Man was ever a materialist. Right from the start he seized what lay around him to fashion into tools with which to hack, carve, pound and sew his way though life. But as he shaped the wood, horn, stone and iron that nature had provided, he had little understanding of why each had particular strengths and weaknesses. Nor did he need it. It is only when you make materials from scratch that knowing why things are as they are begins to matter; before that, trial and error will suffice.
The array of materials around today shows that trial and error has done pretty well. But scientists, of course, are not satisfied with it. They want to know why. In the past century they have developed a wide field of material science that seeks to answer the question, explaining what arrangements of matter at a micro-scopic level give rise to a properties of everyday substances. This knowledge has its practical side. When corporate research directors gathered at the California Institute of Technology to hear about its work in material science, it was not just the knowledge that drew them; it was the fact that the researchers presenting it can use that knowledge to build new properties into matter.
One of the materials they have been working on is glass. This Glass is extremely hard and can be shaped fairly easily, encouraging scientists to look for ways to use its strength while eliminating, or getting around, its tendency to shatter. The usual way to toughen glass, developed in the 1920s, is to draw it into fibres that can reinforce other materials.
A different approach was on display at recently held Caltech in the form of a handful of shiny lozenges cooked up in the material science laboratories. They were glasses, but were made of metal. This metallic glass provides a combination of strength and flexibility nothing else can match.
The fact that these grey lozenges are called glass demonstrates the differences between the way normal people and scientists think about materials. To most people a material is a mixture of properties and substance; glass is transparent and made of sand. To scientists, materials are characterised by thier microscopic structure; glasses are solid with no internal order to the arrangements of their atoms.
In this they are more like liquids than other solids, such as specks of sand or ingots of metal, which are crystalline. The atoms in crystals forma regular pattern that is repeated all the way through; one little bit will look like any other one, far away. Glass is made when a liquid cools so quickly that its molecules, which can jiggle around freely in the molten state, do not get a chance to make such orderly lines.
Instead, they screech to a halt wherever they happen to be: the liquid’s randomness is frozen. The freezing is not complete. Look at a really old window and you can see the effects of the glass’s slow, sagging flow.
Old-fashioned glass is easy to make form the silicate crystals in sand; once they melt, it is hard for silica to do anything except cool into a glass. Making crystalline forms, such as quartz and chrysolite, requires slow cooling under tremendous pressure.
But metals are just the opposite. Metal atoms are extremely amenable to being ordered, so metals tend to crystallines quickly and easily.
To get them to keep their liquid disorder in a solid takes real skill.
The first vitreous metals had to be made by super-quenching small droplets in a old bath that cooled them at a rate equivalent to one million degree Cellcius in a second. With the energy sucked out this quickly, even metals will not crystallise.
That was done in the 1950s. It was not until 1990 that a Japanese group found how to make metallic glass that could be cooled much more gently. The secret was in the mix; alloys of zirconium and lanthanum spiked with aluminium, nickel and copper. The atoms in these mixtures differ widely in size, which thwarts their urge to pack themselves into a regularly configured crystal. Following the Japanese lead, two scientists have produced a zirconium, titanium, copper nickel and beryllium all0y that forms glass when cooled at less than 10^C a second. The advantage of slow cooling is that you can make lumps of metal, rather than specks.
The material thus produced is twice as strong and half as dense as the best steel. It is hard, resistant to wear and corrosion, and smooth – it slides across a surface as if it were Teflon. It is also ductile; a strip of the glass can be rolled into a ribbon a hundred tomes thinner without cracking.
And because the molecular structure is not locked rigidly, the glass does not have a fixed melting point, but gradually softens like toffee as it warms up, making it easy to mould into useful shapes that set when the glass is cooled again. There is already commercial interest, and that the material could be used to make everything from springy golf clubs to stamping dies to mirrors to car bodies to even window panes.
The Japanese team that did the earlier work in now investigating a further refinement, using an aluminium-based alloy that incorporates both glassy and crystalline forms too achieve the strength of steel at a much lower weight.
By carefully regulating the recipe and controlling the cooling, they create a kind of composite, in which tiny crystals of aluminium a few billionths of a metre across start to form in a predominantly glassy mass. The crystals occupy about a quarter of the whole volume, and bind it internally like straw in clay bricks. It is glass that grows its own internal fibres.
Zeolites are porous minerals made of aluminium and silicon. Industry likes them because they act as chemical sieves. Their pores allow only molecules below a certain a size to pass through, which means they can be sued to filter things out of and gases and liquids. They are also used too soften water, by absorbing into their pores the ions that make water hard. This technique allows to control the size of the sieve’s mesh. An organic molecule of the right size is found and grown to a zeolite crystal with this molecule in suspension.
The zeolite grows with its porous spaces. filled by the organized molecule. When it is finished the organic material is burnt away, to leave a bespoke silicate skeleton.
Anyone undertaking this technique, should know how to control the zeolites because he knows their structures; the more he controls it, the greater his understanding of it.
Ductile materials are attractive to manufactures because of the ease with which a mould can shape them to their final form.
With today’s tools, finding out how something works at microscopic level necessarily means learning how too make it work differently.
Trial and error is still around, if at a smaller scale; its impact could be bigger than ever
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