Physical aging of plastoferrites under tensile stress and its effect on microwave properties
Authors: Brosseau C. , NDong W. 
Published: September 2008

Aging phenomena in soft viscoelastic materials have been used as an important tool to investigate the physics of complexity for both scientific and practical purposes. At ambient conditions, microwave frequency-domain spectroscopy (gigahertz-FDS) is employed to investigate the electromagnetic properties at continuum length scales for two commercial plastoferrites (PFs) subjected to a uniaxial tension during a long time t of application. PFs are an attractive prototype for investigating the interactions between polarization and magnetization phenomena, and time. For both PFs we have measured simultaneously the time evolution of the effective complex permittivity ε=ε′−jε″ and magnetic permeability μ=μ′−jμ″ spectra during tensile-loading experiments. These changes allow a determination of how physical aging affects the microwave properties of PFs with tensile elongation in the range of 3%–17% on the time scale of hundreds of hours. At short times of strain application the ε and μ measurements under stress can be explained in terms of a Gaussian molecular network model (affine behavior) in the limit of low strain. However, the large-strain mechanical response does show nonaffine behavior. The key achievement is that we present observational evidence for a three-stage aging in which (“zone I”) ε′ and ε″ increase as the ln(t) and reach a peak value at tI, then (“zone II”) there is a slower decrease in ε′ and ε″∝−ln(t) as time moves from tI up to tII, and finally, (“zone III”) as time moves to higher values, ε′ and ε″ saturate for long times. We have provided experimental evidence that, while tI is found to be constant with the initial elongation ratio λ0 that is applied to the PF sample, the data revealed that tII∝λ0. In sharp contrast, there is no significant change in μ′ and μ″ as a function of the elongation ratio. It can be argued that the decoupling of the dielectric properties from the magnetic properties may be regarded as a consequence of rigidity of the ferrite grains. To probe the influence of aging that alters the initial morphology of the PF samples, scanning electron microscopy (SEM) was used. The results of the SEM studies indicate the existence of cracks at the PF surface and disagglomeration of the ferrite grains. A tentative phenomenological model is proposed to explain the three-stage evolution of the elasticity network which has been tracked by gigahertz-FDS during the isothermal application of tension. Because the structure of soft materials is scale dependent, the response to a mechanical load also depends on the length and time scales of the probing excitation. We argued that the effective permittivity increase during stage I can be attributed to the electric dipole reorientation with respect to the direction of the polarization under the tensile stress applied to the material. The physical mechanism driving the effective permittivity decrease in stage II is thought to be a consequence of the breakup of the adsorbed ferrite particle network due to the detachment of chains from the surface of the particles. Stage III reflects the eventual stable steady state for which local stresses are redistributed in the elasticity network of the filled polymer under strain. The understanding of the physical aging mechanisms is significant because it can offer a strategy for materials selection and PF-based device performance optimization.