Tuning hydrogen storage properties and reactivity: Investigation of the LiBH4-NaAlH4 system

Tuning the hydrogen storage properties of complex metal hydrides is of vast interest. Here, we investigate the hydrogen release and uptake pathways for a reactive hydride composite, LiBH₄−NaAlH₄ utilizing in situ synchrotron radiation powder X-ray diffraction experiments. Sodium alanate transforms to sodium borohydride via a metathesis reaction during ball milling or by heating at T∼95 °C. NaBH₄ decomposes at ∼340 °C in dynamic vacuum, apparently directly to solid amorphous boron and hydrogen and sodium gas and the latter two elements are lost from the sample. Under other conditions, T=400 °C and p(H₂)=∼1 bar, NaBH₄ only partly decomposes to B and NaH. On the other hand, formation of LiAl is facilitated by dynamic vacuum conditions, which gives access to the full hydrogen contents in the LiBH₄−NaAlH₄ system. Formation of AlB₂ is observed (T∼450 °C) and other phases, possibly AlB_x or Al_1−xLi_xB₂, were observed for the more Li-rich samples. This may open new routes to the stabilization of boron in the solid state in the dehydrogenated state, which is a challenging and important issue for hydrogen storage systems based on borohydrides.     

High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Electrical properties of KTiOPO4 single crystal in the temperature range from −100 °C to 100 °C

The electrical property of a KTiOPO₄ single crystal was studied by means of a dielectric spectroscopy method in the temperature range from −100 to 100 °C. Dielectric dispersion began at a temperature, T_S=−80 °C. It is believed that this dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of K⁺ ions. The activation energy concerned with hopping conduction is E_a∼0.20 eV above T_S. T_S=−80 °C can be the minimum temperature for the hopping K⁺ ion.     

Thermal, electrical, and electrochemical properties of Lanthanum-doped Ba0.5Sr0.5 Co0.8Fe0.2O3-δ

Crystal structure, thermogravimetry (TG), thermal expansion coefficient (TEC), electrical conductivity and AC impedance of (Ba_0.5Sr_0.5)_1-xLa_xCo_0.8Fe_0.2O_3-δ (BSLCF; 0.05?x?0.20) were studied in relation to their potential use as intermediate temperature solid oxide fuel cell (IT-SOFC) cathode. A single cubic pervoskite was observed by X-ray diffraction (XRD). The TEC of BSLCF was increasing slightly with the increasing content of La, and all the compounds showed abnormal expansion at high temperature. Proved by the TG result, it was associated with the loss of lattice oxygen. The electrical conductivity, which is the main defect of Ba_0.5Sr_0.5 Co_0.8Fe_0.2O_3-δ (BSCF), was improved by La doping, e.g., the compound of x=0.20 demonstrated a conductivity of σ=376 S cm⁻¹ at 392 °C. The increase of electrical conductivity resulted from the increased concentration of charge carrier induced by La doping. In addition, the AC impedance revealed the better electrochemical performance of BSLCF. For example, at 500 °C, the sample with composition x=0.15 yielded the resistance values of 2.12 Ω cm², which was only 46% of BSCF.     

Chemical spray pyrolysis deposition and characterization of p-type CuCr1−xMgxO2 transparent oxide semiconductor thin films

A chemical spray pyrolysis technique for deposition of p-type Mg-doped CuCrO₂ transparent oxide semiconductor thin films using metaloorganic precursors is described. As-deposited films contain mixed spinel CuCr₂O₄ and delafossite CuCrO₂ structural phases. Reduction in spinel CuCr₂O₄ fraction and formation of highly crystalline films with single phase delafossite CuCrO₂ structure is realized by annealing at temperatures ?700 °C in argon. A mechanism of synthesis of CuCrO₂ films involving precursor decomposition, oxidation and reaction between constituent oxides in the spray deposition process is presented. Post-annealed CuCr_0.93Mg_0.07O₂ thin films show high (?80%) visible transmittance and sharp absorption at band gap energy with direct and indirect optical band gaps 3.11 and 2.58 eV, respectively. Lower (∼450 °C) substrate temperature formed films are amorphous and yield lower direct (2.96 eV) and indirect (2.23 eV) band gaps after crystallization. Electrical conductivity of CuCr_0.93 Mg_0.07O₂ thin films ranged 0.6-1 S cm⁻¹ and hole concentration ∼2×10¹⁹ cm⁻³ determined from Seebeck analysis. Temperature dependence of conductivity exhibit activation energies ∼0.11 eV in 300-470 K and ∼0.23 eV in ?470 K region ascribed to activated conduction and grain boundary trap assisted conduction, respectively. Heterojunction diodes of the structure Au/n-(ZnO)/p-(CuCr_0.93Mg_0.07O₂)/SnO₂ (TCO) were fabricated which show potential for transparent wide band gap junction device.     

Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La_1−xCa_xW_1/6O₂ and La_1−yW_1/6O₂ (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H⁺/D⁺ isotope exchange, and thermogravimetry in the temperature range 300-1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW_1/6O₂ with values reaching 3×10⁻³ S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.     

Preparation, structure and oxide ion conductivity in Ce6−xYxMoO15−δ (x=0.1-1.4) solid solutions

The Ce_6−xY_xMoO_15−δ solid solution with fluorite-related structure have been characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), IR, Raman, scanning electric microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods. The electric conductivity of samples is investigated by Ac impedance spectroscopy. An essentially pure oxide-ion conductivity of the oxygen-deficiency was observed in pure argon, oxygen and air. The highest oxygen-ion conductivity was found in Ce_5.5Y_0.5MoO_15−δ ranging from 5.9×10⁻⁵ (S cm⁻¹) at 300 °C to 1.3×10⁻² (S cm⁻¹) at 650 °C, respectively. The oxide-ion conductivities remained stable over 80 h-long test at 800 °C. These properties suggested that significant oxide-ionic conductivity exists in these materials at moderately elevated temperatures.     

Thermoelectric properties of sol-gel derived cobaltite Bi2Ca2.4Co2Oy

Layered misfit cobaltite Bi₂Ca_2.4Co₂O_y has been synthesized by a sol-gel method. This compound exhibits large thermoelectric (TE) power (S_300 K∼170 μV K⁻¹), low resistivity (ρ_300 K∼42 mΩ cm) and relatively small thermal conductivity (κ_300 K∼2.8 W K⁻¹ m⁻¹) at room temperature. Furthermore, the resistivity of this compound displays a metallic behavior above T^?∼150 K with a semiconducting behavior below this temperature. This abnormal behavior in resistivity is analogous to those observed in Sr and Ba based misfit cobaltites. The observed features of the TE have been discussed based on the narrow band model.     

Fluoride glasses in the InF3-GaF3-YF3-PbF2-CaF2-ZnF2 system

New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35−x) InF₃-xGaF₃-10YF₃-25PbF₂-15CaF₂-15ZnF₂. T_g lies between 260 and 296 °C while melting starts around 480 °C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.     

Structural, magnetotransport and morphological studies of Sb-doped La2/3Ba1/3MnO3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La_2/3Ba_1/3Mn_1−xSb_xO₃ perovskite manganites. Pristine material La_2/3Ba_1/3MnO₃ (LBMO) shows two insulator-metal (I-M) transitions in the electrical resistivity-temperature (ρ-T) behavior. While the higher temperature transition (T_P1) at ∼340 K is reminiscent of the usual I-M transition in manganites, the lower temperature transition (T_P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La³⁺ and Ba²⁺). With Sb-doping T_P1 shifts to lower temperatures while T_P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T_P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump (T_P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T>T_P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron-magnon scattering is found to dominate in the metallic regime (T<T_P1).     

Electrical properties vs. microstructure of nanocrystallized V2O5-P2O5 glasses — An extended temperature range study

An electronically conducting nanomaterial was synthesized by nanocrystallization of a 90V₂O₅·10P₂O₅ glass and its electrical properties were studied in an extended temperature range from − 170 to + 400 °C. The conductivity of the prepared nanomaterial reaches 2 ? 10^− 1 S cm^− 1 at 400 °C and 2 ? 10^− 3 S cm^− 1 at room temperature. It is higher than that of the original glass by a factor of 25 at room temperature and more than 100 below − 80 °C. A key role in the conductivity enhancement was ascribed to the material's microstructure, and in particular to the presence of the large number of small (ca. 20 nm) grains of crystalline V₂O₅. The observed conductivity dependencies are discussed in terms of the Mott's theory of the electronic hopping transport in disordered systems. Since V₂O₅ is known for its ability to intercalate lithium, the presented results might be helpful in the development of cathode materials for Li-ion batteries.     

Effect of fabrication parameters on the electrical conductivity of YxSr1−xTiO3 for anode materials

Yttrium-doped strontium titanate (Y_xSr_1−xTiO₃), as probable anode material for SOFC, was prepared by solid-state reaction. The solubility of yttrium in SrTiO₃ at different temperatures was examined and the electrical conductivities of Y_xSr_1−xTiO₃ were measured from 500 to 1000 °C. The effects of doping amount, fabrication atmosphere, and sintering temperature on the electrical conductivity of Y_xSr_1−xTiO₃ were investigated. Y_xSr_1−xTiO₃ with x=0.08 was found to give the maximum electrical conductivity, 71 S/cm at 800 °C in pure hydrogen. Reducing atmospheres and appropriate sintering temperatures play a positive role in improving the electrical conductivity.     

Synthesis and magnetic properties of Au-coated amorphous Fe20Ni80 nanoparticles

Gold-coated nanoparticles of Fe₂₀Ni₈₀ (permalloy) have been synthesized by a microemulsion process. The as-prepared samples consist of ∼5 nm diameter particles of amorphous Fe₂₀Ni₈₀ that are likely encapsulated in B₂O₃. One or more Fe₂₀Ni₈₀@B₂O₃ particles are subsequently encapsulated in 8-20 nm gold nanospheres, as determined by TEM and X-ray powder diffraction (XRD) line broadening. The gold shells were found to be under expansive strain. Magnetic data confirm the existence of a superparamagnetic phase with a blocking temperature, T_B, of ∼33 K. The saturation magnetization, M_S, of the as-prepared, Au-coated sample is ∼65 emu g⁻¹ at 5 K and ∼16 emu g⁻¹ at 300 K. The coercivity, H_C, is ∼280 Oe at 5 K.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

Preparation and multi-properties of insulated single-phase BiFeO3 ceramics

Six types of BiFeO₃ ceramic samples, with subtle differences in synthesis conditions, were prepared. The comparison of their phases, electrical resistivity, and porosity revealed that the use of Bi₂O₃ and Fe₂O₃ powders of <1 μm size and a rapid liquid-phase sintering process of 855 °C for 5 min at 100 °C/s is beneficial to synthesize poreless single-phase BiFeO₃ samples with high electrical resistivity of ∼5×10¹² Ω cm. Deoxygenated Bi_xFe_yO_{1.5x+1.5y−δ} (x≠y, δ≥0) impurities were identified and found to be the main cause of low electrical resistivity and high porosity in the multi-phase samples. Large saturation polarization of 16.6 μC/cm² and low leakage current density of 30 mA/m², both at a high electric field of 145 kV/cm, were measured in the optimized single-phase samples at room temperature besides a large piezoelectric d₃₃ coefficient of 27 pC/N and an obvious canted antiferromagnetic behavior.     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Dielectric relaxation in single crystal NH4H2PO4 in the high-temperature regime

The dispersion curves of the dielectric response in single crystal NH₄H₂PO₄ were obtained in the radio frequency range and below the high-temperature transition at T_p−160 °C. The results reveal dielectric relaxation at low frequency, which is about 10⁵ Hz at 70 °C, and it shifts to higher frequencies (∼3×10⁶ Hz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E_a=0.55 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that cause distortion and change the local lattice polarizability inducing dipoles like      

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.