Enhancing hydrogen-generation performance of γ-Al2O 3 modified al powder by ultrasonic dispersion

Article abstract of DOI:10.1111/j.1551-2916.2012.05105.x

Previous work indicated that γ-Al₂O₃ modified Al powder could react with water and generate hydrogen under ambient conditions. In this work, an ultrasonic dispersion procedure was used to disperse and mix the Al and γ-Al₂O₃ powders. It was found that γ-Al₂O₃ modified Al powder prepared by the ultrasonic procedure has a higher hydrogen-generation speed than that prepared by a traditional ball-milling method. Microstructure analyses showed that the ultrasonic procedure greatly eliminated the agglomeration of Al and γ-Al₂O₃ powders so that γ-Al₂O ₃ grains covered Al particle surfaces more uniformly. This implies that a favorable processing procedure is the key to obtain high performance modified Al powder.

Full text of DOI:10.1111/j.1551-2916.2012.05105.x

J. Am. Ceram. Soc., 1–4 (2012)
DOI: 10.1111/j.1551-2916.2012.05105.x
©
2012 The American Ceramic Society
ournal
J
Enhancing Hydrogen-Generation Performance of c-Al O Modified
2
3
Al Powder by Ultrasonic Dispersion
‡
‡
‡,†
§
Wen-Hui Liu, Wei-Zhuo Gai, Zhen-Yan Deng, and Junwang Tang
‡
Energy Materials and Physics Group, Department of Physics, Shanghai University, Shanghai 200444, China
§
Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, U.K.
Previous work indicated that c-Al O3 modified Al powder
could react with water and generate hydrogen under ambient
conditions. In this work, an ultrasonic dispersion procedure
fied Al powders prepared by the ultrasonic procedure and
ball milling was performed.
2
was used to disperse and mix the Al and c-Al
2 3
O powders. It
II. Experimental Procedure
was found that c-Al O modified Al powder prepared by the
2
3
ultrasonic procedure has a higher hydrogen-generation speed
than that prepared by a traditional ball-milling method. Micro-
structure analyses showed that the ultrasonic procedure greatly
Three kinds of Al powders with the average sizes of
72.94 nm, 0.98 lm (Shanghai St-nano Sci. & Tech. Co.,
Shanghai, China), 1.23 lm (99.9% purity; Henan Yuan
eliminated the agglomeration of Al and c-Al O3 powders so
Yang Aluminum Industry Co., China), respectively, and c-
2
2
powder (99.99% purity, surface area 190 m /g, Taimei
that c-Al
formly. This implies that a favorable processing procedure is
2
O
3
grains covered Al particle surfaces more uni-
Al
2
O
3
Chemical Co., Nagano, Japan) were used in the present
research. Two processing procedures, i.e., ultrasonic disper-
sion and ball milling were used to prepare the mixtures of
the key to obtain high performance modified Al powder.
2 3 2 3
Al + c-Al O . For ultrasonic dispersion, the Al and c-Al O
I. Introduction
powders with a total weight of 5 g were put into a beaker
with 150 mL of highly pure ethanol, and then the mixture
solution was ultrasonically dispersed in an ultrasonic bath
(Type: SCQ100, frequency 40 kHz, power 100 W, Shanghai
Shengpu Equipment Co., Shanghai, China) with water for a
time period. For ball milling, the Al and c-Al O powders
NERGY is the basis for human survival and development.
However, now, the fossil fuels which people mainly
depends on is on the verge of depletion, so developing new
clean and renewable energy technologies is essential and
urgent. Hydrogen is an ideal fuel due to its high calorific
E
1
2
3
2
value and environmentally benign oxidation product.
with a total weight of 5 g were put into a Teflon bottle with
Hydrogen materials become an important research field
because storage and transportation of hydrogen remain a
problem.
150 mL of highly pure ethanol and 120 g of Al balls, and
then ball-milled for a time period using a tumble milling
machine (Hefei Kejing Equipment Co., Hefei, China) with a
2 3
O
2,3
Metal Al is a particularly interesting hydrogen material
due to its light atomic weight, high electron density, abun-
dance in the earth and low cost relative to other hydrogen
2 3
rotational speed of 90 rpm. The new Al O balls were ball-
milled in highly pure ethanol for 48 h and then washed
before they were used in the experiment. Three kinds of
3–6
materials. However, direct reaction of Al with pure water
is difficult because of a dense passive oxide film covering on
2 3
highly pure Al O balls (High Purity Chemical Co., Tokyo,
Japan) with the sizes of 2.0, 1.5, and 1.0 cm, respectively,
were mixed with a weight ratio of 12:3:2 and then used in
the present ball-milling experiment. Both of the Al + c-Al O
7
Al particle surfaces. Recently, Deng et al. found that metal
Al particle surfaces could be modified by fine c-Al O grains
2
3
2
3
using a ceramic processing procedure, and the c-Al
fied Al powder can continuously react with water and gener-
2
O
3
modi-
mixture solutions prepared by the above procedures were
dried. The dried mixtures of Al and c-Al were sieved
2 3
O
ate hydrogen under an ambient condition.
using a 100-mesh nylon sieve and then pressed under a unidi-
rectional pressure of 60 MPa to form the green compacts.
The compacts were heat-treated at a heating rate of 1°C/min
and held at a temperature of 400°C under vacuum for 1 h.
The heat-treated compacts were crushed into powder and
sieved using a 100-mesh nylon sieve, and finally became c-
8
,9
Previous results indicated that a uniform distribution of
modification agents over Al particle surfaces is important for
the Al-water reaction dynamics. However, usually fine pow-
ders are strongly agglomerated, especially for nanometer
powders. In the previous works, the mixtures of Al + c-
Al
to mix and disperse micrometer Al particles and nanometer
2
O
3
were prepared by ball milling, which is not sufficient
2 3
Al O modified Al powders (GMAPs). All the GMAPs in
this research have the same composition of 70 vol% Al
+ 30 vol% c-Al O .
9
c-Al O3 grains. To effectively eliminate the agglomeration,
2
2
3
1
0,11
an ultrasonic processing procedure
was used to disperse
The hydrogen-generation experiment of GMAPs with
water was carried out in a closed glass reactor with a volume
of ~1.4 L, which was airtight and could keep a constant gas
pressure of <3 bar for more than 1 week. One gram of
GMAPs was used in each test, which was suspended in
Al and c-Al powders in the present work. A comparative
2 3
O
2 3
study for the hydrogen-generation behavior of c-Al O modi-
2
50 mL of deionized water. At the beginning, the reactor
E. Dreizin—contributing editor
was filled with the ambient air and the initial gas pressure in
the reactor was 1 bar. The hydrogen evolution could be
determined by the gas pressure in the reactor, which was
measured by a manometer with an accuracy of 0.2 kPa
(Type: LEO 2, Keller Co., Switzerland), using the ideal-gas
Manuscript No. 30723. Received November 25, 2011; approved January 13, 2012.
Author to whom correspondence should be addressed. e-mail: zydeng@shu.edu.cn
†
1

2
Rapid Communications of the American Ceramic Society
equation
a ¼
(arrows), and it seems that the agglomeration of nanometer
powders [Figs. 1(a) and (d)] is more serious than that of
micrometer powders [Figs. 1(b) and (c)].
ðP ꢀ PvaporÞðVreactor ꢀ VH O ꢀ VAlÞ
2
;
ð1Þ
n0RT
Figure 2 shows the H evolution from the deionized water
2
at 25°C containing the GMAPs with three different Al parti-
cle sizes (72.94 nm, 0.98 lm, 1.23 lm), where all the
GMAPs were ultrasonically dispersed or ball milled for the
same time period, i.e., 1 h. It can be seen that generally, the
hydrogen-generation speed of GMAP increases with decreas-
ing the average Al particle size. For all the Al powders with
different particle sizes, the hydrogen-generation speed of the
GMAPs prepared by ultrasonic dispersion is faster than that
prepared by ball milling, indicating that the ultrasonic pro-
cessing procedure is more effective to disperse and mix Al
where a is the hydrogen yield; P is the total gas pressure in
the reactor; Pvapor is the initial pressure in the reactor, which
was measured before the hydrogen-generation test; Vreactor
VH2O, and VAl are the volumes of the reactor, water, and Al
,
powder, respectively; n is the theoretical hydrogen moles by
0
reacting all of metal Al; R is the gas constant and T is the
reaction temperature. Moreover, at least two same tests were
carried out to check the reproducibility of each hydrogen-
generation curve.
An X-ray diffractometer (XRD; Model No. D/max-2550,
Rigaku Co., Japan) was used to analyze the phases in pure Al
powders and the GMAPs after reaction with water. Scanning
electron microscopy (SEM; Model No.JSM-6700F, JEOL,
Japan) was used to observe the morphologies of pure Al pow-
ders and the cross-sections of the modified Al compacts.
and c-Al O grains. Figure 2 also shows that the hydrogen-
2
3
generation speed of the GMAP with nanometer Al particles
[72.94 nm, Fig. 2(a)] is not considerably faster than that of
the GMAPs with micrometer Al particles [0.98, 1.23 lm,
Figs. 2(b) and 2(c)], though the size of nanometer Al powder
is much smaller than that of micrometer Al powders. This
suggests that the agglomeration in nanometer Al powder is
stronger and agglomerates are not easy to be completely
eliminated, as evidenced in Fig. 1. In fact, among the above
three Al powders, the improvement in hydrogen-generation
performance by ultrasonic dispersion is best for submicrome-
III. Results and Discussion
Figure 1 shows SEM micrographs of three as-received Al
2 3
powders and c-Al O powder. It can be seen that all of the
micrometer and nanometer powders have many agglomerates
(
a)
(b)
(
c)
(d)
Fig. 1. SEM micrographs of as-received Al powders with the average sizes of (a) 72.94 nm, (b) 0.98 lm, (c) 1.23 lm, and c-Al
2 3
O powder (d),
showing the agglomeration of the Al and c-Al particles (arrows).
2 3
O
Fig. 2.
and ball milling for 1 h (○), where the average Al particle sizes in GMAPs are 72.94 nm (a), 0.98 lm (b), and 1.23 lm (c), respectively. All the
GMAPs have the same composition of 70 vol% Al + 30 vol% c-Al and 1 g of GMAPs was used in each test.
2
H evolution (a) from deionized water at 25°C using the GMAPs prepared from two different processes: ultrasonic treating for 1 h (■)
2 3
O

Rapid Communications of the American Ceramic Society
3
ter Al powder [0.98 lm, Fig. 2(b)], indicating that the effect
of ultrasonic dispersion on nanometer Al agglomerates is
limited.
those prepared by ball milling. For example, the agglomer-
ates of nanometer Al particles and c-Al grains can be still
clearly seen in the GMAP with the average Al size of
2 3
O
Figure 3 shows the H evolution from the deionized water
2
72.94 nm prepared by ball milling for 1 h, as shown in
at 25°C using the GMAPs with submicrometer Al particles
2 3
Fig. 4(b), where the arrows point to the area of c-Al O
(
0.98 lm) prepared from ultrasonic dispersion for 1 h and
agglomeration (very fine black-gray grains) and outside this
area there are only the large white- gray nanometer Al parti-
cles. Figure 4(d) shows that some of the Al particle surfaces
are smooth (arrows) and the large c-Al O agglomerates
2 3
(double arrows) are clearly seen in the GMAP with the aver-
age Al size of 1.23 lm prepared by ball milling for 1 h,
ball milling for 1, 5, 10, and 20 h, respectively. It can be seen
that the hydrogen generation speed increases with increasing
ball-milling time. However, even if the ball-milling time is up
to 20 h, the hydrogen-generation speed of the GMAP pre-
pared by ball milling is still slower than that prepared by
ultrasonic dispersion for 1 h.
To analyze the causes that result in the difference in
hydrogen generation behavior of the GMAPs prepared by
two different processing procedures, microstructure morpho-
logies of the fresh fracture surfaces of different GMAP com-
pacts are shown in Fig. 4. Figure 4 indicates that for the
same processing time, the microstructures of the GMAPs
prepared by ultrasonic dispersion are more uniform than
implying that the mixing of Al particles and c-Al
is not so good. In contrast, the mixing of Al particles and c-
Al grains by ultrasonic dispersion are relatively uniform
2 3
O grains
2 3
O
and most of the agglomerates were eliminated in the
GMAPs, as shown in Figs. 4(a) and 4(c).
Ball milling and ultrasonic procedure are used to disperse
and mix different powders in material processing. Ball milling
is more popular in fabricating ceramic materials. The effect
of ultrasonic dispersion and ball milling on the breakage of
agglomerates is different due to their different dispersion
mechanisms. When high-intensity ultrasonic waves are
launched into the liquid-powder mixture, alternative low-
and high-pressure waves in liquids lead to the formation and
violent collapse of small vacuum bubbles, i.e., “cavitation
1
1
effect”, which produces shock waves and micro-jet liquid
flow, leading to the very large shear and strain gradients that
could break the agglomerates. However, the breakage of
agglomerates by ball milling results from the impact and
liquid shear fields produced by the low-speed cylinder rota-
tion. This is why ultrasonic dispersion is more effective to
break the agglomerates of Al and c-Al O powders and form
2 3
high performance GMAPs.
X-ray diffraction analyses indicate the reaction byproducts
of all GMAPs with water are bayerite and the reaction equa-
tion can be written
2
Al þ 6H2O ! 2AlðOHÞ þ 3H2 "
ð1Þ
Fig. 3.
H
2
evolution (a) from deionized water at 25°C using the
3
GMAPs with the average Al particle size of 0.98 lm prepared from
the following five processes: ultrasonic treatment (UST) for 1 h and
ball milling (BM) for 1, 5, 10, 20 h, respectively, where 1 g of
GMAPs was used in each test.
This reaction produces 3.7 wt% hydrogen (weight ratio of
generated H to Al and H O).
2
2
(a)
(b)
(c)
(d)
Fig. 4. SEM micrographs of the fresh fracture surfaces of the GMAP compacts fabricated by ultrasonic treating for 1 h (a, c) and ball milling
for 1 h (b, d), where the GMAPs in (a, b) are with the average Al particle size of 72.94 nm and those in (c, d) are with the average Al particle
size of 1.23 lm. The arrows in (b) and (d) show the nonuniform microstructures by ball milling.

4
Rapid Communications of the American Ceramic Society
2
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2
h