Cool-down Procedure Research Articles

Factors influencing the atomization of vanadium in graphite furnace AAS

We have made the determination of V conform to the requirements of the modern (stabilized temperature) furnace technology where the integrated absorbance (A·s) signals are used to quantitate analyte volatilized into a chamber whose temperature is relatively constant during the period when the analyte peak is measured. Graphite tubes with good pyrolytic coating and fast (maximum power) heating are required. We explored the advantages of specially designed tubes, of a cool-down procedure between the char and atomization steps and of very thin platforms. We found that Mg(NO 3) 2 was advantageous as a matrix modifier. With these conditions we found no problems with several matrices reported by earlier workers to be troublesome, for example HNO 3, phosphate, Fe, Mg and Ca. However metals that form very refractory carbides, such as La, Mo, W and Zr may remain troublesome for V, probably because mixed carbides result which include VC. A group of geological samples was analysed for V. Our recommendation is the use of wall atomization from tubes with good pyrolytic coatings, Mg(NO 3) 2 as a matrix modifier and the cool-down procedure to establish a nearly constant temperature along the tube.

High-Efficiency Superconducting Homopolar dc Generators

The immediate requirement for flux pumps appears to be a design capable of producing 104 A at 100 W and high efficiency to energize large stabilized magnets. We have, however, successfully operated homopolar superconducting dc generators and also stabilized solenoids in helium vapor rather than liquid, and it appears reasonable to suggest that large magnets could be energized during the latter stages of cooldown, so that the power requirements for reasonable energization times may be less than this. Two designs for homopolar superconducting dc generators will be discussed and their performances reported on; both use permanent magnet and iron rotors. The first design has produced 1600 A and 20 W. The most significant feature of the design is the magnetic circuit used to generate in excess of 6 kG in the normal spot crossing the sheet. This enables reasonable power to be generated in a low-speed (300 rpm) design. There are, however, large losses attributed to sloshing of liquid helium. The second design is evolved from the first by enclosing the rotor in a vacuum container; special cooldown procedures are required. High efficiency is obtained (measured in excess of 100% at the time of writing, the excess being due to measurement error rather than a triumph over the second law of thermodynamics!). The design promises heavy currents and reasonable power outputs. The design allows topological manipulations of the sheet to allow adjustments of the current-carrying capacity of the sheet or output voltage (and hence power) or output impedance (and hence efficiency).