Composites Science and Technology 61 (2001) 885±888
`
`www.elsevier.com/locate/compscitech
`
`Carbon-®ber/polymer-matrix composites as capacitors
`
`Xiangcheng Luo, D.D.L. Chung *
`
`Composite Materials Research Laboratory, State University of New York at Bu€alo, Bu€alo, NY 14260-4400, USA
`
`Received 8 February 1999; received in revised form 18 October 1999; accepted 18 July 2000
`
`Abstract
`
`A continuous carbon-®ber/epoxy-matrix composite with a paper interlayer (0.04 mm thick after composite fabrication) was
`found to exhibit a capacitance of 1.2 mF/m2 at 2 MHz, in contrast to a value of 0.21 mF/m2 for epoxy-impregnated paper (0.10 mm
`thick). The high capacitance is partly a consequence of the large area of the surface of a ®ber layer sandwiching the paper interlayer.
`This area is twice the ¯at area. Without a paper interlayer, the composite failed to serve as a capacitor, because of the conductivity
`in the through-thickness direction. # 2001 Elsevier Science Ltd. All rights reserved.
`
`Keywords: A. Carbon ®bers; A. Polymer-matrix composites (PMCs); A. Structural materials; B. Electrical properties; Capacitor
`
`1. Introduction
`
`Capacitors are important elements in electrical cir-
`cuits, although they tend to be more bulky than other
`elements, such as resistors, diodes and transistors, and
`their high-frequency performance remains an issue. The
`bulkiness is of particular concern when a large capaci-
`tance is required. Capacitors based on electrical double
`layers cannot operate at a high frequency, so capacitors
`based on dielectrics are most common for electronics.
`This paper is focused on capacitors based on dielectrics.
`Capacitors based on dielectrics are conventionally
`parallel-plate capacitors in which a dielectric is sand-
`wiched by electrically conducting plates. The dielectric
`material can be paper [1], polymer [1], a high dielectric
`constant ceramic such as barium titanate in thin ®lm or
`thick ®lm forms [2], or other electrically insulating
`materials. The conducting plates are commonly metals
`in foil, thick ®lm or thin ®lm forms [1,2]. To achieve a
`high capacitance, the conducting plates are large in
`area, the dielectric is low in thickness, and numerous
`layers of dielectric and conducting plates are alter-
`natively stacked (and usually wound to save space).
`This paper provides a new type of parallel-plate capa-
`citor, namely carbon ®ber polymer-matrix composites in
`
`* Corresponding author. Tel.: +1-716-645-2593; fax: +1-716-645-
`3875.
`E-mail address: ddlchung@acsu.bu€alo.edu (D.D.L. Chung).
`
`the
`serve as
`layers
`which continuous carbon-®ber
`conducting plates and paper (placed between the ®ber
`layers), together with the polymer matrix, serves as the
`dielectric. Carbon-®ber polymer-matrix composites are
`structural materials that are important for lightweight
`structures,
`such as aircrafts, automobiles,
`sporting
`goods, wheel chairs, etc. The ability of these composites
`to serve as capacitors and other circuit elements means
`that the structure is itself the electronics, so that the
`electronics `vanish' into the structure. Electronics made
`from structural materials such as carbon-®ber polymer-
`matrix composites and concrete constitute a new ®eld
`of electronics called structural electronics [3]. In the
`case of continuous carbon ®ber polymer-matrix com-
`posites, the carbon ®bers are the conductors (resistors)
`and they can be intercalated to become electron metals
`or hole metals [3]. By having the electronics vanish into
`the structure, space is saved. The space saving is parti-
`cularly valuable for capacitors of large capacitance. As
`a result of the large area of a structure and the numerous
`®ber layers in a composite laminate, the capacitance in a
`composite structure can be very large. In addition to
`space saving, structural electronics have the advantage
`of being mechanically rugged and inexpensive, since
`structural materials are necessarily rugged and inexpen-
`sive. The use of a structure as a capacitor is particularly
`valuable
`in conjunction with structures
`that are
`powered by solar cells, as the structure (capacitor) can
`be used to store the electrical energy generated by the
`solar cells.
`
`0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
`P I I : S 0 2 6 6 - 3 5 3 8 ( 0 0 ) 0 0 1 6 6 - 4
`
`Exhibit 1033
`IPR2016-00636
`AVX Corporation
`
`000001
`
`

`

`886
`
`X. Luo, D.D.L. Chung / Composites Science and Technology 61 (2001) 885±888
`
`2. Experimental methods
`
`Epoxy-matrix composites comprising four continuous
`unidirectional carbon-®ber layers were constructed from
`individual layers cut from a 12 inch wide unidirectional
`carbon ®ber prepreg tape manufactured by ICI Fiberite
`(Tempe, AZ). The product used was Hy-E 1076E, which
`consisted of 976 epoxy matrix and 10E carbon ®bers.
`The ®ber and matrix properties are shown in Table 1.
`The matrix was electrically insulating, whereas the ®bers
`were electrically conducting, with a resistivity of
`2.2103
` cm.
`The composite laminates were laid up in a 0.5 inch (13
`mm) diameter steel compression mold with laminate
`con®guration [0/90]2 (i.e. four unidirectional ®ber layers
`stacked in the sequence [0/90/0/90]). The individual ®ber
`layers were cut from the prepreg tape. A liquid mold
`release was used. The laminates were cured using a cycle
`based on the ICI Fiberite C-5 cure cycle. Curing occur-
`red at 35510F (1796C) and 89 psi (0.61 MPa) for
`120 min. Then the samples were sanded to a rectangular
`shape (about 88 mm; the exact dimensions were mea-
`sured for each sample in order to calculate the area of
`the rectangle) for capacitance measurement.
`Composites without interlayer (additive between the
`®ber layers) and with various types of dielectric inter-
`layers were fabricated. In a composite with an inter-
`layer, the interlayer was placed between the second and
`third ®ber layers in the stack of four ®ber layers. The
`interlayers were paper (Table 2) and barium titanate
`thick ®lm (0.13±0.25 mm thick). The interlayer thickness
`after composite fabrication was determined by cross-
`sectional optical microscopy. The barium titanate thick
`®lm was applied as a paste, which was made by mixing
`barium titanate powder (1 mm size, from TAM Cera-
`mics Inc., Niagara Falls, NY) and epoxy (Epon Resin
`862 and EPI-Cure 3274 Curing Agent from Shell Che-
`mical Co., Houston, TX). The barium titanate powder
`was in amounts ranging from 17 to 80 vol.% of the
`paste. The epoxy from the prepreg layers penetrated the
`
`Table 1
`Carbon ®ber and epoxy matrix properties (according to ICI Fiberite)
`
`10E Ð Torayca T-300 (6K) untwisted, UC-309 sized
`7 mm
`Diameter
`1.76 g cm3
`Density
`2.2103
` cm
`Electrical resistivity
`Tensile modulus
`221 GPa
`Tensile strength
`3.1 GPa
`
`976 epoxy
`Process temperature
`Maximum service temperature
`
`Flexural modulus
`Flexural strength
`Tg
`Density
`
`350F (177C)
`350F (177C) dry
`250F (121C) wet
`3.7 GPa
`138 MPa
`232C
`1.28 g cm3
`
`Table 2
`Thickness of paper (mm, 0.01) before and after composite fabrica-
`tion
`
`Tissue paper
`Writing paper
`Bond paper
`
`Before
`
`0.05
`0.08
`0.10
`
`After
`
`0.02
`0.04
`0.08
`
`paper (tissue, bond or writing) interlayer, which was
`porous, during the composite fabrication.
`Capacitance (for the sample capacitance and sample
`resistance in parallel) measurement was made using a
`precision RLC capacitance meter (Model 7600, QuadTech,
`Inc., Marlborough, MA) at frequencies ranging from 10
`Hz to 2 MHz. During the measurement, the rectangular
`sample (mechanically polished on the rectangular faces)
`was sandwiched by two copper disks (mechanically
`polished on the circular faces) of diameter 0.5 inch (13
`mm). The sandwich was held together by pressure pro-
`vided by a clip. The contact resistance of the interface
`between a copper disk and a sample was only a few
`ohms, so the interface contributed negligibly to the
`measured capacitance.
`
`3. Results and discussion
`
`As a consequence of the touching of the ®bers of the
`two ®ber layers, the through-thickness conductivity was
`too high (impedance too low; Table 3) for meaningful
`capacitance measurement for the composite without
`interlayer, the composite with tissue paper interlayer
`and the composite with barium titanate thick ®lm
`(without paper) interlayer. The use of a writing paper or
`bond paper interlayer was sucient for avoiding the
`®bers of one layer to touch those of the other layer, but
`the use of tissue paper or barium titanate thick ®lm
`interlayer was not. The ®bers were not able to go
`through the writing paper or bond paper, even though
`the epoxy matrix was able to penetrate the paper. (The
`penetration of epoxy is desirable for the mechanical
`integrity of the composite.) As a result, only composites
`with a writing paper or bond paper interlayer could
`serve as capacitors.
`Table 3 shows the dielectric behavior of various com-
`posites in the through-thickness direction. The highest
`capacitance per unit area was attained by using writing
`paper alone as the interlayer. The use of bond paper as
`the interlayer gave lower capacitance per unit area
`because of the larger interlayer thickness. The combined
`use of writing paper and barium titanate as the interlayer
`also gave lower values of the capacitance per unit area
`than the use of writing paper alone. By using a paste with
`80 vol.% barium titanate, the relative dielectric constant
`
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`
`

`

`X. Luo, D.D.L. Chung / Composites Science and Technology 61 (2001) 885±888
`
`887
`
`Table 3
`Dielectric behavior of carbon ®ber epoxy-matrix composites in the through-thickness direction
`
`Interlayer
`
`Capacitance per unit area (mF/m2)
`
`Relative dielectric
`constanta at 2 MHz
`
`Interlayer
`thickness (mm, 0.01)
`
`Impedance
`at 10 Hz (
`)
`
`2 kHz
`
`2 MHz
`
`None
`33 vol.% BaTiO3
`Tissue paper
`Bond paper
`Writing paper
`Writing paper+17 vol.% BaTiO3
`Writing paper+33 vol.% BaTiO3
`Writing paper+65 vol.% BaTiO3
`Writing paper+80 vol.% BaTiO3
`
`±
`±
`±
`0.54
`1.23
`0.90
`1.06
`0.99
`0.84
`
`±
`±
`±
`0.45
`1.17
`0.78
`0.92
`0.87
`0.71
`
`±
`±
`±
`4.1
`5.3
`8.9
`13.6
`14.2
`19.8
`
`0
`0.11
`0.02
`0.08
`0.04
`0.10
`0.13
`0.14
`0.25
`
`33
`30
`33
`2.34106
`1.89106
`2.84106
`1.78106
`1.59106
`2.54106
`
`a Calculated from the capacitance, interlayer thickness, and the ¯at area of the laminate (the actual area of the conductor surface sandwiching the
`dielectric is much larger than the ¯at area as a result of the fact that a lamina consists of ®bers of diameter 7 mm).
`
`attained the highest value of 19.8, but the large thick-
`ness of the interlayer (resulting from the low workability
`of this paste) caused the capacitance per unit area to be
`low. Therefore, the use of a barium titanate interlayer is
`not attractive, whatever is the volume fraction of
`BaTiO3 in the paste.
`For all samples in Table 3 exhibiting dielectric (rather
`than conducting) behavior in the through-thickness
`direction, the capacitance decreased with increasing fre-
`quency. For example, for the composite with writing
`paper alone as the interlayer, the capacitance at 2 MHz
`was 95% of that at 2 kHz.
`Table 4 shows the dielectric behavior of paper and
`epoxy-impregnated paper in the absence of ®bers. The
`relative dielectric constant is higher for writing paper
`(used in some of the composites in Table 3) than bond
`paper. This is caused by contamination in the writing
`paper. The contamination is shown by dispersed dark
`spots on the optical micrograph. However, such con-
`tamination (spots) was not observed for the bond paper.
`Table 4 shows that the relative dielectric constant of
`epoxy-impregnated writing paper (2.4) is below that of
`writing paper (2.7) and that of epoxy (3.0). The relative
`dielectric constant of epoxy-impregnated bond paper
`(2.1) is the same as that of bond paper, but less than
`that of epoxy. These e€ects are believed to be a result of
`the reactions between epoxy and paper.
`
`Table 4
`Dielectric behavior (at 2 MHz) of epoxy, paper and epoxy impreg-
`nated paper in the absence of ®bers (all have thickness 0.10 mm)
`
`Capacitance per
`unit area (mF/m2)
`
`Relative dielectric
`constant
`
`Writing paper
`Writing paper+epoxy
`Bond paper
`Bond paper+epoxy
`Epoxy
`
`0.24
`0.21
`0.19
`0.19
`0.10
`
`2.7
`2.4
`2.1
`2.1
`3.0
`
`The relative dielectric constant of the writing paper
`interlayer (5.3) in Table 3 (in the presence of ®bers) is
`much higher than that (2.4) of the epoxy-impregnated
`writing paper in Table 4 (in the absence of ®bers). This
`is partly attributed to the fact that the surface of a ®ber
`layer is not ¯at, so that the actual area of the surface is,
`from simple geometry, as much as p/2 or 1.6 times the
`¯at area used in calculating the relative dielectric con-
`stant of Table 3. Since the relative dielectric constant is
`inversely related to the area, the ratio of the actual area
`to the ¯at area is 5.3/2.4=2.2, if the dielectric behavior
`is assumed to be the same for the interlayer part of the
`composite and the epoxy-impregnated paper. In other
`words, the actual relative dielectric constant of the
`writing paper interlayer is only 2.4, but
`the large
`(actual) area makes the relative dielectric constant
`appear high (5.3). That this ratio exceeds 1.6 is probably
`because of the di€erence in dielectric behavior between
`the interlayer and the epoxy-impregnated paper resulting
`from the di€erences in the extent of reaction between
`epoxy and paper and in the extent of cure of the epoxy.
`Hence, the e€ectiveness of carbon-®ber epoxy-matrix
`composites for capacitors is partly caused by the large
`area provided by the lamina surface, which is not ¯at.
`The capacitance per unit area is 1.17 mF/m2 for the
`writing paper interlayer (Table 3), but just 0.21 mF/m2
`for the epoxy impregnated writing paper (Table 4). This is
`partly a result of the large actual area of the conducting
`surface sandwiching the writing paper interlayer, but is
`caused by the small thickness of the writing paper
`interlayer (0.04 mm) compared to the thickness of the
`epoxy-impregnated writing paper (0.10 mm). The pressure
`during composite fabrication probably resulted in the
`decrease of the thickness of the writing paper.
`For a typical composite structure with a substantially
`large surface area and numerous ®ber layers, the e€ective
`area is at least 1000 m2, and the associated capacitance
`(for the case of the writing paper interlayer) is at least 1.2
`mF. Hence a large capacitor is built in to the structure.
`
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`
`

`

`888
`
`X. Luo, D.D.L. Chung / Composites Science and Technology 61 (2001) 885±888
`
`Although carbon-®ber polymer-matrix composites are
`much more conductive in the ®ber direction than in the
`through-thickness direction,
`the conductivity in the
`through-thickness direction is substantial. The through-
`thickness conductivity is a consequence of the contact
`between ®bers of adjacent ®ber layers. The contact
`occurs in spite of the presence of the epoxy matrix
`because of the ¯ow of the epoxy resin during composite
`fabrication and the waviness of the ®bers. The contact
`cannot be stopped by the use of a tissue paper interlayer
`given the porosity of the tissue paper. For the same
`reason, the contact cannot be stopped by the use of a
`thick ®lm interlayer. However, the contact can be e€ec-
`tively stopped by the use of a writing paper or bond
`paper interlayer, which has enough porosity for the
`epoxy resin to go through and has small enough a por-
`osity for the ®bers to be not able to go through.
`
`4. Conclusion
`
`Carbon-®ber epoxy-matrix composite was found to
`be a parallel-plate capacitor with capacitance 1.2 mF/m2
`at 2 MHz, if the composite contained a writing paper
`interlayer of thickness 0.04 mm. The further addition of
`
`a BaTiO3 thick ®lm to the interlayer decreased the
`capacitance as a results of the increase in interlayer
`thickness. Without an interlayer or with a more porous
`paper interlayer, the composite was conducting in the
`through-thickness direction. The capacitance of
`the
`epoxy-impregnated paper (0.10 mm thick) was 0.21 mF/
`m2. The high capacitance for the composite with paper
`interlayer is partly a consequence of the large area of the
`surface of a ®ber layer; this area is 2 times that of the
`¯at area. The high capacitance is partly a result of the
`reduction in the paper thickness during composite fab-
`rication.
`
`References
`
`1 Sarjeant WJ, MacDougall FW. Capacitors for high power elec-
`tronics. In: IEEE Annual Report Ð Conference on Electrical
`Insulation and Dielectric Phenomena, IEEE Dielectrics and
`Electrical Insulation Society, Vol. I, 1997. p. 113±120.
`2 Yoo IK, Burton LC, Stephenson FW. Electrical conduction
`mechanisms of barium-titanate-based thick-®lm capacitors. IEEE
`Trans Components, Hybrids, and Manufacturing Technology
`1987;CHMT-10(2):274±82.
`3 Chung DDL, Wang S. Carbon ®ber polymer-matrix structural
`composite as a semiconductor and concept of optoelectronic and
`electronic devices made from it. Smart Mater Struct 1999;8:161±6.
`
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`
`