PROGRAM | Materials Science & Engineering

By: Chunyan Zhang Chair: Chaoying Ni

ABSTRACT

High thermal conductive materials, such as diamond and carbon nanotube (CNT), have numerous applications in the fields of electronics, optics, and the energy sectors. Significant efforts have been made to develop high thermal conductive materials for the design and fabrication of next-generation devices. This investigation focuses on the structural attributes and thermal properties of two material systems: chemical vapor deposition (CVD) diamond thin film and multi-walled CNT (MWCNT) reinforced polymer composites fabricated by additive manufacturing (AM).

Diamond exhibits exceptional thermal conductivity and unique electronic, optic, acoustic and mechanical properties, and diamond films attract significant research efforts for a broad range of applications, such as heat sink, high-power chips, high speed drill bits, and cosmological mirrors where outstanding heat transfer and structural stability are vital. This study focuses on the structural and thermal-property correlations of diamond-film deposited on Si, including the relationship between structures and thermal conductivity and interfacial thermal conductance of diamond-film, and the effect of the interfacial structure on the interfacial thermal conductance of diamond film. The measured thermal conductivity values for 0.63 μm diamond and 2.2 μm 3C-SiC film were 241 W/(mK) and 297 W/(mK), respectively. The relatively low thermal conductivity value in the polycrystalline diamond film is probably attributed to grain size and grain boundaries, which increase phonon scattering rate and reduce thermal conductivity. The measured interfacial thermal conductance values for diamond-film/Si (100) and 3C-SiC-film/Si (100) were 18 MW/(m²K) and 77 MW/(m²K), respectively. The low interfacial thermal conductance value at the diamond-film/Si (100) interface can be attributed to the presence of a significant interfacial amorphous layer, as well as factors like a large Debye temperature mismatch, lattice constant mismatch and phonon frequency mismatch between diamond and Si (100). Results obtained so far on the structure and thermal properties of the 0.63 μm diamond-film/Si (100) and the 2.2 μm 3C-SiC-film/Si (100) suggest that the film structures and interfacial microstructure have significant impacts on the thermal conductivity values of the films and  values of the interfaces.

Just like diamond, CNT is another essential carbon polymorphs with unique and exceptional physical and mechanical properties. CNT-reinforced polymer composites have garnered significant attention due to their processability and cost-effectiveness in addition to the synergistic properties and enhancement afforded by CNT. In this work, we prepared a high-loading printable filament for 3D printing and printed MWCNT-reinforced nanocomposites with the alignment of CNT along a specified direction. The as-printed vertically (or through-plane) aligned structure, composed of 20 wt.% MWCNT/PLA, has a through-plane thermal conductivity of ~0.575 W/(mK), which is around 2.64 times that of horizontally aligned structure (~0.218 W/(mK)). The thermal conductivity results revealed that the maximum through-plane thermal conductivity for 20 wt.% MWCNT/PLA nanocomposites reached around 6 times that of neat PLA at 35 °C. The through-plane thermal conductivity of the vertically aligned structure, with heat flow parallel to filler alignment, exhibits a significant improvement compared to the horizontally aligned structure with heat flow perpendicular to the filler alignment, indicating the importance of filler orientation in enhancing thermal conductivity. Furthermore, 3D printed nanocomposites with high MWCNT loading show outstanding thermomechanical properties. The results indicate that the 20 wt.% MWCNT/PLA structure exhibits much better heat distribution or dissipation, while the temperature gradient in the neat PLA structure is much higher when heated from bottom, and the neat PLA starts to collapse at 200 °C.

The thermomechanical stability of the MWCNT/PLA nanocomposites with high MWCNT loading and entanglement of themselves are essential to manufacture a full carbon architecture. By subjecting the printed object to controlled heating environment, a 3D carbon scaffold was achieved and it preserves the original alignment of MWCNTs and exhibits a thermal conductivity of 0.148 W/(mK) in the in-plane direction and 0.359W/(mK) in the out-of-plane direction. The compressive strength of the 3D carbon scaffold along the MWCNT direction was 2.95 MPa, surpassing that of the perpendicular direction at 2.48 MPa. This anisotropic property in compressive strength remains preserved after the removal of the polymer.

The 3D carbon scaffold can serve as a highly valuable preform for the preparation of composites by incorporating desired matrix components such as thermosets, metals, and ceramics, enabling the design and application of advanced structural and functional materials. In this work, a novel method for the manufacturing of composites called composite architected mesoscale process (CAMP) has been explored. The MWCNT/epoxy nanocomposites produced through the CAMP process demonstrated enhanced mechanical properties.

Back >