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Transcript
Scale, structure and behaviour Lecture 2 MTX9100 Nanomaterjalid OUTLINE -How are crystals structured? -Why and how does nanoworld differ from the world we live in? -When does size matter? What is the smallest particle in matter? What are things made of? Everything is made of atoms. In the Bohr atomic model, there is a nucleus consisting of protons with a positive charge and a mass of 1.67 × 10−27 kg; and neutrons with no charge but with the same mass as the protons. The nucleus is surrounded by electrons with a negative charge and a mass of 9.11 × 10−31 kg that revolve around the nucleus in discrete orbits. What is particular about electrons? (1) the number of electrons in an electrically neutral atom depends on the number of protons in the nucleus, (2) an electron will enter the orbital possessing the least possible energy, and (3) only two electrons can fit into any one of the energy states. How do atoms bond together? Energy of bonding Ionic bonding • Occurs between strongly electronegative and strongly electropositive atoms • Electron(s) are transferred from electropostive atom to electronegative atom, thereby forming a cation (positively charged) and an anion (negatively charged) Covalent bonding Atoms form stable electron structures, i.e. those of inert gases, by sharing of electrons with other atoms • F2, Cl2,… – Group VII – diatomic molecules • Resulting bonds are strongly directional Metal bonding Valence electrons ( in outer shell) leave atoms and form a “sea” of free electrons Positively charged ion cores are shielded from one another by the free electrons Free electron acts as the “glue” that hold positive cores together Non-directional High thermal and electrical conductivity Van der Waals bonding Isolated Ar atom Due to statistical nature of electron motion, occasionally the center of negative charge is spatially different than the center of positive charge => temporary dipole Secondary bond • Dipole moment produced by instantaneousasymme try of electron charge distribution • Coulombic attraction occurs between positive end of one dipole and negatively charged end of another Hydrogen bonding Atomic interactions Interatomic interactions energy Interatomic forces Energy – Force Crystal structure In solids, atoms are often arranged on a periodic lattice, forming 3D crystals with many atoms Crystal structures of metals Compounds structure Materials Packing Energy and packing Polycrystalline structure Three-dimensional structures or bulk materials with a nanometer-sized microstructure are assembled of nanometer-sized building blocks or grains that are mostly crystallites. Engineering materials There are currently over 50,000 engineering materials! Schematic classification of nano – materials: (a) three – dimensional structures; (b) two – dimensional; (c) one – dimensional; and (d) zero – dimensional structures. Classes of materials Metallic materials (consist principally of one or more metallic elements, although in some cases small additions of nonmetallic elements are present; When a particular metallic element dissolves well in one or more additional elements, the mixture is called a metallic alloy. Ceramic materials (are composed of at least two different elements). Polymeric materials (consist of long molecules composed of manyorganic molecule units) Composites (are formed of two or more materials with verydistinctive properties, which act synergistically to create propertiesthat cannot be achieved by each single material alone) Electronic materials Biomaterials Nanomaterials Metals Characteristics: – High electrical and thermal conductivity – Ductile/malleable – Moderate to high strength – Atoms arranged on periodic lattice, i.e. crystalline Ceramics and polymers Stoichiometric compounds made of electropositive (metallic) and electronegative (nonmetallic) elements • Examples: Al2O3, SiC, ZrO2 , WC • Characteristics: – Low electrical and thermal conductivity – High melting point – Very hard – Brittle (flaw-sensitive) Made of long molecules, with very strong intramolecular bonds but weak intermolecular bonds. • Examples: Polyethylene (PE), polymethylmethacrylate (PMMA, aka acrylic or plexiglass), polystyrene (PS), polyvinylchloride (PVC), epoxy, elastomers Characteristics: – Low electrical and thermal conductivity – Low melting point – Relatively weak (compared to metals and ceramics) Composites Composite materials are made of two or more distinct phases, often from dissimilar material categories, e.g. polymer + ceramic, metal + ceramic • Examples: Glass fiber-reinforced polymers (GFRP), carbon fiber-reinforced polymers (CFRP), WC/Co (“cermets”), C/C, nanotube reinforced composites • Characteristics: – Properties usually intermediate to those of the constituents Scale changes everything There are enormous scale differences in our universe! At different scales – Different forces dominate – Different models better explain phenomena Four important ways in which nanoscale materials may differ from macroscale materials – Gravitational forces become negligible and electromagnetic forces dominate – Quantum mechanics is the model used to describe motion and energy instead of the classical mechanics model – Greater surface area to volume ratios – Random molecular motion becomes more important Dominance of electromagnetic forces • Because the mass of nanoscale objects is so small, gravity becomes negligible – Gravitational force is a function of mass and distance and is weak between (low-mass) nanosized particles – Electromagnetic force is a function of charge and distance is not affected by mass, so it can be very strong even when we have nanosized particles –The electromagnetic force between two protons is 1036 times stronger than the gravitational force! Sources: http://www.physics.hku.hk/~nature/CD/regular_e/lectures/images/chap04/newtonlaw.jpg http://www.antonine-education.co.uk/Physics_AS/Module_1/Topic_5/em_force.jpg Quantum Effects • Large ZnO particles – Block UV light – Scatter visible light – Appear white • Nanosized ZnO particles – Block UV light – So small compared to the wavelength of visible light that they don’t scatter it – Appear clear The following are among the most important things that quantum mechanical models can describe (but classical models cannot): • Discreteness of energy • The wave-particle duality of light and matter • Quantum tunneling • Uncertainty of measurement Sources: http://www.apt owders.com/images/zno/im_zinc_oxide_particles.jpg http://www.abc.net.au/science/news/stories/s1165709.htm ; http://www.4girls.gov/body/sunscreen.jpg Discreteness of energy It is the fact that electrons can only exist at discrete energy levels that prevents them from spiraling into the nucleus, as classical models predict. This quantization of energy, along with some other atomic properties that are quantized, give quantum mechanics its name. In 1901, Max Planck published an analysis that succeeded in reproducing the observed spectrum of light emitted by a glowing object. To accomplish this, Planck had to make an ad hoc mathematical assumption of quantized energy of the oscillators (atoms of the blackbody) that emit radiation. It was Einstein who later proposed that it is the electromagnetic radiation itself that is quantized, and not the energy of radiating atoms. In 1905, Albert Einstein provided an explanation of the photoelectric effect, a hitherto troubling experiment that the wave theory of light seemed incapable of explaining. He did so by postulating the existence of photons, quanta of light energy with particulate qualities. Extended internal surface Surface to Volume Ratio Increases Since reactions occur at the interface of two substances, when a large percentage of the particles are located on the surface, we get maximum exposed surface area, which means maximum reactivity! So nanosized groups of particles can make great catalysts. Random molecular motion is significant • Tiny particles (like dust) move about randomly – At the macroscale, we barely see movement, or why it moves – At the nanoscale, the particle is moving wildly, batted about by smaller particles • Analogy – Imagine a huge (10 meter) balloon being batted about by the crowd in a stadium. From an airplane, you barely see movement or people hitting it; close up you see the balloon moving wildly. At the nanoscale, these motions can be on the same scale as the size of the particles and thus have an important influence on how particles behave.