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Maintaining the mechanical strength of La-, Y-co-substituted zirconia porous ceramics through the superplastically foaming method
Materials Science & Engineering A 581 (2013) 98–103
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Maintaining the mechanical strength of La-, Y-co-substituted zirconia porous ceramics through the superplastically foaming method Akira Kishimoto n, Masanori Okada, Takashi Teranishi, Hidetaka Hayashi Division of Chemistry and Biotechnology, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2013 Received in revised form 10 May 2013 Accepted 28 May 2013 Available online 14 June 2013
The superplastically foaming method was adopted to make closed-pore inclusive zirconia-based ceramics. Lanthanum oxide was added to monoclinic or tetragonal yttria-stabilised zirconia to reduce the thermal conductivity of the matrix. Sintering and superplastic deformation led to a solid solution and transformation to the cubic phase. The resulting superplastically foamed porous ceramics having a porosity of 45% had only 40% of the thermal conductivity of the fully densified ceramics having the same composition. This value was comparable to that of conventionally fabricated porous ceramics with the same composition and porosity. The superplastically foamed ceramics had 60%, while conventionally fabricated ceramics had only 20%, of the mechanical strength of the fully dense ceramics. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zirconia Porous ceramics Thermal insulator Superplasticity
1. Introduction We have previously reported the fabrication of porous ceramics by introducing pores after full densification of the matrix [1–8]. In that method, granular foaming agents were arranged at regular intervals in the powdered matrix and then the mixture was compacted. After densification of the matrix, the pores expanded by utilising the superplastic deformation driven by the gas pressure evolved from the foaming agents [9]. We called this novel process “superplastically foaming method”. Almost all the internal pores are closed pores surrounded by fully densified pore walls. Consequently, high mechanical strength, gastightness and good thermal insulation are expected and indeed have already been demonstrated to a certain extent. These properties contrast with conventional porous ceramics fabricated through a partial sintering process. Porous ceramics containing high numbers of closed pores are promising as high-temperature gastight thermal insulators. The foaming of the zirconia-based ceramics is believed to arise from the swelling of gas phases formed from the active oxidation of silicon carbide [9] as SiC þ O2 ðgÞ-SiOðgÞ þ COðgÞ This reaction has proved to occur under conditions determined by the oxygen partial pressure and temperature [9]. When the n
Corresponding author. Tel./fax: +81 86 251 8069. E-mail address: [email protected] (A. Kishimoto).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.084
amount of gas evolution is constant, the degree of pore expansion depends on the superplastic deformation rate and limit. To improve the porosity of superplastically foamed ceramics, some dispersoids such as silica [2,8] and spinel [6] are added into the matrix. However, these dispersoids can sometimes negatively affect other properties of the porous ceramics. We have previously fabricated superplastically foamed silicadispersed 3YSZ (3 mol% yttria-stabilised zirconia) porous ceramics. The thermal conductivity of one superplastically foamed ceramic having a porosity of 25% was 1.7 W/mK, which is about half of the 3.5 W/mK value for the fully dense case [4]. However, the silicadispersed 3YSZ had a somewhat greater thermal conductivity than 3YSZ without dispersoids, which was due to silica present at the gain boundaries. Zirconia-based ceramics show promise as thermal barrier coatings [10–12]. Various additives have been examined [12–15], among which lanthanum oxide has proven to be particularly effective at improving the thermal insulation properties because it enhances phonon scattering while concurrently suppressing the sinterability [15,16]. As a result, lanthanum oxide added zirconia ceramics show smaller mechanical strength compared with the other oxide added ones [16]. Further degradation of mechanical strength was a serious problem for practical use when incorporating pores to reduce the thermal conductivity. In the present study, we employed lanthanum oxide as an additive to make porous YSZ-based ceramics suitable as thermal insulators. At this time, our innovated superplastically foaming method was adopted to maintain the mechanical strength. The added lanthanum ions substituted for zirconium ones and
A. Kishimoto et al. / Materials Science & Engineering A 581 (2013) 98–103
transformed the YSZ ceramic from a tetragonal to a cubic structure having lower thermal conductivity. The cubic phase, however, has been shown to have relatively poor superplastic deformation, resulting in lower porosity in superplastically foamed ceramics [3,17]. To overcome this drawback, our innovative poststabilisation method was developed, in which sintering and superplastic deformation was first conducted on a monoclinic or tetragonal YSZ structure containing dispersed lanthanum oxide, followed by formation of a solid solution having the cubic phase with its lower thermal conductivity.
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ions. In one case, lanthanum oxide was dissolved in yttriastabilised-zirconia through calcining, crushing, grinding and then compacting and heating. In the other approach, lanthanum oxide was simply mixed with the powder before compaction; in this case, a solid solution formed during or after the sintering/foaming process. By comparing these two types of processing, the influence of the existing form of lanthanum ion on the superplastic deformation was evaluated.
2.2. Fabrication of porous ceramics 2. Experimental procedure 2.1. Fabrication of mono-foam To determine the conditions under which ceramic foaming could be achieved, macroscopic single foams were fabricated using a method similar to that previously reported [1–6]. In short, ß-SiC (AZmax Co., Ltd, Ichihara, Chiba, Japan) as foaming agent was dispersed into an aqueous solution containing 10 wt% methyl cellulose with a solid/liquid weight ratio of 0.10. This SiC slurry was spread on a polyethylene sheet using a glass bar to adjust the thickness. Disk-like pieces of this foaming agent about 12 mm in diameter were obtained after drying. Commercial zirconia without additives, and 2-, 3- and 6-mol %-yttria-stabilised zirconia (TZ-0, TZ-2Y, TZ-3Y, TZ-6Y; Tosoh, Tokyo, Japan) were used as matrices. Predetermined amounts of silica (Snow Tex S; Nissan Chemical, Tokyo, Japan) or lanthanum oxide (Kojundo Chemical Co., Ltd, Tokyo, Japan) were added to make the powder mixtures. Samples (ca. 4 g) of each powder mixture were weighed. Half of the powder mixture, discs of SiC foaming agent, and the remaining half of the powder mixture were placed into a 20-mm-diameter steel die, pressed uniaxially at 15 MPa for 1 min and then hydrostatically at 124 MPa. The resulting powder compacts were heated to 1600 1C for 8 h in air and then cooled. Fabrication procedures are illustrated in Fig. 1 together with the following superplastically foamed porous ceramics. The commercial zirconia matrix powder contained dissolved yttrium ions. We investigated two ways to introduce lanthanum
Porous ceramics containing numerous pores were fabricated according to a previously reported method [4,5]. Composite granules that were composed of the ß-SiC foaming agent surrounded by the matrix powder were prepared. Superplastically foamed porous ceramics were obtained after compaction and sintering. Specifically, 15 wt% of ß-SiC powder was dispersed in an aqueous solution containing 1 wt% of methylcellulose to provide a low-viscosity slurry. After spraying this slurry on the matrix powder and then ball-milling, the resulting mixture provided composite granules with a primary core size of 15 μm. The granules were placed in a 15-mm-diameter steel die and pressed uniaxially at 31 MPa for 1 min and then hydrostatically at 124 MPa. The resulting powder compacts were heated at 1600 1C for 8 h in air. Conventional porous ceramics were prepared, and their porosity was controlled by compaction pressure or by the addition of different amounts of carbon as the pore foaming agent. The heating scheme used for the composite granules was also used for these reference materials [7].
2.3. Evaluation The external dimensions of a mono-foam such as its foam height [1] and foam diameter were measured using a calliper. The apparent density of the outer shell and the entire ceramic foam was measured using Archimedes' method with water as the medium. The porosity of the ceramic foam was estimated from
Fig. 1. Schematic illustration of superplastically foamed ceramics. Mono-foam (upper) and porous ceramics (lower).
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the apparent density of the ceramic foam and the relative density of the outer shell. The apparent densities of the superplastically foamed porous ceramics were similarly measured by Archimedes' method using water. Cross sections were observed by scanning electron microscopy (SEM; S-4300; Hitachi, Ltd, Tokyo, Japan). The formed crystalline phases were identified by X-ray diffraction (XRD; Multiflex; Rigaku Co., Tokyo, Japan). Thermal diffusivity was measured by the laser flash method (LFA447 NanoFlash; Bruker AX, Tokyo, Japan), which provided the thermal conductivity from the specific heat and density.
3. Results and discussion The effect of lanthanum oxide addition on superplastic deformation was examined in detail. Changing the composition of the matrix while the amount of the foaming agent remained constant affected the height of the mono-foam. The relationship between foam height and the kind of dispersoid is shown in Table 1. With lanthanum oxide addition, pore expansion was more dominant during the process of sintering/foaming followed by solid solution formation than in the process of solid solution formation followed by the inverse process. Fig. 2 shows side views of both mono-foams. The first case (right side) clearly shows the better expansion, indicating that the first process is more favourable for obtaining highly porous ceramics. The crystalline phase transformed from tetragonal to cubic after formation of the solid state solution of lanthanum oxide with 3YSZ. The superplastic deformation speed of the cubic phase in zirconia-based ceramics has been reported to be lower than that of the tetragonal phase [8,10,11]. In the present case, lanthanum oxide particles remained as dispersoids after full densification because of the slow rate of the solid state reaction, which facilitated sliding of the grain boundaries during the foaming or pore-expansion process. At the same time, the crystalline phase remained tetragonal because of incomplete formation of the solid solution, which also contributed to Table 1 Matrix composition dependence of foam height of a 3YSZ based mono-foam fabricated through sinter/foaming at 1600 1C for 8 h. Composition of matrix
Foam height (mm)
3YSZ 10 mol% Silica added 3YSZ 5 mol% Lanthanum oxide added 3YSZ Silica and lanthanum oxide added 3YSZ
4.66 8.83 7.03 4.11
Fig. 2. Side view of the superplastically foamed 5 mol% lanthanum oxide added 3 mol% yttria stabilized zirconia (mono-foam) fabricated through (left) solid solution followed by supreplastically foaming (right) superplastically foaming followed by solid solution.
enhancing the superplastic deformation. We called this the “poststabilisation method”, in which sintering and superplastic deformation were first conducted in a monoclinic or tetragonal YSZ dispersed with lanthanum oxide, then followed by forming a solid solution to transform the ceramic into the cubic phase. Silica is commonly used to enhance superplastic deformation in zirconia. In this work, lanthanum oxide led to comparable foam heights and porosities for the same addition level. Co-dispersion of silica and lanthanum oxide, however, reduced the deformation. We also studied the effect of different amounts of lanthanum oxide on yttria-stabilised zirconia. Fig. 3 shows the relationship between the amount of stabilising atoms added and the porosity in the mono-foam ceramics. Lanthanum oxide was added to 2-, 3- and 6-mol% yttria-stabilised zirconia powders (2Y, 3Y and 6Y, respectively) or zirconia with no additive (0Y). In Fig. 3, the total amounts (mol%) of yttrium and lanthanum atoms are indicated in the abscissa. Cracks formed in the 0Y and 6Y ceramics without added lanthanum oxide, resulting in no pore expansion. These results indicated that the monoclinic (0Y) or cubic (6Y) zirconia without hetero-additives showed only small superplastic deformations compared with tetragonal (2Y or 3Y) ones. Monoclinic zirconia (0Y) to which lanthanum oxide was added expanded only slightly although it transformed into a cubic phase via a tetragonal one. The grain growth would have proceeded in the monoclinic phase, leading to a small deformation in the transitional tetragonal phase because of the larger grain size. SEM photos of the surfaces of superplastically foamed 3YSZbased ceramics with and without lanthanum oxide are shown in Fig. 4. Pore walls were dense and the grain sizes were smaller when lanthanum oxide was present; both properties helped to improve the superplastic deformation. XRD patterns are shown in Fig. 5 for 3YSZ-based mono-foam ceramics dispersed with 5 mol% of lanthanum oxide with a holding time of 1 and 8 h. The original tetragonal phase remained at a holding time of 1 h, while the cubic phase predominated after 8 h as zirconium ions were replaced by lanthanum ions. The foam height changed from 5.8 mm at 1 h to 7.0 mm at 8 h, indicating that most of the foam had been formed by 1 h of holding time; the remaining tetragonal phase contributed to increasing the foam height until 8 h.
Fig. 3. Porosity of the superplastically foamed YSZ based mono-foam. Lanthanum oxide was added into 0, 2, 3, or 6 mol% yttria stabilized zirconia. Total rare earth contents are taken as the abscissa.
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Fig. 5. XRD profiles of 5 mol% lanthanum oxide added 3 mol% yttria stabilized zirconia heat treated at 1600 1C for 1 h and for 8 h.
Fig. 6. Thermal conductivity of the YSZ based dense and superplastically foamed porous ceramics. Lanthanum oxide was added into 0, 2, 3, or 6 mol% yttria stabilized zirconia. Total rare earth contents are taken as the abscissa.
was much lower than when added to 3YSZ, indicating the strong effect of co-doping on decreasing the thermal conductivity. Cracks are often reported in lanthanum oxide-doped YSZ [15,16]. However, the examples described here were gastight. Thermal conductivity k is expressed by the following equation: k¼ Fig. 4. SEM photos of zirconia based ceramics sintered at 1600 1C for 8 h. 3 mol% yttria stabilized zirconia (3YSZ), 5 mol% lanthanum oxide added 3YSZ (5La3YSZ), 10 mol% silica added 3YSZ (Silica added 3YSZ).
The effect of lanthanum addition on the thermal conductivity of the matrix and porous YSZ-based ceramics was examined. The results are plotted in Fig. 6. Adding lanthanum oxide transformed monoclinic zirconia into a tetragonal or cubic phase which showed only a small volume change on cooling. The thermal conductivities of the ceramics were large, up to 3.5 W/mK, when only lanthanum oxide was added. However, co-addition of lanthanum oxide into 2YSZ or 3YSZ led to a drastic decrease in the thermal conductivities (i.e. down to 2 W/mK, about two-thirds that of the original YSZ ceramics). With the same addition amount of 3 mol%, the thermal conductivity of 1 mol% of lanthanum oxide added to 2YSZ
1 lC p v; 3
where l is the phonon mean free path, Cp is the specific heat and v is the velocity of sound. The velocity of sound is strongly affected by the bonding quality of the matrix. Because the bonding was not degraded in our materials, any decreases in the matrix thermal conductivity were due to decreases in the mean free paths. The effect of lanthanum addition on the thermal conductivity of YSZ ceramics has been interpreted in several ways, from the micro to the macroscale. One interpretation concerns the substitution effect of a constituent atom. Substitution of Zr by La has been suggested to reduce the phonon mean free path, even for the same fluorite structure, an effect ascribed to the different ionic radii and masses [12,18]. Adding lanthanum oxide also induces the formation of a pyrochrore phase, which scatters the phonon as a heterophase [19,20]. This phase was identified in our materials.
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Fig. 7. Comparison of thermal conductivity and bending strength of 5 mol% lanthanum oxide added 3YSZ ceramics. Dense, conventionally fabricated porous, and superplastically foamed porous ceramics.
The formed pyrochrore phase contains defects that also play the role of phonon scattering centres. The addition of lanthanum oxide degrades the sinterability of YSZ ceramics, leading to the formation of mesoscopic pores. Macroscopic pores that form through the superplastic forming method should contribute to reducing the thermal conductivity [4,8]. The mechanical strengths and thermal conductivities of three kinds of 3YSZ ceramics with 5 mol% of lanthanum oxide added are illustrated in Fig. 7. The pyrochrore phase sometimes creates cracks at the boundary between the fluoride and pyrochrore in lanthanumdoped zirconia-based ceramics, decreasing the mechanical strength [20,21]. In the present case, the mechanical strength of 5La3YSZ was as low as 50 MPa, which is one-fifth the value for 8YSZ ceramics, although both types of ceramics contain the same 8 mol% total rareearth atom content. Conventionally fabricated porous ceramics with a porosity of 45% had an average mechanical strength of 10 MPa, which is only 20% that of the fully dense 5La3YSZ ceramics. Remarkably, the mechanical strength of the superplastically foamed porous ceramics having the same porosity was as high as 30 MPa, which corresponds to 60% of the value for the fully densified ceramics. As described in the previous section, the thermal conductivity of the superplastically formed porous ceramics decreased to 1 W/mK, which is 40% that of the fully dense one. Conventionally fabricated porous ceramics have thermal conductivities of about 1.1 W/mK, which is almost equivalent to that of the superplastically porous ceramics. In summary, the strength degradation was suppressed in superplastically porous ceramics having three times greater mechanical strength than conventionally fabricated ones. Both types of ceramics have comparable thermal insulation properties at the same porosity. Microstructural feature for conventional and superplastically porous ceramics were compared by SEM observation on the cross sections (Fig. 8). Open pores are predominant in the former while the closed pores are predominant in the latter. In contrast to conventional porous ceramics, the location and amount of the foam agents are controlled to make closed pores. During the pore expansion process utilising superplastic deformation, surface tension can make the pores spherical, reducing the stress concentration. Then, a strong pore wall network should form, resulting in the simultaneous realisation of strong pore walls and high
Fig. 8. Cross sectional view of 5 mol% lanthanum oxide added 3YSZ (5La3YSZ) ceramics sintered at 1600 1C for 8 h. Conventionally fabricated porous ceramics (porosity: 45%) and superplastically fabricated porous ceramics (porosity: 45%).
porosity. The high mechanical strengths of the latter are due to fully densified pore walls and closed pores. Superplastically foamed ceramics characteristically possess low thermal conductivities. Further investigation is needed to interpret this coupling of thermal insulation with high mechanical strength, taking into account the presumably lower gas pressure in the pores.
4. Conclusions Our innovated superplastically foaming method was adopted to make closed pore inclusive zirconia based ceramics. Lanthanum oxide was added to reduce the matrix thermal conductivity. The sintering and superplastic deformation was first conducted in a monoclinic or tetragonal YSZ dispersed with lanthanum oxide followed by solid solution to cubic phase. The resultant superplastically foamed porous ceramics with porosity of 45% showed only 40% of thermal conductivity compared with the fully densified ceramics with the same composition. This thermal conductivity was comparable to that of conventionally fabricated porous ceramics with the same composition and porosity. The superplastically foamed ceramics maintained 60% of mechanical strength, however, conventionally fabricated ceramics degraded to 20% of the full dense one.
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Acknowledgements This work was partially supported by the Canon Foundation. References [1] A. Kishimoto, T. Higashiwada, H. Asaoka, H. Hayashi, Adv. Eng. Mater. 8 (2006) 708–711. [2] T. Higashiwada, H. Asaoka, H. Hayashi, A. Kishimoto, J. Eur. Ceram. Soc. 27 (2007) 2217–2222. [3] Y. Hashida, H. Hayashi, A. Kishimoto, J. Jpn. Soc. Powder Powder Metall. 54 (2007) 740–743. [4] M. Hanao, H. Hayashi, A. Kishimoto, J. Jpn. Soc. Powder Powder Metall. 54 (2008) 732–737. [5] A. Kishimoto, M. Obata, K. Waku, H. Hayashi, Ceram. Int. 35 (2009) 1441–1445. [6] M. Obata, A. Kishimoto, H. Hayashi, J. Alloys Compd. 471 (2009) 32–35. [7] H. Yamaoka, H. Hayashi, A. Kishimoto, J. Ceram. Soc. Jpn. 117 (2009) 1233–1235. [8] A. Kishimoto, T. Nakagawa, T. Teranishi, H. Hayashi, Mater. Sci. Forum 735 (544–545) (2013) 109–112.
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[9] T. Narushima, T. Goto, Y. Iguchi, T. Hirai, J. Am. Ceram. Soc. 74 (1991) 2583–2586. [10] O. Unal, T.E. Mitchell, A.H. Heuer, J. Am. Ceram. Soc. 77 (1994) 984–992. [11] D.E. Wolfe, J. Singh, R.A. Miller, J.I. Eldridge, T. Zhu, Surf. Coat. Technol. 194 (2005) 31–35. [12] J. Wu, X. Wei, N.P. Padture, G. Klemens, M. Gell, E. Garcia, P. Miranzo, M.I. Osendi, J. Am. Ceram. Soc. 85 (2002) 3031–3035. [13] J.R. Nicholls, K.J. Lawson, A. Johnstone, D.S. Rickerby, Surf. Coat. Technol. 151– 152 (2002) 383–391. [14] K.S. Lee, K.I. Jung, Y.S. Heo, T.W. Kim, Y.G. Jung, U. Paik, J. Alloys Compd. 507 (2010) 448–455. [15] M. Matsumoto, T. Kato, N. Yamaguchi, D. Yokoe, H. Matsubara, Surf. Coat. Technol. 203 (2009) 2835–2840. [16] M. Matsumoto, K. Aoyama, H. Matsubara, K. Takayama, T. Banno, Y. Kagiya, Y. Sugita, Surf. Coat. Technol. 194 (2005) 31–35. [17] S. Tekeli, T.J. Davies, Mater. Sci. Eng. A 297 (2001) 168–175. [18] R. Mevrel, J. Laizet, A. Azzopardi, B. Leclercq, M. Poulain, O. Lavigne, D. Demange, J. Eur. Ceram. Soc. 24 (2004) 3081–3089. [19] J. Xiang, S. Chen, J. Huang, H. Zhang, X. Zhao, Ceram. Int. 38 (2012) 3607–3612. [20] B.K. Jang, S. Kim, Y.S. Oh, H.T. Kim, Y. Sakka, H. Murakami, J. Ceram. Soc. Jpn. 119 (2011) 929–932.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Maintaining the mechanical strength of La-, Y-co-substituted zirconia porous ceramics through the superplastically foaming method Akira Kishimoto n, Masanori Okada, Takashi Teranishi, Hidetaka Hayashi Division of Chemistry and Biotechnology, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2013 Received in revised form 10 May 2013 Accepted 28 May 2013 Available online 14 June 2013
The superplastically foaming method was adopted to make closed-pore inclusive zirconia-based ceramics. Lanthanum oxide was added to monoclinic or tetragonal yttria-stabilised zirconia to reduce the thermal conductivity of the matrix. Sintering and superplastic deformation led to a solid solution and transformation to the cubic phase. The resulting superplastically foamed porous ceramics having a porosity of 45% had only 40% of the thermal conductivity of the fully densified ceramics having the same composition. This value was comparable to that of conventionally fabricated porous ceramics with the same composition and porosity. The superplastically foamed ceramics had 60%, while conventionally fabricated ceramics had only 20%, of the mechanical strength of the fully dense ceramics. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zirconia Porous ceramics Thermal insulator Superplasticity
1. Introduction We have previously reported the fabrication of porous ceramics by introducing pores after full densification of the matrix [1–8]. In that method, granular foaming agents were arranged at regular intervals in the powdered matrix and then the mixture was compacted. After densification of the matrix, the pores expanded by utilising the superplastic deformation driven by the gas pressure evolved from the foaming agents [9]. We called this novel process “superplastically foaming method”. Almost all the internal pores are closed pores surrounded by fully densified pore walls. Consequently, high mechanical strength, gastightness and good thermal insulation are expected and indeed have already been demonstrated to a certain extent. These properties contrast with conventional porous ceramics fabricated through a partial sintering process. Porous ceramics containing high numbers of closed pores are promising as high-temperature gastight thermal insulators. The foaming of the zirconia-based ceramics is believed to arise from the swelling of gas phases formed from the active oxidation of silicon carbide [9] as SiC þ O2 ðgÞ-SiOðgÞ þ COðgÞ This reaction has proved to occur under conditions determined by the oxygen partial pressure and temperature [9]. When the n
Corresponding author. Tel./fax: +81 86 251 8069. E-mail address: [email protected] (A. Kishimoto).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.084
amount of gas evolution is constant, the degree of pore expansion depends on the superplastic deformation rate and limit. To improve the porosity of superplastically foamed ceramics, some dispersoids such as silica [2,8] and spinel [6] are added into the matrix. However, these dispersoids can sometimes negatively affect other properties of the porous ceramics. We have previously fabricated superplastically foamed silicadispersed 3YSZ (3 mol% yttria-stabilised zirconia) porous ceramics. The thermal conductivity of one superplastically foamed ceramic having a porosity of 25% was 1.7 W/mK, which is about half of the 3.5 W/mK value for the fully dense case [4]. However, the silicadispersed 3YSZ had a somewhat greater thermal conductivity than 3YSZ without dispersoids, which was due to silica present at the gain boundaries. Zirconia-based ceramics show promise as thermal barrier coatings [10–12]. Various additives have been examined [12–15], among which lanthanum oxide has proven to be particularly effective at improving the thermal insulation properties because it enhances phonon scattering while concurrently suppressing the sinterability [15,16]. As a result, lanthanum oxide added zirconia ceramics show smaller mechanical strength compared with the other oxide added ones [16]. Further degradation of mechanical strength was a serious problem for practical use when incorporating pores to reduce the thermal conductivity. In the present study, we employed lanthanum oxide as an additive to make porous YSZ-based ceramics suitable as thermal insulators. At this time, our innovated superplastically foaming method was adopted to maintain the mechanical strength. The added lanthanum ions substituted for zirconium ones and
A. Kishimoto et al. / Materials Science & Engineering A 581 (2013) 98–103
transformed the YSZ ceramic from a tetragonal to a cubic structure having lower thermal conductivity. The cubic phase, however, has been shown to have relatively poor superplastic deformation, resulting in lower porosity in superplastically foamed ceramics [3,17]. To overcome this drawback, our innovative poststabilisation method was developed, in which sintering and superplastic deformation was first conducted on a monoclinic or tetragonal YSZ structure containing dispersed lanthanum oxide, followed by formation of a solid solution having the cubic phase with its lower thermal conductivity.
99
ions. In one case, lanthanum oxide was dissolved in yttriastabilised-zirconia through calcining, crushing, grinding and then compacting and heating. In the other approach, lanthanum oxide was simply mixed with the powder before compaction; in this case, a solid solution formed during or after the sintering/foaming process. By comparing these two types of processing, the influence of the existing form of lanthanum ion on the superplastic deformation was evaluated.
2.2. Fabrication of porous ceramics 2. Experimental procedure 2.1. Fabrication of mono-foam To determine the conditions under which ceramic foaming could be achieved, macroscopic single foams were fabricated using a method similar to that previously reported [1–6]. In short, ß-SiC (AZmax Co., Ltd, Ichihara, Chiba, Japan) as foaming agent was dispersed into an aqueous solution containing 10 wt% methyl cellulose with a solid/liquid weight ratio of 0.10. This SiC slurry was spread on a polyethylene sheet using a glass bar to adjust the thickness. Disk-like pieces of this foaming agent about 12 mm in diameter were obtained after drying. Commercial zirconia without additives, and 2-, 3- and 6-mol %-yttria-stabilised zirconia (TZ-0, TZ-2Y, TZ-3Y, TZ-6Y; Tosoh, Tokyo, Japan) were used as matrices. Predetermined amounts of silica (Snow Tex S; Nissan Chemical, Tokyo, Japan) or lanthanum oxide (Kojundo Chemical Co., Ltd, Tokyo, Japan) were added to make the powder mixtures. Samples (ca. 4 g) of each powder mixture were weighed. Half of the powder mixture, discs of SiC foaming agent, and the remaining half of the powder mixture were placed into a 20-mm-diameter steel die, pressed uniaxially at 15 MPa for 1 min and then hydrostatically at 124 MPa. The resulting powder compacts were heated to 1600 1C for 8 h in air and then cooled. Fabrication procedures are illustrated in Fig. 1 together with the following superplastically foamed porous ceramics. The commercial zirconia matrix powder contained dissolved yttrium ions. We investigated two ways to introduce lanthanum
Porous ceramics containing numerous pores were fabricated according to a previously reported method [4,5]. Composite granules that were composed of the ß-SiC foaming agent surrounded by the matrix powder were prepared. Superplastically foamed porous ceramics were obtained after compaction and sintering. Specifically, 15 wt% of ß-SiC powder was dispersed in an aqueous solution containing 1 wt% of methylcellulose to provide a low-viscosity slurry. After spraying this slurry on the matrix powder and then ball-milling, the resulting mixture provided composite granules with a primary core size of 15 μm. The granules were placed in a 15-mm-diameter steel die and pressed uniaxially at 31 MPa for 1 min and then hydrostatically at 124 MPa. The resulting powder compacts were heated at 1600 1C for 8 h in air. Conventional porous ceramics were prepared, and their porosity was controlled by compaction pressure or by the addition of different amounts of carbon as the pore foaming agent. The heating scheme used for the composite granules was also used for these reference materials [7].
2.3. Evaluation The external dimensions of a mono-foam such as its foam height [1] and foam diameter were measured using a calliper. The apparent density of the outer shell and the entire ceramic foam was measured using Archimedes' method with water as the medium. The porosity of the ceramic foam was estimated from
Fig. 1. Schematic illustration of superplastically foamed ceramics. Mono-foam (upper) and porous ceramics (lower).
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the apparent density of the ceramic foam and the relative density of the outer shell. The apparent densities of the superplastically foamed porous ceramics were similarly measured by Archimedes' method using water. Cross sections were observed by scanning electron microscopy (SEM; S-4300; Hitachi, Ltd, Tokyo, Japan). The formed crystalline phases were identified by X-ray diffraction (XRD; Multiflex; Rigaku Co., Tokyo, Japan). Thermal diffusivity was measured by the laser flash method (LFA447 NanoFlash; Bruker AX, Tokyo, Japan), which provided the thermal conductivity from the specific heat and density.
3. Results and discussion The effect of lanthanum oxide addition on superplastic deformation was examined in detail. Changing the composition of the matrix while the amount of the foaming agent remained constant affected the height of the mono-foam. The relationship between foam height and the kind of dispersoid is shown in Table 1. With lanthanum oxide addition, pore expansion was more dominant during the process of sintering/foaming followed by solid solution formation than in the process of solid solution formation followed by the inverse process. Fig. 2 shows side views of both mono-foams. The first case (right side) clearly shows the better expansion, indicating that the first process is more favourable for obtaining highly porous ceramics. The crystalline phase transformed from tetragonal to cubic after formation of the solid state solution of lanthanum oxide with 3YSZ. The superplastic deformation speed of the cubic phase in zirconia-based ceramics has been reported to be lower than that of the tetragonal phase [8,10,11]. In the present case, lanthanum oxide particles remained as dispersoids after full densification because of the slow rate of the solid state reaction, which facilitated sliding of the grain boundaries during the foaming or pore-expansion process. At the same time, the crystalline phase remained tetragonal because of incomplete formation of the solid solution, which also contributed to Table 1 Matrix composition dependence of foam height of a 3YSZ based mono-foam fabricated through sinter/foaming at 1600 1C for 8 h. Composition of matrix
Foam height (mm)
3YSZ 10 mol% Silica added 3YSZ 5 mol% Lanthanum oxide added 3YSZ Silica and lanthanum oxide added 3YSZ
4.66 8.83 7.03 4.11
Fig. 2. Side view of the superplastically foamed 5 mol% lanthanum oxide added 3 mol% yttria stabilized zirconia (mono-foam) fabricated through (left) solid solution followed by supreplastically foaming (right) superplastically foaming followed by solid solution.
enhancing the superplastic deformation. We called this the “poststabilisation method”, in which sintering and superplastic deformation were first conducted in a monoclinic or tetragonal YSZ dispersed with lanthanum oxide, then followed by forming a solid solution to transform the ceramic into the cubic phase. Silica is commonly used to enhance superplastic deformation in zirconia. In this work, lanthanum oxide led to comparable foam heights and porosities for the same addition level. Co-dispersion of silica and lanthanum oxide, however, reduced the deformation. We also studied the effect of different amounts of lanthanum oxide on yttria-stabilised zirconia. Fig. 3 shows the relationship between the amount of stabilising atoms added and the porosity in the mono-foam ceramics. Lanthanum oxide was added to 2-, 3- and 6-mol% yttria-stabilised zirconia powders (2Y, 3Y and 6Y, respectively) or zirconia with no additive (0Y). In Fig. 3, the total amounts (mol%) of yttrium and lanthanum atoms are indicated in the abscissa. Cracks formed in the 0Y and 6Y ceramics without added lanthanum oxide, resulting in no pore expansion. These results indicated that the monoclinic (0Y) or cubic (6Y) zirconia without hetero-additives showed only small superplastic deformations compared with tetragonal (2Y or 3Y) ones. Monoclinic zirconia (0Y) to which lanthanum oxide was added expanded only slightly although it transformed into a cubic phase via a tetragonal one. The grain growth would have proceeded in the monoclinic phase, leading to a small deformation in the transitional tetragonal phase because of the larger grain size. SEM photos of the surfaces of superplastically foamed 3YSZbased ceramics with and without lanthanum oxide are shown in Fig. 4. Pore walls were dense and the grain sizes were smaller when lanthanum oxide was present; both properties helped to improve the superplastic deformation. XRD patterns are shown in Fig. 5 for 3YSZ-based mono-foam ceramics dispersed with 5 mol% of lanthanum oxide with a holding time of 1 and 8 h. The original tetragonal phase remained at a holding time of 1 h, while the cubic phase predominated after 8 h as zirconium ions were replaced by lanthanum ions. The foam height changed from 5.8 mm at 1 h to 7.0 mm at 8 h, indicating that most of the foam had been formed by 1 h of holding time; the remaining tetragonal phase contributed to increasing the foam height until 8 h.
Fig. 3. Porosity of the superplastically foamed YSZ based mono-foam. Lanthanum oxide was added into 0, 2, 3, or 6 mol% yttria stabilized zirconia. Total rare earth contents are taken as the abscissa.
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Fig. 5. XRD profiles of 5 mol% lanthanum oxide added 3 mol% yttria stabilized zirconia heat treated at 1600 1C for 1 h and for 8 h.
Fig. 6. Thermal conductivity of the YSZ based dense and superplastically foamed porous ceramics. Lanthanum oxide was added into 0, 2, 3, or 6 mol% yttria stabilized zirconia. Total rare earth contents are taken as the abscissa.
was much lower than when added to 3YSZ, indicating the strong effect of co-doping on decreasing the thermal conductivity. Cracks are often reported in lanthanum oxide-doped YSZ [15,16]. However, the examples described here were gastight. Thermal conductivity k is expressed by the following equation: k¼ Fig. 4. SEM photos of zirconia based ceramics sintered at 1600 1C for 8 h. 3 mol% yttria stabilized zirconia (3YSZ), 5 mol% lanthanum oxide added 3YSZ (5La3YSZ), 10 mol% silica added 3YSZ (Silica added 3YSZ).
The effect of lanthanum addition on the thermal conductivity of the matrix and porous YSZ-based ceramics was examined. The results are plotted in Fig. 6. Adding lanthanum oxide transformed monoclinic zirconia into a tetragonal or cubic phase which showed only a small volume change on cooling. The thermal conductivities of the ceramics were large, up to 3.5 W/mK, when only lanthanum oxide was added. However, co-addition of lanthanum oxide into 2YSZ or 3YSZ led to a drastic decrease in the thermal conductivities (i.e. down to 2 W/mK, about two-thirds that of the original YSZ ceramics). With the same addition amount of 3 mol%, the thermal conductivity of 1 mol% of lanthanum oxide added to 2YSZ
1 lC p v; 3
where l is the phonon mean free path, Cp is the specific heat and v is the velocity of sound. The velocity of sound is strongly affected by the bonding quality of the matrix. Because the bonding was not degraded in our materials, any decreases in the matrix thermal conductivity were due to decreases in the mean free paths. The effect of lanthanum addition on the thermal conductivity of YSZ ceramics has been interpreted in several ways, from the micro to the macroscale. One interpretation concerns the substitution effect of a constituent atom. Substitution of Zr by La has been suggested to reduce the phonon mean free path, even for the same fluorite structure, an effect ascribed to the different ionic radii and masses [12,18]. Adding lanthanum oxide also induces the formation of a pyrochrore phase, which scatters the phonon as a heterophase [19,20]. This phase was identified in our materials.
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Fig. 7. Comparison of thermal conductivity and bending strength of 5 mol% lanthanum oxide added 3YSZ ceramics. Dense, conventionally fabricated porous, and superplastically foamed porous ceramics.
The formed pyrochrore phase contains defects that also play the role of phonon scattering centres. The addition of lanthanum oxide degrades the sinterability of YSZ ceramics, leading to the formation of mesoscopic pores. Macroscopic pores that form through the superplastic forming method should contribute to reducing the thermal conductivity [4,8]. The mechanical strengths and thermal conductivities of three kinds of 3YSZ ceramics with 5 mol% of lanthanum oxide added are illustrated in Fig. 7. The pyrochrore phase sometimes creates cracks at the boundary between the fluoride and pyrochrore in lanthanumdoped zirconia-based ceramics, decreasing the mechanical strength [20,21]. In the present case, the mechanical strength of 5La3YSZ was as low as 50 MPa, which is one-fifth the value for 8YSZ ceramics, although both types of ceramics contain the same 8 mol% total rareearth atom content. Conventionally fabricated porous ceramics with a porosity of 45% had an average mechanical strength of 10 MPa, which is only 20% that of the fully dense 5La3YSZ ceramics. Remarkably, the mechanical strength of the superplastically foamed porous ceramics having the same porosity was as high as 30 MPa, which corresponds to 60% of the value for the fully densified ceramics. As described in the previous section, the thermal conductivity of the superplastically formed porous ceramics decreased to 1 W/mK, which is 40% that of the fully dense one. Conventionally fabricated porous ceramics have thermal conductivities of about 1.1 W/mK, which is almost equivalent to that of the superplastically porous ceramics. In summary, the strength degradation was suppressed in superplastically porous ceramics having three times greater mechanical strength than conventionally fabricated ones. Both types of ceramics have comparable thermal insulation properties at the same porosity. Microstructural feature for conventional and superplastically porous ceramics were compared by SEM observation on the cross sections (Fig. 8). Open pores are predominant in the former while the closed pores are predominant in the latter. In contrast to conventional porous ceramics, the location and amount of the foam agents are controlled to make closed pores. During the pore expansion process utilising superplastic deformation, surface tension can make the pores spherical, reducing the stress concentration. Then, a strong pore wall network should form, resulting in the simultaneous realisation of strong pore walls and high
Fig. 8. Cross sectional view of 5 mol% lanthanum oxide added 3YSZ (5La3YSZ) ceramics sintered at 1600 1C for 8 h. Conventionally fabricated porous ceramics (porosity: 45%) and superplastically fabricated porous ceramics (porosity: 45%).
porosity. The high mechanical strengths of the latter are due to fully densified pore walls and closed pores. Superplastically foamed ceramics characteristically possess low thermal conductivities. Further investigation is needed to interpret this coupling of thermal insulation with high mechanical strength, taking into account the presumably lower gas pressure in the pores.
4. Conclusions Our innovated superplastically foaming method was adopted to make closed pore inclusive zirconia based ceramics. Lanthanum oxide was added to reduce the matrix thermal conductivity. The sintering and superplastic deformation was first conducted in a monoclinic or tetragonal YSZ dispersed with lanthanum oxide followed by solid solution to cubic phase. The resultant superplastically foamed porous ceramics with porosity of 45% showed only 40% of thermal conductivity compared with the fully densified ceramics with the same composition. This thermal conductivity was comparable to that of conventionally fabricated porous ceramics with the same composition and porosity. The superplastically foamed ceramics maintained 60% of mechanical strength, however, conventionally fabricated ceramics degraded to 20% of the full dense one.
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