Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz

A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98 mol.% ZnO–2 mol.% (Bi₂O₃, Sb₂O₃, Co₃O₄, MnO₂) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45 GHz. The effect of the sintering temperature (900–1200 °C), holding time (5–120 min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation Gⁿ = kte^(?Q/RT). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n = 3.06–3.27, Q = 206–214 kJ mol^?1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100 °C for a 5 min soaking time for both H and E field processed samples, where respectively densities of 99.2 ± 0.5% theoretical density (TD) and 98.3 ± 0.5% TD along with grain size values of G = 7.2 ± 0.36 μm and G = 6.6 ± 0.33 μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500–570 V mm^?1 were obtained, together with high non-linear coefficients α = 29–39 and low leakage currents (J_l ≈ 5 × 10^?3 mA cm^?2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.     

Optical properties of films of polycarbazolyldiacetylene PDCHD-HS for photonic applications

Spun films of polycarbazolyldiacetylene polyDCHD-HS were prepared from toluene solutions with a thickness ranging from 9 nm up to 3.6 μm. Thicker films were characterized as waveguides at 849 and 1321 nm; 5 dB/cm propagation losses were measured at 1321 nm. The off-resonance third-order nonlinearity of ultra-thin samples (9–14 nm) was measured at 1064 nm with picosecond pulses and using surface plasmon spectroscopy. The very large value χ⁽³⁾=4.4×10⁻¹⁷ m²/V² was detected for the real part of the third-order susceptibility.     

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.     

Hydrogen trapping in an API 5L X60 steel

Electrochemical hydrogen permeation tests were performed on an API 5L X60 steel to study trapping and diffusion properties in the as-received (AR) and in a cold-rolled (CR) state, at 30, 50 and 70 °C. Hydrogen trapping was characterized by assuming saturable traps in local equilibrium. Binding energies (ΔG) and trap densities (N) were determined by fitting a trapping theoretical model to experimental data. Both conditions AR and CR present a high density of weak traps |ΔG| < 35 kJ/mol, namely N = 1.4 × 10^?5 and 7.9 × 10^?4 mol/cm³ respectively. Strong trapping sites were detected (?72 < ΔG < ?57 kJ/mol) and their densities increased markedly after cold-rolling.     

Study of the effect of α irradiation on the microstructure and mechanical properties of nanocrystalline Ni

The effect of irradiation damage on the microstructure evolution and mechanical properties in nanocrystalline (nc) Ni with an average grain size of ～60 nm was studied. Samples were irradiated at doses of 1.6 × 10¹⁵, 2.3 × 10¹⁵, 5 × 10¹⁵ and 2.3 × 10¹⁶ He²⁺ cm^?2 by 12 MeV He ions at room temperature. Microstructural parameters like domain size and microstrain were studied in detail using the X-ray diffraction (XRD) technique with the simplified breadth method and Williamson–Hall analysis. The average domain size was found to decrease systematically with the increase in irradiation dose. Microscopic observations made with transmission electron microscopy showed dislocation loops and dislocation networks within the grain interior of the irradiated sample. In addition, irradiation at a higher dose showed small amounts of the sand-like black dots inside some grains, which could be due to the accumulation of point defects. Tensile tests performed on the irradiated sample were compared with unirradiated samples. The irradiated sample showed higher ultimate tensile strength and average work-hardening rate as compared to the unirradiated sample. Nanoindentation studies were performed to study the effect of irradiation on deformation parameters like strain rate sensitivity (m) and activation volume (V^*). The value of m was found to increase whereas V^* was found to decrease with increase in irradiation dose. Fracture surfaces of the tensile sample were investigated by SEM. The fracture morphology of the unirradiated sample showed dimpled rupture with much large dimple diameter and depth. The irradiated sample also showed dimple rupture but with much finer dimple diameter with wide size distribution and shallow depths.     

Effect of hydrogen on internal friction and elastic modulus in titanium alloys

The effect of hydrogen on the variation with temperature of internal friction (Q^?I) and elastic modulus (E) of a number of Ti-based alloys has been studied in the Hz and kHz frequency ranges. A relaxation peak of internal friction with a high degree of relaxation (Q^?I_max ～ 10^?1) and with a ΔE effect is observed in all hydrogen-doped samples at T ～ 600 K at ～1 kHz, and at T ～ 500 K at ～1 Hz. Such a peak is not present in samples without hydrogen. The activation energy W and the frequency factor v₀ of the observed relaxation are determined to be W ～ 1.55 eV, v₀ ～ 10¹⁷ s^?1. It is shown that the observed effects are connected with the mechanism of grain boundary relaxation, as the introduction of hydrogen into titanium alloys leads to the formation of fine-grained structures.     

Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10⁻⁶ to 1.18 × 10⁻⁴ ms⁻¹ and at a constant temperature gradient at the solid–liquid interface of G_L = 12 × 10³ K m⁻¹. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni₃Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q_a = 401 kJ/mol.     

Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Tensile experiments on a fine-grained single-phase Mg–Zn–Al alloy (AZ31) at 673 K revealed superplastic behavior with an elongation to failure of 475% at 1 × 10^?4 s^?1 and non-superplastic behavior with an elongation to failure of 160% at 1 × 10^?2 s^?1; the corresponding strain rate sensitivities under these conditions were ～0.5 and ～0.2, respectively. Measurements indicated that the grain boundary sliding (GBS) contribution to strain ξ was ～30% under non-superplastic conditions; there was also a significant sharpening in texture during such deformation. Under superplastic conditions, ξ was ～50% at both low and high elongations of ～20% and 120%; the initial texture became more random under such conditions. In non-superplastic conditions, deformation occurred under steady-state conditions without grain growth before significant flow localization whereas, under superplastic conditions, there was grain growth during the early stages of deformation, leading to strain hardening. The grains retained equiaxed shapes under all experimental conditions. Superplastic deformation is attributed to GBS, while non-superplastic deformation is attributed to intragranular dislocation creep with some contribution from GBS. The retention of equiaxed grain shapes during dislocation creep is consistent with a model based on local recovery related to the disturbance of triple junctions.     

Void initiation in fcc metals: Effect of loading orientation and nanocrystalline effects

It is shown, through molecular dynamics simulations, that the emission and outward expansion of special dislocation loops, nucleated at the surface of nanosized voids, are responsible for the outward flux of matter, promoting their growth. Calculations performed for different orientations of the tensile axis, [0 0 1], [1 1 0] and [1 1 1], reveal new features of these loops for a face-centered cubic metal, copper, and show that their extremities remain attached to the surface of voids. There is a significant effect of the loading orientation on the sequence in which the loops form and interact. As a consequence, the initially spherical voids develop facets. Calculations reveal that loop emission occurs for voids with radii as low as 0.15 nm, containing two vacancies. This occurs at a von Mises stress approximately equal to 0.12G (where G is the shear modulus of the material), and is close to the stress at which dislocation loops nucleate homogeneously. The velocities of the leading partial dislocations are measured and found to be subsonic (～1000 m s^?1). It is shown, for nanocrystalline metals that void initiation takes place at grain boundaries and that their growth proceeds by grain boundary debonding and partial dislocation emission into the grains. The principal difference with monocrystals is that the voids do not become spherical and that their growth proceeds along the boundaries. Differences in stress states (hydrostatic and uniaxial strain) are discussed. The critical stress for void nucleation and growth in the nanocrystalline metal is considerably lower than in the monocrystalline case by virtue of the availability of nucleation sites at grain boundaries (von Mises stress ～0.05G). This suggests a hierarchy of nucleation sites in materials, starting with dispersed phases, triple points and grain boundaries, and proceeding with vacancy complexes up to divacancies.     

Microstructure evolution of 6061 O Al alloy during ultrasonic consolidation: An insight from electron backscatter diffraction

6061 O Al alloy foils were welded to form monolithic and SiC fibre-embedded samples using the ultrasonic consolidation (UC) process. Contact pressures of 135, 155 and 175 MPa were investigated at 20 kHz frequency, 50% of the oscillation amplitude, 34.5 mm s^?1 sonotrode velocity and 20 °C. Deformed microstructures were analysed using electron backscatter diffraction (EBSD). At all contact pressures deformation occurs by non-steady state dislocation glide. Dynamic recovery is active in the upper and lower foils. Friction at the welding interface, instantaneous internal temperatures (0.5–0.8 of the melting temperature, T_m), contact pressure and fast strain rates result in transient microstructures and grain size reduction by continuous dynamic recrystallization (CDRX) within the bonding zone. Bonding occurs by local grain boundary migration, which allows diffusion and atom interlocking across the contact between two clean surfaces. Textures weaken with increasing contact pressure due to increased strain hardening and different grain rotation rates. High contact pressures enhance dynamic recovery and CDRX. Deformation around the fibre is intense within 50 μm and extends to 450 μm from it.     

Reduced thermal conductivity in Pb-alloyed AgSbTe2 thermoelectric materials

Pb-alloyed AgSbTe₂ (Pb_xAg₂₀Sb_30?xTe₅₀ (x = 3, 4, 5 and 6)) composites were synthesized using a modified Bridgman method with a graphite mold to form plate-like samples. The Bridgman-grown specimens were dense, with few solidification cavities, and were sufficiently mechanically robust for a variety of electronic/thermal transport measurements. Inhomogeneity was found on the grain boundary, and was embedded with the nanoprecipitates of δ-Sb₂Te with a feature size of 100 nm of the 5 at.% Pb and 6 at.% Pb specimens. A combined effect of alloying, inhomogeneity and nanoprecipitates leads to a low thermal conductivity of 0.3–0.4 W m^?1 K^?1, which approaches the theoretical minimum thermal conductivity of the amorphous material (κ_min ～ 0.36 W m^?1 K^?1). A peak of the zT value, ranging from 0.7 to 0.8, is achieved at 425 K. Further annealing at 673 K increases the grain size and causes a reduction in the value of the zT peak to 0.4.     

Microwave and infrared dielectric response of tunable Ba1−xSrxTiO3 ceramics

The dielectric properties, infrared (IR) dielectric response and Raman spectra of tunable microwave (MW) Ba_1?xSr_xTiO₃ (x = 0.4, 0.5, 0.6 and 0.7) ceramics were studied systematically as functions of composition and temperature. It was found that, with increasing the content of Sr from 40% to 70%, the permittivity of the Ba_1?xSr_xTiO₃ ceramics decreased from 7200 to 640 while the dielectric tunability decreased from 50.1% to 5.4% measured at 10 kHz. It is particularly interesting that the MW dielectric loss tangent was significantly decreased from 1.5 × 10^?2 to 7.4 × 10^?4. Complex permittivity spectra obtained by fitting the infrared data were also extrapolated to MW frequency and compared with the dielectric data near 1 GHz. For the samples with x = 0.4 and x = 0.5, the dielectric loss measured at ～1 GHz was much higher than that calculated at 10 GHz, which is presumably due to the Debye-type dielectric relaxation in GHz region. However, the dielectric loss of the samples with x = 0.6 and x = 0.7, extrapolated from IR range, was in agreement with that measured at MW frequency. Low-temperature Raman scattering showed that the band at 760 cm^?1 assigned to the Ti?O₃ torsional mode markedly sharpens and shifts upward with increasing content of Sr. This is an indication that the bonding between cations and anions was tightened due to the substitution of Ba with Sr, which explains well the decrease in dielectric permittivity, loss and tunability with increasing concentration of Sr.     

Ion tracks and microstructures in barium titanate irradiated with swift heavy ions: A combined experimental and computational study

Tetragonally structured barium titanate (BaTiO₃) single crystals were irradiated using 635 MeV ²³⁸U⁺ ions to fluences of 1 × 10⁷, 5 × 10¹⁰ and 1.4 × 10¹² ions cm^?2 at room temperature. Irradiated samples were characterized using ion channeling, X-ray diffraction, helium ion microscopy and transmission electron microscopy. The results show that the ion-entry spot on the surface has an amorphous core of up to ～10 nm in diameter, surrounded by a strained lattice structure. Satellite-like defects around smaller cores are also observed and are attributed to the imperfect epitaxial recrystallization of thermal-spike-induced amorphization. The critical value of the electronic stopping power for creating observable amorphous cores is determined to be ～22 keV nm^?1. Molecular dynamics simulations show an amorphous track of ～1.2 nm in radius under thermal energy deposition at 5 keV nm^?1; the radius increases to ～4.5 nm at 20 keV nm^?1. A linear fit of the core diameter as a function of the square root of the energy deposition rate suggests a reduction in the diameter by an average of ～8.4 nm due to thermal recrystallization if electron–phonon coupling efficiency of 100% is assumed. The simulation also reveals details of the bonding environments and shows different densities of the amorphous zones produced at different energy deposition rates.     

Effect of Gd substitution on the crystal structure and multiferroic properties of BiFeO3

The crystal structure, magnetic and local ferroelectric properties of polycrystalline Bi_1?xGd_xFeO₃ (x = 0.1, 0.2, 0.3) samples were investigated at room temperature. Gadolinium substitution was found to induce a polar-to-polar R3c → Pn2₁a structural phase transition at x ≈ 0.1. Increasing the content of the substituting element suppressed the spontaneous polarization in Bi_1?xGd_xFeO₃, resulting in a ferroelectric–paraelectric Pn2₁a → Pnma phase transition at 0.2 < x < 0.3. The substitution caused the appearance of a weak ferromagnetic moment. The data presented are consistent with the hypothesis that the most effective way to induce spontaneous magnetization in antiferromagnetic BiFeO₃ seems to be related to A-site substitution by rare-earth ions with a large difference in ionic radius with respect to Bi³⁺. The effect of the magnetically active Gd³⁺ substitution on the low-temperature magnetic properties of Bi_1?xGd_xFeO₃ samples is also discussed.     

Pure red electroluminescence from novel heteroleptic cyclometalated platinum(II) emitters embedded in polyvinylcarbazole

We fabricated molecularly doped, polymer-based light-emitting diodes possessing a single emitting layer containing a hole-transporting host polymer poly(N-vinylcarbazole) and an electron-transporting auxiliary, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, doped with novel phosphorescent cyclometalated Pt(II) complexes bearing arylpyridine and 1,3-diketone ligands. These novel cyclometalated Pt(II) complexes emit pure red color both in steady-state emissions (poly(methyl methacrylate) films)) and electrophosphorescence. They exhibited pure red emissions with the Commission Internationale de l’Eclairage coordinates (X = ～0.67, Y = ～0.33), which is almost identical to the coordinates of standard red (0.66, 0.34) demanded by the National Television System Committee. The color coordinates remained unchanged over a range of operating voltages, even at luminances greater than 1 × 10⁴ cd/m². The maximum external quantum efficiency of these devices exceeded 3.6% and the maximum brightness was greater than 1 × 10⁴ cd/m².     

Oxygen grain-boundary transport in polycrystalline alumina using wedge-geometry bilayer samples: Effect of Y-doping

Novel wedge-geometry, dual-layer alumina samples, both undoped and 500 ppm Y³⁺-doped, were studied in the temperature regime 1250–1400 °C to determine the effect of Y³⁺ on oxygen grain-boundary transport in alumina. The samples consisted of a wedge-shaped, single-phase alumina top layer, diffusion bonded to an alumina/Ni substrate containing a fine, uniform dispersion of Ni marker particles (0.5 vol.%). The extent of the alumina spinel oxidation layer was measured as a function of the wedge thickness for a series of heat-treatment conditions. Models of the transport behavior were used to derive values for the rate constants (k) in both the alumina top layer and the alumina/Ni substrate. It was found that the presence of yttrium slows oxygen grain-boundary diffusion in alumina by a factor of ～5 (at 1300 °C), and increases the corresponding activation enthalpy for oxidation from 407 ± 20 to 486 ± 34 kJ mol^?1. Microstructural observations suggested that yttrium also slows Ni outward diffusion. A comparison of the different k values revealed that, at 1300 °C, the presence of Ni alone enhances transport by a factor of ～2 relative to undoped alumina.     

Mechanical behavior and microstructural evolution of a Mg AZ31 sheet at dynamic strain rates

The mechanical behavior of an AZ31 Mg sheet has been investigated at high strain rate (10³ s^?1) and compared with that observed at low rates (10^?3 s^?1). Dynamic tests were carried out using a Hopkinson bar at temperatures between 25 and 400 °C. Tensile tests were carried out along the rolling and transverse directions and compression tests along the rolling and the normal directions in both strain rate ranges. The tension–compression yield asymmetry as well as the yield and flow stress in-plane and out-of-plane anisotropies were investigated. The microstructure of the initial and tested samples was examined by electron backscatter diffraction. The dynamic mechanical behavior is characterized by the following observations. At high temperatures the yield asymmetry and the yield anisotropies remain present and twinning is highly active. The rate of decrease in the critical resolved shear stress of non-basal systems with temperature is smaller than at quasi-static rates. Rotational recrystallization mechanisms are activated.     

Thermoelectric properties of Sb-doped Mg2Si semiconductors

The thermoelectric properties of Sb-doped Mg₂Si (Mg₂Si:Sb = 1:x(0.001 ≦ x ≦ 0.02)) fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. Sb-doped Mg₂Si samples are n-type in the measured temperature range. The electron concentration of Sb-doped Mg₂Si at 300 K ranges from 2.2 × 10¹⁹ for the Sb concentration, where x = 0.001, to 1.5 × 10²⁰ cm⁻³ for x = 0.02. First-principles calculation revealed that Sb atoms are expected to be primarily located at the Si sites in Mg₂Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Sb concentration. The sample x = 0.02 shows a maximum value of the figure of merit ZT, which is 0.56 at 862 K.     

Twinned dendrite growth in binary aluminum alloys

The formation of twinned dendrites (feathery grains) in binary Al–Zn, Al–Mg, Al–Cu and Al–Ni alloys has been studied in specimens directionally solidified under identical thermal conditions, i.e. G ≈ 100 K cm⁻¹, v ≈ 1 mm s⁻¹, and with slight natural convection in the melt. The influence of the solute element nature and content has been found to be of less importance than previously reported since feathery grains were formed in all four alloys, regardless whether the alloying elements are hexagonal close packed (Zn and Mg) or face-centered cubic with a high (Ni) or low (Cu) stacking fault energy. A detailed analysis confirmed that twinned dendrites grow along 〈1 1 0〉 directions in all four cases, with a complex branch morphology made of up to six to nine arms. Surprisingly, at high Zn or Mg compositions for which regular dendrites grow along 〈1 1 0〉 instead of 〈1 0 0〉, [Gonzales F, Rappaz M. Metall Trans A 2006; 37: 2797. [1]] no twinned dendrites could be formed. In terms of both the growth kinetics advantage of twinned dendrites over regular ones and the associated tip shape, some experimental evidence seems to contradict the doublon conjecture suggested by Henry [Henry S. PhD thesis, Ecole Polytechnique Fédéral de Lausanne, 1999. [21]], at least for the solute compositions studied in the present work.     

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH^? and H₂O in the KN unit cell, tetragonal KNbO₃ (KN) nanowires were formed when the synthesis was carried out at 120 °C for 48 h. However, when the fabrication was conducted at high temperatures (?150 °C) or at 120 °C for a long period of time (?72 h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120 °C for 60 h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb⁵⁺-doped SrTiO₃ substrate, and their d₃₃ values were measured for the first time. A tetragonal KN nanowire exhibited a d₃₃ value of 23.5 pm V^?1, which is larger than that of the orthorhombic KN nanowire (11.6 pm V^?1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d₃₃ value of 40.0 pm V^?1. The d₃₃ values of KN nanowires increased to 104.5, 137.1 and 146.0 pm V^?1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [1 0 0] direction by application of a DC voltage of 10 V.