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Electrical and shielding properties of ABS resin filled with nickel-coated carbon fibers
Composites Science and Technology 56 (19%) 193-200 0 1996 Elsevier Science Limited
Printed in Northern Ireland. All rights reserved 0266-3538/96/$15.00
0266-3538(95)00143-j
ELSEVIER
ELECTRICAL AND SHIELDING PROPERTIES OF ABS RESIN FILLED WITH NICKEL-COATED CARBON FIBERS
Guanghong
Lu, Xiaotian Li & Hancheng Jiang
Department of Materials Science, Jilin University, 119 Jie Fang Road, Changchun
130023, Peoples
Republic of China
(Received 24 July 1995; accepted 23 November 1995) computers and television sets to shield electromagnetic waves and thus make them work safely. However, an obvious weakness of the shielding materials filled with carbon fibers is that the conductivity of carbon fibers is much lower than the meta1,6 so a greater volume fraction of carbon fibers is needed in order to provide the same shielding effect. Although composites filled with more carbon fibers have good shielding, both the toughness of the composites and the rheological properties of the fibers in extrusion or injection molding decrease significantly. At the same time, the increase in filler content means an increase in production cost because of the high cost of carbon fibers. The conductivity of carbon fibers cannot therefore satisfy the requirements for shielding materials for highly demanding applications. In order to decrease the fiber content and still retain a good shielding effect, a layer of metal is usually plated on the surface of the carbon fiber. Nickel and copper are the two main metals used as coatings. Nickel is generally preferred because of its stability and because of the readiness with which copper is oxidized. Some authors have studied the electrical and shielding properties of Ni-coated carbon-fiber-filled polymeric composites,‘-” but few, to our knowledge, have focused on aspects such as the oxidation problem of Ni-coated carbon fibers and comparisons of: (i) electrical conductivity of carbon fibers and Ni-coated carbon-fiber-filled composites; and (ii) theoretical and experimental shielding effectiveness of fiber-filled composites. In the present paper emphasis is laid on the electrical conductivity and shielding effectiveness of nickel-coated carbon fibers and their composites in relation to these aspects.
Abstract and shielding effectiveness of nickel-coated carbon fibers and their composites have been studied and the results are presented in this paper. A continuous electroplating device was used to plate nickel over the surface of the carbon fibers. The resistivity of Ni-coated carbon fibers decreases with an increase in plating time. The optimum thickness of the coating is 0.2-0.5 pm. A solvent method and Brabender mixing were used to prepare the composites of Ni-coated carbon fibers in ABS resin, the resistivity of which decreases with an increase in fiber content. At the same fiber content, the conductivity of the composites filled with Ni-coated carbon fibers is much greater than that of ordinary carbon-fiber-filled composites. To reach the same resistivity as carbonfiber-filled composites, the use of Ni-coated carbon fibers as filler can greatly decrease the required volume fraction of fibers. The shielding effectiveness of the composites was obtained by experiment and by theoretical calculation; at high frequencies, the two correspond very well. From experimental measurement, the shielding effectiveness of ABS resin filled with 10 vol. % of Ni-coated carbon fibers is about 50 dB. Electrical
conductivity
Keywords:
electrical conductivity, shielding effectiveness, Ni-coated carbon fibers, composites
1 INTRODUCTION Polymeric composites for electromagnetic shielding have been developed very rapidly in recent years, especially conductive polymeric composites filled with carbon fibers,14 which have many advantages, such as their good mechanical properties, their significant reinforcing effect, their low density which makes them suitable for light-weight shielding materials, and their ease of forming by extrusion or injection molding, which makes them suitable for being processed in batches.5 These materials have a wide range of applications in military, industrial and commercial fields. For example, they can be used as casings for
2 EXPERIMENTAL 2.1 Materials The materials used in this work were acrylonitrilebutadiene-styrene (ABS) resin and PAN-based T300 carbon fibers of density 1.60g/cm3, fiber diameter of 7 pm, and volume resistivity of 1.6 X lop3 Szcm. 193
G. Lu, X. Li, H. Jiang
194 2.2 Nickel electroplating
An electroplating device was constructed which can continuously plate nickel onto the surface of the fibers. The speed of carbon fibers can be changed by a gearbox, the normal speed being 1 m min-‘. Nickel sulphate was the main salt used in the electroplating solution, and electrolytic nickel plate was used as the anode. Carbon fibers are conductive, so they could be used directly as the cathode to be plated. Before being plated, carbon fibers were activated in a colloidal solution of palladium in order to enhance the adhesion force between the nickel coating and the carbon fibers.
2.3 Hydrogen reduction The problem of nickel oxidation can be solved by reduction. Dry hydrogen was used and the reduction temperature was 200°C for a treatment time of 3 h. A layer of soluble polyphenylamine was applied to the surface of N&coated carbon fibers to avoid reoxidation in the manufacturing process.
2.4 Mixing method and sample preparation Solvent method of mixing: ABS resin was mixed with chloroform to form a ‘paste’, and Ni-coated carbon fibers, chopped into 5 mm lengths, were dispersed into the paste by mechanical stirring to the desired proportion. The fiber-filled paste was then dried at room temperature. The composite was then molded by hot pressing at 200-210°C for about 5 min. Brabender mixing method: Ni-coated carbon fibers were chopped into 5 mm pieces and mixed
with ABS resin in a Brabender mixer operating at about 210°C for 3 min followed by hot compression molding also at 200-210°C for about 5 min. In order to determine the aspect ratio, L/D, of the fibers after mixing, the composites were treated with chloroform to dissolve out the resin phase. The fibers were extracted and observed under an optical microscope. 2.5 Resistance measurement Resistance was measured by the volt-ampere method illustrated in Fig. 1. The samples were of dimensions 60 mm X 3 mm X 2 mm and four samples were prepared for each composite. The resistance of each material was measured and average values were obtained. A type 2575 eight digital millivoltmeter and a type 2553 DC voltage current standard (YEW Co., USA) were used. The contact resistance was eliminated by the double-lead-wire method. 2.6 Shielding effectiveness measurement The shielding effectiveness of the specimens was measured by the coaxial-cable method illustrated in Fig. 2. A Fluk signal generator, 6071 (lOOHz1.5 GHz), HP3336 A/B/C (100 Hz-l-5 GHz), and frequency spectrograph HP8566A (10 Hz-26.6 GHz) were used to make these measurements. The sample was tested in the form of an annular ring with an outside diameter of 115 mm, an inside diameter of 12mm, and a thickness of 3 mm. Silver conductive paint was applied to the surface and the edge of the samples to allow a continuous contact to be made between the sample and the steel tubing or the steel chamber. To make the measurements, shielding effectiveness values were recorded with and without the sample in the apparatus. The difference between the two values is the real shielding effectiveness of the sample.
8’ Fig. Fig. 1.
Illustration of the method of resistance measurement by the volt-ampere method (press direction is vertical to the plane of plastic plate). 1, insulating plastic plate; 2, copper electrode; 3, sample; 4, millivoltmeter; 5, milliammeter.
8’ 2.
Coaxial set-up for measuring the shielding effectiveness of the composites. 1, coaxial switch GH 101-000; 2, coaxial connector L27/L16-503K; 3, coaxial line pedestal; 4, connecting tight nut; 5, coaxial line; backplate; 7, basement; 8, lea1 rail; 9, switch box.
6,
195
Electrical and shielding properties of ABS resin 3 RESULTS
AND
3.1 Conductivity
DISCUSSION
of nickel-coated
carbon fibers
Variations of the resistivity of Ni-coated carbon fibers are given in Table 1, which shows that the resistivity of Ni-coated carbon fibers is about lop4 Qcm, an order of magnitude less than that of the uncoated fiber (1.6 x lo-” Qcm). When the electric current is 2 A and electroplating time is 15 min, the resistivity of the Ni-coated carbon fibers can reach 6.4 X lo-’ Qcm. It is also shown that the resistivity of the Ni-coated carbon fibers decreases with increase in plating time as a consequence of the increase in coating thickness. If considering the conductivity of the fibers only, the thicker the coating the better the conductivity. But there is a limiting thickness, since if the coating thickness exceeds 0.5 pm, the Ni-coated carbon fibers have defects, as follows:
(b)
1. the toughness of the fibers markedly decreases, the fibers become stiff and are easily broken in the mixing process; 2. the fibers easily adhere to each other and are hard to separate in the mixing process, so that the uniformity of the fiber distribution in the composites is lost. On the other hand, if the coating is thinner than 0.2 ,um, not only is the nickel coating not continuous, but the required conductivity of the fibers cannot be achieved. The optimum thickness therefore ranges from 0.2 to 0.5 pm. In this range the nickel coating is uniform and continuous, the toughness of the fibers is good, and the fiber dispersion in the composites after mixing is also satisfactory. Figure 3 shows SEM micrographs of Ni-coated carbon fibers. Surface observation of a bunch of fibers is shown in Fig. 3(a) and a cross-section observation of a single Ni-coated fiber is shown in Fig. 3(b). The thickness of the coating in the figures is about 0.3 pm. The micrographs show that the coating surface is uniform and smooth which indicates that electroplating is satisfactory.
Fig. 3. SEM micrographs of Ni-coated carbon fibers: (a) surface of a bunch; (b) cross-section of a single fiber.
Figure 4 is an X-ray photoelectron spectroscopy (XPS) analysis of Ni-coated fiber which has been left in air for one day. The binding energy of nickel and nickel oxide are given, which indicates that the nickel
Table 1. The resistlvity of Ni-coated carbon fibers Fiber
Electric current (A)
Plating time (min)
Carbon Ni-plated carbon
Resistivity @cm) 1.6 x lo-”
2
4
5 8 12 15 3 3.5 4
5.2 x 1om4 2.6 x lo-“ 1.2 x 1om4 0.64 x 10m4 5.2 x 1om4 4.7 x 1o-4 4.4 x 1o-4
I
892.400
882.400
I
872.400
Binding
I
I
862.400
energy
852.400
\,
842.400
(eV)
Fig. 4. X-ray photoelectron spectrograph carbon fibers.
of nickel-coated
196
G. Lu, X. Li, H. Jiang
Table 2. Variation of the resistivity of Ni-coated carbon fiber before and after hydrogen reduction (current = 15 A)
Time (min)
1.5 3.5 4.5
Resistivity before reduction (km) 1.3 x 1o-3 1.1 x 1om3 2.4 x lo-’
Resistivity after reduction (Qcm) 4.2 x 10m4 1.7 x 1o-4 1.4 x 1om4
coating is oxidized. A very thin oxide layer (1 nm) can decrease the conductivity of the fibers significantly, so hydrogen reduction was used to eliminate the oxidized layer. Table 2 gives the variation of the resistivity of Ni-coated carbon fibers before and after reduction. It is shown that before reduction the resistivity of the Ni-coated fibers increased markedly, some of them exceeding the resistivity of the plain fiber (1.6 X 10e3 Qcm). After reduction the resistivity returned to normal (about 1O-4 Qcm), which indicates that the hydrogen reduction has achieved its purpose. 3.2 The conductivity of ABS resin filled with Nicoated carbon fibers 3.2.1 Variation of the resistivity of the composites with fiber content Figures 5 and 6 show the relationship between the resistivity of the composites and the volume fraction of Ni-coated carbon fibers. Figure 5 represents results for the composites mixed by the solvent method, and Fig. 6 for the composites mixed by the Brabender process. In both figures, line a, as a comparison, is for carbon fiber for plain-filled ABS resin composites, and line b is for Ni-coated carbon fiber-filled ABS resin composites. It is clear that whichever method is used the resistivity of the composites decreases with an increase in fiber content. Compared with the resistivity of ABS resin filled with plain carbon fibers, the resistivity of ABS resin filled with Ni-coated fibers decreases by about l-2 orders of magnitude for the same volume fraction of fibers. The decrease of the filler resistivity directly brings about the decrease in resistivity of the composites. It is shown in Fig. 5 (composites mixed by the solvent method) that the resistivity of ABS resin with 12.5 vol.% of Ni-coated fiber is equal to that of a composite with 27.5 vol.% of plain fibers, a saving of 15 vol.% of filler; similarly, the resistivity of the ABS resin filled with 5 vol.% of Ni-coated fiber is equal to that of the composite with 14.5 vol.% of plain carbon fiber, a saving of about 10 vol.% of filler. In Fig. 6 (composites mixed by the Brabender process) it is shown that the resistivity of resin with 12.5 vol.% of Ni-coated fiber is equal to that of the ABS with 21.5 vol.% of plain fiber, a saving of 9 vol.% of filler; and for 5 vol.% of Ni-coated fiber, the resistivity of the
I IO Content
I 20 of fibres (~01%)
Fig. 5. Variation of the resistivity of Ni-coated carbonfiber/ABS composites mixed by the solvent method with fiber volume content: (a) carbon-fiber filler; (b) Ni-coated carbon-fiber filler.
composite is equal to that of the composite with 13 vol.% of plain carbon fiber, saving about 8 vol.% of filler. To reach the same resistivity as the plain fiber-filled composites, the required volume fraction of the Ni-coated fiber decreases greatly. This decrease in filler content can improve both the toughness of the composite and the rheological properties of the fibers during extrusion or injection molding. This also decreases the production costs, with significant benefit. 3.2.2 Aspect ratio offibers after mixing Comparing Figs 5 and 6, it can be seen that the resistivity of the composites mixed by the Brabender method is higher than that of the composites mixed by the solvent method, the difference between them
0
10 Content
Fig. 6. Variation
20 of fibres (~01%)
of the resistivity of Ni-coated carbonfiberlABS composites mixed by the Brabender method with fiber volume fraction: (a) carbon-fiber filler; (b) Ni-coated carbon fiber as filler.
Electrical and shielding properties of ABS resin
being about one order of magnitude. By the method illustrated in Section 2 of this paper, the aspect ratio of fibers can be obtained. The fiber aspect ratio of the composites mixed by the solvent method is about 600, i.e. the fiber length is about 4 mm, which indicates that when mixed by this method the fibers are slightly damaged. This is an ideal situation for the fibers in the composites since the composites can achieve lower resistance (higher conductivity). On the other hand, the fiber aspect ratio in the composites mixed by the Brabender method is about 40, i.e. the length of the fibers is about 0.3 mm. This leads to a decrease in the conductivity of the composites. The aspect ratio of the fibers is the main factor to affect the conductivity of the fiber-filled composites. A high fiber aspect ratio can be achieved by control of the mixing technology. The basic rule is that the mixing temperature should be as high as possible, but not so high as to cause the ABS to be degraded, and the mixing time should be as short as possible consistent with good fiber dispersion in the composites. Good (a)
Fig. 7. SEM cross-section observation of Ni-coated carbonfiber/ABS composites with 3.5 vol.% fibers: (a) X400; (b) x 1000.
197
fiber distribution in the composites and short mixing times are naturally mutually exclusive, however, so it is important to seek the optimum processing conditions. In the present work we found that the most suitable conditions for the ABS resin matrix are a mixing temperature of 210°C and a mixing time of 3 min. It should be noted that the filler consists of two parts, the nickel plating and the carbon fibers, and we have not distinguished between these in the above discussion. Two indications are given now: when the thickness of the nickel coating is 0.6 pm, the relative proportions, by weight, of Ni and carbon fiber are both 50%, and when the thickness of the nickel coating is 0.2 pm, the proportions are 2.5 and 75%, respectively. The thicker the coating, the less the proportion of carbon in the Ni-coated fibers. The thickness of the nickel coating used in this work was about 0.5 pm.
3.2.3 SEM analysis Figure 7 shows SEM micrographs of cross-sections of Ni-coated carbon-fiber/ABS resin composites mixed by the Brabender method with 3.5 vol.% of fibers, in which the bright part is the fibers and the dark part is the resin phase. The micro-distribution of Ni-coated fibers in the composites can be observed from the two micrographs. It is shown that the fiber distribution is uniform and has no orientation, indicating that the mixing is satisfactory under the conditions discussed above.
3.2.4 X-ray analysis An SEM photograph is a micro-observation of a certain point in the composite. In order to determine the macrosopic distribution of Ni-coated fiber in the composites, X-radiography was used. Figure 8 shows macrophotographs obtained by this method of the composites mixed by the Brabender method with different fiber contents. It is seen from Fig. 8(a) that the fiber content in this composite is lower compared to that in Fig. S(b), and the interfiber contacts are also fewer, so that the resistivity of this composite is higher. On the other hand, Fig. 8(b) shows that the interfiber contacts in this composite have formed a conductive network, so the resistivity of this composite is much lower. The results above correspond to the conductive mechanism of fiber-filled composites, i.e. ‘percolation theory’. This theory suggests that it is the existence of conductive passages (interfiber contacts) that results in the conductivity of the composites. The number of conductive passages increases gradually with increase in fiber content, so that the resistivity of the composite decreases, as indicated by Fig. 8.
198
G. Lu, X. Li, H. Jiang
(a)
Absorpsion loss, A, can be evaluated equations as follows:
from
A(dB) = 3.34 x 10-3tV&i
the
(5)
In eqns (3)-(5), f is the incidental frequency of the electromagnetic wave (Hz), r is the distance from the emission source to the shielding material, u is the electrical conductivity, p is the magnetic transmissivity, and t is the thickness of the shielding material. For composites with a polymeric matrix, SE can be calculated from the electrical resistivity of the composite as follows: 0))
SE(dB)=50+lOlog(;)+l.7t$
(6)
where p is the volume resistivity of the composites (Qcm), f is the incidental frequency of the electromagnetic wave (MHz), and t is the thickness of the composite (cm). This equation is also called the Simon formalism and is generally used in the calculation of SE. In the present work, t = 3 mm so that SE of the composites could be compared with the experimental work. The unit of electromagnetic shielding (dB) is defined as follows: Fig. 8. X-radiographs (enlarged 20 times) of Ni-coated carbon-fiber/ABS composites with fiber content of: (a) 3.5, and (b) 12.5 vol.%.
or dB=20log
3.3 Shielding effectiveness N&coated carbon fibers 3.3.1 Electromagnetic
of composites
filled with
shielding mechanism
According to Schelkunoff’s theory, the shielding effectiveness, SE, of a material can be divided into three parts, R, A and M, as follows: SE(dB)=R+A+M
(1)
where A is absorption loss of the energy of the electromagnetic wave, R is the first reflection loss of energy, and M is a multi-reflection loss of energy. When A exceeds lOdB, M can be ignored in the calculation. So eqn (1) becomes: SE=R+A
i
2 2
>
where P, and P2 are input and output power of the electromagnetic wave, respectively, and El and E2 are input and output electric fields of the electromagnetic wave, repectively. 3.3.2 Shielding effectiveness of Ni-coated carbon fiber/A BS composites By calculation from eqn (6) Fig. 9 shows the relationship between the shielding effectiveness of
(2)
R can be expressed in two forms:
R(dB) = 354 + 10 log R (dB) = 20 log =+
O-316++
0.3541
(4)
rm
R can be evaluated
from eqn (3) for high impedance, electric field component in the i.e. a greater electromagnetic wave, and from eqn (4) at low impedance, i.e. a greater magnetic field component in the electromagnetic wave.
301
IO'
I
I
I
I
I
102
103
IO4
IO5
106
Frequency
(KHz)
Fig. 9. Shielding effectiveness of Ni-coated carbon-fiberfilled composites by theoretical calculation (fiber content = 10 vol.%): (a) solvent method; (b) Brabender mixing.
199
Electrical and shielding properties of ABS resin 140 130
SE
120 110 100 90
Ni-coated
CF
80 70 60
a”
01 10’
I
I
I
I
102
103
104
105
Frequency
I I06
(KHz)
Fig. 10.
Shielding effectiveness of Ni-coated carbon-fiberfilled composites measured by coaxial-cable method (fiber content = 10 vol.%).
composites filled with Ni-coated fiber and the incidental frequency of the electromagnetic wave. Line a is the shielding effectiveness of composites mixed by the solvent method filled with 10 vol.% of Ni-coated fiber; and line b is for composites mixed by the Brabender method filled with the same fiber content. Figure (9) shows that the shielding effectiveness of line a is higher than that of line b as a consequence of the lower resistivity of the composite which line a represents. This indicates that by theoretical calculation, the lower the resistivity, the higher the shielding effectiveness. Figure 10 shows the variation of the shielding effectiveness of the composites mixed by the Brabender method, also filled with 10 vol.% of Ni-coated carbon fiber, with electromagnetic wave frequency. The SE values are between 40 and 60 dB, which indicates a better shielding effect. Compared to Fig. 9(b), in the high-frequency region from lo3 to lo6 kHz, the experimental SE values shown in Fig. 10 are roughly the same as the theoretical values, but in the low-frequency region from 10 to lo3 kHz, the two sets of SE values do not coincide with each other. A value of 70dB is exceeded at 10 kHz by the theoretical calculation, and varies from 53 to 72 dB in this frequency range; however, a constant value of approximately 50 dB is obtained for the composite by measurement, which is lower than that by calculation. So in the high-frequency range which is generally used, the experimental and theoretical SE values correspond well. Thus, at least for high frequencies, the shielding effectiveness of the composites can be predicted by theoretical calculation.
4 CONCLUSIONS
The resistivity of Ni-coated carbon fibers decreases with increase in electroplating time and is about one order of magnitude lower than that of plain carbon fiber. The optimum thickness of the nickel coating is 0.2-0.5 pm. The resistivity of Ni-coated carbon-fiber/ABS composites decreases with an increase in volume fraction of the Ni-coated fibers. The aspect ratio of fibers is a main factor affecting the conductivity of fiber-filled composites. At the same fiber content, the conductivity of composites filled with Ni-coated fibers is much better than that of plain fiber-filled composites. To reach the same resistivity as that of plain fiber-filled composites, the use of Ni-coated fibers as filler can greatly decrease the required fiber volume fraction. This reduction in filler content can improve both the toughness of the composite and the rheological properties of the fibers during extrusion or injection molding and it also reduces the production cost, with resultant benefits in production. The shielding effectiveness of composites filled with 10 vol.% of Ni-coated carbon fiber is about 50 dB, which gives a better shielding effect. At high
ranging
from
ld
to lo6 kHz,
REFERENCES 1. Baker, Z. Q., Abdelazeez, M. K. & Zihlif, A. M., Measurements of the Magnex DC characteristics at microwave frequencies. J. Muter. Sci., 23 (1988) 2995-3000. 2. Jana, P. B., Mallick, A. K. & De, S. K., Electromagnetic interference shielding by carbon fiber-filled polychloroprene rubber composites. Composites, 22 (1991) 451-S. 3. Chiou, J.-M., Zheng, Q. & Chung, D. D. L., Electromagnetic interference shielding by carbon fiber reinforced cement. Composites, 20 (1989) 379-81. 4. Jana, P. B. & De, S. K., Electrical conductivity of barium-ferrite-vulcanized polychloroprene filled with short carbon fibers. Rubber Chem. Technol., 65, pp. 7-23. 5. Wu, G. & Schultz, J. M., Processing, microstructure, and failure behavior in short fiber reinforced PEEK composites. Polym. Comp., 11 (1990) 126-32. 6. Bigg, D. M., Mechanical, thermal and electrical properties of metal fiber filled polymer composites. Polym. Eng. Sci., 19 (1979) 1188-92.
7. Abraham,
From the above discussion the following conclusions can be drawn:
frequencies
the SE obtained by measurement approximately corresponds to that obtained by theoretical calculation, so that the shielding effectiveness of the composites can be predicted in this most frequently used range.
S. & Pai, B. C., Studies on nickel coated carbon fibers and their composites. J. Muter. Sci., 25 (1990) 2839-45. 8. Ahmad, M. S. & Zihilif, A. M., The electrical
G. Lu, X. Li, H. Jiang
conductivity of polypolypropylene and nickel-coated carbon fiber composites. Polym. Comp., 13 (1992) 53-7. 9. Ahmad, M. S. & Abdelazeez, M. K., Some properties of nickel-coated carbon fiber-polypropylene composites at
microwave frequencies. J. Muter. Sci., 25 (1990) 3083-8. 10. Di Liello, V., Tensile properties and fracture behaviour of polypropylene-nickel-coated carbon-fiber composites. J. Mater. Sci., 25 (1990) 706-12.
Printed in Northern Ireland. All rights reserved 0266-3538/96/$15.00
0266-3538(95)00143-j
ELSEVIER
ELECTRICAL AND SHIELDING PROPERTIES OF ABS RESIN FILLED WITH NICKEL-COATED CARBON FIBERS
Guanghong
Lu, Xiaotian Li & Hancheng Jiang
Department of Materials Science, Jilin University, 119 Jie Fang Road, Changchun
130023, Peoples
Republic of China
(Received 24 July 1995; accepted 23 November 1995) computers and television sets to shield electromagnetic waves and thus make them work safely. However, an obvious weakness of the shielding materials filled with carbon fibers is that the conductivity of carbon fibers is much lower than the meta1,6 so a greater volume fraction of carbon fibers is needed in order to provide the same shielding effect. Although composites filled with more carbon fibers have good shielding, both the toughness of the composites and the rheological properties of the fibers in extrusion or injection molding decrease significantly. At the same time, the increase in filler content means an increase in production cost because of the high cost of carbon fibers. The conductivity of carbon fibers cannot therefore satisfy the requirements for shielding materials for highly demanding applications. In order to decrease the fiber content and still retain a good shielding effect, a layer of metal is usually plated on the surface of the carbon fiber. Nickel and copper are the two main metals used as coatings. Nickel is generally preferred because of its stability and because of the readiness with which copper is oxidized. Some authors have studied the electrical and shielding properties of Ni-coated carbon-fiber-filled polymeric composites,‘-” but few, to our knowledge, have focused on aspects such as the oxidation problem of Ni-coated carbon fibers and comparisons of: (i) electrical conductivity of carbon fibers and Ni-coated carbon-fiber-filled composites; and (ii) theoretical and experimental shielding effectiveness of fiber-filled composites. In the present paper emphasis is laid on the electrical conductivity and shielding effectiveness of nickel-coated carbon fibers and their composites in relation to these aspects.
Abstract and shielding effectiveness of nickel-coated carbon fibers and their composites have been studied and the results are presented in this paper. A continuous electroplating device was used to plate nickel over the surface of the carbon fibers. The resistivity of Ni-coated carbon fibers decreases with an increase in plating time. The optimum thickness of the coating is 0.2-0.5 pm. A solvent method and Brabender mixing were used to prepare the composites of Ni-coated carbon fibers in ABS resin, the resistivity of which decreases with an increase in fiber content. At the same fiber content, the conductivity of the composites filled with Ni-coated carbon fibers is much greater than that of ordinary carbon-fiber-filled composites. To reach the same resistivity as carbonfiber-filled composites, the use of Ni-coated carbon fibers as filler can greatly decrease the required volume fraction of fibers. The shielding effectiveness of the composites was obtained by experiment and by theoretical calculation; at high frequencies, the two correspond very well. From experimental measurement, the shielding effectiveness of ABS resin filled with 10 vol. % of Ni-coated carbon fibers is about 50 dB. Electrical
conductivity
Keywords:
electrical conductivity, shielding effectiveness, Ni-coated carbon fibers, composites
1 INTRODUCTION Polymeric composites for electromagnetic shielding have been developed very rapidly in recent years, especially conductive polymeric composites filled with carbon fibers,14 which have many advantages, such as their good mechanical properties, their significant reinforcing effect, their low density which makes them suitable for light-weight shielding materials, and their ease of forming by extrusion or injection molding, which makes them suitable for being processed in batches.5 These materials have a wide range of applications in military, industrial and commercial fields. For example, they can be used as casings for
2 EXPERIMENTAL 2.1 Materials The materials used in this work were acrylonitrilebutadiene-styrene (ABS) resin and PAN-based T300 carbon fibers of density 1.60g/cm3, fiber diameter of 7 pm, and volume resistivity of 1.6 X lop3 Szcm. 193
G. Lu, X. Li, H. Jiang
194 2.2 Nickel electroplating
An electroplating device was constructed which can continuously plate nickel onto the surface of the fibers. The speed of carbon fibers can be changed by a gearbox, the normal speed being 1 m min-‘. Nickel sulphate was the main salt used in the electroplating solution, and electrolytic nickel plate was used as the anode. Carbon fibers are conductive, so they could be used directly as the cathode to be plated. Before being plated, carbon fibers were activated in a colloidal solution of palladium in order to enhance the adhesion force between the nickel coating and the carbon fibers.
2.3 Hydrogen reduction The problem of nickel oxidation can be solved by reduction. Dry hydrogen was used and the reduction temperature was 200°C for a treatment time of 3 h. A layer of soluble polyphenylamine was applied to the surface of N&coated carbon fibers to avoid reoxidation in the manufacturing process.
2.4 Mixing method and sample preparation Solvent method of mixing: ABS resin was mixed with chloroform to form a ‘paste’, and Ni-coated carbon fibers, chopped into 5 mm lengths, were dispersed into the paste by mechanical stirring to the desired proportion. The fiber-filled paste was then dried at room temperature. The composite was then molded by hot pressing at 200-210°C for about 5 min. Brabender mixing method: Ni-coated carbon fibers were chopped into 5 mm pieces and mixed
with ABS resin in a Brabender mixer operating at about 210°C for 3 min followed by hot compression molding also at 200-210°C for about 5 min. In order to determine the aspect ratio, L/D, of the fibers after mixing, the composites were treated with chloroform to dissolve out the resin phase. The fibers were extracted and observed under an optical microscope. 2.5 Resistance measurement Resistance was measured by the volt-ampere method illustrated in Fig. 1. The samples were of dimensions 60 mm X 3 mm X 2 mm and four samples were prepared for each composite. The resistance of each material was measured and average values were obtained. A type 2575 eight digital millivoltmeter and a type 2553 DC voltage current standard (YEW Co., USA) were used. The contact resistance was eliminated by the double-lead-wire method. 2.6 Shielding effectiveness measurement The shielding effectiveness of the specimens was measured by the coaxial-cable method illustrated in Fig. 2. A Fluk signal generator, 6071 (lOOHz1.5 GHz), HP3336 A/B/C (100 Hz-l-5 GHz), and frequency spectrograph HP8566A (10 Hz-26.6 GHz) were used to make these measurements. The sample was tested in the form of an annular ring with an outside diameter of 115 mm, an inside diameter of 12mm, and a thickness of 3 mm. Silver conductive paint was applied to the surface and the edge of the samples to allow a continuous contact to be made between the sample and the steel tubing or the steel chamber. To make the measurements, shielding effectiveness values were recorded with and without the sample in the apparatus. The difference between the two values is the real shielding effectiveness of the sample.
8’ Fig. Fig. 1.
Illustration of the method of resistance measurement by the volt-ampere method (press direction is vertical to the plane of plastic plate). 1, insulating plastic plate; 2, copper electrode; 3, sample; 4, millivoltmeter; 5, milliammeter.
8’ 2.
Coaxial set-up for measuring the shielding effectiveness of the composites. 1, coaxial switch GH 101-000; 2, coaxial connector L27/L16-503K; 3, coaxial line pedestal; 4, connecting tight nut; 5, coaxial line; backplate; 7, basement; 8, lea1 rail; 9, switch box.
6,
195
Electrical and shielding properties of ABS resin 3 RESULTS
AND
3.1 Conductivity
DISCUSSION
of nickel-coated
carbon fibers
Variations of the resistivity of Ni-coated carbon fibers are given in Table 1, which shows that the resistivity of Ni-coated carbon fibers is about lop4 Qcm, an order of magnitude less than that of the uncoated fiber (1.6 x lo-” Qcm). When the electric current is 2 A and electroplating time is 15 min, the resistivity of the Ni-coated carbon fibers can reach 6.4 X lo-’ Qcm. It is also shown that the resistivity of the Ni-coated carbon fibers decreases with increase in plating time as a consequence of the increase in coating thickness. If considering the conductivity of the fibers only, the thicker the coating the better the conductivity. But there is a limiting thickness, since if the coating thickness exceeds 0.5 pm, the Ni-coated carbon fibers have defects, as follows:
(b)
1. the toughness of the fibers markedly decreases, the fibers become stiff and are easily broken in the mixing process; 2. the fibers easily adhere to each other and are hard to separate in the mixing process, so that the uniformity of the fiber distribution in the composites is lost. On the other hand, if the coating is thinner than 0.2 ,um, not only is the nickel coating not continuous, but the required conductivity of the fibers cannot be achieved. The optimum thickness therefore ranges from 0.2 to 0.5 pm. In this range the nickel coating is uniform and continuous, the toughness of the fibers is good, and the fiber dispersion in the composites after mixing is also satisfactory. Figure 3 shows SEM micrographs of Ni-coated carbon fibers. Surface observation of a bunch of fibers is shown in Fig. 3(a) and a cross-section observation of a single Ni-coated fiber is shown in Fig. 3(b). The thickness of the coating in the figures is about 0.3 pm. The micrographs show that the coating surface is uniform and smooth which indicates that electroplating is satisfactory.
Fig. 3. SEM micrographs of Ni-coated carbon fibers: (a) surface of a bunch; (b) cross-section of a single fiber.
Figure 4 is an X-ray photoelectron spectroscopy (XPS) analysis of Ni-coated fiber which has been left in air for one day. The binding energy of nickel and nickel oxide are given, which indicates that the nickel
Table 1. The resistlvity of Ni-coated carbon fibers Fiber
Electric current (A)
Plating time (min)
Carbon Ni-plated carbon
Resistivity @cm) 1.6 x lo-”
2
4
5 8 12 15 3 3.5 4
5.2 x 1om4 2.6 x lo-“ 1.2 x 1om4 0.64 x 10m4 5.2 x 1om4 4.7 x 1o-4 4.4 x 1o-4
I
892.400
882.400
I
872.400
Binding
I
I
862.400
energy
852.400
\,
842.400
(eV)
Fig. 4. X-ray photoelectron spectrograph carbon fibers.
of nickel-coated
196
G. Lu, X. Li, H. Jiang
Table 2. Variation of the resistivity of Ni-coated carbon fiber before and after hydrogen reduction (current = 15 A)
Time (min)
1.5 3.5 4.5
Resistivity before reduction (km) 1.3 x 1o-3 1.1 x 1om3 2.4 x lo-’
Resistivity after reduction (Qcm) 4.2 x 10m4 1.7 x 1o-4 1.4 x 1om4
coating is oxidized. A very thin oxide layer (1 nm) can decrease the conductivity of the fibers significantly, so hydrogen reduction was used to eliminate the oxidized layer. Table 2 gives the variation of the resistivity of Ni-coated carbon fibers before and after reduction. It is shown that before reduction the resistivity of the Ni-coated fibers increased markedly, some of them exceeding the resistivity of the plain fiber (1.6 X 10e3 Qcm). After reduction the resistivity returned to normal (about 1O-4 Qcm), which indicates that the hydrogen reduction has achieved its purpose. 3.2 The conductivity of ABS resin filled with Nicoated carbon fibers 3.2.1 Variation of the resistivity of the composites with fiber content Figures 5 and 6 show the relationship between the resistivity of the composites and the volume fraction of Ni-coated carbon fibers. Figure 5 represents results for the composites mixed by the solvent method, and Fig. 6 for the composites mixed by the Brabender process. In both figures, line a, as a comparison, is for carbon fiber for plain-filled ABS resin composites, and line b is for Ni-coated carbon fiber-filled ABS resin composites. It is clear that whichever method is used the resistivity of the composites decreases with an increase in fiber content. Compared with the resistivity of ABS resin filled with plain carbon fibers, the resistivity of ABS resin filled with Ni-coated fibers decreases by about l-2 orders of magnitude for the same volume fraction of fibers. The decrease of the filler resistivity directly brings about the decrease in resistivity of the composites. It is shown in Fig. 5 (composites mixed by the solvent method) that the resistivity of ABS resin with 12.5 vol.% of Ni-coated fiber is equal to that of a composite with 27.5 vol.% of plain fibers, a saving of 15 vol.% of filler; similarly, the resistivity of the ABS resin filled with 5 vol.% of Ni-coated fiber is equal to that of the composite with 14.5 vol.% of plain carbon fiber, a saving of about 10 vol.% of filler. In Fig. 6 (composites mixed by the Brabender process) it is shown that the resistivity of resin with 12.5 vol.% of Ni-coated fiber is equal to that of the ABS with 21.5 vol.% of plain fiber, a saving of 9 vol.% of filler; and for 5 vol.% of Ni-coated fiber, the resistivity of the
I IO Content
I 20 of fibres (~01%)
Fig. 5. Variation of the resistivity of Ni-coated carbonfiber/ABS composites mixed by the solvent method with fiber volume content: (a) carbon-fiber filler; (b) Ni-coated carbon-fiber filler.
composite is equal to that of the composite with 13 vol.% of plain carbon fiber, saving about 8 vol.% of filler. To reach the same resistivity as the plain fiber-filled composites, the required volume fraction of the Ni-coated fiber decreases greatly. This decrease in filler content can improve both the toughness of the composite and the rheological properties of the fibers during extrusion or injection molding. This also decreases the production costs, with significant benefit. 3.2.2 Aspect ratio offibers after mixing Comparing Figs 5 and 6, it can be seen that the resistivity of the composites mixed by the Brabender method is higher than that of the composites mixed by the solvent method, the difference between them
0
10 Content
Fig. 6. Variation
20 of fibres (~01%)
of the resistivity of Ni-coated carbonfiberlABS composites mixed by the Brabender method with fiber volume fraction: (a) carbon-fiber filler; (b) Ni-coated carbon fiber as filler.
Electrical and shielding properties of ABS resin
being about one order of magnitude. By the method illustrated in Section 2 of this paper, the aspect ratio of fibers can be obtained. The fiber aspect ratio of the composites mixed by the solvent method is about 600, i.e. the fiber length is about 4 mm, which indicates that when mixed by this method the fibers are slightly damaged. This is an ideal situation for the fibers in the composites since the composites can achieve lower resistance (higher conductivity). On the other hand, the fiber aspect ratio in the composites mixed by the Brabender method is about 40, i.e. the length of the fibers is about 0.3 mm. This leads to a decrease in the conductivity of the composites. The aspect ratio of the fibers is the main factor to affect the conductivity of the fiber-filled composites. A high fiber aspect ratio can be achieved by control of the mixing technology. The basic rule is that the mixing temperature should be as high as possible, but not so high as to cause the ABS to be degraded, and the mixing time should be as short as possible consistent with good fiber dispersion in the composites. Good (a)
Fig. 7. SEM cross-section observation of Ni-coated carbonfiber/ABS composites with 3.5 vol.% fibers: (a) X400; (b) x 1000.
197
fiber distribution in the composites and short mixing times are naturally mutually exclusive, however, so it is important to seek the optimum processing conditions. In the present work we found that the most suitable conditions for the ABS resin matrix are a mixing temperature of 210°C and a mixing time of 3 min. It should be noted that the filler consists of two parts, the nickel plating and the carbon fibers, and we have not distinguished between these in the above discussion. Two indications are given now: when the thickness of the nickel coating is 0.6 pm, the relative proportions, by weight, of Ni and carbon fiber are both 50%, and when the thickness of the nickel coating is 0.2 pm, the proportions are 2.5 and 75%, respectively. The thicker the coating, the less the proportion of carbon in the Ni-coated fibers. The thickness of the nickel coating used in this work was about 0.5 pm.
3.2.3 SEM analysis Figure 7 shows SEM micrographs of cross-sections of Ni-coated carbon-fiber/ABS resin composites mixed by the Brabender method with 3.5 vol.% of fibers, in which the bright part is the fibers and the dark part is the resin phase. The micro-distribution of Ni-coated fibers in the composites can be observed from the two micrographs. It is shown that the fiber distribution is uniform and has no orientation, indicating that the mixing is satisfactory under the conditions discussed above.
3.2.4 X-ray analysis An SEM photograph is a micro-observation of a certain point in the composite. In order to determine the macrosopic distribution of Ni-coated fiber in the composites, X-radiography was used. Figure 8 shows macrophotographs obtained by this method of the composites mixed by the Brabender method with different fiber contents. It is seen from Fig. 8(a) that the fiber content in this composite is lower compared to that in Fig. S(b), and the interfiber contacts are also fewer, so that the resistivity of this composite is higher. On the other hand, Fig. 8(b) shows that the interfiber contacts in this composite have formed a conductive network, so the resistivity of this composite is much lower. The results above correspond to the conductive mechanism of fiber-filled composites, i.e. ‘percolation theory’. This theory suggests that it is the existence of conductive passages (interfiber contacts) that results in the conductivity of the composites. The number of conductive passages increases gradually with increase in fiber content, so that the resistivity of the composite decreases, as indicated by Fig. 8.
198
G. Lu, X. Li, H. Jiang
(a)
Absorpsion loss, A, can be evaluated equations as follows:
from
A(dB) = 3.34 x 10-3tV&i
the
(5)
In eqns (3)-(5), f is the incidental frequency of the electromagnetic wave (Hz), r is the distance from the emission source to the shielding material, u is the electrical conductivity, p is the magnetic transmissivity, and t is the thickness of the shielding material. For composites with a polymeric matrix, SE can be calculated from the electrical resistivity of the composite as follows: 0))
SE(dB)=50+lOlog(;)+l.7t$
(6)
where p is the volume resistivity of the composites (Qcm), f is the incidental frequency of the electromagnetic wave (MHz), and t is the thickness of the composite (cm). This equation is also called the Simon formalism and is generally used in the calculation of SE. In the present work, t = 3 mm so that SE of the composites could be compared with the experimental work. The unit of electromagnetic shielding (dB) is defined as follows: Fig. 8. X-radiographs (enlarged 20 times) of Ni-coated carbon-fiber/ABS composites with fiber content of: (a) 3.5, and (b) 12.5 vol.%.
or dB=20log
3.3 Shielding effectiveness N&coated carbon fibers 3.3.1 Electromagnetic
of composites
filled with
shielding mechanism
According to Schelkunoff’s theory, the shielding effectiveness, SE, of a material can be divided into three parts, R, A and M, as follows: SE(dB)=R+A+M
(1)
where A is absorption loss of the energy of the electromagnetic wave, R is the first reflection loss of energy, and M is a multi-reflection loss of energy. When A exceeds lOdB, M can be ignored in the calculation. So eqn (1) becomes: SE=R+A
i
2 2
>
where P, and P2 are input and output power of the electromagnetic wave, respectively, and El and E2 are input and output electric fields of the electromagnetic wave, repectively. 3.3.2 Shielding effectiveness of Ni-coated carbon fiber/A BS composites By calculation from eqn (6) Fig. 9 shows the relationship between the shielding effectiveness of
(2)
R can be expressed in two forms:
R(dB) = 354 + 10 log R (dB) = 20 log =+
O-316++
0.3541
(4)
rm
R can be evaluated
from eqn (3) for high impedance, electric field component in the i.e. a greater electromagnetic wave, and from eqn (4) at low impedance, i.e. a greater magnetic field component in the electromagnetic wave.
301
IO'
I
I
I
I
I
102
103
IO4
IO5
106
Frequency
(KHz)
Fig. 9. Shielding effectiveness of Ni-coated carbon-fiberfilled composites by theoretical calculation (fiber content = 10 vol.%): (a) solvent method; (b) Brabender mixing.
199
Electrical and shielding properties of ABS resin 140 130
SE
120 110 100 90
Ni-coated
CF
80 70 60
a”
01 10’
I
I
I
I
102
103
104
105
Frequency
I I06
(KHz)
Fig. 10.
Shielding effectiveness of Ni-coated carbon-fiberfilled composites measured by coaxial-cable method (fiber content = 10 vol.%).
composites filled with Ni-coated fiber and the incidental frequency of the electromagnetic wave. Line a is the shielding effectiveness of composites mixed by the solvent method filled with 10 vol.% of Ni-coated fiber; and line b is for composites mixed by the Brabender method filled with the same fiber content. Figure (9) shows that the shielding effectiveness of line a is higher than that of line b as a consequence of the lower resistivity of the composite which line a represents. This indicates that by theoretical calculation, the lower the resistivity, the higher the shielding effectiveness. Figure 10 shows the variation of the shielding effectiveness of the composites mixed by the Brabender method, also filled with 10 vol.% of Ni-coated carbon fiber, with electromagnetic wave frequency. The SE values are between 40 and 60 dB, which indicates a better shielding effect. Compared to Fig. 9(b), in the high-frequency region from lo3 to lo6 kHz, the experimental SE values shown in Fig. 10 are roughly the same as the theoretical values, but in the low-frequency region from 10 to lo3 kHz, the two sets of SE values do not coincide with each other. A value of 70dB is exceeded at 10 kHz by the theoretical calculation, and varies from 53 to 72 dB in this frequency range; however, a constant value of approximately 50 dB is obtained for the composite by measurement, which is lower than that by calculation. So in the high-frequency range which is generally used, the experimental and theoretical SE values correspond well. Thus, at least for high frequencies, the shielding effectiveness of the composites can be predicted by theoretical calculation.
4 CONCLUSIONS
The resistivity of Ni-coated carbon fibers decreases with increase in electroplating time and is about one order of magnitude lower than that of plain carbon fiber. The optimum thickness of the nickel coating is 0.2-0.5 pm. The resistivity of Ni-coated carbon-fiber/ABS composites decreases with an increase in volume fraction of the Ni-coated fibers. The aspect ratio of fibers is a main factor affecting the conductivity of fiber-filled composites. At the same fiber content, the conductivity of composites filled with Ni-coated fibers is much better than that of plain fiber-filled composites. To reach the same resistivity as that of plain fiber-filled composites, the use of Ni-coated fibers as filler can greatly decrease the required fiber volume fraction. This reduction in filler content can improve both the toughness of the composite and the rheological properties of the fibers during extrusion or injection molding and it also reduces the production cost, with resultant benefits in production. The shielding effectiveness of composites filled with 10 vol.% of Ni-coated carbon fiber is about 50 dB, which gives a better shielding effect. At high
ranging
from
ld
to lo6 kHz,
REFERENCES 1. Baker, Z. Q., Abdelazeez, M. K. & Zihlif, A. M., Measurements of the Magnex DC characteristics at microwave frequencies. J. Muter. Sci., 23 (1988) 2995-3000. 2. Jana, P. B., Mallick, A. K. & De, S. K., Electromagnetic interference shielding by carbon fiber-filled polychloroprene rubber composites. Composites, 22 (1991) 451-S. 3. Chiou, J.-M., Zheng, Q. & Chung, D. D. L., Electromagnetic interference shielding by carbon fiber reinforced cement. Composites, 20 (1989) 379-81. 4. Jana, P. B. & De, S. K., Electrical conductivity of barium-ferrite-vulcanized polychloroprene filled with short carbon fibers. Rubber Chem. Technol., 65, pp. 7-23. 5. Wu, G. & Schultz, J. M., Processing, microstructure, and failure behavior in short fiber reinforced PEEK composites. Polym. Comp., 11 (1990) 126-32. 6. Bigg, D. M., Mechanical, thermal and electrical properties of metal fiber filled polymer composites. Polym. Eng. Sci., 19 (1979) 1188-92.
7. Abraham,
From the above discussion the following conclusions can be drawn:
frequencies
the SE obtained by measurement approximately corresponds to that obtained by theoretical calculation, so that the shielding effectiveness of the composites can be predicted in this most frequently used range.
S. & Pai, B. C., Studies on nickel coated carbon fibers and their composites. J. Muter. Sci., 25 (1990) 2839-45. 8. Ahmad, M. S. & Zihilif, A. M., The electrical
G. Lu, X. Li, H. Jiang
conductivity of polypolypropylene and nickel-coated carbon fiber composites. Polym. Comp., 13 (1992) 53-7. 9. Ahmad, M. S. & Abdelazeez, M. K., Some properties of nickel-coated carbon fiber-polypropylene composites at
microwave frequencies. J. Muter. Sci., 25 (1990) 3083-8. 10. Di Liello, V., Tensile properties and fracture behaviour of polypropylene-nickel-coated carbon-fiber composites. J. Mater. Sci., 25 (1990) 706-12.