Characterization of reticulated ceramic foams with mercury intrusion porosimetry and mercury probe atomic force microscopy

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Characterization of reticulated ceramic foams with mercury intrusion porosimetry and mercury probe atomic force microscopy ⁎

Claudia Voigta, , Jana Hubálkováa, Lisa Ditscherleinb, Ralf Ditscherleinb, Urs Peukerb, Herbert Gieschec, Christos G. Anezirisa a

Technische Universität Bergakademie Freiberg, Institute of Ceramic, Glass and Construction Materials, Agricolastr. 17, 09599 Freiberg, Germany Technische Universität Bergakademie Freiberg, Institute of Mechanical Process Engineering and Mineral Processing, Agricolastraße 1, 09599 Freiberg, Germany c NYSCC, Alfred University, McMahon 349, 2 Pine Street, Alfred, NY 14802, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Mercury intrusion porosimetry Reticulated ceramic foams Hysteresis Atomic force microscopy

This study addresses the question of whether mercury intrusion porosimetry is an appropriate measurement devise for the porosity characterization of reticulated ceramic foams which features three kinds of pores: functional pores, material pores and strut cavities remaining after decomposition of the polymeric foam. Reticulated ceramic foam samples made of Al2O3 and Al2O3-C and with functional pore sizes between 10 and 60 ppi (pores per inch) were investigated. The results show that it is feasible to measure the strut cavities and the entryways of the material pores with the help of the mercury intrusion porosimeter. The results for the strut cavity diameter depends strongly on the ppi number of the polyurethane foam used for the preparation of the ceramic foam. For the reticulated Al2O3-C foam samples no extrusion of the mercury is observed. Reasons for the missing extrusion of mercury are discussed. Furthermore, atomic force microscopy (AFM) measurements on Al2O3 and Al2O3-C samples are carried out using a recent AFM method to examine interactions between a mercury droplet and rough surfaces. Factors like approach/retrace speed, reproducibility and applied force are investigated. It is seen that for Al2O3 larger attractive forces are measured than for Al2O3-C.

1. Introduction The replica process is an established processing route for preparation of cellular ceramic foams and was patented by Schwartzwalder et al. in 1963 [1]. The polymeric foam template is coated with ceramic slurry by soaking into followed by removing of the excess slurry with preset rollers or a centrifugation process [2]. After drying, the polymer is burned out and the ceramic material is sintered, whereby a replica of the original polymeric foam is produced. This process allows the manufacturing of open celled ceramic foams with a high porosity whereby a high flow-through with a low pressure drop, a high tortuosity and a high surface area is reached. Due to these properties reticulated ceramic foams manufactured with the replica process are used or tested in the area of metals melt filtration and gas filtration (diesel particulate filters), porous burners and bone replacement material [3,4, pp. 403–618]. The ceramic foams manufactured by the replica technique features three kinds of pores [5,4, pp. 33–40]: 1) functional pores (pores

⁎

surrounded by the struts), 2) material pores (pores in the struts) and 3) strut cavities remaining after decomposition of the polymeric foam, see Fig. 1. Since the porosity exerts a dominating influence on the properties of ceramic components, it is essential to characterize and quantify the porosity. There are different methods for the determination of the porosity for example buoyancy method, 2D and 3D image analysis (with the help of microscope or computer tomography), the nitrogen adsorption/desorption as well as mercury porosimetry. The buoyancy method measured according to DIN EN 993-1 [6] allows to evaluate the bulk density and the ratio of the apparent porosity. Contrary to the buoyancy method, the other mentioned methods give not only information about the open pore volume but also about the pore sizes. The image analysis allows the determination of open and closed pores. The size of pores detectable with image analysis depends on the resolution of the imagining equipment. Nitrogen adsorption/desorption can measure pores sizes (only open pores) in the microporous (< 2 nm) and mesoporous range (2–50 nm) [7,8], whereas mercury intrusion porosimeter detect pores between 3.5 nm and 500 µm (only open pores)

Corresponding author. E-mail address: [email protected] (C. Voigt).

https://doi.org/10.1016/j.ceramint.2018.09.094 Received 15 August 2018; Received in revised form 10 September 2018; Accepted 10 September 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Voigt, C., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.094

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Fig. 1. Structure of reticulated ceramic foams a) foam structure and b) structure of a strut.

pore network for example the ratio between pore opening and cavity, the pore shape and the ratio between pore length and pore opening [9]. The aim of this work was to investigate the feasibility and limitations of mercury intrusion porosimetry (MIP) for measuring of different kinds of pores using the example of two different reticulated ceramic foams – exclusive oxidic (Al2O3) and carbon containing (Al2O3-C). Furthermore, interactions between mercury and the two tested materials (Al2O3 and Al2O3-C) were evaluated with the help of liquid probe atomic force microscopy (AFM).

[9]. The mercury intrusion porosimetry uses non-wetting mercury which has to be forced into the pores with elevated pressures. The intruded mercury volume is measured as a function of pressure. With the help of the Washburn equation, which assumes cylindrical pores, [10] the imposed pressure p is converted into the corresponding pore radius r.

p=

−

2γ cosθ r

(1)

whereas γ is the surface tension and θ the contact angle. The mercury intrusion porosimetry provides the option to measure the intruded mercury volume (intrusion) while increasing the pressure and the extruded mercury volume (extrusion) while decreasing the pressure. The intrusion gives information about the pore volume and size of the pores whereas the extrusion is used to get information about the pore form. A hysteresis between intrusion and extrusion is detected in nearly all samples. Despite intensive research on the reason for the hysteresis there are still several open questions [9]. Two main reasons for the occurrence of the hysteresis are stated in the literature [9]. The first reason is the contact angle hysteresis which is caused by differences in the advancing and receding contact angle and differences in the contact angle depending on the radius of curvature. These differences in the contact angles are not considered when using the Washburn equation for the transformation of the pressure into the pore size. There is an empirically determined alternative for the application of the Washburn equation which includes differences in the contact angle and was established by Kloubek [11–13]. However, the equations are valid only for pore sizes between 6 and 99.75 nm (intrusion) and 4–68.5 nm (extrusion). Hence, the Kloubek equation is not suitable for reticulated ceramic foams with broader pore size distribution. The second reason for the hysteresis is the shape of real pores which consist often of a pore cavity and a smaller pore entryway opening (pore throat). The small pore entrance opening causes a breakage of the mercury network at the throats during the extrusion process resulting in a remaining of trapped mercury in the sample and therefore in a hysteresis between intrusion and extrusion measurement. Besides, the small pore entrance opening influences on the other hand the measured pore size because the mercury enters the pore cavity at a pressure determined by the size of the pore entrance opening and not the size of the cavity. The amount of entrapped mercury in the pores depends on the

2. Materials and methods 2.1. Preparation of the ceramic foams Ceramic foams made of two different materials were investigated: alumina (Al2O3) and carbon bonded alumina (Al2O3-C). For the preparation of the ceramic foams polymeric foams and ceramic slurries are needed. The used commercial polymeric foams made of polyurethane with different ppi (pores per inch) numbers, manufactured by Eurofoam (Germany), had a size of 17 mm × 17 mm × 17 mm and, cell widths between 850 µm and 5000 µm which corresponds to 60 pores per inch (ppi) to 10 ppi, see Table 1. The coating of the polyurethane foams with the ceramic slurries was carried out in two steps and for both steps a combined dip centrifugation process was used. The foams were dipped into slurry 1 followed by removal of excess slurry with a spinning process using a modified stirrer RZR 2102 control mixer (Heidolph, Germany). After drying of the foams coated with slurry 1 at room temperature for at least 24 h, the process was repeated with slurry 2. The rotation speed was between 500 and 1000 rotations per minute depending on the used slurry, the solid content of the slurry and the cell width of the polyurethane foams. The spinning process took at least 5 s. The ratio of slurry 1 and slurry 2 of the final product is roughly 1:1. The compositions of the ceramic slurries are summarized in Tables 2, 3. The solid contents of the Al2O3 and Al2O3-C slurries are given as a range due to the necessity of a variation of the slurry rheology because of the differences in the cell size of the polyurethane foams of 10–60 ppi. After drying of the second coating a sintering/coking step followed - for more information, see [14,15]. The Al2O3 ceramic foams were coated after sintering a further time and sintered a second time to close cracks formed during the burn-

Table 1 Cell widths and ppi numbers of the used polyurethane foams, information of datasheets for polyurethane foams provided by Eurofoam Deutschland GmbH Schaumstoffe on 2018. Bulpren® C

28720

31410

32240

32175

32138

38113

38093

Cell width according to data sheet/µm Pores per inch/ppi

≥ 5000 10

3800–4400 20

2100–2700 25

1600–1900 33

1260–1500 45

1050–1210 50

850–1010 60

2

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out of the polyurethane foams. The coking of the Al2O3-C foams took place in a steel retort filled with pet coke to provide a reducing atmosphere. The masses of the final ceramic foams pieces are roughly 3 – 5 g (Al2O3) and 2–3.5 g (Al2O3-C). For the preparation of material without polymeric template (as reference material) slurry 2 (of Al2O3) and slurry 1 (of Al2O3-C) was slip casted to small discs with a diameter of around 10 mm. The sintering and coking conditions were the same as the ceramic foams. The substrates, used in the sessile drop and atomic force experiments consisted of pressed Al2O3 and Al2O3-C substrates (uniaxially pressed with 60 MPa) and have been coated with the same slurries which were used to coat the ceramic foams tested with the mercury intrusion porosimetry. The coating step ensured a disc surface quality comparable to the ceramic foam samples which is essential for the measurement of the contact angles and the AFM measurements. After the coating step, the substrates (diameter 12 mm) were sintered or coked at temperatures given in Tables 2, 3.

The ceramic foam structure and the functional pores were evaluated with a micro focus X-ray computer tomograph CT-ALPHA (ProCon XRay, Germany) equipped with a 160 kV X-ray tube and a Dexela detector (Dexela 1512, Perkin Elmer, Germany) with 1944 × 1526 active pixels. For every material and every ppi number one ceramic foam sample was measured. The reconstructed CT data were analyzed with the software Modular Algorithms for Volume Images MAVI (Fraunhofer, Germany) with regard to functional pore size and porosity. The sample size was 17 × 17 × 17 mm3 and the reached voxel (volume pixel element) sizes were between 13 µm (Al2O3-C) and 26 µm (Al2O3). Data processing started with a cropping step followed by a binarisation with Otsu's threshold. Due to the fact, that the polyurethane foams generate voids after debindering (sharp edged pores inside the struts and nodes) a morphological operation “closing” was applied to remove these voids and analyze so the real functional porosity and pore size. The mean functional pore size was determined using the feature “complex morphology” of the MAVI software. The strut size of the ceramic foams was evaluated with a digital microscope VHX-2000D. For each ppi number at least 40 struts were measured. The material pores of the samples were analyzed with a 3D microscope Xradia 510 Versa (Zeiss, Germany). It has an X-ray source with a maximum acceleration voltage of 160 keV and a maximum power of 10 W. Compared to the normal tomographic setup it has a magnifying optics (after conversion from X-ray to visual spectrum via scintillator) to increase the possible resolution. The sample with 25 ppi and the smallest functional pores (60 ppi) made of Al2O3 and Al2O3-C were measured with an acceleration voltage of 80 keV. The sample size was in the range of 1–1.5 mm in diameter and 2–3 mm in height and the reached voxel sizes were between 2 and 3 µm for the overview and around 0.9 µm for the detail image. Additionally, samples after the mercury intrusion porosimetry measurements were investigated. These samples were embedded in an epoxy resin before the CT measurement to avoid evaporation of the mercury inclusions due to the X-ray exposure. Because of the remaining mercury in the pores and the large density differences between the mercury and the ceramic materials (Al2O3 and Al2O3-C) the occurrence of a large number of artifacts in the final projections and the tomographic reconstructions afterwards are unavoidable, see Davis et al. [16]. Due to the higher attenuation coefficient of mercury the acceleration voltage had to be increased to 140 keV. In order to minimize the artifacts and to ensure sufficient transmission, the mercury droplets in the functional pores were removed with a centrifugation step at 6000 rpm (Megafuge 1.0, Heraeus, Germany). Furthermore, areas without large mercury droplets were chosen for the detail scan. The voxel sizes were similar to the measurement without mercury. The sintered/cooked ceramic foams were characterized with the help of a scanning electron microscope (SEM) Philips XL 30 (Philips, Germany) equipped with an energy-dispersive X-ray microanalysis (EDX) device (Phoenix, USA).

2.2. Sample characterization

2.3. Mercury intrusion porosimetry measurement (MIP)

The contact angles between Al2O3 (Al2O3-C) and mercury were evaluated with sessile drop measurements at room temperature using a G10 (Kruess, Germany) at temperatures between 20 and 21 °C with the software G40. For the measurements pressed and coated substrates were used, see chapter 2.1. The drop shape was automatically recorded, contact angles of left and right sides of the three phase contact were determined with a tangent and the average value was calculated. For each material two different substrates with each 15 mercury droplets were measured. The volume of the mercury droplets was 70 µL. The sessile drop measurements were performed at substrates before and after mercury intrusion porosimetry in order to evaluate if the intruded mercury had an influence on the contact angle.

The porosity of the samples was characterized with a mercury intrusion porosimeter Autopore 5 (Micromeritics, USA). Penetrometer with 15 cm3 cup volume (15 bulb) were used which allow the measurement of the entire samples (as prepared), around 17 × 17 × 17 mm3. The volumes of the capillary (stem volume) were 0.392 cm3 stem volume for the Al2O3 samples and 1.13 cm3 for the Al2O3-C samples. For the measurement of crushed reticulated foam samples a powder penetrometer with a 5 cm3 cup volume and a 1.13 cm3 stem volume was used. The crushing of ceramic foams was realized with a uniaxial manual press. The broken fragments were separated into two fractions: > 2.5 mm and < 2.5 mm with the help of a sieve.

Table 2 Composition and sintering temperature for the preparation of the Al2O3 foams. Al2O3 Slurry 1 Al2O3 CT 9 FG (Almatis. Germany)/wt% Al2O3 CT 3000 SG (Almatis, Germany)/wt% Al2O3 T60/T64 45 µm (Almatis, Germany)/wt% Thickener Axilat RH 50 MDa (C.H. Erbslöh, Germany)/wt% Binder Optapix AC 170a (Zschimmer & Schwarz, Germany)/ wt% Dispersant Dolapix CE 64a (Zschimmer & Schwarz, Germany)/wt% Solid content/wt% Sintering temperature of the coating/°C a

33.3 33.3 33.3 0.5 1.0

Slurry 2

0

0.6 80–85 1600

Based on sum of solids.

Table 3 Composition and coking for the preparation of the Al2O3-C foams. Al2O3-C Slurry 1 Al2O3 Martoxid MR-70 (Martinswerk, Germany)/wt% Coal pitch Carbores P (Rütgers, Germany)/wt% Carbon black N991 (Lehmann & Voss & Co., Germany)/wt% Graphite AF 96–97 (Graphit Kropfmühl, Germany)/wt% Binder and Dispersant Ammonium ligninsulfonatea (Otto Dille. Germany)/wt% Dispersant MelPers® 9360a (BASF, Germany)/wt% Anti-foaming agent Contraspum K 1012a (Zschimmer & Schwarz, Germany)/wt% Solid content/wt% Coking temperature of the coating/°C a

Slurry 2

66.0 20.0 6.3 7.7 1.5 0.3 0.1 70–81.1 800

65–70

Based on sum of solids.

3

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Most mercury intrusion measurements consist of three intrusion and extrusion cycles with 60 measuring points per decade. This corresponds to 294 measuring points for the first intrusion and 199 measuring points for the following extrusions and intrusions). The samples were evacuated to a vacuum of < 50 µm Hg before the penetrometer was filled with mercury. The equilibrium time was varied between 5 s (majority of the measurements) and 300 s. Additionally, rate controlled measurements with a rate of 0.001 µL g−1 s−1 were performed. The pressure was converted into the corresponding pore size with the help of the Washburn Eq. (1) whereas a surface tension of 0.485 N m−1 and the measured contact angle (see 2.1 and 3.1) were used.

Table 4 Contact angle of mercury on Al2O3 and Al2O3-C (15 measurements per value).

2.4. Liquid probe atomic force microscopy with mercury droplets

speed and applied force as well as repetition were examined.

Contact angle/° Al2O3

Average Standard deviation

Average Standard deviation

Liquid probe adhesion experiments were done using an atomic force microscope XE-100 (Park Systems, South Korea). Because of the limitations of the AFM, Al2O3 and Al2O3-C samples before and after mercury intrusion in the form of tablets (similar to contact angle experiments) were investigated. As liquid probe cantilever a 46 µm sized pure mercury droplet was attached on a tipless AIOAL-TL C cantilever with a spring constant of 4.15 N/m (Budget Sensors, Bulgaria) using a method similar to the method developed by Escobar et al. [17]. Here, purified mercury was sprayed via a syringe onto a rough surface. The cantilever was firstly carefully pressed onto adhesive tape and then a mercury droplet was attached. An exemplary SEM image of a liquid probe cantilever can be found in Fig. 2a. Adhesion measurements were realized by recording force distance curves (example in Fig. 2b): During trace, the force was zero unless any interactions occur. When adhesive forces overcame the stiffness of the cantilever, it snaped onto the surface. The cantilever was moved further until it reaches the final z-position or applied forces. The cantilever was then retraced from the sample. Because of hysteresis (e.g. caused by deformation and contact line pinning) the pull off force was in most cases larger than a snap-in force. The pull off force was the adhesive force of the droplet onto the sample surface just before snap off. In the case of distinct line pinning, several peaks during retrace might occur. Line pinning did not necessarily increase measured adhesion force, but increased adhesion energy (the area below the baseline), so it was a more effective value to describe the influence of impacts. For each sample > 100 force distance curves were measured to get statistical valid results. A MatLAB routine was used to get adhesion forces (pull offs), energies and snapins. Besides the investigation of the different samples, the impacts of

Sample Sample 141 3 Sample Sample 144 3

Al2O3-C before mercury intrusion porosimetry 1 Sample 2 Sample 1 138 150 2 2 after mercury intrusion porosimetry 1 Sample 2 Sample 1 136 153 1 2

Sample 2 148 3 Sample 2 152 2

3. Results 3.1. Sample characterization The contact angle measurement of mercury on Al2O3 and Al2O3-C samples were conducted on pressed and coated substrates. The contact angle measurement of mercury on Al2O3 samples (before mercury intrusion measurement) showed mean values of 140° (see Table 4) and confirmed the often used contact angle of 140° for mercury measurements. The Al2O3-C samples showed a higher contact angle with a mean value of 149° (before mercury intrusion measurement). The measured contact angles were in good agreement with contact angles measured by Groen et al. [18], who measured a static angle of 141° for Al2O3 and 151° for a Novacarb mesoporous carbon which is comparable to the Coal pitch Carbores P, used for the Al2O3-C. The contact angle measurements of samples after the mercury intrusion measurements were done to verify if the entrapped mercury in the sample had an influence on the contact angle. The entrapped mercury in the disc samples was verifiable due to the darker color of the Al2O3 samples and an increase of the disc sample masses by roughly 126% (Al2O3) and 370% (Al2O3-C) after the mercury intrusion measurement. The mean contact angles after the mercury intrusion measurement of 140° (for Al2O3) and 153° (Al2O3-C) showed no significant changes to the contact angles before mercury intrusion measurement. These somewhat unexpected results can be explained by a missing penetration of the mercury into the pore cavities of the samples due to the high surface tension of the mercury and only gravity of the mercury

Fig. 2. AFM measurements a) SEM image of liquid probe cantilever with mercury droplet and b) exemplary force distance curve between mercury droplet and rough ceramic surface. 4

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Table 5 Results of the characterization of the Al2O3 foams with the help of the CT. ppi numbers of polyurethane foams

Functional porosity (without strut cavities)/%

Mean strut diameter ± standard deviation/mm

Mean pore size of functional pores ± standard deviation/mm

10 20 25 33 45 50 60

66 64 66 66 67 67 63

1.10 0.65 0.32 0.26 0.13 0.16 0.08

5.0 2.6 1.7 1.6 1.1 1.0 0.8

± ± ± ± ± ± ±

0.23 0.14 0.04 0.05 0.03 0.04 0.02

± ± ± ± ± ± ±

1.1 0.7 0.9 0.4 0.3 0.2 0.2

Table 6 Results of the characterization of the Al2O3-C foams with the help of the CT. ppi numbers of polyurethane foams

Functional porosity (without strut cavities)/ %

Mean strut diameter ± standard deviation/mm

Mean pore size of functional pores ± standard deviation/mm

10 20 25 33 45 50 60

70 70 64 66 65 69 70

0.83 0.49 0.39 0.26 0.16 0.10 0.11

4.8 2.7 2.3 1.8 1.3 1.0 1.0

± ± ± ± ± ± ±

0.18 0.07 0.09 0.07 0.02 0.02 0.01

± ± ± ± ± ± ±

Fig. 3. Cumulative pore volume in dependence on the pore diameter of reticulated Al2O3 foams with 45 ppi (Equilibrium time 5 s).

standard blank-correction sometimes overcompensates this and as such no blank correction was applied for the Al2O3 samples. The measurement of negative intruded mercury volumes as well as a hysteresis between intrusion and extrusion curves in this pressure range can be traced back to heating / cooling due to compression / decompression, which in turn causes expansion and contraction effects of the mercury relative to the glass-penetrometer. Those effects are strongest if measurements are done under short (5 s) equilibration times, since the introduced temperature changes have less time to equilibrate with the surrounding environment and even negative intrusion volumes were recorded for several samples. Heating will expand the mercury and essentially less actual pore-volume and cooling will contract the mercury and as such a “negative” pore-volume is measured by the instrument. The used stem volume of the presented measurements was between 54% and 75%. The stem volume is the volume of the capillary of the penetrometer and the used stem volume gives information about the utilization of the measurement range. According to the literature the used stem volume can be between 10% and 90% but ideally should be between 70% until 80% [9]. However, most of the presented measurements were performed using the entire original sample and as such an adjustment of the sample weight was not possible. In order to determine the repeatability, 45 ppi samples were measured four times with an equilibrium time of 5 s The curve shape of the four measurements possessed a good comparability whereas there were small differences in the intruded mercury volume, see Fig. 4. The shape of the curves was typical for intrusion/extrusion trials

1.7 0.8 0.4 0.2 0.2 0.1 0.1

droplet but no external force affecting the droplet. The pore openings (throats) did not contain mercury which could affect the contact angle measurement. The structure of the prepared ceramic foams was characterized with the help of a micro focus X-ray computer tomograph CT-ALPHA. The Tables 5, 6 show the results of the CT analysis. As expected the pore size of the functional pores decreased with increasing ppi number of the used polyurethane foam. Besides, the strut diameter decreased with increasing ppi number as well. The porosity fraction of the functional pores varied between 0.63 and 0.67 (Al2O3) and 0.64 and 0.70 (Al2O3C). The relatively large differences in the porosity fraction can be traced back to differences in the sample weight. 3.2. Mercury intrusion porosimetry measurement 3.2.1. Al2O3 samples The mercury intrusion porosimeter allows the measurement of open pores with a diameter between 3.5 nm and 500 µm [9]. According to the results of the foam structure analysis, the functional pores (see Tables 5, 6) were not measurable with the help of the mercury intrusion porosimeter. Nevertheless, it was feasible to measure the strut cavities and the material pores with the help of the mercury intrusion porosimeter. Fig. 3 shows the cumulative pore volume in dependence on the pore diameter of an Al2O3 foam with 45 ppi and presents the range of low and high pressure measurements, whereby the application of cyclic measurement is only possible in the area of the high pressure measurement (between 0.15 MPa and 420 MPa which corresponds to a pore size of around 9.5 µm to 3.5 nm). Fig. 3 presents three cycles (intrusion and extrusion steps). Obviously no additional entrapment of mercury was visible after the first cycle and the following curves for intrusion (Intrusion 3) and extrusion (Extrusion 2 and 3) resembled the intrusion 2 and the extrusion 1 respectively. For that reason, only intrusion 1, 2 and extrusion 1 were presented in the following diagrams. Artificial pore volume effects were observed in the high pressure range of these measurements without a “correct” blank-correction. Unfortunately, the

Fig. 4. Repeatability of the cumulative pore volume in dependence on the pore diameter of reticulated Al2O3 foams with 45 ppi (Equilibrium time 5 s). 5

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Fig. 5. Cumulative pore volume in dependence on the pore diameter of reticulated Al2O3 foams with 20 ppi measured with different equilibrium conditions a) without blank correction, b) with blank correction and c) penetrometer without sample.

of the blank measurement to the measurement with sample. For the 0.392 cm3 penetrometer the ratio was 0.07 and for the 1.13 cm3 penetrometer it was 0.02. The Fig. 6b) plotted the normalized cumulative volume in milliliter mercury per gram sample and showed a smaller hysteresis area (for small pore sizes) for the crushed 20 ppi sample measured with the penetrometer with the higher stem volume. However, after the blank correction a negative mercury volume was observed. The comparison of the reticulated Al2O3 foam with the Al2O3 casted reference material showed large differences especially a pronounced increase in the cumulative pore volume in the area of 50 until 7.5 µm for the reticulated Al2O3 foam, see Fig. 7. Furthermore, the hysteresis was less pronounced indicating a larger amount of entrapped mercury. The pronounced increase in the cumulative mercury volume could be attributed to the filling of the volume of the strut cavities formed by the burn-out of the polyurethane. According to Fig. 4 the pore size of the filling of the strut cavities had a good repeatability. This explained the scatter of the measured pore volume, see Figs. 4 and 8: the pore volume (given in ml/g) of the material pores did not depend on the sample mass, whereas the volume of the strut cavities depended on the sample mass or actually the ratio between coating thickness and amount of initial polymer sample. This meant that the standard scaling of the intruded mercury volume in ml/g of sample mass introduceed a pore volume variation for the strut-pores depending on the exact sample preparation conditions. Obviously, there was a correlation between the progress of the cumulative mercury volume curves (for a pore size between 10 µm and 150 µm) and the ppi size of the used polyurethane foam, see Fig. 8 which implied a decrease of the polyurethane strut diameter with increasing ppi number. Between 10 ppi and 33 ppi the pore size of the pronounced increase in the cumulative mercury volume decreased from around 90 µm to around 30 µm. Samples with ppi numbers between 45 ppi and 60 ppi showed a different behavior in comparison to the 10–33 ppi samples. They possessed a comparable pore size of the main increase in the cumulative pore volume (pore size of around 20 µm), but with a beginning of the increase of the cumulative pore volume at larger pore sizes (about 40 µm for 45 ppi, about 40 µm for 50 ppi and about 145 µm for 60 ppi). Fig. 8 showed also a good repeatability for the pore size of the main increase of the cumulative pore volume.

except for the hysteresis area for small pore sizes (3.5–200 nm) which could be traced back to the influence of the compression and decompression of the mercury and the penetrometer. The measured blankruns (mercury filled penetrometer without sample) was plotted in Fig. 5c) under various run-conditions. It showed for the measurement with an equilibrium time of 5 s an intruded volume of up to 0.02 ml mercury. The influence of the penetrometer and the mercury was particularly high for small pore sizes (< 100 nm) and for the extrusion step and could be explained by the heating and cooling effect during compression and decompression of the mercury as described in the previous paragraph. According to literature, the measurement of a negative intruded volume can be reduced by increasing the measurement time [9, pp 320–327]. Measurements with an equilibrium time of 300 s and a rate controlled measurement with a rate of 0.001 µL g−1 s−1 at 20 ppi Al2O3 foams were conducted, see Fig. 5. Typically, blank-runs are then subtracted from the actual sample data as shown in Fig. 5b). The measurements with different equilibrium times were corrected with the corresponding blank measurement. The hysteresis area for small pore sizes (3.5–200 m) was eliminated for all measurements. However, after the blank correction the intrusion curves contained negative intruded volume of mercury. This could be explained by “over-compensation”, since the expansion or contraction was primarily due to the liquid phase (mercury) and the amount of mercury in the sample cell was different (smaller) for the sample measurement as compared to the corresponding blank-run. The hysteresis area for small pore sizes (3.5–200 nm) was also measured for an equilibrium time of 300 s although it was considered as a temperature related effect which should decrease with longer measuring times. In the next step it should be tested if the stem volume of the penetrometer had an effect on the hysteresis area for small pore sizes. So ceramic foams with 20 ppi were measured with two penetrometers with a stem volume of 0.392 cm3 and 1.13 cm3. To reach a sufficient sample mass for the usage of the 1.13 cm3 penetrometer, the 20 ppi foam was crushed in pieces with > 2.5 mm and the sample cup was filled completely (around 10 g of crushed Al2O3 foam). The measurement with the penetrometer with 0.392 cm3 was conducted with crushed 20 ppi foam samples (around 4.5 g of crushed Al2O3 foam), too. In Fig. 6a) the cumulative volume in ml mercury is shown and additionally the cumulative volume of the blank measurements which showed a different ratio of the maximum cumulative mercury volume 6

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Fig. 6. Cumulative pore volume in dependence on the pore diameter of reticulated Al2O3 foams with 20 ppi (crushed sample) measured with different penetrometer with a stem volume of 0.392 cm3 and 1.13 cm3, a) cumulative volume in milliliter of the penetrometer without and with sample and b) cumulative volume in milliliter per gram of the penetrometer with sample.

powder. After an adjustment of the starting pressure from 0.005 MPa to 0.21 MPa even the fraction < 2.5 mm revealed a behavior comparable to the whole sample or the fraction > 2.5 mm. However, the information about the strut cavities is no longer available. In addition, it is only a matter of how fine the material is being crushed before interparticle porosity is also visible in the pore size range below 10 µm. As a general guide, interparticle pore sizes are about ¼ of the particle size. As such all sample pieces below a critical size need to be removed or otherwise the separation of material pores and interparticle voids is no longer possible.

3.2.2. Al2O3-C The Al2O3-C samples possessed a significant higher porosity than the Al2O3 samples which was in accordance with the computer tomographic images, see Figs. 10 and 11. According to Fig. 11 a) and c), the Al2O3 ceramic foam featured a denser micro structure with smaller pore sizes in comparison with the Al2O3-C ceramic foam, see Fig. 11 b) and d). The structure of the Al2O3-C ceramic foam was inhomogeneous – two different layers were visible. The first zone was an area with large angular pores next to the strut cavities and the second layer was covering the first layer and possessed small and homogeneous pores. The large pores in the first layer were caused by larger particles of the Carbores P which had a broad particle size distribution with particles up to 200 µm. During the coking step at 800 °C the Carbores P softened, melted, flowed in the surrounding structure and left the large angular pores behind. The reason for the different structures of the two layers was the production process, which consisted of a two-step coating/centrifugation process. The first coating step used a slurry with a higher viscosity as compared to the slurry used for the second coating layer. The first slurry was prepared in a high shear mixer ToniMix (ToniTechnik, Germany). The second coating step used a lower viscosity slurry, which was prepared in a ball mill with Al2O3 grinding ball for at least 20 h. Apparently the large Carbores P particles were reduced in size during this milling process. So the layer prepared with the second slurry features a micro structure with smaller and more homogeneous pores. For the ceramic foams with smaller functional pore sizes (higher ppi numbers) the difference between the two zones was smaller due to a smaller strut size, see Fig. 11 c) and d). Comparing the mercury intrusion porosimetry curves with the computer tomography images, there were some discrepancies related to the pore size distribution: The pore sizes measured with the mercury intrusion porosimeter were comparable for Al2O3 and Al2O3-C foam

Fig. 7. Cumulative pore volume in dependence on the pore diameter of the casted reference material and a reticulated Al2O3 foam with 45 ppi (Equilibrium time 5 s).

This finding might allow the characterization of the strut cavities of reticulated ceramic foams. In summary, the measurement of the whole samples allowed the measurement of the strut cavities in combination with the material pores. Another set of experiments now studied if crushing the samples would affect the analysis results. For this, the crushed foam samples were separated into two fractions (< 2.5 mm and > 2.5 mm), see Fig. 9. For the fraction > 2.5 mm the shape of the mercury intrusion curves were comparable to the measurement of the whole samples except for a broadening of the cumulative pore volume curve at larger pore sizes, which could be explained by a better accessibility of those pores in the crushed pieces as compared to one “long” single strand of pore, which was only accessible from either end. The increase of the cumulative volume for the fraction > 2.5 mm was less steep which showed an opening and a facilitated access of the strut cavities due to the crushing of the samples. For a characterization of the strut cavities the usage of whole samples was more suitable. If the focus is on the measurement of the material porosity the usage of crushed materials with a larger fraction size was possible as well. The measurement of the fraction < 2.5 mm consisting mainly of single struts (see Fig. 9), showed a higher volume for the larger pores (> 10 µm), but with rather poor repeatability. The strong increase at large pore sizes was caused by the interparticle void filling at the breakthrough pressure and can be typically observed when measuring 7

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Fig. 8. Cumulative pore volume in dependence on the pore diameter of different ppi numbers between 10 ppi till 60 ppi (Equilibrium time 5 s).

porosimeter only open pores are measurable whereas CTs can detect open and closed porosity but keeping in mind that the measurable pore size depends on the resolution of the CT. Furthermore, in mercury intrusion porosimetry pores in the interior of the specimen can only be reached by mercury only through a chain of intermediate pores with

samples, whereas the CT measurements revealed large differences, for example pore sizes of up to 170 µm were seen in CT, whereas they were not observed in mercury porosimetry tests. This could be explained by differences in the way how mercury intrusion porosimetry and computer tomography detect the pores. With the mercury intrusion

Fig. 9. Cumulative pore volumes in dependence on the pore diameter of the reticulated Al2O3 foams with 33 ppi with different sample sizes a) with a starting pressure of 0.005 MPa (~ 300 µm pore size), b) with a starting pressure of 0.21 MPa (~ 60 µm pore size) and images of the different sample sizes (Equilibrium time 5 s). 8

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porosimeter. The evaluation of the pores caused by the melting of the Carbores P pores was apparently not possible with the mercury intrusion porosimeter. The 3D microscope Xradia 510 Versa allowed the detection of the two layers caused by the differences in the slurry preparation technique. Comparable with the ceramic foams made of Al2O3, the ceramic foam made of Al2O3-C showed pronounced increase in the cumulative volume provoked by the filling of the strut cavities, see Fig. 10. Both Al2O3-C reference (casted and pressed) samples possessed no porosity in the relevant area (20–100 µm). Another noticeable effect was the missing extrusion of mercury for the Al2O3-C 25 ppi foam and casted reference, whereas the reference material produced by uniaxial pressing showed a clear mercury extrusion combined with a hysteresis curve, see Fig. 10. In the literature a missing mercury extrusion is linked to amalgamation [20] or blocking of the mercury withdrawal caused by the destruction of the pore structure [21]. Despite of missing information in the literature about the chemical reaction between carbon and mercury, the amalgamation of the Al2O3-C and mercury can be excluded due to the results of the Al2O3-C uniaxial pressed reference which was made of the same material and possess a typical hysteresis curve. The computer tomography images of the ceramic foam samples after mercury intrusion porosimetry showed clearly the entrapped mercury but did not show significant differences between the Al2O3 and the Al2O3-C foams which might explain the different extrusion behavior, see Fig. 12. It must be noticed that the mercury extrusion stops at a pressure of 0.16 MPa which is slightly higher than the ambient pressure during the recording of the computer tomography image and the samples were centrifuged before the CT measurement to remove the mercury droplets from the functional pores. The centrifugation was a necessary step due to image changing artefacts introduced by large

Fig. 10. Cumulative pore volume in dependence on the pore diameter of reticulated Al2O3 and Al2O3-C foams and pressed and casted reference samples (with blank correction) (Equilibrium time 5 s).

different size and shape [19]. So the large pores in the center of the specimen are not intruded by mercury until pressures are reached to penetrate narrower entryways. So the mercury intrusion porosimeter measures the largest entrance of a pore and not the pore cavities itself [9]. This explains the comparable pore sizes for Al2O3 and Al2O3-C foam samples measured by mercury intrusion porosimetry despite large differences in the CT images of Al2O3 and Al2O3-C ceramic foam samples. The entryway between the pore cavities for Al2O3 and Al2O3-C foam samples were comparable whereas the pore cavities showed large differences which were not measurable with the mercury intrusion

Fig. 11. Reconstructed images of a 3D microscope Xradia 510 Versa, ceramic foam samples before mercury intrusion porosimetry a) Al2O3 25 ppi, b) Al2O3-C 25 ppi, c) Al2O3 60 ppi and d) Al2O3-C 60 ppi. 9

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Fig. 12. Reconstructed images of a 3D microscope Xradia 510 Versa, ceramic foam samples after mercury intrusion porosimetry a) Al2O3 25 ppi, b) Al2O3-C 25 ppi, c) Al2O3 60 ppi and d) Al2O3-C 60 ppi.

conductive metal drops. Additionally, it should be noted, that the filling of the strut cavities took place at pressures below the ambient pressure in the low pressure step and so the extrusion of the strut cavities was not recordable. Nevertheless, the strut cavities will be emptied after the removal of the sample from the mercury. In order to evaluate the repeatability of the mercury intrusion measurement, 25 ppi Al2O3-C foam samples were measured three times with an equilibrium time of 5 s. Additionally, one measurement with an equilibrium time of 120 s and one measurement with a rate of 0.001 µL g−1 s−1 were conducted, see Fig. 13. All five measurements

showed comparable behavior with no significant differences. So it could be stated that the missing mercury extrusion was not affected by too short equilibrium times. The differences in the extrusion behavior between pressed and casted reference samples of the same Al2O3-C material indicated differing microstructure of the Al2O3-C as reason for the missing mercury extrusion. A SEM investigation of the foam samples showed large differences in the inner pore surface between the Al2O3 and the Al2O3-C foam samples, see Fig. 14 c) and d). Fig. 14 c) showed large and sintered Al2O3 grains due to a sintering step at 1600 °C. On the contrary, the Al2O3-C foam samples possessed an almost nanostructured pore surface consisting of spherical carbon containing particles which were precipitated on the pore surface during the coking step at 800 °C. This nanostructured pore surface increased the roughness of the surface. According to Wenzel et al. [22] an increase in roughness should increase the wetting angle of the mercury on the Al2O3-C as the mercury Al2O3-C/mercury system is a non-wetting system. It was conceivable that a further increase in the wetting angle to values larger than 150° hindered the mercury to leave the pores during extrusion step when the pores possessed an ink bottle structure. However, the uniaxial pressed reference samples showed such a nanostructured surface as well and possessed a notable extrusion of mercury. This extrusion might be caused by the presence of canal-like pores and cracks forming due to the alignment of the plate-like graphite particles during the pressing of graphite containing compositions. Furthermore, it should be mentioned that the surface of the foam especially in the area of the strut cavities was very dense. The formation of such a dense layer at the surface was supported by the usage of fluid slurry due to a particle alignment during the drying. Such an alignment was not expectable when preparing samples by pressing. A dense sample surface might hinder the extrusion of the mercury out of the sample. Hence, further investigations to determine the reasons for the

Fig. 13. Repeatability of cumulative pore volume in dependence on the pore diameter of reticulated Al2O3-C foams with 25 ppi with different equilibrium criterions (with blank correction) (Equilibrium time 5 s). 10

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Fig. 14. SEM images of the foam samples a) Al2O3 strut, b) Al2O3-C strut, c) inner pore surface of Al2O3 foam and d) inner pore surface of Al2O3-C foam.

was used as a standard condition. A different behavior was seen for the variation of applied force (the maximum force with which a droplet was pressed onto the surface). With a higher applied force an increase in adhesion energy was recorded. The reason was a jump of the three phase contact line while the droplet was pressed with increasing force onto the rough surface, see Fig. 2, and a larger surface area would come into contact with the mercury drop. The surface energy of the sample was higher on asperities than on smooth parts due to the smaller number of adjacent atoms of the same material. The droplet kept pinned onto the asperity until the supplied energy (applied force) was large enough to overcome this energy of contact line pinning. Because of the large contact angle of mercury on alumina, the CASSIE-BAXTER case [23] – pores were not fully wetted by the liquid – was assumed: The three phase contact line jumped to the next asperity and did not wet holes or any other kinds of valleys during the jump [24]. This also occured during mercury intrusion porosimetry, but with higher pressure, the mercury-free pore volume was reduced depending on pressure stages. During droplet retrace, pinning occured now in the changed direction. Due to a stronger droplet deformation, depinning of breakpoints could be seen more easily during retrace. Each jump of the three phase contact line was equal with a significantly larger adhesive energy. A very interesting result was that adhesion energy was more meaningful as adhesion force, since the highest pull off force needed not necessarily to be applied at the last equilibrium stage, see Fig. 16. An example can be seen in the Table 7. While adhesion force was nearly constant, adhesion energy had increased with larger applied force. For the poorer wetted system, smaller applied forces were necessary to move the three phase contact line. For that reason, applied force was kept constant for further investigations at 500 nN. In Fig. 17, the results of the Al2O3 and Al2O3-C samples before and after mercury intrusion are shown. Clearly visible is a larger adhesive force for the Al2O3 samples, due to the increased interactions. An explanation might be the smaller contact angle measured onto the Al2O3 sample, in other words, the mercury droplet wet the Al2O3-surface better than the Al2O3-C-surface and thereby interactions

missing mercury extrusion are necessary. The measurement of the Al2O3-C foams with different ppi numbers featured a similar behavior with regard to the filling of the strut cavities – the pore size of the main step in the cumulative pore volume for larger pore sizes (20–250 µm) decreased with increasing ppi number, see Fig. 15. 3.3. Liquid probe atomic force microscopy with mercury droplets The impact of velocity and repetition of the experiment could be neglected for trace and retrace; the absolute deviation for all repetitions was below 2.3% and for different velocities below 8.6%. This was valid for both samples Al2O3 and Al2O3-C before and after mercury intrusion. It meant in effect that the method was robust and a speed of 10 µm/s

Fig. 15. Cumulative pore volume in dependence on the pore diameter of Al2O3C foams with different ppi numbers between 10 and 60 ppi (with blank correction) (Equilibrium time 5 s). 11

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Fig. 16. Exemplary force distance curves for a) Al2O3 and b) Al2O3-C sample with different applied forces.

topographical scans of the samples show Rrms values of 0.235 µm (before Hg-intrusion) and 0.254 µm (after mercury intrusion). This explained why larger forces were measured on Al2O3-C before mercury intrusion (for Al2O3 samples Rrms values were nearly the same: 0.345 µm (before Hg-intrusion) and 0.340 µm (after)). A repulsive force might also explain the lower adhesive forces of Al2O3-C after Hg-intrusion. Assuming a pore network filled with remaining mercury, only one side of the pore was freely accessible for air (contrary to the Al2O3 sample, where more mercury had been removed). During scanning the Al2O3-C surface with a ContAl-G cantilever (apex angle: 25°), no interaction with any mercury droplets had been registered, so the mercury-free height of a pore with a diameter of 500 nm was at least 550 nm. The inner pressure of the liquid probe and the mercury remaining inside of a pore was calculated via the YOUNG-LAPLACEequation to 20.7 kPa (probe) and 1.9 MPa (droplet inside pore). If the liquid probe was pressed with 500 nN onto the surface with one pore, this would compress the air inside of it because of the high inner pressure of the mercury droplets and the significantly higher compressive module. The maximum compression was calculated to 0.648 MPa. The gas filled volume was lowered by reducing the height to 85 nm. The compressed air inside of the pore generates a counterforce onto the interfaces that had a maximum value of about 86 nN, 1.874 mN/m respectively. Although this was only roughly estimated, we believed that this counterforce was the main reason for the lower adhesive interactions. For mercury intrusion porosimetry, the repulsive force could be neglected because the samples were vented.

Table 7 Applied forces and exemplary results for different samples. Al2O3

Al2O3-C

Applied force/nN

Adhesive force/nN

Adhesive energy/ J·10–15

Applied force/nN

Adhesive force/nN

Adhesive energy/ J·10–15

147.0 586.0 1130.4 1637.1 2159.3

102.0 98.8 95.4 95.0 98.2

12.0 39.0 40.2 38.5 38.9

6.9 98.1 380.4 1140.0 1990.8 2769.1

25.5 32.8 35.2 35.2 39.1 34.8

4.6 7.6 7.3 6.7 6.7 6.8

4. Conclusions This study addresses the question of whether mercury intrusion porosimetry is an appropriate measurement devise for the porosity characterization of reticulated ceramic foams which features three kinds of pores: functional pores, material pores and strut cavities remaining after decomposition of the polymeric foam. Reticulated ceramic foam samples made of Al2O3 and Al2O3-C and with functional pore sizes between 10 and 60 ppi (pores per inch) were investigated. According to the results of the foam structure analysis, the functional pores were not measurable with the help of the mercury intrusion porosimeter. Nevertheless, it was feasible to measure the strut cavities and the entryways of the material pores with the help of the mercury intrusion porosimeter. The results for the strut cavity diameter depended strongly on the ppi number of the polyurethane foam used for the preparation of the ceramic foam. Furthermore, the influence of the sample preparation (in particularly sample size) was investigated. The mercury intrusion porosimetry with too fine fragments lead to imprecise results due to the simultaneous acquisition of the material pores as well as inter particle pores. For the reticulated Al2O3-C foam samples no extrusion of the mercury was observed. This behavior did not depend on the equilibrium

Fig. 17. Results of adhesion force measurements of the samples Al2O3 and Al2O3-C before and after mercury intrusion.

were enlarged. Here, the same mercury droplet wet more surface area than in the case of Al2O3-C. Interestingly, no snap in was recorded for the Al2O3-C samples, whether before or after mercury intrusion. For the Al2O3 samples, no visible difference could be detected before or after mercury intrusion which indicated that residual mercury in small pores had no significant influence on adhesive interactions. For Al2O3-C this was different: Adhesive interactions, whether adhesion force or energy, was reduced by 50% after mercury intrusion although there was no significant change in contact angle measurements. According to the literature, there are several possible explanations for this behavior, such as a reaction between mercury and substrate material, a difference of surface morphology of the samples or repulsive forces due to compressed air in pores. A reaction (e.g. by amalgamation) would not explain similar contact angles. Surface morphologies of Al2O3-C samples before and after mercury intrusion were slightly different; 12

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time and was very reproducible. Reasons for the missing extrusion of mercury were discussed. Computer tomography measurements on ceramic foam samples before and after the mercury intrusion porosity measurement showed large differences in the amount of porosity and in the pore sizes. The differences in the amount of porosity was confirmed by mercury intrusion porosimetry measurements whereas the differences in the pore size was not detectable by the mercury intrusion porosimeter. Furthermore, atomic force microscopy (AFM) measurements on Al2O3 and Al2O3-C samples were carried out using a recent AFM method to examine interactions between a mercury droplet and rough surfaces. Factors like approach/retrace speed, reproducibility and applied force were investigated. It was seen that for Al2O3 larger attractive forces were measured than for Al2O3-C. Contrary to Al2O3, a difference in adhesive interactions between Al2O3-C before and after mercury intrusion was observed due to a higher amount of trapped mercury inside of the pores.

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Acknowledgements The authors would like to thank the German Research Foundation (DFG) for supporting these investigations as part of the Collaborative Research Centre 920 “Multi-Functional Filters for Metal Melt Filtration – A Contribution towards Zero Defect Materials.” subprojects A02 and B01. The authors would also like to acknowledge the support of U. Querner, G. Schmidt, A. Schramm (C06) as well as the assistance of M. Dieper (Eurofoam Deutschland GmbH). References [1] K. Schwartzwalder, Patent – Method of Making Porous Ceramic Articles, May 21, 1963, 3.090.094. [2] T. Fey, U. Betke, S. Rannabauer, M. Scheffler, Reticulated replica ceramic foams: processing, functionalization, and characterization, Adv. Eng. Mater. 19 (2017) 1700369. [3] J. Adler, G. Standke, Offenzellige Schaumkeramik, Teil 2. Keramische Zeitschrift, 55(10), 2003, pp. 786–792. [4] M. Scheffler, P. Colombo, Cellular Ceramics: Structure, Manufacturing, Properties and Applications, Wiley-VCH Verlag GmbH, 2005 (ISBN: 3-527-31320-6).

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