Anisotropic elastic constants and thermal expansivities in monocrystal CrB2, TiB2, and ZrB2

The elastic constants and thermal expansivities in monocrystals of three transition-metal diborides with the AlB₂ structure, CrB₂, TiB₂, and ZrB₂, have been investigated in the temperature ranges from 300 to 1373 K and from 300 to 1073 K. The anisotropic parameters deduced from the measured elastic constants and thermal expansivities indicate that of the three diborides, the anisotropy is the most and least significant in CrB₂ and ZrB₂, respectively. The factors determining the significance in anisotropy in atomic bonding in AlB₂-type diborides are analyzed by an approach similar to the valence-force-field method and are discussed in terms of the deformation of the electronic charge around the metal atoms occurring to fit themselves in the (0 0 0 1) basal plane.     

Anisotropic elastic and thermal properties of the double perovskite slab–rock salt layer Ln2SrAl2O7 (Ln = La,Nd, Sm,Eu, Gd or Dy) natural superlattice structure

The anisotropic elastic and thermal properties of layered compounds in the series Ln₂SrAl₂O₇ (Ln = La, Nd, Sm, Eu, Gd or Dy) are calculated from first principles using density functional theory combined with the Debye quasi-harmonic approximation. The polycrystalline values of the elastic constants and bulk, shear and Young’s moduli are consistent with those determined experimentally. All compounds in the compositional series have weakly anisotropic elastic and thermal properties. For instance, thermal expansion in the [0 0 1] direction of the tetragonal unit cell is slightly larger than along the [1 0 0] or [0 1 0] directions for most Ln₂SrAl₂O₇ compounds and the calculated in-plane thermal conductivity is always larger than that along the c-axis, parallel to the layer stacking direction.     

First-principles investigation of electronic,mechanical and thermodynamic properties of L12 ordered Co3(M,W) (M = Al,Ge, Ga) phases

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co₃(M, W) (M = Al, Ge, Ga) phases in terms of first-principles calculations. The results of the ground-state elastic constants revealed that Co₃(M, W) phases are mechanically stable and possess intrinsic ductility. It was found that the elastic heat-resistant properties of Co₃(Ge, W) phase are inferior to those of Co₃(Al, W) and Co₃(Ga, W). Analyzing the charge density difference provides the explanation that the sharp decrease in mechanical properties is mainly due to the weakening of Co–Ge bonding at elevated temperatures for Co₃(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co₃(M, W) phases show a high degree of elastic anisotropy. The degree of elastic anisotropy could be significantly decreased by an increase in temperature for Co₃(M, W). The lattice vibration is treated with the quasiharmonic phonon approach, considering both the vibrational and thermal electronic contributions. The thermodynamic properties as a function of temperature are computed without any adjustable parameters, including heat capacity, entropy, enthalpy and thermal expansion coefficient.     

Thermal expansion of the V5Si3 and T2 phases of the V–Si–B system investigated by high-temperature X-ray diffraction

The thermal expansion anisotropy of the V₅Si₃ and T₂-phase of the V–Si–B system were determined by high-temperature X-ray diffraction from 298 to 1273 K. Alloys with nominal compositions V_62.5Si_37.5 (V₅Si₃ phase) and V₆₃Si₁₂B₂₅ (T₂-phase) were prepared from high-purity materials through arc-melting followed by heat-treatment at 1873 K by 24 h, under argon atmosphere. The V₅Si₃ phase exhibits thermal expansion anisotropy equals to 1.3, with thermal expansion coefficients along the a and c-axis equal to 9.3 × 10^?6 K^?1 and 11.7 × 10^?6 K^?1, respectively. Similarly, the thermal expansion anisotropy value of the T₂-phase is 0.9 with thermal expansion coefficients equal to 8.8 × 10^?6 K^?1 and 8.3 × 10^?6 K^?1, along the a and c-axis respectively. Compared to other isostructural silicides of the 5:3 type and the Ti₅Si₃ phase, the V₅Si₃ phase presents lower thermal expansion anisotropy. The T₂-phase present in the V–Si–B system exhibits low thermal expansion anisotropy, as the T₂-phase of the Mo–Si–B, Nb–Si–B and W–Si–B systems.     

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

Half-Heusler thermoelectric materials Hf_1?xZr_xNiSn_1?ySb_y (x = 0, 0.25, 0.4, 0.5; y = 0.02, 0.04, 0.06) have been prepared by levitation melting followed by spark plasma sintering or hot pressing. X-ray diffraction analysis and scanning electron microscopy observation show that single-phased half-Heusler compounds without compositional segregations have been obtained by levitation melting in a time-efficient manner. A small amount of Sb doping can improve the electrical power factor but undesirably increases the thermal conductivity due to the increased carrier thermal conductivity. The isoelectronic substitution of Zr for Hf substantially decreased the lattice thermal conductivity. A state-of-the-art ZT value of 1.0 has been attained at 1000 K for the levitation-melted and spark-plasma-sintered Hf_0.6Zr_0.4NiSn_0.98Sb_0.02, which is one of the highest achieved ZT values for half-Heusler thermoelectric alloys.     

Thermal expansion and melting temperature of the half-Heusler compounds: MNiSn (M = Ti,Zr, Hf)

Half-Heusler compounds: MNiSn (M = Ti, Zr, Hf) are considered as a candidate of environmentally friendly and low-cost thermoelectric (TE) materials. Although the thermomechanical properties are quite important when utilizing the half-Heusler compounds in TE devices, such properties have been scarcely reported. In the present study, we tried to collect the data of the thermal expansion coefficient and the melting temperature (T_m) of MNiSn. The thermal expansion coefficient was evaluated by means of two methods: the dilatometer measurement and the high temperature X-ray diffraction analysis. The T_m was evaluated from the differential thermal analysis. The relationship between the thermal expansion coefficient and the T_m of the half-Heusler compounds was studied.     

Low-temperature specific heat of the superconductor Mo3Sb7

The low-temperature specific heat of a superconductor Mo₃Sb₇ with T_c = 2.2 ± 0.05 K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient γ_n, and the Debye temperature Θ_D are found to be 34.5(2) mJ mol⁻¹ K⁻² and 283(5) K, respectively. The enhanced γ_n value is interpreted as due to a narrow Mo-4d band pinned at the Fermi level. The electronic specific heat in the superconducting state can be analyzed in terms a phenomenological two BCS-like gap model with the gap widths 2Δ₁/k_BT_c = 4.0 and 2Δ₂/k_BT_c = 2.5, and relative weights of the molar electronic heat coefficients γ₁/γ_n = 0.7 and γ₂/γ_n = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at T_c, ΔC(T_c)/γ_nT_c, the electron–phonon coupling constant, λ_e−ph, the upper H_c2 and thermodynamic critical H_c0 fields, the penetration depth λ, coherence length ξ and the Ginzburg–Landau parameter κ are evaluated. The estimated values of parameters such as 2Δ₀/k_BT_c, ΔC(T_c)/γ_nT_c, N(E_F) and λ_e−ph suggest that Mo₃Sb₇ belongs to an intermediate-coupling regime. The electronic band structure calculations indicate that the density of states near the Fermi level is formed mainly by the Mo-4d orbitals and that there is no overlap between the Mo-4d and Sb-sp orbitals.     

Effect of Te additions on the glass transition and crystallization kinetics of (Sb15As30Se55)100−xTex amorphous solids

Differential scanning calorimetry results under non-isothermal conditions of chalcogenide (Sb₁₅As₃₀Se₅₅)_100?xTe_x (where 0 ≤ x ≤ 10 at.%) glasses are reported and discussed. The dependence of the characteristic temperatures “glass transition temperature (T_g), the crystallization onset temperature (T_c) and the crystallization temperature (T_p)” on the heating rate (β) utilized in the determination of the activation energy for the glass transition (E_g), the activation energy for crystallization (E_c), the glass thermal stability (ΔT = T_c ? T_g) and the Avrami exponent (n). The composition dependence of the T_g, E_g, and E_c were discussed in terms of the chemical bond approach, the average heats of atomization and the cohesive energy (CE). The diffractogram of the transformed material shows the presence of some crystallites of AsSb, Sb₄Te₆, As₂Se₃ and Sb₂Se₃ in the residual amorphous matrix.     

Hardness of hexagonal AlB2-like diborides of s,p and d metals from semi-empirical estimations

The high-level ab initio calculations of elastic moduli of materials now become a routine procedure, and the renewed attention is paid to the semi-empirical macroscopic models, providing the correlations between the Vickers hardness H_V and some elastic moduli. Such correlations, building up a bridge between Vickers hardness and elastic parameters, seem a helpful way to semi-quantitative estimations of hardness and to theoretical search of novel hard materials, based on the calculations of their elastic properties. Here, using the results of our calculations of elastic parameters of 14 hexagonal AlB₂-like diborides of s, p and d metals MB₂ (where M = Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W), we have probed six recently proposed correlations between H_V and bulk (B), shear (G), Young's moduli (E), as well as Poisson's ratio (ν), and the parameter k (the G/B ratio), and we compare the obtained results with the available experimental data. Our analysis reveals that for this family of related materials the correlations H_V ~ G, and H_V ~ E will be the most suitable approximations which enable us to roughly evaluate the Vickers hardness of diborides numerically.     

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.     

Thermal expansion of the W5Si3 and T2 phases of the W–Si–B system investigated by high-temperature X-ray diffraction

The coefficients of thermal expansion (CTE) of the W₅Si₃ and T₂ phases of the W–Si–B system were determined using high-temperature X-ray diffraction in the 298–1273 K temperature interval. Alloys with nominal compositions 62.5W37.5Si (at%) and 58W21Si21B (at%) were prepared from high-purity materials through arc melting followed by heat treatment at 2073 K for 12 h under argon atmosphere. The highly different thermal expansion coefficients of W₅Si₃ along the a (5.0 × 10⁻⁶ K⁻¹) and c (16.3 × 10⁻⁶ K⁻¹) axes lead to a high thermal expansion anisotropy (α_c/α_a ≅ 3.3). On the other hand, the T₂-phase exhibits similar thermal expansion coefficients along the a (6.9 × 10⁻⁶ K⁻¹) and c (7.6 × 10⁻⁶ K⁻¹) axes, indicating a behavior close to isotropic (α_c/α_a ≅ 1.1).     

Ion-irradiation-induced structural transitions in orthorhombic Ln2TiO5

The response of a material to a high radiation field is important when selecting materials for nuclear applications, such as structural materials, nuclear waste forms and inert matrix fuels. In the present study, the radiation response of orthorhombic, rare-earth titanates, Ln₂TiO₅ (Ln = La, Nd, Sm, Gd, Dy and Y), was investigated by 1 MeV Kr²⁺ ion bombardment at temperatures ranging from 50 to 1073 K. In situ transmission electron microscopy revealed that the radiation tolerance and irradiation-induced structural transitions vary largely with composition. Y₂TiO₅ exhibits the lowest critical amorphization temperature (T_c = 623 K), above which crystals cannot be amorphized, which is consistent with its use in the form of nanoclusters in radiation-resistant oxide-dispersion-strengthened steels. The disordered fluorite structure type of Ln₂TiO₅, with smaller Ln cations, formed as an intermediate phase prior to becoming fully amorphous. The fluorite structure type of Ln₂TiO₅, containing more vacancies as compared with that of Ln₂Ti₂O₇, may exhibit enhanced ionic conductivity, which highlights an effective way of using ion beams to modify the properties of materials.     

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.     

Reduced thermal conductivity in Pb-alloyed AgSbTe2 thermoelectric materials

Pb-alloyed AgSbTe₂ (Pb_xAg₂₀Sb_30?xTe₅₀ (x = 3, 4, 5 and 6)) composites were synthesized using a modified Bridgman method with a graphite mold to form plate-like samples. The Bridgman-grown specimens were dense, with few solidification cavities, and were sufficiently mechanically robust for a variety of electronic/thermal transport measurements. Inhomogeneity was found on the grain boundary, and was embedded with the nanoprecipitates of δ-Sb₂Te with a feature size of 100 nm of the 5 at.% Pb and 6 at.% Pb specimens. A combined effect of alloying, inhomogeneity and nanoprecipitates leads to a low thermal conductivity of 0.3–0.4 W m^?1 K^?1, which approaches the theoretical minimum thermal conductivity of the amorphous material (κ_min ～ 0.36 W m^?1 K^?1). A peak of the zT value, ranging from 0.7 to 0.8, is achieved at 425 K. Further annealing at 673 K increases the grain size and causes a reduction in the value of the zT peak to 0.4.     

Microstructure,martensitic transformation and anomalies in c′-softening in Co–Ni–Al ferromagnetic shape memory alloys

The morphology, microstructure and elastic softening in single crystals of Co–Ni–Al ferromagnetic shape memory alloy were studied to clarify the conditions for martenstic transformation in this alloy. We used two-phase (β matrix + γ particles) samples with different heat treatments, as-cast and annealed at temperatures from 1523 to 1623 K, and a sample of pure β (B2) phase. A complete set of elastic coefficients at room temperature and the temperature dependence of the softest shear coefficient (c′) of the Co₃₈Ni₃₃Al₂₉ austenite was measured by a combination of pulse echo and resonant ultrasound spectroscopy in the range 208–398 K. All examined materials exhibit anomalous c′-softening for the whole temperature range except the interval 258– 328 K, in which a change in the slope appears. However, only annealed samples transformed to martensite. The change in the slope is ascribed to (i) magnetoelastic softening with the absence of a sharp Curie point; (ii) structural stiffening that prevents the martensitic transition in both the as-cast and single-phase alloys. No signature of the premartensite phenomenon was found.     

Crystal structure,morphology and physical properties of LaCo1−xTixO3±δ perovskites prepared by a citric acid assisted soft chemistry synthesis

The influence of the partial substitution of Co by Ti in the LaCoO₃ perovskite system is studied by evaluating the electrical conductivity, the Seebeck coefficient and the thermal conductivity of the compounds up to T = 1273 K. The X-ray diffraction patterns of the LaCo_1?xTi_xO_3±δ (0.01 ? x ? 0.5) phases show two structural modifications depending on the Ti content. Compounds with x < 0.3 crystallize in the rhombohedrally distorted perovskite structure while samples with x ? 0.3 possess an orthorhombic unit cell. The oxidation state of the Co ions is studied by X-ray absorption near edge structure (XANES) spectroscopy. A negative thermoelectric power is found in the LaCoO₃ system for low level Ti substitution (x = 0.01). In contrast, samples with higher Ti content show a large positive Seebeck coefficient, indicating positive majority charge carriers in the system. The electrical resistivity of the studied materials reveals a semiconducting-like behaviour. The lattice thermal conductivity was found to be low and nearly temperature-independent. The samples exhibit very small crystallite sizes in the range of few nanometres. Therefore, the low thermal conductivity can be assigned to an enhanced phonon scattering on grain boundaries.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

Elastic constants of binary Mg compounds from first-principles calculations

Elastic constants (C_ij's) of 25 compounds in the Mg–X (X = As, Ba, Ca, Cd, Cu, Ga, Ge, La, Ni, P, Si, Sn, and Y) systems have been predicted by first-principles calculations with the generalized gradient approximation and compared with the available experimental data. Ductility and the type of bonding in these compounds are further analyzed based on their bulk modulus/shear modulus ratios (B/G), Cauchy pressures (C₁₂–C₄₄), and electronic structure calculations. It is found that MgNi₂ and MgCu₂ have very high elastic moduli. Mg compounds containing Si, Ge, Pb, Sn, and Y, based on their B/G ratios, are inferred as being brittle. A metallic bonding in MgCu₂ and a mixture of covalent/ionic bond character in Mg₂Si, as inferred from their electronic structures, further explain the corresponding mechanical properties of these compounds.     

Structure–conductivity relationships in chemical polypyrroles of low,medium and high conductivity

Chemically synthesized polypyrroles of low (σ < 75 S/cm), medium (75 < σ < 200 S/cm) and high (σ > 200 S/cm) electrical conductivity (σ) with the same dopant and degree of doping have been investigated by means of Wide Angle X-ray Scattering (WAXS), ¹³C Cross Polarized Magic Angle Spinning Nuclear Magnetic Resonance (¹³C CP/MAS NMR) spectroscopy and Fourier Transform Infrared (FTIR) Spectroscopy to establish structure–conductivity relationships useful for industrial applications. A similar amorphous structure was found by WAXS even for the higher conducting PPy (σ = 288 S/cm). WAXS spectra for polypyrroles of medium and high conductivity showed a weak peak at 2θ = 10–11° due to improved order of the counterions in these materials. The effect of the counterion size in the asymmetry of the PPy main WAXS peak was elucidated by performing ion exchange of the Cl⁻ dopant with counterions of larger size such as BF₄⁻ and ClO₄⁻. From ¹³C CP/MAS NMR measurements predominantly α–α′ bonding was found in these materials. The main ¹³C CP/MAS NMR resonance peak of PPy located at 126–128 ppm was broadened upon increasing conductivity. Interestingly, a linear relationship was observed between the half-width at half-height (HWHH) of the ¹³C CP/MAS NMR peak and conductivity where a doubling of the polypyrrole conductivity leads to an increase of HWHH by 6–7 ppm. FTIR data of these materials were analysed in the framework of the Baughman–Shacklette theory describing the dependence of conductivity on conjugation length. By comparison of model predictions and experimental results, the PPy samples were found to be in the regime of long conjugation lengths, L ≫ K₂/k_BT, where K₂ is a parameter related to the energy change on going from j − 1 to j charges on a conjugated segment of conjugation length L, k_B the Boltzman constant and T is the absolute temperature.     

Elastic constants of amorphous and single-crystal Pd40Cu40P20

We have measured the adiabatic second-order elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ (isotropic, two independent elastic constants) and single-crystal Pd₄₀Cu₄₀P₂₀ (tetragonal, six independent elastic constants) over the range 3.9 < T < 300 K. Below ∼20 K, the elastic constants of single-crystal Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0)[1 − bT² − dT⁴], which is the same temperature dependence found in metallic elements. In contrast, below ∼20 K the elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0¹)[1 − aT]. The bulk modulus of amorphous Pd₄₀Cu₄₀P₂₀ is approximately 2% lower than that of crystalline Pd₄₀Cu₄₀P₂₀, whereas the shear modulus of the glass is 13–36% lower than the four independent shear moduli of the tetragonal crystal. The lower moduli of the glass are best explained by additional atomic displacements, beyond those experienced by the atoms in a crystal, that occur because the atoms in glasses do not occupy centers of symmetry.