Dense and gastight dead-end tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes were fabricated by thermoplastic extrusion. Optimal feedstocks for the extrusion of dead-end tubular structures were obtained for a feedstock consisting of 89.83 wt% BSCF powder, 5.63 wt% ethylene vinyl acetate (EVA), 3.56 wt% paraffin wax (PW), and 0.98 wt% stearic acid (SA). Thermogravimetric analysis (TGA) was used to determine the thermal characteristics of the feedstock and to characterize the amount of binder removed during the wicking step. Straight and round dead-end tubular membranes with wall thickness of 0.5 mm could be achieved after sintering. The gastightness test showed that the membranes were leak-tight up to a pressure of 35 bar

Clemens, Frank

Graule, Thomas

Grobéty, Bernard

Kaletsch, Anke

Pfaff, Ewald M.

Salehi, Mehdi

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Manufacturing of Tubular Dead-End Membranes by ContinuousThermoplastic ExtrusionMehdi SalehiLaboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology,Ueberlandstrasse 129, Duebendorf CH-8600, SwitzerlandThe Fribourg Center for Nanomaterials (FriMat), Department of Geosciences, University of Fribourg, Perolles Ch. du Musee 6,Fribourg CH-1700, SwitzerlandEwald M. Pfaff and Anke KaletschInstitute for Materials Applications in Mechanical Engineering, RWTH Aachen University, Nizzaallee 32, Aachen 52072,GermanyThomas Graule and Frank Clemens*Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology,Ueberlandstrasse 129, Duebendorf CH-8600, SwitzerlandBernard GrobetyThe Fribourg Center for Nanomaterials (FriMat), Department of Geosciences, University of Fribourg, Perolles Ch. du Musee 6,Fribourg CH-1700, SwitzerlandDense and gastight dead-end tubular Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) membranes were fabricated by thermoplastic extru-sion. Optimal feedstocks for the extrusion of dead-end tubular structures were obtained for a feedstock consisting of 89.83 wt%BSCF powder, 5.63 wt% ethylene vinyl acetate (EVA), 3.56 wt% parafﬁn wax (PW), and 0.98 wt% stearic acid (SA). Thermo-gravimetric analysis (TGA) was used to determine the thermal characteristics of the feedstock and to characterize the amount ofbinder removed during the wicking step. Straight and round dead-end tubular membranes with wall thickness of 0.5 mm couldbe achieved after sintering. The gastightness test showed that the membranes were leak-tight up to a pressure of 35 bar.IntroductionDue to 100% oxygen permselectivity at elevatedtemperatures, dense ceramic membranes with highoxygen ionic and electronic conductivity have becomeincreasingly competitive and are already under investiga-tion as oxygen separation membranes, catalytic partialoxidation reactors, and cathodes in solid oxide fuel cells(SOFCs).1–4 BSCF has one of the highest oxygen perme-ability, and it has been investigated extensively foroxygen separation membranes.5,6High-purity oxygen can be obtained using densemixed ionic conductor ceramic membranes when there isa higher oxygen partial pressure on one side of the mem-brane (feed side) than the other side (permeate side).The oxygen pressure gradient can be increased by com-pressing the air on feed side and/or by decreasing theoxygen partial pressure on the permeate side. The lattercan be achieved using an inert sweep gas, for example,argon or helium, or by evacuating the permeate side.The use of a sweep gas requires a second separation pro-cess to separate the latter from the oxygen. In recentyears, the use of sweep gas was abandoned, and the evac-uation of the permeate side by vacuum pumps hasbecome the method of choice to produce high-purityoxygen. Lowering the overall pressure on the permeateside enhances signiﬁcantly the oxygen permeability.7–9One of the major problems in using tubular membranesfor the oxygen separation is sealing of the tube end.Because thermal expansion coefﬁcients of most suitablesealing materials differs largely from the coefﬁcients of*Frank.clemens@empa.ch; chemehdi3333@gmail.com1Published in ,QWHUQDWLRQDO-RXUQDORI$SSOLHG&HUDPLF7HFKQRORJ\(±(which should be cited to refer to this work.http://doc.rero.chthe perovskite-based membrane materials, heating up ofthe membrane to operating temperatures may inducehigh thermal stresses, which may cause leakage across thejoints interface.10 Therefore, shaping tubular membranesin form of the dead-end structures may obviate the sealingproblems.Fabricating dead-end tubular membrane is not asimple process, and different methods are used. Lianget al.7 have closed open BSCF tubes on one end by athick disk of the same material before sintering, using apaste based on copper oxide to keep tube and platetogether. The seal between tubes and plate was estab-lished during reaction-diffusion sintering in which thecopper oxide functioned as sintering aid. BaCoxFeyZr1-x-yO3-d (BCFZ) hollow ﬁbers have been closed by goldplugs using Au paste.8 Wei et al.9 reported a novelmethod to produce the dead-end hollow ﬁber using awet spinning/sintering method. After forming hollowﬁbers with two open ends by a phase inversion spinningprocess, one end of the ﬁbers was coated with the samespinning solution followed by a second phase inversionprocess. However, they could solve the complexity andhigh cost of sealing a disk, but they still needed a two-step process to fabricate the dead-end hollow ﬁbermembrane.In this study, thermoplastic extrusion method wasused to fabricate the dead-end Ba0.5Sr0.5Co0.8Fe0.2O3-d(BSCF) tubular membranes in a single-step process.ExperimentalFor the preparation of membranes, commerciallyavailable Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) powder (Trei-bacher Industrie AG, Althofen, Austria), with a meanparticle size of 3.8 lm and a speciﬁc surface area of1.55 m2/g, was used. Ethylene vinyl acetate (EVA) (EL-VAX250A, DuPontTM; Geneva, Switzerland), polyethyl-ene (PE) (1700MN18C Lacqtene PEBD, ArkemaGroup, Cedex, France), and parafﬁn wax (PW) (76231Fluka AG, Buchs, Switzerland) were used as binders(Table I). Typically, a parafﬁn content for thermoplasticprocessing route is in the range between 40 and 60vol.%. In a previous study, a parafﬁn content of 40vol.% was used to achieve dimensionally stable thintubular structures after extrusion process.11Stearic acid (SA) was added as a surfactant(Fluka AG). The desired SA content was calculated basedon the area occupied by one SA molecule and the spe-ciﬁc surface area of the ceramic powder. According topractical results, this value has been tripled to achievesufﬁcient feedstock properties (e.g., low viscosity andhomogeneity).12 Two different binder systems (Table II)were tested for the preparation of the thermoplastic feed-stocks. Both formulations are based on a ceramic-poly-mer composite with 60% volume fraction of ceramic.Prior to feedstock preparation, the BSCF powderwas precoated with stearic acid. For the compounding, ahigh shear mixer (HAAKE PolyLab Mixer, Rheomix600, Thermo Scientiﬁc, Karlsruhe, Germany) equippedwith a pair of roller blades was used. The torque wasrecorded as a function of mixing time. The mixingprocess is described elsewhere.13The rheology of the feedstocks was analyzed with atwin-bore capillary rheometer (RH7-2, Malvern,Herrenberg, Germany). The appeared viscosity wascalculated from the total extrusion pressure and pistonspeeds. For the rheological investigations, a long capillarywith a diameter of 1 mm and a length of 16 mm wasused.Tubular membranes were extruded through a10-mm-diameter die and with wall thicknesses of 1 andTable I. Physical Properties of Binders ComponentsUsed in this StudyBinder ComponentDensity(g/cm3)Meltingpoint, Tm (°C)Stearic acid 0.845 68Ethylene vinylacetate (EVA)0.95 72Polyethylene (PE) 0.92 109Parafﬁn wax 0.91 60Table II. Formulations of the Two Different Feedstocks and Extrusion ConditionsFeedstock Composition (wt%) Extrusion Rate (mm/s)ExtrusionTemperature (°C)1 89.83% BSCF powder, 5.63% EVA,3.56% PW, 0.98% SA5–10 682 89.83% BSCF powder,5.58% PE, 3.61% PW, 0.98% SA50 952http://doc.rero.ch0.5 mm, using a capillary rheometer (RH7-2, Malvern,Germany). After the extrusion, one end of the tubes wasclosed manually with pliers, and the leakage can besimply investigated by a bubble point test. Solventextraction and wicking followed by a thermal treatmentwere tested to remove the binder. The solvent extractionof parafﬁn was performed by immersing the green sam-ples into an ethanol bath at 40–45°C. The wicking stepwas carried out at 150°C and 4 h holding time toremove the parafﬁn. Thermal treatment was carried outat a heating rate of 10°C/h up to 600°C in air. Afterbinder removal, the tubular membranes were sintered at1000°C for 2 h in air, using a heating rate of 60°C/h.The tensile strength of sintered tubes(douter = 10 mm; dwall = 0.9 mm; Ltube = 25 mm) wasmeasured by a Zwick Z005 universal testing machine(Zwick GmbH, Ulm, Germany). The test settings wereas follows: preload 0.5 N, preload speed 0.5 mm/min,and test speed 1 mm/min. The tubes were appliedlengthwise between two dies and loaded by compressionforces. Leather clothes (very thin) were used as pressurepads to decrease frictional stresses and to avoid slippageof the membrane tube and the stainless steel platesduring compressive strength testing. For calculation thefailure tensile strength the following equation describedelsewhere18 was usedr ¼ P  K4ðD0  DiÞ  twhere P is the load to failure, D0 and Di are the outsideand inner diameter, t the width of the tubes, and K4 isan empirical determined stress constant, which is a func-tion of the ratio Di/D0 and the manufacturing method.For our extruded tubes K4 was estimated as 7.The sintered membranes were tested for gastightnessby feeding the tubes with pressurized air (2 bar, Fig. 1).The pressurized tube was submerged in water. Leakscould be detected by the appearance of bubbles. After test-ing the gastightness, the tubes were integrated in a bursttest device to evaluate at which pressure the tubes fail,especially the dead-end part. In the test device, the tubeswere pressurized with external air in steps of 2 bars atintervals of 1 min up to a maximal pressure of 35 bar.Results and DiscussionTorque evolution for feedstock 1 and 2 is shown inFig. 2. The binder components were ﬁrst ﬁlled into thechamber. BSCF powder coated with stearic acid wasthen added and kneaded until the torque was stabilized.An increase in torque is associated with the initial addi-tion of the powder. As soon as the particles are wettedby binder, torque decreases until reaching an equilibriumstate.Fig. 1. Device for gas-tightness tests of the dead-end tubularmembrane.Fig. 2. Change of torque with mixing time for feedstock 1 and 2.Fig. 3. Viscosity of feedstocks 1 (EVA+PW binder system) andfeedstock 2 (PE + PW binder system) as a function of shear rate.3http://doc.rero.chFigure 3 illustrates the rheological properties offeedstock 1 and 2 at different temperatures. Due to thedifference in the melting point of PE and EVA(Table I), rheological measurements of the individualfeedstocks were performed at different temperatures. Itcan be seen that the viscosity of the feedstocks decreaseswith increasing shear rate. This behavior is well knownas the pseudoplastic or shear-thinning behavior.Extrusion experiments, which have been carried outat the same temperatures used for the rheological mea-surements, led to strong deformation of the tubes duringextrusion. Therefore, the extrusion temperatures werelowered for both binder systems (Table II). In this study,the best results were obtained for feedstock 1 (EVA-based system). For extrusion of feedstock 1, tubularmembranes were extruded into ethanol (~30°C) belowthe nozzle to achieve quick solidiﬁcation and thereforeshape stability by quenching. The following debindingand sintering experiments were, therefore, only contin-ued with samples prepared with feedstock 1 (see Fig. 4).N-hexane or n–heptane is typically used for theremoval of PW by solvent extraction.14,15 Preliminarytests with feedstock 1 revealed that the solubility of thebinder system consisting of EVA, PW, and SA was toohigh in both solvents to use them for debinding (Fig. 5).This result is somewhat contradicting to others wheren-hexane or n-heptane have successfully been used as sol-vent to remove a binder system consisting of EVA, PW,and SA.15,16Instead ethanol was used at 40°C/5 h for solventextraction. When raising the temperature above 45°C,slumps and cracks appeared in the membrane walls. Afterdrying at room temperature, the samples showed deforma-tion in axial direction (Fig. 6). This could be due to thecontraction of leftover backbone polymer (EVA).Owing to the negative results with solvent debindingmethod, wicking was therefore performed at 150°C/5 h(a)(b)(c)Fig. 4. Images of: (a) green tubes, feedstock 2 (b) green tubes,feedstock 1 (c) sintered tubes, feedstock 1.Fig. 5. Samples after solvent debinding in n-hexane.Fig. 6. Dead- end tube (feedstock 1) after solvent debinding inethanol at 40°C/5 h and drying at room temperature.Fig. 7. Thermogravimetric analysis of feedstock 1: before andafter wicking process.4http://doc.rero.chto remove SA and PW and subsequent thermal treatmentup to 600°C to eliminate EVA and the leftover SA andPW. Figure 7a,b shows the mass loss for both continuousthermal treatment and combined wicking/thermal anneal-ing of feedstock 1. When continuously heated up, the bin-der decomposition starts at 190°C and is completed at550°C with a total weight loss of 8.54 wt%.The weight loss (Fig. 7b) in samples, which under-went a wicking debinding prior to the analysis, initiates atabout 240°C, around 50°C higher than the sample with-out wicking step. The difference between the two thermo-gravimetric analysis (TGA) curves shows (Fig. 7) thatduring the wicking debinding, about 70% of the SA andPW content in green sample is removed. This helps to cre-ate a pore network through which the gaseous breakdownproducts of the remaining binder components can diffuseto the surface of sample. There is an increase in mass withincreasing temperature from 420°C to approximately550°C due to carbonatization of BSCF. Infrared spectrataken from quenched samples contain two peaks character-istic for carbonate groups (Fig. 8).The grain size distribution, grain orientation, anddensity of the sintered BSCF structures by thermoplasticprocessing have been published elsewhere.11,13The Weibull modulus (m) determined from themechanical tests is 3.1 and the characteristic strength(r0) 15 MPa (Fig. 9). Both values are obvious smallercompared to literature data for BSCF tubular membranesfabricated by isostatic pressing (m = 6.7 andr0 = 130MPa).17 But it has to be considered that thetest methods and the loaded volumes are not directlycomparable. The details of determining compressivestrength and Weibull plot are described elsewhere.18,19All the tubes were gastight up to a pressure of2 bar. Two of four tubes withstand the maximal appliedpressure up to 35 bar (Table III). The remaining twotubes broke at pressures of 10 and 22 bar, but none ofthem broke at the dead-end part. One broke in the mid-dle of the tubes, and the other at the metal connection.ConclusionDead-end dense tubular BSCF membranes were suc-cessfully prepared using thermoplastic extrusion. Tubesprepared with a binder system based on PE and PWshowed axial deformation during extrusion, which couldbe avoided by using an EVA-based binder system. Sam-ples were completely destroyed during solvent debindingin n-hexane. Samples treated in ethanol showed deforma-tion in axial direction during drying. Therefore, a wick-Fig. 8. FTIR spectra of feedstock 1 after thermal treatment at550°C in air.Fig. 9. Weibull plot for sintered tubes made of feedstock 1.Table III. Gastightness Test of the Dead-End Tubesat Different PressuresPressure (bar) Tube 1 Tube 2 Tube 3 Tube 42 U U U U4 U U U U6 U U U U8 U U U U10 U ✗ U U12 U ✗ U U14 U ✗ U U16 U ✗ U U18 U ✗ U U20 U ✗ U U22 U ✗ ✗ U24 U ✗ ✗ U26 U ✗ ✗ U28 U ✗ ✗ U30 U ✗ ✗ U32 U ✗ ✗ U34 U ✗ ✗ U35 U ✗ ✗ U✗ = failed; U = gas tight.5http://doc.rero.ching process was applied to extract PW and SA. TGAshowed that about 70% of PW and SA are removedduring the wicking step. The remaining binder wasremoved by a subsequent thermal annealing up to600°C. Nondeformed dead-end tubular membranes withwall thickness of 0.5 mm were obtained with EVAbinder system after sintering. The gastightness test con-ﬁrmed that the membranes were leak-tight to nitrogenup to the pressure of 35 bar.AcknowledgmentsThe authors would like to acknowledge the ﬁnancialcontribution provided by Swiss Electric Research and theCompetence Center for Energy & Mobility (CCEM) inSwitzerland.References1. Z. Chen, R. Ran, Z. Shao, H. Yu, J. C. D. D. Costa, and S. Liu, Ceram.Int., 35 [6] 2455–2461 (2009).2. M. D. Mat, X. Liu, Z. Zhu, and B. Zhu, Int. J. Hydrogen Energy, 32 [7]796–801 (2007).3. Z. Shao, G. Xiong, H. Dong, W. Yang, and L. Lin, Sep. Purif. Technol., 25[1–3] 97–116 (2001).4. S. Svarcova, K. Wiik, J. Tolchard, H. J. M. Bouwmeester, and T. Grande,Solid State Ionics, 178 [35–36] 1787–1791 (2008).5. J. P. Kim, J. H. Park, E. Magnone, and Y. Lee, Mater. Lett., 65 [14]2168–2170 (2011).6. Q. Jiang, S. Faraji, D. A. Slade, and S. M. Stagg-Williams, Membr. Sci.Technol., 14 235–273 (2011).7. F. Liang, H. Jiang, H. Luo, R. Kriegel, and J. Caro, Catal. Today, 193 [1]95–100 (2012).8. F. Liang, H. Jiang, T. Schiestel, and J. R. Caro, Ind. Eng. Chem. Res., 49[19] 9377–9384 (2010).9. Y. Wei, J. Tang, L. Zhou, Z. Li, and H. Wang, Chem. Eng. J., 183473–482 (2012).10. K. Li, Ceramic Membranes for Separation and Reaction. John Wiley, Chiches-ter, U.K. and Hoboken, NJ, 2007.11. M. Salehi, et al., J. Membr. Sci., 443 237–245 (2013).12. J. Heiber, F. Clemens, T. Graule, and D. H€ulsenberg, Adv. Eng. Mater., 7[5] 404–408 (2005).13. M. Salehi, et al., J. Membr. Sci., 382 [1–2] 186–193 (2011).14. D.-S. Tsai and W.-W. Chen, Ceram. Int., 21 [4] 257–264 (1995).15. M. T. Zaky, F. S. Soliman, and A. S. Farag, J. Mater. Process. Technol., 209[18–19] 5981–5989 (2009).16. M. T. Zaky, J. Mater. Sci., 39 [10] 3397–3402 (2004).17. E. M. Pfaff, A. Kaletsch, and C. Broeckmann, Chem. Eng. Technol., 35 [3]455–463 (2012).18. J. M. Villora, P. Callejas, M. F. Barba, and C. Baudı́n, J. Eur. Ceram. Soc.,24 [3] 589–594 (2004).19. W. Weibull, A Statistical Representation of Fatigue Failures in Solids by W.Weibull, Henrik Lindstahl, Stockholm, Sweden, 1949.6http://doc.rero.ch

Manufacturing of tubular dead-end membranes by continuous thermoplastic extrusion

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