実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report
July 24, 2012

課題番号 Project No.

装置責任者 Name of responsible person

2012A0089

Yasuhiro Miyake

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Static/dynamic behavior in MgH2 at high temperatures

D1

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Jun Sugiyama

June 06, 2012 - June 10, 2012

所属 Affiliation
Toyota Central Research and Development Laboratories, Inc.
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
Two powder samples (MgH2 and milled MgH2), which were prepared at Toyota CRDL, were packed intoa gold O-ring sealed titanium cell in an Ar-filled glove box. Then, the Ti cell was mounted on a graphite
sample holder in order to measure wTF-, ZF- and wLF-µSR spectrum at temperatures from 300 to 650
K under high vacuum. In order to avoid an unexpected explosion at high T due to the desorption
reaction, we made small holes on the window— i.e. a Ti foil with 50 µm thickness, of the Ti cell justbefore the measurements.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

Following upon the µSR measurements on a hydrogen storage material, MgH2 below 500 K, we havemeasured wTF-, ZF-, and wLF-µSR spectra for an “as prepared MgH2” and “milled MgH2” sample until
750 K, in order to study the change in µSR parameters by the H-desorption reaction. Particularly,
although there are no crucial changes in crystal structure between the “as prepared MgH2” sample and“milled MgH2” sample, the hydrogen desorption temperature (Td) for the latter sample decreases by
about 50 K compared with Td for the former sample [1].
Since the hydrogen desorption reaction of MgH2 is an endothermal reaction, it was very difficult tocontrol the sample temperature around Td. In particular, the infrared ｌamp used as a heater of thepresent oven was found to respond too fast to stabilize the temperature around Td. Therefore, for themilled sample, we obtained reliable data only below 570 K, whereas Td = 620 K. Figures 1(a) and 1(b)show the temperature dependences of the relaxation rate (λTF) of the wTF-spectrum, whichcorresponds to a spin-spin relaxation rate, i.e., the field distribution width (Δ) due to 1H nuclear field, andthe field fluctuation rate (ν) for the two samples. For the as prepared sample, both λTF and ν are almost
temperature independent up to 630 K, and then become very small, eventually 0 above Td. This means

12. 実験方法及び結果（つづき） Experimental method and results (continued)that the H-desorption reaction abrupty occures and completes at Td. In fact, at temperatures above Td,
the wTF-spectrum exhibited a non-relaxing oscillatory signal due to the applied wTF. Furthermore, thewTF-spectrum obtained in cooling mode from 750 K did not change even below Td, suggesting that all
the hydrogen atoms were removed from the sample.
On the other hand, for the milled sample, λTF gradually decreases with temperature above 470 K, whileν apparently increases with temperature also above 470 K. This suggests that the H-desorption
reaction starts to occur far below Td (=620 K) for the milled sample. Moreover, the increase in ν implies
rapid diffusion of H+ and/or µ+ in the sample, which is proposed to be a significant factor to decrease Tdfor the other hydrogen storage materials,onTdsuch as LiAlH4 [2] and M(BH4)n [2].

and/or by adding a small amount of Nb2O5,
we need more systematic measurements athigh temperatures, particularly below thevicinityofTdusing

amore

reliable

0.2

MgH2

λTF, heating
λTF, cooling 0.2
ν, heating

0.1

0.1(a)

0temperature control system.

0onTd[2] R. Kadono, et al., Phys. Rev. Lett. 100,026401 (2008).
[3] J. Sugiyama, et al., Phys. Rev. B 81,092103 (2010).

0.2

Td

milled MgH2

0.1

0.2

ν (106 s-1)

Compounds 446-447, 67 (2007).

λTF (106 s-1)

REFERENCES[1] N. Hanada, et al., J. Alloys andν (106 s-1)

mechanism on the decrease in Td by millingλTF (106 s-1)

However, in order to further elucidate theTd

λTF, heating
0.1
ν, heating(b)00300 400 500 600 700 800
TEMPERATURE (K)Fig. 1: The temperature dependences of the wTF
relaxation rate (λTF) and the field fluctuation rate (ν)
for (a) the as prepared MgH2 and (b) milled MgH2
sample. The data of λTF were obtained by fitting thewTF-spectrum using an exponentially relaxing cosine
oscillation function, and that of ν were obtained byfitting the ZF- and wLF-spectra using a dynamic
Kubo-Toyabe function. Here, wTF was 20 Oe andwLF was 10 Oe.

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