No abstract

It is known that the optical absorption property of a material is linked to its crystal structure. Therefore, it can be modified or displaced when the material is subjected to thermal treatments, generating displacements of the unit cell, atomic positions and bond angles of its atoms. In this work, the analysis of three samples of BiFeO3 powders subjected to different heat treatment temperatures was carried out to study their optical properties by analyzing the crystal structure, diffuse reflectance, Raman scattering spectroscopy, density of state and band structure. In the results, the pure phase and the hexagonal structure of the samples were identified, showing an increase in the angles between the Fe-O-Fe atoms when the material is thermally treated. The band gap (Egexperimental) of the BiFeO3 samples without and with treatment temperature showed direct and indirect transitions according to the Tauc equation, these values were 2.36, 2.11 and 2.19 eV for direct band gaps and 2.10, 1.58 and 1.85 eV for indirect band gaps. In the Raman scattering spectroscopy analysis, overtones were observed in the samples when a green laser with a wavelength of 532nm (2.33 eV) was applied. In the density of states, the maximum valence band formed by the O(2p) state and the minimum of the conduction band formed by the hybridization of the Fe(3d) and O(2p) states were identified. In the electronic band structure, indirect band gap values (Egtheoretical) of 2.274, 2.216 (500 °C) and 2.217 eV (600 °C) were found for the samples without and with heat treatment. It is concluded that BiFeO3 absorbs visible light and that its band gap can decrease by increasing the bond angle of Fe-O-Fe atoms when the sample is subjected to heat treatment. These results demonstrate the semiconductor property of BiFeO3 and can be applied to solar cells, batteries, hydrogen production, among other applications.

Bismuth ferrite ceramic powders (BiFeO3) were synthesized by a combustion reaction in solution, using the mixture of urea and glycine as fuels, whose objectives were to improve their optical, electrical and photovoltaic response properties. X-ray diffraction analysis indicated the formation of the single phase BiFeO3. Transmission electron microscopy (TEM) shows images of uniform nanoparticles of 70 nm on average. Through diffuse reflectance we obtain a band gap with a direct electronic transition of 2.14 eV, this value presents an improvement compared to that reported by other authors in BiFeO3 ceramic powders. The value of the resistivity obtained by the 4-pointed method was ρ = 0.486 × 106 Ω.cm, checking that our material is a semiconductor. Finally, by building a prototype of Ag/BiFeO3/Ag, with visible light (λ = 405 nm), a photovoltaic response of 0.2 nA at room temperature was achieved.

In this work, the multilayer perceptron model was used to forecast the time series of global solar radiation for a near future about a week. Different architectures of this model were built through varying its different hyperparameters such as optimizers, activation functions, number of neurons and neuron dropout in which their performance was evaluated using error metrics. It was found that the architectures (60, SGD, Sigmoid), (10, Adam, Relu) and (60, SGD, Sigmoid) presented an R2 around 0.877, 0.873 and 0.872, respectively. The architecture with neuron dropout (150, SGD, Sigmoid, 0.2) presented a higher performance among all the architectures evaluated and its R2 value was 0.884. Architectures with higher performance are used to predict future values of solar radiation.