Electrospun Ca3Co4−xO9+δ nanofibers and nanoribbons: Microstructure and thermoelectric properties

Oxide-based ceramics offer promising thermoelectric (TE) materials for recycling high-temperature waste heat, generated extensively from industrial sources. To further improve the functional performance of TE materials, their power factor should be increased. This can be achieved by nanostructuring and texturing the oxide-based ceramics creating multiple interphases and nanopores, which simultaneously increase the electrical conductivity and the Seebeck coefficient. The aim of this work is to achieve this goal by compacting electrospun nanofibers of calcium cobaltite Ca3Co4−xO9+δ, known to be a promising p-type TE material with good functional properties and thermal stability up to 1200 K in air. For this purpose, polycrystalline Ca3Co4−xO9+δ nanofibers and nanoribbons were fabricated by sol–gel electrospinning and calcination at intermediate temperatures to obtain small primary particle sizes. Bulk ceramics were formed by sintering pressed compacts of calcined nanofibers during TE measurements. The bulk nanofiber sample pre-calcined at 973 K exhibited an improved Seebeck coefficient of 176.5 S cm−1 and a power factor of 2.47 μW cm−1 K−2 similar to an electrospun nanofiber-derived ceramic compacted by spark plasma sintering

Complex formation and degradation in poly(acrylonitrile-co-vinyl acetate) containing metal nitrates

Abstract Polymers containing metal-nitrates have been proposed as advantageous precursors for high temperature superconductors such as yttriumbarium-copper-oxide (YBCO). The advantage lies in using conventional polymer processing such as fiber spinning or microlithography before pyrolysis. This research investigated complex formation and degradation in poly(acrylonitrile-co-vinyl acetate) (P(AN-VA)) containing either yttrium nitrate (YN) or barium nitrate (BaN) and follows a similar investigation for copper nitrate. Complex formation was observed in P(AN-VA)/YN but not in P(AN-VA)/BaN. The exothermic nitrate degradation below the P(AN-VA)) cyclization temperature involved the release of NO 3 , a reaction with the nitrile group that disrupted cyclization, and, for BaN, P(AN-VA) degradation. For a P(AN-VA)/nitrate ratio of 2/1 there was no cyclization and the degradation temperature was reduced by about 200 8C. The pyrolysis of P(AN-VA)/BaN yielded largely BaCO 3 , which is likely to impede the formation of YBCO.

Superior Thermoelectric Performance of Textured Ca3Co4−xO9+δ Ceramic Nanoribbons

Calcium cobaltite Ca3Co4−xO9+δ (CCO) is a promising p-type thermoelectric (TE) material for high-temperature applications in air. The grains of the material exhibit strong anisotropic properties, making texturing and nanostructuring mostly favored to improve thermoelectric performance. On the one hand multitude of interfaces are needed within the bulk material to create reflecting surfaces that can lower the thermal conductivity. On the other hand, low residual porosity is needed to improve the contact between grains and raise the electrical conductivity. In this study, CCO fibers with 100% flat cross sections in a stacked, compact form are electrospun. Then the grains within the nanoribbons in the plane of the fibers are grown. Finally, the nanoribbons are electrospun into a textured ceramic that features simultaneously a high electrical conductivity of 177 S cm−1 and an immensely enhanced Seebeck coefficient of 200 µV K−1 at 1073 K are assembled. The power factor of 4.68 µW cm−1 K−2 at 1073 K in air surpasses all previous CCO TE performances of nanofiber ceramics by a factor of two. Given the relatively high power factor combined with low thermal conductivity, a relatively large figure-of-merit of 0.3 at 873 K in the air for the textured nanoribbon ceramic is obtained

Complex Formation and Degradation in Poly(Acrylonitrile- co-Vinyl Acetate) Containing Copper Nitrate

ABSTRACT: The pyrolysis of polymers containing metal nitrates may provide a relatively simple, rapid, and advantageous method of producing high-temperature superconductors (HTSCs). The advantage lies in the ability to use conventional polymer processing or microlithographic patterning before pyrolysis. A copolymer of acrylonitrile and vinyl acetate [P(AN-VA)], a well-known fiber-forming polymer, was investigated as a potential HTSC precursor. Complex formation with the highly polar acrylonitrile groups was expected to enhance atomic-level mixing and hinder nitrate recrystallization. The metal nitrates were found to have a profound effect on P(AN-VA) pyrolysis. P(AN-VA) containing copper nitrate (CuN) exhibited complex formation and an exothermic decomposition that began at about 170°C (reaction 1-CuN). Reaction 1-CuN had a heat of about 3.5 kJ/g NO3 and a mass loss of about 0.99 g/g NO3 . As reaction 1-CuN also involved the nitrile groups, it disrupted the nitrile cyclization reaction at about 290°C. For a P(AN-VA)/CuN ratio of 2/1, there was no nitrile cyclization, and the thermooxidative degradation temperature was reduced by approximately 200°C

Thermal degradation of poly(acrylic acid) containtig cooper nitrite, Polymer Degradation and Stability 86

ABSTRACT: The use of polymers containing metal salts as ceramic high-temperature superconductor (HTSC) precursors may provide a relatively simple and rapid method for producing materials that can take advantage of advanced polymer processing and then be pyrolyzed to HTSCs. The mechanisms of thermal degradation in these precursors, which have not been characterized, can be used to optimize the pyrolysis conditions for HTSC production. This article describes the degradation of a precursor based on poly (acrylic acid) (PAAc) containing yttrium, barium, and copper nitrates in the proportions needed for the formation of the HTSC YBa 2 Cu 3 O 7Àx (YBCO). This article also describes the effects of the pyrolysis process on the resulting materials. The degradation of the precursor is a complex, multistage process. The presence of the metal ions and HNO 3 reduces the thermal stability of PAAc and increases the degradation rate. The results indicate that the initial stages of the pyrolysis should be conducted in argon or nitrogen to inhibit BaCO 3 formation and that the final stages should be conducted in air/oxygen to enhance oxidation. Optimization of the pyrolysis conditions produces a YBCO film with minimal contamination.