Magnetoelectric coupling in multiferroic BiFeO3 by co-doping with strontium and nickel

We report the effects of the Sr²⁺ and Ni²⁺ co-doping of BiFeO₃ on the crystal structure and multiferroic properties of Bi_1?xSr_xFe_1-yNi_yO₃ (x?=?0.05, 0.0?≤?y?≤?0.10, and Δy?=?0.05) that is synthesized using assisted high-energy ball milling. The mixtures of Bi₂O₃, Fe₂O₃, SrO and NiO were milled for 5?h, pressed at 900?MPa, and sintered at 800?°C in order to obtain cylindrical test pieces. X-ray diffraction and Rietveld refinement elucidated the effects of Sr²⁺ and Ni²⁺ on the crystal structure. Co-doping with SrNi in suitable proportions stabilizes rhombohedral BiFeO₃. High contents of Ni²⁺ promote the precipitation of secondary phases in the forms of NiFe₂O₄ and Bi₂₅FeO₄₀. The magnetic behavior was examined by means of vibrating sample magnetometry. The results showed a change in the magnetic order from antiferromagnetic for the undoped sample to the ferromagnetic order for the co-doped samples. This change is attributed to the modulations in the magnetic moment due to crystal structure distortions. All samples show high relative permittivity values, which were enhanced by doping with Sr²⁺. Ni²⁺ cations increase the dielectric dissipation factor; this enhancement is related to their interactions with cations of a different oxidation state, such as Fe³⁺, Fe²⁺, Ni²⁺, Bi³⁺ and Sr²⁺ in the crystal structure of BiFeO₃. The magnetoelectric coupling that was evaluated using magnetodielectric measurements was above 4% at 1?kHz for the higher applied magnetic field of 18?kOe.     

Preparation and absorbing properties of flaky GR/Fe95Si1B2P0.5Cu1.5 composite powders

In this study, flaky Fe₉₅Si₁B₂P_0.5Cu_1.5 powders with a particle size of 48–75?μm were compounded with graphene (GR) by high-energy ball milling to prepare GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders. The electromagnetic parameters of GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders in the frequency band of 0.3–8.5?GHz were determined by a vector network analyser. The influence of milling time and graphene content on the electromagnetic parameters of GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders was studied. The results showed that with the increase of graphene content, magnetic permeability and dielectric constant increase simultaneously. The 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite powders prepared by milling for 24?h exhibited the best absorbing property. In the frequency band of 0.3–8.5?GHz, for 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite, the dielectric constant was 182.4–18.2, and the magnetic permeability was 7.3–1.0. The dielectric loss tangent was 3.40–6.60, the magnetic loss tangent was 0.48–3.46, and their sum was 5.28–8.60; therefore, 1.0?wt% GR/Fe₉₅Si₁B₂P_0.5Cu_1.5 composite exhibited excellent absorbing properties.     

High Ag2S-containing amorphous materials in the system Ag2S-SiS2 and their electrical properties

Concentration of conducting ions is known to be a main factor affecting ionic conductivities of glasses. Therefore, the melt-quenched Ag₂S-SiS₂ glasses reported by Pradel et al. (J. Non-Cryst. Solids 188 (1995) 75) were difficult to show high ionic conductivity owing to the low Ag₂S content of 40-50 mol.%. We recently reported that 60Ag₂S·40SiS₂ amorphous sample showed electrochemical potential window of more than 2.5 V vs. Ag/Ag⁺ electrode and silver ion conductivity of 7×10⁻² Sm⁻² at room temperature (J. Mater. Chem. 12 (2002) 1094). The ionic conductivity is somewhat low for practical uses. In order to further increase the ion conductivity, we tried to prepare high Ag₂S-containing amorphous samples in the system Ag₂S-SiS₂ by employing the high-energy ball-milling process. Our results proved that amorphous samples containing 70 and 80 mol.% Ag₂S can be synthesized in a suitable milling time range, and that the amorphous samples partially transformed into Ag₈SiS₆ crystalline phases with further milling. The high Ag₂S-containing amorphous samples showed high ionic conductivity, and the highest ion conductivity of 2.1×10⁰ Sm⁻¹ was obtained at 298 K for the 80Ag₂S·20SiS₂ amorphous sample. Those samples, however, showed very poor electrochemical stability.     

Conductivity study of a gelatin-based polymer electrolyte

Natural polymers are particularly interesting due to their richness in nature, very low cost and principally biodegradation properties. For these reasons different solid polymeric electrolytes (SPE) have been obtained using cellulose derivatives, starch, chitosan and rubber. This work presents the results of gelatin-based protonic SPEs, which were characterized by impedance spectroscopy, X-ray diffraction, UV-vis-NIR spectroscopy and scanning electron microscopy (SEM). The ionic conductivity results obtained for these SPEs were 4.5 × 10⁻⁵ S/cm and 3.6 × 10⁻⁴ S/cm at room temperature and 80 °C, respectively. Temperature-dependent ionic conductivity measurements were taken to analyze the mechanism of ionic conduction in polymer electrolytes. Good conductivity results combined with transparency and good adhesion to the electrodes have shown that gelatin-based SPEs are very promising materials to be used as solid electrolyte in electrochromic devices.     

Effect of lattice structure evolution on the thermal and mechanical properties of Cu–Al2O3/GNPs nanocomposites

In this study, high-energy ball milling accompanied by compaction and sintering were employed for manufacturing Cu-based hybrid nanocomposite reinforced by Al₂O₃ and GNPs. This hybrid nanocomposite is proposed to meet the specification of heat sink applications, where excellent mechanical and thermal performance is demanding. Different processing parameters were experimentally considered such as sintering temperature and weight percentage of GNPs, 0, 0.25, 0.50, 0.75, and 1 wt %. The weight percentage of Al₂O₃ was fixed at 10%. The results demonstrated that the mechanical and thermal performance of the fabricated nanocomposites were superior for nanocomposite containing 0.5% GNPs and sintered at 1000 °C. The hardness, the thermal conductivity and the coefficient of thermal expansion (CTE) were improved by 21%, 16.7%, and 55.2%, respectively, compared to composite without GNPs addition. The improved mechanical and thermal properties were attributed to the low stacking fault energy, small crystallite size, high dislocation density, and low lattice strain of the composite prepared at this composition. Moreover, the better dispersion of the nano-particles of GNPs and Al₂O₃ inside the matrix helped for the strength and thermal conductivity improvement while maintaining low CTE.     

Crystal structure and multiferroic behavior of perovskite YFeO3

We present a study of multiferroic properties of YFeO₃ synthesized by means of high-energy ball milling assisted by annealing at low temperature. Fe₂O₃ and Y₂O₃ powders were mixed in a stoichiometric ratio, milled for 5?h, pressed and annealed at temperature from 773 to 1073?K. X-ray diffraction (XRD) analysis confirmed the formation of single-phase orthorhombic structure. Magnetic hysteresis loops, at room temperature, from vibrating sample magnetometry show the transition from ferromagnetic order to G-antiferromagnetic order, related to the transformation from amorphous to crystalline orthorhombic single phase. The value of Néel temperature of single phase YFeO₃ was obtained at 595?K, lower than previously reported. Dielectric behavior at room temperature of YFeO₃ single-phase sample shows a direct dependence with frequency of both dielectric constant and dielectric loss, in good agreement with Maxwell-Wagner effect. A fit made using Cole-Cole equation shows that the Low Temperature Dielectric Relaxation, LTDR, corresponds to a Debye-type relaxation. Finally, it was found that AC conductivity (σ_AC) increases linearly with frequency. All results show that YFeO₃ synthesized by high-energy ball milling assisted with annealing possess a multiferroic behavior.     

Stabilization of α-BiFeO3 structure by Sr2+ and its effect on multiferroic properties

We report a study on the effect of the substitution of Bi³⁺ by Sr²⁺ on the stabilization of R3c structure of Bi_1?xSr_xFeO₃ (0 ≤ x ≤ 0.3, Δx = 0.05), and its effect in the magnetic and dielectric behavior. Stoichiometric mixtures of Bi₂O₃, Fe₂O₃ and SrO were mixed and milled for 5?h using a ball to powder weight ratio of 10:1 by high-energy ball milling. The obtained powder were pressed at 900?MPa to obtain cylindrical pellets and sintered at 800?°C for 2?h. X-ray diffraction and Rietveld refinement were used to evaluate the effect of Sr²⁺ on the crystal structure. In addition, vibrating sample magnetometry (VSM) and dielectric tests were used for describing the multiferroic behavior. The results show that Sr-doped BiFeO₃ particles present rhombohedral structure (R3c) characteristic of α-BiFeO₃ when the doping is below 0.10?mol of Sr. Additionally, a gradual decrease in the amount of secondary phases with the increase of the amount of strontium is observed. For doping concentration higher than 0.15?mol of Sr, a phase transition to an orthorhombic symmetry (β-BiFeO₃, Pbnm) is detected. Besides, changes in relative intensities of reflection peaks planes (110) and (104) are associated with the phase transformations and with the magnetic and dielectric behavior. The α-BiFeO₃ phase show antiferromagnetic behavior and high values of dielectric permittivity, whereas the β-BiFeO₃ phase show a ferromagnetic behavior and low dielectric permittivity.     

Electrochemical performances of Al-based composites as anode materials for Li-ion batteries

Al-C, Al-Fe and Al-Fe-C composite materials have been prepared by high-energy ball milling technique. The electrochemical measurements demonstrated that the Al-Fe-C composites have greatly improved electrochemical performances in comparison with Al, Al-C and Al-Fe anode. For example, Al₇₁Fe₉C₂₀ can deliver the reversible capacity of 436 mAh g⁻¹ at first cycle and 255 mAh g⁻¹ at 15th cycle. This improved electrochemical performance could be attributed to the alloying formation of Al with Fe and the buffering effect by the graphite matrix. This suggests that the Al-Fe-C composite has a potential possibility to be developed as an anode material for lithium-ion batteries.     

Spark plasma sintering of Ti1−xAlxN nano-powders synthesized by high-energy ball milling

The present study focused on the fabrication of bulk materials from Ti_1−xAl_xN nano-powders using a spark plasma sintering (SPS) apparatus. Super-saturated Ti_1−xAl_xN solid solutions containing differing fractions of AlN (10, 20, 30 and 50 mol%) were synthesized by high-energy ball milling (HEBM) of pure nitrides. The complete dissolution of AlN in TiN was achieved after 100 h of milling. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-filtered transmission electron microscopy spectra imaging and energy dispersive X-ray spectroscopy. The crystalline size of the mechanically alloyed powders after 100 h of milling was about 12–14 nm. Ti_1−xAl_xN powders of various compositions were sintered by SPS under pressure of 63 MPa at 1673 K. Maximal hardness and bending strength values (610 MPa and 18.6 GPa, respectively) were obtained for composites containing 20 mol%AlN. Powder with 20% mole%AlN was consolidated under pressure of 500 MPa in the 1273–1423K temperature range by high pressure SPS (HPSPS). A fully dense nano-structured specimen, processed at 1423 K, displayed a Young modulus of 420 GPa, hardness of 20.5 GPa, bending strength of 670 MPa and fracture toughness of 7.1 MPa m^0.5.     

The effect of high-energy ball milling on the electromechanical strain properties of Bi-based lead-free relaxor matrix ferroelectric composite ceramics

The crystal structure, ferroelectric, and electric-field-induced strain (EFIS) properties of Bi-based lead-free ferroelectric/relaxor composite materials are investigated. Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ as a ferroelectric material and 0.98Bi_1/2(Na_0.78K_0.22)_1/2TiO₃‒0.02LaFeO₃ as a relaxor were synthesized via conventional ceramic processing routes while the relaxor (matrix phase) was prepared via high-energy ball milling (HEBM) after calcination. The average particle size was decreased via HEBM treatment. As a result, a high d₃₃^* value of over 600 pm/V was obtained at 4 kV/mm for 30-min HEBM-treated composites. This demonstrates that HEBM treatment is effective in enhancing the strain properties of lead‒free piezoelectric composite materials.     

Solid-state electrochromic devices with Nb2O5:Mo thin film and gelatin-based electrolyte

A gelatin-based electrolyte has been developed and characterized by impedance spectroscopy, X-ray diffraction, UV-vis-NIR spectroscopy and atomic force microscopy (AFM). The heat treatment temperature was found the key factor affecting its ionic conductivity that increases from 1.5 × 10⁻⁵ S/cm to 4.9 × 10⁻⁴ S/cm by heating from room temperature up to 80 °C. The temperature dependence of the ionic conductivity exhibits an Arrhenius behavior. EC-devices with the configuration K-glass/Nb₂O₅:Mo EC-layer/gelatin-based electrolyte/(CeO₂)_x(TiO₂)_1−x ion-storage (IS) layer/K-glass, have been assembled and characterized. They show a good long time cyclic stability, but the change of the optical density measured at 550 nm after 25 000 cycles was only 0.13.     

Hard and easy sinterable B4C-TiB2-based composites doped with WC

B₄C-TiB₂ composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6?wt%Co spheres or directly as sintering aid to 50?vol% B₄C / 50?vol%TiB₂ mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting TiB₂-B₄C powders. As a result, the sintering temperature necessary for full densification decreased to 1860?°C and both strength and hardness benefited from the microstructure refinement, 860?±?40 MPa and 28.5?±?1.4?GPa respectively. High energy milling was then adopted for producing 75?vol% B₄C/25?vol% TiB₂ and 25?vol% B₄C/ 75vol%TiB₂ mixtures. The B₄C-rich composition showed the highest hardness, 32.2?±?1.8?GPa, whilst the TiB₂-rich composition showed the highest value of toughness, 5.1?±?0.1?MPa?m^0.5.     

Reaction mechanism and size control of CaB6 micron powder synthesized by the boroncarbide method

Reaction synthesis mechanism of Calcium hexaboride (CaB₆) powder was investigated by using CaCO₃-B₄C-C system. Micron-scale CaCO₃ and B₄C powders were used as main raw materials. The synthesized powder was determined by X-ray diffraction, showing no left reactants if enough CaCO₃ was added to compensate the evaporation of calcium atoms at high temperature. The powder morphology was observed through SEM. The synthesized CaB₆ powder formed hard agglomerates which consisted of cubic CaB₆ crystallites when the reaction completely finished. Reaction process was illustrated indicating it was a solid-state reaction occurred from B₄C surface to the centre. The dry high-energy ball milling was used to investigate the influence of ball-milling time on the shape and size of powder particles. The particle granularity was measured by laser size analysis method. It is obvious that the particles were refined greatly after ball milling for 8 h. However, the CaB₆ powder could not been refined markedly after 16 h. Finally, optimized parameters for size controlling were given in this paper.     

Synthesis and multiferroism in mechanically processed BiFeO3–PbTiO3 ceramics

In this study, (1 − x)BiFeO₃–(x)PbTiO₃ multiferroic ceramics, with x = 0, 0.1, 0.2, 0.25, 0.3 and 0.4, were processed through high-energy ball milling followed by reactive sintering in air atmosphere. The optimization of the procedure for the preparation of highly-dense (1 − x)BiFeO₃–(x)PbTiO₃ ceramics was carefully investigated and structural/microstructural effects on ferroic properties were carefully addressed. Shrinkage dilatometric measurements revealed an expansion related to a sintering reaction that has occurred before densification. This sintering behaviour was highly PbTiO₃ concentration-dependent. The sintering mechanism was found to be directly related with the aliovalent substitution of Pb and Ti ions on A and B sites of the perovskite structure. The obtained ceramics were confirmed as ferroelectric ordered in ferroelectric characterizations. Remnant polarizations and coercive fields greatly dependent on grain size distribution and aliovalent substitutions were revealed. The magnetic hysteresis displayed a weak-ferromagnetic behaviour in all studied samples.     

Structural and optical properties of MgМоO4 prepared by mechanochemical technique

MgMoO₄ with hexagonal particles were prepared by combining high energy ball milling with heat treatment technique. The influence of the mechanochemical activation/heat-treatment on the phase, structural and morphology transformation were investigated by X-ray powder diffraction analysis (XRD), infrared spectroscopy (IR), differential scanning calorimetry (DSC), particle size distribution (PSD) and scanning electron microscope (SEM). Optical properties of the final product were studied by UV–Vis and photoluminescence (PL) measurements. Mechanochemical activation of the initial oxides for 10 h ball milling leads to a full amorphization of MoO_3, only. The heat-treatment at different temperatures after 10 h milling time results to the phase formation of MgMoO₄. The reaction started at 400 °C for 5 h and completed at 800 °C for 5 h. The calculated energy band gap value as prepared MgMoO₄ is 2.03 eV and exhibits photoluminescence emission above 600 nm. The CIE chromaticity coordinates (x = 0.53 and y = 0.43) were found to lie in the orange region.     

Li ion conduction of perovskite Li0.375Sr0.4375Ti0.25Ta0.75O3 and related compounds

In this work, perovskite-structured Li_0.375Sr_0.4375M_0.25N_0.75O₃ (M=Ti, Sn, N=Nb, Ta) solid electrolytes were synthesized by conventional solid state reaction method. Phase compositions, fractured morphologies and conductivities of these compounds were investigated by X-ray diffraction, scanning electron microscope and AC-impedance spectroscopy, respectively. X-ray diffraction analysis confirms that all of Li_0.375Sr_0.4375M_0.25N_0.75O₃ (M=Ti, Sn, N=Nb, Ta) ceramics present perovskite structure. Pure Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Ta_0.75O₃ perovskite ceramics were obtained. But impurities were detected in Li_0.375Sr_0.4375Ti_0.25Nb_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Nb_0.75O₃. Among all investigated compounds, Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ shows the highest total ionic conductivity of 2.60 × 10^?4 S cm^?1 at room temperature and the lowest activation energy of 0.347 eV. Conductivities of Li_0.375Sr_0.4375Sn_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Sn_0.25Nb_0.75O₃ were 4.4 × 10^?5 S cm^?1 and 1.82 × 10^?6 S cm^?1, respectively. Their conductivities were much lower than Li_0.375Sr_0.4375Ti_0.25Ta_0.75O₃ and Li_0.375Sr_0.4375Ti_0.25Nb_0.75O₃.