Carbon encapsulation of elemental nanoparticles

---

Tezin Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Mühendislik Fakültesi, Metalurji ve Malzeme Mühendisliği Bölümü, Türkiye

Tezin Onay Tarihi: 2018

Tezin Dili: İngilizce

Öğrenci: PELİN LİVAN

Danışman: Tayfur Öztürk

Özet:

There is a considerable interest in the encapsulation of nanoparticles into the carbon cages for a variety of purposes e.g. providing stability in detrimental environments, preventing agglomeration, controlling surface modifications and improving in electrical conductivity etc. The current study deals with the encapsulation of elemental particles and is made up of two parts. In the first part, the encapsulation of elemental particles was studied using a spark discharge generator where an elemental rod used against a carbon electrode under a constant argon flow. The source of carbon was further enriched by methane which was co-fed with argon into the reaction chamber. The study showed that elements W, V, Ti and Si were converted into carbide and were encapsulated successfully by graphitic layers producing sound core-shell structure. Cu yielded a partially filled core-shell structure. The resulting structure in the case of Mg was quite different. Here Mg did not produce encapsulated structure; rather it occurred as elemental particles embedded in graphitic matrix. Simple calculation based on empty volume in the partially filled Cu core-shell structure indicated that the process of encapsulation is probably complete at around 1900 K. Considering that carbon condenses at around 4000 K, elements may be divided into three categories. Elements/carbides with condensation temperature higher than 4000 K e.g. W, V or Ti produce a sound core-shell structure. Elements/carbides whose condensation vi temperature is between 4000 K and 1900 K yield sound or partially filled core-shell structure depending on the volume shrinkage during solidification. Elements/compounds whose condensation temperature is below 1900 K fail to develop core-shell structure. Instead, they form embedded composite structure where particles are embedded into a graphitic matrix. In the second part of this study, encapsulation of silicon nanoparticles was studied by thermal plasma. Feeding Si powders together with methane into a 25 kW RF reactor yielded nanopowders which comprised SiC, Si, and graphite. The particles were successfully encapsulated with 7-10 nm thick graphitic layers with a high degree of crystallinity. Co-feeding of silicon with methane therefore did not yield pure Si@C as carbide formation was unavoidable. It is proposed that it might be possible to obtain Si@C i.e. carbon encapsulated Si nanoparticles, but for this an alternative approach may be adopted, i.e. first to produce Si nanoparticles and encapsulating them by post feeding of methane at lower temperature.