SAXS and in-situ SAXS to follow the structural evolution in hybrid materials
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SAXS and in-situ SAXS to follow the structural evolution in hybrid materials Silvia Pabisch1, Harald Rennhofer2, Nicola Hüsing3 and Herwig Peterlik2 1 University of Vienna, Faculty of Physics, Boltzmanngasse 5, Vienna, Austria 2 University of Natural Resources and Applied Life Sciences Vienna, Institute of Physics and Material Science, Peter-Jordan-Strasse 82, Vienna, Austria 3 Paris-Lodron University Salzburg, Materials Chemistry, Hellbrunnerstrasse 34, Salzburg, Austria ABSTRACT The paper focuses on the evolution of oriented nanostructures: An orientation in real space leads to scattering intensities with a preferred orientation with respect to the azimuthal angle in reciprocal space. Thus, the macroscopic orientation of nanostructures can be obtained from SAXS patterns. The additional advantage of in-situ SAXS is that one can directly follow the development of orientated nanostructures during thermal treatment, under extreme conditions or during processing. This is shown in the following for an orientational change of pores in two very different systems, the first being the formation of pores within carbon fibers during loading at high temperatures up to 2000 oC and the second is the development of macroscopically aligned pores in mesostructured silica in the sol-gel process during shear. INTRODUCTION Orientation of elongated structures or pores can result in multiple influences on material’s properties, including the mechanical behavior as well as several functional properties, i.e. nature optimizes the crack resistance by orientation of mineral crystals and collagen fibrils [1]. As an example for technical materials, mechanical properties, among others, are controlled by the orientation of fibers [2]. Mechanical tensile stretching during production increases the orientation of crystallites within carbon fibers [3-5]. Another method to orient nanostructures is shear, which induced oriented crystallization in polymers [6]. Orientation of the structure often leads, as a consequence, to long and oriented pores: Considering carbon fibers, the structure is formed by stacks of graphene layers, which form long and elongated crystallites. These crystallites are ribbon-like shaped and form a network preferentially oriented with respect to the fiber axis. These ribbons consist of multi-layer graphene. Within the network, pores are enclosed between the crystallites. Because the boundaries are long and elongated ribbons with a high preferential orientation, the pores are usually needle- or slit-like and incline only a small angle with the fiber axis [7]. The degree of orientation controls the mechanical properties: The Young’s modulus depends mainly on the orientation of the crystallites [8] and can be calculated for different structural organisations of the material, such as the series or parallel elements, the undulating ribbon, zigzag or the mosaic model [9]. The ultimate tensile strength, however, is dependent not only on the pores and their size, but also on the misalignment and entanglement of crystallites [10]. For c
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