Some Helps with the Concepts! Material Engineering Questions 9. Structure-proper
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Some Helps with the Concepts!
Material Engineering Questions
9. Structure-property of polymer (6 points) Please compare the difference between linear and branched polyethylene (assume: they have similar molecular weight) in terms of glass transition temperature (3 points) and crystallinity (3 points) with an explanation. Why? 10. pn junction diodes (16 points) pn junction diodes have many applications; for example, (a) solar cell, (b) light emitting diodes (LEDs), and etc. (a) What are the pn junction diode structure and the principle of solar cell? (8 points) (b) What are the pn junction diode structure and the principle of light emitting diode? (8 points) (Hint: type of semiconductor, level of doping, and their relative thickness)Explanation / Answer
9) Linear polyethylene has better packed chains seperated by liquid like regions, it is crystalline in nature. Pulling a fiber out of it makes the chain line up and shows the crystallinity. When we pull down polyethylene necks down because chains align along linearly and also the chains willl not have time to orient. For branched polyethylene due to bulky sidegroups or irregular side groups the chains will not organize properly into crystalline solid.
In terms of glass transition temperature, Since the branched polyethylene cannot find the crystalline arrangement before solidification the become glass like structure and are not mobile. They cannot crystallize and find the right orientation. On glass transition temperature Tg the chains become frozen into glass.But in the case of linear polyethyle they become crystalline on cooling and has no glass like structure.
10) a) The configuration of solar cell is a large area p-n junction. It is made from silicon by diffusing n- type dopant into one side of p-type wafer. Diffusion of electrons occur from n-type junction (high concentration to p-type junction (low concentration). During diffusion the electrons on n-type side recombine with holes of p-type side. During diffusion the electrons recombine with holes on p- type. Electric field builds up due to this diffusion of carriers and it promotes charge flow which is called drift current which balances diffusion. The region of diffusion is called depletion region. The region also does not contain any mobile charge carriers.
The main principle is the conversion of sunlight into electricity using photovoltaic modules or solar modules. These modules are made out of semiconductors made of silicon which are explained above. Light striking silicon crystals induces photovoltaic effect generating electricity.
b) It operates like normal p-n junction diodes and operates only in forward bias condition.N-type material is connected to negative terminal of battery and p-type is connected to positive terminal. N-type is negatively charged and p-type is positively charged. It uses gallium,phosphorus and arsenic materials for construction. Trivalent impurities are added to pure semiconductor to form p-type semiconductor. Here holes are majority charge carriers and free electrons are minority charge carriers. During addition of pentavalent impurities n-type semiconductor is formed. Free electrons are majority charge carriers and holes are minority charge carriers. Depletion region is between p type and n-type semiconductor. When voltage greater than barrier of depletion layer is applied electric current starts flowing. LED works in forward bias, free electrons reach junction and some electrons combine with holes and holes from p-side combine with electrons in depletion region.
Electrons are released in the form of photons in LED when fitting voltage is applied and an effect called electroluminescence occurs. The color of light is is determined by energy band gap.
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