CVD SYNTHESIS OF CARBON NANOMATERIALS Doctoral Dissertation

OF DOCTORAL DISSERTATION AALTO UNIVERSITY SCHOOL OF SCIENCE AND TECHNOLOGY P.O. BOX 11000, FI-00076 AALTO http://www.aalto.fi Author Prasantha Reddy Mudimela Name of the dissertation CVD synthesis of carbon nanomaterials Manuscript submitted 24.08.2010 Manuscript revised 25.10.2010 Date of the defence 18.11.2010 Monograph Article dissertation (summary + original articles) Faculty Faculty of Information and Natural Sciences Department Department of Applied Physics Field of research Nanotechnology Opponent(s) Dr. Nicole Grobert Supervisor Professor Esko I. Kauppinen Instructor Docent Albert G. Nasibulin Abstract This thesis describes the development of methods for the controlled synthesis of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on flat substrates and on micron sized particles. The carbon nanomaterials were synthesized using three different types of CVD reactors at atmospheric pressure. The vertical CVD reactor was used to study CNT formation on thermally oxidized silicon wafers and to synthesize individual single-walled CNTs on Si3N4 substrates. By using CO as the carbon source and iron as the catalyst, the temperature dependence of CNT growth on the silica substrate was studied by changing the CO2 concentration and catalyst sputtering time. A variation of the number of CNT walls from 1 to 4 was found by changing the synthesis temperature from 590 to 1070 °C, together with CO2 concentration. This incremental variation of the number of CNT walls as well as the CNT lengths with temperature was explained by an enhancement of carbon solubility and diffusivity. Individual single-walled CNT synthesis was performed across 1.5 m wide Si3N4 slits for studying the chirality dependent properties of individual tubes. By employing a horizontal laboratory scale CVD reactor, we reported for the first time the growth of CNFs on cement and clinker particles without additional catalyst. CNFs and CNTs were also synthesized on silica particles impregnated by iron salt. To synthesize CNTs and CNFs on cement particles on a semi-industrial scale, a screw feeder reactor was designed and constructed to provide a continuous feed of the particles. FT-IR measurements and XRD characterization revealed that no major changes were found in the crystallinity of cement at optimized synthesis temperatures below 750 °C. This allowed us to get a good dispersion of the carbon nanomaterials in a cement matrix to improve the properties of composites. The compressive strength and electrical conductivity measurements using this novel cement hybrid material (CHM) revealed a 2-fold increase in the compressive strength and 40-fold increase in the electrical conductivity after 28 days curing in water.