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From conventional ac flash-sintering of YSZ to hyper-flash and double flash
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Journal of the European Ceramic Society 33 (2013) 2093–2101
From conventional ac flash-sintering of YSZ to hyper-flash and double flash Marlu Cesar Steil ∗ , Daniel Marinha, Yann Aman, Jonas R.C. Gomes, Michel Kleitz Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces de Grenoble (LEPMI), UMR 5279 CNRS, Université de Grenoble, BP 75, 38402 Saint Martin d’Hères, France Received 5 February 2013; received in revised form 4 March 2013; accepted 5 March 2013 Available online 6 April 2013
Abstract Flash-sintering experiments were performed on conventional 8 mol% Y2 O3 -doped ZrO2 . By varying the initial power spike, the steady state limited current density, the flash duration and furnace temperature, various microstructures corresponding to different sintering stages were observed. It is shown that operating the flash-sintering under the power dissipation regime may lead to microstructure heterogeneity. Under a quasi-adiabatic regime a better homogeneity is obtained. The major part of the shrinkage strain occurs within a short time interval of about 1 s, after the incubatory period. The remaining part appears to be a conventional shrinkage process induced by Joule heating. Hyper-flashes were performed over this short time interval, yielding relative densities up to 90%. Additional flashes can be subsequently applied up to full density. It is stressed that densification is not an absolute quality parameter in the optimization of the experimental conditions; inappropriate conditions may lead to fairly dense but brittle materials. © 2013 Elsevier Ltd. All rights reserved. Keywords: Sintering; ZrO2 ; Microstructure; Field assisted sintering; Flash-sintering
1. Introduction The so-called flash-sintering process has already demonstrated its capability in terms of investment cost, energy and time savings. Currently, researches aimed at understanding the basic mechanisms are focused on Zirconia-based oxide ion conductors (doped either with 3 or 8 mol% Y2 O3 ). The main emphasis is on the magnitude and location of the temperature increase resulting from the internal Joule effect.1–6 The alteration of the grain boundary space charge layer by the applied electric field, can explain a smaller grain growth.7,8 In this work, we focused our attention on the microstructures that can be obtained by varying the flash-sintering parameters, i.e. the initial power spike, the steady state limited current density, the flash duration and the furnace temperature. Most of the published results were obtained by applying a selected electric field to the sample, at low temperature, and then ramping up its temperature.5,9,10 In contrast to this procedure, we first heated the sample to the selected temperature, for instance 900 ◦ C, and then applied various ac electric
signals. The general features of the response, as a function of time, were described by Muccillo et al.11 It includes an incubatory period before the flash-sintering runaway. To avoid any electrical breakdown we used equipment which enabled a current limitation, already implemented by Cordier et al.12 In a first set of results, we describe the microstructures resulting from moderate electrical polarizations, at various furnace temperatures. In this case, the electric signal was maintained till the shrinkage strain reached its steady-state value, typically after a time of the order of 10 min. This allowed us to select appropriate parameters for the flash-sintering experiments proper. Then, an initial temperature of 900 ◦ C was selected and more conventional flash conditions were applied, varying the current density and flash duration over the 10–50 s range. The third part of this work is focused on the initial abrupt shrinkage which occurs over a 1–2 s period. We will call it the hyper-flash. Double flashes were also applied. 2. Experimental 2.1. Sample processing and characterization
∗
Corresponding author. Tel.: +33 4 76 82 65 98. E-mail address: [email protected] (M.C. Steil).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.03.019
We used a commercial powder (TZ-8Y from Tosoh, Japan) formed of granules (diameter range: 15–140 m) composed of
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Fig. 1. Experimental set-up for flash sintering experiments.
30 nm particles. Its specific surface area is 16 m2 g−1 . It was used without any further treatment. The samples were uniaxially and isostatically cold pressed under 100 and 250 MPa, respectively, resulting in green relative densities of about 50% TD (Theoretical Density). The pellets were 8 mm in diameter (0.5 cm2 in area) and about 2 mm thick. The post-sintering densities were determined geometrically and by the Archimedes method. Obviously, when the samples exhibit an open porosity, these two densities markedly differ. All the current densities and electric fields mentioned in the following text were calculated using the pre-sintered sample dimensions. Micro-structures were examined both on fracture as well as on polished (1 m diamond paste) and thermally etched (1200 ◦ C, 30 min) surfaces. SEM images were obtained, either in the “in lens” detection mode, or in the conventional secondary electron mode (SE), in a Field Emission Gun SEM (Zeiss Ultra 55). For the impedance spectroscopy analysis, an Impedancemeter HP 4194A was used in the frequency range 100–1.5 × 107 Hz with a 100 mV signal. All the impedance diagrams were plotted at 350 ◦ C and were normalized by the pellet geometrical factor after sintering. (The numbers on the diagrams indicate the logarithm of the measuring ac frequency). The crystallographic structure of a few samples, randomly selected, were X-ray controlled using a Philips X’Pert with 2θ ranging from 15◦ to 135◦ , with 0.033◦ steps and a 45 s step time. They all exhibited the expected cubic phase. 2.2. Flash-sintering equipment and procedures The experimental set-up which was assembled in our laboratory is sketched in Fig. 1. It is equipped with a displacement sensor (GEFRAN, PY2 Rectilinear displacement transducer) capable of measuring a shrinkage of 10 m. To improve the uniformity of the current distribution through the samples, some pellets had their bases covered with Pt paint (Metalor® 6926). The current collectors made of Pt grids were slightly pressed against the pellet bases. When platinum paint was used, the grid was 0.75 mm mesh wide and made of a wire 0.25 mm in diameter. Without platinum paint, we used a thinner grid 0.50 mm mesh wide, made of a 0.12 mm wire. These grids were connected to the power generator by 1 mm diameter Pt wires capable of sustaining
Fig. 2. Variations of the CF900-14A-30s sample flash parameters: furnace temperature 900 ◦ C; limited current density 14 A cm−2 ; time under limited current (t) 30 s. The post-flash relative density of the sample was 97%TD.
high currents. The experiments were performed under air. The furnace temperature ramps were 10 ◦ C min−1 both on heating and cooling. All temperatures mentioned henceforth refer to the furnace temperature (it is the pre-heated sample temperature). For a qualitative evaluation, a thermocouple was placed close to the sample (∼1 mm). Before each experiment, the sample temperature was stabilized during 30 min. The power generator was a Pacific Smart Source 115ASX AC, controlled by the UPC Manager V.1.4 software. It enabled us to apply to the sample, an ac voltage of 1000 Hz, adjustable between 0 and 115 V, which corresponds, in our case, to a field of about 0–500 V cm−1 , while limiting the resulting alternating electric current to a selected value up to 15 A (corresponding to a current density of 30 A cm−2 ). Its response time is of the order of 1 s. It is to be stressed that the long and medium time flashes were performed under constant current densities and not under constant electric fields as in most of the published experimental conditions. In practice, the flash was triggered by applying an appropriate electric field. This resulted in an abrupt current density increase which was limited by the power supply to the selected value, within about 1 s. This limitation was evidently obtained at the expense of the applied field which accordingly decreased. Before the current limitation, over a transient period, the applied electric field induced a larger current upsurge that resulted in a power spike. An illustration of this behavior is shown in Fig. 2. All the electrical parameter variations were continuously recorded. The flash duration t is defined in Fig. 2. It is that of the current limited period.
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Fig. 4. Relative densities as a function of the furnace temperature, for various current densities in moderate flash conditions.
Fig. 3. Green sample MEB micrographs. (a) In lens SE detector mode; (b) conventional SE mode.
3. Results Fig. 3 shows two micrographs of a compacted sample, before sintering, recorded in the “in lens” SE mode and in the conventional mode. The grains are smaller than 50 nm and form agglomerates of about 150 nm. 3.1. Moderate flash conditions Table 1 presents all the experiments which were performed under the moderate flash conditions and their associated acronyms. Thin grids were used without any Pt paint. At this stage, the electric polarization was maintained till shrinkage stabilization. The initial electric field applied to the sample was gradually increased by subsequent 20 V cm−1 steps lasting 1 min, until the flash runaway. The initial field mentioned in Table 1 is that producing the flash runaway. After completion of the experiment, the current was gradually decreased to avoid any thermal shock. The large differences in some of the reported geometrical and Archimedes densities point to open porosities. Fig. 4 summarizes the results in terms of relative geometrical densities. For a low current density of 0.3 A cm−2 , no significant sintering has yet started. The porosity closes only at a current density of 6 A cm−2 . As expected, whatever the furnace temperature, an increase in the current density plays a favorable part in the densification (Table 1).
Fig. 5 shows microstructures obtained with different current densities, at a furnace temperature of 900 ◦ C. In the initial stage (up to 1.5 A cm−2 ), the sintering proceeds mostly by coalescence of closely neighboring grains in the initial agglomerates (cf. Fig. 3), and form grains of about 100 nm. Over 3 A cm−2 , a densification by intergrains sintering occurs with à small grain growth at 6 A cm−2 . This observation is in agreement with our previous results obtained on the flash welding of pre-sintered YSZ granules which show the neck formation starting at 1.3 A cm−2 .12 Fig. 6 shows the influence of the furnace temperature on the sample microstructure, under the most “efficient” current density of 6 A cm−2 . As can be seen, the higher the furnace temperature, the higher the densification and the grain growth. This could appear in contradiction with the commonly accepted Joule effect analysis. As the furnace temperature increases, the sample resistance decreases and therefore the Joule power supplied to the sample decreases (RI2 with a constant I). This observed variation could result from the grain faceting mentioned below and most probably from the initial power shot. Additional observations that were made are the following: - With current densities lower than 6 A cm−2 , a large part of the porosity remains open as shown by the marked differences between the geometrical and Archimedes densities. - The impedance diagrams (Fig. 7) which appear highly sensitive to the densification, show a fairly continuous variation, consistent with previously published results13 on samples partially densified by conventional sintering. As shown in Fig. 7, the overall resistance which is given by the rightmost intercept of the diagram curve with the real axis (electrode contribution was extracted), markedly varies with the sintering degree. This certainly contributes to the flash runaway. - Grain faceting can occur under the experimented conditions, as shown in samples MF800-3A and MF800-6A (Fig. 8). This is usually regarded as a detrimental evolution impeding a further densification. This would be in agreement with the leveling off of the densification observed at 800 and 900 ◦ C (Table 1). A way to overcome this difficulty is to accelerate the densification by increasing the furnace temperature as
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Table 1 Experiments performed under the moderate flash conditions. Sample
MF800-0.3A MF800-1.5A MF800-3A MF800-6A MF900-0.3A MF900-1.5A MF900-3A MF900-6A MF975-0.3A MF975-1.5A MF975-3A MF975-6A
Furnace temperature T (◦ C)
800 800 800 800 900 900 900 900 975 975 975 975
Initial electric field (V cm−1 )
160 180 190 190 100 120 120 120 40 80 80 80
Current density limited (A cm−2 )
0.3 1.5 3.0 6.0 0.3 1.5 3.0 6.0 0.3 1.5 3.0 6.0
t (s) under limited current
1761 510 770 760 2000 470 320 250 1300 1300 260 165
Relative density (%TD) Geometrical
Archimedes
48 49 63 85 48 50 64 88 48 51 68 94
48 82 85 89 49 83 90 92 48 90 87 97
confirmed by the densities obtained at a furnace temperature of 975 ◦ C (Table 1). - Nice images of spiral steps associated with the grain growth were observed in sample MF975-6A (Fig. 9). - The power spike supplied to the sample at the onset of the flash runaway, before the current limitation sets in (Cf. Fig. 2), is fairly constant and equal to about 2000 W cm−3 (the corresponding experimental data vary by about 50% around this average value, without showing any significant trend). This appears to be the determining parameter of the flash runaway, instead of the electric field.
3.2. Conventional flash sintering
The main practical conclusion of this fairly systematic preliminary investigation is that the sample density levels off at 94%TD, under the conditions selected in this first part. To obtain better densifications, current densities higher than 6 A cm−2 have to be used.
- The investigated experimental conditions (Table 2) cover a broad field of sintering stages. At one end, the CF900-10A-7s sample has a density of only 81%TD and its grain size is about 0.5 m. At the other end, with the CF900-20A-30s sample, a local fusion is reached before 30 s (Fig. 10).
In this second experimental part, the initial sample temperature (furnace temperature) was fixed at 900 ◦ C and the initial field electric was fixed at 180 V cm−1 . Three constant limited current densities of 10, 14 and 20 A cm−2 were applied and the flash time spanned the 7–45 s range. No Platinum paint was used. The polarization was applied abruptly, without any pre-polarization steps. Table 2 summarizes the investigated experimental conditions and gives the corresponding sample acronyms. The main observations we made are the following:
Fig. 5. Influence of the current density on the microstructure. Furnace temperature 900 ◦ C.
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Table 2 Experiments performed under the conventional flash conditions. The initial electrical field was fixed at 170 V cm−1 . Sample
CF900-10A-7s CF900-10A-30s CF900-10A-45s CF900-14A-7s CF900-14A-14s CF900-14A-30s CF900-10A-7s CF900-20A-14s CF900-20A-20s CF900-20A-30s
Current density limited (A cm−2 )
10
14
20
t (s) under limited current
7 30 45 7 14 30 7 14 20 30
Grain size (m)
Relative density (%TD) Geometrical
Archimedes
81 89 96 92 92 97 95 94 94 Fusion
82 87 98 94 93 97 98 98 94
0.5 0.3 0.5–1 0.5 1–2 1–2 3–5 Heterogeneity Heterogeneity
- Densities between 95 and 98%TD can be reached. - As shown in Table 2, under the relatively small current density of 10 A cm−2 , the flash time plays a significant part on the densification, without leading to a marked increase in the grain size which stays around 0.5 m. - On the other hand, over a constant short sintering time of 7 s, the limited current density is a determining parameter for the grain size which varies from 0.5 to 4 m (Table 2). - Fig. 11 compares the impedance diagrams of some dense samples (CF-900-10A-45s and CF-900-20A-14s) to that of a material of similar grain size, conventionally sintered at 1400 ◦ C.13 The diagrams are quasi identical. This important result undoubtedly shows that flash sintering does not
Fig. 6. Influence of the furnace temperature on the microstructure at constant current density (6 A cm−2 ). Fig. 7. Influence of the current density on the sample impedance diagram for a furnace temperature of 900 ◦ C. In this figure and in Fig. 11 numbers indicate the logarithm of signal frequency.
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Fig. 10. Example of local fusion in the sample CF900-30A-30s (flash conditions: furnace temperature 900 ◦ C; limited current density 20 A cm−2 ; time under current (t) 30 s).
Fig. 8. SEM images (SE in lens mode) of MF800-3A and MF800-3A ceramics showing the grain faceting. Fig. 11. Impedance diagrams, obtained at 350 ◦ C, of flash-sintered samples to quasi full density (CF900-10A-45s and CF900-20A-14s) and a conventionally sintered sample (1400 ◦ C).
Fig. 9. Crystal growth spiral steps.
introduce any peculiar electric properties in the 8 mol% Y2 O3 doped ZrO2 , especially in their grain boundaries. - As shown in Fig. 12, under relatively severe conditions, the microstructures reveal heterogeneities, with grains significantly smaller at the sample periphery than in its center. This observation brings about a new argument in the current debate on the Joule effect and more specifically on the steady state power dissipation regime. According to this model, the supplied power is dissipated from the sample surface (essentially by radiation). This implies a calorie flux from the center to the surface of the sample and therefore a temperature gradient inside the sample. The observed
Fig. 12. Microstructure heterogeneity in sample CF900-20A-14s.
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3.3. Hyper-flash sintering
Fig. 13. Examples of trans-granular and inter-granular fractures.
heterogeneities (and the fusion of the center of the sample shown in Fig. 10) appear to confirm this analysis. The risk of obtaining such heterogeneities would be an inherent drawback of the flash-sintering technique. One way to overcome this difficulty (at least partly) could be to operate the flash-sintering under a quasi adiabatic regime, in other words, over a very short time (see the following hyper-flash chapter). - Under high current densities, we frequently observed a sample splitting into parts. This led us to suspect that minor cohesion ruptures could occur under slightly less “violent” experimental conditions. The comparison of fracture surfaces made in Fig. 13 shows that clear-cut trans-granular or inter-granular fractures can be obtained depending on the flash conditions. This indicates a weakening of the grain cohesion can occur under severe flash conditions.
Fig. 2 shows a sharp initial power spike and a corresponding shrinkage step over a short period of the order of 1 s. This was observed whatever the flash conditions. It is reasonable to assume that over such a short period, the energy lost by radiation from the sample surface remains small with respect to that supplied by the Joule effect. To evaluate an upper limit of the temperature jump which would be observed under this condition, let us use the value of the power spike reported above, i.e. 2000 W cm−3 and suppose that the flash duration is 1.5 s. Under these conditions the energy supplied to one cubic centimeter of the sample is 3000 J. The thermal capacity of Zirconia is about 0.65 J g−1 K−1 , as reported by Raj2 who also examined this assumption. With these figures (and a YSZ density of 5.97 g cm−3 ), one gets an adiabatic temperature step of 810 K leading to a sample temperature jump up to 1710 ◦ C. The fact that Zirconia fusion can be easily obtained confirms that very high temperatures can be reached during a flash. The thermocouple located close to the sample (Fig. 1), at times, gives temperatures of about 1400 ◦ C (the recorded thermocouple temperatures are not quoted in details, because of the irreproducibility in the thermocouple location). One can also mention that a fusion of the platinum grid was occasionally observed. All these experimental observations indicate that an unexpectedly high temperature can be reached during a hyper flash. Could this very fast temperature increase be the determining parameter of the observed shrinkage step? To gather more experimental data on this initial part of the flash, and to evaluate its potential technological interest, measurements were performed at a furnace temperature of 900 ◦ C, varying the initial polarization from 150 to 300 V cm−1 and the power spike duration from 1 to 2 s (the limited current density was also varied from 10 to 20 A cm−2 , but, in the case of an hyper flash, it was just a technical parameter for the power generator). The polarization was also applied abruptly, without any pre-polarization steps. Platinum paint was used. As it favors the reproducibility and homogeneity of the sample. Table 3 summarizes these experimental conditions and show the relative densities obtained. The main observation we made is that densifications of the order of 80–90%TD can easily be obtained in about 1 s. Fig. 14 shows the corresponding micrographs. Grains remain very small, of the order of 200 nm. None of the microstructure heterogeneities reported above were observed. 3.4. Double flash sintering From all the above observations, we deduced that the flashsintering experiments described in Sections 3.2 and 3.3 are likely to be composed of two mechanisms: a very fast one which would be the true flash-sintering process and a second one which would be a more conventional sintering process induced by the Joule effect described in the literature.1–6 Quite obviously, the initial power spike (which corresponds in our experiments to a transient constant voltage regime) results from an abrupt drop in the sample resistance associated with
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Table 3 Experiments performed under the hyper flash conditions. The initial furnace temperature was 900 ◦ C. Sample
Initial electric field (V cm−1 )
Current density limited (A cm−2 )
t (s) under limited current
Relative density (%TD) geometrical
HF900 HF900 HF900
150 300 150
10 10 20
1 1 2
78 80 88
Table 4 Experiments performed under the double flash conditions. Sample
Initial furnace temperature (◦ C)
Initial Electric field (V m−1 )
Current density limited (A cm−2 ) t (s) Under limited current
First flash Second flash DF900 DF975
900 975
130 150
90 120
10 10
Fig. 14. Microstructures of samples hyper-flash sintered during about 1 s at a furnace temperature of 900 ◦ C. (a) Initial field 150 V cm−1 ; (b) initial field 300 V cm−1 .
a rapid shrinkage (see Fig. 7 and Ref. 13) and a temperature increase in a sort of chain reaction.11 Let us examine these two assumptions. If the shrinkage parameter is determining, then the hyper-flash sintering would have a marked effect only in green bodies. If the temperature effect is prevalent, then the hyper-flash could also be triggered in partially sintered materials. In the latter case, the effects of subsequent flashes should be cumulative. To verify this assumption, we applied a second conventional flash on samples partially densified by hyper-flashes. Details are given in Table 4. The experiments described in this table were duplicated and reproducibility of about 1% was obtained.
Relative density (%TD)
First flash Second flash
Geometrical Arch.
1 1
96 97
130 130
97 99
Fig. 15. Variations of the double-flash-sintering parameters. Sample DF975 (furnace temperature: 975 ◦ C).
Fig. 15 shows an example of the resulting shrinkage strain, voltage and current variations which were recorded. The second flash has the same structure as the conventional flashes applied to green bodies and also includes the initial retraction step associated with the power spike. This is an additional argument which confirms, should it be necessary, that the thermal effect is the determining parameter of the hyper flash. As shown in Table 4, the effects of subsequent flashes are cumulative and double (or multiple) flashes enable us to reach full densification. Fig. 16 shows the micrograph of the DF975 sample, densified by double flash.
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References
Fig. 16. Microstructure of a doubly flash sintered sample (DF975).
4. Conclusions Besides being a promising fabrication technique, ac flashsintering appears to also be a versatile investigation tool for the sintering studies. By appropriately selecting the experimental parameters, it allows us to get a large variety of microstructures. A sharp electric hyper-flash over a short period of the order of 1 s, injecting about 2000 W cm−3 into the sample, can sinter YSZ up to 88%TD with only a minor grain growth. A transient temperature jump above 1600 ◦ C could explain this performance. Additional subsequent flashes can be applied and easily lead to full densification. Acknowledgements The authors thank R. Martin and Dr. F. Charlot (CMTC, INP Grenoble) for SEM observations.
1. Grasso S, Sakka Y, Rendtorff N, Hu C, Maizza G, Borodianska H, et al. Modeling of the temperature distribution of flash sintered zirconia. J Ceram Soc Jpn 2011;119(2):144–6. 2. Raj R. Joule heating during flash-sintering. J Eur Ceram Soc 2012;32:2293–301. 3. Holland TB, Anselmi-Tamburini U, Quach DV, Tran TB, Mukherjee AK. Effect of local Joule heating during field-asssisted sintering of ionic ceramics. J Eur Ceram Soc 2012;32:3667–74. 4. Baraki R, Schwartz S, Guillon O. Effect of electrical field/current on sintering of fully stabilized zirconia. J Am Ceram Soc 2012;95(1):75–8. 5. Francis JSC, Cologna M, Raj R. Particle size effect in flash sintering. J Eur Ceram Soc 2012;32:3129–36. 6. Holland TB, Tran TB, Quach DV, Anselmi-Tamburini U, Groza JR, Mukherjee AK. Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of externally applied field. J Eur Ceram Soc 2012;32:3675–83. 7. Yang D, Raj R, Conrad H. Enhanced sintering rate of zirconia (3Y-TZP) through the effect of a weak dc electric field on grain growth. J Am Ceram Soc 2010;93(10):2935–7. 8. Cologna M, Rashkova B, Raj R. Flash sintering of nanograin zirconia in <5 s at 850 ◦ C. J Am Ceram Soc 2010;93(11):3556–9. 9. Conrad H, Yang D. Influence of an applied dc electric field on the plastic deformation kinetics of oxide ceramics. Philos Mag 2010;90(9): 1141–57. 10. Yang D, Conrad H. Enhanced sintering rate and finer grain size in yttriastabilized zirconia (3Y-TZP) with combined DC electric field and increase heating rate. Mat Sci Eng A 2011;528:1221–5. 11. Muccillo R, Muccillo ENS, Kleitz M. Flash grain welding in yttria stabilized zirconia. J Eur Ceram Soc 2011;31:1517. 12. Cordier A, Kleitz M, Steil MC. Welding of yttrium-doped zirconia granules by electric current activated sintering (ECAS): protusion formation as a possible intermediate step in the consolidation mechanism. J Eur Ceram Soc 2012;32:1473–9. 13. Steil MC, Thevenot F, Kleitz M. Densification of yttria stabilized zirconia, impedance spectroscopy analysis. J Electrochem Soc 1997;144(1): 390–8.
Journal of the European Ceramic Society 33 (2013) 2093–2101
From conventional ac flash-sintering of YSZ to hyper-flash and double flash Marlu Cesar Steil ∗ , Daniel Marinha, Yann Aman, Jonas R.C. Gomes, Michel Kleitz Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces de Grenoble (LEPMI), UMR 5279 CNRS, Université de Grenoble, BP 75, 38402 Saint Martin d’Hères, France Received 5 February 2013; received in revised form 4 March 2013; accepted 5 March 2013 Available online 6 April 2013
Abstract Flash-sintering experiments were performed on conventional 8 mol% Y2 O3 -doped ZrO2 . By varying the initial power spike, the steady state limited current density, the flash duration and furnace temperature, various microstructures corresponding to different sintering stages were observed. It is shown that operating the flash-sintering under the power dissipation regime may lead to microstructure heterogeneity. Under a quasi-adiabatic regime a better homogeneity is obtained. The major part of the shrinkage strain occurs within a short time interval of about 1 s, after the incubatory period. The remaining part appears to be a conventional shrinkage process induced by Joule heating. Hyper-flashes were performed over this short time interval, yielding relative densities up to 90%. Additional flashes can be subsequently applied up to full density. It is stressed that densification is not an absolute quality parameter in the optimization of the experimental conditions; inappropriate conditions may lead to fairly dense but brittle materials. © 2013 Elsevier Ltd. All rights reserved. Keywords: Sintering; ZrO2 ; Microstructure; Field assisted sintering; Flash-sintering
1. Introduction The so-called flash-sintering process has already demonstrated its capability in terms of investment cost, energy and time savings. Currently, researches aimed at understanding the basic mechanisms are focused on Zirconia-based oxide ion conductors (doped either with 3 or 8 mol% Y2 O3 ). The main emphasis is on the magnitude and location of the temperature increase resulting from the internal Joule effect.1–6 The alteration of the grain boundary space charge layer by the applied electric field, can explain a smaller grain growth.7,8 In this work, we focused our attention on the microstructures that can be obtained by varying the flash-sintering parameters, i.e. the initial power spike, the steady state limited current density, the flash duration and the furnace temperature. Most of the published results were obtained by applying a selected electric field to the sample, at low temperature, and then ramping up its temperature.5,9,10 In contrast to this procedure, we first heated the sample to the selected temperature, for instance 900 ◦ C, and then applied various ac electric
signals. The general features of the response, as a function of time, were described by Muccillo et al.11 It includes an incubatory period before the flash-sintering runaway. To avoid any electrical breakdown we used equipment which enabled a current limitation, already implemented by Cordier et al.12 In a first set of results, we describe the microstructures resulting from moderate electrical polarizations, at various furnace temperatures. In this case, the electric signal was maintained till the shrinkage strain reached its steady-state value, typically after a time of the order of 10 min. This allowed us to select appropriate parameters for the flash-sintering experiments proper. Then, an initial temperature of 900 ◦ C was selected and more conventional flash conditions were applied, varying the current density and flash duration over the 10–50 s range. The third part of this work is focused on the initial abrupt shrinkage which occurs over a 1–2 s period. We will call it the hyper-flash. Double flashes were also applied. 2. Experimental 2.1. Sample processing and characterization
∗
Corresponding author. Tel.: +33 4 76 82 65 98. E-mail address: [email protected] (M.C. Steil).
0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.03.019
We used a commercial powder (TZ-8Y from Tosoh, Japan) formed of granules (diameter range: 15–140 m) composed of
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Fig. 1. Experimental set-up for flash sintering experiments.
30 nm particles. Its specific surface area is 16 m2 g−1 . It was used without any further treatment. The samples were uniaxially and isostatically cold pressed under 100 and 250 MPa, respectively, resulting in green relative densities of about 50% TD (Theoretical Density). The pellets were 8 mm in diameter (0.5 cm2 in area) and about 2 mm thick. The post-sintering densities were determined geometrically and by the Archimedes method. Obviously, when the samples exhibit an open porosity, these two densities markedly differ. All the current densities and electric fields mentioned in the following text were calculated using the pre-sintered sample dimensions. Micro-structures were examined both on fracture as well as on polished (1 m diamond paste) and thermally etched (1200 ◦ C, 30 min) surfaces. SEM images were obtained, either in the “in lens” detection mode, or in the conventional secondary electron mode (SE), in a Field Emission Gun SEM (Zeiss Ultra 55). For the impedance spectroscopy analysis, an Impedancemeter HP 4194A was used in the frequency range 100–1.5 × 107 Hz with a 100 mV signal. All the impedance diagrams were plotted at 350 ◦ C and were normalized by the pellet geometrical factor after sintering. (The numbers on the diagrams indicate the logarithm of the measuring ac frequency). The crystallographic structure of a few samples, randomly selected, were X-ray controlled using a Philips X’Pert with 2θ ranging from 15◦ to 135◦ , with 0.033◦ steps and a 45 s step time. They all exhibited the expected cubic phase. 2.2. Flash-sintering equipment and procedures The experimental set-up which was assembled in our laboratory is sketched in Fig. 1. It is equipped with a displacement sensor (GEFRAN, PY2 Rectilinear displacement transducer) capable of measuring a shrinkage of 10 m. To improve the uniformity of the current distribution through the samples, some pellets had their bases covered with Pt paint (Metalor® 6926). The current collectors made of Pt grids were slightly pressed against the pellet bases. When platinum paint was used, the grid was 0.75 mm mesh wide and made of a wire 0.25 mm in diameter. Without platinum paint, we used a thinner grid 0.50 mm mesh wide, made of a 0.12 mm wire. These grids were connected to the power generator by 1 mm diameter Pt wires capable of sustaining
Fig. 2. Variations of the CF900-14A-30s sample flash parameters: furnace temperature 900 ◦ C; limited current density 14 A cm−2 ; time under limited current (t) 30 s. The post-flash relative density of the sample was 97%TD.
high currents. The experiments were performed under air. The furnace temperature ramps were 10 ◦ C min−1 both on heating and cooling. All temperatures mentioned henceforth refer to the furnace temperature (it is the pre-heated sample temperature). For a qualitative evaluation, a thermocouple was placed close to the sample (∼1 mm). Before each experiment, the sample temperature was stabilized during 30 min. The power generator was a Pacific Smart Source 115ASX AC, controlled by the UPC Manager V.1.4 software. It enabled us to apply to the sample, an ac voltage of 1000 Hz, adjustable between 0 and 115 V, which corresponds, in our case, to a field of about 0–500 V cm−1 , while limiting the resulting alternating electric current to a selected value up to 15 A (corresponding to a current density of 30 A cm−2 ). Its response time is of the order of 1 s. It is to be stressed that the long and medium time flashes were performed under constant current densities and not under constant electric fields as in most of the published experimental conditions. In practice, the flash was triggered by applying an appropriate electric field. This resulted in an abrupt current density increase which was limited by the power supply to the selected value, within about 1 s. This limitation was evidently obtained at the expense of the applied field which accordingly decreased. Before the current limitation, over a transient period, the applied electric field induced a larger current upsurge that resulted in a power spike. An illustration of this behavior is shown in Fig. 2. All the electrical parameter variations were continuously recorded. The flash duration t is defined in Fig. 2. It is that of the current limited period.
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Fig. 4. Relative densities as a function of the furnace temperature, for various current densities in moderate flash conditions.
Fig. 3. Green sample MEB micrographs. (a) In lens SE detector mode; (b) conventional SE mode.
3. Results Fig. 3 shows two micrographs of a compacted sample, before sintering, recorded in the “in lens” SE mode and in the conventional mode. The grains are smaller than 50 nm and form agglomerates of about 150 nm. 3.1. Moderate flash conditions Table 1 presents all the experiments which were performed under the moderate flash conditions and their associated acronyms. Thin grids were used without any Pt paint. At this stage, the electric polarization was maintained till shrinkage stabilization. The initial electric field applied to the sample was gradually increased by subsequent 20 V cm−1 steps lasting 1 min, until the flash runaway. The initial field mentioned in Table 1 is that producing the flash runaway. After completion of the experiment, the current was gradually decreased to avoid any thermal shock. The large differences in some of the reported geometrical and Archimedes densities point to open porosities. Fig. 4 summarizes the results in terms of relative geometrical densities. For a low current density of 0.3 A cm−2 , no significant sintering has yet started. The porosity closes only at a current density of 6 A cm−2 . As expected, whatever the furnace temperature, an increase in the current density plays a favorable part in the densification (Table 1).
Fig. 5 shows microstructures obtained with different current densities, at a furnace temperature of 900 ◦ C. In the initial stage (up to 1.5 A cm−2 ), the sintering proceeds mostly by coalescence of closely neighboring grains in the initial agglomerates (cf. Fig. 3), and form grains of about 100 nm. Over 3 A cm−2 , a densification by intergrains sintering occurs with à small grain growth at 6 A cm−2 . This observation is in agreement with our previous results obtained on the flash welding of pre-sintered YSZ granules which show the neck formation starting at 1.3 A cm−2 .12 Fig. 6 shows the influence of the furnace temperature on the sample microstructure, under the most “efficient” current density of 6 A cm−2 . As can be seen, the higher the furnace temperature, the higher the densification and the grain growth. This could appear in contradiction with the commonly accepted Joule effect analysis. As the furnace temperature increases, the sample resistance decreases and therefore the Joule power supplied to the sample decreases (RI2 with a constant I). This observed variation could result from the grain faceting mentioned below and most probably from the initial power shot. Additional observations that were made are the following: - With current densities lower than 6 A cm−2 , a large part of the porosity remains open as shown by the marked differences between the geometrical and Archimedes densities. - The impedance diagrams (Fig. 7) which appear highly sensitive to the densification, show a fairly continuous variation, consistent with previously published results13 on samples partially densified by conventional sintering. As shown in Fig. 7, the overall resistance which is given by the rightmost intercept of the diagram curve with the real axis (electrode contribution was extracted), markedly varies with the sintering degree. This certainly contributes to the flash runaway. - Grain faceting can occur under the experimented conditions, as shown in samples MF800-3A and MF800-6A (Fig. 8). This is usually regarded as a detrimental evolution impeding a further densification. This would be in agreement with the leveling off of the densification observed at 800 and 900 ◦ C (Table 1). A way to overcome this difficulty is to accelerate the densification by increasing the furnace temperature as
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Table 1 Experiments performed under the moderate flash conditions. Sample
MF800-0.3A MF800-1.5A MF800-3A MF800-6A MF900-0.3A MF900-1.5A MF900-3A MF900-6A MF975-0.3A MF975-1.5A MF975-3A MF975-6A
Furnace temperature T (◦ C)
800 800 800 800 900 900 900 900 975 975 975 975
Initial electric field (V cm−1 )
160 180 190 190 100 120 120 120 40 80 80 80
Current density limited (A cm−2 )
0.3 1.5 3.0 6.0 0.3 1.5 3.0 6.0 0.3 1.5 3.0 6.0
t (s) under limited current
1761 510 770 760 2000 470 320 250 1300 1300 260 165
Relative density (%TD) Geometrical
Archimedes
48 49 63 85 48 50 64 88 48 51 68 94
48 82 85 89 49 83 90 92 48 90 87 97
confirmed by the densities obtained at a furnace temperature of 975 ◦ C (Table 1). - Nice images of spiral steps associated with the grain growth were observed in sample MF975-6A (Fig. 9). - The power spike supplied to the sample at the onset of the flash runaway, before the current limitation sets in (Cf. Fig. 2), is fairly constant and equal to about 2000 W cm−3 (the corresponding experimental data vary by about 50% around this average value, without showing any significant trend). This appears to be the determining parameter of the flash runaway, instead of the electric field.
3.2. Conventional flash sintering
The main practical conclusion of this fairly systematic preliminary investigation is that the sample density levels off at 94%TD, under the conditions selected in this first part. To obtain better densifications, current densities higher than 6 A cm−2 have to be used.
- The investigated experimental conditions (Table 2) cover a broad field of sintering stages. At one end, the CF900-10A-7s sample has a density of only 81%TD and its grain size is about 0.5 m. At the other end, with the CF900-20A-30s sample, a local fusion is reached before 30 s (Fig. 10).
In this second experimental part, the initial sample temperature (furnace temperature) was fixed at 900 ◦ C and the initial field electric was fixed at 180 V cm−1 . Three constant limited current densities of 10, 14 and 20 A cm−2 were applied and the flash time spanned the 7–45 s range. No Platinum paint was used. The polarization was applied abruptly, without any pre-polarization steps. Table 2 summarizes the investigated experimental conditions and gives the corresponding sample acronyms. The main observations we made are the following:
Fig. 5. Influence of the current density on the microstructure. Furnace temperature 900 ◦ C.
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Table 2 Experiments performed under the conventional flash conditions. The initial electrical field was fixed at 170 V cm−1 . Sample
CF900-10A-7s CF900-10A-30s CF900-10A-45s CF900-14A-7s CF900-14A-14s CF900-14A-30s CF900-10A-7s CF900-20A-14s CF900-20A-20s CF900-20A-30s
Current density limited (A cm−2 )
10
14
20
t (s) under limited current
7 30 45 7 14 30 7 14 20 30
Grain size (m)
Relative density (%TD) Geometrical
Archimedes
81 89 96 92 92 97 95 94 94 Fusion
82 87 98 94 93 97 98 98 94
0.5 0.3 0.5–1 0.5 1–2 1–2 3–5 Heterogeneity Heterogeneity
- Densities between 95 and 98%TD can be reached. - As shown in Table 2, under the relatively small current density of 10 A cm−2 , the flash time plays a significant part on the densification, without leading to a marked increase in the grain size which stays around 0.5 m. - On the other hand, over a constant short sintering time of 7 s, the limited current density is a determining parameter for the grain size which varies from 0.5 to 4 m (Table 2). - Fig. 11 compares the impedance diagrams of some dense samples (CF-900-10A-45s and CF-900-20A-14s) to that of a material of similar grain size, conventionally sintered at 1400 ◦ C.13 The diagrams are quasi identical. This important result undoubtedly shows that flash sintering does not
Fig. 6. Influence of the furnace temperature on the microstructure at constant current density (6 A cm−2 ). Fig. 7. Influence of the current density on the sample impedance diagram for a furnace temperature of 900 ◦ C. In this figure and in Fig. 11 numbers indicate the logarithm of signal frequency.
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Fig. 10. Example of local fusion in the sample CF900-30A-30s (flash conditions: furnace temperature 900 ◦ C; limited current density 20 A cm−2 ; time under current (t) 30 s).
Fig. 8. SEM images (SE in lens mode) of MF800-3A and MF800-3A ceramics showing the grain faceting. Fig. 11. Impedance diagrams, obtained at 350 ◦ C, of flash-sintered samples to quasi full density (CF900-10A-45s and CF900-20A-14s) and a conventionally sintered sample (1400 ◦ C).
Fig. 9. Crystal growth spiral steps.
introduce any peculiar electric properties in the 8 mol% Y2 O3 doped ZrO2 , especially in their grain boundaries. - As shown in Fig. 12, under relatively severe conditions, the microstructures reveal heterogeneities, with grains significantly smaller at the sample periphery than in its center. This observation brings about a new argument in the current debate on the Joule effect and more specifically on the steady state power dissipation regime. According to this model, the supplied power is dissipated from the sample surface (essentially by radiation). This implies a calorie flux from the center to the surface of the sample and therefore a temperature gradient inside the sample. The observed
Fig. 12. Microstructure heterogeneity in sample CF900-20A-14s.
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3.3. Hyper-flash sintering
Fig. 13. Examples of trans-granular and inter-granular fractures.
heterogeneities (and the fusion of the center of the sample shown in Fig. 10) appear to confirm this analysis. The risk of obtaining such heterogeneities would be an inherent drawback of the flash-sintering technique. One way to overcome this difficulty (at least partly) could be to operate the flash-sintering under a quasi adiabatic regime, in other words, over a very short time (see the following hyper-flash chapter). - Under high current densities, we frequently observed a sample splitting into parts. This led us to suspect that minor cohesion ruptures could occur under slightly less “violent” experimental conditions. The comparison of fracture surfaces made in Fig. 13 shows that clear-cut trans-granular or inter-granular fractures can be obtained depending on the flash conditions. This indicates a weakening of the grain cohesion can occur under severe flash conditions.
Fig. 2 shows a sharp initial power spike and a corresponding shrinkage step over a short period of the order of 1 s. This was observed whatever the flash conditions. It is reasonable to assume that over such a short period, the energy lost by radiation from the sample surface remains small with respect to that supplied by the Joule effect. To evaluate an upper limit of the temperature jump which would be observed under this condition, let us use the value of the power spike reported above, i.e. 2000 W cm−3 and suppose that the flash duration is 1.5 s. Under these conditions the energy supplied to one cubic centimeter of the sample is 3000 J. The thermal capacity of Zirconia is about 0.65 J g−1 K−1 , as reported by Raj2 who also examined this assumption. With these figures (and a YSZ density of 5.97 g cm−3 ), one gets an adiabatic temperature step of 810 K leading to a sample temperature jump up to 1710 ◦ C. The fact that Zirconia fusion can be easily obtained confirms that very high temperatures can be reached during a flash. The thermocouple located close to the sample (Fig. 1), at times, gives temperatures of about 1400 ◦ C (the recorded thermocouple temperatures are not quoted in details, because of the irreproducibility in the thermocouple location). One can also mention that a fusion of the platinum grid was occasionally observed. All these experimental observations indicate that an unexpectedly high temperature can be reached during a hyper flash. Could this very fast temperature increase be the determining parameter of the observed shrinkage step? To gather more experimental data on this initial part of the flash, and to evaluate its potential technological interest, measurements were performed at a furnace temperature of 900 ◦ C, varying the initial polarization from 150 to 300 V cm−1 and the power spike duration from 1 to 2 s (the limited current density was also varied from 10 to 20 A cm−2 , but, in the case of an hyper flash, it was just a technical parameter for the power generator). The polarization was also applied abruptly, without any pre-polarization steps. Platinum paint was used. As it favors the reproducibility and homogeneity of the sample. Table 3 summarizes these experimental conditions and show the relative densities obtained. The main observation we made is that densifications of the order of 80–90%TD can easily be obtained in about 1 s. Fig. 14 shows the corresponding micrographs. Grains remain very small, of the order of 200 nm. None of the microstructure heterogeneities reported above were observed. 3.4. Double flash sintering From all the above observations, we deduced that the flashsintering experiments described in Sections 3.2 and 3.3 are likely to be composed of two mechanisms: a very fast one which would be the true flash-sintering process and a second one which would be a more conventional sintering process induced by the Joule effect described in the literature.1–6 Quite obviously, the initial power spike (which corresponds in our experiments to a transient constant voltage regime) results from an abrupt drop in the sample resistance associated with
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Table 3 Experiments performed under the hyper flash conditions. The initial furnace temperature was 900 ◦ C. Sample
Initial electric field (V cm−1 )
Current density limited (A cm−2 )
t (s) under limited current
Relative density (%TD) geometrical
HF900 HF900 HF900
150 300 150
10 10 20
1 1 2
78 80 88
Table 4 Experiments performed under the double flash conditions. Sample
Initial furnace temperature (◦ C)
Initial Electric field (V m−1 )
Current density limited (A cm−2 ) t (s) Under limited current
First flash Second flash DF900 DF975
900 975
130 150
90 120
10 10
Fig. 14. Microstructures of samples hyper-flash sintered during about 1 s at a furnace temperature of 900 ◦ C. (a) Initial field 150 V cm−1 ; (b) initial field 300 V cm−1 .
a rapid shrinkage (see Fig. 7 and Ref. 13) and a temperature increase in a sort of chain reaction.11 Let us examine these two assumptions. If the shrinkage parameter is determining, then the hyper-flash sintering would have a marked effect only in green bodies. If the temperature effect is prevalent, then the hyper-flash could also be triggered in partially sintered materials. In the latter case, the effects of subsequent flashes should be cumulative. To verify this assumption, we applied a second conventional flash on samples partially densified by hyper-flashes. Details are given in Table 4. The experiments described in this table were duplicated and reproducibility of about 1% was obtained.
Relative density (%TD)
First flash Second flash
Geometrical Arch.
1 1
96 97
130 130
97 99
Fig. 15. Variations of the double-flash-sintering parameters. Sample DF975 (furnace temperature: 975 ◦ C).
Fig. 15 shows an example of the resulting shrinkage strain, voltage and current variations which were recorded. The second flash has the same structure as the conventional flashes applied to green bodies and also includes the initial retraction step associated with the power spike. This is an additional argument which confirms, should it be necessary, that the thermal effect is the determining parameter of the hyper flash. As shown in Table 4, the effects of subsequent flashes are cumulative and double (or multiple) flashes enable us to reach full densification. Fig. 16 shows the micrograph of the DF975 sample, densified by double flash.
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References
Fig. 16. Microstructure of a doubly flash sintered sample (DF975).
4. Conclusions Besides being a promising fabrication technique, ac flashsintering appears to also be a versatile investigation tool for the sintering studies. By appropriately selecting the experimental parameters, it allows us to get a large variety of microstructures. A sharp electric hyper-flash over a short period of the order of 1 s, injecting about 2000 W cm−3 into the sample, can sinter YSZ up to 88%TD with only a minor grain growth. A transient temperature jump above 1600 ◦ C could explain this performance. Additional subsequent flashes can be applied and easily lead to full densification. Acknowledgements The authors thank R. Martin and Dr. F. Charlot (CMTC, INP Grenoble) for SEM observations.
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