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Center for Material Science

The Center for Materials Science is devoted to the chemical materials analysis and to mathematical modelling in the context materials science.

Thematic Fields

Material Analysis

The team of the thematic field of ​​materials analysis focuses on carrying out chemical material analysis using chemical spectroscopic methods available in-house. In addition to chemical characterization of oxidation processes and other material changes, we can determine very precise flow properties of liquids and pasty substances, also in combination with detection of molecular material changes in the course of shearing processes. We also offer advice and contac persons on suitable analysis methods.

Methodes

Spectroscopic characterization of the chemical bonds of oils, fats and polymer materials using vibrational spectroscopy methods

  • Infrared (IR) spectroscopy
  • Raman spectroscopy

Both methods supplement each other in terms of information content.

Spectroscopic determination of metallic atoms (all metals heavier than sodium) in liquid and solid samples using X-ray fluorescence analysis (XRF).

Rheological-spectroscopic determination of the pot life of resins and paints. The parallel detection of the Raman vibration spectra and the change in viscosity of the mixed substances allows a very precise determination of the polymerization time under constant temperature conditions.

Rheological determination of oil and fat viscosities.

Determination of friction values ​​for material pairings.

Computed tomographic (CT) determination of material inhomogeneity (foreign bodies / air pockets).

Projects

Vib-HVDC: Vibration spectroscopic analysis of E-field induced transformer oil movement

Using Raman spectroscopy, non-polar mode oscillations can be detected without contact. Transformer oil molecules that are exposed to an external electric field align themselves in the field. This dipole-induced alignment of the hydrocarbon molecules is expressed in changes in the intensity of certain Raman vibration bands.

The research project aims to investigate the alignment behavior of transformer oils depending on the water content and oil composition. Furthermore, it contributes to clarifying the influence of paper as an electrode cover on oil molecule mobility. Results of this study are complementary to optical Kerr measurements and contribute to the interpretation of oil conductivity behavior.

Measurement setup. The Raman spectrum of the transformer oil is detected before the HVDC field effect (0 kV/mm) and after 60 min of polarization with 1 kV/mm.
Resulting spectra with two excitation sources (wavelength 532 nm and 735 nm). There is a clear decrease in the CH vibration band intensity at 2900 cm-1 and an increase at 1450 cm-1. The intensity changes are due to field-induced molecular alignment of the oil molecules. Time-resolved measurements allow conclusions to be drawn about molecular, polarization-driven oil dynamics depending on the ageing state and water content.

Relaxed: Raman-based detection of the relaxation time of shear-induced material stresses

Mechanical shearing or high-frequency E-fields induce molecular movements in the material, which lead to an increase in the Raman background signal. Through time-resolved detection of the Raman signal intensity, we can determine the relaxation time due to shear without contact. The aim of the research project is to explain the currently unknown phenomenon of the shear-induced Raman signal increase.

Raman spectra of a Li-soap grease during rotational shearing (light blue) and at multiples of 2 min time intervals after shearing. An exponential decrease in the Raman background signal with relaxation time can be seen.
Integrated Raman spectra as a function of relaxation time, as well as the exponential approximation function (fit). To a good approximation, the decrease in area of ​​the Raman spectra is simply exponential. The constant τ therefore corresponds to the relaxation time of the induced shear stress in the grease.

Contact:

Prof. Dr. Dagmar Unz

Phone
+49 931 3511-8521
E-Mail
dagmar.unz[at]thws.de
Visiting Address
Tiepolostraße 6
97070 Würzburg
Room
T.2.05
Function(s)
Examination Committee for Media Management (B.A.)
Areas of Duty

Dean

Profile picture Prof. Dr. Dagmar Unz

Multiscale Modeling

Multiscale modeling aims for descriptions of materials properties across length scales, starting with chemical processes via molecular dynamics to macroscopic properties such as thermal conductivities. Our expertise: we can simulate chemical, micro- and macroscopic dynamic processes under the influence of external fields, e.g. electric fields, such as those occuring in battery cells, electrolysis / fuel cells and insulating materials. The insight from the modeling results lead to targeted materials optimisation. A powerful computing cluster is available for the numerical implementation.

Methods

The partial differential equations of elasticity theory or (electro)hydrodynamics lead to macroscopic descriptions of solids and fluids. The corresponding materials constants are typically spatial average values or correlation functions of microscopic quantities.

In the case of electrolyte solutions, e.g. contaminated insulator oils or battery fluids, the Poisson-Nernst-Planck theory provides a mesoscopic description of the dynamics. This provides access to quantities such as the DC-conductivity or the impedance so that macroscopic RC models are dispensable.

A microscopic description of the spatial structure of non-uniform liquids on the length scale of molecular diameters is possible using classical density functional theory (DFT), within which, in partiular, short-range intermolecular interactions can be taken into account. This description on molecular length scales requires technically complex nonlinear integral equations, the solution of  methods for which we have many years of experience in.

Chemical reactions, such as redox reactions at metal-fluid interfaces or dissociation reactions, are modeled in the working group by using quantum mechanical density functional theory (DFT), which can resolve the molecular structure on atomic length scales. This allows for the calculation of reaction rates and reaction paths, even under the influence of external E-fields.

Projects

First global modeling of the conductivity behavior of transformer oils from the molecule to the current curve

The properties of application-relevant material systems are often characterized by an interaction of processes on different length scales. For example, if the Joule heat from high-voltage transformers (dimensions 0.1-10 m) is to be dissipated using insulating oil, the latter is usually separated from the metal of the transformer windings using insulating paper (thickness 10-100 µm). The electric field present in the pores of the insulating paper (diameter 10-100 nm) leads to various molecular processes (chemical bond length 100 pm) such as redox reactions, field-enhanced dissociation and electrical breakdown. The correct functioning of the transformer is determined, among other things, by the quality of the oil, whose insulating properties can decrease over time due to the molecular processes mentioned.

In order to technically control (if necessary avoid) such changes in properties, a comprehensive understanding of the interplay of the processes relevant to materials science on the individual length scales is of crucial importance.

Example of quantum chemical modelling of a paraffin molecule. Shown are the highest occupied molecular orbitals (blue/red) and the lowest unoccupied molecular orbitals (green/orange) of the C28H58 molecule in the absence of an electric field (a) as well as in HVDC fields parallel to the molecular axis (b) and parallel to one terminal C-H bond (c) at 10 kV/mm and 100 kV/mm electric field strength. The direction of the external field is symbolized by the blue arrow, the strength of the field by the arrow thickness.
The greater the spatial separation of the occupied and unoccupied molecular orbitals under consideration, the more strongly the molecule is polarized by the electric field, which in turn causes the tendency to split into ionic compounds. At 100 kV/mm the strongest polarization results when the field acts parallel to the terminal bond (c).

Structure of ionic fluids on inhomogeneously charged surfaces

With the help of electric fields that arise between charged surfaces, the structure of fluids, i.e. the distribution of molecules, can be easily influenced, which can be used, e.g., to modify the interfacial tension (electrowetting) or to control chemical reactions (electrolysis or batteries). Since the concentration of ionic components in fluids can be tiny but it does not vanish exactly, electrostatic fields in the absence of currents are shielded inside the fluid. For uniform surface charge distributions, the relevant decay length is given by the Debye length λ, which depends on the ion concentration and which can be, e.g., 1 µm in pure water, many 100 µm in purified organic solvents and less than 1 nm in concentrated electrolyte solutions. The fundamental question arises as to how far an arrangement of fluid molecules created by a uniform distribution of surface charges can extend into the interior. Information about this distribution of fluid molecules is crucial, e.g., for the design of supercapacitor, in which the capacitive properties of the arrangement of ions close to the surface are exploited.

(a) Schematic representation of a planar, non-uniformly charged surface in contact with a fluid containing ionic components. (b) A detailed analysis provides a dependence of the decay length ℓ(L) of the electric fields into the interior of the fluid on the length scale L of the charge inhomogeneities on the surface. For length scales L < 2πλ the decay length ℓ(L) is proportional to the length scale L, while for L > 2πλ it saturates at the Debye length λ. (c) Since small surface structures (small L) decay faster (smaller ℓ(L)) than large ones (see (b)), structural variables of the fluid, here the electrostatic potential, become less detailed as the distance z from the non-uniformly charged surface increases. This convergence towards the fluid structure of a uniformly charged surface takes place at the decay length ℓ(L).

Contact

Prof. Dr. Dagmar Unz

Phone
+49 931 3511-8521
E-Mail
dagmar.unz[at]thws.de
Visiting Address
Tiepolostraße 6
97070 Würzburg
Room
T.2.05
Function(s)
Examination Committee for Media Management (B.A.)
Areas of Duty

Dean

Profile picture Prof. Dr. Dagmar Unz

Numerical Simulation

The team of the thematic field of numerical simulation deals with the development and implementation of reliable and efficient algorithms for the numerical treatment of innovative non-classical material laws and with the integration of such algorithms into the framework of existing general simulation software systems. The current focus of work is on memory-based material models, such as models for viscoelastic materials (polymers, biological tissue, etc.) based on differential equations of fractional order. The use of our algorithms allows users, particularly from structural mechanics and related areas, to precisely predict the behavior of the components they have designed and to optimize the design of these components.

Methods

Creep behaviour of a polymer (time domain): Comparison between a classical model with integer derivative in the constitutive law (blue) and the use of fractional differential operators (red). The creep modulus in N/mm² is plotted over time in seconds. The integer model only exhibits creep behaviour over a very short time interval and therefore cannot reproduce the actual material behaviour over longer periods of time with sufficient accuracy, especially in the sense of extrapolation. Both models have approximately the same number of parameters. (Source: A. Schmidt, Univ. Stuttgart)

The finite element method is an established and well-understood standard tool for simulating structural mechanical processes. In order to use the method in practice, one needs software systems that, in addition to the general mathematical framework, also incorporate the material laws of those materials that are represented in the structures to be simulated. While corresponding material algorithms exist for numerous established material classes, this is e.g. hardly the case for viscoelastic materials. An important aspect here is that proven mathematical models for such materials exhibit memory effects, i.e. the current state of deformation depends not only on the current load, but on the entire previous history. This is a significant difference to common material models which has significant software engineering implications for the algorithms to be used.

In view of this background, the Numerical Simulation team is concerned with the development and implementation of numerical methods with which such memory-based models can be treated reliably and efficiently. The current focus of work is on mathematical models based on differential equations of fractional (i.e. non-integer) order. Experience has shown that such models are particularly well suited to accurately describing the behavior of viscoelastic materials over longer periods of time. From a theoretical point of view, the so-called diffusive representation of the occurring differential and integral operators has significant advantages because, compared to traditional representations, it leads to algorithms that require less computing time, have a significantly lower memory requirement for handling the process history and can be integrated into existing, proven finite element packages with little software effort.

Projects

ProVerB

As part of the ProVerB joint project funded by the BMBF from 2018 to 2021, we developed material models for the behavior of concrete over extremely long periods of time together with the Gesellschaft für numerische Simulation mbH (Braunschweig) and the Institute for Nonlinear Mechanics at the University of Stuttgart. The background was the use of concrete as a material to produce barriers and closure systems for final storage sites for radioactive waste.

MuSiK

As part of the MuSiK joint project, which began in 2022 and is expected to run until 2025 and is also funded by the BMBF, we are once again devoting ourselves, together with the Institute for Nonlinear Mechanics at the University of Stuttgart, to the development of material models and associated numerical methods for the description of fiber-reinforced plastics and synthetic resins. The specific application here is reinforcing bars to be made from such materials for concrete in building construction and civil engineering, which are intended to serve as a replacement for the steel reinforcements previously used. Because fiber-reinforced plastics are substantially less susceptible to corrosion than structural steel, the service life of structures constructed with them can be significantly increased in this way.

Contact

Prof. Dr. Dagmar Unz

Phone
+49 931 3511-8521
E-Mail
dagmar.unz[at]thws.de
Visiting Address
Tiepolostraße 6
97070 Würzburg
Room
T.2.05
Function(s)
Examination Committee for Media Management (B.A.)
Areas of Duty

Dean

Profile picture Prof. Dr. Dagmar Unz