MIT OpenCourseWare | Physics | 8.02 Electricity and Magnetism, Spring 2002 | Video Lectures: "Video Lectures
RealOne™ Player software is required to run the .rm files in this section. Learn how to save the RealPlayer video filesto a disk or to your hard drive."
Wednesday, December 19, 2007
Monday, September 17, 2007
Technology Review: How to Make an Object Invisible
Technology Review: How to Make an Object Invisible: "Wednesday, April 11, 2007 How to Make an Object Invisible A new theoretical design using nanowires provides a way to hide devices from visible light. By Duncan Graham-Rowe
Technology Review: Superlenses and Smaller Computer Chips
Technology Review: Superlenses and Smaller Computer Chips: "Friday, March 23, 2007 Superlenses and Smaller Computer Chips Researchers report using metamaterials to make devices that could transform computing, data storage, and optical microscopy. By Kevin Bullis"
Technology Review: TR10: Invisible Revolution
Technology Review: TR10: Invisible Revolution: "TR10: Invisible Revolution Artificially structured metamaterials could transform telecommunications, data storage, and even solar energy, says David R. Smith."
Monday, July 16, 2007
Star cracks
Glassware and Equipment: "Standard taper glassware, especially round bottom flasks, are susceptible to 'star cracks'. These are tiny, radiating defaults in the glassware. Before setting up a reaction, carefully check your glassware for star cracks. If you find one, you can trade (no charge!) for a new piece of glassware. To prevent star cracks, always handle the glassware carefully."
Tuesday, July 3, 2007
Thursday, June 28, 2007
BASF - Product Information the Chemicals Catalog - Pluronic ®
BASF - Product Information the Chemicals Catalog - Pluronic ®: "Pluronic ®
The Pluronic® types are block copolymers based on ethylene oxide and propylene oxide. They can function as antifoaming agents, wetting agents, dispersants, thickeners, and emulsifiers. Use of the unique 'Pluronic Grid' system can help a formulator decide which Pluronic® surfactant is right for his needs. The Pluronic® R types tend to generate less foam than the standard Pluronic® products, but otherwise provide similar functions."
The Pluronic® types are block copolymers based on ethylene oxide and propylene oxide. They can function as antifoaming agents, wetting agents, dispersants, thickeners, and emulsifiers. Use of the unique 'Pluronic Grid' system can help a formulator decide which Pluronic® surfactant is right for his needs. The Pluronic® R types tend to generate less foam than the standard Pluronic® products, but otherwise provide similar functions."
Thursday, May 31, 2007
Thursday, May 17, 2007
Wiley InterScience: Search Results
Wiley InterScience: Search Results: "There are 504 results for: ''mesoporous' in Keywords, in all subjects, in product type Journals' of which the first 500 are returned"
Wiley InterScience :: Hot Topics :: Mesoporous Materials
Wiley InterScience :: Hot Topics :: Mesoporous Materials: "Interest in mesoporous materials (IUPAC definition: pore size 2–50 nm) has developed dramatically over the last few years, not least because the pore structure of these materials provides an extremely large surface area within a relatively small volume of material. This makes the materials suitable, for example, for catalysis, chemical sensors, and molecular separation.
The following is a selection of recent articles in this field from Angewandte Chemie, Chemistry—A European Journal, Advanced Materials, Advanced Functional Materials, and the European Journal of Inorganic Chemistry."
The following is a selection of recent articles in this field from Angewandte Chemie, Chemistry—A European Journal, Advanced Materials, Advanced Functional Materials, and the European Journal of Inorganic Chemistry."
Thursday, May 10, 2007
CHAPTER-7
CHAPTER-7: "Field Lock
In order to produce a high resolution NMR spectrum of a sample, especially one which requires signal averaging or phase cycling, you need to have a temporally constant and spatially homogeneous magnetic field. Consistency of the Bo field over time will be discussed here; homogeneity will be discussed in the next section of this chapter. The field strength might vary over time due to aging of the magnet, movement of metal objects near the magnet, and temperature fluctuations. Here is an example of a one line NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. The field lock can compensate for these variations.
The field lock is a separate NMR spectrometer within your spectrometer. This spectrometer is typically tuned to the deuterium NMR resonance frequency. It constantly monitors the resonance frequency of the deuterium signal and makes minor changes in the Bo magnetic field to keep the resonance frequency constant. The deuterium signal comes from the deuterium solvent used to prepare the sample. The animation window contains plots of the deuterium resonance lock frequency, the small additional magnetic field used to correct the lock frequency, and the resultant Bo field as a function of time while the magnetic field is drifting. The lock frequency plot displays the frequency without correction. In reality, this frequency would be "
In order to produce a high resolution NMR spectrum of a sample, especially one which requires signal averaging or phase cycling, you need to have a temporally constant and spatially homogeneous magnetic field. Consistency of the Bo field over time will be discussed here; homogeneity will be discussed in the next section of this chapter. The field strength might vary over time due to aging of the magnet, movement of metal objects near the magnet, and temperature fluctuations. Here is an example of a one line NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. The field lock can compensate for these variations.
The field lock is a separate NMR spectrometer within your spectrometer. This spectrometer is typically tuned to the deuterium NMR resonance frequency. It constantly monitors the resonance frequency of the deuterium signal and makes minor changes in the Bo magnetic field to keep the resonance frequency constant. The deuterium signal comes from the deuterium solvent used to prepare the sample. The animation window contains plots of the deuterium resonance lock frequency, the small additional magnetic field used to correct the lock frequency, and the resultant Bo field as a function of time while the magnetic field is drifting. The lock frequency plot displays the frequency without correction. In reality, this frequency would be "
Free induction decay - Wikipedia, the free encyclopedia
Free induction decay - Wikipedia, the free encyclopedia: "In Fourier Transform NMR, a free induction decay (FID) is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the magnetic field (conventionally along z). This non-equilibrium magnetisation is generally created by applying a pulse of resonant radio-frequency close to the Larmor frequency of the nuclear spins.
If the magnetisation vector has a non-zero component in the xy plane, then the precessing magnetisation will induce a corresponding oscillating voltage in a detection coil surrounding the sample. This time-domain signal is typically digitised and then Fourier transformed in order to obtain a frequency spectrum of the NMR signal i.e. the NMR spectrum.
The duration of the NMR signal is ultimately limited by T2 relaxation, but mutual interference of the different NMR frequencies present also cause the signal to be damped more quickly. When NMR frequencies are well-resolved, as is typically the case in the NMR of dissolved samples (solution-state NMR), the overall decay of the FID is relaxation-limited and the FID is approximately exponential (with a time constant T2 or more accurately T2*). FID durations will then be of the order of seconds for nuclei such as 1H. If NMR lineshapes are not relaxation-limited (as is commonly the case in solid-state NMR), then the NMR will generally decay much more quickly e.g. microseco"
If the magnetisation vector has a non-zero component in the xy plane, then the precessing magnetisation will induce a corresponding oscillating voltage in a detection coil surrounding the sample. This time-domain signal is typically digitised and then Fourier transformed in order to obtain a frequency spectrum of the NMR signal i.e. the NMR spectrum.
The duration of the NMR signal is ultimately limited by T2 relaxation, but mutual interference of the different NMR frequencies present also cause the signal to be damped more quickly. When NMR frequencies are well-resolved, as is typically the case in the NMR of dissolved samples (solution-state NMR), the overall decay of the FID is relaxation-limited and the FID is approximately exponential (with a time constant T2 or more accurately T2*). FID durations will then be of the order of seconds for nuclei such as 1H. If NMR lineshapes are not relaxation-limited (as is commonly the case in solid-state NMR), then the NMR will generally decay much more quickly e.g. microseco"
Category:Nuclear magnetic resonance - Wikipedia, the free encyclopedia
Category:Nuclear magnetic resonance - Wikipedia, the free encyclopedia: "Category:Nuclear magnetic resonance"
Shim - Wikipedia, the free encyclopedia
Shim - Wikipedia, the free encyclopedia: "Nuclear magnetic resonance / Magnetic resonance imaging
In NMR or MRI, 'shimming' is used prior to the operation of the magnet to eliminate inhomogeneitites in its field.
Initially magnetic field inside a MR scanner is far from being homogeneous. It could be even 100 times worse with respect to its homogeneity than an 'ideal' field of the scanner. This is a result of the production tolerances and magnetic field of the 'environment' - iron constructions in walls and floor of the examination room gets magnetized and disturb field of the scanner.
There are two types of shimming: active, and passive. The active shimming is done using coils with adjustable current. The passive shimming involves pieces of steel with good magnetic qualities. The steel is placed in the neighbourhood of the permanent of superconducting magnet. It gets magnetized and produces its own magnetic field. Additional magnetic field (produced by coils or steel) adds to the magnetic field of the magnet in such a way that total field is getting more homogeneous."
In NMR or MRI, 'shimming' is used prior to the operation of the magnet to eliminate inhomogeneitites in its field.
Initially magnetic field inside a MR scanner is far from being homogeneous. It could be even 100 times worse with respect to its homogeneity than an 'ideal' field of the scanner. This is a result of the production tolerances and magnetic field of the 'environment' - iron constructions in walls and floor of the examination room gets magnetized and disturb field of the scanner.
There are two types of shimming: active, and passive. The active shimming is done using coils with adjustable current. The passive shimming involves pieces of steel with good magnetic qualities. The steel is placed in the neighbourhood of the permanent of superconducting magnet. It gets magnetized and produces its own magnetic field. Additional magnetic field (produced by coils or steel) adds to the magnetic field of the magnet in such a way that total field is getting more homogeneous."
Subscribe to:
Comments (Atom)