Superconducting and metallurgical properties of bronze processed Nb3Sn wires with Ni and Zn additions and comparison with Ta and Ti additions

Critical current density measurements up to 23 T of nineteen core Nb₃Sn wires with simultaneous addition of Ni to the core and Zn to the Cu-Sn bronze matrix have revealed a considerable increase of J_c at fields above 11 T. For a wire with the composition Nb-0.6 wt% Ni/Cu-10 wt% Sn-3 wt% Zn, reacted at 750°C for 64 h, J_c in the layer was determined to 1.3 × 10⁵A cm^?2 at 14 T and to 4 × 10⁴A cm^?2 at 19 T. Comparison with Ta and Ti core-alloyed Nb₃Sn wires, also performed in the present study, shows very similar results in J_c and J_c vs. ε up to the highest fields.Composition profiles for Sn and for Ni, Ta and Ti additives in the A15 layers were studied by Auger spectroscopy. For Sn a concentration gradient across the layer (from ≈ 25 at% to ≈ 22 at% Sn) was observed, with the highest Sn content occurring at the interface with the bronze. The presence of the additives in the layer was detected as well by Auger analysis as by X-ray diffractometry.     

Superconducting ceramic/metal composite layers

Composite layers consisting of a YBa₂CU₃O_y superconducting ceramic and Ag or Al-clad substrates are prepared by rolling and pressing. The procedure for preparing an Al-clad Cu substrate is described. The superconducting transition temperature of the ceramic layers is 89-90 K, and the critical current densityis j_c > 10³ A/cm². The j_c data are correlated with the surface microstructure of the ceramic layers.     

Structural phase transformations in tin dioxide under the action of a nanosecond high-power ion beam

Structural phase transformations in polycrystalline tin dioxide under the action of a nanosecond high-power ion beam with a current density of 50–150 A/cm² are studied. It is found that the effect of a beam with a current density of 150 A/cm² on SnO₂ leads to the formation of submicron particles of tetragonal SnO with an average size of 210 nm on the exposed surface. The particles have a pronounced crystallographic facet pattern. Possible mechanisms of the observed transformations are discussed.     

Low energy O2 and N2O ion beam modification of polyimide for improving adhesive strength of Cu/PI in roll-to-roll process

To enhance the weak mechanical durability of directly deposited copper layers on polyimide (PI) film due to their poor adhesive strength, a continuous roll-to-roll process involving surface modification using a reactive ion beam irradiation and in-situ deposition process is studied. The polyimide film is modified by an ion source with a linear stationary plasma thruster (LSPT) in the vacuum roll-to-roll process. An O₂ ion beam, with beam energy of 214 eV and beam current density of 0.78 mA/cm², and N₂O ion beam, with 220 eV and 0.69 mA/cm², irradiate PI film in winding speed of 0.5 m/min. The surface energy increases from 38 mN/m for the pristine PI film to 80 mN/m after beam irradiation at an ion fluence of 3.5 × 10¹⁶ ions/s. After beam irradiation, a 10 nm thick tie layer and 200 nm thick copper layer are successively deposited by in-situ DC magnetron web coating. The peel strength of the copper layer on the PI film is enhanced from 0.4 kgf/cm without ion beam treatment to 0.71 kgf/cm after O₂ beam treatment and to 0.75 kgf/cm after N₂O beam treatment. This enhancement is closely related to the increase in the polar force originating from the formation of hydrophilic CO (carbonyl) groups on the modified PI surface.     

High-current electron beam in a hollow-cathode gas discharge at elevated pressures (∼100 Torr)

A discharge with plasma filling a flat-bottom cavity of depth δ in the cathode, partly closed by a dielectric plate with a hole (determining the aperture of the discharge between the cavity bottom and the anode), has been studied. In a discharge cell of type 1 with δ = 0.5 mm and a hole diameter of 22 mm, a pulsed electron beam was obtained with a duration of t _EB = 700 ns and a beam current j _EB approximately 10 times greater than that (j _AD) of the equivalent anomalous discharge (at fixed discharge voltage U and gas pressure p _He = 3.5 Torr). An electric field with the direction opposite to the field of applied voltage appeared at the cathode that was related to a space charge formed at the cathode plasma boundary, which could not follow a rapid drop of voltage across the discharge gap. In a discharge cell of type 2 with δ = 0.5 mm and a narrow slit (S = 0.1 × 5 cm²) in the dielectric plate, a pulsed electron beam was obtained with a duration of t _EB = 2 ns and a beam current of j _EB = 0.7 kA/cm² (j _EB/j _AD = 1.5) at U = 4.2 kV and p _He = 50 Torr.     

Experimental modeling of fast neutron-beam impact on Pt using Ar+ beams

Using the methods of field ion microscopy, we studied radiation-induced defects on an atomically clean surface and within a subsurface volume of platinum initiated by the interaction of neutron (E > 0.1 MeV) and Ar⁺ beams (E = 30 keV). It is shown that interaction of fast neutrons (E > 0.1 MeV, F = 6.7 × 10²¹ m^?2, and 3.5 × 10²² m^?2) with platinum leads to the same radiation damage in the volume of Pt as that produced by beams of Ar⁺ ions (E = 30 keV, F = 10¹⁶ ion/cm²) and is observed at a depth of about 1.5–2 nm beneath the irradiated Pt surface. Thus, we have carried out modeling of neutron impact with matter when replacing the neutron beam by an ion beam that causes the same radiation damage in the bulk of the material.     

The formation of nanostructured carbon material on a ferrocene-containing polymer surface induced by a high-power ion beam

The surface morphology and the composition of polymer layers based on chlorinated polyvinylchloride with addition of ferrocene (up to 10% of the polymer mass) subject to the action of a nanosecond high-power ion beam are studied. It is demonstrated that carbon material in the form of nanofibers with an average diameter of 80 nm and a length of up to 10 μm is formed on a surface singly irradiated by such beam with a current density of ～100 A/cm². A possible mechanism of the observed phenomenon is discussed.     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

Optical properties of chalcogenide glasses with ion-synthesized copper nanoparticles

Substrates of chalcogenide glassy semiconductors As₂S₃ and Ge_15.8As₂₁S_63.2 are implanted with Cu⁺ ions (energy 40 keV, radiation dose 1.5 × 10¹⁷ ion/cm², fixed current density in the ion beam 1 μA/cm²). The composite layers are analyzed by measuring linear optical transmittance and recording nonlinear optical absorption using the Z-scan technique at 780 nm (probe laser radiation with 150-fs pulses; intensity of 25–100 mW). It is ascertained for the irradiated materials that (1) the linear transmission characteristic of the optical surface plasmon resonance (SPR) band, which indicates the formation of copper nanoparticles in the near-surface region, has emerged and (2) there are simultaneously saturated and two-photon nonlinear absorption types; the latter prevails as the intensity of laser irradiation is increased.     

Ion bombardment of polyethylene—influence of polymer structure

Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (M_w), molecular weight distribution (M_w/M_n) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate polymer targets ∼2 mm were bombarded with 100 keV He⁺ or 130 keV Ar⁺ ions (dose of 10¹⁴-10¹⁶ ions/cm²; ion energy stream density <0.1 μA/cm²), micromechanical properties of their surface layer (hardness, mechanical modulus and elastic recovery) determined and compared to the virgin materials.Ar⁺ ion beam bombardment generally lowers micromechanical properties of the polyethylenes, whereas He⁺ ion beam treatment makes them higher. The effect is the stronger the higher the molecular weight of polyethylene. However, a long chain branching adversely affects the modification. The degree of crystallinity facilitates an ion beam bombardment from the point of view of micromechanical properties of the materials, however, also the morphology of a crystalline phase was found to play a role.     

High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire,gallium arsenide,and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa₂Cu₃O₇ with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al₂O₃, Si, and GaAs with buffer layers of CeO₂, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredT _c andj _c (77 K) were 87.5±0.2 K and >1×10⁶ A/cm², respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si₃N₄.     

Electrical properties of electrochemically prepared thin polytetrahydrofuran films II: Characterization under D.C. conditions

The electrical characteristics of electrochemically prepared polytetrahydrofuran (poly-THF) films were studied under d.c. conditions. At low fields (?10⁵ V cm^?1) a phonon-assisted hopping conduction occurs in poly-THF films 2000 Å thick. The low temperature dependence of the conductivity (T< 243 K) is related to interchain hopping, whereas the high temperature dependence of the conductivity (>320 K) is ascribed to trap hopping. At high fields (E>10⁵ V cm^?1) and high temperatures (T>320 K) Schottky conduction is observed and the Schottky barrier height depends both on the electrode metal and on the field direction. At temperatures below 320 K, Poole-Frenkel conduction occurs and the current intensity is independent of the nature of the electrodes and of the field direction. At very low temperatures (T<100 K) and high fields (E>10⁶ V cm^?1), thermally assisted tunnel effect conduction (δj∞T²) is observed. These changes in conduction with the temperature and the field are ascribed to changes in the polymer structure.     

X-ray photoelectron spectroscopy studies of hard coatings formed by titanium implantation on 304 stainless steel

Titanium ions are implanted (at 30 keV) in 304 stainless steel to a dose of 1.8 × 10¹⁷ ions cm⁻² using 15 μA cm⁻² and 5 μA cm⁻² beam current densities for specimens 2 and 3 respectively. X-ray photoelectron spectroscopy (XPS) measurements are performed at different temperatures. The microhardness of implanted and unimplanted specimens is also measured. In specimen 2 the microhardness does not increase significantly and XPS measurements give evidence of carburized surface alloy formation. At 250°C TiO₂ is detected on the surface and it migrates into the bulk phase above 350°C. In specimen 3 the XPS measurements exhibit an absence of iron owing to the radiation-induced segregation of titanium on the surface. This specimen shows an increase in microhardness. The XPS measurements reveal a layer of (TiC_x-C) on the surface which is suggested to be responsible for the increase in microhardness. Upon heating, TiC_x is seen to move into the bulk phase and the carbon concentration is increased. These changes occuring at higher temperatures are suggested as having an effect on the wear-resistant properties of titanium-implanted 304 stainless steel.     

Amorphization of the subsurface regions in ion-implanted alloys

The effect of partial amorphization of the subsurface regions in atomically ordered alloys upon the implantation of 20 keV positive argon ions (D=10¹⁸ion/cm² and j=200 μA/cm²) has been established by the method of field ion microscopy. The phenomenon of partial amorphization is observed at distances not less than 90 nm from the irradiated surface of materials.     

Numerical study on effects of secondary electrons generated by TEMP II accelerator

A pulsed voltage of magnetically insulated diode and intense pulsed ion beam (IPIB) models have been built according to the experimental results. A model of secondary electrons yield while IPIB irradiated metal target has also been established. The temporal law of secondary electrons induced by IPIB irradiated titanium target according to the energy deposition at surface layer has been calculated based on the models. The maximum number of secondary electrons reached is 10¹³/cm² while the energy density of IPIB is 4 J/cm². And near the surface of target, the electric field rises up to 10¹⁰ V/m with the same IPIB energy density irradiation. The evaporation ions will be accelerated near the target, therefore it is important to the formation of films.     

Degradation processes in the aluminum-silicon system induced by pulsed electric signals

Thermal conditions in a metallization layer deposited onto a single crystal silicon substrate were studied during the passage of single electric pulses with a current density of j=(1–8)×10¹⁰ A/m² and a duration of τ=50–800 μs. Mechanisms of the irreversible degradation in the aluminum-silicon contact under the pulsed current action are established. The degradation is manifested by the contact melting and the metallization layer fusion. Methods for the identification of these phenomena and determination of the critical current densities j _k are proposed. The critical current density depends on the current pulse duration as described by the relationship $j_k \sim 1/\sqrt[4]{\tau }$ . It is established that the passage of single current pulses with j≥5×10¹⁰ A/m² and τ≥200 μs leads to the formation of linear defects in the region of maximum temperature gradient in the test structure.     

Diffusion of Pb in carbon ion-implanted silicon: Discovery of a new crystalline phase after electron beam annealing

A novel phase has been discovered by dual low-energy ion implantation and high vacuum electron beam annealing. (100) p-type Si was implanted with (a) 20 keV ¹²C⁺ ions to the fluence of 6 × 10¹⁶ cm⁻² and (b) 7 keV Pb⁺ ions to the fluence of 4 × 10¹⁵ cm⁻². The ¹²C ion implantation results in an understoichiometric shallow SiC_x layer that intersects with the surface. The implanted Pb ions decorate a shallow subsurface region. High vacuum electron beam annealing at 1000 °C for 15 s using a temperature gradient of 5 °C s⁻¹ leads to the formation of large SiC nanocrystals on the surface with RBS measurements showing Pb has diffused into the deeper region affected by the ¹²C implantation. In this region, a new crystalline phase has been discovered by XRD measurements.     

Localization effect of a current-path in amorphous In-Ga-Zn-O thin film transistors with a highly doped buried-layer

The highly-doped buried layer (carrier concentration of ~ 10¹⁹ cm^− 3) in an amorphous indium-gallium-zinc oxide (a-IGZO) channel layer of thin film transistor (TFT) led to dramatic improvements in the performance and prolonged bias-stability without any high temperature treatment. These improvements are associated with the enhancement in density-of-states and carrier transport. The channel layer is composed of Ga-doped ZnO (GZO) and a-IGZO layers. Measurements performed on GZO-buried a-IGZO (GB-IGZO) TFTs indicate enhanced n-channel active layer characteristics, such as V_th, μ_FE, I_off, I_on/off ratio and S.S, which were enhanced to 1.2 V, 10.04 cm²/V·s, ~ 10⁻¹³A, ~ 10⁷ and 0.93 V/decade, respectively. From the result of simulation, a current path was well defined through the surface of oxide active layer especially in GB-IGZO TFT case because the highly-doped buried layer plays the critical role of supplying sufficient negative charge density to compensate the amount of positive charge induced by the increasing gate voltage. The mechanism underlying the high performance and good stability is found to be the localization effect of a current path due to a highly-doped buried layer, which also effectively screens the oxide bulk and/or back interface trap-induced bias temperature instability.     

Parallel-plate resonator of variable spacer thickness for accurate measurements of surface impedance of high-T c superconductive films

Measurements of microwave surface impedance of high-T _c films at gigahertz frequencies and nitrogen temperature are performed. A simple technique employing a parallel-plate resonator with liquid nitrogen as a dielectric spaces is suggested. The use of a precise mechanical device provides smooth changing of distance between films from 200μm down to zero. Coupling to the resonator is accomplished by means of two small antennas-half-wave vibrators for frequency 10 GHz. The method for determining resistivity and magnetic field penetration depth was based on the analysis of spacer thickness dependences of the resonator quality factor and frequency. YBa₂Cu₃O₇ films produced by a laser deposition technique on CaNdAlO₄ substrates withT _c =91 K andj _c =10⁷ A/cm² and on NdGaO₃ substrates withT _c =91 K andj _c =10⁶ A/cm² are examined, and the valuesR _s =0.6 mΩ,λ=348 nm atf=8.97 GHz andR _s =0.5 mΩ,λ=250 nm atf=10.12 GHz, respectively, are obtained at 77 K.     

Growing nanocrystalline silicon on sapphire by molecular beam epitaxy

It is established that an array of silicon nanoislands is formed on the surface of a sapphire substrate at the initial stages of molecular beam epitaxy. Silicon islands formed at low temperatures of the substrate (below 650°C) exhibit a pyramidal shape, while the islands grown at T > 650° possess a dome shape. The maximum density of islands was 2 × 10¹¹ cm^?2, their lateral dimensions were within 20 nm, and their heights did not exceed 3 nm.