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JOURNAL OF APPLIED PHYSICS
VOLUME 93, NUMBER 10
15 MAY 2003
Structure and magnetic properties of NiFeÕSiO2 and CoÕSiO2
nanocomposites consolidated by detonation compaction
Y. D. Zhang,a) X. Q. Ma, S. Hui, Mingzhong Wu, and S. H. Ge
Inframat Corporation, Willington, Connecticut 06279
W. A. Hines and J. I. Budnick
Department of Physics and Institute of Materials Science, University of Connecticut, Storrs,
Connecticut 06269
B. M. Cetegen and S. Y. Semenov
Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269
共Presented on 12 November 2002兲
In this article, the structural and magnetic properties of NiFe/SiO2 and Co/SiO2 nanocomposites
fabricated via powder processing are presented. The NiFe/SiO2 and Co/SiO2 nanoparticles were
both synthesized by a wet chemistry approach. The as-synthesized nanoparticles were characterized
by scanning electron microscopy, x-ray diffraction, and superconducting quantum interference
device magnetometer, yielding detailed information concerning the structure and size of NiFe or Co
particles, coating, and composition purification. The nanoparticles were then consolidated into solid
components via detonation compaction. Depending on the powder morphology and detonation
conditions, the density of the consolidated sample can reach over 91% of the theoretical density of
the conventional materials. X-ray diffraction experiments on the samples both before and after
consolidation indicate that the crystal structure of the nanocomposites remains unchanged during
detonation consolidations; however, a rather large increase in particle size for the magnetic
constituent was observed. Static magnetic property studies carried out on the samples both before
and after detonation showed no change in the saturation magnetization after consolidation,
indicating that the detonation consolidation does not cause oxidation of the nanopowders. Initial
complex permeability measurements on the consolidated samples showed an essentially flat ␮ ⬘
versus frequency response over a wide frequency range between 5 kHz and 13 MHz, implying a
high resistivity of the consolidated sample. These experiments show that detonation compaction is
a promising approach for consolidating magnetic nanoparticles, and NiFe/SiO2 and Co/SiO2 are
good candidates for high frequency soft magnetic materials. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1558606兴
I. INTRODUCTION
high strength, high hardness, and high density is required. In
the D-gun process, the combustion of oxygen and acetylene
is ignited using a sparking device and the ejected powder
feedstock is partially melted by the combustion heat. The
detonation wave in the gun barrel drives the powder particles
toward the substrate at high speed of about 800–1000 m/s.
Thus, a resultant layer can be formed with high bond
strength and very low porosity due to the high kinetic energy
and high velocity. In this work, a consolidation study of
NiFe/SiO2 and Co/SiO2 using detonation compaction is presented.
Nanocomposite fabrication provides an opportunity to
develop soft magnetic materials. The resistivity of a metal/
insulator nanocomposite system can be dramatically increased compared with conventional metallic alloys, leading
to significantly reduced eddy current loss. Meanwhile, the
exchange coupling between neighboring magnetic nanoparticles can overcome the anisotropy and demagnetizing effects, resulting in much better soft magnetic properties than
conventional materials.1,2 In pursuit of this idea, a large number of metal/insulator nanocomposite powder systems have
been explored based on powder processing, which is advantageous for mass production of nanocomposite materials.3–5
However, densifying the nanoparticles into a bulk material
while retaining the nanostructure of these particles is a big
challenge. Among consolidation approaches,6,7 detonation
compaction is promising.8 –12 The detonation gun spray process, referred to as the D-gun process, has been widely used
in various industrial applications where a coating layer with
II. EXPERIMENTAL PROCEDURE, RESULTS,
AND ANALYSIS
(Ni75Fe25) v /(SiO2 ) 1⫺ v and Cov /(SiO2 ) 1⫺ v nanocomposites, where v denotes the volume fraction of metal phase
in the composites, were synthesized using a wet chemical
solution technique.13 Solutions of Ni-, Fe-, and Si-containing
precursors were prepared separately in alcohol with controlled pH, reaction time, and temperature, to form precomposite precipitates in solution. The precipitated nanoparticle
suspensions were dried in an oven to obtain a precomposite
a兲
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© 2003 American Institute of Physics
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J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003
Zhang et al.
FIG. 1. SEM image of D-gun consolidated (Ni75Fe25) 0.5 /(SiO2 ) 0.5 sample.
The layer boundaries are marked by the arrows.
FIG. 2. XRD patterns of (Ni75Fe25) 0.5 /(SiO2 ) 0.5 samples before and after
D-gun consolidation.
powder. The precomposite powder was then thermochemically converted into a metal–ceramic (NiFe/SiO2 or
Co/SiO2 ) nanocomposite powder in the presence of a reducing agent at temperatures from 400 to 700 °C. The nanocomposites were passivated before removing from the furnace to
prevent oxidation.
The consolidation of NiFe/SiO2 and Co/SiO2 was performed using a custom-built D-gun facility. This facility consists of a 2.25-m-long, 26.4-mm-inner-diameter 共I.D.兲,
stainless-steel detonation tube closed at one end. The closed
end of the tube contains a spark plug for ignition of the gas
mixture that was energized with an ignition circuit. Small
quantities of NiFe/SiO2 or Co/SiO2 powders were placed at
distances from the open end of the detonation tube. An aluminum disk 共10–15 mm in diameter兲 with a central post 共6
mm in diameter兲 was used as the substrate. The substrate was
degreased in acetone and the surface was coarsened by sand
blasting prior to D-gun spraying. The substrate was inserted
into a cylinder holder 共20 mm I.D.兲 that was fixed on a stand.
After placement of the powder in the detonation tube, the
tube was purged slowly with mixture of oxygen and acetlylene and the mixture was ignited by means of a spark plug.
The ignition resulted in initiation of the combustion wave
which quickly transitioned to a detonation wave. Detonation
wave passing over the powder entrained the powder into the
detonation wave and forced the nanopowder onto the Al substrate to form a deposit. In this process, the main variables
included stoichiometry of O2 and C2 H2 flow rate and the
distance between the powder feedstock and substrate. Figure
1 shows a scanning electron microscopy 共SEM兲 image of a
D-gun consolidated (Ni75Fe25) 0.5 /(SiO2 ) 0.5 sample.
The consolidation was characterized by a packing density fraction, which is defined as the ratio of the measured
density, d, of the densified sample to its theoretical density,
d TD . The value of d was measured using Archimedes’ law
while the theoretical density of (NiFe) v /(SiO2 ) 1⫺ v or
Cov /(SiO2 ) 1⫺ v was calculated according to the volume fractions of the metal and SiO2 phases. It was found that the
resulting packing density is sensitively dependent on the stoichiometry of O2 and C2 H2 mixture, powder morphology
and the distance between powder and the substrate. By adjusting these parameters, a packing density as high as 0.91
was achieved on the (NiFe) v /(SiO2 ) 1⫺ v , while 0.85 was
reached on Cov /(SiO2 ) 1⫺ v .
The crystal structure was determined by x-ray diffraction
共XRD兲 experiments. Also, the particle sizes of the assynthesized nanoparticles and the grain sizes of the consolidated samples were calculated from the linewidth of the
main XRD line using the Scherrer equation. Figure 2 shows
typical XRD patterns for (Ni75Fe25) 0.5 /(SiO2 ) 0.5 . As shown
in Fig. 2, the XRD peak 2␪ values for the samples before and
after consolidation are the same, indicating that the D-gun
operation does not cause any change in the structure. However, a significant increase in the grain size was observed
after consolidation as seen by the sharper peaks. For the
sample shown in Fig. 2, the average particle size changed
from 16 to 44 nm. Similarly, an increase of grain size from
30 to about 50 nm was observed in the Co0.5 /(SiO2 ) 0.5 system.
Static magnetic studies indicated essentially no change
共within 5%兲 of saturation magnetization for the materials
before and after D-gun compaction. This indicates that
the D-gun operation does not cause an oxidation of the
nanopowders.
Initial complex permeability, ␮ ⫽ ␮ ⬘ - j ␮ ⬙ , versus frequency curves were measured using an impedance meter on
the as-compacted samples and annealed samples. Figure 3
shows the initial complex permeability of the D-gun consoli-
FIG. 3. Initial permeability as a function of frequency for D-gun consolidated n-Co0.5 /(SiO2 ) 0.5 .
Zhang et al.
J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003
dated Co0.5 /(SiO2 ) 0.5 sample as a function of frequency. For
this sample, the packing density fraction is about 0.87.
III. DISCUSSION AND CONCLUSION
Consolidation of nanocomposites is required not only for
manufacturing bulk components, but also to satisfy a fundamental requirement for obtaining novel magnetic properties
from the nanomaterials.14 The key to dramatically improving
the soft magnetic properties of a metal/insulator nanocomposite magnetic material is to make the exchange coupling
between neighboring magnetic grains dominate the demagnetizing and magnetocrystalline anisotropy energies in the
material.15 In order to achieve this, the particle size, which is
the size of the metal grain plus the thickness of the insulator
coating layer, has to be significantly smaller than the exchange coupling length, and the system has to be consolidated to full density. Consolidation of nanoparticles without
the deterioration of their nanostructure is a big challenge to
date. The consolidation of conventional powder magnetic
materials is achieved by sintering the powder at a certain
temperature at which a solid-phase reaction process takes
place.6,7 For conventional materials, grain growth during sintering does not damage the performance of magnetic materials. In fact, in most cases, it is favorable. However, grain
growth has to be avoided when densifying nanoparticles. If
the grain size is increased during consolidation process and
becomes larger than the exchange coupling length for the
material, the intergrain exchange interaction will no longer
be dominant, and the material behaves essentially like a conventional one. Furthermore, the NiFe/SiO2 or Co/SiO2 nanocomposite system with core 共metal兲–shell 共insulator兲 nanostructure is heterogeneous in nature. When the composites
are heated to near the softening temperature of one of the
constituent phases, intergrain atomic diffusion starts for both
constituents and leads to form separated NiFe 共or Co兲 and
SiO2 phases with enlarged grain sizes. Such a change destroys the core–shell nanostructure of the powder and, consequently, the required magnetic and electrical properties.
Therefore, a conventional ceramic sintering approach cannot
be used for consolidating nanocomposite powder; consolidating a nanocomposite powder has to be performed at a
lower temperature than that for sintering its bulk powder
counterpart. To achieve high density compaction for nanocomposites, high pressure is needed. Concerning the applicability of the detonation approach for nanoparticle compaction, several issues have to be addressed. 共1兲 During the
detonation process, depending on the mixing condition of O2
and C2 H2 gases, the metallic 共NiFe, Co兲 nanopowder can be
in an oxygen-rich atmosphere and the ambient temperature is
very high during the D-gun consolidation process, consequently, phase transitions and oxidation of the metal nanoparticle are possibilities. 共2兲 The high temperature may cause
a deterioration of the core 共metal兲–shell 共insulator兲 nanostructure. 共3兲 Upon reaching the substrate, the nanoparticles
flatten into a layered structure, which may further deteriorate
the nanostructure of the nanocomposite. This problem exists
in all approaches which use uniaxial pressure.
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Figure 1 shows that the D-gun consolidated sample possesses a layered structure with a thickness of about 0.1 mm.
Each layer corresponds to one detonation. Within each layer,
the powder is compacted densely, while the pores exist
mostly in the interface region between the two neighboring
layers. The highest packing density fraction achieved is 92%,
and there appears to be possibility to further densify the nanopowder. The thickness of the layer is determined by the
mass of powder in each individual detonation. By increasing
the mass of the powder for each detonation, the thickness of
each layer, and thus the packing density and mechanical
strength, can be significantly improved. It is shown in Fig. 2
that the detonation consolidated samples retain essentially
nanostructured materials. As to the grain size, although a
significant increase was observed, it can be controlled by
reducing the effective temperature and the flight time of the
particle in the barrel. Figure 3 illustrates the high frequency
magnetic properties of the detonation consolidated nanocomposites. If the SiO2 coating were destroyed, direct metalmetal contact would take place, the resulting eddy current
would cause ␮ ⬘ to decrease with frequency much faster than
that shown in Fig. 3, and ␮ ⬙ would reach a maximum at a
lower frequency, roughly in the kHz range for a 0.1-mmthick layer. As shown in Fig. 3, however, ␮ ⬘ is essentially
flat and ␮ ⬙ is still small up to 13 MHz. This indicates a good
insulation of the metal particles by SiO2 .
In summary, the work presented here demonstrates that
detonation compaction is a promising approach in nanoparticle consolidation for its ability in obtaining high density
while retaining the nanostructure. More work is needed to
develop this processing further.
ACKNOWLEDGMENT
This work was supported by NASA Contract No. NAS301013.
1
Y. Hayakawa, A. Makino, H. Fujimori, and A. Inoue, J. Appl. Phys. 81,
3747 共1997兲.
2
H. Fujimori, Scr. Metall. Mater. 33, 1625 共1995兲.
3
G. C. Hadjipanayis and G. A. Prinz, Science and Technology of Nanostructured Magnetic Materials 共Plenum, New York, 1991兲.
4
J. F. Loffler, H. B. Braun, W. Wagner, G. Kostorz, and A. Wiedenmann,
Mater. Sci. Eng., A 304-306, 1050 共2001兲.
5
Y. D. Zhang, S. H. Wang, T. D. Xiao, J. I. Budnick, and W. A. Hines, IEEE
Trans. Magn. 37, 2275 共2001兲.
6
W. D. Jones, Fundamental Principles of Powder Metallugy 共Arnold, London, 1960兲.
7
R. M. German, Powder Metallurgy of Iron and Steel 共Wiley, New York,
1998兲.
8
X. Q. Ma, Y. M. Rhyim, H. W. Jin, C. G. Park, and M. C. Kim, Mater.
Manuf. Processes 14, 195 共1999兲.
9
R. C. Tucker, Jr., J. Vac. Sci. Technol. 11, 725 共1974兲.
10
P. Saravanan, V. Selvarajan, M. P. Srivastava, D. S. Rao, S. V. Joshi, and
G. Sundararajan, J. Therm. Spray Technol. 9, 505 共2000兲.
11
P. Saravanan, V. Selvarajan, D. S. Rao, S. V. Joshi, and G. Sundararajan,
Surf. Coat. Technol. 123, 44 共2000兲.
12
Z. E. Erkmen, J. Biomed. Mater. Res. 48, 861 共1999兲.
13
Y. D. Zhang, S. H. Wang, and T. D. Xiao, U. S. Patent Appl. No. 60/243,
649 共October 26, 2000兲.
14
Z. Turgut, N. T. Nuhfer, H. R. Piehler, and M. E. McHenry, J. Appl. Phys.
85, 4406 共1999兲.
15
G. Herzer, in Handbook of Magnetic Materials, edited by K. H. J. Buschow 共Amsterdam, the Netherlands, 1997兲, Vol. 10, p. 415.