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Report on the Royal Military College Swindon

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Introduction

Report on the Royal Military College Swindon - Cranfield University On Tuesday 12th March 2002, I visited Shrivenham science laboratories at Cranfield University. Cranfield is a research institute for post-graduates and they have now stopped enrolling under-graduates. This was a group visit with my Year 12 Physics class as part of our syllabus; to focus on two ways Physics is applie in the outside world. On arrival we were introduced to a Dr. Keith Rogers who is the Director of Material and Medical Science. Dr. Rogers then proceeded with a slide show, which gave us background knowledge about what is undertaken at the University. There are five main areas of study at Cranfield University. These are as follows: * Medical Science - Diagnostics This could be using physical methods to examine tissues for example, and the new study of refraction imaging. * Renewable Energy Sources - Solar Power and Fuel Cells Looking at solar and wind energy (there was actually an ongoing experiment at Cranfield whilst we were there which was trying to determine which renewable energy source would be more beneficial to our country's needs), studying photovoltaic cells and heterojunction solar cells. * Forensic Science - Materials Identification This area can concentrate on identifying different materials and quite often drug detection. * Exploring the Nature of Materials - physical, electrical, optical, mechanical, structural. Often links in a bit of chemistry in exploring the different properties of different materials. * Development of New Materials - Biomimetics This is to do with mimicking the processes of nature. For example bone mechanics - relating age and disease to bone mechanical properties and looking at possible solutions to today's standard of hip replacements. Dr. Rogers said that at Cranfield they look at how professional Physicists can apply Physics in other areas than Physics. He also spoke about Physics being a discipline; e.g. medical spanning where doctors / radiographers / physiotherapists etc all become involved in the processes of Physics. ...read more.

Middle

Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in 3 different shells. The first 2 shells, those closest to the centre, are completely full. The outer shell, however, is only half full, having only 4 electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have 8 electrons). To do this, it will share electrons with 4 of its neighbour silicon atoms. It's like every atom holds hands with its neighbours, except that in this case, each atom has 4 hands joined to 4 neighbours. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell. What I have described is pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about as electrons are in good conductors like copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell. Our cell has silicon with impurities - other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in this case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has 5 electrons in its outer shell, not 4. It still bonds with its silicon neighbour atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. ...read more.

Conclusion

There are a few more steps left before we can really use the cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to below 5%. The final step is the glass cover plate, which protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a cover glass and positive and negative terminals on the back. Figure 3. Basic structure of a generic silicon PV cell Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower energy photons to be absorbed by the lower band gap material beneath. This technique can result in much higher efficiencies. Such cells, called multi-junction cells, can have more than one electric field. That is the end of my report. I have concentrated on two main aspects of Physics which I am particularly interested in. I enjoyed the trip and look forward to furthering my studies. Appendix Sources of Information: o Figures 1,2,3 from Science Explained on the Internet o Encarta Encyclopaedia o Hutchinson's Encyclopaedia CD-Rom o Salter's Physics Textbook o Dr. Rogers slide show ...read more.

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