Bart Soenen

Characterization of Strain Induced Precipitation of Nb in Microalloyed Austenite Using Classical and Novel Techniques

In this contribution strain induced precipitation of niobium carbides has been analyzed making use of different hot-rolling simulators and combining the advanced precipitation characterization methods of selective chemical extraction and transmission electron microscopy. A laboratory cast Fe-0.1C-0.07Nb alloy has been employed for the study. Thermomechanical simulations were carried out by torsion, plastodilatometry and plane strain compression techniques. The results have shown that, in spite of the different deformation modes a relatively good correlation is obtained between the measurements of the precipitate size and the amount of Nb precipitated in the different experiments.

Aging of ZnS:Mn ALE ac thin-film electroluminescent devices

The aging behavior of ZnS:Mn ACTFEL devices, driven with asymmetrical voltage waveforms, was analyzed. Samples were aged with asymmetrical waveforms, so a time-averaged electric field exists in the phosphor layer during operation. Transient measurements were used to determine the voltage polarity dependence of the aging process. While with symmetrical aging and waveforms which cause a positive time-averaged field the positive polarity is mainly affected, a negative time-averaged field affects both polarities. A simple model to explain these measurements is proposed.

Static Strain Aging in Cold Rolled Metastable Austenitic Stainless Steels

Transformation of austenite to martensite during cold rolling operations is widely used to strengthen metastable austenitic stainless steel grades. Static strain aging (SSA) phenomena at low temperature, typically between 200°C and 400°C, can be used for additional increase in yield strength due to the presence of α’-martensite in the cold rolled metastable austenitic stainless steels. Indeed, SSA in austenitic stainless steel affects mainly in α’-martensite. The SSA response of three industrial stainless steel grades was investigated in order to understand the aspects of the aging phenomena at low temperature in metastable austenitic stainless steels. In this study, the optimization of, both, deformation and time-temperature parameters of the static aging treatment permitted an increase in yield strength up to 300 MPa while maintaining an acceptable total elongation in a commercial 301LN steel grade. Deformed metastable austenitic steels containing the “body-centered” α’-martensite are strengthened by the diffusion of interstitial solute atoms during aging at low temperature. Therefore, the carbon redistribution during aging at low temperature is explained in terms of the microstructural changes in austenite and martensite.

KINETICS AND AGING IN ATOMIC LAYER EPITAXY ZNS:MN AC THIN-FILM ELECTROLUMINESCENT DEVICES

The kinetics of the aging of atomic layer epitaxy ac thin-film electroluminescent devices was studied. In a first series of experiments, we aged devices at different temperatures from 50 to 190 °C, and measured the steady-state transferred charge versus voltage characteristics. From monitoring Q145 V, the charge transferred at 145 V, we could trace the relationship between Q145 V and the aging time. The aging process was found to be temperature dependent, and we could deduce an activation energy of 0.34 eV. In a second series of experiments, devices were aged 16 h at room temperature and subsequently heat treated at different temperatures from 250 to 450 °C. Monitoring again Q145 V, we found that the devices recover from aging following the relationship −krecot=ln[Q145 V/Q145 V(t=0)], where t is the heat treatment time. The recovery rate constant kreco was found to have an activation energy of 1.3 eV. In a last series of experiments we found the aging rate to be proportional with the transferred charge. P...

Electrical modeling of interface roughness in thin film electroluminescent devices

If two dielectric materials with different permittivities are in contact with each other and the interface between them is rough, then the electric field near this interface will be very inhomogeneous. In thin film electroluminescent devices, light is generated when electrons move back and forth in the phosphor layer under the influence of a strong ac electric field. At high electric fields, the electrons trapped in deep states at the interface between phosphor and insulator layer tunnel into the conduction band of the phosphor. This tunnel process is very sensitive to the electric field at the interface, so for a rough interface the electron flow will be very inhomogeneous. The relation between the interface roughness and the inhomogeneous charge transfer in thin film electroluminescent devices is investigated, based on an analytical flux tube model. The importance of the inhomogeneous current for the use of gray levels and aging is discussed.

The study of the low-field behavior of optically generated charges in thin-film electroluminescent devices

— The measurement of photo-stimulated currents (PSC) or thermally stimulated currents (TSC) in ACTFEL devices is sometimes used as a method to identify energy levels of trap centers in the phosphor layer. These methods are based on the fact that optically or thermally generated charges are able to move in the polarization field in the phosphor layer of an EL device, which remains after charging the device with a voltage pulse. Similar information about space charge can be obtained by the measurement of loss factor and capacitance of uncharged EL devices under illumination during a low ac-voltage excitation. This new method avoids one of the major disadvantages of the other methods, namely, the continuous change of the field situation throughout the measurement. The measurement of low-voltage photo-stimulated currents (LVPSC) creates an identical and reproducible field situation at each wavelength, which makes interpretation more reliable. Moreover, this new method allows the extraction of information about the low-field conduction behavior of EL thin films.

Experimental Determination of the Space Charge in AC Thin-Film Electroluminescent Devices

We developed equations for the center of mass and the amount of space charge in the phosphor layer of AC thin-film electroluminescent devices. The equations are function of the mean electric field in the phosphor layer and the electric field at both interfaces of the phosphor layer. We describe an electrical measurement method to determine these electric fields, assuming a constant space charge during steady state operation. Results are given for an undoped device based on ZnS.