Materials Science/Analytical Techniques

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Rutherford backscattering

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Rutherford backscattering (or RBS, for Rutherford Backscattering Spectrometry) is an analytical technique in materials science. It is named for Ernest Rutherford who in 1911 first explained Geiger and Marsden's experimental results for alpha particle scattering from a very thin gold foil in a backward direction by using the Coulomb electrostatic force between the positively charged nucleus and the positively charged alpha particle. Rutherford first correctly described the atom as a tiny positive nucleus surrounded by negatively charged electrons (essentially the Bohr atom) on the basis of this experiment. This contradicted J.J. Thomson's "plum pudding model," the popularly accepted model of the atom at that time. Rutherford famously expressed his surprise at this experiment: "It was as though one fired a bullet at a piece of paper, and it bounced back at you!"

A high energy beam ( 2 to 4 MeV ) of low mass ions ( e.g. He++ ) is directed at a sample. A detector is placed such that particles which scatter from the sample at close to a 180 degree angle will be collected. The energy of these ions will depend on their incident energy and on the mass of the sample atom which they hit, because the amount of energy transferred to the sample atom in the collision depends on the ratio of masses between the ion and the sample atom. Thus, measuring the energy of scattered ions indicates the chemical composition of the sample.

Additionally, in the case that the incident ion doesn't hit any of the atoms near the surface of the sample, but instead hits an atom deeper in, the incident ion loses energy gradually as it passes through the solid, and again as it leaves the solid. This means that RBS can be used as a means to perform a depth profile of the composition of a sample. This is especially useful in analysis of thin-film materials. For example, films about half a micrometre in thickness can be profiled using a 2 MeV He beam, or films up to about 10 micrometres thick can be profiled with a 2 MeV H beam.

RBS is now a very widely used analytical technique, which has the great advantage that it is absolute, requiring no standards for quantification (since the probability of interaction - the cross-section - is given by the Coulomb potential). It is one of a family of techniques, collectively known as Ion beam analysis.

Neutron diffraction

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Neutron diffraction is a crystallographic method for the determination of the atomic structure of a material. This is a form of elastic scattering where the neutrons exiting the experiment have more or less the same energy as the incident neutrons. The technique is similar to X-ray diffraction but the different type of radiation gives complementary information. A sample to be examined is placed in a beam of thermal or cold neutrons and the intensity pattern around the sample gives information of the structure of the material.

Neutrons are particles found in the atomic nucleus. In a nuclear reactor, neutrons can be set free when nuclei decay (fission, radioactivity). All quantum particles can exhibit wave phenomena we typically associate with light or sound. Diffraction is one of these phenomena; it occurs when waves encounter obstacles whose size is comparable with the wavelength. If the wavelength of a quantum particle is short enough, atoms or their nuclei can serve as diffraction obstacles. When neutrons from a reactor are slowed down and selected properly, their wavelength lies near one angstrom (0.1 nanometer), the typical separation between atoms in a solid material.

A neutron diffraction measurement requires a neutron source (e.g. a nuclear reactor or spallation source), a sample (the material to be studied), and a detector. At a research reactor other components such as crystal monochromators or filters may be needed to select the desired neutron wavelength. Some parts of the setup may also be movable. At a spallation source the time of flight technique is used to sort the energies of the incident neutrons, so no monochromator is needed, just a bunch of electronics. (Higher energy neutrons are faster - v. simple)

Neutrons interact with matter differently than x-rays. X-rays interact primarily with the electron cloud surrounding each atom. The contribution to the diffracted x-ray intensity is therefore larger for atoms with a large atomic number (Z) than it is for atoms with a small Z. On the other hand, neutrons interact directly with the nucleus of the atom, and the contribution to the diffracted intensity is different for each isotope; for example, regular hydrogen and deuterium contribute differently. It is also often the case that light (low Z) atoms contribute strongly to the diffracted intensity even in the presence of large Z atoms. Non-magnetic neutron diffraction is directly sensitive to the positions of the nuclei of the atoms. Although neutrons are uncharged, they carry a spin, and therefore interact with magnetic moments, including those arising from the electron cloud around an atom. Neutron diffraction can therefore reveal the microscopic magnetic structure of a material.

Neutron diffraction can be used to establish the structure of low atomic number materials like proteins and surfactants much more easily with lower flux than at a synchrotron radiation source. This is because some low atomic number materials have a higher cross section for neutron interaction than higher atomic weight materials.

The first neutron diffraction experiments were carried out in 1945 by Ernest O. Wollan using the Graphite Reactor at Oak Ridge. He was joined shortly thereafter by Clifford Shull, and together they established the basic principles of the technique, and applied it successfully to many different materials, addressing problems like the structure of ice and the microscopic arrangements of magnetic moments in materials. For this achievement Shull was awarded one half of the 1994 Nobel Prize in Physics. Wollan had passed away in the 1990's. (The other half of the 1994 Nobel Prize for Physics went to Bert Brockhouse for development of the inelastic scattering technique at the Chalk River facility of AECL. This also involved the invention of the triple axis spectrometer). Brockhouse and Shull jointly take the somewhat dubious distinction of the longest gap between the work being done (1945) and the Nobel Prize being awarded (1994).

One unusual application of elastic neutron scattering/diffraction is that the lattice constant of metals can be very accurately measured. Together with an accurately aligned micropositioner a map of the lattice constant through the metal can be derived. This can easily be converted to the stress field experienced by the material. This has been used to analyse stresses in aerospace and automotive components to give just two examples.

X-ray diffraction

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X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystallographic structure, chemical composition, and physical properties of materials and thin films. These techniques are based on observing the scattered intensity of an x-ray beam hitting a sample as a function of incident and scattered angle, polarization, and wavelength or energy.

Nuclear reaction analysis

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Nuclear reaction analysis (NRA) is a nuclear method in materials science to obtain concentration vs. depth distributions for certain target chemical elements in a solid thin film.

If irradiated with select projectile nuclei at kinetic energies Ekin these target elements can undergo a nuclear reaction under resonance conditions for a sharply defined resonance energy. The reaction product is usually a nucleus in an excited state which immediately decays, emitting ionizing radiation.

To obtain depth information the initial kinetic energy of the projectile nucleus (which has to exceed the resonance energy) and its stopping power (energy loss per distance travelled) in the sample has to be known. To contribute to the nuclear reaction the projectile nuclei have to slow down in the sample to reach the resonance energy. Thus each initial kinetic energy corresponds to a depth in the sample where the reaction occurs (the higher the energy, the deeper the reaction).

Analytical chemistry

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Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition, structure and function.

Analytical chemistry is a sub discipline of chemistry that has the broad mission of understanding the chemical nature of all matter. This differs from other sub disciplines of chemistry in that it is not intended to understand the physical basis for the observed chemistry as with physical chemistry and it is not intended to control or direct chemistry as is often the case in organic chemistry. Analytical chemistry generally does not attempt to use chemistry or understand its basis; however, these are common outgrowths of analytical chemistry research. Analytical chemistry has significant overlap with other branches of chemistry, especially those that are focused on a certain broad class of chemicals, such as organic chemistry, inorganic chemistry or biochemistry, as opposed to a particular way of understanding chemistry, such as theoretical chemistry. For example the field of bioanalytical chemistry is a growing area of analytical chemistry that addresses all analytical questions in biochemistry, (the chemistry of life). Analytical chemistry and experimental physical chemistry, however, have a unique relationship in that they are very unrelated in their mission but often share the most in common in the tools used in experiments.

Analytical chemistry is particularly concerned with the questions of "what chemicals are present, what are their characteristics and in what quantities are they present?" These questions are often involved in questions that are more dynamic such as what chemical reaction an enzyme catalyzes or how fast it does it, or even more dynamic such as what is the transition state of the reaction. Although analytical chemistry addresses these types of questions it stops after they are answered. The logical next steps of understanding what it means, how it fits into a larger system, how can this result be generalized into theory or how it can be used are not analytical chemistry. Since analytical chemistry is based on firm experimental evidence and limits itself to some fairly simple questions to the general public it is most closely associated with hard numbers such as how much lead is in drinking water.

Modern analytical chemistry is dominated by instrumental analysis. There are so many different types of instruments today that it can seem like a confusing array of acronyms rather than a unified field of study. Many analytical chemists focus on a single type of instrument. Academics tend to either focus on new applications and discoveries or on new methods of analysis. The discovery of a chemical present in blood that increases the risk of cancer would be a discovery that an analytical chemist might be involved in. An effort to develop a new method might involve the use of a tunable laser to increase the specificity and sensitivity of a spectrometric method. Many methods, once developed, are kept purposely static so that data can be compared over long periods of time. This is particularly true in industrial quality assurance (QA), forensic and environmental applications. Analytical chemistry plays an increasingly important role in the pharmaceutical industry where, aside from QA, it is used in discovery of new drug candidates and in clinical applications where understanding the interactions between the drug and the patient are critical.

Analytical methods rely on scrupulous attention to cleanliness, sample preparation, accuracy and precision.

Many practitioners will keep all their glassware in acid to prevent contamination, samples will be re-run many times over, and equipment will be washed in specially pure solvents.

A standard method for analysis of concentration involves the creation of a calibration curve.

If the concentration of element or compound in a sample is too high for the detection range of the technique, it can simply be diluted in a pure solvent. If the amount in the sample is below an instrument's range of measurement, the method of addition can be used. In this method a known quantity of the element or compound under study is added, and the difference between the concentration added, and the concentration observed is the amount actually in the sample.