Morphological control of nanocrystalline aluminum nitride: Aluminum chloride-assisted nonowhisker growth [7]

Full text of DOI:10.1021/ja963368y

J. Am. Chem. Soc. 1997, 119, 5455-5456
5455
Transmission electron microscopy (TEM) revealed 0.5-5 µm
aggregates of 10-200 nm Al crystallites (Figure 1a).
Morphological Control of Nanocrystalline
Aluminum Nitride: Aluminum Chloride-Assisted
Nanowhisker Growth
Reaction of the nano-Al with N2 at 1000-1100 °C resulted
in complete conversion to nano-AlN (25-50 nm mean coher-
ence length by XRD).8 The morphology of the nano-AlN was
almost entirely equiaxed (Figure 1b) when pure nano-Al or when
nano-Al mixed with an inert additive was heated at 20 °C/min
to 1000 or 1100 °C. The morphological distribution of the
nano-AlN was altered to favor the formation of nanowhiskers
by addition of AlCl3 to the nano-Al before heating or by
increasing the heating rate. Addition of AlCl3 also improved
the purity of the AlN produced.9 Increasing the amount of AlCl3
added and increasing the heating rate produced larger fractions
of nanowhiskers (up to ca. 90%, Figure 1c).
Nanocrystalline AlN was also obtained by nitridation of
commercial 2 and 20 µm Al10 under identical conditions. The
changes in the morphological distribution of the nano-AlN
produced from the commercial powders mirrored the changes
observed with nano-Al, but lower fractions of nanowhiskers
were obtained, and the nitridation did not go to completion.11
Microscopy of partially reacted samples revealed a significant
fraction of nanowhiskers in all cases and that addition of AlCl3
increased the whisker fraction.
The whiskers were single crystalline as shown by selected
area diffraction (SAD) and were generally 20-100 nm in
diameter with aspect ratios (length/diameter) of 20 to >100.
When 10 wt % of AlCl3 was added and a heating rate of 20
°C/min was used, most of the whiskers were straight, but
unusual growth morphologies were also observed such as bent,
curved, axe-shaped, hexagonal cone, and a hollow tube. When
93 wt % of AlCl3 was added and/or when a heating rate of
>50°C/min was used branched and comblike crystals were
common, indicating a significant change in growth kinetics that
made additional growth directions accessible.
Among the known whisker-growth mechanisms, the vapor-
liquid-solid (VLS) and vapor-solid (VS) mechanisms are the
most likely to function under the present conditions.12 In the
VLS mechanism, whiskers grow from liquid flux droplets
attached to whisker tips. We observed no such flux droplets,
and addition of various potential flux materials did not promote
whisker formation.12 Additionally, the variety of observed
crystallite morphologies and whisker-growth directions is more
Joel A. Haber,† Patrick C. Gibbons,‡ and
William E. Buhro*,†
Departments of Chemistry and Physics
Washington UniVersity
St. Louis, Missouri, 63130-4899
ReceiVed September 25, 1996
Herein we report a morphologically selective synthesis of
nanocrystalline aluminum nitride (nano-AlN) by low-temper-
ature nitridation of nanocrystalline aluminum (nano-Al).1 Par-
ticle morphologies are varied from predominately equiaxed to
predominately whisker-like, apparently by the presence of vapor-
transport species during nitridation. Whisker formation appears
to be due to an increased volatility of aluminum induced by
the large surface area of nano-Al and the action of volatile
aluminum chlorides. The altered electronic, magnetic, and
mechanical properties of nanocrystalline materials have received
much emphasis;2 our results illustrate their altered reactivities.3
To our knowledge, procedures allowing selective synthesis of
various nanoparticle morphologies by purposeful variations in
reaction conditions are rare.4
Air-sensitive nano-Al was produced by the catalytic decom-
position of H3Al(NMe2Et)5 with Ti(O-i-Pr)4 (0.05-1.2 mol %,
eq 1). The nano-Al powders were 99 wt % Al by elemental
1,3,5-Me3C6H3
H3Al(NMe2Et) Ti(O-i-Pr)4 cat.8 nano-Al + NMe2Et + 3/2H2 (1)
∼164 °C
analysis,6 and mean crystallite dimensions of 40-180 nm were
determined by Scherrer analysis of XRD line broadening.7
† Department of Chemistry.
‡ Department of Physics.
(1) For other syntheses of nano-AlN, see: (a) Vissokov, G. P.; Stefanov,
B. I.; Gerasimov, N. T.; Oliver, D. H.; Enikov, R. Z.; Vrantchev, A. I.;
Balabanova, E. G.; Pirgov, P. S. J. Mater. Sci. 1988, 23, 2415-2418. (b)
Chow, G. M.; Xiao, T. D.; Chen, X.; Gonsalves, K. E. J. Mater. Res. 1994,
9, 168-174. (c) Wade, T.; Park, J.; Garza, E. G.; Ross, C. B.; Smith, D.
M.; Crooks, R. M. J. Am. Chem. Soc. 1992, 114, 9457-9464. (d) Bolt, J.
D.; Tebbe, F. N. In AdVances in Ceramics; American Ceramic Society:
Westerville, OH, 1989; Vol 26, pp 69-76. (e) Ramesh, P. D.; Rao, K. J.
AdV. Mater. 1995, 7, 177-17. (f) Adjaottor, A. A.; Griffin, G. L. J. Am.
Ceram. Soc. 1992, 75, 3209-3214. (g) Jung, W.-S.; Ahn, S.-K. J. Mater.
Chem. 1994, 4, 949-953. (h) Johnston, G. P.; Muenchausen, R. E.; Smith,
D. M.; Foltyn, S. R. J. Am. Ceram. Soc. 1992, 75, 3465-3468. (i) Sood,
R. R.; Southam, F. W.; Raghavan, N. S. Eur. Pat. 88308209.1, 1989,
document number 0 308 116 A1, Chem. Abstr. an 111:9752 ca. (j) Pratsinis,
S. E.; Wang, G.; Panda, S.; Guiton, T. Weimer, A. W. J. Mater. Res. 1995,
10, 512-520.
(8) In a typical run, the MgO tube was loaded with nano-Al, 2 µm Al,
20 µm Al, or an Al/AlCl3 mixture and inserted into a fused-silica tube sealed
on one end and fitted with an Ultratorr valve assembly on the other end.
The assembly was removed from the glovebox, and the MgO tube was
placed into a tube furnace. After appropriate purging of the connecting lines,
the valve was opened to N2 (1 atm). The sample was heated to 900-1100
°C for 15 min to 10 h to produce nano-AlN.
(9) nano-AlN was prepared by heating nano-Al under N2 at 1100 °C
for 10 h. Elemental analysis performed by Galbraith laboratories with airless
handling found (wt %): Al, 63.5; N, 32.2; Ti, 0.32; C, <0.5; Mg, <0.06;
O, <4.6 (by difference). nano-AlN was also prepared by mixing nano-Al
with 9.3 wt % AlCl3 and treating it identically to the previous sample. Anal.
found (wt %): Al, 66.4; N, 31.9; Cl, 0.27; Ti, <0.09; C, 0.56; Mg, <0.09;
O, <0.9 (by difference). Calcd (wt %): Al, 65.8; N, 34.2.
(10) The 2 µm Al (Strem) ranged from 0.5 to 20 µm in diameter, and
the 20 µm powder (Aldrich) ranged from 2 to 75 µm in diameter, as
determined by SEM.
(11) Aluminum powder (2-3 µm mean particle size) was reacted with
N2 at 1100 °C as received and with 13 wt % added AlCl3. XRD patterns of
both products showed peaks for nano-AlN only; however, TEM showed
that both products contained spherical aggregates (g200 nm diameter) of
equiaxed particles (e20 nm diameter) with excess Al by energy-dispersive
X-ray spectroscopy. Significantly, the Al without AlCl3 produced very few
whiskers, whereas the Al with added AlCl3 produced a large yield of
nanowhiskers. We believe that the 2-3 µm Al powder is at the crossover
point between nano-Al, which nitrides completely, and coarse Al powder,
which does not nitride completely under these conditions.
(12) (a) Givargizov, E. I. In Current Topics in Materials Science; Kaldis,
E., Ed; North-Holland Publishing: New York, 1978; Vol. 1, p 22. (b)
Givargizov, E. E. J. Cryst. Growth 1975, 31, 20-30. (c) Campbell, W. B.
In Whisker Technology; Levitt, A. P., Ed.; Wiley-Interscience: New York,
1970; pp 17, 27, and 28.
(2) (a) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.;
Goddard, W. A., III; Kaldor, A.; Louie, S. G.; Moscovits, M.; Peercy, P.
S.; Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J. Mater. Res. 1989,
4, 704-736. (b) Gleiter, H. Prog. Mater. Sci. 1989, 33, 223-315.
(3) Rieke, R. D. Science 1989, 246, 1260-1264.
(4) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-
Sayed, M. A. Science 1996, 272, 1924-1926. (b) Tanori, J.; Pileni, M. P.
AdV. Mater. 1995, 7, 862-864.
(5) Frigo, D. M.; van Eijden, G. J. M.; Reuvers, P. J.; Smit, C. J. Chem.
Mater. 1994, 6, 190-195. A powerful explosion occurred during the
synthesis of H3Al(NMe2Et), which extensively deformed a steel fume-hood
enclosure. Details are in the Supporting Information.
(6) nano-Al (28 nm) produced using 0.5 mol % Ti(O-i-Pr)4 was
consolidated by hot pressing at 100 °C for 1 h at 350 MPa in an Ar-filled
glove box. Elemental analysis of a portion of this pellet was performed by
Glow Discharge Mass Spectrometry analyzing for 25 elements. The primary
impurities were (wt %): C, 0.23; O, 0.25; N, 0.055; Cl, 0.14; Ti, 0.32; Al,
99.0 (by difference). Minor impurities were (ppm by wt): Li, 1.3; B, 0.15;
F, <0.05; Na, 2.9; Mg, 0.37; Si, 40; S, 1.8; K, 4.8; Ca, 0.61; Cr, 0.5; Mn,
0.01; Fe, 1.5; Ni, 0.35; Cu, 0.4; Zn, 0.27; Ga, 0.12; Mo, 0.9; In, 1.9; Ta,
<1; W, 0.05.
(7) The coherence length of Al produced by decomposition of H3Al-
(NMe2Et) changes as a function of mol % Ti(O-i-Pr)4.
S0002-7863(96)03368-9 CCC: $14.00 © 1997 American Chemical Society

5456 J. Am. Chem. Soc., Vol. 119, No. 23, 1997
Communications to the Editor
Figure 1. TEM micrographs of nano-Al from eq 1 (a) and the two principal nano-AlN morphologies produced from it: nano-AlN equiaxed
crystallites (b), and AlN nanowhiskers and a highly branched crystallite produced by adding 93 wt % AlCl3 and using a heating rate of 100 °C/min
(c).
consistent with VS growth than with VLS growth.12
VS growth is apparently supported by the large nano-Al
surface area, which increases the effective steady-state Al vapor
pressure. The equilibrium Al vapor pressure predicted by the
Kelvin equation13 is increased by at most 2-3% for the nano-
Al (44 nm mean coherence length) used. However, the system
is not in equilibrium because the Al vapor is reacting to form
AlN. The increased rate of evaporation afforded by the large
Figure 2. Proposed vapor-transport process for whisker growth.
surface area of nano-Al likely produces a larger steady-state
vapor pressure than is present over coarser Al particles. The
Rapid heating rates and small Al particle sizes increase the
whisker fraction by maximizing the surface area and the AlCl3
concentration when the nitridation temperature is reached. The
reaction between Al(l) and N2 to form AlN begins at ca. 1073
K, 140 K above the melting point of Al.7 A faster heating rate
reduces the time during which Al particles may coalesce before
nitridation begins, resulting in smaller droplet sizes. Once
nitridation begins further droplet growth is prevented by
formation of a shell of AlN particles. A faster heating rate also
decreases the amount of AlCl3 that sublimes out of the system
before nitridation begins, effectively increasing the concentration
of AlCl3 in the reacting system.
Thus, the enhanced reactivity of nano-Al allows the low-
temperature synthesis of AlN with morphological control.
Direct surface nitridation of coarse-grained Al results in an AlN
surface coating that inhibits complete nitridation at the temper-
atures we employed.11,16 In contrast, the small particles in nano-
Al are completely nitrided. Additionally, the high surface area
of nano-Al amplifies the proposed AlCl3/AlCl VS whisker-
growth mechanism, producing large yields of AlN nanowhiskers
that are among the smallest known.17
increased Al vapor pressure enables growth of a small number
of whiskers even when AlCl3 is not added. However, in the
absence of AlCl3, we observe that the whisker fraction decreases
significantly as the Al particles sinter and melt into larger
droplets, which become coated with AlN.
The dramatic increase in whisker fraction upon addition of
AlCl3 supports a mechanism in which AlCl3 acts as a transport
agent. AlCl3 did not maintain Al particle sizes at nanometer
dimensions; partially reacted Al/AlN droplets ranged from 5-15
µm with or without added AlCl3, and nitridation of 2 or 20 µm
Al with AlCl3 added produced a large whisker fraction.
However, AlCl3 changed the morphology of the partially reacted
droplets. When no AlCl3 was added, all of the partially reacted
Al formed spherical balls covered with a stubble of AlN
particles.14 When 10 wt % of AlCl3 was added, about half of
the partially reacted Al formed spherical balls covered with long
fibers or whiskers and half of the Al formed starfish-like
particles which were partially nitrided, but had smooth surfaces.
In completely reacted samples to which 10 wt % AlCl3 had
been added, some of the AlN shells had holes revealing a
hollowed interior, indicating that Al was removed from the core
during the reaction. AlCl3 was likely thus a transport agent for
Al removal from partially nitrided droplets.
AlCl3(g) and Al(l) are known to form significant equilibrium
quantities of AlCl(g) at the nitridation temperatures employed.15
Therefore, it is likely that AlCl participates in the formation of
AlN whiskers as proposed in Figure 2. The mixture of nano-
Al and AlCl3 produced a greater fraction of whiskers than did
the mixture of 2 or 20 µm Al and AlCl3 because the nano-Al
affords a much larger surface area, which increases the rate of
the interfacial reaction between Al(l) and AlCl3.
Acknowledgment. This work was funded by an NSF-PYI award
to W.E.B. (CHE-9158369), generously supported by the Exxon
Education Foundation, Emerson Electric, Monsanto, and Eastman
Kodak. J.A.H. was supported by a Department of Education GAANN
grant (P200A40147). We thank W. L. Gladfelter, T. P. Hanusa, and
T. Turney for suggesting the participation of AlCl in the VS process.
We thank W. L. Gladfelter for suggesting use of the Ti(O-i-Pr)4 catalyst.
Supporting Information Available: Precautions for the preparation
of H3Al(NMe2Et), XRD patterns, and synthetic procedures (6 pages).
See any current masthead page for ordering and Internet access
instructions.
(13) (a) Atkins, P. W. Physical Chemistry, 4th ed.; W. H. Freeman: New
York, 1990; p 158. (b) The equilibrium vapor pressure can also be calculated
using the Gibbs-Thompson equation.20c (c) Jena, A. K.; Chaturvedi, M.
C. Phase Transformations in Materials; Prentice-Hall: Englewood Cliffs,
NJ, 1992; p 59.
JA963368Y
(14) The “stubble” coating the partially reacted Al droplets appears to
be fibrous aggregates of equiaxed particles. It is not possible to distinguish
single-crystal whiskers from dense polycrystalline fibers by SEM; however,
the bent, conelike appearance of much of the “stubble” and “hair” on the
Al droplets suggests that they are not whiskers, but rather polycrystalline.
Fragments of similarly shaped structures are seen to be polycrystalline by
TEM.
(15) Roa, D. B.; Dadape, V. V. J. Phys. Chem. 1966, 70, 1349-1353.
(16) Weimer, A. W.; Cochran, G. A.; Eisman, G. A.; Henley, J. P.; Hook,
B. D.; Mills, L. K.; Guiton, T. A.; Knudsen, A. K.; Nicholas, N. R.;
Volmering, J. E.; Moore, W. G. J. Am. Ceram. Soc. 1994, 77, 3-18.
(17) (a) Tadashi, Y.; Katsutoshi, N.; Akihisa, I.; Tsuyoshi, M. Eur. Pat.
94114147.5, 1994. (b) Yamaguchi, T.; Inoue, A.; Nosaki, K.; Nakane, H.
Mater. Trans. JIM 1994, 35, 538-542.