Impact of extrusion parameters on the mechanical performance of tubular BSCF-supports for asymmetric oxygen transporting membranes

Author’s Accepted Manuscript Impact of extrusion parameters on the mechanical performance of tubular BSCF-supports for asymmetric oxygen transporting membranes R. Hoffmann, U. Pippardt, R. Kriegel www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)31752-6 https://doi.org/10.1016/j.memsci.2018.10.019 MEMSCI16535

To appear in: Journal of Membrane Science Received date: 25 June 2018 Revised date: 27 September 2018 Accepted date: 3 October 2018 Cite this article as: R. Hoffmann, U. Pippardt and R. Kriegel, Impact of extrusion parameters on the mechanical performance of tubular BSCF-supports for asymmetric oxygen transporting membranes, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Impact of extrusion parameters on the mechanical performance of tubular BSCF-supports for asymmetric oxygen transporting membranes R. Hoffmann*1, U. Pippardt2, R. Kriegel3

*

Corresponding author: Tel.: +49 036601 9301-5014; fax:+49 036601 9301-3921,

[email protected] [email protected] [email protected]

Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Michael-Faraday-Str. 1, D-07629 Hermsdorf, Germany

Abstract

Tubular asymmetric membranes based on BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-δ) are well-known for high O2 permeability, but there use in first pilot plants for small scale O2 production is constricted by low fracture strength. Therefore, ceramic shaping of porous support tubes by extrusion was optimized to improve mechanical properties, especially fracture strength. Accordingly, content of pore forming agent, kneading time and rotational speed of the main extrusion screw were varied, systematically. Microstructure and fracture strength were characterized and compared to reference samples manufactured using a common set of extrusion parameters. It is demonstrated that a notable improvement of mechanical strength could be achieved by even small adjustments of the individual parameters mentioned in this work. In addition, the received results allowed to discuss influence of rotational speed of the main extrusion screw on the behavior of the extruded mixture, as well as, sintering behavior of ceramic structures containing large pores, in more detail.

Keywords ceramic extrusion, oxygen, BSCF, membrane, asymmetric

1

orcid.org/0000-0002-2201-5911 Tel: +49 036601 9301-4919; fax: +49 036601 9301-3921, orcid.org/0000-0002-2164-613x 3 Tel.: +49 036601 9301-4870; fax: +49 036601 9301-3921,orcid.org/0000-0001-8326-6004 2

1

Introduction

Oxygen production capacity amounted to appr. 400 million tons per year in 2006 and is growing with 5 – 6 % per year [1]. Oxygen is largely used for combustion processes [2] in metallurgy, glass melting or fabrication of ceramics to enhance combustion temperatures [3] and efficiency of combustion [2 - 5]. Accordingly, oxygen enrichment saves primary fuel entailed by lowering of CO2 emissions. A combustion with pure oxygen frequently called Oxyfuel® enables an easy and efficient CO2 capture after steam condensation. Accordingly, Oxyfuel® combustion is a well-known approach for a coal power plant with CO2 capture [6]. Oxygen is typically produced by cryogenic air separation in large plants. This process is characterized by a minimal theoretical energy demand of 0.33 kWh/m³ O 2 (STP – standard temperature and pressure) [7], but real energy consumption amounts to at least 0.46 kWh/m³ O2 (STP) [8]. Customers with a demand less than 1.000 m³/h (STP) will be delivered with liquid O2 or compressed gaseous O2 entailed by costs increasing steeply with decreasing demand, especially below 100 m³/h O2 (STP). Alternative techniques like PSA (Pressure Swing Adsorption) or VPSA (Vacuum Pressure Swing Adsorption) are able to reach a maximal purity of 93 – 95 vol.-% O2, which is not acceptable for CO2 capture because of the remaining air gases N2 and Ar. Ceramic membranes with high oxide ion conductivity but also good conductivity for electronic charge carriers are able to separate pure O2 at high temperatures. Such membranes typically abbreviated as MIEC membranes (Mixed Ionic Electronic Conductor), ITM (Ion Transport Membranes) or OTM (Oxygen Transport Membranes) are suited for local O2 production from small [9,10] to large scale [11]. Competitiveness of a pressure driven membrane process for local O2 production depends on investment costs and operational costs. If thermal energy of a combustion process is used for membrane or air heating, energy demand for gas compression determines operational costs. Correspondingly, an overpressure applied at feed side for the entire air needs much more energy than a vacuum extraction of O 2 separated at permeate side [12]. Therefore, the former process needs always a very efficient recovery of compression energy not necessary for the vacuum process [13,14]. During last years, Fraunhofer IKTS has realized different pilot plants or demonstration units working in vacuum mode and typically characterized by a growing O2 production rate and a decreasing energy demand [9,15,16]. All plants realized were equipped with tubular BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-δ) membranes and electricity was used for vacuum generation but also for membrane or air heating. BSCF was preferred because of its famous high O2 permeation [17,18], its relatively low chemical expansion [19] and a tolerable mechanical stress level calculated for vacuum operation [20]. Last membrane plant built up was equipped with monolithic BSCF capillaries. The device produced 9.6 m³/h O2 (STP) and needed 0.72 2

kWh/m³ O2 (STP) [21]. Electricity for gas compression amounted to only 0.2 kWh/m³ O2 (STP), but heat recovery was higher than expected because of some thermal bridges and air leakages of the housing [21]. Competitiveness of small MIEC membrane plants for local O2 production is typically dominated by investment costs and entailed depreciation, especially if energy demand and operational costs are already low. Presently, membrane costs cause around a fourth of total investment costs and have to be decreased for an easier market entry. This could be done by a more efficient manufacturing of tubular membranes or by a significant higher O 2 throughput decreasing the number of membrane tubes needed for production of a distinct amount of O2 as already described [20]. The most promising way for the latter route is the use of asymmetric BSCF membranes with a much higher O2 flux density [22] compared to monolithic ones [23]. However, fracture strength of asymmetric BSCF membranes [24] is much lower and seems to be critical for membrane pilot plants. According to that, mechanical strength of asymmetric BSCF membranes has to be improved distinctly to enable utilization in pilot plants for O2 production. Fracture strength of ceramic components is typically affected by powder properties, organic additives used for plasticizing and by manufacturing conditions during ceramic shaping. Even in dense BSCF, remaining pores are typically an origin of micro cracks or failures affecting mechanical strength [25,26]. Therefore, much more failures can be expected for porous ceramics used as support for tubular asymmetric MIEC membranes. Similar issues relating the influence of support porosity or pore size on membrane performance and stability as well as the effect of varying pore forming materials and other organic additives have been discussed by several groups for deviant membrane types. Flat membranes [27,28], multichannel tubes produced by plastic extrusion [29] or thermoplastic extruded membrane supports [30,31] consisting of alumina-based, YTZ-based or perovskite-based materials were often focused in these surveys. A summary of the development and technical state of the art regarding membrane support manufacturing due to extrusion of non-perovskite materials is given in [29]. Despite the more complex material behavior of materials like BSCF compared to alumina-based materials and the varying archetypes of membrane supports, general relations between gas transporting properties and pore content and pore size are always valid. For this reason support porosities above 30-40 vol.-% and pore sizes in the range of 115 µm are often preferred to reach good permeation properties of the membranes which is entailed with quite lowered fracture strengths (20-50 MPa, depending on porosity and detection method) compared to monolithic components [27-31]. Accordingly, the aim of this work was an improvement of mechanical strength of tubular asymmetric BSCF membrane supports without significant changes of gas transport related properties by a stepwise adjustment of manufacturing conditions. An important step before extrusion is the 3

preparation of plasticized powder batches containing the ceramic material and several organic additives. In order to ensure sufficient homogeneity and a good flow behavior, mixtures have to be kneaded whereby kneading time is a critical parameter [32]. Accordingly, kneading time was varied for one group of the batches fabricated, hereafter called ktbatches. During extrusion, mixtures are exposed to thermal and mechanical forces mainly determined by the rotational movement of the extruder screw (or screws) and corresponding pressure and friction [32]. Therefore, another group of batches was prepared applying different rotational speeds for the extruder screw, hereafter called rs-batches. A last group of batches was prepared by adding different amounts of pore forming agent, since porosity of supports should be a determining factor for their mechanical performance. This group of batches was called ap-batches, hereafter. The state of the art for manufacturing asymmetric BSCF membranes at Fraunhofer IKTS was used as a reference state characterized by a commonly used parameter set which was already described in [23]. In this context, an open porosity of ~35 % and a fracture strength of ~34 MPa were taken as target values for the used membrane supports, as they roughly represent the average values reported by Fraunhofer and other groups cited in this work.

2

Materials and methods

A commercially available BSCF powder delivered by Treibacher Industrie AG (Althofen, Austria)

was

used

for

manufacturing

of

plasticized

powder

batches.

PMMA

(polymethylmethacrylate) Spheromeres® CA15 with median diameter of 15 µm from Microbeads AG (Brugg, Switzerland) were used as pore forming agent and volume part of PMMA to ceramic powder was varied between 0 and 60 vol.-%. An aqueous binder solution containing various organic additives was prepared and used for plasticizing by kneading. It contained hydroxypropyl methylcellulose (HPMC) from former DOW Chemical (Michigan, USA) and methyl hydroxypropylcellulose (MHPC) delivered by Kremer Pigmente GmbH & Co. KG (Aichstetten, Germany) as binder together with water. Table 1 Raw materials for preparation of BSCF-tubes with some relevant properties (* theoretical density of BSCF, calculated according values from [19]). raw material

d10 [µm]

d50 [µm]

d90 [µm]

density [g/cm³]

BSCF

1.20

2.86

5.18

5.57*

PMMA Spheromeres®

12.34

15.86

20.39

1.18

MHPC

-

-

-

0.42

HPMC

-

-

-

1.34

Hydrowax X

-

-

-

0.99

Polyglycol D21/700

-

-

-

1.07

4

Furthermore, HydroWax X from Sasol Germany GmbH (Hamburg, Germany) and Polyglycol D21/700 produced by Clariant Masterbatches (Deutschland) GmbH (Lahnstein, Germany) were added. Table 1 summarizes the materials used and some of their relevant properties, as specified by manufacturers. The mixtures of raw materials were kneaded and homogenized while adding small amounts of water using a kneading unit (W350) from Brabender GmbH & Co. KG. Rotational speed of the kneading blade was set on 20 rpm for all mixtures. For kt-batches kneading time was varied between 90 min and 300 min, while it was set on 90 min for all mixtures from rsbatches and ap-batches. After kneading, tubes were formed by stiff-plastic extrusion of the prepared mixtures using a lab extruder from Brabender GmbH & Co. KG equipped with a chamber for degassing, a vertical oriented main extrusion screw and an extrusion die with outer diameter of 12.25 mm and inner diameter of 9.75 mm. For rs-batches, rotational speed of the main extrusion screw was varied between 20 and 36 rpm while the rotational speed of the initial screw was adjusted simultaneously. A constant rotational speed of 22 rpm was used for the extrusion of the other batches. Table 2 summarizes the various batches with their individual kneading and extrusion parameters. Table 2: Summarized kneading and extrusion parameters for the batches prepared in this work, batches with * represent the usual set of parameters resulting in insufficient mechanical stability.

batch group

ap

kt

rs

Rotational speed

Content PMMA

Kneading time

[vol.-%]

[min]

ap-0

0

90

22

ap-45

45

90

22

ap-50

50

90

22

ap-54*

54

90

22

ap-60

60

90

22

kt-90*

54

90

22

kt-120

54

120

22

kt-300

54

300

22

rs-20

54

90

20

rs-22*

54

90

22

rs-30

54

90

30

rs-36

54

90

36

batch

5

(extrusion screw) [rpm]

The tubes extruded were dried in air for 48 h at least. After drying, averaged inner and outer diameters of 9.5 mm and 12 mm were obtained. For the investigation of properties samples with lengths (green state) of 40 mm and 12 mm were prepared by cutting the dried tubes. All samples were fired using the same sintering regime consisting of a multi-step debindering in flowing nitrogen at temperatures below 600 °C and a firing temperature of 1130 °C with a dwelling time of 2 h in flowing air at the main firing step. For all steps smooth heating and cooling rates below 2 k/min were chosen. After firing samples with lengths of 33 mm and 10 mm, thicknesses of 0.9 mm and outer diameters of 9.5 mm were received. Shrinking behavior was analyzed according to occurred variations of samples volumes and masses before and after firing process. Volume was determined by measuring sample dimensions and using basic geometric calculations for tubes. Archimedean buoyancy principle referring to DIN EN 1389:2004-03 [33] was used for determination of porosity and densification. For each batch five samples (l  33 mm) were boiled in water for 2 h with subsequent cooling in water for 1 h. Mass of infiltrated samples was determined in water and air using a balance equipped with a special attachment for uplift-measurements. Archimedean matrix density M and open porosity PO were calculated using equations 1 and 2, where mdry is the mass in dry state, minf,air and minf,H2O are the masses of the infiltrated samples determined in air and water, respectively, and H2O represents density of water.

Grain size, pore size and pore content at outer surface of as-fired samples were determined using images from scanning electron microscopy taken by SEM (ULTRA 55 plus) produced by Carl Zeiss AG. There was no need for additional preparation steps like using conductive coatings or special sample attachment. For taking the images excitation voltage of 15 kV, working distances between 8 and 12 mm and secondary electron detector (SE2) were used. The average two-dimensional grain sizes G2D were obtained from the printed pictures referring to DIN EN 623-3 [34] following equation 3 where n represents the number of lines with the length L that were drawn in and N represents the total number of grain boundaries that were intercepted by the lines. M is the respective magnification and Lp is the total length of pore sections intercepted by the lines. For the calculation of each value a number of about 100 grains was analyzed. 6

∑ Average three-dimensional grain sizes D3D were calculated from measured average twodimensional grain sizes D2D by multiplying a factor of 1.57 according to equation 4 [35].

For the calculation of pore content and two-dimensional pore size equations 5 and 6 [34] were used, where NP represents the total number of pores intercepted by the lines.

In addition, determination of pore size and pore content, as well as determination of grain size for ap-0, was carried out for several samples per batch evaluating images of the bulk microstructure taken by VLM (visible light microscopy). Individual specimens were prepared by grinding in several steps while adding water (last step – 21 µm), followed by lapping using a diamond lubricant (9 µm) and polishing in several steps using diamond suspension (last step – 1 µm) to obtain clean and smooth surfaces. Afterwards, samples were chemical etched for 60 s using a solution containing ethanol and 2 vol.-% of nitric acid. The pictures were taken using the optical microscope Axioplan equipped with Epiplan-objectives for magnifications of 200x, 500x and 1000x, produced by Carl Zeiss AG. Values for grain size, pore content and pore size were determined using equations 3-6. Fracture strength was obtained by vertical loading of 20 samples (l  10 mm) for each batch using a testing machine Zwick 100 produced by Franz Wohl & Partner Prüfmaschinen GmbH. Samples were placed on a flexible mounted base and loaded with a fixed stamp until cracking applying a traversal velocity of 1 mm/min, as displayed in Fig. 1. The prevalent forces were recorded using a force transducer and converted into values for fracture strength following basic calculations relating vertical bending of ring shaped components [36].

7

Fig. 1: Schematic illustration of the loaded sample and geometrical values used for calculation of bending stresses. Bending stress in z-direction σz, which can be described due to bending moment in zdirection Mz and moment of inertia in x-direction Ix, as given in equation 7, was assumed to be main trigger for cracking.

Bending moment in z-direction was calculated using equation 8 where F is the maximum force detected before cracking and R represents the distance between center and half of the material thickness w according to y-z cross section of the samples (see fig. 1).

The results of the bending experiments were statistically verified using Weibull-statistics according DIN EN 843-5:2007-03 [37]. Estimated values for m (Weibull-modulus) and σ0 (stress relating to fracture probability of 63.21 %) were received by numerical solution of equations 9 and 10 using interval bisection procedure with 20 iterations.

̂

∑

̂

∑

̂

∑

8

̂

̂

̂

∑

For all following diagrams data points were given together with error intervals calculated from standard deviation of sample values within the same dataset in combination with respective equation-related error propagation. 3

Results

Only slight variations of shrinking behavior were observed according to the applied kneading times during mixture homogenization and the applied rotational speed of the extrusion screw during extrusion. Figure 2 shows shrinking behavior of the extruded samples for varying kneading time of the mixtures and rotational speed of the main screw during extrusion. A small decrease of volume shrinkage for increasing kneading time from 90 to 120 min was observed, while no further variation occurred with rising kneading time from 120 to 300 min. A small increase of volume shrinkage for rising rotational speed during extrusion between 20 and 22 rpm and a linear decrease of 5 % between 22 and 36 rpm was found.

rotational speed [rpm] 16

20

24

28

32

36

40

250

300

350

55

V/V0 [%]

53

51

49

47

kneading time

rot. speed

45 50

100

150

200

kneading time [min] Fig. 2: Shrinking behavior in dependence on kneading time of the mixture and rotational speed of the main screw during extrusion.

Accordingly, only small variations of matrix density (below 2 %) were observed in dependence on the applied kneading times and rotational speeds, as visible in figure 3, which is why impact of these parameters on density can be neglected.

9

rotational speed [rpm] 16

20

24

28

32

36

40

300

350

98

M (rel.) [%]

97 96 95 94

kneading time (mat.)

rot. speed (mat.)

93 50

100

150

200

250

kneading time [min] Fig. 3: Relative Archimedean matrix density (related to theoretical density of BSCF of 5.57 g/cm³ at room temperature determined according values from [19]) in dependence on kneading time of the mixture and rotational speed of the extrusion screw.

rotational speed [rpm] 16

24

32

40

80 open porosity (Arch.) - kt porosity (VLM) - kt porosity (SEM) - kt open porosity (Arch.) - rs porosity (VLM) - rs porosity (SEM) - rs

P [%]

60

40

20

0 50

150

250

350

kneading time [min] Fig. 4: Pore content determined using Archimedean buoyancy principle, VLM and SEM in dependence on kneading time of the mixture and rotational speed of the extrusion screw.

10

In Figure 4 the impact of kneading time and rotational speed during extrusion on pore content is presented. Open porosity determined with Archimedean buoyancy principle showed no variation in relation to both parameters. No impact of kneading time on bulk pore content determined using VLM was observed, while an increase from 40 % to 70 % was detected for rising rotational speed between 20 and 36 rpm. For surface pore content analyzed using SEM a decrease with rising kneading time between 120 and 300 min and with rising rotational speed between 22 and 36 rpm was observed. A small increase of surface pore content was detected for rising kneading time between 90 and 120 min and rising rotational speed between 20 and 22 rpm.

rotational speed [rpm] 16

24

32

40

25

grain size (VLM) - kt grain size (SEM ) - kt pore size (VLM) - kt pore size (SEM) - kt pore size (VLM) - rs pore size (SEM) - rs grain size (VLM) - rs grain size (SEM) - rs

D3D, DP [µm]

20

15

10

5

0 50

150

250

350

kneading time [min Fig. 5: Grain sizes and pore sizes determined using images taken by VLM and SEM in dependence on kneading time of the mixture and rotational speed of the extrusion screw.

Three-dimensional surface grain sizes of as-fired samples determined using images taken by SEM showed a small increase with rising kneading time between 90 and 300 min and with rising rotational speed between 22 and 36 rpm, as depicted in figure 5. A small decrease of surface grain size was detected for rising rotational speed between 20 and 22 rpm. In contrast, bulk grain size determined using VLM showed lower average values and a decrease with rising kneading time. Because of the comparable large pores inside material grain size determination was more difficult and al larger scattering of the values occurred. Two-dimensional bulk pore size determined using VLM was not affected by the applied kneading times but showed an increase with rising rotational speed of the main screw during extrusion between 20 and 36 rpm. No significant variation of surface pore size determined 11

using SEM in relation to kneading time or rotational speed was observed. Discussion towards obvious differences of pore size and pore content at the surface compared to bulk pore size and pore content determined using the varying methods follows in the next chapter.

1.0

0.8

0.6

Pf

σ (kt-90 - standard) σ* (kt-90 - standard)

0.4

σ (kt-120) σ* (kt-120) 0.2

σ (kt-300) σ* (kt-300) 0.0

0

10

20

30

40

50

60

70

σ [MPa] Weibull-

Fig. 1: Weibull-distribution of fracture strength values for kt-batches.

distribution of fracture strength values for kt-batches is shown in figure 6. For rising kneading time between 90 and 120 min the distribution was shifted to higher stresses entailed with higher values for fracture strength. In addition slope sharpening of the linear section occurred going along with rising Weibull-modulus. Further increase of kneading time from 120 to 300 min led to decreasing stress values and slope smoothening of the linear section, resulting in lower values for fracture strength and Weibull-modulus. The stress dependent fracture probability for rs-batches is presented in figure 7. Only small variations of mechanical behavior were detected in dependence on rotational speed during extrusion. A decrease of rotational speed during extrusion from 22 to 20 rpm, as well as an increase between 22 and 36 rpm led to similar shifting of Weibull-distribution to higher stress values. A smoothened slope of the linear section was detected for rotational speeds greater than 20 rpm.

12

1.0

0.8

σ (rs-20)

0.6

Pf

σ* (rs-20) σ (rs-22 - standard) 0.4

σ* (rs-22 - standard) σ (rs-30) σ* (rs-30)

0.2

σ (rs-36) σ* (rs-36) 0.0 5

15

25

35

45

55

65

σ [MPa] Table

3 Fig. 2: Weibull-distribution of fracture strength values for rs-batches.

summarizes the values for average fracture strength σZ determined using equation 7 and the estimated values for the parameters σ0 and m derived from Weibull-distribution for kt-batches and rs-batches.

batch

σZ [MPa]

̂ [MPa]

̂

kt-90/ rs-22 (standard)

30.3  9.3

33.6

3.4

kt-120

38.6  7.5

41.5

6.2

kt-300

35.8  10.2

39.5

4.2

rs-20

33.5  9.3

36.9

4.5

rs-30

36.1  11.5

40.1

3.8

rs-36

34.3  10.8

38.2

3.7

Table 3: Average fracture strength in z-direction and parameters derived from Weibull-distribution for ktbatches and rs-batches.

A strong influence of kneading time on fracture stress and Weibull-modulus is visible while smaller variations were found in dependence of rotational speed of the main screw. 13

100

55

95

50

90

45

85

40

80

volume shrinkage

ρM (rel.) [%]

ΔV/V0 [%]

60

rel. matrix density

35

75 0

0.1

0.2

0.3

0.4

0.5

0.6

VPMMA/(VBSCF+VPMMA) Increasing

amount

of

pore

forming agent (PMMA) in the

Fig. 3: Volume shrinkage and relative Archimedean matrix density in dependence on PMMA-content in the extruded mixture.

extruded mixture between 0 and 60 vol.-% led to small decrease of relative Archimedean matrix density by 1-2 % according to a theoretical density of BSCF from 5.57 g/cm, as depicted in figure 8. Volume shrinkage increased from 40 to 55 % for rising PMMA-content between 0 and 60 vol.-%. A change of the slope for increase and decrease was observed at PMMA-contents above 50 vol.-%. Figure 9 shows impact of PMMA-content in the extruded mixture on porosity of the samples determined by various methods. In general, increasing values were found with rising amount of PMMA for surface pore content determined using SEM, bulk pore content analyzed using VLM and open porosity received from Archimedean buoyancy principle. Open porosity increased almost linearly from 1 % to 40 % with rising PMMA-content between 0 and 60 vol.-%, while a rise of 5 % was found for bulk porosity and surface porosity with rising amount of PMMA between 45 and 60 vol.-%. According to the described behavior of shrinkage and density, the behavior of porosity of samples prepared with amounts of PMMA below and above 50 vol.-% differs obviously.

14

70

open porosity (Arch.)

60

porosity (VLM)

50

porosity (SEM)

P [%]

40 30 20 10

0 0

0.1

0.2

0.3

0.4

0.5

0.6

VPMMA/(PBSCF+VPMMA)

50

12

40

10

30

8

20

6

10

grain size (VLM)

grain size (SEM)

pore size (VLM)

pore size (SEM)

DP [µm]

D3D [µm]

Fig. 9: Open porosity received from Archimedean buoyancy principle and pore content determined using images taken by VLM and SEM in dependence on PMMA-content in the extruded mixture.

4

0

2 0

0.1

0.2

0.3

0.4

0.5

0.6

VPMMA/(VBSCF+VPMMA) Fig. 10: Three-dimensional grain size and two-dimensional pore size determined using images obtained from VLM and SEM in dependence on amount of PMMA in the extruded mixture.

Three-dimensional surface grain size obtained from evaluation of images taken by SEM decreased almost linearly by 50 % with rising amount of PMMA, as visible in figure 10. At 15

high PMMA contents smaller variation occurred. In contrast, bulk grain size showed lower average values, with a similar decreasing rate for PMMA-content in the mixture between 0 and 50 vol.-% and increased scattering of values at larger PMMA-contents. For the ap-0 batch a surface grain size of 43.3  0.9 µm was determined, which was approximately twice as large compared to the value for bulk grain size of 22.2  2.3 µm. Only small variations of two-dimensional pore size were observed for rising PMMA-content between 45 and 60 vol.%. Surface pore size determined using SEM showed values in the range of 4-6 µm, whereas pore sizes of 9-10 µm were found in the bulk using VLM. Weibull-distribution of fracture strength values for ap-batches (except batch ap-0) is shown in figure 11. Shifting to higher stress values occurred with decreasing amount of PMMA in the extruded mixture. The distributions for ap-60 and ap-54 showed only small differences while great shifting occurred for decreasing PMMA-content between 54 and 50 vol.-%. No significant change of distribution was found for further reduction of PMMA-content to 45 vol.%.

1.0

0.8

0.6

Pf

σ (ap-45) σ* (ap-45) σ (ap-50)

0.4

σ* (ap-50)

σ (ap-54 - standard) σ* (ap-54 - standard)

0.2

σ (ap-60) σ* (ap-60) 0.0 5

15

25

35

45

55

65

σ [MPa] Fig. 11: Weibull-distribution of fracture strength values for ap-batches (except batch ap-0).

Table 4 summarizes fracture strength in z-direction and the parameters derived from Weibulldistribution for ap-batches. 16

Table 4: Average fracture strength in z-direction and parameters derived from Weibulldistribution for ap-batches. batch

σZ [MPa]

̂ [MPa]

̂

ap-0

139.0  41.2

154.0

4.0

ap-45

37.3  10.1

41.1

4.0

ap-50

39.5  8.8

42.9

5.2

ap-54/ kt-90/ rs-22 (standard)

30.3  9.3

33.6

3.4

ap-60

28.8  9.5

32.2

3.5

Samples from ap-0 batch showed high average values but also great deviations for fracture strength. For batches containing PMMA as pore forming agent in general lower fracture strength values, decreased up to a factor of 4 were found. Only small variations of Weibullmodulus with a slightly rising value between 45 and 50 vol.-% PMMA and a decrease with rising PMMA-content between 50 and 54 vol.-% were found. The normalized standard deviation for the detected fracture strength values behaved nearly proportional according to Weibull-modulus (see Fig. 12). Interpolation of the fitting curve towards coefficient of variation of 0 resulted in a maximum modulus between 9 and 10, in dependence on fitting parameters, which could be a characteristic value relating material and manufacturing conditions.

coefficient of variation (σ) [%]

40

30

20

10

values

fit

0 2

3

4

5

6

7

8

m Fig. 12: Correlation of Weibull-modulus and deviation of fracture strength

17

9

10

4

Discussion

A strong influence of the parameters for the manufacturing of porous BSCF tubes applied in this work on the properties of the ceramic structure and the resulting mechanical stability was demonstrated. Comparison of the results for varying parameters, like content of pore forming agent, kneading time of the mixture or rotational speed of the main screw during extrusion revealed great potential for optimization of the mechanical performance towards normally used parameter sets. Increasing kneading time up to a certain level resulted in rising values for fracture strength and Weibull-modulus while properties of ceramic structure like density, porosity or shrinkage were almost unaffected. Using a kneading time of 120 min instead of 90 min led to improvement of average fracture strength by 27 % and rise of Weibullmodulus from 3.4 to 6.2, which can be assorted with a raised homogeneity of the mixture, due to longer times of kneading. The decrease of fracture strength for kneading times of 300 min was most-likely related to eminently softening of the mixture which resulted in negative impact on flowing behavior. A clear correlation between the minimum torsional moment reached during kneading and the decrease of pressure detected inside extrusion die for rising kneading times gave additional evidence for changes in mixtures behavior (see Table 5). Table 5: Minimum torsional moment reached during kneading for kt-batches and pressure detected inside extrusion die for all batches fabricated in this work. min. torsional moment

pressure

(kneading) [Nm]

(extrusion die) [bar]

kt-90/ rs-22/ ap-54 (standard)

81.7

80

kt-120

79.8

76

kt-300

61.3

55

rs-20

-

75

rs-30

-

81

rs-36

-

83

ap-0

-

72

ap-45

-

79

ap-50

-

78

ap-60

-

88

batch

Since decreased density of ceramic components caused by the presence of pores generally results in lowered mechanical stability, the amount of pore forming agent in the mixture had a clear effect on porosity and hence on fracture strength for the fabricated batches. In this 18

context, adding an amount of 45 vol.-% PMMA even results in lowered fracture stresses by 70 % compared to monolithic BSCF-tubes with comparable wall thickness. Nevertheless, porosity of the support for asymmetric oxygen transporting membranes is a main aspect for its performance, which is why a certain loss of mechanical stability has to be accepted to ensure functionality. For this reason impact on ceramic structure has to be taken into account, intensively. Reasonable impact on density and shrinkage was found for rising content of PMMA related to a rise of porosity. Various analyzing methods revealed different results for pore content and pore size, which can be explained due to the character of the individual analyzing principles and measuring conditions. Pore sizes determined on the samples surfaces using images taken by SEM were notable lower compared to values received for the bulk material using VLM images, as depicted in figure 13.

10 µm

Fig. 13: VLM image (left) showing bulk pore content/ pore size and SEM image (right) showing surface pore content/ pore size for samples from rs-20.

At the last step of extrusion the ceramic mixture is formed into tubes by pressing through the extrusion die. Because of the tight shaped die, the mixture is densified and PMMA-spheres were pushed inside the material. Therefore, direct contact area between microspheres and tube/ die surfaces is minimized, resulting in lower pore sizes on the surfaces. For the same reason, the pore content nearby the surface determined using SEM is also decreased, whereas values for bulk pore content determined using VLM are in relatively good agreement with the volume part of PMMA added. Open porosity analyzed using Archimedean buoyancy principle represents an overall porosity for the whole tube, resulting in a mixed value, which is lower compared to bulk porosity and higher compared to surface porosity. A rising amount of PMMA between 45 and 60 vol.-% lead to an increase of surface porosity from ~5 to 10 % while bulk porosity raised between 48 and 55 % and Archimedean open porosity increased from ~30 to ~38 %. Sensitivity of BSCF in water or aqueous vapor atmosphere is often 19

claimed in relation to solubility of Barium and Strontium. However, only small mass changes of the analyzed samples in the range of 0.01-0.7 % were detected after boiling in water for two hours, which is why a significant effect on matrix density and open porosity determined using Archimedean buoyancy principle could be excluded. Moreover, the detected mass loss seemed to be related to residuals of the debindering process during sintering, rather than to dissolving of BSCF. Decreasing of PMMA content from 54 to 50 vol.-% led to a remarkable increase of average fracture strength by 30 %, while open porosity is changed only slightly. That indicates that a small reduction of PMMA content can be a promising way for great improvement of mechanical stability, without drastic loss of gas transporting paths and hence of functionality. In addition, lower pressures were detected inside extrusion die for decreasing content of PMMA (Table 5), which could be related to an improvement of flowing behavior for the extruded mixture. A decreasing grain size occurred with rising PMMA-content, which can be explained by a grain growth inhibiting effect of large pores in ceramic structures. The growth of the grains is limited due to size and content of the larger pores, as schematically shown in figure 14.

Fig. 14: Schematically illustration of grain size limitation with estimated maximum grain size marked by dotted lines (1/2 – unfavorable diffusion paths across large pores and pore channels, 3 – favorable diffusion paths for grain growth and densification).

During debindering the pore forming agent and other organic additives are vaporized leading to development of channels linking individual large pores with each other and with the surface of the ceramic component resulting in the generation of an open pore network, before sintering of the ceramic structure actually occurs. When sintering starts, diffusion 20

paths for the transfer of atoms were energetically limited. Diffusion across the large pores is thermodynamically unfavorable which is why densification and grain growth mainly take place within the areas generated due to the presence of the pore network. For this reason maximum grain size is limited by size of the large pores and pore channels, as well as, by their distance to each other which is determined by the added amount of pore forming agent. Hence, a rising content of PMMA in the extruded mixture led to decreasing pore distances inside the ceramic structure resulting in lowered grain sizes for the sintered components compared to samples without pore forming agent from batch ap-0. Accordingly this effect is much stronger for bulk grain size since bulk pore size is approximately twice as large compared two surface pore size. Rotational speed of the main screw influenced the behavior of the extruded mixture significantly. Rising pressures inside extrusion die were detected for rising rotational speed of the main screw (Table 5) as a result of stronger forces of friction exerted to the mixture during transport inside the extruder. The in this context reduced water content, therefore caused an increase of pre-densification for the extruded mass which led to lower shrinkage during sintering. Due to a rise of forces caused by increasing rotational speed of the main screw the PMMAspheres were pulled away from the surface of the extruded mixture, resulting in lowered PMMA-content at the surface of the as-extruded tubes. Therefore, a rise of bulk pore content was detected using images taken by VLM while a slight decrease of surface pore content was found for rising rotational speed of the main screw during extrusion. The increase of bulk pore size was most likely related to the lowered distances between pores and pore channels resulting in rising affinity for the reduction of inner surface by coalescences of pores. Degradation of homogeneity caused by changed pore distribution could also be concluded from the decreased Weibull-modulus for rising rotational speed of the main screw. Nevertheless, the improved pre-densification due to higher forces exerted on the mixture during extrusion in combination with lowered pore content at the surface is the most probable reason for a slightly increased fracture strength detected for batches fabricated applying high rotational speeds of 30 or 36 rpm. 5

Conclusion

The results have shown that each of the parameters applied in this work for the fabrication of porous tubular BSCF-supports could be varied in order to optimize mechanical stability. Increase of kneading time up to a certain level and slightly decreasing the content of pore forming agent in the mixture appeared to be the most effective routes due to improvement of fracture strength by up to 30 % within the applied ranges. Nevertheless, changing of pore 21

distribution in relation to increased rotational speed of the main screw during extrusion could be also used. No critical changes of porosity or pore size were observed which suggested that there was no strong influence on permeation performance of finally coated membranes but this should be analyzed more precisely in a further work. Because of the great potential revealed in this survey, further investigations applying even wider parameter ranges should also be performed in order to evaluate some of the occurring effects in more detail. Since only individual parameters were varied during this experiments, another approach could be the combination of different optimized parameters in one route in order to reach further improvements. However, the presented results represent an amazing step towards industrial application of tubular asymmetric oxygen transporting BSCF-membranes and could be also helpful for investigations on differing material systems and their respective applications.

Acknowledgements

This work was supported by German Federal Environmental Foundation.

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