Dr. Robert E. Salomon
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Physical ChemistryResearch InterestsLIQUID CRYSTALS Professor Salomon has been working on developing new methods for the preparationand subsequent characterization of existing and new high Temperature Superconductors.The thrust of this preparative work is the development of techniques whichwill produce near atomic mixtures of normally insoluble solid constituents. Three of the most developed of these techniques are 1) freeze dryingof aerosolyzed solutions; 2)use of liquid ammonia as a solvent system forthe initial dissolution of precursors; 3)xerogel methods in which the precursormetal ions dispersed in an aqueous gel are immobilized while water is removedfrom this gel. High temperature superconductors can not be prepared by simply meltingthe components because peritectic decomposition occurs and other more stable,but non-superconductive phase are produced. Solid state diffusion, whichwas the initial method of choice, is slow even at the relatively high temperatures(ca. 900 C) which are used in the preparation of YBa2Cu3O7-x. The longtimes required for this diffusion process leads to samples of poor phasepurity and contamination from the containers. The origin of superconductivity in these ceramic materials remains amatter of much speculation and interest. Insights into the origins of thissuperconductive behavior can often be obtained by doping pure samples withimpurities which lower the transition temperature or which affect the magneticproperties. We have participated and continue to participate, with collaboratorsat other institutions, in these experiments. The freeze drying method begins with a moderately dilute aqueous solutionof the precursor ions in the correct stoichiometry and with a decomposableanion such as nitrate. This solution is ultrasonically converted to anaerosol with small particle size and these particles are rapidly frozenin liquid nitrogen. This prevents unwanted phase separation. This metal ion doped ice isthen freeze dried to remove water while it is frozen. When suitably dehydrated,the atomic mixture of precursor nitrates is thermally decomposed usingpredetermined temperature-time profiles. A relatively short heat treatmentleads to phase pure oxide which shows high volume % superconductivity.Additional support for the thermal treatment is obtained from thermal analysismeasurements on precursors. This includes TGA and DSC. Superconductor samplesare characterized by powder x-ray diffraction, magnetic susceptibility,electrical conductivity vs temperature and more recently by measurementsof the Meissner force created between the superconductor and a permanentmagnet. The liquid ammonia method is based on the relatively good solubilityof precursor salts in liquid ammonia and the relative ease of removal ofammonia (compared to water) from the solution. Liquid ammonia solutionsof precursor metal ions have been thermally decomposed, combusted in hightemperature flames and solidified by addition of dry ice to form carbamates.The use of ammonia as the solvent system allows great flexibility and hasmore recently been used by us to prepare superconductor films by flamespraying onto a heated ceramic substrate. The xerogel method developed in our lab begins with an aqueous solutionof precursor ions which is formed into a gel by the addition of a gellingagent followed by subsequent heating. After the formation of a clear andcontinuous gel, the solution is dehydrated under vacuum at room temperature.During the course of this dehydration, the metal ions are immobilized andthe eventually dried material is close to an atomic mixture which can beheat treated to yield desired product using the mildest conditions. A fourth method, which we label an acetate method, is based on the formationof an acetate slurry using acetic acid and metal acetates in the rightstoichiometry. With a simple heat treatment, a highly viscous solutionis obtained which ultimately forms a glass. This method, while highly empirical,is rapid and is useful for the preparation of new materials. We are pioneeringin the use of the Meissner force to demonstrate or detect superconductingphases. When the fraction of superconducting phase is very low, direct electricaldemonstration of superconductivity is not possible because of the lackof continuity. We have designed and tested some experiments which can directlymeasure the Meissner force as a function of temperature. In addition, weseek to relate the Meissner force to the traditionally measured volumesusceptibility by making both type of measurements on the same samples.These methods, which have been developed in our laboratory, are describedin the literature1-19.
Superconducting and Magnetic Phase Boundaries in Bi2Sr2Ca1-xMxCu2O8with M=Y, Gd and Pr. (with Y. Gao, P. Pernambuco-Wise, J. E. Crow, J. O'Reilly,N. Spencer and H. Chen) Phys. Rev. B, 45, iss. 13, 7436-7443 (1992).
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