Electrophoretic deposition of boehmite on additively- manufactured

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Electrophoretic deposition of boehmite on additively-manufactured, interpenetrating periodic open cellular structures for catalytic applications Giang Do, Thomas Stiegler, Markus Fiegl, Lucas Adler, Carolin Körner, Andreas Bösmann, H. Freund, Wilhelm Schwieger, and Peter Wasserscheid Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02453 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Industrial & Engineering Chemistry Research

Electrophoretic deposition of boehmite on additivelymanufactured, interpenetrating periodic open cellular structures for catalytic applications Giang Doa,b, Thomas Stieglera, Markus Fiegla, Lucas Adlerc,d, Carolin Körnerc, Andreas Bösmanna, Hannsjörg Freunda, Wilhelm Schwiegera*, Peter Wasserscheid a,e* a

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, D-91058 Erlangen, Germany b

Anwenderzentrum VerTec, Zentralinstitut für Neue Materialien und Prozesstechnik, FriedrichAlexander-Universität Erlangen-Nürnberg, Dr.-Mack-Str. 81, D-90762 Fürth, Germany

c

Lehrstuhl Werkstoffkunde und Technologie der Metalle (WTM), Friedrich-Alexander-Universität, Erlangen-Nürnberg, Martensstraße 5, 91058 Erlangen, Germany d

e

Energie Campus Nürnberg, Fürther Str. 250, 90429 Nürnberg, Germany

Forschungszentrum Jülich, „Helmholtz-Institute Erlangen-Nürnberg for Renewable Energies“ (IEK 11), Nägelsbachstraße 59, 91058 Erlangen, Germany (*Corresponding Author’s E-mail’s: [email protected], [email protected]) Dedicated to Professor Tapio Salmi, Åbo Akademi University, Turku, Finland on the occasion of his 60th birthday.

Keywords: Electrophoretic deposition, selective electron beam melting SEBM, periodic open cellular structures, interPOCS, structured reactors

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Our contribution demonstrates that the combination of additive manufacturing and electrophoretic deposition offers great potential for the future manufacturing of tailor-made catalytic structures for continuous flow applications. A new protocol for the controlled and homogeneous coating of both electrodes of interpenetrating porous open cell structures (interPOCS) with layers of boehmite is presented. Moreover, it has been found that by applying different coating voltages in an Electrophoretic Deposition (EPD) process, the properties of the obtained coating can be fine-tuned with respect to layer thickness, density and porosity. This offers very interesting options to optimise the catalysis-relevant properties from two sides, by the use of the special interPOCS support design and by adjusting the coating through the parameters of the EPD coating process.

1

INTRODUCTION

For the field of catalysis, open-cell foams exhibit interesting properties compared to conventional packings in terms of heat transfer1, 2, 3, 4 and hydrodynamics5, 6, 7, 8, 9, 10. Next to traditional monolithic foams (sometimes also named sponges, which is due to their open structure the more correct name), which have been investigated intensely as catalyst supports11, 12, 13, 14, 15, 16, additively fabricated, metallic periodic open-cellular structures (POCS) represent a next generation of very interesting open-cellular structures17 and offer an almost unlimited range of different geometries within the building resolution limits. In contrast to the traditional monolithic foams, the POCS can be produced in a very well defined way and characterized by at least two geometric parameters, strut thickness and characteristic cell size, which can be varied independently. Thus, in contrast to the traditional, so-called reticulate foams, specific surface area and porosity can be predicted and adjusted separately over a wide range just based on the knowledge of their geometric properties. For reaction engineering applications, this facilitates overcoming packed bed limitations, such as high pressure drop and ACS Paragon Plus Environment

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the always existing temperature gradients often resulting in reduced selectivities. In addition, the high heat conductivity of the metallic structure and the regular, well defined structure of the POCS are highly beneficial properties for their use as model structures in scientific investigations and also of particular importance for catalytic reactions with high heat production/consumption and high volume expansion/reduction. In order to use catalytically functionalized POCS in heterogeneous catalysis, the original metal structure has to be functionalized in the first step with a porous coating to increase the active surface for hosting the active metal nanoparticles. The right coating methodology is crucial for catalytic performance as it determines the homogeneity of the layer and of the layer properties, such as layer thickness, porosity and mechanical strength. The incorporation of the active metal nanoparticles into this coated layer can be carried out during or after the coating step via impregnation, deposition-precipitation or ion exchange18. The established coating methods are chemical vapor deposition19, spraying20, 21, dip coating14, anodic oxidation22 or electrophoretic deposition (EPD)23, 24, 25. For catalytic applications spraying or dip coating of suspensions of ceramic or carbon powders are applied due to the efficient layer formation realized with these processes13, 26, 27, 28, 29, 30. In all mentioned cases, layer deposition is followed by a drying step and layer thickness is adjusted by repeated deposition/drying cycles. One very relevant problem with the traditional coating methodologies is, however, that during these cycles layer gradients along the radial or axial structure dimension are unavoidable. These lead to an inhomogeneous deposition of the catalyst and thus to inhomogeneous catalytic performance along the axes of the structure. Electrophoretic Deposition (EPD) has been applied for a wide range of coating applications and offers great variability and controllability. Still the main drawback of EPD, as reported in the


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literature27, 31, 32, 33, 34, 35, 36, 37, is the fact that only small parts can be coated homogeneously and thus many small parts have to be merged after coating if larger devices are to be fabricated. In this contribution, we demonstrate that suitable POCS manufactured by additive manufacturing processes (either Selective Electron Beam Melting (SEBM)17, 38, 39 or Selective Laser Melting (SLM)40 allow for a homogeneous coating of even large structures by EPD. This is demonstrated in the present paper for the deposition of boehmite (porous gamma-alumina) onto Ti6Al4V. Additive manufacturing of metal structures has attracted strongly increasing interest over the recent years41. The almost complete freedom in geometric design allows optimization of process equipment with regard to mixing and heat transfer properties42. Metal devices additively fabricated by powder bed fusion processes usually exhibit rough surfaces supporting the EPD coating procedure. Additional etching or roughening steps are usually not required prior to the coating step. As we apply SEBM in the here-presented work, this technique should be shortly presented. SEBM is a powder bed-based additive manufacturing process for metals, which uses an electron beam as energy source. To enhance the process stability, SEBM is typically carried out under a defined He atmosphere with a pressure of 2·10-1 Pa. The movement and focusing of the electron beam is realized by electromagnetic lenses, which enable high beam deflection speeds of up to 8000 m s-1 and a high usable power within the process of 3500 W17, 39, 43, 44, 45. Prior to the SEBM fabrication, the CAD (computer-aided design) model of the object to be built is sliced into layers with a defined thickness (typically 50 µm – 100 µm). These layers are fabricated one by one following the four step cycle shown in Figure 117.


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step 1

e-beam

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step 4

step 2

step 3

Figure 1: Schematic representation of the four process steps that lead to the generation of one layer during additive manufacturing via SEBM; here the “VerTec” logo is manufactured. In step 1, a layer of metal powder with defined thickness (typically 50 µm) is spread over the building area by a rake system. This powder layer is then heated with a defocused electron beam in step 2. This preheating step leads to a slight sintering of the powder within the top powder layers. This sintering improves electrical conductivity and mechanical stability of the powder bed preventing instabilities through electrostatic charging of the powder particles induced by the electron beam. The stabilized powder is selectively melted afterwards by a focused electron beam in step 3. The regions to be melted are specified through the sliced CAD model. The cycle ends in step 4 by lowering the building area by a defined layer thickness. These distinct steps are repeated until the whole CAD model is generated. The fabricated parts are embedded in a sinter


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cake as a result of the preheating steps. The partly sintered powder is removed via blasting with the same powder used for manufacturing. This ensures a recyclability of the powder and prevents changes of the manufactured parts at the surface. This contribution deals with the deposition of boehmite onto SEBM-fabricated POCS by EPD. Various mechanisms have been proposed to describe the EPD deposition mechanism. The best known model was suggested by Sarkar and Nicholson46. Due to the existence of a solid-liquid interface between particle and surrounding liquid an electric double layer is formed. The first layer contains the ions adsorbed onto the particle. In case of amphoteric alumina in acid media, protons form this layer. The second layer comprises a collective of free ions that are loosely attached to the particle. If the particle moves due to sedimentation or an electric field between two electrodes (inter-electrode electric field), one part of the double layer wanders parallel to the particle whereas the other part rests in its former position. The slipping plane separating both layers is defined as the zeta potential. Further EPD influencing parameters comprise the particle size and suspension properties (conductivity, viscosity and stability)23. The following steps characterize the EPD process (see Figure 2):


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i)

ii) b)

iii)

Figure 2: Schematic illustration of the main mechanisms in the EPD process according to Sakar and Nicolson46: i) Charged particles are surrounded by the diffusive double layer. Applying an electric field between two electrodes, the particles are attracted to the counter electrode. Thus the double layer undergoes a distortion becoming thinner ahead and wider behind; ii) While the particles wander through the suspension, ions of the same charge will also move to the counter electrode and neutralize counter ions in the double layer. Hence, the thinning of the latter is promoted; iii) Due to thinning of double layer, the attractive van-der-Walls forces outweigh the repulsive forces at the counter electrode leading to coagulation of particles.


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Concerning the solvent choice for the EPD process, the relative dielectric constant and therefore ionic strength should be in a moderate range to establish an appropriate size of double layer and thus electrophoretic mobility47.Stability and high deposition rates of alumina in ethanol have been achieved at a pH value of 2.2, where a high zeta potential was measured48. Under acidic conditions, protons adsorb on the AlOH-surface leading to a positively charged surface. According to Chen, the absolute zeta potential is higher using acid than bases. Aluminium isopropoxide was often added as a binder to control the suspension conductivity and enhance the adhesion of the deposited aluminium31 or alumina particles49, 50. EPD offers the possibility of monitoring electric current, voltage and therefore resistance. Tassel et al.51 showed that the first dense uniform alumina layer exhibits a depleted ion concentration and thus a high force on the particles enhancing density and coagulation strength. The effective resistance measured between both electrodes depends on the compound contributing the highest resistance. Hence, if layer resistance is the dominating part, normalized deposition can be measured and controlled by monitoring the effective resistance. EPD can be performed via direct, pulsed or alternating current (AC) depositions. Usually AC deposition is used to reduce nucleation of gas bubbles produced by water electrolysis. According to Table 1 there are various ways to apply EPD on open cellular structures. Table 1 a) shows the structure without any counter electrodes. Applying conventional EPD leads to a strong radial gradient in layer thickness due to the change of distance to the external counter electrode and therefore interelectrode electric field within the structure (see Table 1 b). To overcome this gradient, a multitude of cylindrical electrodes could be inserted from one side as shown in Table 1 c). Compared to the reticulate foams with a not ordered openings and cages, in the case of the POCS – due to their regular construction – such systematic implementation of an electrode array will be


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possible. However, the insertion procedure of the cylindrical electrode array into the cells of the POCS is complicated and time-consuming as long as the size is below a certain unit cell size. This contribution reports on the development of a new coating concept that combines the advantages and the design-freedom offered by both, EPD and additive manufacturing. In our new concept, the counter electrode for the EPD process is at least one further, geometrically optimized structure that is simultaneously built during the additive manufacturing progress as shown in Table 1 d). Both electrode structures are designed as interpenetrating periodic open cell structures (interPOCS) which allows to keep the distance between the electrodes almost constant over the entire structural element. In addition, this approach allows even to move the two interpenetrating elements relative to each other from the outside adjusting distance even during the coating process itself.


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Table 1: Comparison of different electrode arrangements. Isometric Unit cell

Side view

---

c) Inserted electrodes

b) Ext. electrode

a) Single structure

view

d) interPOCS

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2

EXPERIMENTAL

2.1 SEBM structure manufacturing via selective electron beam melting CAD files were designed and evaluated with respect to geometric properties using Autodesk Inventor 2015. For the fabrication of the cylinders and cellular structures an Arcam S12 (Arcam AB, Sweden) SEBM machine was used. The powder material of choice was spherical Ti6Al4V powder with a mean value in particle size of d50 = 69 µm. As building platform a steel start plate (170 x 170 mm²) was used. The process temperature was 660 °C. The structures were built with hatching in snake mode with a line offset of 0.1 mm. The deflection velocity of the electron beam was set to 1600 mm s-1 with a beam current of 4.8 mA (power: 288 W). This set of parameters was found to produce homogeneous struts over the whole structure with a thickness in accordance to the designed thickness in the CAD model (from 0.6 mm up to 1.2 mm). To compensate the beam diameter of about 0.4 mm, a scaling factor off 1.1 for the struts in the CAD model was applied. After production the structures were freed from the sinter cake via blasting with Ti6Al4V powder. 2.2 Chemicals All suspensions contained commercial boehmite powder (obtained by Sasol Germany GmbH) in ethanol (Staub & Co.-Silbermann GmbH). Nitric acid (Carl Roth GmbH & Co. KG) and Alisopropoxide (Molekula GmbH) were added in order to optimize suspension stability and zeta potential. Boehmite particles had the particle size d50 of 25 µm and the surface area of 180 m2g-1 after the calcination procedure.


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2.3 Preparation of suspensions For the coating process, suspensions were prepared by dissolving Al-isopropoxide in ethanol and stirring for 15 min. After that, the respective amount of boehmite (5 wt-%) was added and stirred for another 15 min. Finally, the desired amount of nitric acid was added and the suspension was then stirred again for 10 min. For conductivity measurements the digital conductivity meter GMH 3430 (GHM Messtechnik GmbH) was applied. Zeta-potential was determined using a Nano Zetasizer ZS from Malvern Instruments at 25 °C. 2.4 EPD setup For EPD of SEBM cylinders, the distance between both cylinders was kept at 1 mm by placing a teflon plate between the cylinders on both ends. In case of interPOCS according to Table 1 d), the distance was kept maximal by placing rubber band in each x and y direction into the upper grid of the structures as shown in Figure 3. The structures and cylinders were contacted by clamps to the respective pole (Figure 3). Electric current was recorded using a Voltcraft VC850 multimeter. The electrode arrangement according to Table 1 c) using inserted counter electrodes results in a set-up as shown in Figure 3 (right).


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+ -

R

+ -

R

e)

a)

b)

c)

+

d)

R a) b) c) d) e)

-

Structure Inserted counter electrodes Non-conductive brackets Non-conductive plates Steel wool connecting all inserted counter electrodes

Figure 3: Contacting method of both electrodes for simple cylinders (upper left), POCS with inserted cylindrical electrodes (right) and interPOCS (bottom left). 2.5 Layer characterization Coated layers where characterized optically using a confocal laser scanning microscope (Olympus LEXT OLS4000) after embedding and cutting the coated interPOCS under investigation. According to Figure 4, embedded interPOCS were cut perpendicularly to one of the four possible tetraeder axial directions for the evaluation of the layer thickness in five different layers for both structures, respectively.


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A-A A

A

Al2O3

A ̶ A

TiAl6V4

sLayer

Figure 4: Schematic description of layer thickness characterization perpendicular to axial direction of one strut group (upper left: interPOCS with one section layer A-A; upper right: isometric view of one tetraeder; lower left: layer A-A of whole interPOCS; lower right: layer A-A of one strut). 3

RESULTS AND DISCUSSION

3.1 Suspension properties and zeta potential Suspension stability was determined over a range of nitric acid concentrations at constant boehmite mass fraction with and without Al-isopropoxide (Al(-OiPr)3) as shown in Figure 5. It was found that the absence of Al(-OiPr)3 in the system leads to particle sedimentation at acid concentrations above 15 mmol l-1. Hence, the application of Al(-OiPr)3 lead to additional repulsive forces between the particles avoiding sedimentation. Obviously, addition of Al(-OiPr)3 is crucial to maintain suspension stability at higher acid concentrations.


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Figure 5: Sedimentation test; cBoehmite,t = 0 min = 5 wt-%; cHNO3 varied for suspensions with and without Al(-OiPr)3. To achieve the maximal electrophoretic mobility of the alumina particles, the zeta potential should be adjusted as high as possible at acceptable suspension parameters. Figure 6 shows the zeta potential of boehmite in ethanol as a function of the amount of nitric acid added. In addition, the suspension conductivity is depicted on the right axis. Due to the detection limits of the zeta potential measurement, the boehmite concentration was limited to 0.1 wt-% while the standard concentration in the conductivity and also during coating experiments was 5 wt-%. In order to make conductivity and zeta potential measurements comparable, the acid loading (mHNO3/mboehmite) was adjusted accordingly. It was found that the zeta potential increases from 1 mmol l-1 to 10 mmol l-1 of nitric acid concentration. This is attributed to protons adsorbed onto the particle surface. The conductivity increases slightly and this indicates that protons added to the suspension are primarily adsorbed


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on the particle surface. In the range above 10 mmol l-1 nitric acid concentration, the zeta potential decreases. This is due to the increase of the bulk potential without further surface potential increase which is also indicated by a strong increase of conductivity. Zeta potential and conductivity measurements without Al(-OiPr)3 could not be performed at HNO3 concentrations higher than 15 mmol l-1 due to severe particle agglomeration and sedimentation.

Figure 6: Zeta potential and conductivity for various nitric acid concentrations/loadings (cboehmite,t = 0 min = 5 wt-%). According to Figure 3, three electrode-configurations were applied in this work. The following results (section 3.1 to 3.3) refer to the two-cylinder-configuration, whereas the other two


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configurations were applied in section 3.5. Deposition experiments with varying acid loadings are shown in Figure 7. The highest normalized depositions (mboehmite/mTiAl6V4&boehmite) are achieved for an acid range between 10 and 20 mmol l-1, which is in-line with the range of high zeta potential. However, post-calcination layer quality was found to be low for acid loadings up to 20 mmol l-1. High deposition rates obviously correlate with unordered particle placing leading to poor mechanical stability of the obtained coating. An acid concentration of 25 mmol l-1 including Al(-OiPr)3 was chosen as best compromise of coating rate and coating quality for our further investigations.

Figure 7: Optical and gravimetric evaluation of deposition quantity and quality after coating and calcination step (E-field = 20 V mm-1; tEPD = 10 min; cBoehmite,t = 0 min = 5 wt-%, cAP= 0.5 wt-%). 3.2 Electrical Resistance Electrical resistance was determined by the ratio between adjusted voltage and measured current and compared to normalized deposition after 2.5, 5.0, 7.5, 10 min for 10 – 30 V mm-1, respectively. Three regimes can be recognized from the normalized deposition/resistance profile


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shown in Figure 8. During the first 2.5 min, the normalized deposition/resistance slope is highest, following a linear trend between 2.5 and 7.5 min to finally reach a plateau in normalized deposition remains while resistance keeps increasing (7.5 min to 20 min). This behavior can be explained by a mass transfer restriction for the conductive ions due to the increasing density of the coated layer.

Figure 8: Relation between electrical resistance and normalized deposition; (tEPD = 20 min, distance of electrodes: 1 mm).


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3.3 Alternating coating: Influence of AC-frequency and profile form While in the previous chapter only one of the two electrodes was coated with porous boehmite by using DC (direct current), the objective of the following set of experiments is a homogeneous coating of boehmite onto both electrode surfaces of the interPOCS structure via the EPD process. For this purpose, the coating process has been modified by adapting AC-frequency (alternating current) and waveform. In order to obtain a homogeneous and stable coating of both electrodes, different combinations of AC-frequency and waveform were tested in the following experiments. 3.3.1

Variation of AC-frequency

Experiments with varying AC-frequency at constant absolute voltage and rectangular waveform showed that the minimal AC-frequency is most appropriate for a maximum deposition on both cylinders as shown in Figure 9.


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Figure 9: Application of alternating current for the alternating coating of SEBM cylinders - Influence of AC-frequency on normalized deposition (E-field = 20 V mm-1; tEPD = 10 min). In one deposition interval, particles are deposited on the cathode, leading to a charge-driven compression procedure promoting a stable particle agglomeration at the cathode. Alternating the charge of the electrodes afterwards leads to a loss of loosely attached particles due to the same charge of deposited particles and electrode after charge alternation. With increasing ACfrequency of charge alternation the share of loosely bound particles increases. As expected, the electrode that is coated last in the process shows always a slightly higher coating thickness. For all subsequently described investigations, the charges of the electrodes were alternated only once and both electrodes were coated subsequently for 10 min.


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3.3.2

Particle distribution

In the following experiments, the alternate coating of both electrodes was investigated in more detail with respect to the deposited masses as a function of coating time. For this purpose, a total coating time tcoating of 20 min was applied and the polarity of the two electrodes was switched after 10 min. Figure 10 shows the normalized deposition xi on both cylinders after the switching point at tcoating = 10 min for the subsequent coating period tcoating = 10 to 20 min. It can be seen that the deposited mass decreases sharply on the first cylinder especially in the first seconds after polarization switch while deposition on the second cylinder grows. Both deposited masses reach a plateau 5 – 8 min after the polarization switch. This plateau can be shifted to a higher level of deposition by increasing the inter-electrode electric field.


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Figure 10: Normalized deposition on both cylinders in secondary coating period (10 min to 20 min experimental time) for 10, 20 and 30 V mm-1.

3.3.3

Influence of waveform profile for counter electrode coating

In a next set of experiments, we aimed at a higher efficiency of the deposition process by testing electric field gradients instead of a sudden change during the switch of the inter-electrode electric field. This should maximize the mass of deposition on both cylinders reached per deposition cycle. Figure 11 shows the applied variation of field gradients in the switching step.


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Figure 11: Tested variation of the inter-electrode electric field gradients at polarity switch during the coating process of both electrodes of an interPOCS structure.


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Figure 12: Effect of inter-electrode electric field gradients at polarity switch during the coating process of both electrodes of an interPOCS structure (electric field profiles according to Figure 11). According to Figure 12, a more moderate switch of the inter-electrode electric field reduces the abrupt repulsive forces at the counter electrode on the freshly deposited particles. As a consequence, the modification of the inter-electrode electric field leads to a higher mass of deposition on both electrodes. The normalized deposition shifts from around 7.5 wt-% under abrupt switching conditions to more than 10 wt-% at milder gradients of 2.5 V min-1 and 7.5 V min-1. From Figure 12 it can also be seen that convergence of deposited masses is reached earlier for steeper switching ramps.


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3.4 Design of interpenetrating POCS The design of interpenetrating POCS according to Table 1 d) requires identical outer diameters for both structures in order to fit well into a reactor pipe. In this contribution, we applied structures based on the diamond lattice. As shown in Figure 13 a), the unit cell can be characterized by the strut diameter dstrut and the cell size aunit

cell.

Considering two

interpenetrating diamond POCS, the second structure is shifted by half of the unit cell size from the first structure in z-direction. Each strut of the one structure is surrounded by six struts of the other structure. Considering a repetitive unit geometry, each strut of the one electrode structure faces one sixth of the second structure`s strut as shown in Figure 13 b) and c). For aunit cell = 5 mm and dStrut = 0.9 mm, sgap (Figure 13 d)) is in the range of sgap,min = 0.87 mm to sgap,max = 1.14 mm according to our Autodesk Inventor calculation. This corresponds well to sgap for the cylinder experiments shown above.


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Figure 13: Structural features of the applied interPOCS design: a) Unit cell; b) Single strut surrounded by six counter electrode struts; c) Counter electrode surfaces facing single strut; d) Repetitive unit respresenting the gap distance between both electrodes. 3.5 Layer deposition and characterization According to the methodology described above, the layer thickness was determined for several contacting methods and electrode arrangements. In the simplest case a POCS is placed inside a cylindrical tubular electrode. Figure 14 a) shows the thickness of the coating around the respective struts over the radius of the POCS. It can be clearly seen that the layer thickness


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decreases approaching the center of the structure. This can be attributed to the electrical field which is lowest in the center of the structure. This problem can be circumvent by inserting a multitude of cylindrical electrodes into the void spaces of the POCS (see Figure 3).As shown in Figure 14 b), this makes the coating result much more homogeneous as the penetrating electrodes result in a more homogeneous inter-electrode electric field. However, the insertion of many counter electrodes is tedious and lead to a more complex coating system. In contrast, the interPOCS according to Table 1 d) is manufactured in one additive manufacturing step, with the complexity of the part having no significant influence on building time. In the here applied interPOCS structure all struts, i.e. electrodes, have almost the same distance to each other as shown in 3.4. Figure 14 c) shows the distribution of the layer thickness for both structures in the interPOCS. Regarding structure A, the same homogeneous coating results were achieved, while structure B showed some outliers with higher layer depositions at 2 mm and 4 mm. This can be attributed to some additional metal agglomerations, caused in the additive manufacturing process. Hence, at this strut position the distance to the next counterelectrode could have below-average leading to a higher inter-electrode electric field and thus particle deposition. Looking closer to the details, it is also found that structure B showed an overall higher layer thickness, due to the fact that deposition on structure B took place last. Compared to conventional dip-coating our new interPOCS EPD process has a number of advantages: a) For realizing a comparable coating thickness by dip coating, the immersing procedure has to be coupled with a subsequent drying step and repeated many times28. b) The given pore sizes of the metal structures applied in this work would lead to pore blocking in case


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of dip coating. c) Dip-coated structures would exhibit a serious thickness gradient in the direction of dipping.


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a)

b)

c)


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Figure 14: Layer thickness across radial direction: a) simple POCS with external electrode (according to Table 1 b), b) simple POCS with inserted electrodes (according to Table 1 c), c) interPOCS consisting of structure A and B (according to Table 1 d).

4

CONCLUSIONS AND OUTLOOK The combination of additive manufacturing and EPD coating technologies offers new

possibilities for the homogeneous coating of complex metallic structures with porous aluminium oxide layers. For the first time, we have reported here the successful preparation of a new class of support materials, namely a system of two, fully interpenetrated periodic open cellular structures, the interPOCS. In addition, a new protocol for the controlled and homogeneous coating of these interpenetrating Periodic Open Cell Structures (interPOCS) with layers of boehmite has been presented. By applying voltage switch protocols with mild gradients, layer deposition on both electrodes could be realized. We propose to apply the so-coated interPOCS as novel catalyst supports in heterogeneous catalysis. In the near future, we intend to complete the catalytic activation of these oxide layers by a followed-up impregnation step in order to validate the coated interPOCS’ feasibility for catalytic applications. In order to verify the contribution of the strong heat conductivity among the continuous solid’s matrix, strong exothermic or endothermic reactions, such as hydrogenation or dehydrogenation are favoured candidates.


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AUTHOR INFORMATION Corresponding Authors Wilhelm Schwieger and Peter Wasserscheid Corresponding Author’s E-mail’s: [email protected], [email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors acknowledge financial support by the Bavarian state (application center VerTec in Fürth) and by the Erlangen Excellence Cluster “Engineering of Advanced Materials”. Notes Dedicated to Professor Tapio Salmi, Åbo Akademi University, Turku, Finland on the occasion of his 60th birthday. ACKNOWLEDGMENT The authors acknowledge financial support by the Bavarian state (application center VerTec in Fürth) and by the Erlangen Excellence Cluster “Engineering of Advanced Materials”. Also, the authors gratefully acknowledge the possibility to use services and facilities of the Energie Campus Nürnberg and financial support through the “Aufbruch Bayern” initiative of the state of Bavaria.


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TABLE OF CONTENTS GRAPHIC


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Electrophoretic deposition of boehmite on additively- manufactured

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