Study on dynamic strain aging phenomenon of 3004 aluminum alloy

3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10⁻⁵ to 5.56 × 10⁻³ s⁻¹) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, T_t, was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than T_t. According to the activation energy, it is believed that the process at the temperature region lower than T_t is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.     

Electroresistance and magnetoresistance in La0.9Ba0.1MnO3 thin films

The electroresistance and magnetoresistance effects have been investigated in La_0.9Ba_0.1MnO₃ epitaxial thin films. Tensile strain caused by substrate mismatch makes the Curie temperature T_C of the film at ∼300 K. The influence of an applied dc-current on the resistance in the absence of a magnetic field was studied. Significant change of the peak resistance at different currents was found. The reduction of the peak resistance reaches ∼27% with an electric current density up to 1.3 × 10⁵ A cm⁻². We also studied colossal magnetoresistance (CMR) effect in the films. Applying a magnetic field of 2 T could lead to a magnetoresistance as large as 42%. The reduction of resistance caused by a current density ∼1.3 × 10⁵ A cm⁻² was found to be equivalent to the CMR effect caused by 1.5 T near T_C. The phenomenon that the resistance in CMR manganites could be easily controlled by the electric current should be of high interest for both fundamental research and practical applications.     

Negative thermal expansion in silicalite-1 and zirconium silicalite-1 having MFI structure

In situ high temperature X-ray diffraction (HTXRD) studies on monoclinic silicalite-1 (S-1, silica polymorph of ZSM-5) and an orthorhombic metallosilicate molecular sieve, zirconium silicalite-1 (ZrS-1) with MFI structure (Si/Zr = 50) have been carried out using a laboratory X-ray diffractometer with an Anton Parr HTK 1600 attachment. While the structure of the S-1 collapsed at 1123 K forming α-cristobalite. S-1 and ZrS-1 showed a complex thermal expansion behavior in the temperature range 298–1023 K, ZrS-1 was stable. Powder X-ray diffraction (PXRD) data taken in this region have shown strong negative lattice thermal expansion coefficient, α_V = −6.75 × 10⁻⁶ and −17.92 × 10⁻⁶ K⁻¹ in the temperature range 298–1023 K⁻¹ for S-1 and ZrS-1 samples, respectively. The thermal expansion behavior of S-1 and ZrS-1 is anisotropic, with the relative strength of contraction along a axis is more than that along b and c axes. Three different thermal expansion regions could be identified in the overall temperature range (298–1023 K) studied, corroborating with the three steps of weight loss in the TG curve of ZrS-1 sample. While the region between 298 and 423 K, displays positive thermal expansion coefficient with α_V = 2.647 × 10⁻⁶ and 4.24 × 10⁻⁶ K⁻¹, the second region between 423 and 873 K shows strong negative thermal expansion (NTE) coefficient α_V = −7.602 × 10⁻⁶ and −15.04 × 10⁻⁶ K⁻¹, respectively, for S-1 and ZrS-1 samples. The region between 873 and 1023 K, shows a very strong NTE coefficient with α_V = −12.08 × 10⁻⁶ and −45.622 × 10⁻⁶ K⁻¹ for S-1 and ZrS-1, respectively, which is the highest in the whole temperature range studied. NTE seen over a temperature range 298–1023 K could be associated with transverse vibrations of bridging oxygen atoms in the structure which results in an apparent shortening of the Si–O distances.     

Synthesis,crystal structure refinement,electrical and magnetic properties of BaV13O18 and SrV13O18

Polycrystalline samples of BaV₁₃O₁₈ and SrV₁₃O₁₈ were prepared by solid-state reaction of BaCO₃, SrCO₃, V₂O₅ and V at 1773–2073 K in flowing Ar. The crystal structures of BaV₁₃O₁₈ (R-3, a_h=12.6293(10) Å, c_h=7.0121(4) Å) and SrV₁₃O₁₈ (a_h=12.5491(7) Å, c_h=6.9878(3) Å) were refined by the Rietveld method using X-ray diffraction data. BaV₁₃O₁₈ exhibited semiconducting behavior with electrical resistivity from 5.8×10⁻³ to 2.7×10⁻³ Ω cm at 100–300 K. Electrical resistivity of SrV₁₃O₁₈ ranged from 1.5×10⁻³ to 1.8×10⁻³ Ω cm, and it increased slightly up to around 250 K and decreased above 250 K with increasing temperature. Negative Seebeck coefficients of both compounds at 100–300 K indicated that electron was the dominant carrier. BaV₁₃O₁₈ and SrV₁₃O₁₈ showed paramagnetism with the effective magnetic moment of 0.11μ_B and 0.15μ_B, respectively, at 10–100 K.     

Optical and thermal properties of crystalline Tb: KLu(WO4)2

A single crystal of Tb: KLu(WO₄)₂ with dimensions of 40 mm × 40 mm × 18 mm has been grown by the top-seeded solution growth (TSSG) method. The color of the crystal is brown. Absorption and fluorescence spectra were measured at room temperature. The measured specific heat is a little lower than that of Yb: KLW (0.365 J/g K) at 90 °C. The measured mean linear coefficients of thermal expansion are α_a = 17.1643 × 10^− 6 K^− 1_, α_a^⁎ = 14.0896 × 10^− 6 K^− 1, α_b = 8.7938 × 10^− 6 K^− 1, α_c = 23.1745 × 10^− 6 K^− 1, α_c^⁎ = 20.2866 × 10^− 6 K^− 1. The results indicate that the crystal has a large anisotropy. The refractive index was measured.     

Effect of Zn addition,strain rate and deformation temperature on the tensile properties of Sn–3.3 wt.% Ag solder alloy

Stress–strain characteristics of the binary Sn–3.3 wt.% Ag and the tertiary Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys were investigated at various strain rates (SR, ε^·) from 2.6 × 10^− 4 to 1.0 × 10^− 2 s^− 1 and deformation temperatures from 300 to 373 K. Addition of 1 wt.% Zn to the binary alloy increased the yield stress σ_y and the ultimate tensile stress σ_UTS while a decrease of ductility (total elongation ε_T) was observed. Increasing the strain rate (ε^·) increased both σ_y and σ_UTS according to the power law σ = C ε^·m. A normal decrease of ε_T with strain rate was observed according to an empirical equation of the form ε_T = A exp (− λε^·); A and λ are constants. Increasing the deformation temperature decreased both σ_y and σ_UTS in both alloys, and decreased the total elongation ε_T in the Zn-free binary alloy, whereas ε_T was increased in the Zn-containing alloy. The activation energy was determined as 41 and 20 kJ mol^− 1 for these alloys, respectively. The results obtained were interpreted in terms of the variation of the internal microstructure in both alloys. The internal microstructural variations in the present study were evaluated by optical microscopy, electron microscopy and X-ray diffraction. The results show the importance of Zn addition in enhancing the mechanical strength of the Sn–3.3 wt.% Ag base alloy.     

Carbon fiber polymer–matrix structural composites exhibiting greatly enhanced through-thickness thermoelectric figure of merit

The through-thickness thermoelectric behavior of continuous carbon fiber epoxy-matrix composites is greatly improved by adding tellurium particles (13 vol.%), bismuth telluride particles (2 vol.%) and carbon black (2 vol.%). The thermoelectric power is increased from 8 to 163 μV/K, the electrical resistivity is decreased from 0.17 to 0.02.Ω.cm, the thermal conductivity is decreased from 1.31 to 0.51 W/m.K, and the dimensionless thermoelectric figure of merit ZT at 70 °C is increased from 9 × 10⁻⁶ to 9 × 10⁻². Tellurium increases the thermoelectric power greatly. Bismuth telluride decreases the electrical resistivity and thermal conductivity. Carbon black decreases the electrical resistivity.     

Structure and magnetism in Pr3RuO7

The crystal structure of the ordered fluorite, Pr₃RuO₇, was refined from powder neutron diffraction data in Cmcm. An interesting structural feature is the presence of relatively well separated zig-zag chains of corner sharing RuO₆ octahedra, Ru–Ru interchain distance 6.61 Å vs. Ru–Ru intrachain distance of 3.76 Å. Magnetic susceptibility data show a Curie–Weiss behavior for T>225 K with C=5.96(4) emu K mol⁻¹ and θ_c=+11(2) K. In an attempt to separate the contributions of Pr(3+) and Ru(5+), the properties of isostructural Pr₃TaO₇ were also measured, yielding C=4.63(3) emu K mol⁻¹. Thus, the contribution of Ru(5+), 4d³, S=3/2, to the measured Curie constant is estimated to be 1.33 emu K mol⁻¹, not far from the spin-only value of 1.87 emu K mol⁻¹. This supports the view that the Ru 4d electrons are localized and magnetic, not itinerant. A susceptibility maximum at about 50 K is attributed to long-range magnetic order and this is substantiated by neutron diffraction data. There is little evidence for one-dimensional antiferromagnetic correlations in this material but behavior characteristic of short-range ferromagnetic correlations attributed to Pr–Ru exchange interactions are found in the temperature range 50–200 K, consistent with the positive θ_c.     

A new low temperature modification of TaTe2—Comparison to the room temperature and the hypothetical 1T-TaTe2 modification

A new low temperature modification of TaTe₂ is reported (structural data at T = 150 K: C2/m (no. 12), Z = 18, a = 14.792(1) Å, b = 10.8829(9) Å, c = 9.3005(7) Å, β = 110.693(1)°). Temperature dependent single crystal X-ray diffraction, magnetic susceptibility, resistivity, specific heat capacity and differential scanning calorimetry measurements all show a phase transition at T_PT ≈ 170 K. The structural change is mainly reflected by a shift of only 2/9 of the Ta atoms by 0.283 Å, which leads to clustering of the Ta atoms. The bonding patterns were derived from the Crystal Orbital Hamiltonian Population and from the Electron Localization Function based on Density Functional Theory calculations, respectively. Differences to the room temperature modification and to the hypothetical 1T-TaTe₂ structure are elaborated. It is suggested, that the room temperature structure might show a dynamic type of disorder, with the X-ray structure analysis reflecting the time averaged structure.     

Effects of high-temperature annealing on magnetic properties of (Zn0.91Mn0.09)O thin films grown by RF magnetron sputtering

The (Zn_0.91Mn_0.09)O thin film annealed at 1000 °C for 60 min in N₂ atmosphere showed two kinds of Curie temperature (T_C) values of T_C1 around 130 K and T_C2 above 300 K while the as-grown (Zn_0.91Mn_0.09)O thin film showed typical ferromagnetism with the T_C of 108 K. It is expected that the increase of T_C up to T_C1 might be attributed to the enhancement of crystal magnetic anisotropy because the increases of (0 0 0 l) peak for X-ray diffraction patterns and Mn²⁺-related emission for photoluminescence spectra were observed for annealed (Zn_0.91Mn_0.09)O thin films. The increase of T_C above T_C2 might be originated from the formation of nano-sized (Zn_0.91Mn_0.09)O islands because it was confirmed that the nano-sized (Zn_0.91Mn_0.09)O islands were formed after annealing treatment and that they revealed clearly the magnetic domains at 300 K for the measurements of atomic force microscopy and magnetic force microscopy, respectively. The room temperature ferromagnetism in the nano-sized (Zn_0.91Mn_0.09)O islands is considered to be originated from the inhomogeneous distribution of Mn ions.     

Low-temperature superplasticity of Al2O3(p)/Ni–Co nanocomposites

Nanocomposites of Al₂O₃/Ni–Co prepared using Al₂O₃ of various particle sizes were fabricated by pulse current electrodeposition. Their superplastic tensile deformation was investigated at strain rates of 8.33 × 10⁻⁴ s⁻¹ and 1.67 × 10⁻³ s⁻¹ and temperatures of 723–823 K. The Al₂O₃ particle sizes and the deformation temperature had significant influence on the elongation of the deposited materials. The optimal superplastic condition and the maximum elongation were determined. A low temperature superplasticity with elongation of 632% was achieved at a strain rate of 1.67 × 10⁻³ s⁻¹ and 823 K. Scanning electron microscopy and transmission electron microscopy were used to examine the microstructures of the deposited and deformed samples. The grains grew to a micrometer dimensions and were elongated along the tensile direction after superplastic deformation. Superplasticity in electrodeposited nanocomposites is related to the presence of S at grain boundaries and to deformation twinning.     

Oxygen partial pressure dependence of the structural properties of CdO thin films deposited by a modified reactive vacuum evaporation process

Thin films of cadmium oxide were thermally deposited on glass substrates at partial pressures of oxygen, pO₂ in the range 1.33×10⁻² to 0.133 Pa at a substrate temperature of 160 °C. Energy dispersive analysis of X-ray fluorescence (EDAX) revealed that the CdO films deposited at pO₂ value of 4.00×10⁻² Pa were nearly stoichiometric. X-ray diffractometry (XRD) confirmed the polycrystalline nature of the film structure. All the films showed an fcc structure of the NaCl-type, as the dominant phase. The films exhibited preferred orientation along the (1 1 1) diffraction plane. The texture coefficients calculated for the various planes at different oxygen partial pressures (pO₂) indicated that the maximum preferred orientation of the films occurred along the (1 1 1) plane at an oxygen partial pressure of 4.00×10⁻² Pa. This was interpreted in terms of oxygen chemisorption and desorption processes. The lattice parameters determined from the diffraction peaks were in the range 4.655–4.686 Å. The average lattice parameter a₀ found by extrapolation using the Nelson–Riley function was 4.696 Å. Both the lattice parameter and the crystallite size were found to increase with increased partial pressure of oxygen. On the other hand, the strain and dislocation density were found to decrease as the partial pressure of oxygen was raised. A maximum (80%) in the optical transmittance at λ=600 nm and minimum in the electrical resistivity (9.1×10⁻⁴ Ω cm) of the films occurred at an optimum partial pressure of oxygen of 4.00×10⁻² Pa. The results are discussed.     

Compression behaviour and micro-structure evaluation of Zr57Nb5Cu15.4Ni12.6Al10 bulk metallic glass under high pressure

Compression behaviour and micro-structure evaluation of Zr₅₇Nb₅Cu_15.4Ni_12.6Al₁₀ bulk metallic glass is investigated at room temperature up to 32.8 GPa using in-situ high pressure energy dispersive X-ray diffraction with a synchrotron radiation source. The equation of state of the bulk metallic glass is − ΔV / V = 0.012P − 2.49 × 10^− 4P² − 9.5 × 10^− 7P³ + 5.02 × 10^− 8P⁴. The result shows that the nearest atom pair of the as-quenched bulk metallic glass corresponds to Zr–Zr correlations. And with pressure increasing, the nearest atom pair changes to a new one at 32.8 GPa.     

The rock salt oxide Li2MgTiO4: Type I dielectric and ionic conductor

The exploration of the Li–Ti–Mg–O system, using both sol–gel technique and solid state reaction method, allowed a new phase, Li₂MgTiO₄, with disordered rock salt structure (a = 4.159 Å) to be synthesized. The latter is shown to be a good type I dielectric material, with a relative constant of 15 at high frequency and low dielectric loss (tanδ < 10⁻³) over the temperature range −60 to 160 °C. It is also observed that the sintering temperature of this phase is strongly lowered by adopting the sol–gel technique compared to solid state reaction (1150 °C instead of 1300 °C). Finally we show that this phase exhibits cationic conductivity above 400 °C (σ_600 °C = 9 × 10⁻⁵ S cm⁻¹).     

Hot top design and its influence on feeder channel segregates in 100-ton steel ingots

The influence of hot top design on feeder channel segregates (F-CS) and centerline shrinkage porosities (C-SP) were investigated both experimentally and numerically. Two 100-ton 30Cr2Ni4MoV steel ingots with different insulating hot tops were longitudinally sectioned. The experimental results showed few channel segregates but severe shrinkage porosities appeared in the ingot with poorly insulated hot top, while it was the opposite case after the improved hot topping practice. By employing the finite element numerical simulation, the critical condition for the formation of F-CS in 30Cr2Ni4MoV steel was verified to be R^2.1G ≤ 1.0 × 10^− 5 °C mm^1.1 s^− 2.1. Through coupling with the published C-SP criterion (GR^− 0.5 ≤ 2.5 °C mm^− 1.5 s^0.5), it was found out that the increase of hot-top height and preheating temperature would aggravate F-CS while alleviate C-SP contrarily. Hence, to comprehensively control those two defects, the optimum hot-top height and preheating temperature for 100-ton ingot were suggested to be 700 mm and 600 °C, respectively. Ultimately, the ratio of the solidification time for the whole ingot to the ingot body (t_f/t_b) was proposed as a novel criterion for hot top design. This practical criterion has been successfully utilized to optimize the hot top of a 5-ton steel ingot.     

Nonlinear optical materials: Synthesis,characterizations, thermal stability and electro-optical properties

Two series of thermal stable polyimides and polyimide/silica hybrid materials have been synthesized. The silica content in the hybrid materials was varied from 0 to 22.5 wt.% and SiO₂ phase was well dispersed in the polymer matrix. The glass transition temperature (T_g) and the decomposition temperature (T_d) were in the range 226–382 °C and 345–499 °C, respectively, for materials. The polymer solutions could be spin coated on the indium–tin–oxide (ITO) glass or other substrates to form optical quality thin films. The electro-optic coefficients (γ₃₃) at the wavelength of 832 nm for polymer thin films poled were in the range of 12–25 pm/V and the values remained well (retained > 85% for more than 100 h). The heat capacities of some materials were in the range of 0.9755–1.1821 J K^− 1 g^− 1 for the temperature 273 and 363 K. No thermal anomaly was found in this temperature range. Most of them showed high thermal stability.     

The thermal behaviour of the tritium source in KATRIN

The tritium source in the Karlsruhe Tritium Neutrino Experiment (KATRIN) will deliver 10¹¹ β decay electrons per second, in order to determine the mass of the electron antineutrino through analysing the tritium β spectrum. The source is built of a 10 m long beam tube of 90 mm inner diameter, which is operated at 30 K. Gaseous tritium is injected through a central injection chamber and diffuses towards the tube ends, where it is pumped by large turbomolecular pumps and further processed in a closed tritium loop. In order to achieve the KATRIN sensitivity of 0.2 eV/c², the decay rate in the source (and hence the tritium density profile) must be stable to a level of ±0.1%. As the density profile is influenced by the beam tube conductance, both the temperature stability and the temperature homogeneity must be within a range of ±0.03 K at 30 K. A thermosiphon with saturated neon was developed for this purpose, with horizontal evaporator tubes connected all along the 10 m beam tube. The system behaviour was tested in a 12 m long test cryostat, containing the original beam tube with the adjacent pumping chambers, as well as the cooling circuits and the thermal shields. The so-called “Demonstrator” was operated in the Tritium Laboratory Karlsruhe (TLK) being connected to the cryogenic infrastructure of KATRIN. The temperature stability was found a factor 20 better than specified, achieving a standard deviation of only 1.5 mK/h, which corresponds to ΔT/T = 5 × 10⁻⁵ h⁻¹ relative stability at 30 K. The ±0.03 K temperature homogeneity along the 10 m beam tube was not yet reached, because of an increased heat load through the pump ports. The repeatability of the temperature measurement with vapour pressure sensors was within ± 0.004 K.     

Structural analysis and magnetic properties of nanocrystalline NiCuZn ferrites synthesized via a novel gelatin method

In this study, nano-crystalline Ni_0.6−xCu_xZn_0.4Fe₂O₄ ferrite (0.0 ⩽ x ⩽ 0.5) with grain sizes of 16–38 nm have been prepared through a novel method using gelatin. The proper calcination temperature for the precursors which corresponding to the ferrites formation is estimated through thermal analysis measurement. X-ray diffraction patterns confirmed the formation of single phase cubic spinel structure. The grain size was estimated using transmission electron microscopy (TEM) technique. The lattice constant hardly changed with increasing copper concentration while the crystallite size increases. Fourier transform infrared spectra (FT-IR) indicate that the portion of Fe³⁺ ions at the tetrahedral sites move to the octahedral sites as some of the substituted Cu²⁺ ions get into the tetrahedral sites. The effect of Cu-substitution on the magnetic properties was studied through the temperature dependence of the molar magnetic susceptibility. The Curie temperatures (T_C) were found to decrease with increasing the Cu concentration while molar magnetic susceptibilities remain constant.     

Temperature influence on the voltage-controlled diffractive property of Mn-doped potassium sodium tantalate niobate crystal

We report the temperature influence on the voltage-controlled diffractive property of Mn-doped potassium sodium tantalate niobate crystal. The crystal was grown by the top seeded solution growth method. Its quadratic electro-optic coefficients achieved as high as R₁₁ = 3.50 × 10⁻¹⁵ m²/V² and R₁₂ = −0.44 × 10⁻¹⁵ m²/V² near the Curie temperature, while they declined with the increasing temperature. The external electric field which correspond to the maximum diffraction efficiency of photorefractive grating moved from 166 V/mm to 512 V/mm as the temperature increased from 25 °C to 32.5 °C. The maximum diffraction efficiencies all reached the maximum value of 60% at different temperatures. The results were discussed and compared with the theoretical equations.     

Above room temperature magnetocaloric effect in perovskite Pr0.6Sr0.4MnO3

The dependence of magnetization M on the applied magnetic field H and temperature T was measured carefully, near the Curie temperature T_C for the perovskite manganite sample Pr_0.6Sr_0.4MnO₃. The experimental results indicate that this specimen exhibit ferromagnetic (FM) to paramagnetic (PM) transition at T_C ~ 320 K. In the 200 K–45 K temperature range the spontaneous magnetization was decreasing, probably due to spin canted state between manganese and praseodymium spin systems. At 46 K the magnetization presents a second little transition, which can be ascribed to very weak traces of secondary Mn₃O₄ phase, and remains constant between 10 K and 46 K. The maximum value of the magnetic entropy change obtained from the M(H) plot data is |ΔS_M^max| = 2.3 Jkg^? 1 K^? 1 for applied magnetic field of 2.5 T. At this value of magnetic field the relative cooling power (RCP) is 34.5 Jkg^? 1. The temperature corresponding to ΔS_M maximum value is almost equal to T_C. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. It is suggested by the results that this compound can be used as the working material in an active magnetic regenerative refrigerator above room temperature.