Three different lithium-containing raw materials were used in this work for wetting tests and the investigations of the influence on the hydrogen porosity (i.e., filtration tests): (i) spodumene 1 from the ceramic supplier Keramik Kraft (Germany), (ii) spodumene 2 (Albemarle), and (iii) a lithium aluminate (Sigma Aldrich). Additionally, spodumene 1 was used for the basic investigations regarding the preparation of ceramic foam filters with a lithium-containing surface.
In the first step, the lithium-containing raw materials were analyzed with an ICP-OES Spectrometer 725 (Varian Inc.) regarding their Li content after a chemical digestion with HNO3/HF. Every raw material was analyzed two times. Additionally, the raw materials were measured by the differential scanning calorimetry (DSC) STA 409 PC Luxx (Netzsch, Germany) between 20 and 1400 °C with a heating rate of 10 K/min in platinum crucibles. Furthermore, the raw material spodumene 1 was heated in a hot stage microscope (Raczek, Germany) to examine the softening point until a temperature of 1500 °C with a heating rate of 10 K/min. Before the test, the spodumene powder was pressed with 60 MPa to a cylindrical sample with a diameter of 10 mm and exhibiting a height of 3 mm placed on a sintered alumina substrate.
Preparation of ceramic foam filters exclusively from spodumene is not possible due to the low melting point of the spodumene. Therefore, the alumina skeleton foams (50 × 50 × 20 mm3 with 10 pores per inch) as well as Al2O3 substrates for hot stage microscopy (diameter 12 mm and height 3 mm), and Al2O3 substrates for hot stage microscopy with capillary purification setup (diameter 55 mm and height 3 mm) were coated with slurries containing different lithium sources listed in Table I. Uncoated Al2O3 skeleton foams and uncoated Al2O3 substrates were used as reference material. The slurries consisting of the lithium-containing raw material, deionized water, Optapix AC 170 (temporary binder), and Gießfix 162 (dispersant) were homogenized by ball milling for at least 20 hours. In the next step, the coating slurries were applied using a combined dip-spin technique where the Al2O3 skeleton was immersed into the slurry completely, followed by a centrifugation step for the removal of excess slurry. In order to investigate the influence of the applied amount of spodumene 1 on the bending strength, the solid content of the slurries was varied between 55 and 80 mass pct. After drying the slurry, the coated ceramic foam filters and substrates were thermally treated at temperatures from 1375 °C to 1600 °C regarding to the utilized lithium-containing raw material. The corresponding temperatures of the thermal treatment are given in Table I. By measuring the masses of the coated and thermally treated foam filters and the corresponding uncoated alumina skeletons, the actual mass of the applied lithium-containing material was calculated.
Table I Slurry Compositions of the Coating Slurries (*Based on the Sum of Solids)
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The bending and compression strength of the ceramic foam filters coated with spodumene 1 (thermal treatment at 1375 and 1400 °C) was tested using the mechanical testing device Tiratest 2420 (TIRA GmbH, Germany) at a loading rate of 20 mm/min and a span of 36 mm between the supports for the ceramic foam filters (50 × 50 × 22 mm3). The measurement is aborted when a force loss of 70 pct of the maximum force is reached and the maximum force was used for the calculation of the bending strength. At least 25 samples were tested for each batch of samples treated thermally at the respective selected temperature.
In the next step, the presence of lithium was verified by a plasma-based secondary neutral mass spectrometry (SNMS) to make sure that the sintering step of the applied coating did not lead to a complete evaporation of the lithium.[12] This method is based on ion bombardment of the sample and subsequent ionization of the sputtered neutrals. Experiments were performed using the INA-X equipment (SPECS GmbH, Germany).[13] In this configuration, an electron cyclotron wave resonance (ECWR) plasma serves both as source for primary ions and for post-ionization. After passing an ion optic, the post-ionized neutrals are separated by a quadrupole mass analyzer and counted by a secondary electron multiplier. The SNMS measurements were done in the so-called high-frequency mode (HFM) due to the dielectric properties of the investigated samples.[14]
The measurements were conducted at the substrates prepared for the sessile drop measurements which are flat and possess a diameter of ~ 12 mm. Each sample type was measured three times by using a copper mask (5 mm in diameter) positioned on top of the sample. For the measurements, a krypton plasma with 152 W, 4.5 A and working pressure of 2 × 10−3 mbar was used. The applied voltage was set to 500 V at a frequency of 91 kHz and a duty cycle of 60 pct. The distance between the sample surface and the molybdenum aperture during measurements was 1.5 mm. The sputter time for each mass spectrum was 210 seconds.
For the investigation of the wetting behavior between the ceramic substrates and metal melts, sessile drop tests are often used. In this work, two different sessile drop procedures were applied. In the first step, conventional sessile drop tests were conducted in a high-temperature tube furnace, equipped with a high vacuum pump and an inert gas system (Carbolite Gero, Germany) located at the Institute for Nonferrous Metallurgy and Purest Materials (TU Bergakademie Freiberg, Germany).
The used aluminum alloys were produced by Trimet Aluminium, Germany, and cut into shape immediately before the testing to avoid the buildup of a massive oxide layer.[15] Metal samples weighing < 100 mg were prepared, placed on the substrates coated with the lithium-containing raw materials (diameter 12 mm), and positioned in the furnace at room temperature. The alloy AlSi7Mg was used in combination with substrates made of pure Al2O3 as reference, spodumene 2, and lithium aluminate, and AlSi5Mg was investigated in combination with spodumene 1.
Previous experience showed that the different silicon content of both used alloys has no significant influence on their wetting behavior. Every lithium-containing raw material was tested twice. As a reference, a sintered alumina substrate was used. Before starting the heating procedure at 350 K/h to the temperature of 950 °C, the furnace was evacuated to attain a pressure of p ≤ 1.5 × 10−5 mbar. After a dwell time of 180 minutes at the maximum temperature, a pressure lower than p < 8 × 10−6 mbar was achieved.
For evaluating the contact angle θcal, following equation, valid for small droplets (m < 100 mg), was used[16]:
$$ \theta_{{{\text{cal}}}} = {2}\;{\text{arctan}}\;\left( {{2}\;{\text{hour}} / {\text{day}}} \right). $$
(4)
The height h and the base diameter d of the aluminum droplet were obtained from digital images recorded with a digital camera (The Imaging Source, Germany).
The used test temperature of 950 °C is the lowest usable temperature for conventional sessile drop tests with aluminum due to the strong affinity of aluminum to oxygen. An oxide layer covering the aluminum droplet prevents a direct contact between the liquid aluminum and the substrate and thus falsifies the measured contact angle. The oxide skin decomposes only at temperatures ≥ 950 °C and sufficient vacuum, making the contact angles attained with the conventional sessile drop technique considerably more reliable under these conditions. As the aluminum melt filtration took place at temperatures between 690 and 730 °C, the temperature of the sessile drop tests at 950 °C is significantly higher. In the case of reactive systems (substrate and aluminum), this can lead to falsification of the occurring reactions and incorrect results of the contact angle measurements.[17,18]
For sessile drop tests at lower temperatures, removal of the oxide skin is necessary. This can be accomplished by the application of an adapted dropping device, as, for example, in capillary purification[19] or improved sessile drop technique.[20] In this work, a simplified capillary purification setup with hopper for melting of aluminum and a drop pressing plunger standing on the aluminum sample was used as described by Malczyk et al.[21] This capillary setup made of aluminum inert boron nitride (Henze Boron Nitride Products AG, Germany) ensured the mechanical removal of oxide skin from the initial aluminum sample. This capillary setup was placed on top of the substrate (diameter 55 mm and a height of 3 mm) and then positioned in the hot stage microscope. The hot stage was heated at a rate of 10 K/min to a temperature of 730 °C with a holding time of 10 minutes. During heating and holding, the furnace chamber was flushed with argon to decrease the oxygen level. The AlSi7Mg alloy (Trimet Aluminium, Germany) was used for the tests. The drop weight was between 380 and 530 mg, which is significantly higher than in the conventional sessile drop tests, and the deviation was higher. The drawback of the setup shows no adjustability of the dropping temperature due to the missing of a mechanical trigger. The contact angles were measured with a using a FIJI plugin called drop snake.[22]
Due to the differences in the measuring temperature and drop masses between the two utilized testing setups, their measured contact angles were not compared.
For conducting the casting trials with a focus on the hydrogen porosity of the resulting aluminum alloy castings, a special steel mold with a wedge-like geometry and a metal core in the central area has been used as described by Fankhänel et al.[5] The uppermost part of the casting solidifies first (fast) thus disallowing significant amounts of hydrogen to escape upon cooling of the rest of the cast. On the contrary, the upper part of the mold is cooled in such a way that promotes a specific solidification behavior (as a result of the metal core presence) resulting in an accumulation of pores in this upper part of the casting if hydrogen is present in the aluminum melt. Before each casting process, the inner surface of the mold is spray-coated with a BN spray release agent HeBoCoat SL-E 125 (Henze Boron Nitride Products AG, Germany).
The prepared ceramic foam filters (alumina, Al2O3 + spodumene 1, Al2O3 + spodumene 2, and Al2O3 + lithium aluminate) were used for filtering the molten AlSi7Mg alloy. Three casting trials with respective filters were performed for each filter type to ensure repeatability of the results.
Before filtration, the sides of the ceramic foam filters were closed off using the refractory paste Kerathin® (Rath Group, Austria), containing ceramic fibers, and dried at room temperature for at least 3 days. This sealing step should avoid the spilling of liquid metal outside of the mold. In the next step, the ceramic foam filter was topped with a ceramic hopper and placed on the top of the mold. Strips of ceramic fiber paper were placed between the mold’s top surface and the ceramic foam filter to ensure the evenness of the filter’s placement.
The mold and the ceramic foam filters with ceramic hopper were preheated in an electric resistance furnace (MLW VEB Elektro Bad Frankenhausen, GDR) to a temperature of 250 °C to avoid cold shuts and filter clogging during the casting process.
The aluminum alloy was melted down in a graphite-coated crucible, which was placed in an electric furnace (Carbolite Gero, Germany) set to a temperature of 750 °C. According to the measurements of the melt temperature, the casting temperature was always kept at approximately 745 °C.
After removing the oxide and impurity layer from the melt surface with a stainless steel tool, the aluminum melt was manually poured through the respective ceramic foam filter into the preheated mold. In the last step, the aluminum casting was solidified and cooled down to room temperature and unpacked from the casting mold.
The evaluation of the porosity of the cast samples was performed with the help of light microscopy and computed tomography.
The cast samples (Figure 1(a)) were first cut into two pieces (Figure 1(b)). The right piece of the casting sample (see Figure 1(b)) was entirely analyzed by a micro-focus X-ray computer tomograph CT-ALPHA (ProCon X-Ray, Germany) equipped with a 160 kV X-ray tube and a Dexela detector 1512. The resolution (voxel size) of the reconstructed volume images was 41 µm indicating that the smallest detectable pore size is about 100 µm. The reconstructed CT data were analyzed with the software Modular Algorithms for Volume Images (MAVI, Fraunhofer, Germany). The data processing started with a cropping step to choose the region of interest whereby a volume of 360 × 680 × 1040 pixel (correspond to approximately 14 × 27 × 42 mm3) was selected for further analysis, see Figure 1 (red areas).
Fig. 1
Aluminum casting sample (a) sample after casting, (b) sample cut in two pieces—frontal view, (c) side view, (d) further cut and polished sample for analysis with light microscope, and (e) blue areas are the marked pores, the red area was analyzed by computed tomography (Color figure online)
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The following binarization step transformed the grayscale image into a black image (background) and white image (foreground) see Figures 2(a) and (b), which was a particularly critical step due to the strong influence on the detected pore size and the challenging definition of the threshold. Furthermore, the occurrence of ring artifacts, due to the size of the aluminum samples, made the selection of a suitable threshold more complicated. After binarization, a labeling step (see Figure 2(c)) on the background and the feature “object analysis” allows the determination of the number and size of the pores. For the visualization of the pore distribution in the analyzed sample, the background was made transparent by the feature “object filter.”
Fig. 2
Data processing from reconstructed computed tomography data (a) cropped data, (b) binarized, and (c) labeled data
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For the investigation of the pores smaller than 100 µm, analysis by light microscope was conducted for one sample per cast, see Figure 1 (d)), whereby the sample was taken from the upper part of the wedge-shaped region above the core of the mold. The surface of this sample was then polished and evaluated with regard to its pores using a digital optical microscope Axio Imager M2m (Carl Zeiss AG, Germany) along with the evaluation software AxioVision SE64 V4.8 (Carl Zeiss Microscopy GmbH, Germany). Utilizing the contrast between the pores and the surrounding metal, the pores can be quantified and evaluated considering their quantity, size, and shape. Per sample, an area of nearly 200 mm2 (consisting of 33 measured and merged images with a size of 2829 × 2132 µm2) was evaluated. The pores were automatically detected, and their number and size were analyzed. With the help of the pore area, an area-equivalent circle diameter was calculated.