 11. 0 creating a two-dimensional model with elements of singularity around the tip crack. The results were compared with the experiments and have been found to be in good correlation.

Abstract: This study aims to identify the effects caused by diffusion through heat treatment in diamonds, brazed components with metals and its effects on mechanical properties and microstructure. It will be used diamond films produced by CVD process, brazed by active filler metals with metallic substrate, using a high vacuum furnace at MAT / LNLS / CNPEM; the samples were heat treated in the atmosphere furnace at MAT too. The characterizations are carried out by hardness testing, vacuum sealing equipment at the MAT / LNLS / CNPEM facilities, and chemical and microstructural characterization will be performed using the scanning electron microscopes and EDS at LNNano / CNPEM.

Abstract: We investigated two coatings, Pt:Al and Pt3:Al, deposited onto pure Al substrates. After annealing at 500°C for 2 hours in vacuum, the Pt:Al coating became “matt silver” while the Pt3:Al colour was “old gold”. Although the deposited coatings were relatively smooth, rope-and ridge-like morphologies were observed in the annealed Pt:Al and Pt3:Al coatings, respectively. X-ray diffraction analyses revealed the presence of Al₂₁Pt₆ and Al₂Pt intermetallics in annealed Pt:Al while only the Al₂Pt intermetallic phase was observed in the annealed Pt3:Al coatings. RBS spectra showed an increase of coating thickness by four times in the Pt:Al whilst the coating thickness was doubled in the Pt3:Al coatings. The changes to color, surface morphology, and thickness caused by thermal annealing were attributed to the structural characteristics of the most dominant phases.

Abstract: The effect of Na₂S on the phase composition and microstructure of tempered Cu₂S-Ni₃S₂ alloys was studied by X-ray diffraction, optical microscopy and electron probe microanalysis (EPMA). It was found that quick crystallization of the sulfide melt leads to separation into two phases - Ni₃S₂ and a solid solution of Cu₂S with Na₂Cu₄S₅, moreover, nickel is concentrated in large particles and copper – in small ones. In contrast to the fine dendrite solidification of granular Cu₂S-Ni₃S₂ alloys, in the ternary system there is a well-defined two-phase microstructure with rounded borders of the interface. Friability and a low microhardness of Cu₂S - Na₂Cu₄S₅ solid solution provide an autodecomposition of the sulfides melt by quenching into water (granulation). The degree of separation of copper and nickel depends on the overheating temperature and a quantity of Na₂S in melt. The results can be used to hydrometallurgical processing of copper-nickel convert matt.