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Lorentz force velocimetry (LFV) is a contactless velocity measurement technique for electrically conducting fluids. When a liquid metal or a molten glass flows through an externally applied magnetic field, eddy currents and a flow-braking force are generated inside the liquid. This force is proportional to the velocity or flow rate of the fluid and, due to Newton's third law, a force of the same magnitude but in opposite direction acts on the source of the applied magnetic field which in our case are permanent magnets. According to Ohm's law for moving conductors at low magnetic Reynolds numbers, an electric potential is induced which ensures charge conservation. In this paper, we analyze the contribution of the induced electric potential to the total Lorentz force by considering two different scenarios: conducting walls of finite thickness and aspect ratio variation of the cross-section of the flow. In both the cases, the force component generated by the electric potential is always in the opposite direction to the total Lorentz force. This force component is sensitive to the electric boundary conditions of the flow of which insulating and perfectly conducting walls are the two limiting cases. In the latter case, the overall electric resistance of the system is minimized, resulting in a considerable increase in the measured Lorentz force. Additionally, this force originating from the electric potential also decays when the aspect ratio of the cross-section of the flow is changed. Hence, the sensitivity of the measurement technique is enhanced by either increasing wall conductivity or optimizing the aspect ratio of the cross-section of the flow.

Local Lorentz force velocimetry (LFV) is a contactless local velocity measurement technique for liquid metals. Due to the interaction between an electrically moving liquid and an applied magnetic field, eddy currents and flow-braking Lorentz forces are induced within the fluid. Due to Newton’s third law, a counter force of the same magnitude acts on the source of the applied magnetic field which is in our case a permanent magnet. The magnet is connected to a new generation force/torque sensor that has been especially developed to record all force and torque components. This sensor has already been tested at a continuous casting model with a 15 mm cubic magnet providing an insight of the 3-D velocity distribution of GaInSn near the wide face of the mold [1]. For a better understanding of these results, especially regarding torque sensing, we propose a kinematic approach of ohm’s law where the velocity field is already given. In this case we avoid the variability and noise of the measurements meaning that the force depends just on the velocity distribution of the liquid metal. In this paper we present a comparison between our numerical model and the previous experiments.

Local Lorentz force velocimetry (LFV) is a local velocity measurement technique for liquid metals. Due to the interaction between an electrically moving liquid and an applied magnetic field, eddy currents and flow-braking Lorentz forces are induced within the fluid. Due to Newtons’s third law, a force of the same magnitude acts on the source of the applied magnetic field which is in our case a permanent magnet. The magnet is connected to a new generation L2F2 that has been especially developed to record all these three force and three torque components. This sensor has already been tested at a continuous casting model with a 15 mm cubic magnet providing an insight of the 3-D velocity distribution of GaInSn near the wide face of the mold. For a better understanding of these results, especially regarding torque sensing, we propose dry experiments which consist on replacing the flowing liquid by a moving solid. In this kinematic approach, where the velocity field is already given, we are able to decrease considerably the variability and the noise of the measurements providing an accurate calibration of the system. In this paper we present the numerical results using a rotating disc made of aluminum and two different magnet systems that move across the plane of its axis of rotation.

The flow in a circular pipe subjected to a non-uniform magnetic field is considered. The flow is set into rotation at the inlet section. The non-uniform transverse magnetic field with strong leading and fron gradients is imposed at a certain offset from the inlet. We apply direct numerical simulation (DNS) to study the flow transformation, including transition to turbulentce at a subcritical Reynolds number for pipe flow. This process is studied for various intensities of the swiling and the magnetic field.

Local velocity measurement of liquid metals continues to be an unsolved issue. Contact or even contactless measurement techniques cannot be used due to the fact that metal melts, like liquid steel, are usually at high temperatures, aggressive and opaque. Fortunately, there is a contactless velocity measurement technique called Lorentz force velocimetry in which a static magnetic field is applied on the electrically conductive metal stream. This static magnetic field is produced by permanent magnets, and if their size is smaller in comparison with the cross-section of the flow, a localized magnetic field distribution on the liquid metal is achieved. As a result and according to the principles of magnetohydrodynamics, eddy currents are generated within the liquid giving rise to a localized flow-breaking Lorentz force. Additionally and owing to Newtons third law, a force of the same magnitude but in the streamwise direction acts on the permanent magnet system which is connected to an optical interference force measurement device, giving access to local velocity information. This paper presents the results of local Lorentz force velocimetry at the mini-Limmcast facility at Helmholtz-Zentrum Dresden - Rosendorf using a 10mm cubic magnet and having as test fluid Galinstan in eutectic composition. In addition, partial results of Lorentz LFV using a multi-degree-of-freedom force/torque sensor are presented.

Solar cells based on AlxGa1-xAs in its direct bandgap range were fabricated and analyzed. We show that state-of-the-art metalorganic vapor phase epitaxy systems and precursors are capable of growing AlxGa1-xAs solar cells with defect concentrations up to 1 × 1014 cm-3 and less determined by deep level transient spectroscopy. However, for the n-doped material, inevitable DX-centers exist. These dopant-related defects limit the performance of the investigated AlxGa1-xAs solar cells with x > 0.20. To overcome this issue, the n-doped AlxGa1-xAs As emitter can be replaced by an n-doped Ga0.51In0.49P heteroemitter. This heterojunction solar cell again shows overall defect concentrations below 1 × 1014 cm-3 and significantly improved cell characteristics.

Lorentz force velocimetry is a contactless velocity measurement technique for electrically conductive liquids like molten steel. In this technique, a localized static magnetic field is applied on a flowing metalmelt generating eddy currentsand,therefore, a flow-braking Lorentz force withinthe fluid. Hence, due to Newton’s third law, a force of same magnitude acts on the magnet system which is connected to a force sensor. According to the principles of magnetohydrodynamics, this force is proportional to the velocity of the liquid metal. Depending on the volume subset of the flow which interacts withthe magnetic field lines produced by permanent magnets, we have local Lorentz force velocimetry (LFV) or Lorentz force flow meter. In the case of the Lorentz force flow meter, the magnetic field lines penetrate the entire cross-section on the flow given access to flow rate information. Inregard to local LFV, significantly smaller magnets are used for local velocity measurements. This paper presents the possibility of increasing the spatial resolution of the model experimentand acquiring more information of the flow, e.g. local velocity gradient, by introducing a novel arrangement of small-size permanent magnets connected to a multi-degree-of-freedom force/torque sensor.

Light spectrometers are highly versatile state-of-the-art measurement devices. However, using these systems, e.g., in semiconductor device characterization, creates challenging obstacles with respect to measurement time. We present a new, flexible and accurate approach to either characterize optical properties of arbitrary photosensitive devices or examine the spectral components of light reliably. Using a spatial light modulator (SLM) in combination with frequency division multiplexing methods, it is possible to significantly improve signal-to-noise ratios and decrease measurement times. Moreover, the use of SLM ensures a greater reliability of the setup because conventional moving parts are replaced. The feasibility and experimental setup are described in detail. The setup has been validated for various applications by comparative measurements.

Direct numerical simulations (DNS) of natural convection in liquid metal within closed boxes heated from below with uniform strong vertical magnetic fields are conducted. The main aim is to explore the possibilities and mechanisms of convection instabilities in such flows, which are referred as MHD convection. It is shown that vertical magnetic field can suppress convection at the center and reside the convection rolls attach to side walls. Stronger magnetic field can even generate new convection rolls which are aligned with the direction of magnetic field.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1975-1978, Triple-junction (3J) solar cells will soon be history. The next generation of multi-junction (MJ) devices are now reaching efficiencies far beyond the record levels of 3J cells on Germanium. In this paper we present results of a 4J wafer-bonded solar cell with bandgaps 1.88 / 1.45 // 1.10 / 0.73 eV measured with an improved efficiency of 46.5% at 324x by Fraunhofer ISE. Design for cost has, from the outset, been a priority with the development of engineered substrates to replace costly and low yielding InP substrates, a product building on Soitec’s proprietary SmartCut® technology. Wafer bonding enables the electro-mechanical combination of lattice and thermal expansion mismatched materials with electrically conductive, transparent bonds, enabling concentrator solar cells to be built from high quality 2J GaInP/GaAs absorbers lattice-matched to GaAs bonded to high quality 2J GaInPAs/GaInAs absorbers lattice matched to InP that operate well at high concentration.

Triple-junction solar cells from III-V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III-V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III-V multi-junction solar cells having four and in the future even more junctions.

The spectral electroluminescence of a monolithic triple-junction solar cell reveals the sub-cell open-circuit voltages under variation of temperature and carrier concentration. We present an electroluminescence set-up that is able to acquire a full spectrum in less than 10s and give insight into the voltage-extraction process. The sum of the sub-cell voltages is in excellent agreement to open-circuit voltages acquired under concentrated illumination. The temperature-induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub-cell bandgaps helps to identify a temperature rise with increasing current density.

28th European Photovoltaic Solar Energy Conference and Exhibition; 107-112, The spectral electroluminescene (EL) of a monolithic triple-junction solar cell reveals the sub cell open circuit voltages under variation of temperature and carrier concentration. We present an EL-setup which is able to acquire a full spectrum in less than 10 seconds and give insight into the voltage-extraction process. The sum of the sub cell voltages is in excellent agreement to open circuit voltages acquired under concentrated illumination. The temperature induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub cell bandgaps helps to identify a temperature rise with increasing current density.