Thermoelectric properties of Cr1−xMoxSi2

The thermoelectric properties of Mo-substituted CrSi₂ were studied. Dense polycrystalline samples of Mo-substituted hexagonal C40 phase Cr_1−xMo_xSi₂ (x=0–0.30) were fabricated by arc melting followed by spark plasma sintering. Mo substitution substantially increases the carrier concentration. The lattice thermal conductivity of CrSi₂ at room temperature was reduced from 9.0 to 4.5 W m⁻¹ K⁻¹ by Mo substitution due to enhanced phonon–impurity scattering. The thermoelectric figure of merit, ZT, increases with increasing Mo content because of the reduced lattice thermal conductivity. The maximum ZT value obtained in the present study was 0.23 at 800 K, which was observed for the sample with x=0.30. This value is significantly greater than that of undoped CrSi₂ (ZT=0.13).     

Thermoelectric properties of the Nd2−x Ce x CuO4−y system

The temperature dependence of the Seebeck coefficient (in region 300–900 K) and the lattice constants of Nd_2–x Ce_xCuO_4–y (x=0, 0.05, 0.10, 0.15) with different oxygen content were measured. The value of S is always negative and it decreases with both Ce content and oxygen non-stoichiometry. At a certain concentration of Ce and oxygen in the material, the Seebeck coefficient becomes temperature-independent.The authors would like to thank Professor J. Horák of University of Chemical Technology, Pardubice for helpful discussions.     

Properties of Cu2ZnSn(SxSe1−x)4 thin films prepared by one-step sulfo-selenization of alloyed metal precursors

The pentenary system, Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe), is a promising alternative for thin film solar cells. In this study, CZTSSe thin films were prepared using a two-stage process involving the thermal diffusion of sulfur (S) and selenium (Se) vapors into sputtered metallic precursors at approximately 450 °C. The effects of the sulfur content on the composition, structure, optical and electrical characteristics of the CZTSSe thin films were investigated. The films showed a kesterite structure with a predominant (112) orientation. X-ray diffraction and Raman spectroscopy confirmed the formation of a single phase CZTSSe compound. The band gap was dependent on the sulfur content and was calculated to be 1.25 eV, 1.33 eV and 1.40 eV for CZTSSe films with a S/(S + Se) ratio of 0.3, 0.5 and 0.7, respectively. All films exhibited p-type semiconductor properties.     

Effects of Poly(Propylene Oxide)–Poly(Ethylene Oxide)–Poly(Propylene Oxide) Triblock Copolymer on the Gelation of Poly(Ethylene Oxide)–Poly(Propylene Oxide)–Poly(Ethylene Oxide) Aqueous Solutions

Poly(ethylene oxide)-poly(propylene oxide)–poly(ethylene oxide) ((EO)_n–(PO)_m–(EO)_n) block copolymers, commercially available as Pluronics (BASF Corp.) and Poloxamers (ICI Corp.), have been widely applied in medicine, biochemistry, and other fields because of their ability to form reversible micelles and physical gels in aqueous solution. Generally, for PEO–PPO–PEO block copolymers with higher ethylene oxide concentration, the micellization and gelation in aqueous solution are easier. However, if we introduce the reverse block copolymer PPO–PEO–PPO into PEO–PPO–PEO aqueous solutions, the micellization and gelation of the system will be more complex. In this work, the reverse block copolymer PO₁₄–EO₂₄–PO₁₄ (17R4) was added to the Pluronics EO₂₀–PO₇₀–EO₂₀ (P123), EO₁₀₀–PO₆₅–EO₁₀₀ (F127), and EO₁₃₃–PO₅₀–EO₁₃₃ (F108) aqueous solutions with different molar ratios. The rheological properties of different mixtures were measured to study the additive effect on the gelation behavior. The sol–gel transition temperature of the P123, F127, and F108 solutions shifted to a higher temperature when 17R4 was added to the solutions. In addition, the existence of 17R4 greatly affected the stability of gels. These results help to better understand the gelation of Pluronic aqueous solutions.     

Thermoelectric properties of Ca3−xYxCo4O9+δ ceramics

The Ca_3?xY_xCo₄O_9+δ (x=0, 0.15, 0.3) ceramics were prepared by combining the polyacrylamide gel method and the spark plasma sinter (SPS) technology in order to improve the thermoelectric properties of Ca₃Co₄O_9+δ ceramics. The Seebeck coefficients and the resistivities of the Y-doped samples were obviously enhanced due to the decrease of carrier concentration, and their thermal conductivities were decreased due to the impurity scattering effect. The thermoelectric properties were improved at high temperature by Y-doping according to the power factor analysis and the thermoelectric figure of merit (ZT) data. The optimized figure of merit ZT=0.22 at 973 K was obtained for Ca_2.7Y_0.3Co₄O_9+δ.     

Spectral Properties of CuGaxIn1−xSe2 Solid Solutions

We have investigated the absorption spectra in the region of the fundamental absorption band edge and the photoluminescence spectra of single crystals of the ternary compounds CuInSe₂ and CuGaS₂, and of solid solutions based on them that were grown by the method of chemical transport reaction. The forbidden band width and the positions of the maxima of the radiation bands are determined. The concentration dependences of the indicated quantities are plotted and they are shown to be linear.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Properties of RF sputtered ZnxCd1−xS thin films

Zn _x Cd_1–x S thin films (0x0.20) were prepared using rf sputtering in argon atmosphere and characterized using X-ray diffraction, optical transmission, electrical resistivity and photoconductive decay measurements. The films were found to possess hexagonal structure. The crystallite size and degree of preferential orientation were found to decrease with the increase ofx and to improve upon annealing in vacuum at 250 °C. The transmission edge shifted towards shorter wavelengths with the increase ofx in agreement with the expected shift in the energy band gap. The films were found to exhibit room temperature resistivity in the range 100–1000 cm. The obtained values of long wavelength transmission (70–80%) and minority carrier diffusion length (30 m) are high enough for the application of these films in the field of solar cells.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Far-infrared spectroscopy of CdTe1−xSex(In): Phonon properties

The far-infrared reflectivity spectra of CdTe_0.97Se_0.03 and CdTe_0.97Se_0.03(In) single crystals were measured at different temperatures. The analysis of the far-infrared spectra was carried out by a fitting procedure based on the dielectric function which includes spatial distribution of free carriers as well as their influence on the plasmon–phonon interaction. We found that the long wavelength optical phonon modes of CdTe_1−xSe_x mixed crystals exhibit a two-mode behavior. The local In mode at about 160 cm⁻¹ is observed. In both sample, a surface layer with a low concentration of free carriers (depleted region) are formed.     

Synthesis and luminescent properties of Na5La1−xEux(WO4)4 red phosphors

A series of Na₅La_1?xEu_x(WO₄)₄ (x=0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0) phosphors have been synthesized successfully by a solid-state reaction technique and characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Rietveld refinement. The photoluminescent properties have also been systematically investigated under the near-ultraviolet light. High concentration of doped Eu³⁺ enhanced the UV absorption and consequently red emission without concentration quenching. The Na₅Eu(WO₄)₄ phosphor was observed a strong red emission under 395 nm excitation, bearing with 47% quantum efficiency and the CIE chromaticity coordinates (x=0.67, y=0.33) being close to the National Television Standard Committee (NTSC) standard values. The primary studies indicate that Na₅Eu(WO₄)₄ is a promising candidate as the red phosphor for white light-emitting diodes (LEDs).     

Thermoelectric power of Ba(Fe1−x Co x )2As2 (0 ≤ x ≤ 0.05) and Ba(Fe1−x Rh x )2As2 (0 ≤ x ≤ 0.171)

Temperature-dependent, in-plane, thermoelectric power data are presented for single crystals of Ba(Fe_1?x Co _x )₂As₂ (0?≤?x?≤?0.05) and Ba(Fe_1?x Rh _x )₂As₂ (0?≤?x?≤?0.171). Given that previous thermoelectric power and angle resolved photoemission spectroscopy studies of Ba(Fe_1?x Co _x )₂As₂ delineated a rather large Co-concentration range for Lifshitz transitions to occur, and the underdoped side of the phase diagram is poorly explored, new measurements of thermoelectric power on tightly spaced concentrations of Co, 0?≤?x?≤?0.05, were carried out. The data suggest evidence of a Lifshitz transition, but instead of a discontinuous jump in thermoelectric power in the range 0?≤?x?≤?0.05, a more gradual evolution in the S(T) plots as x is increased was observed. The thermoelectric power data of Ba(Fe_1?x Rh _x )₂As₂ show very similar behavior to that of Co substituted BaFe₂As₂. The previously outlined T–x phase diagrams for both systems are further confirmed by these thermoelectric power data.     

An−1(1) reflection K-matrices

We investigate the possible regular solutions of the boundary Yang–Baxter equation for the vertex models associated with the A_n−1⁽¹⁾ affine Lie algebra. We have classified them in two classes of solutions. The first class consists of n(n−1)/2 K-matrix solutions with three free parameters. The second class are solutions that depend on the parity of n. For n odd there exist n reflection K-matrices with 2+[n/2] free parameters. It turns out that for n even there exist n/2 K-matrices with 2+n/2 free parameters and n/2 K-matrices with 1+n/2 free parameters.     

Vibrational and Light-Emitting Properties of Si/Si1−xSnx Heterostructures

Multilayer heterostructures Si/Si_1−xSn_x grown by molecular beam epitaxy on Si (001) substrates have been studied by Raman and photoluminescence spectroscopy. Raman spectra exhibit peaks corresponding to the vibrations of Si-Sn and Sn-Sn bonds; the vibrations of Sn-Sn bonds imply the presence of tin nanocrystals in heterostructures. Two photoluminescence bands at 0.75 eV (1650 nm) and 0.65 eV (1900 nm) have been observed in heterostructures at low temperatures. The former band can be attributed to optical transitions in quantum wells in the type II heterostructure Si/Si_1−xSn_x. The latter band can be associated with excitons localized in tin nanocrystals.

     

Spectromicroscopic investigation of (NH4)2S treated polycrystalline Cu(In1−xGax)Se2

Device-grade polycrystalline thin-film Cu(In_1−xGa_x)Se₂ was treated with (NH₄)₂S at 60°C to determine the resulting microscopic surface composition/morphology. Scanning electron microscopy was used to evaluate the resultant macroscopic surface morphology. Modification of the surface and grain boundary chemistry of the Cu(In_1−xGa_x)Se₂ polycrystalline films was investigated with scanning photoemission spectromicroscopy. The submicrometer lateral resolution of this technique allows us to directly characterize not only the surface chemistry of the treated films on the submicron scale, but also to probe the grain boundary chemistry. Chemical maps depicting the distribution of chemical species on the surface and at grain boundaries were obtained by monitoring the S 2p, Se 3d, In 4d/Ga 3d and Cu 3d (valence band) photoelectrons while scanning the sample. Background maps were also acquired of each of the peak energies to separate chemical contrast from topographic contrast. Results show that S has been incorporated at the surface, possibly creating a wider bandgap Cu(In_1−xGa_x)(Se_1−yS_y)₂ surface layer, and along the grain boundaries. The purpose of this investigation is to find an environmentally safe replacement for the toxic CdS overlayer commonly used for heterojunction devices without sacrificing overall device performance and reliability.     

Synthesis,microstructure and dielectric properties of (1−x)PSN−xPLuN

Ceramic lead niobates and their solid solutions PSN–PLuN (pure lutecium niobate) were synthesized by solid state reactions. The sequence of phases formed at PSN–PLuN synthesis has been studied by X-ray analysis. Their symmetry changed from rhombohedral for PSN to pseudo-monoclinic for the 0.75PSN–0.25PLuN compositions. The performed EDS investigations revealed that the samples PSN–PLuN are perfectly sintered. They contain a little glassy phase and their grains are well shaped. The increase of lutecium content in the examined solid solution caused downward shift of the temperature of the phase transition. The decrease of the achieved permittivity values ? was observed as well.     

X-ray photoelectron spectroscopy analysis of (Ln1−xSrx)CoO3−δ (Ln: Pr,Nd and Sm)

The chemical states of the surface of (Ln_0.5Sr_0.5)CoO_3?δ (Ln (lanthanides) = Pr, Nd and Sm) used for cathode materials of intermediate temperature operating solid oxide fuel cells (IT-SOFCs) were investigated by X-ray photoelectron spectroscopy (XPS). Oxygen peaks comprised of lower binding energy (LBE) and higher binding energy (HBE) peaks from (Ln_0.5Sr_0.5)CoO_3?δ and Pr_0.3Sr_0.7CoO_3?δ (PSC37) showed that some merged oxygen peak behavior is a function of the Sr and lanthanide concentrations. By investigating the oxygen peaks, it was determined that more oxygen vacancies were generated on the surface of the cathodes when the lanthanides and Sr were substituted into perovskite oxides. When comparing the binding energies (BEs) of PSC37 with Pr_0.5Sr_0.5CoO_3?δ (PSC55), the LBE and HBE of the Sr peaks both increased when Sr was substituted at the A-site of a perovskite. Surface analysis of the Co peak on the surface of the cathode materials showed that the Co exists mainly as Co³⁺ and partially oxidized to Co⁴⁺ on the cathode materials. The partial existence of Co⁴⁺ can provide some polaron hopping providing electronic conduction for the solid oxide fuel cell.     

Solution of functional equations of restricted An−1(1) fused lattice models

Functional equations, in the form of fusion hierarchies, are studied for the transfer matrices of the fused restricted A_n−1⁽¹⁾ lattice models of Jimbo, Miwa and Okado. Specifically, these equations are solved analytically for the finite-size scaling spectra, central charges and some conformal weights. The results are obtained in terms of Rogers dilogarithm and correspond to coset conformal field theories based on the affine Lie algebra A_n−1⁽¹⁾ with GKO pair A_n−1⁽¹⁾ ⊕ A_n−1⁽¹⁾ ⊃ A_n−1⁽¹⁾.     

Properties of Hg1−xZnxSe (0.09 ⩽ x ⩽ 1.0) thin films prepared by flash evaporation technique

Flash evaporated mercury zinc selenide films are observed to grow as single phase ternary alloys of the type Hg_1−xZn_xSe in the composition range 0.09 ⩽ x ⩽ 1.0, with a f.c.c./sphalerite structure on substrates maintained at T_s between 30°C and 175°C. The grain size is observed to increase with increase in T_s for all compositions. The films are observed to have a direct optical band gap which increases from 0.01 to 2.60 eV as the Zn concentration x is varied between 0.09 and 1.0. The band gap V_scomposition shows a bowing, typical of pseudobinary solid solutions. Zn rich films were observed to be p-type whereas Hg-rich films were n-type. Room temperature resistivity was observed to increase with Zn concentration x, which can be attributed to the increase in the band gap of the semiconductors. Higher resistivity in films deposited at high substrate temperatures is due to the decrease in contribution to conduction from the highly conducting grain boundaries.     

Low-temperature heat capacity of ErxY1−xRh4B4

Resistivity and specific heat data have been used to establish the presence of antiferromagnetic transitions in the Er_xY_1−xRh₄B₄ system for concentrations x=0.1 and 0.3. The relationship of these transitions to the general magnetic and superconducting phase diagram for this pseudoternary system is discussed.