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.     

Synthesis, thermal stability and properties of [(Fe1−xCox)72Mo4B24]94Dy6 bulk metallic glasses

A series of [(Fe_1−xCo_x)₇₂Mo₄B₂₄]₉₄Dy₆ (x = 0.1, 0.2, 0.3, 0.4 and 0.5 at.%) bulk metallic glasses (BMGs) in rod geometries with critical diameter up to 3 mm were fabricated by copper mold casting method. This alloy system exhibited good thermal stability with high glass transition temperature (T_g) 860 K and crystallization temperature (T_x) 945 K. The addition of Co was found to be effective in adjusting the alloy composition deeper to eutectic, leading to lower liquidus temperature (T_l). The [(Fe_0.8Co_0.2)₇₂Mo₄B₂₄]₉₄Dy₆ alloy showed the largest supercooled liquid region (ΔT_x = T_x − T_g = 92 K), reduced glass transition temperature (T_rg = T_g/T_l = 0.622) and gamma parameter (γ = T_x/(T_g + T_l) = 0.424) among the present system. Maximum compressive fracture strength of 3540 MPa and micro-Vickers hardness of 1185 kg/mm² was achieved, resulting from the strong bonding structure among the alloy constituents. The alloy system possessed soft magnetic properties with high saturation magnetization of 56.61-61.78 A m²/kg and coercivity in the range of 222-264.2 A/m, which might be suitable for application in power electronics devices.     

Photoluminescent properties of LiSrxBa1−xPO4:RE (RE = Sm, Eu) f-f transition phosphors

Rare-earth ions (Sm³⁺ or Eu³⁺) doped LiSr_xBa_1−xPO₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) f-f transition phosphor powders were prepared by a high temperature solid-state reaction. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the phase structure of the sample changes from LiBaPO₄ to LiSrPO₄ when x changes from 0 to 1.0. The excitation spectra indicate that only direct excitation of rare earth ions (Sm³⁺ or Eu³⁺) can be observed. The doped rare earth ions show their characteristic emission in LiSr_xBa_1−xPO₄, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Sm³⁺⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2), respectively. The dependence of the emission intensities of the LiSr_xBa_1−xPO₄:Sm³⁺ and LiSr_xBa_1−xPO₄:Eu³⁺ phosphors on the x value and Ln³⁺ (Ln³⁺ = Sm³⁺, Eu³⁺) concentration is also investigated.     

Electronic structure and optical property of As2(Te1−xSx)3 and As2(Te1−xSex)3 crystals

Single crystals of S- and Se-incorporated As₂Te₃ have been grown by vertical Bridgman method. The electronic structure and optical property of As₂(Te_1−xS_x)₃ [ATS] and As₂(Te_1−xSe_x)₃ [ATSe] series compounds have been characterized experimentally by thermoreflectance (TR) measurements in a wide energy range of 0.7-6 eV. X-ray diffraction measurements showed that the diffraction peaks of sulfur- and selenium-incorporated As₂(Te_1−xS_x)₃ 0 ≤ x ≤ 0.3 and As₂(Te_1−xSe_x)₃ 0 ≤ x ≤ 0.6 crystals shift to higher diffraction angles with the increase of the sulfur or selenium incorporations. The analysis of X-ray measurement revealed similar crystalline phase for the As₂Te₃ and those of the S- or Se-incorporated As₂Te₃. The experimental TR spectra of As₂(Te_1−xS_x)₃ (0 ≤ x ≤ 1) and As₂(Te_1−xSe_x)₃ (0 ≤ x ≤ 1) exhibit a lot of derivative-like spectral features in the vicinity of band edge as well as in the higher-lying bands. Transition energies and broadening parameters of the TR features at 40 and 300 K were analyzed. Compositional dependences of band gap and interband transition energies of the ATS and ATSe series were evaluated. The origins for the interband transitions in the ATS and ATSe are assigned. Based on the experimental analyses, the electronic structure of the diarsenic trichalcogenides, As₂(Te_1−xSe_x)₃ and As₂(Te_1−xS_x)₃, is hence being realized.     

Luminescent properties of Na3Gd1−xEux(PO4)2 and energy transfer in these phosphors

A series of Eu³⁺ activated Na₃Gd_1−xEu_x(PO₄)₂ (0 ≤ x ≤ 1) phosphors were synthesized by solid-state reaction method. The structures and photo-luminescent properties of these phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the orthorhombic Na₃Gd(PO₄)₂. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The intensities of magnetic dipole transition ⁵D₀ → ⁷F₁ and forced electric dipole transition ⁵D₀ → ⁷F₂ are comparable, and the energy ratio (⁵D₀ → ⁷F₁/⁵D₀ → ⁷F₂) is 1.1. The emission spectra exhibit strong reddish orange performance (CIE chromaticity coordinates: x = 0.62, y = 0.38), which is due to the ⁵D₀ → ⁷F_J transitions of Eu³⁺ ions. The correlation between the structure and the photo-luminescent properties of the phosphors was studied. The energy transfer and concentration quenching of the phosphors were discussed. Na₃Gd_1−xEu_x(PO₄)₂ has a potential application for white light-emitting diodes.     

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.     

Crystal structure and magnetic properties of DyCuxGa2−x (x = 0-2.0) antiferromagnetic compounds

DyCu_xGa_2−x (x = 0-2.0) compounds have been synthesized; meanwhile, their crystal structure and magnetic properties have been investigated by X-ray diffraction and magnetic measurements. The result shows that the continuous solid-solution series crystallize in three phases, with the structure types of AlB₂ (x = 0-0.2), DyCuGe (x = 0.3-0.6) and CeCu₂ (x = 0.7-2.0), respectively. The main reason to form the three structure types is considered to be the average atomic radius ratio of R to Cu/Ga. Magnetic-ordering transition of the compounds with x = 0.2-0.6 takes place at about 20 K and 113 K, while those of other compounds only takes place at about 20 K, which is attributed to the change of the near Dy-Dy distances and the ordered substitution of Ga by Cu.     

Magnetocaloric effect in the Ce2Fe17−xMnx helical magnets

The Ce₂Fe_17−xMn_x (x = 0-2) compounds demonstrate a complex temperature dependence of the magnetocaloric effect MCE, which is inverse in a narrow temperature interval just below Néel temperature T_N and normal at higher or lower temperatures. The normal MCE exhibits two peaks in the vicinity of temperatures of ferromagnetic ordering Θ_T and T_N for compositions x = 0-0.35, 1.3-2 or one peak near T_N for antiferromagnets with x = 0.5-1. The maximal change of the peak entropy −S_M is about 3 J/kg K in a field of 5 T for the compounds with x = 0-0.5 at T ∼230 K close to T_N. The drastic decrease of the MCE, by half, in the Ce₂Fe_17−xMn_x system is traceable to a decrease of the spontaneous magnetization and the helical type of magnetic states in the compounds.     

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.     

Phase evolution, crystal structure and dielectric behavior of (1 − x)Nd(Zn0.5Ti0.5)O3 + xBi(Zn0.5Ti0.5)O3 compound ceramics

The phase evolution, crystal structure and dielectric properties of (1 − x)Nd(Zn_0.5Ti_0.5)O₃ + xBi(Zn_0.5Ti_0.5)O₃ compound ceramics (0 ≤ x ≤ 1.0, abbreviated as (1 − x)NZT-xBZT hereafter) were investigated. A pure perovskite phase was formed in the composition range of 0 ≤ x ≤ 0.05. The B-site Zn²⁺/Ti⁴⁺ 1:1 long range ordering (LRO) structure was detected by both XRD and Raman spectra in x ≤ 0.05 samples. However, this LRO structure became gradually degraded with an increase in x. The dielectric behaviors of the compound ceramic at various frequencies were investigated and correlated to its chemical composition and crystal structure. A gradually compensated τ_f value was obtained in (1 − x)NZT-xBZT microwave dielectrics at x = 0.03, which was mainly due to the dilution of dielectric constant in terms of Claussius-Mossotti differential equation.     

Effect of the sintering temperature on the properties of nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) powders

Nanocrystalline Ca_1−xSm_xMnO₃ (0 ≤ x ≤ 0.4) manganites were prepared by a soft chemical method (Pechini method) followed by auto-combustion and sintering in air at 1073 or 1473 K. Single-phase powders with general composition Ca_1−xSm_xMnO₃ were obtained after 18 h annealing. The particle and grain sizes of the substituted Sm-manganites did not exhibit variation with samarium content, but increase with increasing the sintering temperature. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹ characteristic of the BO₆ octahedron vibrations.For the samples sintered at T_s = 1473 K, the partial substitution of calcium by samarium in the CaMnO₃ phase induces a marked decrease in the electrical resistivity, in the temperature range of 300-900 K, and at the same time a metal-to-insulator transition occurs; for T_s = 1073 K all the samples present semiconductor behaviour. With the increase of the annealing temperature the grain size increases and a metal-semiconductor transition appears. The results can be ascribed to the Mn⁴⁺/Mn³⁺ ratio and particle grain size. The effects of particle size on the electrical properties can be attributed to the domain status, changes in the Mn-O-Mn bond angle and Mn-O bond length.     

Luminescent properties of UV excitable blue emitting phosphors MSr4(BO3)3:Ce (M = Li and Na)

A series of Ce³⁺ doped novel borate phosphors MSr₄(BO₃)₃ (M = Li or Na) were successfully synthesized by traditional solid-state reaction. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. The optimal concentrations of dopant Ce³⁺ ions in compound MSr₄(BO₃)₃ (M = Li or Na) were determined through the measurements of photoluminescence spectra of phosphors. Ce³⁺ doped phosphors MSr₄(BO₃)₃ (M = Li or Na) show strong broad band absorption in UV spectral region and bright blue emission under the excitation of 345 nm light. In addition, the temperature dependences of emission spectra of M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors with optimal composition x = 0.05 for Li and x = 0.09 for Na excited under 355 nm pulse laser were also investigated. The experimental results indicate that the M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors are promising blue emitting phosphors pumped by UV light.     

Preparation and characterization of La1−xKxFeO3 (x = 0-1) by self-propagating high-temperature synthesis for use as soot combustion catalyst

This paper proposes La_1−xK_xFeO₃ prepared by self-propagating high-temperature synthesis (SHS) as an alternative to platinum catalysts for promoting diesel soot combustion. The catalytic property of eleven products SHSed with different substitution ratios of potassium (x = 0-1) was experimentally evaluated using a thermobalance. In the mass loss curves of the product, T₅₀ was defined as the temperature at which the weight of the reference soot decreases to half its initial weight. The BET specific surface area of SHSed La_1−xK_xFeO₃ depended on x strongly. All the products showed good oxidation catalytic activity. Despite having the smallest surface area (0.11 m²/g) among the obtained products, La_0.9K_0.1FeO₃ (x = 0.1) was found to be the best catalyst with the lowest T₅₀ (442 °C). T₅₀ of La_1−xK_xFeO₃ decreased with increasing x for x > 0.2. The products with x = 0.6 and 0.8 were the second-best catalysts in terms of their T₅₀. Moreover, average apparent activation energy of La_0.9K_0.1FeO₃ (x = 0.1) calculated by Friedman method using TG was as much as 61 kJ/mol lower than that of Pt/Al₂O₃ catalyst. In conclusion, potassium-substituted SHSed La_1−xK_xFeO₃ can be used as an alternative to Pt/Al₂O₃ for soot combustion.     

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.     

Near room temperature magnetic entropy changes in as-cast Gd100−xMnx (x = 0, 5, 10, 15, and 20 at.%) alloys

A series of Gd_100−xMn_x (x = 0, 5, 10, 15, and 20 at.%) alloys were prepared by arc-melting. The Curie temperature (T_C) associated with the ferromagnetic-paramagnetic transitions, derived from M-T curves, show decrease in T_C for as-cast alloys (∼279 K) as compared to as-cast Gd (∼292 K). No appreciable decrease in the |ΔS_M|_max values ∼4.6 J/kg K (0-2 T) and ∼8.6 J/kg K (0-5 T) were observed upon alloying Gd with Mn up to x ≤ 15 at.%. Refrigerant capacity (q) showed negligible variation ∼195 J/kg (0-2 T) and ∼450 J/kg (0-5 T) with increasing Mn (up to x ≤ 15 at.%) content. Similar values of |ΔS_M|_max and q coupled with ∼13 K decrease in T_C for as-cast Gd_100−xMn_x (0 ≤ x ≤ 15) alloys as compared to Gd, suggests expansion of working temperature region of Gd upon alloying with Mn up to 15 at.%. Low cost, adjustable T_C, favorable magnetocaloric properties make Gd_100−xMn_x alloys potential candidates as second-order transition based magnetic refrigerants for near room temperature air-conditioning and magnetic refrigeration.     

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).     

Magnetic properties, crystal and electronic structure of GdNi5−xCux series

The effect of Ni/Cu substitution on the magnetic properties, crystal and electronic structure of the polycrystalline GdNi_5−xCu_x series has been studied. All compounds crystallize in the hexagonal CaCu5 type of crystal structure (space group P6/mmm). The temperature dependence of magnetic phase transition (T_mag) estimated from χ_AC(T) susceptibility as well as magnetization M(T) below room temperature indicates the maximum for x = 1.0 copper concentration. In the paramagnetic range (above 300 K) the magnetic susceptibilities follow a Curie-Weiss-type dependence. The effective magnetic moments are higher than theoretical value for free Gd³⁺.From X-ray photoelectron spectroscopy (XPS) data the valence band as well as the core level spectra have been analyzed. The filling of Ni3d band in the GdNi_5−xCu_x system by charge transfer of Gd conduction electrons is revealed by a reduction of the satellite intensities in the Ni2p core level spectrum. The obtained results exhibit that the valence bands at the Fermi level are dominated by hybridized Ni3d and Gd5d states, when Cu3d states are rather localized about 3 eV below the Fermi level. Quite good relation between the magnetic properties and electronic structure has been found.     

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.     

Structure and thermal behavior of multicomponent Fe68−xNixZr15Nb5B12 (x = 5, 10, 15, 20) alloys

Multicomponent Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloy powders milled for 60 h were prepared by mechanical alloying (MA). The structure and crystallization behavior were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). Ni enhances the amorphisation of alloy powders. Particle size increases with increasing Ni content. Both onset crystallization temperature T_x and the first crystallization peak temperature T_p of the four alloys shift to a higher temperature with increasing heating rate while melting temperature (T_m) is just the opposite. Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloys all have a large supercooled liquid region ΔT_x. The supercooled liquid region ΔT_x increases and the crystallization activation energy E decreases with increasing Ni content.     

Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3

Superconductors Ba_1−xK_xBiO₃ and body-centered double perovskites Ba_1−xK_xBi_1−yNa_yO₃ have been selectively synthesized by a facile hydrothermal route. The appropriate ratio and adding sequence of initial reagents, alkalinity, reaction temperature and time are the critical factors that influence the crystal growth of the compounds. The purity and homogeneity of the crystals were detected by the ICP, SEM, EDX and TEM studies. Magnetic measurements show that the superconducting transition temperatures T_C of Ba_1−xK_xBiO₃ decrease from 22 K (for x = 0.35) to 8 K (for x = 0.55) with increasing the K doping level.