Jürgen Almanstötter

Grain growth phenomena in tungsten wire

Abstract This paper describes a three-dimensional lattice model for the study of the recrystallization and grain growth of tungsten wires in comparison to additional experiments. Microstructures similar to experimental ones were obtained. The experimentally obtained grain size data was used to determine the grain growth exponent and the activation energy for grain boundary migration at temperatures ranging from 1500 to 1900 °C. The evolution with time and temperature of the average grain size has been calculated by the Monte-Carlo method. For this, a relation between length and time scale in simulation and experiment, based on intrinsic material parameters, was developed. In general, the results agree very well with the experimental observations.

Modeling the optical properties of fluorescent powders: Y1.91Eu0.09O3

A numerical model is developed to study the optical properties of phosphor powder coatings. The method derived uses a combination of ray tracing and Monte Carlo modeling which allows us to calculate the absorption, reflection, and transmission for the exciting ultraviolet radiation as well as for the emitted visible light. Size, shape, morphology, density of packing of the phosphor grains, and the roughness of the coating can be studied in detail with our model. Calculations for Y1.91Eu0.09O3 red phosphor powder layers used in commercial fluorescent lamps are presented. Good agreement with experiment was found.A numerical model is developed to study the optical properties of phosphor powder coatings. The method derived uses a combination of ray tracing and Monte Carlo modeling which allows us to calculate the absorption, reflection, and transmission for the exciting ultraviolet radiation as well as for the emitted visible light. Size, shape, morphology, density of packing of the phosphor grains, and the roughness of the coating can be studied in detail with our model. Calculations for Y1.91Eu0.09O3 red phosphor powder layers used in commercial fluorescent lamps are presented. Good agreement with experiment was found.

Sodium monolayers on thermionic cathodes

Under certain conditions alkali vapours form dipole monolayers on metallic electrodes that can lower the work function of the bulk material. In this case, the power balance of the electrode, the electrode fall voltage and the electrode loss power can change considerably. To verify this effect a pyrometric technique was adapted and optimized for the diagnostics of tungsten electrodes in high pressure sodium discharges. Using an already verified model of thermally emitting cathodes the effect was observed in a Na DC discharge and the range of existence was investigated. An interpretation of the results is given using a Langmuir description of forming the Na monolayers and first-principles electronic structure calculations using a pseudopotential plane wave method to solve the Kohn-Sham equations of density-functional theory.

ELECTRONIC STRUCTURE OF FLUORESCENT LAMP CATHODE SURFACES : BAO/W(001)

The BaO/W interaction is responsible for the emission properties of barium–oxide coated tungsten electrode coils in fluorescent lamps. The electronic structure of the BaO/W(001) interface is investigated by first-principles calculations within the local-density approximation of the density functional theory using the full-potential linearized augmented plane wave method. Results are presented for the total density of states (DOS), the atom- and orbital-resolved partial DOS and charge density distributions. Partial covalent character in the W–O and W–Ba bonding is shown. The main contribution to chemical bonding is caused by the tungsten d states and the adsorbate valence states, which interestingly also involve barium d states. This leads to a stabilization of the adsorbed configuration, with respect to cathode operation temperatures. The calculated work function is in agreement with experimental data.