Vol.:(0123456789) 1 3 Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-019-01482-y Niobium oxide prepared by sol–gel using powder coconut water J. M. F. Lucas 1  · S. Soreto Teixeira 1  · S. R. Gavinho 1  · P. R. Prezas 1  · C. C. Silva 1  · A. J. M. Sales 1  · M. A. Valente 1  · A. F. Almeida 2  · F. N. Freire 2  · C. C. M. Salgueiro 3  · J. F. Nunes 3  · M. P. F. Graça 1 Received: 13 November 2018 / Accepted: 7 May 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Niobium oxides are presently very important materials for electronic device applications. In this work, a new approach, using the proteic sol–gel route, was used to prepare niobium pentoxide powders. This approach uses powdered coconut water (PCW) for the powders production. The obtained powders, with an amorphous structure, were heat-treated at difer- ent temperatures and their structural and morphological properties were studied and related to their dielectric properties. Dielectric measurements as a function of frequency (100 Hz–1 MHz) and temperature (200–400 K) were performed. The heat-treatment promotes the formation of crystalline structures, mainly Nb 2 O 5 with orthorhombic structure, which transforms to monoclinic with treatments at 1000 °C. The formation of CaNb 2 O 6 and NaNbO 3 crystallites, in the samples treated at 800 and 1000 °C is related to the PCW base composition. The samples revealed a quasi-independence of the dielectric constant with temperature and frequency, presenting a   ≈ 70 and tan ≈ 0.006, at room temperature and 100 kHz. 1 Introduction There is currently a great deal of interest in materials with high dielectric constant values and low dielectric losses, as these are a hotspot of research in microelectronics. Nio- bium pentoxide is a very promising dielectric material that fulflls these desired attributes. One of the main potential applications of niobium pentoxide is in the production of solid electrolyte capacitors [1–4], as this is an extremely promising substitute for tantalum pentoxide, which is the most used material in this technology. The reasons for this change to happen resides in the fact that amorphous niobium pentoxide is a dielectric material with higher dielectric con- stant (   ≈ 41) than tantalum pentoxide (   ≈ 27) [4–6], in addition to niobium being more abundant in nature and less expensive. The need of a solid electrolyte capacitors with higher ε, has led researchers and industrials to explore this potential application of niobium pentoxide. Furthermore, it possesses a vast range of interesting properties which can be explored for other various applications. Considering the photo and electrochromic properties of niobium pentoxide it is reported that by applying an appropriate voltage the colors of thin flms can be changed [7–10]. Other applica- tions include its use in lithium batteries [8, 11], humidity sensors [12], photoelectrochemical DNA biosensors [13], catalysis of various reactions [14, 15], being the catalysis of hydrogen storage in MgH 2 an example [16–19]. There are diferent structural polymorphs of niobium oxide that can be obtained, and consequently, some of the physical properties difer between them [3, 20–22]. Addi- tionally, diferent synthesis methods can cause the same polymorph to show diferent physical properties [21–26]. In general, niobium pentoxides structures consist of NbO 6 octahedra which can be more or less distorted as a conse- quence of the nature of the linkage between octahedra [27]. The multiplicity of Nb 2 O 5 polymorphs originates from the diferent geometrical combinations of one or both types of links between octahedra, which can occur by edge-sharing or corner-sharing [20, 27]. The most employed methods for the preparation of dif- ferent niobium pentoxide polymorphs comprehend the oxidation of lower stoichiometry niobium oxides, by heat- ing in air, or by heat treatment of other niobium pentoxide * S. R. Gavinho silvigavinho@ua.pt * M. P. F. Graça mpfg@ua.pt 1 I3 N and Physics Department, Aveiro University, 3800-193 Aveiro, Portugal 2 Mechanics Engineering Department, Ceará Federal University, Fortaleza, Brazil 3 Faculty of Veterinary Medicine, Integrated Nucleus of Biotechnology, Ceará State University, Fortaleza, Brazil