Application of the Transient Hot-Wire Technique for Measurement of Effective Thermal Conductivity of Catalyzed Sodium Alanate for Hydrogen Storage

Sodium alanate, or the Na-Al-H system, has been the focus of intense research over the past decade due to its ability to hold almost 5 wt% of hydrogen. In this research, the effective thermal conductivity, k, of a sample of titanium-doped sodium alanate is studied over a range of operating conditions pertinent to practical on-board hydrogen storage. A transient technique employing a platinum hot-wire is used to make the measurements. A cylindrical experimental apparatus was designed with the aide of a finite element model that was used to quantify the cylinder boundary effects. The apparatus dimensions were optimized based on the finite element results with the goal of minimizing measurement uncertainty and temperature rise during testing. Finite element results were also used to predict test times and current requirements. A sample of sodium alanate was obtained and loaded into the experimental apparatus which was enclosed in a pressure vessel with a controlled atmosphere. Effective thermal conductivity was measured as a function of pressure at the fully-hydrided and fully-dehydrided states. The results from the pressure-dependence investigation were compared to an existing study that utilized an alternate measurement technique. The results matched well qualitatively — the effective thermal conductivity was highly dependent on pressure, and was found to be significantly higher in the fully-dehydrided state. However, the results of this study were 20 to 30% lower than the existing available data. Additionally, an exploratory investigation used the PCI technique to study the effect of varying composition between the fully-hydrided state and the intermediate decomposition step at a relatively constant pressure. Effective thermal conductivity did not vary significantly over this range of compositions.