Current-ramp assisted sintering of 3YSZ: Electrochemical and microstructural comparison to flash and thermal sintering

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

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Original Article

Current-ramp assisted sintering of 3YSZ: Electrochemical and microstructural comparison to flash and thermal sintering Kent Harrison Christiana, , Harry Charalambousa, Shikhar Krishn Jhaa,b, Thomas Tsakalakosa ⁎

a b

Department of Materials Science and Engineering, Rutgers University, Piscataway, NJ, 08854, United States Department of Material Science and Engineering, Indian Institute of Technology, Kanpur, UP 208016, India

ARTICLE INFO

ABSTRACT

Keywords: Flash sintering Current assisted sintering Yttria-stabilized zirconia Impedance spectroscopy Space charge layer

Flash sintering features an unoptimized and uncontrolled rise in current density and sample conductivity. By using a controlled current-ramp technique with a predetermined ramp function, microstructure and electrochemical properties can be improved. This current-ramp method is investigated through use of test functions that follow square-root, linear, and parabolic time dependence with comparison to conventional flash sintering and thermal sintering. Steeper ramp functions during the sintering result in higher activation energy, suggesting a change in the vacancy concentration for both the bulk and grain boundary regions. Estimation of the grain boundary domain width suggests a grain size dependence of the unique space charge contribution to conduction independent of sintering method. Contrary to conventional wisdom, flash sintering can actually result in enhanced grain growth compared to controlled current-ramps and conventional sintering, implying that uncontrolled rise in current to a set cutoff may not be the optimal method for densification.

1. Introduction Pure zirconia is in monoclinic phase at room temperature, but transforms to tetragonal phase upon heating to 1170 °C, and to cubic phase at 2370 °C. The consequent volume expansion in the unit-cell induces large shear stresses which results in the specimen crumbling into pieces [1], necessitating stabilization via doping with other oxides such as yttria [2,3]. Doped zirconia is a widely used ceramic with applications including structural ceramics [4], solid oxide fuel cells [5], and dental crowns [6]. Since 3 mol% yttria stabilized zirconia (3YSZ) ceramics were the first to be studied under flash conditions and were found to densify well with limited grain growth, this material was used for this study. Yttria stabilized zirconia usually require furnace temperatures greater than 1400 °C for conventional, pressureless sintering. However, a new electric current activated sintering (ECAS) technique, called flash sintering [7], has been used to lower the required furnace temperature dramatically with the help of an externally applied DC electric field and has been extended successfully to densify a wide variety of compositions [8–10]. Although applying an electric field to internally heat up the specimen is simple, how one applies an electric field and how the current surges through the specimen is critical in determining the final physical density and microstructural distribution of the specimens. In the most widely used method of flash sintering, an electric field is ⁎

applied while the sample is heated at a constant heating rate. Under this field, a small current flows through the sample. As the specimen heats up, the intrinsic conductivity of the ceramic rises. At sufficiently high temperature, the current begins to rise non-linearly, which further heats up the specimen as a consequence of Joule heating. This sets a feedback loop which results in mutually dependent current-thermal runaway. To avoid this unintended runaway, which could cause melting, an artificial limit is placed on current, beyond which the power supply providing the voltage switches to current control mode. This feedback event up to the current limit corresponds to a sudden, dramatic increase in brightness due to Joule heating-induced rise in blackbody radiation of the ceramic [11], referred to as the “flash” [12,13]. Since its discovery, a wide variety of studies have been performed to study the mechanisms of flash sintering and the properties of sintered materials including, but not limited to, the effect of initial grain size [14], effect of field and current limit (isothermal) [15], electrochemical reduction [16–18] electronic conductivity [19], complex impedance properties [20], photoemission [21], pseudocubic phase generation [22], thermal runaway effect [23], microstructural inhomogeneity [24,25], effect of heating rate [26], mechanical properties [27], anisotropic lattice expansion [28], effect of processing environment [29,30], densification and grain growth kinetics [31,32], and superplasticity under flash conditions [33]. A few proposed mechanisms have gained prominence in order to

Corresponding author. E-mail address: [email protected] (K.H. Christian).

https://doi.org/10.1016/j.jeurceramsoc.2019.09.036 Received 31 July 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Kent Harrison Christian, et al., Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.09.036

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heated to 1000 °C at 10 °C min−1, and held for 5 min to equilibrate to the furnace temperature before applying an electric field. A Pacific Power Source 118-ACX power supply was used to apply an AC voltage at 1000 Hz. AC power was used to avoid cracking, which is observed when DC electric field is applied, potentially related to a partial pseudo-cubic phase transition [22]. An initial field of 75 Vrms cm−1 was applied, in voltage control mode, for the case of conventional flash, which was capped at a maximum current density of 10 Arms cm−2 and kept under current control for 10 s. For the current-ramp experiments, an initial current limit of 1 Arms cm−2 was quickly reached with an applied field of 100 Vrms cm−1 to ensure the starting current was hit quickly, and thereafter the current limit was stepped with 0.3 s time resolution intervals from 1 to 10 Arms cm−2 under varying ramp profiles: square-root ramp, linear ramp, and parabolic ramp. These current-ramp controlled experiments were compared to conventional flash experiments. In all cases the ramp time from 1 Arms cm−2 to 10 Arms cm−2 was approximately 18 s, and the dwell time at 10 Arms cm−2 was 10 s in order to match the corresponding rise and hold times of the uncontrolled conventional flash case. The rationale for this currentramp procedure is outlined in other works, in which only linear currentramp was used [50,51]. These linear current-ramps may not be the most optimized profile, because rise in conduction in the sample is nonlinear. The densities of all samples were measured via Archimedes method.

explain the rapid sintering kinetics characteristic of flash, which can be broadly divided into two categories: thermal effects and non-thermal effects. According to the thermal effect of current induced Joule heating, specimens heat up rapidly creating a fast firing effect [34,35] comparable to conventional fast firing, but using internal resistive heating (volumetric heating) to achieve a similar effect [23,36]. However, questions have been raised as to whether the temperature rise is sufficient to explain the kinetics. An alternative hypothesis suggests that local heating at the grain boundaries is considerably higher than the bulk, which pins grain boundaries by reducing the interfacial energy and eliminating the driving force for grain growth [37,38]. This temperature gradient has been proposed to be so severe in the case of flash sintering as to cause local melting of the grain boundaries and a self-wetting liquid phase sintering [39]. Alternatively, it has been suggested that the electric field, in addition to heating the specimen, also has non-thermal effects on material by inducing an avalanche of Frenkel defect pairs, which have their excess charges stripped with the resulting electron-hole pairs as the charge carriers [40,41]. This process lowers the energy barrier to diffusion as the neutral atoms are less strongly repelled by ions in the lattice [42]. This understanding can explain the luminescence effect, rise in electronic conductivity, and high kinetics of diffusion required for sintering at relatively lower temperature [12]. It is tough to resolve this debate of thermal/non-thermal effects of electric field since it requires very accurately determined local temperatures, which could be significantly different from the bulk temperature measured by pyrometers or thermocouples. Thermocouples have an additional problem of being electrically loaded if brought into contact with the current carrying specimens. An alternative method of determining the local temperature is to measure the change in lattice parameters using high energy synchrotron X-ray diffraction and correlate it to thermal expansion. The estimated temperature (in bulk and on surface) was found by several studies to be comparable to conventional sintering temperatures [43–46]. This suggests that heating rate is a critical parameter of flash sintering, albeit there may be other factors that cannot be ruled out. The densification rate depends on the average electrical powerdissipation into the sample, which suggests that control of the average power dissipation rate can allow for finer control over the sintering kinetics, microstructures, and, as a result, the mechanical and electrical properties of the final product. In this work, we explore a method to control the rise in current during the onset of the flash event by applying an initial constant voltage, and then incrementally raising the current limit when it is reached. In doing so, the current is made to undergo a controlled ramp instead of the runaway one commonly seen in flash sintering under a constant current limit [47,48]. Using this technique, the sharp power spike, as seen during transition from voltage control to current control [49], can also be avoided. This power spike has been associated with very high temperature and is a point of instability.

2.2. Impedance spectroscopy After sintering, both faces of each sample were manually polished and coated with silver paste before performing electrochemical impedance spectroscopy (EIS) measurements. The samples were connected with silver electrodes to a Gamry Reference 600 + potentiostat. EIS data was collected with a voltage of 1000 mV from 0.1 to 5 MHz over a temperature range 310–450 °C. Responses from impedance spectroscopy were deconvoluted into bulk, grain boundary, and electrode-interface components, which were fit with a capacitor-resistor brick circuit model. The impedance plots were compared directly for each sample by accounting for size differences by normalizing to a geometric factor (L = H /A ) where L is length, H is height, and A is surface area of one face. Fitting of the experimental results to the circuit model provided values for resistive and capacitive elements. 2.3. Microstructural characterization Vertically cut slices of the cylindrical pellets were mechanically polished down to 0.25 μm with diamond polishing paste. Specimens were thermally etched at 1250 °C for 1 h, followed by 20 nm gold sputter coating to the polished surfaces. Specimens were then loaded into a Zeiss Sigma Field Emission SEM and micrographs were taken using the in-lens detector to exaggerate the contrast between bulk and grain boundaries. The sampled areas were the axial and radial center of each pellet. The average grain size for each sample was calculated by using the linear intercept method using Lince242e software. A linear least squares fitting method was applied with greater than 100 grains per micrograph and a factor of 1.56 was applied to correct for the shape factor [52].

2. Materials and methods 2.1. Sample preparation Tetragonal 3YSZ nanopowder, sourced from Tosoh Corporation (TZ3YB), was pressed into cylindrical pellets with 6 mm diameter and 4.5 ± 1 mm thickness resulting in an average green density of 47–48%. A conventionally heated standard was placed in a box furnace, heated to 1350 °C at 10 °C min−1, held for 10 s, and then cooled to room temperature. For the flash experiments, the pellet-faces were coated with platinum paste, loaded into an alumina stage with platinum electrodes, and placed into a custom built furnace. The sample was then

3. Results and discussion According to the progression of current indicated in Fig. 1, it is apparent that the total current flowing through the specimen depends on the shape of the curve, with lowest area under the curve for conventional flash and highest for square-root ramp. By controlling the ramp rate the power spike evident in the transition from flash onset to

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Fig. 1. Root mean square current density curves under conventional flash (CF) as well as current-ramp functions: square-root (SR), linear (LR), and parabolic (PR). Average power spike magnitudes indicated on right side of plot. Constant electric field was applied to instigate each flash.

steady state has also been drastically reduced. Treating the samples as resistive loads during flash, we take the average power dissipated in them by an alternating current to be Pavg = Vrms Irms . Thereby finding that maximum average power density was reduced from 783.3 W cm−3 for conventional flash to 337.4–457.8 W cm−3 for controlled currentramps (Table 1). By reducing the power spike, the corresponding increase in temperature [53] can be eliminated, avoiding the formation of a liquid phase or decomposition to secondary phases in ceramics with narrow sintering windows. Using an approximation that incorporates blackbody radiation and thermal conduction [53] the final temperature of each specimen in current-controlled steady state was calculated (Eq. 1):

P= Pradiation + Pconduction =

(T 4

T o4 )2 RH+

2k (T TC)2 R2 H

which meets the threshold of the conventional thermal sintering temperature. Attempts to densify at lower current limits resulted in lower final density, indicating that 10 Arms cm−2 is a minimum current density required for near full densification with flash and current-ramp sintering. Estimation of the temperature during the prominent power spike is difficult as the sample is in a non-equilibrium state. However, as higher average power dissipation corresponds to higher resistive heating, the power spike temperature can be safely assumed to be higher than the steady state temperature. It had been previously shown that weak AC [56] and DC [57] electric fields resulted in enhanced sintering rate with limited grain growth. The considerably larger cross sectional area to surface area ratio of the pellets sintered in this study compared to the dog boneshaped specimens sintered in other experiments [7,20,58] may result in partial reduction that cannot be easily replenished from the air [59]. In addition, inhomogeneous microstructure across the specimen thickness has been observed in CeO2 [45], TiO2 [44,59], ZnO [60], MgAl2O3 [58], Al2O3 [61], 3YSZ [25], and 8YSZ [62,63], attributed to currentinduced differential heating under DC flash conditions. Since an AC power supply is used in the present study, the grain-size distribution was symmetric across the height of the specimens. Representative micrographs of the central region of each pellet are shown in Fig. 2 with corresponding grain size distributions in Fig. 3. The sintering conditions and resulting average grain size and density are listed in Table 1. The average grain size under conventional flash conditions was comparable to those measured in other works with nanopowder provided by the same supplier [30,64,65] and the coarser grain morphology is likely due to the high temperature reached during the power spike in stage II, resulting in a sudden temperature rise and enhanced grain growth. We found conventional heating conditions (with no electric field) achieved the finest microstructure by far through careful

(1)

Blackbody radiation and thermal conduction model for calculating temperature in the steady state. P is average power dissipation (W) into the sample, ε is emissivity (estimated ∼0.9 for oxide ceramics [54]), σ is Boltzmann’s constant (5.67·10−8 W m−2 K−4), To is temperature of the furnace, R is radius of the pellet, H is height of the pellet (in cm), k is 3YSZ thermal conductivity (0.023 W cm−1 K−1 at 1000 °C) [27,55], and TC is electrode contact temperature due to contact resistance (∼174 °C based on a previous work [53]). It is critical to point out that this model simplifies the situation by assuming that the temperature is homogeneous in the sample and thus provides an approximation of the average sample temperature, accounting for the radiative and conductive heat transfer through the electrodes. Additionally, the equation combines the radiative and conductive parts of the equation as independent terms in order to make an approximation of the temperature. The calculated sample temperature is a reflection of the central region of the pellet. The temperature was approximated to 1420 °C in the steady state,

Table 1 Summary of sintering conditions including root mean square current density (j), ramp function, and estimated steady state sample temperature as well as average grain size and density. Sintering Condition

j(t) (Arms·cm−2·s−1)

jmax (Arms·cm−2)

Pmax (W·cm−3)

tdwell (s)

Tfurnace (°C)

Tsample (°C)

Grain size (μm)

Density (%)

CS CF SR

0 uncontrolled a1 t + b1 a2 t + b2 a3 t2 + b3

0 10 10

0 783.3 337.4

10 10 10

1350 1000 1000

1350 1420 1420

0.22 ± 0.09 1.18 ± 0.64 0.55 ± 0.29

96.3 92.2 96.5

LR PR

10 10

457.8 401.6

10 10

3

1000 1000

1420 1420

0.62 ± 0.35 0.56 ± 0.32

94.3 96.5

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Fig. 2. Micrographs of specimens sintered under conventional sintering (CS), conventional flash (CF), square-root ramp (SR), linear ramp (LR), and parabolic ramp (PR) conditions. Shallow pores correspond to grain pullouts while dark pores are true pores.

determination of optimal sintering temperature, in contrast to other works which have used higher temperatures and resulted in higher grain growth. The findings contradict the claims of reduced grain growth under flash conditions and highlight the importance of careful selection of sintering conditions. It is clear that flash sintering methods do not always result in finer grain sizes than conventional sintering, and future study is required on the mechanisms of grain growth or lack thereof in flash sintering. The current-ramp sintering experiments resulted in comparable microstructure with each other for all rate functions. Complex impedance plots in the temperature range of 310–450 °C are shown in Fig. 4 for all sintering conditions, with overlapping simulated patterns and equivalent circuit also indicated in the figure. The data fits closely to a series of three resistor-constant phase elements corresponding to high frequency bulk, medium frequency grain boundary, and low frequency electrode-ceramic interface. The real and

imaginary components of the impedance have an inverse relationship with specimen temperature as expected in general for ceramics. The highest magnitude complex impedance is observed under conventional sintering and conventional flash conditions while the three currentramp sintering conditions produce specimens with lower impedance magnitudes. Assuming that impedance comes only from oxygen ions and that electronic conduction is negligible, this suggests that better ionic conduction was achieved at the same temperature. This could happen for several reasons, such as the current-ramp samples having higher density, larger grain size, or a more ordered crystal structure. In this case we expect larger grain size to explain why the current-ramp samples have smaller magnitude impedances than the conventional sintering sample, since the increase in average grain size is evident. The conventional flash sample has a higher magnitude of impedance despite its large grain size, most likely due to being relatively porous and low

Fig. 3. Representative grain size distributions for sintering conditions (a) conventional sintering (CS), (b) conventional flash (CF), (c) square-root ramp (SR), (d) linear ramp (LR), (e) parabolic ramp (PR). (f) Mean grain size and standard deviation.

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to have activation energies close to or lower than that of conventional sintering; however, parabolic ramp and conventional flash had activation energies that exceeded those of conventional sintering. Currentramp conditions suggest a trend of progressively higher activation energy corresponding to steeper ramp and, as conventional flash sintering can be approximated to an exponential current-ramp condition, this would further corroborate the aforementioned trend. Maqueda, et al. observed a similar result of the change in activation energy in BiFeO3 ceramic sintered under flash conditions [73]. The trend may be explained as an effect of disorder of the lattice caused by increased concentration of dislocations as observed by Cho, et al. [27]. Published results on the effect of flash sintering on grain boundary conductivity are somewhat contradictory. M’Peko et al. and Vendrell et al. found that conductivity significantly increased for grain boundaries of flash sintered specimens (with DC field) of 3YSZ and 8YSZ, respectively. This was said not to be the result of densification or microstructural changes, and was instead attributed to accommodation of oxygen vacancies in the space-charge region [20,74]. On the other hand, Du et al. found that flashed (with AC field) and conventional specimens of 8YSZ had similar characteristic responses, suggesting that sintering method does not play a significant role in characteristic electrochemical behavior [75]. The present study shows agreement with Du’s results that flash sintered and conventionally sintered specimens share similar impedance spectra, however the lower magnitude impedance spectra for the current-ramped specimens suggests that decreased impedance is also possible under certain flash conditions. Estimation of the grain boundary thickness was performed by applying the model of M’Peko, et al. [20], who utilize the bricklayer model and assume a series model circuit of grains and grain boundaries along the conduction path. The average grain size, Dg, is of a much higher magnitude than the average grain boundary thickness in submicron grains, δgb, such that Dg > > δgb. The average grain boundary thickness is highly grain size dependent in this regime (Fig. 6), ranging from 2.69 nm to 15.45 nm for conventional sintering and conventional flash, respectively, which are the two extreme cases in our study. However, the ratio of grain boundary thickness to grain size is approximately constant, ranging within a narrow band of 0.012 - 0.014, for all temperatures and sintering conditions. In other words, for all current-ramp profiles, the grain boundary thickness, which incorporates grain boundary core and space charge layer, is proportional to grain size, which is further linked to average power dissipation. These results contradict the findings of M’Peko et al. who reported that flash sintered materials are characterized by thinner grain boundaries [20], albeit in that study a DC electric field was used with a non-alternating direction for vacancy migration. Additionally, axial microstructural inhomogeneity is characteristic of DC flash sintering and the average grain size estimation depends on the position where the micrograph was acquired. This also seems to be at odds with conventional wisdom which expects grain boundary thickness to depend on the thickness of the oxygen vacancy depletion layer, not the grain size. Although we have found a correlation between grain size and grain boundary thickness in this study, the evidence is not sufficient to imply causation. As an alternative explanation, the conventional flash may cause a greater depletion of the oxygen vacancies, creating a thick depletion layer. On the other hand, conventional thermal sintering may have the lowest level of oxygen vacancy depletion, creating a thin depletion layer.

Fig. 4. Complex impedance spectra and temperature dependence of specimens sintered under conventional (CS) and flash (CF) conditions as well as currentramp functions: square-root (SR), linear (LR), and parabolic (PR). Theoretical plots of the proposed circuit model are shown in black.

density. Simulation of the data allows for the calculation of conductivity, relaxation frequency, and permittivity (see Appendix A) for each sample at all measured temperatures. Data has been fit linearly to an Arrhenius function relating these values to the inverse temperature (Fig. 5). For each case, grain boundaries have slightly higher activation energy compared to the bulk, indicating that the space charge layers formed around the grain boundaries act as blocking layers to oxygen diffusion [66,67]. In YSZ ceramics, oxygen ions diffuse by hopping from vacancy to vacancy. The energy required to make this jump, the activation energy, depends on the distance between the vacancy sites and the local crystallographic environment in the ion’s path [68–70]. Due to depletion of the concentration of oxygen vacancies as well as increase of the concentration of yttrium interstitials in the space charge layer [71] we expect the activation energy to increase in the vicinity of the grain boundaries. Activation energies were calculated for bulk and grain boundary regions based on Arrhenius fitting of the relaxation frequency and AC conductivity curves are summarized in Table 2 . Bulk activation energies for conductivity range from 0.84 to 1.04 eV and grain boundary activation energies for the same conductivity range from 1.02 to 1.22 eV, with good agreement between the values calculated using the relaxation frequencies. Another study of conventionally sintered 3YSZ ceramics estimates activation energies of 0.85 eV and 1.09 eV for the bulk and grain boundary contributions, respectively, showing adequate agreement with the results of the conventionally sintered sample in the present study [72]. Square-root and linear ramp specimens were found

4. Conclusions The application of a controllable current-ramp function introduces new possibilities for tuning ceramic properties during the sintering

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Fig. 5. Arrhenius dependence of relaxation frequency, conductivity, and permittivity for bulk and grain boundary components. Table 2 Calculated activation energy (Ea) for bulk and grain boundary regions based on Arrhenius fitting of relaxation frequency and AC conductivity. sintering condition

relaxation frequency

AC conductivity

CS CF SR LR PR

Ea,bulk (eV) 0.92 1.04 0.88 0.94 1.00

Ea,bulk (eV) 0.89 1.00 0.84 0.89 0.96

Ea,gb (eV) 1.07 1.22 1.03 1.12 1.16

Ea,gb (eV) 1.06 1.19 1.02 1.05 1.12

process and is advantageous over conventional flash sintering, which is characterized by a potentially undesirable power spike. Current-ramp assisted sintering has been shown to achieve higher density and lower grain growth than conventional flash sintering, achieving properties more closely related to those of conventional thermal sintering. Impedance spectroscopy indicates that all conditions result in distinct bulk, grain boundary, and electrode-interface regimes, but the bulk and grain boundary activation energy correlates to the acceleration of the current during the current rise to maximum. The trend suggests that the presence of high dislocation density observed in other studies of flash sintered 3YSZ, which results in more ductile behavior, also increases the activation energy for oxygen ion mobility. The space charge region, which corresponds to a separate conduction regime from the bulk, is estimated to have a thickness proportional to the bulk grain size.

Fig. 6. Estimation of the grain boundary thickness for conventional (CS), conventional flash (CF) and current-ramp samples under square-root (SR), linear (LR), and parabolic (PR) ramping functions. Right axis shows ratio of grain boundary thickness to average grain size.

Acknowledgements The authors wish to acknowledge the support provided by the Office of Naval Research and Dr. Antti Makinen under Contract No. N0001415-1-2492.

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Appendix A Conductivity (σ), relaxation frequency (f0), and permittivity (ε) values were calculated with the following equations: i

L (1 + Y0i Ri (j i ) i ) Ri

=

f0i =

i

=

(2)

1 1

2 (Ri Y0i )

(3)

i

1 + Yi Ri (j 0i ) jRi 0i C0

i

(4)

Where L is the geometric factor mentioned in Section 2.2, Y0 and α are empirical constants relating the constant phase element to an ideal capacitor, R is the resistance, ω is applied AC frequency, ω0 is relaxation frequency, C0 is empty cell capacitance, and j2 = 1. i is an index indicating a component of the system, either bulk, grain boundary, or electrode-interface. As values for electrode-interface effects are not sample specific, they were not of interest in the present study but are included in the circuit model for deconvolution of the experimental impedance curves.

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