A 3D-weaving machine from the company MAGEBA (Figure 2(a)) was used to weave preforms with highly thermal high conductive carbon fibers in the out-of-plane direction. The in-plane fabric structure was designed similar to a plain weave with additional vertical yarns. The 3D-weaving-machine works with four shuttles to transport weft yarns. These shuttles are a special in-house development that avoids the frequent breakage of carbon fibers caused by twisting and bending of the yarn in commonly-used shuttles. However, the weaving process was only satisfactory stable with a z-fiber spacing of 6 mm and cabled pitch-fibers. Warp-yarns are attached to Jacquard-threads mounted on a Unival from Stäubli. Contrary to conventional Jacquard-devices, the Unival is able to lift the warp-yarns to four different height-positions with motors ca-

pable to lift 160 g. From 1024 available Jacquard-threads, 750 were used to move the same number of the light Tenax-warp-yarns. 14 Jacquard-threads were attached to the 14 heavy pitch-carbon rovings. Preforms were made with attempted preform z-fiber contents 0%, 3% and 6%. The woven fabrics had the dimensions 140 × 140 × 5 mm.

The high modulus pitch fibers used have an ultimate elongation of 0.4%. Thus, they are not primarily suitable to be processed by weaving or stitching due to the small bending radii incorporated by these methods. Braiding is used for a cabling process as known from carpet and tire industries. A supporting polyester thread was braided around the pitch-fiber roving [18] [20] . The braiding machine used was manufactured by Muratec Murata in Japan (Figure 2(b)). For the cabling process, it was necessary to implement twelve spools. The pulling speed of the roving was 0.263 m/min while the braider was spinning with a speed of 5.7 rev/min.

Six panels were produced by VARTM. Each panel contained either 0%, 3% or 6% of z-fibers and 0% or 3.6% Al-particles. The entire lay-up was assembled in accordance to Figure 3. Before infusion, the mixed resin was degassed in a pressure pot under vacuum for 10 min to reduce the air content in the resin. The maximum vacuum level was reached after a slow gradual decrease of pressure to avoid boiling of the resin. After degassing, ambient pressure was applied, forcing the resin to move into the still clamped inlet tube. Upon unclamping of the inlet, the resin flowed through the distribution media and impregnated transversely the preform. Once the flow front had reached the end of preform, the post-filling stage was initiated and pressure at the inlet and vent was set to 500 mbar. The laminate thickness and resin pressure gradients

gradually dissipated, and the pressure boundary conditions were maintained until the resin was cured and subsequently post-cured at 65˚C.

It could be noticed that the Al-particles were filtered out within the first 10 mm of the flow path making these panels not worth for further consideration. Thus, a preform with no z-fibers was manufactured, allowing introducing the Al-particles directly into the preform. This preforms was infused as described above.

Four circular specimens with a diameter of 50 mm were cut out of each previously described infused panels by water-jet cutting. The panels had since been milled in order to create plano-parallel surfaces with accessible z-fibers (Figure 4). For additional comparison, two samples each were manufactured of neat resin discs and discs consisting of resin with 3.6 vol.-% Al-particles.

The thermal conductivity measurements were performed within a measuring cell (Figure 5) built in-house at the Institut für Kunststofftechnik Westpfalz according to ASTM E 1225-04, which allows measurements of circular samples with a diameter of 50 mm [21] [22] . Sample thicknesses can vary from 2 until 50 mm. Three thermistors in the top meter bar are used to measure heat flux which is assumed to be constant as the heat traveling through the sample and into the lower meter bar where a fourth temperature is read. Both meter bars are made out of stainless steel (k_mat = 15.9 W/(m∙K). The top meter bar is insulated with foam and additionally shielded by a guard heater to prevent radial heat loss. The cell was placed on a water-cooled plate to provide a heat sink with a constant temperature T_sink beneath the bottom plate. Thermistor data were evaluated by a LabView-based program. Conductivity paste (OT-201 from Omega with k = 2.3 W/(m∙K)) was used in order to facilitate coupling and to reduce interfacial thermal resistance. The amount of coupling paste was weighed in order to apply a constant amount for each measurement. Comparing calibration measurements were performed on isotropic specimens to determine the accuracy of the measuring cell.

The thermal conductivity for a particle filled resin can be calculated for particle volume fraction up to 30% with a sufficient accuracy using a model suggested by Maxwell in the 19^th century [23] .

with ν _P―particle volume fraction.

k_p―thermal conductivity of particle.

k_m―thermal conductivity of matrix.

For a thermal conductivity of a particle much higher than that of the matrix (, , Equation (1) can be simplified to

More elaborate models have been published by Bruggemann, Cheng-Vachon, and Nielsen.

Bruggemann [24] :

Cheng-Vachon [25] :

with

Nielsen [26] for dispersed hemispherical particles with data from [27] :

with

and

Other, very theoretical models have been developed. However, they are not practical to use [28] - [30] .

The out-of-plane thermal conductivity for laminated and woven composites consists of contributions of vertical z-fibers plus the transverse thermal conductivity of the composite without vertical fibers. It can be approximated as a parallel connection of both components [19] .

with δ_Z-Fibers = ν_Fz/ν_F.

ν_Fz―fiber volume fraction of z-fibers.

ν_F―fiber volume fraction of x-, y-, z-fibers.

k₃₃―out-of-plane thermal conductivity of a 3D-woven fabric composite.

k₁₁―longitudinal thermal conductivity of a laminated composite.

k₂₂―transverse thermal conductivity of a laminated composite.

The longitudinal z-fiber contribution can be described according to the longitudinal thermal conductivity modeled by Thornburgh and Pears using the simple rule of mixture [31] .

with k_Fa―axial fiber thermal conductivity.

The transverse thermal conductivity has been modeled by several researchers. A well accepted model was published by Hatta and Taya [32] .

with k_Fr―radial thermal conductivity of fiber.

It has been found out that Equation (9) overestimates the thermal conductivities of composite samples with fibers of high thermal conductivity in perpendicular direction [19] [33] . Three different effects due to the presence of high conductive heat flux entry points could be identified for consideration:

Ÿ Distribution of the high conductive areas (number of entry points)

Ÿ Thermal conductivity of an attaching material to deviate an incoming homogeneous heat flux into an inhomogeneous one (or vice versa for an existing heat flux)

Ÿ Thickness of composite part or sample

with ξ_Kmat―material factor.

ξ_TD―thickness-grid density factor.

with k_mat―Thermal conductivity of adjacent material.

t_sample―thickness of composite part or sample.

d_rov―diameter of roving.

The thermal conductivity of the loaded resin was calculated using the Equations (2)-(8) with particle volume content of ν_p = 3.6%. All models delivered quite similar results (Table 1).

The out-of-plane thermal conductivity k₃₃ can be predicted according to Equations (9)-(14). One key parameter to obtain high k₃₃-values is the z-fiber content, which depends on the distance between the z-fibers as shown in Figure 6.

With the parameters given above, the out-of-plane thermal conductivity was calculated depending on the distance between the z-fibers for a 5 mm thick sample and matrix conductivities for neat resin (k_m = 0.2 W/m∙K) and resin loaded with 3.6% Al-par- ticles (Table 1). It turns out that the impact of the matrix loading on the out-of-plane-

thermal conductivity is about 0.1 W/(m∙K) higher than for neat resin and is thus negligible for higher z-fiber contents. As depicted in Figure 7, an out-of-plane thermal conductivity of about 4 W/(m∙K) can be achieved with a spacing of 6 mm resulting in a z-fiber content of 6.3%.