Advanced Materials Research Vol. 409

Abstract: Manganese-Zinc ferrites belong to the group of soft ferrite materials characterized by high magnetic permeability and low power loses. These materials are mainly used as cores for inductors, transformers, recording heads and noise filters among others. In this study, nanocrystalline Mn-Zn ferrite with the chemical formula Mn_1-xZn_xFe₂O₄ with x=0.2, 0.4, 0.6, 0.8 has been successfully synthesized by glycine-nitrate auto-combustion process using glycine as a fuel and nitrates as oxidants. The structures and magnetic properties of the resulting powder were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). It is revealed from the XRD pattern than a significant amount nanocrystalline Mn_1-xZn_xFe₂O₄ ferrite with average crystallite size in the range 43.25-66.7 nm has been formed. The magnetic measurement gives a typical value of saturation magnetic of 34-69 emu/g and coercivity of 40-60 Oe.

Abstract: In this study, alternatives to corrosion-resistant cadmium coatings on high strength steel fasteners are explored. Nanocrystalline Zn-Ni and Zn-Ni-Co electrodeposits, synthesized by electrochemical deposition, were analyzed by potentiodynamic polarization in a 1M NaCl aqueous solution, to determine the corrosion behaviour of the alloys. Sample coatings were characterized using scanning electron microscopy to examine differences in surface morphology that can affect corrosion resistance. Energy dispersive x-ray spectroscopy was used to relate composition to corrosion resistance and to characterize the corrosion behavior during polarization. All coatings showed corrosion potentials at equivalent, or lower values than that of cadmium, suggesting that both alloys are viable as cadmium replacements. Specific compositions showed slightly passivating regions, but the brittle film formed during polarization is unstable. Comparatively, Zn-Ni shows a better resistance to corrosion than Zn-Ni-Co. The effect of composition on the corrosion resistance can be explained on the basis of the sacrificial depletion of Zn over Ni.

Abstract: The electronic states of Sr_1-xNd_xMnO₃ with the simple perovskite structure are characterized by a three-dimensional highly-correlated electronic system. To understand the detailed features of the A-and CE-type antiferromagnetic states in this system, their crystallographic features for x = 0.47, 0.48, and 0.50 have been investigated mainly by both x-ray powder diffraction and transmission electron microscopy. It was found at room temperature that the crystal structure for x = 0.47 was determined to have the monoclinic C2/m symmetry, while the orthorhombic Imma structure was confirmed for x = 0.48 and 0.50. The in-situ observation for x = 0.47 indicated that, in the heating process from room temperature, the C2/m-to-Imma transition occurred in the paramagnetic state, and that the A-type antiferromagnetic state appeared below about 200 K on cooling. In addition, the cooling from room temperature for x = 0.48 and 0.50 resulted in the direct transitions from the orthorhombic Imma state to the A-and CE-type antiferromagnetic states, respectively. Based on these features, we simply discussed the physical origin of the appearance of the paramagnetic state with the monoclinic symmetry for x = 0.47.

Abstract: Oligothiophene derivatives, 4,4’-bis-(2-aminoethylthienyl)-3,4-ethylenedioxythiophene dihydrohalide (AET-EDOT·HX: X = I, Br), were synthesized by Grignard coupling reactions. The hybrid films of oligothiophene and lead halide were fabricated by casting and spin-coating methods. The effect of halogen species on the optical properties of the perovskites was investigated. The X-ray diffraction profiles of (AET-EDOT)PbX₄ cast films indicated that the layered structures were formed by the self-organization. After hybridization of AET-EDOT·HX with PbX₂, new absorption peaks due to the confined excitons were observed at 400 nm for (AET-EDOT)PbBr₄ film, and at 395 nm for (AET-EDOT)PbI₄ film. These results indicated that the diamino-type oligothiophenes were successfully incorporated into the organic-inorganic layered perovskites. AFM images of (AET-EDOT)PbX₄ cast films showed polycrystalline grains, indicating that the polycrystalline structures covered on the substrate surface.

Abstract: In order to reduce CO₂ emissions, the cement industry has developed a new class cements. The Calcium-Silicate-Hydrates (CSH) that form are generally characterized by a low stoichiometric ratio for CaO and SiO₂. This low C/S ratio affects the C-S-H layer structure and has a significant effect on the mechanical properties. This work exploits a novel statistical nanoindentation technique (SNT) to study the effect of the C/S ratio on the mechanical properties of synthetic CSH. Different CSH types were prepared by varying the C/S ratio of the starting materials. After undertaking a grid nanoindentation approach for each sample, the statistical analysis allowed extracting the mechanical properties, such as elastic modulus, hardness and creep. The results of this preliminary work shed new light on the implications of C-S-H stoichiometry on mechanical properties.

Abstract: The work hardening behavior of electrodeposited nanocrystalline (grain size: 100 nm) and fully annealed polycrystalline nickel (grain size: 160 µm) was examined by hardness indentation analysis. First, plastic strain was introduced into the materials through large Rockwell hardness indentations. A series of Vickers micro-hardness traces below and away from the Rockwell indentation then measured the change in hardness as a function of distance from the plastic zone. The results showed that polycrystalline nickel exhibited considerable strain hardening, with micro-hardness values closest to the Rockwell indentation averaging twice the hardness value of the bulk material. On the other hand, for the nanocrystalline nickel the Vickers micro-hardness values changed only by a few percent indicating a limited strain hardening capacity.

Abstract: The mixed-oxide system Ba (Ti_1-xZr_x)O₃ (BTZ) with the simple perovskite structure was reported to exhibit the remarkable dielectric response around x = 0.08. In order to understand the origin of the response, we have investigated the crystallographic features of ferroelectric states in BTZ with 0 ≤ x ≤ 0.15 mainly by transmission electron microscopy. According to the constructed phase diagram of BTZ, when the Zr content increases from x = 0 around 300 K, the ferroelectric tetragonal state in BaTiO₃ is changed into the ferroelectric orthorhombic state around x = 0.03, and then into the ferroelectric monoclinic state around x = 0.06. That is, the monoclinic state with a polarization vector in the {110}_PC planes is present in BTZ with 0.07 ≤ x ≤ 0.15, where the subscript PC denotes the paraelectric cubic state. The notable feature of the monoclinic state is that it exhibits the herringbone-type domain structure consisting of nanometer-sized ferroelectric domains. It is thus understood that the remarkable dielectric response in BTZ should be associated with the presence of nanometer-sized domains with the monoclinic symmetry.

Abstract: The grain-size dependences of thermal conductivity and electrical resistivity of polycrystalline and nanocrystalline nickel were measured by the flash method and four-point probe method, respectively. Nanocrystalline nickel (grain size: 28 nm) was made by the pulsed-current electrodeposition process, while polycrystalline nickel (grain size: 57 μm) was the same material in fully annealed condition. Noticeable differences in thermal conductivity and electrical resistivity were observed for both materials. These results can be explained on the basis of the rapid increase in the intercrystalline grain boundary and triple junction volume fractions at very small grain sizes. The relationship between thermal conductivity and electrical resistivity of nanocrystalline nickel follows the classic Wiedemann-Franz law.

Abstract: This work was devoted to the study of a copper based shape memory alloy using the neutron diffraction method. We present the crystallographic texture evolution of the austenite phase of the Cu-11%Al-0.62%Be (wt. %) alloy which was subjected to cold and hot rolling processes. The texture of the cold rolled samples at reduction rates of 5%, 10% and 15% respectively was measured and compared to the raw material. A <001> partial fiber is observed for the raw material and for cold rolled samples whereas the texture of the hot rolled sample is mainly characterized by a <111> fiber. On the second part, we have studied the influence of the rolling treatment on the transition temperatures by following the integrated intensity of the {220} austenite reflection. The increase of the rolling rate seems to shift the characteristic temperatures towards low temperatures.

Abstract: The feasibility of measuring residual stresses in Self-Pierce Riveted (SPR) joints by neutron diffraction was evaluated in this study. Despite the small dimensions involved, meaningful results were obtained. It was observed that residual stress in the rivet head was higher in the centre and lower at the edge. For the SPR joints examined, the maximum value of residual stress evaluated was 550MPa, compressive and occurred in the rivet leg. Stresses in material adjacent to the rivet wall and at a distance of three times the rivet radius from the rivet axis were not significant. The results are discussed with respect to the physical events involved during SPR.