Preparation of copper powder by glycerol process

Article abstract of DOI:10.1016/S0025-5408(01)00819-4

A simple, least energy intensive, efficient, low temperature chemical process for the production of copper powder of narrow size distribution is described. It involves reduction of copper salts by glycerol under atmospheric conditions at a temperature below 240°C. The powders produced by this process have been characterized by X-ray diffraction, chemical composition, BET surface area analysis and scanning electron microscopy. The powder is well crystalline and contains oxygen, carbon, and hydrogen as impurity elements. The purity of the powder produced by the process depends on the starting compound of copper. The glycerol process using copper acetate as starting compound yields copper powder having a purity of 99.7%.

Full text of DOI:10.1016/S0025-5408(01)00819-4

Materials Research Bulletin 37 ꢀ2002) 407±416
Preparation of copper powder by glycerol process
Amit Sinha*, B.P. Sharma
Powder Metallurgy Division, Bhabha Atomic Research Center, Vashi Complex,
Navi Mumbai 400 075, India
ꢀRefereed)
Received 15May 2001; received in revised form 20 August 2001; accepted 29 November 2001
Abstract
A simple, least energy intensive, ef®cient, low temperature chemical process for the
production of copper powder of narrow size distribution is described. It involves reduction of
copper salts by glycerol under atmospheric conditions at a temperature below 2408C. The
powders produced by this process have been characterized by X-ray diffraction, chemical
composition, BET surface area analysis and scanning electron microscopy. The powder is
well crystalline and contains oxygen, carbon, and hydrogen as impurity elements. The purity
of the powder produced by the process depends on the starting compound of copper. The
glycerol process using copper acetate as starting compound yields copper powder having a
purity of 99.7%. # 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Metals; A. Nanostructures; A. Electronic materials; B. Chemical synthesis
1. Introduction
Conductive pastes have been used for the formation of thick ®lm conductors such
as electrodes or conductive patterns in multilayered electronic parts and printed
circuit boards. Conductive paste includes metal powder dispersed in an organic
vehicle along with glass frit. Replacement of precious metal powders like gold, silver
or palladium by copper powder for production of conductive paste for hybrid
integrated circuit and for the metallization of multilayer ceramic capacitor ꢀMLCC)
is a current trend in the electronic industry. Copper offers several economic and
* Corresponding author. Fax: ‡91-22-766-6684.
E-mail address: amit97@yahoo.com ꢀA. Sinha).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ꢀ 0 1 ) 0 0 8 1 9 - 4

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A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
performance advantages over precious metals. The conductivity of copper is as good
as silver and 40% better than that of gold. The stability of copper at high frequencies is
better than gold or silver and it is compatible with most solder materials [1].
For conductive paste application, it is required to use high purity crystalline non-
agglomerated copper powder, which is free from surface oxidation. The oxidation of
the powder leads to the formation of a thick ®lm conductor with bad appearance and
poor solderability. In addition to the purity, it is highly desirable that the particles
should have narrow size distribution in order to precisely control the thickness of the
electrode formed with the conductive paste and also facilitate intimate metal bonding
[2].
Reduction of metallic compounds by gases is a widely employed method of
producing metal powders. Gases such as hydrogen, dissociated ammonia, carbon
monoxide, partially combusted hydrocarbon are few of the examples of the reducing
agents used. Copper powder can be produced by atomization, electrolysis, reduction
of sulfate solution using hydrogen and by solid-state reduction. Each of these pro-
cesses, however, produces particles of varying physical properties. Among the various
methods of preparation of copper powders, chemical routes facilitate molecular/
atomic level controls and also ef®cient scale up for processing and production [3].
Precipitation of metallic copper is accomplished through chemical reduction of a salt,
oxide, or hydroxide of copper in solution. Various reducing agents like starch [4],
hydrazine [5], hydroxylammine hydrochloride [6] etc. have been used for the
production of copper powder.
One of the successful methods for preparation of metal powders belonging to
groups VIII and IB is the so-called polyol process [7]. The polyol process essentially
involves preparation of metal powders by heating a suitable inorganic/organic
metallic salt in a polyol, which acts as a solvent as well as a reducing agent.
In the polyol process metal particles are formed through nucleation and growth
steps from the solution. The synthesis of metal powders of well-de®ned particle
shape and size with controlled size distribution is facilitated by the kinetic control of
the nucleation and growth steps. De®nite separation between the nucleation and the
growth stages is a prerequisite for the formation of mono-size particles [7]. One of
the measures to separate growth from nucleation may be addition of seed particles
where seed crystals act as foreign nuclei and homogeneous nucleation is thereby
replaced by heterogeneous nucleation.
Fievet et al. [7] has described a process for preparation of metal powder using
ethylene glycol/diethylene glycol as a polyol. In the present study, a process for
preparation of copper powder having narrow size distribution is described using
glycerol. Glycerol is having higher boiling point and dielectric constant compared to
other polyols ꢀTable 1). Due to its higher boiling point, it can provide higher reaction
temperature at atmospheric pressure. Due to the presence of one secondary and two
primary alcoholic groups in its structure, glycerol can reduce salts of easily reducible
metals. Though ethylene glycol, diethylene glycol, or their mixtures have been
studied for preparation of metal powders, the use of glycerol to prepare metal powders
has not been investigated.

A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
409
Table 1
Boiling point and dielectric constant ꢀat 258C) of different alcohols at one atmospheric pressure [8]
Alcohol
Boiling point ꢀ8C)
Dielectric constant ꢀe)
Methanol
Ethanol
64.7
78.3
32.70
24.55
19.92
17.93
37.70
42.50
2-Propanol
Isobutyl alcohol
Ethylene glycol
Glycerol
82.3
107.7
197.3
290.0
2. Experimental
For the preparation of copper powder three salts were selected. These were AR
grade copper oxide, copper hydroxide, and copper acetate. A predetermined amount
of starting salt was suspended in glycerol ꢀAR grade). The molar ratio ꢀR) between
starting metal compound and glycerol was ®xed at 0.02. The suspension was heated,
while stirring, by using a hot plate. The copper salts started to dissolve with increase
in temperature. The reaction temperature was measured using a thermometer inserted
in a glass port that was submerged in the solution.
The suspension of copper powders obtained was cooled down to room temperature
and then diluted with ethanol. The precipitated metal powders were separated from
the solution by repeated centrifugation using ethanol as a washing agent. The washed
powder was dried under vacuum at 808C for 2 h.
Powder samples were characterized by X-ray diffraction ꢀXRD, Philips Analytical,
Model PW1710) using Cu Ka radiation at a scan rate of 0.058/s. The powder
morphology was studied by scanning electron microscope ꢀSEM, Cambridge,
Stereoscan-240, UK). Speci®c surface area measurement of the powders was carried
out by using nitrogen adsorption BET surface area analyzer ꢀCE Sorptomatic 1990).
Before carrying out the surface area analysis the powder samples were thoroughly
dried under vacuum at 4008C for 2 h to remove any glycerol or its oxidation product
from the surface of the powder particles.
3. Results and discussion
The basis of selection of metal precursor salts for the preparation of copper powder
lies in the fact that it should be dissolved and reduced to metal by glycerol at the
reaction temperature and form by-product that would not add any impurity to the
powder. Though the solubility of the copper precursor compounds in glycerol is low at
room temperature, it increases with temperature leading to a transparent colored
solution, color of the solution being dependent on the starting precursor. Complete
dissolution of the copper precursors takes place before reduction by glycerol. The metal
particles form by homogeneous nucleation and growth mechanism. The morphology of
the metal powders produced through this route can be manipulated by heterogeneous

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nucleation by adding nucleating agents such as palladium powder. In the present study,
no external nucleating agents have been incorporated in the suspension.
When copper oxide is used as the precursor compound, the precipitation of copper
from the solution starts at a temperature of 2108C and completes at 2208C, which is
lower than the boiling point of glycerol. In the case of preparation of copper powder
from hydroxide, the precipitation completes at 2308C. The copper powder precipi-
tates at a temperature of 2348C when acetate salt is used. The total incubation period
for the precipitation of metal powder is less than an hour, which is much less than
other polyol processes as described by Chow et al. [3].
The XRD patterns of the copper powders prepared through glycerol route are
shown in Figs. 1±3. The pattern of the three powders corresponds to the FCC copper
phase. The powders are well crystallized as evidenced by the XRD pattern. The
broadening of peaks of the pattern correlates to the ®neness of the copper particle. The
crystallite size of the powder was calculated by line broadening of ꢀ1 1 1) peak of
XRD pattern using Scherrer's formula
0:9l
Crystallite size ˆ
b cos y
where b is the broadening due to the crystallite size which is obtained after correcting
the measured value ꢀB) with instrumental broadening ꢀb) as per the formula:
b2 ˆ B2 À b2. The instrumental broadening was measured using silicon as standard.
The BET surface area of the powders produced through glycerol route is shown in
Table 2. With the assumption of monodispersed spherical shaped crystal particles, the
particle size ꢀmm) was calculated based on the relation
6
Particle size ˆ
r SBET
Fig. 1. XRD pattern of copper powder produced through glycerol route using copper oxide as
precursor.

A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
411
Fig. 2. XRD pattern of copper powder produced through glycerol route using copper hydroxide as
precursor.
Fig. 3. XRD pattern of copper powder produced through glycerol route using copper acetate as
precursor.
Table 2
BET surface area measurement and calculated crystallite size of copper powders produced through
glycerol process
Copper
compound
BET specific surface
area ꢀSBET) ꢀm2/g)
Calculated particle
size from SBET ꢀnm)
Calculated crystallite
size from XRD ꢀnm)
Oxide
Hydroxide
Acetate
5.48
9.56
3.70
123
70
71
53
182
105

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A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
where r is density of copper ꢀg/cm3) and SBET is the measured value of specific
surface area in m2/g. The crystallite sizes of copper powders as calculated from
specific surface area data as shown in Table 2 reveal the potential of glycerol process
to produce nanostructured copper powder.
The SEM photomicrographs of the copper powders produced from copper
oxide are shown in Fig. 4a and b. The micrographs reveal that the powder is made
up of particles of narrow size distribution having an average particle size of 1 mm.
Fig. 4. ꢀa) and ꢀb) SEM photomicrographs of the copper powder produced through glycerol route from
copper oxide as precursor.

A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
413
Fig. 5. ꢀa) and ꢀb) SEM photomicrographs of the copper powder produced through glycerol route using
copper hydroxide as precursor.
Fig. 5a and b show, the photomicrographs of the copper powder produced through
glycerol route using copper hydroxide as starting material. The powder is spheroidal
having narrow size distribution. The average particle size of the powder is 3.5 mm.
The SEM photomicrographs of the copper powder produced from copper acetate
reveal that the powder is made up of spheroidal particles with a faceted structure
ꢀFig. 6a and b). The powder produced from copper acetate as precursor salt comprises
of particles of narrow size distribution having an average particle size of 5.6 mm.

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A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
Fig. 6. ꢀa) and ꢀb) SEM photomicrographs of the copper powder produced through glycerol route from
copper acetate as precursor.
The above results suggest that dense, polycrystalline copper powder with narrow
size distribution is produced through glycerol route. For a given molar ratio ꢀR), the
average particle size as well as crystallite size of the powder depends on the type of
copper precursor used. The particle size of the powders produced from different
precursors, as calculated from SBET data are higher than the respective crystallite
size obtained from X-ray line broadening. However, these calculated particle sizes
are lower as compared to the particle sizes estimated from SEM observations. The
particle size of the powder is calculated from speci®c surface area data with the

A. Sinha, B.P. Sharma / Materials Research Bulletin 37 -2002) 407±416
415
Table 3
Chemical analysis of copper powders produced through glycerol process
Copper compound
Impurity content ꢀwt.%)
Oxygen
Carbon
Hydrogen
Oxide
Hydroxide
Acetate
0.50.3
0.2
0.2
0.044
0.11
0.44
0.016
0.009
assumption that the powder comprises of spherical monosized particles. However,
the SEM photomicrographs of the copper powders reveal that glycerol process yields
spheroidal particles with faceted structure. Hence, the lower calculated particle size of
the copper powders obtained from the SBET measurements may be attributed to the
spheroidal faceted structure of powder particles and spread in the size distribution.
Since the process utilizes glycerol as reducing agent and AR grade copper salts as
starting compounds, the copper powder is expected to contain only oxygen, carbon,
and hydrogen. The typical chemical analysis of the powders produced from different
starting salts is given in Table 3. The process produces high purity copper powder
having a total level of impurity content less than 1 wt.%. The glycerol process using
copper acetate as starting salt yields powder with 99.7% purity. The powder produced
by this process contains a thin organic surface layer, which helps to prevent the
agglomeration and surface oxidation.
4. Conclusions
A simple chemical process for the production of copper is described where copper
precursor powder is reduced by glycerol at a temperature below the boiling point of
glycerol yielding nanostructured copper powder of narrow size distribution. Out of
the three compounds used, copper acetate produce copper powder of 99.7% purity
having a crystallite size below 110 nm.
Acknowledgments
The authors wish to thank A.R. Biswas of Atomic Fuels Division, BARC, Mumbai
for carrying out the scanning electron microscopy.
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