Phenomenological understanding of flash sintering in MnCo2O4

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Original Article

Phenomenological understanding of flash sintering in MnCo2O4 ⁎

Anshu Gaura,b, , Mahamad Ahamad Mohiddonb, Vincenzo M. Sglavoa a b

Department of Industrial Engineering, University of Trento, Via sommarive 9, Trento 38123, Trentino-Alto Adige, Italy Department of Science and Humanities, National Institute of Technology Andhra Pradesh, Tadepalligudem, 534102, Andhra Pradesh, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Flash sintering MnCo2O4 Phase stability Micro-Raman spectroscopy Activation mechanism

The dual role of electric field in the flash sintering process of conducting MnCo2O4 is demonstrated. The flash and conventionally sintered MnCo2O4 samples produced at different temperatures are characterized using energy dispersive X-ray and micro-Raman spectroscopy to elucidate the micro-level spatial distribution of evolved phases. Raman signal mapping over the two ways sintered samples exposes differently grown areas of cobalt oxide based secondary phase. Electrical conductivity of conventionally sintered sample is recorded as a function of temperature and E-field and is utilized to discover the charge carrier activation mechanism during the flash effect. The conductivity before the flash-onset is shown to be comparable to that occurs by Poole-Frenkel effect and Phonon-assisted tunneling i.e. by the mechanism that occurs before the dielectric breakdown of semiconductors and insulators. The observed results, finally, confirm that catalyst like drift action of E-field on cobalt oxide formation is responsible for enhancement in the flash-sintering.

1. Introduction In the family of electric field assisted sintering techniques, flash sintering method has a unique identity due to the utilization of ‘moderate’ electric field and current density that sinters the powder compact in a very short time. The essential electric field and electric current for driving the sintering is substantially lower (and thus more practical) compared to those used in the field based electric discharge compaction (EDC) and the current based spark plasma sintering (SPS) respectively which are other electrical effect based fast sintering techniques [1,2]. This new and ‘almost’ transient process, thus, has become popular in the field of ceramic processing with the name of ‘flash’ sintering. In this sintering event, the material on a critical combination of electric field and furnace temperature undergoes rapid increase in the conductivity and the specimen temperature which draws the physical shrinkage of the specimen in few seconds [3]. Such conductivity based sintering occurs for the materials having negative coefficient of resistance. The two intermediate processes, the rise of the conductivity and the specimen temperature in this paper will be collectively termed as flash effect. There are some unique features of this flash sintering effect: in comparison to breakdown field of typical semiconductors, a fairly low value of electric field is applied in the flash effect that leads to electrical and thermal runway kinds of effect. For example, good conducting MnCo2O4 and (La, Sr)(Co, Fe)O3 [4,5] can undergo the flash effect under 10–15 V cm−1 (100–200 °C), weakly conducting Y2O3 stabilized ⁎

ZrO2 (YSZ) [6], Gd doped CeO2 [7] etc and insulating Al2O3 [8,9], BiFeO3 [10], MgAl2O4 [11] etc demand 100–200 V cm-1 (700–800 °C) and 1–2 kV cm-1 (1100–1200 °C) respectively. These values are very low compared to the electric breakdown fields of tens of kV cm−1 for typical semiconductors and dielectrics such as silicon, germanium [12], barium titanate [13]. It is, however, not established that the flash effect is typical electrical breakdown like phenomenon. Recently, Mattia et al has compared the flash effect with the electrical breakdown of alumina and shown that the conductivity of alumina at high temperature shows dependence on electric field described by Poole Frenkel effect (PFE) [9]. This effect describes the conductivity of the insulating materials at high electric fields before the electrical breakdown event [14]. Zafar et al also has shown that a relatively conducting (La, Sr)FeO3 experiences the non-linear rise in the conductivity following PFE [15]. No literature, however, is available on the electrical breakdown kind of effects on conducting materials. On the other hand, Todd et al has reported that the conduction in 3YSZ under the electric field before (and after) the flash event follow the its usual mechanism that is described by Arrhenius relation of type, ρ = ρ0 exp(Q/ kT ) [16,17]. Dong et al has reported that the flash onset temperature can be predicted solely by considering the Joule heating in the sample. Under the electric field, the developed electrical power dissipation has been decoded as the specimen temperature with the help of heat capacity data [18,19]. Likewise, many groups working in the flash sintering area believe that it is the Joule heating after the breakdown like increase in conductivity that drives the cationic diffusion and the physical shrinkage [7,20]. Further,

Corresponding author at: Department of Science and Humanities, National Institute of Technology Andhra Pradesh, Tadepalligudem, 534102, Andhra Pradesh, India. E-mail address: [email protected] (A. Gaur).

https://doi.org/10.1016/j.jeurceramsoc.2018.06.006 Received 3 March 2018; Received in revised form 31 May 2018; Accepted 3 June 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Gaur, A., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.06.006

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

as another distinctive feature, the degree of sintering in the flash process is reported to be enhanced compared to that in the conventional method [4,5]. For the enhanced sintering results, the predictions are extended to electric field drift effects on ionic diffusion [16], additional defects formation [8] etc. Raj et al suggested the formation of lattice defects, such as Frankel pair, during flash sintering of YSZ as the cause of sintering enhancement wherein the electrons will justify for the high conductivity and the ions for the sintering [8]. Narayana observed large number of dislocations and grain boundaries in the transmission electron microscopy study of high electric field treated Ni doped MgO [21]. He suggested that these defects which form additionally under the influence of an electric field play crucial role in the cationic diffusion and sintering. In addition to these, some of the works relies on the belief that the temperature local to the specimen is significantly higher than that is used during conventional process. However, there is no experimental demonstration that confirms the validity of such a higher inner temperature that can lead to sintering in few seconds. Also, formation of a second phase during the sintering process of MnCo2O4, SrTiO3 [22] and YSZ is also underlined as playing a critical role in the growth of the sintered microstructure. The phenomenological understanding of the flash effect is diverse for different materials and there is a search of unified mechanism that can broadly be accepted for the flash sintering effect. Moreover, the role of the electric field for the sintering process remained explicitly un-discussed. In a previous work, the electric field assisted flash sintering of MnCo2O4 under different electric fields is reported where on the basis of the local temperature a correlation between the microstructure and phase stability characterization results is highlighted [4]. The microstructure was found to be significantly grown after 1080 °C where MnCo2O4 starts reducing into pure cobalt oxide. The secondary phase formation is considered as having the deriving role in the flash sintering process. However, no direct experimental evidence was pointed out about the role of electric field for controlling the sintering or diffusion process. In the present work, two different roles of the electric field occurring at different stages of the flash sintering effect are discussed. The secondary phase formation analysis is extended to uncover the differences in the area distribution of the phases in the flash and conventionally sintered samples employing energy dispersive x-ray and micro-Raman spectroscopy. Other side, from the current-voltage characteristics and the systematic changes in these with respect to furnace temperature it was suggested that its inherent polarons of MnCo2O4 which are activated during the rapid conductivity increase by the electric field caused flash effect. The electrical characteristics are utilized in the present work to find out the charge carrier activation mechanism which is shown to rely majorly on the electric field. On these bases, the flash sintering mechanism proposed in Ref [4] is revised in the ‘Discussion’ section.

2. Materials and method Commercial MnCo2O4 powder with average particle size ∼1.17 μm and surface area = 4.22 g cm−2) is used in the present work. Flash sintering is performed on dog bon shaped pellets (gauge section: 20 × 3.0× (1.65 ± 0.05) mm3) by applying a constant electric field (10.0–17.5 V cm-1) across the sample while being heated with a constant rate (5 °C min-1) in a furnace. Details of the flash sintering experimental set up and sample fabrication/arrangement are mentioned in a previous work [4]. In the electrical circuitry, a fixed maximum current density (1.4–1.6 A mm−2) was set in the power supply in order to avoid excessive heating during electrical runway. At the maximum current point, power supply turns from constant voltage mode to constant current mode. A 60 s hold at the specified maximum currents was considered as the period of flash sintering. Local temperature during the flash sintering event is recorded using pyrometer (Ultimax Infrared Thermometer, UX-20/600–3000 °C), which was calibrated up to 1100 °C prior to the measurement. MnCo2O4 pellets were also sintered in conventional manner heated with a rate of 5 °Cmin-1 up to the similar temperatures and time that were used in the flash process. Morphology and elemental composition of the produced samples is obtained with scanning electron microscopy (Model Jeol JSM 5500 SEM) coupled with energy dispersive x-ray spectroscopy (EDXS) respectively. MicroRaman spectroscopy (Model Witech alpha 200, Germany made) is employed to investigate the structural information. Raman imaging is also performed with Witech alpha 200 by mapping a specific Raman shift of the phase of interest. In the Raman microscopy/spectroscopy experimental setup a 532 nm Nd-YAG laser is fiber coupled to a microscope, a 100X objective with an approximate spot size of 680 nm is used to focus on to the sample. A maximum laser power of 40 mW is applied for Raman spectroscopy and imaging experiments. Raman signal is collected in back scattering mode and the scattered signal is send to CCD based spectrograph through 600 grooves/mm grating. 100/125 μm optical fiber is used for fiber coupling from microscope to spectrograph. Spectral range of 120 nm equivalent to 3000 cm-1 is selected for micro-Raman and Raman imaging experiments. To find out the activation mechanism, current voltage characteristics were recorded on 1300 °C-sintered specimen in a similar to flash sintering experimental set up at furnace temperatures of 200–700 °C in the steps of 100 °C. The electric field was increased with a constant rate of ∼ 8 mV cm-1 s-1 up to approximately 7.0 V cm-1. Different features of recorded current-voltage characteristics are discussed in our previous report [4]. 3. Results 3.1. Flash sintering of MnCo2O4 A typical sharp rise in power dissipation of a specimen as function of furnace temperature is demonstrated in Fig. 1 for MnCo2O4 subjected to

Fig. 1. Power dissipation, specimen temperature and shrinkage of MnCo2O4 under 10 V cm−1 as a function of furnace temperature. 2

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

concentration at the darker regions (73.6%) of Fig. 2(e) is observed to be higher than that of the pure MnCo2O4 which contains 68.2% of Co and 31.8% of Mn. The other portions complement the actual spinel by containing lower Co concentration. These changes in the atomic fractions of Co and Mn are more significant for FMC1320 (Fig. 2(e)) where cobalt percentage at darker place is increased to 75.5% from 73.6% of previous FMC1160 sample. These spatial stiochiometric deviations indicate the possibility of coexisting secondary phases within MnCo2O4 having different Co and/or Mn ion concentrations. It is predicted that these changes are associated with the cobalt oxide based secondary phase which forms by the reduction of MnCo2O4 at high temperature [23]. A different plateau like morphology (no dark like region) on the surface of background grains is recorded in the CMC1320 sample (Fig. 2(f)). These plateaus contain high percentage of cobalt (∼90%) strongly indicating the existence of cobalt oxide at these places. However, such cobalt rich areas are not clearly distinguished in the conventional treatment at 1100–1160 °C. The observed differences between the distribution and morphology of such Co-rich regions for flash and conventional process are presumably associated with the electric field effect which may modulate the ions diffusion and cobalt oxide growth activity.

Table 1 Flash sintering parameters of MnCo2O4 samples for different electric fields. S. No.

Sample Name

Electric Field (V cm−1)

Current Density (A mm−2)

Processing Temperature (oC)

Specimen Temperature (oC)

1 2 3

FMC1320 FMC1160 FMC1100

17.5 17.5 15

1.6 1.4 1.4

120 120 145

1320 1160 1100

a field of 10 V cm−1 and a constant heating rate of 5 °C min−1. Figure depicts the three typical features of flash sintering effect, namely, the abrupt rise in the power dissipation, the abrupt increase of the local temperature and subsequent rapid physical shrinkage. Power dissipation is the product of the circuit current and the voltage drop across the sample and normalized over the specimen’s volume. The flash event, under 10 V cm−1 applied at 100 °C, occurred at a furnace temperature of 207 °C and raised the specimen temperature to 990 °C within a short time of 4–5 s. On increasing the electric field, the flash effect occurs at lower furnace temperature and to a higher specimen temperature (under the same current density). In addition, the increase in the maximum current limit at a constant field also leads to a higher specimen temperature. A specimen temperature of 1320 °C from 1160 °C is achieved by increasing the current density from 1.4 to 1.6 A mm-2 at 17.5 V cm−1. The flash sintering parameters of MnCo2O4 samples sintered under different field and current values are presented in the Table 1.

3.3. Structural characterization: micro-Raman spectroscopy Further investigation of the secondary phase formation and its distribution over the selected region of the sample is carried out using micro-Raman imaging which gives the details of the phase distribution over the surface of the sample at the microscopic level. The Raman spectra of MnCo2O4 samples sintered at 1320 °C by the flash and the conventional sintering processes are shown in Fig. 3. Spectrum collected at different positions on the samples exhibits different features, indicates the different phase distribution across the sample surface. The spectra of similar features are collected at 8–10 different position of the sample and its average is presented in the figure. Raman spectrum of the starting untreated sample (pre-sintered at 900 °C) is also included for correlation. MnCo2O4 phase in the untreated sample is identified from three broad peaks situated at 180, 490 and 640 cm−1 which correspond to F12g, (F22g, Eg) and (F2g, A1g) modes of the cubic MnCo2O4 respectively [24,25]. The mode F12g at 180 cm−1 represents vibration of Co2+ ions sitting at tetrahedral site whereas rest of the modes corresponds to the vibration of octahedral sites of Co2+[Co2+ Co3+, Mn3+ Mn4+]O4 distribution where Mn and Co’s mixed valency terms in the square bracket represents octahedral positions [25,26]. The Raman spectrum collected over different position of the green sample has same features confirming the single phase of the green sample and it is expected as no heat treatment is applied over it. On the other hand spectrum collected over different regions of flash sintered sample has random features as shown in Fig. 3 (spots A, B and C). The spectrum denoted by ‘spot A’ has quite similar features of green MnCo2O4 pellet Raman spectra except the occurrence of a small peak at 680 cm−1 and a shifting in the 640 cm-1 peak (∼10 cm−1). The evolution of the 680 cm-1 peak is associated with the removal of Mn or substitution of Mn by Co into MnCo2O4 matrix which will change the composition towards Co-rich (Mn, Co)3O4 [24]. The shift in 640 cm−1 peak is associated with the complementary changes in which rest of the matrix turns towards Mn-rich (Mn, Co)3O4 causing Raman peak to shift towards higher wave number [24]; the extreme end compound, Mn3O4, has its characteristic peak at 660 cm−1 [27]. The spectrum collected at other different regions of the sample denoted by ‘spot C’ in Fig. 3 has totally different features than the previous Raman spectrum. Peaks are sharper and are more in numbers (188, 475, 515, 610 and 680 cm−1) relative to the spectra collected from other regions denoted by ‘spot A’ and expected to represent CoO phase. At room temperature CoO oxide holds face-centered cubic crystal structure, in which Co2+ ions are octahedrally coordinated to six O2- ions [28–30]. Theoretically, the Oh symmetry of CoO should lead to the observation of three Raman active

3.2. Elemental characterization: energy dispersive x-ray spectroscopy The fractured surface scanning electron micrographs of flash sintered MnCo2O4 samples corresponding to the specimen temperatures of 1100, 1160 °C and 1320 °C that are produced by employing the electric fields of 15.0–17.5 V cm−1 respectively are depicted in Fig. 2. Their conventional counterparts produced at the same temperatures are also shown in the same figure for comparison. The flash sintered and conventionally sintered MnCo2O4 samples from here onwards will be referred as FMC and CMC respectively. The microstructures clearly show the differences in the two kinds of treatments. The inter-particle connectivity and grain growth is found to be enhanced in the flash sintering over its conventional counterpart. For example, at 1160 °C in the Fig. 2(c), FMC grains are completely surrounded with the neighboring ones without visible porosity; the CMC grains though are well connected however the micrograph is not free from porosities (Fig. 2(d)). These differences in microstructures appear more magnificent when we recall the fundamental differences in flash and conventional sintering processing. A huge time gap is involved between the two kinds of treatments wherein the flash one involves a single minute like short period whereas conventional indulges into 100 min of sintering time (for 800–1300 °C range acquired with 5 °C min−1 [4]). Further, the continuous increase in the dimension of the microstructure with temperature indicates the significance of Joule heating in the flash process. However, if it were only by Joule heating, the rapid flash process would not have led to the pore-free sintering of MnCo2O4 at 1160 °C temperature. On the Fig. 2, it is further observed that there are some irregularly arranged darker regions within the surface of the FMC1100 and FMC1160 samples’ grains (Fig. 2(a) and (c), respectively). On increasing the temperature to 1320 °C (Fig. 2(e)), the darker regions are observed to be spread over larger area. Elemental composition of these regions is determined by means of EDXS, and is given in Table 2 along with the concentration profiles of conventionally produced and untreated (pre-sintered) pellets. The composition is calculated in terms of atomic weight fraction of Mn and Co elements in Mn-2Co formula structure. The percentage of Mn and Co elements at two different contrast regions of FMC is different from that of pure MnCo2O4. Cobalt 3

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

Fig. 2. SEM micrograph of MnCo2O4 samples, flash sintered and conventionally sintered at the specimen temperatures of (a, b) at 1100 °C, (c, d) 1160 °C and (e, f) 1320 °C respectively.

local heating by laser power. It is shown from their work that the laser power of 4 mW converts CoO into Co3O4 [36]. Similarly Thibeau et al. and de Faria et al. have reported the conversion of FeO into Fe3O4 by local heating by laser used for probing Raman measurement [37,38]. In our present investigation we have used 40 mW of laser for Raman spectra measurements. We tried to collect the Raman spectra with lower laser power, but due to the large signal to noise ratio we were constrained to use optimum laser power, which is far beyond 4 mW. This is due to the fact that the present work is of bulk pellet, where the scattering over the rough surface dominates. At very low laser power, even after long integration time, we did not obtain good signal to noise ratio of Raman signal. The local heating due to laser power reported by D. Gallant et al. was carried on the thin film, where one can observe a good signal to noise ratio of Raman spectra even at very low laser powers (∼4 μW). In the present work, XRD data confirms the presence of CoO additional phase along with MnCo2O4 phase in both conventional and flash sintered samples. However, due to relatively high laser power used for measuring the Raman spectra, pre-existing CoO phase has converted to Co3O4 phase. Thus the Raman signal observed for Co3O4 phase in our Raman spectra measurements is considered as indirect evidence for the presence of CoO phase. The data presented in

Table 2 Elemental concentration profile of flash and conventionally sintered MnCo2O4 samples as determined by EDXS. Position on the microstructure

Position 1 Position 2

Co and Mn concentrations (weight %) FMC1160

FMC1320

CMC1320

MC (untreated)

73.6, 26.4 45.8, 54. 2

75.5, 24.5 41.3, 58.7

90.7, 8.3 61.7, 39.3

68.2, 31.8 68.2, 31.8

modes (A1g, Eg and T2g) [31]. Therefore, the Raman shift observed at 610, 475 and 188 cm−1 can be assigned to the above three modes respectively. However the same peaks can also be assigned to Co3O4 phase of the cobalt oxides [24]. The Co3O4 oxide crystallizes in the normal spinel structure with Co2+ and Co3+ located at tetrahedral and octahedral sites, respectively and possesses five Raman active modes (A1g, Eg, and three T2g) [32–35]. D. Gallant et al. have proposed from their study that the Raman spectra of CoO and Co3O4 compound exhibit almost similar shape with a slightly higher (5-10 cm−1) vibrational wave numbers for Co3O4 than that for CoO [36]. They carried an extensive Raman spectra study of CoO and Co3O4 oxides as function of 4

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

which may confuse the outcome of the experiment. To overcome this ambiguity the sample surfaces were polished to reach a good quality of smoothness. The Raman imaging was carried over the flash and conventionally sintered samples by selecting a region of 25 × 25 μm2 area which is divided into 125 × 125 data points with each data point integration time of 2 s. The data is collected overnight and is analyzed using the Witec project 2.10 software. In order to estimate the distribution of cobalt oxide phase over the selected region, a filter near the characteristic Raman shift of cobalt oxide (i.e. 680 cm−1) is selected and Raman image is extracted. The resulting Raman maps are shown in Fig. 4(a–b) for the field assisted and conventionally sintered MnCo2O4 samples respectively. The contrast of the image indicates the distribution of cobalt oxide. Therefore, higher the brightness, larger is the fraction of cobalt oxide present in the region. It is observed in Fig. 4(a–b) that there are brighter areas with respect to remaining part of the images which indicates the assured presence of cobalt oxide at those bright colored places. For the flash sintering case as shown in Fig. 4(a), the contrast is higher and the bright region is extended along a unidirectional straight line. This observation suggests the directional growth of cobalt oxide across the surface of the sample, in the same direction of applied field. The Raman spectra collected across the bright regions of the image is depicted in the inset of Fig. 4(a) to confirm the argument that the bright regions are mainly composed of cobalt oxide. The surrounding regions have low concentration of cobalt oxide and are probably made of dominant MnCo2O4 phase. Conversely, no such preferred directional growth for cobalt oxide regions are observed in conventionally sintered sample. The irregular distribution of bright regions across the image as shown in Fig. 4(b) show that the bright regions are random in dimension with no preferred shape, direction and distribution. This observation confirms the clustered growth of cobalt oxide phase in the localized regions with no preferential growth direction as found in the flash sintered samples. In order to overcome the ambiguity of roughness induced contrast change in the Raman map (as discussed earlier), a filter near the zero shift i.e. Rayleigh signal is selected and extracted map is shown in Fig. 4(c–d). The figures shows homogeneous intensity distribution in both the cases for Rayleigh signal which means that, the contrast in the intensity of the Raman map is not because of morphological in-homogeneity or surface roughness. Thus the brightness/contrast variation is solely due to the presence of the cobalt oxide phase. The observation of unidirectional growth of cobalt oxide across the surface of the sample is correlated with the observations drawn from SEM micrograph data. The unidirectional cobalt oxide channel in Fig. 4(a) has approximate width of 2–3 μm. This is approximately in same dimension as the average diameter (2–3 μm) of darker spots in SEM morphology which is recorded on the sample cross section and contains higher cobalt concentration. Thus the straight line channels of cobalt oxide in the flash sintered sample suggest the directed-growth of the oxide phase developed from the directional effect of electric field. On the other hand, the irregular pattern of conventional method supports that the sintered specimen is produced from random thermal effect (on sintering and reduction reaction). Therefore, the Raman map gave direct evidence to the directional effect of electric field on the cations diffusions.

Fig. 3. Raman spectra of MnCo2O4 samples, flash and conventionally sintered at the specimen temperature of 1320 °C.

the Fig. 3 denoted by ‘spot C’ confirms the cobalt oxide phase presence at different regions of the sample. The spectrum denoted by ‘spot B’ in the figure is the intermediate situation where both MnCo2O4 and cobalt oxide phase pre-exists and hence the spectra has overlapped features of both phases. Similar observation of two different kinds of Raman spectra based distinguishable regions are recorded for the sample subjected to conventional sintering. The Raman spectra collected at different position of the sample are presented in the Fig. 3 denoted by ‘spot A1′ and ‘spot B1′. Both these spectra have similar features as discussed in the above case. Due to the microscopic featured data of Raman signal we have emphasized that there are some localized regions over the surface of the samples, as predicted by SEM and EDS characterizations, where pure single phase cobalt oxide and MnCo2O4 regions exists along with the regions where both phases are present together. This observation is common for both the samples annealed in flash and conventional sintering techniques. The existence of pure cobalt oxide (CoO) in the sintered MnCo2O4 samples is observed in the X-ray diffractogram also, reported in the Ref. [4]. It was observed that the samples treated to 1100 °C and higher temperatures undergo phase loss by CoO formation whereas lower temperature sintered samples were pure in spinel phase. The concentration of this CoO is observed to increase with the electric field (and the specimen temperature) and was relatively higher in case of flash sintering. The implication made from such observation is that if the flash sintering process is totally a thermal effect due to Joule heating, the high heating rate of flash process would have resulted to lower concentration of cobalt oxide. This is because the slow heating rates give sufficient time for conversion of MnCo2O4 into cobalt oxide. However, this is not the case and therefore the observation suggests that additional effect of, possibly, electric field is involved which causes higher degree of reduction reaction. To investigate the spatial distribution of MnCo2O4 and cobalt oxide phases over the surface of the samples Raman imaging was employed for both FMC and CMC samples. Before proceeding to the Raman imaging, samples were polished sequentially with silicon carbide sandpaper of 60, 120, 180, 320, 600 grit followed by diamond paste of 9 and 5 μm. The Raman imaging is sensitive to the roughness of the sample. This is because, in Raman imaging laser is focused with a 100X objective whose spot size at the focal point will be approximately 680 nm. If the sample surface is highly rough the intensity of laser interacting with sample at the focal points will change across the surface and hence there will be a change in the intensity of the collected data,

3.4. Activation mechanism It is well established that the flash sintering effect begins with an abrupt increase in the spinel’s conductivity. It infers that the charge carriers at this event are generated in a large concentration (unusual for that working temperature) under the reign of sufficiently high electric field. The electrical conductivity of MnCo2O4, from current-voltage characteristics, is established to be a function of both, the electric field and the temperature [4]. The I–V characteristics in Ref. [4] clearly showed that the conductivity of MnCo2O4 at a specific temperature is not constant at applied electric fields (0–7 V cm−1). The constant conductivity becomes the case during ohmic conduction ( J = σE ). The 5

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

Fig. 4. Mapping of the Raman signal of cobalt oxide over MnCo2O4 samples a) flash sintered and b) conventionally sintered at 1320 °C. Rayleigh signal mapping is correspondingly shown in c–d.

presence of electric field is modified by ΔU = 2eEr0 = 2e (eE / ε )1/2 [40], where e is the electronic charge, r0 is the maximum distance from the positive ion. This decrease in the potential barrier facilitates the charge carriers generation, and thus occurring conductivity increase by PooleFrankel effect is described by the relation [39,40],

conductivity changes in nonlinear fashion and the degree of nonlinearity is higher for lower furnace temperatures [4]. True thermal response of the MnCo2O4 spinal’s conductivity is given by Arrhenius E relation of type, σT = σ0exp − kTa , Ea is the activation energy, σ0 material’s constant, k Boltzmann’s constant and T is the absolute temperature [4]. For the electric field control of the conductivity it is considered that the field may activate the charge carriers of MnCo2O4 in a flash-like event (as shown in the Fig. 1) following Poole-Frankel (PF) or tunneling (T) mechanism as mentioned by Ganichev et al [39,40] for insulating materials. As mentioned in the introduction, these mechanisms define the higher conductivity of insulating materials before they enter into electrical breakdown event. In the Poole-Frenkel effect, it is mentioned that when the potential that binds the electron with the cation of a material matrix is of long range and the barrier is not too high, the applied electric field acts for lowering the barriers for the carrier generation and the rate of thermal activation is enhanced [41]. This potential barrier situation occurs with semiconductors and insulators where electrons are not freely available and the requirement is same with the flash sintering also. Height of the potential barrier (that is related to the activation energy) in the

( )

1

⎡⎧ ⎤ e3E ⎞ 2⎫ σ = σ0 exp ⎢ −Ea + ⎛ / kT⎥ ⎢⎨ ⎥ ⎝ ε ⎠ ⎬ ⎭ ⎣⎩ ⎦ ⎜

⎟

(1)

1

lnσ = lnσ0−

Ea (e3/ ε ) 2 1/2 + E kT kT

(2)

where Ea activation energy for the electrical conduction, k Boltzmann’s constant, e is the electronic charge and ε is the dielectric constant. On the other hand in case of short range potential when the barrier is high, electric field can generate the charge carriers by quantum mechanical tunneling process. This probabilistic tunneling can occur in two ways, namely, by the assistance of phonon or by direct tunneling (very high field and low temperature situation). This situation occurs again in a band gap material but at relatively lower temperatures compared to that in above mentioned PF effect. Phonon assisted 6

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

Fig. 5. Natural logarithm of MnCo2O4’s conductivity as function of E 2 and E1/2 at furnace temperatures of a) 700 °C, b) 600 °C, c) 500 °C, d) 400 °C, e) 300 °C and f) 200 °C.

and electric field. The tunneling will be the dominated mechanism for higher electric fields and lower temperatures (due to square dependence of E ). On the other hand, the electric field and temperature criteria are relatively relaxed for Poole-Frankel mechanism and can occur with relatively lower electric field and higher temperature (due to inverse T dependence). To demonstrate the carrier’s activation mechanism, natural log of the electrical conductivity is plotted against square root and square of the electric field and are shown in the Fig. 5 for different furnace temperatures (200–700 °C). At the first sight, two distinct features are

tunneling of carriers is described by the conductivity relation as [39],

σ=

σ0 exp [e 2τ23E 2/(3m*ħ)]

lnσ = lnσ0 +

e 2τ23 2 E 3m*ħ

(3)

(4)

where m* is the effective mass of the charge carriers, h the Planck’s constant, τ2 is the tunneling time which contains temperature term as 1/ kT [42]. From the two electric conductivity governing equations, one therefore can infer that the conductivity is a function of temperature 7

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

observed in loge σ versus E 2 and E1/2 graphs. At lower electric field, loge σ increases rapidly with the electric field; it is the regime where the activation mechanism is sought. At higher fields, the graph gets saturated suggesting that the changes in the conductivity are relatively insignificant in this regime of electric field. Out of the two plotted powers of electric field, the variation of loge σ with E 2 is more close to linear fit than with E1/2 for 400–700 °C temperature cases (Fig. 5(a–d)); loge σ maintains approximately parabolic relation with E1/2 . This observation suggests that at these temperatures the conductivity increases with the electric field described by Eq. (3) following phonon assisted tunneling mechanism for charge carrier activation i.e. at much faster rate than by the Poole-Frenkel effect, generally suggested mechanism for insulating materials [9]. Slope of these fitted lines is calculated to be 0.32, 0.22, 0.13 and 0.10 for 400, 500, 600 and 700 °C respectively. The slope therefore systematically increases with the decrease of the temperature suggesting that loge σ and so, the conductivity of MnCo2O4 increases with electric field at a faster rate at lower temperatures (the field was increased with a constant rate). For an example, at 700 °C the time taken to increase the conductivity by 0.22 S/cm is 301 s whereas at 400 °C the same increase on an average was achieved in much lower time of 28 s. In addition, these temperatures (400–700 °C) which do not involve the flash effect (Tth = 210°C [4]) the rise of the conductivity is so rapid that the charge carriers in the considered range of electric field are activated by tunneling like mechanism. At lower temperatures of 200 and 300 °C, the situation appears relatively different (Fig. 5(e–f)). loge σ versus E 2 graph for 300 °C deviates from the linearity in the considered regime (before the saturation type response) and turns to totally non linear for 200 °C. The response looks strange for 200 °C where the conductivity data moved back to the lower electric fields though the experiment was set to perform with monotonically increasing electric field. It happened because rise of the conductivity at 5.18 V cm−1 field was so rapid and high that the voltage drop across the specimen has decreased due to constant current setting and the conductivity turned towards lower electric fields. The current was set to a constant maximum to avoid the electrical/thermal runway. If the power supply was not limited to such setting, the conductivity would have displayed by a vertical line at the field value of 5.18 V/cm or (E 2 = 26.5225 V2 cm-2). This fast growing conductivity by the electric field categorizes itself into the flash effect (Tth = 200°C , Eth = ∼ 5.18 V cm−1). The deviation of loge σ versus E 2 and E1/2 graphs from linear behavior suggests that loge σ now has higher order (> 2) dependence on electric field and the activation of charge carriers is categorized under direct tunneling effect of the electric field. It is therefore, concluded that during the flash effect of MnCo2O4 the charge carriers are activated by direct tunneling phenomenon. The 300 °C-case where loge σ versus E 2 graph is also not linear, but the conductivity rise is not as rapid as in 200 °C case, is considered as under the transition from Phonon assisted to direct tunneling effects. In case of insulating samples, the range of electric field for Poole-Frenkel effect or the later occurring electrical breakdown is very high, of the order of kV cm−1 and MV cm−1 for example. On the contrary, in the present case of conducting MnCo2O4 the breakdown like increase in the conductivity is observed with small values of electric fields, e.g. with 5–10 V cm−1. With this, it is established that the flash effect of MnCo2O4 is similar to electrical breakdown of insulators which involve Poole-Frenkel and tunneling kinds of mechanisms. Being higher in the conductivity, these unique mechanisms are observed at smaller electric fields.

heating phenomenon [16,18,19]. A nonlinear event which triggers by the support of Joule heating and is in close correspondence with the flash effect also, is the abrupt rise of the current observed in the perovskite type oxides (barium and strontium titanate). The current is shown to increase rapidly with respect to time on constant application of electric field [45–47]. Such nonlinearity is firstly discussed in the weakly conducting oxides for long term stability concern of capacitors where these are used as dielectrics; in the application context the event is commonly referred as resistance degradation. The degradation is shown to become severe with the increase of the temperature and the electric field as is the case with the flash effect. This nonlinear characteristics is due to concentration polarization of oxygen vacancies between the two electrodes [45–47]. Physical mechanism based on experimental observation and qualitative analysis of resistance degradation is as follows [48]: The degradation in barium and strontium titanate ceramics starts from the de-mixing of oxygen vacancy from the regular lattice of their single phase. Acceptor doped and undoped forms of these oxides have plenty of oxygen vacancies which are highly mobile at even room temperature. The sites with missing oxygen (oxygen vacancy) behave as positively charged with respect to the regular lattice and in the electric field drift towards cathode creating effective deficiency towards anode. With time there is a pile of such oxygen vacancy sites at the cathode and its deficiency at the anode (and accumulation of acceptor-sites in acceptor doped composition) due to electrodes blocking. The excess electrons of oxygen vacancy site coming from the metal cations make the cathodic region as n-conducting, vice versa anodic region behaves as p-conducting. Such de-mixing leads to the formation of a p-n junction like structure with cathode-side as n-conducting, anode side as p-conducting and central region having gradient of components. In such p-n junction, simultaneous Joule heating caused by the movement of charged species accompanies the field in overcoming the barrier. The degradation process is similar to rapid current flow in a forward biased p-n junction after the barrier is overcome. At constant temperature, higher electric field drifts the oxygen vacancies (and acceptor ions) in faster manner leading to strong barrier and more Joule heating, leading to strong and early occurrence of the nonlinearity. Similar accumulation (reducing atmosphere) and deficiency of oxygen vacancies is shown to cause gradient in grain growth in zirconia and ceria based ceramics, having enormously large grains at the cathode and almost pre-sintered like at the anode [49,50]. Same phenomenological explanation is suggested for the flash sintering of zirconia based compositions [51]. With a similar oxygen based structure, flash sintering of MnCo2O4 can be thought of as the result of similar oxygen vacancy activity. However, it is important to recollect that in these weakly conducting oxides, the movement of oxygen vacancy (and acceptor ions) will have profound effect on the lattice which has low mobility species. Unlikely, MnCo2O4 is an electronic conductor based on polaron hopping mechanism; the electrons associated with Co and Mn cations hop between their different oxidation states in a remarkably easy manner. The conductivity is such high that the nonlinear flash effect is observed with 5 V/cm like small electric field, and with 15-20 V/cm the effect is seen at 100 °C like low temperatures (in the context of flash sintering). The electrical conductivity of MnCo2O4 is dominated by the large concentration of polarons and its high mobility. The contribution of mobility/concentration of oxygen vacancy to the total conductivity is relatively small. Hence, it is difficult to conceive a picture of oxygen vacancy segregation and p-n regions/junction in MnCo2O4 lattice which have high mobility electrons (in the form of polarons). At any point of time, namely, start of the segregation, p-n unction formation, overcoming of the junction barrier, the high electronic conduction will suppress these effects. Moreover, unlike to reduction and grain growth of zirconia based phases we have not noticed a color variation with respect to cathodic and anodic position in the samples treated to lower or higher electric fields. MnCo2O4 treated to higher temperature/field such that it is partially reduced to CoO phase turn to grey color whereas lower temperature sintered spinel remains

4. Discussion i) Apart from the above mentioned electric field based mechanism, the onset of abrupt rise in the circuit current during the flash effect under constant voltage, and, in i) constant rate heating [20,43] and ii) isothermal experiments [44] is investigated by self-accelerated Joule 8

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

References

black as like untreated sample. ii) Question regarding the flash sintering effect is open for the discussion that how an event starting with breakdown like increase in the conductivity and resulting rapid increase in the specimen temperature leads to sintering at ‘lower’ temperatures. If the flash sintering which proceed at a very high heating rate (roughly 200–300 °C/s) is an accelerated process, it must be due to higher concentrations of defects and/or due to higher temperature. In our work, the specimen temperature recorded by pyrometer is not observed to be unconventionally high (Table 1 and [4]). For the same reason, it is considered that flash sintering is enhanced over conventional because of higher concentration of defects. We have proposed (detailed in our previous work [4]) that these defects are formed during MnCo2O4’s inherent polaron hopping based conduction process (I–V curves recorded at different temperature). During the electrical conduction, the polarons (bound electrons associated with transition metal cations) hop between two cobalt sites (and the two Mn-sites also). At these sites, cobalt and manganese are in different oxidation states or bonded with different number of oxygen atoms [26]. While hopping, the (Co or Mn) cation site left behind acts like a defect structure and can participate in the diffusion process leading to the sintering. It was observed that the flash sintering was enhanced over conventional one only after 1080 °C [4]. This observation indirectly suggests that the formed defects structures are available actively (in order to give flash sintering) for diffusion and sintering when cobalt starts stabilizing into +2 oxidation state or cobalt oxide phase. However, no proof was provided for this proposition. From the oriented growth of the cobalt oxide from this work, it is emphasized that electric field acts for drifting the cations and orienting grains of secondary phase, and not for the parent/primary phase. Though there is significant change in the microstructure of flash and conventional sintered samples, no sign of oriented grain growth for flash sintered MnCo2O4 is observed even when temperature was insufficient for reduction reaction. Therefore, directed growth occurred for cobalt oxide phase provides support to the proposed mechanism mentioned above and discussed in Ref. [4].

[1] J.R. Groza, A. Zavaliangos, Sintering activation by external electrical field, Mater. Sci. Eng. A 287 (2000) 171–177. [2] R. Orrù, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Consolidation/synthesis of materials by electric current activated/assisted sintering, Mater. Sci. Eng. R Rep. 63 (2009) 127–287. [3] R. Raj, Joule heating during flash-sintering, J. Eur. Ceram. Soc. 32 (2012) 2293–2301. [4] A. Gaur, V.M. Sglavo, Flash-sintering of MnCo2O4 and its relation to phase stability, J. Eur. Ceram. Soc. 34 (2014) 2391–2400. [5] A. Gaur, V.M. Sglavo, Densification of La0.6Sr0.4Co0.2Fe0.8O3 ceramic by flash sintering at temperature less than 100 °C, J. Mater. Sci. 49 (2014) 6321–6332. [6] M. Cologna, A.L.G. Prette, R. Raj, Flash-sintering of cubic yttria-stabilized zirconia at 750°C for possible use in SOFC manufacturing, J. Am. Ceram. Soc. 94 (2011) 316–319. [7] X. Hao, Y. Liu, Z. Wang, J. Qiao, K. Sun, A novel sintering method to obtain fully dense gadolinia doped ceria by applying a direct current, J. Power Sour. 210 (2012) 86–91. [8] M. Cologna, J.S.C. Francis, R. Raj, Field assisted and flash sintering of alumina and its relationship to conductivity and MgO-doping, J. Eur. Ceram. Soc. 31 (2011) 2827–2837. [9] M. Biesuz, P. Luchi, A. Quaranta, V.M. Sglavo, Theoretical and phenomenological analogies between flash sintering and dielectric breakdown in alpha-alumina, J. Appl. Phys. 120 (2016). [10] L.A. Perez-Maqueda, E. Gil-Gonzalez, A. Perejon, J.M. Lebrun, P.E. SanchezJimenez, R. Raj, Flash sintering of highly insulating nanostructured phase-pure BiFeO3, J. Am. Ceram. Soc. 100 (2017) 3365–3369. [11] H. Yoshida, P. Biswas, R. Johnson, M.K. Mohan, Flash-sintering of magnesium aluminate spinel (MgAl2O4) ceramics, J. Am. Ceram. Soc. 100 (2017) 554–562. [12] J. Zhang, W. Cui, M. Juda, D. McCammon, R. Kelley, S. Moseley, et al., Non-ohmic effects in hopping conduction in doped silicon and germanium between 0.05 and 1 K, Phys. Rev. B 57 (1998) 4472–4481. [13] A. Branwood, J.D. Hurd, R.H. Tredgold, Dielectric breakdown in barium titanate, J. Appl. Phys. 528 (1962) 4–5. [14] C. Neusel, H. Jelitto, G.A. Schneider, Electrical Conduction Mechanism in Bulk Ceramic Insulators at High Voltages Until Dielectric Breakdown, (2015), p. 154902. [15] A. Zafar, Z. Imran, M.A. Rafiq, M.M. Hasan, Evidence of Pool-Frankel conduction mechanism in Sr-doped Lanthanum Ferrite La1-xSrxFeO3 (0≤ x≤ 1) system Saudi Int. Riyadh, Electron. Commun. Photonics Conf. (SIECPC), 2011 3 (2011) 1–4 Riyadh: IEEE. [16] R.I. Todd, E. Zapata-Solvas, R.S. Bonilla, T. Sneddon, P.R. Wilshaw, Electrical characteristics of flash sintering: thermal runaway of joule heating, J. Eur. Ceram. Soc. 35 (2015) 1865–1877. [17] E.N.S. Mucccillo, M. Kleitz, Impedance spectroscopy of Mg-partially stabilized zirconia and cubic phase decomposition.Pdf, J. Eur. Ceram. Soc. 16 (1996) 453–465. [18] Y. Dong, I.W. Chen, Predicting the onset of flash sintering, J. Am. Ceram. Soc. 98 (2015) 2333–2335. [19] Y. Dong, I.W. Chen, Onset criterion for flash sintering, J. Am. Ceram. Soc. 98 (2015) 3624–3627. [20] S.K. Jha, R. Raj, The effect of electric Field on sintering and electrical conductivity of titania, J. Am. Ceram. Soc. 97 (2013) 527–534. [21] J. Narayan, A new mechanism for field-assisted processing and flash sintering of materials, Scr. Mater. 69 (2013) 107–111. [22] A. Karakuscu, M. Cologna, D. Yarotski, J. Won, J.S.C. Francis, R. Raj, et al., Defect structure of flash-sintered strontium titanate, J. Am. Ceram. Soc. 95 (2012) 2531–2536. [23] E.-J. Yi, M.-Y. Yoon, J.-W. Moon, H.-J. Hwang, Fabrication of a MnCo2O4/gadolinia-doped ceria (GDC) dual-phase composite membrane for oxygen separation, J. Korean Ceram. Soc. 47 (2010) 199–204. [24] F. Kovanda, T. Rojka, J. Dobešová, V. MacHovič, P. Bezdička, L. Obalová, et al., Mixed oxides obtained from Co and Mn containing layered double hydroxides: preparation, characterization, and catalytic properties, J. Solid State Chem. 179 (2006) 812–823. [25] N.E. Rajeevan, R. Kumar, D.K. Shukla, R.J. Choudhary, P. Thakur, A.K. Singh, et al., Magnetoelectric behavior of ferrimagnetic BixCo 2-xMnO4 (x=0, 0.1 and 0.3) thin films, J. Magn. Magn. Mater. 323 (2011) 1760–1765. [26] H. Bordeneuve, A. Rousset, C. Tenailleau, S. Guillemet-Fritsch, Cation distribution in manganese cobaltite spinel Co3-xMnxO4 (0 < x < 1) determined by thermal analysis, J. Therm. Anal. Calorim 101 (2010) 137–142. [27] J. Zuo, C. Xu, Y. Liu, Crystallite size effects on the Raman soectra of Mn3O4, J. Nanostruct. Mater. 10 (1998) 1331–1335. [28] A. Bewick, C. Gutiérrez, G. Larramona, C. Gutierrez, An in-situ IR spectroscopic study of the anodic oxide film on cobalt in alkaline solutions, J. Electroanal. Chem. 333 (1992) 165–175. [29] Q. Guo, H.-K. Mao, J. Hu, J. Shu, R.J. Hemley, The phase transitions of CoO under static pressure to 104 GPa, J. Phys. Condens. Matter 14 (2002) 11369–11374. [30] P.J. Gielisse, J.N. Plendl, L.C. Mansur, R. Marshall, S.S. Mitra, R. Mykolajewycz, et al., Infrared properties of NiO and CoO and their mixed crystals, J. Appl. Phys. 36 (1965) 2446–2450. [31] K. Nakamoto, John wiley sons, Infrared and Raman Spectra of Inorganic and Coordination Compounds, (1986), p. 2009. [32] M.P. O’Horo, A.L. Frisillo, W.B. White, Lattice vibrations of MgAl2O4 spinel, J. Phys. Chem. Solids 34 (1973) 23–28. [33] G.A. de Wijs, C.M. Fang, G. Kresse, G. de With, First-principles calculation of the

5. Conclusion Different roles of electric field in the flash sintering process are demonstrated. Flash sintering of MnCo2O4 is carried out under constant electric field and constant furnace heating rate. The MnCo2O4 phase evolution with electric field/specimen temperature is confirmed utilizing energy dispersive X-ray spectroscopy and micro-Raman Spectroscopy. The two techniques which give local information identified the spatial distribution of MnCo2O4 and CoO bases secondary phase. Raman signal mapping over the sample surface exposes the secondary phase growth profile (preferred or distributed) in the two different kinds of treatments, the flash and conventional. The observation provides experimental support to the proposition that the cations are actively available for the diffusion leading to enhanced sintering when they are involved in the reduction reaction. I–V characteristics recorded at different temperatures are utilized to find out the activation mechanism during the flash effect. It is shown that for the temperatures higher than threshold also, the material involves a fairly fast increase in the conductivity (not as fast as the flash effect) can undergo Poole Frenkel and tunneling like mechanisms. During the flash effect the electric field is such high and temperature is such low that the rapid conductivity increase occur through direct tunneling phenomenon. With these observations, two roles of electric field occurring at different stages of flash sintering process are emphasized, namely, 1) for charge carrier activation in quantum mechanical tunneling event and 2) for controlling the cations diffusion and acting over the secondary phase formation. These two events, together, impact over the microstructure resulting into accelerated flash sintering phenomenon.

9

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

A. Gaur et al.

located in an electric-field, I Teor. Fiz. 2331 (1986) 1376–1383. [43] A. Gaur, V.M. Sglavo, Tuning the flash sintering characteristics of ceria with MnCo2O4, J. Mater. Sci. Eng. B 228 (2018) 160–166. [44] K. Naik, R. Raj, V.M. Sglavo, Flash sintering as a nucleation phenomenon and a model thereof, J. Eur. Ceram. Soc. (2014). [45] R. Waser, T. Baiatu, K.‐H. Härdtl, Dc Electrical degradation of perovskite‐type titanates: II, single crystals, J. Am. Ceram. Soc. 73 (1990) 1654–1662. [46] T. Baiatu, R. Waser, K.‐H. Härdtl, Dc electrical degradation of perovskite‐type titanates: III, a model of the mechanism, J. Am. Ceram. Soc. 73 (1990) 1663–1673. [47] R. Waser, Dc electrical degradation of perovskite-type titanates:i, ceramics, J. Am. Ceram. Soc. 73 (1990) 1645–1653. [48] R. Waser, T. Baiatu, K.H. Härdtl, Degradation of dielectric ceramics, Mater. Sci. Eng. A 109 (1989) 171–182. [49] Y. Dong, H. Wang, I.-W. Chen, Electrical and hydrogen reduction enhances kinetics in doped Zirconia and Ceria: I. Grain growth study, J. Am. Ceram. Soc. 100 (2017) 876–886. [50] Y. Dong, I.-W. Chen, Electrical and hydrogen reduction enhances kinetics in doped zirconia and ceria : II. Mapping electrode polarization and vacancy condensation in YSZ, J. Am. Ceram. Soc. 101 (2018) 1058–1073. [51] R. Kirchheim, On the mixed ionic and electronic conductivity in polarized yttria stabilized zirconia, Solid State Ionics 320 (2018) 239–258.

phonon spectrum of MgAl2O4, Phys. Rev. B 65 (2002) 094305. [34] O.N. Shebanova, P. Lazor, Vibrational modeling of the thermodynamic properties of magnetite (Fe3O4) at high pressure from raman spectroscopic study, J. Chem. Phys. 119 (2003) 6100–6110. [35] O.N. Shebanova, P. Lazor, Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum, J. Solid State Chem. 174 (2003) 424–430. [36] D. Gallant, M. Pézolet, S. Simard, Optical and physical properties of cobalt oxide films electrogenerated in bicarbonate aqueous media, J. Phys. Chem. B 110 (2006) 6871–6880. [37] R.J. Thibeau, C.W. Brown, R.H. Heidersbach, Raman spectra of possible corrosion products of iron, Appl. Spectrosc. 32 (1978) 532–535. [38] D.L. de Faria, S.V. Silva, M.T. de Oliveira, Raman microspectroscopy of some iron oxides and oxyhydroxides, J. Raman Spectrosc. 28 (1997) 873–878. [39] S. Ganichev, E. Ziemann, W. Prettl, I. Yassievich, A. Istratov, E. Weber, Distinction between the Poole-Frenkel and tunneling models of electric-field-stimulated carrier emission from deep levels in semiconductors, Phys. Rev. B 61 (2000) 10361–10365. [40] J. Frenkel, On pre-breakdown phenomena in insulators and electronic semi-conductors [3], Phys. Rev. 54 (1938) 647–648. [41] M. Zaidi, J. Bourgoin, H. Maaref, Poole-frenkel-assisted emission from deep levels in electron-irradiated germanium, Semicond. Sci. 739 (1989) 5–9. [42] V. Karpus, V. Perel, Multiphoton ionization of deep centers in semiconductors

10