Introduction to Nanoscience and Nanotechnology. Chris Binns
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Название: Introduction to Nanoscience and Nanotechnology
Автор: Chris Binns
Издательство: John Wiley & Sons Limited
Жанр: Отраслевые издания
isbn: 9781119172253
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4 4 The high magnetization was first observed in Fe67Co33 alloys, but the magnetically soft variant Fe50Co50 that is used commercially was patented in 1929 by G. W. Elmer.
5 5 Bacterium that infects the lungs of cystic fibrosis sufferers and causes inflammation and breathing problems.
6 6 You should find that your estimate is much larger than the critical size (~100 nm) for a single‐domain particle given in the text. The reason is that the exchange energy in a domain boundary can be reduced by a large factor by spreading it over a number of atomic layers. That is, instead of having an abrupt 180ª reversal of the magnetization across a single atomic plane, the magnetization rotates a fraction of 180° across each plane. This brings the critical size down to ~100 nm.
2 Nanoparticles and the Environment
In the previous chapter, the special properties of matter at nanoscale dimensions were presented and how this novel behavior could be exploited in technologies as diverse as advanced engineering materials and health care. This chapter is about nanoparticles and the environment, which encompasses naturally occurring nanoparticles and environmental applications of nanotechnology. Sections 2.1–2.7 discuss naturally occurring nanoparticles, both in our immediate environment, that is, the Earth's crust, oceans and atmosphere and out into deep space, where nanoparticles with their unique properties have helped shape the observable Universe. Section 2.7 describes the use of nanoparticles in addressing environmental issues via two important examples, that is, removing toxins from groundwater and recycling plastics.
2.1 Nanoparticles in the Atmosphere
The particles in the Earth's atmosphere have an important influence on the climate, but also have a poorly understood effect on life and our health. Improving our understanding of the effect of airborne nanoparticles is becoming increasingly important in a world where nanotechnology is poised to become a major activity. Clearly, the amount of manufactured nanoparticles will increase, so it is wise to be aware of how they interact with life and with the environment. It is important to emphasize, however, that manufactured nanoparticles are normally bound up in some material and the number of “loose” particles produced by nanotechnology will not necessarily become significant compared to those produced by the natural processes described below. In this chapter, the discussion is extended to encompass nanoparticles that are generated by existing human activities not directly involving nanotechnology, such as power generation, transport, etc. Obviously, these are not naturally occurring in the normal sense of the phrase, but they are a component of a pre‐nanotechnology background of nanoparticles in which we live. The effect of naturally occurring nanoparticles on the environment is an enormous multidisciplinary subject and a rigorous discussion is well beyond the scope of this book. It is an important hot topic, however, as it encompasses climate change and nanoparticles are implicated in many of the feedback mechanisms involved in the Gaia hypothesis that treats the Earth as a living organism. The aim of this chapter is to describe, in general terms, where the nanoparticles come from and, as in the previous chapter, emphasize the special nature of particles belonging to the nanoworld (<100 nm – Figure I.1).
Figure 2.1 Sources of background nanoparticles. (a) Volcanoes and (b) forest fires produce nanoparticles in the atmosphere (aerosols). (c) Hydrothermal vents produce nanoparticles in the ocean (hydrosols). (d) Some bacteria produce nanoparticles such as this river‐dwelling bacterium that manufactures magnetic nanoparticles (black dots). (e) Supernova explosions such as the crab nebula shown here spread nanoparticles through space.
Source: (a) US Geological Survey. (b) Reproduced with permission from the government of British Columbia. (d) Reproduced with the permission of the Spanish Society for Microbiology from D. Schüler [1]. (e) NASA.
Naturally occurring nanoparticles are ubiquitous in land, sea and air and come from a number of processes (Figure 2.1), including volcanic activity, forest fires, ocean bed hydrothermal vents,1 geological processes, living creatures, and human industrial activity. They are also to be found in space (though normally called dust by astronomers) and produced by, among other things, supernova explosions.2
To begin with, we will focus on atmospheric nanoparticles as these probably have the most immediate effect on living things. The general term for a tiny (solid or liquid) particles suspended in a gas is an aerosol. This term was first used in the 1920s to distinguish air‐suspended particles from liquid suspensions, or hydrosols. The term suspension implies that the particles are defying gravity, but this, of course, is not the case. The particles are falling through the gas (a viscous medium), but their terminal velocity due to gravity is so low that it may take years for them to settle (see Advanced Reading Box 2.1). In this regime, for all practical purposes, we can consider them to be suspended. Other processes can, however, remove nanoparticles from an aerosol. To begin with, if they have a sufficiently high concentration they will agglomerate and the larger particles will settle much more rapidly. In addition, in a humid atmosphere, nanoparticles will act as nuclei for the formation of water droplets (see below) and if these grow large enough to fall as rain, this will act as a removal mechanism (“rainout”). Alternatively, the particles can be incorporated into existing raindrops and removed (“washout”).
Advanced Reading Box 2.1 Terminal Velocity of Aerosol Particles
It is easy to show [2] that for large (micron‐sized or more) particles with a diameter d and a density ρp, their terminal velocity due to gravity in a still gas with a density ρg is:
(2.1)
where η is the viscosity of the gas (η = 1.81 × 10−5 Pa s for air at Standard Conditions) and g is the acceleration due to gravity. For 1 μm diameter particles with a typical density (1000–5000 kg/m3), this gives ~0.1 mm/s. The equation, however is only valid for relatively large particles. In its derivation, it is assumed that the gas velocity at the particle surface is zero, which is invalid for very small particles whose size is less than the mean‐free path of the gas molecules. To put it crudely, very small particles “slip” through the gaps between the gas molecules and fall faster than predicted by the equation. As the particles get smaller, an increasing slip correction factor needs to be applied and this can get to be a factor of 10 or more. Even so, the fact that the terminal velocity decreases as d2 ensures that small particles do drop more slowly. Applying the slip correction factor to 10 nm diameter СКАЧАТЬ