Electrically conducting formulations based on ultradispersed powders of copper, obtained by reduction of its salts with the hypophosphite ion

Article abstract of DOI:10.1023/B:RJAC.0000031275.81993.ff

Powders of copper were obtained by reducing its various salts with hypophosphorous acid and sodium hypophosphite. The influence exerted by various factors on the stability and electrical conductivity of composites on their base was studied.

Full text of DOI:10.1023/B:RJAC.0000031275.81993.ff

Russian Journal of Applied Chemistry, Vol. 77, No. 3, 2004, pp. 380 384. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 3,
004, pp. 386 390.
Original Russian Text Copyright
2
2004 by Obraztsova, Simenyuk, Eremenko.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrically Conducting Formulations Based on Ultradispersed
Powders of Copper, Obtained by Reduction of Its Salts
with the Hypophosphite Ion
I. I. Obraztsova, G. Yu. Simenyuk, and N. K. Eremenko
Kemerovo Branch, Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division,
Russian Academy of Sciences, Kemerovo, Russia
Received July 29, 2003
Abstract Powders of copper were obtained by reducing its various salts with hypophosphorous acid and
sodium hypophosphite. The influence exerted by various factors on the stability and electrical conductivity
of composites on their base was studied.
Thanks to unique physicochemical, electrical,
magnetic, and optical properties, polymeric formula-
tions with an ultradispersed metallic filler find wide
application in modern science and technology in de-
veloping current-conducting pastes, photo- and X-ray-
sensitive resists, shielding coatings, and electrodes.
Silver is most commonly used in electrically conduct-
ing formulations because of its high electrical conduc-
tivity and resistance to oxidation [1, 2]. Recently,
steadily increasing attention has been given to devel-
opment of copper-based formulations [3], whose
production cost is lower. In this context, the problems
associated with stabilization of ultradispersed powders
acetylsalicylic acids) leads to an increase in the reac-
tion rate by a factor of 5 10, decrease in the process
temperature by 20 40 C, and improvement of the
stability and electrical conductivity of the pastes.
It was found that epoxy formulations with addition
of stabilizers (1-naphthol and p-aminophenol) in
amount of 1% (relative to the mass of the copper
UDP) retain high electrical conductivity without
changes for more than 10 years, and formulations with
1
-naphthol have the lowest resistivity of (0.8 1.2)
6
10
m, which is close to that characteristic of
metals. It was also found that 1-naphthol is the best
oxidation inhibitor for the novolak resin, and addition
of glycerol as a plasticizer in a 1 : 3 ratio yields for-
mulations whose electrical conductivity (resistivity
(
UDP) of copper and improvement of the service life
and electrical conductivity of composites on their base
are of current interest.
1
.2 10 6 m) is comparable with that of the best
Previously, methods were developed for obtaining
copper UDP with average particle size of 20 40 nm
by reducing various copper salts with glycerol and
making formulations on their base with epoxy and
novolak resins [4 8]. The most efficient is the reduc-
tion of basic carbonate, acetate, and tartrate of copper
with glycerol, and formulations on their base with
novolak resin exhibit higher electrical conductivity
samples with epoxy resin. Pastes based on the novo-
lak resin are only slightly inferior to the above formu-
lations in stability, but they are less expensive and
more readily available, and, therefore, primary atten-
tion will be given here to novolak formulations.
The study is concerned with reduction of various
copper salts with hypophosphorous acid and sodium
hypophosphite at a molar ratio (Cu) : (H PO ) =
= (0.6 1.9) 10 6
2
2
[
(
resistivity
more than 6 years).
m] and stability
1
: (1.5 4.0). The promise of this method is in that
the reduction with hypophosphite ions is carried out
under milder conditions (reaction temperature not
higher than 95 C, reaction time 2 35 min) than those
in reduction with glycerol (130 200 C, 15 240 min).
The copper UDP obtained was used to prepare formu-
lations with phenol formaldehyde novolak resin. The
influence exerted by various factors on the stability
and electrical conductivity of the formulations ob-
In addition, it was found that, with appropriate
stabilizing agents, it is also possible to prepare formu-
lations based on copper sulfate with the electrical
conductivity and stability comparable with those of
pastes based on copper UDP obtained by reduction of
readily decomposing copper salts. Introduction of
organic acid reductants (ascorbic, citric, formic, and
1
070-4272/04/7703-0380 2004 MAIK Nauka/Interperiodica

ELECTRICALLY CONDUCTING FORMULATIONS BASED ON ULTRADISPERSED POWDERS
381
tained was studied. These materials were compared
with samples obtained previously by reduction with
glycerol and with industrially used pastes based on
noble metals.
EXPERIMENTAL
The copper salts were reduced with sodium hypo-
phosphite as follows: into a vessel with a solution of
a copper(II) salt heated to 80 90 C, we gradually
d, nm
poured with continuous agitation a solution of sodium
hypophosphite heated to 50 60 C. Then the reaction
mixture was heated to 80 95 C [depending on a
copper(II) salt used] and kept at this temperature until
an ultradispersed copper powder precipitated. After
that, the UDP was washed with water to neutral reac-
tion of the washing water, treated with solutions of
hydroquinone and stearic acid in ethanol, and stored
under a layer of the solution of stearic acid in ethanol.
According to small-angle X-ray scattering data
Fig. 1. Mass function D (d) of the size (d) distribution
of ultradispersed copper particles, calculated from small-
angle X-ray scattering curves in the approximation of
uniform spherical particles.
m
log P [ m]
,
years
(
Fig. 1), the copper UDP obtained has particle size in
the range 8 100 nm, with 45% of the particles (by
mass) being less than 30 nm in size. The formulations
prepared from SF-010 phenol formaldehyde resin and
copper UDP (with 80 wt % metallic phase) were kept
at 140 145 C for 4 h. The dc resistance of the pastes
was measured (12 V, 2.5 mA) at room temperature
Fig. 2. Electrical resistivity
UDP + novolak resin SF-010) (1) in the initial state and
2) after additional treatment with formic acid, vs. the
testing time
of the formulation (copper
(20 25 C).
(
To improve their resistance to oxidation, all the
.
copper UDP samples were treated with solutions of
hydroquinone and stearic acid in ethanol, as suggested
in [4]. The ultradispersed powders of copper obtained
by reduction of copper(II) salts (and CuSO 5H O,
at a lower rate because of their lower solubility in
water. With CuCl used, copper UDP was formed, in
2
contrast to the case of reduction with glycerol [8], but
formulations on its base could not be obtained because
of the decreased stability and increased reactivity of
the powder. Before being introduced into formula-
tions, all the copper UDP samples obtained were
treated with formic acid.
4
2
in particular) with sodium hypophosphite are highly
active and are readily oxidized in air because of their
increased dispersity. As a result, the formulations on
their base have relatively high resistivity (10 2
m)
and are very unstable in storage (Fig. 2, curve 1).
However, it was found that treatment of copper UDP
with formic acid leads to a dramatic increase in the
electrical conductivity and stability of the formula-
tions (Fig. 2, curve 2). This is due to the fact that
treatment with formic acid removes the oxide film
from the surface of UDP particles and stabilizes
metallic copper against further oxidation.
The copper UDP samples obtained were used to
prepare formulations with a novolak resin at a copper
UDP to novolak resin ratio of 100 : 25. The highest
electrical conductivity and stability were observed for
a formulation based on copper formate, since in this
case the reaction products (formate ion etc.) stabilize
copper (sample no. 8). Only slightly inferior in elec-
trical conductivity to this sample are formulations
based on copper sulfate (sample nos. 5, 6) and copper
citrate (sample no. 9), but they are unstable in storage.
The formulations based on copper nitrate and acetate
have even lower electrical conductivity and stability.
Inorganic (nitrate, sulfate, chloride) and organic
acetate, formate, citrate) salts of copper(II) were
(
reduced with sodium hypophosphite. The results are
listed in Table 1. It can be seen that the highest reduc-
tion rate is observed for copper nitrate, sulfate, and
formate (Table 1, sample nos. 1, 2, 5, 6, 8). Copper
acetate and nitrate (sample nos. 3, 4, 9) are reduced
Apparently, the presence of an excess amount of
sodium hypophosphite leads to higher electrical con-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

382
OBRAZTSOVA et al.
Table 1. Reduction of copper(II) salts with sodium hypophosphite in an aqueous medium.* SF-010 novolak resin : copper
UDP = 25 : 100
1
06, m, at indicated testing time, years
Sample
no.
Cu(II) : NaH PO ,
r,
min
Tr,
C
2
2
Copper salt
mol : mol
1
day
1
2
3
4
1
2
3
4
5
6
7
8
9
Cu(NO3)2 3H O
1 : 1.5
1 : 2
1 : 1.5
1 : 2
1 : 1.5
1 : 2
1 : 3
7
5
35
30
10
7
10
10
15
85
85
90
90
85
85
90
85
95
11.2
8.5
18.2
10.8
8.2
15.6
11.8
22.6
14.3
10.6
8.2
22.5
16.9
32.4
19.8
16.8
12.0
31.8
22.4
44.2
27.2
24.4
16.4
40.2
28.2
55.8
34.6
32.6
21.2
2
Cu(OAc)2 H O
2
CuSO4 5H O
2
6.8
CuCl2 6H O
2
Cu(COOH)2
1 : 2
1 : 3
4.2
7.4
4.4
8.6
4.6
11.8
4.7
14.5
Cu (Citr) 2.5H O
2
2
*
and T are the time and temperature of the reduction of copper(II) salt with sodium hypophosphite.
r
r
ductivity and stability of copper formulations. This is
presumably due to an additional stabilizing effect of
unreacted hypophosphite ions, which decompose in
the course of the reaction under the action of water,
to give hydrogen [9].
the ratio between UDP, novolak resin, 1-naphthol, and
glycerol on the stability and electrical conductivity of
pastes with novolak resin (Fig. 3). Ultradispersed
copper powders were obtained by reducing copper
sulfate with sodium hypophosphite at a ratio (Cu) :
(
H PO ) = 1 : 1.5 (Table 1, sample no. 5). It was
We also studied the influence exerted by stabilizers
introduced directly into the novolak resin on the elec-
trical conductivity and stability of copper powders.
It was noted in [8] that the best stabilizer (for both
epoxy and novolak resins) is 1-naphthol, and addi-
tional introduction of glycerol as a plasticizer leads
to a considerable increase in the stability and electrical
conductivity. In this work, we analyzed the effect of
2 2
found that, after introduction of 1-naphthol and gly-
cerol, phenol formaldehyde novolak formulations
based on copper UDP obtained by the hypophosphite
method become comparable in stability and electrical
conductivity with copper ultradispersed powders
prepared by reduction with glycerol.
It can be seen that the optimal content of 1-naphthol
in the formulations is 2 wt % relative to the amount of
copper UDP (sample nos. 5, 6). A smaller amount of
6
1
0 ,
m
1
-naphthol is insufficient for obtaining stable formula-
tions with nearly metallic electrical conductivity
sample nos. 1 4). As for the use of a larger amount,
(
it is unfeasible because the electrical conductivity
and stability increase in this case only slightly. Ap-
parently, the stability and electrical conductivity also
become higher as the amount of glycerol is raised
(sample nos. 2, 4, 6). However, a large excess of gly-
cerol may impair the properties of the pastes, because
they show poorer hardening and become less stable in
storage. Introduction of glycerol in an amount of 1.5
3
% (relative to the mass of copper UDP) diminishes
,
years
the brittleness of the novolak resins. The highest sta-
bility and electrical conductivity are exhibited by
sample no. 6, stabilized with 2 wt % naphthol and
Fig. 3. Effect of stabilizers on the electrical resistivity
of formulations with an SF-010 phenol formaldehyde
novolak resin. ( ) Testing time. Weight ratio of copper
UDP, SF-010, 1-naphthol, and glycerol (plasticizer):
1) 100 : 25 : 0.5 : 1.5, (2) 100 : 25 : 0.5 : 3, (3) 100 : 25 :
.0 : 1.5, (4) 100 : 25 : 1.0 : 3, (5) 100 : 25 : 2.0 : 1.5, and
6) 100 : 25 : 2.0 : 3.
3
1
wt % glycerol. The higher optimal content of
-naphthol in the formulations, compared to that in
(
1
(
formulations prepared on the basis of ultradispersed
copper powders obtained by reduction with glycerol,
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ELECTRICALLY CONDUCTING FORMULATIONS BASED ON ULTRADISPERSED POWDERS
383
Table 2. Reduction of copper(II) salts with sodium hypophosphite in a water hexanol medium. SF-010 novolak
resin : copper UDP = 25 : 100
Sam-
ple
no.
106, m, at indicated testing time, years
Water : hexa- Cu(II) : NaH PO ,
r,
min
Tr,
C
2
2
Copper salt
nol, v/v
mol : mol
1 day
1
2
3
4
1
2
3
4
5
6
7
8
9
0
Cu(NO3)2 3H O
10 : 1
5 : 1
10 : 1
5 : 1
10 : 1
5 : 1
5 : 1
10 : 1
10 : 1
5 : 1
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 4
1 : 2
1 : 2
2
4
35
30
5
4
4
4
7
85
85
90
90
85
85
85
90
85
85
6.4
4.2
7.8
5.6
3.8
2.6
2.8
7.4
4.8
8.9
6.2
4.2
2.8
3.0
8.6
5.6
10.2
7.1
5.1
3.1
9.8
6.2
11.8
7.9
5.8
3.4
10.7
6.8
13.2
9.2
6.4
3.8
2
Cu(OAc)2 H O
2
CuSO4 5H O
2
*
3.2
3.5
CuCl2 6H O
2
Cu(COOH)2
2.1
1.7
2.2
1.8
2.4
2.0
2.5
2.1
1
5
*
Formulation prepared on the basis of copper(II) (sample no. 6) a year after UDP synthesis.
is due to the higher dispersity and reactivity of the
UDP synthesized by reduction with sodium hypo-
phosphite.
unstable, are rapidly oxidized, and formulations based
on these UDP samples are not electrically conducting.
This is probably due to the presence of chloride ions
adsorbed on the UDP surface.
Copper(II) compounds were reduced with sodium
hypophosphite both in an aqueous medium and in
a mixture of solvents, polar (distilled water) and less
polar (hexyl alcohol), at a water : hexanol volume
ratio of 5 : 1 and 10 : 1. The reduction was carried out
as follows: into a vessel containing a copper(II) salt
solution was poured a sodium hypophosphite solution,
the mixture was agitated, a necessary amount of hexyl
alcohol was added, and the resulting mixture was
heated to the reaction temperature of 80 90 C and
kept at this temperature for 2 35 min, depending on
the copper(II) salt used. Then, the hexanol layer was
decanted, and the UDP formed was washed with water
and treated with solutions of hydroquinone and stearic
acid in ethanol. It was found that the stability of the
UDP is enhanced as the amount of hexyl alcohol is
increased.
Copper(II) acetate, sulfate, formate, basic carbo-
nate, and oxide were reduced with hypophosphorous
acid in a water hexanol medium at a water : hexanol
volume ratio of 5 : 1. It was found that basic copper
carbonate and copper oxide can only be reduced in the
presence of excess hypophosphorous acid [ (H PO ) :
3
2
(Cu) = 4], and, being poorly soluble in water, these
compounds do not react with sodium hypophosphite.
The results obtained are listed in Table 3. Apparently,
the reduction of copper(II) compounds with H PO2
3
occurs more actively than that with sodium hypo-
phosphite.
Comparison of Tables 2 and 3 shows that the high-
est electrical conductivity is exhibited by formulations
based on copper UDP samples obtained by reduction
with hypophosphorous acid. This is due to the fact
that, according to the results of X-ray phase analysis,
virtually no oxides could be found in the samples
reduced with hypophosphorous acid even in an aque-
ous medium, in contrast to the samples synthesized by
reduction with sodium hypophosphite. Thus, the high-
est electrical conductivity and stability of formulations
based on copper UDP are attained in the case of
reduction of copper(II) formate or sulfate with hypo-
phosphorous acid, and also upon introduction of
The copper UDP samples synthesized were used to
prepare formulations with novolak resin. The results
obtained are listed in Table 2. It can be seen that
all the samples prepared in the system water hexanol
have higher electrical conductivity and stability, com-
pared to the samples synthesized in an aqueous medi-
um (Table 1). The highest stability and electrical con-
ductivity were observed for the UDP samples based
on copper(II) formate and sulfate (even when the UDP
was stored for a year). This is due to the action of
hexanol, which, being adsorbed on the surface of
copper UDP, prevents its oxidation and aggregation.
However, the copper UDP samples obtained from
copper chloride (Table 2, sample no. 10) are rather
2
wt % 1-naphthol and 3 wt % glycerol into the
formulations.
These formulations are only slightly inferior in
electrical conductivity and stability to those ob-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

384
OBRAZTSOVA et al.
Table 3. Reduction of copper(II) salts with hypophosphorous acid in a water hexanol medium. Volume ratio
water : hexanol = 5 : 1, SF-010 novolak resin : copper UDP = 25 : 100
6
1
0 ,
m, at indicated testing time, years
Sample
no.
Cu(II) : H PO ,
r,
min
Tr,
C
3
2
Copper salt
mol : mol
1
day
1
2
3
4
1
2
3
4
5
Cu(OAc)2 H O
1 : 2
1 : 2
1 : 2
1 : 4
1 : 4
20
3
4
15
25
90
85
85
90
90
4.2
4.6
2.2
1.6
2.7
2.7
5.4
2.5
1.7
3.1
3.0
6.2
2.8
1.8
3.6
3.4
6.6
3.2
2
CuSO4 5H O
2.0
1.5
2.4
2.5
2
Cu(COOH)2
CuCO3 Cu(OH)2
CuO
4.1
3.9
tained by reduction with glycerol and stabilized with
-naphthol and glycerol, but their synthesis is charac-
terized by lower consumption of energy and materials
milder conditions, low expenditure of the reducing
agent, lower expenditure of a solvent for UDP wash-
ing).
conductive upon treatment with formic acid.
3) The ultradispersed copper powders synthesized
in a water hexanol medium show higher stability and
electrical conductivity.
1
(
(
(4) The increased stability and low production cost
of these materials make them promising for use in
modern nanoelectronics instead of composites based
on noble metals.
Thus, it is preferable to perform the reduction of
copper(II) compounds with sodium hypophosphite
and hypophosphorous acid in the system water
hexanol, rather than in the aqueous medium. Adsorp-
tion of hexyl alcohol on the surface of copper particles
substantially improves their stability and raises the
electrical conductivity, so that formulations based on
the UDPs of this kind become comparable with those
prepared on the basis of UDPs synthesized by reduc-
tion with glycerol [4 8] and can compete with similar
materials based on silver [1 2].
REFERENCES
1
2
3
4
. US Patent 5891367.
. US Patent 5929141.
. JPN Patent Appl. 3239767.
. RF Patent 2031759.
5. RF Patent 2115516.
6
. Obraztsova, I.I., Efimov, O.A., Eremenko, N.K., and
Simenyuk, G.Yu., Izv. Ross. Akad. Nauk, Neorg.
Mater., 1995, vol. 31, no. 4, pp. 798 799.
CONCLUSIONS
(
1) Ultradispersed copper powders and formula-
7
8
. Obraztsova, I.I., Simenyuk, G.Yu. and Eremenko, N.K.,
Inorg. Mater., 1999, vol. 35, no.. 8, pp. 792 794.
tions based on them were synthesized by reducing
various copper salts with sodium hypophosphite and
hypophosphorous acid.
. Obraztsova, I.I., Simenyuk, G.Yu., and Eremen-
ko, N.K., Zh. Prikl. Khim., 2002, vol. 75, no. 11,
pp. 1772 1775.
(2) The ultradispersed copper powders obtained by
reduction with sodium hypophosphite and hypophos-
phorous acid become more stable and electrically
9. Chernogorenko, V.B. and Tasybaeva, Sh.B., Zh. Prikl.
Khim., 1995, vol. 68, no. 4, pp. 529 533.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004