Influence of SnO2 addition on the thermoelectric properties of Zn1 − xSnxO (0.01 ≤ x ≤ 0.05)

The grain size and the density of the Zn_1 − xSn_xO (0 ≤ x ≤ 0.05) samples decreased with increasing SnO₂ content. The addition of a small amount of SnO₂ (x ≤ 0.01) to ZnO led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in a significant increase in the power factor. The thermoelectric power factor was maximized to a value of 1.25 × 10⁻³ Wm⁻¹ K⁻² at 1073 K for the Zn_0.99Sn_0.01O sample.     

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.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

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.     

Effect of the Mn ion on electrical and magnetic properties of orthorhombic perovskite-type Ca(Mn1−xTix)O3−δ

Orthorhombic perovskite-type Ca(Mn_1−xTi_x)O_3−δ (0 ≤ x ≤ 0.7) was synthesized at 1173 K for 12 h in a flow of oxygen from a precursor gel prepared using citric acid and ethylene glycol. The Mn³⁺ ion was generated by substituting a Ti⁴⁺ ion in CaMnO₃. The average particle size was 100-300 nm and did not depend on x. The lattice constants and the (Mn, Ti)-O distance increased linearly with increasing x. The variation in global instability index (GII) indicated that the instability of the structure increases monotonically with increasing x. Ca(Mn_1−xTi_x)O_3−δ was an n-type semiconductor that had its minimum values of electrical resistivity (ρ) and activation energy (E_a) at x = 0.1. Ca(Mn_1−xTi_x)O_3−δ (x = 0 and 0.1) exhibited a weak ferromagnetic behavior. The variation in μ_eff indicated that the spin state of the Mn³⁺ ion changes from low to high at x = 0.1, then reverts to low in the range of 0.2 ≤ x ≤ 0.7. The variations in ρ and E_a are explained by the number of electrons according to the change in the spin state of the Mn³⁺ ion.     

Materials with layered structures XIII: electrical and optical properties of layered chalcogenides in the system CoIn2S4-CoIn2Se4

The system CoIn₂S_4xSe_4(1−x) has been investigated by X-ray powder methods on samples quenched at 700 °C. The spinel type phase has a phase width of 1≥x>0.9. A new layered compound is formed for 0.9>x>0.45 which crystallizes with the α-FeGa₂S₄-type with a=392.6 pm and c=1270.3 pm (x=0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750→700 °C. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10⁻⁵ Ω⁻¹ cm⁻¹.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4

Sn doping in an n-type transparent conducting oxide MgIn₂O₄ is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg_1+xIn_2−2xSn_xO₄ for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10⁻⁴ K⁻¹ for the parent compound at 600 K.     

Electrical properties of crystalline PbxSn1−xTe0.5Se0.5 thin films

Electrical conductivity in the dark, σ, and thermoelectric power, S, of Pb_xSn_1−xTe_0.5Se_0.5 films with x = 0.4, 0.6, 0.8, and 1 were studied for films annealed at 473 K in the temperature range 300-473 K, while the Hall voltage was investigated at room temperature. The temperature dependence of σ revealed an intrinsic conduction mechanism above 370 K, while for temperatures less than 370 K an extrinsic conduction is dominant. Both activation energy, ΔE₁, and the energy gap, E_g, were found to decrease with increasing Sn content. This decrease of E_g with increasing Sn content revealed that band inversion exists. The variation of S with temperature revealed that the investigated samples are non-degenerate semiconductors with p-type conduction. Also, the Fermi energy, E_F, was determined from the linear variation of S with 1/T in the intrinsic range. The compositional dependence of the room temperature Hall constant, R_H (0.21-0.38 cm³/Coul.), hole carrier's concentration, p (2.9-1.6 × 10¹⁹ cm⁻³), Hall mobility, μ_H (0.88-0.03 cm²/V s), and effective mass, m^∗/m_e (0.28-0.78) are given.     

Synthesis and characterization of the colossal magnetoresistance manganite La1.2(Sr1.4Ca0.4)Mn2O7 by citrate gel

Single-phase La_1.2(Sr_1.4Ca_0.4)Mn₂O₇ has been synthesized from the aqueous solution of metal nitrates and citric acid by the sol-gel technique. Small particle size with high homogeneity of the powder was obtained. The valence of Mn is determined to be 3.45±0.05 by chemical titration. The MR ratio [ρ−ρ(H)]/ρ(H), is 115% (102 K, 1.5 T) for the composition prepared by the citrate route, and is nearly three times larger than that of the sample prepared by solid state reaction.     

Effect of the cooling rate on the thermoelectric properties of p-type (Bi0.25Sb0.75)2Te3 and n-type Bi2(Te0.94Se0.06)3 after melting in the bismuth-telluride system

The p-type (Bi_0.25Sb_0.75)₂Te₃ and n-type Bi₂(Te_0.94Se_0.06)₃ ingots were prepared by cooling at various cooling rates C after melting so that they have an intermediate state between the polycrystalline and Bridgman ingots which lowers their thermal conductivity κ, where C was changed from 0.10 to 2375 K/min in an evacuated glass tube. When the ingots were cooled at C = 0.50 K/min under the uniaxial temperature gradient of 5 K/cm, it was observed that the c axis of some grains points to the freezing direction. The electrical resistivity ρ, Seebeck coefficient α and κ of ingots were measured at 298 K along the freezing direction, so that ρ and κ at C = 0.50 K/min were lower by 20-30% and 9% than those of the corresponding Bridgman ingots. The thermoelectric figure of merits ZT(=α²T/ρκ) estimated for the p- and n-type ingots then reached high values of 1.27 and 1.25 at 298 K, respectively.     

Transport properties and Mössbauer spectra of Fe-substituted La10−x(Si,Al)6O26 apatites

Increasing iron content in apatite-type La_9.83Si_4.5Al_1.5−yFe_yO_26+δ (y=0.5-1.5) leads to increasing unit cell volume, fraction of Fe⁴⁺, partial oxygen ionic and p-type electronic conductivities, and ceramics sinterability. The oxygen ion transference numbers, determined by Faradaic efficiency (FE) measurements at 973-1223 K in air, are in the range 0.986-0.994. Data on total conductivity and Seebeck coefficient as functions of the oxygen partial pressure, varying in the range 10⁻² Pa to 70 kPa, confirm that under oxidizing conditions the ionic conduction in Fe-substituted La_9.83(Si,Al)₆O_26+δ apatites is dominant. Due to stabilization of Fe³⁺, substantially worse transport properties are observed for A-site stoichiometric La₁₀Si₄Fe₂O₂₆, having activation energy for ionic conductivity of 107 kJ/mol and electron transference numbers close to 0.03. The correlation between partial ionic and electron-hole conductivities suggests a significant role of Fe⁴⁺ formation compensated by extra oxygen incorporation into the vacant sites, which are formed due to Frenkel-type disorder induced by La vacancies. The average thermal expansion coefficients of Fe-doped La_10−x(Si,Al)₆O_26+δ ceramics, calculated from dilatometric data in air, are 8.9×10⁻⁶ to 9.9×10⁻⁶ K⁻¹ at 300-1250 K.     

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl_0.5Fe_0.5O_2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10⁻³ to 5×10⁻³. The analysis of structural parameters showed that the ionic transport in the CaAl_0.5Fe_0.5O_2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl_0.5Fe_0.5O_2.5+δ lattice. The average thermal expansion coefficients of CaAl_0.5Fe_0.5O_2.5+δ ceramics in air are 16.7×10⁻⁶ and 12.6×10⁻⁶ K⁻¹ at 370-850 and 930-1300 K, respectively.     

Synthesis of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

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.     

Influence of Cr deficiency on sintering character and properties of SOFC interconnect material La0.7Ca0.3Cr1−xO3−δ

The SOFC interconnect materials La_0.7Ca_0.3Cr_1−xO_3−δ (x = 0-0.09) were prepared using an auto-ignition process and characterized. XRD analysis indicated that all the samples displayed a pure perovskite phase after sintered at 1400 °C for 4 h. The relative density increased from 67% (x = 0) to 95.8% (x = 0.02) and reached to about 97% (x > 0.02), as sintered at 1400 °C for 4 h. The electrical conductivity in air dramatically increased and then lowered slowly with x values. The sample with 0.03 Cr deficiency got a maximum conductivity of 61.7 S cm⁻¹ at 850 °C in air, which is about three times as high as that of the sample with no Cr deficiency (20.6 S cm⁻¹). The sample with 0.06 Cr deficiency exhibited the highest electrical conductivity of 3.9 S cm⁻¹ at 850 °C in pure H₂. The thermal expansion coefficient (TEC) were below 11.8 × 10⁻⁶ K⁻¹ for samples of x = 0.02-0.09, that was of well compatibility with other components in SOFCs. Results indicate that the materials with 0.02-0.06 Cr deficiency have high properties and are much suitable for SOFC interconnect.     

Effect of BiFeO3 additions on the dielectric and piezoelectric properties of (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 ceramics

(1 − x) (K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.1Sb_0.06)O₃ − x BiFeO₃ (x = 0, 0.002, 0.004, 0.006, 0.008, 0.01) lead-free piezoelectric ceramics were prepared by the conventional ceramic processing. The compositional dependence of the phase structure and the electrical properties of the ceramics were studied. A morphotropic phase boundary between the orthorhombic and tetragonal phases was identified in the composition range of 0.004 < x < 0.006. The ceramics near the morphotropic phase boundary exhibit a strong compositional dependence and enhanced piezoelectric properties. The ceramics with 0.6 mol.% BiFeO₃ exhibit good electrical properties (d₃₃ ∼ 246 pC/N, k_p ∼ 43%, T_c ∼ 285 °C, ?_r ∼ 1871, and tan δ ∼ 1.96%). These results show that the (1 − x) (K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.1Sb_0.06)O₃ − x BiFeO₃ ceramic is a promising lead-free piezoelectric material for applications in different devices.     

Influence of Mn-site doped with Ru on the high-temperature thermoelectric performance of CaMnO3−δ

Thermoelectric properties were measured from room temperature to 1000 K to study the effects of doping of Ru for Mn-site on the thermoelectric performance of CaMnO_3−δ. The electrical resistivity shows a sharp decrease upon Ru doping. The Seebeck coefficients of all the samples are negative, and their absolute values remain fairly large even after Ru doping. Among the samples of CaMn_1−xRu_xO_3−δ (x=0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15, 0.18), CaMn_0.96Ru_0.04O_3−δ has the largest power factor, 1.85×10⁻⁴ W/mK² at 1000 K. An increase or a decrease of x value results in a marked decrease of the power factor. The thermal conductivity and the figure of merit Z of CaMn_0.96Ru_0.04O_3−δ is 2.88 W/mK and 0.64×10⁻⁴ K⁻¹ at 1000 K, respectively.