High-temperature stability: principles

High-temperature stability principles D E PACKHAM Molecular structure and thermal stability... 

Two basic principles are commonly used for the preparation of OLEDs the sublimation method, in which the organic layers are prepared by vapor deposition results in well-defined layers of excellent purity but tolerates only low molecular mass molecules with high temperature stability [16]. The less expensive preparation out of solution, requires soluble substances or precursors [17] and is therefore widely used in combination... 

Sodium is also a very reactive metal, and with a melting point even lower than that of lithium, presents in principle problems similar to those of lithium. However, the fortunate discovery of ceramic materials which show high stability to molten sodium together with good sodium ionic conductivity at high temperature has permitted the reliable fabrication of sodium-based cells. In some sodium high temperature cells, the liquid metal is housed in closed, shaped ceramic containers. In the others, the... 

Most of these ternary compounds can, in principle, be prepared by high-temperature reactions, e.g., heating the respective elements or binary components (or both) together in sealed glass or silica ampuls. A certain knowledge of the thermal stabilities of the respective compounds is required. Separation from other phases often causes difficulties. 

An entirely different selectivity principle known as phase equilibrium comes into play in high-temperature ionic conductors. Many important gases dissolve in ionic solids at elevated temperatures. However, the solubility is rather sharply defined for the gas and the solid by the lattice parameters and the size of the gas molecule. The best example is the solubility of oxygen in zirconium dioxide. When Z1O2 is doped with yttrium ions, it exhibits a high mobility for the O anion. The solubility and anion mobility then become the basis for several electrochemical gas sensors, using yttria-stabilized zirconia (YSZ). 

Closely related to DSC is the much older technique of differential thermal analysis (DTA). DTA works on the simpler principle of measurement (via thermocouple) of temperature difference between a sample and reference as the same heating power is supplied to both. The DTA trace therefore represents a temperature effect, which is related only semiquantitatively to AH. A combined DTA-TGA trace for the Werner clathrate (Section 9.4) [Ni(NCS)2(4-phenylpyridine)4] CgHg is shown in Figure 9.22. The DTA trace shows that all three thermal events observed are endothermic. The first is associated with loss of the benzene guest, while the second and third relate to loss of the coordinated 4-Phpy ligands. Note the high temperature (about 350 °C) required to remove the enclathrated benzene. This is a clear indication of the thermal stability of the Werner clathrate family.18... 

In all of the discussion of this chapter we have used an aqueous solution as the electrolyte, and electrodes suitable to those aqueous solutions. However, cells are not limited to aqueous solutions. Indeed, other solvents have been used for which liquid ammonia would be an example. Molten salts, such as mixtures of lithium chloride and potassium chloride, have been used for the study of cells at high temperatures. Some studies have been made at higher temperatures, in which solid electrolytes were used. Electrodes compatible with such solvents have also been devised. For example, a zirconium-zirconium oxide electrode stabilized with calcium oxide was used to measure the oxygen potential in nonstoichiometric metal oxides. However, no matter what the electrolytes or the electrodes are, the principles discussed in this chapter such as reversibility and proper measurement must be followed. 

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