Experimental studies of the effect of formation conditions on the microstructural state of hydrogen storage alloys based on zirconium :: SPE RESST

Experimental studies of the effect of formation conditions on the microstructural state of hydrogen storage alloys based on zirconium

One of the most likely substitutes for fossil fuels for transport and energy is hydrogen. For the transition to hydrogen energy, it is necessary to solve a number of scientific and technological problems of obtaining rather cheap hydrogen on an industrial scale, its storage, delivery and efficient use.

Among the technological solutions to the problem of hydrogen storage, the most promising is the metal hydride technology, based on the accumulation in so-called hydrogen storage alloys (HSAs), and followed by the desorption when the alloy is heated. Now the main efforts of the researchers are aimed at achieving greater hydrogen capacity of metal hydrides, improving sorption-desorption conditions and increasing the cyclic stability of hydrogen accumulators.

Earlier, the optimization of the properties of Hydrogen Storage Alloys was carried out by manipulation with its chemical consist. However, in recent years, the connection of their microstructure (amorphous, micro- and nanocrystalline, quasi-crystalline, nanocomposite) with the storage capacities of HSA has been actively studied. For example, due to amorphization of the structure, researchers achieve an increase in the hydrogen capacity of zirconium-based alloys by 1.8 times in comparison with samples of the same consist in the microcrystalline state.

The most perspective direction for increasing the hydrogen content is the creation in HSA of a structure with nanoscale cluster elements and the synthesis of alloy-composites with ultradisperse inclusions.

To create a nanostructured state in zirconium and its alloys, the method of intensive plastic deformation (IPD by precipitation-extrusion followed by drawing) and the high-energy milling method were chosen. Zirconium was used as a model material for studying the influence of the degree of deformation on the processes of structure formation and the properties of a highly deformed material.

Effect of IPD conditions on the structure and properties of zirconium

For experiments we obtain the initial ingots of high purity zirconium by the method of electron-beam melting of iodide zirconium. The ingots had a coarse-grained structure (grain size of the order of 1 mm).

Electron microscope studies showed (see Figure 1) that after one and a half cycles of precipitation-extrusion, the structure had a high degree of homogeneity, the grain size (subgrain) being 0.4 μm (mikrometer). Further heat treatment at 580 °C for 3 hours resulted in complete recrystallization with the formation of grains with an average size of 15 μm.

Figure 1 - Electronically microscopic structure of intensely deformed zirconium

In Table. 1 it is showen the effect of annealing regimes on the relative electrical resistivity and the average grain size of zirconium.

Table 1 - Relative electrical resistance and grain size in zirconium, depending on the production regimes

State of Sample	Relative electrical resistance R₂₉₃/R₇₇	Average grain size
Original rod Ø 10 mm, ТО 580 °С/3h	6,22	15 μm
Original strip thickness h=250 μm	4,41	70 нм
Strip after annealing 325 °С/1h	5,62	75 nm
Strip after annealing 580 °С/1h	6,32	20 μm

Measurements of the relative electrical resistance indicate the ability of this method to reduce the average grain size in the material from 15 μm to 70 nm. It is maintained at annealing temperatures up to 325 °C, exceeding the boundary of the required operating temperature range of hydrogen storage alloys.

Investigations of the hydrogenation parameters of samples of ZrV₂ alloys ground in a ball mill under various conditions have been carried out. Earlier, we showed that these samples are either amorphous (long-range order is absent), or the size of the crystallites (coherent scattering regions) is less than 15 nm. The results of studies of the activation temperature and the amount of hydrogen absorption are shown in Table 2.

Table 2 - Parameters of hydrogen sorption by ZrV₂ alloys under different milling conditions

Sample	Т activation, °С	Amount of sorbed Н₂
Milled in Ar, t=30min	385	0.6 mass.%
Milled in ethanol, t=2h	280	0.83 mass.%

We determine that the environment and the process of manufacturing the hydrogen storage alloy have a significant effect on its operating parameters (up to 35% growth of the hydrogen capacity, up to 20% decrease in the activation temperature of hydrogen absorption).

As a result of experiments on the example of the ZrV₂ alloy it was established that the conditions of intensive deformation by the ball grinding method significantly affect the structural state of zirconium alloys and the parameters of their sorption by hydrogen. In connection with this, further research is needed on the conditions of obtaining the structure and properties HSA.