Non-equilibrium atmospheric pressure radio-frequency glow-like discharges

At atmospheric pressure uniform radio-frequency (RF) glow-like discharges can be generated in helium in gap spacings down to 0.1 mm. In larger gaps an α discharge is present, which exhibits a decreasing bulk plasma region with decreasing gap spacing. At gap spacings < 0.5 mm the discharge changes to a sheath-only structure. At gap spacings > 0.4 mm two regimes can be observed, a normal regime with partial coverage and an abnormal regime with full coverage of the electrodes. In all cases impedance measurements have been performed and compared with equivalent circuit models. There is a good agreement for the α discharges in the normal and abnormal regime, but still a lack of knowledge in the case of the sheath-only structure.     

Optimum eddy current excitation frequency for subsurface defect detection in SQUID based non-destructive evaluation

Detailed experimental studies have been carried out for the determination of optimum eddy current excitation frequencies for the defects located at different depths below the top surface of an aluminum plate. These subsurface defects were detected by using a highly sensitive superconducting quantum interference device (SQUID) based eddy current non-destructive evaluation (NDE) system. The signal to noise ratio was found to be significantly higher at the optimum excitation frequency, which depended on the depth of the defect. The optimum excitation frequencies have been evaluated for defects located at different depths from 2 to 14 mm below the top surface of the plate. The defect depth was varied in steps of 2 mm, while the overall total thickness of the stack of plates was kept constant at 15 mm. Each defect represented a localized loss of conductor volume, which was 60 mm in length, 0.75 mm in width and 1 mm in height. The experimental results show that the square root of the optimum excitation frequency is inversely proportional to the depth of defect.     

SANS characterization of particle dispersions in W-Ti and W-V alloys

SEM analyses of W-Ti and W-V alloys produced by a powder metallurgy route have revealed the presence of Ti or V pools with different sizes and shapes. The larger pools have sizes between ~ 0.3 and several microns and are embedded between the matrix grains. The smaller Ti-rich (V-rich) particles, with sizes below ~ 0.3 μm, are spherical and are dispersed inside the matrix grains. The characteristics of the second phase nanoparticles have been studied using the SANS technique. The scattering curves have been analyzed in terms of a polydisperse particle system following the Beaucage approach, and the maximum entropy method to obtain the particle size distributions. Beaucage analyses suggest the presence of two particle populations, characterized by radii of gyration from 50 to 80 Å (population I) and between 650 and 760 Å (population II). The volume distribution obtained by the maximum entropy approach reveals a prominent peak with particle diameter of 60 Å and ~ 70 Å for the W-Ti and W-V alloys, respectively. Secondary peaks in the size range 100–300 Å could be associated with the particle population with the larger radius of gyration.     

Inhibitive properties of 2,5-bis(n-methylphenyl)-1,3,4-oxadiazole and biocide on corrosion,biocorrosion and scaling controls of brass in simulated cooling water

2,5-Bis(n-methylphenyl)-1,3,4-oxadiazole with (n = 2,3,4), denoted n-MPOX, have been investigated by using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) methods, to evaluate the electrochemical behavior of brass in simulated cooling water system. The PP study revealed that oxadiazole inhibited both cathodic and anodic reactions, indicating a mixed type control of inhibition. It was shown that, 3-MPOX as well as 4-MPOX, were the best inhibitors and the inhibition efficiency followed the sequence: 3-MPOX ⩾ 4-MPOX ≫ 2-MPOX. The interference between non-oxidizing biocide CTAB and 3-MPOX and 4-MPOX against corrosion and biocorrosion has also been studied.     

Forming characteristics of tubular product through the rotary swaging process

Experiments for the forming characteristics of a tube by a rotary swaging process have been carried out to obtain a tubular product of desirable quality taking into account the process variables such as the forming feed and reduction of diameter of the product using the developed rotary swaging machine and four-split dies. From these experimental results, it is found that the process variables affect the quality of the tube such as the dimensional precision, hardness, surface roughness and microstructure of the product. It is also found that defects can occur at the forming feed of more than 2.0 mm/rev. The thickness of the product increased about 63% from its initial thickness of 2.3 mm to 3.75 mm. The hardness measured on the surface of the tube depended on the reduction of the diameter and is rarely affected by the variation of the forming feed from 0.5 to 2.0 mm/rev. The surface roughness of the product after swaging is about three times better than that before swaging. Based on these results a tubular rod shift for an automobile steering part was shown to be effectively manufactured by the rotary swaging process.     

Magnetic and transport properties of Laves phase Dy1−xMmxCo2 (x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy_1?xMm_x)Co₂ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (T_C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T_C and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed.     

Effect of HIP’ping on deformation anisotropy in a single-crystal Ni-based superalloy

The effect of HIP’ping on plastic anisotropy of a Ni-based superalloy was studied. The orientation dependence of tensile properties was evaluated experimentally at room temperature at a nominal strain rate of 10⁻³ s⁻¹. The results indicate that HIP’ping eliminates the non-Schmid effect by increasing the strength of the 〈1 1 1〉 loaded samples. This phenomenon is attributed to the influence of stress fields of pores and eutectic pools on the yielding behavior of the 〈1 1 1〉 loaded samples, whose deformation is confined to the γ-channels. When deformation takes place by the shearing of γ′-particles, the softening effect dominates and the presence of discontinuities has less or no effect on the deformation path. A new plastic anisotropy was discovered in this work. It is shown that the tensile properties along the primary dendrite growth ([0 0 1] orientation) is significantly different than along the secondary dendrite growth direction ([1 0 0] orientation).     

Structure and properties of 6061 + 26 mass% Si aluminum alloy produced via coupled rapid solidification and KOBO-extrusion of powder

Experiments on mechanical consolidation of rapidly solidified (RS) powder of 6061 + 26 mass% Si alloy were performed using the oscillating-die extrusion method. The RS powder was wrapped in thin-wall 6061-alloy cup 35 mm in diameter and vacuum-compressed by means of 100 ton press. Bars 8 mm in diameter were extruded with cross-section reduction of λ = 19 without any preheating of the charge. Tubes with a diameter/wall thickness of 14 mm/1 mm and cross-section reduction of λ = 33 were also manufactured with success. TEM/STEM observations revealed a very fine structure of as-extruded material and bimodal distribution of quasi-spherical silicon particles. Statistical analysis revealed a silicon fine fraction of 0.1–0.7 μm and a coarse fraction 2.1–2.5 μm in diameter. Examination by means of TEM did not reveal any significant changes in the morphology of the silicon particles, even when a high extrusion ratio and the material annealing after deformation were used. Hot compression tests on as-extruded rods (λ = 19) and preliminary annealed samples were performed at a constant true strain rate of 5 × 10^?3 s^?1 within the temperature range of 293–823 K. High strength of the material and relatively high ductility of samples deformed by compression up to ?_t ? 0.4 were observed. The maximum flow stress value for as-extruded material was reduced with deformation temperature from ～390 to ～3.5 MPa for 293 and 823 K, respectively. Annealing of the samples at 773 K/30 min was found to reduce the maximum flow stress by 30–40%. Tensile strengths of similar as-cast alloys and materials manufactured by means of other powder metallurgy methods were shown for the purpose of comparison.     

Characterization of mobile type I and type II twin boundaries in 10M modulated Ni–Mn–Ga martensite by electron backscatter diffraction

Mobile type I and type II twin boundaries mediating the magnetic field-induced strain in five-layered modulated (10M) Ni–Mn–Ga martensite were analyzed by electron backscatter diffraction. Taking into account the slight monoclinic distortion of the pseudo-tetragonal lattice, the electron backscatter diffraction study reveals domains of 0.01–1 mm thickness adjacent to the type I and type II twin boundaries. The domains differing in the modulation direction are {1 0 0) compound twins and their effect on twinning stress is discussed. Detailed analysis of type II twin boundary reveals that the domains are further internally twinned by compound {1 1 0) twins 1–15 μm in size. An additional example of a complex twin microstructure combining type I and type II twin boundaries is presented.     

The formation and evolution of oxide particles in oxide-dispersion-strengthened ferritic steels during processing

The fabrication of oxide-dispersion-strengthened (ODS) steels is a multi-stage process involving powder ball milling, degassing and consolidation by hot isostatic pressing. Y is introduced by mechanical alloying (MA) with either Y₂O₃ or Fe₂Y so a high density of Y–Ti–O-based oxide nanoparticles is formed. The evolution of ～2 nm oxide nanoparticles and larger >5 nm grain boundary oxides has been studied at each step of the processing. The nanoparticle dispersions produced in material MA with Fe₂Y were comparable to those produced by alloying with Y₂O₃. Hence the majority of oxygen which forms the nanoparticles must be incorporated from the atmosphere during MA, rather than being introduced via the alloying additions. During mechanical alloying, a high density of subnanometer particles are formed (2.5 ± 0.5 × 10²⁴ m^?3). The oxygen content of the nanoparticles decreases slightly on annealing, and then the composition of the nanoparticles remains constant throughout subsequent processing stages. The nanoparticle size increases during processing up to ～2 nm radius, while the number density decreases to 4 ± 0.5 × 10²³ m^?3. Grain boundary oxides (>5 nm) have a Ti–Cr–O-rich shell, and contain no Y before consolidation, but have similar core composition to the matrix nanoparticles after consolidation. The formation of the larger grain boundary oxides is shown to take place during the degassing and consolidation stages, and this occurs at the expense of the nanoparticles in the matrix. This work provides a mechanistic understanding of the importance of controlling the oxygen content in the powder during MA, and the resulting impact on the formation of the ODS microstructure.     

Variation of atomic spacing and thermomechanical properties in Ni–Mn–Ga/alumina film composites

Ni_51.4Mn_28.3Ga_20.3 thin films deposited on alumina ceramics have been studied by X-ray powder diffraction and dynamic mechanical analysis. Substantial temperature vs. film thickness dependencies of interatomic spacing measured in the direction of the film normal are observed in the range of 25–200 °C and 0.1–5 μm, respectively. The coefficient of thermal expansion (CTE) of the film in the paramagnetic cubic phase has been determined to be equal to (15 ± 1) × 10⁻⁶ K⁻¹ for all the films, in agreement with the CTE of bulk material. The thickness dependent shrinkage of the pseudo-cubic lattice along the film normal direction is attributed to the thermally induced tensile stress in the film plane. The thickness dependence of the elastic modulus of submicron films is obtained. It is shown that the internal stresses result in both the thickness dependence of martensitic transformation temperature and the reversible, thermally induced change in shape of the Ni–Mn–Ga/alumina cantilever actuator.     

Fast trilayer polypyrrole bending actuators for high speed applications

The synthesis and performance evaluation of trilayer bending type polymer actuators in terms of frequency response and step response are described. The actuators are shown to achieve mechanical resonance at up to 90 Hz and giving tip displacements up to 60 mm for an input voltage of ±1 V at 4 Hz. Polypyrrole doped with trifluoromethanesulfonimide (TFSI) had a considerably higher speed of response when compared to polypyrrole-hexafluorophosphate (PF₆) actuator with the same dimensions. The TFSI doped polypyrrole showed faster charging rates, likely due to higher ion diffusion rates through a gel-like polymer structure, thereby producing faster actuation response. These new actuators may be useful for applications requiring a high speed of response, e.g. a propulsion element for a swimming device/robotic fish.     

The characterization of structure,thermal stability and magnetic properties of Fe–Co–B–Si–Nb bulk amorphous and nanocrystalline alloys

Recently bulk amorphous alloys have attracted great attention due to their excellent magnetic properties. The glass-forming ability of bulk amorphous alloys depends on the temperature difference (ΔT_x) between glass transition temperature (T_g) and crystallization temperature (T_x). The increase of ΔT_x causes a decrease of the critical cooling rate (V_c) and growth of the maximum casting thickness of bulk amorphous alloys. The aim of the present paper is to characterize the structure, the thermal stability and magnetic properties of Fe₃₆Co₃₆B₁₉Si₅Nb₄ bulk amorphous alloys using XRD, Mössbauer spectroscopy, DSC and VSM methods. Additionally the magnetic permeability μ_i (at force H ≈ 0.5 A/m and frequency f ≈ 1 kHz) and the intensity of disaccommodation of magnetic permeability Δμ/μ(t₁) (Δμ = μ(t₁ = 30 s) ? μ(t₂ = 1800 s)), have been measured, where μ is the initial magnetic permeability measured at time t after demagnetisation, the Curie temperature T_C and coercive force H_c of rods are also determined with the use of a magnetic balance and coercivemeter, respectively.Fe–Co–B–Si–Nb bulk amorphous alloys were produced by pressure die casting with the maximum diameters of 1 mm, 2 mm and 3 mm.The glass transition temperature (T_g) of studied amorphous alloys increases from 807 K for a rod with a diameter of 1 mm to 811 K concerning a sample with a diameter of 3 mm. The crystallization temperature (T_x) has the value of 838 K and 839 K for rods with the diameters of 1 mm and 3 mm, respectively. The supercooled liquid region (ΔT_x = T_x ? T_g) has the value of about 30 K. These values are presumed to be the origin for the achievement of a good glass-forming ability of the Fe–Co–B–Si–Nb bulk amorphous alloy. The investigated amorphous alloys in the form of rods have good soft magnetic properties (e.g. M_s = 1.18–1.24 T). The changes of crystallization temperatures and magnetic properties as a function of the diameter of the rods (time of solidification) have been stated.     

Temperature dependence of sound attenuation and shear modulus of ultra fine grained copper produced by equal channel angular pressing

The influence of temperature on shear modulus and internal friction in ultrafine-grained copper processed by equal channel angular pressing (ECAP) was investigated in the temperature range from 150 to 520 K. Acoustic measurements were performed on the inverted torsion pendulum at the frequencies of ∼18 and ∼45 Hz. An irreversible shear modulus increase and a concurrent decrease in sound attenuation were observed in the temperature region from ∼350 to 450 K on the first heating of specimens. The activation energy E ≈ 1.05 eV and the attempt frequency ν₀ ≈ 10¹⁰ s⁻¹ of the irreversible relaxation process were determined using the measurements at different heating rates. The overall decrease in the shear modulus in ECAP-processed copper was shown to be made up by two components: a temperature-independent and a temperature-dependent ones. The latter is accompanied with an additional internal friction of the relaxation type, which is reversible up to ∼350 K. An estimate of the activation energy for this reversible relaxation process was obtained. Possible mechanisms responsible for the anomalous behavior of the shear modulus and the sound attenuation in ultrafine-grained copper are discussed.     

Pure carbon microwave absorbers from anion-exchange resin pyrolysis

A series of carbon materials [C_F-x (where x denotes carbonization temperature)] have been prepared by pyrolysis of an anion-exchange resin at different temperatures (500–700 °C). X-ray diffraction and Raman spectroscopy suggest the presence of tiny crystalline domains in these materials, whose content is strongly determined by carbonization temperature. The microwave absorption of these materials is examined in the frequency range of 2–18 GHz, and it is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. At a thickness of 2 mm, C_F-600 exhibits the best microwave absorbing ability with a maximum reflection loss of ?20.6 dB at 16 GHz, and a bandwidth exceeding ?10 dB in the range 13.5–18 GHz. It is concluded that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of C_F-600. Furthermore, a reflection loss exceeding ?10 dB can be obtained in the range of 7–18 GHz by manipulating the thickness from 2 to 3.5 mm, and the maximum can reach ?37.0 dB at 10.8 GHz with a thickness of 2.8 mm. These materials may be used as light-weight and highly effective microwave absorbers over a wide frequency range.     

Eddy current analysis of mid-bore and corner cracks in bolt holes

Fatigue cracks are prone to develop around fasteners found in multi-layer aluminum structures on aging aircraft. Probability of Detection (POD) studies using eddy current techniques within the bolt holes contribute to risk assessments used in evaluating the serviceability of these aircraft. The signal response to corner and mid-bore cracks by eddy current testing using standard split-D differential probes has been examined. The data indicate that split-D probes are primarily sensitive to cracks at the corners of the Ds and that this gives rise to a double maximum for small mid-bore cracks as the corners pass over the crack. The double signal is not seen for corner cracks because the excitation coil, which is almost completely out of the hole, excites almost no eddy currents when the second corner of the D is over the crack. However, the eddy current response is more sensitive to corner cracks. The a_90/95 for corner cracks was measured to be 0.22 mm (length) compared to 0.34 mm (depth) and 0.62 mm (length) for mid-bore cracks. The enhanced detectability of corner cracks is attributed to the presence of the edge, which restricts passage of eddy currents around the crack.     

Polypyrrole actuators working at 2–30 Hz

“Soft actuators” based on the conducting polymer polypyrrole (PPy) may be especially suitable for use in combination with human limbs. A research project under the European Union Quality of Life program (DRIFTS, Dynamically Responsive Intervention for Tremor Suppression, http://www.drifts.org/) focuses on the development of practical tremor suppression orthoses prototypes [M. Manto, M. Topping, M. Soede, J. Sanchez-Lacuesta, W. Harwin, J. Pons, J. Williams, S. Skaarup, L. Normie, IEEE Eng. Med. Biol. 22 (2003) 120]. One of the choices of actuation mechanism is to use conducting polymers.The main challenge is to provide significant forces at the frequencies relevant to tremor in upper limbs: 2–16 Hz. Forces in the range of 0.1–1 kg are required. It has earlier been shown that utilizing the stiffness change instead of the length change may extend the useable maximum frequency by about a factor of 10 [J.D. Madden, R.A. Cush, T.S. Kanigan, I.W. Hunter, Solid State Ionics 113 (2000) 185]. The maximum frequency reached was, however, only about 1 Hz.By optimizing the synthesis method, and the choice of counterion in PPy and in the electrolyte, a polypyrrole actuator able to yield significant force at up to 30 Hz has been made. The stiffness change turns out to be approximately 20 times faster than the change in length. Simple scaling up of the present data leads to a required total thickness of PPy (30 mm wide film) of 0.13 mm at 2 Hz, and 0.32 mm at 15 Hz for the 1 kg limit. The required mass of the actuator itself at 15 Hz is ∼100 mg. The results indicate the feasibility of using PPy actuators for tremor suppression.     

Manufacture of brightness enhancement films (BEFs) by ultraviolet (UV) irradiation and their applications for organic light emitting diodes (OLEDs)

By ultraviolet (UV) irradiation, brightness enhancement films (BEFs) have been successfully manufactured with UV-curable polymers and applied for organic light emitting diodes (OLEDs).With BEFs, either green OLEDs (BEF/ITO glass/NPB (30 nm)/Alq₃ (65 nm)/LiF (0.5 nm)/Al (100 nm)) or white OLEDs (BEF/ITO glass/TAPC (40 nm)/mCP:Os:Firpic mixture (weight ratio = 82:17:1; 25 nm)/BCP (15 nm)/Alq₃ (30 nm)/LiF (0.5 nm)/Al(150 nm)) exhibit better electroluminescent performances than those without BEFs. In case of green OLEDs, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.51-fold and 1.42-fold (at 9 V, 60 mA/cm²) larger than those without BEFs. In case of white OLEDs, moreover, the luminance and electroluminescent yield with 45° compound BEFs are, respectively, 1.28-fold and 1.21-fold (at 9 V, 16 mA/cm²) larger than those without BEFs.     

Synthesis of LaCe-based bulk metallic glasses with low glass transition temperature

LaCe-based bulk metallic glasses (BMGs) with diameter up to 10 mm have been synthesized in copper mold. The BMG alloys have low glass transition temperatures, about 403 K and large supercooled liquid regions up to 58 K. The hardness of the BMG alloys is around 1.9 GPa. They also exhibit large plasticity and imprintability feature in the supercooled liquid region, which make the alloy a potential imprinting material. In addition, the BMG alloys might offer an ideal model system for the study of glass transition with a large experimentally accessible time and low temperature windows.     

Exchange bias and magneto-resistance in an all-oxide spin valve with multi-ferroic BiFeO3 as the pinning layer

Modern microscopy techniques indicate that the electrical switching of magnetic domains in multi-ferroic materials is possible. However, the application of such functionality in a real device has yet to be proven. In this work we fabricated an all-oxide spin valve with the ferroelectric anti-ferromagnet BiFeO₃ (BFO) as the pinning layer. The multi-layered heterostructure was grown epitaxially on a (0 0 1) SrTiO₃ substrate and magneto-resistance was achieved at room temperature, which was switchable magnetically in a similar way to conventional metallic spin valves. Some key physical and material issues for building up such a novel device were addressed, in particular the hetero-epitaxy-induced strain effects on the electrical and magnetic properties of each layer and the establishment of exchange bias between BFO and an oxide ferrimagnet, e.g. Zn_0.7Ni_0.3Fe₂O₄ (ZNFO). The strains caused a significant increase in the coercivity but a decrease in the saturation magnetization of the ferrimagnet used. The former is particularly undesirable because it increases the required switching field. The all-oxide architecture allowed the spin valve to be field annealed from a temperature above the high Néel point of BFO (～660 K), after which a very large exchange bias field (H_ex) was achieved at 5 K and kept at a decent value at room temperature. The H_ex–T curve did not follow the widely observed (1 ? T/T_N)^β temperature dependence, but could be explained by the random field model with one-dimensional (1-D) anti-ferromagnetic sublattice magnetization derived from the spin wave theory. Based on the observed 1-D spin wave behavior and the geometric arrangements of the paramagnetic ions at the (0 0 1) surface we propose an atomic model in which only a part of the spin along the diagonal lines in the BFO (0 0 1) surface was strongly exchange coupled with ZNFO.