Fabrication and magnetic properties of Fe-Pd nanowire arrays

Ordered 20 nm Fe-Pd nanowire arrays with different compositions have been fabricated by alternating current electrodeposition into nanoporous anodic alumina. The structural and magnetic properties of the arrays were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). When Fe content is lower than 46 at.%, Fe-Pd alloy phase with fcc structure forms for the as-deposited. After annealing the alloy structure remains unchanged, but the coercivity (H_C) and squareness (M_r/M_s) increase. When Fe content is up to 60 at.%, α-Fe and Fe-Pd phases with fcc structure coexist for the as-deposited. After annealing the nanowires consist of a uniform Fe-Pd phase with fcc structure and the coercivity and squareness decrease. The change of the structure and magnetic properties with the alloy composition and annealing are explained reasonably.     

Evolution of structural and magnetic properties of FePt/C granular films with thermal annealing

We report the structural and magnetic properties of as-deposited and thermally annealed FePt/C granular multilayer films. The as-deposited system exhibits a disordered fcc FePt phase with an average grain size of 3 nm. Thermal annealing at 650 °C results in partial L1₀ ordering and an associated grain growth to 7 nm. Mössbauer measurements show that there is no non-magnetic component present, suggesting that carbon resides only in the grain boundary region. The ferromagnetic grains are magnetically decoupled.     

Structure and electrical conductivity of ultrathin Ni-Cu films

The structure, phase composition, morphology, and electrical conductivity of Ni-Cu alloy ultrathin films having a thickness of d = 1?10 nm and a Cu concentration of 10–95 at % have been studied. All films are shown to be fcc Ni-Cu alloys; they have an island structure with an island size of 1.5–2 nm in the as-deposited films and of about 20 nm in the films annealed to 700 K. The electrical conductivity of the films depends on their thickness and morphology. For films with d ≈ 1 nm, the electrical conductivity is thermally activated with an activation energy E _a ≈ 0.086?0.095 eV. Films with d > 3 nm exhibit the metallic temperature dependence of electrical conductivity with a positive temperature coefficient of resistivity.     

Study of the microstructure influence on the transport properties of Bi86.5Sb13.5 polycrystalline alloy

This study is focused on the investigation of the transport properties of Bi_86.5Sb_13.5 polycrystalline alloys. Bulk materials were prepared by cold pressing ultrafine powders of alloy and by annealing the resulting pellets. Special care was taken to avoid contamination of the powders. Starting with powders of average grain size of 0.06 μm bulk semi-conducting sample with mean grain size respectively of 0.1, 0.8, 2.5 and 200 μm were obtained. The influence of the grain size on both electrical resistivity, thermal conductivity, thermoelectric power, thermoelectric figure of merit is presented within the range 80-330 K. The thermoelectric properties are discussed and compared with those of single crystals presented in previous studies.     

Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys

Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained by hardening of the amorphous matrix due to changes in its chemical composition. Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998)     

Growth,optical, and electrical properties of silicon films produced by the metal-induced crystallization process

Amorphous Si (a-Si) and Ni films were deposited by electron beam evaporation on to borosilicate glass (BSG) substrate maintained at ambient temperature. The BSG/a-Si/Ni stack was subjected to post deposition annealing in air at various temperatures from 200 to 500 °C for 1 h. Electron diffraction was employed to characterize the crystallographic phases appearing on the stacks that were depending on initial conditions. Clear evidence of the formation of hexagonal Si and fcc NiSi₂ was shown by TEM. In parallel, an increase of refraction index was observed. Electrical resistivity measurements showed that resistance is of the order of kilo ohms in the as-deposited films, increasing sharply to giga ohms in films annealed at T higher than 300 °C. A large band gap of 2.23 eV which is the combined contribution from a-Si, wurtzite-Si, and Ni silicide phases, is observed.     

Effects of peak current density on the mechanical properties of nanocrystalline Ni-Co alloys produced by pulse electrodeposition

Cobalt content, grain size, microhardness and tensile strength of nanocrystalline Ni-Co deposits produced from a solution containing saccharin and cobalt sulfate at constant electrodeposition conditions (pulse on-time T_on at 1 ms and pulse off-time T_off at 15 ms) but varying the peak current density J_p were investigated. It is found that an increase in J_p makes the deposit Co content lower, colony-like morphology more obvious, grain size smaller, and hardness and tensile strength higher. All of the facts are believed to result from the higher overpotential and nucleation rates caused by the J_p increase. But its further increase could lead to reduction in the hardness and tensile strength. Peak current densities in the range of 100-120 A dm⁻² are recommended for the preparation of nanostructured Ni-Co alloy deposits with grain sizes in the range of 15-20 nm, containing 7-8% Co, possessing hardness of 590-600 kg mm⁻² and tensile strength of 1180-1200 MPa—significantly higher than the strength of pure nickel deposit which is produced by the similar method and gets similar grain size.     

Effect of annealing upon the structure and adhesion properties of sputtered bio-glass/titanium coatings

Bio-glass films were deposited by radio-frequency magnetron sputtering technique onto medical grade Ti6Al7Nb alloy substrates from prepared silica based bio-glass target. A low deposition temperature was used (150 °C) and three different working pressures, followed by annealing in air at 550 and 750 °C. A quasi-stoichiometric target to substrate atomic transfer was found for Si, Ca and P, along with strong enrichment in Na and depletion in K and Mg, as evidenced by the energy dispersive microanalysis. The best results, taking into account stoichiometry and surface roughness, were obtained for the BG layers deposited at 0.3 Pa argon working pressure. The infrared spectroscopy of the as-sputtered and of the annealed films evidenced the characteristic molecular vibrations of silicate, phosphate and carbonate functional groups. The as-deposited films are amorphous and became partly crystalline after annealing at 750 °C, as evidenced by X-ray diffraction. The pull-out measurements, performed with a certified pull-test machine, gave very strong film–substrate adhesion strength values. For the non-crystalline layers, the pull-out strength is higher than 85 MPa, and decreases after annealing at 750 °C to 72.9 ± 7.1 MPa. The main objective of this work was to establish the influence of the working pressure upon the composition and morphology of the as-deposited films, and of the annealing temperature upon structure and film–substrate adhesion.     

Structure and magnetic state of the films deposited by laser ablation of composite nickel and palladium targets

Transmission electron microscopy, electron diffraction, and vibrating-sample magnetometry are used to show that a metastable hcp structure can form in both nickel and Ni-Pd alloy films during alternating sputtering of the Ni and Pd components of composite targets. The hcp lattice parameters increase monotonically when the palladium content in a sputtered target increases in the range 0–75%. The ratio of the hcp lattice parameters c/a is close to the ideal ratio for the hcp lattice (1.63) within the limits of experimental error. In the as-deposited state, nickel and Ni-Pd alloy films with an hcp structure have no magnetic moment. Upon annealing, the films transform into a ferromagnetic state with an fcc structure. The concentration dependence of the lattice parameter of the fcc solid solution a ₀ is found to exhibit a positive deviation from Vegard’s law, which is characteristic of alloys with a concave liquidus line.     

Heat treatment effects on the structural and electrical properties of thermally deposited AgIn5S8 thin films

The heat treatment effects on structural and electrical properties of thermally deposited AgIn₅S₈ thin films have been investigated. By increasing the annealing temperature of the sample from 450 to 500 K, we observed a change in the crystallization direction from (420) to (311). Further annealing of the AgIn₅S₈ films at 550, 600 and 650 K resulted in larger grain size in the (311) preferred direction. The room temperature electrical resistivity, Hall coefficient and Hall mobility were significantly influenced by higher annealing temperatures. Three impurity levels at 230, 150, and 78 meV were detected for samples annealed at 350 K. The electrical resistivity decreased by four orders of magnitude when the sample annealing temperature was raised from 350 to 450 K. The temperature dependent electrical resistivity and carrier concentration of the thin film samples were studied in the temperature ranges of 25-300 K and 140-300 K, respectively. A degenerate-nondegenerate semiconductor transition at approximately 180 was observed for samples annealed at 450 and 500 K. Similar type of transition was observed at 240 K for samples annealed at 600 and 650 K.     

Microstructural and compositional studies of liquid-phase deposition derived PbTiO3 thin films on LaNiO3 substrates

PbTiO₃ thin films were successfully deposited on the LaNiO₃ (LNO) substrates by the liquid-phase deposition (LPD) method and post-annealing at various temperatures. The structure, morphology and composition of the films were investigated by some analytical techniques. The as-deposited films are amorphous and composed of densely packed spherical particles. The films with the grain size of 180 nm start to decompose and crystallize into perovskite structure at 450 °C and show a perovskite single phase with tetragonal structure after annealing at 650 °C. X-ray photoelectron spectroscopy (XPS) analysis reveals that the as-deposited film contains fluorine and carbon as major impurities. Fluorine could be completely eliminated by annealing at 650 °C in air.     

Ni-Co-TiO2 nanocomposite coating prepared by pulse and pulse reversal methods using acetate bath

Ni-Co/nano TiO₂ (Ni-Co-TiO₂) composite coatings were prepared under pulse current and pulse reverse current methods using acetate bath. The microstructure and corrosion resistance of the coatings were characterized by means of XRD, SEM and EIS. Both the Ni-Co alloy and composite coatings exhibited single phase of Ni matrix with face centered cubic (fcc) crystal structure. The crystal orientation of the Ni-Co-TiO₂ composite coating was transformed from crystal face (2 0 0) to (1 1 1) compared with Ni-Co alloy coatings. The results showed that the microstructure and performances of the coatings were greatly affected by TiO₂ content on the deposits prepared by PC and PRC methods. The microhardness and corrosion resistance were enhanced in the optimum percentage of TiO₂ composite coatings. The PRC composite coatings were exhibited from compact surface, higher microhardness and good corrosion resistance compared with that of the PC composite coating.     

Effects of annealing time on infrared emissivity of the Pt film grown on Ni alloy

Platinum films were sputter-deposited on polished nickel alloy substrates. The platinum thin films were applied to serve as low-emissivity layers to reflect thermal radiation. The platinum-coated samples were then heated in the air at 600 °C to explore the effects of annealing time on the emissivity of platinum films. The results show that the grain size of the Pt films increased with the increasing annealing time while their dc electrical resistivity decreased. Besides, the IR emissivitiy of the films gradually decreased with the increasing annealing time. Especially, when the annealing time reached 150 h, the average IR emissivity at the wavelength of 3-14 μm was only about 0.1. Moreover, the chemical analysis indicated that the Pt films on Ni-based alloy exhibit a good resistance against oxidation at 600 °C.     

The influence of substrate temperature on the morphology, optical and electrical properties of thermal-evaporated ZnTe Thin Films

The structural, morphological, optical and electrical properties of ZnTe films deposited by evaporation were investigated as a function of substrate temperature (at −123 and 27 °C) and post-deposition annealing temperature (at 200, 300 and 400 °C). It was determined that films deposited at both substrate temperatures were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. A small Te peak was detected in XRD spectra for both substrate temperatures, indicating that as-deposited ZnTe films were slightly rich in Te. Larger grains and a tighter grain size distribution were obtained with increased substrate temperature. Scanning electron microscopy (SEM) studies showed that the microstructures of the as-deposited films agreed well with the expectations from structure zone model. Post-deposition annealing induced further grain growth and tightened the grain size distribution. Annealing at 400 °C resulted in randomization in the texture of films deposited at both substrate temperatures. Optical spectroscopy results of the films indicated that the optical band gap value increased from 2.13 to 2.16 eV with increased substrate temperature. Increasing the annealing temperature sharpened the band-edge. Resistivity measurements showed that the resistivity of films deposited at substrate temperatures of −123 and 27 °C were 32 Ω cm, and 1.0 × 10⁴ Ω cm, respectively with corresponding carrier concentrations of 8.9 × 10¹⁵ cm⁻³ and 1.5 × 10¹⁴ cm⁻³. Annealing caused opposite changes in the film resistivity between the samples prepared at substrate temperatures of −123 and 27 °C.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

Synthesis, characterization, and microwave absorption properties of Fe-40 wt%Ni alloy prepared by mechanical alloying and annealing

Fe-40 wt%Ni alloys with granular shape and flake shape were prepared by a mechanical alloying (MA) and annealing method. The phase composition and morphology of the FeNi alloys, electromagnetic parameters, and microwave absorbing properties of the silicone rubber composite absorbers filled with the as-prepared FeNi alloy particles were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and vector network analyzer. The XRD results indicate that the crystalline structures of the Fe-40 wt%Ni alloys prepared by both one-step and two-step MA processes are face-centered cubic (fcc) Ni (Fe) solid solutions, and the structures can be retained after annealing at 600 °C for 2 h. SEM images show that the FeNi alloy powders for one-step process have a granular shape; however the particles turned into flake form when they were sequentially milled with absolute ethyl alcohol. With the increase in thickness of composite absorber, the reflection loss (RL) decreases, and the peak for minimum reflection loss shifts towards the lower frequency range. Compared to the absorbers filled with the granular FeNi alloy, the absorbers filled with flaky FeNi alloys possess higher complex permittivities and permeabilities and have a lower RL and peak frequency under the same thickness. Microwave absorbing materials with a low reflection loss peak in the range of 1-4 GHz are obtained, and their microwave absorbing properties can be adjustable by changing their thicknesses.     

Influence of complexing agent on the electrodeposited Co-Pt-W magnetic thin films

Complexing agents are often used to improve the quality of electrodeposited alloys. Influence of different complexing agents with hydroxycarboxylic acid group on the electrodeposited Co-Pt-W thin films has been investigated. Cathodic polarization curves show that the polarization behaviors of electroplating bath with different complexing agents are very different. Surface morphology, phase composition and magnetic properties are observed by means of FESEM, XRD and vibrating sample magnetometer (VSM), respectively. It has been found out that, if citrate was used as complexing agent, the Co-Pt-W thin films were homogeneous and the granular crystals with the average grain size of 2 μm have been observed. Co-Pt-W thin films exhibited hexagonal close packed (hcp) lattice and strong perpendicular anisotropic magnetic behavior (H_c⊥ = 215.5 kA/m; H_c∥ = 55.4 kA/m). In the presence of gluconate, needle-like deposits were obtained and a strong face centered cubic (fcc(1 1 1)) texture was measured. The Co-Pt-W thin films showed isotropic magnetic behavior. In the case of tartate and malate, the coexistence of needle-like deposits and cellular deposits appeared. The XRD patterns showed that the mixed fcc and hcp phase formed. Perpendicular anisotropic magnetic behaviors of thin films, from malate or tartate baths, were not obvious.     

Magnetic and structural characterizations on nanoparticles of FePt,FeRh and their composites

The various compositions of FePt and FeRh nanoparticles, and their composite particles have been fabricated by the solution-phase chemical method and their magnetic properties characterized. High-resolution transmission electron microscopic observations indicate that mono-dispersed FeRh and FePt/FeRh nanoparticles are fabricated with the average size of 3–5 nm. However, larger size particles are distributed in the annealed state. From X-ray diffraction results, the as-deposited FeRh nanoparticles reveal a chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. Similarly, the annealed FePt nanoparticles show the L1₀-phase fct structure although the fcc structure is apparent in the as-deposited state. It is also found that the first time in the exchange bias effect in the composite of ferromagnetic (FePt) and anti-ferromagnetic (FeRh) nanoparticles; result in a shift of the hysteresis loop after field cooling process.     

Low temperature thermoelectric properties of Pb- or Sn-doped Bi-Sb alloys

Pb- or Sn-doped Bi₈₈Sb₁₂ alloys were prepared by direct melting, quenching, and annealing. The Bi-Sb alloy phase was predominant in all samples. Pb or Sn atoms were distributed almost uniformly in Bi₈₈Sb_12, while some segregation was confirmed at the grain boundaries when Pb or Sn was involved heavily. The thermoelectric properties of these doped materials were investigated by measuring the Hall coefficient, electrical resistivity, and Seebeck coefficient between 20 K and 300 K. The Hall and Seebeck coefficients of Pb- or Sn-doped samples were positive at low temperatures, indicating that the doping element acted as an acceptor. Temperatures resulting in positive Hall and Seebeck coefficients further increased with increasing doping amount and with respect to the annealing process. As a result, a large power factor of 1.2 W/mK² could be obtained in the 3-at% Sn-doped sample at 220 K, with a large positive Seebeck coefficient.     

Influence of grain boundary state on electrical resistivity of ultrafine grained aluminium

An ultrafine grained (UFG) structure has been obtained in commercial purity Al by high-pressure torsion (HPT). Changes in microhardness and electrical resistivity of the UFG material after annealing at various temperatures within a range of 363–673 K have been investigated in correlation with the microstructure evolution. It has been shown that annealing at 363 K leads to substantial decrease in the electrical resistivity while keeping high microhardness level and approximately the same average grain size. The contributions from the various microstructural units (vacancies, dislocations, grain boundaries (GBs)) to the electrical resistivity were analysed. It was shown for the first time that a non-equilibrium state associated with strain-distorted grain boundary (GB) structure strongly affects electrical resistivity of UFG Al. The resistivity of non-equilibrium GBs in UFG structure formed by HPT was evaluated to be at least 50% higher than the resistivity of the thermally equilibrium GBs in a coarse-grained structure.