#neutron diffraction

Revealed: High-Entropy Alloy Multi-Stage Deformation Process at Ultra-Low Temperatures

Pave the way to design new structural materials with superior mechanical properties.

An international research team led by scientists from City University of Hong Kong (CityU) has recently discovered that high-entropy alloys (HEAs) exhibit exceptional mechanical properties at ultra-low temperatures due to the coexistence of multiple deformation mechanisms. Their discovery may hold the key to design new structural materials for applications at low temperatures.

Professor Wang Xunli, a newly elected Fellow of the Neutron Scattering Society of America, Chair Professor and Head of Department of Physics at CityU, joined hands with scientists from Japan and mainland China in conducting this challenging study on HEAs’ deformation behaviors at ultra-low temperatures. Their research findings were published in the latest issue of the scientific journal Science Advances, titled “Cooperative deformation in high-entropy alloys at ultralow temperatures.”

Molecular structure predicted by early Nobel Laureate found after a century

In the journal Nature published overnight, researchers from Imperial College, London have reported a transition metal complex with a geometric arrangement of atoms that was predicted in 1893 by the 1913 Nobel Prize recipient.

This important development in inorganic chemistry demonstrates the existence of hexagonal planar geometry in a transition metal complex with potential significance for catalysis, synthesis, materials science, photophysics and bioinorganic chemistry.

"Six coordinate complexes are ubiquitous in coordination chemistry. Remarkably, Alfred Werner in his seminal research in the 19th and early 20th century, using only observable properties, established the existence of various possible isomers for six coordination and derived three proposed geometries," said ANSTO co-author Dr. Alison Edwards.

"These were trigonal prismatic, in which the substituents lie on two parallel triangles either side of the metal atom, octahedral where the parallel triangles don't overlap (the dominant geometry) and a third possibility identified was hexagonal planar. These late 18th century observations predated X-ray diffraction studies by 20 years and Werner received the Nobel prize for his studies the year after Laue received the Nobel Prize for his observation of the diffraction of X-rays by crystals."

Taking the stress out of residual stress mapping

Researchers from the University of Virginia (UVA) are using neutrons to explore fundamental work in residual stress mapping that promises more precise science down the road for Oak Ridge National Laboratory (ORNL) and similar facilities around the world.

Led by Sean Agnew, the group aims to obtain more accurate reflections of concentrated stress levels within a material using neutron diffraction. Residual stresses are stresses that remain within a solid material even after the original cause of the stress is removed. These kinds of stresses can occur through a variety of mechanisms, such as inelastic deformations, temperature gradients, or structural changes.

Using the Neutron Residual Stress Mapping Facility instrument at ORNL's High Flux Isotope Reactor, HFIR beamline HB-2B, researchers are able to study residual stresses in steel, aluminum, superalloys, and other structural materials. The team's research will provide insight into the accuracy of residual stress mapping measurements in such materials when the neutron beam must travel large distances through the sample. Team members include UVA's Robert Klein, Matthew Steiner (now with University of Cincinnati), and John Einhorn (now with National Grid).

"What we're interested in, with residual stress mapping, is obtaining the most accurate measurements possible," said Steiner. "So we have a very small neutron beam that we locate inside the sample, then we map the changes in the lattice spacing which correspond to stress in the material."

Nuclear techniques unlock the structure of a rare type of superconducting intermetallic alloy

Nuclear techniques have played an important role in determining the crystal structure of a rare type of intermetallic alloy that exhibits superconductivity.

The research, which was recently published in the Accounts of Chemical Research, was a undertaking led by researchers from the Max Planck Institute for Chemical Physics of Solids, with the collaboration of the Ivan-Franko National University of Lviv, the Technical University Freiberg, the Helmholtz-Zentrum Dresden-Rossendorf, and ANSTO.

Complex metallic alloys (CMAs) have the potential to act as catalysts and serve as materials for devices that covert heat into energy (thermoelectric generators) or use magnetic refrigeration to improve the energy efficiency of cooling and temperature control systems.

Thermoelectric generators are used for low power remote applications or where bulkier but more efficient heat engines would not be possible.

The unique properties of CMAs stem from their intricate superstructure, with each repeating unit cell comprising hundreds or thousands of atoms.

Neutron diffraction experiments of materials with structures comprising multiple metal elements

Materials containing multiple metal elements are important for various applications as the combination of different metal cations provides new or enhanced properties, which cannot be obtained through the use of just one metal. A recent study involving neutron diffraction experiments has enabled the development of a new general strategy to produce complex materials with metal cation arrangements that can be virtually controlled on demand for desired applications; a result that will be of great importance in various fields.

The preparation of complex materials with structures made up of multiple metal cations occupying specific sites is a challenging task, as it involves simultaneously addressing the incorporation of different elements at exact positions. However, these multimetal materials are important in various different fields, as the combination of metal cations provides new or enhanced properties; something that cannot be achieved through the use of just one metal.

A frequent application of mixed-metal oxides and salts is being used as anode materials in batteries, due to superconductivity demonstrated by several multimetal families whose structures are composed of several cations combined. Other applications include doped metal oxides being used in optical devices and mixed-metal oxides being catalysts in key chemical transformation. However, the use of multimetal materials for these applications does not come with ease; synthesising new materials with structures where the disposition of the metal elements is highly controllable still remains a challenge. In fact, control over the arrangement of the elements in most existing multimetal materials has been limited or even inexistent to date. Moreover, there are limitations concerning the amount and nature of the elements that can be combined within one structure.

Neutron diffraction studies reveal origins of deterioration in lithium batteries

Researchers use diffraction data from pulsed high-intensity neutrons to understand the reactions that deteriorate lithium batteries during operation. The findings are published in Scientific Reports, 30 June 2016.

Despite the commercialisation of rechargeable lithium batteries in 1991, improvements to the reliability, safety, and long-term stability are still needed for their use in a wider range of large-scale applications. Now a collaboration of researchers in Japan have developed a technique they can use while the battery is in operation, which may shed light on some of the reactions that occur in these systems, providing insights for better battery design.

While techniques exist to monitor reaction dynamics, so far they have been difficult to apply on commercial batteries because of the stainless steel cans around them and their size. Ryoji Kanno worked with researchers at Tokyo Institute of Technology, High Energy Accelerator Research Organization (KEK), Sokendai (the Graduate University for Advanced Studies), and Kyoto University. They developed an in operando diffraction technique that uses pulsed high-intensity neutrons and a high-resolution high-intensity time-of-flight diffractometer to monitor the processes in the battery while it is in action.

Problem #1:

The kinetic energy of neutrons emerging in thermal equilibrium from a reactor is given by 3/2 kT where k = Boltzmann’s constant, 1.38 × 10^-23 J/K and T is the Kelvin temperature. Given that the (rest) mass of a neutron = 1.67x10^-27 kg and Planck’s constant h = 6.63 × 10^-34 Js, estimate the wavelength of neutrons in thermal equilibrium at 100 °C.

Solution: To answer this problem, we have to consider first that our concern particle is neutron, meaning that this problem talks about neutron diffraction and not x-ray diffraction.

The quantity that we have to find is the wavelength. We knew that there is an existing relationship between the wavelength of a particle and its velocity. If a stream of particles can behave like wave motion, it must have a wavelength associated with it. The theory of wave mechanics indicates that this wavelength is given by the ratio of Planck's constant h to the momentum of the particle. This wavelength is also known as the de Broglie wavelength and is given by (1)

               λ = h/mv                    (1)

where h is the Planck's constant (for which value was given on the problem above), m is the rest mass of the particle, and v is the velocity of the particle.

Now, what we don't know is the velocity of the particle. Yet, given its kinetic energy, we can calculate its velocity using formula (2)

              KE = 1/2 (mv^2)         (2)

where m is the rest mass of the particle and v is its velocity.

Combining the two equations, we get the relationship of the wavelength and the kinetic energy, and is given by (3)

              λ = h/sqrt(2mKE)        (3)

where h is the Planck's constant, m is the particle mass, and KE is the kinetic energy.