Fabrication of Ni-coated Al2O3 powders by the heterogeneous precipitation method

Article abstract of DOI:10.1016/S0025-5408(01)00628-6

Ni-coated Al₂O₃ powders were prepared by the heterogeneous precipitation method using Al₂O_3, Ni(NO₃)₂·6H₂O and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O was uniformly coated on the surface of Al₂O₃ particles with the thickness of 20 nm. The amorphous coating layer was crystallized to NiO with the size of about 15 nm at 500°C for 2 hours in air, meantime, the dispersant was eliminated. NiO was completely reduced to Ni at 700°C for 4 hours in hydrogen atmosphere. Ni with the size of about 20 nm in the coating layer was spherical and weakly agglomerated. The continuous coating layer became discontinuous during heating treatment.

Full text of DOI:10.1016/S0025-5408(01)00628-6

Materials Research Bulletin 36 (2001) 1307–1315
Pergamon
Fabrication of Ni-coated Al2O3 powders by the
heterogeneous precipitation method
Guo-jun Li*, Xiao-xian Huang, Jing-kun Guo
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-xi Road, Shanghai 200050,
People’s Republic of China
(Refereed)
Received 23 October 2000; accepted 6 February 2001
Abstract
Ni-coated Al2O3 powders were prepared by the heterogeneous precipitation method using Al2O3,
Ni(NO3)2⅐6H2O and NH4HCO3 as the starting materials. The amorphous NiCO3⅐2Ni(OH)2⅐2H2O was
uniformly coated on the surface of Al2O3 particles with the thickness of 20 nm. The amorphous
coating layer was crystallized to NiO with the size of about 15 nm at 500°C for 2 hours in air,
meantime, the dispersant was eliminated. NiO was completely reduced to Ni at 700°C for 4 hours in
hydrogen atmosphere. Ni with the size of about 20 nm in the coating layer was spherical and weakly
agglomerated. The continuous coating layer became discontinuous during heating treatment. © 2001
Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; A. Metals; B. Chemical synthesis; D. Microstructure
1. Introduction
Al2O3 ceramics are used in many fields. However, the utility of a ceramic material in an
engineering application is critically determined by its brittleness. Thus, the toughening of
ceramic materials is very necessary, and has been investigated for the years. The dispersion
of a ductile metallic phase (Ni, W, Mo, Cu etc.) in a brittle ceramic matrix is found to be a
promising way [1–4]. Reinforcement models show the importance of the homogeneity and
fine size of the metallic inclusion [5]. The control of the microstructure of Al2O3-metal
* Corresponding author. Fax: ϩ86-21-62513903.
E-mail address: guojunlee@netease.com (G.J. Li).
0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(01)00628-6

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composites is generally difficult to achieve by traditional techniques involving mechanical
mixing of ceramic and metallic powders followed by hot pressing. A fine-scale homogeneity
can be obtained using the sol-gel route [6–8]. However, the relatively high cost of some
reactants and the difficulty to control the gel drying step are drawbacks of this method.
In recent years, coating of processing aids on ceramic particles has been investigated for
producing homogeneous ceramics. It not only improves the green density and sintering
activity [9], but also enhances different phase uniformity and mechanical properties of
sintered body [10]. A few methods were used to prepare the coated powder, such as
co-precipitation method [11], sol-gel method [12] and electroless plating method etc. [13].
Co-precipitation method is a promising way for fabricating the coated power of ceramic in
above methods. Thus, a large number of studies have been done and got the exciting results
[14,15]. However, few literature on fabrication of Al2O3-metal composite materials from
metal particles coated Al2O3 have been reported yet.
In the present work, Ni-coated Al2O3 powder was prepared by the heterogeneous precip-
itation method. This paper addresses the difference in the preparation process with K. Niihara
and his co-workers [16]. The purpose of this paper is to expect to obtain much better
microstructure and properties of material.
2. Experimental Procedure
Ni-coated Al2O3 powder in the composition of Al2O3-10vol% Ni was prepared using
Al2O3 with diameter of 0.35 ␮m (Shanghai Songjiang Fertilizer, Co. China), Ni(NO3)2⅐6H2O
(Analytically Pure) and NH4HCO3 (Analytically Pure) as starting materials. Ni(NO3)2⅐6H2O,
Al2O3 and 0.5wt% (equivalent to Al2O3 weight) polyacrylic acid as dispersant were first
mixed in distilled water by ball milling for 48 hours. Next, NH4HCO3 solution of 1.0M was
added dropwise to the homogeneous slurry obtained above under vigorous stirring. Its
reaction was following equation [17],
3Ni2ϩ ϩ 6HCO31Ϫ ϩ H2O ϭ NiCO3⅐2Ni͑OH͒ ⅐2H2O2 ϩ 5CO21
(1)
2
The precipitates were not dissolved in ammonia solution [18]. To ensure complete
reaction, an excess of NH4HCO3 was used and pH value of the mixed solution was between
8–9 during precipitation. The resulting precipitates were filtered and thoroughly washed with
distilled water three times. Finally, the precipitates were washed with ethyl alcohol and dried
at room temperature for 24 hours. The as-dried precipitates were calcined in air at 450°C and
500°C for 2 hours, and then the as-calcined samples were reduced at 700°C for 4 hours in
hydrogen [16].
The thermal decomposition behavior of as-precipitated composite powder was studied by
thermogravimetric analysis (TGA) and differential thermal analysis (DTA) on Netzsch-STA
429 thermal analysis device. TGA-DTA determination was carried out in the air in a Al2O3
crucible at a heating rate of 10°C/min in range of room temperature to 750°C. Infrared
absorption spectroscopy (FTIR) spectra were determined on Nicolet 7900 spectrometer using
the KBr pellet technique and used to determine the dispersant content in the coated powders.
X-ray diffraction (XRD) patterns were obtained at a scanning rate of 4°/min with 2␪ range

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Fig. 1. DTA and TGA curves of as-precipitated powder.
from 10° to 70° using a fully automated diffraction (Rigaku RAX-10, Japan) with CuK␣
(0.15406 nm) radiation. A transmission electron microscope (TEM) coupled with energy-
dispersive spectroscope (EDS) (Model JEM-200CX, JEOL, Tokyo, Japan) was applied to
investigate the particle size, shape and thickness of the coated powders. Selected area
diffraction pattern (SAD) was used to identify the crystalinity of the powder.
3. Results and discussion
Fig. 1 shows DTA and TGA curves of as-precipitated powder. As observed from the TGA
curve, there are two major weight losses. The first major weight loss from 40°C to 155°C is
attributed to the desorption of physisorbed water and alcohol and the elimination of crys-
talline water. This is supported by an endothermic peak at 122°C on the DTA curve. A sharp
exothermic with a broad shoulder at 310°C is due to decomposition of the precipitates
together with the removal of the dispersant, and it coincides with the second weight loss over
the temperature range from 155°C to 450°C on the TGA curve. A stable weight is attained
for as-precipitated powder at about 450°C.
The presence of C in the powder is disadvantage to sintering of Al2O3-Ni powder, so the

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Fig. 2. FTIR spectra of (a) the uncoated (ϭ as-received) Al2O3 powder (b) as-precipitated powder (c) as-calcined
powder at 450°C for 2 hours in air (d) as-calcined powder at 500°C for 2 hours in air.
Fig. 3. TEM images of the uncoated and coated Al2O3 particles: (a) the uncoated (ϭ as-received) Al2O3 particles,
(b) the coated Al2O3 particles.

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Fig. 4. EDS spectra of the coated Al2O3 particles (uncalcined).
dispersant must be eliminated. To monitor the elimination of residual dispersant phase from
as-precipitated powder with increasing calcinations temperature, spectroscopic analysis
using FTIR was conducted on as-precipitated powder and those calcined at 450°C and
500°C, respectively. As shown in Fig. 2, the peak at 1379 cmϪ1 corresponding to CH2
stretching vibration [19] indicates the presence of the organic dispersant. As expected, these
spectra had the same feature, they showed a relatively broad but strong vibrational band over
the range of 3200 ϳ 3700 cmϪ1 and the weak peak at 1630 cmϪ1, corresponding to the

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Fig. 5. XRD patterns of (a) uncoated (ϭ as-received) Al2O3 powder (b) as-precipitated powder (c) as-calcined
powder at 500°C for 2 hours in air (d) as-reduced powder at 700°C for 4 hours in hydrogen atmosphere (after
calcinations at 500°C for 2 hours in air). }–Al2O3, F–NiO, ■–Ni.
stretching vibration of the absorbed water and a hydroxyl group [20,21]. The two peaks at
1405 cmϪ1 and 1464 cmϪ1 are attributed to the stretching vibrational mode of carbonate
group [21]. Upon calcination at 450°C for 2 hours, at which as-calcined powder has attained
a stable weight on the TGA curve (Fig. 1), the stretching vibrational peaks of carbonate
group completely disappear and the band intensities relating to both hydroxyl and organic are
reduced drastically although they are still visible. They have completely disappeared from
the FTIR spectrum when as-precipitated powder was calcined at 500°C for 2 hours and only
visible bands over the range of 400–900 cmϪ1 correspond to metal-O bond.
Fig. 3a is a TEM image of the uncoated Al2O3 powder. It is observed that the uncoated
Al2O3 particles have considerably smooth and dense surface. There are no other particles

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Fig. 6. TEM micrograph of NiO-coated Al2O3 particle calcined at 500°C for 2 hours in air.
adhered on the surface. However, a TEM image of as-precipitated powders (Fig. 3b)
indicates there is coating layer with thickness of about 20 nm on the surface of as-received
Al2O3 particles. EDS (Fig. 4) of the coated powder show the intensity ratio of Ni peak to Al
peak in the center field are much lower than those at the edge field. The presence of C mainly
results from organic dispersant adhered on the particles and the precipitates. This is a coated
structure. The core particles are Al2O3 and the coating layer is NiCO3⅐2Ni(OH)2⅐2H2O. The
coating layer is continuous. XRD result (Fig. 5b) shows the coating layer is amorphous phase
because no new diffraction peaks appear except for those of Al2O3 (Fig. 5a). Above results
suggest the nuclei of NiCO3⅐2Ni(OH)2⅐2H2O in the solution grow on surface of the Al2O3
core particles instead of forming discrete precipitates in the presence of ultrafine Al2O3
powder. Al2O3 surface acts as the heterogeneous nucleation site. In Fig. 6, the coating layer
of the Al2O3 powder was crystallized after calcining at 500°C for 2 hours in air. XRD (Fig.
5c) indicates the amorphous layer was converted to NiO with cubic phase during the
calcination. The diffraction peaks of NiO with markedly broadening effect show the particle
size of NiO is fine. According to the Scherrer formula (particle is spheric):
D ϭ 0.89␭/B cos͑␪͒
(2)
Where D is the average particle size, B is the full-width at half-height of the peak, ␭ is the
wavelength of X-ray and ␪ is the diffraction angle of the peak, the average particle size of
NiO calculated is 14.7nm.This agrees with an average particle size of about 15nm from TEM
observation. Nanocrystalline NiO in the coating layer was completely reduced to Ni after
as-calcined composite powder was treated at 700°C for 4 hours in hydrogen atmosphere by
XRD analysis (Fig. 5d). Its TEM and SAD are shown in Fig. 7. TEM image shows that Ni
in the coating layer is spherical and weakly agglomerated and the particle size is about 20nm.
It is evident that the coating layer becomes discontinuous. From SAD, there are two sets of
diffraction patterns in Ni-coated Al2O3 particles, the spots and the ring patterns correspond-
ing to Al2O3 crystal and Ni polycrystal, respectively.

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Fig. 7. TEM image and corresponding SAD pattern of Ni-coated Al2O3 particle reduced at 700°C for 4 hours in
hydrogen (after as-precipitated powder was calcined 500°C for 2 hours in air).
4. Conclusions
Nanocrystalline Ni particles are coated on the surface Al2O3 particles very successfully by the
heterogeneous precipitation method. In the presence of Al2O3 powder, Al2O3 surface acted as the
heterogeneous nucleation site, so the nuclei of the precipitates in slurry grew on the surface of
Al2O3 core particles. After the as-precipitated powder was calcined in air and reduced in
hydrogen atmosphere, Ni with the size of about 20 nm in the coating layers was spherical and
weakly agglomerated. The continuous coating layer became discontinuous during treatment.
References
[1] W.H. Tuan, R.J. Brook, J. Euro. Ceram. Soc. 6 (1990) 31–37.
[2] T. Sekino, K. Niihara, J. Mater. Sci. 32 (1997) 3943–3949.

G.J. Li et al. / Materials Research Bulletin 36 (2001) 1307–1315
1315
[3] S.-C. Wang, W.-C.J. Wei, Nanostuc. Mater. 10 (6) (1998) 938–1000.
[4] S.-T. Oh, T. Sekino, K. Niihara, J. Eruo. Ceram. Soc. 18 (1998) 31–37.
[5] A.G. Evans, Mat. Sci. Eng. A 105/106 (1988) 65–75.
[6] E. Breval, G. Dodds, C.G. Pantano, Mat. Res. Bull. 20 (1985) 1191–1205.
[7] E. Breval, Z. Deng, et al, J. Mater. Sci. 27 (1992) 1464–1468.
[8] E.D. Rodeghiero, O.K. Tse, et al, Materials Science and Engineering A 195 (1995) 151–161.
[9] C.M. Wang, F.L. Riley, J. Eur. Ceram. Soc. 10 (1992) 82–93.
[10] W.H. Tuan, H.H. Wu, T.J. Yang, J. Mater. Sci. 30 (1995) 855–859.
[11] D. Kapolnek, C. De Jonghe, J. Eur. Ceram. Soc. 7 (1991) 345–351.
[12] C.Y. Chai, W.H. Shih, J. Am. Ceram. Soc. 82 (2) (1999) 436–440.
[13] A.C.D. Chaklader, K.R. Linger, Comopsite 10 (1976) 239–243.
[14] S.W. Wang, X.X. Huang, J.K. Guo Mater. Lett. 28 (1996) 43–46.
[15] Micchael D. Sacks, Nazim Bozkurt, Gary W. Scheiff, J. Am. Ceram. Soc. 74 (10) (1991) 2428–2437.
[16] T. Sekino, T. Nakajima, S. Ueda, K. Niihara, J. Am. Ceram. Soc. 80 (5) (1997) 1139–1148.
[17] Li Yadong, Li Chengwei, Duan Xianfeng et al, Jounal of China University of Science and Technology 27
(3) (1997) 346–349.
[18] J.A. Dean Ed. Lange’s Handbook of Chemistry, thirteenth Edition, McGraw-Hill Book Company, 1985.
[19] Shangnong Zhu, Analysis of High Molecular Compound Materials, The Publishing House of Sciences,
Beijing, 1988 (in Chinese).
[20] G.A. Nazari, C. Julien, Solid State Ionics 80 (1995) 271–275.
[21] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, John Wiley & Sons. Inc. USA,
1963.