Fundamentals

The physical and structural fundamentals of polymorphism are introduced, including a review of the phase rule and the thermodynamic relations in polymorphs. The latter are used to introduce energy–temperature diagrams, leading to the definition of the concepts enantiotropism and monotropism describing the thermodynamic relationships between and among polymorphs with appropriate examples. The alternate representation of phase diagram in terms of pressure and temperature is also presented. These lead to a number of rules regarding the relationships between polymorphs and ways to understand and predict some important physical properties: the heat-of-transition rule, the heat-of-fusion rule, the entropy-of-fusion rule, the heat-capacity rule, the density rule, and the infrared rule. Structural aspects include the distinction between crystal form and crystal habit and methods for characterizing and comparing structures in polymorphic systems. Current developments are discussed that deal with the ramifications of nanoscale situations on structural concepts and thermodynamic relationships.

Superconductivity

The Meissner effect is discussed. The latent heat of transition is derived by a thermodynamic approach. The concept of surface energy is introduced, leading to the distinction between Type I and Type II superconductors. The differential equation for the superconducting wave function is derived. The energy gap and the phenomenon of tunnelling are discussed. The difference between superconducting tunnelling and Josephson tunnelling is explained. The significance of high transition temperature superconductors is discussed. It is shown that an important application of superconductors is to produce high-field magnets.

Phase change materials for thermal stabilization of composite thermistors

The objective of this investigation was to develop a triphasic PTC thermistor composite which incorporated a phase capable of absorbing heat at a critical temperature, and thus limiting deleterious effects associated with thermal runaway. The system chosen for study was pentaerythritol incorporated into a carbon black–polyethylene thermistor system. Pentaerythritol exhibits a first order tetragonal to cubic phase transition at 185 °C, with a 1.87 to 3.18 J/°C · g change in specific heat and a 425 J/cm3 heat of transition. Composites with room temperature resistivities as low as 0.1 Ω · m, a PTCR effect of up to six orders of magnitude, and reproducible temperature-cycling behavior were developed. The pentaerythritol introduced thermal delays up to 7 min at 185 °C and substantially increased the electrical and mechanical stability of the composites with temperature and voltage cycling. High fields imparted irreversible effects in these composites as reflected by an increase in the room temperature and high temperature resistivity.