In Situ Observations of Phase Transitions in Metastable Nickel (Carbide)/Carbon Nanocomposites

Abstract

Nanocomposite thin films comprised of metastable metal carbides in a carbon matrix have a wide variety of applications ranging from hard coatings to magnetics and energy storage and conversion. While their deposition using nonequilibrium techniques is established, the understanding of the dynamic evolution of such metastable nanocomposites under thermal equilibrium conditions at elevated temperatures during processing and during device operation remains limited. Here, we investigate sputter-deposited nanocomposites of metastable nickel carbide (Ni3C) nanocrystals in an amorphous carbon (a-C) matrix during thermal postdeposition processing via complementary in situ X-ray diffractometry, in situ Raman spectroscopy, and in situ X-ray photoelectron spectroscopy. At low annealing temperatures (300 °C) we observe isothermal Ni3C decomposition into face-centered-cubic Ni and amorphous carbon, however, without changes to the initial finely structured nanocomposite morphology. Only for higher temperatures (400–800 °C) Ni-catalyzed isothermal graphitization of the amorphous carbon matrix sets in, which we link to bulk-diffusion-mediated phase separation of the nanocomposite into coarser Ni and graphite grains. Upon natural cooling, only minimal precipitation of additional carbon from the Ni is observed, showing that even for highly carbon saturated systems precipitation upon cooling can be kinetically quenched. Our findings demonstrate that phase transformations of the filler and morphology modifications of the nanocomposite can be decoupled, which is advantageous from a manufacturing perspective. Our in situ study also identifies the high carbon content of the Ni filler crystallites at all stages of processing as the key hallmark feature of such metal–carbon nanocomposites that governs their entire thermal evolution. In a wider context, we also discuss our findings with regard to the much debated potential role of metastable Ni3C as a catalyst phase in graphene and carbon nanotube growth.

Elastic and piezoresistive properties of nickel carbides from first principles

“…A meta-study on this subject can be found in reference 4. A recent study 6 confirmed, that Ni 3 C in such films only decomposes at temperatures well above 250 • C.…”

Growth, structure and stability of sputter-deposited MoS2 thin films

“…Possible degradation scenarios include exposure to visible light radiation or heating to elevated temperatures in ambient atmosphere [30]. In order to study the stability of our PVD MoS 2 films, we employed in situ laser annealing [39–40] in ambient air coupled with Raman spectroscopic investigations (Fig. 4).…”

Granular metal–carbon nanocomposites as piezoresistive sensor films – Part 1: Experimental results and morphology

“…In between are metals like Ni, Co, and Fe with a low affinity to carbon. These metals are still able to interact with carbon but the reaction products depend on many parameters like, for example, the deposition technique, substrate temperature, and metal content (Bayer et al, 2016). For instance, nickel is a prominent catalyst in the production of carbon nanotubes and graphene (Garlow et al, 2016).…”