At superconductivity

If it enters in 1,2 or 3,4 fashion, chiral centers are introduced and relative stereoisomerism must be considered. Actually, as it turns out, things are not so complicated as they might be because of a strong preference for one mode of addition. Complications ate quite sufficient, however, as they include a head-to-head head-to-tail option not normally open to vinyl monomers but available to unsymmetrically substituted dienes adding in 1,4-fashion. In fact, little progress can be made in structural determination except by use of C-13 nmr at superconducting frequencies. 

I have discussed two cases of chain microstructure determination in SO2 copolymers. First, the styrene-S02 system, which exhibits the general kinetic and compositional behavior of such systems, particularly as a function of polymerization temperature. Second, and considerably more complicated, is the chloroprene-S02 system. This one represents the limit of what can be handled and more or less completely solved at the present time. To do so requires about all o ir resources at superconducting frequen-... 

Finally, a very large and exciting field of polymer nmr studies is that of the structure and dynamics of biomolecules. Here, proton spectroscopy (at superconducting frequencies) remains dominant. I illustrate this by an example of Dinshaw Patel s studies of nucleic acids and oligonucleotides, which shows the use of H— H nuclear Overhauser enhancement (NOE) to explore the binding site of the antibiotic netropsin... 

Materials which become super conductive at higher temperatures than the boiling point of helium could have a major impact on the demand for helium. These less costly refrigerant materials could replace the present need to cool superconductive materials to the boiling point of helium. 

It is widely used as filaments for mass spectrographs and ion gauges. Rhenium-molybdenum alloys are superconductive at 10 K. 

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described. 

Properties. Pure thorium metal is a dense, bright silvery metal having a very high melting point. The metal exists in two allotropic modifications. Thorium is a reactive, soft, and ductile metal which tarnishes slowly on exposure to air (12). Having poor mechanical properties, the metal has no direct stmctural appHcations. A survey of the physical properties of thorium is summarized in Table 1. Thorium metal is diamagnetic at room temperature, but becomes superconducting below 1.3—1.4 K. 

Titanium alloyed with niobium exhibits superconductivity, and a lack of electrical resistance below 10 K. Composition ranges from 25 to 50 wt % Ti. These alloys are P-phase alloys having superconducting transitional temperatures at ca 10 K. Thek use is of interest for power generation, propulsion devices, fusion research, and electronic devices (52). 

---