Synthesis and dc conductivity of Nd2/3TiO2.988

The A-site deficient perovskite Nd_2/3TiO_3 − δ was synthesized under an H₂–CO₂ gas mixture. The sample was found to have slight oxygen deficiency of δ∼0.012. The crystal structure was assigned to a double perovskite structure with orthorhombic space group Pmmm, as in the case of La_2/3TiO_3 − δ. Electrical conductivity measurement has also been performed. The temperature dependence of conductivity shows that electronic transport in Nd_2/3TiO_2.988 is well described by Emin–Holstein adiabatic small polaron model. The polaron density extracted from the conductivity measurement is ∼1.96 × 10²⁰ cm^− 3. This result agrees well with nominal polaron density for Nd_2/3TiO_2.988, ∼2.1 × 10²⁰cm^− 3. We have also derived important quantities for transport in Nd_2/3TiO_2.988.     

Electrical transport and magnetoresistance in La2 / 3Ca1 / 3MnO3/CuO composites

The electrical transport and magnetoresistance properties were experimentally studied for composites according to the nominal composition (1 − x)La_2 / 3Ca_1 / 3MnO₃/xCuO which were fabricated using a special chemical route (x is molar ratio). The composites display different transport behaviors for the range of x < 6% and > 6%. The introduction of CuO causes a large shift of the insulator–metal transition temperature (T_p) toward low temperature for x < 6%, while the T_p is almost independent on x for x > 6%. We also show that the magnetoresistance near T_p can be substantially increased through the introduction of CuO. The largest magnetoresistance with a value as high as ∼90% is obtained in the x = 20% composite for a rather small magnetic field (0.3 T). It is interesting to observe another transition in MR₀ vs. T curve at temperature range of 170∼200 K for the composites with CuO addition. Based on structural and microscopic analysis, unusual observations of transport and magnetoresistance are discussed.     

Plasticized alkaline solid polymer electrolyte system

Alkaline solid polymer electrolytes were prepared by utilizing poly(vinyl alcohol) (PVA), potassium hydroxide (KOH), α-Al₂O₃ and different amounts of propylene carbonate (PC). The addition of PC to the PVA:KOH:α-Al₂O₃:H₂O increased its conductivity by three orders of magnitude to the reading of ∼ 10^− 4 S cm^− 1. The plot for log σ − 1 / T showed a transformation from liquid-like conductivity to Arrhenius type. The dielectric constant (ε_r) of the samples increases with increasing PC concentrations and temperatures. Scanning electron microscopy also shows the effect of PC on the polymer electrolytes surface. Thermogravimetric studies show that the thermal stability of the polymer electrolytes decreases with the addition of PC.     

Electrical properties of lead zirconate titanate thick films prepared by hybrid sol–gel method with multiple infiltration steps

High-quality ferroelectric thick films are required for various piezoelectric applications including high-frequency transducers and microelectromechanical systems. In this work, we report the fabrication of dense crack-free lead zirconate titanate (PZT) thick films on Pt-coated Si substrates using commercial PZT powder dispersed in a sol–gel precursor solution without viscous additives. Preannealed films were infiltrated with the same solution and heat treated at 500 °C. Dielectric and ferroelectric properties of the films are found to be strongly dependent on the number of infiltration steps reaching sufficiently high values: dielectric constant ∼2270 and remanent polarization ∼35 μC cm⁻². Moderate coercive field of 60 kV cm⁻¹ and low dielectric loss ∼0.04 are observed in these films. Effective longitudinal piezoelectric coefficient d₃₃ also depends on the number of infiltrations demonstrating remanent value of ∼80 pm V⁻¹ for eight infiltration steps. The results show the suitability of hybrid sol–gel method for the fabrication of PZT thick films for dielectric and piezoelectric applications.     

Neutron flux measurements with a Li2B4O7 crystal

The effect of neutron irradiation on a lithium tetraborate (Li₂B₄O₇, LBO) single crystal has been investigated. The crystals of high optical quality are found to be quite stable under high neutron fluence. This study shows that LBO crystals can be used as a proportional counter for neutron fluxes of the order 10⁹ cm⁻² s⁻¹ and higher. The detectors fabricated were found to have a sensitivity of ∼3×10⁻¹⁸ A (nv)⁻¹.     

On the synthesis and dielectric studies of (1 − x)Bi(Mg1 / 2Zr1 / 2)O3 − xPbTiO3 piezoelectric ceramic system

Scandium free piezoelectric ceramics of the composition (1 − x)Bi(Mg_1 / 2Zr_1 / 2)O₃ − xPbTiO₃ (BMZ − xPT) were fabricated by the solid state reaction method. Dielectric and structural properties were measured and phase diagram was constructed from the temperature dependent dielectric and impedance data. The morphotropic phase boundary (MPB) was found to be located in the range 0.55 < x < 0.60 with paraelectric–ferroelectric phase transition temperature, T_C (∼ 280 °C). The ceramics near the MPB showed high room temperature dielectric constant (∼ 1387). The room temperature values of the remanent polarization (P_r) and coercive filed (E_C), were ∼ 29 μC/cm² and ∼ 23 kV/cm, respectively.     

Electrical transport and magnetoresistance properties in (1 − x)La2/3Ca1/3MnO3/xSb2O5 composites

The composites with the nominal composition of (1 − x)La_2/3Ca_1/3MnO₃ (LCMO)/xSb₂O₅ were fabricated, and their electrical transport and magnetoresistance (MR) behavior were investigated. Experimental results show that the metal–insulator transition temperature (T_MI) and resitivity (ρ) of the composites are largely dependent on Sb₂O₅ addition level x. T_MI shifts downwards and ρ increases with the increase of x at the range of x ≤ 3%, but T_MI shifts upwards and ρ decreases with further increasing of x ≥ 3%. The high temperature (T > T_MI) semi-conducting part of the ρ data follow a small polaron hopping (SPH) conduction mechanism, and the metallic behavior of the samples (T < T_MI) fits the model in terms of electron–magnon scattering of the carriers. Furthermore, the MR effect can be largely enhanced over a wide temperature range in the composites compared with pure LCMO. At 3 T field, the MR rises from a base value ∼46%, for pure LCMO, to a maximum value of ∼86.7% for the case of x = 3%. It is argued that the enhancement of MR effect is attributed to the enhanced spin-polarized tunneling, which is manipulated by magnetic disorder especially at LCMO grain boundaries caused by Sb₂O₅ addition.     

Cobalt–carbon derived from zeolitic imidazolate framework on Ni foam as high-performance supercapacitor electrode material

ZIF-67 (Co(MeIM)₂, MeIM = 2-methylimidazole) was selected as a precursor to synthesize the metallic cobalt/carbon composites on Ni foam directly and intimately. The results show that ZIF-67 forms a uniform membrane and densely covers the Ni foam substrate. Besides, the cobalt/carbon composites obtained are composed of cobalt nanoparticles with a diameter up to around 2.6 nm, which are anchored on carbon homogeneously. The specific capacitance of the composites material is up to 512 F g⁻¹ at a current density of 1 A g⁻¹ in 1.0 M KOH electrolyte. Furthermore, the specific capacitance loss is ∼46.4% with increase at the current density from 1 to 20 A g⁻¹. More significantly, the specific capacitance retention rate is 87.5% at a current density of 5 A g⁻¹ after 1000 cycles. These results suggest that the electrochemical performance can be greatly enhanced by the well-controlled size and distribution of the cobalt nanoparticles, which is mainly attributed to the unique structure and composition of ZIF-67.     

Synthesis of ytterbium bisphthalocyanines: Photophysicochemical properties and nonlinear absorption behavior

Herein we report on the syntheses, photophysico-chemical properties and nonlinear absorption parameters of bis-{1(4), 8(11), 15(18), 22(25)-(tetrapyridin-2-yloxy phthalocyaninato)} ytterbium (III) (3) and bis-{1(4), 8(11), 15(18), 22(25)-(tetrapyridin-4-yloxy phthalocyaninato)} ytterbium (III) (4). The fluorescence and singlet oxygen quantum yields obtained for complexes 3 and 4 are low. The triplet quantum yield obtained for complex 3 is high at Φ_T = 0.89 whereas for complex 4 Φ_T = 0.48. The third order optical susceptibility values are of the order: 10⁻¹¹ esu (for complex 3), and 10⁻¹³ esu (for complex 4) while the hyperpolarizability values are of the order: 10⁻²⁸ esu (for complex 3) and 10⁻³¹ esu (for complex 4). Complexes 3 and 4 show two-photon absorption coefficients of the order of 10⁻⁴⁶ cm⁴ s/photon and 10⁻⁴⁸ cm⁴ s/photon, and threshold intensities as low as 0.3 J cm⁻² and 0.0045 J cm⁻², respectively.     

Effect of magnetic fields on the abnormal electroresistance behavior in epitaxial thin films of La0.8Ca0.2MnO3

We report an unusual electroresistance (ER) behavior induced by a current and its response to magnetic fields in La_0.8Ca_0.2MnO₃ epitaxial thin films. These thin films were fabricated on SrTiO₃ (1 0 0) substrate using pulsed laser deposition (PLD) technique. It is found that the electric resistivity in these films is significantly enhanced by applying a dc current over a threshold value. Simultaneously, an abnormal electroresistance behavior appears in the temperature range from 10 to 300 K. The enhanced resistance turns out to be very sensitive to a weak current. Even a very small dc current can remarkably depress the high resistance, showing an unusual colossal ER effect. The ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.72 to 10.5 μA. The influence of magnetic fields on the transport was also studied. The I–V curves can be strongly influenced by a low magnetic field even at room temperature. The deduced magnetoresistance (MR) reaches 120% at 300 K upon applying a magnetic field of 0.25 T. An interesting phenomenon is that the observed MR is current dependent.     

Study on 2.0 μm fluorescence of Ho-doped water-free fluorotellurite glasses

Ho³⁺-doped water-free fluorotellurite glasses with composition of 60TeO₂–30ZnF₂–10NaF (mol%, TZNF60) were made by using specially-designed physical and chemical dehydration technique. 2.04 μm fluorescence (Ho³⁺: ⁵I₇ → ⁵I₈) was observed experimentally and presented in this paper: A broad bandwidth of ∼149 nm, large simulated emission cross-section of 7.2 × 10⁻²¹ cm², and the longest reported fluorescence lifetime of ∼10 ms among all the reported Ho³⁺-doped oxide glasses. Thanks to the absence of OH groups and low phonon energy with the addition fluorides into tellurite oxide glasses, 1.00Ho-TZNF60 glass demonstrates the maximum figure of merit (σ_em × τ_f) of 7.13 × 10⁻²⁷ m² s, thus regarded as a promising optical material for the development of 2.0 μm fiber lasers.     

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.     

Crystal growth and spectral properties of pure and Co2+-doped Mg3B2O6 crystal

Novel pure and cobalt-doped magnesium borate crystals (Mg₃B₂O₆) have been grown successfully by the Czochralski technique for the first time. Crystal growth, X-ray powder diffraction (XRD) analysis, absorption spectrum, fluorescence spectrum as well as fluorescence decay curve of Co²⁺:Mg₃B₂O₆ (MBO) were described. From the absorption peaks for the octahedral Co²⁺ ions, the crystal-field parameter Dq and the Racah parameter B were estimated to be 943.3 cm⁻¹ and 821.6 cm⁻¹, respectively. The fluorescence lifetime of the transition ⁴T₁(⁴P) → ⁴T₂ centered at 717 nm was measured to be 9.68 ms.     

High-temperature electrical resistivity and heat capacity studies on nano-crystalline geikielite

We report here for the first time the temperature dependence of the electrical resistivity and heat capacity of nano-crystalline MgTiO₃ geikielite of up to 1000 K. The temperature dependence of heat capacity of nano-crystalline geikielite expressed as C_p = 46.44(5) + 0.0502(2)T − 4.56 × 10⁶T² + 1.423 × 10³T^− 0.5 − 8.672 × 10^− 6T^− 2, where C_p = is specific heat expressed in J/mol. K and T is the temperature in K. Both the electrical resistivity and heat capacity behaviour show that the geikielite (both the natural and synthetic nano-crystalline samples) are stable and remains electrically insulating up to 1000 K.     

Effects of grain boundaries on electrical and magnetic properties of melt-processed SmBa2Cu3Ox superconductors

In the present study, we examine the energy dissipation mechanisms and grain boundary effects on the superconducting properties of high-T_c SmBa₂Cu₃O_7?x samples. Sm-123 was synthesized with the top-seeded-melt-growth technique. Grain sizes, as determined by optical microscopy, ranged between 0.5 mm and 1.5 mm. Thus, each specimen, on which the electrical and magnetic measurements were carried out, contained several grains. Some of our findings are as follows: (1) the intragrain T_c is 93.5 K and the intergrain T_c is 87.5 K. Superconducting transition width is narrow and is not affected much by the magnetic field. (2) The intragrain J_c is 110,000 A cm^?2 while the intergrain J_c is 210 A cm^?2 at 10 K. (3) The intergrain J_c shows a secondary peak with the increase in temperature from 10 K to 30 K. The main reasons behind these observations are discussed in detail.     

Influence of W5+ ions on the optical properties of doped Bi12TiO20

There have been studied single crystals of undoped and doped Bi₁₂TiO₂₀ with two concentrations of W⁵⁺ (2.62 × 10¹⁷ cm⁻³ and 2.62 × 10¹⁸ cm⁻³). There have been obtained absorption spectra in the energy range of 10,482–15,408 cm⁻¹ by classical measurements. There have been determined the cross-section (σ_a) of the impurity absorption and the oscillator strength of d–d transitions. There have been calculated the refractive index of doped crystals and the concentration of Ti³⁺ ions in an undoped sample through an experiment.     

Tuning colossal magnetoresistance response at roomtemperature by La2/3+ySr1/3−yMn1−yCryO3

The special formula of La_2/3+ySr_1/3−yMn_1−yCr_yO₃ with [Mn³⁺]/([Mn⁴⁺] + [Cr³⁺]) ratio fixed at optimal proportion 2:1 was designed to tune colossal magnetoresistance (CMR) response around room temperature and test the possibility of ferromagnetic (FM) interaction of hetero-ionic coupling between Mn³⁺ and Cr³⁺. The polycrystalline bulk samples were fabricated by the traditional solid-state reaction method. The structural, magnetic, electrical transports and magnetoresistance (MR) properties were investigated. An enhancement of CMR at room temperature with appropriate content y has been observed, meanwhile, substituting Cr for Mn in manganites shows inefficiency in lowering ferromagnetic–paramagnetic (FM–PM) transition temperature T_c and metal–insulator (M–I) transition temperature T_p. Experimental results indicate that there might exist a FM coupling between Mn³⁺ and Cr³⁺.     

Effect of high temperature sintering schedule for enhanced CMR properties of La0.67Ca0.33MnO3 close to room temperature

Magnitude of [Δρ(T,H)/ρ(0)] and insulator to metal transition temperature (T_IM) in the colossal magnetoresistance (CMR) bulk ceramics response of La_0.67Ca_0.33MnO₃ (LCMO) manganites are observed to be changed significantly as a function of higher sintering temperature (close to melting) and long hours sintering schedule. Magnetoresistance and magnetization (up to 5 T) of LCMO are found to be decreased considerably as the sintering temperature is increased from 1400 to 1600°C. Further reduction of CMR response is also observed for long time (a few tens of hours in O₂) at higher temperature. However, on the other hand the value of T_IM is significantly enhanced and reaching close to room temperature under the same sintering schedule to achieve drastic reduction of CMR response. Microstructural analysis revealed the presence of clean and clear intergrain strong connectivity at 1600°C sintering temperature for period of 17 h. This process enhances the physical densification of the bulk and promotes the creation of new conduction channels and reduction of intergrain effective contact area for transport current under magnetic field. Hence, drastic reduction of resistance associated with the CMR response is due to this reason. The disruption in the crystalline order observed by TEM at the grain (formulation of anti-phase) boundaries promotes local spin disorder which may alter the conductivity of polarized electrons. It is also observed that short-range local magnetic order is disrupted by the presence of grain boundaries which may be responsible for the reduction of magnetoresistance and increase of T_IM point towards room temperature.     

Structural characterization of β-SiC nanowires synthesized by direct heating method

Crystal structure of β-SiC nanowires was investigated using Raman spectroscopy, FT-IR, XRD, transmission electron microscopy and selected area electron diffraction. Cubic β-SiC nanowires were synthesized by heating NiO catalyzed Si substrates with WO₃ and graphite mixed powders in the growth temperature of 1000–1100 °C. HRTEM image showed atomic arrangements of the grown SiC nanowires with a main growth direction of [111]. Raman spectra showed two characteristic peaks at 796 cm^− 1 and 968 cm^− 1, which are corresponding to transversal optic mode and longitudinal optic mode of β-SiC, respectively. Also, FT-IR absorption spectroscopy showed a SiC characteristic absorption band at ∼792 cm^− 1.     

A2−αAα′BO4-type oxides as cathode materials for IT-SOFCs (A = Pr,Sm; A′ = Sr; B = Fe,Co)

Preparation, thermal expansion, electrical conductivity and polarization of A_2−αA _α′BO₄-type oxides (A = Pr, Sm, A′ = Sr, B = Fe, Co) were investigated systematically to evaluate their potential as cathode materials for IT-SOFCs. Within 0.8 ≤ α ≤ 1.5, A_2−αSr_αBO_4−δ (A = Pr, Sm, B = Fe, Co) could be obtained as a single K₂NiF₄-structural phase. Thermal expansion coefficients (TECs) of the specimens increase with increasing Sr²⁺ content, TECs of cobaltites are much higher than that of ferrites. The electrical conductivity of cobaltites is in the order of 10² S cm^− 1 near 800 °C, which is acceptable for the cathode of IT-SOFC. Polarization measurements showed that Sm_0.5Sr_1.5CoO_4−δ exhibited the lowest cathodic overpotential at 700–900 °C (72 mV at 500 mA/cm² at 800 °C), being a high potential candidate of cathode material for IT-SOFCs.