domingo, 21 de marzo de 2010

Application of Neutron Diffraction in everyday life

Understanding colossal magnetoresistance

The complex manganites are derived from a structure consisting of a rare-earth element such as lanthanum combined with manganese and oxygen (LaMnO3).These electrically conducting compounds have caused great excitement in the past decade because they show a huge change in resistivity when a magnetic field is applied.This colossal magnetoresistance (CMR) effect could be the key to the next generation of magnetic memory devices, magnetic-field sensors, or transistors.

Like their chemical cousins, the high-temperaturesuperconductors (see p.14), they possess complicated electronic structures.Yet unravelling their behaviour – over a range of temperatures or in magnetic fields – is essential to understanding CMR and finding suitable CMR materials for commercial application.

Hidden electron orbitals

Our contribution to this active research area has been to investigate, in these materials, an attribute of electronic structure that has hitherto been difficult to probe experimentally – the electron orbitals. Electrons involved in chemical bonding have three defining properties laid down by quantum mechanics, their charge, spin and orbital.The orbital refers to the shape of the volume that the electron is likely to occupy (the electron cloud). In transition metals such as manganese, it is the outer 'd' orbitals that are involved in bonding and give heavy metal compounds their electronic and structural characteristics.

In the CMR manganites, the manganese atom is triply charged (Mn3+) and is bound to six neighbouring oxygen atoms in an octahedral shape as shown in Figure 1.Mn3+ has one d electron involved in bonding. This 'valence' electron occupies one of two possible d orbitals, as shown in Figure 2.These orbitals are thought to be significant in CMR. Each can mix, or 'hybridise' with the oxygen orbitals to form bonding orbitals; each is geometrically different and is associated with differing bond lengths.

The lanthanum ions, which are also triply charged (La3+), sit between the layers of the manganese-oxygen octahedra. If a percentage of the lanthanum ions is replaced with metal ions such as strontium (Sr2+) which are doubly charged, then there is a redistribution of electrons resulting in the formation of some Mn4+ ions, now with an empty d orbital (a hole).This is called 'hole-doping' and allows the remaining valence electrons to hop from manganese to manganese atom so that the material conducts like a metal. Furthermore, when the 30 or 40 per-cent hole-doped manganite is cooled below a certain temperature, the spins of these electrons align so that the material becomes ferromagnetic. It is this that increases the resistivity of the material because the aligned electrons scatter oppositely-aligned electrons trying to pass through.

Using a combination of X-ray and neutron diffraction,we investigated the interplay between the ordering of the spins, the charge and the orbital occupancy in the 50 per-cent hole-doped manganite (LaSr2MnO7). Neutron studies enabled us to measure the manganese-oxygen bond lengths, which indicated which d orbitals were occupied.We found that this compound has equal numbers of Mn3+ and Mn4+ ions arranged alternately, and a 'staggered' arrangement of bonding orbitals (see Figure 3) below 225K. At 170K this charge-orbital order 'melts' as antiferromagnetic order (spins alternately aligned) sets in.

The structure of lanthanum strontium manganiteThe d electronorbitals of manganese

The structure of 50 per-cent hole-doped
lanthanum strontium manganite
(LaSr2Mn2O7), at a temperature of 165K,
looking down through the manganese-oxygen
octahedra. Neutron diffraction allowed us to
determine the position of both manganese
and oxygen atoms and thus the Mn–O bond
distances.This revealed the ordering of the
Mn3+ and Mn4+ ions (A) and the ordering of
the hybridised orbitals (B)

Application of Neutron Diffraction in everyday life

Magnetic shape memory alloys

The shape memory effect is the ability of some alloys to remember, and return to, the form which they had at one temperature after being plastically deformed into another shape at a lower temperature. This property can be exploited in many ways, for example, in actuators controlled by heat.Devices based on shape memory alloys are being developed in fields as far apart as astronautics and medicine.

In order for an alloy to have the shape-memory property, it must undergo what is called a martensitic transition.This derives its name from a change in crystal structure when steel is cooled rapidly to form so-called martensite (after its discoverer Adolf Martens), which has a variety of characteristic microstructures. The transition involves small displacements, or slips, between planes of atoms in the crystal at a certain temperature.

The shape memory alloy is first formed at a temperature above that of the transition and then deformed below it.The memory arises because residual stresses in the structure introduced during the forming process, influence which slips then occur in the martensitic transformation, and thus which variants of the martensitic phase are present at the lower temperature.

Shape memory alloys scatter neutrons very effectively, so neutrons are an almost ideal probe with which to view the martensitic transformation at a microstructural level. Neutron diffraction can follow the evolution of different martensite variants as the temperature is changed.

Magnetic possibilities

Combining the shape memory property with ferromagnetism, vastly increases the range of applications.Magnetic fields can also influence the martensitic transition, and the possibility of controlling shape memory properties using a magnetic field is currently receiving much attention. An alloy made of nickel, manganese and gallium (Ni2MnGa) is one of the rare ferromagnetic shape memory alloys. It undergoes a martensitic transition from a cubic structure to a tetragonal variant at a temperature around 200K.

At ILL,we carried out neutron diffraction experiments on single crystals of this alloy using the 4-circle diffractometer (D9). On this instrument a small position-sensitive detector records the scattered neutrons.The figure below shows how the diffraction pattern evolves before, during and after the martensitic transition. At 235K, above the transition, the scattering shows a single compact peak associated with the cubic phase. At 206K, the martensitic transformation is under way, and the original single peak has broken up into seven smaller peaks, each corresponding to a different martensite variant. On further cooling, two of the variants grow at the expense of the others so that at 200K there are just two peaks in the pattern. On reheating the process is reversed and the original cubic single crystal restored.

Experiments of this kind, carried out on crystals which have been subjected to different mechanical treatment, allow the transformation processes that give rise to the shape memory effect to be studied in great detail

A 3D representation of the neutron scattering
peaks as the crystal of the ferromagnetic
shape memory alloy is cooled through the
martensitic transition

Positioning a sample on the 4-circle diffractometer D9


Abel A. Colmenares E.

Application of Neutron Diffraction in everyday life

New ceramics for jet engines

Titanium silicon carbide (Ti3SiC2) is a layered material. Its crystal structure, shown left, has double layers of titanium carbide interleaved with single layers of silicon.This laminated structure leads to an unusual combination of properties that is both ceramic and metallic. It is stable at high temperatures, as in the case of ceramics, but also conducts heat and electricity like a metal. Furthermore, because the layers can slide over each other, the material is not brittle like other ceramics and is thus readily machined with ordinary machine tools.Other useful mechanical properties include excellent resistance to sudden changes in temperature, good strength at high temperatures and resistance to oxidation. It is also relatively resistant to fracturing. All these attributes make titanium silicon carbide an excellent candidate material for high-temperature applications such as jet-engine turbine blades.

One drawback, however, is that the material is difficult to make in a pure form. It tends to contain unacceptably large amounts of other compounds – titanium carbide (TiC) and titanium silicides (TixSiy). This makes it harder, less machinable and difficult to determine the properties of the pure material.

Ceramics like titanium silicon carbide are traditionally made by heating the components, either as elements or compounds, in the correct ratios. However, a large amount of heat is generated in the synthesis of titanium silicon carbide, and this heat of reaction can actually be exploited to make the material.This is called selfpropagating high-temperature synthesis (SHS). Once ignited by an ignition source (laser, electron beam, furnace or electric arc), the reaction becomes self-sustaining and converts the reactants to the product very rapidly – in less than 100 seconds.

Reactions caught in the act

We have been studying SHS reactions in the Ti-Si-C system so as to understand better how to make the pure layered material. Until recently, the extreme speed of the reactions had prevented us from obtaining a clear understanding of the reaction mechanism. However, using the powder-diffraction instrument – D20 – at ILL,we have been able to follow the detailed structural changes as the reaction proceeded.

We first heated cold-pressed pellets of the reactants (titanium, carbon and silicon carbide) in a furnace at a rate of 100°C per minute through the critical range of 800 to 1000°C – to initiate the reaction – and simultaneously recorded diffraction patterns every 0.9 seconds, as shown below.

The SHS reaction proceeded in five stages.Up to 880°C, the reactants simply heat up.Then, there is a change in the crystal structure of the titanium, which goes on to react with the free carbon in the sample. It is this reaction that gives off the heat needed to start ignition.The fourth stage is the formation of a single solid intermediate phase (in less than 0.9 seconds) by the true SHS reaction. It is stable for only about 6 seconds.The reaction temperature was estimated at 2200°C.We believe that the intermediate phase is a solid solution of silicon in titanium carbide which is relatively stable at the combustion temperature but becomes unstable as the temperature drops.The final stage is the rapid development and growth of the product phase, titanium silicon carbide.We are now working on measuring the rates of the different reaction steps.

"Neutrons explore how to make high-performance
materials that behave both like ceramics and metals"
Erich Kisi

A 3-D plot of a portion of the
diffraction patterns during SHS of Ti3SiC2
as the reaction progresses (left to right)

Application of Neutron Diffraction in everyday life

The benefits of stress

Every mechanical component is stressed to some extent, even when it is not experiencing a load. This is due to various stages in its manufacture like rolling or machining, heat treatment or welding.These residual stresses remain inside the material, influencing its characteristics, and thus the performance and lifetime of the component.

For ever-more reliable hi-tech products – especially in applications where safety is a priority, it is important to know the distribution and the size of stresses in each component.Tensile stresses can cause cracks to develop, while compressive stresses can prevent them starting. In fact, hardening surfaces by various surface treatments such as 'peening' (shot is fired at a surface so that it is plastically deformed) introduces beneficial compressive stresses.

Neutron strain imaging

Neutrons provide a unique tool for determining residual stresses deep inside matter.The principle is quite simple: compressive stresses reduce the spacing between atoms in the material while tensile stresses stretch them. Since neutron diffraction can measure atomic distances, it can probe the stresses in an object. This 'neutron strain imaging' technique does not destroy the material and can map stresses with a spatial resolution of a cubic millimetre, or even smaller. The high penetration power of neutrons allows measurements to be carried out to several centimetres deep in steel.

ILL's neutron strain imaging instrument is used in many applications for examining alloys and composites, and various engineering components.The stresses in joints introduced by different welding techniques can also be characterised.We have studied the tungstencopper brazed components for a fusion reactor, as well as titanium tanks and new composite materials for satellites.

One everyday application, which illustrates how the neutron strain imaging technique helps to develop better products, is in strengthening the crankshafts of car engines. A crankshaft can oscillate when rotating, thus shortening its lifetime. One way of overcoming this problem is to use a process called deep rolling, in which three wheels are turned around the workpiece while applying force in the radial direction.This introduces compressive residual stresses into the material which strengthen the piece and may reduce the amplitude of oscillation by 200 times, provided that the force and number of turns in the process are optimised. Engineers perform calculations to find the optimum parameters, but only experiment can verify their exactness and the efficiency of the deep rolling procedure.The neutron strain imaging technique provides the determination of the stress distribution, which can be directly compared with calculations.

Because the potential of the technique is so high, ILL is constructing a new strain imager with improved characteristics and an extended range of applications. It will enable measurements to be carried out on specimens from 1 centimetre up to 2 metres in length and weighing more than 500 kilograms.

"A unique neutron technique for mapping residual stresses in engineering components can help extend their life". Thilo Pirling

Thilo Pirling aligning the neutron strain imaging instrument with a cog as a sample

The  picture  below  shows the result of a neutron   strainimaging  measurement ± 1.5 m rolled region and down to a depth of 4 millimetres into the material. Compressive stresses are marked in blue and it shows that the deep rolling process modifies the stress field of the workpiece down to a depth of about 2 millimetres.


Abel A. Colmenares E.

Application of Neutron Diffraction in everyday life

Metal hydrides hold the key to green energy

"Hydrogen could be an ideal ecological fuel once researchers have found the right material in wich to store it". Klaus Yvon

For several decades, people have been exploring alternative fuels to petroleum that could be used to provide power. Hydrogen is one possibility. It is environmentally friendly, burning in an internal combustion engine to produce only water. It is also possible to extract hydrogen from water (H2O) with electricity made, say, with solar energy.

Hydrogen has one big snag, however. As a gas, it takes up a large volume and is very explosive, so is difficult to store and handle. Fortunately, there are a range of metal alloys (based on copper, manganese and titanium, for example) that can soak up huge amounts of hydrogen gas, which is then released on gentle heating.They provide a safe, effective way of storing and transporting hydrogen.

The hydrogen-storage materials tested so far are expensive and rather heavy, so our research team in Geneva is investigating alternatives which are lighter and can absorb even more hydrogen.These are the so-called complex metal hydrides which may contain up to four times as much hydrogen as the conventional hydrogen-absorbing alloys. In fact, two of our compounds, the barium-rhenium and magnesium-iron complexes shown below, hold the world record for hydrogen absorption. However, the complex hydrides studied so far have disadvantages too.They do not release their hydrogen easily but require heating to 300°C, which is commercially uneconomic; the bariumrhenium compound does release its hydrogen at just above room temperature but is too heavy and expensive.

We haven't yet found the ideal storage material, but neutron experiments play an important part in our search by helping us to characterise the compounds we make and predict more effective ones.

High resolution at ILL

Neutron diffraction is ideal for investigating the crystal structures of metal compounds containing light elements like hydrogen which scatter neutrons strongly but are almost invisible to X-rays. Nevertheless, our polycrystalline complex hydrides, which are analysed using ILL's powder diffraction instruments, present a formidable challenge.The smallest repeating unit of each crystal (unit cell) contains many atoms, resulting in very crowded, overlapping diffraction patterns.The ILL neutron source is the only facility in the world that provides an intense-enough neutron beam to resolve these patterns and pinpoint the location of the atoms.

On the practical side, several car companies have been experimenting with hydrogen-fuelled vehicles employing metal hydrides. In Switzerland,we have been demonstrating the technology at home! I have been using a hydrogen-powered lawnmower for 10 years, and an architect,Markus Friedli, has equipped his house in central Switzerland with solar panels to generate the electricity for recovering hydrogen from water.This is then used to power the family van.

So far, none of the applications of hydrogen gas storage materials has been commercialised.This is partly because we are still searching for that 'miracle material'. However,we have barely started to look at all possible combinations of elements, and neutron experiments will continue to make a significant contribution.

The  neutron  diffraction  patterns  and  structures  for  (A)  magnesium  iron  hydride  (Mg2FeH6)  and  (B)  barium rhenium hydride (BaReH9). Note that the magnesium-iron compound contains  twice the density  of  hydrogen atoms (150 grams per itre) compared with that in liquid hydrogen (70 grams per litre). The barium-rhenium compound contains 4.5 hydrogen atoms per metal atom, which is greater than the ratio of hydrogen to carbon in natural gas, methane (CH4).


Abel A. Colmenares E.

Application of Neutron Diffraction in everyday life

Marvellous Zeolites

Zeolites are remarkable minerals with a crystal structure consisting of a porous aluminosilicate framework which creates a system of linked channels and cavities. Atoms, ions (charged atoms) and molecules can enter the framework, and this property also makes zeolites extremely useful.

In naturally-occurring zeolites, the cavities are filled with water and possibly lightweight metal ions such as those of sodium, potassium or calcium, which can be exchanged for other ions. One of the first applications of zeolites was as water softeners to remove calcium ions from hard water.The minerals' absorptive and ionexchange habilities are, in fact, the basis for many washing powders.

Zeolites, however, have many other important uses, particularly in the chemical industry. Zeolites with straight channels less than a nanometre across are widely used as 'molecular sieves' to separate molecules of different sizes and shapes such as similar hydrocarbons but with either a straight-chain or branched structure.

Perhaps the most exciting applications are as catalysts, bringing about specific and selected chemical reactions.They are used in the petroleum industry to break down or 'crack' heavy, oily hydrocarbons into lightweight products like gasoline. Zeolites are considered to be much more environmentally friendly than other traditional catalysts.They are also increasingly being used to make various organic chemicals. Ionic exchange can introduce catalytically active metal ions like those of copper or the rare earths, which are then constrained by the geometry of the zeolite framework to carry out only specific reactions giving compounds in high yields.

The new synthetics

Natural zeolites are not adequate for that many applications. A large number of synthetic zeolites have therefore emerged on the market, the most common being zeolites X and Y (with the structure of the mineral faujasite) and ZSM-5. A great deal of effort has also gone into fabricating new materials with different pore systems, like the mesoporous MCM-systems developed by the American company Mobil.

Establishing how and where molecules are absorbed in the zeolite structure is the basis for understanding the behaviour of such systems and their applications.Neutrons are particularly suited to studying both the structure and the dynamics of water or organic molecules in the voids and channels of zeolites.We can incorporate different 'probe' molecules and study them. For example,we used neutron diffraction to determine the location of an organic molecule 7,7,8,8-tetracyanoquinodimethane in the supercage of a zeolite Y, shown opposite.You can see how the molecule is attached to the zeolite framework.

The sensitivity of neutrons for hydrogen and other light atoms allowed us to locate the organic molecule. The results of structure determination is supported by molecular dynamics calculations.The dynamics of these 'guest' molecules can be followed by inelastic incoherent scattering, therefore complementing nuclear magnetic resonance experiments, which are on a different time-scale. Neutron scattering thus provides not only basic knowledge but also supports important industrial processes.

Abel A. Colmenares E.

Application of Neutron Diffraction in everyday life

Towards a better Battery

Materials for storing electrical energy have been the subject of intense research in recent years, as a result of the rapid development of portable electronic equipment such as mobile phones and laptop computers.These devices depend for their operation on rechargeable batteries, which work by converting chemical energy stored in electrodes, into electrical energy, via electrochemical reactions.These reactions correspond to the loss or gain of electrons and ions in the electrode materials.The electrical current is carried by electrons outside the battery and by ions (charged atoms) in the electrolyte between the electrodes inside the battery.The electrochemical reactions are reversible, so when all the energy has been delivered the battery can be charged up again from an external electrical source.
There are two types of rechargeable batteries currently used in portable devices: nickel-metal hydride (Ni-MH) batteries, which involve hydrogen ions, and are replacing the kind based on nickel and toxic cadmium; and lithium-ion batteries.
These batteries still have shortcomings, however, and are continually being improved in terms of performance – power capacity and output, and lifetime (the number of charge-discharge cycles the battery goes through without degrading). One of the keys to making improvements is a detailed understanding of the crystal structure of the electrodes and how the ions diffuse in it.Here, neutron powder diffraction (see p.4) is a powerful tool and gives more information than X-ray diffraction. Neutrons can easily penetrate the sample so as to give accurate information about the bulk material, and the hydrogen and lithium ions trapped in the electrodes scatter neutrons well so they can be readily located.
Studies in real time

We have been carrying out extensive studies on nickellanthanum hydrides used as electrode materials in Ni-MH batteries.The continual charge-discharge of the standard electrode involves a cyclic transformation between two crystalline phases called αand β.These have unit (cell)volumes that differ by 20 per cent, which induces heavy constraints in the material and causes its crystal structure to fragment.That leads to surface corrosion and reduces the battery lifetime.

We carried out experiments using the D1B diffractometer at ILL, which followed the structural changes during cycling on different electrode materials.These experiments showed that a transitory intermediate γphase with a cell volume between that of the αand βphases may appear (see left) for some peculiar alloy compositions.The appearance of this intermediate γ phase at the boundary between the α and βphases significantly reduces the constraints during the cycling process, leading to a better lifetime.

Another problem is related to the high charge/discharge rates required by many applications of these batteries.Using a newly designed electrochemical cell (above), with an electrode arrangement similar to that in real batteries – in conjunction with the high neutron flux available on the D20 diffractometer – we could also follow changes in the electrode at high cycle rates. We showed that the cycle rate was limited by the speed of associated transitions in the electrode’s crystal form, rather than the rate of diffusion of hydrogen ions in the electrode material.

The next challenge will be similar studies of lithiumion battery electrode materials.
Abel A. Colmenares E.