Online Annual Report 2011
- Better Quality in Steel Casting
- Corrosion Contains Radioactive Substances in Repositories
- A New Model to Simulate Countercurrent Flow Limitation
- Quicker Processors through Indium Arsenide Quantum Dots
- High Magnetic Field Researchers Heading towards 100 Teslas
- How Long Do Electrons Survive in Graphene?
- Exploring the Origin of the Universe
Magnetic fields can influence electroconductive fluids and liquids – a characteristic feature which has been harnessed in industrial steel production for a long time now in order to improve steel quality. Yet, such magnetic brakes do not always have the desired effect: Instead of improving the quality of the steel, it can sometimes actually deteriorate it. Dr. Sven Eckert and Klaus Timmel from the Institute of Fluid Dynamics have examined with the help of the unique LIMMCAST facility, which simulates steel casting, why magnetic fields do not always have the anticipated effect.
Water models are often used to investigate the flow of liquid steel. Even though these models are easier to set up, they are not capable of reproducing the interactions between magnetic fields and liquid metal flows. After all, when compared to metal, water does not conduct electric currents very well. So far, the assumptions on the impact of magnetic fields have actually been based on the simplified concepts of a pipe flow. That is why the LIMMCAST facility was installed at the HZDR. It even simulates the decisive moment when the liquid steel is discharged from the tundish and pours into the mold where it is formed into a strand. According to the researchers, that’s where the magnetic field may give rise to large-scale anisotropic turbulence in the liquid steel; thus, losing the intended damping function.
Eventually, the magnetic field significantly changes the steel movement, and the molten material is actually excited instead of being calmed. These findings help understand the interactions between magnetic fields and liquid metal flows more thoroughly, and they also help improve the numeric models which are designed to simulate continuous steel casting. They are also a vital prerequisite to optimize previously used magnetic systems which, in turn, will further improve the quality of steel.
- Publication: Klaus Timmel et. al., „Experimental investigation of the flow in a continuous casting mould under the influence of a transverse DC magnetic field”, Metallurgical and Materials Transactions B 42 (2011), 68-80 (DOI-Link: http://dx.doi.org/10.1007/s11663-010-9458-1 )
- Contact: Dr. Sven Eckert / Klaus Timmel, Institute of Fluid Dynamics
In order to protect the environment from radionuclides which are produced in repositories for highly radioactive waste, it is necessary to use a number of technical, geotechnical, and geological barriers which surround the waste like the skins of an onion. Steel containers form the innermost part of this structure. Over the geologically long periods of time during which radioactive waste has to be stored away safely and securely, even the best steel will corrode which is why it might actually lose its barrier function. Thus, scientists from the Institute of Resource Ecology have studied the interactions between iron minerals, which rust is composed of, and plutonium 242, one of the most long-lived and most toxic radionuclides.
A major challenge was to conduct the experiments under the same atmospheric conditions that prevail in repositories: They are designed to be located far below the surface where there is virtually no oxygen. This state had to be maintained over the entire chain of experimental investigations. The experiments also included strict safety precautions for handling plutonium. The iron minerals were prepared at the University of Grenoble and brought to a reaction in a special glove box which had been developed at the Karlsruhe Institute of Technology. They were then returned to Grenoble in a container which had been designed at the Paul Scherrer Institute specifically for this purpose before they were finally analyzed at the Rossendorf Beamline ROBL with the ESRF’s synchrotron light. This was the result: Plutonium accumulates on the rust minerals’ surface or is precipitated as a mineral compound that is not very soluble. So even if the steel containers rust through, the plutonium will not escape. Such investigations under conditions similar to repositories are essential for assessing the safety of future repositories.
- Publication: Regina Kirsch et. al., „Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite”, Environmental Science & Technology 45 (2011), 7267-7274 (DOI-Link: http://dx.doi.org/dx.doi.org/10.1021/es200645a )
- Contact: Dr. Andreas Scheinost / Dr. Regina Kirsch, Institute of Resource Ecology
Together with the software developer ANSYS, scientists from the Institute of Fluid Dynamics have developed a new model which describes the interactions between a gas and a liquid inside a channel. This permits, for example, predictions about the so called countercurrent flow limitation. It plays an important role in energy technology processes. That is why there is a great demand for the new model when it comes to analyzing the safety of nuclear power plants or designing fuel cells.
Countercurrent flow limitation means that a water current flowing through a pipe is impeded by steam flowing from the opposite direction. In extreme cases, the water can be blocked completely. In order to test whether the new model reflects the real phenomena, the flow was experimentally examined in the HZDR’s large-scale facility TOPFLOW. The experimental set-up reproduces the hot leg of a nuclear reactor. While water was introduced from one direction, the scientists added steam from the other side. The steam flow was gradually increased until it blocked the water current completely.
It took about three months for a computer cluster consisting of several processors to simulate the experiment on the computer. Specifically, the interactions at the interfaces between water and steam, which were simulated more precisely, played a decisive role in this process. The simulations were conducted, for example, by Dr. Deendarlianto from the Gadjah Mada University in Yogyakarta/Indonesia who was working as a Humboldt fellow at the Institute of Fluid Dynamics for two years. The simulation results corresponded quite well to the data obtained from the experiments so that the countercurrent flow limitation model can now be used in real life.
- Deendarlianto et. al., „CFD studies on the phenomena around counter-current flow limitations of gas/liquid two-phase flow in a model of a PWR hot leg”, Nuclear Engineering and Design 241(2011), 5138-5148 (DOI-Link: http://dx.doi.org/10.1016/j.nucengdes.2011.08.071 )
- Höhne, T.; Deendarlianto; Lucas, D., "Numerical simulations of counter-current two-phase flow experiments in a PWR hot leg model using an interfacial area density model", International Journal of Heat and Fluid Flow 32(2011), 1047-1056 (DOI-Link: http://dx.doi.org/doi:10.1016/j.ijheatfluidflow.2011.05.007)
- Contact: Dr. Dirk Lucas / Dr. Thomas Höhne, Institute of Fluid Dynamics
Scientists from the Institute of Ion Beam Physics and Materials Research are able to produce quantum dots from indium arsenide on silicon wafers. Since a combination of these materials is considered to be very promising for microelectronics, it has been in the focus of various research activities for quite a while now. For example, the semiconductor material indium arsenide – also known as III-V semiconductor – permits higher operation frequencies because electrons move 30 times faster in it than in a silicon wafer. In addition, transistors based on this material consume less power. And the material is also presumed to be a good candidate for efficient lasers which transmit signals and information not electrically, but optically. This permits even faster transmission rates than before.
While the process that has been used in indium arsenide production so far has not been compatible with the conventional procedures of the semiconductor industry, the HZDR researchers employ a method which is already being used in semiconductor production today. They enrich the arsenic and indium ions on the silicon surface and use ion accelerators which are also applied in the chip industry for doping. With the help of the flash lamp annealing technique, ions form tiny little islands which are shaped like nanopyramids. Subsequently, the silicon is removed from the surface through etching with potassium hydroxide. What remains are small, approximately 100 nanometer high silicon columns on which the exposed pyramids rest. III-V semiconductors might further advance the miniaturization of transistors.
- Publication: Slawomir Prucnal et. al., „n-InAs Nanopyramids Fully Integrated into Silicon“, Nano Letters 2011, 11 (7), Seiten 2814–2818 (DOI-Link: http://dx.doi.org/10.1021/nl201178d )
- Contact: Dr. Slawomir Prucnal / Dr. Wolfgang Skorupa, Institute of Ion Beam Physics and Materials Research
Very high pulsed magnetic fields are made available in a non-destructive manner for materials-science research at the Dresden High Magnetic Field Laboratory. The objective is to achieve a magnetic field of 100 teslas which would be about two million times stronger than the magnetic field of the earth. Last year, the scientists actually took a big step in this direction: In June, they generated a magnetic field of 91.4 teslas and, thus, established a new world record. In January 2012, they managed to increase this value even further to 94.2 teslas. The National High Magnetic Field Laboratory in Los Alamos, USA, was the world’s first facility to reach more than 100 teslas in March 2012, but only in a magnet with inner bore diameter of 10 mm instead of the 16 mm of the Dresden coils. The Dresden researchers continue to pursue the 100 tesla benchmark with their next coil system.
The Dresden High Magnetic Field Laboratory operates a unique capacitor bank. It is capable of retrieving lots of energy for producing high magnetic fields in a very efficient manner and within a very short period of time. It also allows the researchers to use rather compact high-performance coils at low investment costs. Dresden’s record coils have been used in many different scientific projects with magnetic fields of up to 90 teslas, for example, to investigate high-temperature superconductors and carbon nanostructures.
Last year, the “life cycle” of electrons in graphene at lower energy ranges was analyzed in experiments conducted at the HZDR’s free-electron laser. This permits the scientists at the Institute of Ion Beam Physics and Materials Research to make a vital contribution towards researching the fundamental physical properties of graphene. The project was subsidized by the German Science Foundation and the EU.
The researchers managed to observe the behavior of those charge carriers which are close to the point where the energy bands touch each other. This is one of the properties in which graphene is fundamentally different from many other solids. Typically, the band structure of semiconductors is characterized by a conduction band and a valence band, which are separated by an energy gap. The band structure determines what energy levels can be adopted by the electrons and which not. With graphene, this is unique: Here, the energy bands touch each other without the appearance of any gap. That is why the material is capable of absorbing the radiation of lower energies below the visible spectrum, such as terahertz and infrared light; thus, making it a superb material for detectors.
The researchers discovered that the energy of the photons exciting the electrons and the vibrations of the atomic lattice influence the life cycle of the electrons: When the energy of the photons is greater than the energy of the lattice vibrations, then the electrons will alter their energy state more rapidly and have a shorter life time. Conversely, the electrons will linger longer at a specific energy level if the excitation energy is lower than the energy of the lattice vibrations. The researchers, thus, made a valuable contribution towards a better understanding of the electronic and optical properties of graphene, which is crucial for the development of new, rapid electronic and optoelectronic components.
- Publication: Stephan Winnerl et. al., „Carrier dynamics in epitaxial graphene close to the Dirac point“, Physical Review Letters 107, 237401 (2011) (DOI-Link: http://dx.doi.org/10.1103/PhysRevLett.107.237401 )
- Contact: Dr. Stephan Winnerl, Institute of Ion Beam Physics and Materials Research
Scientists from the Institute of Radiation Physics are working on a large-scale international project in which more than 400 researchers from 50 scientific institutions of 15 countries participate. They are jointly developing and building the detector system CBM (compressed baryonic matter) at the future FAIR accelerator center in Darmstadt. Its purpose will be to track the most elementary particles which, in part, do not exist freely and which are the building blocks of our universe. The scientists seek to explore the origin of our universe after it emerged from the big bang. In their experiments, they have to create the specific state of highly compressed matter which was produced by the big bang and is referred to as quark-gluon plasma.
The giant detectors measure several properties of the elementary particles which are flying through the spectrometer. The HZDR physicists are engaged in the construction of the central part of the 120 square meter time-of-flight detector. The prototypes built so far are among the best detectors on the globe: They measure the time of flight of single charged particles with an accuracy of better than 100 picoseconds (the time during which a particle at the speed of light moves 30 millimeters). The high particle flux amounts up to one million particles per second and per square centimeter and represents a new world record which the scientists reached during test experiments at the radiation source ELBE.
The development of the CBM experiment is accompanied by new theoretical and experimental findings on the behavior of elementary particles. These findings have been published by the Springer Verlag in the 960 page volume entitled The CBM Physics Book – Compressed Baryonic Matter in Laboratory Experiments which appeared in the Lecture Notes in Physics series. HZDR physicists participated here as well. The volume is considered to be both an introduction and a guide to future research on the properties of elementary particles in dense nuclear matter and the assumed quark-gluon plasma.
- B. Friman et. al. (Ed.), „The CBM Physics Book – Compressed Baryonic Matter in Laboratory Experiments“, Lecture Notes in Physics 814 (2011)
- L. Naumann et. al., „Ceramics high rate timing RPC”, Nuclear Instruments and Methods in Physics Research A 628 (2011), 138-141, DOI-Link: http://dx.doi.org/10.1016/j.nima.2010.09.121
- Contact: Dr. Lothar Naumann, Institute of Radiation Physics