Insight into the BiFeO3 flash sintering process by in-situ energy dispersive X-ray diffraction (ED-XRD)

Author’s Accepted Manuscript Insight into the BiFeO3flash sintering process by insitu energy dispersive X-ray diffraction (ED-XRD) Luis A. Perez-Maqueda, Eva Gil-Gonzalez, Mary Anne Wassel, Shikhar K. JHA, Antonio Perejon, Harry Charalambous, John Okasinski, Pedro E. Sanchez-Jimenez, Thomas Tsakalakos PII: S0272-8842(18)32035-2 DOI: https://doi.org/10.1016/j.ceramint.2018.07.293 Reference: CERI19015 To appear in: Ceramics International Cite this article as: Luis A. Perez-Maqueda, Eva Gil-Gonzalez, Mary Anne Wassel, Shikhar K. JHA, Antonio Perejon, Harry Charalambous, John Okasinski, Pedro E. Sanchez-Jimenez and Thomas Tsakalakos, Insight into the BiFeO3flash sintering process by in-situ energy dispersive X-ray diffraction (ED-XRD), Ceramics International, https://doi.org/10.1016/j.ceramint.2018.07.293 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. www.elsevier.com/locate/ceri 1 Insight into the BiFeO3flash sintering process by in-situ energy dispersive X-ray diffraction (ED-XRD) Luis A. Perez-Maqueda1,2* Eva Gil-Gonzalez1, Mary Anne Wassel2, Shikhar K. JHA2, Antonio Perejon1,3, Harry Charalambous2, John Okasinski4, Pedro E. Sanchez-Jimenez1, Thomas Tsakalakos2 1Instituto de Ciencia de Materiales de Sevilla, (C.S.I.C. - Univ. Sevilla), Americo Vespucio 49, 41092 Sevilla, Spain 2Department of Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, USA 3Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Sevilla 41071, Spain 4X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA *Corresponding author: Tel.: +(34)954489501; fax: [+34] 954460165, [email protected] 2 Abstract The sintering mechanism of BiFeO3 has been investigated in-situ by energy dispersive X-ray diffraction (ED-XRD) using high-energy white collimated X-ray beam from Advanced Photon Source (Argonne National Laboratories). Such radiation is very penetrating thereby allowing measurements of the sample even when placed inside the flash sintering set up. Additionally, the fast ED-XRD measurements permit monitoring the flash sintering process by providing information about phase composition and sample temperature in real time. Moreover, profile scans, obtained by moving the stage vertically while recording the ED-XRD spectra, permits investigating the homogeneity of the flash for the entire length of the sample. All experiments have been complemented by ex-situ studies. It has been concluded that flash sintering of BiFeO3 is a homogeneous process without any directionality effects. Furthermore, flash sintering takes place at quite low temperatures (below the Tc 830ºC), which may be related to the high quality of the samples, as pure ceramics without evidence of secondary phases with an homogenous nanostructured grain size distribution and highly insulating are obtained by this technique. Moreover, it is also evidenced that the rapid heating of the sample does not seem to justify, at least by itself, the densification process. Therefore, it appears that the electric current should play a role in the enhanced mobility during sintering process. 1. INTRODUCTION Flash sintering implies applying a small DC current through a ceramic pellet, producing sintering within seconds at furnace temperatures much lower than those required for conventional sintering. Since its discovery in 2010, flash sintering has become a hot topic in ceramic processing with an increasing number of publications (from two in 3 2010 to more than forty in the last year) [1-4]. It has been successfully applied to a number of ceramic materials, including single and mixed oxides [1, 2, 5-12], carbides [13, 14], and composites [15, 16]. Recently, pure highly-insulating nanostructured BiFeO3 dense pellets have been prepared by flash sintering [17] and directly synthesized by reaction flash-sintering [18]. This is an interesting result considering that BiFeO3 is a metastable phase that easily decomposes into secondary phases, i.e. Bi25FeO39 and Bi2Fe4O9 [19-22], which increase its leakage current [20-23], deteriorating the electrical properties of the material. Moreover, above the Curie Temperature (Tc) transition (about 830ºC) the material destabilizes into secondary phases [23, 24]. Unfortunately, sintering usually requires temperatures above 830ºC. Thus, most BiFeO3 ceramics have certain amounts of secondary phases [19]. Furthermore, although Spark Plasma Sintering (SPS) has been effectively employed to sinter BiFeO3 ceramics at low temperatures, the BiFeO3 ceramics obtained by the SPS process, exhibit extreme conductivity due to the highly reducing conditions, and an oxidative post-sintering anneal is required [25-27]. Despite growing interest in flash sintering, the densification mechanism is still unknown and controversial [28-33]. However, it is quite clear that a sharp increase in temperatures of several hundreds of degrees, in one or two seconds, is produced by the Joule heating of the sample. The onset temperature of such an event depends on the applied voltage and on the electrical conductivity of the sample. Although ultrafast heating plays a significant role on the sintering process [30], it is unclear whether it is the only effect of flash sintering [29]. In fact, there are issues that do not seem to be justified by Joule heating alone. It appears the electrical current may have some additional effects. 4 To clarify these open questions, different experimental techniques have been used to perform in-situ studies around the effect of the electric field on sintering process. Thus, by in-situ high-resolution TEM analysis, it has been observed that electric field changes grain boundary mobility [34] and improves sintering [35]. Besides, optical emission measurements during flash sintering experimentation have shown electroluminescence with fixed wavelength, independent of specimen temperature, corresponding to the band-gap for electron-hole recombination [36]. Moreover, it has been reported that an applied electric field shifts the temperature of phase transitions, such as Curie temperature, Tc [38-41]. One of the most powerful experimental procedures for studying flash sintering under in-situ conditions is synchrotron energy dispersive X-ray diffraction (ED-XRD). This method allows determination during the flash event of specimen temperature [36], temperature uniformity [37], lattice expansion [38, 39], and phase composition [40]. Thus, ED-XRD has shown, under simultaneous applied electric and thermal potentials, anomalous lattice expansion in YSZ [38] and TiB2 powder with TiO2 and TiBO3 as secondary phases [41]. Moreover, during the application of electric field during flash sintering of 3YSZ, a new crystallographic phase which disappears when the field is shut off, was observed [40]. The aim of the present article is to utilize the unique capabilities of the in-situ synchrotron ED-XRD to provide an insight of the flash sintering mechanism of BiFeO3 that yields pure and highly insulating materials unlike other sintering procedures. Furthermore, this study aims to explore the role of the electric potential used during the flash-sintering 5 process on the reaction kinetics, beyond the Joule heating effect. Understanding the sintering mechanism of a metastable compound such as BiFeO3 offers new opportunities for applying flash sintering for the densification of other relevant metastable compounds. 2. EXPERIMENTAL Mechanosynthesised BiFeO3 powders were prepared from stoichiometric amounts of the starting oxides, Bi2O3 (Sigma-Aldrich 223891-500G, 10 μm, 99.9% purity) and Fe2O3 (Sigma-Aldrich 310050-500G, <5 μm, ≥99% purity), which were milled for 8 hours in a high energy planetary ball mill (Fritsch Pulverisette 7, Fritsch GmbH, Idar-Oberstein, Germany) under 7 bar of oxygen pressure [42]. For the preparation of green pellets for flash sintering experiments, the mechanosynthesised BiFeO3 powders were uniaxially pressed into either cylindrical (¼” diameter, 5 mm thickness) and dog bone shaped (1 mm thickness) pellets. For in-situ energy dispersive X-ray diffraction (ED-XRD), the green cylindrical pellets were placed on a special hot stage, which consists of a spring loaded ceramic specimen holder that is resistively heated. The hot stage was mounted on a positioning stage so that the sample could be stationary for exposure to an ultrahigh energy polychromatic radiation of photons of 200 keV at a fixed Bragg angle. A detailed description of the experimental set up and the technique has been previously described by Akdoğan et al. and Özdemir et al. [38, 41]. The experiments were carried out in the National Synchrotron Light Source (NSLS) of Argonne National Laboratory (ANL). For the study of densification during conventional heating, a sample pellet was heated at 10ºC min-1 while the 6 diffraction patterns were collected every 1 second. Flash sintering experiments were performed under isothermal conditions at 350ºC, while a DC potential of 100 V cm-1 was applied between two wires attached to opposite ends of the BiFeO3 pellets. Platinum paste was used to increase the electrical contact between the electrodes and the specimen. A typical flash sintering experiment consists of three well-defined stages. Firstly, a small incubation stage (stage I) is followed by a non-linear rise of the specimen conductivity up to the predefined limit, which corresponds to the flash event (stage II). At this point, the power supply is switched from voltage to current controlled mode signaling the beginning of stage III, which is maintained for 75 seconds approximately. As in the conventional heating experiment, in-situ ED-XRD data were collected every 1 second. Ex-situ flash sintering experiments were performed using dog bone pellets and an experimental set up similar to that originally proposed by Cologna et al [1]. The density of the samples was measured by the Archimedes’ method using distilled water at room temperature as the immersion liquid. Laboratory X-ray diffraction (XRD) patterns were collected in a Miniflex diffractometer (Rigaku, Japan) working at 45 kV and 40 mA using CuKα radiation. The data were acquired from 15º to 60º using a step size of 0.02º and a scan speed of 0.24 seconds per step. The microstructure of the samples was studied by scanning electron microscopy (SEM) employing a Hitachi S-4800 SEM-FEG using a field-emission gun operated at 2 kV. In order to reveal the grain structure, the sintered pellets were prepared for SEM by polishing the samples and using a thermal etching for 30 min at 90% of temperature 7 reached during the sintering experiment. Due to the insulating nature of the samples, the samples were Au sputter-coated using an Emithech K550 Sputter (Telstar, Spain). Dilatometric studies were performed in cylindrical pellets in air at a heating rate of 10ºC min-1 using a TMA PT100 (Linseis, Germany). Relative permittivity and tan δ measurements as a function of temperature were carried out using a 4263B LCR meter (Agilent Technologies, California, USA) at a fixed frequency of 100 kHz. Opposite pellet faces were previously Au sputter-coated using the Emitech K550 Sputter (Telstar, Spain) analogous to the samples prepared for SEM. 3. RESULTS The governing equations for ED-XRD results from the combination of the Bragg’s Law and Planck’s equation to relate the scattered energy of the (hkl) reflections with the interplanar distance dhkl (in KeV and Å for Eq. 1, respectively): Figure 1a includes a contour plot of diffraction peak intensities as a function of temperature for the conventional sintering experiment. Values of d-spacing for (104) and (110) reflections (calculated from equation 1) as a function of the temperature are depicted in Fig. 1b. These figures show the crystal lattice thermal expansion in the temperature range up to about 830ºC where there is a change in the pattern that corresponds to the α–β or Curie (Tc) transition for BiFeO3 (from rhombohedral R3c to orthorhombic Pnma). Fig. 1c includes the evolution of the volumetric thermal expansion 8 coefficient (β) as a function of temperature in the range from 500ºC to 825ºC, which has been calculated from the cell volume expansion that the sample undergoes in that temperature range (see Fig. S1 of the supporting information). As expected, it is observed an increase of the volumetric thermal expansion coefficient with temperature that can be approximated to the following parabolic function: (2) The dilatometric curve in Fig. 1d has been corrected considering the thermal expansion coefficients (eq. 2) determined from diffraction measurements. This figure (Fig. 1d) shows sample densification in the range from 425ºC up to about 870ºC, that is accompanied by the reversible volume decrease due to the Curie transition. The overlap of the Curie transition with the densification of the material, has been postulated as one of the reasons for the formation of secondary phases during sintering as the material is very unstable above Curie transition [23]. Figure 2 shows a flash sintering experiment at 350ºC furnace temperature at a constant electric field of 100 V cm-1 and a current limit of 2.5 A cm-2. This figure includes the evolution of electric field (Fig. 2a), current intensity (Fig. 2b), and power density dissipated (Fig. 2c) as a function of time. The experiment has three stages that have been marked in the figure. First, a small incubation stage (marked as I in Fig. 2c) is followed by a sharp increase in current intensity up to the predefined limit that corresponds to the flash event (marked as II); Second, the power supply shifts to current-controlled mode and it is maintained for 75 seconds (marked as III) and; Third, the power supply was switched off. 15 of the sample during the flash process. Thus, it is speculated that current should play some role, facilitating the mobility during the sintering. Furthermore, in a profile scan obtained by moving the stage vertically it was concluded that the flash is quite homogeneous suggesting a constant temperature throughout the samples. Ex-situ studies confirmed the homogeneity in terms of structure and microstructure all over the entire length of the sample (from cathode to anode). All in all, it is worthy to note that in-situ energy dispersive X-ray diffraction (EDXRD) is proven to be a powerful technique for providing insight into the flash-sintering behavior of a metastable compound such as BiFeO3. This methodology can be extrapolated to the study of other relevant metastable compounds. ACKNOWLEDGEMENT The authors wish to express their gratitude for the financial support provided by the Office of Naval Research (ONR) under Contract No. N00014-10-1-042 and Contract No. N00014-17-1-2087 Sub-Award No. 4104-78982 from Purdue. The authors wish to thank Dr. Antti Makinen of the ONR for his support of this project valuable technical feedback. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 16 Financial support from Projects CTQ2014-52763-C2-1-R, CTQ2017-83602-C2 (MINECOFEDER) and TEP-7858 (Junta Andalucía-FEDER) is acknowledged. AP thanks VPPI-US for his contract. 17 REFERENCES [1] M. Cologna, B. Rashkova, R. Raj, Flash Sintering of Nanograin Zirconia in <5 s at 850°C, Journal of the American Ceramic Society, 93 (2010) 3556-3559. [2] A.L.G. Prette, M. Cologna, V. Sglavo, R. Raj, Flash-sintering of Co2MnO4 spinet for solid oxide fuel cell applications, Journal of Power Sources, 196 (2011) 2061-2065. [3] M. Cologna, J.S.C. Francis, R. 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Feldmann, P.C. Dowden, R.F. Depaula, S.R. Foltyn, Q.X. Jia, Temperature-dependent leakage mechanisms of Pt/BiFeO3/SrRuO3 thin film capacitors, Applied Physics Letters, 91 (2007). 21 Figure 1. a) Contour plot of diffraction peak intensities as a function of temperature for the conventional sintering experiment of BiFeO3. b) Corresponding d-spacing values for (104) and (110) reflections. c) Evolution of the volumetric thermal expansion coefficient as a function of temperature. d) Corrected dilatometric curve as a function of temperature for the conventional sintering experiment of BiFeO3 along with the linear thermal expansion coefficient calculated during cooling. 22 Figure 2. Evolution of a) the electric field, b) current density and b) power density dissipated as a function of time for a flash sintering experiment of a BiFeO3 pellet at a furnace temperature of 350ºC. The constant electric field applied was 100 V cm-1 and the current limit was set at 2.5 A cm-2. 23 Figure 3. a) Contour plot of diffraction peak intensities and b) Evolution of the interplanar d-spacing values for the (104) and (110) diffractions as a function of time during the flash sintering experiment shown in Fig. 2 for a BiFeO3 pellet. 24 Figure 4. a) SEM micrograph and b) XRD pattern of the BiFeO3 pellet prepared by mechanosynthesis and flash sintered under the conditions given in Fig. 2. The solid line corresponds to the fit from the Rietveld refinement. Bragg reflactions for R3c space group are indicated by sticks.