Fabrication and thermoelectric properties of highly textured NaCo2O4 ceramic

Highly textured NaCo₂O₄ polycrystalline sample was fabricated by means of the cold high-pressure compacting followed by the solid-state reaction. X-ray diffraction and scanning electron microscope were employed to show that the plate-like grains within the sample are aligned along the pressing direction. The resistivity ρ and thermoelectric power S along the preferred {0 0 1} plane were measured in the whole temperature range from 15 to 973 K in air and the correlation between thermoelectric properties and texture was investigated. It was found that both ρ and S exhibit metallic behavior in the whole temperature range and the above sample exhibits lower ρ and higher S due to high texture and density. The power factor exhibits a steep rise above 400 K and reaches 761 μW m⁻¹ K⁻² at 973 K, suggesting a promising candidate for thermoelectric application at higher temperature. The change of slope in both resistivity and thermoelectric power curves at about 450 K might arise from the spin-state transition of Co ions in the CoO₂ blocks.     

Crystal structure and properties of U2Co3Al9

A new ternary compound with stoichiometry U₂Co₃Al₉ has been synthesized. It adopts the orthorhombic Y₂Co₃Ga₉-type structure (space group Cmcm, Z = 4, a = 12.824(2) Å, b = 7.515(1) Å, c = 9.249(2) Å). Measurements of dc- and ac-magnetic susceptibility, electrical resistivity, and magnetoresistivity on polycrystalline samples have been performed. The Curie–Weiss law is strictly followed, with θ_CW = −48 K and μ_eff = 3.2 μ_B. A small kink observed in the temperature dependence of the resistivity is attributed to a phase transition at T_t = 8 K. The magnetoresistivity was found to be negative at all temperatures examined below 45 K, with a sharp minimum at T_t = 8 K.     

Microstructure and mechanical properties of MoSi2/TaSi2 two-phase alloys

Effects of pre-strain on the compressive mechanical behavior were investigated on the alloys with an aligned lamellar microstructure consisting of C11_b MoSi₂ and C40 TaSi₂ prepared through optical floating zone (OFZ) method and annealing in the two-phase MoSi₂/TaSi₂ region. Single crystals of C40 TaSi₂ were successfully grown at solidification rate of 5 or 10 mm/h, and well-aligned lamellar microstructure can be achieved by selecting an MoSi₂–17 mol% TaSi₂ alloy. Firstly, compression tests were conducted to examine the effect of the angle ϕ between the aligned lamellae and the loading axis on strength and ductility. Results indicate that ϕ = 0, lamellae parallel to the axis, is a hard-orientation and ϕ = 54 and 40 (identically 45) are soft orientations. The alloy with ϕ = 0 shows higher strength but lower ductility than those with ϕ = 54 and 40 where ductility is evaluated in terms of brittle-to-ductile transition temperature (BDTT). Then effects of pre-straining at 1773 K on the mechanical behavior at 1573 K were investigated using soft-oriented lamellar alloys with ϕ = 40 whose BDTT is determined at least lower than 1673 K. Pre-straining to a few to several percent at 1773 K improves ductility of the alloy at 1573 K and also raises 0.2% flow stress, compared with the absence of the pre-strain. We believe that dislocations can be generated and stored in the alloy at 1773 K, and these dislocations are mobile even at 1573 K, although the analyses of operative slip systems and characteristics of dislocations remain as future works.     

The structure and magnetic properties of Th4Mn13Sn5

We have synthesized the new intermetallic compound Th₄Mn₁₃Sn₅: it crystallizes with the Th₄Fe₁₃Sn₅ type structure, tetragonal tP44, space group P4/mbm, a = 8.423(1) Å and c = 11.972(2) Å. The Mn ions in Th₄Mn₁₃Sn₅ order ferro/ferrimagnetically around 110 K. A low magnetization of 0.13 μ_B/Mn at 4.2 K in a field of 70 kOe and a large fractional temperature independent contribution to susceptibility in the paramagnetic regime suggest an appreciable itinerancy of the Mn 3-d state in this compound. The electrical resistivity shows an anomaly at the magnetic transition and varies as T² below 30 K with a coefficient of 8.38 × 10^?3 μΩ cm/K². The low temperature heat capacity data furnish a Sommerfeld coefficient of 16.9 mJ/g atom K². In the course of present work we have also identified the ThMn_0.2Sn₂ phase which adopts a defective CeNiSi₂ type structure, orthorhombic oS16, space group Cmcm with a = 4.476(2) Å, b = 17.059(3) Å and c = 4.398(1) Å.     

Shape memory behavior and internal structure of Ti–Ni–Cu shape memory alloy thin films and their application for microactuators

Internal structure and shape memory behavior of Ti–38.3Ni–9.3Cu (at.%) thin films heat-treated at 873 K, 923 K, 973 K and 1023 K were investigated by TEM observation and thermal cycling tests under various constant stresses. The thin film heat-treated at 873 K contained two types of precipitates, i.e., fine platelets and Ti₂Ni particles. The density of the platelets decreased with increasing heat-treatment temperature and annihilated completely when the heat-treatment temperature reached 973 K. The Ti₂Ni precipitates increased in volume fraction with increasing heat-treatment temperature from 873 K to 923 K, then their volume fraction was almost kept constant above 923 K. The recoverable strain decreased and the M_s increased with increasing heat-treatment temperature from 873 K to 923 K. Both the recoverable strain and the M_s became almost constant when the heat-treatment temperature was above 923 K. A diaphragm-type microactuator utilizing a Ti–38.0Ni–10.0Cu (at.%) thin film was fabricated. The diaphragm was square with the width of 500 μm. The actuation properties were investigated under conditions of both quasi-static and dynamic actuation. The M_s and the transformation temperature hysteresis of the microactuator were determined to be 352 K and 6 K, respectively. The microactuator operated at a working frequency of 100 Hz.     

Structure of a new ternary compound with high magnesium content,so-called Gd13Ni9Mg78

The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h^?1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h^?1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a¹ and b¹ axis with vectors of modulation q1 = 0.42a¹ and q2 = 0.42b¹ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.     

High-temperature thermoelectric properties of Sr2RuYO6 and Sr2RuErO6 double perovskites influenced by structure and microstructure

The high-temperature thermoelectric properties of Sr₂RuYO₆ and Sr₂RuErO₆ double perovskites were evaluated and reported for the first time. These compounds show high Seebeck coefficients not only at room temperature, but also at high temperature (for Sr₂RuYO₆, S_RT ≈ ?475 μV K^?1 and S_1200K ≈ ?250 μV K^?1; Sr₂RuErO₆, S_RT ≈ ?400 μV K^?1 and S_1200K ≈ ?250 μV K^?1). The n-type semiconducting behaviour dominates the resistivity values. Both compounds crystallize in a monoclinic unit cell (space group P2₁/n). The lattice parameters are a = 5.7761(2), b = 5.7804(1), c = 8.1689(1), α = γ = 90° and β = 90.2087(8)° for the Sr₂RuYO₆, and a = 5.7760(1), b = 5.7722(0), c = 8.1544(4), α = γ = 90° and β = 90.2099(7)° for Sr₂RuErO₆. The unit cell can be described approximately as √2a_p × √2a_p × 2a_p, where a_p is the unit cell parameter of the ideal cubic perovskite structure. High-resolution transmission electron microscopy shows an interesting three-dimensional micro-twin-domain texture where the c axis is placed in the three space directions. Structural transitions at high temperatures (T_t(Sr₂RuYO₆) ≈920 K and T_t(Sr₂RuErO₆) ≈890 K) are observed by specific heat measurement in both compounds, which are found to have a strong influence on the Seebeck coefficient and electrical conductivity.     

Texture and orientation characteristics of α-Al2O3 films prepared by laser chemical vapor deposition using Nd:YAG laser

α-Al₂O₃ films were prepared by laser chemical vapor deposition (LCVD) and the effects of precursor vaporization temperature (T_vap), total chamber pressure (P_tot), laser power (P_L) and deposition temperature (T_dep) on the phase, orientation and texture of Al₂O₃ film were investigated. At P_tot = 0.93 kPa, α-Al₂O₃ films were obtained in the region of T_vap > 423 K and T_dep > 1100 K. The orientation of α-Al₂O₃ film changed from (1 1 0) to (0 1 2) to (1 0 4) to (0 0 6) with increasing P_tot. Porous α-Al₂O₃ films were formed at high T_vap (443 K) and low P_tot (0.47 kPa). At T_vap = 413 K, α-Al₂O₃ film had hexagonal and rectangular plate-like grains with finely faceted edges. With increasing P_tot = 0.93–1.4 kPa, (0 0 6)-oriented α-Al₂O₃ film with a hexagonal terrace texture was obtained.     

Thermoelectric properties of Sb-doped Mg2Si semiconductors

The thermoelectric properties of Sb-doped Mg₂Si (Mg₂Si:Sb = 1:x(0.001 ≦ x ≦ 0.02)) fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. Sb-doped Mg₂Si samples are n-type in the measured temperature range. The electron concentration of Sb-doped Mg₂Si at 300 K ranges from 2.2 × 10¹⁹ for the Sb concentration, where x = 0.001, to 1.5 × 10²⁰ cm⁻³ for x = 0.02. First-principles calculation revealed that Sb atoms are expected to be primarily located at the Si sites in Mg₂Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Sb concentration. The sample x = 0.02 shows a maximum value of the figure of merit ZT, which is 0.56 at 862 K.     

Thermoelectric properties of Ti1+xS2 prepared by CS2 sulfurization

Titanium disulfide (TiS₂) powder was prepared by sulfurizing TiO₂ powder with CS₂ gas at 1073 K for 4 h. Because CS₂ gas is a powerful sulfurizing agent for TiO₂, CS₂ sulfurization can be performed at a low temperature. The TiS₂ powder thus prepared was first mixed with sulfur powder in a mass ratio of 1:0.04 and pressure sintered at 973 K for 1 h under a uniaxial pressure of 50 MPa in vacuum. The addition of a small amount of sulfur powder to the TiS₂ powder prevents sulfur deficiency in the sintered compact, resulting in the formation of a near-stoichiometric Ti_1.008S₂ composition. X-ray diffraction patterns show that the crystalline ab-axis is preferentially oriented perpendicular to the pressing direction. The Seebeck coefficient, electrical resistivity and thermal conductivity of the oriented TiS₂ sintered compacts with near-stoichiometric and sulfur-poor (titanium-rich) compositions were measured in the temperature range 300–723 K. The thermoelectric figure of merit ZT was enhanced by prevention of sulfur deficiency and formation of the oriented texture. The highest ZT of 0.34 was observed at 663 K in Ti_1.008S₂ for the direction perpendicular to the pressing axis.     

Crystal structure and physical properties of a magnetic molecular conductor (EDO-TTFVODS)2FeCl4

Crystal structure, and electrical conducting and magnetic properties of a radical cation salt of EDO-TTFVODS with magnetic FeCl₄^? ion, (EDO-TTFVODS)₂FeCl₄ (EDO-TTFVODS = ethylenedioxytetrathiafulvalenoquinone-1,3-diselenolemethide) are reported. In this salt, there are two independent donor molecules formed two different layers A and B, and the counter FeCl₄^? ions layer is sandwiched between two donor layers A and B along the b-axis. The donor molecules form the one-dimensional columns along the a-axis in both donor layers. This salt shows high conductivity at room temperature (σ_RT = 25 S cm^?1) and a metallic behavior down to ca. 80 K, where a metal–insulator transition however occurs. The magnetic susceptibility obeys a Curie–Weiss law (Curie constant C = 4.42 emu K mol^?1 and Weiss temperature Θ = ?1.5 K), without any magnetic ordering down to 1.8 K. This result suggests the weak antiferromagnetic interaction between the d spins of FeCl₄^? ions.     

Neutron structural and vibrational studies of dipotassium selenate tellurate

At room temperature the potassium selenate tellurate K₂SeO₄·Te(OH)₆ (KSeTe) was prepared from water solution of H₆TeO₆ and K₂SeO₄. Its structure has been determined from single crystal using neutron diffraction data. KSeTe is monoclinic with C2/c space group. The unit cell parameters are: a = 11.572(9) Å, b = 6.437(7) Å, c = 13.938(1) Å, β = 106.07(7)°, V = 997.7(1) Å³ and Z = 4. The main feature of this structure is the presence of two different and independent anions (TeO₆⁶⁻ and SeO₄²⁻) in the same unit cell. The structure is built by layers of TeO₆ octahedra altering with planes of SeO₄ tetrahedra linked by a network of hydrogen bonds performed by protons belonging to the OH hydroxide groups. DSC measurements indicate that the crystals of K₂SeO₄·Te(OH)₆ undergo three endothermal peaks at 433, 480 and 495 K.Raman scattering measurements on KSeTe material taken between 300 and 620 K are reported in this paper. The spectra indicate clearly these phase transitions.     

Solidification behavior of γ′-Ni3Al-containing alloys in the Ni–Al–O system

The chemical activities of Al and Ni in γ′-Ni₃Al-containing alloys were measured using the multi-cell Knudsen effusion-cell mass spectrometry technique, over the composition range 8–32 at.% Al and temperature range T = 1400 to 1750 K. From these measurements a better understanding of the equilibrium solidification behavior of γ′-Ni₃Al-containing alloys in the Ni–Al–O system was established. Specifically, these measurements revealed that (i) γ′-Ni₃Al forms via the peritectiod reaction, γ + β (+Al₂O₃) = γ′ (+Al₂O₃), at 1633 ± 1 K; (ii) the {γ + β + Al₂O₃} phase field is stable over the temperature range 1633–1640 K; and (iii) equilibrium solidification occurs by the eutectic reaction, L (+Al₂O₃) = γ + β (+Al₂O₃), at 1640 ± 1 K and a liquid composition of 24.8 ± 0.2 at.% Al (at an unknown oxygen content). When projected onto the Ni–Al binary, this behavior is inconsistent with the current Ni–Al phase diagram and a new diagram is proposed. This new Ni–Al phase diagram explains a number of unusual steady-state solidification structures reported previously and provides a much simpler reaction scheme in the vicinity of the γ′-Ni₃Al phase field.     

Intermetallic compounds in the M–Cu–Sn systems with M = Eu,Sr, Ba

Several samples were prepared in the title systems, starting from the elements sealed under argon in Ta crucibles, melting in an induction fornace and annealing at 823 K. Six ternary phases were found: EuCu₂Sn₂, a = 11.100 (3), b = 4.307 (1), c = 4.824 (1) Å, β = 108.88 (1)°, and SrCu₂Sn₂, a = 11.197 (4), b = 4.322 (2), c = 4.859 (1) Å, β = 108.43 (1)°, C2/m, CaCu₂Sn₂-type, closely related to the BaAl₄ structure; Sr₃Cu₈Sn₄, a = 9.3280 (2), c = 7.8826 (4) Å, P6₃mc, Nd₃Co₈Sn₄-type, ordered variant of the BaLi₄-type; SrCu₄Sn₂, a = 8.176 (1), c = 7.799 (1) Å, I4/mcm, CaNi₄Sn₂-type; SrCu₉Sn₄, a = 8.663 (1), c = 12.457 (2) Å, and BaCu₉Sn₄, a = 8.717 (1), c = 12.545 (2) Å, I4/mcm, LaFe₉Si₄-type, ordered variant of the NaZn₁₃ structure. The structure of the first compound was refined by the Rietveld method, while single crystal data were used for the others. Some remarks are given on the crystal chemistry of the encountered and related structure types.     

Crystal structure of the new ternary germanide Tb4Zn5Ge6

The Tb₄Zn₅Ge₆ phase was prepared by arc melting in an argon atmosphere and then annealed at 670 K for 400 h. The structure of orthorhombic Tb₄Zn₅Ge₆ was determined by single-crystal X-ray diffraction (Cmc2₁, Z = 4, a = 4.2330(10) Å, b = 18.576(4) Å, c = 15.275(3) Å, R₁ = 0.0272 and wR₂ = 0.1076). The structure is isostructural to Gd₄Zn₅Ge₆ and composed of edge- and corner-sharing ZnGe₄ tetrahedra, Tb-atoms forms trigonal prisms filled by Ge-atom.     

Electrical,structural and magnetic properties of polyaniline/pTSA-TiO2 nanocomposites

Polyaniline (PANI)/para-toluene sulfonic acid (pTSA) and PANI/pTSA-TiO₂ composites were prepared using chemical method and characterized by infrared spectroscopy (IR), powder X-ray diffraction (XRD), scanning electron microscopy (SEM). The electrical conductivity and magnetic properties were also measured. In corroboration with XRD, the micrographs of SEM indicated the homogeneous dispersion of TiO₂ nanoparticles in bulk PANI/pTSA matrix. Conductivity of the PANI/pTSA-TiO₂ was higher than the PANI/pTSA, and the maximum conductivity obtained was 9.48 (S/cm) at 5 wt% of TiO₂. Using SQUID magnetometer, it was found that PANI/pTSA was either paramagnetic or weakly ferromagnetic from 300 K down to 5 K with H_C ≈ 30 Oe and M_r ≈ 0.015 emu/g. On the other hand, PANI/pTSA-TiO₂ was diamagnetic from 300 K down to about 50 K and below which it was weakly ferromagnetic. Furthermore, a nearly temperature-independent magnetization was observed in both the cases down to 50 K and below which the magnetization increased rapidly (a Curie like susceptibility was observed). The Pauli susceptibility (χ_pauli) was calculated to be about 4.8 × 10^?5 and 1.6 × 10^?5 emu g^?1 Oe^?1 K for PANI/pTSA and PANI/pTSA-TiO₂, respectively. The details of these investigations are presented and discussed in this paper.     

Complex magnetic properties of Ho3Cu4Sn4

The Ho₃Cu₄Sn₄ compound exhibits multiple magnetic transitions below its Néel temperature of 7.6 K. As the holmium ions occupy two different crystallographic positions (2d and 4e) a complex magnetic phase diagram was expected. Former neutron diffraction measurements revealed the existence of at least three different magnetic phases. In order to investigate the phase diagram of the Ho₃Cu₄Sn₄ compound in more detail magnetometric, specific heat, transport and high-resolution neutron diffraction measurements were carried out. The specific heat data indicate the occurrence of five magnetic transitions at temperatures of 2.3 K, 3.3 K, 4.4 K, 5.5 K and 7.6 K, which turned out to be in a strict agreement with our new neutron diffraction data. The 2d and 4e rare-earth sublattices order independently at 7.6 K and 3.3 K, respectively. The sublattices are described by different propagation vectors which change with the temperature.     

Electronic properties and crystal structures of RE3Rh2Ga2 and RE3Rh3Si2 (RE = La,Ce)

Polycrystalline samples of Ce₃Rh₂Ga₂, Ce₃Rh₃Si₂ and their La-based isostructural analogues were studied by means of magnetic susceptibility, magnetization and electrical resistivity measurements. The crystal structure of La₃Rh₃Si₂ (Ce₃Rh₃Si₂ type) was investigated by single-crystal X-ray diffraction. The cerium gallide was found to order ferromagnetically at T_C = 3.5 K, whereas for the silicide more complex magnetic behaviour was established with antiferromagnetic order setting at T_N = 6.5 K and subsequent change in the magnetic structure occurring at T_t = 5.5 K. Both cerium compounds exhibit weak Kondo effect. The L_III-edge XAS data indicated rather stable 4f¹ character of cerium in both compounds. The electronic band structures of Ce₃Rh₂Ga₂ and Ce₃Rh₃Si₂ were calculated by the LMTO method and compared with those of Ce₃Rh₂Ge₂ and La₃Rh₂Ge₂. For each Ce-based compound the total electronic DOS is dominated by a peak of Ce 4f states near the Fermi level and a Rh 4d band located about 2 eV below the Fermi energy.     

Martensitic transformation and thermal cycling effect in Cu–Co–Zr alloys

Cu_50?xCo_x Zr₅₀ (x = 0, 0.5, 1, 2, 3, 4 and 5 at.%) alloys were cast in a cylindrical copper mold by a suction casting device. In order to investigate the thermal behavior of the as-cast rods, the samples were heated from 313 to 573 K and then cooled down to about 253 K. The structure of the samples was studied by X-ray diffraction and optical microscopy. Thermal cycling measurements were also done for alloys with 2, 3, 4 and 5 at.% cobalt. It was found that increasing the cobalt content decreases martensite (M_s) and austenite (A_s) start temperatures, while it increases the temperature region in which austenite is stable. Thermal cycling measurements revealed that by increasing the number of cycles, the austenite start temperature increases while martensite start temperature shifts to lower temperatures.     

Wetting behavior of Al alloys on a TiH2 substrate

The wetting behavior of Al–Ge alloys on TiH₂ substrates was investigated by an improved sessile drop method under high vacuum and in a temperature range of 773–818 K. Results indicate that the equilibrium contact angles of Al–Ge/TiH₂ increase linearly with temperature according to the following formula: θ = 0.2882T ? 85.04, and decrease linearly as the Ge content increases from 25.2% to 36.2% according to the formula: θ = 214 ? 200Ge (wt.%). The worst wetting behavior was obtained for a pure Al/TiH₂ system at its foaming temperature (973 K). TiH₂ particles were prone to aggregate and were thus difficult to disperse. This could be one of the reasons for closed-cell aluminum foam products having non-uniform pores.