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Comments on ” Evidence of the hydrogen release mechanism in bulk MgH 2 ”

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ABSTRACT

The effect of an electron beam induced dehydrogenation of MgH2 in the transmission electron microscope (TEM) is largely underestimated by Nogita et al., and led the authors to a misinterpretation of their TEM observations. Firstly, the selected area diffraction (SAD) pattern is falsely interpreted. A re-evaluation of the SAD pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only. Secondly, the transformation of the sample upon in-situ heating in the TEM cannot be ascribed to dehydrogenation, but is rather to be explained by the (nanoscale) Kirkendall effect, which leads to the formation of hollow MgO shells without any metallic Mg in their cores. Hence, the conclusions drawn from the TEM investigation are invalid, as the authors apparently have never studied MgH2.

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(a–c) Still frame TEM images. These images are selected from the in-situ heating video S1 (see Supplementary information) of the Mg particle that was obtained through electron beam induced dehydrogenation during the former acquisition of SAD patterns (cf. Fig. 1). Temperature and time stamp (format mm:ss) are indicated in the images, respectively. (d) SAD pattern of the sample at the end of the heating process at T = 500 °C. (e) Simulated electron diffraction ring pattern of MgO.
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f2: (a–c) Still frame TEM images. These images are selected from the in-situ heating video S1 (see Supplementary information) of the Mg particle that was obtained through electron beam induced dehydrogenation during the former acquisition of SAD patterns (cf. Fig. 1). Temperature and time stamp (format mm:ss) are indicated in the images, respectively. (d) SAD pattern of the sample at the end of the heating process at T = 500 °C. (e) Simulated electron diffraction ring pattern of MgO.

Mentions: We have also conducted an in-situ TEM heating experiment comparable to the one reported by Nogita et al.8. For this, we have studied exactly the same meanwhile fully dehydrogenated MgH2 particle that was priorily studied by electron diffraction (cf. Fig. 1). For this experiment we have used the Wildfire S3 heating holder from DENS solutions. The constant heating rate of 13 K/min is the same as the one applied by Nogita et al.8. Figure 2 shows three still frame TEM images from the in-situ heating video S1 that can be found in the Supplementary information. The images show a relative high noise level because of the reduced electron current density that was used in order to again minimize any further impact of the electron beam. Figure 2a shows a TEM image of the particle at T = 33 °C. The relatively strong contrast variations within the particle indicate a high density of defects in the nanocrystalline Mg particle. Upon annealing the sample to T = 480 °C, the contrast variations are reduced due to partial recrystallization. This becomes also obvious from video S1. At T = 486 °C, a region of bright contrast begins to form and discontinuously grows in the centre of the particle. The temperature was then kept constant at T = 500 °C, thereby allowing this reaction to complete, which took about 2 min. Figure 2c shows the fully transformed particle, which is characterized by a much brighter contrast in the core and a shell with a dark contrast. This observation is identical to the results presented by Nogita et al.8, however, with the exception that in the present case, the transformation occurs at temperatures which are roughly 50–60 K higher than in the report of Nogita et al.8. This discrepancy can either be ascribed to the uncertainty with which temperatures can be measured in a TEM heating holder. It can, however, also be due to the impact of the electron beam (the dose rate of which must be clearly higher during the experiments of Nogita et al.8), which is known to promote solid state diffusion processes10.


Comments on ” Evidence of the hydrogen release mechanism in bulk MgH 2 ”
(a–c) Still frame TEM images. These images are selected from the in-situ heating video S1 (see Supplementary information) of the Mg particle that was obtained through electron beam induced dehydrogenation during the former acquisition of SAD patterns (cf. Fig. 1). Temperature and time stamp (format mm:ss) are indicated in the images, respectively. (d) SAD pattern of the sample at the end of the heating process at T = 500 °C. (e) Simulated electron diffraction ring pattern of MgO.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5384078&req=5

f2: (a–c) Still frame TEM images. These images are selected from the in-situ heating video S1 (see Supplementary information) of the Mg particle that was obtained through electron beam induced dehydrogenation during the former acquisition of SAD patterns (cf. Fig. 1). Temperature and time stamp (format mm:ss) are indicated in the images, respectively. (d) SAD pattern of the sample at the end of the heating process at T = 500 °C. (e) Simulated electron diffraction ring pattern of MgO.
Mentions: We have also conducted an in-situ TEM heating experiment comparable to the one reported by Nogita et al.8. For this, we have studied exactly the same meanwhile fully dehydrogenated MgH2 particle that was priorily studied by electron diffraction (cf. Fig. 1). For this experiment we have used the Wildfire S3 heating holder from DENS solutions. The constant heating rate of 13 K/min is the same as the one applied by Nogita et al.8. Figure 2 shows three still frame TEM images from the in-situ heating video S1 that can be found in the Supplementary information. The images show a relative high noise level because of the reduced electron current density that was used in order to again minimize any further impact of the electron beam. Figure 2a shows a TEM image of the particle at T = 33 °C. The relatively strong contrast variations within the particle indicate a high density of defects in the nanocrystalline Mg particle. Upon annealing the sample to T = 480 °C, the contrast variations are reduced due to partial recrystallization. This becomes also obvious from video S1. At T = 486 °C, a region of bright contrast begins to form and discontinuously grows in the centre of the particle. The temperature was then kept constant at T = 500 °C, thereby allowing this reaction to complete, which took about 2 min. Figure 2c shows the fully transformed particle, which is characterized by a much brighter contrast in the core and a shell with a dark contrast. This observation is identical to the results presented by Nogita et al.8, however, with the exception that in the present case, the transformation occurs at temperatures which are roughly 50–60 K higher than in the report of Nogita et al.8. This discrepancy can either be ascribed to the uncertainty with which temperatures can be measured in a TEM heating holder. It can, however, also be due to the impact of the electron beam (the dose rate of which must be clearly higher during the experiments of Nogita et al.8), which is known to promote solid state diffusion processes10.

View Article: PubMed Central - PubMed

ABSTRACT

The effect of an electron beam induced dehydrogenation of MgH2 in the transmission electron microscope (TEM) is largely underestimated by Nogita et al., and led the authors to a misinterpretation of their TEM observations. Firstly, the selected area diffraction (SAD) pattern is falsely interpreted. A re-evaluation of the SAD pattern reveals that no MgH2 is present in the sample, but that it rather consists of Mg and MgO only. Secondly, the transformation of the sample upon in-situ heating in the TEM cannot be ascribed to dehydrogenation, but is rather to be explained by the (nanoscale) Kirkendall effect, which leads to the formation of hollow MgO shells without any metallic Mg in their cores. Hence, the conclusions drawn from the TEM investigation are invalid, as the authors apparently have never studied MgH2.

No MeSH data available.