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.     

Internal stresses in cold-deformed Cu–Ag and Cu–Nb wires

The co-deformation of Cu–Ag or Cu–Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic–plastic response, which is related to both magnet design and service life. The second derivative ?² σ/?² ε of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. The internal stresses in Cu–Nb are higher than in Cu–Ag and, consequently, the absolute values of (?² σ/?² ε)_Cu–Nb are higher than those of (?² σ/?² ε)_Cu–Ag at low strains.     

Electrodeposited Co–Cu/Cu multilayered microposts

X-ray lithography and electrodeposition were combined to deposit an array of Co–Cu/Cu multilayer microposts of 500 μm tall into deep recesses for novel giant magnetoresistance (GMR) architectures. A citrate-boric acid electrolyte was used with pulsed potential. The applied potential was determined through inspection of the polarization curve from linear sweep voltammetry, and scanning electron microscopy (SEM)/transmission electron microscope (TEM) confirmed the micropost layered structure. Room temperature magnetoresistance was reported for different bilayer sizes of the micropost, and up to 4% current perpendicular-to-plane giant magnetoresistance (CPP-GMR) with saturation values less than 1 T was observed.     

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.     

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     

Molecular dynamics simulation of thermal conductivity of Cu–Ar nanofluid using EAM potential for Cu–Cu interactions

Mechanism of heat conduction in copper-argon nanofluids is studied by molecular dynamics simulation and the thermal conductivity was obtained using the Green–Kubo method. While the interatomic potential between argon atoms is described using the well-known Lennard–Jones (L–J) potential, a more accurate embedded atom method (EAM) potential is used in describing the interatomic interaction between copper atoms. It is found that the heat current autocorrelation function obtained using L–J potential to describe the copper-copper interatomic interaction fluctuates periodically due to periodic oscillation of the instantaneous microscopic heat fluxes. Thermal conductivities of nanofluids using EAM potentials were calculated with different volume fractions but the same nanoparticle size. The results show that thermal conductivity of nanofluids are almost a linear function of the volume fraction and slightly higher than the results predicted by the conventional effective media theory for a well-dispersed solution. A solid-like base fluid liquid layer with a thickness of 0.6 nm was found in the simulation and this layer is believed to account for the small discrepancy between the results of MD simulation and the conventional effective media theory.     

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.     

Hume-Rothery electron concentration rule across a whole solid solution range in a series of gamma-brasses in Cu–Zn,Cu–Cd,Cu–Al,Cu–Ga,Ni–Zn and Co–Zn alloy systems

The aim of the present work is to examine if the Hume-Rothery stabilisation mechanism holds across whole solid solution ranges in a series of gamma-brasses with especial attention to the role of vacancies introduced into the large unit cell. The concentration dependence of the number of atoms in the unit cell, N, for gamma-brasses in the Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga, Ni–Zn and Co–Zn alloy systems was determined by measuring the density and lattice constants at room temperature. The number of itinerant electrons in the unit cell, e/uc, is evaluated by taking a product of N and the number of itinerant electrons per atom, e/a, for the transition metal element deduced earlier from the full-potential linearised augmented plane wave (FLAPW)-Fourier analysis. The results are discussed within the rigid-band model using as a host the density of states (DOS) derived earlier from the FLAPW band calculations for the stoichiometric gamma-brasses Cu₅Zn₈, Cu₉Al₄ and TM₂Zn₁₁ (TM = Co and Ni). A solid solution range of gamma-brasses in Cu–Zn, Cu–Cd, Cu–Al, Cu–Ga and Ni–Zn alloy systems is found to fall inside the existing pseudogap at the Fermi level. This is taken as confirmation of the validity of the Hume-Rothery stability mechanism for a whole solute concentration range of these gamma-brasses. An exception to this behaviour was found in the Co–Zn gamma-brasses, where orbital hybridisation effects are claimed to play a crucial role in stabilisation.     

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.     

Study of inverse a.c. Josephson effect in Y–Ba–Cu–O and Y–8a–Sr–Cu–O

Microwave induced d.c. voltage due to inverse a.c. Josephson effect has been observed across bulk samples of Y-Ba-Cu-O and Y-Ba-Sr-Cu-O. The d.c. voltage is found to vary with microwave power, frequency and also with small external magnetic fields. Although the resistivity curve of Y-Ba-Cu-O does not show any appreciable resistance drop around 230 K, the microwave induced d.c. voltage due to the inverse a.c. Josephson effect has been found to exist upto 230 K. The resistivity behaviour of Y-Ba-Sr-Cu-O shows a sharp resistivity drop above 230 K. In this sample the inverse Josephson effect is found to exist upto +26 °C, indicating the presence of a phase having a superconducting onset around this temperature.     

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.     

High-Temperature superconductivity in the Y–Ba–Cu–O system

High-temperature superconductivity in the Y-Ba-Cu-O system has been discussed with special reference to the identification and characterization of the pure monophasic compound responsible for the superconductivity. The crucial role of oxygen has been examined in the light of the structure and thermogravimetric analysis.     

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.     

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.     

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.     

Phonon- and phason-dynamics in Al–Cu–Fe quasicrystals

The lattice dynamics of quasicrystals includes local phason jumps as well as phonons. Phason dynamics is important for the understanding of both the structure and atomic motion in quasicrystals, leading to short-ranged atomic motion not involving vacancies in addition to diffusion. We have studied the phason and phonon dynamics of icosahedral i-Al₆₂Cu_25.5Fe_12.5. Quasielastic Mössbauer spectroscopy (QMS) was used to probe the iron phason dynamics. Inelastic nuclear-resonant absorption (INA) of synchrotron radiation and inelastic neutron scattering (INS) were used to study the iron-partial as well as the total vibrational DOS (VDOS). We find from preliminary QMS studies that iron atoms jump on a time scale about two orders of magnitude slower than that found for copper. The EFG shows an abrupt change in slope at ca. 825 K which may be related to a transition from simple (isolated) to more complicated (co-operative) phason jumps. From INA we find that the iron-partial VDOS differs radically from that of the total (neutron-weighted) generalised VDOS measured by INS. Both these properties are related to the specific local environments of Fe and Cu in i-AlCuFe.     

Microstructural evolution in Al–Mn–Cu–(Be) alloys

This study investigates the effects of alloying elements on the microstructural evolution of Al-rich Al–Mn–Cu–(Be) alloys during solidification, and subsequent heating and annealing. The samples were characterised using scanning electron microscopy, energy dispersive X-ray spectroscopy, synchrotron X-ray diffraction, time-of-flight secondary-ion mass spectroscopy, and differential scanning calorimetry. In the ternary Al₉₄Mn₃Cu₃ (at%) alloy, the phases formed during slower cooling (≈1?K?s^?1) can be predicted by the known Al–Mn–Cu phase diagram. The addition of Be prevented the formation of Al₆Mn, decreased the fraction of τ₁-Al₂₉Mn₆Cu₄, and increased the fraction of Al₄Mn. During faster cooling (≈1000?K?s^?1), Al₄Mn predominantly formed in the ternary alloy, whereas, in the quaternary alloys, the icosahedral quasicrystalline phase dominated. Further heating and annealing of the alloys caused an increase in the volume fractions of τ₁ in all alloys and Be₄Al (Mn,Cu) in quaternary alloys, while fractions of all other intermetallic phases decreased. Solidification with a moderate cooling rate (≈1000?K?s^?1) caused considerable strengthening, which was reduced by annealing for up to 25% in the quaternary alloys, while hardness remained almost the same in the ternary alloy.     

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.     

Disorder in liquid Cu–Pd alloys

The disorder in thermodynamic and microscopic structure of liquid Cu–Pd alloy at 1350?K has been studied using regular associated solution model. For this, we have calculated free energy of mixing (G_M ), activity (a), concentration fluctuation in long wavelength limit [S_CC (0)] and chemical short-range order parameter (α ₁) of liquid Cu–Pd alloy at 1350?K. The energetic and structural asymmetry of liquid Cu–Pd alloys has been successfully explained on the basis of regular associated solution model.     

Magnetic resonance in a gallium-doped Cu–Cr–S structure

A layered Cu–Cr–S structure doped with Ga ions and consisting of single-crystal CuCrS₂ layers, embedded with thin plates of spinel phases CuCr₂S₄ and CuGa_xCr_2–xS₄, has been studied using the magnetic resonance and magnetic susceptibility methods. The Curie temperature and the saturation magnetization of the spinel phases of the samples have been determined. The spinel phase layer thickness has been estimated.