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Ceramic microspheres with controlled porosity by emulsion-ice templating
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JECS-11075; No. of Pages 10
Journal of the European Ceramic Society xxx (2017) xxx–xxx
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Ceramic microspheres with controlled porosity by emulsion-ice templating Valentina Naglieri a,∗ , Paolo Colombo a,b a b
Department of Industrial Engineering, University of Padova, via Marzolo 9, 35131 Padova, Italy Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA
a r t i c l e
i n f o
Article history: Received 6 December 2016 Received in revised form 15 February 2017 Accepted 17 February 2017 Available online xxx Keywords: Freeze-casting Preceramic polymers Microbeads Hierarchical porosity Emulsion templating
a b s t r a c t Silicon Carbide porous microspheres were fabricated using a preceramic polymer by emulsion-ice templating. An oil-in-water macroemulsion was prepared by adding an organic solution, comprising polycarbosilane and cyclohexane, to the aqueous phase containing a nonionic surfactant. Upon directional freezing and freeze drying, microspheres with aligned pores were obtained. The influence of processing parameters was assessed. In particular, the solidification temperature affected size and morphology of the macroporosity. Our strategy enables an independent control of macro and nanoporosity. Indeed, the thermal treatment can be optimized to tune the micro-porosity at the nanoscale as well as the specific surface area of the samples. The emulsion-ice templating technique was optimized to produce microspheres with multimodal macroporosity up to 90 vol.% in the range 1–30 m, with micro- and meso-pores with diameter up to 6 nm, and specific surface area as high as 117 m2 g−1 . © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Porous ceramics find applications in many industries where they are used as filters, membranes, thermal insulators, catalytic supports, burners, biomedical devices, etc. [1]. Their versatility is achieved by simultaneously tuning chemical composition and architecture. By tailoring the architecture and characteristics of the porosity (e.g. pore volume, size and size distribution, tortuosity and interconnectivity), such properties as thermal and electrical conductivity, specific strength and modulus, permeability, and corrosion rate can be optimized to meet the requirements of each application [2]. Among the many porous ceramic components, beads and microspheres have received great attention for their potential use as drug carriers, adsorbents, microreactors, etc. [3–7]. Spherical particles offer good mobility, flowability, high packing density, ease of separation and reuse after regeneration. However, undesired pressure drop and poor permeability are often observed in conventional packed beds. Finding a compromise between high surface area and pores accessibility is crucial [8–10]. Exemplary is the case of catalytic supports, where the introduction of channels, serving as the
∗ Corresponding author. E-mail address: [email protected] (V. Naglieri).
main paths for fast mass transport, decreases the amount of active sites available for the reaction and thus limits the overall yield [11]. Consequently, a rational design of the structure at all lengthscales is necessary. Coppens and coworkers have modeled pore configurations optimized for maximum yield, demonstrating that a hierarchical network of meso/macropores performs better than monomodal porous systems in many processes, such as heterogeneous catalysis and separation [11–14]. Their results, although case-specific, emphasize the importance of a multiscale design approach and motivate the development of processing techniques that yield multimodal porosity, in order to achieve optimum mass diffusion and a high active surface, and to overcome such drawbacks as high pressure drop or poor heat transfer. Aerosol routes [15], double emulsion [16,17], polymer templating [18,19], electrohydrodinamic forming [20], microfluidic processing [21], ionotropic gelation [4,5], freeze drying [22,23] and phase separation-assisted pyrolysis [24] are just some of the techniques proposed to produce porous ceramic microspheres. As diverse as they are, these strategies can yield spheres with diameter ranging from 100 nm to few millimeters. The pores inside the beads−obtained, for example, by self-assembly, polymer or emulsion templating−are often spherical cavities partly connected to each other through smaller windows. However, macro-channels spanning the entire bead and connected to smaller pores in a treelike fashion can be better suited for some specific applications, such
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Please cite this article in press as: V. Naglieri, P. Colombo, Ceramic microspheres with controlled porosity by emulsion-ice templating, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.02.033
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as processes that involve the deposition of reaction species onto the catalyst surface [25,26]. Although the advantages of complex pore networks have been illustrated by modeling studies [11,27,28] and are testified by the frequent occurrence of such patterns in nature [14], manufacturing the desired pore geometry remains a challenge − especially for porous microspheres. Here we propose a strategy that merges different techniques: by processing a preceramic polymer solution via emulsion and ice templating we obtained porous ceramic microspheres with complex pore networks. Despite the popularity of these techniques, in the literature there are just few examples of porous microspheres obtained by combining emulsion and ice templating [29,30]. These studies are limited to the production of polymeric spheres [29], or utilize ceramic powders [30]. What sets our approach apart is the use of a preceramic polymer that, upon a proper thermal treatment, yields silicon based inorganic components that can perform better than polymers in a number of applications where high chemical, thermal, and mechanical resistance are required. Preceramic polymers, moreover, offer many advantages over other ceramic precursors or ceramic powders. Their development has prompted innovation in many field [31], such as in the production of porous ceramics [32]. Of particular relevance for this work is the possibility of tuning the porosity of polymer-derived ceramics at multiple length-scales by combining proper forming techniques and thermal treatments. Indeed, the optimization of the pyrolysis step has been shown to play a pivotal role in retaining the nanoscale porosity up to high temperature or in developing it through carbothermal reduction reactions [33–36]. Ice templating of preceramic polymers has been investigated in a few studies concerning the fabrication of monoliths with aligned pores [37–40]. This technique, also referred to as freeze casting, has been mostly used with colloidal suspensions and with water-soluble polymers [41–45], while fewer examples of polymeric honeycomb monoliths obtained by freezing organic solutions have also been reported [46,47]. Ice templating of porous ceramic beads has been carried out [23], but the proposed method (directly freezing a preceramic polymer solution in liquid nitrogen) did not offer a full control on the pore morphology, and in particular could yield a dense outer skin blocking the access to the inside of the beads. Here, we show that such drawbacks can be overcome by performing a controlled freezing of o/w emulsions.
2. Experimental As a precursor of silicon carbide (SiC), we selected a commercial polycarbosilane (PCS, Type S, Nippon Carbon Co. Ltd., Yokohama, Japan). Cyclohexane (Sigma–Aldrich, St. Louis, MO) was used as organic solvent for the PCS. A polyoxyethylene-sorbitan20-monooleate (also known as polysorbate 80 or commercially as Tween 80, Sigma–Aldrich, St. Louis, MO) was employed as a nonionic surfactant, dissolved in deionized water. Fig. 1 illustrates the details of our emulsion-ice templating method. After dissolving the PCS in the cyclohexane (weight ratio of 1:9) and the Tween 80 in deionized water (weight ratio of 5:100), the o/w emulsion was prepared by slowly adding the oil phase to the aqueous solution (volume ratio 1:1). Upon gentle stirring (stirring speed of 200 rpm), an o/w macroemulsion formed that soon creamed when agitation stopped (Fig. 1a). The cream phase, which comprises oil droplets separated by a continuous film of saturated surfactant, was then placed in a mold whose copper bottom was kept at a low temperature to induce directional freezing (Fig. 1b). By applying a unidirectional thermal gradient, both the dispersed and the continuous phases solidified. Since the oil droplets comprised a solution of polycarbosilane in cyclohexane, a phase separation occurred during solidification: cyclohexane dendrites grew along
a preferred orientation while polycarbosilane concentrated in the interdendritic space (Fig. 1c). In order to investigate the influence of the bottom temperature on the macroporosity, we produced microsphere by freezing at −30, −60 and −100 ◦ C. Once the o/w emulsion was completely solid, the samples were transferred in a freeze dryer (FreeZone 2.5 l Benchtop Freeze Dry System, Labconco, Kansas City, MI) where a vacuum level of 0.133 mBar and a collector temperature of −50 ◦ C were maintained in order to sublimate water and cyclohexane. Loose microspheres possessing aligned pores which are the replica of cyclohexane dendrites were obtained. After oxidation curing at 200 ◦ C for 1 h in air (heating rate of 0.3 ◦ C/min) polycarbosilane microsphere were heat treated at higher temperature in argon atmosphere, in order to convert the polymer into a ceramic. In the perspective of producing microspheres with hierarchical porosity, thermal treatments between 600 ◦ C and 1400 ◦ C were carried out in order to find an optimal pyrolysis temperature which would enable to retain the porosity at nanoscale without compromising the mechanical strength of the beads. Heating rate and dwelling time were 2 ◦ C/min and 1 h, respectively, for all the treatments. For comparison, we also produced porous microspheres by directly freezing in liquid nitrogen droplets of the same preceramic polymer solution used for the o/w emulsions. The morphology of the microspheres was observed by Scanning Electron Microscopy (SEM, JSM-6490, JOEL, Tokyo, Japan) and their size distribution was assessed by image analysis on the SEM micrographs. X Ray Diffraction analyses (XRD, Bruker AXS D8 Advance, Karlsruhe, Germany) were performed to investigate the phase assemblage after pyrolysis at 1400 ◦ C in argon atmosphere, by using Cu K␣1 radiation (0.15418 nm, 40 kV, 40 mA, step scan of 0.05◦ , counting time 2 s/step). Phase identification was performed using ® the software Match! (Crystal Impact GbR, Bonn, Germany), supported by data from PDF-2 database> (ICDD-International Centre for Diffraction Data, Newtown Square, PA). The influence of the freezing temperature on the macroporosity was investigated by mercury intrusion porosimetry, using the Pascal 140 and Pascal 240 porosimeters (Thermo Finnigan, Walthman, USA), with pressure up to 200 MPa, in order to measure the total open porosity and the pore size distribution in the range 0.007–120 m. The evolution of porosity during pyrolysis was investigated by nitrogen gas adsorption, using a Quantachrome Nova 1200e Surface Area and Pore Analyzer. Values of the specific surface area (SSA) were calculated by a multipoint Brunauer-Emmett-Teller (BET) analysis of the nitrogen adsorption/desorption isotherms at −196 ◦ C. A density functional theory (DFT) method was used to calculate the pore size distribution curves on both the adsorption and desorption branches of the isotherms. Before the analysis, the samples were outgassed under vacuum at 250 ◦ C for 12 h.
3. Results By performing emulsion and ice templating as schematized in Fig. 1, porous microspheres (Fig. 2a) with diameter ranging between 70 and 400 m, as measured by image analysis on the SEM micrographs, were produced after freeze drying. During pyrolysis, the polycarbosilane converted into silicon carbide; thus, the microspheres treated at 1400 ◦ C (Fig. 2b) exhibited a significant shrinkage, their diameter being in the range 30–310 m. The size distributions for microspheres before and after pyrolysis are compared in Fig. 2c. The XRD pattern (Fig. 2d) of the microspheres pyrolyzed at 1400 ◦ C in argon atmosphere shows that -SiC was the only crys-
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Fig. 1. Schematic of the emulsion-ice templating technique. PCS dissolved in cyclohexane is slowly added to the aqueous solution. Upon gentle stirring, an o/w macroemulsion forms that soon creams when agitation stops (a). The cream phase is then placed in a mold whose copper base is kept at a low temperature to induce unidirectional thermal gradient (b). During solidification, cyclohexane dendrites grow along a preferred orientation while polycarbosilane is concentrated in the interdendritic space (c).
talline phase detected, with fine crystallites of about 2–3 nm, calculated by XRD data by applying the Scherrer’s equation. While the ice templating technique can be used to form a macroporous scaffold, optimization of the thermal treatment offers control of the pores at the nanometric level, opening up the possibility of tuning the porosity at multiple length-scales [34–36]. The influence of the pyrolysis temperature on the nitrogen adsorption isotherms is shown in Fig. 3a. The values of the specific surface area, the total pore volume and the mean pore size are reported in Table 1. We compared thermal treatments in argon for 1 h, at temperature between 600 and 1400 ◦ C. Samples pyrolyzed at 600 ◦ C had the highest SSA (492 m2 g−1 ). The micro- and meso-porosity formed at low temperature during pyrolysis was eliminated as the temperature increased above 600 ◦ C, in accordance with published literature [34]. Indeed, microspheres heat treated at 800, 1000, and 1200 ◦ C possessed limited SSA (less than 5 m2 g−1 ). Samples pyrolyzed at 1400 ◦ C had a much higher SSA value (117 m2 g−1 ), because of the carbothermal reduction reaction occurring at high temperature creating micro- and meso-porosity [33–36]. The pore size distributions obtained from nitrogen adsorption/desorption isotherms are reported in Fig. 3b for the microspheres pyrolyzed at 600 and 1400 ◦ C. Both the total pore volume and the size distribution were influenced by the heat treatment temperature. The
Table 1 Specific Surface Area (BET), total pore volume, and average pore diameter, a function of pyrolysis temperature for microspheres that were obtained by freezing an o/w suspension at −60 ◦ C and were cross-linked at 200 ◦ C in air before pyrolysis. Pyrolysis temperature (◦ C)
SSA (BET) (m2 /g)
Total pore volume (cm3 /g)
Average pore diametera (nm)
600 800 1000 1200 1400
492 3 5 2 117
0.193 0.006 0.008 0.003 0.079
∼1 – – – ∼ 2.5
a The average pore diameters were obtained from the cumulative pore volume curves reported in Fig. 3b, measuring the diameters corresponding to 50% of the total pore volume.
total pore volumes for the low and the high temperature samples were 0.193 and 0.079 cm3 g−1 , respectively (see Table 1). While the microspheres pyrolyzed at 600 ◦ C possessed mainly micropores with diameter <2 nm, the sample heat treated at 1400 ◦ C showed both micropores and mesopores (see Fig. 3b). Thus, in order to produce hierarchical porosity, the temperature selected to treat the microspheres was 1400 ◦ C, since it yielded a good compromise between specific surface area and high temperature stability.
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Fig. 2. SEM micrographs of microspheres obtained by freezing an o/w emulsion at −60 ◦ C after freeze drying and oxidation curing at 200 ◦ C in air (a) and after pyrolysis at 1400 ◦ C in argon (b). Microspheres diameter distributions obtained by image analysis (c). XRD pattern of a sample treated at 1400 ◦ C for 1 h in argon (d).
The SEM observations, presented in Fig. 4, show the pore architecture of the microspheres fabricated via emulsion-ice templating, compared to that of the beads obtained by direct freezing of the preceramic polymer solution. Fig. 4a shows a microsphere that was gently crushed to reveal the inner pore architecture. Continuous and parallel channels corresponding to cyclohexane primary den-
drites run throughout the sphere, and smaller pores left by the secondary dendrite arms opened onto the ceramic walls. Fig. 4b shows a detail of the pores at higher magnification: an important feature was the absence of a dense skin on the microsphere surface, so that the pores are readily accessible. Conversely, the beads obtained from direct freezing (Fig. 4c and d) clearly show a
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Fig. 3. Influence of the pyrolysis temperature on the specific surface area of microspheres obtained by emulsion-ice templating. Nitrogen adsorption − desorption isotherms for microspheres treated at temperatures between 600 and 1400 ◦ C (a) and DFT pore size distributions for the samples treated at 600 and 1400 ◦ C (b).
dense skin on the surface and many cracks. Moreover, the inner porosity was highly inhomogeneous, as if very fine cyclohexane crystals grew inward in concomitance with larger and randomly oriented crystals possessing the characteristic symmetry of cyclohexane dendrites. To appreciate the influence of the freezing temperature on pore morphology, we compared microspheres obtained from emulsions frozen at −30, −60, and −100 ◦ C. The SEM micrographs reported in Fig. 5 document the role that this parameter played during solidification. Pore shape and size changed with the freezing temperature. Although the three samples present some similarities, such as the preferential orientation of the main pore channels, the pore morphology changed with the freezing temperature. The microspheres that were frozen at −30 ◦ C have larger channels and thicker ceramic walls. Instead of the characteristic pore morphology that would be expected to remain from cyclohexane dendrites, we observed a less ordered and highly interconnected porosity (Fig. 5a). The sample processed at −60 ◦ C showed the typical pore morphology associated with the cyclohexane dendrites, as already seen in Fig. 4a and b and here in Fig. 5b. This sample presented finer ceramic walls, smaller and straighter channels, and pores left from secondary den-
Table 2 Total volume of intrusion intruded mercury, open porosity and values of the pore size distribution peaks, as a function of the freezing temperature. Before the mercury intrusion porosimetry analyses, the microspheres were cross-linked at 200 ◦ C in air and treated at 1400 ◦ C for 1 h in argon. Freezing temperature (◦ C)
Total volume of intruded Hg (mm3 /g)
Open porosity (%)
Pore size distribution main peaks (m)
−30 −60 −100
2892 2530 2033
∼ 90 ∼ 89 ∼ 86
25.7 5.9 4.4
4.8 3.3 3.6
3.6 – –
drite arms were often evident. In Fig. 5c, the third sample, from emulsion frozen at −100 ◦ C, exhibited a refined morphology similar to that of microspheres prepared at −60 ◦ C. Mercury intrusion porosimetry results are shown in Fig. 6, the main data are summarized in Table 2. The total volume of intruded mercury (Fig. 6a) slightly changed for the samples frozen at different temperature, resulting in a similar open porosity of 90, 89, and 86 vol.% (Table 2) for the microspheres frozen at −30, −60 and −100 ◦ C, respectively. The pore size distributions (Fig. 6b) exhib-
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Fig. 4. SEM micrographs of microspheres obtained by emulsion-ice templating (a and b) and of beads obtained by direct freezing of droplets of preceramic polymer solution in liquid nitrogen (c and d). Both the microspheres and the beads were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
ited multiple peaks for all the three samples. Since the tail (towards large values) of the distributions can be associated with interparticle pores, i.e. voids occurring between the microspheres, pores larger than 30 m were not considered when analyzing the intraparticle porosity. From the comparison of pore size distributions and the SEM micrographs (Fig. 6c–e), we saw a further evidence that samples frozen at lower temperature possessed smaller channels. In particular, for the microspheres frozen at −30 ◦ C, the pore size distribution exhibited a broad peak centered at 25.7 m, and two narrow peaks at 4.8 and 3.6 m. The microspheres frozen at intermediate temperature showed a pore size distribution with two main peaks at 5.9 and 3.3 m. Similarly, the last sample obtained at −100◦ C showed two main peaks centered at 4.4 and 3.6 m.
4. Discussion As schematically illustrated in Fig. 1, our method comprises two techniques that were used to template the polycarbosilane preceramic polymer at different length-scales. First, we performed emulsion templating to prepare microspheres. The macroemulsion tended to cream, as expected for the significant droplets size and the density difference between the oil and water phases. The observed characteristics of o/w macroemulsion were in agreement with the theory that correlates the type and stability of macroemulsions to the equilibrium phase behavior of oil-watersurfactant mixtures [48,49]. The type of emulsion that forms when a mixture is stirred depends on the curvature of the nonionic surfactant monolayer at the oil − water interface. Surfactants of the polyethoxylated family have a spontaneous positive curvature
(with hydrophilic heads larger than the lipophilic tails) at temperatures below the phase inversion temperature (PIT), and thus the mixture will separate into a surfactant-rich aqueous phase, comprising direct micelles swollen by oil, and an almost pure oil phase. Under stirring, an o/w emulsion develops in this temperature range [48,49]. At temperature higher than the PIT, w/o emulsions are predicted to be stable because inverse micelles form spontaneously, as the dehydrated polar heads of the surfactant favor a negative curvature [48,49]. Although the measurement of the PIT of our oil- water-surfactant mixture is beyond the scope of this study, the temperature at which the emulsions were prepared (25 ◦ C) is assumed to be lower than the PIT of the system. Indeed, for similar nonionic surfactants in water/cyclohexane systems, a PIT of about 70 ◦ C is reported in literature [49,50]. Moreover, despite macroemulsions tend to cream and to coalesce, it has been shown that coalescence is faster at temperatures close to the PIT of the system, while the stability is improved at temperatures that are 20 ◦ C (or more) below PIT [49,50]. In our case, macroemulsions creamed when not agitated, but the coalescence rate was slow enough to enable the freezing of the system. Indeed, the cream phase was placed in the mold whose low temperature prompted the rapid formation of cyclohexane crystals, which templated the pores inside the microspheres after sublimation. Solidification conditions are key parameters for ice templating, as both the freezing temperature and the thermal gradient affect the pore morphology, orientation, and size [51–54]. In our experiments, the mold was a cylinder made of silicone rubber that ensured thermal insulation through the lateral walls. It was placed on a copper base that was kept at a constant temperature below
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Fig. 5. SEM micrographs of microspheres obtained by emulsion-ice templating, frozen at −30 ◦ C (a), −60 ◦ C (b) and −100 ◦ C (c). The three samples were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
the solidification temperature of the systems. This setup ensures the control on the temperature and the unidirectional thermal gradient, so that the cyclohexane dendrites grew along a preferential orientation. After oxidative cross-linking, the high temperature pyrolysis led to a size decrease, that corresponded to a linear shrinkage of up to 45% for samples pyrolysed at 1400 ◦ C. Shrinkage is inherently associated with the polymer-to-ceramic conversion because of the difference between the density of the precursor (∼1–1.5 g cm−3 ) and that of the ceramic product (∼2–3 g cm−3 ).
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Moreover, the ceramic yield of the polycarbosilane used in this study was 75%, as reported elsewhere [55], which means that also the polymer degradation contributed to shrinkage with loss of volatile species [56]. Polymer decomposition and gas evolution are also the causes of the transient porosity that developed at the nanometric level inside the microspheres, as demonstrated by the analyses of nitrogen adsorption isotherms (Fig. 3a and b). Our results are in agreement with what reported in the literature: it has been shown that when the pyrolysis is stopped at low temperature, for example at 600 ◦ C, the polymer-to-ceramic transformation is only partial, and hybrid materials with SSA of about 600 m2 g−1 can be obtained [34,57]. The transient porosity closes up if the samples are treated at or above 800 ◦ C, when the polymer-to-ceramic conversion is complete and shrinkage and viscous flow of the amorphous ceramic occur [34]. However, it has been reported that for high temperatures above 1300 ◦ C, polymer-derived ceramics can develop an open nanoporous structure, as crystallization and grain growth of SiC take place [33–36]. At such high temperatures the carbothermal reduction reaction, which occurs due to the presence of oxygen introduced into the polymer by the oxidative cross-linking (from ∼10 to 20 wt.% [58]), yields evolution of gaseous CO and can cause open nano-scale porosity. This is also the case of the sample treated at 1400 ◦ C, which showed an increase in SSA compared to samples treated 800, 1000 and 1200 ◦ C, and exhibited the presence of micro- and meso-pores with diameter up to 6 nm (see Fig. 3 and Table 1). It is worth noting that a pyrolysis for longer times at 1400 ◦ C gave similar SSA and pore size distribution values, suggesting the completion of the carbothermal reduction reaction already after 1 h (samples treated at 1400 ◦ C for 3 h had a SSA of 83 m2 g−1 ). With respect to the macroporosity left by cyclohexane crystals, SEM observations (Fig. 4) demonstrated that, by performing a controlled freezing of o/w emulsions, we could obtain a homogeneous pore morphology and overcome such drawbacks as the formation of a dense outer skin on the beads and inhomogeneous porosity, defects that are invariably obtained when droplets of a preceramic polymer solution were directly frozen in liquid nitrogen. We posit that the reason why such skin developed under these last condition, is that freezing was so fast that the phase separation between cyclohexane crystals and PCS did not occur, and a dense polymeric skin solidified almost instantaneously. Moreover, during the following drying step, the solvent sublimated and the skin shrunk and cracked extensively, as shown in Fig. 4c and d. Conversely, the emulsion/ice templating method ensured that the solidification of the emulsion occurred under controlled conditions and unidirectionally, thus obtaining continuous channels running inside the microspheres and open to the outside through the surface, features that are fundamental for applications when pore accessibility is concerned. The concentration of the preceramic polymer in the starting solution is a key parameter to control the total volume of the macro-pores, which mainly depends on the amount of solvent that crystallizes into dendrites. This is a great advantage of the ice − templating method, as open porosity can be easily tailored by varying the PCS/cyclohexane ratio in order to meet the requirements of the specific application. In their study on macroporous SiOC monoliths [39], Naviroj et al. indeed showed that the percentage of open porosity decreased from 91 to 61 vol.% when the content of preceramic polymer in cyclohexane increased from 5 to 40 wt.%. In the present study, all the samples were obtained from starting polymer solutions at the same concentration, and consequently similar values of open porosity were measured (86–90 vol.%, see Table 2). However, as shown in the literature, the volume of open porosity in the microspheres could be easily tuned by increasing the PCS content in the oil phase, if required, for example, to increase their mechanical strength.
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Fig. 6. Mercury intrusion porosimetry for the microspheres obtained by emulsion-ice templating, and corresponding SEM images of samples obtained by using a different freezing temperature. Comulative volume of intruded mercury (a) and pore size volume distribution (b) for microsperes frozen at −30, −60 and −100 ◦ C. SEM micrographs showing details of the pore morphology for the microspheres frozen at −30 ◦ C (c), −60 ◦ C (d) and −100 ◦ C (e). The three samples were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
The size of macro-pore could be tuned by changing the freezing temperature, as mercury intrusion porosimetry results (Fig. 6a and b) and SEM micrographs (Fig. 6c–e) show. In particular, the mercury intrusion porosimetry gave important information about the total porosity and the pore size distribution. However, when particles are analyzed by this technique, the results include both inter and intraparticle pores. The former ones are pores between particles and depend on the particle size and packing, while the latter ones are those located inside the single microspheres, and depend on the processing conditions. In general, the contributions of these two types of porosity to the total volume of intruded mercury can be discerned when the diameters of interparticle pores significantly exceed those of the intraparticle ones, yielding a clear bi-modal pore size distribution. When the demarcation between inter and intraparticle pores is less obvious, knowing the particle size distribution can help to approximate the size range where inter and intraparticle pores overlap. Indeed, some models have been developed to quantify the size distribution of dense spheres from
intrusion mercury porosimentry data [59]. In our case, we used the size distribution of the microspheres obtained by image analysis (Fig. 2c) to approximate the size limits of the interparticle pores. Since having a precise quantification of the interparticle porosity was beyond the scope of this study, we could use a simplified model known as the Meyer-Stone theory, which relates the breakthrough pressure, required to force mercury to penetrate the interparticle void spaces, to the diameter of the packed spheres [58]. From this simplified evaluation, the distribution of interparticle pore diameter results centered at around 45 m and overlapped with the intraparticle pore size distribution in the range 30–120 m. It is worth noticing that the curves of the intruded mercury volume for the three samples of microspheres were almost identical for pore diameters larger than 40 m, suggesting that, in such interval, the three samples had a similar porosity that was independent from the freezing temperature. This observation further supports the assumption that the distribution tail was associated with inter-
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particle pores, depending only on the microsphere size, which was affected by the emulsification conditions. The freezing temperature affected the pores at lower diameters, associated with ice templating. By comparing the SEM images of the samples obtained by freezing at different temperature (Fig. 6c–e), we observed that microspheres from emulsions frozen at lower temperature had smaller pores, as expected [41,42]. Moreover, the pore size distributions of the three samples showed multiple peaks (Fig. 6b). We associated the peaks centered at larger diameters (at 25.7, 5.9 and 4.4 m for the microspheres processed at −30, −60 and −100 ◦ C, respectively) to the channels left by the primary arms of cyclohexane dendrites. The peaks at lower diameters, following a similar trend, may correspond to the pores left by secondary dendrite arms, or to the interconnection between the main channels. Therefore, by controlling both the freezing conditions and the pyrolysis temperature, we were able to develop ceramic components possessing hierarchical porosity and variable pore architecture. 5. Conclusions The method presented here to produce porous ceramic microspheres is a simple approach that combines two templating methods at different length-scales: an oil-in-water (o/w) emulsion was prepared to shape spheres with diameter ranging from 50 to 300 m, while cyclohexane crystals formed during solidification and subsequently removed by sublimation were employed to template aligned channels inside the microbeads. We demonstrated that emulsion-ice templating of preceramic polymer solutions can be used to obtain defect-free ceramic microspheres with remarkably high open porosity (up to 90 vol.%). The pores were easily accessible because no dense skin formed on the microspheres surface. Crushed microspheres revealed a regular inner structure, with pores mainly aligned in one direction, running throughout the particles. We produced samples with multimodal porosity in the range 1–30 m. By decreasing the freezing temperature, the structure was refined and the main peaks of the pore size distributions shifted to smaller pore diameters. The use of a preceramic polymer, in our case polycarbosilane, offers the possibility to tune the phase composition and add porosity at the nano-scale, by optimizing the thermal treatment during which the polymer-toceramic conversion occurs. In particular, microspheres pyrolyzed at 1400 ◦ C comprised only SiC as crystalline phase, had a specific surface area of 117 m2 g−1 and exhibited nano-scale pores (both micro- and meso-pores) with diameter up to 6 nm. Concluding, the emulsion-ice templating of preceramic polymer solutions proved to be successful to produce ceramic microspheres with porosity spanning over multiple length-scales. This method opens up interesting possibilities to fabricate tailored catalyst supports, microreactors, adsorbents, etc., in which nano and macroporosity can be independently tuned for the specific applications. Acknowledgments The research leading to these results received fundings from the European Commission, Seventh Framework Programme, under the Grant Agreement no. 600376. VN was a Marie Curie-Piscopia Fellow of the University of Padua. References [1] Cellular Ceramics: Structure, Manufacturing, Properties and Applications, in: M. Scheffler, P. Colombo (Eds.), Wiley–VCH Verlag GmbH, Weinheim, Germany, 2005. [2] P. Colombo, In praise of pores, Science 322 (2008) 381–393.
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Ceramic microspheres with controlled porosity by emulsion-ice templating Valentina Naglieri a,∗ , Paolo Colombo a,b a b
Department of Industrial Engineering, University of Padova, via Marzolo 9, 35131 Padova, Italy Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, USA
a r t i c l e
i n f o
Article history: Received 6 December 2016 Received in revised form 15 February 2017 Accepted 17 February 2017 Available online xxx Keywords: Freeze-casting Preceramic polymers Microbeads Hierarchical porosity Emulsion templating
a b s t r a c t Silicon Carbide porous microspheres were fabricated using a preceramic polymer by emulsion-ice templating. An oil-in-water macroemulsion was prepared by adding an organic solution, comprising polycarbosilane and cyclohexane, to the aqueous phase containing a nonionic surfactant. Upon directional freezing and freeze drying, microspheres with aligned pores were obtained. The influence of processing parameters was assessed. In particular, the solidification temperature affected size and morphology of the macroporosity. Our strategy enables an independent control of macro and nanoporosity. Indeed, the thermal treatment can be optimized to tune the micro-porosity at the nanoscale as well as the specific surface area of the samples. The emulsion-ice templating technique was optimized to produce microspheres with multimodal macroporosity up to 90 vol.% in the range 1–30 m, with micro- and meso-pores with diameter up to 6 nm, and specific surface area as high as 117 m2 g−1 . © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Porous ceramics find applications in many industries where they are used as filters, membranes, thermal insulators, catalytic supports, burners, biomedical devices, etc. [1]. Their versatility is achieved by simultaneously tuning chemical composition and architecture. By tailoring the architecture and characteristics of the porosity (e.g. pore volume, size and size distribution, tortuosity and interconnectivity), such properties as thermal and electrical conductivity, specific strength and modulus, permeability, and corrosion rate can be optimized to meet the requirements of each application [2]. Among the many porous ceramic components, beads and microspheres have received great attention for their potential use as drug carriers, adsorbents, microreactors, etc. [3–7]. Spherical particles offer good mobility, flowability, high packing density, ease of separation and reuse after regeneration. However, undesired pressure drop and poor permeability are often observed in conventional packed beds. Finding a compromise between high surface area and pores accessibility is crucial [8–10]. Exemplary is the case of catalytic supports, where the introduction of channels, serving as the
∗ Corresponding author. E-mail address: [email protected] (V. Naglieri).
main paths for fast mass transport, decreases the amount of active sites available for the reaction and thus limits the overall yield [11]. Consequently, a rational design of the structure at all lengthscales is necessary. Coppens and coworkers have modeled pore configurations optimized for maximum yield, demonstrating that a hierarchical network of meso/macropores performs better than monomodal porous systems in many processes, such as heterogeneous catalysis and separation [11–14]. Their results, although case-specific, emphasize the importance of a multiscale design approach and motivate the development of processing techniques that yield multimodal porosity, in order to achieve optimum mass diffusion and a high active surface, and to overcome such drawbacks as high pressure drop or poor heat transfer. Aerosol routes [15], double emulsion [16,17], polymer templating [18,19], electrohydrodinamic forming [20], microfluidic processing [21], ionotropic gelation [4,5], freeze drying [22,23] and phase separation-assisted pyrolysis [24] are just some of the techniques proposed to produce porous ceramic microspheres. As diverse as they are, these strategies can yield spheres with diameter ranging from 100 nm to few millimeters. The pores inside the beads−obtained, for example, by self-assembly, polymer or emulsion templating−are often spherical cavities partly connected to each other through smaller windows. However, macro-channels spanning the entire bead and connected to smaller pores in a treelike fashion can be better suited for some specific applications, such
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as processes that involve the deposition of reaction species onto the catalyst surface [25,26]. Although the advantages of complex pore networks have been illustrated by modeling studies [11,27,28] and are testified by the frequent occurrence of such patterns in nature [14], manufacturing the desired pore geometry remains a challenge − especially for porous microspheres. Here we propose a strategy that merges different techniques: by processing a preceramic polymer solution via emulsion and ice templating we obtained porous ceramic microspheres with complex pore networks. Despite the popularity of these techniques, in the literature there are just few examples of porous microspheres obtained by combining emulsion and ice templating [29,30]. These studies are limited to the production of polymeric spheres [29], or utilize ceramic powders [30]. What sets our approach apart is the use of a preceramic polymer that, upon a proper thermal treatment, yields silicon based inorganic components that can perform better than polymers in a number of applications where high chemical, thermal, and mechanical resistance are required. Preceramic polymers, moreover, offer many advantages over other ceramic precursors or ceramic powders. Their development has prompted innovation in many field [31], such as in the production of porous ceramics [32]. Of particular relevance for this work is the possibility of tuning the porosity of polymer-derived ceramics at multiple length-scales by combining proper forming techniques and thermal treatments. Indeed, the optimization of the pyrolysis step has been shown to play a pivotal role in retaining the nanoscale porosity up to high temperature or in developing it through carbothermal reduction reactions [33–36]. Ice templating of preceramic polymers has been investigated in a few studies concerning the fabrication of monoliths with aligned pores [37–40]. This technique, also referred to as freeze casting, has been mostly used with colloidal suspensions and with water-soluble polymers [41–45], while fewer examples of polymeric honeycomb monoliths obtained by freezing organic solutions have also been reported [46,47]. Ice templating of porous ceramic beads has been carried out [23], but the proposed method (directly freezing a preceramic polymer solution in liquid nitrogen) did not offer a full control on the pore morphology, and in particular could yield a dense outer skin blocking the access to the inside of the beads. Here, we show that such drawbacks can be overcome by performing a controlled freezing of o/w emulsions.
2. Experimental As a precursor of silicon carbide (SiC), we selected a commercial polycarbosilane (PCS, Type S, Nippon Carbon Co. Ltd., Yokohama, Japan). Cyclohexane (Sigma–Aldrich, St. Louis, MO) was used as organic solvent for the PCS. A polyoxyethylene-sorbitan20-monooleate (also known as polysorbate 80 or commercially as Tween 80, Sigma–Aldrich, St. Louis, MO) was employed as a nonionic surfactant, dissolved in deionized water. Fig. 1 illustrates the details of our emulsion-ice templating method. After dissolving the PCS in the cyclohexane (weight ratio of 1:9) and the Tween 80 in deionized water (weight ratio of 5:100), the o/w emulsion was prepared by slowly adding the oil phase to the aqueous solution (volume ratio 1:1). Upon gentle stirring (stirring speed of 200 rpm), an o/w macroemulsion formed that soon creamed when agitation stopped (Fig. 1a). The cream phase, which comprises oil droplets separated by a continuous film of saturated surfactant, was then placed in a mold whose copper bottom was kept at a low temperature to induce directional freezing (Fig. 1b). By applying a unidirectional thermal gradient, both the dispersed and the continuous phases solidified. Since the oil droplets comprised a solution of polycarbosilane in cyclohexane, a phase separation occurred during solidification: cyclohexane dendrites grew along
a preferred orientation while polycarbosilane concentrated in the interdendritic space (Fig. 1c). In order to investigate the influence of the bottom temperature on the macroporosity, we produced microsphere by freezing at −30, −60 and −100 ◦ C. Once the o/w emulsion was completely solid, the samples were transferred in a freeze dryer (FreeZone 2.5 l Benchtop Freeze Dry System, Labconco, Kansas City, MI) where a vacuum level of 0.133 mBar and a collector temperature of −50 ◦ C were maintained in order to sublimate water and cyclohexane. Loose microspheres possessing aligned pores which are the replica of cyclohexane dendrites were obtained. After oxidation curing at 200 ◦ C for 1 h in air (heating rate of 0.3 ◦ C/min) polycarbosilane microsphere were heat treated at higher temperature in argon atmosphere, in order to convert the polymer into a ceramic. In the perspective of producing microspheres with hierarchical porosity, thermal treatments between 600 ◦ C and 1400 ◦ C were carried out in order to find an optimal pyrolysis temperature which would enable to retain the porosity at nanoscale without compromising the mechanical strength of the beads. Heating rate and dwelling time were 2 ◦ C/min and 1 h, respectively, for all the treatments. For comparison, we also produced porous microspheres by directly freezing in liquid nitrogen droplets of the same preceramic polymer solution used for the o/w emulsions. The morphology of the microspheres was observed by Scanning Electron Microscopy (SEM, JSM-6490, JOEL, Tokyo, Japan) and their size distribution was assessed by image analysis on the SEM micrographs. X Ray Diffraction analyses (XRD, Bruker AXS D8 Advance, Karlsruhe, Germany) were performed to investigate the phase assemblage after pyrolysis at 1400 ◦ C in argon atmosphere, by using Cu K␣1 radiation (0.15418 nm, 40 kV, 40 mA, step scan of 0.05◦ , counting time 2 s/step). Phase identification was performed using ® the software Match! (Crystal Impact GbR, Bonn, Germany), supported by data from PDF-2 database> (ICDD-International Centre for Diffraction Data, Newtown Square, PA). The influence of the freezing temperature on the macroporosity was investigated by mercury intrusion porosimetry, using the Pascal 140 and Pascal 240 porosimeters (Thermo Finnigan, Walthman, USA), with pressure up to 200 MPa, in order to measure the total open porosity and the pore size distribution in the range 0.007–120 m. The evolution of porosity during pyrolysis was investigated by nitrogen gas adsorption, using a Quantachrome Nova 1200e Surface Area and Pore Analyzer. Values of the specific surface area (SSA) were calculated by a multipoint Brunauer-Emmett-Teller (BET) analysis of the nitrogen adsorption/desorption isotherms at −196 ◦ C. A density functional theory (DFT) method was used to calculate the pore size distribution curves on both the adsorption and desorption branches of the isotherms. Before the analysis, the samples were outgassed under vacuum at 250 ◦ C for 12 h.
3. Results By performing emulsion and ice templating as schematized in Fig. 1, porous microspheres (Fig. 2a) with diameter ranging between 70 and 400 m, as measured by image analysis on the SEM micrographs, were produced after freeze drying. During pyrolysis, the polycarbosilane converted into silicon carbide; thus, the microspheres treated at 1400 ◦ C (Fig. 2b) exhibited a significant shrinkage, their diameter being in the range 30–310 m. The size distributions for microspheres before and after pyrolysis are compared in Fig. 2c. The XRD pattern (Fig. 2d) of the microspheres pyrolyzed at 1400 ◦ C in argon atmosphere shows that -SiC was the only crys-
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Fig. 1. Schematic of the emulsion-ice templating technique. PCS dissolved in cyclohexane is slowly added to the aqueous solution. Upon gentle stirring, an o/w macroemulsion forms that soon creams when agitation stops (a). The cream phase is then placed in a mold whose copper base is kept at a low temperature to induce unidirectional thermal gradient (b). During solidification, cyclohexane dendrites grow along a preferred orientation while polycarbosilane is concentrated in the interdendritic space (c).
talline phase detected, with fine crystallites of about 2–3 nm, calculated by XRD data by applying the Scherrer’s equation. While the ice templating technique can be used to form a macroporous scaffold, optimization of the thermal treatment offers control of the pores at the nanometric level, opening up the possibility of tuning the porosity at multiple length-scales [34–36]. The influence of the pyrolysis temperature on the nitrogen adsorption isotherms is shown in Fig. 3a. The values of the specific surface area, the total pore volume and the mean pore size are reported in Table 1. We compared thermal treatments in argon for 1 h, at temperature between 600 and 1400 ◦ C. Samples pyrolyzed at 600 ◦ C had the highest SSA (492 m2 g−1 ). The micro- and meso-porosity formed at low temperature during pyrolysis was eliminated as the temperature increased above 600 ◦ C, in accordance with published literature [34]. Indeed, microspheres heat treated at 800, 1000, and 1200 ◦ C possessed limited SSA (less than 5 m2 g−1 ). Samples pyrolyzed at 1400 ◦ C had a much higher SSA value (117 m2 g−1 ), because of the carbothermal reduction reaction occurring at high temperature creating micro- and meso-porosity [33–36]. The pore size distributions obtained from nitrogen adsorption/desorption isotherms are reported in Fig. 3b for the microspheres pyrolyzed at 600 and 1400 ◦ C. Both the total pore volume and the size distribution were influenced by the heat treatment temperature. The
Table 1 Specific Surface Area (BET), total pore volume, and average pore diameter, a function of pyrolysis temperature for microspheres that were obtained by freezing an o/w suspension at −60 ◦ C and were cross-linked at 200 ◦ C in air before pyrolysis. Pyrolysis temperature (◦ C)
SSA (BET) (m2 /g)
Total pore volume (cm3 /g)
Average pore diametera (nm)
600 800 1000 1200 1400
492 3 5 2 117
0.193 0.006 0.008 0.003 0.079
∼1 – – – ∼ 2.5
a The average pore diameters were obtained from the cumulative pore volume curves reported in Fig. 3b, measuring the diameters corresponding to 50% of the total pore volume.
total pore volumes for the low and the high temperature samples were 0.193 and 0.079 cm3 g−1 , respectively (see Table 1). While the microspheres pyrolyzed at 600 ◦ C possessed mainly micropores with diameter <2 nm, the sample heat treated at 1400 ◦ C showed both micropores and mesopores (see Fig. 3b). Thus, in order to produce hierarchical porosity, the temperature selected to treat the microspheres was 1400 ◦ C, since it yielded a good compromise between specific surface area and high temperature stability.
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Fig. 2. SEM micrographs of microspheres obtained by freezing an o/w emulsion at −60 ◦ C after freeze drying and oxidation curing at 200 ◦ C in air (a) and after pyrolysis at 1400 ◦ C in argon (b). Microspheres diameter distributions obtained by image analysis (c). XRD pattern of a sample treated at 1400 ◦ C for 1 h in argon (d).
The SEM observations, presented in Fig. 4, show the pore architecture of the microspheres fabricated via emulsion-ice templating, compared to that of the beads obtained by direct freezing of the preceramic polymer solution. Fig. 4a shows a microsphere that was gently crushed to reveal the inner pore architecture. Continuous and parallel channels corresponding to cyclohexane primary den-
drites run throughout the sphere, and smaller pores left by the secondary dendrite arms opened onto the ceramic walls. Fig. 4b shows a detail of the pores at higher magnification: an important feature was the absence of a dense skin on the microsphere surface, so that the pores are readily accessible. Conversely, the beads obtained from direct freezing (Fig. 4c and d) clearly show a
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Fig. 3. Influence of the pyrolysis temperature on the specific surface area of microspheres obtained by emulsion-ice templating. Nitrogen adsorption − desorption isotherms for microspheres treated at temperatures between 600 and 1400 ◦ C (a) and DFT pore size distributions for the samples treated at 600 and 1400 ◦ C (b).
dense skin on the surface and many cracks. Moreover, the inner porosity was highly inhomogeneous, as if very fine cyclohexane crystals grew inward in concomitance with larger and randomly oriented crystals possessing the characteristic symmetry of cyclohexane dendrites. To appreciate the influence of the freezing temperature on pore morphology, we compared microspheres obtained from emulsions frozen at −30, −60, and −100 ◦ C. The SEM micrographs reported in Fig. 5 document the role that this parameter played during solidification. Pore shape and size changed with the freezing temperature. Although the three samples present some similarities, such as the preferential orientation of the main pore channels, the pore morphology changed with the freezing temperature. The microspheres that were frozen at −30 ◦ C have larger channels and thicker ceramic walls. Instead of the characteristic pore morphology that would be expected to remain from cyclohexane dendrites, we observed a less ordered and highly interconnected porosity (Fig. 5a). The sample processed at −60 ◦ C showed the typical pore morphology associated with the cyclohexane dendrites, as already seen in Fig. 4a and b and here in Fig. 5b. This sample presented finer ceramic walls, smaller and straighter channels, and pores left from secondary den-
Table 2 Total volume of intrusion intruded mercury, open porosity and values of the pore size distribution peaks, as a function of the freezing temperature. Before the mercury intrusion porosimetry analyses, the microspheres were cross-linked at 200 ◦ C in air and treated at 1400 ◦ C for 1 h in argon. Freezing temperature (◦ C)
Total volume of intruded Hg (mm3 /g)
Open porosity (%)
Pore size distribution main peaks (m)
−30 −60 −100
2892 2530 2033
∼ 90 ∼ 89 ∼ 86
25.7 5.9 4.4
4.8 3.3 3.6
3.6 – –
drite arms were often evident. In Fig. 5c, the third sample, from emulsion frozen at −100 ◦ C, exhibited a refined morphology similar to that of microspheres prepared at −60 ◦ C. Mercury intrusion porosimetry results are shown in Fig. 6, the main data are summarized in Table 2. The total volume of intruded mercury (Fig. 6a) slightly changed for the samples frozen at different temperature, resulting in a similar open porosity of 90, 89, and 86 vol.% (Table 2) for the microspheres frozen at −30, −60 and −100 ◦ C, respectively. The pore size distributions (Fig. 6b) exhib-
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Fig. 4. SEM micrographs of microspheres obtained by emulsion-ice templating (a and b) and of beads obtained by direct freezing of droplets of preceramic polymer solution in liquid nitrogen (c and d). Both the microspheres and the beads were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
ited multiple peaks for all the three samples. Since the tail (towards large values) of the distributions can be associated with interparticle pores, i.e. voids occurring between the microspheres, pores larger than 30 m were not considered when analyzing the intraparticle porosity. From the comparison of pore size distributions and the SEM micrographs (Fig. 6c–e), we saw a further evidence that samples frozen at lower temperature possessed smaller channels. In particular, for the microspheres frozen at −30 ◦ C, the pore size distribution exhibited a broad peak centered at 25.7 m, and two narrow peaks at 4.8 and 3.6 m. The microspheres frozen at intermediate temperature showed a pore size distribution with two main peaks at 5.9 and 3.3 m. Similarly, the last sample obtained at −100◦ C showed two main peaks centered at 4.4 and 3.6 m.
4. Discussion As schematically illustrated in Fig. 1, our method comprises two techniques that were used to template the polycarbosilane preceramic polymer at different length-scales. First, we performed emulsion templating to prepare microspheres. The macroemulsion tended to cream, as expected for the significant droplets size and the density difference between the oil and water phases. The observed characteristics of o/w macroemulsion were in agreement with the theory that correlates the type and stability of macroemulsions to the equilibrium phase behavior of oil-watersurfactant mixtures [48,49]. The type of emulsion that forms when a mixture is stirred depends on the curvature of the nonionic surfactant monolayer at the oil − water interface. Surfactants of the polyethoxylated family have a spontaneous positive curvature
(with hydrophilic heads larger than the lipophilic tails) at temperatures below the phase inversion temperature (PIT), and thus the mixture will separate into a surfactant-rich aqueous phase, comprising direct micelles swollen by oil, and an almost pure oil phase. Under stirring, an o/w emulsion develops in this temperature range [48,49]. At temperature higher than the PIT, w/o emulsions are predicted to be stable because inverse micelles form spontaneously, as the dehydrated polar heads of the surfactant favor a negative curvature [48,49]. Although the measurement of the PIT of our oil- water-surfactant mixture is beyond the scope of this study, the temperature at which the emulsions were prepared (25 ◦ C) is assumed to be lower than the PIT of the system. Indeed, for similar nonionic surfactants in water/cyclohexane systems, a PIT of about 70 ◦ C is reported in literature [49,50]. Moreover, despite macroemulsions tend to cream and to coalesce, it has been shown that coalescence is faster at temperatures close to the PIT of the system, while the stability is improved at temperatures that are 20 ◦ C (or more) below PIT [49,50]. In our case, macroemulsions creamed when not agitated, but the coalescence rate was slow enough to enable the freezing of the system. Indeed, the cream phase was placed in the mold whose low temperature prompted the rapid formation of cyclohexane crystals, which templated the pores inside the microspheres after sublimation. Solidification conditions are key parameters for ice templating, as both the freezing temperature and the thermal gradient affect the pore morphology, orientation, and size [51–54]. In our experiments, the mold was a cylinder made of silicone rubber that ensured thermal insulation through the lateral walls. It was placed on a copper base that was kept at a constant temperature below
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Fig. 5. SEM micrographs of microspheres obtained by emulsion-ice templating, frozen at −30 ◦ C (a), −60 ◦ C (b) and −100 ◦ C (c). The three samples were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
the solidification temperature of the systems. This setup ensures the control on the temperature and the unidirectional thermal gradient, so that the cyclohexane dendrites grew along a preferential orientation. After oxidative cross-linking, the high temperature pyrolysis led to a size decrease, that corresponded to a linear shrinkage of up to 45% for samples pyrolysed at 1400 ◦ C. Shrinkage is inherently associated with the polymer-to-ceramic conversion because of the difference between the density of the precursor (∼1–1.5 g cm−3 ) and that of the ceramic product (∼2–3 g cm−3 ).
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Moreover, the ceramic yield of the polycarbosilane used in this study was 75%, as reported elsewhere [55], which means that also the polymer degradation contributed to shrinkage with loss of volatile species [56]. Polymer decomposition and gas evolution are also the causes of the transient porosity that developed at the nanometric level inside the microspheres, as demonstrated by the analyses of nitrogen adsorption isotherms (Fig. 3a and b). Our results are in agreement with what reported in the literature: it has been shown that when the pyrolysis is stopped at low temperature, for example at 600 ◦ C, the polymer-to-ceramic transformation is only partial, and hybrid materials with SSA of about 600 m2 g−1 can be obtained [34,57]. The transient porosity closes up if the samples are treated at or above 800 ◦ C, when the polymer-to-ceramic conversion is complete and shrinkage and viscous flow of the amorphous ceramic occur [34]. However, it has been reported that for high temperatures above 1300 ◦ C, polymer-derived ceramics can develop an open nanoporous structure, as crystallization and grain growth of SiC take place [33–36]. At such high temperatures the carbothermal reduction reaction, which occurs due to the presence of oxygen introduced into the polymer by the oxidative cross-linking (from ∼10 to 20 wt.% [58]), yields evolution of gaseous CO and can cause open nano-scale porosity. This is also the case of the sample treated at 1400 ◦ C, which showed an increase in SSA compared to samples treated 800, 1000 and 1200 ◦ C, and exhibited the presence of micro- and meso-pores with diameter up to 6 nm (see Fig. 3 and Table 1). It is worth noting that a pyrolysis for longer times at 1400 ◦ C gave similar SSA and pore size distribution values, suggesting the completion of the carbothermal reduction reaction already after 1 h (samples treated at 1400 ◦ C for 3 h had a SSA of 83 m2 g−1 ). With respect to the macroporosity left by cyclohexane crystals, SEM observations (Fig. 4) demonstrated that, by performing a controlled freezing of o/w emulsions, we could obtain a homogeneous pore morphology and overcome such drawbacks as the formation of a dense outer skin on the beads and inhomogeneous porosity, defects that are invariably obtained when droplets of a preceramic polymer solution were directly frozen in liquid nitrogen. We posit that the reason why such skin developed under these last condition, is that freezing was so fast that the phase separation between cyclohexane crystals and PCS did not occur, and a dense polymeric skin solidified almost instantaneously. Moreover, during the following drying step, the solvent sublimated and the skin shrunk and cracked extensively, as shown in Fig. 4c and d. Conversely, the emulsion/ice templating method ensured that the solidification of the emulsion occurred under controlled conditions and unidirectionally, thus obtaining continuous channels running inside the microspheres and open to the outside through the surface, features that are fundamental for applications when pore accessibility is concerned. The concentration of the preceramic polymer in the starting solution is a key parameter to control the total volume of the macro-pores, which mainly depends on the amount of solvent that crystallizes into dendrites. This is a great advantage of the ice − templating method, as open porosity can be easily tailored by varying the PCS/cyclohexane ratio in order to meet the requirements of the specific application. In their study on macroporous SiOC monoliths [39], Naviroj et al. indeed showed that the percentage of open porosity decreased from 91 to 61 vol.% when the content of preceramic polymer in cyclohexane increased from 5 to 40 wt.%. In the present study, all the samples were obtained from starting polymer solutions at the same concentration, and consequently similar values of open porosity were measured (86–90 vol.%, see Table 2). However, as shown in the literature, the volume of open porosity in the microspheres could be easily tuned by increasing the PCS content in the oil phase, if required, for example, to increase their mechanical strength.
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Fig. 6. Mercury intrusion porosimetry for the microspheres obtained by emulsion-ice templating, and corresponding SEM images of samples obtained by using a different freezing temperature. Comulative volume of intruded mercury (a) and pore size volume distribution (b) for microsperes frozen at −30, −60 and −100 ◦ C. SEM micrographs showing details of the pore morphology for the microspheres frozen at −30 ◦ C (c), −60 ◦ C (d) and −100 ◦ C (e). The three samples were treated under the same conditions: freeze dried, cured in air at 200 ◦ C, and pyrolyzed at 1400 ◦ C for 1 h in argon.
The size of macro-pore could be tuned by changing the freezing temperature, as mercury intrusion porosimetry results (Fig. 6a and b) and SEM micrographs (Fig. 6c–e) show. In particular, the mercury intrusion porosimetry gave important information about the total porosity and the pore size distribution. However, when particles are analyzed by this technique, the results include both inter and intraparticle pores. The former ones are pores between particles and depend on the particle size and packing, while the latter ones are those located inside the single microspheres, and depend on the processing conditions. In general, the contributions of these two types of porosity to the total volume of intruded mercury can be discerned when the diameters of interparticle pores significantly exceed those of the intraparticle ones, yielding a clear bi-modal pore size distribution. When the demarcation between inter and intraparticle pores is less obvious, knowing the particle size distribution can help to approximate the size range where inter and intraparticle pores overlap. Indeed, some models have been developed to quantify the size distribution of dense spheres from
intrusion mercury porosimentry data [59]. In our case, we used the size distribution of the microspheres obtained by image analysis (Fig. 2c) to approximate the size limits of the interparticle pores. Since having a precise quantification of the interparticle porosity was beyond the scope of this study, we could use a simplified model known as the Meyer-Stone theory, which relates the breakthrough pressure, required to force mercury to penetrate the interparticle void spaces, to the diameter of the packed spheres [58]. From this simplified evaluation, the distribution of interparticle pore diameter results centered at around 45 m and overlapped with the intraparticle pore size distribution in the range 30–120 m. It is worth noticing that the curves of the intruded mercury volume for the three samples of microspheres were almost identical for pore diameters larger than 40 m, suggesting that, in such interval, the three samples had a similar porosity that was independent from the freezing temperature. This observation further supports the assumption that the distribution tail was associated with inter-
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particle pores, depending only on the microsphere size, which was affected by the emulsification conditions. The freezing temperature affected the pores at lower diameters, associated with ice templating. By comparing the SEM images of the samples obtained by freezing at different temperature (Fig. 6c–e), we observed that microspheres from emulsions frozen at lower temperature had smaller pores, as expected [41,42]. Moreover, the pore size distributions of the three samples showed multiple peaks (Fig. 6b). We associated the peaks centered at larger diameters (at 25.7, 5.9 and 4.4 m for the microspheres processed at −30, −60 and −100 ◦ C, respectively) to the channels left by the primary arms of cyclohexane dendrites. The peaks at lower diameters, following a similar trend, may correspond to the pores left by secondary dendrite arms, or to the interconnection between the main channels. Therefore, by controlling both the freezing conditions and the pyrolysis temperature, we were able to develop ceramic components possessing hierarchical porosity and variable pore architecture. 5. Conclusions The method presented here to produce porous ceramic microspheres is a simple approach that combines two templating methods at different length-scales: an oil-in-water (o/w) emulsion was prepared to shape spheres with diameter ranging from 50 to 300 m, while cyclohexane crystals formed during solidification and subsequently removed by sublimation were employed to template aligned channels inside the microbeads. We demonstrated that emulsion-ice templating of preceramic polymer solutions can be used to obtain defect-free ceramic microspheres with remarkably high open porosity (up to 90 vol.%). The pores were easily accessible because no dense skin formed on the microspheres surface. Crushed microspheres revealed a regular inner structure, with pores mainly aligned in one direction, running throughout the particles. We produced samples with multimodal porosity in the range 1–30 m. By decreasing the freezing temperature, the structure was refined and the main peaks of the pore size distributions shifted to smaller pore diameters. The use of a preceramic polymer, in our case polycarbosilane, offers the possibility to tune the phase composition and add porosity at the nano-scale, by optimizing the thermal treatment during which the polymer-toceramic conversion occurs. In particular, microspheres pyrolyzed at 1400 ◦ C comprised only SiC as crystalline phase, had a specific surface area of 117 m2 g−1 and exhibited nano-scale pores (both micro- and meso-pores) with diameter up to 6 nm. Concluding, the emulsion-ice templating of preceramic polymer solutions proved to be successful to produce ceramic microspheres with porosity spanning over multiple length-scales. This method opens up interesting possibilities to fabricate tailored catalyst supports, microreactors, adsorbents, etc., in which nano and macroporosity can be independently tuned for the specific applications. Acknowledgments The research leading to these results received fundings from the European Commission, Seventh Framework Programme, under the Grant Agreement no. 600376. VN was a Marie Curie-Piscopia Fellow of the University of Padua. References [1] Cellular Ceramics: Structure, Manufacturing, Properties and Applications, in: M. Scheffler, P. Colombo (Eds.), Wiley–VCH Verlag GmbH, Weinheim, Germany, 2005. [2] P. Colombo, In praise of pores, Science 322 (2008) 381–393.
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Please cite this article in press as: V. Naglieri, P. Colombo, Ceramic microspheres with controlled porosity by emulsion-ice templating, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.02.033