Saturday, 28 February 2015

Beyond ‘Fun with Flags’

The character of Dr Sheldon Cooper in the long-running series The Big Bang Theory produces as one of his hobbies a YouTube video series – or should that be a vlog – entitled Fun with Flags (e.g. here). I’m not going to attempt to ape that; indeed, I couldn't. However, for a long time now I have been meaning to place on record something about my favourite elements in the Periodic Table: those types of atoms that have shaped and even led my science career through four decades of research. I can’t make it as hilarious as the script-writers and actors manage in Big Bang, and there’ll be a little more overt science in this post than is usual for me. However, the scientific detail, such as it is, is almost incidental in a way. This post will be more like a collection of milestones: a potted summary of how I found myself moving seamlessly, almost imperceptibly, between Physics, the subject in which I was formally educated and trained, and other fields such as Chemistry and Materials Science. What became evident to me early on is that my driving motivation is not a subject or discipline as such but, rather, a desire to explore and to understand a particular material – usually an amorphous one such as a liquid or a glass (see here). If, in order to ‘scratch that itch’, I needed to learn something new or to join forces with those already expert in that area then so be it; this approach eventually attracted the epithet of a ‘materials-led methodology’ (see my earlier post, here). In essence, I have sought to understand and to be able to explain, at the atomic scale, why a given material has the properties and attributes it does. The narrow ‘tags’ we place on subject areas cease to have any real significance in this context. Thus, as if to illustrate this truth, I have emerged at this closing stage of my serendipity-rich research career as a Fellow of both the Institute of Physics and the Royal Society of Chemistry, and my University label me as a Materials Physicist. (In passing, I’d have preferred Chemical Physicist since that’s mostly what I do – an admixture of both disciplines, devoted to the understanding of novel materials at the level of the arrangement of their atoms.)

The teddy bear my son won in a
raffle many years ago: today sporting
a rather geeky Periodic Table tie.
In earlier posts I alluded to some of the more recent areas of my research activity (e.g. here), but this was built on a track record of experience gained through the investigation of liquid metals, molten salts, thin-film and narrow-gap semiconductors and photovoltaics, ultra-hard coatings and non-linear optical glasses. One might perhaps track further back in time to the beginnings of my love for science, which extend deep into primary school – but that’s a far broader topic than I’m willing to tackle here and now. What does occur to me however is the fact that, in pursuit of these varied scientific goals, I have, with my research team members and other partners and collaborators, synthesised and/or studied materials comprising elements from across the Periodic Table. Now, there are 92 naturally occurring elements in the Periodic Table – all the way from hydrogen to uranium – and then a slew of man-made elements beyond, such as plutonium*. Even this long list is but the tip of the proverbial iceberg in the sense that most of these elements – types of atom – can exist as one of several isotopes. The humble hydrogen atom, for example, is most often found in its simplest form comprising one positively charged proton as its tiny nucleus with a single negatively charged electron somewhere within a much larger ‘cloud’ surrounding it; but one may also find it in the form of deuterium or tritium. All three isotopes have but one proton in the nucleus, but deuterium and tritium respectively have in addition one or two neutrons in there with the proton; tritium also happens to be radioactive. Whilst it is the number of protons that dictates which element we’re talking about, it is the number of neutrons in its nucleus that determines which particular isotope of that element is at our focus; if one adds both numbers together we get the atomic mass number (- the electrons contribute less than a thousandth of an atom’s mass, so we’ll ignore them in this regard). Were there no isotopes, each element would therefore have an integer mass number; if we consider again our example of hydrogen, it is listed as having an average atomic mass not of 1 but 1.008 – the difference arising from the presence of small percentages of deuterium and tritium. My story is about particular isotopes as much as it is about their parent elements. 

There have been quite a few depictions of the Periodic Table over the years: the figure above illustrates the variety. From the geologist’s view of the relative abundance of elements in the Earth’s crust (top left) through a naïve historical display of the nations within which each was originally isolated (top right) to my personal favourite variations which show the elements set out in the style of the London Underground map (lower left, by fellow scientist and public engagement enthusiast Mark Lorch) and an edible depiction in chocolate I recently spotted on Twitter (lower right)
Of course, one might instead break into song … 

A rather more conventional and contemporary layout for the Periodic Table is shown in the figure below. The design is such that it brings out similarities in chemical behaviour (within the columns) and trends and differences (along the rows). The elements that have figured in the catalogue of materials/samples I and my co-workers have studied are all tan-shaded, and in each and every case there is a story to tell if I had the time (and you had the patience). In addition I have shaded in aqua a few more elements that have, at one time or another, been vital in some other particular way in the context of experiments – perhaps as part of bespoke, ‘hand-made’, equipment. However, to keep this post short enough to be within the realms of the readable we’ll focus only on the small sub-set I have circled in red; even then, there is time only to skim across the surface. Between them, these elements – and some of their isotopes – exemplify important and/or particularly enjoyable aspects of my research. Carbon (C, 6), for example, has on occasions been at the very heart of things or a downright nuisance. For the decade or so when I was primarily focused on studying catalytically-active and then bio-active glasses, a key step in their synthesis was the removal of ‘left-over’ organic compounds through a process of heat-treatment: get this wrong and our final product was contaminated with residual carbon and therefore useless. By contrast, there was a period of five or six years when we were studying ultra-hard coatings made by depositing carbon atoms onto various substrates from a plasma gun; these materials became known as diamond-like carbon. Moreover, by adding a little hydrogen, we could generate a hard but bio-compatible coating with potential uses in prosthetic surgery.

Gadolinium (Gd, 64) is a metal within a series called the Lanthanides (so-named because of the element at the start of the row, lanthanum). I have singled it out for two reasons: as an example of the extensive study we undertook into a family of lanthanide-doped silicate and phosphate glasses which exhibit fascinating electronic, optical and magnetic properties, and because it sits at the core of some novel but particularly tricky data that is still in need of analysis some 15 years after the original experiments. Lanthanide-doped glasses are already used in the fibre optics industry as optical amplifiers and switches, and one aspect of our research was supported by industry; there is interest way beyond this realm however. Some of our more challenging experiments have been associated with this work, using neutron research facilities in the UK and in France and x-ray sources in the UK, France and the USA, and it is from a particular ground-breaking experiment at the latter that we have data which is still not fully analysed. When this task is completed – and I’m confident it will be, one day – we shall have a pretty complete picture of where the lanthanide atoms sit within their glass host matrix.

The Mary Rose, Henry VIII’s favourite warship, by William Bishop
The inclusion of iron (Fe, 26) in my selection is representative of the sheer fun that’s possible sometimes when one plays a 'wild card'. A few years ago a colleague of mine, Alan Chadwick, and I were approached by the Mary Rose Trust and asked to help them understand key issues of marine timber conservation and preservation. I have written a little about this project in an earlier post (here) but suffice it to say that iron turned out to be a culprit in that it greatly accelerated the conversion of sulfur present in the wood into sulfuric acid – never a good thing when one wants unique artifacts to survive. Using an experimental technique based on the absorption of high-energy x-rays and then some clever software written by one of my PhD students we were able to reveal for the first time the finer details of this process. After sharing the results with our friends at the Mary Rose Trust, they were published in my one and only paper in an archaeological journal. As part of all this we got to visit behind the scenes in Portsmouth and to see the amazing collection of artifacts not on public display, and of course it enabled me to do what I love – to learn about very different areas of endeavour by working with the experts, and thereby to make a contribution.

Most of us will have been told, even if only in childhood, that drinking milk is good for us because it contains the calcium (Ca, 20) which helps us to build strong bones. One can now take this to a new level: if one incorporates calcium into a glass, and gets the whole thing just right, it’s possible to make a bio-active scaffold for the regeneration of bone. In terms of the sheer scale of the undertaking (see here) and the degree to which all of those involved had to learn new things outside their professional comfort zones this project collects the prize. However, this is also the point at which I can begin to introduce the usefulness of isotopes. By conducting experiments on two samples of our bio-active glass which differed solely in that the calcium in one of them was a particular isotope only, Ca-44, we gained new insights. In essence, by subtracting the results from the sample with natural calcium from the one containing only the one calcium isotope, we eliminated the effects of the majority of the atoms in the glass – beautifully isolating the subtle, and hitherto hidden, environment of the calcium. In this one experiment we managed to offer an atomic-scale explanation for the remarkable behaviour of the material as it ‘grew’ bone mineral when placed in a body fluid (blood plasma or saliva will do). I have already used this experiment as an example in an article written for the non-expert readers of the magazine Laboratory News, so follow the link here if you want to know more.
Part of a poster campaign on local buses for public engagement with science

Titanium (Ti, 22) and nickel (Ni, 28) are in my highlighted selection for similar reasons, not least because they have both cropped up over and over again as my career has progressed, but there are enough differences to warrant separate mentions. Nickel was, if you’ll forgive the phrase, my first love as an element. It figured in my PhD work in the ’70s when I was studying the electronic properties of liquid metals, and in the research into the atomic-scale structure of molten salts (nickel chloride, as distinct from table salt) I undertook during my first postdoctoral contract. In that latter case, the rich choice of stable nickel isotopes available allowed me to design some particularly sophisticated experiments and led to the first measurements I conducted outside of the UK (at this place – still going strong). It cropped up again early in my academic career when looking at metal-doped semiconductors and more recently as an additive to the bio-active glasses mentioned above which may help promote the growth of new blood vessels through our bone-regenerating scaffold. When added to a suitable porous glass host, titanium, which offers some wonderfully useful ‘wrinkles’ my team could exploit in the context of x-ray absorption spectroscopy (the exact analogue of the iron case mentioned above), acts as a catalyst able to break down large hydrocarbon molecules. In the form of a doped silicate glass it also allows us to make transparent low-expansion protective coatings on optical components; we were again able to explain what the titanium does and why there is a maximum concentration that one can use. Most recently, within a collaborative team including a dentist and a metals corrosion expert, we were able to explore the way in which the titanium in medical implants gets into the surrounding tissue and even into living cells.

Perhaps, at some stage in the future, I’ll pick the above stories up and explore some of them in more detail: ‘Fun with Elements’ if you will. In the meantime, if you want to dig deeper I suggest that you use one of the many repositories of my team’s research papers (e.g. here, here, here or here). Until then, …

* In truth, this statement is in need of clarification: things are not quite so clear-cut in that even some of those 92 exist only because they are created in small quantities as a result of nuclear reactions. Astatine (At, 85) is one of the more extreme examples: it exists for short periods only before it decays, and even then in the tiniest of quantities; by contrast, plutonium (Pu, 94) now exists in relatively large amounts following its first creation within nuclear reactors in the last quarter of the twentieth century. For the complete low-down I would recommend looking here or here. I must also record my indebtedness to a chemistry colleague at my University, Robert Benfield, who generously proof-read the particular table I assembled for this post and made some characteristically good suggestions.

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