Background

A polymeric composite material which can exhibit an active response to its environment is considered to be somewhat novel. Based on the response type, factor and the physical properties of the polymer based system such a polymer has a wide range of potential applications. LLNL’s polymer/carbon composites exhibit a large, low hysteresis temperature dependent conductivity response and in contrast to other related systems. The conductivity of LLNL’s system increases markedly as a function of increasing temperature and is directly linked to thermal phase transition of the polymeric component.

There are two kinds of polymer-based non-linear temperature responsive conductors and semiconductor composite systems, referenced with respect to the increase or decrease in resistance with increasing temperature. Both PTC (positive temperature coefficient) and the less common NTC (negative temperature coefficient) materials show a range of responses in terms of absolute magnitude of change, cycling hysteresis and temperature of transition. LLNL’s polymer/carbon composites is classed as an NTC type material.

Description

LLNL’s polymer/carbon composites exhibit a strong temperature dependent conductivity response. Below a critical temperature such as the glass transition temperature ( Tg) or melting temperature, Tm of the polymeric network, the composite material is electrically insulating, having measured conductivities in the range of 1E-10 S cm-1. Upon being heated through a phase transition, the conductivity abruptly increases; this transition has been shown to be fully reversible and with a low hysteresis upon thermal cycling. Due to the nature of the switching behavior (semiconductor gating) the polymeric component of the composite is not limited to single polymer type and may be a variety of polymer systems including various elastomers, thermoplastics or thermosets, furthermore the critical switching temperature may be altered by tuning the temperature of the polymeric component phase change.

Advantages

LLNL’s invention may be the first example of a high performance, low hysteresis negative thermal coefficient material based on a true semiconductor gating mechanism and not conductive phase percolation effects.

As 3D printing and advanced, alternative manufacturing technologies become more sophisticated, there is both a desire and a requirement for mixed material and more sophisticated feedstocks with improved, better defined properties and multi-functionality. LLNLs polymer/carbon composites represent a class of multifunctional and processable polymeric material which are thermally/electronically responsive and can be incorporated into 3D printed objects to provide temperature dependent electronic functionality (such as low hysteresis, thermally triggered, large magnitude electronic switching) in a flexible format. Examples include a rheostat, thermal ‘fuse’ sensor or even a component of a temperature responsive capacitor.

Potential Applications
  • Polymeric sensors (temperature, mechanical stress, damage, ingress or egress of solvents, water, environmental and atmospheric contaminants)
  • New material for polymeric NTC thermistors
  • Electronic switching material for flextronics
  • Self-sensing/self-reporting polymeric systems (new material for stress cushions, gaskets, seals etc. capable of self-sensing a range of phase transitions and mechanical/environmental changes.
Development Status

LLNL has filed for patent protection on this invention.

Reference Number
31616