Laser-zone Growth in a Ribbon-to-ribbon (RTR) Process Silicon Sheet Growth Development for the Large Area Silicon Sheet Task of the Low Cost Solar Array Project

A technique for growing limited-length ribbons continually was demonstrated. This Rigid Edge technique can be used to recrystallize about 95% of the polyribbon feedstock. A major advantage of this method is that only a single, constant length silicon ribbon is handled throughout the entire process sequence; this may be accomplished using cassettes similar to those presently in use for processing Czochralski waters. Thus a transition from Cz to ribbon technology can be smoothly affected. The maximum size being considered, 3 inches x 24 inches, is half a square foot, and will generate 6 watts for 12% efficiency at 1 sun. Silicon dioxide has been demonstrated as an effective, practical diffusion barrier for use during the polyribbon formation

An N-port system model for multiwinding transformer based multilevel converters in DC-autotransformer configuration

This paper proposes a time-domain modelling approach for a multi-winding transformer-based multilevel DC/DC converter. Within previous research, multi-winding transformers have been modeled using cantilever models. These models, while simple, have no direct intuitive interpretation. Furthermore, they do not include currents circulating within each individual module and external load currents affecting only some of the modules at the same time. In this paper, a DC/DC converter model that includes the transformer as N-port network with magnetization and stray inductance and conduction losses is proposed. The model is verified using simulations showing both accurate predictions of both, circulating currents within each module and load currents affecting multiple modules

Nanoscale intermittent contact-scanning electrochemical microscopy

A major theme in scanning electrochemical microscopy (SECM) is a methodology for nanoscale imaging with distance control and positional feedback of the tip. We report the expansion of intermittent contact (IC)-SECM to the nanoscale, using disk-type Pt nanoelectrodes prepared using the laser-puller sealing method. The Pt was exposed using a focused ion beam milling procedure to cut the end of the electrode to a well-defined glass sheath radius, which could also be used to reshape the tips to reduce the size of the glass sheath. This produced nanoelectrodes that were slightly recessed, which was optimal for IC-SECM on the nanoscale, as it served to protect the active part of the tip. A combination of finite element method simulations, steady-state voltammetry and scanning electron microscopy for the measurement of critical dimensions, was used to estimate Pt recession depth. With this knowledge, the tip-substrate alignment could be further estimated by tip approach curve measurements. IC-SECM has been implemented by using a piezo-bender actuator for the detection of damping of the oscillation amplitude of the tip, when IC occurs, which was used as a tip-position feedback mechanism. The piezo-bender actuator improves significantly on the performance of our previous setup for IC-SECM, as the force acting on the sample due to the tip is greatly reduced, allowing studies with more delicate tips. The capability of IC-SECM is illustrated with studies of a model electrode (metal/glass) substrate

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process, silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project

The objective of this research is to fully investigate the Ribbon-To-Ribbon (R-T-R) approach to silicon ribbon growth. Initial work has concentrated on modification and characterization of an existing R-T-R apparatus. In addition, equipment for auxiliary heating of the melt is being evaluated and acquired. Modification of the remote viewing system and mechanical staging are nearly complete. Characterization of the laser and other components is in progress and several auxiliary heating techniques are being investigated

Variable Switching Point Model Predictive Control for DC-Link Voltage Regulation of Back-to-Back Converters

In this paper, a novel control method for back-toback converters used in grid-to-motor connections is explored. To increase the robustness of low DC-link capacitances, a control method based on variable switching point model predictive control is proposed. While previous model predictive control methods for the back-to-back converter selected a certain switching state to fulfill all control goals, we use the switching time in addition to the switching state in order to minimise deviations from the target voltage. Choosing a variable switching point provides an additional degree of freedom to the control framework and allows the system to cope with the large number of control variables. In this case, the variable switching point is used to minimize the effects of low DC-link capacitances on the system. This can either be achieved by selecting a switching point that yields low DClink capacitor charging or by selecting a switching point that aims to keep the DC-link voltage close to the reference. The proposed method is verified through numerical simulations and hardware-in-the-loop (HIL) experiments and compared to existing approaches. The results show that it is possible to control the DClink using only the switching point of the converter

Multi-active bridge based DC-link balancing of three-level NPC inverters

A common topic in multilevel converter research is the question of DC-link capacitor voltage balancing. For the three-level neutral point clamped converter the balancing is difficult due to the nonlinear nature of the DC-link capacitor voltages. Previous papers solved this problem using sophisticated controllers or additional circuitry connected in parallel to the load. Those solutions however restrict the performance of the output voltage since the controller has to provide a trade-off between output voltage performance and DC-link balancing while the load-connected balancing circuit puts limitations on the applied modulation scheme. In this paper, we connect a multi-active bridge circuit to the DC-link of the converter to overcome this problem. The proposed method is independent of the modulation scheme of the neutral point clamped converter and allows to use of all possible switching states to control the output voltage without taking the effect on the DC-link balance into account. The efficiency of the proposed method is verified using experimental results

Thermal design of superconducting cryogenic rotor: Solutions to conduction cooling challenges

This paper describes the design and testing of the first cryogenic rotor based on conductively cooled superconducting stacked tape pseudo bulks used in a rotating machine with various magnetisation methods. The rotor design demanded a number of unusual features and constraints that required novel, innovative solutions that may be applicable in other designs. The aim of this work is not to create a complete design manual, rather, to suggest some ideas that could be useful to others who are looking to solve similar problems. The outline of the proposed design is followed by a detailed description of the key systems and their interactions. Several innovative design characteristics are discussed, including calculation of liquid cryogenic cooling. Finally, experimental thermal results indicate that the design calculations are reliable and provide reaffirmation of the overall success of the design

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process. Silicon sheet growth development for the large area sheet task of the low-cost solar array project

A new calculation of the effects of thermal stresses during growth on silicon ribbon quality is reported. Thermal stress distributions are computed for ribbon growth under a variety of temperature profiles. A growth rate of 55 cu cm/min with a single ribbon was achieved. The growth of RTR ribbon with a fairly uniform parallel dendritic structure was demonstrated. Results with two approaches were obtained for reducing the Mo impurity level in polycrystalline feedstock. Coating the Mo substrate with Si3N4 does not effect thermal shear separation of the polyribbon; this process shows promise of improving cell efficiency and also increasing the useful life of the molybdenum substrate. A number of solar cells were fabricated on RTR silicon grown from CVD feedstock

Laser-zone growth in a Ribbon-To-Ribbon (RTR) process. Silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project

The Ribbon-to-Ribbon (RTR) approach to silicon ribbon growth is investigated. An existing RTR apparatus is to be upgraded to its full capabilities and operated routinely to investigate and optimize the effects of various growth parameters on growth results. A new RTR apparatus was constructed to incorporate increased capabilities and improvements over the first apparatus and to be capable of continuous growth. New high power lasers were implemented and this led to major improvements in growth velocity -- 4 inch/min. growth has been demonstrated. A major step in demonstration of the full feasibility of the RTR process is reported in the demonstration of RTR growth from CVD polyribbon rather than sliced polyribbon ingots. Average solar cell efficiencies of greater than 9% and a best cell efficiency of 11.7% are reported. Processing was shown to provide a substantial improvement in material minority carrier diffusion length. An economic analysis is reported which treats both the polyribbon fabrication and RTR processes

Chelator free gallium-68 radiolabelling of silica coated iron oxide nanorods via surface interactions

The commercial availability of combined magnetic resonance imaging (MRI)/positron emission tomography (PET) scanners for clinical use has increased demand for easily prepared agents which offer signal or contrast in both modalities. Herein we describe a new class of silica coated iron–oxide nanorods (NRs) coated with polyethylene glycol (PEG) and/or a tetraazamacrocyclic chelator (DO3A). Studies of the coated NRs validate their composition and confirm their properties as in vivo T₂ MRI contrast agents. Radiolabelling studies with the positron emitting radioisotope gallium-68 (t1/2 = 68 min) demonstrate that, in the presence of the silica coating, the macrocyclic chelator was not required for preparation of highly stable radiometal-NR constructs. In vivo PET-CT and MR imaging studies show the expected high liver uptake of gallium-68 radiolabelled nanorods with no significant release of gallium-68 metal ions, validating our innovation to provide a novel simple method for labelling of iron oxide NRs with a radiometal in the absence of a chelating unit that can be used for high sensitivity liver imaging