Boron system, high-temperature

The electrochemical window of pure molten cryolite has not been expressly stated, but a voltammogram of purified cryolite recorded at a graphite working electrode exhibits very little residual current over the range of potentials extending from 0.4 to -1.9 V vs. a nickel wire quasi-reference electrode [7]. Physical property data for molten cryolite and phase equilibria for the AlF3-NaF melt system have been summarized [31,32]. The extremely high temperature of cryolite places severe constraints on the materials that can be used for cells. Platinum and boron nitride are the materials of choice. 

In addition to thin-film electrodes, compact diamond single crystals grown at high temperature and high pressure have become the object of electrochemical study in recent years. These so-called HTHP crystals can be obtained by crystallization from a carbon solution in a metal melt (e.g., based on the Ni-Fe-Mn system) at /arranges that correspond to the conditions of thermodynamic stability of diamond. These crystals can be also doped with boron in the course of growth. 

Sluggish reactions are often met with in boride systems even at high temperatures, especially at boron-rich compositions. This often hinders phase equihbria to be established and phase transformations under equihbrium conditions to be investigated. 

The synthesis of processable precursors for Si-B-N-C ceramics became a goal of intensive investigations as soon as the outstanding thermal and mechanical properties of this system were reported [1,2]. The amorphous phase of Si-B-N-C ceramics can show excellent thermal stability up to 2000 °C without mass loss or crystallization. The role of boron is believed to be to increase the high-temperature stability and to prevent the crystallization and decomposition of silicon nitride above 1500 °C. Primarily, the atomic ratio and chemical environment of boron in Si-B-N-C precursors seem to affect the thermal behavior of resulting ceramic materials. 

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