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Cost-effective solid-state reactive sintering method for high conductivity proton conducting yttrium-doped barium zirconium ceramics
Solid State Ionics 181 (2010) 496–503
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Cost-effective solid-state reactive sintering method for high conductivity proton conducting yttrium-doped barium zirconium ceramics Jianhua Tong ⁎, Daniel Clark, Michael Hoban, Ryan O'Hayre ⁎ Metallurgical & Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA
a r t i c l e
i n f o
Article history: Received 29 July 2009 Received in revised form 26 January 2010 Accepted 8 February 2010 Keywords: Proton conductivity Ionic conductivity Perovskite Ceramic electrolyte Fuel cell
a b s t r a c t Using cost-effective precursors of BaCO3, ZrO2, and Y2O3, proton conducting ceramic pellets of BaZr0.8Y0.2O3 − δ (BZY20) were successfully fabricated with the help of a range of sintering aids including LiF, NiO, Al2O3, and SnO2. This simple and cost-effective solid-state reactive sintering (SSRS) method involved only a single high-temperature sintering step. The effect of various experimental conditions on the crystal structure, relative density, morphology, and total conductivity of the as-prepared BZY20 ceramic pellets were investigated in detail. Tunable experimental parameters included the type of sintering aid, the amount of aid, and the sintering temperature. NiO was determined to be the most effective sintering aid investigated. Under optimized conditions using 1–2 wt.% NiO as a sintering aid, dense BZY20 ceramic pellets (N 95% relative density) with grain sizes as large as 5 μm were successfully prepared at a relatively low sintering temperature of 1400 °C. In comparison, most alternative sintering techniques for BZY require temperatures in excess of 1700 °C. Total conductivities as high as 2.2 × 10− 2 and 3.3 × 10− 2 S cm− 1 were obtained from the resulting pellets at 600 °C under dry- and wet-argon atmospheres respectively. These are among the highest values reported for BZY20, highlighting the potential of the NiO-modified reactive sintering approach to provide a simple, cost-effective, reduced-temperature route to achieve dense, large-grained parts for protonic-ceramic applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Proton conducting ceramics are intriguing candidate electrolyte materials for solid oxide fuel cells (SOFCs) owing to their unique ability to provide high efficiency, flexible fuel operation, and high fuel utilization [1–3]. In recent studies, proton conducting ceramics have also been suggested for use as steam permeation membranes based on an ambipolar co-ionic diffusion mechanism [4,5]. In such a configuration, proton conducting ceramics could find applications in many chemical processes including hydrocarbon steam reforming, operation of SOFCs, coal gasification, and heat recovery from steam [4–7]. Among candidate proton conducting electrolyte materials, yttriumdoped barium zirconate (BZY) has garnered particular attention because of its chemical stability, mechanical robustness, and high bulk proton conductivity [8–12]. However, the refractory nature of BZY results in significant challenges to its implementation in commercial applications. First, it is difficult to process BZY to the high density (N93%) that is usually needed for most applications. Typically, extreme conditions, such as high temperatures (1700–2100 °C), long sintering times (N24 h), and nanometer-sized powders are needed to prepare
⁎ Corresponding authors. E-mail addresses: [email protected] (J. Tong), [email protected] (R. O'Hayre). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.02.008
fully densified BZY pellets. This leads to high costs, process incompatibilities, and often causes barium volatilization which is detrimental to the proton conductivity of the resultant material [13–16]. Second, it is difficult to process BZY with sufficiently large grain size. Achieving a large grain size is critical in BZY and in many other proton conducting ceramics because the grain boundaries are significantly more resistive than the grain bulk. Enhanced densification and grain growth has been previously demonstrated in BZY pellets with careful optimization of sintering conditions, modification of powder properties, change in dopants, and addition of sintering aid materials, etc. [15–23]. Increasing the sintering temperature or sintering time, while improving grain growth and densification, normally results in the barium deficiency and the emergence of deleterious second phases such as Y2O3, which inevitably cause low total conductivity [17–19]. The use of nanocrystalline BZY powders, which can be prepared via polymeric sol–gel or combustion methods, has provided relatively good performance [15,16]. However, the synthesis of nanocrystalline BZY powder is economically unfavorable for industrial application. Alternative dopants such as Sc2O3 can also improve grain growth and densification, but have been shown to cause increased bulk and/or grain boundary resistivity [20]. Sintering aids such as ZnO, NiO, MgO, and Al2O3 have been added to BZY powders prepared by wet chemistry methods and the grain size and density of the BZY pellets have been increased to some degree [21–23]. Although the sintering mechanism is not clear and sometimes the sintering aids
J. Tong et al. / Solid State Ionics 181 (2010) 496–503
have a negative effect on the total conductivity of BZY, the addition of sintering aids can often decrease the sintering temperature to as low as 1350 °C, which makes it possible to prepare SOFCs stacks by inexpensive co-fired techniques. Therefore, increasing the grain size and density of BZY pellets by the means of a sintering aid is a very promising method. Correct choice of sintering aid is crucial. In the present paper, we therefore examine the effect of a range of different potential sintering aids on the phase-formation and densification of BZY. However, in contrast to previous sintering aid studies which mixed a sintering aid with already phase-pure BZY powder to encourage grain growth and densification, we mix our sintering aids with inexpensive BaCO3, ZrO2, and Y2O3 precursor powders to drive a simple and inexpensive singlestep solid-state reactive sintering process. This aid-assisted reactive sintering approach involves only a single high-temperature (≤1500 °C) sintering step, where both reactions of the precursors to single-phase BZY and the sintering/densification of the BZY to a fully dense pellet occur simultaneously. We examine four different potential sintering aids: LiF, NiO, Al2O3 and SnO2. These specific aids are chosen to enable a comparison between cation valence (Li+ vs. Ni2+ vs. Al3+ vs. Sn4+) and aid effectiveness. Furthermore, all four aids have specific properties which can potentially contribute to the sintering ability or proton conductivity of the resulting BZY20 ceramic. For example, LiF is a typical sintering aid used in the sintering of spinel and perovskite oxides. LiF exhibits many interesting properties such as specific surface activity and the ability to form oxygen vacancies which promotes densification. Also, because of its high volatility, it is a non-residual sintering aid when used in high-temperature (N1300 °C) sintering processes. NiO is a transition metal oxide sintering aid, and it has previously been proven to enhance the sintering of BZY powder compacts synthesized by the combustion method [21]. Al2O3 has been used as sintering aid for barium metazirconate BaZrO3 ceramic. Pure BaZrO3 pellets with bulk density ∼92% of theoretical were successfully obtained using 5 wt.% Al2O3 additive after sintering at 1600 °C for 6 h [23]. SnO2 has a relatively low melting point, and the same valence as zirconium (4+), but a larger cation radius. The introduction of the Sn4+ cation into the B site of perovskite-type BZY may therefore increase the conductivity while improving the sintering ability.
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Fig. 1. Summary of XRD patterns for BZY20 pellets (grinding into powder) obtained under different experimental conditions.
produced by the SSRS method. Conventional EDTA and citric acid combined complexing process was used for the synthesis of the BZY20 sol–gel derived powder. Briefly, nitrate salts of barium, yttrium, and zirconium were dissolved into DI water. The complexing organics of EDTA and citric acid were dissolved into ammonium hydroxide aqueous solution. The metal source solution was then gradually added to the complexing solution. The resulting solution was continuously stirred on a hot-plate at 80 °C, gradually evaporating the water from the mixture and converting the clear solution into a dark sticky gel. The sticky gel
2. Experimental Barium carbonate (Alfa Aesar, item number 14341, 99.8%, BaCO3), zirconium(IV) oxide (Alfa Aesar, item number 41528, 99.7%, ZrO2), and yttrium(III) oxide (Alfa Aesar, item number 44286, 99.9%, Y2O3) were used as raw materials for the synthesis of the yttrium-doped zirconate BaZr0.8Y0.2O3 − δ (BZY20) ceramic pellets examined in this study. Lithium fluoride (Alfa Aesar, item number 36359, 99.85%, LiF), nickel(II) oxide (Alfa Aesar, item number 45094, green, Ni 78.5%, NiO), aluminum oxide (Alfa Aesar, item number tem 12553, 99.5%, Al2O3), and tin(IV) oxide (Alfa Aesar, item number 12283, 99.9% SnO2) were used as sintering aids, respectively. Stoichiometric amounts of BaCO3, ZrO2, and Y2O3 were weighed and mixed with 2-propanol (SigmaAldrich, product number 190764, 99.5%) as a ball milling medium to produce a BZY20 precursor mixture. The sintering aid was then added to the BZY20 precursor in varying weight percentages of 0.5–3%. Specifically, pellet samples of BZY20–LiF1, BZY20–LiF2, BZY20–LF3, BZY20–NiO0.5, BZY20–NiO1, BZY20–NiO2, BZY20–Al2O30.5, BZY20– Al2O31, BZY20–Al2O32, BZY20–SnO21, and BZY20–SnO22 were prepared by this SSRS method followed by ball milling for 24 h, drying at 80 °C for 48 h, hydraulic dry pressing under 375 MPa pressure for 120 s (in a circular carbon-aided steel dry pressing die with diameter of 19 mm), and sintering at temperatures from 1200 to 1500 °C for 24 h. Control pellet samples of unadulterated BZY20 (without any sintering aids) were also prepared using the same process. As an additional point of reference, high quality BZY20 powder was prepared by a polymeric sol–gel method [24] and BZY20 pellets were prepared from this sol–gel BZY20 powder for comparison with the pellets
Fig. 2. The higher magnification for the XRD patterns in Fig. 1 in the 2theta range of 28–33°.
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Table 1 Impurity ratio in the cubic perovskite BZY20 for samples shown in Fig. 2.
Sol–gel-1450 Control-1450 LiF1-1450 NiO1-1450 Al2O31-1450 SnO21-1450 NiO0.5-1500 NiO1-1500 NiO2-1500 NiO2-1450 NiO2-1400 NiO2-1350 NiO2-1300 NiO2-1250
Second cubic perovskite
Y2O3
BaY2NiO5
Ba4Al2O7
0 0.579(7) 1.070(11) 0 0 0.625(6) 0 0 0 0 0 0 0 0
0 ∼0 0.0623(8) 0 0.0392(5) 0 0 0 0 0 0 0 0 0
0 0 0 0.0159(3) 0 0 ∼0 0.0172(3) 0.0408(6) 0.0389(5) 0.0348(5) 0.0501(6) 0.0446(6) 0.0493(6)
0 0 0 0 0.1322(15) 0 0 0 0 0 0 0 0 0
The impurity ratios were defined as follows: I110 (second cubic perovskite)/I110(cubic perovskite BZY20), I222(Y2O3)/I110(cubic perovskite BZY20), I112(BaY2NiO5)/I110(cubic perovskite BZY20), and I226(Ba4Al2O7)/I110(cubic perovskite BZY20).
was dried at 150–190 °C for 24–48 h and then calcined at 1250 °C to achieve pure BZY20 powders. The sol–gel BZY20 powder was also pressed at 375 MPa pressure for 120 s (with the same circular carbonaided steel dry pressing die) and sintered at temperatures of 1400 °C, 1450 °C, 1500 °C and 1600 °C for 24 h in order to obtain sol–gel BZY20 pellets for comparison with the SSRS BZY20 pellets. X-ray diffraction (XRD) analyses of the sintered pellet samples (after grinding into powders) were performed at room temperature using a Philips diffractometer (X'Pert Pro) with CuKα radiation, tube voltage 45 kV, and tube current 40 mA. Intensities were collected in the 2θ range between 20° and 120° with a step size of 0.008° and a measuring time of 5 s at each step. Relative densities of the sintered pellets were determined by the Archimedes method using water as the liquid medium and the direct measurements of the regular geometric volume and the corresponding mass. The microstructure and chemical composition of the sintered pellets were investigated by means of scanning electron microscopy (SEM, FEI Quanta 600) in conjunction with energy dispersive X-ray (EDX, Princeton GammaTech Prism). Cylindrical test bars (diameter: 4 mm and length: 25 mm) for fourprobe conductivity measurements of the BZY20–NiO2 were prepared by hydraulic dry pressing with a cylindrical stainless steel die (diameter: 6.4 mm) under 375 MPa for 60 s and sintering at 1500 °C for 24 h. Four silver wires (Alfa Aesar, item number 12187, 0.25 mm in diameter) were bound to the cylindrical sample as electrodes with the help of silver paste (Alfa Aesar, item number 44075). The distance between the two inner “voltage-sense” electrodes was 10 mm, with additional 5 mm spacing on either side for the two outer current-imposing electrodes. Four-probe DC technique was used to measure conductivities of the asprepared bars using Gamry Reference 600 Potentiostat/Galvanostat/ZRA with constant DC current from 3 mA to 180 mA. The conductivity data
were collected at temperatures from 350 to 750 °C under the atmospheres of dry and water saturated argon with a flow rate of 100 sccm. 3. Results and discussion 3.1. X-ray diffraction studies on crystal structure Fig. 1 compares the XRD patterns for the SSRS-derived BZY20 pellets (after grinding into powders) using different sintering aid types, amounts, and sintering temperatures against a SSRS-derived BZY20 control pellet (without any sintering aid) as well as a phase-pure BZY standard derived using a sol–gel method. The main phase of the cubic perovskite structure based on barium zirconate (PM3-M, ICSD 97459) has been successfully formed for all the BZY20 pellets under our experimental conditions (sintering aid type: LiF, NiO, Al2O3, or SnO2; sintering aid amount: 0–2 wt.%; and sintering temperature: 1250 to 1500 °C). However, phase separation (two cubic perovskite phases with same structure but different lattice constants) occurs for the BZY20 pellets obtained by sintering at 1450 °C for 24 h with 1 wt.% LiF or SnO2 as sintering aids and the SSRS BZY20 control pellet without any sintering aid. The existence of the second cubic perovskite phase indicates that sintering at 1450 °C for 24 h is not sufficient to attain the pure cubic perovskite single-phase structure using SSRS without a sintering aid, nor do the LiF or SnO2 sintering aids provide help in this regard. The second cubic perovskite phase is only slightly observable in BZY20 pellet with Al2O3 as sintering aid. Thus, the addition of Al2O3 may have a slight positive contribution to the formation of phase-pure cubic perovskite because it results in significantly lower second-phase peak intensities compared to the control BZY20 pellet obtained under the same sintering conditions. On the other hand, the BZY20 pellets obtained by sintering at 1250–1500 °C for 24 h with NiO as a sintering aid and the BZY20 pellet
Table 2 Experimental condition effect on lattice constant for the resulting BZY20 pellets. Aid type effect on lattice constant while aid amount is 1 wt.% and sintering temperature is 1450 °C Aid type Lattice constant, Å
Sol–gel 4.218(2)
Control 4.217(2) 4.194(2)
LiF1 4.221(2) 4.183(2)
NiO1 4.218(2)
Al2O31 4.186(2)
SnO21 4.222(2) 4.198(2)
Aid amount effect on lattice constant while aid is NiO and sintering temperature is 1500 °C Aid amount, wt.% Lattice constant, Å
NiO0.5 4.220(2)
Ni1.0 4.212(2)
NiO2.0 4.203(2)
Sintering temperature effect on lattice constant while aid is 2 wt.% NiO Sintering temperature, °C Lattice constant, Å
1250 4.209(2)
1300 4.201(2)
1350 4.205(2)
1400 4.203(2)
1450 4.211(2)
1500 4.203(2)
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Fig. 3. The summary of relative densities for BZY20 pellets obtained under different experimental conditions.
derived from sol–gel BZY20 powder by sintering at 1450 °C for 24 h have no second cubic perovskite phase, demonstrating that these two approaches can both yield phase-pure cubic BZY20. Fig. 2 provides a detailed view in the angle range 2θ = 28–33° for the same group of XRD patterns in Fig. 1. In order to quantify the magnitude of the second-phase impurities present in the samples, relative
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impurity-peak intensities were calculated using the ratios between the highest peak intensities for the impure phases and the 110 peak intensity for cubic perovskite BZY20. These are summarized in Table 1. From this detailed analysis, it is observed that the BZY20 SSRS control pellet as well as the pellets fabricated using LiF or Al2O3 show evidence of Y2O3 impurity (IA3-, ICSD 86814; the Y2O3 222 peak is at 2θ = 29.2°). It is well-known that Y2O3 impurities have a significant negative effect on the proton conductivity of BZY20 [21,22]. Moreover, Ba4Al2O7 (PDF 041-0165) second-phase impurity was found for the Al2O3 modified BZY20 pellet, which is also reported to have a negative effect on proton conductivity [12]. Fortunately, the BZY20 pellets prepared with NiO or SnO2 show no signs of Y2O3 impurity. However, the NiO-modified BZY20 pellets do show indications of BaY2NiO5 impurities (PDF 041-0463), which increase with increasing wt.% NiO additive. Because the effect of BaY2NiO5 on the sintering ability and proton conductivity of BZY20 has not yet been reported, we are currently investigating this issue in our ongoing work. In order to provide further insight into the influence of the sintering aids on the BZY structure, trends in the lattice constant of the primary cubic perovskite barium zirconate phase for all of the pellet samples were analyzed from the XRD data. For the pellets which incurred phase separation into two cubic perovskite phases, the lattice constants for the second cubic perovskite phase were also calculated. This data is presented in Table 2. Calculated lattice constant uncertainties, also provided in Table 2, were estimated from the measurement step size uncertainty of 0.008°. Systematic uncertainty from flatness and instrumental systematic error are not considered. As shown in Table 2, the control BZY20 pellet without any sintering aids and the sol–gel
Fig. 4. Sintering aid type effect on BZY20 pellet morphology (additive amount 1 wt.%, sintering temperature 1500 °C, sintering time 24 h).
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Fig. 5. Sintering temperature effect on the BZY20 pellet morphology while using 1.0 wt.% NiO as sintering aid (BZY20–NiO1, sintering time 24 h).
BZY20 pellet after sintering at 1450 °C for 24 h have lattice constants of 4.218(2) and 4.217(2) Å respectively, which are very close to the reported value of 4.228 Å for BZY20 pellets sintered at temperatures between 1400 and 1500 °C [25,26]. The addition of LiF or SnO2 resulted in two cubic perovskite phase with different lattice constants. In both materials, the larger lattice constant phase is again close to that of BZY20 (∼4.22 Å), while the lattice constant for the second phase (4.19–4.20 Å) is very close to that of pure barium zirconate [25,26]. The addition of Al2O3 into the BZY20 pellet did not lead to phase-separation, but did result in a very small lattice constant of 4.1861 Å, which may be a result of Al3+ substitution on the Zr4+ or Y3+ sites in the cubic perovskite structure (Al3+ has a smaller ionic radius compared with both Zr4+ and Y3+). The lattice constant also decreased as a function of NiO addition, again perhaps indicating substitution on the Zr4+ or Y3+ sites with a smaller cation (Ni2+ or Ni3+). The temperature effect on the lattice constants of the BZY20–NiO2 pellets was negligibly small, which can be ascribed to the saturated solubility of nickel in the BZY crystal structure. 3.2. Sintering behavior 3.2.1. Relative density The relative densities of the sintered BZY20 pellets are summarized in Fig. 3 vs. sintering aid type, sintering aid amount, and sintering temperature. Relative densities for the BZY20 pellets sintered from sol– gel powders are also included for comparison. From this figure, it is clear that the NiO sintering aid had the greatest positive contribution to the sintering ability, and this effect increases with increasing amounts of NiO addition. Relative densities greater than 95% were obtained for the
BZY20 pellets by sintering at temperatures higher than 1400 °C for 24 h with 2 wt.% NiO. The highest relative density of 97% was achieved for a 2 wt.% NiO-modified pellet sintered at 1500 °C. In comparison, the unmodified SSRS BZY20 control pellet achieved only 35% relative density when sintered at 1500 °C, and even pellets sintered from sol–gel derived BZY20 powder (the standard approach to achieve high-density BZY at lower sintering temperatures) achieved only about 70% relative density at 1500 °C. In contrast to NiO, the addition of Al2O3 or SnO2 had almost no effect (or perhaps even a slight negative effect) on densification. In fact, for the Al2O3 modified pellets, relative density measurements could only be obtained for the 0.5 wt.% Al2O3 sample, as the 1 wt.% and 2 wt.% Al2O3; sintered pellets did not even obtain sufficient mechanical strength for relative density measurement. The LiF aid did appear to have a positive effect on the sintering ability of BZY20 pellets, although the effect was small and did not lead to higher densities than what could be achieved from the phase-pure sol–gel route. Specifically, relative densities of around 40% and 50% were obtained for the 1 wt.% and 2 wt.% LiF modified samples, respectively, after sintering at 1500 °C for 24 h. 3.2.2. SEM cross-section images SEM microstructure analysis provides perhaps the most compelling evidence for the remarkable effect of NiO aid on BZY20 sinterability. Fig. 4 provides SEM fracture cross-section images from the BZY20 pellets with 1 wt.% LiF, NiO, and SnO2, respectively, after sintering at 1500 °C for 24 h. For comparison, a cross-section for a control SSRS BZY20 pellet (without any additive) prepared under the same experimental
J. Tong et al. / Solid State Ionics 181 (2010) 496–503
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Fig. 6. NiO aid concentration effect on BZY20 pellet morphology (sintering temperature 1450 °C, sintering time 24 h).
conditions is also included. From this SEM comparison, pellet density and grain size can be observed to increase in the order of control BZY20 ≤ BZY20–SnO21 b BZY20–LiF1 b BZY20–NiO1. For the control BZY20 and the BZY20–SnO21 pellets, the grain size is too small to be observed clearly under our experimental conditions. Although the grain size for the BZY20–LiF1 pellet is also not clear from the SEM crosssection, this pellet does show microstructural evidence of improved
intergranular connectivity. Anecdotally, the BZY20–LiF1 pellet also showed relatively higher mechanical strength and crush resistance than the unmodified BZY20 and BZY20–SnO21 pellets, providing further evidence for enhanced sintering. The most dramatic changes, however, were observed for the 1 wt.% NiO-modified BZY20 pellet, which exhibited a non-porous cross-section with equiaxed grains as large as 5 μm, corroborating well with the 95% relative density measurement
Fig. 7. Morphology comparison for BZY20 pellets from sol–gel powders (sintering temperature 1600 °C, sintering time 24 h) and from solid-state reactive sintering directly (sintering aid 1 wt.% NiO, sintering temperature 1500 °C, sintering time 24 h), respectively.
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and grain size continued to improve for the 1 wt.% NiO-modified sample, although increasing the NiO concentration from 1 wt.% to 2 wt.% did not appear to provide any substantial further enhancement. As a final point of comparison, Fig. 7 compares typical SEM crosssection images obtained from a NiO-modified reaction sintered BZY20 pellet (1.0 wt.% NiO, sintering temperature 1500 °C, sintering time 24 h) vs. an unmodified sol–gel powder derived BZY20 pellet (sintering temperature 1600 °C, sintering time 24 h). Despite being sintered at lower temperature, the NiO-modified BZY20 pellet shows a fully dense cross-section and large grain size (d ∼ 5 μm), while the unmodified sol– gel BZY20 pellet manifests appreciable porosity and 10× smaller grain size (d ∼ 0.5 μm). This comparison highlights the potential of the NiOmodified reaction sintering approach to provide a cost-effective, reduced-temperature route to achieve dense, large-grained BZY20 ceramic pellets. 3.3. Electrical conductivity
Fig. 8. Arrhenius plots of total conductivity for BZY20 obtained by SSRS method with 2 wt.% NiO as sintering aid by sintering at 1500 °C for 24 h, and summary comparison with total or bulk conductivities recently reported for BZY20.
previously estimated for this pellet. Importantly, the large grain size and high density of this pellet should bode well for its protonic conductivity and use in functional ionic-ceramic applications. Given the significant potential of the NiO-modified BZY20, the effect of sintering temperature on cross-section morphology was studied for BZY20–NiO1 at sintering temperatures between 1250 and 1500 °C as shown in Fig. 5. Based on this analysis, it is clear that the sintering temperature has a marked effect on the pellet density and grain size. For the pellet sintered at 1250 °C, significant porosity is clearly observed and the grain size is less than 1 μm. Increasing the sintering temperature to 1350 °C results in a significant reduction in pore volume and an increase in the grain size to about 1.5–2 μm. Further increasing the sintering temperature to 1450 and 1500 °C results in nearly complete elimination of obvious porosity and causes the grain size to increase to about 3 μm and 5 μm, respectively. Because the NiO additive may have unintended effects on BZY20 proton conductivity, it is clearly desirable to reduce the concentration as much as possible while still maintaining the beneficial sintering enhancement effects. To this end, the effect of NiO concentration on BZY20 pellet morphology was also studied by sintering samples with different NiO concentration (0.0 wt.%, 0.5 wt.%, 1.0 wt.%, and 2.0 wt.%) at 1450 °C for 24 h. The resulting SEM cross-section images for these samples are shown in Fig. 6. Importantly, even the addition of only 0.5 wt.% NiO had a tremendous influence on the sintered BZY20 morphology, significantly improving both density and grain size. Density
Because of its promising grain size and density, the conductivity of the 2 wt.% NiO-modified BZY20 material was subsequently evaluated using a DC four-point probe technique in both dry and wet argon and compared to the conductivities of the BZY20 ceramics reported in the recent literature. As shown in Fig. 8, the NiO-modified BZY20 demonstrates the highest total conductivities ever reported for this material. Table 3 provides further details comparing the conductivities obtained in this study (at 600 °C) to previous reports. The 2 wt.% NiOmodified BZY20 obtained in this report using the SSRS method achieves a maximum conductivity of 3.3× 10− 2 S cm− 1 at 600 °C in wet argon. As is typically observed in BZY, the conductivity response exhibits two distinct slopes when viewed in an Arrhenius plot. For conduction in the dry-argon atmosphere, the activation energy in the lower temperature region (350–600 °C) is about 0.82 eV, while the activation energy in the high-temperature region (N600 °C) is about 0.14 eV. The oxygen partial pressure in the dry-argon atmosphere is ∼10− 5 atm, while the water partial pressure and hydrogen partial pressure are negligibly small. The main defect formation reaction under these conditions can be described by Eq. (1): 1= O 2 2
••
X
•
þ VO ↔OO þ 2h :
ð1Þ
Therefore, oxygen vacancies and electron holes will be the dominant charge-carriers under these conditions. At low temperatures (b600 °C), in dry-argon atmosphere, the total conductivity of BZY20 is often dominated by oxygen vacancy transport [11,17,28]. Our measured activation energy (∼0.82 eV) in this region is typical for oxygen vacancy conduction in BZY (0.7–1.1 eV) [17]. At higher temperatures (N600 °C), electron hole conductivity begins to dominate with a commensurate decrease in the activation energy. In a wet-argon atmosphere, hydration of the BZY lattice by water molecules leads to substantial proton conductivity, as illustrated by Eq. (2). ••
X
•
H2 O þ VO þ OO ↔2OHO :
ð2Þ
At low temperatures (b600 °C), the reaction equilibrium expressed by Eq. (2) favors proton (OH•O) formation and the material exhibits
Table 3 Detailed experimental conditions of electrolytes reported in Fig. 8. BaZr0.8Y0.2O3 − δ name
Preparation condition
Measurement condition
Conductivity at 600 °C, S/cm
Ref.
A B C D E F
1500 °C 1500 °C 1700 °C 1600 °C 1600 °C 1325 °C
Wet Ar (pH2O = 0.031 atm) Dry Ar Wet N2 (pH2O = 0.023 atm) Wet N2 or Ar (pH2O = 0.031 atm) Wet N2 (pH2O = 0.031 atm) Wet 5% H2 in Ar
3.3 × 10− 2 (total) 2.2 × 10− 2 (total) 2.5 × 10− 2 (bulk) N1.5 × 10− 2 (total) 7.9 × 10− 3 (total) 1.2 × 10− 4(total)
This work This work [9] [27] [11] [22]
for 24 h in air for 24 in air for 24 h in O2, special bury, sol–gel power (850 °C) for 24 h in O2, special bury, combustion powder (1250 °C) for 10 h with 1 wt.% ZnO aid
J. Tong et al. / Solid State Ionics 181 (2010) 496–503
primarily proton-dominated conductivity. Due to the higher mobility of proton defects compared to oxygen vacancies, the total conductivity of the BZY material increases substantially in the wet-argon atmosphere compared to the dry-argon condition. The Arrhenius activation energy of 0.66 eV is obtained in this low-temperature range under wet-argon atmosphere. While this is significantly lower than the activation energy obtained for the dry-argon atmosphere condition, it is still somewhat higher than is typically observed for proton transport in BZY ceramics (typically 0.45–0.55 eV). The discrepancy may indicate that only partial hydration is obtained in this material and that the conductivity is due to a combination of both protons and oxygen vacancies. At temperatures above 600 °C, the conductivity under wet argon actually decreases. A “leveling off”, “hump” or decrease in the total conductivity of BZY and other proton conductors is often observed at intermediate temperatures [17], and is frequently attributed to the dehydration of the material with increasing temperature. Dehydration (representing a shift to the right in the defect equilibrium expressed by Eq. (2)) results in a decrease in the proton conductivity, and often a decrease in the overall total conductivity as the higher-mobility protons are replaced by lowermobility oxygen vacancies [3,11]. Future work must consider the fate of the NiO additive under strongly reducing atmospheres, where the reduction of nickel from the cubic perovskite structure and the BaY2NiO5 phase impurity is a possibility. It is likely that nanoparticles of nickel metal may be formed in the grain boundaries under reducing conditions, in analogy to recent studies of NiO-modified yttrium-doped barium cerate pellets [29]. In a future paper, the role of nickel under these conditions, including the nickel reduction behavior, nickel location, nickel oxidation state, and the effect of these properties on the conductivity, structure, and stability will be reported in detail. 4. Conclusions This study demonstrates that dense, large grain-size BZY20 can be achieved via a simple SSRS method from inexpensive BaCO3, ZrO2, and Y2O3 precursors with the help of 1–2 wt.% NiO sintering aid. By adding 1–2 wt.% NiO into BaCO3, ZrO2, and Y2O3 precursor powders, BZY20 pellets with N95% density and grain size of d ∼ 5 μm were obtained after sintering at 1400 °C for 24 h. In contrast, unmodified sol–gel BZY20 control pellets (sintered at 1600 °C for 24 h) manifest appreciable porosity and 10× smaller grain size (d ∼ 0.5 μm). Moreover, the total
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conductivity obtained from NiO-modified SSRS BZY20 pellets is among the highest ever reported for BZY20 (3.3 × 10− 2 S cm− 1 at 600 °C in wet-argon atmosphere). These results highlight the potential of the NiOmodified reaction sintering approach to provide a cost-effective, reduced-temperature route to achieve dense, large-grained, high conductivity BZY20 ceramic pellets. Acknowledgements This work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract DEFG3608GO88100 and by the National Science Foundation MRSEC program under Grant No. DMR-0820518 at the Colorado School of Mines. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. S.M. Haile, Acta Mater. 51 (2003) 5981. K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. W.G. Coors, J. Power Sources 118 (2003) 150. W.G. Coors, J. Electrochem. Soc. 151 (2004) A994. T. Schober, W.G. Coors, Solid State Ionics 176 (2005) 357. W.G. Coors, Solid State Ionics 178 (2007) 481. H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65. K.D. Kreuer, S. Adams, W. Münch, A. Fuchs, U. Klock, J. Maier, Solid State Ionics 145 (2001) 295. A. Magrez, T. Schober, Solid State Ionics 175 (2004) 585. P. Babilo, T. Uda, S.M. Haile, J. Mater. Res. 22 (2007) 1322. S.B.C. Duval, P. Holtappels, U. Stimming, T. Graule, Solid State Ionics 179 (2008) 1112. K.H. Ryu, S.M. Haile, Solid State Ionics 125 (1999) 355. T. Schober, H.G. Bohn, Solid State Ionics 127 (2000) 351. A. Sin, B.E. Montaser, P. Odier, J. Am. Ceram. Soc. 85 (2002) 1928. G. Taglieri, M. Tersigni, P.L. Villa, C. Mondelli, Int. J. Inorg. Mater. 1 (1999) 103. H.G. Bohn, T. Schober, J. Am. Ceram. Soc. 83 (2000) 768. F. Iguchi, T. Yamda, N. Sata, T. Tsurui, H. Yugami, Solid State Ionics 177 (2006) 2381. S.B.C. Duval, P. Holtappels, U.F. Vogt, E. Pomjakushina, K. Conder, Solid State Ionics 178 (2007) 1437. S. Imashuku, T. Uda, Y. Nose, K. Kishida, S. Harada, H. Inui, Y. Awakura, J. Electrochem. Soc. 155 (2008) B581. P. Balilo, S.M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362. S.W. Tao, J.T.S. Irvine, J. Solid State Chem. 180 (2007) 3493. A.M. Azad, S. Subramaniam, T.W. Dung, J. Alloys Compd. 334 (2002) 118. J.H. Tong, W.S. Yang, R. Cai, B.C. Zhu, L.W. Lin, Catal. Lett. 78 (2002) 129. R.B. Cervera, Y. Oyama, S. Yamaguchi, Solid State Ionics 178 (2007) 569. A. Magrez, T. Schober, Solid State Ionics 175 (2004) 585. Y. Yamazaki, R. Hernandez-Sanchez, S.M. Haile, Chem. Mater. 21 (2009) 2755. T. Schober, W. Schilling, H. Wenzl, Solid State Ionics 86–88 (1996) 653. R. Costa, N. Grunbaum, M.H. Berger, L. Dessemond, A. Thorel, Solid State Ionics 180 (2009) 891.
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Cost-effective solid-state reactive sintering method for high conductivity proton conducting yttrium-doped barium zirconium ceramics Jianhua Tong ⁎, Daniel Clark, Michael Hoban, Ryan O'Hayre ⁎ Metallurgical & Materials Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA
a r t i c l e
i n f o
Article history: Received 29 July 2009 Received in revised form 26 January 2010 Accepted 8 February 2010 Keywords: Proton conductivity Ionic conductivity Perovskite Ceramic electrolyte Fuel cell
a b s t r a c t Using cost-effective precursors of BaCO3, ZrO2, and Y2O3, proton conducting ceramic pellets of BaZr0.8Y0.2O3 − δ (BZY20) were successfully fabricated with the help of a range of sintering aids including LiF, NiO, Al2O3, and SnO2. This simple and cost-effective solid-state reactive sintering (SSRS) method involved only a single high-temperature sintering step. The effect of various experimental conditions on the crystal structure, relative density, morphology, and total conductivity of the as-prepared BZY20 ceramic pellets were investigated in detail. Tunable experimental parameters included the type of sintering aid, the amount of aid, and the sintering temperature. NiO was determined to be the most effective sintering aid investigated. Under optimized conditions using 1–2 wt.% NiO as a sintering aid, dense BZY20 ceramic pellets (N 95% relative density) with grain sizes as large as 5 μm were successfully prepared at a relatively low sintering temperature of 1400 °C. In comparison, most alternative sintering techniques for BZY require temperatures in excess of 1700 °C. Total conductivities as high as 2.2 × 10− 2 and 3.3 × 10− 2 S cm− 1 were obtained from the resulting pellets at 600 °C under dry- and wet-argon atmospheres respectively. These are among the highest values reported for BZY20, highlighting the potential of the NiO-modified reactive sintering approach to provide a simple, cost-effective, reduced-temperature route to achieve dense, large-grained parts for protonic-ceramic applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Proton conducting ceramics are intriguing candidate electrolyte materials for solid oxide fuel cells (SOFCs) owing to their unique ability to provide high efficiency, flexible fuel operation, and high fuel utilization [1–3]. In recent studies, proton conducting ceramics have also been suggested for use as steam permeation membranes based on an ambipolar co-ionic diffusion mechanism [4,5]. In such a configuration, proton conducting ceramics could find applications in many chemical processes including hydrocarbon steam reforming, operation of SOFCs, coal gasification, and heat recovery from steam [4–7]. Among candidate proton conducting electrolyte materials, yttriumdoped barium zirconate (BZY) has garnered particular attention because of its chemical stability, mechanical robustness, and high bulk proton conductivity [8–12]. However, the refractory nature of BZY results in significant challenges to its implementation in commercial applications. First, it is difficult to process BZY to the high density (N93%) that is usually needed for most applications. Typically, extreme conditions, such as high temperatures (1700–2100 °C), long sintering times (N24 h), and nanometer-sized powders are needed to prepare
⁎ Corresponding authors. E-mail addresses: [email protected] (J. Tong), [email protected] (R. O'Hayre). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.02.008
fully densified BZY pellets. This leads to high costs, process incompatibilities, and often causes barium volatilization which is detrimental to the proton conductivity of the resultant material [13–16]. Second, it is difficult to process BZY with sufficiently large grain size. Achieving a large grain size is critical in BZY and in many other proton conducting ceramics because the grain boundaries are significantly more resistive than the grain bulk. Enhanced densification and grain growth has been previously demonstrated in BZY pellets with careful optimization of sintering conditions, modification of powder properties, change in dopants, and addition of sintering aid materials, etc. [15–23]. Increasing the sintering temperature or sintering time, while improving grain growth and densification, normally results in the barium deficiency and the emergence of deleterious second phases such as Y2O3, which inevitably cause low total conductivity [17–19]. The use of nanocrystalline BZY powders, which can be prepared via polymeric sol–gel or combustion methods, has provided relatively good performance [15,16]. However, the synthesis of nanocrystalline BZY powder is economically unfavorable for industrial application. Alternative dopants such as Sc2O3 can also improve grain growth and densification, but have been shown to cause increased bulk and/or grain boundary resistivity [20]. Sintering aids such as ZnO, NiO, MgO, and Al2O3 have been added to BZY powders prepared by wet chemistry methods and the grain size and density of the BZY pellets have been increased to some degree [21–23]. Although the sintering mechanism is not clear and sometimes the sintering aids
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have a negative effect on the total conductivity of BZY, the addition of sintering aids can often decrease the sintering temperature to as low as 1350 °C, which makes it possible to prepare SOFCs stacks by inexpensive co-fired techniques. Therefore, increasing the grain size and density of BZY pellets by the means of a sintering aid is a very promising method. Correct choice of sintering aid is crucial. In the present paper, we therefore examine the effect of a range of different potential sintering aids on the phase-formation and densification of BZY. However, in contrast to previous sintering aid studies which mixed a sintering aid with already phase-pure BZY powder to encourage grain growth and densification, we mix our sintering aids with inexpensive BaCO3, ZrO2, and Y2O3 precursor powders to drive a simple and inexpensive singlestep solid-state reactive sintering process. This aid-assisted reactive sintering approach involves only a single high-temperature (≤1500 °C) sintering step, where both reactions of the precursors to single-phase BZY and the sintering/densification of the BZY to a fully dense pellet occur simultaneously. We examine four different potential sintering aids: LiF, NiO, Al2O3 and SnO2. These specific aids are chosen to enable a comparison between cation valence (Li+ vs. Ni2+ vs. Al3+ vs. Sn4+) and aid effectiveness. Furthermore, all four aids have specific properties which can potentially contribute to the sintering ability or proton conductivity of the resulting BZY20 ceramic. For example, LiF is a typical sintering aid used in the sintering of spinel and perovskite oxides. LiF exhibits many interesting properties such as specific surface activity and the ability to form oxygen vacancies which promotes densification. Also, because of its high volatility, it is a non-residual sintering aid when used in high-temperature (N1300 °C) sintering processes. NiO is a transition metal oxide sintering aid, and it has previously been proven to enhance the sintering of BZY powder compacts synthesized by the combustion method [21]. Al2O3 has been used as sintering aid for barium metazirconate BaZrO3 ceramic. Pure BaZrO3 pellets with bulk density ∼92% of theoretical were successfully obtained using 5 wt.% Al2O3 additive after sintering at 1600 °C for 6 h [23]. SnO2 has a relatively low melting point, and the same valence as zirconium (4+), but a larger cation radius. The introduction of the Sn4+ cation into the B site of perovskite-type BZY may therefore increase the conductivity while improving the sintering ability.
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Fig. 1. Summary of XRD patterns for BZY20 pellets (grinding into powder) obtained under different experimental conditions.
produced by the SSRS method. Conventional EDTA and citric acid combined complexing process was used for the synthesis of the BZY20 sol–gel derived powder. Briefly, nitrate salts of barium, yttrium, and zirconium were dissolved into DI water. The complexing organics of EDTA and citric acid were dissolved into ammonium hydroxide aqueous solution. The metal source solution was then gradually added to the complexing solution. The resulting solution was continuously stirred on a hot-plate at 80 °C, gradually evaporating the water from the mixture and converting the clear solution into a dark sticky gel. The sticky gel
2. Experimental Barium carbonate (Alfa Aesar, item number 14341, 99.8%, BaCO3), zirconium(IV) oxide (Alfa Aesar, item number 41528, 99.7%, ZrO2), and yttrium(III) oxide (Alfa Aesar, item number 44286, 99.9%, Y2O3) were used as raw materials for the synthesis of the yttrium-doped zirconate BaZr0.8Y0.2O3 − δ (BZY20) ceramic pellets examined in this study. Lithium fluoride (Alfa Aesar, item number 36359, 99.85%, LiF), nickel(II) oxide (Alfa Aesar, item number 45094, green, Ni 78.5%, NiO), aluminum oxide (Alfa Aesar, item number tem 12553, 99.5%, Al2O3), and tin(IV) oxide (Alfa Aesar, item number 12283, 99.9% SnO2) were used as sintering aids, respectively. Stoichiometric amounts of BaCO3, ZrO2, and Y2O3 were weighed and mixed with 2-propanol (SigmaAldrich, product number 190764, 99.5%) as a ball milling medium to produce a BZY20 precursor mixture. The sintering aid was then added to the BZY20 precursor in varying weight percentages of 0.5–3%. Specifically, pellet samples of BZY20–LiF1, BZY20–LiF2, BZY20–LF3, BZY20–NiO0.5, BZY20–NiO1, BZY20–NiO2, BZY20–Al2O30.5, BZY20– Al2O31, BZY20–Al2O32, BZY20–SnO21, and BZY20–SnO22 were prepared by this SSRS method followed by ball milling for 24 h, drying at 80 °C for 48 h, hydraulic dry pressing under 375 MPa pressure for 120 s (in a circular carbon-aided steel dry pressing die with diameter of 19 mm), and sintering at temperatures from 1200 to 1500 °C for 24 h. Control pellet samples of unadulterated BZY20 (without any sintering aids) were also prepared using the same process. As an additional point of reference, high quality BZY20 powder was prepared by a polymeric sol–gel method [24] and BZY20 pellets were prepared from this sol–gel BZY20 powder for comparison with the pellets
Fig. 2. The higher magnification for the XRD patterns in Fig. 1 in the 2theta range of 28–33°.
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Table 1 Impurity ratio in the cubic perovskite BZY20 for samples shown in Fig. 2.
Sol–gel-1450 Control-1450 LiF1-1450 NiO1-1450 Al2O31-1450 SnO21-1450 NiO0.5-1500 NiO1-1500 NiO2-1500 NiO2-1450 NiO2-1400 NiO2-1350 NiO2-1300 NiO2-1250
Second cubic perovskite
Y2O3
BaY2NiO5
Ba4Al2O7
0 0.579(7) 1.070(11) 0 0 0.625(6) 0 0 0 0 0 0 0 0
0 ∼0 0.0623(8) 0 0.0392(5) 0 0 0 0 0 0 0 0 0
0 0 0 0.0159(3) 0 0 ∼0 0.0172(3) 0.0408(6) 0.0389(5) 0.0348(5) 0.0501(6) 0.0446(6) 0.0493(6)
0 0 0 0 0.1322(15) 0 0 0 0 0 0 0 0 0
The impurity ratios were defined as follows: I110 (second cubic perovskite)/I110(cubic perovskite BZY20), I222(Y2O3)/I110(cubic perovskite BZY20), I112(BaY2NiO5)/I110(cubic perovskite BZY20), and I226(Ba4Al2O7)/I110(cubic perovskite BZY20).
was dried at 150–190 °C for 24–48 h and then calcined at 1250 °C to achieve pure BZY20 powders. The sol–gel BZY20 powder was also pressed at 375 MPa pressure for 120 s (with the same circular carbonaided steel dry pressing die) and sintered at temperatures of 1400 °C, 1450 °C, 1500 °C and 1600 °C for 24 h in order to obtain sol–gel BZY20 pellets for comparison with the SSRS BZY20 pellets. X-ray diffraction (XRD) analyses of the sintered pellet samples (after grinding into powders) were performed at room temperature using a Philips diffractometer (X'Pert Pro) with CuKα radiation, tube voltage 45 kV, and tube current 40 mA. Intensities were collected in the 2θ range between 20° and 120° with a step size of 0.008° and a measuring time of 5 s at each step. Relative densities of the sintered pellets were determined by the Archimedes method using water as the liquid medium and the direct measurements of the regular geometric volume and the corresponding mass. The microstructure and chemical composition of the sintered pellets were investigated by means of scanning electron microscopy (SEM, FEI Quanta 600) in conjunction with energy dispersive X-ray (EDX, Princeton GammaTech Prism). Cylindrical test bars (diameter: 4 mm and length: 25 mm) for fourprobe conductivity measurements of the BZY20–NiO2 were prepared by hydraulic dry pressing with a cylindrical stainless steel die (diameter: 6.4 mm) under 375 MPa for 60 s and sintering at 1500 °C for 24 h. Four silver wires (Alfa Aesar, item number 12187, 0.25 mm in diameter) were bound to the cylindrical sample as electrodes with the help of silver paste (Alfa Aesar, item number 44075). The distance between the two inner “voltage-sense” electrodes was 10 mm, with additional 5 mm spacing on either side for the two outer current-imposing electrodes. Four-probe DC technique was used to measure conductivities of the asprepared bars using Gamry Reference 600 Potentiostat/Galvanostat/ZRA with constant DC current from 3 mA to 180 mA. The conductivity data
were collected at temperatures from 350 to 750 °C under the atmospheres of dry and water saturated argon with a flow rate of 100 sccm. 3. Results and discussion 3.1. X-ray diffraction studies on crystal structure Fig. 1 compares the XRD patterns for the SSRS-derived BZY20 pellets (after grinding into powders) using different sintering aid types, amounts, and sintering temperatures against a SSRS-derived BZY20 control pellet (without any sintering aid) as well as a phase-pure BZY standard derived using a sol–gel method. The main phase of the cubic perovskite structure based on barium zirconate (PM3-M, ICSD 97459) has been successfully formed for all the BZY20 pellets under our experimental conditions (sintering aid type: LiF, NiO, Al2O3, or SnO2; sintering aid amount: 0–2 wt.%; and sintering temperature: 1250 to 1500 °C). However, phase separation (two cubic perovskite phases with same structure but different lattice constants) occurs for the BZY20 pellets obtained by sintering at 1450 °C for 24 h with 1 wt.% LiF or SnO2 as sintering aids and the SSRS BZY20 control pellet without any sintering aid. The existence of the second cubic perovskite phase indicates that sintering at 1450 °C for 24 h is not sufficient to attain the pure cubic perovskite single-phase structure using SSRS without a sintering aid, nor do the LiF or SnO2 sintering aids provide help in this regard. The second cubic perovskite phase is only slightly observable in BZY20 pellet with Al2O3 as sintering aid. Thus, the addition of Al2O3 may have a slight positive contribution to the formation of phase-pure cubic perovskite because it results in significantly lower second-phase peak intensities compared to the control BZY20 pellet obtained under the same sintering conditions. On the other hand, the BZY20 pellets obtained by sintering at 1250–1500 °C for 24 h with NiO as a sintering aid and the BZY20 pellet
Table 2 Experimental condition effect on lattice constant for the resulting BZY20 pellets. Aid type effect on lattice constant while aid amount is 1 wt.% and sintering temperature is 1450 °C Aid type Lattice constant, Å
Sol–gel 4.218(2)
Control 4.217(2) 4.194(2)
LiF1 4.221(2) 4.183(2)
NiO1 4.218(2)
Al2O31 4.186(2)
SnO21 4.222(2) 4.198(2)
Aid amount effect on lattice constant while aid is NiO and sintering temperature is 1500 °C Aid amount, wt.% Lattice constant, Å
NiO0.5 4.220(2)
Ni1.0 4.212(2)
NiO2.0 4.203(2)
Sintering temperature effect on lattice constant while aid is 2 wt.% NiO Sintering temperature, °C Lattice constant, Å
1250 4.209(2)
1300 4.201(2)
1350 4.205(2)
1400 4.203(2)
1450 4.211(2)
1500 4.203(2)
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Fig. 3. The summary of relative densities for BZY20 pellets obtained under different experimental conditions.
derived from sol–gel BZY20 powder by sintering at 1450 °C for 24 h have no second cubic perovskite phase, demonstrating that these two approaches can both yield phase-pure cubic BZY20. Fig. 2 provides a detailed view in the angle range 2θ = 28–33° for the same group of XRD patterns in Fig. 1. In order to quantify the magnitude of the second-phase impurities present in the samples, relative
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impurity-peak intensities were calculated using the ratios between the highest peak intensities for the impure phases and the 110 peak intensity for cubic perovskite BZY20. These are summarized in Table 1. From this detailed analysis, it is observed that the BZY20 SSRS control pellet as well as the pellets fabricated using LiF or Al2O3 show evidence of Y2O3 impurity (IA3-, ICSD 86814; the Y2O3 222 peak is at 2θ = 29.2°). It is well-known that Y2O3 impurities have a significant negative effect on the proton conductivity of BZY20 [21,22]. Moreover, Ba4Al2O7 (PDF 041-0165) second-phase impurity was found for the Al2O3 modified BZY20 pellet, which is also reported to have a negative effect on proton conductivity [12]. Fortunately, the BZY20 pellets prepared with NiO or SnO2 show no signs of Y2O3 impurity. However, the NiO-modified BZY20 pellets do show indications of BaY2NiO5 impurities (PDF 041-0463), which increase with increasing wt.% NiO additive. Because the effect of BaY2NiO5 on the sintering ability and proton conductivity of BZY20 has not yet been reported, we are currently investigating this issue in our ongoing work. In order to provide further insight into the influence of the sintering aids on the BZY structure, trends in the lattice constant of the primary cubic perovskite barium zirconate phase for all of the pellet samples were analyzed from the XRD data. For the pellets which incurred phase separation into two cubic perovskite phases, the lattice constants for the second cubic perovskite phase were also calculated. This data is presented in Table 2. Calculated lattice constant uncertainties, also provided in Table 2, were estimated from the measurement step size uncertainty of 0.008°. Systematic uncertainty from flatness and instrumental systematic error are not considered. As shown in Table 2, the control BZY20 pellet without any sintering aids and the sol–gel
Fig. 4. Sintering aid type effect on BZY20 pellet morphology (additive amount 1 wt.%, sintering temperature 1500 °C, sintering time 24 h).
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Fig. 5. Sintering temperature effect on the BZY20 pellet morphology while using 1.0 wt.% NiO as sintering aid (BZY20–NiO1, sintering time 24 h).
BZY20 pellet after sintering at 1450 °C for 24 h have lattice constants of 4.218(2) and 4.217(2) Å respectively, which are very close to the reported value of 4.228 Å for BZY20 pellets sintered at temperatures between 1400 and 1500 °C [25,26]. The addition of LiF or SnO2 resulted in two cubic perovskite phase with different lattice constants. In both materials, the larger lattice constant phase is again close to that of BZY20 (∼4.22 Å), while the lattice constant for the second phase (4.19–4.20 Å) is very close to that of pure barium zirconate [25,26]. The addition of Al2O3 into the BZY20 pellet did not lead to phase-separation, but did result in a very small lattice constant of 4.1861 Å, which may be a result of Al3+ substitution on the Zr4+ or Y3+ sites in the cubic perovskite structure (Al3+ has a smaller ionic radius compared with both Zr4+ and Y3+). The lattice constant also decreased as a function of NiO addition, again perhaps indicating substitution on the Zr4+ or Y3+ sites with a smaller cation (Ni2+ or Ni3+). The temperature effect on the lattice constants of the BZY20–NiO2 pellets was negligibly small, which can be ascribed to the saturated solubility of nickel in the BZY crystal structure. 3.2. Sintering behavior 3.2.1. Relative density The relative densities of the sintered BZY20 pellets are summarized in Fig. 3 vs. sintering aid type, sintering aid amount, and sintering temperature. Relative densities for the BZY20 pellets sintered from sol– gel powders are also included for comparison. From this figure, it is clear that the NiO sintering aid had the greatest positive contribution to the sintering ability, and this effect increases with increasing amounts of NiO addition. Relative densities greater than 95% were obtained for the
BZY20 pellets by sintering at temperatures higher than 1400 °C for 24 h with 2 wt.% NiO. The highest relative density of 97% was achieved for a 2 wt.% NiO-modified pellet sintered at 1500 °C. In comparison, the unmodified SSRS BZY20 control pellet achieved only 35% relative density when sintered at 1500 °C, and even pellets sintered from sol–gel derived BZY20 powder (the standard approach to achieve high-density BZY at lower sintering temperatures) achieved only about 70% relative density at 1500 °C. In contrast to NiO, the addition of Al2O3 or SnO2 had almost no effect (or perhaps even a slight negative effect) on densification. In fact, for the Al2O3 modified pellets, relative density measurements could only be obtained for the 0.5 wt.% Al2O3 sample, as the 1 wt.% and 2 wt.% Al2O3; sintered pellets did not even obtain sufficient mechanical strength for relative density measurement. The LiF aid did appear to have a positive effect on the sintering ability of BZY20 pellets, although the effect was small and did not lead to higher densities than what could be achieved from the phase-pure sol–gel route. Specifically, relative densities of around 40% and 50% were obtained for the 1 wt.% and 2 wt.% LiF modified samples, respectively, after sintering at 1500 °C for 24 h. 3.2.2. SEM cross-section images SEM microstructure analysis provides perhaps the most compelling evidence for the remarkable effect of NiO aid on BZY20 sinterability. Fig. 4 provides SEM fracture cross-section images from the BZY20 pellets with 1 wt.% LiF, NiO, and SnO2, respectively, after sintering at 1500 °C for 24 h. For comparison, a cross-section for a control SSRS BZY20 pellet (without any additive) prepared under the same experimental
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Fig. 6. NiO aid concentration effect on BZY20 pellet morphology (sintering temperature 1450 °C, sintering time 24 h).
conditions is also included. From this SEM comparison, pellet density and grain size can be observed to increase in the order of control BZY20 ≤ BZY20–SnO21 b BZY20–LiF1 b BZY20–NiO1. For the control BZY20 and the BZY20–SnO21 pellets, the grain size is too small to be observed clearly under our experimental conditions. Although the grain size for the BZY20–LiF1 pellet is also not clear from the SEM crosssection, this pellet does show microstructural evidence of improved
intergranular connectivity. Anecdotally, the BZY20–LiF1 pellet also showed relatively higher mechanical strength and crush resistance than the unmodified BZY20 and BZY20–SnO21 pellets, providing further evidence for enhanced sintering. The most dramatic changes, however, were observed for the 1 wt.% NiO-modified BZY20 pellet, which exhibited a non-porous cross-section with equiaxed grains as large as 5 μm, corroborating well with the 95% relative density measurement
Fig. 7. Morphology comparison for BZY20 pellets from sol–gel powders (sintering temperature 1600 °C, sintering time 24 h) and from solid-state reactive sintering directly (sintering aid 1 wt.% NiO, sintering temperature 1500 °C, sintering time 24 h), respectively.
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and grain size continued to improve for the 1 wt.% NiO-modified sample, although increasing the NiO concentration from 1 wt.% to 2 wt.% did not appear to provide any substantial further enhancement. As a final point of comparison, Fig. 7 compares typical SEM crosssection images obtained from a NiO-modified reaction sintered BZY20 pellet (1.0 wt.% NiO, sintering temperature 1500 °C, sintering time 24 h) vs. an unmodified sol–gel powder derived BZY20 pellet (sintering temperature 1600 °C, sintering time 24 h). Despite being sintered at lower temperature, the NiO-modified BZY20 pellet shows a fully dense cross-section and large grain size (d ∼ 5 μm), while the unmodified sol– gel BZY20 pellet manifests appreciable porosity and 10× smaller grain size (d ∼ 0.5 μm). This comparison highlights the potential of the NiOmodified reaction sintering approach to provide a cost-effective, reduced-temperature route to achieve dense, large-grained BZY20 ceramic pellets. 3.3. Electrical conductivity
Fig. 8. Arrhenius plots of total conductivity for BZY20 obtained by SSRS method with 2 wt.% NiO as sintering aid by sintering at 1500 °C for 24 h, and summary comparison with total or bulk conductivities recently reported for BZY20.
previously estimated for this pellet. Importantly, the large grain size and high density of this pellet should bode well for its protonic conductivity and use in functional ionic-ceramic applications. Given the significant potential of the NiO-modified BZY20, the effect of sintering temperature on cross-section morphology was studied for BZY20–NiO1 at sintering temperatures between 1250 and 1500 °C as shown in Fig. 5. Based on this analysis, it is clear that the sintering temperature has a marked effect on the pellet density and grain size. For the pellet sintered at 1250 °C, significant porosity is clearly observed and the grain size is less than 1 μm. Increasing the sintering temperature to 1350 °C results in a significant reduction in pore volume and an increase in the grain size to about 1.5–2 μm. Further increasing the sintering temperature to 1450 and 1500 °C results in nearly complete elimination of obvious porosity and causes the grain size to increase to about 3 μm and 5 μm, respectively. Because the NiO additive may have unintended effects on BZY20 proton conductivity, it is clearly desirable to reduce the concentration as much as possible while still maintaining the beneficial sintering enhancement effects. To this end, the effect of NiO concentration on BZY20 pellet morphology was also studied by sintering samples with different NiO concentration (0.0 wt.%, 0.5 wt.%, 1.0 wt.%, and 2.0 wt.%) at 1450 °C for 24 h. The resulting SEM cross-section images for these samples are shown in Fig. 6. Importantly, even the addition of only 0.5 wt.% NiO had a tremendous influence on the sintered BZY20 morphology, significantly improving both density and grain size. Density
Because of its promising grain size and density, the conductivity of the 2 wt.% NiO-modified BZY20 material was subsequently evaluated using a DC four-point probe technique in both dry and wet argon and compared to the conductivities of the BZY20 ceramics reported in the recent literature. As shown in Fig. 8, the NiO-modified BZY20 demonstrates the highest total conductivities ever reported for this material. Table 3 provides further details comparing the conductivities obtained in this study (at 600 °C) to previous reports. The 2 wt.% NiOmodified BZY20 obtained in this report using the SSRS method achieves a maximum conductivity of 3.3× 10− 2 S cm− 1 at 600 °C in wet argon. As is typically observed in BZY, the conductivity response exhibits two distinct slopes when viewed in an Arrhenius plot. For conduction in the dry-argon atmosphere, the activation energy in the lower temperature region (350–600 °C) is about 0.82 eV, while the activation energy in the high-temperature region (N600 °C) is about 0.14 eV. The oxygen partial pressure in the dry-argon atmosphere is ∼10− 5 atm, while the water partial pressure and hydrogen partial pressure are negligibly small. The main defect formation reaction under these conditions can be described by Eq. (1): 1= O 2 2
••
X
•
þ VO ↔OO þ 2h :
ð1Þ
Therefore, oxygen vacancies and electron holes will be the dominant charge-carriers under these conditions. At low temperatures (b600 °C), in dry-argon atmosphere, the total conductivity of BZY20 is often dominated by oxygen vacancy transport [11,17,28]. Our measured activation energy (∼0.82 eV) in this region is typical for oxygen vacancy conduction in BZY (0.7–1.1 eV) [17]. At higher temperatures (N600 °C), electron hole conductivity begins to dominate with a commensurate decrease in the activation energy. In a wet-argon atmosphere, hydration of the BZY lattice by water molecules leads to substantial proton conductivity, as illustrated by Eq. (2). ••
X
•
H2 O þ VO þ OO ↔2OHO :
ð2Þ
At low temperatures (b600 °C), the reaction equilibrium expressed by Eq. (2) favors proton (OH•O) formation and the material exhibits
Table 3 Detailed experimental conditions of electrolytes reported in Fig. 8. BaZr0.8Y0.2O3 − δ name
Preparation condition
Measurement condition
Conductivity at 600 °C, S/cm
Ref.
A B C D E F
1500 °C 1500 °C 1700 °C 1600 °C 1600 °C 1325 °C
Wet Ar (pH2O = 0.031 atm) Dry Ar Wet N2 (pH2O = 0.023 atm) Wet N2 or Ar (pH2O = 0.031 atm) Wet N2 (pH2O = 0.031 atm) Wet 5% H2 in Ar
3.3 × 10− 2 (total) 2.2 × 10− 2 (total) 2.5 × 10− 2 (bulk) N1.5 × 10− 2 (total) 7.9 × 10− 3 (total) 1.2 × 10− 4(total)
This work This work [9] [27] [11] [22]
for 24 h in air for 24 in air for 24 h in O2, special bury, sol–gel power (850 °C) for 24 h in O2, special bury, combustion powder (1250 °C) for 10 h with 1 wt.% ZnO aid
J. Tong et al. / Solid State Ionics 181 (2010) 496–503
primarily proton-dominated conductivity. Due to the higher mobility of proton defects compared to oxygen vacancies, the total conductivity of the BZY material increases substantially in the wet-argon atmosphere compared to the dry-argon condition. The Arrhenius activation energy of 0.66 eV is obtained in this low-temperature range under wet-argon atmosphere. While this is significantly lower than the activation energy obtained for the dry-argon atmosphere condition, it is still somewhat higher than is typically observed for proton transport in BZY ceramics (typically 0.45–0.55 eV). The discrepancy may indicate that only partial hydration is obtained in this material and that the conductivity is due to a combination of both protons and oxygen vacancies. At temperatures above 600 °C, the conductivity under wet argon actually decreases. A “leveling off”, “hump” or decrease in the total conductivity of BZY and other proton conductors is often observed at intermediate temperatures [17], and is frequently attributed to the dehydration of the material with increasing temperature. Dehydration (representing a shift to the right in the defect equilibrium expressed by Eq. (2)) results in a decrease in the proton conductivity, and often a decrease in the overall total conductivity as the higher-mobility protons are replaced by lowermobility oxygen vacancies [3,11]. Future work must consider the fate of the NiO additive under strongly reducing atmospheres, where the reduction of nickel from the cubic perovskite structure and the BaY2NiO5 phase impurity is a possibility. It is likely that nanoparticles of nickel metal may be formed in the grain boundaries under reducing conditions, in analogy to recent studies of NiO-modified yttrium-doped barium cerate pellets [29]. In a future paper, the role of nickel under these conditions, including the nickel reduction behavior, nickel location, nickel oxidation state, and the effect of these properties on the conductivity, structure, and stability will be reported in detail. 4. Conclusions This study demonstrates that dense, large grain-size BZY20 can be achieved via a simple SSRS method from inexpensive BaCO3, ZrO2, and Y2O3 precursors with the help of 1–2 wt.% NiO sintering aid. By adding 1–2 wt.% NiO into BaCO3, ZrO2, and Y2O3 precursor powders, BZY20 pellets with N95% density and grain size of d ∼ 5 μm were obtained after sintering at 1400 °C for 24 h. In contrast, unmodified sol–gel BZY20 control pellets (sintered at 1600 °C for 24 h) manifest appreciable porosity and 10× smaller grain size (d ∼ 0.5 μm). Moreover, the total
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conductivity obtained from NiO-modified SSRS BZY20 pellets is among the highest ever reported for BZY20 (3.3 × 10− 2 S cm− 1 at 600 °C in wet-argon atmosphere). These results highlight the potential of the NiOmodified reaction sintering approach to provide a cost-effective, reduced-temperature route to achieve dense, large-grained, high conductivity BZY20 ceramic pellets. Acknowledgements This work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract DEFG3608GO88100 and by the National Science Foundation MRSEC program under Grant No. DMR-0820518 at the Colorado School of Mines. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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