1. Carbon-Bonded Filter Materials and Filter Structures with Active and Reactive Functional Pores for Steel Melt Filtration

In a first approach, the production of Al₂O₃-C filters composed of alumina, fine natural graphite, carbon black powder as well as modified coal tar pitch Carbores®P was explored. In comparison to other pitches, Carbores®P is characterized by a lower B(a)P value (<500 ppm), a softening point of 200 °C and a RCC of approx. 85 wt%. The carbon bonds are obtained by the graphitization of an intermediate, liquid pitch-mesophase, which is formed during the pyrolysis below 1000 °C [3]. To determine the interactions between the individual raw materials, the impact of three compositions (AC1, AC2 and AC3) presented in Table 1.1 on the filter producibility were analyzed.

The filter manufacturing was performed in multi-stage processes based on the replica technique. For this purpose, the raw materials, deionized water as well as the additives polypropylene glycol (wetting agent), ligninsulfonate (temporary binder), polycarboxylate ether (dispersing agent) and polyalkylene glycolether (antifoam agent) were admixed in a high shear Hobart-type mixer (ToniTechnik, Germany) to obtain the impregnation, centrifugation and spraying slips. These slips were applied on reticulated polyurethane (PU) foams with a 10 pores per inch (ppi) macrostructure, according to the process routes shown in Fig. 1.1. Afterwards, the Al₂O₃-C filters were placed in a steel retort embedded in pet coke (MÜCO Mücher & Enstipp GmbH, Germany) and fired at 800 °C for 3 h. The heating regime was carried out with 1 K min⁻¹ and 30 min dwell times each 100 K.

An illustration of a manufacturing route for carbon-bonded alumina filters. The polyurethane foams are split into impregnation by rolling and impregnation by centrifugation. They are followed by coating by slip spraying and coating by centrifugation, respectively.

The comparison of the impact of the slip compositions on the filter producibility revealed a considerable effect of the amount of Carbores®P on the slip properties. It was observed that an increase of the Carbores®P content related to the total solid content led to a considerable decrease in the water amount, which is required for the preparation of coating slips with the same viscosity. This was attributed to the fact that Carbores®P possesses a smaller specific surface area compared to alumina, graphite and carbon black and thus a higher packing density was achieved. However, pure Carbores®P exhibited a shear-thickening behavior, wherefore a minimum amount of graphite and carbon black must be added to obtain stable and shear-thinning slips for the coating procedure [4].

Figure 1.2 presents the filter structure as well as the cross-section of a hollow filter strut after the pyrolysis. Due to the decomposition of the PU foam during the heat treatment, a triconcave cavity with sharp edges was formed, which passed through the entire filter structure. The wall thickness of the filter struts ranged between 300 and 400 µm when the “rolling and spraying” preparation was used. Although the appearance of the AC1, AC2 and AC3 filter structures were similar to each other, the filter composition exerted a significant impact on the filter properties.

2 illustrations. A. An illustration of a A C 3 filter structure. It looks similar to the honeycomb structure. B. An illustration of a raised cross section of a filter after pyrolysis. The raised part looks like a hill with a flat top.

As presented in Table 1.2, the RCC increased with decreasing alumina content due to the higher amount of carbon sources. Furthermore, the increase of the Carbores®P content related to the total solid content increased the volumetric shrinkage, whereas the open porosity decreased. Both phenomena were attributed to the high temperature behavior of Carbores®P. On the one hand, the higher amount of Carbores®P caused a higher release of organics bond within the Carbores®P structure and the emergence of a higher amount of liquid pitch-mesophase, which both contributed to a higher volumetric shrinkage and a reduced occurrence of defects. On the other hand, the higher amount of the liquid pitch-mesophase effected a higher infiltration rate of pores and thus decreased the porosity. Also a considerable increase of the CCS with increasing Carbores®P content was determined. Besides the decrease of defects and porosity, a higher cross-linking degree of the particles was obtained, which improved the structure bonding and the mechanical filter properties [4]. These effects could be further amplified by increasing the Carbores®P content up to 30 wt%. However, a higher Carbores®P content would impair the coating slip properties and raise the B(a)P value, which is not expedient for the filter production. Considering the improved mechanical properties as well as the higher RCC, which is essential for the carbothermal reaction, AC3 turned out to be the most suitable Al₂O₃-C filter composition for the steel melt filtration.

Apart from the filter composition, a strong dependence of the filter structure and the mechanical filter properties on the coating procedure was found. While the “centrifugation + centrifugation” method led to a homogenous Al₂O₃-C structure over the entire filter volume, the “rolling + spraying” procedure exhibited a decreasing wall thickness with increasing distance to the filter surface. This was traced back to the fact that the inner filter struts were obscured by the outer filter struts during spraying and hence impeded the access of the spraying slip to the interior filter volume. In comparison, the immersion of the impregnated foam into the centrifugation slip ensured an even distribution of the slip on the filter surface. Although the median wall thickness was lower in case of “centrifugation + centrifugation” due to the higher water amount used, the more homogeneous application of the coating resulted in a higher CCS (cp. Table 1.2). Especially for larger filter sizes, this effect was intensified and caused a CCS increase of about 2.5 times [6].

Since Carbores®P still exceeds the maximum B(a)P value regulated by European Parliament (<50 ppm), an environment-friendly binder system based on lactose and condensed tannin was examined for the Al₂O₃-C filter production. Both substances are used in the pharmaceutical and food industry demanding a high level of environmental compatibility. Due to the reactive nature of condensed tannins, interlinked carbon structures similar to phenolic resins are formed during the pyrolysis, which should ensure the filter stability [7]. In order to evaluate the usability of the environment-friendly binder system, the AC3 filter composition was adjusted, whereby Carbores®P was completely replaced by lactose/tannin (L/T) with a mixing ratio of 5:1. Besides the raw materials of AC3, Hexamethylenetetramine (hardening agent), aluminum powder (antioxidant), micro silica (antifoam agent), rutile as well as P-doped n-silicon or silicon carbide (stabilizer of carbon matrix) were added. The slip compositions (AC4 and AC5) are listed in Table 1.1.

The filter fabrication was conducted by the “rolling + spraying” method, as described in Sect. “Carbon-Bonded Alumina Filters Based on Carbores®P”. Afterwards, the filters were hardened in a drying chamber up to 180 °C to initialize the polymerization of tannin monomers and pyrolyzed at 1000 °C under a reducing atmosphere. Both heat treatments corresponded to those of Himcinschi et al. [5].

Figure 1.3 shows the filter structure of AC4. On top of the filter surface, bright spots were detected, which most likely originate from the interactions of SiO₂ and carbon resulting in metallic silicon formation. The filters exhibited a rough surface with a large number of cracks, which were formed during the heat treatments due to the additional admixture of fine solid additives. Consequently, a lower CCS was obtained compared to the AC3 filters, as presented in Table 1.3.

2 illustrations. A. An illustration of a honeycomb like the A C 4 structure. It is at a scale of 1000 micrometers. B. An illustration of a cross-section of a filter with a rough surface on top.

Another fact for the decrease of the CCS consisted in the increase of the open porosity. In comparison to Carbores®P, which converts into a liquid pitch-mesophase during the pyrolysis, L/T creates a rigid carbon matrix similar to phenolic resins. Hence, the pores remained unaltered within the Al₂O₃-C structure and affected the mechanical filter properties. Furthermore, a significant decrease in the RCC content (19.3 wt%) was observed. While Carbores®P has a RCC of 85 wt%, the L/T mixture achieved only 41 wt%. Based on the lower RCC, the carbon matrix evinced a weaker carbon bonding and hence a less stable carbon matrix. However, a positive impact on the filter properties provoked the addition of n-Si and SiC. Besides an improvement of the CSS, the RCC increased up to 20.6 wt%, whereby SiC exerted a slightly higher effect. This observation was attributed to the ability of both substances to emit electrons into the carbon matrix at elevated temperatures. In analogy to the mechanisms described by Yamaguchi et al. [8], the graphitization of the carbon matrix was enhanced resulting in a stronger carbon bonding. In order to evaluate the usability of the AC4 and AC5 filters for steel melt filtration, impingement tests were conducted. The AC5 filters with n-Si or SiC survived the thermal shock, whereas the AC4 filters were destroyed.

Consequently, it can be concluded that the AC5 filter composition with SiC is most suitable for steel melt filtration. Although Al₂O₃-C filters with Carbores®P obtained better filter properties, the new binder system based on lactose and tannin offers a great potential for manufacturing environment-friendly Al₂O₃-C filters.

Similar to the established Al₂O₃-C filter production, spray coating was the first technology applied to deposit functional coatings. For this purpose, coating slips were admixed and homogenized in a barrel with alumina grinding balls (d = 1.5 mm). Afterwards, the Al₂O₃-C filters were sprayed with an airgun of the type SATAjet B (Sata, Germany) with a nozzle diameter of 1.5 mm and a spraying distance of approx. 15 cm. The coated filters were placed in an alumina retort embedded in pet coke and sintered at 1400 °C for 3 h. An overview of the coating slip composition is presented in Table 1.4.

In order to enhance the filtration efficiency, pure oxidic coatings composed of alumina (“A”, CL 370, Almatis Germany, d₅₀ = 2.5 µm), spinel (“S-1”, AR 78, Almatis, Germany, d₅₀ = 2.0 µm) and mullite (“M”, SYMULOX M 72 K0C, Nabaltec, Germany, d₅₀ = 5.0 µm) were applied on thermally pretreated and untreated AC3 filters, respectively. After the pyrolysis at 1400 °C, macrocracks emerged within the filter structure of the untreated filter samples. This was attributed to the partial absorption of Carbores®P of the Al₂O₃-C substrate by the oxidic coating due to capillary forces and hence led to a deterioration of the binder matrix. In contrast, Fig. 1.4 shows the thermally pretreated filter samples, which evinced a stable oxidic coating with a thickness of 50 to 80 µm after the pyrolysis. The intermediate layer without solid content (marked with a dashed arrow in Fig. 1.4a) indicated that the coating was not chemically bonded to the Al₂O₃-C substrate, but adhered due to mechanical bonding within the coating. This observation can be explained by the sintering behavior of both media. The Al₂O₃-C substrate underwent a thermal expansion until 850 °C due to the dehydrogenation process resulting in a slight detachment of the coating and the formation at the gap. Afterwards, the oxidic coatings shrank on the filter surface due to further heating and cooling. As a result, the particles interlocked mechanically with each other and compression stresses were generated within the coatings, which ensured the coating stability. Since a higher shrinkage leads to an increase in the mechanical bond as well as to a decrease of the porosity, the alumina-coated Al₂O₃-C filters exhibited the highest CCS of 0.63 MPa, followed by spinel- (0.54 MPa) and mullite-coated (0.47 MPa) Al₂O₃-C filters. Impingement tests revealed that the coated filters withstood the thermal shock of molten steel and led to a deposition of inclusions on the filter surface, which underlines their suitability for steel melt filtration [9, 17].

3 illustrations. A. An illustration of A L 2 O 3 carbon bonded alumina. It is coated with alumina. The width A is labeled. B. An illustration of A L 2 O 3 carbon bonded alumina coated with spinel labeled S 1. C. An illustration of A L 2 O 3 carbon bonded alumina coated with mullite labeled M.

However, an excessive shrinkage is negative. On the one hand, too high compressive stresses initiated the formation of cracks and defects, which caused a complete detachment of the coating. On the other hand, the carbothermal reaction and the gas exchange were affected, as the reduction of the porosity contributed to a permeability decrease of the coatings. For this reason, carbon was added to the oxidic coatings due to its flexible structure as well as its influence on the coating properties and analyzed regarding the interactions with the steel melt.

In the first instance, the wettability behavior of molten steel on sintered alumina (“A-C”, Martoxid MR 70, Martinswerk, Germany, d₅₀ = 1.8 µm) coatings with 0, 4 and 30 wt% carbon (after the pyrolysis) was determined between 1500 and 1520 °C within 30 min. Since the steel melt exhibits a poorer wetting on carbon compared to alumina, an increase in the contact angle with increasing carbon content was observed. Additionally, the larger particle size of the carbon materials as well as the higher open porosity formed by the removal of volatiles effected an increase of the coating surface roughness, which resulted in a further increase of the contact angle up to 144°. For all samples, a decrease in the contact angle occurred with increasing wetting time. After 15 min, an almost constant value was detected, which was attributed to the formation of an in-situ formed layer on the coating surface due to the carbothermal reaction. Hence, it was concluded that the addition of carbon enhances the steel melt purification, as a higher contact angle encourages the deposition of solid, non-metallic inclusions due to the more favorable interfacial energy situation [10]. Furthermore, an improvement of the coating structure was achieved. Besides a slight decrease in the shrinkage, the admixture of carbon provoked a higher open porosity due to the release of volatiles during the sintering. For this purpose, carbonaceous coatings (6.25 wt% carbon) based on alumina (“A-C”, Martoxid MR 70, Martinswerk, Germany, d₅₀ = 1.8 µm) and calcium aluminates (“C–C”, Kerneos, France) were applied on Al₂O₃-C filters, pyrolized at 1400 °C under reducing atmosphere and brought into contact with molten steel. The results evinced distinct interactions between the coating materials and the steel melt, whereby a significant amount of inclusions was removed due to the “reactive” and “active” filtration mechanisms. Especially calcium aluminates turned out to be most effective due to the reactive nature of CaO under steelmaking conditions and the associated gas bubble formation [11, 12].

In another approach, the feasibility of an in-situ spinel synthesis during the filtration process was investigated. Since the spinel formation is accompanied by a volume expansion, the excessive shrinkage could be counteracted. Therefore, filter samples composed of alumina (Martoxid MR 70, Martinswerk, Germany, d₅₀ = 1.8 µm), magnesia (Refratechnik, Germany, d = 0–63 µm) and with or without carbon sources, respectively, were manufactured according to “S-2” and “S-3” (cp. Table 1.4) and thermally pretreated at 800 °C. Afterwards, the filters were immersed in molten steel at 1640 °C. Preliminary examinations of the sintering behavior revealed that the spinel formation started at a temperature of 925 °C and caused a volume expansion of 0.7% (S-2) and 2.4% (S-3). The spinel formation proceeded with increasing temperature and was almost completed at 1600 °C for S-3, whereby an overall shrinkage of 0.4% was obtained after cooling. In comparison, the presence of carbon inhibited the spinel formation and led to a slightly higher shrinkage of approx. 0.55% in the case of S-2. A contrary development was found after the immersion into the steel melt. Here, the alumina/magnesia mixture of S-2 was completely transferred into the spinel phase, with only a small amount of alumina and magnesia remaining unaltered. It was concluded that the carbothermal reaction lowers the reaction temperature and promotes the in-situ spinel formation under casting conditions [13]. Although the in-situ spinel formation was successfully conducted, the filter samples exhibited lower mechanical properties before the immersion attempt due to the absence of a sintering step. For this purpose, the Al₂O₃-C filter surface was coated with presynthesized spinel compounds MgAl₂O₄, FeAl₂O₄, MnAl₂O₄, Fe_0.5Mg_0.5Al₂O₄ and Fe_0.5Mn_0.5Al₂O₄ (“S-C”, AGH University of Science and Technology, Poland) in combination with carbon and sintered at 1400 °C under reducing atmosphere. The qualitative analysis of the phase composition before and after sintering indicated that a partial decomposition of the FeAl₂O₄, MnAl₂O₄, Fe_0.5Mg_0.5Al₂O₄, and Fe_0.5Mn_0.5Al₂O₄ took place during the pyrolysis most likely due to the low oxygen pressure, which encouraged the decomposition of the spinel compounds at elevated temperatures. MgAl₂O₄ was not affected by the decomposition due to higher thermal resistance. The carbonaceous spinel coatings withstood the thermal shock by the immersion into the steel melt and entailed a considerable removal of non-metallic inclusions, whereby metallic manganese formed during the decomposition was assumed to exert a catalytic effect similar to iron [14].

While the oxidic coatings presented were applied to enhance the inclusion deposition rate on the filter surface, the possibility of improving the filter reactivity was focused on within the next step. Therefore, nano-scaled materials were adopted, since their high specific surface could promote the “reactive” filtration behavior. The used raw materials were multi-walled carbon nanotube (“MWCNT”, TNM8, Chengdu Organic Chemicals, China) and graphene oxide (“GO”, VSCHT Praha, Czech Republic), which were prepared in advance according to a modified Tour’s method [18]. For the preparation of the nano materials dispersions, xanthan stock solutions were dissolved in deionized water under high-shear mixing for 15 min to ensure the total hydration of the biopolymer [15, 19]. Afterwards, an appropriate mass of MWCNT was added and the resulting mixtures were ultrasonicated with an ultrasonic probe (Sonopuls HD 2200, 20 kHz, 200 W) for 3 min at 50% amplitude to force unbundling of the nanotubes and promote the interaction with the surfactant molecules. Finally, the suspensions were thoroughly stirred for 30 min to achieve proper homogeneity [20]. The preparation of the GO-based dispersion was performed similarly [16, 21]. For the binding of carbon nanotubes and graphene oxide to the ceramic foams during the heat treatment, a separate Carbores®P dispersion was prepared in deionized water (50 wt%) using horizontal ball milling for 1 h. Furthermore, 0.3 wt% ligninsulfonate (T11B, Otto-Dille, Germany) and 0.1 wt% polyalkylene glycolether (Contraspum K 1012, Zschimmer & Schwarz, Germany) were used to promote the wettability of the pitch powder with the water and to prevent foaming during the preparation. In order to improve the stability of the system and achieve a proper rheological behavior, Xanthan gum (0.3 wt%) was added. After the different dispersions were mixed in various weight ratios and stirred for 5 min, the resulting coating slips (“Nanos”, cp. Table 1.2) were applied on Al₂O₃-C filters. Subsequently, the filters were pyrolized at 800 °C under a reducing atmosphere for 3 h. The heating was realized with 0.8 K min⁻¹, whereby a heating rate of 0.3 K min⁻¹ between 100 and 250 °C was conducted in the case of a GO-coated filter to prevent sudden exfoliation and allow a slow transformation of GO into thermally reduced graphene.

The examination of the mechanical filter properties revealed that an increasing proportion of nano materials related to the Carbores®P content entailed a progressively worse CCS of the filter samples. On the one hand, this observation was attributed to the oxidation of the coating, which cannot be completely avoided during the pyrolysis. Thereby, the oxidation was promoted by the higher surface area, which increased with increasing nano material proportion. On the other hand, the decreasing amount of Carbores®P impaired the crack and porosity cure, achieved by the infiltration of the liquid pitch mesophase and the subsequent graphitization. Nonetheless, the mechanical properties were sufficient to withstand the thermal shock of molten steel at 1650 °C.

Figure 1.5 presents the microstructure of the Al₂O₃-C filters after the pyrolysis functionalized with MWCNT, GO and MWCNT-GO. While the presence of cracks was observed for the GO filters, the GO-MWCNT filters exhibited an apparently smooth surface covered by reduced graphene oxide. In the case of MWCNT, clusters of Carbores®P residuals and MWCNT agglomerates with dimensions of a few microns were found resulting in an irregular filter surface. Since no such large agglomerates were detected in the dispersions before spraying, they likely developed during the pyrolysis [16]. Although a rougher surface should contribute to an increase in the wetting angle between molten steel and filter material, the GO-MWCNT coating exhibited the highest contact angle (>150°), followed by GO and MWCNT [21]. It was presumed that the high specific surface of the nano materials provoked a more intense carbothermal reaction, whereby the roughness increased due to the reaction of the nano materials and oxygen dissolved molten steel. Hence, the application of carbon-based nano materials constitutes an excellent opportunity to improve the filter reactivity and purification degree of molten steel.

6 illustrations of 6 microstructures labeled from a to f. A and b are 2 microstructures of A l 2 O 3 carbon bonded with a graphene oxide coating over them. C and D are also microstructures, but they are coated with multi walled carbon tubes. E and f illustrate the combined coating of graphene oxide and carbon nanotubes.

In order to avoid a second pyrolysis step and the associated impairments of the filter and coating structure mentioned in the previous section, the application of functional coatings by flame spraying was investigated. Compared to slip spraying, the flame spraying technique is based on the application of melted ceramic particles on the filter surface, where the particles rapidly solidify, sinter and form a dense coating. As a result, the functional coating reaches immediately its final state after the application and further thermal treatments can be omitted.

For this purpose, sintered rods composed of 100% Al₂O₃ (“A-FS”) and Al₂O₃-ZrO₂-TiO₂ (AZT, “90-5-5”, “85-5-10” and “80-10-10”) were prepared to be utilized as feedstock. The raw materials were mixed with a thermoplastic binder, extruded with a heatable extruder of the type DSE 25 (Brabender, Germany), and sintered at 1500 °C. Furthermore, an Al₂O₃ flexicord feedstock was used, which was composed of an alumina powder material in a paste coextruded with a flexible organic skin.

The A-FS and AZT rods as well as the alumina flexicord were fed to the Master Jet® Gun (Saint-Gobain Coating Solutions, France) to coat the Al₂O₃-C filters or produce free-standing structures. Inside the flame spraying gun, oxygen and acetylene as fuel gas combusted to melt the feedstock. The combustion temperature (max. approx. 3160 °C) depends on the flow rate of the oxygen and acetylene gases and can be controlled by adjusting the gas pressure and the ball height at the flowmeter [22]. During the flame spraying process, compressed air was used to fluidize, transport and at the same time homogenize the melted drops. Thereby, several traverses were necessary until the overlaying splats formed an effective coating that nearly completely covered the substrate.

Before characterizing the flame-spray coated Al₂O₃-C filters, promising free-standing flame-sprayed structures composed of A-FS and AZT were investigated. Flame-sprayed alumina was characterized by a typical surface with splats of varying morphology such as almost circular discs, pancakes, and splashes of so-called flower structures as well as microcracks. At polished cross-sections, the typical lamellar structure composed of flat Al₂O₃ grains with the highest extension perpendicular to the spray direction, pores and additional phases inside the grains became visible. In addition, grain boundaries were observed in the case of the 90-5-5 and 85-5-10 samples. Based on the 85-5-10 samples, two different microcrack patterns were identified: (1) A primary pattern with major propagating cracks with widths between 150 and 300 nm, which was formed by the rapid cooling/solidification (cooling rate of approx. 10⁶ K s⁻¹) was partially interconnected with (2) a secondary pattern with microcracks in the range of 120 nm and less, which resulted from slow crack growth during further cooling of the droplets after their solidification. Thereby, the feedstock composition played an important role. Due to its ability to create multiple phases during the flame spraying process, the 85-5-10 showed a higher level of interconnection between the microcracks compared to A-FS. The A-FS samples exhibited a comparatively low open porosity of 7.1%. The porosity increased with the addition of titania and zirconia (9.7 and 9.3% in sample 90-5-5 and 85-5-10, respectively), most likely due to the incomplete melting of the zirconia particles. Controlled crack propagation experiments of bar-shaped samples yielded a distinct R-curve behavior for A-FS and 80–10-10 samples with initial values of K_I0 of approx. 0.5 MPa m^1/2, while plateau values reached about 1.8–2.5 MPa m^1/2 [23]. The rising portion between those characteristics spanned a range of approximately 100–400 μm of the crack extension. It is assumed that the raising crack resistance was related to the microstructure as a result of energy dissipating processes like crack deflection and crack branching on the one hand and crack wake interactions such as microcrack toughening and crack bridging on the other hand [24]. In order to evaluate the thermomechanical properties, the B3B method was conducted before and after thermal shock trails. It was found that A-FS discs evinced a strength of 83.46 MPa with a Weibull modulus m of 5.48 before and 42.27 MPa with a Weibull modulus m of 1.88 after thermal shock at 1000 °C to room temperature [25]. Since the B3B method is also suitable for flexural tests at elevated temperatures, an increasing ductile behavior was identified with increasing temperature. The determination of the residual bending strengths of band-shaped samples after thermal shock at rising temperature gradients revealed that all A-FS and AZT compositions presented a similar thermal shock behavior with an average strength plateau kept up to a specific temperature gradient, whereby a significant strength loss was registered after a critical temperature gradient was applied. Due to a microcrack toughening effect, the A-FS and AZT compositions demonstrated a slightly increasing strength up to quenching temperature difference in the range of 800 (A-FS) to 900 K (85-5-10). Hence, flame sprayed A-FS and AZT provide outstanding thermal shock resistance.

For this purpose, the application and impact of flame-sprayed A-FS coatings on commercial Al₂O₃-C filters were examined. After four traversing steps, an average coating thickness of 90 µm was obtained. As a very high cooling rate occurred during the solidification of the molten alumina droplets, the formation of a high portion of amorphous phases and metastable transition γ- and η-Al₂O₃ as well as a low portion of corundum was detected [22].

Figure 1.6 presents the filter structure after the coating procedure. It was observed that an irregular and patchy coating predominantly present at the top of the struts and filter parts facing the flame-spray gun arose, since the particles did not flow across the surface of the filter struts due to the rapid cooling. Thereby, the typical structure of overlaid molten droplets became visible. At some positions of the flame-spray coated surface, the subjacent Al₂O₃-C microstructure as well as microcracks crossing the droplets could be seen. The CT analysis revealed a thick, crack-free and dense coating in particular at the outside area of the filter. However, a continuous coating could only be detected in the outer regions of the filter, covering approx. one-third of the filter towards the center. The core of the filter was not well coated, since the outer filter struts obscured the inner part of the filter volume. Regarding the mechanical filter properties, a CCS reduction of approx. 30% was measured. This was traced back to an emerging gap of up to 25 µm between the filter and the coating. Due to the heat transfer from the molten droplets to the filter substrate, its temperature increased to max. 450 °C, expanded during flame spraying and shrank after finishing the spray process. As the molten particles immediately cooled to ambient/ substrate temperature after impacting onto the filter and hence did not shrink, a gap between filter and coating was formed. Nonetheless, the flame-spray coatings survived the immersion attempts in molten steel and resulted in an inclusions deposition due to an improved “active” filtration behavior.

3 illustrations. A. An illustration of a honeycomb like structure represents A F S coated A L 2 O 3 carbon. B. A S E M image of the structure with a very rough surface. C. A C T scan illustrates dispersed small structures.

In a further approach, the feasibility of producing filters completely composed of flame-sprayed alumina structures was investigated to encounter the disadvantages of sintered oxide and carbon-bonded filters. Therefore, Al₂O₃-C filters were flame-spray coated with A-FS and subsequently sintered at 800, 1100 and 1400 °C, respectively, to burn the carbon-bonded filter substrate. As long as there is no notable sintering effect, the remaining alumina flame-spray coating covering the decomposed carbon-free residual alumina filter was too thin and weak to act as a reliable load-bearing structure. This led to a negligible CCS of 0.01 and 0.02 MPa of the coated filters after sintering at 800 and 1100 °C, respectively, whereas sintering at 1400 °C led to a higher CCS of 0.16 MPa. Casting tests with molten steel will reveal whether this strength is high enough for the application of such new filters for molten metal filtration.

Refractory fibers represent a kind of 1D materials with many advantages such as light-weight, good thermal stability, low thermal conductivity, small specific heat and a high surface-to-volume ratio. A large number of advanced techniques have been developed to fabricate 1D nanostructures with well-controlled morphology and chemical composition. Unfortunately, most of these methods are multistep, require high energy or generate secondary products. Electrospinning provides a simple and versatile process capable of generating continuous fibers with diameters ranging from a few tens of nanometers to several micrometers. Based on this, functional fiber coatings produced by electrospinning constitute a promising opportunity to increase the filter reactivity and improve the steel melt purification.

In order to evaluate the feasibility of the coating fabrication by electrospinning, fiber mats composed of magnesium borate (“MB”, Mg₂B₂O₅) and calcium zirconate (“CZ”) were analyzed. For this purpose, the raw materials boron ethoxide (C₆H₁₅BO₃) and magnesium ethoxide (C₄H₁₀MgO₂) as well as zirconium basic carbonate (Zr(OH)₂CO₃ · ZrO₂) and calcium nitrate tetrahydrate (Ca(NO₃)₂ · 4H₂O) were used as precursors. Furthermore, polyvinyl-pyrrolidone powder, 2-Methoxyethanol, N,N-dimethylformamide, glacial acetic acid and methanol were utilized as solvents to generate proper fibrous structures. The raw materials were purchased from Sigma–Aldrich (USA), while the solvents were derived from Carl Roth GmbH (Germany). The production of the fibers was performed in a NE100 unit (Inovenso, Turkey). The prepared electrospinning solutions were loaded into a 10 ml syringe connected to a 0.8 mm diameter conductive brass needle and fed at 0.5 ml h⁻¹ with a syringe pump (NE-300, New Era Pump Systems, USA). Electrospinning was carried out at room conditions and at a voltage of 15 kV using a DC power supply. An aluminum plate covered with aluminum foil was used as the collector and the distance between nozzle and collector was 10–12 cm. After the process, the as-spun fiber mat was removed from the aluminum foil. The magnesium borate fibers were then pyrolized at 800 °C under a reducing atmosphere for 3 h or at 900 °C in air for 1 h, whereas the calcium zirconate fibers were heat-treated at 800 or 1000 °C in air for 1 h. At least, Al₂O₃-C filters were used instead of the aluminum plate to apply the fibrous coating and sintered at 800 °C under reducing atmosphere for 3 h [26, 35].

The SEM analysis of the MB fibers revealed that a matt of continuous fibers without any droplets or beads was created, whereby the fibers exhibited a main diameter of 500–700 µm. After the pyrolysis at 900 °C in air, the fibers consisted of small particles, which joined together to form a thin filament and are known as “necklace” structure. Thereby, a decrease of the fiber diameter to 300–500 µm took place due to the thermal release of aqueous and organic precursors. The fibers were primarily composed of Mg₂B₂O₅ with Mg₃B₂O₆ as a secondary phase. In comparison, the pyrolysis under a reducing atmosphere caused a similar shrinkage, but the occurrence of the “necklace” structure was avoided. However, the specific surface was 10-times higher and the main phase consisted of Mg₃B₂O₆ due to the evaporation of boron. The CZ fibers evinced a diameter between 400 and 800 µm in the green state, which was halved after the pyrolysis. Thereby, the formation of a “necklace” structure was not observed. Regarding the phase composition, a higher conversion of the precursors to CaZrO₃ was achieved with increasing pyrolysis temperature, reaching a purity above 98% at 1000 °C [26, 35].

As presented in Fig. 1.7, the fiber mats were successfully applied to the filter surface. However, the fibers were deposited only on the first conductive surface in reach, but not in the filter pore volume. Due to the shrinkage and lack of bonding between fibers and filter substrate, a detachment and loss of the fiber mat was inevitable after the pyrolysis. Hence, the application of functional coatings by electrospinning is possible, but not expedient for the steel melt filtration.

2 images. A. A photograph of square shaped A L 2 O 3 carbon bonded alumina. It is non porous in nature. B. A photograph of porous and square shaped A L 2 O 3 carbon bonded alumina after pyrolysis.

In a first approach, the feasibility of producing filter structures with full-strut cross-sections by in-situ gel casting was investigated. Therefore, sustainable alginates were admixed to ceramic slips and cast in an aqueous solutions enriched with divalent ions such as Ba²⁺ or Ca²⁺. Since alginates are composed of two of the monomers D-mannuronic and L-guluronic which exhibit a ribbon and buckle geometry, a polymerization of the monomers is initialized by the integration of divalent ions in the structure. As a result of the cross-linkage, a stiff gel is formed ensuring the dimensional stability of the ceramic component.

In order to fabricate Al₂O₃-C filter structures, carbonaceous alumina slips based on the AC3 slip composition (cp. Sect. “Carbon-Bonded Alumina Filters Based on Carbores®P”) with a total solid content of 50.0, 52.6 and 55.6 wt%, respectively, were admixed with 0.35, 0.50 or 0.65 wt% sodium alginate (C.E. Roeper GmbH, Germany). Furthermore, the additives polyvinyl alcohol (temporary binder, C.T.S, Italy), polyacrylic acid (dispersing agent, Dolapix PC21, Zschimmer & Schwarz, Germany) and polyalkylene glycol ether (antifoam agent, Contraspum K 1012, Zschimmer & Schwarz, Germany) were used. The resulting slips were continuously pumped through a tube with a diameter of 2 mm and a length of 1 m into an aqueous cross-linking solution enriched with 1 wt% Ba(NO₃)₂ or CaCl₂ using a peristaltic pump of the type Pumpdrive 5206 (Heidolph Instruments, Germany). The tube was fixed to a robot arm (Robot armMover 4, Commonplace Robotics, Germany), which was controlled by the software RobotExpress (Commonplace Robotics, Germany). Based on this setup, droplet beads and filter samples with a periodic grid structure were manufactured and dried at room temperature. Subsequently, the filter samples and the beads were pyrolized at 800 °C for 3 h under a reducing atmosphere. Besides the rheological behavior of the slips, the dimensional stability and mechanical properties of the samples were analyzed.

All slips exhibited a shear-thinning behavior without major thixotropic effects. The dynamic viscosity increased with increasing alginate and total solid content. However, the viscosity increase caused by the increase of alginate content can be counteracted by increasing the water content. Thereby, the addition of 10 wt% water related to the solid content countered an alginate increase of 0.15 wt% to obtain slips with the same rheological behavior. Furthermore, a strong dependency between the filter dimensions and the slip composition was observed. While low alginate and total solid contents led to an increased shrinkage and sagging of the filter struts, the shrinkage in the filter height plateaued at 30% for high contents. Hence, exceedance of the optimal amount would only result in an increase of the viscosity, but not in dimensional stability. Additionally, a radial strut shrinkage up to 13.6% occurred, which has to be taken into account for the modelling of the filter macrostructure. Since the determination of the mechanical properties for all pyrolized bead samples did not reveal any noticeable differences, an alginate content of 0.65 wt% and a total solid content of 55.6 wt% turned out to be the most suitable composition for the alginate-based robo gel casting. In the case of the cross-linking solutions, the utilization of Ba²⁺ ions entailed a stiffer gel structure and increased filter properties compared to Ca²⁺ ions. As shown in Fig. 1.8, the filter exhibited a periodic grid structure with full-strut cross-sections. However, as the single slip immediately gels after the contact with the cross-linking solution, the single strands lie on top of each other but are not connected at the knot points. Therefore, the filters were coated by an AC3 spraying slip before the pyrolysis, which avoided a shift of the strands. As a result, Al₂O₃-C filter structures with an improved resistance against external load and a high interaction performance with molten steel were fabricated. Since the procedure is not restricted to dimensional limits, this novel approach enables the feasibility of easy upscaling and thus offers a promising alternative for the fabrication of ceramic filters [28].

2 illustrations. A. A photograph highlights the grid like structure of the A l 2 O 3 carbon-bonded alumina filter. B. A S E M image of the full strut cross sections, which look like a rod like structure.

In a further approach, the adjustment of the filter template geometry was explored. In order to avoid the disadvantages of commercial PU foams mentioned before, filter template geometries with round filter struts were designed, modelled and evaluated regarding their impact on the steel melt filtration efficiency [29, 30]. The filter designs with the highest filtration potential were selected and fabricated using additive manufacturing methods. In particular, fused filament fabrication (“FFF”) and selective laser sintering (“SLS”) were focused on due to the variety of available raw materials. The examination of the resulting filter templates revealed that the filter templates produced by SLS exhibited a rough surface, whereas the FFF generated templates with a smooth extruded surface. Since a rougher surface encouraged a better adhesion of coating slips to the template, the SLS technique based on the device SnowWhite (Sharebot S.r.l., Italy) with a printing area of 100 × 100 × 100 mm³ and a 14 W CO₂ laser was considered for further experiments [31]. As most polymers undergo a high volume expansion at evaluated temperatures, which cause macrocracks in the ceramic structure and could lead to a filter breakdown [32], the fabrication of filter structures based on water-soluble raw materials was investigated. Besides an improvement of the filter structure, water-soluble raw materials offer the possibility for recycling in the following processes. Thereby, two variants were conducted to produce either “reactive” or “active” filter structures.

“Reactive” Al₂O₃-C filter structures with 5 ppi macrostructure were manufactured using polyvinyl alcohol (“PVA”) filter templates as described by Bock-Seefeld et al. [33]. Since conventional coating slips would lead to premature dissolution of the template and hence to a deformation of the filter structure, the filter templates were dip-coated into an alginate-based slip, which corresponded to the final slip composition mentioned in Sect. “Alginate-Based Robo Gel Casting”. Subsequently, the coated filters were placed in an aqueous cross-linking solution for 48 h, which was enriched with 1 wt% Ba(NO₃)₂. Simultaneously to the formation of the alginate network which avoided a collapse of the filter structure, the filter template was completely dissolved and removed from the filter structure. However, the dissolution was accompanied by gas bubble formation, which caused breaches and cracks in the Al₂O₃-C filter structure. Nonetheless, the filter structures exhibited sufficient dimensional stability in the green state, which is crucial for further processing. To cure the defects and to increase the coating thickness, the filters were spray-coated twice with alginate-free slips (AC3 spraying slip, cp. Sect. “Carbon-Bonded Alumina Filters Based on Carbores®P”) and pyrolized at 800 °C for 3 h under a reducing atmosphere.

As shown in Fig. 1.9, most of the macroscopic defects were successfully sealed after the pyrolysis. However, the filter evinced a porous structure, especially in the inner part of the filter strut, which was traced back to the removal of the higher water and alginate content of the alginate-based slip. Due to the multiple application of the coatings, the initial functional pore volume of the filter template diminished. Furthermore, a shrinkage in the filter height of approx. 18% occurred so that the final filter structure almost corresponded to an Al₂O₃-C filter with a 10 ppi macrostructure. After the thermal treatment, the filter structure achieved a CCS of approx. 0.13 MPa, which was slightly lower compared to conventional Al₂O₃-C filters. Although the generation of round cavities within the filter structure should inhibit the growth of critical cracks and hence improve the mechanical filter properties, it was assumed that the comparably larger cavity size superposed the beneficial effect. Furthermore, a decrease in the coating thickness with increasing distance from the filter surface due to the spraying process was observed, which also affected the CCS. Nonetheless, the novel method of Al₂O₃-C filter production using water-soluble PVA templates coated by alginate-based slips provides a promising alternative for “reactive” filter systems [33].

3 illustrations. A. An illustration of the sponge like structure of an A l 2 O 3 carbon bonded alumina filter. B. A S E M image of the same structure illustrates small, scattered parts. C. A C T scan illustrates the porous like structure of the filter.

For the production of “active” filter structures, filter templates composed of water-soluble polysaccharide were additively manufactured. The filter templates were coated with Al₂O₃ employing flame spraying, whereby the spraying parameters corresponded to those described in Sect. “Surface Functionalization by Flame Spraying”. Although the polysaccharide exhibits a higher thermal resistance compared to polymeric raw materials, the spraying procedure was performed with multiple spray shots pausing for several seconds after each shot to avoid thermal damage. This procedure was repeated from all directions until a dense alumina coating was formed on the filter template struts. Afterwards, the filters were placed in a stirred water bath for 24 h to remove the polysaccharide template. Since the flame-sprayed alumina coating reached its final state after the application, no thermal treatment at high temperatures had to be carried out.

After the removal from the water bath and subsequent drying at room temperature, a dense free-standing filter structure was achieved, which can be used immediately. The coating procedure turned out to be a challenging process, as the spraying have to be carried out by very quick shots to avoid melting the template. Hence, their number and the required coating time increased significantly. The median wall thickness at the filter surface averaged 300 µm. However, the wall thickness decreased with increasing distance from the filter surface, which was already observed in previous studies for flame-sprayed structures (cp. Sect. “Surface Functionalization by Flame Spraying”). Nonetheless, the resulting filter structures evinced good mechanical stability after the complete removal of the polysaccharide template. Due to the excellent thermal shock resistance of flame-sprayed Al₂O₃, the filter withstood the thermal shock by immersion into molten steel at 1650 °C [34]. For this reason, “active” filters based on water-soluble filter templates and flame spraying could play an important role in the purification of steel melts.