Thermoelectric properties of A-site doped perovskites (Sr0.6Ba0.4)1−xMxPbO3 (M = La, K)

La- and K-doped perovskite-type ceramics, (Sr_0.6Ba_0.4)_1−xLa_xPbO₃ with x = 0.0−0.1 and (Sr_0.6Ba_0.4)_1−xK_xPbO₃ with x = 0.00−0.15, were prepared to modify thermoelectric properties of semi-metallic Sr_0.6Ba_0.4PbO₃ via the doping of electrons and holes, respectively. The electrical conductivity σ and Seebeck coefficient S for the ceramics were measured at temperatures of 373–1073 K in air. With the La doping, electron carriers were successively doped and the material changed from a semi-metal for the undoped Sr_0.6Ba_0.4PbO₃ to a metal for the (Sr_0.6Ba_0.4)_0.9La_0.1PbO₃. With the K doping, the thermoelectric properties were essentially unchanged probably due to the carrier compensation effect by the generation of oxygen deficiencies. The thermoelectric power factor S²σ was maximized to a value of 3.1 × 10⁻⁴ Wm⁻¹ K⁻² at 773 K for the undoped Sr_0.6Ba_0.4PbO₃ ceramic.     

Thermoelectric properties of Ni- and Zn-doped Nd2CuO4

The electrical resistivity, Seebeck coefficient, and thermal conductivity of Nd₂(Cu_0.98M_0.02)O₄ (M: Ni and Zn) have been measured in the temperature range from room temperature to about 1000 K. Ni- and Zn-doping decreases the electrical resistivity and the absolute values of the Seebeck coefficient. The thermal conductivity decreases with increasing temperature, showing phonon conduction, and also decreases by doping. The power factor of Nd₂(Cu_0.98Ni_0.02)O₄ reaches 1.02×10⁻⁴ W m⁻¹ K⁻² and the figure of merit is 1.35×10⁻⁵ K⁻¹ at 320 K. The relatively low figure of merit compared with that of the state-of-the-art thermoelectric materials is due to the high thermal conductivity.     

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Kinetic study on Li2.8(V0.9Ge0.1)2(PO4)3 by EIS measurement

Kinetics for lithium ion transfers in the fast ionic conductor Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ prepared by solid-state reaction method has been studied by electrochemical impedance spectroscopy (EIS) at various temperatures and the results were correlated with observed cathodic behavior. The specific conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ (x = 0.9–2.8) versus temperatures were analyzed from blocking-electrodes by Wagner's polarization method and the activation energy was calculated. It was observed that electronic conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ increased with lithium contents in the materials. The compounds show a reversible capacity of 131 mAh g⁻¹ at low current density (13 mA g⁻¹). Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer and surface film resistances. The Li ion diffusion coefficient (D_Li⁺) versus voltage plot shows three valleys during the first charge cycle coinciding with the irreversible plateau of the voltage versus lithium content profiles reflecting the irreversible phase change in the compound. The obtained D_Li⁺ from EIS varies within 10⁻⁸ to 10⁻⁷ cm² s⁻¹, so Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ shows excellent chemical diffusion performance.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).     

Preparation and bulk thermal expansion studies in M1−xCexSiO4 (M = Th, Zr) system, and stabilization of tetragonal ThSiO4

Two series of compositions with the general formula M_1−xCe_xSiO₄ (M = Th, Zr; x = 0.0–0.5; 1.0) were prepared by a standard solid state route and characterized by powder XRD. About 10 mol% of ceria could be dissolved in the lattice of ThSiO₄. A striking observation was the stabilization of tetragonal modification of ThSiO₄, which is metastable, by ceria substitution. There was no solubility of ceria in zircon (ZrSiO₄) lattice. The average linear thermal expansion coefficient (293–1123 K) of ZrSiO₄, ThSiO₄ and Th_0.9Ce_0.1SiO₄ are 4.65 × 10⁻⁶, 4.97 × 10⁻⁶ and 5.14 × 10⁻⁶ K⁻¹, respectively.     

Thermal and electrical properties of perovskite-type strontium molybdate

Polycrystalline samples of perovskite-type strontium molybdate, SrMoO₃, have been prepared and the thermal and electrical properties have been measured from room temperature to about 1000 K. The electrical resistivity is of an order of magnitude of 10⁻⁵ to 10⁻⁶ (Ω m) in the whole temperature range. The Seebeck coefficient is around 4–9 μV K⁻¹. At room temperature, the thermal conductivity is about 30 W m⁻¹ K⁻¹, and it decreases with increasing temperature.     

Spectral properties of Yb ions in LiNbO3 single crystals: influences of other rare-earth ions, OH ions, and γ-irradiation

Absorption spectra have been studied in 190–3100 nm region at various temperatures from 16 to 292 K for Yb³⁺-doped and Yb³⁺/Nd³⁺-, Yb³⁺/Er³⁺- and Yb³⁺/Pr³⁺-co-doped LiNbO₃ single crystals before and after γ-irradiation with a dose of 10⁵–10⁷ Gy. Intense 400 and 500 nm absorption bands were observed after γ-irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb⁴⁺ polaron, respectively. Different change was observed in the 2870 nm OH⁻ absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li⁺ or Nb⁵⁺ host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated ²F_7/2 Stark levels to the excited ²F_5/2 level. The position of the Yb^{3+ 2}F_7/2→²F_5/2 first resonant line was observed to be slightly different among the co-doped crystals. This is due to the perturbation of Yb³⁺ by co-doped rare earth ion which is located at the neighborhood of theYb³⁺.     

掺杂sc的CaZrO3的制备及电学性能

采用掺杂Sc₂O₃,通过固相反应法制备了CaZr_1-xSc_xO_3-α（x=0,0.1,0.15）材料.在610-850℃采用交流阻抗谱法测定了CaZr_1-xSc_xO_3-α的电导率及电导激活能,并对CaZrO₃掺Sc及掺In样品的电学性能进行了比较.结果表明:在610-850℃,CaZrO₃的电导率为4.3×10^-19-1.4×10^-6S/cm,CaZr_1-xSc_xO_3-α（x=0.1,0.15）的电导率为1.16×10^-4-1.4×10^-3S/cm,CaZr_1-xIn_xO_3-α（x=0.1,0.15）的电导率为0.34×10^-4-4.33×10^-4S/cm,且随着温度的升高而提高;掺杂能极大提高CaZrO₃的电导率,并随着掺杂量的增加,电导激活能降低,电导率增加;温度及掺杂量相同时,掺Sc材料电导率明显高于掺In材料,说明掺Sc对提高材料的电导率更有效.     

ICP-MS direct determination of trace amounts of rare earth impurities in various rare earth oxides with only one standard series

An inductively coupled plasma mass spectrometry (ICP-MS) method with just one standard series for direct determination of trace rare earth impurities in various rare earth oxides was developed. The spectral interference in ICP-MS analysis of high-purity neodymium (Nd₂O₃) was thoroughly estimated. For the investigation of matrix effect, high-purity Y₂O₃ was used as model sample and the experimental results showed that the maximal matrix tolerant amount obtained by stepwise dilution method is comparable to that obtained by conventional method with the use of higher purity Y₂O₃ as matrix. Under the selected conditions, no obvious matrix effect can be found with the matrix (Y) concentration of less than 500 μg mL⁻¹. For real sample analysis, 100 μg mL⁻¹ of matrix was chosen as the sample concentration. The proposed method was applied to the analysis of trace rare earth impurities in different high-purity rare earth oxides (Y₂O₃, Pr₆O₁₁, Nd₂O₃, Dy₂O₃, Er₂O₃), and the analytical results obtained were in good agreement with the recommended values. The detection limits of the method for rare earth elements were 1–21 ng L⁻¹ with the R.S.D varying between 2.9 and 7.8%, and the percentage recovery ranged from 93 to 115% for the spiked samples. This method was characterized with simplicity, rapidity, sensitivity, small sample amount required, and no internal standard/matrix matching requirements.     

Ionic conductivities of polymeric solid electrolyte films containing rare earth ions

Polymeric solid electrolyte films containing rare earth metal ions (Ce³⁺, La³⁺, Yb³⁺) have been prepared, with a view to applying them to solid-state electrochemical devices. The films, composed of poly(ethylene oxide)-grafted poly(methylmethacrylate) (PEO–PMMA), were prepared by dissolving rare earth salts with appropriate amounts of poly(ethylene glycol) dimethyl ether (PEG) that have ethylene oxide units in their structure. The ionic conductance behaviour of the polymeric composite electrolyte systems was investigated by AC impedance and DC polarization methods in an ambient temperature range. The oligo(ethylene oxide) units in the polymer matrix and the PEG components ensured the dissociation of the salts and the high mobility of the resulting ionic species in the solid films. About 10⁻⁵ S cm⁻¹ or above of ionic conductivity was obtained for a PEO–PMMA/PEG/Ce(ClO₄)₃ system at room temperature. The addition of a liquid plasticizer in the composite improved the conductivity by about two orders of magnitude.     

Structural, magnetic and spectroscopic investigations of europium oxychloride, EuOCl

The reaction of the ammonium halides (NH₄X, X⁻ = F⁻, Cl⁻, Br⁻) with the rare earth (R) oxides (e.g. R₂O₃) is a convenient way to obtain the trihalides (RX₃) or stoichiometrically pure oxyhalides (ROX). Problems may arise with rare earths which can be easily reduced by the ammonia produced during the reaction. In this study, the products of the reaction between Eu₂O₃ and NH₄Cl were investigated with the X-ray powder diffraction, Mössbauer and EPR spectroscopies and magnetic susceptibility measurements. The tetragonal crystal structure of EuOCl was refined with the Rietveld method. The Mössbauer and EPR spectra as well as the paramagnetic susceptibility measurements were analysed to detect the presence of divalent europium. The evolution of the paramagnetic susceptibility as a function of temperature was measured and compared to that calculated from previous spectroscopic measurements. Only the paramagnetic susceptibility measurements revealed beyond doubt the presence of divalent Eu²⁺ impurity.     

Optical dephasing of rare earth ions in mixed crystalline and size-restricted systems

High resolution optical hole burning spectroscopy of rare earth ions is used to study the dynamics of disordered and size-restricted crystalline solids. Materials described include mixed crystals of Ca_1−xLa_xF_2+x and Ca_1−xY_xF_2+x with Eu³⁺ and Pr³⁺ and sol–gel prepared nanometer scale γ-Al₂O₃:Eu³⁺. All of these materials exhibit, at low temperatures, a temperature dependence of their spectral hole linewidth which is similar to the dynamical properties observed in amorphous materials such as glasses. This suggests an enhanced density of states at low frequencies. The results on the mixed alkaline earth fluorides are discussed in relation to far infrared and heat capacity measurements, which indicate the presence of low frequency excitations. It appears that while the density of defect states responsible for the dynamics increases with Y or La content, the density saturates at low concentrations. Both persistent and transient hole burning mechanisms occur for nanoscale sol–gel γ-Al₂O₃:Eu³⁺. The nearly exponential increase in the linewidth above 7 K is explained by Raman scattering by phonons associated with the size-restricted nature of the phonon density of states.     

Internal friction of Cu–CuAlO alloys

Polycrystalline internal oxidize copper alloy with CuAlO particles has been studied by isothermal mechanical spectrometry, above room temperature. The frequency range was 10⁻⁴ to 30 Hz and the maximum strain 2×10⁻⁵. Experiments were carried out on a sample after 18% cold-rolling and successive annealings at various temperatures. The internal friction spectra obtained between 290 and 400 K exhibit only a low frequency background superimposed to a non thermally activated maximum. After annealing at 400 K, the background decreases, a relaxation peak appears and its maximum rises progressively as temperature increases. The relaxation parameters, obtained from the frequency shift of the maximum with temperature, leads to E_a=0.9 eV, and τ₀=6.9×10⁻¹⁰ s. These values are similar to those described in the literature for a relaxation peak assigned to dislocation motion inside grains.     

Optical study of praseodymium dicarboxylate [Pr(H2O)]2[O2C(CH2)3CO2]3.4H2O

Praseodymium dicarboxylate, [Pr(H₂O)]₂[O₂C(CH₂)₃CO₂]₃.4H₂O]–glutarate, Pr[glut], is synthesized by hydrothermal techniques. The title compound crystallizes in the monoclinic space group C2/c (No. 15). The rare earth cation is coordinated by nine oxygen atoms, eight oxygen atoms from the carboxylate groups and one from the water molecule. The local symmetry of Pr site is low, C_s. The absorption spectra of Pr[glut] are recorded from the visible to the far IR domain at 300, 77 and 9 K. Under various Ar⁺ laser excitations no emission is detected from ³P₀ and ¹D₂ excited levels of Pr³⁺ ion. In the low temperature absorption spectra only one electronic line is recorded for ³H₄→³P₀ transition. It confirms a unique local environment for the rare earth ion in Pr[glut]. The utility of the ‘barycenter curves’ in the attribution of electronic lines is demonstrated. Energy level scheme of 36 Stark components is deduced from the absorption spectra. The parametric calculation was performed on the whole 4f² (Pr³⁺) configuration with the starting set of crystal field parameters obtained previously for the Eu³⁺ ion in the isostructural compound. Eight free ion and nine phenomenological crystal field parameters in C_2v symmetry reproduce quite well several electronic levels of Pr³⁺ ion experimentally observed in Pr[glut]. A good r.m.s. standard deviation of 14.8 cm⁻¹ is obtained.     

Thermal stability and electronic specific heat of GaN

The thermal stability of GaN single crystals obtained by Li-based flux was investigated by DTA–TG, XRD, Raman and infrared spectroscopy. The results evidence that pure GaN becomes unstable above 900 °C under N₂ of 0.1 MPa. The specific heat of GaN was determined at temperatures ranging from 1.9 K to 80 K. Its electronic specific heat coefficient is 0.47 mJ K⁻² mol⁻¹ and the Debye temperature is 278 K.     

Self-propagating high-temperature synthesis of La(Sr)Ga(Mg)O3−δ for electrolyte of solid oxide fuel cells

This paper describes self-propagating high-temperature synthesis (SHS) of an electrolyte for solid oxide fuel (SOFC), in comparison to a conventional solid-state reaction method (SRM). Doped-lanthanum gallate: La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM9182) and LSGM9173 as the SOFC electrolyte, was prepared by the SHS and sintered at different temperatures, for measuring the electrical conductivity of the sintered LSGM and the power generating performance at 1073 K, in comparison to the SRM. In the SHS, the LSGM powders with smaller size were obtained and easily sintered at the 100 K-lower temperature, 1673 K, than in the SRM. Most significantly, the electrical conductivity of the sintered LSGM9182 was as high as 0.11 S cm⁻¹ and its maximum power density was a value of 245 mW cm⁻² in the cell configuration of Ni/LSGM9182 (0.501 mm in thickness)/Sm_0.5Sr_0.5CoO₃. The conclusion was that the proposed SHS-sintering method with many benefits of minimizing the energy requirement and the processing time in the production, easing temperature restriction for the sintering, and improving the electrolyte performance up to a conventional level is practicable for producing the LSGM-electrolyte of SOFC at an intermediate-temperature application.     

Luminescent properties of Sr2MgSi2O7 and Ca2MgSi2O7 long lasting phosphors activated by Eu, Dy

Long lasting afterglow phosphors Sr₂MgSi₂O₇:Eu,Dy and Ca₂MgSi₂O₇:Eu,Dy were prepared by solid-state reaction method. It is revealed by the results that both phosphors show two emission peaks that are caused by the different emitting centers Eu²⁺ held in the host lattice. Wavelength shift was observed due to the slight differences in the structure parameters of the two hosts. Investigation on afterglow property shows that Sr₂MgSi₂O₇:Eu,Dy phosphor held a better afterglow property than Ca₂MgSi₂O₇:Eu,Dy phosphor. Thermal simulated luminescence study indicates that the long afterglow is generated by a higher trap concentration formed by the co-doped rare earth ions within the host.     

EXAFS Debye–Waller factors of La and Ni in LaNi5

XAFS measurements of the La L₃ and Ni K-edges for LaNi₅ were done at temperatures of 20, 100, 200 and 293 K. The temperature dependences of the Debye–Waller factors e^−2σ2k2 for La and Ni are deduced. The results show that mean square fluctuation in interatomic distance σ² of La for LaNi₅ is fairly large and the Debye temperature Θ_D of La changes with temperature. These results indicate that the La atom with large atomic size changes its atomic position easily in LaNi₅. The large σ² value of A (rare earth) elements for AB₅ compounds might be correlated to their high capability of the hydrogen absorption and desorption.