D. Stöver

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (291)515.56 Total impact

  • I.V. Drozdov · V. Kochubey · Li Meng · G. Mauer · R. Vaßen · D. Stöver ·
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    ABSTRACT: A simple model of the hydrogen desorption kinetics of metal hydrides is formulated and solved analytically. The particle surface reaction is assumed to be a rate-controlling-step. Then a volumetric measurement of hydrogen desorption process is evaluated on an example of wet ball milled magnesium hydride, and can be applied generally for any metal hydride. The solution reproduces the shape of experimental curves for desorption process. In the case of surface-controlled kinetics, a volumetric measurement requires a special evaluation of results, predicted by the solution of the model. An improved evaluation of the volumetric measurement of hydrogen desorption from magnesium hydride powders using the model has been demonstrated. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    International Journal of Hydrogen Energy 08/2015; 40(32):10111-10122. DOI:10.1016/j.ijhydene.2015.05.119 · 3.31 Impact Factor
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    ABSTRACT: Inkjet printing was applied to manufacture silica-based gas separation membranes, which were coated on a pore-graduated alumina substrate with a mesoporous γ-alumina interlayer. A silica sol diluted by 1-propanol was used to print the membrane layer followed by thermal treatment in a rapid thermal processing furnace. The membrane thickness was varied between 30 and 110 nm by conducting one, two, and three coating steps. In the latter case, H2 permeance in the range of 2.0 × 10−8–3.3 × 10−8 mol/s·m2·Pa combined with H2/CO2 selectivities in the range of 15–25 were achieved, proving the concept that inorganic gas separation membranes can be successfully processed by inkjet printing.
    Journal of the American Ceramic Society 05/2015; 98(8). DOI:10.1111/jace.13657 · 2.61 Impact Factor
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    ABSTRACT: Cavitation is a severe wear mechanism in technical applications where parts are in contact with rapidly flowing liquids. Examples are turbine blades in hydropower plants or pump components. Coating exposed surfaces with wear-resistant materials is an effective measure for extending lifetime in the case of cavitation attack. NiTi is an attractive material for such coatings considering its clearly pronounced damping behaviour based on its pseudoelastic properties. A promising processing route for coating net-shaped components with NiTi is low-pressure plasma spraying (LPPS). In the present work, NiTi layers were produced by LPPS, starting from pre-alloyed NiTi powder. Cavitation resistance was investigated in relation to LPPS parameters, layer thickness and specific surface treatment. Increased cavitation resistance was demonstrated compared to UTP 730, an established cavitation protection material. The study was accompanied by comprehensive characterization of microstructure and phase transformation behaviour of the NiTi coatings.
    Wear 03/2015; 328-329. DOI:10.1016/j.wear.2015.03.003 · 1.91 Impact Factor
  • I. V. Drozdov · R. Vaßen · D. Stöver ·
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    ABSTRACT: The ‘shrinking core’ model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for micro- and nano-powders.
    RSC Advances 12/2014; 5(7). DOI:10.1039/C4RA08089K · 3.84 Impact Factor
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    ABSTRACT: A key to the development of metal-supported solid oxide fuel cells (MSCs) is the manufacturing of gas-tight thin-film electrolytes, which separate the cathode from the anode. This paper focuses the electrolyte manufacturing on the basis of 8YSZ (8 mol.-% Y2O3 stabilized ZrO2). The electrolyte layers are applied by a physical vapor deposition (PVD) gas flow sputtering (GFS) process. The gas-tightness of the electrolyte is significantly improved when sequential oxidic and metallic thin-film multi-layers are deposited, which interrupt the columnar grain structure of single-layer electrolytes. Such electrolytes with two or eight oxide/metal layers and a total thickness of about 4 μm obtain leakage rates of less than 3 × 10−4 hPa dm3 s−1 cm−2 (Δp: 100 hPa) at room temperature and therefore fulfill the gas tightness requirements. They are also highly tolerant with respect to surface flaws and particulate impurities which can be present on the graded anode underground. MSC cell tests with double-layer and multilayer electrolytes feature high power densities more than 1.4 W cm−2 at 850 °C and underline the high potential of MSC cells.
    Journal of Power Sources 06/2014; 256:52–60. DOI:10.1016/j.jpowsour.2014.01.043 · 6.22 Impact Factor
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    ABSTRACT: Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200 °C in long-term applications. Typical abradable coating thicknesses are above 1 mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer.
    Journal of the European Ceramic Society 12/2013; 33(15-16):3335-3343. DOI:10.1016/j.jeurceramsoc.2013.06.021 · 2.95 Impact Factor
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    ABSTRACT: Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La6-xWO12-y (with y = 1.5x + δ and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La6-xWO12-y and for all substituted La6-xWO12-y compounds was measured in the temperature range of 400-900 °C in wet (2.5% H2O) and dry mixtures of 4% H2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H2 at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM.
    Inorganic Chemistry 09/2013; 52:10375. DOI:10.1021/ic401104m · 4.76 Impact Factor
  • Y. Zhang · J. Malzbender · D. E. Mack · M. O. Jarligo · X. Cao · Q. Li · R. Vaßen · D. Stöver ·
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    ABSTRACT: Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550 degrees C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3MPa.m(0.5) for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection.
    Ceramics International 09/2013; 39(7):7595-7603. DOI:10.1016/j.ceramint.2013.03.014 · 2.61 Impact Factor
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    ABSTRACT: 2-Component-Metal Injection Moulding (2-C-MIM) is a technique derived from plastics industry which has been adapted to metal powders. In the present work, the production of titanium implants with a gradient in porosity was demonstrated by using this technology, starting from titanium feedstocks with and without space holder particles (NaCl, 350–500 µm). Binder systems specially tailored for the application were developed. Compared to established production routes, the net-shape fabrication of titanium implants by 2-C-MIM promises a significant reduction of cost if aiming at large scale production. The feasibility study was accompanied by a detailed characterisation of each production step of 2-C-MIM process including influence of MIM processing on mechanical properties.
    Advanced Engineering Materials 06/2013; 15(6). DOI:10.1002/adem.201200289 · 1.76 Impact Factor
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    ABSTRACT: Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) is widely known as a promising candidate material for oxygen transport membranes (OTMs). In order to maximize the oxygen permeation through such a membrane, the membrane layer should be as thin as possible, which requires a porous support. Because of the expansion behavior of BSCF, porous supports of the same material were developed to avoid failure due to mismatches in the thermal expansion coefficients. For the purpose of minimizing concentration polarization in the support pores, the microstructure of these support-layers has been optimized. For that reason supports with a porosity of up to 41% were developed. Membrane curvature caused by different shrinkage rates during co-firing could be minimized by the use of corn starch as pore former. By increasing the support porosity from 26% to 41%, the oxygen permeation of a supported 20 μm membrane in an air–argon gradient at 800 °C was increased by 50%. Compared to a disc membrane of 0.9 mm thickness the permeation enhancement is 90%.
    Journal of Membrane Science 04/2013; 433:121–125. DOI:10.1016/j.memsci.2013.01.028 · 5.06 Impact Factor
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    ABSTRACT: Asymmetric perovskite membranes have an attractive potential in the application of O2/N2 gas separation for future membrane-based power plants using oxyfuel technology. In this study – a metal-supported membrane structure with a thin-film perovskite layer and porous ceramic interlayers was developed. Porous NiCoCrAlY sintered at 1225 °C in H2 was selected as the substrate based on a sufficient permeability and corrosion resistance in co-firing conditions. According to the oxidation behaviour of NiCoCrAlY, the temperature for co-firing of the substrate and the interlayers was defined as 1100 °C for 5 h in air. Two interlayers of La0.58Sr0.4Co0.2Fe0.8O3−δ were applied by screen printing. The top layer was deposited by magnetron sputtering with a thickness of 3.8 μm. While gas-tightness was improved considerably, significant air-leakage was still detected. In summary, the successful development of a metal-perovskite-composite is shown, which acts as a basis for further development of a gas-tight metal-supported oxygen transport membrane structure.
    Journal of the European Ceramic Society 02/2013; 33(2):287–296. DOI:10.1016/j.jeurceramsoc.2012.08.025 · 2.95 Impact Factor
  • M. Bram · M. Bitzer · H. P. Buchkremer · D. Stöver ·
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    ABSTRACT: Powder metallurgy (P/M) is an attractive manufacturing process for net-shaped NiTi parts considering the limited machinability of NiTi alloys. Nevertheless, the industrial implementation of P/M processing for NiTi alloys is not trivial. To become competitive to manufacturing of NiTi alloys based on established ingot metallurgy, combination of fully pronounced shape memory behavior with sufficient mechanical properties is required. Successful use of P/M technology is strongly influenced by high affinity of NiTi alloys for uptake of oxygen and carbon, which leads to the formation of oxygen-containing Ti2Ni and TiC phases coupled with increase of Ni content in the matrix. In the case of Ni-rich NiTi alloys, this increase leads to a shift of phase transformation temperatures to lower values. Furthermore, precipitation of Ni4Ti3 during cooling from sintering temperature is difficult to avoid. Even if these precipitates might be used to decrease the Ni:Ti ratio of the matrix balancing oxygen and carbon uptake, significant loss of ductility arises, especially in the case of finely dispersed Ni4Ti3 precipitates. In the present work, each step of P/M manufacturing is discussed regarding its influence on the specific properties of NiTi alloys. The work is based on the application of prealloyed, gas atomized NiTi powders. Metal injection molding was used for net-shaped manufacturing of tensile samples, which enabled detailed study of sintering behavior combined with investigation of shape memory and mechanical properties depending on particle size, oxygen and carbon content as well as precipitation of Ni4Ti3 phase.
    Journal of Materials Engineering and Performance 12/2012; 21(12). DOI:10.1007/s11665-012-0264-6 · 1.00 Impact Factor
  • M. Bitzer · M. Bram · H. P. Buchkremer · D. Stöver ·
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    ABSTRACT: Owing to their unique properties, NiTi-based shape memory alloys (SMAs) are highly attractive candidates for a lot of functional engineering applications like biomedical implants (stents), actuators, or coupling elements. Adding a third element is an effective measure to adjust or stabilize the phase transformation behavior to a certain extent. In this context, addition of alloying elements, which are low soluble or almost insoluble in the NiTi matrix is a promising approach and—with the exception of adding Nb—has rarely been reported in the literature so far, especially if the manufacturing of the net-shaped parts of these alloys is aspired. In the case of addition of elemental Nb, broadening of hysteresis between austenitic and martensitic phase transformation temperatures after plastic deformation of the Nb phase is a well-known effect, which is the key of function of coupling elements already established on the market. In the present study, we replaced Nb with additions of elemental Ag and W, both of which are almost insoluble in the NiTi matrix. Compared with Nb, Ag is characterized by higher ductility in combination with lower melting point, enabling liquid phase sintering already at moderate temperatures. Vice versa, addition of W might act in opposite manner considering its inherent brittleness combined with high melting temperature. In the present study, hot isostatic pressing was used for manufacturing such alloys starting from prealloyed NiTi powder and with the additions of Nb, Ag, and W as elemental powders. Microstructures, interdiffusion phenomena, phase transformation behaviors, and impurity contents were investigated aiming to better understand the influence of insoluble phases on bulk properties of NiTi SMAs.
    Journal of Materials Engineering and Performance 12/2012; 21(12). DOI:10.1007/s11665-012-0375-0 · 1.00 Impact Factor
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    ABSTRACT: The overall performance of ionic conducting electrolyte layers is a key factor for determining the power density of solid oxide fuel cells (SOFCs). The aim of this work is to investigate high performance SOFC electrolyte layers developed in our lab via a low cost wet-chemical processing method. In this paper, SOFCs with bulk ionic conductivity dominated thin-film electrolyte demonstrate superior electrochemical performances. Conventional materials for SOFCs are applied in this work: Ni–YSZ cermet as the anode, yttria-stabilized zirconia (YSZ) as the electrolyte, gadolinia-doped ceria (CGO) as the Sr-diffusion barrier layer, and LSCF or LSC as the cathode. At 0.7 V and 600 °C, single cells with an active LSCF and LSC cathode area of 4 × 4 cm2 obtain a power density of 0.7 and 1.4 W cm−2, respectively. According to electrochemical impedance spectroscopy (EIS), the ohmic resistance of the single cells is almost one order of magnitude lower than the conventionally fabricated SOFCs. Due to the improved performance of the electrolyte, SOFCs are able to deliver high power output at reduced operating temperature and increased cell voltage.
    Journal of Power Sources 11/2012; 218:157–162. DOI:10.1016/j.jpowsour.2012.06.087 · 6.22 Impact Factor
  • Alexandre Guignard · Georg Mauer · Robert Vaßen · Detlev Stöver ·
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    ABSTRACT: In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young’s modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400 °C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400 °C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening.
    Journal of Thermal Spray Technology 06/2012; 21(3-4). DOI:10.1007/s11666-012-9762-1 · 1.34 Impact Factor
  • Andreas Hospach · Georg Mauer · Robert Vaßen · Detlev Stöver ·
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    ABSTRACT: The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150 kW) and very low pressure (~100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO2, Al2O3 or MgAl2O4, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications.
    Journal of Thermal Spray Technology 06/2012; 21(3-4). DOI:10.1007/s11666-012-9748-z · 1.34 Impact Factor
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    ABSTRACT: The morphology of layers of fully yttria-stabilised zirconia (YSZ) deposited by reactive magnetron sputtering was studied with regard to their application as thin electrolytes for solid oxide fuel cells (SOFC). A thin layer of YSZ was deposited on top of anode substrates for SOFC. The substrate comprises the warm-pressed anode itself, which supports the complete cell, and an anode functional layer deposited by vacuum slip casting, which is in direct contact with the electrolyte. From previous experiments it is known that non-assisted reactive DC magnetron sputtering produces layers with rather high leak-rate even when depositing at high temperatures. Residual pores on the substrates’ surfaces are responsible for the incomplete coverage by the thin electrolyte and are detrimental to the cell's performance. In the present paper, the effect of increasing bias power applied to the substrate is studied. A clear improvement of the layer morphology and gas-tightness can be observed with increasing bias power. SOFC single cell-tests show an improved performance with regard to standard wet-ceramic processing routes.
    Journal of Power Sources 05/2012; 205:157–163. DOI:10.1016/j.jpowsour.2012.01.054 · 6.22 Impact Factor
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    ABSTRACT: Yttria partially stabilized zirconia (YSZ) coatings are widely used for thermal barrier coatings (TBCs) to increase operating temperature of gas turbines. In the wavelength range where most of the radiation by walls and combustion gas is emitted within the gas turbine YSZ is semitransparent leading to increasing radiation heat flows into the components at increasing service temperatures. The objective of this work is to optimize the diffuse reflectance of plasma‐sprayed TBCs by improving the coating microstructure such that the reflectance of radiation is increased. As a result, a more efficient thermal screening of the underlying metallic substrate is achieved. In this work, air plasma‐sprayed and suspension plasma‐sprayed (SPS) coatings of 7% YSZ using powder of different grain size distributions and different spray parameters were deposited. The reflectance and transmittance has been investigated in the wavelength range from 0.3 to 2.5 μm. The SPS‐coatings showed the highest reflectance up to 94% at 1.5 μm wavelength. In addition, the scattering and absorption coefficients of the sprayed TBCs calculated with the Kubelka–Munk two flux model showed strong correlation with the measured porosity. By improving the microstructure, we were able to reduce thermal conductivity while increasing scattering of radiation, resulting in lower heat flow and lower temperature at the metallic substrate. These results are strengthened by numerical calculations.
    International Journal of Applied Ceramic Technology 05/2012; 9(3). DOI:10.1111/j.1744-7402.2011.02689.x · 1.32 Impact Factor
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    ABSTRACT: ZrO2-7 wt.% Y2O3 plasma-sprayed (PS) coatings were applied on high-temperature Ni-based alloys precoated by physical vapor deposition with a thin, dense, stabilized zirconia coating (PVD bond coat). The PS coatings were applied by atmospheric plasma spraying (APS) and inert gas plasma spraying (IPS) at 2 bar for different substrate temperatures. The thermal barrier coatings (TBCs) were tested by furnace isothermal cycling and flame thermal cycling at maximum temperatures between 1000 and 1150 °C. The temperature gradients within the duplex PVD/PS thermal barrier coatings during the thermal cycling process were modeled using an unsteady heat transfer program. This modeling enables calculation of the transient thermal strains and stresses, which contributes to a better understanding of the failure mechanisms of the TBC during thermal cycling. The adherence and failure modes of these coating systems were experimentally studied during the high-temperature testing. The TBC failure mechanism during thermal cycling is discussed in light of coating transient stresses and substrate oxidation.
    Journal of Thermal Spray Technology 04/2012; 9(2):191-197. DOI:10.1361/105996300770349917 · 1.34 Impact Factor
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    ABSTRACT: Perovskite Ba(Mg1/3Ta2/3)O3, BMT has promising bulk properties (thermal conductivity ~2W/m-K and coefficient of thermal expansion ~11×10-6/K at 1473K) for thermal barrier coating (TBC) applications at high temperature. However, during atmospheric plasma spraying (APS), such material was found to lose constituents due to the differences of vapor pressures resulting to non-stoichiometric composition of deposited coatings. To investigate the extent of phase decomposition at spray distance and varying electric arc current, different feedstock powders were plasma sprayed into water and collected for chemical, microstructural and phase analyses. When the electric arc current was decreased from 500 A to 300 A, the decomposition of the powders was reduced and the microstructure of the deposited coatings were improved. The thermal cycling lifetime of the deposited coatings at~1250°C surface temperature is also higher.
    Surface and Coatings Technology 01/2012; 206(8-9). DOI:10.1016/j.surfcoat.2011.11.003 · 2.00 Impact Factor

Publication Stats

7k Citations
515.56 Total Impact Points


  • 1992-2015
    • Forschungszentrum Jülich
      • • Institute of Energy and Climate Research (IEK)
      • • Materials Synthesis and Processing (IEK-1)
      Jülich, North Rhine-Westphalia, Germany
  • 2009
    • Pierre and Marie Curie University - Paris 6
      Lutetia Parisorum, Île-de-France, France
  • 2008
    • University of Bonn
      Bonn, North Rhine-Westphalia, Germany
  • 2003
    • Technische Universität Dortmund
      • Institute of Machining Technology
      Dortmund, North Rhine-Westphalia, Germany
  • 2002
    • Ruhr-Universität Bochum
      • Institut für Experimentalphysik II
      Bochum, North Rhine-Westphalia, Germany