- 3149 - SUPERCONDUCTORS - and MRIs? Superconductor magnets are used in hospital MRIs today. MRIs are Magnetic Resonance Imaging. They actually work on the principle of NMRs, Nuclear Magnetic Resonance. But, they call them MRIs in hospitals because the word “nuclear” would scare patients. They would not put their body in that giant cylinder if they thought it was radioactivity. MRIs are totally safe
- ----------------- 3149 - SUPERCONDUCTORS - and MRIs
- Atoms in a solid are a lattice structure held together by electric and magnetic forces. Gravitational and nuclear forces are negligible influence in this context and are ignored.
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- Electric and magnetic forces are attractive and repulsive forces between spinning charges of negative electrons and positive nuclei. ( See Review 3016 “ Phonons, Plasmons and Magnons” for further learning on the atomic structure of solids.)
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- Your solid feet standing on the solid floor are not held up by the material in the atoms. Both solids are mostly empty space. You are held up by these electric and magnetic charges. This review focuses on superconductivity which is a special state where electron charges are not repelled or attracted but flow through the atomic lattice with zero resistance. Zero!
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- If a superconductor wire was made in a circle and electricity started flowing in it electricity would continue to flow forever without additional power, as long as superconductivity was maintained.
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- The electrical resistance of metallic conductors decreases as temperatures are lowered. Most materials like copper and silver reach a lower limit where resistance no longer decreases as temperatures decrease. However, some materials mysteriously drop to zero electrical resistance when they get below a “ critical temperature”.
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- This superconductivity was first discovered in 1911 when playing around with liquid helium the metallic mercury was found to have zero resistance at 4.1 Kelvin. Lead was super conductive at 7K. Niobium Nitride at 16K. Below this critical temperature these superconductors change to a totally different state.
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- It is like liquid water changing to a crystal ice structure at 273 Kelvin. It was not until 1957 that physics came up with an explanation for how zero resistance could occur in this state. The theory has to do with spinning electrons pairing up in what is called “Cooper Pairs”.
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- Until 1986 the highest temperature superconductor was at 23 Kelvin. In 1986 other materials were found to reach critical temperatures up to 120K. This allowed liquid nitrogen to be used that liquefies at 77 Kelvin.
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- Nitrogen is air and a lot cheaper to liquefy than helium. Cupate-Provskite ceramic was found to be superconductive at 90 K. Magnesium Dibroide (MgB2) at 39K. Cuprate YBa2Cu3O7 at 92K. Mercury Cuprates at 130K.
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- Most recent superconductors have to do with newly discovered iron compounds. So, these are very exciting discoveries for scientists and engineers. The idea of levitating trains that could travel at the speed of sound. Of transmitting electricity around the country at near zero cost. Of building particle accelerators. Of building fusion reactors. But, things have not moved as fast as we like and more discoveries are needed still.
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- In a normal metal that is an electric conductor it can be viewed as a stream of electrons flowing across a lattice structure of nuclei. However, the flow is inhibited by collisions between electrons and ions. These collisions generate heat, kinetic energy transformed into thermal energy.
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- This is called electrical resistance. It is what makes a toaster work. However, in many other applications it is lost energy. In superconductors these collisions don’t happen. Why?
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- The superconductor’s electrons do not flow as individual charges. They flow as bound pairs of two electrons. This pairing is caused by the attractive force between electrons from the exchange of Phonons. ( See Review 3016).
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- These paired electrons flow as a super fluid that can flow without any energy dissipation. All the current carried by a superconductor is carried on the surface of the material. Inside the superconductor there is zero electric field and likewise zero magnetic field.
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- In fact, an external magnetic field applied can easily stop a superconductor from working. Unfortunately the pairing theory of electrons with Phonons is only part of the explanation of superconductivity. The theory does not work at these higher critical temperatures. More learning is needed. In the meantime we can use what we know.
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- Superconductor magnets are used in hospital MRIs today. MRIs are Magnetic Resonance Imaging. They actually work on the principle of NMRs, Nuclear Magnetic Resonance. But, they call them MRIs in hospitals because the word “nuclear” would scare patients. They would not put their body in that giant cylinder if they thought it was radioactivity. MRIs are totally safe.
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- In Review 3016 we learned that electrons spin but only in one of two directions, clockwise or counter-clockwise. Physicists call this spin up or spin down. Nuclei also have a ½ spin like electrons, however, protons are 1,836 times more massive than electrons. Because they are positive spinning charges they are intrinsic magnetic dipoles just like negative spinning electrons.
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- Because they are more massive it takes a much bigger magnetic field to change their spin from up to down. MRIs use a trick to do this. If you add a small oscillating magnetic field to the large fixed magnetic field you can get the magnetic moments of the nuclei to precess, wobble like a spinning top.
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- The wobble traces out a cone called the precession of the axis of rotation. The angular frequency of this precession exactly matches the frequency of the oscillating small magnetic field which can cause the direction of the magnetic moment of the nuclei to flip.
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- NRMs can be tuned for many different nuclei that have many different precession frequencies. MRIs are tuned for hydrogen. Hydrogen nuclei is a single proton. At resonance the proton will absorb the exact amount of magnetic energy to flip its spin from up to down.
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- It turns out that fat in the body has a high concentration of hydrogen, muscle has a lesser concentration. Tumors and bone have an even smaller concentration of hydrogen. MRIs can measure the flips of hydrogen protons by tuning the magnetic oscillations to the exact precession resonance for hydrogen.
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- Measuring these flips of absorbed magnetic energy measures the concentration, or density, of hydrogen throughout the body. Tissues show up on the computer image much differently that tumors and bones allowing smart doctors to make a medical diagnosis.
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- So, where do superconductors come in to all of this?
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- It has to do with the large magnetic field needed for the MRI to work. If a copper wire magnetic solenoid was built to go around your body the current flow needed to create the magnetic field would be 300 amperes per cm^2 of wire.
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- This would create the needed 4 Tesla of magnetic field. (A Tesla is a unit of magnetic flux density of a kilogram of magnetic force per ampere of current per second squared.) The copper winding would have to be over one meter thick and the power dissipation would be hundreds of kilowatts per meter of winding length. The energy and cooling required would make this MRI impractical.
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- However, a superconductor of tin-niobium wire cooled to liquid helium temperatures could run the solenoid with a persistent current and without additional power to maintain the magnetic field.
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- To get magnetic resonance small solenoids are placed around the larger magnetic field . The small solenoids emit an oscillating magnetic field that exactly matches the resonant frequency for hydrogen.
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- This resonant frequency source is rotated around the boy to get a cross-section 3-dimensional map of the soft tissue densities inside your body. A computer is used to create the 3-D map image. The patient never feels a thing while the doctor is looking at your insides fully exposed.
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- Another use for superconductor magnet is in large particle accelerators. (See Review 955 “The Large Hadron Collider at CERN” for further learning). Superconductors have left us many mysterious yet to be solved. How fun is that?
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- May 5, 2021 SUPERCONDUCTORS - and MRIs? 963 3149
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