Application of new materials for integrated low-energy ignition elements in airbag systems - characterization and simulation

The direction of current research activities by international producers of ignition elements for airbag systems is developing cheap elements which are possible to be integrated into an ignition microsystem. These integrated ignition element is the most important actuator in the whole airbag system.
In conformity with the integration of ignitor structure and electronic parts Si became the basic material. Related materials were selected for functional coatings.
The investigations involved preparation, characterization, testing, simulation and reliability of the functional coatings for ignition structures.
Poly-Si was used as reference material to compare our results with the literature. Furthermore hydrides of titanium, zirconium and hafnium were investigated. Optimaly conversions of the metallic coatings (deposited by sputtering) after a heat treatment in a hydrogen atmosphere could be observed by X-Ray Diffraction and the nuclearphysical 15N-method (Figure 1).
After a heat treatment (110°C, on air) the evaluation of stability over the time was performed by analysing the sheet resistance for TiHx- and HfHx - probes. The divergence of the resistance values related to the start value (without heat treatment) amounts 5% for TiHx and 1% for HfHx. Accordingly to /1/ the thermal stability of the metal hydrides increases from Ti to Hf.
To obtain suitable design parameters for the ignition elements, a coupled electrical and thermal simulation was performed by the commercial ANSYS FEM program.
Figure 2 shows an example of the good agreement between the curves of the simulated thermal behavior and the measured current.

/1/ Streb, Wasserstoff in den Metallen Titan, Zircon und Hafnium, Thesis, Frankfurt am Main, 1984
/2/ Weiß U., Löbner B., Abschlußbericht SIFA-MST, "Schneller Anzünder", Januar 1999
/3/ Weiß U., Dünnschichtanzündelement für pyrotechnische Wirkmassen, Patent DE 19721929