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Polymer matrix composites filled with nanoporous metal powders: Preparation and electrical properties
NanoStructwed
Pergamon
Materials,
Vol.
12, pp. 531-534, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-w front matter
PI1 SO9659773(99)00176-2
POLYMER MATRIX COMPOSITES FILLED WITH NANOPOROUS METAL POWDERS: PREPARATION AND ELECTRICAL PROPERTIES Hans-Gerd Busmann, Bernd Giinther, Udo Meyer Fraunhofer-IFAM, Lesumer Heerstrasse 36, D-287 17 Bremen, GERMANY
Abstract -- The production of a new kind of silver powder for the preparation qf electrically conducting polymers is described. This comprises the method of inert-gascondensation with forced gas flow in a closed-loop system and a quasi continuous collection of the nanoporous metal deposits via filter separation. After sieving, powders of various morphologies are obtained. The electrical resistance of polymers filled with two kinds of such powders as a function of filler content is presented, showing percolation at filler contents below 5 ~01% In a two component epoxy resin system, the metal-like resistance observed for samples cured at room temperature changes into a semiconductor-like resistance upon thermal annealing. However, a thermal pretreatment of the powder retains the metal-like conductivity. 01999 Acta Metallurgica hc.
INTRODUCTION The electrical properties of composite materials of metallic filler particles embedded in polymer matrices strongly depend on the concentration and morphology of the particles (1). The electrical resistance shifts from dielectric to metallic behavior with increasing metal content (2), whereas solid filler materials in general tend to deteriorate mechanical properties of such composites. Therefore, filler systems with a low percolation threshold are preferred in applications like isotropically conductive adhesives and injection molding feedstocks. Commercially available adhesives for uses in electronics are generally filled with more than 25 ~01% of Ag-flakes. Recently, a novel nanoscale metal / polymer matrix composite made by the dispersion of a porous silver powder into a polymer matrix has been developed and led to a reduction of the percolation threshold to less than 7 ~01% (3,4). This powder is made from porous silver obtained by the technique of inert gas condensation. This paper reports on the development of a continuously operating apparatus for the production of porous metals of aggregated nanoparticles and on the influence of the morphology and thermal treatments of the hereof produced powders on the electrical resistance of the final composite.
531
532
.. . ,‘.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .ii;.;efio..&.it
.i
MATERIALS
PRODUCTION OF POROUS SILVER
The porous silver has been produced by inert gas .. . . . . . . . . . . . . condensation in two different devices. In both cases, a Agwire is continuously fed onto a tungsten crucible and evaporated into an inert gas atmosphere ( lo3 Pa to lo4 Pa, argon or helium). The Agcompressor nanoparticles formed hereby coagulate and form highly porous aggregates. In the first device, these aggregates are deposited on liquid nitrogen substrates. During cooled Fig. 1: Schematic drawing of an apparatus for continuous warm-up to room temperature, production of highly porous metals by aggregation of sintering leads to highly metallic nanoparticles. conductive Ag-networks (5). With respect to increase the efficiency of production, a new device with forced gas flow in a closed-loop cycle and continuous powder collection was set-up - Fig. 1. The main components are the Ag-crucible, a filter with a gas-reflow unit, a gas compressor (oil-free vacuum pump) and a heat exchange box. The latter counterbalances the heating of the gas at the crucible. The silver is evaporated into the gas and collect& on the outer surface of the filter. The porous Ag-deposit is periodically removed from the filter by pulsed gas reflow and collected at the bottom of the chamber. ~!izYs%2:“./a”to : ji+; :~....F.........,................,
POWDER
:
MORPHOLOGY
With the goal of producing polymer matrix composites with well-defined and reproducible mechanical and electrical properties, the porous powder deposits were Cuther processed. Several methods like vibrational sieving, air jet milling and air jet sieving have been applied, whereupon the resulting powders were dispersed into epoxy resins. Fig. 2 shows micrographs taken with a transmission electron microscope of two powders of principally different morphologies. Powder 1 - Fig. 2a - was obtained by vibrational sieving using a mesh size of 50 pm. Powder 2 - Fig. 2b - was obtained by air-jet sieving and exhibits a size distribution of the powder particles with a much smaller mean size (< 10 urn). Obviously different powder morphologies result from the two types of processing methods: The surfaces of the particles of powder 1 are well defined and compressed and those of powder 2 are hayed.
FOURTH INTERNATIONAL
CONFERENCE
ON NAN~STRUCTURED
MATERIALS
533
Fig.2: Transmission electron micrographs (bright field, 200 kV) of polymer matrix composites filled with nanoporous silver powders. The powder was prepared by a) mechanical sieving and b) by air-jet sieving of aggregated silver nanoparticles. ELECTRICAL
RESISTANCE
Fig. 3 shows the DC-resistance of composites of powder 1 and 2 in Vitralit and Araldit 2020, respectively, as a function of filler content. They exhibit a specific resistance of - 15 Ran and 3300 Rem at 2 ~01% and drop down to 0,005 Rem and 0,5 Rem at 10 vol%, respectively. Since the unfilled polymer matrix has a resistance of - 1015 !&II, a steep decrease of the resistance has to be assumed for an increase of the Ag-content from 0 ~01% up to 2 ~01%. Thus, the transition regime from high to low resistance is much broader for powder 2 than for powder 1. This is to be expected owing to the dependence of percolation behavior on the morphology of the powder (1). Important for all applications is the thermal stability of the composites. Fig. 4 shows the resistance of powder 1 in Araldit 2020 (10 ~01%) as a function of temperature in the range from -20 “C to 40 “C! before and after a thermal treatment of the composite at 85 “C for 2 h. The as-prepared sample cured at RT for 24 h shows a positive temperature-coefficient of 0,017 LI/K (pure Ag: 0,004 !XK) indicating a conductivity determined by a continuous Agnetwork with metal-like contacts within and between the agglomerates. After the heattreatment at 85 “C / 2 h, the resistance increases drastically to 2600 Rem and the temperature-coefficient becomes semiconductor-like (-7,4 0/K). This indicates a loss of metal-like contacts between and/or within the agglomerates. The large ditference in the coefficients of thermal expansion of the polymer matrix (- 200 x 10&/K) and the Ag-filler (20 x 10v6/K) might lead to an interruption of the metal-like agglomerate-agglomerate interfaces
534
FOURTH
O,Wl J+---b--0
INTERNATIONAL
-. + 2
4 saw
-
-.-+.
6
&INFERENCE
ON NANOSTRUCTURED
MATERIALS
-.- -4 8
10 40
/ vol%
-20
“II tenldhlre
Fig.3: DC- resistance as a Iimction of filler content for different powders - cf. Fig.2. Standard four probe I-U-measurements, sample size 1 mm x lmm x 18 mm.
/ Otp
Fig.4: Resistance of powder 1 in Araldit (10 ~01%) as a function of temperature of an ascured and a post-annealed sample. Measured with a HP Impedance Analyzer at 1 MHz.
and to a tearing of the agglomerates. Subsequent covering of hereby newly formed metallic surfaces with the polymer would prevent the recovery of metal-like contacts after the treatment. In an attempt to stabilize powder 1 against this thermal instability, the powder was heat treated at 100 “C for 2 h prior to its dispersion into the Araldit matrix. The resistance of this composite after a heat treatment at 85 “C for 2 h was found to be 100 S2cm. This value is only slightly higher than that of the untreated powder and shows that the thermal pretreatment of the powder retains the metal-like resistance of the composite.
CONCLUSIONS
The technology of inert gas condensation can be used for the production of a nanoporous Agfiller in a quantity that is sufficient for its commercial use specifically in electrically conducting polymers. By means of different processing technologies, the morphology and size distribution of the nanoporous aggregates can be varied, thermally stabilized, and tailored to specific demands. E.g., a percolation behavior as in the system powder1 in Araldit 2020 may be preferred if the filler content must be kept as low as possible, whereas a behavior as in the system powder2 in Vitralit may be preferred if the absolute conductivity should be adjustable over a large range of resistivity. REFERENCES
1. For an overview, see e.g.: Gul, V.E., Structure and Properties of Conducting Polymer Composites, Utrecht VSP, 1996, ISBN 90-6764-204-L 2. R.Pelster, P.Marquardt, G.Nimtz, A.Enders, H.Eifert, K.Friedrich, and F.Petzoldt, Phys.Rev.B 45( 16),8929,1992. 3. H.Eifert, B.Gtinther, pat. DE 42 28 608 C 2,1994. 4. S.Kotthaus, B.Giinther, RHaug, H.SchZifer, IEEE Trans. A22 15,1997. 5. H.Eifert, B.Giinther, J.Neubrand, Int.J.Electronics 73(5),945,1992.
Pergamon
Materials,
Vol.
12, pp. 531-534, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-w front matter
PI1 SO9659773(99)00176-2
POLYMER MATRIX COMPOSITES FILLED WITH NANOPOROUS METAL POWDERS: PREPARATION AND ELECTRICAL PROPERTIES Hans-Gerd Busmann, Bernd Giinther, Udo Meyer Fraunhofer-IFAM, Lesumer Heerstrasse 36, D-287 17 Bremen, GERMANY
Abstract -- The production of a new kind of silver powder for the preparation qf electrically conducting polymers is described. This comprises the method of inert-gascondensation with forced gas flow in a closed-loop system and a quasi continuous collection of the nanoporous metal deposits via filter separation. After sieving, powders of various morphologies are obtained. The electrical resistance of polymers filled with two kinds of such powders as a function of filler content is presented, showing percolation at filler contents below 5 ~01% In a two component epoxy resin system, the metal-like resistance observed for samples cured at room temperature changes into a semiconductor-like resistance upon thermal annealing. However, a thermal pretreatment of the powder retains the metal-like conductivity. 01999 Acta Metallurgica hc.
INTRODUCTION The electrical properties of composite materials of metallic filler particles embedded in polymer matrices strongly depend on the concentration and morphology of the particles (1). The electrical resistance shifts from dielectric to metallic behavior with increasing metal content (2), whereas solid filler materials in general tend to deteriorate mechanical properties of such composites. Therefore, filler systems with a low percolation threshold are preferred in applications like isotropically conductive adhesives and injection molding feedstocks. Commercially available adhesives for uses in electronics are generally filled with more than 25 ~01% of Ag-flakes. Recently, a novel nanoscale metal / polymer matrix composite made by the dispersion of a porous silver powder into a polymer matrix has been developed and led to a reduction of the percolation threshold to less than 7 ~01% (3,4). This powder is made from porous silver obtained by the technique of inert gas condensation. This paper reports on the development of a continuously operating apparatus for the production of porous metals of aggregated nanoparticles and on the influence of the morphology and thermal treatments of the hereof produced powders on the electrical resistance of the final composite.
531
532
.. . ,‘.
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .ii;.;efio..&.it
.i
MATERIALS
PRODUCTION OF POROUS SILVER
The porous silver has been produced by inert gas .. . . . . . . . . . . . . condensation in two different devices. In both cases, a Agwire is continuously fed onto a tungsten crucible and evaporated into an inert gas atmosphere ( lo3 Pa to lo4 Pa, argon or helium). The Agcompressor nanoparticles formed hereby coagulate and form highly porous aggregates. In the first device, these aggregates are deposited on liquid nitrogen substrates. During cooled Fig. 1: Schematic drawing of an apparatus for continuous warm-up to room temperature, production of highly porous metals by aggregation of sintering leads to highly metallic nanoparticles. conductive Ag-networks (5). With respect to increase the efficiency of production, a new device with forced gas flow in a closed-loop cycle and continuous powder collection was set-up - Fig. 1. The main components are the Ag-crucible, a filter with a gas-reflow unit, a gas compressor (oil-free vacuum pump) and a heat exchange box. The latter counterbalances the heating of the gas at the crucible. The silver is evaporated into the gas and collect& on the outer surface of the filter. The porous Ag-deposit is periodically removed from the filter by pulsed gas reflow and collected at the bottom of the chamber. ~!izYs%2:“./a”to : ji+; :~....F.........,................,
POWDER
:
MORPHOLOGY
With the goal of producing polymer matrix composites with well-defined and reproducible mechanical and electrical properties, the porous powder deposits were Cuther processed. Several methods like vibrational sieving, air jet milling and air jet sieving have been applied, whereupon the resulting powders were dispersed into epoxy resins. Fig. 2 shows micrographs taken with a transmission electron microscope of two powders of principally different morphologies. Powder 1 - Fig. 2a - was obtained by vibrational sieving using a mesh size of 50 pm. Powder 2 - Fig. 2b - was obtained by air-jet sieving and exhibits a size distribution of the powder particles with a much smaller mean size (< 10 urn). Obviously different powder morphologies result from the two types of processing methods: The surfaces of the particles of powder 1 are well defined and compressed and those of powder 2 are hayed.
FOURTH INTERNATIONAL
CONFERENCE
ON NAN~STRUCTURED
MATERIALS
533
Fig.2: Transmission electron micrographs (bright field, 200 kV) of polymer matrix composites filled with nanoporous silver powders. The powder was prepared by a) mechanical sieving and b) by air-jet sieving of aggregated silver nanoparticles. ELECTRICAL
RESISTANCE
Fig. 3 shows the DC-resistance of composites of powder 1 and 2 in Vitralit and Araldit 2020, respectively, as a function of filler content. They exhibit a specific resistance of - 15 Ran and 3300 Rem at 2 ~01% and drop down to 0,005 Rem and 0,5 Rem at 10 vol%, respectively. Since the unfilled polymer matrix has a resistance of - 1015 !&II, a steep decrease of the resistance has to be assumed for an increase of the Ag-content from 0 ~01% up to 2 ~01%. Thus, the transition regime from high to low resistance is much broader for powder 2 than for powder 1. This is to be expected owing to the dependence of percolation behavior on the morphology of the powder (1). Important for all applications is the thermal stability of the composites. Fig. 4 shows the resistance of powder 1 in Araldit 2020 (10 ~01%) as a function of temperature in the range from -20 “C to 40 “C! before and after a thermal treatment of the composite at 85 “C for 2 h. The as-prepared sample cured at RT for 24 h shows a positive temperature-coefficient of 0,017 LI/K (pure Ag: 0,004 !XK) indicating a conductivity determined by a continuous Agnetwork with metal-like contacts within and between the agglomerates. After the heattreatment at 85 “C / 2 h, the resistance increases drastically to 2600 Rem and the temperature-coefficient becomes semiconductor-like (-7,4 0/K). This indicates a loss of metal-like contacts between and/or within the agglomerates. The large ditference in the coefficients of thermal expansion of the polymer matrix (- 200 x 10&/K) and the Ag-filler (20 x 10v6/K) might lead to an interruption of the metal-like agglomerate-agglomerate interfaces
534
FOURTH
O,Wl J+---b--0
INTERNATIONAL
-. + 2
4 saw
-
-.-+.
6
&INFERENCE
ON NANOSTRUCTURED
MATERIALS
-.- -4 8
10 40
/ vol%
-20
“II tenldhlre
Fig.3: DC- resistance as a Iimction of filler content for different powders - cf. Fig.2. Standard four probe I-U-measurements, sample size 1 mm x lmm x 18 mm.
/ Otp
Fig.4: Resistance of powder 1 in Araldit (10 ~01%) as a function of temperature of an ascured and a post-annealed sample. Measured with a HP Impedance Analyzer at 1 MHz.
and to a tearing of the agglomerates. Subsequent covering of hereby newly formed metallic surfaces with the polymer would prevent the recovery of metal-like contacts after the treatment. In an attempt to stabilize powder 1 against this thermal instability, the powder was heat treated at 100 “C for 2 h prior to its dispersion into the Araldit matrix. The resistance of this composite after a heat treatment at 85 “C for 2 h was found to be 100 S2cm. This value is only slightly higher than that of the untreated powder and shows that the thermal pretreatment of the powder retains the metal-like resistance of the composite.
CONCLUSIONS
The technology of inert gas condensation can be used for the production of a nanoporous Agfiller in a quantity that is sufficient for its commercial use specifically in electrically conducting polymers. By means of different processing technologies, the morphology and size distribution of the nanoporous aggregates can be varied, thermally stabilized, and tailored to specific demands. E.g., a percolation behavior as in the system powder1 in Araldit 2020 may be preferred if the filler content must be kept as low as possible, whereas a behavior as in the system powder2 in Vitralit may be preferred if the absolute conductivity should be adjustable over a large range of resistivity. REFERENCES
1. For an overview, see e.g.: Gul, V.E., Structure and Properties of Conducting Polymer Composites, Utrecht VSP, 1996, ISBN 90-6764-204-L 2. R.Pelster, P.Marquardt, G.Nimtz, A.Enders, H.Eifert, K.Friedrich, and F.Petzoldt, Phys.Rev.B 45( 16),8929,1992. 3. H.Eifert, B.Gtinther, pat. DE 42 28 608 C 2,1994. 4. S.Kotthaus, B.Giinther, RHaug, H.SchZifer, IEEE Trans. A22 15,1997. 5. H.Eifert, B.Giinther, J.Neubrand, Int.J.Electronics 73(5),945,1992.