Jan Fikar

Searching for the proper law of dislocation multiplication in covalent crystals

A simple model based on dislocation theory allows the construction of a fully defined system of differential equations and the calculation of curves that correspond to different mechanical tests such as stress relaxation, the creep test and the imposed strain rate test. Various multiplication and exhaustion rates of mobile dislocations have been considered. The numerical solution of the system reproduces satisfactorily experimental curves obtained in Ge single crystals at 750 K.

Atomistic simulation of ½ screw dislocations in BCC tungsten

Atomistic simulations are used to describe the ½<111> screw dislocation in tungsten. Two different embedded atom model (EAM) potentials and one bond-order potential (BOP) are compared. A new analytical approach for constructing asymmetrical screw dislocations is presented.

Spectroscopic Ellipsometry as a Tool for On-Line Monitoring and Control of Surface Treatment Processes

Modern material technology relies increasingly on processes for surface modification and coating. Generally, we are lacking a possibility to monitor the progress of such processes. Thus the outcome can only be analyzed after the end of the whole process cycle. We are proposing to use spectroscopic ellipsometry (SE) as an on-line monitoring tool. SE, as an optical method, is not affected by high temperatures, process gases, plasmas, etc. It can be used as a monitoring tool or a sensor for closed loop control of processes. The main difficulty is the on-line interpretation of SE data. Depending on the nature of the process monitored or controlled, different models are used for the interpretation. These models predict the SE response depending on different parameters describing the surface under investigation. A fitting process is used to solve the inverse problem, i.e. extracting material data from the SE spectra. We expect increased process stability and shorter development time as a practical benefit from the use of SE.

Shape of Small Prismatic Dislocation Loops in Tungsten and Iron

Small prismatic dislocation loops in BCC metals have Burgers vectors either ½<111> or <100> and are usually close to circular shape. In atomistic simulations constructing prismatic dislocation loops of different shapes is straightforward, however, it is difficult to compare their formation energies, since loops of different shapes or different Burgers vectors do not necessarily have exactly the same size. Here we develop a general method to correctly compare loops of similar size but different shapes and the Burgers vectors. This method is combined with molecular statics simulations to identify the most energetically favorable shapes of prismatic dislocation loops in elastically isotropic tungsten and anisotropic α-iron.

Mechanical spectroscopy of decagonal Al-Cu-Fe-Cr quasicrystalline coatings

Mechanical spectroscopy measurements were performed on decagonal quasicrystalline Al–Cu–Fe–Cr coatings of three different thicknesses deposited on a mild steel substrate. The mechanical loss spectra indicate that the internal friction is mostly caused by the quasicrystalline coating and that the contributions of both the steel substrate and the interface are small. The shear modulus measured in torsion increases with temperature, while the Young’s modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as being due to solid friction between cracked segments of the quasicrystalline coating. This phenomenon also explains the broad athermal maximum found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour has been developed. Finally, the reversible high-temperature exponential background was interpreted as being due to the onset of the brittle-to-ductile transition in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed at high temperatures on icosahedral Al–Cu–Fe bulk material.

Generating conjecture and Einstein-Maxwell field of plane symmetry

For the plane symmetry we have found the electro-vacuum exact solutions of the Einstein-Maxwell equations and we have shown that one of them is equivalent to the McVittie solution of a charged infinite thin plane. The analytical extension has been accomplished and the Penrose conformal diagram has been obtained as well.

Anisotropy of Absorption and Luminescence of Multilayer InAs/GaAs Quantum Dots

We report here photoluminescence studies of multilayer InAs quantum dot structures grown by MOVPE on (001)‐oriented GaAs substrates. AFM measurements reveal prolate shapes of the dots, oriented along the [1–10] direction. Different orientations relative to the interfaces between the GaAs matrix and InAs dots are probed using polarized excitation and detection. We suggest a possible role of local fields in models of the matrix‐dot mixtures in the in‐plane anisotropic response.

Low Temperature Plastic Behaviour of Icosahedral AlCuFe Quasicrystals

The mechanism by which dislocations move in the icosahedral quasicrystalline structure, i.e., glide or climb, is still an open question. In order to check whether pure dislocation glide occurs in this quasi-periodic structure, low temperature deformation tests have been performed under confining pressure conditions. These experimental techniques, which superimpose a shear stress on an isostatic component, enable the brittle-to-ductile transition temperature to be shifted to temperatures at which diffusion processes can be assumed to be negligible. Such techniques have been applied to deform plastically AlCuFe poly-quasicrystals at low and intermediate temperatures, using both gas and solid-confining media. Mechanical data as well as microstructural observations associated with this low temperature deformation range are reported. The first results provide new insights into the deformation mechanisms that control plasticity in the icosahedral quasicrystalline phase.