Na-doped Materials Research Articles

Na Doping in PbTe: Solubility, Band Convergence, Phase Boundary Mapping, and Thermoelectric Properties.

Many monumental breakthroughs in p-type PbTe thermoelectrics are driven by optimizing a Pb0.98Na0.02Te matrix. However, recent works found that x > 0.02 in Pb1-xNaxTe further improves the thermoelectric figure of merit, zT, despite being above the expected Na solubility limit. We explain the origins of improved performance from excess Na doping through computation and experiments on Pb1-xNaxTe with 0.01 ≤ x ≤ 0.04. High temperature X-ray diffraction and Hall carrier concentration measurements show enhanced Na solubility at high temperatures when x > 0.02 but no improvement in carrier concentration, indicating that Na is entering the lattice but is electrically compensated by high intrinsic defect concentrations. The higher Na concentration leads to band convergence between the light L and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. This results in a high temperature zT nearing 2 for Pb0.96Na0.04Te, ∼25% higher than traditionally reported values for pristine PbTe-Na. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high zT Na-doped materials. A starting matrix of simple, high performing Pb0.96Na0.04Te synthesized following our guidelines may be superior to Pb0.98Na0.02Te for continued zT optimization in p-type PbTe materials.

Impact of the modification method of Ni/ZrO2 catalyst by alkali and alkaline earth metals on its activity in thermo-chemical conversion of cellulose

The goal of this work is to determine an impact of the modification method of Ni/ZrO2 catalyst by alkali and alkaline earth metals on its activity in thermo-chemical conversion of cellulose to hydrogen-rich gas. MexO-ZrO2 supports (where Me = Ca, Mg, Na or K) were prepared by impregnation, precipitation and sol-gel methods. The obtained results reveal that an introduction of dopants to the zirconia support considerably enhances the H2 yield in comparison to unmodified catalyst. An increase in the hydrogen formation is accompanied by a rise in the total volume of the produced gases. It is demonstrated that the highest amount of hydrogen is formed in the presence of the catalysts containing CaO-ZrO2 support followed by Na doped materials. This phenomenon can be attributed to more efficient incorporation of Ca2+ and Na+ cations in the zirconia lattice making it more stable in the reaction conditions. Moreover, it is observed that an activity order of the investigated catalysts is consistent with the changes in the basic character of their surface.

Development of Li(Ni1/3Mn1/3Co1/3-x Na x )O2 cathode materials by synthesizing with glycine nitrate combustion technique for Li-ion rechargeable batteries

Glycine nitrate combustion technique was investigated for synthesizing Li(Ni1/3Mn1/3Co1/3-x Na (x) )O-2, x = 0-0.11 based transition metal oxide cathode materials for the rechargeable Li-ion battery (LIB) under this study. X-ray diffraction and scanning electron microscopy analysis showed that the synthesized powder samples were well crystalline rather spherical secondary particles. These secondary particles were composed of softly agglomerated nano-scale primary particles. The room temperature electrical conductivity of these Na-doped materials was significantly higher than that of the base material (2.60 x 10(-7) S/cm). Among them, the x = 0.04 material reported the highest electrical conductivity of 1.02 x 10(-03) S cm(-1). The half-cell assembled with cathode fabricated from Li(Ni1/3Mn1/3Co1/3)O-2 base material showed an initial discharge capacity of 187 mA h(-1) g(-1) with 25 mA h(-1) g(-1) irreversible capacity loss and 88.47% columbic efficiency at C/5 rate with a cut-off voltage of 2.5-4.6 V at 25 A degrees C. The electrochemical behavior of the x = 0.04 cathode showed a comparable initial discharge capacity as of the base material but with improved capacity retention.

Enhanced high-rate capability and cycling stability of Na-stabilized layered Li1.2[Co0.13Ni0.13Mn0.54]O2 cathode material

Na-stabilized Li1.2−xNax[Co0.13Ni0.13Mn0.54]O2 is synthesized by introducing larger Na ions into the Li slabs of the layered material through a simple polymer-pyrolysis method. The structural and morphological characterization reveals that the Na doping leads to a more ordered structure with regular cubic morphology and enlarged Li layer spacing. Electrochemical experiments show that the Na-doped Li1.17Na0.03[Co0.13Ni0.13Mn0.54]O2 electrode can deliver larger reversible discharge capacity (307 mA h g−1), higher initial coulombic efficiency (87%), greatly improved rate capability (139 mA h g−1 at 8 C) and cycling stability (89% capacity retention after 100 cycles) in comparison with the undoped Li1.2[Co0.13Ni0.13Mn0.54]O2 electrode. The superior electrochemical performance of the Na-doped material is due to the enhancement of the structural stability and the enlargement of the Li slab space of the layered material, which facilitate the stabilization of the host lattice and allow rapid diffusion of Li ions in the bulk lattice.

Solid-State NMR Study of Na versus K Doping of para-Phenylene Oligomers

13C solid-state nuclear magnetic resonance (NMR) experiments were performed on p-terphenyl, p-quaterphenyl, and p-sexiphenyl either in their pristine or doped with alkali metals form. The 13C NMR spectra of doped materials show new resonances by comparison with pristine compounds. For the K-doped materials, these resonances appear in the 90-135 ppm range, while for Na-doped materials, they are observed in the larger 20-150 ppm range. It suggests that the interaction between the alkali ions and the oligomers depends on the nature of the alkali. It is corroborated by 13C NMR experiments after exposure to air that show different behaviors. As expected, air exposure of K-doped samples restores the pristine spectra. This is not the case for Na doping, where the signature of the doped material persists even after exposure to air. In the latter case, some 13C resonances can be assigned to sp3 hybridized carbons and to the quinoid group. It suggests that Na doping induces a polymerization of the oligophenylenes.

Effect of alkaline-doping on the properties of La2Mo2O9 fast oxygen ion conductor

Synthesis by means of a modified Pechini method of pure and alkaline-doped (Na, K and Rb) La2Mo2O9 (LAMOX) is presented. The electrical conductivity of LAMOX prepared by this method is completely analogous to that of the solid state prepared material. Doping with K and Rb hinders the α → β phase transition found in the pure and Na-doped material around 580 °C. Electrical measurements as a function of pO2 shows that LAMOX has a mainly ionic conductivity down to 10−18 atm. The electrical conductivity of the doped samples is close to that of the pure molybdate for temperatures higher than about 600 °C, while for K- and Rb-doped samples the conductivity is higher for lower temperatures, as a consequence of the absence of the phase transition. Finally, a non-Arrhenius behaviour is observed for all the investigated samples and correlated, at a first approximation, to the structural peculiarities of the LAMOX family.

Characterisation of K-doped Bi4V2O11 by combined XRD and EPMA: A cautionary tale

A comparison of X-ray powder diffraction and electron probe microanalyses on samples of Na- and K-doped Bi4V2O11 (BIMEVOX) solid electrolytes indicates some of the problems associated with using XRD to assess phase purity, especially in materials of variable composition such as BIMEVOXes. In this study both the Na- and K-doped materials appeared phase pure by XRD. EPMA indicated the Na-doped materials to be single phase with their expected compositions while the K-containing materials were not phase pure with very little K present in the main BIMEVOX phase.

Single-Crystal Growth, Alkali Metal Ordering, and Superconductivity in La2-xMxCuO4 (M = Na, K)

Single crystals of La{sub 2-x}M{sub x}CuO{sub 4}, where M is Na or K, have been prepared by precipitation from molten NaOH at 320{degrees}C or from molten KOH at 450{degrees}C. The composition of several individual crystals as determined by wavelength-dispersive X-ray fluorescence analysis averaged to La{sub 1.88}Na{sub 0.12}CuO{sub 4} and La{sub 1.79}K{sub 0.21}CuO{sub 4}. Iodometric titrations of polycrystalline samples with identical compositions showed that the formal oxidation states for the Cu atoms are +2.19 and +2.31, respectively. On the basis of the analysis of single-crystal X-ray diffraction data, it has been determined that the alkali metal doped lanthanum copper oxides crystallize in a new structure type that this distinct from the analogous phases containing alkaline earth metal ions. The space group is Pr/nmm, and lattice parameters are a = 3.766(1) {angstrom}, c = 13.215(5) {angstrom} for the Na-doped material and a = 3.7708(7) {angstrom}, c = 13.321(2) {angstrom} for the K-doped material. In these phases, the alkali metal ions are ordered in the rock-salt blocks, and the Cu atoms are displaced out of the plane toward the alkali metal ions. Despite this distortion of the CuO{sub 2} planes, both phases exhibit superconductivity, near 26 and 6 K, respectively.

Systematic investigation of shallow acceptor levels in ZnSe

A systematic investigation of shallow acceptor levels in ZnSe grown by molecular beam epitaxy (MBE) has been performed using low temperature photoluminescence (PL) measurements as a function of excitation level, temperature, strain, and laser energy (i.e., selectively excited donor-acceptor pair luminescence or SPL). Five of the levels are due to N, Li, As, P, and O, while the chemical origins of two levels, denoted A 1 and A 2, have not yet been determined. The A 1 level is observed in undoped material after annealing using diamond-like C (DLC) caps, while the A 2 level is observed in nominally Na-doped material. The ionization energies of these levels are accurately determined from the temperature dependence of the band-to-acceptor (e-A 0) peak positions, accounting for strain. Those energies are 114.3 +- 0.5, 114.2 +- 0.3, 111.3 +- 0.5, 106.1 +- 0.6, 95.0 +- 0.4, 88.4 +- 0.5, and 83 +- 3 meV, respectively, for As, Li, N, A 1, A 2, P, and O in unstrained material. Several excited states have been observed in SPL measurements for As, A 2, O, and P for the first time. The excited states of As, O, and A 2 fit well to effective mass theory, while those for P do not. A model for the strain splitting of shallow acceptor-bound excitions has been developed and confirmed using measurements on samples whose substrates have been removed. Hayne's Rule is shown to be inapplicable to shallow acceptors in ZnSe. A strain splitting of the (e-A 0) peak fo As or Li acceptors in annealed material is clearly resolved and modeled.

The solubility of sodium in silicon

Silicon crystals were exposed to the liquid solution of Na saturated with Si at 800°C for about one week. Penetration of Na into the crystals was measured by neutron activation analysis. Sodium concentrations of the order of 3 × 10 17 cm −3 were found in a thin (∼ 50 μ thick) surface layer; in the interior of the specimens, the Na concentration was at or below the detection limit of ∼ 10 15 cm −3. No n-type regions could be found in the Na-doped material by thermoelectric type testing, in striking contrast to the case of ion-injected Na, where donor concentrations of ∼ 10 20 cm −3 are observed. We conclude that the solubility of Na in Si at 800°C is of the order of 3 × 10 17 cm −3, but that under the nearly equilibrium conditions of the present experiment less than one per cent of this is present as interstitial donors.