## Abstract

The effective thermal conductivity K_{e} of Mg_{2}NiH_{4} and MmNi_{4}FeH_{5.2} (Mm ≡; misch metal) was measured under steady state conditions as a function of the hydrogen pressure and the temperature. The effective thermal conductivity of the two hydrides increases significantly as a function of the hydrogen pressure up to approximately 40 atm and then remains almost constant. The K_{e} values for Mg_{2}NiH_{4} and MmNi_{4}FeH_{5.2} are 0.83 W m^{-1} K^{-1} and 1.05 W m^{-1} K^{-1} at 373 K and 273 K respectively and a hydrogen pressure of 40 atm. The results were analysed using the Yagi-Kunii model for the effective thermal conductivity in powder-fluid beds. The solid thermal conductivity K_{s} of the above two hydrides was then estimated. The most important and significant result of the present work is the conclusion that K_{e} "saturates" at relatively low values of K_{s}. For example, the maximum K_{e} values (above the hydrogen breakaway pressure) at 300 K and a typical hydride void fraction (ε{lunate} = 0.5) change from about 0.4 to about 1.5 W m^{-1} K^{-1} for a corresponding K_{s} change between 1 and 500 W m^{-1} K^{-1}. A survey of the existing data in the literature for the effective thermal conductivities of powdered metal hydrides supports the above conclusion. The practical meaning of this result is that, in engineering applications of hydrides in which a knowledge of K_{e} is necessary, it can safely be assumed that K_{e} ≈ 1-2 Wm^{-1}K^{-1}.

Original language | English |
---|---|

Pages (from-to) | 287-295 |

Number of pages | 9 |

Journal | Journal of the Less-Common Metals |

Volume | 104 |

Issue number | 2 |

DOIs | |

State | Published - 17 Dec 1984 |