RESEARCH INTERESTS

(1) High pressure research - laser induced dynamic compression

Part of my research is concerned with the petrology of (ultra) high pressure minerals and mineral components such as SiO2, MgO and MgSiO3. The aim of my research is the investigation of high pressure rock materials at conditions of so-called Super Earth's: 

With the help of telescopes on Earth and in space, several thousand planets outside of our solar system have been discovered since 1996. Observation data such as mass, radius, and distance from their central star give only a few details about the composition and origin of these exoplanets. This comprises experimental investigations of materials under extreme conditions, such as those found inside of planets at, among others, the European XFEL and the research centre DESY. 

The Kepler Space Telescope has discovered a large number of planets between one and twenty times the mass of the Earth in orbits close to Sun-like stars. These exoplanets are defined as so-called “super-Earths”, which have a similar density and masses up to ten times that of the Earth, and neptunian planets, which have a similar density as the planet Neptune in our solar system. Neptune has a solid core; a mantle composed of liquid water, ammonia, and methane; as well as an atmosphere made of hydrogen, helium, and methane. In the interiors of all of these types of planets pressures can be many times higher than those inside the Earth and temperatures can reach several thousand degrees Celsius. I want to find out how the principal constituents of these planets—for example, magnesium oxide and silicates for super-Earths as well as water, methane, and ammonia for neptunian planets—behave under these conditions.

The High Energy Density Science instrument at the European XFEL, or HED for short, enables experimental investigations of extreme states of matter like those found inside of planets. The high pressures and temperatures at the HED instrument are generated through a shockwave triggered by an intense laser pulse. If the material decompresses after the shock, it goes through many different combinations of pressures and temperatures with distinctive material characteristics within very small fractions of a second. The short light flashes of the European XFEL enable sharp snapshots of these states and their properties to be taken.

(2) High pressure research - diamond anvil cells

Further research involved the use of a so called dynamic diamond anvil cell. A diamond anvil cell are two diamonds, pushing against each other, with a sample in between. Through X-rays, penetrating the diamonds, X-ray diffraction images can be recorded, and through a piezo motor on the end of one of the diamonds, different ramp compression and decompression rates can be investigated to show in situ the high-pressure phase transitions of materials such as α-cristobalite (SiO2). Our results in these studies suggest that the pressure onset of the phase transformation of α-cristobalite to cristobalite II, cristobalite XI, and ultimately to seifertite (α-PbO2 type SiO2) is dependent on the applied compression rates and stress conditions of the experiment. Increasing compression rates in general shift the studied phase transitions to higher pressures. Furthermore, our results indicate for single crystals under hydrostatic conditions a suppression of a phase transition from cristobalite X-I to seifertite at pressures of up to 82 GPa. 

(3) Lithium diffusion and evaporation in chondrites and chondrite components 

Another part of my research revolves around the very distinct Li abundances in primitive meteorites, so-called chondrites. Chondrites contain small (<1 mm) melt droplets ('chondrules'), which evolved in the very early stages of the solar system (around 4.56 Bya ago). Chondrules are one of the first solids, mainly consisting of olivine and pyroxene, that formed by a high temperature process in the early solar system. This high temperature chondrule forming process led to distinct geochemical and isotopic characteristics. In order to achieve a better understanding of elemental and isotope fractionation during high temperature chondrule formation, my research included a series of evaporation experiments, at chondrule forming conditions in a high temperature furnace. The major aims of this study were to i) determine evaporation rates of moderate volatile elements such as Li, Mg and Fe and more volatile elements such as Na, Mn and Pb, ii) elucidating the Li-Fe relationship in olivine and pyroxenes of different compositions at high temperatures and variable oxygen fugacities and iii) quantifying the Li fractionation at various conditions. The results are used to estimate timescales of the chondrule formation and diffusion rates, to identify the range of temperature and oxygen fugacity conditions in the early solar system and to assess the extent of elemental complementary of chondrules and matrix in chondrites.

(4) Hydrogen isotope fractionation and trace element partitioning within apatite and silicate melt

The chemical composition of apatite can provide an important proxy for volatile processes on planetary processes in the early solar system and yields unique information on the origins of water on Earth. Studies on the hydrogen-deuterium ratio in apatites within planetary materials, for instance, reveal a strong conformity of D/H of planetary bodies such as the asteroid 4-Vesta, lunar samples, Martian meteorites, and Terrestrial water reservoirs. However, these estimates are based on the assumption, that equilibrium fractionation between apatite and melt is most insignificant at the level of analytical precision. In my research, I investigated the hitherto unknown hydrogen isotopic fractionation between apatite and silicate melt as well as partition coefficients of a large number of trace elements (Ga, Rb, Sr, Y, Zr, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Pb, Bi, Th and U). I studied further the correlation between the apatite-melt partition coefficients of REE (DREE), temperature and melt composition. The experiments were done with a Piston Cylinder Press to archive high pressure states.

(5) Fresnel diffractive radiography at laser facilities such as Omega or the National Ignition Facility (NIF)

We developed an x-ray Fresnel diffractive radiography platform for use at the National Ignition Facility. It will enable measurements of micron-scale changes in the density gradients across an interface between isochorically heated warm dense matter materials, the evolution of which is driven primarily through thermal conductivity and mutual diffusion. We use 4.75 keV Ti K-shell x-ray emission, generated from the MJ laser system at the NIF, which focuses on a metal foil. That emission then heats a 1000 μm diameter plastic cylinder, with a central 30 μm diameter channel filled with liquid D2, up to 8 eV. This leads to a cylindrical implosion of the liquid D2 column, compressing it to ∼2.3 g/cm3. After pressure equilibration, the location of the D2/plastic interface remains steady for several nanoseconds, which enables us to track density gradient changes across the material interface with high precision. For radiography, we use Cu He-α x rays at 8.3 keV. Using a slit aperture of only 1 μm width increases the spatial coherence of the source, giving rise to significant diffraction features in the radiography signal, in addition to the refraction enhancement, which further increases its sensitivity to density scale length changes at the D2/plastic interface.