nichtisotherm

Nichtisotherm, or non-isothermal, is a term used in thermodynamics and engineering to describe processes in which the temperature of a system is not constant. It contrasts with isothermal processes, where the temperature remains fixed. Non-isothermal conditions arise when heat exchange with the surroundings is time-dependent, or when external heating or cooling is applied.

In practice, Nichtisotherm applies to any situation where temperature varies with time or position, such as heating a metal billet, cooling a reactor, or curing a polymer under a controlled heating rate. Analyses must account for the temperature history because material properties and reaction rates depend on temperature.

In kinetic and phase-transition studies, non-isothermal conditions affect observed transformation temperatures and rates, and can shift peaks or change their shape. Non-isothermal data are often analyzed with models that integrate over the temperature path or with methods designed for changing temperature, such as non-isothermal Kissinger-type analyses or isoconversion approaches, to extract activation energies and rate parameters.

Common applications include heat treatment of metals, polymer processing, and various materials science experiments, as well as geosciences where temperature changes with time or depth influence processes. Etymology-wise, the term combines nicht (not) and Isotherm (constant temperature).