Gerald A. Zickler

Mechanical properties of pyrolysed wood: a nanoindentation study

The present work focuses on changes of mechanical properties in pyrolysed spruce wood as a function of temperature up to 2400°C. Nanoindentation tests are used for the determination of mechanical properties at the scale of single wood cell walls. Hardness, indentation modulus and elasto-plastic/brittle behaviour of the carbonaceous residues are derived as function of pyrolysis temperature. Hardness values increase continuously by more than one order of magnitude to 4.5 GPa at 700°C. The indentation modulus shows complex changes with a minimum of 5 GPa around 400°C and a maximum of 40 GPa around 1000°C. The deformation induced by the indenter is largely visco-plastic in native wood, but it is almost purely elastic in the carbonaceous residue, with particular low values of the indentation ductility index around 700°C. A low density and a strongly cross-linked carbon structure may explain the mechanical behaviour at these intermediate temperatures. A final decrease of the modulus and a slight decrease of ductility for temperatures above 2000°C can be attributed to a continuous structural transition of the material towards graphite-like stacking of carbon sheets and to preferred carbon orientation along the wood cell axis.

Adsorption in periodically ordered mesoporous organosilica materials studied by in situ small-angle X-ray scattering and small-angle neutron scattering.

Modified periodically ordered mesoporous organosilica materials were prepared starting from a recently introduced type of sol-gel precursor, containing both organic moieties and hydrolyzable Si-OR groups. In order to thoroughly characterize the mesoporosity and its accessibility, different probe gases were used in conventional gas adsorption experiments. Furthermore, in situ small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were applied to study the mesoporosity and the sorption processes, taking advantage of scattering contrast matching conditions. Thereby, the materials were characterized not only by different probe molecules but also at different temperatures (nitrogen at 77 K, dibromomethane at 290 K and perfluoropentane at 276 K). The comparison between the standard and in situ SAXS/SANS adsorption experiments revealed valuable information about the porosity and microstructure of the materials. It is demonstrated that the organic moieties are homogeneously distributed; that is, they do not phase-separate from silica on the nanometer scale.

Pore lattice deformation in ordered mesoporous silica studied by in situ small-angle X-ray diffraction

Sorption and capillary condensation of an organic fluid in ordered mesoporous silica were studied by in situ small-angle X-ray diffraction using synchrotron radiation. The sorption isotherm was calculated from the sample transmission data. The diffraction peaks resulting from the periodic hexagonal arrangement of the pores show characteristic changes of intensity, position and width, and a pronounced hysteresis of these parameters in the capillary condensation/evaporation regime was observed. The change of the peak positions with gas pressure is related to pore lattice deformations due to capillary stresses occurring at the gas–liquid phase transition.

Local microstructure and its influence on precipitation behavior in hot deformed Nimonic 80a

Abstract The influence of local microstructure on the kinetics of heterogeneous carbide precipitation was investigated in hot deformed, commercial nickel-based superalloys Nimonic 80a. The as-deformed alloys were aged at 1073 K for different times, and precipitation characteristics were examined by transmission electron microscopy and small-angle neutron scattering. M 23 C 6 precipitates were found to nucleate at grain boundaries, but in particular also on dislocation lines and boundaries of subgrains. In the early stages of ageing, the kinetics of carbide precipitation is enhanced and the average carbide size is smaller as a result of deformation. An influence of the degree of recrystallization on overall carbide kinetics is also found, which is attributed to differences in the amount of bulk carbides as compared to carbides at grain boundaries. In contrast to the behavior of carbides, the kinetics of the homogeneously distributed γ′ precipitates is not influenced by prior deformation.

Coherent analysis of disordered mesoporous adsorbents using small angle X-ray scattering and physisorption experiments

Characterization of mesoporous adsorbents is traditionally performed in terms of the pore size distribution with bulk methods like physisorption and mercury intrusion. But their application relies on assumptions regarding the basic pore geometry. Although novel tools have enabled the quantitative interpretation of physisorption data for adsorbents having a well-defined pore structure the analysis of disordered mesoporosity still remains challenging. Here we show that small angle X-ray scattering (SAXS) combined with chord length distribution (CLD) analysis presents a precise and convenient approach to determine the structural properties of two-phase (solid-void) systems of mesopores. Characteristic wall (solid) and pore (void) sizes as well as surface areas are extracted without the need to assume a certain pore shape. The mesoporous structure of modern, commercially available fully porous and core-shell adsorbent particles is examined by SAXS/CLD analysis. Mean pore size and surface area are compared with results obtained from nitrogen physisorption data and show excellent agreement.

Kinetics of interstitial segregation in Cottrell atmospheres and grain boundaries

Trapping of interstitial (e.g. carbon) atoms is driven by the reduction in energy in the system. Diffusion of interstitials, together with their trapping in dislocation cores and/or grain boundaries, is studied by the thermodynamic extremal principle. In addition to the total Gibbs energy, a well-established formulation of the total dissipation is applied. Dimension-free evolution equations are derived, whose solution is well approximated by an easy to handle kinetic equation. Cottrell’s power law can be verified in the initial stage.

SANS investigation of phase separation in hot-deformed Nimonic 80a

Abstract Precipitation kinetics in hot-deformed Nimonic 80a alloys is investigated by small-angle neutron scattering. The integrated intensity increases faster and the average size of the precipitates is generally smaller in the deformed alloy as compared to an undeformed reference sample. It is shown that small carbide precipitates contribute considerably to the SANS signal from Nimonic 80a.

Elucidating the Sorption Mechanism of Dibromomethane in Disordered Mesoporous Silica Adsorbents.

The mechanism of dibromomethane (DBM) sorption in mesoporous silica was investigated by in situ small-angle X-ray scattering (SAXS). Six different samples of commercial porous silica particles used for liquid chromatography were studied, featuring a disordered mesoporous structure, with some of the samples being functionalized with alkyl chains. SAXS curves were recorded at room temperature at various relative pressures P/P0 during adsorption of DBM. The in situ SAXS experiment is based on contrast matching between silica and condensed DBM with regard to X-ray scattering. One alkyl-modified silica sample was evaluated in detail by extraction of the chord-length distribution (CLD) from SAXS data obtained for several P/P0. On the basis of this analytical approach and by comparison with ex situ obtained data of nitrogen and DBM adsorption, the mechanism of DBM uptake was studied. Results of average mesopore sizes obtained with the CLD method were compared with pore size analysis using nitrogen physisorption (77 K) with advanced state-of-the-art nonlocal density functional theory (NLDFT) evaluation. The dual SAXS/physisorption study indicates that microporosity is negligible in all silica samples and that surface functionalization with a hydrophobic ligand has a major influence upon the process of DBM adsorption. Also, all of the mesopores are accessible as evidenced by in situ SAXS. The data suggest that no multilayer adsorption occurs on C18-(octadecyl-)modified silica surfaces using DBM as adsorptive, and it is possibly also negligible on bare silica surfaces.

Finite Element Modeling of the Cyclic Wetting Mechanism in the Active Part of Wheat Awns

Many plant tissues and organs are capable of moving due to changes in the humidity of the environment, such as the opening of the seed capsule of the ice plant and the opening of the pine cone. These are fascinating examples for the materials engineer, as these tissues are non-living and move solely through the differential swelling of anisotropic tissues and in principle may serve as examples for the bio-inspired design of artificial actuators. In this paper, we model the microstructure of the wild wheat awn (Triticum turgidum ssp. dicoccoides) by finite elements, especially focusing on the specific microscopic features of the active part of the awn. Based on earlier experimental findings, cell walls are modeled as multilayered cylindrical tubes with alternating cellulose fiber orientation in successive layers. It is shown that swelling upon hydration of this system leads to the formation of gaps between the layers, which could act as valves, thus enabling the entry of water into the cell wall. This supports the hypothesis that this plywood-like arrangement of cellulose fibrils enhances the effect of ambient humidity by accelerated water or vapor diffusion along the gaps. The finite element model shows that a certain distribution of axially and tangentially oriented fibers is necessary to generate sufficient tensile stresses within the cell wall to open nanometer-sized gaps between cell wall layers.

Calibration of phase field parameters demonstrated on kinetics of a shrinking single grain

Abstract The phase field method is frequently employed to simulate evolution of rather complex microstructures. In order to accurately describe the real kinetics of a specific material system, the phase field parameters must be calibrated to standard thermodynamic quantities such as interface mobilities or interface energies. This letter presents a convenient method, based on thermodynamic modelling of shrinking of a single grain embedded in single crystal (matrix), how to calibrate the phase field parameters. It also shows, how the required thermodynamic quantities can be met by proper rescaling of a phase field simulation carried out for given phase field parameters.