Nanomaterials products and functions

Transcription

1 Nanomaterials products and functions Lecture 16 MTX9100 Nanomaterials OUTLINE - Do we really mean nanopaint, for example, or something like polymeric paints containing nanoparticles? -What are functions of nanomaterials? - How many nanoproducts on the market? 1

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3 3 Functional characteristics surface behavior In designing or selecting materials, surface properties are of fundamental importance. Surfaces can now be designed to have many specific properties that are fundamentally important in the design and use of buildings and products. The need for surfaces that are easy to clean (selfcleaning) or that have antibacterial or antimicrobial properties. Other functional needs related to surfaces include abrasion and scratch resistance. A successful quest for self-healing or self-repairing surfaces that recall similar behaviors in the skins of humans and animals would provide an enormous benefit to all. Materials with special behaviors related to light and color also have widespread applicability.

4 4 Fundamental approaches for cleaning Most nano-based products intended for these applications use coatings, paints, or films that contain nanoparticles and that are applied or bonded to conventional materials. Some applications, however, have surface layers in which nanoparticles have been directly intermixed into the base material. Three primary technologies drive applications in areas: the ability to make hydrophobic (water-repelling) surfaces, hydrophilic (water-attracting) surfaces, and photocatalytic surfaces.

5 Photocatalytic properties Nanomaterials with special photocatalytic properties that absorb ultraviolet (UV) energy that in turn causes dirt or other foreign particles on the surface to slowly loosen or break down via oxidation can aid in self- cleaning and can provide antimicrobial action. 5 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

6 Photocatalysis The photocatalysis process can be initiated by UV light present in normal sunlight or from any other kind of UV light source. The process causes oxidizing reagents to form that in turn cause the decomposition of many organic substances that are deposited on or that form on surfaces. 6 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

7 Natural cleaning actions The lotus flower has long been a symbol of purity in Asian countries. The surfaces of the flower are rarely soiled, and that this effect could be observed in many plants that are covered in tiny wax structures that appeared to make the surfaces unwettable. The surfaces are not smooth. The roughness minimizes contact between a water drop and the leaf itself, and surface tension causes drops to form into beads while reducing the wetting of the surface itself. The result of this effect is that water droplets bead up into spheres rather than spread. This characteristic is common in hydrophobic surfaces 7 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

8 Self-cleaning action due to Lotus Effect The droplets roll off the surface easily. As they roll, dirt particles cling to the beads and are removed with them as the beads roll off; hence a natural selfcleaning action occurs 8 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

9 9 What is the shape of water droplet?

10 Applications Photocatalytic processes now based only on UV light are obviously primarily suitable for exterior, not interior, surfaces where sunlight is present. Products based on the Lotus Effect are best used where there is frequent general washing from rain or other sources. Many surfaces are particularly sensitive to degradation when exposed to the kinds of disinfectants commonly used in cleaning. In architecture, the primary intended cleaning action is the removal of everyday dirt, dust, or other things that naturally appear on windows or other surfaces in buildings. 10

11 Self-cleaning glasses 11 The self-cleaning action normally comes from coatings with thicknesses at the nanoscale that have particular photocatalytic and hydrophilic properties. In several applications, the glass is first coated with a photocatalytic material, normally titanium dioxide (TiO2). The photocatalytic coatings produce chemical reactions when subjected to ultraviolet (UV) light that help in oxidizing foreign substances and decomposing them. Coating thicknesses are on the order of 15 nm and are transparent.

12 How does TiO 2 coating work? Thin titanium dioxide coatings exhibit photocatalytic and hydrophilic action. When the coatings are subjected to ultraviolet light, the photocatalysis process oxidizes foreign particles and decomposes them. When the coatings are subjected to washing or rain, the hydrophilic action then causes dirt particles to be carried away. 12 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

13 13 Self-cleaning paints Paints are relatively thick in comparison to the thin coatings on glass and also have self-cleaning properties based on similar technical principles. Titanium dioxide has long been used as a pigment in paints, but nanosized particles show greater photocatalytic actions than do the normal pigmentsized particles because of their greater relative surface area. The subsequent runoff process can be enhanced using either hydrophilic effect. These kinds of paints can be applied to many kinds of base materials, including metals. Applications that occur in a factory circumstance, as is common in metal panels used in vehicles, façade elements in buildings, and other products, are invariably better than simple hand painting or spraying. As with self-cleaning glasses, the self-cleaning processes are slow and do not work equally effectively with all kinds of surface deposits, but they do reduce the effort needed to clean the surfaces when necessary.

14 Self-cleaning textiles Many textiles have been developed that have self-cleaning properties. In everyday textiles such as cotton cloths, the surface tensions of the solid materials are quite high compared to those of water; hence wetting typically occurs. When these same textiles are treated with one or another type of fluorocarbon, the critical surface tensions of the treated textile is less than that of water; hence a waterrepellant action occurs. Fluorocarbons achieve this effect by essentially coating individual cloth fibers. This coating can be more successfully achieved with some solid fibers than others. Synthetic fibers are easier to coat than cotton, for example. 14

15 Antimicrobial materials Surfaces that possess antibacterial properties either kill or inhibit the development of bacteria that have deleterious effects such as discoloration, staining, or odors. Bacterial parasites that cause disease are called pathogens. Antimicrobial surfaces target disease-producing microorganisms with the aim of helping prevent the spread of germs injurious to health. There are several approaches to making improved antibacterial surfaces. One approach is to use copper or silver nanoparticles embedded in some other matrix as a coating on base materials or incorporated directly in the surfaces of the base materials. Another process suitable for certain antimicrobial applications is a photocatalysis process. 15 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

16 Self-healing materials Cracks in materials or scratches on surfaces are ubiquitous and always problematic. Self-healing is an intrinsic part of life 16 Beachamwell Church, Norfolk. Lime mortar was commonly used in churches of this era. Lime mortars were found to have a remarkable selfrepairing capability that would cause cracks to heal over time.

17 Principals of self-healing Embedding into the material small encapsulations (in particle, tube, or layer form) of materials that are released when the encapsulations are ruptured by a crack or scratch. The released materials fill cracks and harden (again involving a chemical reaction) to self-repair the crack, or at least inhibit further growth. 17

18 18 Smart behavior

19 Color changing 19 Smart materials include thermochromics, photochromics, chemochromics, and mechanochromics. In all these materials, input of one form or another of external energy causes an apparent color change in the material. The color of a thermochromic material varies with the temperature of its surrounding environment. M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

20 Shape-memory materials If a piece of shape-memory alloy is deformed from an initial shape into a new shape, it will remain in the new shape. When heated to a certain critical temperature, however, the material will literally go back to its initial shape without mechanical aid. The material remembers its original shape and returns to it. A shape-memory alloy can be bent to a new shape and, on healing, return to its original shape 20

21 Shape-memory process Shape-memory alloys undergo reversible phase transformations at various temperatures 21

22 Nanocoatings Coatings are thin coverings that are deposited on a base material to enhance its surface characteristics or appearance. This broad definition includes coatings used to improve durability or wearing characteristics, provide corrosion resistance, or otherwise protect the base material. 22

23 Nanoadhesives Adhesives are substances that join two surfaces together. Adhesives are used in everything from simple consumer products to automobiles. Nanoadhesives that are bio-inspired notably the gecko nanoadhesives have become meaningful symbols for what nanotechnologies can accomplish. Since the solids content and viscosity of nanoadhesives are low, thinner and more uniform coatings can be produced. General mechanical properties are improved, including basic strengths of set adhesives. Other properties, such as electrical conductivities, can be imparted or enhanced via the use of nanometallic inclusions. 23

26 Nanocosmetics Makeup can be made from organic or inorganic materials. Those that have a coloring function normally employ a variety of pigments that reflect light in certain ways, to generate many colors and color gradations. Zinc, titanium, and iron oxides are in common use as pigments and come in many particle-size variations, including ultrafine. Skin-care or protection products range from sunscreens to moisturizing agents. They either interact directly with the skin or provide some type of active ingredient that provides a function. A whole range of nanomaterial types and forms is being explored for use as vehicles or carriers for active ingredients. Use of nanomaterials can allow carrying various types of active ingredients without degradation as well as offering better control of release rates. Small sizes allow them to more easily be incorporated into outer skin layers. 26

27 Mechanical applications The idea of increasing the strength of conventional materials by creating composite forms that incorporate nanomaterials, such as, for ex., adding nanoparticles into a metal matrix or nanofibers into concrete. There are real possibilities for making structures smaller and lighter via nanocomposites with either the same or even greater performance characteristics than are achievable with conventional materials. There are also opportunities for creating active systems with embedded sensors, control systems, and actuating mechanisms within an integral structure. 27

28 Polymer-matrix nanocomposites Depending on the exact type of polymer host and nanoscale reinforcing phase, both the strength and modulus of elasticity values can be either increased or decreased. When carbon nanotubes are used, carefully formulated composites can show significant elastic modulus and strength increases. Polymer-matrix nanocomposites are being widely explored in relation to many high-end sports, automotive, and aerospace applications. 28

29 Amorphous materials A driver made of Vitreloy, an amorphous metal. Its superior resilience allows a longer drive. 29 Amorphous materials were described as an extreme class of nanostructured matter. As crystal sizes shrink atomic dimensions, the material becomes completely disordered. Ordinary glass is amorphous and is known for its unique properties. Other highly disordered materials with similar structures, such as polymers or metallics, are possible as well and are often also called glasses. M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd. The frame of this racquet is reinforced with Vitreloy, an amorphous metal.

30 Nano-concrete Concrete is fundamentally composed of a mixture of coarse and fine aggregates, cement, and water. Synthetic cements are usually made by grinding calcinated limestone and clay into a fine powder. On mixing with water, an exothermal reaction occurs with the cement that causes time-dependent hardening. The obvious routes for using nanomaterials to improve concrete are generally either in process considerations (ease of mixing, rate of setting, etc.) or in property enhancements. 30

31 31 Nano-engineered concrete.

33 Nanofilms Influence of particle size and viewing angle on color. Influence of strain with flexible crystalline films on color 33

34 Fuel cells 34 Schematic of a proton-exchange membrane fuel cell. Fuel cells are electrochemical devices composed of a conductive ionic membrane, an anode, and a cathode. M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

35 Comparison Between Polymer Membrane Fuel Cells (PEMFCs) and Solid Oxide Fuel Cells (SOFCs) 35