(680e) Directed Covalent Assembly of Nanodiamonds to Form Continuous Films
AIChE Annual Meeting
Monday, November 15, 2021 - 4:50pm to 5:10pm
The morphology of the ND films was examined using atomic form microscopy (AFM) and scanning electron microscopy (SEM), their chemical nature suing X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, and their thermal transport properties via microfabricated test devices. The effect of solvent media and its pH was studied using deionized water, 10 mM KCl solution, and MES buffers with pH 5.5-7. With deionized water or 1 mM KCl as the reaction medium, when the carbodiimide cross-linker was added to a horn-sonicated ND-COOH suspension in deionized water, its hydrochloride content lowers the pH and leads to particle agglomeration. Further, the particle size distribution in DI water varied greatly. The SEM images of nanodiamond films assembled using DI-water based sol prepared by horn sonication showed higher surface coverage and increased density compared to the films formed by bath sonication.
As seen via SEM and AFM, a pH 6.5 or pH 7 buffer leads to a continuous surface coverage and a similar apparent porosity (~30%), while a pH 6 buffer leads to discontinuous films and more porous films (~65%). Further, the films deposited at pH 7 showed smaller pore sizes and a higher thermal conductivity in comparison to films deposited at pH 6.5. The as-deposited films at pH 7 showed a thermal conductivity as high as 12 ± 2.5 W m-1 K-1 at 310 K, which is comparable to that obtained for ultrananocrystalline diamond films obtained via chemical vapor deposition. However, sequential thermal annealing of the nanodiamond films at temperature up to 400 °C led to the aggregation of nanodiamond to segregated islands, loss of surface coverage, and increase in porosity. The thermal conductivity of all the annealed samples was comparable near room temperature regardless of deposition pH. The pH 6.5 and pH 7 samples exhibited a general increase in thermal conductivity with increasing temperature, while pH 6 samples exhibited statistically similar thermal conductivity with increasing temperature. The use of a phonon hopping model to deduce the phonon transfer at the grain-grain boundary indicates that annealing works to homogenize interfaces within the films and reduce sample-to-sample variation but it comes at the expense of less efficient phonon transmission across interfaces. Overall, the results demonstrate a way to achieving porous, low-cost nanocrystalline diamond thin films with tunable film morphology and thermal conductivity for electronics and biomedical applications.