Detailed Mapping of Reaction Diagrams for Metastable Phases
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Detailed Mapping of Reaction Diagrams for Metastable Phases Bertil Sundqvist Department of Physics, Umeå University, SE-90187 Umeå, Sweden
ABSTRACT A method is presented to map reaction diagrams for metastable phases by treating a rodshaped sample under high pressure in a known temperature gradient, such that each point on the sample is treated at a unique, known temperature. After quenching to ambient conditions the structural properties of the sample can be studied by spectroscopic or other techniques. The accuracy and resolution in temperature is mainly limited by the uncertainties in the measured temperature distribution during treatment and the uncertainty in position during analysis. The method is tested by re-mapping the polymerization reaction of C60 under high pressure in the range 0.8 – 2 GPa. Preliminary data show surprisingly sharp "reaction boundaries" at certain temperatures, signalling the onset of dimerization and polymerization of the material. INTRODUCTION One of the largest sub-fields in High Pressure Materials Science is the use of high pressure as a tool to create new materials with novel, interesting and advanced properties. The ultimate goal is usually to be able to bring such materials back to atmospheric pressure in the form of metastable structures or phases. When such metastable phases have been identified, it is useful to make a reaction map to optimize the reaction conditions for practical production of the new material. Making such a map can be extremely time-consuming, since a large number of high pressure experiments need to be carried out, and because of the unavoidable measurement errors in temperature and pressure in each experiment the details of such maps are not always very accurate. As a case in point, the high pressure polymerization reaction of C60 has been studied by many authors (see reviews [1-3]), but there are still many gaps in our knowledge and in many cases there is a strong disagreement as to what polymeric structures, and in what ratio, are produced after treatment at a given pressure and temperature. In this contribution, a simple method to speed up the mapping of such diagrams is presented. In this method a single sample is treated over a wide range of temperatures at constant pressure by submitting a long thick film (or rod) specimen to a known temperature distribution. After quenching, the sample is recovered at atmospheric pressure and any metastable phases can be studied by spectroscopic or other methods. Because the treatment temperature at any point on the sample is known, the sample provides in effect a constant-pressure profile through the pressure-temperature plane along its length. The pressure error should be practically the same at all points, and at least the relative temperatures will always be very well defined compared with those measured in a set of individual single-point experiments.
This contrbution gives a detailed description of a practical realization of the method, discussing many experimental problems. To illustrate the usefulness of th
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