Effect of Cu-doping on the electronic structure and optical properties of LaNi5

Optical properties of intermetallic isostructural compounds LaNi_5−xCu_x (x = 0, 0.6, 1, 1.2) have been studied in the spectral range from 0.22 to 15 μm using the ellipsometry method. It was found that the substitution of copper for nickel leads to local changes in the optical conductivity spectra. Theoretical calculations of the electronic structure and interband optical conductivity of LaNi_5−xCu_x compounds with x = 0, 1, 2, 3 were performed in the generalized gradient approximation within the pseudopotential plane-wave method PWSCF. Both the optical spectroscopic measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated with the Cu 3d → Ni 3d electron transitions and increasing with the growth of copper content.     

Effect of tunneling barrier as spacer on exchange coupling of CoFeB/AlOx/Co trilayer structures

Magnetic tunneling junctions (MTJs) have a sandwiched structure, which comprises a top ferromagnetic (FM1) layer, an insulating tunneling layer (spacer), and a bottom ferromagnetic (FM2) layer. Exchange coupling in MTJs has been extensively widely examined because the effect of spacer thickness on the ferromagnetic spin-coupling can be exploited in read-head sensors, spin-valve structures, and magntoresistance random access memories (MRAMs). In this investigation, MTJs were deposited in the sequence, glass/CoFeB(50 Å)/AlO_x(d)/Co(100 Å), where the thickness of the AlO_x layer d = 12, 17, 22, 26 or 30 Å. Saturation magnetization (M_s) results demonstrate that the exchange coupling strength and coercivity (H_c) can be varied considerably by varying the tunneling barrier AlO_x spacer. The X-ray diffraction patterns (XRD) include a main peak from hexagonal close-packed (HCP) Co with a highly (0 0 2) textured structure at 2θ = 44.7°, and AlO_x and CoFeB are amorphous phases. The full width at half maximum (FWHM) of the Co (0 0 2) peak declines as the AlO_x thickness increases, revealing that the Co layer becomes more crystalline. The magnetic results reveal that the magnetic characteristics are related to the Co crystallinity. The exchange coupling strength increases with AlO_x thickness. The coercivity (H_c) also increases, because the Co crystallinity is eliminated.     

Effects of Pd concentration on the interfacial reaction and mechanical reliability of the Sn-Pd/Ni system

The liquid-solid reaction between Sn-xPd alloy and Ni (x = 0.05-1 wt.%) and the resulting mechanical reliability of the system were examined in this study. The reactions strongly depended on the Pd concentration and the reaction time. When the Pd concentration was low (i.e., x = 0.05 wt.%), the reaction product was only Ni₃Sn₄. In contrast, when the Pd concentration was high (i.e., x ≥ 0.2 wt.%), the reaction product became a dual-layer structure of (Pd,Ni)Sn₄-Ni₃Sn₄. Between 0.05 wt.% and 0.2 wt.% (e.g., x = 0.1 wt.%), discontinuous (Pd,Ni)Sn₄ grains scattered over the Ni₃Sn₄ layer developed. Interestingly, the (Pd,Ni)Sn₄ grains were gradually dispersed in the molten Sn-Pd alloy, leaving the Ni₃Sn₄ at the interface, as the reaction time increased. These Pd-dependent reactions were dictated by thermodynamics and can be rationalized using the Pd-Ni-Sn isotherm. Furthermore, the results of the high-speed-ball-shear (HSBS) test indicated that the mechanical strength of the Sn-Pd/Ni joints dramatically degraded by over one third due to the formation of (Pd,Ni)Sn₄ at the interface. The implication is that the Pd concentration in Sn-Pd solder joints should be reduced to a level below 0.2 wt.% to prevent the creation of an undesired microstructure.     

Magnetic properties and large magnetocaloric effect in Gd-Ni amorphous ribbons for magnetic refrigeration applications in intermediate temperature range

Amorphous Gd_68−xNi_32+x (x = −3, 0, 3) ribbons were prepared by melt-spinning method. The crystallization onset temperatures T_x1 for Gd_68−xNi_32+x amorphous ribbons with x = −3, 0, and 3 are 561, 568, and 562 K, respectively. All the samples undergo the second-order magnetic transition at temperatures between ∼122 (x = −3 and 3) and 124 K (x = 0). The Curie temperature T_C does not change with the composition significantly. The maximum isothermal magnetic entropy changes (−ΔS_M)_max of Gd₇₁Ni₂₉, Gd₆₈Ni₃₂, and Gd₆₅Ni₃₅ amorphous ribbons for a magnetic field change of 0-5 T were 9.0, 8.0, and 6.9 J kg⁻¹ K⁻¹, respectively. Large values of the refrigerant capacity (RC) were obtained in these ribbons. For example, Gd₇₁Ni₂₉ amorphous ribbon has a maximum RC value of 724 J kg⁻¹. Large magnetic entropy change and RC values together with high stability enable the Gd₇₁Ni₂₉ amorphous alloy a competitive candidate among the magnetic refrigeration materials working at temperatures near 120 K.     

Effect of Ta seed layer on crystalline structure and magnetic properties in an exchange-biased Co/IrMn system

Ta-seeded and un-seeded layers of a top-configuration Co/IrMn system were deposited onto glass substrate by DC sputtering. Three sets of deposition conditions for Co(50 Å)/IrMn(t_IrMn Å) and Co(t_Co Å)/IrMn(90 Å), where t_IrMn = 15, 30, 60, 90, 110, and 150 Å, and t_Co = 15, 25, 50, 75, 100, 125, and 150  Å, were: condition (a) substrate temperature (T_s) was kept at room temperature (RT). Condition (b) T_s set to RT, with in-plane magnetic field, H = 500 Oe. In condition (c), condition (b) was followed by post-deposition annealing in the magnetic field at T_A = 250 °C for 1 h, then field cooled to RT. X-ray diffraction (XRD) patterns and grazing incidence scans revealed maximum IrMn (1 1 1) texture to occur for post-deposition annealed Ta seed layer samples. The IrMn (1 1 1) texture-effect significantly influences magnetic properties, including exchange-biasing field (H_ex), interfacial energy (J_k), and coercivity (H_c). The Ta seed layer also significantly influences magnetic properties. Adding a Ta seed layer to the Co/IrMn system increases H_ex, because of the IrMn (1 1 1) texture. For Ta-seeded Co/IrMn under condition (c), H_ex tended to saturate for t_IrMn ≥ 90 Å. Under conditions (a) and (b), H_ex decreased with increasing t_IrMn for t_IrMn ≥ 90 Å. H_ex values for all un-seeded Co/IrMn systems increased with t_IrMn. J_k versus t_IrMn plot is proportional only to H_ex in the Ta-seeded and un-seeded layers of a top-configuration Co/IrMn system, due to the interfacial energy formula, t_Co is fixed, and saturation magnetization (M_s) of the Co layer is constant. Results for the Ta-seeded system showed a strong relationship between H_c and t_IrMn, due to coupling-decoupling interactions between Co spin, and IrMn layers close to the Co/IrMn interface. The H_ex versus t_Co result shows that the H_ex is proportional to (1/t_Co). The H_ex values with the Ta seed layer are almost slightly larger than those without a Ta seed-layer. The dependence of J_k on t_Co is similar to the trend in M_s on t_Co, J_k tends to saturate slowly as t_Co increases. Surface pinning occurred in all systems, revealing an inverse relation between H_c and t_Co. Removing the Ta seed-layer weakens IrMn (1 1 1) texturing, reducing H_ex. The maximum observed H_ex and J_k values were 205 Oe and 0.11 erg/cm², respectively.     

Evidence of room temperature ferromagnetism in Co-doped transparent CuAlO2 semiconductor

We synthesized CuAl_1−xCo_xO₂ (x = 0.00-0.07) thin films by solid-state reaction from Cu₂O and Al₂O₃ on a sapphire (0 0 1) substrate by a simple and cost-effective spin-on technique. X-ray diffraction and Raman spectroscopy confirm the formation of single impurity-free delafossite CuAlO₂ structure for all the compositions. We observed diamagnetism for pristine CuAlO₂ and ferromagnetism for Co-doped CuAlO₂ at room temperature. Specially, the coercivity (H_c) and saturation magnetization (M_s) are significantly enhanced with Co composition from 1 at.% to 5 at.% but show the reverse tendency with higher Co content.     

Function of cobalt in the new type rare-earth Mg-based hydrogen storage electrode alloys

The role of cobalt on the structural and electrochemical properties of the La_0.7Mg_0.3Ni_3.4−xMn_0.1Co_x (x=0–1.15) hydrogen storage alloys were investigated systematically. XRD and Rietveld analyses show that all alloys consist mainly of the (La,Mg)Ni₃ phase and the LaNi₅ phase. P–C isotherms illustrate that the equilibrium pressure for hydrogen decreases monotonically with increasing x. Electrochemical studies indicate that the maximum discharge capacity first increases from 397.5 (x=0) to 403.1 mA h/g (x=0.75) and then decreases to 388.2 mA h/g (x=1.15). Moreover, with increasing Co substitution, the cycling durability of the alloy electrodes was markedly improved due to the decrease in the pulverization of the alloy particles and the corrosion of La in KOH solution. The high rate dischargeability, electrochemical impedance spectrum, linear polarization, Tafel polarization and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases and then decreases with increasing Co content.     

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO3 ceramics

Polycrystalline Bi_1−xBa_xFe_1−yM_yO₃ (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO₃ with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO₃ as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO₃ was observed in the dielectric response curve.     

Structure and electrical properties of MnO2-doped Sr2−xCaxNaNb5O15 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics Sr_2−xCa_xNaNb₅O₁₅ + y wt% MnO₂have been prepared by the conventional solid state reaction method. Our results reveal that Ca²⁺and Mn ions have entered into the Sr₂NaNb₅O₁₅ lattices to form a solid solution with tungsten-bronze structure. The substitution of Ca²⁺ induces a decrease in piezoelectric coefficient d₃₃, electromechanical coupling factors k_p and k_t, while, the addition of Mn ions decreases the sintering temperature and effectively promotes the densification of the ceramics. The effect of substitution of Ca²⁺and Mn ions on the structure, electrical properties and diffused phase changing were investigated systematically. For the ceramic with x = 0.05 and y = 0.5, the piezoelectric, dielectric and ferroelectric properties become optimum, giving a piezoelectric coefficient d₃₃ = 190 pC/N, electromechanical coupling factors k_p = 13.4% and k_t = 36.5%, ?_r = 2123, loss tangent tan δ = 0.038, remanent polarization P_r = 4.76 μC/cm², coercive field E_c = 12.68 kV/cm, and Curie temperature T_c = 260 °C.     

Electrochemical hydrogen storage properties of non-stoichiometric La0.7Mg0.3−xCaxNi2.8Co0.5 (x = 0-0.10) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.7Mg_0.3−xCa_xNi_2.8Co_0.5 (x = 0, 0.05 and 0.10) alloys prepared by arc-melting and subsequent powder sintering method are investigated. The electrochemical measurement results show that the cycle stability after 100 charge/discharge cycles first increases from 46.4% (x = 0) to 54.3% (x = 0.05), then decreases to 43.2% (x = 0.10), and the high rate dischargeability increases from 64.5% (x = 0) to 68.5% (x = 0.10) at the discharge current density of 1200 mA/g. The electrochemical impedance spectroscopy analysis indicates that the electrochemical kinetics of the alloy electrodes is improved by increasing Ca. The entire results exhibit that a suitable content of Ca (x = 0.05) can improve the overall electrochemical hydrogen storage characteristics of the alloys.     

Effect of Al substitution for Co on the hydrogen storage characteristics of Ml0.8Mg0.2Ni3.2Co0.6−xAlx (x=0–0.6) alloys

Ml_0.8Mg_0.2Ni_3.2Co_0.6−xAl_x (x=0–0.6) hydrogen storage alloys were prepared by induction melting and the effect of Al substitution for Co on the structure and hydrogen storage characteristics was investigated. These alloys are confirmed to be composed of LaNi₅ as matrix and (LaMg)Ni₃ as the secondary phase. The Al substitution leads to an expansion of the unit cell volume of matrix and an enhancement of the stability of the alloy hydrides. The thermodynamic properties such as ΔH and ΔS and the diffusion coefficient of hydrogen in these alloys were also evaluated.     

Temperature and field induced strain in polycrystalline Ni50Mn35In15−xSix magnetic shape memory Heusler alloys

Magnetic shape memory properties of polycrystalline Ni₅₀Mn₃₅In_15−xSi_x were investigated. A reversible strain of more than 0.4% was observed for x = 0 at a magnetic field H = 5 T that was found to be associated with a field induced reverse martensitic transformation. The strains were found to increase with the substitution of In by Si and strains larger than 1% were observed for x = 2 at H = 5 T. Both the positive and negative strain changes were observed in the vicinity of martensitic transition temperatures. The strain in Ni₅₀Mn₃₅In_15−xSi_x was found to depend on silicon concentration, and on samples texture.     

Preparation and electrical properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 lead-free piezoelectric ceramics

Lead-free (1 − x)Bi_0.47Na_0.47Ba_0.06TiO₃-xKNbO₃ (BNBT-xKN, x = 0-0.08) ceramics were prepared by ordinary ceramic sintering technique. The piezoelectric, dielectric and ferroelectric properties of the ceramics are investigated and discussed. The results of X-ray diffraction (XRD) indicate that KNbO₃ (KN) has diffused into Bi_0.47Na_0.47Ba_0.06TiO₃ (BNBT) lattices to form a solid solution with a pure perovskite structure. Moderate additive of KN (x ≤ 0.02) in BNBT-xKN ceramics enhance their piezoelectric and ferroelectric properties. Three dielectric anomaly peaks are observed in BNBT-0.00KN, BNBT-0.01KN and BNBT-0.02KN ceramics. With the increment of KN in BNBT-xKN ceramics, the dielectric anomaly peaks shift to lower temperature. BNBT-0.01KN ceramic exhibits excellent piezoelectric properties and strong ferroelectricity: piezoelectric coefficient, d₃₃ = 195 pC/N; electromechanical coupling factor, k_t = 58.9 and k_p = 29.3%; mechanical quality factor, Q_m = 113; remnant polarization, P_r = 41.8 μC/cm²; coercive field, E_c = 19.5 kV/cm.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Interdiffusion in pseudobinary sections of Ni3Al–Co ternary system

Interdiffusion in Ni₃Al intermetallic alloyed by 0–19 at.% Co was studied in three pseudo-binary sections: c_Al=22, c_Co=3.5, and c_Co=16 (c in at.%). The experiments were carried out at temperatures between 1173 and 1473 K. An attempt was made to analyze the interdiffusion coefficient into intrinsic diffusivities of individual components. Since no detectable shift of tungsten inert markers was observed, it was concluded that the intrinsic diffusivities of element pairs with varying concentration (Al/Ni and Co/Ni) in respective pseudo-binary sections are approximately equal one to another. The interdiffusion in pseudo-binary alloys Al/Ni (c_Co=const.) runs slightly faster than that in pseudo-binary alloys Ni/Co (c_Al=const.). The relations between the measured interdiffusion coefficients in the Ni_3−xAlCo_x ordered γ′ phase agree well with literature data for the interdiffusion coefficients in both the ordered γ′ and γ′+Co phases and also for the disordered Al–Ni and Co–Ni solid solutions. They are qualitatively explained by mutual interaction between constituents and by their interaction with vacancies.     

Phase structure and electrical properties of (0.8 − x) BaTiO3-0.2Bi0.5Na0.5TiO3-xBaZrO3 lead-free piezoceramics

Lead-free piezoelectric ceramics (0.8 − x)BaTiO₃-0.2Bi_0.5Na_0.5TiO₃-xBaZrO₃ (BT-BNT-xBZ, 0 ≤ x ≤ 0.08) doped with 0.3 wt% Li₂CO₃ were prepared by conventional solid-state reaction method. With the Li₂CO₃ doping, all the ceramics can be well sintered at 1170-1210 °C. The phase structure, dielectric, ferroelectric and piezoelectric properties of the ceramics were investigated. Results show that a morphotropic phase boundary (MPB) between tetragonal and pseudocubic phases exists at x = 0.03-0.04. The addition of Zr can improve the piezoelectric properties of BT-BNT ceramics. Furthermore, a relaxor behavior is induced and the tetragonal-cubic phase transition shifts towards lower temperatures after the addition of Zr. The ceramics with x = 0.03 possess the optimum electrical properties: d₃₃ = 72 pC/N, k_p = 15.4%, ?_r = 661, P_r = 18.5 μC/cm², E_c = 34.1 kV/cm, T_c = 150 °C.     

High-Q microwave dielectrics in the (Mg1−xZnx)Al2O4 (x = 0-0.1) system

The microwave dielectric properties and the microstructures of (Mg_1−xZn_x)Al₂O₄ (x = 0-0.1) ceramic system prepared by the conventional solid-state route were investigated. The forming of spinel-structured (Mg_1−xZn_x)Al₂O₄ (x = 0-0.1) solid solutions was confirmed by the XRD patterns and the measured lattice parameters, which linearly varied from a = b = c = 8.0815 Å for MgAl₂O₄ to a = b = c= 8.0828 Å for (Mg_0.9Zn_0.1)Al₂O₄. By increasing x, the Q × f of (Mg_1−xZn_x)Al₂O₄ can be tremendously boosted from 82,000 GHz at x = 0 to a maximum of 156,000 GHz at x = 0.05. The Zn substitution was effective in reducing the dielectric loss without detrimental effects on the ?_r and τ_f values of the ceramics.     

Modification of the hydrogenation properties of LaNi5 upon Ni substitution by Rh, Ir, Pt or Au

The hydrogenation properties of the LaNi_5−xM_x (M = Rh, Ir, Au) compounds have been studied. The Ni substitution has several consequences: pressure plateau splitting and increase of plateau pressure. This latter observation disagrees with the general rule that a cell volume increase of the alloy should result in a plateau pressure lowering. In order to elucidate the origin of this anomalous behaviour, DFT calculations have been performed on both LaNi_5−xRh_x and LaNi_5−xPt_x intermetallic compounds, which, according to the present and previous experimental work, present a similar anomaly. We discuss our results in light of the models proposed in the literature. We conclude that, in the case of a Ni substitution by 4d or 5d elements, the size effect alone fails in predicting the hydrogen absorption properties while the rule of reverse stability is obeyed.     

Microstructure, ferroelectric, and piezoelectric properties of (1 − x − y)Bi0.5Na0.5TiO3-xBaTiO3-yBi0.5Ag0.5TiO3 lead-free ceramics

Lead-free (1 − x − y)Bi_0.5Na_0.5TiO₃-xBaTiO₃-yBi_0.5Ag_0.5TiO₃ (BNT-BT-BAT-x/y, x = 0-0.10, y = 0-0.075) piezoelectric ceramics were synthesized by conventional oxide-mixed method. The microstructure, ferroelectric, and piezoelectric properties of the ceramics were investigated. Results show that a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases of BNT-BT-BAT-x/0.04 ceramics is formed at x = 0.06-0.08. The addition of BAT has no obvious change on the crystal structure of BNT-BT ceramics while it causes the grain size of the ceramics to become more homogenous. Near the MPB, the ceramics with x = 0.06 and y = 0.05-0.06 possess optimum electrical properties: P_r ∼ 42.5 μC/cm², E_c ∼ 32.0 kV/cm, d₃₃ ∼ 172 pC/N, k_p ∼ 32.6%, and k_t ∼ 52.6%. The temperature dependences of k_p and polarization versus electric hysteresis loops reveal that the depolarization temperature (T_d) of BNT-BT-BAT-0.06/y ceramics decreases with increasing y. In addition, the polar and non-polar phases may coexist in the BNT-BT-BAT-x/y ceramics above T_d.