3.5.1 Contact mode

Contact mode produces images with the highest resolution. This is because when the probe tip is as close as it can be to the surface, the influence of atoms other than the one directly under the probe tip is relatively small. This is a simple geometrical effect – if the tip were withdrawn a large distance from the surface, a large number of atoms would be at a very similar distance from the tip, and therefore would have a similar contribution to the overall force. In contact mode, the repul
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3.5 Scanning modes of the AFM

One of the interesting effects of scale is the answer to the question of whether the probe needs to come into contact with the surface of the sample being scanned. The cantilever on which the probe tip is mounted is a very compliant structure. The control system of the AFM ensures that the deflection of the cantilever, and hence the force it exerts on the surface, is maintained within very strict limits. Author(s): The Open University

3.4 The atomic force microscope

The most commonly used scanning probe microscope is the AFM – the atomic force microscope. It works in a way much more similar to the gramophone stylus, but instead of detecting the movement of the probe tip electromagnetically, it usually does so optically. As the probe tip is drawn across the sample, a laser beam is reflected off the cantilever on which the tip is mounted. A position-sensitive optical detector picks up the deflection of the beam, converting the angle of bending into a vol
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3.3 The scanning tunnelling microscope

The first scanning probe microscope, the scanning tunnelling microscope (STM), was invented by Heinrich Rohrer and Gerd Binnig in 1981, and used the quantum-mechanical effect of electron tunnelling (in which electrons ‘tunnel’ through an energy barrier that classical physics would suggest is too high to cross). In this instance, the energy barrier is the tendency of the metal of the probe tip to want to hang on to its electrons. In effect, as you try to remove an electron from the surface
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3.2 The principles of scanning probe microscopes

Scanning probe microscopy is a term that is applied to a set of imaging methods based on a common element: a fine stylus. In many ways, what scanning probe microscopes do is similar to what a gramophone does. A gramophone stylus scans a spiral groove (by travelling along it) on which information has been encoded in the form of undulations in the groove wall. Side-to-side and up-and-down movements of the stylus (which is mounted on one end of a rod supported and pivoted at its centre) as it fo
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3.1 Introduction

The atomic force microscope is a key visualisation tool for the ‘invisible’ world of micro and nano technology. Within it, right at the heart, is a probe tip that is itself a triumph of nanotechnology.

This section is going to begin with a fair amount of detail about how scanning probe microscopes of various types work, starting with a description of the scanning tunnelling microscope (STM). After that I want to concentrate on its close relative, the atomic force microscope. Then we
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2.2 The challenge for innovation

For a Pirani sensor, the basic task is to provide a reliable measurement of pressure in a vacuum system as it varies from atmospheric pressure down to a value at least as low as 1 Pa. This statement can be further qualified by saying that unless its performance or cost is a fantastic improvement on the existing type, the micromachined sensor must be compatible with existing interface electronics, such that only minor modifications to its design are needed. This implies an electrically resisti
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2.1 Introduction

One aspect of micro and nano scale engineering that distinguishes it from many other forms of manufacturing is the way it involves building both the devices themselves and the very materials from which they are made, in one place and at more or less the same time. In general, MEMS are made from thin layers of new material produced, and then shaped in some way, on the surface of a silicon wafer. The devices contain several different materials, and have a three-dimensional structu
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Introduction

This unit examines how small features can be etched and cut out of solid materials at a very small scale.

This unit is an adapted extract from the Open University course Engineering small worlds: micro and nano technologies (T356).


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2.4 Early disasters

Many of the earliest bridges were simply a wooden trestle type of construction, an efficient and easy-to-build structure, yet providing a secure and safe passage for heavy metal trains. Although we tend to associate such structures with the United States, they were in fact widely used in Britain in the early days of steam locomotion. However, they had a limited lifetime owing to rot, so were gradually replaced by wrought iron girder bridges, often laid on brick or masonry piers.

Designe
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Introduction

This unit starts by giving an overview of the two main categories of disasters: disasters of natural origin and disasters of human origin. It then analyses the Tay Bridge disaster, which was caused by mechanical failure.

Inevitably, human factors emerge as important in many major disasters. They may involve the failure by engineers, designers or managers to recognise faults in safety-critical products, or managers overriding the design team for other reasons – such as keeping to a dea
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3.1 Introduction

I've an opera here you shan't escape – on miles and miles of recording tape.

Flanders, M. and Swann, D. (1977) ‘The Song of Reproduction’ from The Songs of Michael Flanders and Donald Swann, London, Elm Tree Books and St George's Press, p. 99

Sounds, pictures, measurement data, financial statistics, personal details, etc. can all be recorded and stored on magnetic media, i.e. m
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3 Sound production in musical instruments

Musical instruments come in all shapes and sizes and produce an enormous variety of different sounds. Yet, with the exception of certain electronic instruments, the basic physical principles by which sound is produced are the same for all instruments – including the human voice. In this section, I shall introduce some of these principles. These will then be expanded upon over the rest of the unit.

Remember I told you that when a musician plays an instrument they cause it to vibrate. T
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5.5 Multiple-cause diagrams

Multiple-cause diagrams are another way of using interconnectedness to structure a complex situation. In this case, the interconnectedness is that of causation. Multiple-cause diagrams represent both sufficient and contributory cause, without making a distinction between them. Drawing multiple-cause diagrams allows for the identification of systems of causation. Such a system can be pictured as an interconnected group of events or effects; the effect is of a system that behaves
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2 New products – old failings

This section introduces product ‘usability’. It offers a case study of a vegetable peeler to illustrate how usability issues exist alongside other important concerns in the product development process.

If you look around high-street or shopping-mall stores, you cannot help but notice the number and variety of new products on offer. This year's washing machine or dishwasher, stylish furniture, multi-feature telephones, audio systems, DVD players, digital cameras and camcorders, all b
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5.14.3 Electrodialysis

Electrodialysis is an electrochemical process in which ion transfer separates salt from water. It is effective only for substances that can be ionized: for example, salt (NaCl) becomes, in solution, a mixture of Na+ and Cl ions. (Silica, on the other hand, does not ionize and hence is not removed by electrodialysis. It could, however, be removed by reverse osmosis.) When electrodes, connected to a suitable direct current supply, are immersed in a salt solution, curren
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5.14.2 Reverse osmosis

This technique, explained in Section 3.8.1, is rapidly becoming a major means of desalination, with research producing membranes with lower operating pressures (and hence lower operating costs). Originally a pressure of 14 × 106 Pa was needed to separate pure water from sea water but with newer membranes only half this pressure is required. Reverse osmosis membranes operate at ambient temperature, in contrast to multistage flash distillation, and this lower temperature minimises s
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5.7 Disinfection

Before water can be passed into the public supply, it is necessary to remove all potentially pathogenic micro-organisms. Since these micro-organisms are extremely small, it is not possible to guarantee their complete removal by sedimentation and filtration, so the water must be disinfected to ensure its quality. Disinfection is the inactivation of pathogenic organisms and is not to be confused with sterilisation, which is the destruction of all organisms.

Worldwide, chlorine is the most
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4 Introduction

(Please refer to Reading 4: Learning to act: managing and systems practice, by Andy Lane) This unit teaches some aspects of systems thinking and practice. But what does it mean to be a systems practitioner, and is it different to being a manager? This reading attempts to answer those questions.

First, I believe a good systems practitioner will be more competent at handling complex situations, more capable of managing their working and domestic lives, and more able to learn not only how
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7.1 Some basic principles of religious studies

Remember that in Section 4 I suggested that possible reasons for studying religion could be clustered together under two broad headings:

  1. to understand the society in which we live, the culture we inherit and the wider world of which we are a part;

  2. as part of a personal quest for religious
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