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Dielectric Behavior of Α-Ag2wo4 and Its Huge Dielectric Loss Tangent 1. Introduction

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Dielectric Behavior of Α-Ag2wo4 and Its Huge Dielectric Loss Tangent 1. Introduction Materials Research. 2019; 22(4): e20190058 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2019-0058 Dielectric Behavior of α-Ag2WO4 and its Huge Dielectric Loss Tangent Natalia Jacomacia* , Euripedes Silva Juniora , Fernando Modesto Borges de Oliveirab , Elson Longob , Maria Aparecida Zaghetea aInstituto de Química, Universidade Estadual Paulista (Unesp), 14800-060, Araraquara, SP, Brasil bDepartamento de Química, Universidade Federal de São Carlos (UFSCar), 13565-905, São Carlos, SP, Brasil Received: January 21, 2019; Revised: May 08, 2019; Accepted: June 29, 2019 The microwave-assisted hydrothermal method was used to obtain α-Ag2WO4. Rietveld refinement confirmed that α-Ag2WO4 is stable in the orthorhombic phase, without secondary phase. However, field- effect scanning electron microscope analysis showed that α-Ag2WO4 nanorods surfaces contain silver nanoparticles, confirmed by the X-ray photoelectron spectroscopy by the peak observed at 374.39 eV. In addition to metallic Ag, other Ag oxidation states were also observed on the surface. Hence, Ag (I) as Ag2O and Ag (I) as Ag2WO4 also were identified. DC measurements exhibited a high capacity of charge storage, nevertheless, with a large loss tangent (0.12 µC.cm-2.V-1) and no residual polarization for the voltage range between -100 V and +100 V. AC measurements at frequencies less than 275 Hz, revealed that ionic polarization is dominant, whereas at frequencies higher than 275 Hz, the electronic behavior predominates. The potential of electromagnetic energy conversion in thermal was observed from loss tangent analysis. Keywords: Silver tungstate nanorods, dielectric behavior, ionic and electronic polarization. 1. Introduction Among transition metal tungstates, silver tungstate (Ag2WO4) has attracted the attention of the scientific and technological Tungsten-silver materials are known to exhibit very good community because of its chemical stability, easy synthesis15, erosion resistance properties in low voltage. However, their and multifunctional applications, which include catalysis16,17, use as arcing contacts in circuit breakers is limited when photocatalysis18-20, electrocatalysis21, photoluminescence22-24, exposed to harsh working environments because of the photoswitches25, chemical sensing 25,26, and antibacterial agents27-29. ease with which they form oxides on the surface, thereby From the structural point of view, Ag2WO4 can be 1-5 leading to increased contact resistance . In addition, the found in three main crystallographic forms: α-Ag2WO4 thermodynamic instability of tungsten-silver materials (orthorhombic; Pn2n), β-Ag2WO4(hexagonal; P63 or P63/m), 30-32 reduces their usefulness at high temperatures near electric and γ-Ag2WO4 (cubic, Fd3m) . Orthorhombic is the most arcing regions1,6, because this might result in the formation stable, and its chemical stability is retained up to ~347 of different phases with distinct physical and chemical ºC, while β- and γ-Ag2WO4 are metastable, being easily 6 properties, such as silver tungstates (Ag2WO4 and AgWO3) , transformed to α-Ag2WO4 under heating at 187°C and 7 4,6-8 30 silver oxide (AgO2) and tungsten oxides (WOx) , which 257 ºC, respectively . Regardless of the crystallographic 1,6 would degrade the circuit breaker performance . Thereby, phase, the lattice crystal of Ag2WO4comprises a system of the oxidation of tungsten-silver compounds become a building-block units formed by a series of allotropic forms 15,22-24,29 laborious process, as well as its resistive behavior, since of [WO6] and [AgOy] clusters Since the physical and silver is normally free from oxides at elevated temperatures, chemical properties are heavily dependent on structural leading to breaking of structural arrangement of primitive features, those properties of each Ag2WO4 structure will structure4,7. However, interesting physical and chemical depend on the order-disorder relationship between these properties of the less stable structures may arise, since clusters, since a symmetry break induced by strain, stress, these exhibit unique properties that are distinct from those and distortions within the Ag2WO4 crystal lattice may create of the thermodynamically stable phase 8-10. In this regard, new and distinct structures, and thereby give rise to new in recent years, several researchers have reported many and different properties15,23,33. According to the published interesting potential applications of transition-metal tungstates, literature, these structural variations are a consequence of the because they exhibit a special symmetry property caused by synthesis method and experimental conditions (temperature, spontaneous polarization, which in turn implies ferroelectric processing time, solvent, heating source, pH, template, behavior, ionic conductivity and photoluminescence 11-14. electron beam exposure, etc.)10,11,16,29,33. *e-mail: [email protected] 2 Jacomaci et al. Materials Research The preparation of α-Ag2WO4 has been successfully The parameters refined were scale factor, background, carried out by several methods which include solid state sample shift, crystal lattice, and peak broadening anisotropic, reaction34-36, co-precipitation22,37-39, laser ablation in liquid preferential orientation, atomic position, and isotropic thermal (LAL)18, hydrothermal19,21-27,29,40-42, sonochemical 22,28 and parameters. The peak profile was modelled by fundamentals microemulsion22 methods. Despite the large number of parameters approach.46 The anisotropic broadening of the synthesis methods available, it has been a challenge to obtain sample was modelled using the Stephens’s model.47 The a fully stable α-Ag2WO4 structure. Nevertheless, in recent structure established by statistics parameters Rwp, Rexp, RBragg, years, promising studies15,24,27,29,39,42-44 exploring the chemical Goodfitness (ꭓ2) and good visual adjust both experimentally 48 and structural stability of α-Ag2WO4 nanostructures under fitting and theoretical patterns. electron beam and UV light exposure, revealed that interfaces The shapes and sizes of the α-Ag2WO4 nanostructures created between Ag and α-Ag2WO4lead to improvements were investigated using a field-emission scanning electron in the properties of the nanocomposites that are formed. microscope (FE-SEM) (Carl Zeiss - Supra 35-VP) operating at 2 kV. Lastly, a large number of studies about α-Ag2WO4 has The chemical environment of the α-Ag2WO4 was been published reporting only the structural description, investigated with the X-ray photoelectron spectroscopy (XPS) optical properties and the growth mechanisms, which risen technique. XPS spectra were collected using a commercial from different synthesis methods. However, in this work we spectrometer (UNI-SPECS-UHV) using the Al Kα line (hν concentrate on the structural and microstructural analysis of = 1486.7 eV) with its analyzer pass energy set up to 50eV. α-Ag2WO4 , that were crucial for a better understanding of its The inelastic noise of the C 1s, O 1s, W 4f, and Ag 3d structural stability and AC/DC electrical properties witch show spectra and the Auger Ag MNN peak were subtracted using up an interesting potential as electrothermal energy conversion. Shirley’s method49. The binding energies were corrected using the hydrocarbon component of adventitious carbon 2. Experimental Details fixed at 285.0 eV. In order to perform the direct current (DC) and alternating current (AC) measurements, two cylindrical 2.1 Synthesis pellets with an area of 182 µm2 and a thickness of 1.98 mm were obtained by isostatic pressing at 200 MPa. The electrical The α-Ag2WO4 nanostructures were prepared by a contacts were fabricated by depositing gold on both faces of microwave-assisted hydrothermal (MAH) method. The first the pellets that had been covered with Kapton® tape to avoid synthesis step consists of the preparation of two solutions, A and electrical conduction between them. The DC measurements B, both of which contain 0.25 g of polyvinylpolypyrrolidone-40 were acquired with a Radiant Precision Multiferroic Tester (PVP40 - Sigma Aldrich) dissolved in 50 mL of deionized water (Radiant Technologies Ltd., Albuquerque, NM), while the AC by mechanical stirring. After complete surfactant solubilization, measurements were carried out by a potentiostat/galvanostat 1 mmol of Na2WO4.2H2O (99% Alfa Aesar) and 2 mmol of (PGSTAT128N-Metrohm Autolab). The pellets were stored AgNO3 (99% Synth) were added to solution A and B, respectively, in a chamber under mechanical vacuum pumping and at under continuous stirring. Once precursor solubilization was room temperature. Impedance analysis as well as spectrum observed, both solutions were mixed together and transferred simulations were performed by electrochemical impedance to a Teflon vessel sealed with a manometer to monitoring the spectroscopy spectrum analyzer software (EIS-SA)50. hydrothermal pressure. The system was heated in a microwave oven furnace (PANASONIC, 2.45 GHz and 800 W) at 160 ºC 3. Results and Discussion with a heating rate of 10 ºC·min-1 for 1 h under constant pressure (~7 bar). Afterwards, the particle precipitate was separated via 3.1 XRD structural analysis centrifugation (10.000 rpm) and washed several times with water and acetone to eliminate Na+ and organic residues. Lastly, the Figure 1 shows the indexation planes of X-ray diffraction precipitate was dried at 70 ºC in a desiccator for 24 h. pattern of the MAH-produced α-Ag2WO4. As can be observed in the diffraction pattern, all diffraction peaks are in good agreement 2.2 Characterization
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