W. Muschik
Internal Variables in Non-Equilibrium Thermodynamics
J. Non-Equilib. Thermodyn. 15, 1990, 127-137

The concept of internal variables is introduced axiomatically into nonequilibrium thermodynamics for avoiding small state spaces showing after-effects. The connection between internal variables, projectors onto equilibrium subspace, and accompanying procecesses is demonstrated for discrete systems. Relaxation variables, chemical reactions, and Kestin-Ponter's model of plasticity are discussed.