High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Galvanomagnetic properties of air-stable and highly conductive potassium-intercalated graphite sheet

Magnetoresistance and Hall coefficient of air-stable potassium-intercalated graphite sheets (hereafter abbreviated as K-PGS) were determined at room temperature. The magnitude of the magnetoresistance and the absolute value of Hall coefficient of K-PGS decreased with increasing potassium content of K-PGS, n_K/n_C. Two-carrier model was used for calculating carrier density and mobility. The electron density increased with increasing n_K/n_C: 3.07×10²⁰ cm⁻³ (n_K/n_C=0.005), 5.67×10²⁰ cm⁻³ (n_K/n_C=0.008) and 6.40×10²⁰ cm⁻³ (n_K/n_C=0.011). The value of the electron density of K-PGS with n_K/n_C=0.011 (nominal composition KC₉₁) was about 80% of the reported value, 7.8×10²⁰ cm⁻³, for KC₄₈ (n_K/n_C=0.021) prepared from HOPG (highly oriented pyrolytic graphite). The mobility decreased with increasing n_K/n_C: 2.11×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.005), 1.42×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.008) and 1.34×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.011). The value of the mobility of K-PGS with n_K/n_C=0.011 was about 60% of the reported value (2300 cm² V⁻¹ s⁻¹) for KC₄₈ prepared from HOPG.     

Investigations on the thermo-chemical stability and electrical conductivity of K-doped Ba3 − xKxCaNb2O9 − δ (x = 0.5, 0.75, 1, 1.25)

In this paper, we report the synthesis, crystal structure and electrical transport properties of new K-doped Ba₃CaNb₂O₉ (BCN) and investigate their chemical stability in H₂O and pure CO₂ at elevated temperature. The powder X-ray diffraction (PXRD) of Ba_2.5K_0.5CaNb₂O_9 − δ, Ba_2.25K_0.75CaNb₂O_9 − δ, Ba₂KCaNb₂O_9 − δ, and Ba_1.75K_1.25CaNb₂O_9 − δ showed the formation of a single-phase double perovskite (A₃BB^/₂O₉)-like cell with a lattice constant of a ∼ 2a_p (where a_p is a simple perovskite cell of ∼ 4 Å). Perovskite-like structure was found to be retained after treating with CO₂ at 700 °C and also after boiling H₂O for 120 h. The lattice constant of CO₂ and H₂O treated samples was found to be comparable to that of the corresponding as-prepared compound. The total electrical conductivity of all the investigated K-doped BCN increases with increasing K content in BCN in various atmospheres, including air, dry H₂, wet N₂ and wet H₂. The electrical conductivity in dry and wet H₂ atmospheres was found to be higher than that of air in the temperature range of 300-700 °C, while in wet N₂ a slightly lower value was observed. Among the compounds investigated in the present study Ba_1.75K_1.25CaNb₂O_9 − δ showed the highest total electrical conductivity of 1 × 10^− 3 S/cm in dry H₂ at 700 °C with an activation energy of 1.28 eV in the temperature range of 300-700 °C.     

Measurements and theoretical calculations of N2-broadening and N2-shift coefficients in the ν2 band of CH3D

In this paper, we report measured Lorentz N₂-broadening and N₂-induced pressure-shift coefficients of CH₃D in the ν₂ fundamental band using a multispectrum fitting technique. These measurements were made by analyzing 11 laboratory absorption spectra recorded at 0.0056 cm⁻¹ resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. The spectra were obtained using two absorption cells with path lengths of 10.2 and 25 cm. The total sample pressures ranged from 0.98 to 402.25 Torr with CH₃D volume mixing ratios of 0.01 in nitrogen. We have been able to determine the N₂ pressure-broadening coefficients of 368 ν₂ transitions with quantum numbers as high as J″ = 20 and K = 16, where K″ = K′ ≡ K (for a parallel band). The measured N₂-broadening coefficients range from 0.0248 to 0.0742 cm⁻¹ atm⁻¹ at 296 K. All the measured pressure-shifts are negative. The reported N₂-induced pressure-shift coefficients vary from about −0.0003 to −0.0094 cm⁻¹ atm⁻¹. We have examined the dependence of the measured broadening and shift parameters on the J″, and K quantum numbers and also developed empirical expressions to describe the broadening coefficients in terms of m (m = −J″, J″, and J″ + 1 in the ^QP-, ^QQ-, and ^QR-branch, respectively) and K. On average, the empirical expressions reproduce the measured broadening coefficients to within 4.7%. The N₂-broadening and pressure-shift coefficients were calculated on the basis of a semiclassical model of interacting linear molecules performed by considering in addition to the electrostatic contributions the atom-atom Lennard-Jones potential. The theoretical results of the broadening coefficients are in good overall agreement with the experimental data (8.7%). The N₂-pressure shifts whose vibrational contribution is derived from parameters fitted in the ^QQ-branch of self-induced shifts of CH₃D, are also in reasonable agreement with the scattered experimental data (20% in most cases).     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Experimental determination of pressure-broadening parameters of millimeter-wave transitions of HNO3 perturbed by N2 and O2, and of their temperature dependences

We report on linewidth measurements on the J=24_K,11−23_K^′,10 and J=38_K,33−37_K^′,32 millimeter wave transitions in the ground vibrational state of nitric acid, located near 470.23 and 544.36 GHz, respectively. Experiments were performed with N₂ and O₂ as perturber molecules, in the 240-350 K temperature range by using a video-type spectrometer. The foreign-gas broadening parameters and their temperature dependence coefficients were determined using the Voigt profile, no narrowing effect being observed. In order to check the reliability of reported values, we carried out measurements on the J=14_K,12−13_K^′,11 transition located near 206.6 GHz, previously observed in two other laboratories. For this last line all the reported values are consistent themselves within one claimed standard deviation.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

Magnetic properties of high-Bs Fe–Cu–Si–B nanocrystalline soft magnetic alloys

In the present study, the magnetic properties and microstructures of newly developed Fe–Cu–Si–B alloys prepared by annealing the melt-spun ribbon have been studied. The average size and number density of nanocrystalline grains were about 20 nm and 10²³–10²⁴ m⁻³, respectively. The saturation magnetic flux density B_s for the present alloy is more than 1.8 T, that is about 10% larger than that of Fe-based amorphous alloys. Moreover, core loss P of the present alloy is about half of that of Si-steel up to B=1.7 T.     

High sensitivity CW-CRDS of O enriched water near 1.6 μm

The absorption spectrum of ¹⁸O enriched water has been recorded by continuous wave cavity ring down spectroscopy between 5905.7 and 6725.7 cm⁻¹ using a series of fibred DFB lasers. The investigated spectral region corresponds to the important 1.55 μm transparency window of the atmosphere where water absorption is very weak. The typical CRDS sensitivity (noise equivalent absorption of 5×10⁻¹⁰ cm⁻¹) allowed for the detection of lines with intensity as low as 10⁻²⁸ cm/molecule while the minimum intensity value provided by HITRAN in the considered spectral region is 1.7×10⁻²⁴ cm/molecule. The line parameters were retrieved with the help of an interactive least squares multi-lines fitting program assuming a Voigt function as line profile. Overall, 4510 absorption lines belonging to the H₂¹⁸O, H₂¹⁶O, HD¹⁸O, HD¹⁶O and H₂¹⁷O water isotopologues were measured. Their intensities range between 3×10⁻²⁹ and 5×10⁻²³ cm/molecule at 296 K and the typical accuracy on the line positions is 1×10⁻³ cm⁻¹. 2074 of the observed lines attributed to H₂¹⁸O, HD¹⁸O and H₂¹⁷O are reported for the first time. The transitions were assigned on the basis of variational calculations resulting in 288, 135 and 38 newly determined rovibrational energy levels for the H₂¹⁸O, HD¹⁸O and H₂¹⁷O isotopologues, respectively. The new data set includes the band origin of the 4ν₂ bending overtone of H₂¹⁸O at 6110.4239 cm⁻¹ and rovibrational levels corresponding to J and K_a values up to 18 and 12, respectively, for the strongest bands of H₂¹⁸O: 4ν₂, ν₁+2ν₂, 2ν₂+ν₃, 2ν₁, ν₁+ν₃, and ν₂+ν₃. The obtained experimental results have been compared to the spectroscopic parameters provided by the HITRAN database and to the recent IUPAC critical review of the rovibrational spectrum of H₂¹⁸O and H₂¹⁷O as well as to variational calculations. Large discrepancies between the 4ν₂ variationally predicted and experimental intensities have been evidenced for the H₂¹⁸O and H₂¹⁶O molecules.     

Measurements and theoretical calculations of self-broadening and self-shift coefficients in the ν2 band of CH3D

In this paper, we report measured Lorentz self-broadening and self-induced pressure-shift coefficients of ¹²CH₃D in the ν₂ fundamental band (ν₀ ≈ 2200 cm⁻¹). The multispectrum fitting technique allowed us to analyze simultaneously seven self-broadened absorption spectra. All spectra were recorded at the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory (NSO) on Kitt Peak, AZ with an unapodized resolution of 0.0056 cm⁻¹. Low-pressure (0.98-2.95 Torr) as well as high-pressure (17.5-303 Torr) spectra of ¹²C-enriched CH₃D were recorded at room temperature to determine the pressure-broadening coefficients of 408 ν₂ transitions with quantum numbers as high as J″ = 21 and K = 18, where K″ = K′ ≡ K (for a parallel band). The measured self-broadening coefficients range from 0.0349 to 0.0896 cm⁻¹ atm⁻¹ at 296 K. All the measured pressure-shifts are negative. The reported pressure-induced self-shift coefficients vary from about −0.004 to −0.008 cm⁻¹ atm⁻¹. We have examined the dependence of the measured broadening and shift parameters on the J″, and K quantum numbers and also developed empirical expressions to describe the broadening coefficients in terms of m (m = −J″, J″, and J″ + 1 in the ^QP-, ^QQ-, and ^QR-branch, respectively) and K. On average, the empirical expressions reproduce the measured broadening coefficients to within 3.6%. A semiclassical theory based upon the Robert-Bonamy formalism of interacting linear molecules has been used to calculate these self-broadening and self-induced pressure-shift coefficients. In addition to the electrostatic interactions involving the octopole and hexadecapole moments of CH₃D, the intermolecular potential includes also an atom-atom Lennard-Jones model. For low K (K ? 3) with |m| ? 8 the theoretical results of the broadening coefficients are in overall good agreement (3.0%) with the experimental data. For transitions with K approaching |m|, they are generally significantly underestimated (8.8%). The theoretical self-induced pressure shifts, whose vibrational contribution is derived from results in the ^QQ-branch, are generally smaller in magnitude than the experimental data in the ^QP-, and ^QR-branches (15.2%).     

Electrical conductivity of 4-tricyanovinyl-N,N-diethylaniline

Physical characterizations of 4-tricyanovinyl-N,N-diethylaniline, TCVA, have been reported. The differential scanning calorimetry measurements of TCVA showed that this compound is stable up to 423 K. The temperature dependence of electrical conductivity, in the temperature range from 298 to 403 K, was studied on pellet samples of TCVA with evaporated ohmic Au electrodes. The electrical conductivity was found to be 7.01×10⁻⁹ Ω⁻¹ cm⁻¹ at room temperature. The temperature dependence of the electrical conductivity is typical for semiconducting compounds. The current density-voltage (J-V) characteristics of TCVA pellet samples have been investigated at different temperatures. In low-voltage region, the conduction current obeys Ohm's law while the charge transport phenomenon appears to be space-charge-limited current in the higher voltage regions.     

Room-temperature broadening and pressure-shift coefficients in the ν2 band of CH3D-O2: Measurements and semi-classical calculations

We report measured Lorentz O₂-broadening and O₂-induced pressure-shift coefficients of CH₃D in the ν₂ fundamental band. Using a multispectrum fitting technique we have analyzed 11 laboratory absorption spectra recorded at 0.011 cm⁻¹ resolution using the McMath-Pierce Fourier transform spectrometer, Kitt Peak, Arizona. Two absorption cells with path lengths of 10.2 and 25 cm were used to record the spectra. The total sample pressures ranged from 0.98 to 339.85 Torr with CH₃D volume mixing ratios of 0.012 in oxygen. We report measurements for O₂ pressure-broadening coefficients of 320 ν₂ transitions with quantum numbers as high as J″ = 17 and K = 14, where K″ = K′ ≡ K (for a parallel band). The measured O₂-broadening coefficients range from 0.0153 to 0.0645 cm⁻¹ atm⁻¹ at 296 K. All the measured pressure-shifts are negative. The reported O₂-induced pressure-shift coefficients vary from about −0.0017 to −0.0068 cm⁻¹ atm⁻¹. We have examined the dependence of the measured broadening and shift parameters on the J″, and K quantum numbers and also developed empirical expressions to describe the broadening coefficients in terms of m (m = −J″, J″, and J″ + 1 in the ^QP-, ^QQ-, and ^QR-branch, respectively) and K. On average, the empirical expressions reproduce the measured broadening coefficients to within 4.4%. The O₂-broadening and pressure shift coefficients were calculated on the basis of a semiclassical model of interacting linear molecules performed by considering in addition to the electrostatic contributions the atom-atom Lennard-Jones potential. The theoretical results of the broadening coefficients are generally larger than the experimental data. Using for the trajectory model an isotropic Lennard-Jones potential derived from molecular parameters instead of the spherical average of the atom-atom model, a better agreement is obtained with these data, especially for |m| ? 12 values (11.3% for the first calculation and 8.1% for the second calculation). The O₂-pressure shifts whose vibrational contribution are either derived from parameters fitted in the ^QQ-branch of self-induced shifts of CH₃D or those obtained from pressure shifts induced by Xe in the ν₃ band of CH₃D are in reasonable agreement with the scattered experimental data (17.0% for the first calculation and 18.7% for the second calculation).     

Broadening, shifting, and line mixing in the 030 ← 010 parallel Q branch of N2O

The 03¹0 ← 01¹0 parallel Q branch of N₂O has been studied at 297 K and over the pressure range 1-130 torr. Absorption spectra were recorded using a high resolution (1.5 MHz or 5 × 10⁻⁵ cm⁻¹) and high signal-to-noise (>3500:1) mid-infrared spectrometer based on difference-frequency infrared generation in AgGaS₂. In the low-pressure range (1-11 torr) we obtained accurate values for the line strengths, the broadening coefficients, the weak mixing coefficients, and the overall shifting of the branch. The medium pressure results, ranging from 23 to 130 torr, were analyzed by treating the band as a whole, using a relaxation matrix formalism, based on an energy gap scaling law. We find, effectively, that only 36% of the rotationally inelastic collisions are associated with Q branch mixing, the rest presumably being associated with Q-P and Q-R mixing in the same vibrational band. The pressure shifting coefficient of the 03¹0 ← 01¹0 Q branch as a whole was also determined and found to be 5.8 × 10⁻³ cm⁻¹/atm towards lower frequencies.     

Synthesis, physical and photo electrochemical characterization of La-doped SrSnO3

The transport properties of Sr_0.98La_0.02SnO_3−δ in the system Sr_1−xLa_xSnO_3−δ, after which the pyrochlore La₂Sn₂O₇ appears, were investigated over the temperature range 4.2-300 K. The oxide was found to be n-type semiconductor with concomitant reduction of Sn⁴⁺ into Sn²⁺. The magnetic susceptibility was measured down to 4.2 K and is less than 3×10⁻⁵ emu cgs mol⁻¹ consistent with itinerant electron behavior. The electron is believed to travel in a narrow band of Sn:5s character with an effective mass ∼4 m_o. The highest band gap is 4.32 eV and the optical transition is directly allowed. A further indirect transition occurs at 4.04 eV. The electrical conductivity follows an Arrhenius-type law with a thermal activation of 40 meV and occurs by small polaron hopping between nominal states Sn^4+/2+. The linear increase of thermo-power with temperature yields an electron mobility μ_300 K (2×10⁻⁴ cm² V⁻¹ s⁻¹) thermally activated. The insulating-metal transition seems to be of Anderson type resulting from random positions of lanthanum sites and oxygen vacancies. At low temperatures, the conduction mechanism changes to a variable range hopping with a linear plot Ln ρ⁻¹ vs. T⁻⁴. The photo electrochemical (PEC) measurements confirm the n-type conductivity and give an onset potential of −0.46 V_SCE in KOH (1 M). The Mott-Schottky plot C⁻²-V shows a linear behavior from which the flat band potential V_fb=+0.01 V_SCE at pH 7 and the doping density N_D=1.04×10²¹ cm⁻³ were determined.     

DC conductivity of silver vanadium tellurite glasses

The mixed electronic-ionic conduction in 0.5[xAg₂O-(1−x)V₂O₅]-0.5TeO₂ glasses with x=0.1-0.8 has been investigated over a wide temperature range (70-425 K). The mechanism of dc conductivity changes from predominantly electronic to ionic within the 30?mol% Ag₂O?40 range; it is correlated with the underlying change in glass structure. The temperature dependence of electronic conductivity has been analyzed quantitatively to determine the applicability of various models of conduction in amorphous semiconducting glasses. At high temperature, T>θ_D/2 (where θ_D is the Debye temperature) the electronic dc conductivity is due to non-adiabatic small polaron hopping of electrons for 0.1?x?0.5. The density of states at Fermi level is estimated to be N(E_F)≈10¹⁹-10²⁰ eV⁻¹ cm⁻³. The carrier density is of the order of 10¹⁹ cm⁻³, with mobility ≈2.3×10⁻⁷-8.6×10⁻⁹ cm² V⁻¹ s⁻¹ at 300 K. The electronic dc conductivity within the whole range of temperature is best described in terms of Triberis-Friedman percolation model. For 0.6?x?0.8, the predominantly ionic dc conductivity is described well by the Anderson-Stuart model.     

Physical properties of the delafossite LaCuO2

High-quality LaCuO₂, elaborated by solid-state reaction in sealed tube, crystallizes in the delafossite structure. The thermal analysis under reducing atmosphere (H₂/N₂: 1/9) revealed a stoichiometric composition LaCuO_2.00. The oxide is a direct band-gap semiconductor with a forbidden band of 2.77 eV. The magnetic susceptibility follows a Curie-Weiss law from which a Cu²⁺ concentration of 1% has been determined. The oxygen insertion in the layered crystal lattice induces p-type conductivity. The electrical conduction occurs predominantly by small polaron hopping between mixed valences Cu^+/2+ with an activation energy of 0.28 eV and a hole mobility (μ_300 K=3.5×10⁻⁷ cm² V⁻¹ s⁻¹), thermally activated. Most holes are trapped in surface-polaron states upon gap excitation. The photoelectrochemical study, reported for the first time, confirms the p-type conduction. The flat band potential (V_fb=0.15 V_SCE) and the hole density (N_A=5.8×10¹⁷ cm⁻³) were determined, respectively, by extrapolating the curve C⁻² versus the potential to their intersection with C⁻²=0 and from the slope of the linear part in the Mott-Schottky plot. The valence band is made up of Cu-3d orbital, positioned at 4.9 eV below vacuum. An energy band diagram has been established predicting the possibility of the oxide to be used as hydrogen photocathode.     

High-resolution FTIR and MMW study of the v4 = 2 (A1, E) excited state of NF3 near 985 cm: the axial ground state rotational constants derived by the “loop-method”

The two substates v₄ = 2⁰ (A₁, 983.702 cm⁻¹) and v₄ = 2^±2 (E, 986.622 cm⁻¹) of the oblate symmetric top molecule, ¹⁴NF₃, have been studied by high-resolution (2.5 × 10⁻³ cm⁻¹) infrared spectroscopy of the overtones and 2ν₄ − ν₄ hot bands. Transitions of the overtone, the hot band, and the previously measured fundamental band were combined to yield 585 ground state combination differences differing in K by ±3, with K_max = 36. Using the “loop-method,” a fit (standard deviation σ = 0.320 × 10⁻³ cm⁻¹) provided a complete set of the hitherto not experimentally known axial ground state constants. In units of cm⁻¹ these have the following values: . Upper state parameters were determined using a vibrationally isolated model. Considering l (2, 2) and l (2, −1) interactions between the v₄ = 2⁰ and v₄ = 2^±2 substates and effects accounting for the l (4, −2) interactions within the kl = −2 levels, 25 upper state parameters were obtained by fitting 2747 IR data (1842 transitions, 905 deduced energies, J_max = 42, K_max = 39) with σ_IR = 0.353 × 10⁻³ cm⁻¹. Moreover, millimeter-wave spectroscopy furnished 86 transitions (J_max = 16, K_max = 13) measured on the v₄ = 2 excited state. A merged fit, refining 24 parameters using the described model gave σ_IR = 0.365 × 10⁻³ cm⁻¹ andσ_MMW = 0.855 × 10⁻⁶ cm⁻¹ (26 kHz). The anharmonicity constants (in cm⁻¹) are x₄₄ = −0.84174 (2) and g₄₄ =  + 0.73014 (1). In addition to this model, the D, Q, and L reductions of the rovibrational Hamiltonian were tested. Standard deviations σ_IR = 0.375 × 10⁻³ cm⁻¹ and σ_MMW = 0.865 × 10⁻⁶ cm⁻¹ were obtained for both D and L reductions, and σ_IR = 0.392 × 10⁻³ cm⁻¹ and σ_MMW = 0.935 × 10⁻⁶ cm⁻¹ for Q reduction. The unitary equivalence of the majority of the 18 tested relations between the derived parameters was satisfactorily fulfilled. This confirms that the v₄ = 2 excited vibrational state can be considered in reasonable approximation to be isolated.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Structural evolution of electroless Ni-P coating on Al-12 wt.% Si alloy during heat treatment at high temperatures

The work is concerned with the high-temperature heat treatment of an Al-12 wt.% Si alloy coated by an electroless Ni-P layer. The electroless deposition took place on a pre-treated substrate in a bath containing nickel hypophosphite, nickel lactate and lactic acid. Resulting Ni-P deposit showed a thickness of about 8 μm. The coated samples were heat-treated at 200-550 °C/1-24 h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adherence to the substrate was estimated from the scratch test and microhardness of the heat-treated layers was also measured. It is found that various phase transformations occur, as both temperature and annealing time increase. These include (1) amorphous Ni-P → Ni + Ni₃P, (2) Al + Ni → Al₃Ni, (3) Ni₃P → Ni₁₂P₅ + Ni, (4) Ni₁₂P₅ → Ni₂P + Ni, and (5) Al₃Ni + Ni → Al₃Ni₂. The formation of intermetallic phases, particularly Al₃Ni₂, leads to significant surface hardening, however, too thick layers of intermetallics reduce the adherence to the substrate. Based on the growth kinetics of the intermetallic phases, diffusion coefficients of Ni in Al₃Ni and Al₃Ni₂ at 450-550 °C are estimated as follows: D(Al₃Ni, 450 °C) ≈ 6 × 10⁻¹² cm² s⁻¹, D(Al₃Ni, 550 °C) ≈ 4 × 10⁻¹¹ cm² s⁻¹, D(Al₃Ni₂, 450 °C) ≈ 1 × 10⁻¹² cm² s⁻¹ and D(Al₃Ni₂, 550 °C) ≈ 1 × 10⁻¹¹ cm² s⁻¹. Mechanisms of phase transformations are discussed in relation to the elemental diffusion.