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Abstract

In this work, the effective thermal conductivity measurements of magnesium-based metal hydrides for heat storage at high temperatures are discussed. To perform measurements under operating conditions measurement autoclaves have been developed which withstand temperatures up to 550 ⁰C and hydrogen gas pressure up to 100 bar. Such an autoclave facilitates a Transient Plane Source (TPS) sensor in order to perform thermal conductivity measurements of the high-temperature metal hydrides. The challenge in developing the thermal conductivity measurement autoclave is the electrical connection from the sensor, embedded in a powder bed of highly reactive High Temperature Metal Hydride (HTMH) under high hydrogen gas pressure and high temperature to the TPS 2500 S Hot Disk Thermal Constants Analyser was successfully accomplished. Additionally, the investigation of the thermal conductivity of packed beds of magnesium particles under hydrogen pressure showed that the Effective Thermal Conductivity (ETC) of a particle bed of smaller particles are more susceptible to the change of the gas pressure than a the ETC of a particle bed of larger particles. Increasing the packing density increases the ETC, regardless the gas pressure. Investigated magnesium hydride was purchased and nickel activated magnesium hydride and magnesium iron hydride were thermally prepared by hydrogenation under gas pressure and temperatures above 350 ⁰C. The measured ETC shows a dependency on the sample temperature and a strong dependency on the gas pressure, which is caused by the Smoluchowski effect. At temperatures above 300 ⁰C the above mentioned metal hydrides dehydrogenate once the gas pressure is below the equilibrium pressure. The thermal conductivity strongly depends on the state of the metal hydride. In dehydrogenated state the ETC values are higher and more strongly dependent on the gas pressure, which is caused by the decreased particle size of dehydrogenated metal hydrides compared to the full hydrogenated metal hydrides. Running a hydrogenation and dehydrogenation cycle test of nickel activated magnesium hydride for more than 450 cycles shows a tremendous enhancement of the measured ETC.
The maximum value for dehydrogenated nickel activated magnesium hydrides was found to be at above 8Wm^-1 K^-1 at 15 bar, 400 ⁰C and after 201 cycles. Investigation by electron microscopy presents a percolated network of dehydrogenated magnesium hydride particles which were formed by coalescence and coarsening during the dehydrogenation steps and which are responsible for this enhanced thermal conductivity. Performing ETC measurements on magnesium iron hydride showed that the thermal conductivity does not change with an increasing cycle number of de/-hydrogenation. The fact that the iron and magnesium are immiscible and do not form any alloys under these conditions are responsible for the absence of a formation of a percolated network. Therefore, no change of the ETC with increasing cycle number can be observed.