Crystal structure and magnetic properties of the compound FeDy6Sb2

The crystal structure of a compound FeDy₆Sb₂ was determined by X-ray powder diffraction using the Rietveld method. The compound crystallizes in the hexagonal, space group P6¯2m (No. 189) with the Fe₂P structure type and lattice parameters a = 0.81449(5) nm, c = 0.41641(3) nm, z = 1 and D_calc = 8.842 g/cm³. The maximum magnetic entropy change ΔS_M for the compound is 3.41 J kg^− 1 K^− 1 near its Curie temperature (143.4 K) on the magnetic field changes of 0-2.0 T.     

Lattice thermal expansion studies of V2GeO4F2: A topaz type oxide-fluoride

A new compound V₂GeO₄F₂ was earlier found to exist in the V₂O₃-VF₃-GeO₂ system and the structure elucidation revealed it to be iso-structural to the mineral topaz. Herein, we report the lattice thermal expansion data of this compound. The lattice thermal expansion of V₂GeO₄F₂ was studied in the temperature range of 298-873 K under a flowing helium atmosphere by the high temperature XRD (HTXRD). The coefficients of axial thermal expansions of V₂GeO₄F₂ were found to be as: α_a = 3.5 × 10⁻⁶, α_b = 6.1 × 10⁻⁶ and α_c = 7.6 × 10⁻⁶ K⁻¹. The coefficient of volume (α_V) thermal expansion was 17.3 × 10⁻⁶ K⁻¹, which is relatively low compared to many analogues silicates.     

Thermal and mechanical properties of novel substrate crystal Mg0.4Al2.4O4

Mg_0.4Al_2.4O₄ single crystal was grown by the Czochralski method. The measured specific heat values are 0.804-1.06 J g^− 1 K^− 1 in the temperature range from 298.15 to 573.15 K. The calculated thermal conductivity components are 11.37, 11.47 and 10.77 W m^− 1 K^− 1 along the [111], [004] and [22?0] direction at 298.15 K. The Vickers microhardness values are 1328-1414 kg mm^− 2. These experimental results show that Mg_0.4Al_2.4O₄ crystal is a promising substrate for GaN-based LEDs.     

Study of GeYSi1 − Y:H films deposited by low frequency plasma

In this work, we report a study of the optical properties measured through spectral transmission and spectroscopic ellipsometry in Ge:H and Ge_YSi_1 − Y:H (Y ≈ 0.97) films deposited by low frequency (LF) PE CVD with hydrogen (H) dilution. The dilution was varied in the range of R = 20 to 80. It was observed that H-dilution influences in a different way on the interface and bulk optical properties, which also depend on incorporation of silicon. The films with low band tail characterized by its Urbach energy, E_U, and defect absorption, α_D, have been obtained in Ge:H films for R = 50 with E_U = 0.040 eV and α_D = 2 × 10³ cm^− 1 (hν ≈ 1.04 eV), and in Ge_YSi_1 − Y:H films for R=75 with E_U = 0.030 eV and α_D = 5 × 10² cm^− 1 (hν ≈ 1.04 eV).     

Enhanced thermoelectric properties of NaCo2O4 by adding ZnO

The primary phase present in the as-sintered Na(Co_1 − xZn_x)₂O₄ (0 ≤ x ≤ 0.1) bodies was the solid solution of the constituent oxides with a bronze-type layered structure. The electrical conductivity of the Na(Co_1 − xZn_x)₂O₄ samples significantly increased with an increase in ZnO content. The sign of the Seebeck coefficient for all samples was positive over the whole temperature range (723-1073 K), i.e., p-type conduction. The power factor of Na(Co_0.95Zn_0.05)₂O₄ showed an outstanding power factor (1.7 × 10^− 3Wm^− 1 K^− 2) at 1073 K. The power factor was above four times superior to that of ZnO-free NaCo₂O₄ (0.4 × 10^− 3Wm^− 1 K^− 2). This originates from an unusually large Seebeck coefficient (415 μVK^− 1) accompanied with high conductivity (127Ω^− 1 cm^− 1) at 1073 K.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

High temperature thermoelectric properties of (Ca, Ce)4Mn3O10

Thermoelectric (TE) properties such as resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of Ca_4−3xCe_3xMn₃O₁₀ (0<x≤0.03) polycrystalline samples were measured from room temperature to 1000 K. ρ shows an obvious decrease with the increment of Ce content. The hopping conduction mechanism is used to explain the conduction behavior of these samples. The negative S values indicate that these materials are n-type. The sample of x=0.03 has the largest power factor, 0.52×10⁻⁴ Wm⁻¹ K⁻² at 1000 K. The value of κ and the dimensionless figure of merit of this sample is 1.51 Wm⁻¹ K⁻¹ and 0.034 at 1000 K, respectively.     

The first bulk synthesis of ReO3-type tungsten trioxide, WO3, from nanometric precursors

A perovskite form of WO₃ has been synthesized in bulk for the first time at 0.66 GPa and 973 K with a=3.7823(4) Å [a₀=3.7719(4) Å, at ambient conditions] from nanometric powder of WO₃ with an average crystallite size of 35 nm. Data collected during tests to determine both the likelihood of retaining the structure at room temperature and the effect of high pressure on distortion have afforded analysis of thermal expansivity and compressibility of this phase. These result in V_T=53.407(5)exp(−3.9(12)×10⁻⁶(T−298)+1.91(9)×10⁻⁸(T²−298²)) Å³ and equation of state parameters of V₀=53.67(4) Å³, K₀=41.8(19) GPa with ∂K/∂P=K′=5.6(12).     

Magnetocaloric properties in Ln0.67Ba0.33Mn1 − xFexO3 (Ln = La or Pr) manganites

We have investigated the magnetocaloric properties of Ln_0.67Ba_0.33Mn_1 − xFe_xO₃ (Ln = La or Pr) manganites with x = 0 and 0.05. All compounds present a maximum and large magnetocaloric effect near the Curie temperature (T_C). The associated maximum value of the magnetic entropy change, at 5 T applied change in the magnetic field, is 4.37 J.kg^− 1.K^− 1 for Pr_0.67Ba_0.33MnO₃ manganite with a T_C value of 205 K. The corresponding relative cooling power (RCP) reaches 230 J.kg^− 1. All the samples present similar RCP values that are relatively high and are promising materials to be used in ecologically friendly magnetic refrigeration technology. Iron doping reduces both T_C and ΔS_M^max and spreads the temperature working range with an almost constant RCP and can then be used to tune the working conditions of a refrigerator device.     

Improved thermoelectric properties by adding Al for Zn in (ZnO)mIn2O3

The grain size and density of the sintered (Zn_1 − xAl_xO)_mIn₂O₃ bodies decreased with the small Al₂O₃ content (≤ 0.012), and then increased gradually by further increasing the Al₂O₃ content. The addition of Al for Zn in the (ZnO)_mIn₂O₃ led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient. This indicates that the power factor was significantly enhanced by adding Al for Zn. The thermoelectric power factor was maximized to 1.67 × 10^− 3 W m^− 1 K^− 2 at 1073 K for the (Zn_0.992Al_0.008O)_mIn₂O₃ sample.     

Synthesis of three dimensional photoluminescent [In10S20 − xSex] supertetrahedral clusters: (x < 0.3)

Two strong solid state luminescence peaks, centered at 457 nm and 538 nm, were observed from the newly synthesized ternary [In₁₀S_20 − xSe_x]¹⁰⁻ T3 clusters, under excitation by UV radiation (λ = 360 nm). The 457 nm peak is due to the porosity property of T3 clusters. Nevertheless, the 538 nm peak has not been reported for the T3 clusters before. In this letter, we demonstrate that the 538 nm peak is attributed to the trace Se atoms confined in the [In₁₀S_20 − xSe_x]¹⁰⁻ clusters. The luminescent output from the ternary [In₁₀S_20 − xSe_x]¹⁰⁻ T3 clusters is independent of temperature from 298 K until 77 K.     

Molten salt synthesis and thermoelectric properties of Ca2Co2O5

A molten salt method was applied to synthesize Ca₂Co₂O₅ powder by using Co₃O₄ and CaCO₃ as raw materials and CaCO₃-CaCl₂ as eutectic salt. The formation process and molten salt mechanism of Ca₂Co₂O₅ were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that sheet-like particles can be obtained at 963 K. Moreover, the electrical conductivity and the Seebeck coefficient of the specimen were measured from 323 to 963 K. The properties increased with increasing temperature and the highest value of power factor (2.0 × 10^− 4 W m^− 1 K^− 2) was obtained at 963 K, which indicates that molten salt synthesis is a promising method to prepare high thermoelectric performance material.     

Exceptionally large third-order optical susceptibility in Ag:SrBi2Nb2O9 composite films

Thin films of SrBi₂Nb₂O₉ with and without the inclusion of Ag nanocrystals were prepared on MgO (001) substrates by alternative pulsed laser ablation of SrBi₂Nb₂O₉ ceramic and Ag metal in a nitrogen atmosphere. The size of the Ag nanocrystals can be brought down to ca. 5.0 nm, as revealed by transmission electron microscopy and X-ray diffraction. We measured the third-order susceptibility χ⁽³⁾ of the deposits by single beam z-scan technique at a wavelength of 532 nm. It is found that SrBi₂Nb₂O₉ is an effectively nonlinear optical medium, exhibiting a very large χ⁽³⁾. With the inclusion of Ag nanocrystals the χ⁽³⁾ was nearly doubled (Reχ⁽³⁾ = 8.052 × 10^− 7 esu, and Imχ⁽³⁾ = − 1.717 × 10^− 7 esu), which comes very close to the maximum value available in the current time. This indicates that the Ag:SrBi₂Nb₂O₉ composite structure can be an excellent candidate for nonlinear optical applications. A general approach for further enhanced third-order susceptibility by tailoring the dielectric constant of the metal inclusion is briefly discussed.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Microstructure and thermoelectric properties of n-type 95%Bi2Te3-5%Bi2Se3 alloy produced by rapid solidification and hot extrusion

n-type SbI₃-doped 95%Bi₂Te₃+5%Bi₂Se₃ compounds were prepared by a rapid solidification and extrusion at the temperature range 420-480 °C using an extrusion ratio of 25:1. The microstructure and thermoelectric properties of the compounds were investigated as a function of extrusion temperature. The fabricated powder consists of homogeneous Bi₂Te₃+Bi₂Se₃ solid solution and the relative density of over 99% was obtained by hot extrusion. The values of Seebeck coefficient for the compounds hot extruded at 420, 450, and 480 °C were −160.8, −170.2, and −165.7 μV K⁻¹, respectively. The values of electrical resistivity (ρ) for the compounds hot extruded at 420, 450, and 480 °C were 0.49, 0.57, and 0.51×10⁻⁵ Ω m, respectively. The maximum power factor value of the compounds hot extruded at 480 °C was 53.8×10⁶ μW cm⁻¹ K⁻².     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.     

Third-order nonlinear optical properties of Bi2S3 nanocrystals doped in sodium borosilicate glass studied with Z-scan technique

The third-order nonlinear optical properties of Bi₂S₃ nanocrystals doped in sodium borosilicate glass are measured by Z-scan technique. The microstructures of the glass are characterized by means of X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy, energy dispersion X-ray spectra, and high-resolution transmission electron microscopy. The results show that the Bi₂S₃ nanocrystals ranging from 10 to 30 nm are determined to be of the orthorhombic crystalline phase, and the third-order optical nonlinear refractive index γ, absorption coefficient β, and susceptibility χ⁽³⁾ of the glass are determined to be 2.56 × 10⁻¹⁶ m² W⁻¹, 4.13 × 10⁻¹⁰ mW⁻¹, and 1.43 × 10⁻¹⁰ esu, respectively.     

Nanosized La filled CoSb3 prepared by a solvothermal-annealing method

Nanostructuring is one of the effective approaches to lower the thermal conductivity of materials. Nanosized La filled CoSb₃ was successfully synthesized by a solvothermal-annealing method. It shows that the La atoms were filled into the lattice cages during the annealing process. The average size of the powders after annealing process is about 20 nm, a little larger than the solvothermally prepared powders. The bulk sample was prepared by hot pressing. Electrical properties measurements were performed and the highest power factor is about 0.43 × 10^− 3 Wm^− 1K^− 2 obtained at 520 K.     

Growth and properties of Ca0.28Ba0.72Nb2O6 single crystals

Calcium barium niobate Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystals were successfully grown by the Czochralski method. X-ray powder diffraction experiments indicated that CBN single crystals are tetragonal with a = 12.432(±0.002) Å and c = 3.957(±0.001) Å, which have almost the same structure as the Sr_0.50Ba_0.50Nb₂O₆ (SBN-50) crystal. The thermal expansion coefficient perpendicular to Z-direction had been measured to be 1.25 × 10⁻⁵ K⁻¹ between 293.15 and 572.15 K, and along Z-axis was negative between 298.15 and 543.15 K. The specific heat of the crystal had been measured by the differential scanning calorimetric experiments. The transmittance spectra from 200 to 3200 nm were also measured. The measured temperature dependence of dielectric constants showed that the Curie temperature of the CBN-28 crystals is 260 °C, which is about 200 °C higher than that of the (SBN) crystal.