preparation, characterization and microwave absorption prop
TRANSCRIPT
www.elsevier.com/locate/apsusc
Available online at www.sciencedirect.com
4 (2008) 4708–4715
Applied Surface Science 25Preparation, characterization and microwave absorption properties
of electroless Ni–Co–P-coated SiC powder
Yongjie Li, Rui Wang, Fengming Qi, Chunming Wang *
Department of Chemistry, Lanzhou University, 730000 Lanzhou, China
Received 14 November 2007; received in revised form 16 January 2008; accepted 16 January 2008
Available online 20 January 2008
Abstract
Silicon carbide particles reinforced nickel–cobalt–phosphorus matrix composite coatings were prepared by two-step electroless plating process
(pre-treatment of sensitizing and subsequent plating) for the application to lightweight microwave absorbers, which were characterized by
scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), vibrating sample magnetometer (VSM) and vector network analyzer,
respectively. The results show that Ni–Co–P deposits are uniform and mixture crystalline of a-Co and Ni3P and exhibit low-specific saturation
magnetization and low coercivity. Due to the conductive and ferromagnetic behavior of the Ni–Co thin films, high dielectric constant and magnetic
loss can be obtained in the microwave frequencies. The maximum microwave loss of the composite powder less than �32 dB was found at the
frequency of 6.30 GHz with a thickness of 2.5 mm when the initial atomic ratio of Ni–Co in the plating bath is 1.5.
# 2008 Elsevier B.V. All rights reserved.
PACS : 41.20.Gz; 61.10.Nz; 68.37.Hk; 78.40.�q
Keywords: Silicon carbide; Microstructure; Complex relative dielectric permittivity; Complex relative magnetic permeability; Reflection loss
1. Introduction
In recent years, microwave absorptive materials have
attracted considerable research interest in the materials science
[1,2] because of its widespread applications for many electro-
magnetic compatibility (EMC) purposes. A number of materials
have been described, which are capable of absorbing electro-
magnetic radiation. However, the conventional absorptive
materials such as metal powders and ferrites are quite heavy,
which restricts their usefulness in applications requiring light-
weight mass [3]. Moreover, those materials have difficulties in
increasing the permeability in GHz region because of Snoek limit
for ferrites [4] or eddy current loss for magnetic metals [5].
As one of the ways to overcome these problems, the use of
embedding particles (metallic, non-metallic or polymeric) in
electroless deposited metals is a convenient method of
preparing composite coatings, and the particles increase its
mechanical and physical properties [6]. The presence of fine
particles as the second phase improves the microhardness, high-
* Corresponding author. Tel.: +86 9318911895; fax: +86 9318912582.
E-mail address: [email protected] (C. Wang).
0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2008.01.076
temperature inertness, wear and corrosion resistance of the
composite coatings [7–9]. High wear resistance, low cost and
chemical inertness of ceramic oxides, carbides and nitrides has
led to their widespread use as distributed phase [8,10]. Within
these ceramics SiC is a kind of useful electronic material, and
has high-strength ceramic material with excellent corrosion and
erosion resistance. The merits of SiC for high-temperature
electronics and short-wavelength optical applications are
compared. The outstanding thermal and chemical stability of
SiC should enable them to operate at high temperatures and in
hostile environments, and also make it attractive for high-power
operation [11].
However, their application still suffers from several
difficulties. One problem is the low wettability between
ceramics and liquid metals. In order to promote wetting
between them, the ceramic surface can be modified by
deposition of metal coatings using different techniques [12].
So investigations have been carried out on electrodeposited
composite coatings comprising of alloy matrixes dispersed with
nanoparticles, e.g. Ni–Fe–nano-Si3N4, Co–Ni–nano-Al2O3,
Zn–Ni–nano-SiC, Ni–P–SiC and Co–P–SiC composite coat-
ings are attractive and have been investigated before [13,14].
Ni–Co alloy coatings are of importance, as they possess
Y. Li et al. / Applied Surface Science 254 (2008) 4708–4715 4709
high-temperature wear and corrosion resistance. Moreover, the
Ni–Co alloy deposition is an anomalous co-deposition and the
hardness of alloy increases as long as they possess fcc lattice
structure. Thus it was necessary to understand the influence of
the matrix on the properties of the composites. The composite-
coating is believed to combine the advantages of both Ni–Co
alloy and nano-particulate SiC. One of the available report
focuses on studying the variation in SiC content in a given Ni–
Co alloy obtained from Watt’s bath [14].
As mentioned above, a lot of research work has been carried
out on the effect of operating conditions on the mechanical
properties of composite coatings containing micron size SiC
particles. Reports on the magnetic and microwave-absorbing
properties of nano-composite coatings are scanty. In resonant
absorbers of quarter wavelength, zero-reflection can be
obtained by access to wave impedance mating at the surface
of the absorbing layer, which requires a proper combination of
magnetic permeability and dielectric permittivity at a given
thickness and frequency. Those material parameters in high
frequencies can be controlled by use of magnetic metals
(permittivity control by electrical property and permeability
control by magnetic property) in a single-layered microwave
absorber [15]. In the present work, the thin Ni–Co–P films on
SiC particles were electrolessly synthesized. It was focused on
understanding the influence of Co content on magnetic and
microwave-absorbing properties of composite coatings. The
surface morphology and structure of Ni–Co–P films on SiC
particles were also investigated.
2. Experimental
2.1. Pre-treatment of SiC particles
The materials employed were pure SiC (97% minimum,
Crystolon, Norton Silicon Carbide) which have a true density of
3.2 g/cm3. The electroless plating was performed on the
particle size powders namely 200 grit, and its particle size is
74 mm. The pre-treatment of SiC particles was followed by the
literature of Wang et al. [16].
2.2. Electroless deposition process
The pre-treated SiC particles were plated in an electroless
nickel–cobalt bath with composition listed in Table 1. The
proportion of elemental powders that added to the plating bath
was 10 g/L, and with continuous stirring, all of the particles
were exposed to the electroless nickel–cobalt solution. The
powders were plated at 90 8C and the pH value adjusted to 8.0
using NaOH. The plating time was 30 min, and the samples 1–4
Table 1
Composition of the electroless nickel–cobalt bath
Nickel sulfate, hexahydrate NiSO4�6H2O 0.05–0.1 mol/L
Cobalt sulfate, heptahydrate CoSO4�7H2O 0.05–0.1 mol/L
Sodium citrate Na3C6H5O7�2H2O 40 g/L
Ammonium sulfate (NH4)2SO4 40 g/L
Sodium hypophosphite, monohydrate NaH2PO2�H2O 20 g/L
is plated from the plating solution in which the initial atomic
ratio of Ni–Co is 1:2, 1:1, 1.5:1, and 2:1, respectively.
2.3. Measurements
The surface morphologies of the coatings and the pre-treated
SiC particles were examined using a scanning electron
microscope (SEM, JSM-5600LV/KEVEX Sigma), while
EDX analysis was performed to identify the metal-coating
layer and its components. Phase analysis of the coating was
studied from 308 to 808 using an X-ray diffraction analyzer
(XRD, Rigaku D/max-2400, Cu K-a, l = 0.1514 nm). The
static magnetic properties of sample were investigated by
vibrating sample magnetometer (VSM, Lakeshore 7304). The
sample containing 75 wt.% sample particles was made into
toroidal-shaped samples with an outer diameter of 7.0 mm and
inner diameter of 3.0 mm for microwave measurement. The
complex permeability and permittivity of composite were
measured using a vector network analyzer (Agilent E8363B) in
the frequency range of 0.1–18 GHz.
3. Results and discussion
3.1. SEM/EDX and XRD analysis
Characterization of the Ni–Co–P-coated particles is neces-
sary to determine the amount, chemical nature, morphology,
uniformity and distribution of the metal coating on the powders.
Typical surface morphologies of pre-treated SiC particles and
Ni–Co–P-coated SiC powders that were heat-treated at 400 8Cfor 1 h are shown in Fig. 1. SEM examination of the starting
powders reveals that SiC particles exhibit predominantly
angular shapes with sharp edges and show a clean, deposit-free
surface (Fig. 1a). The chemical components of these particles
as revealed by EDX analysis show that these particles are
mainly composed of SiC. The morphology of sample 1 show
uniformly distributed Ni–Co–P coatings with spicular grains on
activated SiC particle surface (Fig. 1b). Such powders are
consists of a core of ceramic material and a metallic coating. In
Fig. 1c the Ni–Co–P deposits grew larger as the Co content
decreased. In Fig. 1d and e the form of the electroless Ni–Co–P
deposits turned into polyhedrons. Fig. 1 clearly shows that the
electroless deposits with the form of spicular grains were
deposited dispersively on the surface of SiC particles in the
beginning. The size of spicular grains increased more and more
with the decreased Co content until they touched each other in
the direction parallel to the surface of the substrate, which was
attributed to the magnetic attractive force of the coated Ni–Co–
P films. Then the electroless deposits grew only in the direction
vertical to the SiC surface and their form turned into
polyhedrons.
The EDX analysis of samples 1–4 shows that these particles
are primarily comprised of SiC, Ni, Co and P elements
(Table 2). Contrary to expectations, the components of Ni–Co–
P alloy did not show an ideal atomic ratio of Ni–Co compared
with the initial composition of plating bath. The reasons can be
explained as follows. First of all, it can be understood from the
Fig. 1. Surface morphologies of (a) pre-treated silicon carbide, (b) sample 1, (c) sample 2, (d) sample 3 and (e) sample 4.
Table 2
Ni, Co, and P content in the Ni–Co–P alloy films coated on SiC powders
Sample number Ni (at.%) Co (at.%) P (at.%)
1 21 51 28
2 75. 19 5.4
3 68 27 5.7
4 86 7.1 6.7
Y. Li et al. / Applied Surface Science 254 (2008) 4708–47154710
thermodynamics and the kinetics of catalytic reaction. The Pd-
activation procedure assured elimination of the passive
condition of the SiC surfaces, making them suitable for
electroless plating due to the presence of chemically deposited
Pd nuclei that initiate nickel–cobalt deposition, which is a key
step in the entire electroless-coating process. For electroless
Ni–Co–P plating, the oxidizing and reducing reactions and their
standard Gibbs free energy changes [17] can be represented as
below
H2PO2� þH2O ¼ H2PO3
� þ 2Hþ þ 2e�; DG� ¼ �96:5 kJ
(1)
2H2PO2� þ 4Hþ þ 2e� ¼ 2P þ 4H2O; DG� ¼ þ48:3 kJ
(2)
Ni2þ þ 2e� ¼ Ni; DG� ¼ þ54:0 kJ (3)
Co2þ þ 2e� ¼ Co; DG� ¼ þ48:3 kJ (4)
From the value of DG8 for each reaction, it is clear that Ni, Co
and P can all reduced by sodium hypophosphite and the trend of
reduction for Ni is larger than Co. Additionally, the catalytic
activity of Ni for the reaction of (1) is better than that of Co [18].
Second, this is attributed to the fact that the electroless bath has
become exhausted due to the limited amount of the Ni–Co
plating solution compared with the large surface to be plated. In
addition, the pH of the alkaline solution was altered with the
progress of the plating process: during deposition, byproducts
of the reaction, orthophosphite (HPO32�) and hydrogen ions
accumulate in the solution, affecting the chemistry of the
plating bath. Such events were characterized by a reduced
gas evolution from the bath and an excessive decoloration of
the electroless solution.
Fig. 2. XRD patterns of (a) pre-treated silicon carbide and heat-treated (b)
sample 1, (c) sample 2, (d) sample 3 and (e) sample 4 at 400 8C for 1 h.
Fig. 3. Hysteresis loops of samples 1–4 measured at room temperature.
Table 3
Magnetic properties of electroless Ni–Co–P ternary alloy deposits
Sample number Mr (emu/g) Ms (emu/g) Hc (Oe)
1 1.7 6.8 622
2 0.9 5.2 200
3 0.8 2.9 503
4 0.7 1.7 211
Y. Li et al. / Applied Surface Science 254 (2008) 4708–4715 4711
In order to confirm the presence of Ni–Co–P ternary alloy
within the bulk of the coating, XRD analysis of the Ni–Co–P-
coated SiC particles is investigated. Fig. 2 shows five X-ray
traces related to the pre-treated SiC particles and heat-treated
samples 1–4 at 400 8C for 1 h. X-ray diffraction patterns of the
heat-treated powders (Fig. 2b–e) show sharp peaks centered at
2u of about 36–538, signifying that this particular deposits
contains a mixture of crystalline Ni3P and a-Co. The figure also
shows sharp peaks corresponding to silicon carbide, as well as a
noisy background, characteristic of an amorphous structure. X-
ray diffraction traces obtained from samples 1 to 4 showed
sharp, well-defined peaks corresponding to a fully crystallized
structure (Fig. 2b–e). The five peaks at 2u = 36.38, 41.88, 42.78,46.68, and 52.88 represent the well-defined peaks correspond-
ing to diffraction from (0 3 1), (2 3 1), (3 3 0), (1 4 1), and
(1 3 2) planes, respectively, of the Ni3P (JCPDS: 34-501). And
the two peaks at 2u = 44.48 and 51.78 represent peaks
corresponding to diffraction from (1 1 1) and (2 0 0) planes,
respectively, of the a-Co (JCPDS: 01-1254).
It is evident that the amorphous nature of the deposits still
remains when the temperature of heat treatment is below
300 8C according to Ref. [19]. After the heat treatment at
300 8C for 1 h, XRD pattern reveals the presence of fcc nickel,
bct nickel phosphide (Ni3P) and hexagonal Ni5P2 phases. It can
be seen that the Ni5P2 phase disappeared, but tetragonal Ni12P5
phase was formed after annealing at 400 8C for 1 h. When the
temperature of heat treatment exceeds 400 8C, the Ni12P5 phase
vanished, the Ni phase and the Ni3P phase increase strongly.
The higher intensity for annealed samples pattern indicates to
the higher degree of crystallinity in these samples. In this paper,
during the annealing process, the higher phosphorus regions
were supposed to be further increased by extraction of P
dissolved in nickel grains, which in turn gave rise to
precipitation of the hard inter-metallic Ni3P phase, the most-
prevalent phosphide in the deposits. These transformations
support the earlier observation obtained by Sankara et al. [19]
and Hur et al. [20].
3.2. Magnetic properties of Ni–Co–P deposits
The magnetic properties of electroless Ni–Co–P ternary
alloy deposits were studied in their as-deposited condition. The
hysteresis loop obtained for as-plated electroless Ni–Co–P
deposits (samples 1–4) is shown in Fig. 3. The magnetic
properties, viz., specific saturation magnetization (Ms),
remanence (Mr) and coercivity (Hc), derived from the hysteresis
loop, are given in Table 3. The shape of the hysteresis loops
seems to be very similar to that exhibited by partially or totally
amorphous materials and by an amorphous Ni–Co–P film
[21,22]. It is well known that magnetic characteristics of
amorphous materials are significantly smaller than those of
crystalline materials [23]. Being amorphous in nature, the
electroless Ni–Co–P deposits of the present study exhibit soft
magnetic characteristics. According to Tarozaite et al. [24] the
formation of certain crystallites shape, which determines high
coercivity, takes place only at definite phosphorus content. The
phosphorus content, necessary for the formation of grains
boundaries, differs for the films, deposited under different
conditions, because of the grains with different size and shape
formation. According to them, electroless Ni–Co–P deposits
exhibit a high coercivity when the phosphorus content of the
deposit lies between 4 and 6 wt.%. When the phosphorus
content is lower than 4 wt.%, films of coarse crystallites are
deposited, resulting in lower coercivity. If phosphorus content
exceeds 6 wt.%, films consisting of very fine grains are
deposited which also leads to a decrease in coercivity.
Additionally the phosphorus content in Ni–Co–P alloys is
Fig. 4. Frequency dependence of the real (a) and imaginary (b) part of the
complex permittivity of pre-treated silicon carbide and samples 1–4.
Y. Li et al. / Applied Surface Science 254 (2008) 4708–47154712
17, 2.9, 3.1 and 3.7 wt.% corresponding to samples 1, 2, 3 and
4, respectively. Thus the coercivities of all samples are low and
they are found to decrease and then increase with cobalt content
of the deposits (Table 3). A comparison of the magnetic
characteristics of electroless Ni–Co–P deposits of the present
study reveals that the specific saturation magnetization and
remanence are found to increase firstly and then decrease and
increase finally with increased cobalt content in the deposits
(Table 3), which is due to higher magnetic moment of Co atom.
However, Rivero et al. [25] report a linear increase in specific
saturation magnetization with increase in cobalt content for
electrodeposited Ni–Co–P amorphous ribbons and Matsubara
and Yamada [26] also report an increase in specific saturation
magnetization of electroless Ni–Co–P deposits with increased
cobalt content. Therefore detailed magnetic properties need to
be studied further.
3.3. Microwave absorptive properties
Fig. 4 illustrates the real and imaginary parts of permittivity
of samples 1–4, and SiC particles–olefin composites. It can be
seen from Fig. 4 that electroless Ni–Co–P ternary alloy deposits
of various composition show a similar variety trend and it is
apparent that both e0 and e00 are sensitive to metallic behavior of
Ni and Co in the deposits. The powder–paraffin composites
exhibit that the values of real part e0 of complex permittivity
decrease with increasing frequency except a resonance peak
around 8 GHz. It is found that the values of both real and
imaginary parts of complex permittivity of samples are higher
along with increasing metallic content of Ni and Co in the
deposits, which is attributed to better metallic behavior of Ni
and Co atoms than P atom, but they are comparatively smaller
than those observed for the metal powder or fibre composites as
reported in the literatures [27–29]. The lower real part value of
complex permittivity is a great advantage to strike a balance
between permeability and permittivity, thus decreasing the
reflection coefficient of the absorber compared with other metal
magnetic materials for microwave-absorbing application. It is
interesting that the values of imaginary part of complex
permittivity increase slightly with increasing frequency and the
curves of the real part of permittivity exhibit the abrupt
decrease, while the imaginary part of permittivity show the
sharp peak at the corresponding frequency. This suggests a
resonance behavior, which is expected when the composite is
highly conductive and skin effect become significant. In our
studies, the resonant frequency of the samples is related to the
high conductivity of nickel and cobalt. Additionally, the
decreasing amplitude of e0 and the increasing amplitude of e00
are enhanced with the increasing metallic content of Ni and Co
in the Ni–Co–P films, and they all shift to lower frequency. In
general, the permittivity originates from orientation polariza-
tion, atomic polarization and electronic polarization. Normally,
the resonance that originated from vacancy or pores usually
dominates in the low-frequency regions, provided that there
exist space charges in the materials. High-frequency resonance
is attributed to atomic and electronic polarization [30]. So we
observed a resonance peak in the curves of both the real and
imaginary parts of complex permittivity can be interpreted as
different chemical compositions of Ni–Co–P deposits that is the
intrinsic characteristics of our prepared materials and atomic
polarization.
The real and imaginary parts of permeability of samples 1–4,
and SiC particles–olefin composites are shown in Fig. 5. One
feature of the data is that the values of real part m0 of complex
permeability for all samples all decrease with increasing
frequency as shown in Fig. 5a, this is due to both eddy current
loss and ferromagnetic resonance [31]. The values of imaginary
part m00 of complex permeability for all samples also show a
decrease with increasing frequency as shown in Fig. 5b and
exhibit two distinct resonance peaks around 1 and 8 GHz,
respectively. Usually, for ferrite magnetic materials, the
microwave magnetic loss of magnetic materials originates
mainly from hysteresis loss, domain wall resonance, natural
ferromagnetic resonance, and the eddy current effect. The
hysteresis loss was caused due to irreversible magnetization
and was negligible in a weak applied field. It is possible to
separate the permeability spectra into the spin rotational
component and the domain wall motion contribution, using
numerical fitting [32]. Resonance due to domain wall move-
ment normally occurs at low-frequency region (<2 GHz);
Fig. 5. Frequency dependence of the real (a) and imaginary (b) part of the
complex permeability of pre-treated silicon carbide and samples 1–4.
Fig. 6. Frequency dependence of the reflection loss of sample 3 at various
sample thicknesses.
Y. Li et al. / Applied Surface Science 254 (2008) 4708–4715 4713
however, resonance due to spin rotational component occurs as
high-frequency region. So the first resonance peal (around
1 GHz) may be due to domain wall resonance. It has been
reported that the natural resonance frequency for Co (hcp)
particles was 6.5 GHz [33–35]; that is to say, the magnetic loss
that peaked around 8 GHz could not be explained by natural
ferromagnetic resonance. Thus, it could be concluded that the
magnetic loss around 8 GHz was mainly caused by the eddy
current effect.
It is reasonable that both the dielectric loss and the magnetic
loss can be influenced by the ‘‘core–shell’’ microstructure of
microwave absorbent. In general, the dielectric loss is attributed
to the lags of polarization between the core/shell interfaces as
the frequency is varied. In addition, the ‘‘core–shell’’
microstructure of microwave absorbent has something to do
with eddy current loss that is one of the contributors to magnetic
loss. In our study, the SiC particles may act as a magnetic
inactive layer. The magnetic inactive layer causes the
demagnetizing field and the cut-off of the magnetic connection
between the magnetic components. We believed that the core of
SiC particles increased electromagnetic energy dissipation.
However the contribution of ‘‘core–shell’’ microstructure of
electroless Ni–Co–P-coated SiC powders on the microwave
properties is complicated in this paper, and the exact
mechanism yet is not ascertained, so more experiments need
to be done in the future.
For composites with n magnetic particles embedded in a
non-magnetic matrix, we can obtain the relationship between
theoretical effective permeability of the composites and the
total volume fraction of magnetic particles in the matrix.
Basically, the effective permeability meff increases for
composites with high-volume fraction p. A linear relationship
is also observed between the effective permeability and volume
fraction when the volume fraction p in the composites is small.
The SiC particles are non-magnetic, however, the electroless
Ni–Co–P-coated SiC powders are magnetic, therefore the
weight gain of electroless Ni–Co–P-coated SiC powders in the
matrix increases will increase the effective permeability meff of
composites. Similarly, the SiC particles are semiconductor, and
the thin film of Ni–Co–P is conductor, so the volume fraction p
of electroless Ni–Co–P-coated SiC powders in the matrix
increases will also increase the permittivity of composites.
The normalized input impedance Zin of a single metal-
backed microwave-absorbing layer is given by [36]:
Z in ¼ffiffiffiffiffimr
er
rtanh
2p fdffiffiffiffiffiffiffiffimrerp
c(5)
where mr and er are the relative complex permeability and
permittivity, respectively, of the composite medium, c is the
velocity of electromagnetic waves in free space, f is the
frequency of microwaves, and d is the thickness of the absorber.
The reflection loss is related to Zin by [36]:
RL ¼ 20 log
���� Z in � 1
Z in þ 1
���� (6)
Thus, the surface reflectance of an absorber is a function of six
characteristic parameters, viz, m0r, m00r , e0r, e00r , f , and d. Fig. 6
shows the calculated reflection loss as a function of frequency
for sample 3 at different thicknesses. The calculations use the
Fig. 7. Frequency dependence of the reflection loss of samples 1–4 at thickness
of 2.5 mm.
Y. Li et al. / Applied Surface Science 254 (2008) 4708–47154714
actual values er and mr as shown in Figs. 4 and 5. The reflection
loss is found to depend sensitively on the thickness of the
absorber and the maximum attenuation of the incident wave is
observed with a thickness of 2.5 mm for sample 3, and the
values of reflection loss of the two peaks are �32.6 and �27.3,
respectively. As shown in Fig. 6, when it is related to same
sample the values of reflection loss become minus and they all
shift to lower frequency with increasing sample thickness.
Fig. 7 shows the calculated reflection loss for samples 1–4
at thickness of 2.5 mm. It is found that when it is related to same
sample thickness the values of reflection loss become minus
and they all shift to lower frequency with decreasing weight of
Co in the Ni–Co–P films. It is suggested that the reflection loss
is sensitive to a matching thickness and the composition of
samples.
The difference in microwave absorption properties of the
plated powders is resulted from the different composition
contents of the plated Ni–Co–P layers. The Bohr magnetons
(MB) of Co atom and Ni atom are 1.7 and 0.6, respectively. So,
the magnetism of cobalt is better than the nickel. P atom is non-
magnetic. Although cobalt content in sample 1 is the largest, the
phosphorus content in it exceeds 8 wt.%, which means poor
magnetism of sample 1. For comparison, the Co content of
sample 3 is larger and the P content of it is lower, so sample 3
possesses the better magnetism than the other samples. For
magnetic microwave absorbers, magnetic loss is the main
microwave loss tunnel. The powder with better magnetism
results in larger microwave loss. The frequency of the strongest
loss peak of the powders is different, which may be resulted
from the different magnetism of the powders. In a word, the
prepared samples exhibit good absorption performance in the
2–18 GHz frequencies and appear to be a potential microwave-
absorbing material.
4. Conclusions
In summary, the Ni–Co–P alloys were successfully
deposited on SiC particles by two-step electroless plating
process for the application to lightweight microwave absorbers.
Characterization techniques show that Ni–Co–P deposits are
spicular grains and comprised of a mixture of crystalline Ni3P
and a-Co, and exhibit low-specific saturation magnetization
and low coercivity. Due to the conductive and ferromagnetic
behavior of the Ni–Co thin films, high dielectric constant and
magnetic loss can be obtained in the microwave frequencies. It
is concluded that the higher the Co content in the alloy films, the
higher absorption rates for electromagnetic radiation. Reflec-
tion loss less than �32 dB were found at 6.30 GHz with a
thickness of 2.5 mm. The proposed absorber is well advanced in
both mass and thickness in comparison with conventional
ferrite absorber.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (Grant No. 20577017).
References
[1] R.C. Che, L.-M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Adv. Mater. 16
(2004) 401–405.
[2] A. Wadhawan, D. Garrett, J.M. Perez, Appl. Phys. Lett. 83 (2003) 2683–
2685.
[3] K. Hatakeyama, T. Inui, IEEE Trans. Magn. 20 (1984) 1261–1263.
[4] J. Smit, H.P.J. Wijn, Ferrites, Phillips Technical Library, Eindhoven, 1959,
p. 271.
[5] D. Rousselle, A. Berthault, O. Acher, J.P. Bouchaud, P.G. Zerah, J. Appl.
Phys. 74 (1993) 475–479.
[6] G.O. Mallory, J.R. Hajdu, Electroless Plating, AESF, USA, 1990.
[7] E.A. Pavlatou, M. Stroumbouli, P. Gyftou, N. Spyrellis, J. Appl. Electro-
chem. 36 (2006) 385–394.
[8] P. Gyftou, M. Stroumbouli, E.A. Pavlatou, P. Asimidis, N. Spyrellis,
Electrochim. Acta 50 (2005) 4544–4550.
[9] N.K. Shrestha, M. Masuko, T. Saji, Wear 254 (2003) 555–564.
[10] K.H. Hou, M.D. Ger, L.M. Wang, S.T. Ke, Wear 253 (2002) 994–1003.
[11] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, et al. J. Appl. Phys. 76 (1994)
1363–1398.
[12] R. Asthana, J. Mater. Sci. 33 (1998) 1959.
[13] R. Tarozaite, M. Kurtinaitiene, A. Dziuve, Z. Jusys, Surf. Coat. Technol.
115 (1999) 57–65.
[14] L. Shi, C. Shun, P. Gao, F. Zhou, W. Liu, Appl. Surf. Sci. 252 (2006) 3591–
3599.
[15] S.-S. Kim, S.-T. Kim, J.-M. Ahn, K.-H. Kim, J. Magn. Magn. Mater. 271
(2004) 39–45.
[16] R. Wang, et al., Preparation and characterization of nanodiamond
cores coated with a thin Ni–Zn–P alloy film, Mater. Charact. 59 (2008)
108–111.
[17] O.M. Glenn, B.H. Juan, Electroless Plating: Fundamentals Applications,
American Electroplaters and Surface Finishers Society, Orlando, 1990.
[18] H.H. Chiang, W. Shen, The Fundamentals and Practice of Electroless
Plating, National Defense Industry Publisher, Bejing, 2001.
[19] T.S.N. Sankara Narayanan, S. Selvakumar, A. Stephen, Surf. Coat.
Technol. 172 (2003) 298–307.
[20] K.H. Hur, J.H. Jeong, D.N. Lee, J. Mater. Sci. 26 (1991) 2037.
[21] M.R. Khan, E.L. Nicholsan, J. Magn. Magn. Mater. 54–57 (1986) 1654–
1656.
[22] N. Fenineche, A.M. Chaze, C. Coddet, Surf. Coat. Technol. 88 (1996)
264–268.
[23] K. Huller, M. Sydow, G. Dietz, J. Magn. Magn. Mater. 53 (1985) 269–274.
[24] R. Tarozaite, G. Stalnionis, A. Sudavicius, M. Kurtinaitiene, Surf. Coat.
Technol. 138 (2001) 61–70.
Y. Li et al. / Applied Surface Science 254 (2008) 4708–4715 4715
[25] G. Rivero, M. Multigner, J.M. Garcia, P. Crespo, A. Hernando, J. Magn.
Magn. Mater. 177–181 (1998) 119–120.
[26] H. Matsubara, A. Yamada, J. Electrochem. Soc. 141 (1994) 2386.
[27] M.Z. Wu, H.H. He, Z.S. Zhao, X. Yao, J. Phys. D: Appl. Phys. 33 (2002)
2927–2930.
[28] M.Z. Wu, H.H. He, Z.S. Zhao, J. Phys. D: Appl. Phys. 33 (2002) 2398–
2401.
[29] P. Chen, R.X. Wu, T.E. Zhao, F. Yang, J.Q. Xiao, J. Phys. D: Appl. Phys.
38 (2005) 2302–2305.
[30] A. Verma, A.K. Saxena, D.C. Dube, J. Magn. Magn. Mater. 263 (2003)
228–234.
[31] D. Wan, X. Ma, Physics of Magnetism, Press of University of Electric
Science and Technology of China, Chengdu, 1994, pp. 406–459.
[32] T. Tsutaoka, J. Appl. Phys. 93 (2003) 2789–2796.
[33] L. Olmedo, G. Chateau, C. Deleuze, J.L. Forveille, J. Appl. Phys. 73
(1993) 6992–6994.
[34] G. Viau, F. Ravel, O. Acher, F. Fievet-Vincent, F. Fievet, J. Appl. Phys. 76
(1994) 6570–6572.
[35] G. Viau, F. Ravel, O. Acher, F. Fievet-Vincent, F. Fievet, J. Magn. Magn.
Mater. 140–144 (1995) 377–378.
[36] S.S. Kim, S.B. Jo, K.K. Choi, J.M. Kim, K.S. Churn, IEEE Trans. Magn.
27 (1991) 5467.