Effect of chemical modification of ultradispersed copper powders on electrical conductivity of formulations on their base

Article abstract of DOI:10.1023/A:1022277129015

Ultradispersed powders of copper were obtained by reducing various copper salts with glycerol, and formulations on their base were prepared. The effect of various modifiers on the dispersity, stability, and electrical conductivity of copper powders was ex

Full text of DOI:10.1023/A:1022277129015

Russian Journal of Applied Chemistry, Vol. 75, No. 11, 2002, pp. 1736 1739. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 11,
2002, pp. 1772 1775.
Original Russian Text Copyright
2002 by Obraztsova, Simenyuk, Eremenko.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Effect of Chemical Modification of Ultradispersed Copper
Powders on Electrical Conductivity of Formulations
on Their Base
I. I. Obraztsova, G. Yu. Simenyuk, and N. K. Eremenko
Institute of Coal and Coal Chemistry, Siberian Division, Russian Academy of Sciences, Kemerovo, Russia
Received March 13, 2002
Abstract Ultradispersed powders of copper were obtained by reducing various copper salts with glycerol,
and formulations on their base were prepared. The effect of various modifiers on the dispersity, stability, and
electrical conductivity of copper powders was examined.
Synthesis and study of ultradispersed powders
(UDPs) of metals and development of electrically
conducting materials on their base is a topical direc-
tion of research in modern science. Recently, increas-
ing attention has been given to copper [1, 2], which is
virtually as good as noble metals as regards its elec-
trical conductivity [3 5] and much less expensive.
However, copper powders exhibit rather high activity
and are oxidized in the course of time, which inevita-
bly impairs the conductivity. This problem can be
solved by optimizing the preparation process, modify-
ing the surface of copper UDPs, and creating formula-
tions on their base, whose electrically conducting
properties are preserved for a long time. Data on the
longevity of electrically conducting formulations are
virtually lacking from the literature.
treated with pentane can be stored for a long time not
only in solution, but also in air.
In this work, we prepared UDPs from various
copper(II) salts and examined the influence exerted by
chemical modification of copper UDPs on the elec-
trical conductivity and stability of formulations on
their base. The data obtained in the study are com-
pared with those published previously [8, 9]. New
data on the formulation longevity are presented.
EXPERIMENTAL
Copper UDPs were prepared by reducing various
copper salts with glycerol by the method described in
[9]. However, it was found that complete reduction of
readily decomposing salts (acetate, basic carbonate,
and tartrate) requires smaller amount of glycerol,
lower temperature T , and shorter reaction time . The
Previously, two methods for obtaining copper
UDPs have been developed [6, 7]. These methods are
based on reduction of copper sulfate with glycerol.
The optimal temperature, time, and reagent concentra-
tions were determined. Also, the influence exerted
by organic acids with reducing properties (ascorbic,
citric, etc.) on the reaction of copper sulfate with
glycerol and on the properties of the copper powders
obtained was examined. It was established that addi-
tions of these acids accelerate the reaction by a factor
of 5 6, allow the process temperature to be decreased
by 20 40 C, and ensure enhanced stability and elec-
trical conductivity of the resulting copper UDPs.
r
r
results obtained at different copper salt to glycerol
mass ratios A are presented in Table 1. It can be seen
that the reduction of copper basic carbonate, acetate,
and tartrate is the most effective, and their formula-
tions with novolac show high electrical conductivity
and are stable for a long time (more than 5 years)
without addition of any stabilizers. Here, copper UDP
is presumably stabilized in the course of the reaction
by glycerol itself.
It also follows from Table 1 that, in reduction of
copper(II) sulfate, the electrical conductivity of the
resulting formulations falls dramatically when no ad-
ditional stabilizers are introduced into the resin (ex-
periment no. 1). When, however, modifiers are intro-
duced (experiment no. 2), the formulations obtained
It was also found that, upon an appropriate treat-
ment with solutions of hydroquinone and stearic acid
in ethanol or pentane, copper UDPs possess increased
dispersity (average particle size 20 30 nm), stability,
and electrical conductivity. Furthermore, powders
1070-4272/02/7511-1736$27.00 2002 MAIK Nauka/Interperiodica

EFFECT OF CHEMICAL MODIFICATION OF ULTRADISPERSED COPPER POWDERS
1737
Table 1. Conditions of copper UDP preparation from various copper salts and resistivity of novolac formulations based
on copper UDPs
Experi-
ment
no.
104,
cm, at indicated testing time, years
,
mrin
Compound
A
Tr,
C
1 day 1 month 0.5
1
2
3
4
5
1
2
3
4
5
6
7
CuSO4 5H2O
1 : 10
1 : 10
1 : 3
1 : 4
1 : 5
300
300
40
14
145
350
180 190
180 190
145 155
140 145
160 165
160 170
5.4
1.2
0.6
0.6
1.2
8.0
1.2
1.2
0.8
1.4
18.2 32.4 48.4 57.6 64.8 72.6
CuSO4 5H2O*
Cu(OAc)2 H2O
CuCO3 Cu(OH)2
CuTart 3H2O
Cu(NO3)2 3H2O**
CuCl2 2H2O***
1.3
1.2
0.9
1.4
1.5
1.2
0.9
1.5
1.7
1.2
0.9
1.6
1.8
1.2
0.9
1.7
1.8
1.2
0.9
1.8
1.9
1.2
0.9
1.9
1 : 10
1 : 10
* Formulation stabilized with 1-naphthol and glycerol simultaneously.
** Copper UDPs are rapidly oxidized by reaction products.
*** No copper powder is formed.
compare well in the electrical conductivity and stabil-
ity with pastes based on copper UDPs produced by
reduction of readily decomposing copper salts (ex-
periment nos. 3 5).
(10-fold). Furthermore, it was found that the nature of
the acid also affects the particle size. For example, the
highest dispersity is observed when acetylsalicylic
acid is added as initiating agent, since the major part
of particles are 10 40 nm in size (according to elec-
tron microscopy). In the case when ascorbic acid is
used, a minor amount of 100 200-nm aggregates are
present together with the highly dispersed phase, and
introduction of citric acid leads to formation of 40
300-nm particles. It was found that the dispersity
grows with increasing acid concentration. This is due
to the following facts: (i) particle agglomeration be-
comes weaker under milder reaction conditions; (ii) in
some cases, introduction of acids instantaneously ini-
tiates reduction to give a large number of nuclei of the
metallic phase; and (iii) organic acids may enter into
the composition of intermediate reaction products,
thereby changing the structure of copper UDPs
formed on their base.
In the case of reduction of copper(II) nitrate, the
UDP formed is very rapidly oxidized in the course of
the reaction, which markedly diminishes the yield of
the metallic phase.
In reduction of copper(II) chloride, no metallic
copper is formed, even upon addition of an initiator,
ascorbic acid (up to 0.5 g per gram of copper). In
the reaction with copper chloride, glycerol undergoes
extensive tarring to give a black viscous mass solidi-
fying on cooling.
The aforesaid suggests that it is preferable to use
copper sulfate for commercial preparation of elec-
trically conducting materials. On the one hand, other
copper salts are less readily available and more expen-
Prolonged storage of copper UDPs in air or in
ethanol may lead to partial oxidation of the surface
layer, with the electrical conductivity of formulations
decreasing dramatically. It was found that the elec-
trical conductivity can be easily restored in this case
by treating the copper UDP with formic acid (1 2 ml
of HCOOH per gram of copper). No UDP oxidation is
observed in the case of storage in pentane.
sive, and, on the other, CuSO -based formulations
4
with various modifiers are virtually as good as pastes
prepared from other copper salts, as regards their
stability and electrical conductivity.
The effect of chemical modification of the surface
of copper UDPs was studied in different stages: in
synthesis, treatment, storage, and formulation prepara-
tion.
The copper UDPs obtained were used to prepare
electrically conducting formulations with epoxy or
novolac resins. The formulations were cured in ac-
cordance with the polymerization mode of the resin
employed. A study of the influence exerted by reduc-
tion of copper sulfate with glycerol on the electrical
conductivity of the formulations demonstrated that
the number of conducting chains in formulations with
It was shown previously [9] that introduction of
organic acids with reducing properties (ascorbic,
oxalic, citric, and formic) into the reaction mixture
much intensifies the reduction process. It was found
here that addition of acetylsalicylic acid in amount of
5 wt % relative to copper sulfate pentahydrate makes
it possible to diminish the process time even more
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002

1738
OBRAZTSOVA et al.
104, cm
number of organic acids (initiating agents) at different
mass ratios B (CuSO 5H O : glycerol : initiating
4
2
agent) on the parameters and T of the copper UDP
r
r
preparation process.
As follows from Fig. 1 and Table 2, introduction of
various organic acids (ascorbic, oxalic, citric, formic,
or acetylsalicylic) markedly shortens
of copper
r
reduction with glycerol (by a factor of more than 10 in
the case of acetylsalicylic acid) and yields formula-
tions with novolac resin that are characterized by high
stability (for more than 6 years) and low resistivity
(compared to sample no. 1 prepared without introduc-
tion of initiating agents). It can also be seen that the
electrical conductivity and stability of the formula-
tions grow with increasing concentration of any of
the organic acids (sample nos. 3, 5, 6, and 8).
, years
Fig. 1. Electrical resistivity of formulations comprising
SF-010 novolac resin, copper UDP, and organic acid vs.
testing time . Curve nos. are sample nos. in Table 1.
The effect of various stabilizing additives on the
electrical conductivity and stability of formulations
with epoxy resin is illustrated in Fig. 2a. It can be
seen that all formulations with addition of 1-naphthol
or p-aminophenol retain unchanged high electrical
conductivity for more than 10 years (curves 2 6), and
epoxy resin is sufficient for stable electricity transfer
even at metal concentration of 55%. In the case of
novolac resin, a higher content of metal is necessary
(65 70%). However, at a metal content of 75 80% the
electrical conductivities of composites based on these
resins become comparable.
formulations with 1-naphthol have the lowest elec-
4
trical resistivity of (0.8 1.2) 10
cm, close to that
It was established that copper formulations ob-
tained using different methods and different modifiers
markedly differ in the electrical conductivity and
stability (Figs. 1, 2). Table 2 lists the results obtained
in studying the influence exerted by introduction of a
of the metal. Formulations with Diaphene (curve 7)
have high initial electrical conductivity, but, after
4 years, the properties of the formulation deteriorate
dramatically. With other stabilizers (curves 8, 9),
the longevity of formulations was even lower.
104, cm
(a)
104, cm
(b)
, years
104, cm
, years
, years
Fig. 2. Effect of stabilizers on the dependence of the electrical resistivity of formulations with (a) EDP epoxy resin and
(b) SF-010 novolac resin on the testing time . (a) (1) Without stabilizer; stabilizer: (2 4) 1-naphthol, (5, 6) p-aminophenol,
(7) Diaphene, (8) p-phenylenediamine, and (9) 1-naphthylamine. Copper UDP : EDP : stabilizer mass ratio: (1) 100 : 40 : 0,
(2) 100 : 40 : 1.0, (3) 100 : 35 : 1.0, (4) 100 : 25 : 1.0, (5 7, 9) 100 : 30 : 1.0, and (8) 100 : 32 : 1.0. (b) (1) Without stabilizer;
stabilizer: (2) 1-naphthol, (3) p-aminophenol, (4) ascorbic acid, (5) hydroquinone, (6) formic acid, and (7) 1-naphthol + glycerol
(plasticizer). Copper UDP : SF-010 : stabilizer mass ratio: (1) 100 : 25 : 0, (2 5) 100 : 25 : 1.0, (6) 100 : 25 : 5.0, and
(7) 100 : 25 : 1.0 : 3.0 (3 wt parts of glycerol).
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EFFECT OF CHEMICAL MODIFICATION OF ULTRADISPERSED COPPER POWDERS
1739
Table 2. Effect of initiator on the conditions of preparation
of copper UDPs
tions with addition of organic acids (Fig. 1) or those
obtained from readily decomposing copper salts
(Table 1).
Sam-
ple
no.
,
mrin
B
Initiator (acid)
Tr, C
CONCLUSIONS
(1) Ultradispersed powders prepared from readily
decomposing copper salts and formulations syn-
thesized on their base retain high electrical conduc-
tivity for more than 5 years.
1
2
3
4
5
6
7
8
9
1 : 10.0 : 0
1 : 10.0 : 0.003 Ascorbic
1 : 7.5 : 0.020
300 180 190
85 160 165
40 140 145
40 150 155
70 160 165
100 170 175
80 160 165
55 160 165
25 155 160
1 : 5.0 : 0.010
(2) The best stabilizer for ultradispersed copper
powders obtained from copper sulfate is 1-naphthol
or its mixture with glycerol.
1 : 7.5 : 0.100 Oxalic
1 : 7.5 : 0.060 Citric
1 : 7.5 : 0.030 Formic
1 : 6.7 : 0.120
(3) Chemical modification of ultradispersed copper
powders results in that epoxy formulations prepared
on their base retain their high electrical conductivity
for more than 10 years.
1 : 10.0 : 0.050 Acetylsalicylic
Thus, a conclusion can be made that the best sta-
bilizers for epoxy formulations are 1-naphthol and
p-aminophenol, which are known [10] to be effective
inhibitors of oxidation with molecular oxygen, used to
prevent aging of polymers and stabilize jet-engine fuel
in prolonged storage.
(4) The enhanced stability and low cost of these
copper-based materials makes them promising for use
in modern nanoelectronics instead of composites
based on noble metals.
REFERENCES
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various stabilizers is illustrated in Fig. 2b. It can be
seen that all of the modified samples have lower resis-
tivity and are more stable than the initial sample
(curve 1), with the lowest resistivity observed for the
formulation stabilized with a mixture of glycerol and
1-naphthol (curve 7). The introduction of glycerol,
presumably, makes lower the brittleness of the novo-
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quinone (curve 5), and a mixture of glycerol and
1-naphthol (curve 7). As in the case of epoxy formula-
tions, the best oxidation inhibitor is 1-naphthol, and
addition to this compound of glycerol in 1 : 3 ratio
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4
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 11 2002