Fabrication and characterization of Cu–CdSe–Cu nanowire heterojunctions

The Cu–CdSe–Cu nanowire heterojunctions were fabricated by sequential electrochemical deposition of layers of Cu metal and CdSe semiconductor within the nano-pores of anodic alumina membrane templates. X-ray diffraction reveals the cubic phase for Cu and hexagonal phase for CdSe in the electrodeposited Cu–CdSe–Cu nanowire heterojunctions. The composition of the nanowire heterojunction segments is characterized by energy dispersive X-ray spectroscopy. The morphological study of nanowire heterojunctions has been made using scanning electron microscope and high resolution transmission microscopy. The nanowire heterojunctions grown in 100 and 300 nm nano-pore size templates have been found to have optical band gaps of 1.92 and 1.75 eV, respectively. The absorption spectra of 100 nm nanowire heterojunctions show a blue shift of 0.18 eV. The collective nonlinear current–voltage (I–V) characteristics of the 300 and 100 nm nanowire heterojunctions show their rectifying and asymmetric behaviour, respectively.     

Directional solidification of Al–Cu–Ag alloy

Al–Cu–Ag alloy was prepared in a graphite crucible under a vacuum atmosphere. The samples were directionally solidified upwards under an argon atmosphere with different temperature gradients (G=3.99–8.79 K/mm), at a constant growth rate (V=8.30 μm/s), and with different growth rates (V=1.83–498.25 μm/s), at a constant gradient (G=8.79 K/mm) by using the Bridgman type directional solidification apparatus. The microstructure of Al-12.80-at.%–Cu-18.10-at.%–Ag alloy seems to be two fibrous and one lamellar structure. The interlamellar spacings (λ) were measured from transverse sections of the samples. The dependence of interlamellar spacings (λ) on the temperature gradient (G) and the growth rate (V) were determined by using linear regression analysis. According to these results it has been found that the value of λ decreases with the increase of values of G and V. The values of λ ² V were also determined by using the measured values of λ and V. The experimental results were compared with two-phase growth from binary and ternary eutectic liquid.     

63Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

⁶³Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al₂CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α?+?S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al₂CuMg. Analysis of the hyperfine structure of the ⁶³Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al₂CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α?+?θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al₂Cu. A hyperfine-structured ⁶³Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7?at.%?Cu–0.33?at.%?Mg alloys containing 0.1 and 0.18?at.%?Ag, but not in the same Al–Cu–Mg alloy without Ag.     

Measurements of magnetostriction of paramagnetic Fe–Mo–Cu–B metallic glasses

Magnetostriction of amorphous Fe₇₉Mo₈Cu₁B₁₂, (Fe₁₂Co₁)₇₉Mo₈Cu₁B₁₂ and (Fe₉Co₁)₇₉Mo₈Cu₁B₁₂ prepared by planar flow casting was measured using a direct method. The results indicate that magnetostriction in parallel (_λ∥)$({\lambda }_{\parallel })$ and perpendicular (_λ⊥)$({\lambda }_{\perp })$ directions of applied magnetic field is linearly dependent on magnetic field. In order to determine the influences of chemical composition and the conditions of sample preparation the magnetostriction of pure BCC-Fe, Cu and Mo were also measured. Samples containing Co with Curie temperatures slightly above room temperatures were shown to exhibit a hybrid magnetostriction behaviour with both ferromagnetic and paramagnetic features.     

Origin of tweed in Au–Cu–Al alloys

The premartensitic tweed in Au–Cu–Al alloys, contrary to previous thought that resort to defects, is confirmed to be associated with the coherent embryos of an intermediate phase (I phase) embedded in parent phase. The parent?→?I phase transformation temperature was measured by differential scanning calorimeter and dynamic mechanical analysers, which shifts from 82.3 to 557.6?°C depending on the alloy composition. X-ray diffraction and transmission electron microscopes (TEM) results show that the parent?→?I phase transformation is a charge density wave transition that cannot be suppressed even by melt-spun method, which shows obvious compositional inhomogeneity between I phase and parent. The results imply that the parent?→?I phase transition is a fast displacive transformation coupled with diffusion. Moreover, accompanying the parent?→?I phase transformation, alloys demonstrate diversified microstructure revealed by TEM observation, from tweed to chessboard nanowires or twins. These findings provide the experimental evidence for that parent?→?I phase transformation in Au–Cu–Al alloys is originated from pseudospinodal decomposition as theoretically predicted.     

Superconductivity of Y–Ba–Cu–O and substituted systems

Superconductivity of Y-Ba-Cu-O system is studied in the composition 2:2:3 and 1:2:3 of Y:Ba:Cu. The effect of replacement of Y or Ba by divalent Sr and Ca, trivalent Ce and tetravalent Zr is studied. X-ray diffraction, SEM and TEM techniques are used for materials characterization. Superconducting transition temperatures are measured resistively. Rapid resistance drop observed above 230 K in Y-Ba-Sr-Cu-O and Y-Ba-Ca-Cu-O systems indicate the possible existence of superconductivity above 230 K. Substitution of Ce in place of Y is found to reduce the onset Tc from 95 K to 80 K. For the first time, replacement of Cu by Zr in Y-Ba-Cu-O has yielded the onset Tc of about 105 K.     

Cu2ZnSnSe4 films by selenization of Sn–Zn–Cu sequential films

This paper deals with the formation of Cu₂ZnSnSe₄ (CZTS) in the process of selenization of metal precursor layers in elemental selenium vapour. Metallic precursors were sequentially evaported from Sn, Zn and Cu sources. Precursor Sn–Zn–Cu films have a “mesa-like” structure and consist mainly of Cu₅Zn₈ and Cu₆Sn₅ phases. It was confirmed that the formation of different binary copper selenides is the dominating process of selenization in elemental Se vapour at temperatures up to 300 °C. The formation of kesterite CZTS films begins at 300 °C and dominates at higher temperatures, always resulting in multiphase films that consist of high-quality Cu₂ZnSnSe₄ crystals and of a separate phase of ZnSe.     

Characterization of Ni–Cu mixed spinel ferrite

Crystal structure, X-ray density, porosity, compressive strength of Ni_1−xCu_xFe₂O₄ have been investigated along with scanning electron microscopy (SEM) to study the effect of composition and microstructure on the magnetic and electrical properties. The formation of single-phase ferrite is confirmed by the X-ray diffraction. Tetragonal deformation is observed for the sample of composition x=1, i.e. for pure CuFe₂O₄ Crystal structure for samples of other compositions are face centered cubic (FCC). SEM micrographs exhibit increase in grain size with the increase of copper content. Compressive strength decreases with the increase of Cu. Initial magnetic permeability and saturation magnetization is maximum for the composition of x=0.2, i.e. for Ni_0.8Cu_0.2Fe₂O₄, which can be attributed to the maximum sintered density obtained for this composition. Resistivity decreases with the increase of Cu content.     

Optimization of the properties of codoped single-domain Y–Ba–Cu–O bulk superconductors

The intrinsic effects of nanoscopic MnO₂ powders addition combined with Fe cation substitutions for copper sites on the microstructure and superconductive properties of YBa₂Cu₃O_7?δ (Y123) melt-solidified bulks have been investigated. On the one hand, an increase in Y₂BaCuO₅ (Y211) particle pushing, leading to an inhomogeneous bulk microstructure, is caused by increasing MnO₂ content due to increased net interfacial energy, Δσ₀; and, on the other hand, an addition of MnO₂ powders is effective in enhancing both the δT_c-type and δl-type pinning. It also shows that the Fe addition helps to optimize the high magnetic field performance and Y211 particle distribution in textured pellets. Further, this experiment suggests that a combination of the element substitution and the nanoscopic particle is a beneficial way to optimize the microstructure and superconductive properties of single-domain bulk superconductors.     

Top-seeded infiltration growth of Y–Ba–Cu–O bulk superconductors

A top-seeded melt-growth (TSMG) process is widely used to fabricate single domain YBa₂Cu₃O_y (Y–Ba–Cu–O) bulk superconductors. Pores are often found in the TSMG-processed Y–Ba–Cu–O samples due to the oxygen gas evolution during the molten stage. Recently developed liquid infiltration growth (LIG) process is known to be effective in suppressing the pore evolution and in refining the size of Y₂BaCuO₅ (Y211) particles dispersed in YBa₂Cu₃O_y matrix. The LIG process utilizes the liquid (Ba₃Cu₅O₈) infiltration into a pre- sintered Y211 contact and slow cooling through a peritectic temperature. In this study, we fabricated bulk Y–Ba–Cu–O superconductors by the LIG process combined with top-seeding with SmBa₂Cu₃O_y seed and confirmed that a single-domain bulk can be produced. Trapped field measurements however showed that some distortion in the field distribution was observed in the region near the seed crystal, which was attributed to Y211 density and its relatively large size.     

Magnetic properties of Zn-substituted Li–Cu ferrites

The magnetic properties of Zn-substituted Li–Cu ferrites having the general formula Li_xCu_0.4Zn_0.6−2xFe_2+xO₄ (where, x=0.0, 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3) are investigated as a function of Zn content. The X-ray analysis confirms the formation of single-phase of the samples. The lattice parameter increased linearly with Zn content, which is attributed to ionic size differences of the cations involved. The bond lengths R_A and R_B are found to increase with increase in Zn content. The increase of R_A and R_B is attributed to the increase in lattice parameter. The magnetic moment increases with increase in zinc content up to 0.3 and then it decreases with further addition of Zn. The decrease in magnetic moment beyond Zn=0.3 is due to the presence of a triangular spin arrangement on B-site; the three sublattice model suggested by Yafet and Kittle. The initial permeability (μ_i) has been measured, and it is found that it decreases with increase in Zn content. The Curie temperatures also show a decreasing trend.     

Abnormal resistivity behavior of Cu–Ni and Cu–Co alloys in undercooled liquid state

The resistivity behavior of undercooled liquid Cu–Ni and Cu–Co alloys had been studied in the contactless method, to probe the structure transition in undercooled melts during the cooling process. Over the entire concentration range, linear behavior of resistivity with temperature was obtained in liquid and undercooled liquid Cu–Ni system. It implied that the formation of icosahedral order might not influence the electron scattering in undercooled liquid Cu–Ni alloys. Similar results were obtained in Cu–Co system in the vicinity of liquidus temperature. A turning point was obvious in temperature coefficient of resistivity for undercooled liquid Cu–Co alloys around the bimodal line, which was interpreted to be responsible for metastable liquid–liquid phase separation. During liquid phase separation process, resistivity decreased and the temperature coefficient of resistivity was larger than that of homogeneous melts. In combination with transmission electron microscopy and scanning electron microscope studies on the as-solidified microstructure, this was interpreted as the formation of egg-type structure and concentration change in Cu-rich and Co-rich phases. The mechanism controlling the separation and droplets motion was also discussed in undercooled liquid Cu–Co system.     

Au–Cu nanoparticles produced by laser ablation of mixtures of Au and Cu microparticles

In this study we investigate the possibility of producing alloy nanoparticles (NP) from mixtures of elemental microparticle (MP) powders using the laser ablation of microparticle (LAM) process. Mixtures of Au and Cu particles with a diameter of 1.5–2.0 m were fed in aerosol form into a laser ablation cell and ablated using a pulsed laser. The resulting NP were collected electrostatically and characterized using TEM. The NP were spherical and crystalline with a size that depended on the collection location but ranged from 2 to 15 nm. Using TEM/SAD, it was determined that the NP had a face-centered cubic (fcc) crystal structure and, with EDS, it was found that individual NP consisted of both Au and Cu. These experimental results confirm previous numerical models that suggested that it might be possible to form alloy NP from mixtures of elemental MP using the LAM process.     

Approaches to the synthesis of High-TC superconducting oxides in La–Ba–Cu–O and Y–Ba–Cu–O systems

Abstract

High-T_C superconducting oxides of nominal La_1.85Ba_0.15 CuO₄ and YBa₂ Cu₃ O₇ have been prepared by using nitrate, carbonate, oxalate/malonate and citrate precursors. While the samples in the Y-system are generally monophasic YBa₂Cu₃O_7?δ with T_C around 90K, the preparations in the La-system are biphasic containing K₂NiF₄-like La_1.85Ba_0.15 CuO₄ (T_C = 30K) and a perovskite-like phase with' a much higher T_C (200–300K). Effect of Ca, Zr, Ce as well as S substitution in YBa₂Cu₃O_7?δ has also been investigated     

Modification of the Structure of Ti–Al–Si–Cu–N Gradient Coatings by Mechanical Tests

Technical Physics - The structural state in the zones of indentation and scratch testing of Ti–Al–Si–Cu–N gradient coatings has been studied using dark-field electron...     

Solidification mechanism transition of liquid Co–Cu–Ni ternary alloy

We report a solidification mechanism transition of liquid ternary Co₄₅Cu₄₅Ni₁₀ alloy when it solidifies at a critical undercooling of about 344 K. When undercooling at ΔT<344 K, the solidification process is characterized by primary S (Co) dendritic growth and a subsequent peritectic transition. The dendritic growth velocity of S (Co) dendrite increases with the rise of undercooling. However, once ΔT>344 K, the solidification velocity decreases with the increase of undercooling. In this case, liquid/liquid phase separation takes place prior to solidification. The minor L₂ (Cu) droplets hinder the motion of the solidification front, and a monotectic transition may occur in the major L₁ phase. These facts caused by metastable phase separation are responsible for the slow growth at high undercoolings.     

Tensile behavior of post-irradiation annealed Cu–Ni alloy

The present paper investigates the tensile properties of post-irradiation annealed Cu–Ni alloy. The specimens were irradiated with a 15 MeV electron beam at room temperature and the post-irradiation annealing (PIA) of the specimens was carried out under vacuum at 450 °C for 15–120 min. The yield stress (YS), ultimate tensile stress (UTS), percentage elongation, stress relaxation rate and activation volume of both as-irradiated and post-irradiation annealed specimens were examined at room temperature using a universal testing machine. The results show that PIA of the specimen at 450 °C for 15 min decreases its YS and UTS, whereas the percentage elongation is increased. The changes in the tensile parameters become more pronounced with increases in annealing time. Effects of PIA on the stress relaxation rate and activation volume indicate that the relaxation rate of post-irradiation annealed specimens increases, and the activation volume decreases, with an increase in annealing time.     

Cyclic deformation and fatigue cracking behaviour of polycrystalline Cu,Cu–10 wt% Zn and Cu–32 wt% Zn

The cyclic deformation behaviour of polycrystalline Cu, Cu–10 wt% Zn and Cu–32 wt% Zn was systematically investigated in the plastic strain amplitude range of 1 × 10^?4–4 × 10^?3. The differences in the cyclic stress–strain (CSS) responses and fatigue cracking behaviour between Cu, Cu–10 wt% Zn and Cu–32 wt% Zn were compared. It was found that the occurrence of a cyclic saturation for Cu–10 wt% Zn and Cu–32 wt% Zn strongly depends on the applied strain amplitude, whereas polycrystalline Cu always displays cyclic saturation. Surface deformation morphologies were analyzed by scanning electron microscopy (SEM). One of the major features observed is that the slip bands become increasingly homogenous with Zn addition. The fatigue cracks were found to frequently nucleate along the annealing twin boundaries (TBs) in Cu–10 wt% Zn and Cu–32 wt% Zn, but not in polycrystalline Cu. Based on these experimental results, the cyclic deformation response and fatigue cracking behaviour are discussed, and a developed TB cracking mechanism is proposed to explain the difference in fatigue cracking mechanisms in Cu, Cu–10 wt% Zn and Cu–32 wt% Zn.