- Email: [email protected]
Macroporous alumina structures tailored by freeze-casting using naphthalene–camphor as freezing vehicle
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Macroporous alumina structures tailored by freeze-casting using naphthalene–camphor as freezing vehicle Lucas D. Lacerdaa, Douglas F. Souzab, Eduardo H.M. Nunesc, Manuel Houmardd,
⁎
a
Department of Mechanical Engineering, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 1, Belo Horizonte, MG CEP: 31270-901, Brazil b National Institute of Industrial Property – INPI, Rua Mayrink Veiga, 9 - Centro, Rio de Janeiro, CEP: 20090-910, Brazil c Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 2, Belo Horizonte, MG CEP: 31270-901, Brazil d Department of Materials Engineering and Civil Construction, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 1, Belo Horizonte, MG CEP: 31270-901, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Freeze-casting B. Porosity C. Mechanical properties D. Al2O3 E. Macroporous structures
Freeze-casting is a promising technique for fabricating macroporous ceramics because it is an environmentally friendly, cost effective, and easy scale-up method. Several freezing vehicles have been used in freeze-casting, including water, camphene, tert-butyl alcohol, urea, and naphthalene-camphor (Naph-Camp). This work focuses on the preparation of freeze-cast alumina samples using different Naph-Camp compositions as freezing vehicle, alumina loading ranging from 20 to 40 vol% and various freezing conditions. It was observed that macroporous materials with different pore structures and mechanical behaviors can be obtained by changing the Naph-Camp solvent composition. Moreover, the freezing route also showed a great effect on these properties, besides allowing the preparation of samples with oriented pores. These are important findings because naphthalene and camphor are widely available, have low toxicity, and show an easier sublimation than water, which has been commonly used as the freezing vehicle in many works. The range of pore structures and mechanical strengths obtained in this study demonstrates the versatility of the processing route used herein, which could be used to obtain samples for several applications, including catalysis, fluids filtration, and bioengineering. This study is supported by a series of experimental characterizations, including optical microscopy, scanning electron microscopy, Archimedes measurements, and cold crushing tests.
1. Introduction Ceramic materials with tailored macroporous structures have been used in several applications, including biomaterials, catalysis, phase separation, and thermal insulation [1–5]. It has been reported that the wide use of these materials is strongly associated with properties arising from a controlled substitution of a solid phase by pores [6]. Such properties may include high specific surface area, low density, low thermal conductivity, and high anisotropic mechanical strength. Among the different techniques used to produce macroporous ceramics, the freeze-casting method is a promising approach because it is an environmentally friendly, cost effective, and easy scale-up method [7]. In addition, freeze-cast materials usually show a highly-interconnected pore structure, which gives rise to solids with improved mass transport capacities [8]. A range of freezing vehicles has been used in freeze-casting,
⁎
including water [9,10], camphene [11,12], tert-butyl alcohol [8,13], urea [14], and naphthalene-camphor (Naph-Camp) [15]. Water is obviously an interesting solvent since it is cost effective, versatile, and non-toxic [16]. However, water solidification is carried out at temperatures below 0 °C and its sublimation is performed under reduced pressures, which increases the process complexity due to the need of specific equipments. In agreement to a wide literature in the area, camphene is also an interesting option to be used as solvent for fabricating macroporous materials by freeze-casting due to is low-toxicity and easy sublimation under ambient conditions [17,18]. For these reasons, camphene have been used to produce macroporous structures for biomedical application for instance [19]. Nevertheless, since camphene has a dendritic growth during its solidification [20], the porous structures obtained with this solvent often show dead-ended pores which could decrease the material performance in some applications. To reduce the presence of such inadequate pores, Liu et al. [21]
Corresponding author. E-mail address: [email protected] (M. Houmard).
https://doi.org/10.1016/j.ceramint.2018.06.036 Received 25 April 2018; Received in revised form 4 June 2018; Accepted 5 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lacerda, L.D., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.06.036
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freeze-cast alumina samples was evaluated. The alumina loading in the starting slurry was changed from 20 to 40 vol%. The freezing step was carried out using three different routes. The first route was performed by keeping samples at room temperature (≈ 25 °C) for obtaining a slow cooling rate of Naph-Camp. The second one was carried out by keeping samples in a domestic freezer (≈ −17 °C). The third one was performed by pouring the slurries on a liquid nitrogen-cooled cold support (≈ −196 °C), which led to fast cooling rates. After sintering, the as-prepared samples were examined by scanning electron microscopy (SEM) and Archimedes tests. Cold crushing tests were also carried out at room temperature. 2. Materials and methods CT3000-SG (99.8 wt% / Almatis Brasil) was used as the starting alumina powder. According to the supplier data, the mean particle size is 0.76 µm with D50 and D90 respectively about 0.4 and 1.7 µm. Texaphor 963 (Cognis, Southampton Hampshire, UK) was employed as the dispersing agent, whereas Naph-Camp mixtures were used as the freezing vehicle. Both naphthalene and camphor were obtained from Aldrich and show purity above 96 wt%. As aforementioned, hypoeutectic (60 wt% Camp), eutectic (65 wt% Camp), and hypereutectic (70 wt% Camp) compositions were prepared herein. These compositions are highlighted in Fig. 1, which displays the phase diagram of the Naph-Camp system. Naph-Camp mixtures with these compositions were prepared at 70 °C and then solidified by vapor deposition on transparent glass sheets as follows. The glass sheets were partially dipped into Naph-Camp mixtures kept at 70 °C. Thus, one side of the glass sheet was kept at 70 °C, whereas the other side was maintained at a temperature near the room temperature (25 °C). This temperature gradient led to a directional solidification of the Naph-Camp crystals from the hot side to the cold one. After their complete solidification, the NaphCamp crystals were examined by optical microscopy (OM) with an Olympus CH30 microscope operating in the transmission mode. Besides, it was also evaluated the time needed for the complete sublimation of Naph, Camp, and the aforementioned mixtures. This procedure was carried out by preparing 20 mL of these materials at 70 °C and pouring them on glass substrates. These samples were then kept at room temperature in a fume hood and their masses were registered as a function of time. Freeze-cast alumina samples were prepared as follows. Solutions of Naph-Camp with Texaphor as dispersant agent were initially prepared at 70 °C. The alumina CT3000-SG was subsequently added under vigorous stirring to the solution, and the as-prepared slurry was kept under sonication for additional 15 min to break the ceramic agglomerates. As mentioned before, the alumina loading was varied from 20 to 40 vol%. The dispersant concentration in the slurries was kept at 2 wt% of the alumina loading. The as-obtained slurries were then poured into cylindrical acrylic molds (diameter of 12.2 mm and height of 15 mm) and frozen by different routes. The first freezing route was carried out by keeping samples at ambient temperature, whereas the second one was performed storing them in a domestic freezer kept at − 17 °C. In this second case, since the bottom side of the molds was close to the freezer surface, it reached faster the freezer temperature, inducing a slight vertical temperature gradient during the solidification step. The third freezing route was carried out by pouring the starting slurries into a mold placed on the top of a liquid nitrogen-cooled cold support made of cooper. Thus, the ceramic suspensions were fast cooled from the bottom side to the top one due to the high vertical temperature gradient. The sublimation of Naph-Camp solvent was performed by keeping samples in ambient conditions in a fume hood for times as long as 12 days. The time needed for sublimating each Naph-Camp composition used herein was determined on the basis of the sublimation tests aforementioned. After the sublimation of Naph-Camp, the green bodies were heat-treated in atmospheric furnace at 1500 °C for 2 h. The heating and cooling rates were kept constant at 2 and 10 °C min−1,
Fig. 1. Phase diagram of the Naph-Camp system [15]. The compositions and solidification paths used in this work are highlighted.
Fig. 2. Sublimed mass fraction at room temperature as a function of time for Naph, Camp, and Naph-Camp mixtures. The dashed line represents the time needed to reach a 50% mass loss. The solid lines are used as a guide to the eyes only.
illustrated that the use of annealing treatment and oriented freezing direction can be realized. Thus, as reported elsewhere [15], Naph-Camp is a promising freezing vehicle for the fabrication of macroporous structures. First, such solvents present low viscosity, which allows preparing concentrated slurries to access to denser materials. Moreover, Naph-Camp experiences a shrinkage upon freezing, leading to dense and resistant green bodies. It is also worth stressing the expressive melting temperature and large vapor pressure of Naph-Camp mixtures, which allows freezing and sublimating ceramic slurries under milder condition than when water is used as the freezing vehicle. This behavior allows fabricating Naph-Camp-based samples at ambient conditions. In the work of Araki and Halloran [15], macroporous structures have been fabricated by freeze-casting using different Naph-Camp compositions. They showed that the pore morphology can be tailored by changing the solvent composition. However, such observation was performed for samples with low porosity prepared by only one freezing route. Thus, a systematic study concerning the influence of the freeze-casting parameters on the pore structure of Naph-Camp derived samples is still lacking. In this work we prepared freeze-cast alumina samples using NaphCamp as the freezing vehicle. Hypoeutectic, eutectic, and hypereutectic compositions of Naph-Camp were tested herein. The effect of these compositions on the porous structure and mechanical behavior of 2
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
Fig. 3. Top line: OM images of the crystals obtained by solidifying Naph-Camp mixtures on a glass substrate with (a) hypoeutectic, (b) eutectic, and (c) hypereutectic compositions. Bottom line: SEM micrographs of freeze-cast alumina samples prepared using (d) hypoeutectic, (e) eutectic, and (f) hypereutectic Naph-Camp compositions, frozen in a domestic freezer and prepared keeping the solid loading at 30 vol%. The scale bars shown in the OM micrographs represent 1 mm, whereas those ones displayed in the SEM micrographs correspond to 500 µm. The direction of the freezing orientation is indicated for reference purposes.
Fig. 4. SEM micrographs of freeze-cast samples obtained with hypoeutectic, eutectic and hypereutectic compositions. These materials were frozen at ambient condition, in freezer, or using liquid nitrogen. For samples frozen in freezer or using liquid nitrogen, the freezing direction was vertically oriented from the bottom to the top side of the micrographs. The alumina loading in these materials was kept constant at 30 vol%. The scale bars shown in these images correspond to 500 µm. 3
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freezing route employed in the fabrication step. Such observation is probably due to an isotropic shape of the Naph-Camp crystals obtained during the freezing step. Once again, these alumina structures are closely related to the morphology of the crystals shown in Fig. 3a to c. For instance, the eutectic composition leads to continuous thin plate-shaped crystals aligned according to the freezing direction (Fig. 3b). As aforementioned, the pore network of freeze-cast materials is a direct replica of the freezing vehicle. Therefore, one could expect that freezecast samples obtained from the hypoeutectic and eutectic Naph-Camp mixtures should exhibit ordered pore networks following the freezing direction. Fig. 5 gives the mean pore size of the smallest dimension of the pore shape, i.e. the rod diameter for hypoeutectic pores for instance, measured from SEM micrographs of alumina samples prepared under different conditions. In a general way, the hypoeutectic Naph-Camp composition gave rise to materials with the largest pore sizes, followed by those obtained from hypereutectic and eutectic mixtures, respectively. In addition, for hypoeutectic and eutectic Naph-Camp compositions, it can be inferred that the faster the freezing rate, the finer the pore structure. This behavior is strongly related to the nucleation and growth of Naph-Camp crystals during the freezing step. It is well established that nucleation is favored over growth for fast freezing rates [25]. As a consequence, fast freezing rates lead to small Naph-Camp crystals and, consequently, to fine pore structures. On the other hand, slow freezing rates favor the growth of Naph-Camp crystals and give rise to large pores in the freeze-cast material. Moreover, higher solid
respectively. SEM was carried out using an Aspex Explorer microscope operating at a 20 kV accelerating voltage. The samples used in SEM microscopy were sputter-coated with a 10 nm-thick gold film. Cold crushing tests were performed at room temperature, in a direction parallel to the the freezing direction, using a universal testing machine operating at a cross-head displacement speed of 0.5 mm min−1. At least five samples were taken into account for each batch. 3. Results and discussion Fig. 2 exhibits the sublimed mass fraction as a function of time for Naph, Camp, and Naph-Camp mixtures. The dashed line represents the time needed for reaching a 50% mass loss. One observes that camphor showed the fastest sublimation kinetics, whereas naphthalene exhibited the slowest one. For instance, about 165 h are needed for reaching a 50% mass loss for Naph. On the other hand, 30 h are sufficient for sublimating the same amount of Camp. Among the Naph-Camp compositions tested herein, the hypereutectic one displayed the fastest sublimation kinetics, followed by the eutectic and hypoeutectic mixtures, respectively. This finding reveals that the smaller the Camp concentration, the slower the sublimation kinetics of the Naph-Camp mixture. As discussed before, the sublimation times of the Naph-Camp compositions used in this work were determined from the data shown in Fig. 2. Images 3a to 3c were collected by OM from the solvent crystals obtained after freezing Naph-Camp mixtures with different compositions. It can be clearly observed that the morphology of the solidified crystals is strongly dependent of the Naph-Camp composition. The hypoeutectic composition gave rise to rod-shaped crystals with no preferential orientation (Fig. 3a). The eutectic composition led to continuous thin plates oriented according to the freezing direction (Fig. 3b). The hypereutectic mixture gave rise to a solid phase composed by sphere- and thin rod-shaped crystals (Fig. 3c). It can be inferred from the phase diagram shown in Fig. 1 that the proeutectic Naph-Camp crystals obtained from the hypoeutectic composition are ascribed to a Naph-rich phase (α). The α phase should represent about 8.5 wt% of the solidified solvent whereas the eutectic constituent should correspond to 91.5 wt%. In the case of the hypereutectic mixture, the proeutectic Naph-Camp crystals formed are associated with a Camp-rich proeutectic phase (β) which should represent about 20 wt% of the solid crystals whereas about 80 wt% should result from the eutectic transformation. However, no proeutectic crystals were observed for either the hypoeutectic or hypereutectic compositions after the solidification step (Fig. 3a and c); only crystals with different morphologies were observed when the Naph-Camp composition was modified. Thus, as the pore network of freeze-cast materials is a direct replica of the freezing vehicle [22–24], one could suggest that ceramic samples with distinct porous structures should be obtained by using different Naph-Camp mixtures. Indeed, freeze-cast samples with different pore frameworks were obtained when such compositions were employed in their preparation (Fig. 3d to f). Moreover, the pores morphology, especially the rod- and thin plate-shaped pores observed respectively for the hypoeutectic and eutectic compositions (Fig. 3d and e), correspond well to the shape of the Naph-Camp crystals observed in Fig. 3a and b. This finding reveals the versatility of the Naph-Camp system because materials with different pore structures can be prepared by changing its composition. Fig. 4 depicts SEM micrographs of freeze-cast materials prepared under different conditions keeping the alumina loading at 30 wt%. As it was expected, no pore orientation was observed for the samples frozen at room temperature. Nonetheless, for hypoeutectic and eutectic compositions, the pore structure of these materials started to show a vertical orientation parallel to the freezing direction when the freezing step was carried out either in a domestic freezer or using liquid nitrogen. On the other hand, no preferential pore orientation was noted for materials prepared from a hypereutectic Naph-Camp composition, regardless the
Fig. 5. Mean pore size of freeze-cast alumina samples as a function of the alumina loading for the (A) hypoeutectic, (B) eutectic, and (C) hypereutectic compositions frozen under different conditions (ambient, freezer and liquid nitrogen). The pore sizes were measured from the SEM micrographs. 4
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
loading leads to smaller pores size since it should inhibit the growth of the Naph-Camp crystals. However, in the case of the hypereutectic composition, it seems that the freezing condition and solid loading show no influence on the crystals growth, and consequently on the pores size. Materials with thin and interconnected pores, as the ones present in the oriented eutectic structures, could be interesting for filtration or phase separation applications, whereas structures with higher pore size, as the ones from hypoeutectic mixture, could be used for medical applications as a biomaterial. Fig. 6 exhibits the open and closed porosities for freeze-cast samples obtained under different conditions. The total porosity can be inferred from the open and closed porosities. One observes that the higher the alumina loading, the smaller the total porosity. This result is in agreement with the fact that the pore structure of freeze-cast materials is obtained upon Naph-Camp sublimation. Thus, it is expected that the larger the solid loading, the lower the sample porosity. This behavior is also observed in the SEM micrographs displayed in Fig. 7, which shows the porous structures obtained from a hypoeutectic composition for different alumina loadings and freezing conditions. From Fig. 6, it is worth highlighting the expressive concentration of open pores in the samples prepared herein. This is an important finding since alumina samples with expressive open and interconnected porosities find application in many fields, including catalysis, biomaterials, and fluids filtration [26–28]. It can also be observed that in a general way, specimens obtained from a eutectic Naph-Camp composition, showed the
Fig. 6. Open and closed porosities of freeze-cast samples obtained using different alumina loadings and freezing routes. The total porosity can be inferred by summing the open and closed porosities. Data assessed from Archimedes tests.
Fig. 7. SEM micrographs of freeze-cast samples fabricated from a hypoeutectic Naph-Camp composition. The alumina loading in these materials are 20, 30 and 40 vol %, and the freezing conditions are ambient, freezer and liquid nitrogen. For both solidification in freezer and using liquid nitrogen, the freezing direction is from the bottom to the top side of the images. The scale bars shown in these micrographs correspond to 500 µm.
5
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freezing routes. As depicted in Figs. 4, 5 and 7, the faster the freezing rate, the finer the pore structure. Thus, such results are in agreement with Houmard [29] and Genet [30] who reported that the finer the pore structure, the higher the mechanical strength of a porous ceramic. According to these authors, thin pores show a lower concentration of stresses upon loading and also inhibit better the propagation of cracks. However, the eutectic structures, which have the lowest pore volume and the thinnest pores, are not as mechanically resistant as the hypoeutectic materials toward compression stresses. This behavior could be explained by the pore geometry. For instance, Tang et al. [13] showed, for structures produced by freeze-casting from alumina slurries containing different water and tert-butyl alcohol ratios, that the presence of lamellar pores results in a decrease of the mechanical strength in comparison to macroporous structures with hexagonal pores. In our case, on contrary to the eutectic samples, the hypoeutectic system with non-continuous rod-like pores could present higher compression resistance due to a crack propagation which not destroy entire areas bearing the applied stresses during the compression tests. Therefore, since the porous structure of hypereutectic samples does not depend of the freezing route used, the mechanical response for such materials is only related to the solid loading, and consequently to the pore volume. The findings reported herein demonstrate the versatility of the Naph-Camp system because materials with distinct pore structures and mechanical behaviors can be obtained by changing the Naph-Camp composition, solid loading, and freezing route. Both Naph and Camp show low toxicity and are widely available, which explains their expressive use in the pharmaceutical industry. Moreover, these chemicals show an easy sublimation under ambient conditions in opposition to water, which has been used as the freezing vehicle in many works available in the literature [31–37]. For instance, no freeze-dryer was used in this work, which makes the freeze-casting process simpler and cost effective. These features make Naph and Camp promising freezing vehicles to be employed for preparing macroporous samples by freeze casting.
Fig. 8. Cold crushing strength of freeze-cast alumina samples as a function of the total porosity for the (A) hypoeutectic, (B) eutectic, and (C) hypereutectic compositions frozen under different conditions (ambient, freezer and liquid nitrogen). The solid lines are used as a guide to the eyes only.
4. Conclusions Freeze-cast alumina samples were successfully obtained in this work using different Naph-Camp mixtures as the freezing vehicle, alumina loading ranging from 20 to 40 vol% and various freezing conditions. It was shown that materials with distinct pore structures and mechanical behaviors can be prepared by changing the Naph-Camp composition. This is an important finding because Naph and Camp exhibit low toxicity, wide availability, and an easier sublimation than water, which has been widely used as freezing vehicle in many works. The freezing route of alumina/Naph-Camp suspensions also demonstrated to have a great effect on both the pore structure and crushing strength of the samples prepared herein. It was noted that the faster the freezing rate, the smaller the porosity and the higher the mechanical strength of the freeze-cast materials for both hypoeutectic and eutectic Naph-Camp compositions. On the other hand, samples with larger pores were obtained by performing the freeze-casting step at room temperature. The range of pore sizes and mechanical strengths obtained in this study demonstrates the versatility of the processing route used herein, which could be used to obtain samples for applications ranging from catalysis and fluids filtration to biomaterials.
smallest total porosity for a same alumina loading. Such observation could result from a higher shrinkage of the eutectic solvent composition in comparison to the other ones during the solidification step which, in consequence, leads to a lower pore volume after the solvent sublimation. Besides, it appears that the freezing route has no effect on the volume fraction of pores. Indeed a nearly constant total porosity was observed for the alumina structures regardless the freezing route used in its preparation. For instance, a constant total porosity of about 50% was observed for the material obtained from the hypoeutectic slurry containing 30 vol% of alumina loading, regardless the freezing route used. On the other hand, as already observed in Figs. 4 and 5, Fig. 7 reveals that the freezing condition exhibited a great effect on the pore network structure. Freezing step under ambient conditions led to isotropic porous structures, whereas solidifications in freezer or using liquid nitrogen gave rise to pores oriented according to the freezing direction, where smaller pores were observed for fast freezing rates and high alumina loadings. Fig. 8 exhibits the cold crushing strength of the freeze-cast alumina samples obtained in this work when the stress is applied parallel to the freezing direction, i.e. in the same direction of the pores orientation. As expected, the higher the porosity, the lower the crushing strength. Moreover, for hypoeutectic and eutectic compositions, i.e. for samples showing a pore structure dependent of the freezing conditions, structures frozen in liquid nitrogen usually displayed the highest strength, followed by materials prepared in a domestic freezer and at room temperature, respectively. This behavior is strongly associated with the porous structure shown by the samples obtained from these three
Acknowledgements The authors thank Almatis Brasil for kindly providing the alumina powders used in this study. The authors thank the financial support from CNPq (471817/2013-9), FAPEMIG (APQ-00583-14), and PRPqUFMG (05-2016). Prof. Vasconcelos is acknowledged for the support to this research. The views and opinions of authors expressed herein do not state or reflect those of the INPI (Brazil). 6
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
References [20]
[1] H.T. Wang, X.Q. Liu, G.Y. Meng, Porous α-Al2O3 ceramics prepared by gelcasting, Mater. Res. Bull. 32 (1997) 1705–1712, http://dx.doi.org/10.1016/S00255408(97)00152-9. [2] S. Sokolov, D. Bell, A. Stein, Preparation and characterization of macroporous αalumina, J. Am. Ceram. Soc. 86 (2003) 1481–1486, http://dx.doi.org/10.1111/j. 1151-2916.2003.tb03500.x. [3] F. Tang, H. Fudouzi, Y. Sakka, Fabrication of macroporous alumina with tailored porosity, J. Am. Ceram. Soc. 86 (2003) 2050–2054, http://dx.doi.org/10.1111/j. 1151-2916.2003.tb03607.x. [4] E.C. Hammel, O.L.-R. Ighodaro, O.I. Okoli, Processing and properties of advanced porous ceramics: an application based review, Ceram. Int. 40 (2014) 15351–15370, http://dx.doi.org/10.1016/j.ceramint.2014.06.095. [5] T.T. Dele-Afolabi, M.A.A. Hanim, M. Norkhairunnisa, S. Sobri, R. Calin, Research trend in the development of macroporous ceramic components by pore forming additives from natural organic matters: a short review, Ceram. Int. 43 (2017) 1633–1649, http://dx.doi.org/10.1016/j.ceramint.2016.10.177. [6] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processing routes to macroporous ceramics: a review, J. Am. Ceram. Soc. 89 (2006) 1771–1789, http:// dx.doi.org/10.1111/j.1551-2916.2006.01044.x. [7] S. Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (2008) 155–169, http://dx.doi.org/10.1002/adem. 200700270. [8] D.F. Souza, E.H.M. Nunes, D.S. Pimenta, D.C.L. Vasconcelos, J.F. Nascimento, W. Grava, M. Houmard, W.L. Vasconcelos, Synthesis and structural evaluation of freeze-cast porous alumina, Mater. Charact. 96 (2014) 183–195, http://dx.doi.org/ 10.1016/j.matchar.2014.08.009. [9] B. Han, R. Zhang, D. Fang, Preparation and characterization of highly porous Yb2SiO5 ceramics using water-based freeze-casting, J. Porous Mater. 23 (2016) 563–568, http://dx.doi.org/10.1007/s10934-015-0110-y. [10] P.L. Rachadel, D.F. Souza, E.H.M. Nunes, J.C.D. da Costa, W.L. Vasconcelos, D. Hotza, A novel route for manufacturing asymmetric BSCF-based perovskite structures by a combined tape and freeze casting method, J. Eur. Ceram. Soc. (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.035. [11] A.M.A. Silva, E.H.M. Nunes, D.F. Souza, D.L. Martens, J.C.D. da Costa, M. Houmard, W.L. Vasconcelos, The influence of Fe2O3 doping on the pore structure and mechanical strength of TiO2-containing alumina obtained by freeze-casting, Ceram. Int. 41 (2015) 14049–14056, http://dx.doi.org/10.1016/j.ceramint.2015.07.021. [12] K.H. Kim, D.H. Kim, S.C. Ryu, S.Y. Yoon, H.C. Park, Porous mullite/alumina-layered composites with a graded porosity fabricated by camphene-based freeze casting, J. Compos. Mater. 0 (n.d.) 21998316636460. doi:10.1177/0021998316636460. [13] Y. Tang, S. Qiu, C. Wu, Q. Miao, K. Zhao, Freeze cast fabrication of porous ceramics using tert-butyl alcohol–water crystals as template, J. Eur. Ceram. Soc. 36 (2016) 1513–1518, http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.047. [14] S. Vijayan, R. Narasimman, K. Prabhakaran, Dispersion and setting of powder suspensions in concentrated aqueous urea solutions for the preparation of porous alumina ceramics with aligned pores, J. Am. Ceram. Soc. 96 (2013) 2779–2784, http://dx.doi.org/10.1111/jace.12484. [15] K. Araki, J.W. Halloran, Room-temperature freeze casting for ceramics with nonaqueous sublimable vehicles in the naphthalene–camphor eutectic system, J. Am. Ceram. Soc. 87 (2004) 2014–2019, http://dx.doi.org/10.1111/j.1151-2916.2004. tb06353.x. [16] R. Zhang, Q. Qu, B. Han, B. Wang, A novel silica aerogel/porous Y2SiO5 ceramics with low thermal conductivity and enhanced mechanical properties prepared by freeze casting and impregnation, Mater. Lett. 175 (2016) 219–222, http://dx.doi. org/10.1016/j.matlet.2016.04.051. [17] K. Araki, J.W. Halloran, Porous ceramic bodies with interconnected pore channels by a novel freeze casting technique, J. Am. Ceram. Soc. 88 (2005) 1108–1114, http://dx.doi.org/10.1111/j.1551-2916.2005.00176.x. [18] G.T. Araújo, T.S. Brito, D.F. Souza, A.M.A. Silva, E.H.M. Nunes, M. Houmard, Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures, Ceram. Int. 44 (2018) 580–587, http://dx.doi.org/10.1016/j.ceramint. 2017.09.216. [19] X. Liu, M.N. Rahaman, Q. Fu, Bone regeneration in strong porous bioactive glass (13-93) scaffolds with an oriented microstructure implanted in rat calvarial defects,
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
7
Acta Biomater. 9 (2013) 4889–4898, http://dx.doi.org/10.1016/j.actbio.2012.08. 029. A.M.A. Silva, E.H.M. Nunes, D.F. Souza, D.L. Martens, J.C. Diniz da Costa, M. Houmard, W.L. Vasconcelos, Effect of titania addition on the properties of freeze-cast alumina samples, Ceram. Int. 41 (2015), http://dx.doi.org/10.1016/j. ceramint.2015.04.132. X. Liu, M.N. Rahaman, Q. Fu, A.P. Tomsia, Porous and strong bioactive glass (1393) scaffolds prepared by unidirectional freezing of camphene-based suspensions, Acta Biomater. 8 (2012) 415–423. C. Hong, X. Zhang, J. Han, J. Du, W. Han, Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freeze-casting, Scr. Mater. 60 (2009) 563–566, http://dx.doi.org/10.1016/j.scriptamat.2008.12.011. M.P. Ginebra, M. Espanol, E.B. Montufar, R.A. Perez, G. Mestres, New processing approaches in calcium phosphate cements and their applications in regenerative medicine, Acta Biomater. 6 (2010) 2863–2873, http://dx.doi.org/10.1016/j.actbio. 2010.01.036. Y. Zhou, S. Fu, Y. Pu, S. Pan, M.V. Levit, A.J. Ragauskas, Freeze-casting of cellulose nanowhisker foams prepared from a water-dimethylsulfoxide (DMSO) binary mixture at low DMSO concentrations, RSC Adv. 3 (2013) 19272–19277, http://dx.doi. org/10.1039/C3RA43646B. S. Deville, E. Saiz, A.P. Tomsia, Freeze casting of hydroxyapatite scaffolds for bone tissue engineering, Biomaterials 27 (2006) 5480–5489, http://dx.doi.org/10.1016/ j.biomaterials.2006.06.028. X. Liu, W. Xue, C. Shi, J. Sun, Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceram. Int. 41 (2015) 11922–11926, http://dx.doi.org/10.1016/j.ceramint.2015.05.160. K.P. Furlan, R.M. Pasquarelli, T. Krekeler, M. Ritter, R. Zierold, K. Nielsch, G.A. Schneider, R. Janssen, Highly porous α-Al2O3 ceramics obtained by sintering atomic layer deposited inverse opals, Ceram. Int. 43 (2017) 11260–11264, http:// dx.doi.org/10.1016/j.ceramint.2017.05.176. J.A. Queiroga, E.H.M. Nunes, D.F. Souza, D.C.L. Vasconcelos, V.S.T. Ciminelli, W.L. Vasconcelos, Microstructural investigation and performance evaluation of slipcast alumina supports, Ceram. Int. 43 (2017) 3824–3830, http://dx.doi.org/10. 1016/j.ceramint.2016.12.037. M. Houmard, Q. Fu, M. Genet, E. Saiz, A.P. Tomsia, On the structural, mechanical, and biodegradation properties of HA/β-TCP robocast scaffolds, J. Biomed. Mater. Res. Part B Appl. Biomater. 101 (2013) 1233–1242, http://dx.doi.org/10.1002/ jbm.b.32935. M. Genet, M. Houmard, S. Eslava, E. Saiz, A.P. Tomsia, A two-scale Weibull approach to the failure of porous ceramic structures made by robocasting: possibilities and limits, J. Eur. Ceram. Soc. 33 (2013) 679–688, http://dx.doi.org/10.1016/j. jeurceramsoc.2012.11.001. S.W. Sofie, F. Dogan, Freeze casting of aqueous alumina slurries with glycerol, J. Am. Ceram. Soc. 84 (2001) 1459–1464, http://dx.doi.org/10.1111/j.1151-2916. 2001.tb00860.x. T. Moritz, H.-J. Richter, Ceramic bodies with complex geometries and ceramic shells by freeze casting using ice as mold material, J. Am. Ceram. Soc. 89 (2006) 2394–2398, http://dx.doi.org/10.1111/j.1551-2916.2006.01081.x. T. Moritz, H.-J. Richter, Ice-mould freeze casting of porous ceramic components, J. Eur. Ceram. Soc. 27 (2007) 4595–4601, http://dx.doi.org/10.1016/j.jeurceramsoc. 2007.04.010. Y. Zhang, L. Hu, J. Han, Z. Jiang, Freeze casting of aqueous alumina slurries with glycerol for porous ceramics, Ceram. Int. 36 (2010) 617–621, http://dx.doi.org/10. 1016/j.ceramint.2009.09.036. R. Zhang, D. Fang, Y. Pei, L. Zhou, Microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics produced by a new water-based freeze casting, Ceram. Int. 38 (2012) 4373–4377, http://dx.doi.org/10.1016/j. ceramint.2012.01.012. Y. Shao, M.F. El-Kady, C.-W. Lin, G. Zhu, K.L. Marsh, J.Y. Hwang, Q. Zhang, Y. Li, H. Wang, R.B. Kaner, 3D freeze-casting of cellular graphene films for ultrahighpower-density supercapacitors, Adv. Mater. 28 (2016) 6719–6726, http://dx.doi. org/10.1002/adma.201506157. H. Zhang, C.L. Fidelis, A.L.T. Serva, M. Wilhelm, K. Rezwan, Water-based freeze casting: adjusting hydrophobic polymethylsiloxane for obtaining hierarchically ordered porous SiOC, J. Am. Ceram. Soc. 100 (2017) 1907–1918, http://dx.doi. org/10.1111/jace.14782.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Macroporous alumina structures tailored by freeze-casting using naphthalene–camphor as freezing vehicle Lucas D. Lacerdaa, Douglas F. Souzab, Eduardo H.M. Nunesc, Manuel Houmardd,
⁎
a
Department of Mechanical Engineering, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 1, Belo Horizonte, MG CEP: 31270-901, Brazil b National Institute of Industrial Property – INPI, Rua Mayrink Veiga, 9 - Centro, Rio de Janeiro, CEP: 20090-910, Brazil c Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 2, Belo Horizonte, MG CEP: 31270-901, Brazil d Department of Materials Engineering and Civil Construction, Federal University of Minas Gerais - UFMG, Avenida Presidente Antônio Carlos, 6627, Campus UFMG, Escola de Engenharia, Bloco 1, Belo Horizonte, MG CEP: 31270-901, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Freeze-casting B. Porosity C. Mechanical properties D. Al2O3 E. Macroporous structures
Freeze-casting is a promising technique for fabricating macroporous ceramics because it is an environmentally friendly, cost effective, and easy scale-up method. Several freezing vehicles have been used in freeze-casting, including water, camphene, tert-butyl alcohol, urea, and naphthalene-camphor (Naph-Camp). This work focuses on the preparation of freeze-cast alumina samples using different Naph-Camp compositions as freezing vehicle, alumina loading ranging from 20 to 40 vol% and various freezing conditions. It was observed that macroporous materials with different pore structures and mechanical behaviors can be obtained by changing the Naph-Camp solvent composition. Moreover, the freezing route also showed a great effect on these properties, besides allowing the preparation of samples with oriented pores. These are important findings because naphthalene and camphor are widely available, have low toxicity, and show an easier sublimation than water, which has been commonly used as the freezing vehicle in many works. The range of pore structures and mechanical strengths obtained in this study demonstrates the versatility of the processing route used herein, which could be used to obtain samples for several applications, including catalysis, fluids filtration, and bioengineering. This study is supported by a series of experimental characterizations, including optical microscopy, scanning electron microscopy, Archimedes measurements, and cold crushing tests.
1. Introduction Ceramic materials with tailored macroporous structures have been used in several applications, including biomaterials, catalysis, phase separation, and thermal insulation [1–5]. It has been reported that the wide use of these materials is strongly associated with properties arising from a controlled substitution of a solid phase by pores [6]. Such properties may include high specific surface area, low density, low thermal conductivity, and high anisotropic mechanical strength. Among the different techniques used to produce macroporous ceramics, the freeze-casting method is a promising approach because it is an environmentally friendly, cost effective, and easy scale-up method [7]. In addition, freeze-cast materials usually show a highly-interconnected pore structure, which gives rise to solids with improved mass transport capacities [8]. A range of freezing vehicles has been used in freeze-casting,
⁎
including water [9,10], camphene [11,12], tert-butyl alcohol [8,13], urea [14], and naphthalene-camphor (Naph-Camp) [15]. Water is obviously an interesting solvent since it is cost effective, versatile, and non-toxic [16]. However, water solidification is carried out at temperatures below 0 °C and its sublimation is performed under reduced pressures, which increases the process complexity due to the need of specific equipments. In agreement to a wide literature in the area, camphene is also an interesting option to be used as solvent for fabricating macroporous materials by freeze-casting due to is low-toxicity and easy sublimation under ambient conditions [17,18]. For these reasons, camphene have been used to produce macroporous structures for biomedical application for instance [19]. Nevertheless, since camphene has a dendritic growth during its solidification [20], the porous structures obtained with this solvent often show dead-ended pores which could decrease the material performance in some applications. To reduce the presence of such inadequate pores, Liu et al. [21]
Corresponding author. E-mail address: [email protected] (M. Houmard).
https://doi.org/10.1016/j.ceramint.2018.06.036 Received 25 April 2018; Received in revised form 4 June 2018; Accepted 5 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lacerda, L.D., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.06.036
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freeze-cast alumina samples was evaluated. The alumina loading in the starting slurry was changed from 20 to 40 vol%. The freezing step was carried out using three different routes. The first route was performed by keeping samples at room temperature (≈ 25 °C) for obtaining a slow cooling rate of Naph-Camp. The second one was carried out by keeping samples in a domestic freezer (≈ −17 °C). The third one was performed by pouring the slurries on a liquid nitrogen-cooled cold support (≈ −196 °C), which led to fast cooling rates. After sintering, the as-prepared samples were examined by scanning electron microscopy (SEM) and Archimedes tests. Cold crushing tests were also carried out at room temperature. 2. Materials and methods CT3000-SG (99.8 wt% / Almatis Brasil) was used as the starting alumina powder. According to the supplier data, the mean particle size is 0.76 µm with D50 and D90 respectively about 0.4 and 1.7 µm. Texaphor 963 (Cognis, Southampton Hampshire, UK) was employed as the dispersing agent, whereas Naph-Camp mixtures were used as the freezing vehicle. Both naphthalene and camphor were obtained from Aldrich and show purity above 96 wt%. As aforementioned, hypoeutectic (60 wt% Camp), eutectic (65 wt% Camp), and hypereutectic (70 wt% Camp) compositions were prepared herein. These compositions are highlighted in Fig. 1, which displays the phase diagram of the Naph-Camp system. Naph-Camp mixtures with these compositions were prepared at 70 °C and then solidified by vapor deposition on transparent glass sheets as follows. The glass sheets were partially dipped into Naph-Camp mixtures kept at 70 °C. Thus, one side of the glass sheet was kept at 70 °C, whereas the other side was maintained at a temperature near the room temperature (25 °C). This temperature gradient led to a directional solidification of the Naph-Camp crystals from the hot side to the cold one. After their complete solidification, the NaphCamp crystals were examined by optical microscopy (OM) with an Olympus CH30 microscope operating in the transmission mode. Besides, it was also evaluated the time needed for the complete sublimation of Naph, Camp, and the aforementioned mixtures. This procedure was carried out by preparing 20 mL of these materials at 70 °C and pouring them on glass substrates. These samples were then kept at room temperature in a fume hood and their masses were registered as a function of time. Freeze-cast alumina samples were prepared as follows. Solutions of Naph-Camp with Texaphor as dispersant agent were initially prepared at 70 °C. The alumina CT3000-SG was subsequently added under vigorous stirring to the solution, and the as-prepared slurry was kept under sonication for additional 15 min to break the ceramic agglomerates. As mentioned before, the alumina loading was varied from 20 to 40 vol%. The dispersant concentration in the slurries was kept at 2 wt% of the alumina loading. The as-obtained slurries were then poured into cylindrical acrylic molds (diameter of 12.2 mm and height of 15 mm) and frozen by different routes. The first freezing route was carried out by keeping samples at ambient temperature, whereas the second one was performed storing them in a domestic freezer kept at − 17 °C. In this second case, since the bottom side of the molds was close to the freezer surface, it reached faster the freezer temperature, inducing a slight vertical temperature gradient during the solidification step. The third freezing route was carried out by pouring the starting slurries into a mold placed on the top of a liquid nitrogen-cooled cold support made of cooper. Thus, the ceramic suspensions were fast cooled from the bottom side to the top one due to the high vertical temperature gradient. The sublimation of Naph-Camp solvent was performed by keeping samples in ambient conditions in a fume hood for times as long as 12 days. The time needed for sublimating each Naph-Camp composition used herein was determined on the basis of the sublimation tests aforementioned. After the sublimation of Naph-Camp, the green bodies were heat-treated in atmospheric furnace at 1500 °C for 2 h. The heating and cooling rates were kept constant at 2 and 10 °C min−1,
Fig. 1. Phase diagram of the Naph-Camp system [15]. The compositions and solidification paths used in this work are highlighted.
Fig. 2. Sublimed mass fraction at room temperature as a function of time for Naph, Camp, and Naph-Camp mixtures. The dashed line represents the time needed to reach a 50% mass loss. The solid lines are used as a guide to the eyes only.
illustrated that the use of annealing treatment and oriented freezing direction can be realized. Thus, as reported elsewhere [15], Naph-Camp is a promising freezing vehicle for the fabrication of macroporous structures. First, such solvents present low viscosity, which allows preparing concentrated slurries to access to denser materials. Moreover, Naph-Camp experiences a shrinkage upon freezing, leading to dense and resistant green bodies. It is also worth stressing the expressive melting temperature and large vapor pressure of Naph-Camp mixtures, which allows freezing and sublimating ceramic slurries under milder condition than when water is used as the freezing vehicle. This behavior allows fabricating Naph-Camp-based samples at ambient conditions. In the work of Araki and Halloran [15], macroporous structures have been fabricated by freeze-casting using different Naph-Camp compositions. They showed that the pore morphology can be tailored by changing the solvent composition. However, such observation was performed for samples with low porosity prepared by only one freezing route. Thus, a systematic study concerning the influence of the freeze-casting parameters on the pore structure of Naph-Camp derived samples is still lacking. In this work we prepared freeze-cast alumina samples using NaphCamp as the freezing vehicle. Hypoeutectic, eutectic, and hypereutectic compositions of Naph-Camp were tested herein. The effect of these compositions on the porous structure and mechanical behavior of 2
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
Fig. 3. Top line: OM images of the crystals obtained by solidifying Naph-Camp mixtures on a glass substrate with (a) hypoeutectic, (b) eutectic, and (c) hypereutectic compositions. Bottom line: SEM micrographs of freeze-cast alumina samples prepared using (d) hypoeutectic, (e) eutectic, and (f) hypereutectic Naph-Camp compositions, frozen in a domestic freezer and prepared keeping the solid loading at 30 vol%. The scale bars shown in the OM micrographs represent 1 mm, whereas those ones displayed in the SEM micrographs correspond to 500 µm. The direction of the freezing orientation is indicated for reference purposes.
Fig. 4. SEM micrographs of freeze-cast samples obtained with hypoeutectic, eutectic and hypereutectic compositions. These materials were frozen at ambient condition, in freezer, or using liquid nitrogen. For samples frozen in freezer or using liquid nitrogen, the freezing direction was vertically oriented from the bottom to the top side of the micrographs. The alumina loading in these materials was kept constant at 30 vol%. The scale bars shown in these images correspond to 500 µm. 3
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freezing route employed in the fabrication step. Such observation is probably due to an isotropic shape of the Naph-Camp crystals obtained during the freezing step. Once again, these alumina structures are closely related to the morphology of the crystals shown in Fig. 3a to c. For instance, the eutectic composition leads to continuous thin plate-shaped crystals aligned according to the freezing direction (Fig. 3b). As aforementioned, the pore network of freeze-cast materials is a direct replica of the freezing vehicle. Therefore, one could expect that freezecast samples obtained from the hypoeutectic and eutectic Naph-Camp mixtures should exhibit ordered pore networks following the freezing direction. Fig. 5 gives the mean pore size of the smallest dimension of the pore shape, i.e. the rod diameter for hypoeutectic pores for instance, measured from SEM micrographs of alumina samples prepared under different conditions. In a general way, the hypoeutectic Naph-Camp composition gave rise to materials with the largest pore sizes, followed by those obtained from hypereutectic and eutectic mixtures, respectively. In addition, for hypoeutectic and eutectic Naph-Camp compositions, it can be inferred that the faster the freezing rate, the finer the pore structure. This behavior is strongly related to the nucleation and growth of Naph-Camp crystals during the freezing step. It is well established that nucleation is favored over growth for fast freezing rates [25]. As a consequence, fast freezing rates lead to small Naph-Camp crystals and, consequently, to fine pore structures. On the other hand, slow freezing rates favor the growth of Naph-Camp crystals and give rise to large pores in the freeze-cast material. Moreover, higher solid
respectively. SEM was carried out using an Aspex Explorer microscope operating at a 20 kV accelerating voltage. The samples used in SEM microscopy were sputter-coated with a 10 nm-thick gold film. Cold crushing tests were performed at room temperature, in a direction parallel to the the freezing direction, using a universal testing machine operating at a cross-head displacement speed of 0.5 mm min−1. At least five samples were taken into account for each batch. 3. Results and discussion Fig. 2 exhibits the sublimed mass fraction as a function of time for Naph, Camp, and Naph-Camp mixtures. The dashed line represents the time needed for reaching a 50% mass loss. One observes that camphor showed the fastest sublimation kinetics, whereas naphthalene exhibited the slowest one. For instance, about 165 h are needed for reaching a 50% mass loss for Naph. On the other hand, 30 h are sufficient for sublimating the same amount of Camp. Among the Naph-Camp compositions tested herein, the hypereutectic one displayed the fastest sublimation kinetics, followed by the eutectic and hypoeutectic mixtures, respectively. This finding reveals that the smaller the Camp concentration, the slower the sublimation kinetics of the Naph-Camp mixture. As discussed before, the sublimation times of the Naph-Camp compositions used in this work were determined from the data shown in Fig. 2. Images 3a to 3c were collected by OM from the solvent crystals obtained after freezing Naph-Camp mixtures with different compositions. It can be clearly observed that the morphology of the solidified crystals is strongly dependent of the Naph-Camp composition. The hypoeutectic composition gave rise to rod-shaped crystals with no preferential orientation (Fig. 3a). The eutectic composition led to continuous thin plates oriented according to the freezing direction (Fig. 3b). The hypereutectic mixture gave rise to a solid phase composed by sphere- and thin rod-shaped crystals (Fig. 3c). It can be inferred from the phase diagram shown in Fig. 1 that the proeutectic Naph-Camp crystals obtained from the hypoeutectic composition are ascribed to a Naph-rich phase (α). The α phase should represent about 8.5 wt% of the solidified solvent whereas the eutectic constituent should correspond to 91.5 wt%. In the case of the hypereutectic mixture, the proeutectic Naph-Camp crystals formed are associated with a Camp-rich proeutectic phase (β) which should represent about 20 wt% of the solid crystals whereas about 80 wt% should result from the eutectic transformation. However, no proeutectic crystals were observed for either the hypoeutectic or hypereutectic compositions after the solidification step (Fig. 3a and c); only crystals with different morphologies were observed when the Naph-Camp composition was modified. Thus, as the pore network of freeze-cast materials is a direct replica of the freezing vehicle [22–24], one could suggest that ceramic samples with distinct porous structures should be obtained by using different Naph-Camp mixtures. Indeed, freeze-cast samples with different pore frameworks were obtained when such compositions were employed in their preparation (Fig. 3d to f). Moreover, the pores morphology, especially the rod- and thin plate-shaped pores observed respectively for the hypoeutectic and eutectic compositions (Fig. 3d and e), correspond well to the shape of the Naph-Camp crystals observed in Fig. 3a and b. This finding reveals the versatility of the Naph-Camp system because materials with different pore structures can be prepared by changing its composition. Fig. 4 depicts SEM micrographs of freeze-cast materials prepared under different conditions keeping the alumina loading at 30 wt%. As it was expected, no pore orientation was observed for the samples frozen at room temperature. Nonetheless, for hypoeutectic and eutectic compositions, the pore structure of these materials started to show a vertical orientation parallel to the freezing direction when the freezing step was carried out either in a domestic freezer or using liquid nitrogen. On the other hand, no preferential pore orientation was noted for materials prepared from a hypereutectic Naph-Camp composition, regardless the
Fig. 5. Mean pore size of freeze-cast alumina samples as a function of the alumina loading for the (A) hypoeutectic, (B) eutectic, and (C) hypereutectic compositions frozen under different conditions (ambient, freezer and liquid nitrogen). The pore sizes were measured from the SEM micrographs. 4
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
loading leads to smaller pores size since it should inhibit the growth of the Naph-Camp crystals. However, in the case of the hypereutectic composition, it seems that the freezing condition and solid loading show no influence on the crystals growth, and consequently on the pores size. Materials with thin and interconnected pores, as the ones present in the oriented eutectic structures, could be interesting for filtration or phase separation applications, whereas structures with higher pore size, as the ones from hypoeutectic mixture, could be used for medical applications as a biomaterial. Fig. 6 exhibits the open and closed porosities for freeze-cast samples obtained under different conditions. The total porosity can be inferred from the open and closed porosities. One observes that the higher the alumina loading, the smaller the total porosity. This result is in agreement with the fact that the pore structure of freeze-cast materials is obtained upon Naph-Camp sublimation. Thus, it is expected that the larger the solid loading, the lower the sample porosity. This behavior is also observed in the SEM micrographs displayed in Fig. 7, which shows the porous structures obtained from a hypoeutectic composition for different alumina loadings and freezing conditions. From Fig. 6, it is worth highlighting the expressive concentration of open pores in the samples prepared herein. This is an important finding since alumina samples with expressive open and interconnected porosities find application in many fields, including catalysis, biomaterials, and fluids filtration [26–28]. It can also be observed that in a general way, specimens obtained from a eutectic Naph-Camp composition, showed the
Fig. 6. Open and closed porosities of freeze-cast samples obtained using different alumina loadings and freezing routes. The total porosity can be inferred by summing the open and closed porosities. Data assessed from Archimedes tests.
Fig. 7. SEM micrographs of freeze-cast samples fabricated from a hypoeutectic Naph-Camp composition. The alumina loading in these materials are 20, 30 and 40 vol %, and the freezing conditions are ambient, freezer and liquid nitrogen. For both solidification in freezer and using liquid nitrogen, the freezing direction is from the bottom to the top side of the images. The scale bars shown in these micrographs correspond to 500 µm.
5
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
freezing routes. As depicted in Figs. 4, 5 and 7, the faster the freezing rate, the finer the pore structure. Thus, such results are in agreement with Houmard [29] and Genet [30] who reported that the finer the pore structure, the higher the mechanical strength of a porous ceramic. According to these authors, thin pores show a lower concentration of stresses upon loading and also inhibit better the propagation of cracks. However, the eutectic structures, which have the lowest pore volume and the thinnest pores, are not as mechanically resistant as the hypoeutectic materials toward compression stresses. This behavior could be explained by the pore geometry. For instance, Tang et al. [13] showed, for structures produced by freeze-casting from alumina slurries containing different water and tert-butyl alcohol ratios, that the presence of lamellar pores results in a decrease of the mechanical strength in comparison to macroporous structures with hexagonal pores. In our case, on contrary to the eutectic samples, the hypoeutectic system with non-continuous rod-like pores could present higher compression resistance due to a crack propagation which not destroy entire areas bearing the applied stresses during the compression tests. Therefore, since the porous structure of hypereutectic samples does not depend of the freezing route used, the mechanical response for such materials is only related to the solid loading, and consequently to the pore volume. The findings reported herein demonstrate the versatility of the Naph-Camp system because materials with distinct pore structures and mechanical behaviors can be obtained by changing the Naph-Camp composition, solid loading, and freezing route. Both Naph and Camp show low toxicity and are widely available, which explains their expressive use in the pharmaceutical industry. Moreover, these chemicals show an easy sublimation under ambient conditions in opposition to water, which has been used as the freezing vehicle in many works available in the literature [31–37]. For instance, no freeze-dryer was used in this work, which makes the freeze-casting process simpler and cost effective. These features make Naph and Camp promising freezing vehicles to be employed for preparing macroporous samples by freeze casting.
Fig. 8. Cold crushing strength of freeze-cast alumina samples as a function of the total porosity for the (A) hypoeutectic, (B) eutectic, and (C) hypereutectic compositions frozen under different conditions (ambient, freezer and liquid nitrogen). The solid lines are used as a guide to the eyes only.
4. Conclusions Freeze-cast alumina samples were successfully obtained in this work using different Naph-Camp mixtures as the freezing vehicle, alumina loading ranging from 20 to 40 vol% and various freezing conditions. It was shown that materials with distinct pore structures and mechanical behaviors can be prepared by changing the Naph-Camp composition. This is an important finding because Naph and Camp exhibit low toxicity, wide availability, and an easier sublimation than water, which has been widely used as freezing vehicle in many works. The freezing route of alumina/Naph-Camp suspensions also demonstrated to have a great effect on both the pore structure and crushing strength of the samples prepared herein. It was noted that the faster the freezing rate, the smaller the porosity and the higher the mechanical strength of the freeze-cast materials for both hypoeutectic and eutectic Naph-Camp compositions. On the other hand, samples with larger pores were obtained by performing the freeze-casting step at room temperature. The range of pore sizes and mechanical strengths obtained in this study demonstrates the versatility of the processing route used herein, which could be used to obtain samples for applications ranging from catalysis and fluids filtration to biomaterials.
smallest total porosity for a same alumina loading. Such observation could result from a higher shrinkage of the eutectic solvent composition in comparison to the other ones during the solidification step which, in consequence, leads to a lower pore volume after the solvent sublimation. Besides, it appears that the freezing route has no effect on the volume fraction of pores. Indeed a nearly constant total porosity was observed for the alumina structures regardless the freezing route used in its preparation. For instance, a constant total porosity of about 50% was observed for the material obtained from the hypoeutectic slurry containing 30 vol% of alumina loading, regardless the freezing route used. On the other hand, as already observed in Figs. 4 and 5, Fig. 7 reveals that the freezing condition exhibited a great effect on the pore network structure. Freezing step under ambient conditions led to isotropic porous structures, whereas solidifications in freezer or using liquid nitrogen gave rise to pores oriented according to the freezing direction, where smaller pores were observed for fast freezing rates and high alumina loadings. Fig. 8 exhibits the cold crushing strength of the freeze-cast alumina samples obtained in this work when the stress is applied parallel to the freezing direction, i.e. in the same direction of the pores orientation. As expected, the higher the porosity, the lower the crushing strength. Moreover, for hypoeutectic and eutectic compositions, i.e. for samples showing a pore structure dependent of the freezing conditions, structures frozen in liquid nitrogen usually displayed the highest strength, followed by materials prepared in a domestic freezer and at room temperature, respectively. This behavior is strongly associated with the porous structure shown by the samples obtained from these three
Acknowledgements The authors thank Almatis Brasil for kindly providing the alumina powders used in this study. The authors thank the financial support from CNPq (471817/2013-9), FAPEMIG (APQ-00583-14), and PRPqUFMG (05-2016). Prof. Vasconcelos is acknowledged for the support to this research. The views and opinions of authors expressed herein do not state or reflect those of the INPI (Brazil). 6
Ceramics International xxx (xxxx) xxx–xxx
L.D. Lacerda et al.
References [20]
[1] H.T. Wang, X.Q. Liu, G.Y. Meng, Porous α-Al2O3 ceramics prepared by gelcasting, Mater. Res. Bull. 32 (1997) 1705–1712, http://dx.doi.org/10.1016/S00255408(97)00152-9. [2] S. Sokolov, D. Bell, A. Stein, Preparation and characterization of macroporous αalumina, J. Am. Ceram. Soc. 86 (2003) 1481–1486, http://dx.doi.org/10.1111/j. 1151-2916.2003.tb03500.x. [3] F. Tang, H. Fudouzi, Y. Sakka, Fabrication of macroporous alumina with tailored porosity, J. Am. Ceram. Soc. 86 (2003) 2050–2054, http://dx.doi.org/10.1111/j. 1151-2916.2003.tb03607.x. [4] E.C. Hammel, O.L.-R. Ighodaro, O.I. Okoli, Processing and properties of advanced porous ceramics: an application based review, Ceram. Int. 40 (2014) 15351–15370, http://dx.doi.org/10.1016/j.ceramint.2014.06.095. [5] T.T. Dele-Afolabi, M.A.A. Hanim, M. Norkhairunnisa, S. Sobri, R. Calin, Research trend in the development of macroporous ceramic components by pore forming additives from natural organic matters: a short review, Ceram. Int. 43 (2017) 1633–1649, http://dx.doi.org/10.1016/j.ceramint.2016.10.177. [6] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processing routes to macroporous ceramics: a review, J. Am. Ceram. Soc. 89 (2006) 1771–1789, http:// dx.doi.org/10.1111/j.1551-2916.2006.01044.x. [7] S. Deville, Freeze-casting of porous ceramics: a review of current achievements and issues, Adv. Eng. Mater. 10 (2008) 155–169, http://dx.doi.org/10.1002/adem. 200700270. [8] D.F. Souza, E.H.M. Nunes, D.S. Pimenta, D.C.L. Vasconcelos, J.F. Nascimento, W. Grava, M. Houmard, W.L. Vasconcelos, Synthesis and structural evaluation of freeze-cast porous alumina, Mater. Charact. 96 (2014) 183–195, http://dx.doi.org/ 10.1016/j.matchar.2014.08.009. [9] B. Han, R. Zhang, D. Fang, Preparation and characterization of highly porous Yb2SiO5 ceramics using water-based freeze-casting, J. Porous Mater. 23 (2016) 563–568, http://dx.doi.org/10.1007/s10934-015-0110-y. [10] P.L. Rachadel, D.F. Souza, E.H.M. Nunes, J.C.D. da Costa, W.L. Vasconcelos, D. Hotza, A novel route for manufacturing asymmetric BSCF-based perovskite structures by a combined tape and freeze casting method, J. Eur. Ceram. Soc. (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.035. [11] A.M.A. Silva, E.H.M. Nunes, D.F. Souza, D.L. Martens, J.C.D. da Costa, M. Houmard, W.L. Vasconcelos, The influence of Fe2O3 doping on the pore structure and mechanical strength of TiO2-containing alumina obtained by freeze-casting, Ceram. Int. 41 (2015) 14049–14056, http://dx.doi.org/10.1016/j.ceramint.2015.07.021. [12] K.H. Kim, D.H. Kim, S.C. Ryu, S.Y. Yoon, H.C. Park, Porous mullite/alumina-layered composites with a graded porosity fabricated by camphene-based freeze casting, J. Compos. Mater. 0 (n.d.) 21998316636460. doi:10.1177/0021998316636460. [13] Y. Tang, S. Qiu, C. Wu, Q. Miao, K. Zhao, Freeze cast fabrication of porous ceramics using tert-butyl alcohol–water crystals as template, J. Eur. Ceram. Soc. 36 (2016) 1513–1518, http://dx.doi.org/10.1016/j.jeurceramsoc.2015.12.047. [14] S. Vijayan, R. Narasimman, K. Prabhakaran, Dispersion and setting of powder suspensions in concentrated aqueous urea solutions for the preparation of porous alumina ceramics with aligned pores, J. Am. Ceram. Soc. 96 (2013) 2779–2784, http://dx.doi.org/10.1111/jace.12484. [15] K. Araki, J.W. Halloran, Room-temperature freeze casting for ceramics with nonaqueous sublimable vehicles in the naphthalene–camphor eutectic system, J. Am. Ceram. Soc. 87 (2004) 2014–2019, http://dx.doi.org/10.1111/j.1151-2916.2004. tb06353.x. [16] R. Zhang, Q. Qu, B. Han, B. Wang, A novel silica aerogel/porous Y2SiO5 ceramics with low thermal conductivity and enhanced mechanical properties prepared by freeze casting and impregnation, Mater. Lett. 175 (2016) 219–222, http://dx.doi. org/10.1016/j.matlet.2016.04.051. [17] K. Araki, J.W. Halloran, Porous ceramic bodies with interconnected pore channels by a novel freeze casting technique, J. Am. Ceram. Soc. 88 (2005) 1108–1114, http://dx.doi.org/10.1111/j.1551-2916.2005.00176.x. [18] G.T. Araújo, T.S. Brito, D.F. Souza, A.M.A. Silva, E.H.M. Nunes, M. Houmard, Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures, Ceram. Int. 44 (2018) 580–587, http://dx.doi.org/10.1016/j.ceramint. 2017.09.216. [19] X. Liu, M.N. Rahaman, Q. Fu, Bone regeneration in strong porous bioactive glass (13-93) scaffolds with an oriented microstructure implanted in rat calvarial defects,
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
7
Acta Biomater. 9 (2013) 4889–4898, http://dx.doi.org/10.1016/j.actbio.2012.08. 029. A.M.A. Silva, E.H.M. Nunes, D.F. Souza, D.L. Martens, J.C. Diniz da Costa, M. Houmard, W.L. Vasconcelos, Effect of titania addition on the properties of freeze-cast alumina samples, Ceram. Int. 41 (2015), http://dx.doi.org/10.1016/j. ceramint.2015.04.132. X. Liu, M.N. Rahaman, Q. Fu, A.P. Tomsia, Porous and strong bioactive glass (1393) scaffolds prepared by unidirectional freezing of camphene-based suspensions, Acta Biomater. 8 (2012) 415–423. C. Hong, X. Zhang, J. Han, J. Du, W. Han, Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freeze-casting, Scr. Mater. 60 (2009) 563–566, http://dx.doi.org/10.1016/j.scriptamat.2008.12.011. M.P. Ginebra, M. Espanol, E.B. Montufar, R.A. Perez, G. Mestres, New processing approaches in calcium phosphate cements and their applications in regenerative medicine, Acta Biomater. 6 (2010) 2863–2873, http://dx.doi.org/10.1016/j.actbio. 2010.01.036. Y. Zhou, S. Fu, Y. Pu, S. Pan, M.V. Levit, A.J. Ragauskas, Freeze-casting of cellulose nanowhisker foams prepared from a water-dimethylsulfoxide (DMSO) binary mixture at low DMSO concentrations, RSC Adv. 3 (2013) 19272–19277, http://dx.doi. org/10.1039/C3RA43646B. S. Deville, E. Saiz, A.P. Tomsia, Freeze casting of hydroxyapatite scaffolds for bone tissue engineering, Biomaterials 27 (2006) 5480–5489, http://dx.doi.org/10.1016/ j.biomaterials.2006.06.028. X. Liu, W. Xue, C. Shi, J. Sun, Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method, Ceram. Int. 41 (2015) 11922–11926, http://dx.doi.org/10.1016/j.ceramint.2015.05.160. K.P. Furlan, R.M. Pasquarelli, T. Krekeler, M. Ritter, R. Zierold, K. Nielsch, G.A. Schneider, R. Janssen, Highly porous α-Al2O3 ceramics obtained by sintering atomic layer deposited inverse opals, Ceram. Int. 43 (2017) 11260–11264, http:// dx.doi.org/10.1016/j.ceramint.2017.05.176. J.A. Queiroga, E.H.M. Nunes, D.F. Souza, D.C.L. Vasconcelos, V.S.T. Ciminelli, W.L. Vasconcelos, Microstructural investigation and performance evaluation of slipcast alumina supports, Ceram. Int. 43 (2017) 3824–3830, http://dx.doi.org/10. 1016/j.ceramint.2016.12.037. M. Houmard, Q. Fu, M. Genet, E. Saiz, A.P. Tomsia, On the structural, mechanical, and biodegradation properties of HA/β-TCP robocast scaffolds, J. Biomed. Mater. Res. Part B Appl. Biomater. 101 (2013) 1233–1242, http://dx.doi.org/10.1002/ jbm.b.32935. M. Genet, M. Houmard, S. Eslava, E. Saiz, A.P. Tomsia, A two-scale Weibull approach to the failure of porous ceramic structures made by robocasting: possibilities and limits, J. Eur. Ceram. Soc. 33 (2013) 679–688, http://dx.doi.org/10.1016/j. jeurceramsoc.2012.11.001. S.W. Sofie, F. Dogan, Freeze casting of aqueous alumina slurries with glycerol, J. Am. Ceram. Soc. 84 (2001) 1459–1464, http://dx.doi.org/10.1111/j.1151-2916. 2001.tb00860.x. T. Moritz, H.-J. Richter, Ceramic bodies with complex geometries and ceramic shells by freeze casting using ice as mold material, J. Am. Ceram. Soc. 89 (2006) 2394–2398, http://dx.doi.org/10.1111/j.1551-2916.2006.01081.x. T. Moritz, H.-J. Richter, Ice-mould freeze casting of porous ceramic components, J. Eur. Ceram. Soc. 27 (2007) 4595–4601, http://dx.doi.org/10.1016/j.jeurceramsoc. 2007.04.010. Y. Zhang, L. Hu, J. Han, Z. Jiang, Freeze casting of aqueous alumina slurries with glycerol for porous ceramics, Ceram. Int. 36 (2010) 617–621, http://dx.doi.org/10. 1016/j.ceramint.2009.09.036. R. Zhang, D. Fang, Y. Pei, L. Zhou, Microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics produced by a new water-based freeze casting, Ceram. Int. 38 (2012) 4373–4377, http://dx.doi.org/10.1016/j. ceramint.2012.01.012. Y. Shao, M.F. El-Kady, C.-W. Lin, G. Zhu, K.L. Marsh, J.Y. Hwang, Q. Zhang, Y. Li, H. Wang, R.B. Kaner, 3D freeze-casting of cellular graphene films for ultrahighpower-density supercapacitors, Adv. Mater. 28 (2016) 6719–6726, http://dx.doi. org/10.1002/adma.201506157. H. Zhang, C.L. Fidelis, A.L.T. Serva, M. Wilhelm, K. Rezwan, Water-based freeze casting: adjusting hydrophobic polymethylsiloxane for obtaining hierarchically ordered porous SiOC, J. Am. Ceram. Soc. 100 (2017) 1907–1918, http://dx.doi. org/10.1111/jace.14782.