Multifunctional composites that have the ability to store or generate energy have recently been developed and have attracted a great deal of attention [1] . One type of multifunctional composite is a structural capacitor that uses carbon fiber fabric as electrodes. Although the total energy storage of a capacitor is not larger than that of a battery, a capacitor exhibits less degradation during charge-discharge cycles than a battery. As a result, structural capacitors have been investigated as an energy storage method for energy regeneration systems such as automobiles. For a small, lightweight vehicle, it is cumbersome to load a capacitor in the limited space available, so a structural capacitor is an attractive solution to save space. Structural capacitors have therefore recently been developed by several research groups [2] -[7] .

In a structural capacitor using carbon-fiber cloth as electrodes, the electrodes need to electrically be separated by a separator such as a sheet of paper, film or glass-fiber cloth [2] -[7] . However, using a film or paper as a separator reduces long-term reliability because it is difficult to achieve perfect bonding between the film and electrodes. Therefore, for large-scale structures, glass-fiber cloth is an attractive practical option to separate electrodes. Using glass-fiber cloth as a separator means the minimum distance between the electrodes is larger than the thickness of the cloth, so the electric capacitance is not sufficiently large in a parallel-plate capacitor. The electric capacitance can be improved when an electrical double layer capacitor is employed [6] [7] .

A structural capacitor with a glass-fiber fabric separator is easy to fabricate. However, sometimes the carbon-fiber cloth will electrically short even when glass-fiber cloth is used as a separator. In the present study, electrical shorting is considered first. A mechanism of electrical shorting is proposed, and an improved fabrication process is suggested to prevent electrical shorting between the carbon-cloth electrodes. In the present study, a parallel-plate type capacitor is used to simplify the problem of electrical shorting between carbon-fiber fabric electrodes.

Figure 1 shows the fabrication process of the carbon-fiber fabric structural capacitor used in the present study. Carbon-fiber prepreg sheets (W3101-A/Q112J, Toho Tenux Co., Tokyo, Japan, thickness = 0.198 mm) were used as electrodes. A glass-fiber cloth sheet (E01Z SK#1017, Unitika Ltd., Tokyo, Japan, thickness = 0.013 mm) was attached with epoxy adhesive (NB102, Newport, thickness = 0.052 mm) to the electrodes as a separator to produce a sample with a length of 40 mm and width of 20 mm, which was then cured at 135˚C for 1.5 h under atmospheric pressure in a vacuum bag. Silicone rubber blocks with a thickness of 0.5 mm were used to prevent excessive epoxy resin flow. Six samples were fabricated; a photograph of a typical sample is shown in Figure 2. The electrical impedance of each sample was measured using an LCR meter (3532-50, Hioki Co. Ltd., Tokyo, Japan). To measure electric impedance with small contact resistance, copper foil was attached to the surface of

the polished sample. An alternating voltage of 5 V at 100 kHz was used for the measurements. The phase angle of the impedance was measured to determine electrical shorting. When the phase angle was near –90˚, the sample was judged to have capacitance. When the phase angle was almost 0˚, the sample exhibited electrical shorting because of a lack of electrical capacitance.

The impedance of the six samples was measured; only three samples showed capacitance, with an average magnitude of 200 pF. The other samples exhibited electrical shorting between the carbon-fiber cloth electrodes. This implies that glass-fiber cloth separators are not reliable for use in structural capacitors, so their reliability needs to be improved to fabricate large-scale structural capacitors.

In the previous section, electrical shorting between the carbon-fiber electrode cloth plies was shown to be caused by movement of the glass-fiber cloth separator during curing. In the present section, a method to prevent this movement of the fibers in the separator is proposed and experimentally investigated. To prevent fiber movement during the curing process, we pre-cured the glass-fiber cloth prepreg to prevent epoxy resin flow in it. This prevents the movement of glass fibers so that electrical shorting can be avoided. However, perfectly cured prepreg induces limited bonding between the carbon-fiber cloths. Therefore, a suitable pre-curing level is investigated experimentally.

Separators made from glass-fiber cloth and epoxy adhesive were partially cured. First, the degree of curing was measured by differential scanning calorimetry. The curing temperature and time were varied as shown in Table 1 to give three degrees of curing: 0.15, 0.68 and 0.76. The degree of curing is defined as the fraction of heat generation divided by the total heat generation of the perfect cure for each set of curing conditions shown in Table 1.

After pre-curing of each separator, it was sandwiched between carbon-fiber cloth prepreg layers, and then co-cured under the curing conditions described in Section 2. After co-curing of the samples, an alternating electric current was applied and the impedance phase angle was measured as described in Section 2 to investigate sample capacitance. For curing degrees of 0.15 and 0.76, three samples of each type were made. For a curing degree of 0.68, six samples were fabricated. Figure 12 shows the number of samples exhibiting capacitance. For the lowest degree of curing, all of the carbon-fiber electrodes exhibited electrical shorting. For the samples with a curing degree of 0.68, four samples out of six exhibited capacitance. All of the samples with a curing degree of 0.76 had capacitance. These results suggest that a pre-curing degree of 0.76 allows reliable fabrication of structural capacitors.

Figure 13(a) and Figure 13(b) show samples that exhibited an electrical short between carbon-fiber electrodes and electrical capacitance, respectively. Comparison of these images reveals that the sample with an electrical short has a warped surface, so epoxy resin flow occurred in it. This resin flow causes the fibers to move, allowing an electrical contact to form between the carbon-fiber cloth electrodes. To prevent electrical shorting between the electrodes of a structural capacitor, pre-curing of the glass-fiber cloth separator is required.

Electrical shorting between the carbon-fiber electrodes of a structural capacitor containing glass cloth as a separator was investigated. Electrical shorting between electrodes often happens in such systems because the glass fiber of the separator move through epoxy resin flow during curing. Movement of glass fibers can form a hole to allow contact between the carbon-fiber cloth electrodes, resulting in electrical shorting. Pre-curing of the glass- fiber cloth separator to a suitable degree ensures perfect electrical insulation between electrodes, preventing electrical short formation.