# Growth of Functional FeTi Clusters Covered with Carbon Layer

Authors: Chihiro Kaito; Akihito Kumamoto; Ryoichi Ono; Yoshio Saito; Shigeru Morikawa
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

FeTi clusters with a diameter of less than 10 nm and covered with a graphitic layer have been preferentially produced in anH2 gas atmosphere at pressures of 10 and 26.6 kPa by the simultaneous evaporation of Fe and Ti wires from a concave carbon boat. To compare this result with cluster formation in an inert gas atmosphere, the result for an Ar gas pressure of 10 kPa is also discussed. The formation of disordered FeNi clusters predominately took place in an H2 gas atmosphere.


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## Body

## 1. Introduction

Hydrogenis an ideal fuel for all types of fuel cell. It can be stored as a compressed gas, a liquid, and solid as a combined with other elements.
For hydrogen storage, the  ideal type of hydride is a metal or alloy hydride that reversibly absorbs and desorbs hydrogen.
A typical example is FeTi alloy: FeTi +H2→ FeTiH2  [1].   The range of  pressures
of interest for a hydrogen storage system in fuel cell applications
is about  (1to5)×105Pa.
Also, it is necessary
for hydrogen desorption to occur below 100°C. FeTi and VNb alloys are potentially
suitable alloys. Alloys
are generally preferentially produced by
the mechanical alloying method. The oxide layer on a
metallic
powder surface is a practical problem during the alloy formation. Carbon nanotubes
may also be good hydrogen absorbers [2].
Since nanotubes are based
on the graphite structure, nanographitic carbon particles may also act as a hydrogen absorber. Most
metallic
particles of the order of 50 nm can be
produced by the gas evaporation method in an inert gas [3]. Metallic
clusters covered with an insulator layer of SiO2 or an electrically
conductive carbon layer can be produced by the advanced
gas evaporation method [4, 5].In this paper, a new production
method for the formation of FeTi clusters of less than 10 nm diameter covered with a carbon layer has been reported. In the present study, hydrogen gas has been used to replace the inert gas.


## 2. Experimental

To produce the
FeTi alloy, Fe and Ti wires of 5 mmϕ were inserted into a carbon rod with a concave shape as shown in Figure 1. The length of the concave region
was 10 mm. Fe and Ti wires with a composition
ratio of 50:50 atomic percent were
inserted in the concave region. Two carbon rods were pushed together by springs
inserted the stainless-steel electrodes as schematically
shown in Figure 1(a). The work chamber used was a glass cylinder
17 cm in diameter and 30 cm in height,
covered with stainless steel plate, and
connected to a high vacuum exhaust through a valve at its bottom.
The preset method is very similar to the arc evaporation method of carbon [6]. To compare
the produced materials with those
obtained by inert gas evaporation,
the Fe and Ti wires were evaporated
in Ar gas at 10 kPa and hydrogen gas at 10 and
26.6 kPa. The collected samples were examined using a Hitachi  H-9000NAR electron microscope.Schematic representation of present production method. (a) Two carbon rods were pushed together by springs. (b) Concave region of 3 mmϕ as in the diagram. Ti and Fe wires were set in the concave region.
(a)(b)


## 3. Results and Discussions

Figure2
shows a typical electron microscopic (EM) image and the corresponding
electron diffraction
(ED) pattern for the samples produced in
Ar gas at a pressure of 10 kPa. The ED pattern   shows the formation of a mixture of Fe, Ti, FeTi, and TiC phases. The
large  particles in the EM  image are Fe
and Ti particles [3]. The Ti particle surface was covered with a TiC layer [7]. Each particle was covered with a carbon layer. The
FeTi particles were in an ordered phase.EM image and corresponding ED pattern of the particles produced in Ar gas pressure of 10 kPa. The spotty diffraction spots were Fe and Ti particles. Rings were indexed as FeTi and TiC nanoparticles, which are indicated in (b). An FeTi ordered phase appeared.
(a)(b)When the same experiment was performed in hydrogengas, the collected
particles were indexed as the disordered FeTi phase as shown in Figure3. The dark field image clearly shows that the FeTi cluster formation is less than 10 nm in size. Figure 4 shows a high-resolution
transmission electron microscopic (HRTEM) image of an FeTi nanoscale particle covered with a graphitic carbon layer. The preferential formation of the FeTi disordered
phase in hydrogen gas may be due to the fact that Fe, Ti, and FeTi are typical hydrogen absorbing
materials, and that Fe and Ti particles
produced in Ar gas, as shown in Figure 2, were destroyed by the absorption of hydrogen resulting in the formation
of more stable FeTi. Using the smoke particle formation
process, growth by coalescence
among the particles predominately occurs [8]. Thus, stable FeTi nanocrystallites
can be produced. When the hydrogen pressure was 10 kPa, the FeTi crystallites were slightly larger than those formed at 26.6 kPa, as recognized
by the sharpness of the diffraction
pattern. Growth of the FeTi particles of the order of 10 nm was preferentially observed. This result also suggests that the hydrogen gas restricted the formation and the growth
of alloy particles. The alloy clusters produced
in the H2 gas atmosphere were covered with a carbon layer, thus preventing
oxidation of the metallic
alloy clusters.
Fundamental experiments on hydrogen absorption are now being carried out by another laboratory
group. The results will be presented elsewhere.
The absorbent alloy formed by the present
method is expected to be used as another hydrogen permanent absorbent material.
A systematic
study on the formation of the alloy is now being carried
out by our laboratory group, and the
results will be published elsewhere.EM images ((a) bright and dark field) and corresponding ED pattern of the specimen produced inH2 gas at 26.6 kPa. Nanocrystallites of diameter less than 10 nm were clearly seen in dark field image. (b) The ED pattern can be indexed by disordered FeTi. The appearance of (111) suggests that the FeTi nanoparticles may include hydrogen atoms.
(a)(b)Figure 4
HRTEM image of FeTi particles produced inH2 gas atmosphere. Particles were covered by a graphitic carbon layer as indicated by arrows. The left top enlarged image of (110) FeTi shows the Guinier-Preston zone-like contrast
[8]. This suggests that hydrogen atoms are dissolved in the FeTi crystal.



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*Source: 734940-2008-12-22.xml*