Hydrogen evolution, identified by dissolved gas analysis (DGA), is commonly used for fault detection in oil immersed
electrical power equipment. Palladium (Pd) is often used as a sensing material due to its high hydrogen absorption
capacity and related change in physical properties. Hydrogen is absorbed by Pd causing an expansion of the lattice. The
solubility, and therefore lattice expansion, increases with increasing partial pressure of hydrogen and decreasing
temperature. As long as a phase change is avoided the expansion is reversible and can be utilized to transfer strain into a
sensing element. Fiber Bragg gratings (FBG) are a well-established optical fiber sensor (OFS), mainly used for
temperature and strain sensing. A safe, inexpensive, reliable and precise hydrogen sensor can be constructed using an
FBG strain sensor to transduce the volumetric expansion of Pd due to hydrogen absorption.
This paper reports on the development, and evaluation, of an FBG gas sensing OFS and long term measurements of
dissolved hydrogen in transformer mineral oil. We investigate the effects of Pd foil cross-section and strain transfer
between foil and fiber on the sensitivity of the OFS. Two types of Pd metal sensors were manufactured using modified
Pd foil with 20 and 100 μm thickness. The sensors were tested in transformer oil at 90°C and a hydrogen concentration
range from 20- 3200 ppm.
A study of the link between the infra-red (IR) absorbance and the relative static permittivity Ɛ<sub>r</sub> of liquid hydrocarbons is of special interest for developing in-situ oxidation monitoring tools. In particular, where IR measurements are difficult to implement but cost efficient and durable capacitive probes can be used. This paper will explore this link by exposing a paraffinic hydrocarbon to oxidation in an accelerated degradation process, while measuring the IR absorption and Ɛ<sub>r</sub> values during this process. It is shown to what extent the IR response of the hydrocarbon liquid changes in the 500 to 4000 cm<sup>-1</sup> window, and how this can be translated into a measured increase in Ɛ<sub>r</sub> during oxidation time. The correlation coefficient between IR absorbance at around 1720 cm<sup>-1</sup> and Ɛ<sub>r</sub> increase with oxidation time was 99.7%. This remarkably good agreement shows that capacitive probes have the potential to be used as a substitutional in-field tool for in-situ degradation monitoring of hydrocarbon liquids.
This paper reports on the development, and evaluation, of fiber optic hydrogen sensors based on fiber Bragg gratings (FBG) and an experimental measurement system for long term experiments of fiber optic gas sensors. Two types of palladium metal sensors were manufactured; sputter coated and modified palladium foil with 20 and 100 μm thickness. The responses (at 90 °C with both 1 and 5 % hydrogen) of the coated sensor, the 20 μm and 100 μm foil sensor was found to be 10, 160 and 80 pm respectively to 1 % hydrogen and 25, 480 and 225 pm respectively to 5% hydrogen.
Corrosion is the leading failure mechanism for metallic structures. One of the standard non-destructive techniques to assess the status and predict remaining lifetime and possible failure is based on the excitation with a varying magnetic field and measuring the change of the magnetic field due to eddy currents in the device under test. Since the magnetic field is decaying quickly a large lift-off between the excitation source, magnetic sensors and the test object will reduce the signals considerably. In order to obtain a deep penetration into the test object excitation at low frequency is desirable. In this study an investigation of a high power excitation system in combination with giant magneto resistance (GMR) based sensors was done. GMR sensors have a good sensitivity and are suitable for low frequency eddy current testing due to their low 1/f noise. Finite element analysis was used to evaluate the excitation setup, sensor alignment and positions and study the influence of different parameters of the excitation and sensor setup as well as the device under test. Based on these results a laboratory setup was build and used to study the influence of main measurement parameters.
Electrically doped, organic transport layers are important for today's high efficiency organic (opto-)electronic devices. Doped organic layers have a strongly increased free charge carrier density compared to their undoped counterparts and also improve the charge carrier injection from adjacent electrodes into the organics. For practical applications, especially in optoelectronics, these layers have to have low absorption in the wavelength range of interest. The two nearly colorless p- and n-doping materials, rhenium heptoxide and cesium carbonate, are investigated focusing on their conductivity enhancement, injection improvement, and voltage drop over doped transport layers in organic light emitting diodes. They show very good doping properties already at moderate doping concentrations and prove that they can be used in variable thicknesses without a significant voltage increase. This makes them cheap, low absorbing alternatives to today's, well-established doping systems.
Apart from usage of organic light emitting diodes for flat panel display applications OLEDs are a potential candidate for
the next solid state lighting technology. One key parameter is the development of high efficient, stable white devices. To
realize this goal there are different concepts. Especially by using highly efficient phosphorescent guest molecules doped
into a suitable host material high efficiency values can be obtained. We started our investigations with a single dopant
and extended this to a two phosphorescent emitter approach leading to a device with a high power efficiency of more
than 25 lm/W @ 1000 cd/m<sup>2</sup>. The disadvantage of full phosphorescent device setups is that esp. blue phosphorescent
emitters show an insufficient long-term stability. A possibility to overcome this problem is the usage of more stable
fluorescent blue dopants, whereas, due to the fact that only singlet excitons can decay radiatively, the efficiency is lower.
With a concept, proposed by Sun et al.<sup>1</sup> in 2006, it is possible to manage the recombination zone and thus the
contribution from the different dopants. With this approach stable white color coordinates with sufficient current
efficiency values have been achieved.
The transport of charge carriers in polymer-based Organic Light-Emitting Diodes (OLEDs) as determined by the hopping mobility is an important factor influencing both lifetime and performance of OLED devices. It is strongly dependent on the density and energetic distribution of trap states in the polymer material. Especially in multi-component copolymers single functional groups can act as hole or electron traps determining the optical and electrical characteristics of the device. Transient measurements of the charge carrier mobility together with steady-state current-voltage characteristics are used to investigate the behavior of three blue polyspiro-based light-emitting polymers (LEP) with varying compositions. The first material is a simple homopolymer, the second adds a hole transporting component which is copolymerized into the backbone and the third, most complex, additionally includes a blue chromophore. With some of the added components acting as charge carrier traps the electrical behaviour of the diodes changes significantly.
OLEDs for lighting applications are gaining increasing attention due to the possibility to produce large area, 2-dimensional light sources. In contrast to the existing technology e.g. based on white inorganic LEDs this offers a completely new freedom in design for applications of next generation lighting. Today, different approaches to achieve white broadband emission for organic lighting solutions are investigated ranging from devices with blue emission in combination with conversion layers to RGB-color by lateral patterning with the support of active color tunability. Within this contribution we present results of broadband emitting copolymers to achieve white emission. New requirements arising from the shift of OLEDs in a display configuration to those for lighting applications are discussed with focus on the electro-optical behavior. Furthermore, we describe challenges that result from using large active areas and investigate ways to improve large area lighting tiles.
An attractive approach to full color OLED displays is based on white emitting copolymers and color filters. In this paper the special impact of broadband emitting copolymers based on polyspirobifluorene structures is discussed. The EL spectra of broadband emitters in PLEDs are strongly influenced by interference effects as well as by the driving conditions. Experimental results could be confirmed by modelling. Adjustment of emission spectra to the color filter characteristics lead to improved efficiency.
Due to their outstanding properties organic light-emitting displays based on conjugated polymers are on the verge of commercialization. Two major disadvantages of the current processing technique for polymers, spin-coating of polymer solutions, are the material waste and the difficulties involved in patterning the polymers. Therefore we investigate the screen-printing for the production of polymer displays. Here we present performance data of screen-printed light-emitting diodes of different colors. In the production process of these diodes we printed two layers successively one over the other. Furthermore, we show images of printed multichrome demonstrators and passive matrix displays. Our data indicate that the screen-printing technique has the potential to replace the classical spin-coat process. We observe luminance of 10,000 cd/m<SUP>2</SUP> at 8 V and peak efficiencies exceeding 10 cd/A for green diodes and half lifetime of 170 hours at 80 degree(s)C and 100 cd/m<SUP>2</SUP> for red diodes which corresponds to about 7,000 hours at room temperature. These values of printed devices are comparable to those of spin-coated ones.