2013 Fiscal Year Annual Research Report
Mechanism of Activation of TiFe for Hydrogen Storage by High-Pressure Torsion
| Project/Area Number |
25889043
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| Research Category |
Grant-in-Aid for Research Activity Start-up
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| Research Institution | Kyushu University |
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Principal Investigator |
エダラテイ カベー 九州大学, カーボンニュートラル・エネルギー国際研究所, 学術研究員 (60709608)
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| Project Period (FY) |
2013-08-30 – 2015-03-31
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| Keywords | Hydrogen Storage / Energy Storage / Severe Deformation / High-Pressure Torsion / Activation / Deactivation / Metal Hydrides / Ultrafine Grains |
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| Research Abstract |
TiFe is a potential candidate for the solid-state stationary hydrogen storage systems, but it does not absorb hydrogen without an activation process using heat treatment because of surface oxidation. We have shown that TiFe severely deformed by high-pressure torsion (HPT) absorbs and desorbs hydrogen at room temperature without activation process. The purpose of this research is to understand the activation mechanism of TiFe by HPT and investigate the effect of processing parameters on this activation.
1. Straining by 1/4 turns of HPT was enough to fully activate the material in the first hydrogenation cycle.
2. The surface became finer after HPT and many isolated islands surrounded by dark contrasts appeared. Such islands were found to be Fe-rich and the dark borders were Fe-poor. Many cracks also were detected.
3. Transmission electron microscopy (TEM) has shown that a heterogeneous microstructure including nanograins, coarse-grains, amorphous-like phase and disordered phase as well as cracks were formed after severe plastic deformation using HPT.
4. An activated mechanism of TiFe using HPT processing was proposed such that: after processing with HPT, nanograins with a high volume of grain boundaries and cracks are formed. Ti and Fe atoms tend to segregate on the surface in TiFe. Once the hydrogen atoms reach the fresh material (metallic TiFe) and hydrides form, the sample is fragmented and high density of cracks are formed because of 10-19% volume expansion and absorption moves faster.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
So far many points regarding the activation mechanism and the effect of plastic deformation has been clarified, but there are still some other important points that should be studied. A comparison between the achievements and the research plan indicates that the progress of project in all parts was better than the original plan.
1. Investigate the effect of strain and pressure on activation for hydrogenation. After HPT for different levels of strain, the sample absorbs 1.6-2.1% of hydrogen, despite exposure to air for several hundred days. The absorption pressure decreased in the second hydrogenation cycle because of generation of defects. The P-C isotherms for the second cycle show two plateau-like regions corresponding to TiFeH and TiFeH2 hydrides. P-C isotherms also show that hydrogen atoms stay not only in the form of hydrides but also in trapping sites such as lattice defects.
2. Investigate the oxidation and surface condition of samples before and after HPT. Inspection of the samples using different surface analyzing techniques showed that he average composition and the charge of oxides across the depth was the same before HPT and after HPT. Therefore, activation of sample by HPT could not be attributed to disappearance of oxide.
3. Investigate the activation mechanism. We proposed an activation mechanism for TiFe processed by HPT. Further investigations are needed to investigate the activation mechanism by low plastic deformation during HPT or groove rolling.
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| Strategy for Future Research Activity |
Since our recent experiments showed that low strains are also effective to activate TiFe, we will mainly concentrate on HPT at low strains as well as on groove rolling.
1. Because the activation occurred even after plastic deformation at low strains (N=1/4 turns of HPT), we will investigate the mechanism of activations after HPT at low strains including after groove rolling. The effect of processing pressure is less important than the effect of strain but it is important to find the effect of hydrogenation pressure on the activation. The most effective way to change the hydrogenation pressure is addition of a third element to TiFe and process by HPT. For this part, TiFe ingots and TiFe-Mn ingots will be used as starting materials.
2. Investigate the internal microstructure of samples before and after HPT and groove rolling. Although the activation and deactivation in hydrogen storage materials occur on the surface, the surface modification and the activation must be directly or indirectly influenced by grain refinement and formation of lattice defects (grain boundaries, dislocations, point defects and disordered regions). Therefore, it is essential to investigate the microstructure of samples in detail.
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