Studies on the power factor of (Ba,Sr)Co2+xRu4−xO11 compounds

We have prepared polycrystalline single-phase ACo_2+xRu_4−xO₁₁ (A = Sr, Ba; 0 ≤ x ≤ 0.5) using the ceramic method and we have studied their structure, electrical resistivity and Seebeck coefficient, in order to estimate their power factor (P.F.). These layered compounds show values of electrical resistivity of the order of 10⁻⁵ Ωm and their Seebeck coefficients are positive and range from 1 μV K⁻¹ (T = 100 K) to 20 μV K⁻¹ (T = 450 K). The maximum power factor at room temperature is displayed by BaCo₂Ru₄O₁₁ (P.F.: 0.20 μW K⁻² cm⁻¹), value that is comparable to that shown by compounds such as SrRuO₃ and Sr₆Co₅O₁₅.     

Fabrication and properties of p-type K doped Zn1−xMgxO thin film

A series of K doped Zn_1−xMg_xO thin films have been prepared by pulsed laser deposition (PLD). Hall-effect measurements indicate that the films exhibit stable p-type behavior with duration of at least six months. The band gap of the K doped Zn_1−xMg_xO films undergoes a blueshift due to the Mg incorporation. However, photoluminescence (PL) results reveal that the crystallinity decreased with the increasing of Mg content. The fabricated K doped p-type Zn_0.95Mg_0.05O thin film exhibits good electrical properties, with resistivity of 15.21 Ω cm and hole concentration of 5.54 × 10¹⁸ cm⁻³. Furthermore, a simple ZnO-based p-n heterojunction was prepared by deposition of a K-doped p-type Zn_0.95Mg_0.05O layer on Ga-doped n-type ZnO thin film with low resistivity. The p-n diode heterostructure exhibits typical rectification behavior of p-n junctions.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Thermoelectric properties of Bi2SexTe3−x prepared by Bridgman method

Bi₂Se_xTe_3−x crystals with various x values were grown by Bridgman method. The electrical conductivity, σ, was found to decrease with increasing Se content. The highest σ of 1.6 × 10⁵ S m⁻¹ at room temperature was reached at x = 0.12 with a growth rate of 0.8 mm h⁻¹. The Seebeck coefficient, S, was less dependent on Se content, all with positive values showing p-type characteristics, and the highest S was measured to be 240 μV K⁻¹ at x = 0.24. The lowest thermal conductivity, κ, was 0.7 W m⁻¹ K⁻¹ at x = 0.36. The electronic part of κ, κ_el, showed a decrease with increasing Se content, which implies that the hole concentration as the main carriers was reduced by the addition of Se. The highest dimensionless figure of merit, ZT, at room temperature was 1.2 at x = 0.36, which is attributed to the combination of a rather high electrical conductivity and Seebeck coefficient and low thermal conductivity.     

Transparent p-type CuxS thin films

The effect of different mild post-annealing treatments in air, at 270 °C, for 4-6 min, on the optical, electrical, structural and chemical properties of copper sulphide (Cu_xS) thin films deposited at room temperature are investigated. Cu_xS films, 70 nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu₂S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (∼10²² cm⁻³), low electrical resistivity (∼10⁻⁴ Ω cm) and inconclusive p-type conduction. After the mild post-annealing, these films display increasing values of resistivity (∼10⁻³ to ∼10⁻² Ω cm) with annealing time and exhibit conclusive p-type conduction. An increase of copper content in Cu_xS phases towards the semiconductive Cu₂S (chalcocite) compound with annealing time is reported, due to re-evaporation of sulphur from the films. However, the latter stoichiometry was not obtained, which indicates the presence of vacancies in the Cu lattice. In the most resistive films a Cu₂O phase is also observed, diminishing the amount of available copper to combine with sulphur, and therefore the highest values of optical transmittance are reached (65%). The appearance on the surface of amorphous sulphates with annealing time increase is also detected as a consequence of sulphur oxidation and replacement of sulphur with oxygen. All annealed films are copper deficient in regards to the stoichiometric Cu₂S and exhibit stable p-type conductivity.     

Resistivity, thermopower, and thermal conductivity of nickel doped compounds Cr1−xNixSb2 at low temperatures

Substitutional compounds Cr_1−xNi_xSb₂ (0 ≤ x ≤ 0.1) were synthesized, and the effect of Ni substitution on transport and thermoelectric properties of Cr_1−xNi_xSb₂ were investigated at the temperatures from 7 to 310 K. The results indicated that the magnitudes of the resistivity and thermopower of Cr_1−xNi_xSb₂ decreased greatly with increasing Ni content at low temperatures, owing to an increase in electron concentration caused by Ni substitution for Cr. Experiments also showed that the low-temperature lattice thermal conductivity of Cr_1−xNi_xSb₂ decreased substantially with increasing Ni content due to an enhancement of phonon scattering by the increased number of Ni atoms. As a result, the figure of merit, ZT, of lightly doped Cr_0.99Ni_0.01Sb₂ was improved at T > ∼230 K. Specifically, the ZT of Cr_0.99Ni_0.01Sb₂ at 310 K was approximately ∼29% larger than that of CrSb₂, indicating that thermoelectric properties of CrSb₂ can be improved by an appropriate substitution of Ni for Cr.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

The studies of phase relation, microstructure, magnetic transition, magnetocaloric effect in (Gd1−xErx)5Si1.7Ge2.3 compounds

The phase relation, microstructure, Curie temperatures (T_C), magnetic transition, and magnetocaloric effect of (Gd_1−xEr_x)₅Si_1.7Ge_2.3 (x = 0, 0.05, 0.1, 0.15, and 0.2) compounds prepared by arc-melting and then annealing at 1523 K (3 h) using purity Gd (99.9 wt.%) are investigated. The results of XRD patterns and SEM show that the main phases in those samples are mono-clinic Gd₅Si₂Ge₂ type structure. With increase of Er content from x = 0 to 0.2, the values of magnetic transition temperatures (T_C) decrease linearly from 228.7 K to 135.3 K. But the (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds display large magnetic entropy near their transition temperatures in a magnetic field of 0-2 T. The maximum magnetic entropy change in (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds are 24.56, 14.56, 16.84, 14.20, and 13.22 J/kg K⁻¹ with x = 0, 0.05, 0.1, 0.15, and 0.2, respectively.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

CuIn1−xAlxS2 thin films prepared by sulfurization of metallic precursors

CuIn_1−xAl_xS₂ thin films (x = 0, 0.09, 0.27, 0.46, 0.64, 0.82 and 1) with thicknesses of approximately 1 μm were formed by the sulfurization of DC sputtered Cu-In-Al precursors. All samples were sulfurized in a graphite container for 90 min at 650 °C in a 150 kPa Ar + S atmosphere. Final films were studied via X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-Raman spectroscopy. It was found that all samples were polycrystalline in nature and their lattice parameters varied slightly nonlinearly from {a = 5.49 Å, c = 11.02 Å} for CuInS₂ to {a = 5.30 Å, c = 10.36 Å} for CuAlS₂. No unwanted phases such as Cu_2−xS or others were observed. Raman were recorded at a room temperature and the most intensive and dominant A₁ phonon frequency varied nonlinearly from 294 cm⁻¹ (CuInS₂) to 314 cm⁻¹ (CuAlS₂).     

Electrical properties of binder-free thick film BaYxBi1−xO3 NTC thermistors

The microstructure and electrical properties of BaY_xBi_1−xO₃ thick film negative temperature coefficient thermistors, fabricated by screen printing, were investigated. The sintered thick films were the single-phase solid solutions of the BaY_xBi_1−xO₃ compounds with a monoclinic structure. The added Y₂O₃ led to a significant decrease in the grain size of the thermistors. The resistivity and coefficient of temperature sensitivity for the BaY_xBi_1−xO₃ (0 ≤ x ≤ 0.15) thick film NTC thermistors decreased first with increasing x in the range of x < 0.04 and then increased with further increase in x.     

First principles study of structural, electronic and magnetic properties of Mg1−xMnxTe alloys

Density functional FP-LAPW + lo calculations have been performed to study the structural, electronic and magnetic properties of Mg_1−xMn_xTe for compositional parameter x = 0.25, 0.50, 0.75 and 1. Our calculations reveal the occurrence of ferromagnetism in these compounds in which the transition-metal atom is ordered in a periodical way thereby interacting directly with the host atoms. Results extracted from electronic band structure and density of states (DOS) of these alloys show the existence of direct energy band gap for both majority- and minority-spin cases, while the total energy calculations confirm the stability of ferromagnetic state as compared to anti-ferromagnetic state. The total magnetic moment for Mg_1−xMn_xTe for each composition is found to be approximately 5 μ_B, which indicates that the addition of Mn content does not affect the hole carrier concentration of the perfect MgTe compound. Moreover, the s-d exchange constant (N₀α) and p-d exchange constant (N₀β) are also calculated which are in accordance with a typical magneto-optical experiment. The estimated spin-exchange splitting energies originated by Mn 3d states energies, i.e. Δ_X(s-d) and Δ_X(p-d), show that the effective potential for minority-spin is more attractive than that of the majority-spin. Also, the p-d hybridization is found to cause the reduction of local magnetic moment of Mn and produce small local magnetic moments on the nonmagnetic Mg and Te sites.     

Power factor enhancement in Zn-doped Na0.8CoO2

The thermoelectric properties of Na_0.8Zn_xCo_1−xO₂/(ZnO)_y (x ≤ 0.01, 0 ≤ y ≤ 0.14) have been systematically investigated. The results suggest that doping divalent Zn ions within solubility limit x^* ∼ 0.01 leads to simultaneous reduction in resistivity and enhancement of thermopower. Analysis of the results show that the reduction of resistivity may be attributed to improved mobility of carriers, while the enhancement of thermopower may originate from the geometric relaxation of distorted CoO₆ octahedra caused by partial Zn substitution, leading to a narrower band width in the strongly correlated environment, consequently resulting in a remarkable 20% improvement in power factor.     

Magnetomechanical properties of epoxy-bonded Sm1−xNdxFe1.55 (0 ≤ x ≤ 0.56) pseudo-1-3 magnetostrictive particulate composites

Pseudo-1-3 magnetostrictive particulate composites consisting of light rare earth (Sm and Nd)-based magnetostrictive Sm_1−xNd_xFe_1.55 particles with the Nd content x of 0-0.56 and randomly distributed sizes of 10-180 μm embedded and aligned in a passive epoxy matrix are fabricated using the particulate volume fraction of 0.5. The quasistatic magnetomechanical properties of the composites are investigated and the results are compared with their monolithic alloys for various x. The composites exhibit similar qualitative trends in properties with the alloys for all x. The Sm_0.92Nd_0.08Fe_1.55 composite shows a large unsaturated magnetostriction λ of −530 ppm at 500 kA/m and a high piezomagnetic coefficient d₃₃ of −2.0 nm/A at 100 kA/m as a result of the magnetocrystalline anisotropy compensation between Sm³⁺ and Nd³⁺ ions in the Sm_0.92Nd_0.08Fe_1.55 alloy.     

Structural, elastic and electronic properties of Mg(Cu1−xZnx)2 alloys calculated by first-principles

The structural, elastic and electronic properties of Mg(Cu_1−xZn_x)₂ alloys (x = 0, 0.25, 0.5,and 0.75) were investigated by means of first-principle calculations within the framework of density functional theory (DFT). The calculation results demonstrated that the partial substitution of Cu with Zn in MgCu₂ leaded to an increase of lattice constants, and the optimized structural parameters were in very good agreement with the available experimental values. From energetic point of view, it was found that with increase of Zn content the structural stability of Mg(Cu_1−xZn_x)₂ alloys decreased apparently. The single-crystal elastic constants were obtained by computing total energy as a function of strain, and then the bulk modulus B, shear modulus G, Young's modulus Y and Poisson's ratio ν of polycrystalline aggregates were derived. The calculated results showed that among the Mg(Cu_1−xZn_x)₂ alloys, MgCuZn exhibited the largest stiffness, while Mg₂Cu₃Zn showed the best ductility. Finally, the electronic density of states (DOSs) and charge density distribution were further studied and discussed.     

Transparent conducting oxide films of heavily Nb-doped titania by reactive co-sputtering

Niobium-doped titania (TNO) films of various Nb content were deposited on glass and silicon substrates by reactive co-sputtering of Ti and Nb metal targets. Nb content in the TNO films was varied from 0 to ∼13 at.% (atomic percent), corresponding to Ti_1−xNb_xO₂ with x = 0-0.52, by modulating the Nb target power from 0 to 150 W (Watts). The influence of ion bombardment on the TNO films was investigated by applying an RF substrate bias from 0 to 25 W. The as-deposited TNO films were all amorphous and insulating, but after annealing at 600 °C for 1 h in hydrogen, they became crystalline and conductive. The annealed films crystallized into either pure anatase or mixed anatase and rutile structures. The as-deposited and the annealed films were transparent, with an average transmittance above 70%. Anatase TNO film (Ti_1−0.39Nb_0.39O₂) with Nb 9.7 at.% exhibited a dramatically reduced resistivity of 9.2 × 10⁻⁴ Ω cm, a carrier density of 6.6 × 10²¹ cm⁻³ and a carrier mobility around 1.0 cm² V⁻¹ s⁻¹. In contrast, the mixed-phase Ti_1−0.39Nb_0.39O₂ showed a higher resistivity of 1.2 × 10⁻¹ Ω cm. This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films.     

Superconductors of SmFe1−xCoxAsO synthesized by mechanical alloying

The SmFe_1−xCo_xAsO (x = 0 − 0.25) superconductors were synthesized by mechanical alloying (MA) and rapid sintering method with Co atoms doped into FeAs layers to replace the Fe sites. The phase purity and superconducting properties of the samples were characterized by X-ray diffraction, electrical resistivity, magnetic susceptibility and Hall coefficient. All the samples belong to the tetragonal ZrCuSiAs structure type with the grain size in 1-3 μm. The superconducting critical temperature T_c of SmFe_0.9Co_0.1AsO was 12.5 K, and the structure/SDW transition was suppressed by Co doping. The negative Hall coefficient of SmFe_0.9Co_0.1AsO indicated the electron conduction in the sample. The charge carrier density is about 2 × 10²⁰ cm⁻³ at the temperature lower than 150 K, larger than that of SmFeAsO.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting Mg with Ni

The microwave dielectric properties of La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method at various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.71 g/cm³, dielectric constant (?_r) of 20.19, quality factor (Q × f) of 74,600 GHz, and temperature coefficient of resonant frequency (τ_f) of −85 ppm/°C were obtained for La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Effect of non-magnetic doping on multi-step magnetization behavior of Dy50−xAlxAg50

The electrical transport and magnetization measurements have been carried out on Al-doped polycrystalline intermetallic compounds Dy_50−xAl_xAg₅₀ (x = 0, 0.3, 0.6, 1.2, 1.8) in a pulsed high magnetic field, in which multi-step magnetization is observed. Partial substitute of non-magnetic Al³⁺ for Dy³⁺ ions in the compounds increases the critical magnetic fields and the relative area of the magnetic hysteresis loop, which result from the pinning effect, lattice distortion, the change of coupling strength and dilution effect related to the Al³⁺ doping.The experimental results indicate that non-magnetic Al³⁺ ions and induced amorphous phase can pin the rotation and/or growth of magnetic domains, thus, the critical magnetic field can be enhanced by doping non-magnetic ions in the magnetic materials, especially in the permanent magnet materials.     

Phase, structural, and magnetocaloric properties of high temperature annealed LaFe11.6Si1.4BX

The phase relation, microstructural, hysteresis, Curie temperature, and magnetocaloric effects of LaFe_11.6Si_1.4B_x (x = 0.1, 0.2, 0.3, 0.4, and 0.5) prepared by arc-melting and then annealed at 1373 K (1.5 h) + 1523 K (5 h) were investigated. It was found that the main phase is NaZn₁₃-type phase, the impurity phases include α-Fe, Fe₂B, and small amount of La₅Si₃. The boron atom can dissolve into the crystal lattice of LaFe_11.6Si_1.4B_x to form interstitial solid solution, but the content of solid solution is not up to x = 0.5. For LaFe_11.6Si_1.4B_x (x = 0.1, 0.3, and 0.5) compounds, the Curie temperature T_C increases from 190.6 to 198.3 K with the increasing of B content from x = 0.1 to 0.5. The first order magnetic transition behavior becomes weaker and magnetic entropy change ΔS_M (T, H) drops with the increasing of B content, respectively. However, ΔS_M (T, H) still remains a large value, 11.18 J/kg K, when x reaches to 0.5 at 0-2 T. An attractive feature is that both thermal and magnetic hysteresis can be reduced remarkably by introducing B. The maximum magnetic hysteresis loss near T_C drops from 22.52 to 4.95 J/kg when the content of B increases from x = 0.1 to 0.5.