D. Stöver

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (253)405.11 Total impact

  • 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). · 3.71 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. · 5.21 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. · 4.79 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). · 1.51 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. · 4.91 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. · 2.31 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. · 5.21 Impact Factor
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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). · 1.49 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). · 1.22 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. · 5.21 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. · 1.49 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; · 2.20 Impact Factor
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    ABSTRACT: The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5 μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1–2 μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85 nm, 60 nm, 35 nm, 6 nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500 °C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10−4–10−6 (hPa dm3)/(s cm2) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400 °C to the range 1200–1300 °C, using the presented coating process.
    Journal of the European Ceramic Society 01/2012; 32(1):9–26. · 2.31 Impact Factor
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    ABSTRACT: Tape casting is widely used in industrial scale for production of multilayer ceramic capacitors or substrates for different applications. In 2009, it was successfully introduced as standard shaping technology for 3 (BSCF) are shown. The entire scope from the preparation of the used powders, the different manufacturing steps and their optimization potential up to the final tape-cast product will be discussed. The influence of the use of pore forming agents, heat treatment or other parameters during processing will be described in detail. Finally, the option of sequential tape casting of different materials for graded structures as a future step in shaping technology will be presented for different applications.
    Materials Science Forum 01/2012; 706-709:1035-1040.
  • M. Karger, R. Vaßen, D. Stöver
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    ABSTRACT: Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400μm–500μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9cracksmm−1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350°C) and bondcoat temperatures (up to 1085°C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.
    Surface and Coatings Technology 10/2011; 206(1):16-23. · 2.20 Impact Factor
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    ABSTRACT: Highly porous NiTi alloys with pseudoelastic properties are attractive candidates for biomedical implants, energy absorbers, or damping elements. Recently, a new method was developed for net-shape manufacturing of such alloys combining metal injection molding with the application of suitable space-holder materials. A comprehensive study of mechanical properties was conducted on samples with a porosity of 51% and a pore size in the range of 300-500μm. At low deformations <6%, fully pronounced pseudoelasticity was found. Even at higher strains, a shape recovery of maximum 6% took place, on which the onset of irreversible plastic deformation was superposed. Results of static compression tests were also used to calculate the energy-absorbing capacity. Fatigue of porous NiTi was investigated by cyclic loading up to 230,000 stress reversals. The failure mechanisms responsible for a reduction of shape recovery after an increased number of load cycles are discussed. Keywordsfatigue–mechanical properties–metal injection molding–porous NiTi–powder metallurgy–space-holder technique
    Journal of Materials Engineering and Performance 07/2011; 20(4):522-528. · 0.98 Impact Factor
  • Qianli Ma, Frank Tietz, Detlev Stöver
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    ABSTRACT: Nonstoichiometric yittrium-substituted SrTiO3 samples with a nominal composition of SrxY0.07TiO3±δ (x=0.87–1.01) and Sr0.895Y0.07TixO3−δ (x=1.00–1.20) were prepared and investigated. Phase impurities were found in samples with too high A-site or B-site deficiency. The electrical properties of the samples sintered in reducting atmosphere at different temperatures were analyzed with a view to their potential use as anode materials for solid oxide fuel cells (SOFCs). The redox properties of selected samples were investigated in detail. Samples with small A-site deficiency were considered to be the best candidates for SOFC anode materials.
    Solid State Ionics 06/2011; 192(1):535-539. · 2.11 Impact Factor
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    ABSTRACT: One way to improve the mechanical properties of solid oxide fuel cells is the development of metal supported designs. This type of SOFC offers improved thermal shock resistance, reduced temperature gradients due to the greater thermal conductivity of the metal, and lower operating temperatures. Switching from ceramic supports to metal supports also allows the uses of conventional metal joining and forming techniques and could significantly reduce the material and manufacture costs. However, one persistent problem needs to be solved: inter-diffusion of chemical elements contained in the metal substrates and in the anodes of SOFC leads to degradation, which is to be prevented by protective coatings. In order to address the issues of sintering and delamination for metal supported SOFC, the deposition of gadolinia doped ceria on metal substrates made of Crofer 22 APU has been done by electron beam evaporation and reactive spray deposition technique, as two direct deposition techniques that will not require a sintering step, respectively. Additionally, the effect of ion-assistance on layers made by electron beam evaporation was studied. Because metal supported fuel cells aim at low/intermediate operating temperatures, reducing the thickness of these protective coatings is crucial, since layer thickness is directly correlated to its ohmic resistance. A layer of nickel was applied by magnetron sputtering to prove the effectiveness of the deposited diffusion barrier layers.
    Surface and Coatings Technology 05/2011; 205(16):3999–4004. · 2.20 Impact Factor
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    ABSTRACT: Porous NiTi alloys are highly attractive for energy absorbers, damping devices and biomedical implants. In the present work, metal injection moulding (MIM) in combination with the application of a suitable space holder material was used for the production of NiTi parts with well defined pore sizes and porosities in the range of 30–70vol.%. For comparing the properties, porous titanium and Ti–6Al–4V samples were prepared in the same manner.Focus of the present work was a detailed investigation of the mechanical properties of porous NiTi to estimate its potential regarding the abovementioned applications. For a Ni-rich NiTi alloy with a porosity of 50vol.%, fully pronounced pseudoelasticity after 6% compression was demonstrated. An energy dissipation of 1.5MJ/m3 was measured, which could be directly related to the reversible austenite–martensite phase transformation. At higher deformations, pseudoelasticity becomes more and more superposed by the onset of plastic deformation. Nevertheless, even at deformations of up to 50%, a clearly pronounced amount of pseudoelastic shape recovery still remained. Fatigue of pseudoelasticity was investigated by conducting of up to 230,000 load cycles to 4% compression at a frequency of 1Hz.
    Materials Science and Engineering A 03/2011; 528(6):2454-2462. · 2.41 Impact Factor
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    ABSTRACT: A solid oxide fuel cell (SOFC) with a thin-film yttria-stabilized zirconia (YSZ) electrolyte was developed and tested. This novel SOFC shows a similar multilayer set-up as other current anode-supported SOFCs and is composed of a Ni/8YSZ anode, a gas-tight 8YSZ electrolyte layer, a dense Sr-diffusion barrier layer and a LSCF cathode. To increase the power density and lower the SOFC operating temperature, the thickness of the electrolyte layer was reduced from around 10μm in current cells to 1μm, using a nanoparticle deposition method. By using the novel 1μm electrolyte layer, the current density of our SOFC progressed to 2.7, 2.1 and 1.6A/cm2 at operation temperatures of 800, 700 and 650°C, respectively, and out-performs all similar cells reported to date in the literature. An important consideration is also that cost-effective dip-coating and spin-coating methods are applied for the fabrication of the thin-film electrolyte. Processing of 1μm layers on the very porous anode substrate material was initially experienced as very difficult and therefore 8YSZ nanoparticle coatings were developed and optimized on porous 8YSZ model substrates and transferred afterwards to regular anode substrates. In this paper, the preparation of the novel SOFC is shown and its morphology is illustrated with high resolution SEM pictures. Further, the performance in a standard SOFC test is demonstrated.
    Microsystem Technologies 02/2011; 17(2):233-242. · 0.95 Impact Factor

Publication Stats

3k Citations
405.11 Total Impact Points

Institutions

  • 1992–2014
    • Forschungszentrum Jülich
      • Institute of Energy and Climate Research (IEK)
      Jülich, North Rhine-Westphalia, Germany
  • 2010
    • RWTH Aachen University
      • Institute for Materials Applications in Mechanical Engineering
      Aachen, 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
  • 2002–2008
    • Ruhr-Universität Bochum
      • Institut für Werkstoffe
      Bochum, North Rhine-Westphalia, Germany
    • Advanced Materials and Processes Research Institute
      Bhopal, Madhya Pradesh, India
  • 2007
    • University of Patras
      • Department of Chemical Engineering
      Rhion, West Greece, Greece
  • 2003
    • Technische Universität Dortmund
      • Institute of Machining Technology
      Dortmund, North Rhine-Westphalia, Germany
  • 1999
    • University of Minho
      • Departamento de Física (DF)
      Braga, Distrito de Braga, Portugal