Spectral analysis of heat fl uctuations in KI transient premelting states

Abstract

Nonequlibrium fl uctuations, which are of nonlinear Brownian noise by type, occur in different systems near the phase transition points. As a rule, such nonequlibrium processes are the precursors of materials fracture and degradation. Observation of the transient premelting states near the melting point Tm and anomalous temperature behaviour of some physical parameters indicate changes in the structure and properties of a solid body as it approaches the melting point. As a rule, the changes are nonlinearly dependent on heating rate. It is necessary to calculate the index of the shape of the fl uctuation spectrum to characterise the state of complex dynamic systems. The index has information about the processes
in the system and the interrelations between different subsystems. Changes in the spectral characteristics of fl uctuation processes may indicate the state of the system and also help us to develop the meth ods to predict its evolution. The aim of this study is parametrisation of heat fl uctuations in the premelting states of KI ionic crystals and the study of the dependence of spectral parameters on kinetic modes of heating.
Wavelet-analysis has been used to determine the spectral characteristics of thermal fl uctuations in the KI premelting states in various kinetic modes. Wavelet-analysis combines the capabilities of classical spectral Fourier-analysis with the capabilities of a local study of various fl uctuation and oscillating processes in frequency and time domains. It makes it possible to determine the features of the processes at various times and scales of the evolution of the system.
Wavelet transform of oscillating processes allowed obtaining information about the dynamics of the development of complex systems under various nonequilibrium conditions. It was shown that heat fl uctuations in the KI premelting states are nonlinear Brownian noise with the coeffi cient of selfsimilarity of b ~ 2. Using the Hurst parameter, the type of fl uctuation process was defi ned. It was shown that in dynamic heating modes (v = 5, 10 K/min) the fl uctuation process is characterised by oscillating nature of evolution of the “stable-unstable” type (the property of antipersistency). In quasistatic modes (v =
1 K/min) the initial tendency of the evolution of the system is maintained (the property of persistence).

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