Thermodynamic Phase Change Research Articles

Modelling general anaesthesia as a first-order phase transition in the cortex

Since 1997 we have been developing a theoretical foundation for general anaesthesia. We have been able to demonstrate that the abrupt change in brain state brought on by anaesthetic drugs can be characterized as a first-order phase transition in the population-average membrane voltage of the cortical neurons. The theory predicts that, as the critical point of phase change into unconsciousness is approached, the electrical fluctuations in cortical activity will grow strongly in amplitude while slowing in frequency, becoming more correlated both in time and in space. Thus the bio-electrical change of brain-state has deep similarities with thermodynamic phase changes of classical physics. The theory further predicts the existence of a second critical point, hysteretically separated from the first, corresponding to the return path from comatose unconsciousness back to normal responsiveness. There is a steadily accumulating body of clinical evidence in support of all of the phase-transition predictions: low-frequency power surge in EEG activity; increased correlation time and correlation length in EEG fluctuations; hysteresis separation, with respect to drug concentration, between the point of induction and the point of emergence.

Heat capacity in bits

The temperature T may be expressed as the rate of energy increase per unit increase in the state uncertainty under no-work conditions. The consequences of such a choice for heat capacities are explored. I show that the ratio of the total thermal energy E to kT is the multiplicity exponent (log–log derivative of the multiplicity) with respect to energy, as well as the number of base-b units of mutual information that is lost about the state of the system per b-fold increase in the thermal energy. Similarly, the no-work heat capacity CV is the multiplicity exponent for temperature, making CV independent of the choice of the intensive parameter associated with energy (for example, kT vs 1/kT) to within a constant, and explaining why its usefulness may go beyond the detection of thermodynamic phase changes and quadratic modes.

Phase change behavior of polyethylene glycol in blends with cellulose

Abstract Thermodynamic properties and phase change behaviors of polyethylene glycol (PEG) in blends with cellulose (CELL) were found to be completely different than those of pure PEG. When the CELL fraction of the blend was larger than 5 wt%, PEG within the blend did not melt into a liquid state, as was the case with pure PEG, even at a temperature over 50°C above its melting point. Instead of fusion, a solid-solid phase transition was found in these PEG-CELL blends with an enthalpy as large as 100 J/g.

The metastable states of some amorphous FeB alloys

The interdependence of the electronic and thermal properties and their effect on the stability of some Fe 86.5− x TM x B 13.5 (TM is transition metal, 0≦ x ≦5) alloys is investigated. Differential thermoanalysis and soft X-ray fluorescent spectroscopy were used to measure the thermodynamic phase changes and the partial electronic density of states, respectively. A general role of the electronic effects on the thermal stability is suggested from the trend of the measured data. It is concluded that the thermal stability is effectively controlled by the actual electronic structure of the alloy.

Chemical equilibrium calculations on the molecular-to-nonmolecular transition of shock compressed liquid nitrogen

Chemical equilibrium calculations are reported on the dissociation of shock-compressed liquid nitrogen to infer the monatomic nitrogen–nitrogen potential in the condensed dissociated phase at high temperature. The analysis suggests that two types of interatomic potentials with very different physical characteristics can about equally describe experimental shock-wave data. The two differ in the interatomic potential parameter α, characterizing the stiffness of the exponential repulsion, and the parameter ε which scales the repulsive and attractive interactions. The first type, with a large α and a small ε, produces a continuous shock dissociation without thermodynamic phase change, and the second, with a small α and a large ε, predicts that the shock dissociation can accompany a first-order phase change. The predicted first-order phase change is new and similar to a theoretically predicted plasma phase transition. The calculation suggests that the transition in nitrogen at high compression may be associated with a physical change at the atomistic level from a van der Waals-type to a much stronger (metallic, semimetallic, or covalentlike) interaction. The implications in interpreting shock-wave data and possible future experiments to distinguish the two potentials are also discussed.

Further evidence of the low temperature phase transformation in Nb 3Sn and V 3Si

Low temperature X-ray diffraction studies show that the cubic → tetragonal martensitic transformation in Nb 3Sn and V 3Si is accompanied by a change in volume and is therefore a first order thermodynamic phase change. The axial ratio of the Nb 3Sn martensite is confirmed to be less than unity.