Conductivity In Thickness Direction Research Articles

Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.

Enhancing thermal conductivity of C/SiC composites containing heat transfer channels

In this study, CNTs/SiC micro-pillars at controlled content ratios were introduced into C/SiC composites as heat transfer channels to improve the thermal conductivity in the thickness direction. The thermal conductivities and bending strengths before and after heat treatment at 1650 °C were investigated and the results were discussed. The theoretical calculations and finite element analyses confirmed that CNTs/SiC micro-pillars successfully worked as heat transfer channels. The theoretical thermal conductivity calculated by effective medium theory (EMT) model was 19.25 W/m⋅k and agreed-well with the experimental value. The measured thermal conductivity was estimated to 20.69 W/m⋅k and improved to 22.36 W/m⋅k after heat treatment. The latter was 3.56-fold higher than that of traditional C/SiC and attributed to increased grain growth during heat treatment. The optimal bending strength before heat treatment was recorded as 324.5 ± 23.74 MPa due to microstructure evolution caused by CNTs. After heat treatment, the bending strength improved by 138 % with ductile fracture mode attributed to ordered layer structure of PyC interphase and complex phase composition of the composites. These features benefited the abundant propagation of cracks and energy consumption. In sum, introduction of heat transfer channels into C/SiC composites provided a new way to improve the thermal conductivity in thickness direction of ceramic matrix composites.

Development of thermoplastic films for ultrasonic manufacturing of printed circuit boards

AbstractConventional production processes of PCB involve a high amount of complex production steps and the use of hazardous chemicals. Furthermore, a great effort is required for their recycling. Ultrasonic hot embossing of coextruded films, filled with electrical conductive fillers, could be an economic and ecological alternative for the production of PCB. This study focuses on the identification of suitable filler–matrix combinations for the isotropic and anisotropic conductive film layers. Hence, monolayer films with different fillers and filler contents are extruded. Conductivities of 8.31 and 12 S/m were measured in cross and machine direction, respectively, for a polypropylene‐film with 10 wt.‐% of CNT. This is two magnitudes below the required 535 S/m. With 70 mS/m, the conductivity in thickness direction is 115–170 times lower. This is caused by process‐related surface layers with low filler content. For steel fibers, no electrical conductivity can be measured.

Thermal Conductivity Improvement of Fiber Orientation Control By Sheet Folding Method

In this study, a method improving the thermal conductivity in the sample thickness direction by secondary processing of an intermediate sheet in which carbon fibers were oriented in one direction was proposed. Moreover, the effectiveness of the proposed method was experimentally investigated. When fillers having a large aspect ratio such as carbon fibers are added to the polymer matrix, the thermal conductivity of the composites is largely influenced by the filler orientation. Therefore, it is necessary to orient the filler in the out-plane direction to effectively improve the thermal conductivity in the thickness direction of thermal conductive film or sheet. In the experiment, a zigzag shape was given to the intermediate sheet, and a sample with a thickness of 1 mm was obtained by folding the intermediate sheet. As a result, the thermal conductivity in the sheet thickness direction measured by the laser flash method became five times larger than that of the initial sample, the intermediate sheet.

Improved electrical conductivity of NCF-reinforced CFRP for higher damage resistance to lightning strike

Silver-coated knitting yarns are used in an NCF to increase the electrical conductivity of CFRP-laminates. A test textile containing section-wise four different knitting yarns has been produced. The threads differ in yarn count and degree of silver-coating. Depending on the yarn’s linear resistance, the conductivity in thickness direction increases by up to two orders of magnitude to >100S/m. The in-plane conductivity is significantly higher as well. Combining conductive knitting yarns with conductive toughening interleaves further increases the electrical conductivity. Lightning strike resistance benefits from the conductivity improvement. The damage decreases particularly in the lower fibre layers, e.g. at a depth of 1.0mm by app. 90%. In spite of the higher laminate conductivity, additional conductive interleaves do not further improve lightning strike resistance. The damage is smaller than in the reference, but larger than in a comparative laminate where only the knitting yarn is conductive.

Cr-Cu圧延材とCr-Cu/Cuクラッド圧延材の熱特性

Thermal properties of Cr-Cu for heat-sink application are required to be close to those of Mo-Cu and W-Cu. The rolled and Cu-clad and rolled Cr-Cu infiltrated compacts are investigated to improve the thermal properties. In the rolled Cr-Cu material, Cr phases are significantly elongated and flattened, resembling a fibrous structure. Such a fibrous structure of Cr phases makes the thermal expansion coefficient much lower than that predicted from the rule of mixture. While the flattened Cr structure promotes thermal flow in the in-plane direction, it deteriorates that in the thickness direction. To improve thermal conductivity in thickness direction, the Cr-Cu compact was bonded with a Cu plate and rolled. The clad and rolled structure enables the improvement of thermal conductivity in the thickness direction without degrading the thermal expansion coefficient in the rolling direction.

Sulfonated polyimides bearing benzimidazole groups for direct methanol fuel cell applications

Sulfonated polyimides (SPIs) bearing benzimidazole groups in the main chains were synthesized from 1,4,5,8-naphthalenetetracarboxiylic dianhydride, 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid and 2-( p- or m-aminophenyl)-5-aminobenzimidazole in m-cresol. The resulting polymer solutions were not precipitated to gain the solid polymers but directly cast into films because the precipitated SPIs became insoluble in common organic solvents. The strong interaction between sulfonic acid and benzimidazole groups reduced the water uptake and methanol uptake. The SPI membranes with ion exchange capacities (IECs) of 1.9–2.0 mequiv./g displayed reasonably high mechanical strength, thermal stability, water stability and proton conductivity. They showed anisotropic membrane swelling in water with 2.8 times larger swelling in thickness direction than in plane one and anisotropic proton conductivity with 25% smaller conductivity in thickness direction than in plane one. They suppressed methanol crossover in direct methanol fuel cell (DMFC) operation and displayed fairly high DMFC performances even at high methanol concentrations. The Faraday's efficiency and overall DMFC efficiency at 60 °C and 200 mA/cm 2 for SPI membrane with IEC of 1.9 mequiv./g were 75 and 23%, respectively, at 5 wt% methanol feed concentration and 39 and 10.2%, respectively, at 20 wt%. The SPI membranes have high potential for DMFC applications at mediate temperatures (40–80 °C).

3D Thermal Analysis of Hybrid Composite Structures

In the case of composite structures 3D finite elements or finite difference methods are commonly used for the evaluation of the temperature fields. This leads to a high effort on discretization and computation, especially for transient load cases. Therefore, 2D finite elements have been developed based on a layerwise theory to calculate the full 3D temperature field of hybrid composite structures showing different thermal conductivity in thickness direction for each layer. Using heat transfer equilibrium conditions in thickness direction it is possible to account for linear or quadratic shape functions over the thickness for each layer, leaving the number of functional degrees of freedom independent from the number of layers.