Publications by authors named "Stephan Braxmeier"

7 Publications

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Properties of the Space Charge Layers Formed in Li-Ion Conducting Glass Ceramics.

ACS Appl Mater Interfaces 2021 Feb 25;13(4):5853-5860. Epub 2021 Jan 25.

Physik-Department ECS, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany.

For years, the space charge layer formation in Li-conducting solid electrolytes and its relevance to so-called all solid-state batteries have been controversially discussed from experimental and theoretical perspectives. In this work, we observe the phenomenon of space charge layer formation using impedance spectroscopy at different electrode polarizations. We analyze the properties of these space charge layers using a physical equivalent circuit describing the response of the solid electrolytes and solid/solid electrified interfaces under blocking conditions. The elements corresponding to the interfacial layers are identified and analyzed with different electrode metals and applied biases. The effective thickness of the space charge layer was calculated to be ∼60 nm at a bias potential of 1 V. In addition, it was possible to estimate the relative permittivity of the electrolytes, specific resistance of the space charge layer, and the effective thickness of the electric double layer (∼0.7 nm).
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http://dx.doi.org/10.1021/acsami.0c21304DOI Listing
February 2021

Hierarchically organized materials with ordered mesopores: adsorption isotherm and adsorption-induced deformation from small-angle scattering.

Phys Chem Chem Phys 2020 Jun;22(22):12713-12723

Institute of Physics, Montanuniversität Leoben, Franz-Josef Strasse 18, 8700, Leoben, Austria.

In situ small angle scattering is used to study the pore filling mechanism and the adsorption induced deformation of a silica sample with hierarchical porosity upon water adsorption. The high structural order of the cylindrical mesopores on a 2D hexagonal lattice allows obtaining adsorption induced strains from the shift of the corresponding Bragg peaks measured by in situ small-angle X-ray scattering (SAXS). However, apparent strains due to scattering contrast induced changes of the Bragg peak shapes emerge in SAXS. In contrast, small-angle neutron scattering (SANS) allows determining the real adsorption induced strains by employing a H2O/D2O adsorbate with net coherent scattering length density of zero. This allows separating the apparent strains from the real strains experimentally and comparing them with strains obtained from model calculations of the SAXS intensity. It is shown that the apparent strains cannot be described at all by a simple mesopore model of film growth and capillary condensation. A hierarchical model taking the scattering of the micropores and the outer surface of the mesoporous struts in the hierarchically porous sample properly into account, together with a modified mesopore filling mechanism based on a corona model, leads to satisfactory description of both, the adsorption isotherm and the measured apparent strains as derived by SAXS.
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http://dx.doi.org/10.1039/d0cp01026jDOI Listing
June 2020

In Situ Small-Angle Neutron Scattering Investigation of Adsorption-Induced Deformation in Silica with Hierarchical Porosity.

Langmuir 2019 Sep 22;35(35):11590-11600. Epub 2019 Aug 22.

Institute of Physics , Montanuniversitaet Leoben , Franz-Josef-Str. 18 , 8700 Leoben , Austria.

Adsorption-induced deformation of a series of silica samples with hierarchical porosity has been studied by in situ small-angle neutron scattering (SANS) and in situ dilatometry. Monolithic samples consisted of a disordered macroporous network of struts formed by a 2D lattice of hexagonally ordered cylindrical mesopores and disordered micropores within the mesopore walls. Strain isotherms were obtained at the mesopore level by analyzing the shift of the Bragg reflections from the ordered mesopore lattice in SANS data. Thus, SANS essentially measured the radial strain of the cylindrical mesopores including the volume changes of the mesopore walls due to micropore deformation. A HO/DO adsorbate with net zero coherent neutron scattering length density was employed in order to avoid apparent strain effects due to intensity changes during pore filling. In contrast to SANS, the strain isotherms obtained from in situ dilatometry result from a combination of axial and radial mesopore deformation together with micropore deformation. Strain data were quantitatively analyzed with a theoretical model for micro-/mesopore deformation by combining information from nitrogen and water adsorption isotherms to estimate the water-silica interaction. It was shown that in situ SANS provides complementary information to dilatometry and allows for a quantitative estimate of the elastic properties of the mesopore walls from water adsorption.
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http://dx.doi.org/10.1021/acs.langmuir.9b01375DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6733155PMC
September 2019

Quantifying adsorption-induced deformation of nanoporous materials on different length scales.

J Appl Crystallogr 2017 Oct 14;50(Pt 5):1404-1410. Epub 2017 Sep 14.

Institute of Physics, Montanuniversität Leoben, Franz-Josef Strasse 18, Leoben, 8700, Austria.

A new setup combining small-angle neutron scattering (SANS) and dilatometry was used to measure water-adsorption-induced deformation of a monolithic silica sample with hierarchical porosity. The sample exhibits a disordered framework consisting of macropores and struts containing two-dimensional hexagonally ordered cylindrical mesopores. The use of an HO/DO water mixture with zero scattering length density as an adsorptive allows a quantitative determination of the pore lattice strain from the shift of the corresponding diffraction peak. This radial strut deformation is compared with the simultaneously measured macroscopic length change of the sample with dilatometry, and differences between the two quantities are discussed on the basis of the deformation mechanisms effective at the different length scales. It is demonstrated that the SANS data also provide a facile way to quantitatively determine the adsorption isotherm of the material by evaluating the incoherent scattering contribution of HO at large scattering vectors.
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http://dx.doi.org/10.1107/S1600576717012274DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5627682PMC
October 2017

Setting Directions: Anisotropy in Hierarchically Organized Porous Silica.

Chem Mater 2017 Sep 31;29(18):7969-7975. Epub 2017 Aug 31.

Chemistry and Physics of Materials, Paris Lodron University Salzburg, 5020 Salzburg, Austria.

Structural hierarchy, porosity, and isotropy/anisotropy are highly relevant factors for mechanical properties and thereby the functionality of porous materials. However, even though anisotropic and hierarchically organized, porous materials are well known in nature, such as bone or wood, producing the synthetic counterparts in the laboratory is difficult. We report for the first time a straightforward combination of sol-gel processing and shear-induced alignment to create hierarchical silica monoliths exhibiting anisotropy on the levels of both, meso- and macropores. The resulting material consists of an anisotropic macroporous network of struts comprising 2D hexagonally organized cylindrical mesopores. While the anisotropy of the mesopores is an inherent feature of the pores formed by liquid crystal templating, the anisotropy of the macropores is induced by shearing of the network. Scanning electron microscopy and small-angle X-ray scattering show that the majority of network forming struts is oriented towards the shearing direction; a quantitative analysis of scattering data confirms that roughly 40% of the strut volume exhibits a preferred orientation. The anisotropy of the material's macroporosity is also reflected in its mechanical properties; i.e., the Young's modulus differs by nearly a factor of 2 between the directions of shear application and perpendicular to it. Unexpectedly, the adsorption-induced strain of the material exhibits little to no anisotropy.
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http://dx.doi.org/10.1021/acs.chemmater.7b03032DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5627989PMC
September 2017

Deformation of Microporous Carbon during Adsorption of Nitrogen, Argon, Carbon Dioxide, and Water Studied by in Situ Dilatometry.

Langmuir 2015 Nov 5;31(45):12512-9. Epub 2015 Nov 5.

Bavarian Center for Applied Energy Research, Am Galgenberg 87, 97074 Wuerzburg, Germany.

Adsorption-induced deformation of a monolithic, synthetic carbon of clearly distinguishable micro- and mesoporosity was analyzed by in situ dilatometry with N2 (77 K), Ar (77 K), CO2 (273 K), and H2O (298 K). A characteristic nonmonotonic shape of the strain isotherm showing contraction of the sample at initial micropore adsorption followed by expansion toward completion of micropore filling was found for all adsorbates. However, the extent of contraction and expansion varied significantly with the adsorbate type. The deformation differences observed were compared with the density ratio of the adsorbates within the micropores and the respective unconfined fluids. In particular, CO2 caused the least contraction of the sample, while in parallel adsorbed CO2 molecules were predicted to be considerably compacted inside carbon micropores compared to bulk liquid CO2. On the contrary, the packing of H2O molecules within carbon micropores is less dense than in the bulk liquid and adsorption of H2O produced the most pronounced contraction. N2 and Ar, both exhibiting essentially the same densities in adsorbed and bulk liquid phase, induced very similar deformation of the sample. These findings support theoretical predictions, which correlate adsorption-induced deformation and packing of molecules adsorbed in micropores. Additionally for the first time, we demonstrated with the N2 strain isotherm the existence of two nonmonotonic stages of subsequent contraction and expansion in the regions of micropore and mesopore filling. This characteristic behavior is expected for any micro- and mesoporous material.
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http://dx.doi.org/10.1021/acs.langmuir.5b03184DOI Listing
November 2015

Deformation of porous carbons upon adsorption.

Langmuir 2011 Mar 22;27(6):2553-60. Epub 2011 Feb 22.

Bavarian Center for Applied Energy Research , Am Hubland, 97074 Würzburg, Germany.

N2 and CO2 sorption measurements with in situ dilatometry implemented in a commercial volumetric sorption instrument were performed at 77 and 273 K, respectively. The resolution of the linear deformation was about ±0.2 μm. To separate effects due to microporosity, external surface area and mesopores synthetic porous carbons (xerogels) with different external surface areas and microporosities were applied as a model system. The experimental data show that the relative length change of the monolithic carbon xerogels investigated passes different stages during ad- and desorption, which are connected to micropore-, multilayer- and mesopore-sorption. The length change observed in the range of micropore and surface adsorption was found to be nonmonotonic and to take negative as well as positive values, with the maximum swelling observed being on the order of 4‰. With respect to the length change, the micropore structure seems to have the most significant impact on the overall length change, while the external surface is only of minor importance. Quantiative analysis of the deformation according to the models of Bangham and Scherer for the length change in the range of multilayer- and mesopore-adsorption allows extracting the macrosopic as well as the skeletal Young's modulus.
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http://dx.doi.org/10.1021/la104469uDOI Listing
March 2011