Ultrahigh-purity gases

The noise level gets quite high unless ultrahigh purity gases are used with the Hall detector. What is meant by "ultrahigh" purity ... 

Chapter 10). This not only has the potential to affect the detection capability, but ultrahigh-purity gases are typically 2-3 times more expensive than industrial- or laboratory-grade gases. But at the end of the day, if you are investing in brand new technology, it also depends on what kind of funds are available to solve a particular application problem. 

Thermal treatment of a material in a gas oxidizing atmosphere is the simplest concept. This can be done in air, air diluted in N2, dry air, or in ultrahigh purity O2. In the laboratory practice, calcination is done in flowthrough beds, aided by fluidization, or in static box furnaces. Important aspects are the bed geometry, the removal of the generated gases, and temperature gradients. 

Several conditions must be met for successful ETEM investigations. Thin, electron-transparent samples are necessary—this requirement can usually be met with most catalyst powders. Ultrahigh-purity heater materials and sample grids capable of withstanding elevated temperature and gases are required (such as those made of stainless steel or molybdenum). The complex nature of catalysis with gas environments and elevated temperatures requires a stable design of the ETEM instrument to simulate realistic conditions at atomic resolution. 

Electronic gas purification Production of ultrahigh-purity N2, Ar. He, H2, O2 Purification of fiuorinated gases NF3, CF4, C2F6, Sip4 Purification of hydrides NH3, PH3, ASH3, SIH4, Si2H6... 

Chlorine is shipped as a liquefied compressed gas. Many grades of purity are available, such as high-purity grade with a minimum purity of 99.5% ultrahigh-purity grade with a minimum purity of 99.9% and research grade with a minimum purity of 99.96%. Since chlorine and caustic soda are produced by the same process, sales of caustic soda are dependent on chlorine production. However, caustic soda is not as great an environmental concern as chlorine. 

Ultrahigh (99.999 + %) purity tellurium is prepared by zone refining in a hydrogen or inert-gas atmosphere. Single crystals of tellurium, tellurium alloys, and metal tellurides are grown by the Bridgman and Czochralski methods (see Semiconductors). 

Most labs choose option 2 and install a series of traps that are located external to the GC unless there is some compelling reason to go with a ultrahigh- or research-grade purity carrier gas. A hydrocarbon trap is first... 

Ultrahigh vacuum (UHV), i.e., residual pressures of about 10 mbar, guarantees low concentration of impurities in the gas phase (provided that high-purity gases, say CO and O2, are used). It is also crucial in order to ascertain sufficiently long periods of time for the study of the pure surface as well as measurements of the adsorption processes. (At 10 mbar, it takes several hours to cover an initially virgin surface with particles from the residual gas atmosphere). 

There are several advantages of MOCVD over MBE high purity can be achieved without the need for ultrahigh vacuum since the reaction takes place in the gas phase, the deposition rate can be controlled almost as well as with MBE, production costs are lower, and the higher throughput makes commercial production feasible. 

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