Ice templating of ZrB2–SiC systems

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Ice templating of ZrB2–SiC systems Valentina Medrin, Diletta Sciti, Daniele Dalle Fabbriche, Andreana Piancastelli, Elena Landi CNR-ISTEC, National Research Council of Italy, Institute of Science and Technology for Ceramics, via Granarolo, 64, I-48018 Faenza, Italy Received 3 February 2015; received in revised form 16 March 2015; accepted 19 April 2015

Abstract Ice templating of a binary ZrB2–SiC system was set to obtain porous architectures with lamellar structure. Two commercial ammonium polyacrylates, with different molecular weights and pH range of activity, were used as dispersants. Slurries were produced by varying solid loadings (38–48 vol%) and/or the dispersant concentrations (from 0.4 to 2 wt%). It was found that an excess of dispersant is necessary to achieve a lamellar morphology. The more effective is the dispersant in optimizing electro-steric repulsions between particles, the more effective is in promoting the formation of a lamellar morphology. Preliminary compressive strength tests were carried out. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Zirconium diboride; Silicon carbide; Suspension stability; Freeze casting; Porous ceramics

1. Introduction Zirconium, hafnium and tantalum boride, nitride, and carbide based materials and composites are considered ultra-high temperature ceramics, UHTCs, because they combine outstanding robustness and refractoriness (melting temperatures above 3000 1C) [1] and are widely recognized as the unique materials for harsh service environments, especially for aerospace, rocket propulsion and energy applications. Additional applications of UHTCs include refractory linings and furnace heating elements, high temperature electrodes, substrates for microelectronics, cutting tools, etc. The potential of these compounds was initially explored for dense massive ceramics, however, UHTC porous bodies may also have applications in aerospace, as well as in energy sectors or hot gases and molten metal filtration and catalysis [2,3]. Processing of porous UHTC is possible thanks to wet forming techniques [2,4–6]. Ice templating is a versatile and environmentally friendly forming technique that enables the achievement of complex anisotropic porous microstructures [7–10]. It has been widely used for many porous ceramics such as alumina [7,8], zirconia or alumina–zirconia [11,12], titanium dioxide [13], silica [14] and hydroxyapatite [15]. Ice templating is basically a watern

Corressponding author. Tel.: þ39 0546 699751; fax: þ 39 0546 46381. E-mail address: [email protected] (V. Medri).

based freeze casting, consisting in freezing of an aqueous suspension, followed by sublimation of the ice that acts as pore template. Conventional sintering is then adopted to consolidate the walls of the porous ceramic structure [7,8]. The final microstructure and the properties are determined by the ice crystal growth direction during freezing [7–10]. Unidirectional channel-like (lamellar) porosity is obtained in the case of unidirectional freezing. Some of the authors of the present work, successfully fabricated porous architectures with lamellar structure based on monophasic ZrB2 [16] alternatively using two commercial ammonium polyacrylates, as dispersants. However, when biphasic systems with different particle size ranges are considered, the process should be substantially revised to avoid demixing and particle segregation, as well the rheology should be optimized to ensure unidirectional ice growth [17,18]. In this work, for the first time ice templating of a binary ZrB2–SiC system was studied to obtain composite samples with a dense matrix and lamellar macro-porosity. Other authors [19] adopted camphene-based freeze casting to fabricate ZrB2– SiC porous ceramic, but with a 3-dimensional pore network where camphene dendrites acted as pore formers. The motivation of this work is the development of porous ceramics with application as ceramic solar volumetric absorbers for high temperature (41000 1C) and high-efficiency air heating

http://dx.doi.org/10.1016/j.ceramint.2015.04.098 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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Table 1 Compositions, viscosities at 10 s  1 and y¼ ax  k equations interpolating the viscosity vs. shear rate curves (Fig. 1) of ZrB2–SiC based slurries. Sample code

Group code

Dispersant

Solid loading (vol%)

Dispersanta (wt%)

Viscosity at 10 s  1 cP

y¼ ax  k

DZS38-1 DZS38-2 DZS48-2 PZS48-0.4 PZS48-1 PZS48-1.5

DZS

Duramax

PZS

Dolapix

38 38 48 48 48 48

1 2 2 0.4 1 1.5

1290 363 2130 726 878 892

y¼ 9106.4   0.855 y¼ 3762.8   1.041 y¼ 16534   0.901 y¼ 3918.5   0.72 y¼ 4750.6   0.73 y¼ 4997.5   0.747

Active principle on the powder mixture weight.

with submicrometric geometric features, optimized porosity and pore structure. Realization of these properties requires a novel design of engineered pore structures, in contrast to natural random porosity, in order to enhance convection while maintaining high porosity and suitable effective pore diameter. The addition of SiC is used to improve the densification and oxidation resistance of ZrB2 [20]. This study may have also an impact on the sectors of thermal insulation and high temperature filtering [21]. Taking inspiration from the previous work [16], two commercial ammonium poly-acrylates, namely Duramax D3005 and Dolapix PC33, with different molecular weights and pH range of activity were used as dispersants. The powder suspensions were produced by varying the dispersants amount and/or the solid loading (38–48 vol%) in order to highlight the effect of the slurry formulation on the porous structure. This works is a first study aimed at the realization of architectures with a high fraction of unidirectional porosity.

100000 PZ45-1 DZS48-2 PZS48 1 5 PZS48-1.5

DZ45-1 DZS-38-2 PZS48 1 PZS48-1

DZ35-1 DZS38-1 PZS48 0 4 PZS48-0.4

10000 Viscos sity, cP P

a

1000

100 1

10 Shear rate, s-1

2. Materials and methods Commercial ZrB2 powder with D50 ¼ 2.77 μm and s.s. a.¼ 1 m2/g (H.C. Starck grade B, Karlsrhue, Germany) and βSiC powder with D50 ¼ 0.6 μm and s.s.a.¼ 11.6 m2/g (H.C. Starck BF12, Karlsrhue, Germany) were used to obtain porous composite ZrB2-based samples as final components. Commercial α-Si3N4 powder with D50 ¼ 0.58 μm and s.s.a.¼ 12.2 m2/g (Baysinid, Bayer, Leverkusen, Germany) was added as sintering aid. A mixture with composition: 76 vol% ZrB2-20 vol% SiC4 vol% Si3N4, was prepared by pre-mixing the raw materials for 72 h in ethanol with silicon nitride milling media. The solvent was then eliminated by rotary evaporation. The specific surface area of the final mixed powder was 2.4 m2/g. For the suspension preparation, two commercial anionic poly-electrolytes (ammonium poly-acrylates) were used as dispersants: – Duramax D3005, Rohm & Haas, USA, MW ¼ 2500 g mol  1, pH ¼ 6–7, concentration ¼ 35 wt%. – Dolapix PC33 from Zschimmer and Schwarz, Germany, MW¼ 16,000 g mol  1, pH¼ 9–10, concentration¼ 25 wt%.

According to previous studies [22,23], Duramax D3005 promotes a better stabilization of monophasic ZrB2 and ZrB2-SiC

Fig. 1. Logarithmic plots of the viscosity vs shear rate of DZS and PZS suspensions (Table 1). ZrB2 suspensions are coded DZ35-1 [16], DZ45-1 and PZ45-1 [16] where D and P stand for Dolapix PC33 and Duramax D3005, respectively, the solid loadings are 35 and 45 vol% and the dispersant amount is 1 wt%.

slurries compared to Dolapix PC33. Conversely Dolapix is more effective in stabilizing SiC powder. Electro-acoustical characterization [22] showed that the zeta potential reached a nearly constant value at 1 wt% of dispersant active principle over the powder mixture weight: values of about  48 mV and  66 mV were registered with Dolapix PC33 and Duramax D3005, respectively. On the basis of these results, a series of suspensions was prepared varying either the solid loading (38 and 48 vol%) or the dispersant amount (from 0.4 to 2 wt%). Suspensions (and related samples) with Duramax D3005 or Dolapix PC33 were respectively coded DZS or PZS, as listed in Table 1. Viscosity measurements were carried out by a controlledstress rotational rheometer (Bohlin C-VOR 120, Malvern). Flow curves were determined by increasing the shear rate from 1 to 10 s  1, as in [16]. For the freeze drying process, all the suspensions listed in Table 1 were ball milled for 4 h, de-aired in a vacuum desiccator, poured into cylindrical molds with diameters of 1.5 or 4 cm keeping constant the filling height (1.7 cm), and then placed in the freeze-dryer (Edwards Mod.MFD01, Crawley, UK). A directional freezing was provided by the shelf at

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Table 2 Morphological characteristics and mechanical properties of ZrB2–SiC samples. Sample code

Green total porosity (%)

Shrinkage (%)

Sintered total porosity (%)

Accessible porositya (%)

Bulk densitya g/cm3

Lamellar pore widthb (mm)

Lamella thicknessb (mm)

σc (MPa)

DZS38-1 DZS38-2 DZS48-2 PZS48-0.4 PZS48-1 PZS48-1.5

65 67 61 56 60 60

20 17 19 15 15 14

39 49 34 37 39 41

– – – 25.4 32.1 37.8

– – – 3.88 3.79 3.61

– 50–100 40–50 40–50 50–60 90–100

– 50–400 50–200 50–300 50–200 100–200

– – 297 17 – – 187 7

a

Data from mercury intrusion porosimetry. Data are referred to the microstructure of the top surfaces of the samples: width refers to the distance between two adjacent parallel ceramic lamellae.

b

Freezing direcion

Polished

5 mm

As-sintered sintered 50 µm

1 mm

5 mm

50 µ µm

Fig. 2. SEM micrographs of the sample DZS38-1 (a), (b) and (c): mirror polished cross-section (a), top surface (b), bottom surface (c). In (d) and (e): high resolution images of the top surfaces and sides of DZS38-2 (d) and DZS48-2 (e). SiC particles are in dark contrast.

 40 1C. A chamber vacuum value of 8  10  2 torr was used. At least 4 samples for each formulation were prepared. Green samples were thermally treated in a graphite crucible, first performing the dispersant pyrolysis step at 600 1C for 1 h, using a heating rate of 50 1C/h. Samples were then pressureless sintered at 2100 1C for 1 h, using a heating rate of 600 1C/h and flowing argon. Geometrical density measurements were performed on green and sintered samples to calculate the total porosity volume percentage. The morphological and microstructural features were observed by SEM (E-SEM FEI Quanta 200, FEI Company). Ultra- macro-porosity was investigated by the image analysis (Image Pro Plus 6.0., Media Cybernetics, Inc. Bethesda, MD, USA) of high resolution photos (scanner Sharp JX330, Japan) and scanning electron micrographs of surfaces and cross sections. Mercury intrusion porosimetry was also carried out on as-prepared samples (13 mm diameter and 10 mm height) to determine the amount of porosity in the range

0.0058–100 μm (Thermo Finnigan Pascal 140 and Thermo Finnigan Pascal 240). Compressive strength tests were carried out on cylindrical samples with diameter of 13 mm and height of 10 mm using a Zwick Z050 testing machine (Zwick GmbH, Ulm, Germany). The crosshead speed was set at 1 mm/min. 3. Results and discussion 3.1. Viscosity characterization of the DZS and PZS suspensions It is worth comparing the rheological behavior of monophasic ZrB2 suspensions with the composite suspension used in the present work (Fig. 1). DZS and PZS suspensions showed a viscoplastic behavior [24] (Table 1 and Fig. 1) as in the case of ZrB2 suspensions [16]. The logarithmic plots of viscosity vs. shear rate (Fig. 1) followed a power law with k exponents (Table 1) close to

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Freezing direcion

20 0µ µm

1 mm

500 µm

50 µm

500 µm

50 µm

Fig. 3. SEM micrographs of the sample PZS48-1: mirror polished cross-section (a), top surface (b), (c) and (d), bottom surface (e) and (f). SiC particles are in dark contrast.

 1 for DZS suspensions, indicating mainly a plastic behavior, and close to  0.7 for PZS, indicating a pseudo-plastic behavior. The viscosity of the ZrB2–SiC suspensions is up to 6 times higher than the corresponding monophasic ZrB2 slurries (Table 1, Fig. 1), due to incorporation of submicrometric SiC and Si3N4 particles with higher specific surface area (respectively 11.6 and 12.2 m2/g), compared to ZrB2 (1 m2/g). Moreover, in most cases, the viscosity of DZS suspensions is twice as high as PZS ones, especially when the solid loading is increased to 48 vol%. A significant decrease of DZS viscosity is obtained only doubling the dispersant concentration, whilst keeping the solid loading at 38% (Table 1). This confirms the lower ability of Duramax D3005 in optimizing the electro-steric repulsions between particles because of the lower molecular weight. Conversely, in PZS suspensions the viscosity slightly increased form 726 cP to 892 cP at 10 s  1 (Table 1), respectively with 0.4 wt% and 1.5 wt% of dispersant, because of the osmotic effect in presence of an excess of dispersant that reduces the diffuse layer between the particles and the range of repulsive interactions. 3.2. Morphology and microstructure of the porous samples The morphological characteristics obtained after freeze casting and sintering of the ZrB2–SiC suspensions are resumed in Table 2. The geometrical density of the green samples was in the range 1.8– 2.1 g/cm3 as determined by weight to volume ratio. Using the reference value of 5.4 g/cm3 as the theoretical density of the

starting composition, the green porosities ranged from 56% to 67 % depending on the water content and dispersant type and amount of the starting suspensions (Table 1). After sintering, the linear shrinkage of DZS samples is generally higher than in PZS samples, resulting in the lowest total porosity (34 vol%) in DZS48-2. In PZS samples, higher contents of dispersants increased the sintered total porosity from 37 vol% in PZS48-0.4 up to 41 vol% in PZS48-1.5. Although Duramax and Dolapix belong to the same class of anionic poly-electrolytes, namely ammonium poly-acrylates, they act differently both on suspension stability [22] and rheology (see Section 3.1). The final pore morphologies were affected by the suspension stability and viscosity, which in turn affected the way the ice grew. The effect of the two dispersants will be discussed separately in the following paragraphs. 3.2.1. Porous samples from DSZ suspensions From Fig. 2 it can be appreciated that in samples cast with the DZS38-1 formulation the macro-pores are arranged into dendritic domains (Fig. 2a). The polished top surface (Fig. 2b) shows the typical microstructure of ZrB2-20 vol% SiC composites, with SiC platelets with dark contrast homogeneously distributed in the diboride matrix [25]. On the contrary, on the as-sintered top surface (Fig. 2b) it was observed that SiC volumetric fraction is significantly higher, reaching concentrations of 80 vol%, whilst in the bottom (Fig. 2c) the fraction is lower than 5 vol%. Indeed, the finest SiC particles tend to float on the aqueous phase, which gradually solidifies upward, while ZrB2 particles settle down. In

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5

5 mm

5 mm

5 mm

10 mm

Fig. 4. High resolution images of the top surfaces and sides of samples PZS48-0.4 (a), PZS48-1 (b) and PZS48-1.5 (c) and (d).

DZS38-1 with a disordered porosity, the linear shrinkage was the highest (20%) and the final porosity only 39 vol% (Table 2). By doubling the amount of dispersant, lamellae appeared just on the top surface of sample DZS38-2 (Fig. 2d), while a lamellar morphology was developed throughout the sample DZS48-2 increasing both the solid loading and the Duramax amount (Fig. 2e). It was reported by Lasalle et al. [26] that working with an excess of either dispersant or binder is essential to ensure an integer lamellar structure in the final material, because a monolayer of dispersant is adsorbed on the surface of the particles while the excess concentrates in the inter-crystal region thus favouring ice lamellar growth. With the reduction of the water loading, the simultaneous occurrence of a further excess of dispersant in the water medium [26] and the increase of the freezing rate [7] favoured the formation of a morphology with thin lamellae (50–200 mm) and short lamellae inter-distance (40–50 mm) as in DZS48-2. Furthermore, unlike the monophasic ZrB2 suspension [16], in DZS suspensions a high solid loading of 48 vol% can be used. This is due to the presence of SiC and Si3N4 with fine grain sizes. Indeed, Deville et al. [27] stated the nucleation of ice crystals is controlled by the particle size: since the particles surface acts as nucleation site, a larger surface area allows the formation of a higher number of ice nuclei. It follows that a more uniform lamellar porosity can be achieved throughout the material as a consequence of the lower degree of super-cooling than with larger particles [27].

Unfortunately, due to the very high viscosity of DZS48-2, the results obtained for samples of small dimensions are not reproducible in samples of greater size, because the casting of a larger volume is discontinuous due to an insufficient fluidity of the suspension. 3.2.2. Porous samples from PZS suspensions The microstructural details of the sample PZS48-1 are shown in Fig. 3a–f. A lamellar porosity was achieved throughout the cross section (Fig. 3a), while some flaws were observed, such as structural defects oriented perpendicularly to the ice growth direction (the so called ice-lens-induced defects [26]). Due the cooling gradient [7] the top (Fig. 3b) and bottom (Fig. 3e) surfaces showed different porous morphologies, while the microstructure of ZrB2-20 vol% SiC matrix was homogeneous throughout the sample (Fig. 3c, d and f). As observed in previous works [22,23], Dolapix is less effective than Duramax in stabilizing the composite suspensions, but the effectiveness of the two poly-electrolytes results exchanged for the SiC powder. Moreover, Dolapix PC33 is the most effective in optimizing the electro-steric repulsions between particles, because of the higher molecular weight. This entails that in the case of the use of Dolapix PC33, lamellar pore morphologies were obtained in all samples, irrespective of the dispersant's concentration from 0.4 to 1.5 wt% (Fig. 4). The sintered total porosity passes from 37 vol% in PZS48-0.4 to 41 vol% in PZS48-1.5 and the amount of accessible porosity from about 25% to 38 %, respectively (Table 2). The pore size

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6

30 70

20

50 40

15

30 10

vollume (% Re elative pore %) p

p Cumu volume (m ulative pore mm³/g)

25 5 60

20 5

10 0 0.001

0.01

0.1

1

10

100

Pore diameter (µm)

and PZS48-1. This could be ascribed to the higher degree of alignment of the ceramic lamellae and channel like pores in PZS48-1.5, that are not detected by Hg porosimetry over 100 mm. As a general trend for PZS samples, it was in fact observed that higher contents of dispersant increased the distance between lamellae (namely lamellar pore width, Table 2), their thickness and uniformity and the accessible (open) porosity approached the total one. This is clearly related to the increasing excess of dispersant that concentrated in the intercrystal region favouring ice lamellar growth [26]. As a result, the casting of a larger volume (about 21 ml instead of 3 ml) allowed the reproduction of the morphology of PZS48-1.5 in samples of greater size (3.5 cm in diameter, Fig. 4d) by keeping constant the height.

20

100

15

60 10 40

Relative e pore volume (%) v

Cumula C ative porre volum me (mm³/g)

3.3. Compressive strength 80

5

20

0 0 .001

0.01 0 01

0.1 01

1

10

100

Pore diameter (µm) 20

120

Preliminary evaluation of the compressive strength (Table 2) resulted in strength values of about 29 and 18 MPa for DZS482 and PZS48-1.5, respectively. The values are comparable with those of ZrB2 ice-template samples with similar morphologies (namely lamellar pore width and thickness), but with higher total porosities (48 and 57 vol%) [16]. Even though the ZrB2– SiC lamellae are fully dense, the presence of structural defects such as ice-lens-induced defects (Fig. 3e), which turn into transverse cracks in the final material, adversely affects the compressive strength [26]. The data error is up to over 60% for DZS48-2 because of the very scarce reproducibility due to the high viscosity of the suspension.

15 80

60

10

40

Relativve pore volume e (%)

Cumulatiive pore C e volume (mm³//g)

100

5 20

0 0.001

0.01

0.1

1

10

100

Pore diameter (µm)

Fig. 5. Pore size distribution of samples PZS48-0.4 (a), PZS48-1 (b) and PZS48-1.5 (c).

distributions of PZS samples are reported in Fig. 5a–c. A bimodal distribution was observed in all samples, with the first local maximum always centered at 0.8–0.9 mm. The second local maximum was in the range 10–30 mm. Moving from PZS48-0.4 (Fig. 5a) to PZS48-1(Fig. 5b), i.e. increasing the dispersant content in the starting slurry, the bimodal character diminishes to the advantage of pores of larger size. In PZS48-1.5, the pore size range is narrower, with the second mode (centered at 10 mm) being closer to the first one (Fig. 5c), compared to PZS48-0.4

4. Conclusions ZrB2–SiC samples with a dense matrix and a lamellar macro-porosity were produced by ice-templating, where the pores are the replica of the lamellar ice crystals developed by freeze casting. ZrB2–SiC slurries were produced by varying solid loadings (38–48 vol%) and/or the concentrations of the dispersants (from 0.4 to 2 wt%). Two commercial ammonium polyacrylates (Duramax D3005 and Dolapix PC33) with different molecular weights and pH range of activity were used as dispersants. Duramax D3005 was more effective than Dolapix PC33 in electrostatically stabilizing the suspensions. In spite of this Dolapix PC33 was more effective than Duramax 3005 in optimizing electro-steric repulsions between particles because of the higher molecular weight and finally in promoting the formation of a lamellar morphology. High contents of Dolapix PC33 increased lamellar pore width and thickness and the uniformity of lamellae. The final morphology of PZS48-1.5 can be easily reproduced in samples of 3.5 cm in diameter by keeping constant the height. Further studies are ongoing to improve mechanical properties avoiding structural defects and to insure the reproducibility of samples up to 10 cm in diameter.

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Please cite this article as: V. Medri, et al., Ice templating of ZrB2–SiC systems, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.04.098