Cure Control: Strategies for Using Dielectric Sensors

Dielectric measurements are becoming increasingly important as a means for feedback control in the area of polymer processing. The changes in dielectric response as a function of changing molecular weight or cross link density have been the subject of much research for the last 50 years [1]. Only now is process control through dielectric feedback becoming a reality due to recent advances in dielectric measurement capability [2]. Microdielectric sensors are now available which function down to frequencies characteristic of mechanical measurements (less than 1Hz) and can be inserted directly into curing composite structures. By monitoring the dielectric properties (permittivity and loss factor) at several frequencies, the ionic conductivity can be extracted [3] with the aid of commercial software packages in real time. This paper investigates the use of first and second derivative information (log ionic conductivity with respect to time) for controlling certain key processin-ing steps during composite curing

Robust penetrating microelectrodes for neural interfaces realized by titanium micromachining

Neural prosthetic interfaces based upon penetrating microelectrode devices have broadened our understanding of the brain and have shown promise for restoring neurological functions lost to disease, stroke, or injury. However, the eventual viability of such devices for use in the treatment of neurological dysfunction may be ultimately constrained by the intrinsic brittleness of silicon, the material most commonly used for manufacture of penetrating microelectrodes. This brittleness creates predisposition for catastrophic fracture, which may adversely affect the reliability and safety of such devices, due to potential for fragmentation within the brain. Herein, we report the development of titanium-based penetrating microelectrodes that seek to address this potential future limitation. Titanium provides advantage relative to silicon due to its superior fracture toughness, which affords potential for creation of robust devices that are resistant to catastrophic failure. Realization of these devices is enabled by recently developed techniques which provide opportunity for fabrication of high-aspect-ratio micromechanical structures in bulk titanium substrates. Details are presented regarding the design, fabrication, mechanical testing, in vitro functional characterization, and preliminary in vivo testing of devices intended for acute recording in rat auditory cortex and thalamus, both independently and simultaneously

Parameter estimation in equivalent circuit analysis of dielectric cure monitoring signals using genetic algorithms.

This communication concerns the treatment of dielectric data obtained from experiments following the chemical hardening process (cure) in thermosetting resins. The aim is to follow, in real time, the evolution of the individual parameters of an equivalent electrical circuit that expresses the electrical behavior of a curing thermoset. The article presents a methodology for the sequential inversion of impedance spectra obtained in cure monitoring experiments. A new parameter estimation technique based on genetic algorithms is developed and tested using different objective functions. The influence of the objective functions on the modelling performance is investigated. The new technique models successfully spectra contaminated with high noise levels. The introduction of regularization in the optimization function rationalizes the effects of outliers usually detected in cure monitoring dielectric spectra. The technique was successfully applied to the analysis of a series of spectra obtained during the cure of an epoxy thermosetting resin