Incommensurately modulated crystal structure of α′ (O′3)-type sodium cobalt oxide Na x CoO2 (x ∼ 0.78)

A single-phase sample of α′ (O′3)-type layered sodium cobalt oxide Na x CoO2 (x ∼ 0.78) was prepared and its incommensurately modulated crystal structure was analyzed using the (3+1)-dimensional superspace approach to the powder neutron diffraction data. The crystal structure of the cobaltate is accurately described based on the superspace group C2/m(α0γ)00, wherein the positions of Na atoms are most significantly modulated in the monoclinic a direction to form an ordered arrangement. Such a displacive modulation causes a quasi-periodic shift of Na atoms from the centers of the NaO6 polyhedra between undulated CoO2 sheets, changing the form of the NaO6 polyhedron from an octahedral coordination (O) to a trigonal prismatic (P) one, via an intermediate capped trigonal prismatic NaO7 coordination (C). At the positions where the Na atoms are most significantly shifted, the neighboring Na atoms are located at almost touching distances. However, the occupation factor of Na atoms becomes zero at such positions, yielding Na-deficient sites V Na, sandwiched either between C and P, or C and C-type polyhedra.

Effect of Poly(Vinyl Alcohol) on Thermoelectric Properties of Sodium Cobalt Oxide

Organic polymer composites are relatively simple to process and are therefore used in thermoelectric materials. The organic polymers are used as an adhesive agent between thermoelectric material grains. Thermoelectric effects of poly (vinyl alcohol) (PVA) composited with sodium cobalt oxide (NaxCoO2) were studied in this work. PVA is a low cost and an excellent biocompatibility polymer. High electrical conductivity, high Seebeck coefficient and low thermal conductivity are required in thermoelectric materials. As PVA is an insulating material, the PVA in between NaxCoO2 grain boundaries has an effect on the low electrical conductivity of NaxCoO2 composite. This results in a decrease in thermoelectric efficiency. However, PVA has been utilized to increase the Seebeck coefficient and also enhance thermoelectric efficiency. In order to improve the electrical conductivity of NaxCoO2 composite, PVA removal was produced by furnace heating at 500̊ C to eliminate PVA from NaxCoO2/PVA sample. The general thermoelectric parameters including the Seebeck coefficient, electrical conductivity and power factor of NaxCoO2/PVA and PVA removal sample were compared. X-ray diffraction patterns (XRD) and scanning electron microscope (SEM) images were used to identify the phase identification and morphology study, respectively. The results showed that the PVA removal sample had higher electrical conductivity than the NaxCoO2/PVA sample. However, NaxCoO2/PVA sample had higher thermoelectric performance than the PVA removal sample because the NaxCoO2/PVA sample showed higher Seebeck coefficient and power factor.

Thermoelectric Properties of Nickel and Boron Co-Substituted NaCo2O4 Prepared by Electrospinning Technique

In this study nickel and boron doped sodium cobalt oxide NaCo2-xNixByO4 (0≤x≤0.3, 0≤y≤0.1) nanocrystalline thermoelectric ceramic powders were synthesized using electrospinning techniques and then consolidated into bulk ceramics. The differences in the microstructure and thermoelectric properties of the samples as a result of doping effect have been investigated. The crystalline structures of the powders and nanofibers were characterized using X-ray diffraction and scanning electron microscopy and BET Analysis before and after the calcination process at different temperatures. Nanofibers prepared by the use of electrospinning technique, have a diameter of approximately 300 nm, and the diameter of the grains of calcined powders was observed to range between 150 to 500 nanometers. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system (Lot-Oriel PPMS) in a temperature range of 15–300 K. The calculated values of dimensionless figure of merit at 300 K are 4.25×10-5, 5.3×10-6, 8.6×10-5 and 9×10-6 for sintered powders from undoped, Ni and B doped powders, respectively.