Anelasticity of nanocrystalline metals

Nanocrystalline (n-) materials posses a characteristic ultrafine structure where small crystallites less than 100 nm in diameter are connected by highly disordered grain boundaries (GBs). Although the recent mechanical tests on the high-density n-metals suggest that the dynamic modulus is close to the polycrystalline metals, we have found a large anelastic strain comparable with the elastic strain in the quasi-static tests. The estimated activation energy as low as 0.2 eV suggests that certain cooperative motions of many atoms in the GBs are responsible for this large anelastic strain. The strain amplitude dependence (SAMD) in the resonant frequency, f, was measured by the vibrating reed technique, where f decreased by about 1% with increasing strain, , up to 10⁻⁴ and then turned to increase showing the saturation at =10⁻⁴–5×10⁻³. This strange SAMD in f showed no change by the low-temperature irradiation, in contrast, f showed a large increase due to the accumulation of the point defects probably in the GBs during the irradiation. We surmise that the (an)elastic properties of n-metals are governed by the several processes not only in the GBs but also in the crystallites.     

Anomalous elastic response of amorphous alloys suggesting collective motions of many atoms

The dynamic Young’s modulus, E, of amorphous (a-) Zr₆₀Cu₃₀Al₁₀ (numbers indicate at.%) alloy was measured as a function of frequency, f, with a strain amplitude, _t, of 10⁻⁶, E(10⁻⁶,f), and also as a function of _t for f near 10² Hz, E(_t,10² Hz), by means of the vibrating reed methods. The elasticity study under the passing of electric current (PEC) was carried out too. E(10⁻⁶,f) is lower than E₀ for f between 10 and 10⁴ Hz showing local minima near 5×10, 5×10² and 5×10³ Hz, which are indicative of the resonant collective motion of many atoms, where E₀ is the static Young’s modulus. E(_t,10² Hz) increases showing saturation with increasing _t. Qualitatively, the outlines of E(10⁻⁶,f) and E(_t,10² Hz) observed for a-Zr₆₀Cu₃₀Al₁₀ are similar to those reported for various a-alloys. Quantitatively, a change in E(_t,10² Hz) for a-Zr₆₀Cu₃₀Al₁₀ is smallest among that reported for various a-alloys, presumably reflecting that the crystallization volume, (ΔV/V)_x, is smallest for a-Zr₆₀Cu₃₀Al₁₀. The effective charge number, Z^*, estimated from the change in E(10⁻⁶,10² Hz) due to PEC is 3.0×10⁵, which is comparable with Z^* reported for various a-alloys. We surmise that the number of atoms in the collective motions excited near 10² Hz is similar among various a-alloys. The E(10⁻⁶,f) data suggest that the spatial sizes of the density fluctuations may show a distribution.     

The optical and electrical properties of copper indium di-selenide thin films

Thin films of copper indium di-selenide (CIS) with a wide range of compositions near stoichiometry have been formed on glass substrates in vacuum by the stacked elemental layer (SEL) deposition technique. The compositional and optical properties of the films have been measured by proton-induced X-ray emission (PIXE) and spectrophotometry (photon wavelength range of 300–2500 nm), respectively. Electrical conductivity (σ), charge-carrier concentration (n), and Hall mobility (μ_H) were measured at temperatures ranging from 143 to 400 K. It was found that more indium-rich films have higher energy gaps than less indium-rich ones while more Cu-rich films have lower energy gaps than less Cu-rich films. The sub-bandgap absorption of photons is minimum in the samples having Cu/In ≈ 1 and it again decreases, as Cu/In ratio becomes less than 0.60. Indium-rich films show n-type conductivities while near-stoichiometric and copper-rich films have p-type conductivities. At 300 K σ, n and μ_H of the films vary from 2.15 × 10⁻³ to 1.60 × 10⁻¹ (Ω cm)⁻¹, 2.28 × 10¹⁵ to 5.74 × 10¹⁷ cm⁻³ and 1.74 to 5.88 cm² (V s)⁻¹, respectively, and are dependent on the composition of the films. All the films were found to be non-degenerate. The ionization energies for acceptors and donors vary between 12 and 24, and 3 and 8 meV, respectively, and they are correlated well with the Cu/In ratios. The crystallites of the films were found to be partially depleted in charge carriers.     

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag⁺ and Au⁺ ions of 100 MeV at two different fluencies such as 1 × 10¹² ions/cm² and 1 × 10¹³ ions/cm² at a pressure of 10^− 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi₂. The structural parameters such as crystallite size (D), strain (ε) and dislocation density (δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi₂ have been discussed in this paper.     

Thermal conductivity of BaWO4 single crystal

Barium tungstate (BaWO₄) single crystal has been grown using Czochralski technique. It belongs to the scheelite structure, forming the space group I4₁/a at room temperature and the primitive cell contains two formular units. The thermal expansion, specific heat and thermal diffusivity were measured, and then the thermal conductivity was calculated. These results show that BaWO₄ possesses large anisotropic thermal expansion and its thermal expansion coefficients are _a = 1.10 × 10⁻⁵/K, _b = 1.08 × 10⁻⁵/K, and _c = 3.51 × 10⁻⁵/K in the temperature range from 303 to 1423 K. However, its thermal conductivity shows small anisotropic in the temperature range from 297 to 563 K and even displays isotropic at about 428 K. The calculated thermal conductivities are 2.59 and 2.73 W m⁻¹ K⁻¹ at room temperature, along [1 0 0] and [0 0 1] directions, respectively.     

Glass-free LTCC microwave dielectric ceramics

The sintering behavior, microstructure and microwave dielectric properties of complex pyrophosphate compounds AMP₂O₇ (A = Ca, Sr; M = Zn, Cu) were investigated in this paper. All compounds could be densified below the temperature of 950 °C without any glass addition, and exhibit low permittivity (_r < 8), high Q × f value and negative temperature coefficient of resonant frequency. The Q × f value was discussed from the point of view of bond strength. The chemical compatibility with silver and copper was also investigated. All compounds seriously react with silver at 700 °C. However SrZnP₂O₇ could be co-fired with copper in reduced atmosphere. The microwave dielectric properties of SrZnP₂O₇ sintered at 950 °C in reducing atmosphere are: _r = 7.06, Q × f = 52781 GHz, τ_f = −70 ppm/°C. In terms of its lower sintering temperature, chemical compatibility with copper and good microwave dielectric properties, SrZnP₂O₇ ceramic is very promising for low temperature co-fired ceramic (LTCC) applications.     

Magnetic and electrical characterization of heavily boron-doped diamond

For a heavily boron-doped diamond (BDD) film, temperature variations of the electrical conductivity σ and magnetic susceptibility χ are reported. The room temperature σ 143 (Ω-cm)⁻¹ corresponds to a carrier concentration 10³ ppm, and its temperature variation yields an activation energy E_a 28 meV from 140 to 300 K and E_a0.88 meV from 40 to 80 K. It is argued that larger boron doping leads to lower magnitudes of E_a. The χ vs. T data (1.8–350 K) fits the Curie–Weiss law, with the concentration of paramagnetic species 120 ppm and a diamagnetic susceptibility −0.4×10⁻⁶ emu/g Oe. The results obtained from the measurements of σ and χ are discussed and compared.     

The relationship of the volume fraction of martensite vs. plastic strain in an Fe–Mn–Si–Cr–N shape memory alloy

The volume fractions of stress-induced martensite formed by certain plastic strains were determined by X-ray diffraction and quantitative metallography in an Fe–Mn–Si–Cr–N alloy at room temperature. The results are fitted by least square method and are well consistent with an exponential function f_M=1−exp{−β[1−exp(−η)]ⁿ} deduced by Olson and Cohen, who used it to fit with experimental data for AISI304 stainless steel. The similarity of and β, as well as the difference in n for these two alloys are discussed in relation to their nucleation mechanisms.     

Room temperature indium tin oxide by XeCl excimer laser annealing for flexible display

A XeCl excimer laser (λ=308 nm) has been used to anneal Indium Tin Oxide (ITO) films deposited at 25 °C using DC magnetron sputtering. With increasing laser fluence, the film crystallinity was improved while retaining the as-deposited 111 texture. As a result of laser irradiation, the sheet resistance of 100 nm ITO films decreased from 191 Ω/□ (1.91×10⁻³ Ω cm) to 25 Ω/□ (2.5×10⁻⁴ Ω cm), while the optical transmittance in the visible range increased from 70% to more than 85%. Surface roughness and etching properties were also significantly improved following laser annealing.     

Homogenization kinetics of a cast Ti48A12W0.5Si alloy

The homogenization kinetics of a cast Ti48A12W0.5Si alloy with a duplex microstructure was studied in terms of γ-phase dissolution and -grain growth. It was found that the measured volume fraction of remnant γ grains can be well simulated by a model of interface-controlled dissolution in a dislocation mechanism, instead of a diffusion-controlled one. The activation energy for the /γ interface reaction was found to be Q_int = 476 kJ mol⁻¹, which is much higher than the interdiffusion activation energy in TiAl alloy. The grain growth of phase during homogenization can be categorized into three stages. During the first stage, where the volume fraction of remnant γ grains is higher than about 10%, the growth of grains follows the parabolic law D = k₁t^0.2, and the activation energy for grain growth was calculated to be Q₁ = 442 kJ mol⁻¹, very close to the Q_int for /γ interface reaction. In the second stage, where few fine γ grains (1–10 vol.%) remained, a dramatic grain growth occurs. During the final stage, as the single phase is obtained, the coarsening of grains again satisfies the grain growth law D = k₃t^0.4, with the grain growth activation energy Q₃ of 147 kJ mol⁻¹, lower than the reported interdiffusion energy of γ phase.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

High performance very low frequency forced pendulum

A forced torsion pendulum has been constructed for the measurement of mechanical loss angle (tan δ) and elastic shear modulus in three different modes: (a) as a function of temperature (80–1250 K), at imposed frequency, during heating or cooling at imposed heating or cooling rate (0.1–5 K/min); (b) as a function of frequency (10–10⁻⁴ Hz) in isothermal conditions, and (c) as a function of amplitude (5×10⁻⁶ to 5×10⁻⁵) at imposed frequency and under isothermal conditions. The mechanical part of the pendulum has been designed in such a way that torsional plastic deformation of the specimen can be performed in situ, i.e., at low temperatures to generate fresh dislocations in metals, and at high temperatures to enhance grain boundary sliding, for instance. The whole installation is computer controlled and exhibits not only outstanding performances but also a very high capability of working in different conditions, being a useful tool for studying a large variety of mechanisms in different materials.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage (IV), capacitance–voltage (CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga_0.986In_0.014As heavily doped with Si (6.8 × 10¹⁷ cm⁻³) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10⁻¹⁵ cm² and 8.6 × 10⁻¹⁴ cm² and densities of 2.8 × 10¹⁶ cm⁻³ and 9.6 × 10¹⁵ cm⁻³, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min.

Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10⁻¹⁵ cm² and a density of 1.38 × 10¹⁵ cm⁻³. The concentration of this trap level is also decreased by thermal annealing.     

Analysis of strain rate dependence of tensile elongation for a mechanical milling Al–1.1Mg–1.2Cu alloy tested at 748 K from a dislocation dynamics viewpoint

Tensile deformation was carried out for a mechanically milled and thermo-mechanically treated Al–1.1Mg–1.2Cu (at.%) alloy at 748 K and three nominal strain rates of 10⁻³, 10⁰, and 10² s⁻¹. Despite the prevailing belief that superplasticity occurs by grain boundary sliding which requires slow strain rates at high temperatures, the maximum elongation was observed at the intermediate strain rate of 10⁰ s⁻¹, neither at the lowest nor the highest strain rates. In order to explain this phenomenon, the true stress–true strain behaviors at these three nominal strain rates were analyzed from a viewpoint of dislocation dynamics by computer-simulation with four variables of the thermal stress component σ*, dislocation immobilization rate U, re-mobilization probability of unlocked, immobile dislocations Ω and dislocation density at yielding ρ₀. It can then be concluded that the large elongation (>400% in nominal strain) at the intermediate strain rate is produced by a combination of a very large Ω and a moderate U, resulting in a large strain rate sensitivity m value.     

High polysilicon TFT field effect mobility reached thanks to slight phosphorus content in the active layer

The paper deals with the effect of slightly phosphorus atoms introduced during deposition of polysilicon films. Polysilicon films are used as an active layer in thin film transistors (TFTs) fabricated on glass substrates at a maximum temperature of 600 °C.Three phosphorus atoms contents, determined by the value of the phosphine to silane ratio: Γ (3.7 × 10^− 7, 8 × 10^− 7, 26 × 10^− 6), are used to optimize the active layer quality. The in-situ doped layers induce a better stability of the electrical characteristics, a higher mobility and lower value of the threshold voltage for the slightly doped active layers [M. Zaghdoudi, M.M. Abdelkrim, M. Fathallah, T. Mohammed-Brahim and F. Le-Bihan Control of the weak phosphorus doping in polysilicon, Materials Science and Forum, Vols. 480–481 (2005) pp.305.]. The present work shows that the effect of slightly phosphorus content improves the quality of oxide/polysilicon interface and decreases the defects density. Degradation of electrical properties is shown to originate from the creation of defect at the channel-interface oxide and in the grain boundaries. The effect of temperature change on the electrical properties was studied and the behaviour was also analyzed.     

A low momentum tagged antineutron beam

The operating performances of a low-momentum (< 270 MeV/c) tagged beam are reported. The beam is obtained by means of the charge exchange reaction n on a liquid hydrogen target. The neutron associated to the in the two-body reaction is used for the determination of the energy and direction. The measured total rate of tagged is 8.02 ± 0.03 × 10⁻⁵ per incident at 300 MeV/c.     

Investigation of the dependence of the contact resistance on the external gold layer in AuNiGeAu/n-GaAs

The contact resistance of an Au/Ni/Ge/Au metallization system on n-GaAs exhibits a drop in contact resistance from 13.3 × 10⁻⁶ ω cm² to 8.6 × 10⁻⁶ ω cm² when the external gold layer is varied from 800 Å to 6000 Å in thickness. Secondary ion mass spectroscopy indicates that the improvement in contact resistance is due to the gold's “regulating” the amount of NiAs formed, leading to an increase in the area fraction covered by the Ni₂GeAs. The result shows that the external gold layer deposited to improve bonding should be optimized in thickness. This role played by the external gold layer of the metal system in affecting the contact resistance has not been previously studied.     

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti–46Al–2W–0.5Si alloy

The effect of Al₂O₃ particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al₂O₃ particles and mean ₂–₂ interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10⁻⁶ to 1.18×10⁻⁴ ms⁻¹ in alumina moulds. The ingots with constant volume fraction of Al₂O₃ particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10⁻⁴ ms⁻¹ and subsequent solution annealing followed by cooling at constant rates varying between 0.078 and 1.889 K s⁻¹. The mean ₂–₂ interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ^−0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al₂O₃ particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al₂O₃ particles was proposed for the prediction of the yield stress.