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Engineering: The challenge of temperature
Engineering: The challenge of temperature

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5.2 What's in a phase?

In an engineering and scientific context, a phase is an arrangement of atoms that is identifiable through its recurrence – the same pattern is found time and again. For instance, the compound of hydrogen and oxygen that we call water turns up all over the place in the same form as a runny, colourless liquid; this is a specific phase of the compound H2O. In water, the atoms apparently organise themselves according to what they are and the ambient conditions of temperature and pressure – there is no need for a separate template or a plan. That 'self-organisation' is the key to defining a phase. Water is a particular phase of hydrogen oxide (H2O).

There are three important features of a phase. Changing any one of these three features makes a new, distinct phase.

The first is the physical state of a phase: solid, liquid or gas. Water, ice and steam are different phases of H2O. The normal physical state of a phase depends upon both temperature and pressure.

However, there's more to defining a phase than merely the physical state. The second feature that may distinguish a phase is the arrangement of the atoms. For example, diamond and graphite are different phases of carbon owing to the very different crystal structures that the carbon atoms adopt in these materials (see Figure 22).

Graphite is the normal form for carbon under the conditions of temperature and pressure we are accustomed to, but at extremely high temperature and pressure the diamond structure is the 'natural' one. We know this because somebody has found this out in the first place; every group of carbon atoms 'knows' this because that's the way the inter-atomic forces and thermodynamics dictate it must be. Given enough time, all diamonds that are not at those extreme conditions of temperature and pressure ought to change to graphite, according to the self-organising drive of thermodynamics. However, the transformation is so incredibly slow that they are a safe investment for a few million years – diamonds almost are 'forever'.

Figure 22
Figure 22 Graphite and diamond

Other structural phase changes can be effected and controlled under more modest conditions of temperature and pressure. In fact, manipulating structural phase changes in solids is at the heart of much metallurgy and materials engineering, and that is the basis of the steel industry in particular.

Structural phase changes are also more subtle. For instance, magnetism and various electronic and opto-electronic phenomena are very sensitive to atomic arrangements and are thereby susceptible to sudden changes of phase triggered by temperature and pressure.

Chemical composition can be a third defining feature of a phase. You may have come across 'phase diagrams' that are maps of the various phases that can be expected in simple two-part (binary) alloys over a range of composition and temperature.

Let's look again at my examples of sudden changes and identify the phases involved. I've started this in Table 7 for you to complete as Exercise 6.

Exercise 6

Complete the entries in the second column of Table 7.

Table 7
PhenomenonPhase changes
Water boils at 100 °C…………………………
NdFeB loses magnetisation above 450 °C…………………………
Gold melts at 1063 °C…………………………
Silver spontaneously oxidises below 1530 °CA solid + a gas → another solid
YBCO is superconducting below −180 °COne solid → another solid


Table 8
PhenomenonPhase changes
Water boils at 100 °CLiquid (water) → gas (steam)
NdFeB loses magnetisation above 450 °COne (magnetic) solid → another (non-magnetic) solid
Gold melts at 1063 °CSolid → liquid
Silver spontaneously oxidises below 1530 °CA solid + a gas → another solid
YBCO is superconducting below −180 °COne solid → another solid