On the plasma and electrode erosion processes in spark discharge nanoparticle generators Ph.D. Dissertation Attila Kohut Supervisor: Dr. Zsolt Geretovszky associate professor Doctoral School of Physics Department of Optics and Quantum Electronics Faculty of Science and Informatics University of Szeged Szeged 2017 Table of contents List of abbreviations ............................................................................................................................................. 4 1. Introduction ................................................................................................................................................... 5 1.1 Motivation and aims .......................................................................................................................... 5 1.2 Generation of nanoparticles .......................................................................................................... 7 1.3 The electric spark discharge ....................................................................................................... 11 1.3.1 Classification of gas discharges ........................................................................................ 11 1.3.2 Microsecond-long, atmospheric pressure oscillatory spark discharges ........ 13 1.4 Spark discharge nanoparticle generation ............................................................................. 16 1.4.1 General concept ...................................................................................................................... 16 1.4.2 Nanoparticles generated in the SDG .............................................................................. 18 2. Experimental setup .................................................................................................................................. 24 2.1 Setup of SDGs .................................................................................................................................... 24 3. Results and discussion ............................................................................................................................ 27 3.1 Electrical properties of the spark ............................................................................................. 27 3.1.1 Main electrical characteristics ......................................................................................... 27 3.1.2 Effect of SDG control parameters .................................................................................... 32 3.2 Morphology of the spark plasma .............................................................................................. 34 3.2.1 Experimental ........................................................................................................................... 34 3.2.2 The evolution of plasma morphology ........................................................................... 34 3.2.3 Effect of the gap size on plasma morphology ............................................................. 39 3.3 Spectrally resolved optical emission of the spark plasma ............................................. 41 3.3.1 Experimental ........................................................................................................................... 41 3.3.2 Temporally and spatially resolved emission spectrum of the spark plasma 42 3.3.3 Effect of control parameters on the emission spectra............................................ 47 3.4 OES-based diagnostics of the spark plasma ......................................................................... 50 3.4.1 Methods ..................................................................................................................................... 50 3.4.2 Properties of the spark plasma ........................................................................................ 53 3.5 Morphology of the electrode surface after sparking ........................................................ 60 3.5.1 Experimental ........................................................................................................................... 60 3.5.2 Surface of the electrodes after sparking ...................................................................... 61 3.5.3 Formation of morphological features on the electrode surface ......................... 66 3.5.4 Energy demand of crater formation .............................................................................. 70 3.6 Implications on the generated nanoparticles ...................................................................... 73 2 3.6.1 Experimental ........................................................................................................................... 73 3.6.2 Electrode erosion ................................................................................................................... 75 3.6.3 Particle formation.................................................................................................................. 79 3.6.4 Effect of control parameters on the generated aggregates .................................. 82 4. Summary ...................................................................................................................................................... 86 5. Magyar nyelvű összefoglaló .................................................................................................................. 89 5.1 Bevezetés ............................................................................................................................................ 89 5.2 Szikrakisüléses nanorészecske generálás ............................................................................. 92 5.3 Kísérleti módszerek ....................................................................................................................... 93 5.4 Eredmények....................................................................................................................................... 96 Publications........................................................................................................................................................... 99 Acknowledgements......................................................................................................................................... 100 References .......................................................................................................................................................... 101 3 List of abbreviations 0D: zero-dimensional 1D: one-dimensional 2D: two-dimensional AFM: atomic force microscopy AMS: aerosol mass spectrometer APM: aerosol particle mass analyzer CLSM: confocal laser scanning microscopy CPC: condensation particle counter DC: direct current DMA: differential mobility analyzer ENP: engineered nanoparticle FWHM: full width at half maximum GSD: geometric standard deviation ICCD: intensified charged-coupled device ICP: inductively coupled plasma LTE: local thermodynamic equilibrium MFC: mass flow controller NIST: National Institute of Standards and Technology NP: nanoparticle OES: optical emission spectroscopy OM: optical microscopy RF: radio frequency SDG: spark discharge generator SMPS: scanning mobility particle sizer SRR: spark repetition rate TEM: transmission electron microscopy WD: working distance 4 1. Introduction 1.1 Motivation and aims Today nanotechnology is undoubtedly one of the most important fields of science and technology. Its significance is well reflected by the fact that it probably doesn’t have to be introduced. Most people have heard about “nano” as one of those mysterious things that will save and destroy humanity at the same time. As usually, reality lies somewhere between these two extremes. The ability to manipulate materials on the nanometer scale surely gave us such tools in countless fields of life that were hardly imaginable a few decades ago. The potential for targeted delivery and controlled release of drugs within the human body [1], enhancing the development of energy, food and water supply [2], the fabrication of lightweight yet strong materials [3] or the efficiency-increase of solar cells and batteries [4] are just a few examples that nanotechnology can offer. Although numerous achievements of nanotechnology and nanoscience have already reached the industrial level and hence became part of everyday life, many breakthrough applications still exist only within the walls of laboratories. Several criteria have to be met until a new technology can break out from the lab, including the supply of affordable base material. The basic building blocks of nanotechnology are the nanostructures, which can be either nanometer thick layers, nanorods or nanowires, or nanoparticles, which are usually referred to as 2D, 1D and 0D nanostructures, respectively [5]. The mass-production of such nanostructures by environmentally friendly and cheap means is a crucial technological bottleneck if nanotechnology-based product development should continue to advance. In the present work I focus on some of the fundamental aspects of spark discharge nanoparticle generation, one of the most promising techniques which are capable of producing nanoparticles (NPs) with controlled properties even on the industrial level. NPs can be generated by several different strategies, involving chemical reactions, physical means or mechanical transformations [6]. Chemical synthesis methods are dating back to the middle of the nineteenth century and very