These facilities at Oak Ridge are operated and managed by the Neutron Sciences Directorate, an organization with about 600 staff. Funding is provided by the US Department of Energy (DOE) Office of Basic Energy Sciences. This is by no means the only neutron scattering facility in the US or the world, but it is one of the leading centers.

Oak Ridge National Laboratory. Oak Ridge National Laboratory, originally known as Clinton Laboratories, was established in 1943 to carry out the pilot-scale production and separation of plutonium for the World War II Manhattan Project.

The lab, sited on 58 square miles of land, has grown into a multiprogram science and technology laboratory managed for the US Department of Energy (DOE) by UT-Battelle, LLC. It has an annual budget of around $1.4 billion and has around 4,600 total staff, of which 3,000 are scientists and engineers.

The lab hosts some 3,000 guest researchers each year, about one-fourth from industry. It also has about 30,000 visitors each year, plus 10,000 pre-college students. (One of the lab’s supported initiatives is the First robotics competition.)

ORNL’s current mission is to conduct basic and applied research and development to create scientific knowledge and technological solutions that strengthen the nation’s leadership in key areas of science; increase the availability of clean, abundant energy; restore and protect the environment; and contribute to national security.

ORNL operates nine user facilities—i.e., research facilities for scientists from universities, industry, and other laboratories, as well as to ORNL researchers:

• Building Technologies Research and Integration Center (BTRIC)
• Center for Nanophase Materials Sciences (CNMS)
• Center for Structural Molecular Biology (CSMB)
• High Temperature Materials Laboratory (HTML)
• National Center for Computational Sciences (NCCS)
• National Transportation Research Center (NTRC)
• Shared Research Equipment Collaborative Research Center (SRECRC)
• High Flux Isotope Reactor (HFIR)
• Spallation Neutron Source (SNS)

Neutron scattering was pioneered at ORNL in 1946 by Clifford G. Shull. Shull went on to be a co-recipient of the 1994 Nobel Prize in Physics for “the development of the neutron diffraction technique”. Shull’s co-recipient was Bertram N. Brockhouse “for the development of neutron spectroscopy”.

Neutron scattering. Neutrons (along with protons) are fundamental particles that constitute the nucleus of most atoms (hydrogen being the exception). Neutrons do not possess any electric charge; they are stable inside the nucleus, and outside of it, they decay with a lifetime of about 15 minutes (886 seconds)—a long time for a fundamental particle.

They can be released from the nucleus by fission as in a nuclear reactor (HFIR) or by firing a high-energy beam of protons into a target of a neutron-rich element (a process called spallation). In the case of ORNL’s SNS, the target is mercury. The neutrons are gathered, then beams are sent down beamlines to different specialized instruments. HFIR has 13 instruments currently available to users; SNS currently has 16, with 25 planned.

Neutrons are classified according to their kinetic energy. For neutron imaging, thermal (12 meV - 100 meV; 2.6 - 0.9 Å; 1515 - 4374 m/s) and cold (0.12 meV - 12 meV; 26.1 – 2.6 Å, 152 – 1515 m/s) neutrons are preferred due to their favorable detection reactions and due to their very useful contrast behavior, note the scientists at the Paul Scherrer Institute in Switzerland.

When a beam of neutrons is aimed at a sample, many neutrons will pass through the material, but some will interact directly with atomic nuclei and diffract away at an angle, or scatter. Using detectors in specialized instrument facilities, scientists can count scattered neutrons, measure their energies and the angles at which they scatter, and can map their final position (shown as a diffraction pattern of dots with varying intensities).

Neutron imaging of fuel injection performed at Institu Laue-Langevin, adapted from van Overberghe 2006 and presented by Toops. Neutron imaging is complimentary to current methods: laser-based methods not well suited for dense sprays and unable to penetrate metal; X-ray based methods can penetrate metal but require fuel doping and do not interact with vapor. Click to enlarge.

Neutron imaging has some similarities with x-ray imaging; both, for example, use Bragg’s Law (providing angles for coherent and incoherent scattering from a crystal lattice) to describe scattering.

However, neutrons are much more penetrating than x-rays. While x-ray scattering occurs close to the surface, neutrons can penetrate more deeply. Thus, neutrons can function as non-destructive probes of structures in solids.

Another major difference is that the intensity of x-ray scattering increases with the electron density of a material; light elements such as hydrogen and lithium thus make very little contribution to scattering. Neutrons, on the other hand, can obtain an appreciable scattering signal from light elements. (For some areas of neutron-assisted research, such as the investigation of advanced Li-ion batteries, this is crucial.)

As one result of these differences, most metals typically used for manufacturing purposes are readily penetrated by neutrons (surpassing even the maximum depth of high energy x-rays) while hydrogen atoms have a high probability of scattering neutrons out of the incident beam. This makes neutrons quite suited for the detection of hydrogenous material within engineering materials and structures, noted Oak Ridge researchers in a 1999 paper.

Beam time is granted to researchers via the general user program.

Schematic of the SNS. Click to enlarge.

The SNS. Spallation is a process in which fragments of materials (spall) are ejected from a body due to impact or stress. SNS provides the most intense, pulsed accelerator-based neutron beams in the world for scientific research and industrial development.

SNS produces neutrons with an accelerator-based system that delivers short (microsecond) proton pulses to a target system (mercury), where neutrons are produced at by the collision of high-energy protons with the Hg target: 695 ns pulse with 60 Hz rep. rate. Specs for the 1.4 MW protons beam: 1 GeV, 1.4 mA, 60 Hz rate.

HFIR. Operating at 85 MW, HFIR is the highest flux reactor-based source of neutrons for research in the United States, and provides one of the highest steady-state neutron fluxes of any research reactor. The thermal and cold neutrons produced by HFIR are used to study physics, chemistry, materials science, engineering, and biology.

Instruments. The instruments are the actual measuring devices in the neutron scattering facilities. HFIR and SNS offer researcher two complementary suites of neutron scattering instruments and beam lines. All the instruments are supported by a variety of sample environments and data analysis and visualization capabilities.

The instruments are custom-designed and built for specific types of science; a new instrument runs on the order of $10-$20 million. The SNS has plans for a second cluster of instruments in a new building to be built alongside the existing facility.

Just a few examples of transportation-related neutron-assisted research at ORNL include:

• Using neutron-computed tomography, researchers at the CG-1D neutron imaging instrument at HFIR successfully mapped the 3D spatial distribution of lithium products in electrochemically discharged lithium-air cathodes.

  Recent studies have found that the discharge reaction in Li-air batteries is strongly affected by electrolyte and solvent composition and is driven by complex reaction kinetics. Understanding the complex electrochemical and chemical decomposition of the electrolyte−solvent system and the resulting charge transfer kinetics at various current densities is critical.

  Neutron imaging research at HFIR revealed that there is a nonuniform distribution of the lithium products across the electrode thickness when they are discharged; the lithium concentration is higher near the edges of the lithium-air electrode and more uniform in the center. The origin of such anomalous behavior is the competition between the transport of lithium and oxygen and the accompanying electrochemical kinetics.

  Improved spatial resolution of the neutron imaging technique combined with isotopic substitution methods will further enable scientists to understand spatial distribution of discharge product in the lithium-air cathodes.

• Images of GDI injector. Toops (2013) Click to enlarge.

  ORNL researchers are using neutron imaging to provide fundamental insight into intra-nozzle fluid dynamics for improved simulation and design of GDI injectors. Potential benefits of the work include visualization of cavitation, liquid break-up mechanisms, and evaporation timescales.

• Dr. Hassina Bilheux, a physicist and the lead for developing ORNL’s neutron imaging capabilities (and gracious tour guide for GCC at the SNS facility), has used cold neutrons at HFIR's CG-1D beam line to image automobile engine system components. Projects have included producing two- and three-dimensional images of exhaust gas recirculation (EGR) coolers and images of diesel particulate filters (DPFs). In both cases, the goal is to improve fuel efficiency and, in the case of the DPF project, to consider the emissions and materials impacts of the introduction of biofuels.

  In the EGR coolers project, researchers measured coolers from 10 participating companies. Neutron imaging measures how the hydrocarbon (enriched particulate matter) is deposited within an EGR cooler that shows significant clogging. The role of the coolers is to lower the oxygen content and the combustion temperatures, thereby reducing the formation of NO_x in the cylinder. With the neutron imaging, the researchers can measure the thickness and hydrocarbon content of the deposit and come to understand the spatial and time dynamics of particulate matter deposition. In future measurements, tomography will be used to image complete, intact coolers in three dimensions.

  Ash deposits in section of SiC DPF. Source: Toops (2013) Click to enlarge.

  The DPF work began with measurements at different soot and ash loadings. Because the neutrons are able to penetrate the ceramic filter, they can take measurements of the hydrocarbon-rich particulates (soot) and metal-oxide–based ash. Although these materials are not necessarily highly sensitive to neutrons, they have a high surface area and are very hydroscopic (readily attracting moisture). The adsorbed water allows detection by neutrons.

  Neutron tomography is used to view and measure the thickness of the soot in the channels and the location of ash deposits. Ash, mostly from lubricant additives, affects engine efficiency by clogging the filter and increasing the backpressure on the engine and curtails filter life.

• A team used small-angle neutron scattering (SANS) to help more fully elucidate the process of ionic liquid pretreatment—especially the efficiency of reducing biomass to a liquid state (enzymatic hydrolysis).

  Pretreatment by ionic liquids typically results in a decrease in the crystallinity of cellulose. The native crystal structure, or cellulose I, is changed by pretreatment to cellulose II that is more readily digested. SANS was used to investigate the effect of the ionic liquid pretreatment on the surface roughness of the samples. Neutron scattering is nondestructive and more penetrating to most solid materials than x-ray scattering, which allows better characterization of the internal structure of dense, porous materials.

  X-ray diffraction showed changes in the structure of the crystalline domains on the length scale of angstroms (Å, i.e., tenths of a billionth of a meter), and SANS measured the surface roughness at 10 to 1000 Å.

  The neutron diffraction patterns show the pathway by which the transition in the crystal structure occurs.

• ORNL researchers have been working with General Motors, Dow Kokam and other battery manufacturers, as well as academic researchers to explore the inner workings of batteries to improve their performance. The VULCAN instrument enables researchers to observe the structural evolution of both cathodes and anodes over time.

  Initial experiments with GM, for example, examined battery cells for electric vehicles after hundreds of charge/discharge cycles. Neutrons mapped several points inside the molecular framework of the cells to see how internal stresses and irregularities were distributed. The cells were then charged and mapped again on VULCAN to show structural differences between the charged and discharged state for a degraded battery. They were also probed with neutrons during a series of charge/discharge cycles, allowing the GM team to "see" structural changes inside the batteries as they occurred.

  GM returned for a second set of experiments on a new EV battery, this time observing in real time the movement of lithium ions into the carbide structure of a battery as it charged and discharged. “This was the first time data have been collected in real time at such close intervals using neutron scattering,” noted Ke An, lead ORNL scientist for VULCAN. GM has subsequently returned for several more sets of work.

• Neutron imaging can also be used in conjunction with other advanced manufacturing techniques, such as additive manufacturing. In a recent paper in Advanced Materials & Processes, ORNL researchers and a colleague from NASA Langley illustrated how nondestructive examination of complex additive manufactured components using neutrons is a valuable technique for imaging and measuring residual stress.

  Neutron radiograph (left), volume rendered (center), and transverse slices at top, middle, and bottom (right) of turbine blade fabricated using additive manufacturing. Blade height is ~76 mm. Source: ORNL. Click to enlarge.

In their paper, the team noted that many additive manufacturing (AM) technologies use thermally driven phase change mechanisms to convert the feedstock into functioning material. As the molten material cools and solidifies, the component is then subjected to significant thermal gradients, generating significant internal stresses throughout the part.

As layers are added, they explained, inherent residual stresses cause warping and distortions that lead to geometrical differences between the final part and the original computer generated design. This can also limit the geometries that can be fabricated using AM, such as thin-walled, high-aspect-ratio, and overhanging structures.

The use of neutrons to characterize additive manufactured components is a unique, valuable technique for imaging and measuring residual stress. Neutron and x-ray techniques can be used in a complementary fashion, providing information about the bulk and surface of a component, respectively. Neutrons are useful to characterize complex components, such as those fabricated using additive manufacturing techniques. In this article, the penetrating power of neutrons facilitated unique characterization of aircraft parts fabricated using additive manufacturing, imaging internal structures and passageways and mapping residual strains in a complex turbine blade. Future projects combining these techniques will be required to fully use the potential of additive manufacturing.

—Watkins et al.

Resources

• Roger Pynn, Neutron Scattering Primer, Los Alamos Neutron Science Center

• Neutronsources.org

• Neutron imaging – past, present and future (NMI3: Integrated Infrastructure Initiative for Neutron Scattering and Muon Spectroscopy)

• Todd J. Toops et al. (2011) Neutron Imaging of Advanced Engine Technologies

• Todd J. Toops et al. (2013) Neutron Imaging of Advanced Engine Technologies

• Jagjit Nanda, Hassina Bilheux, Sophie Voisin, Gabriel M. Veith, Richard Archibald, Lakeisha Walker, Srikanth Allu, Nancy J. Dudney, and Sreekanth Pannala (2012) Anomalous Discharge Product Distribution in Lithium-Air Cathodes, J. Phys. Chem. C 116, 8401−8408 doi: 10.1021/jp3016003

• Cheng, G., Varanasi, P., Li, C., Liu, H., Melnichenko, Y. B., Kent, M., Simmons, B., and Singh, S. (2011) Understanding Cellulose: Controlling Crystal Structure and Saccharification Kinetics via Ionic Liquid Processing, Biomacromolecules 12 (4), 933–941 doi: 10.1021/bm101240z

• Thomas Watkins; Hassina Bilheux; Ke An; Andrew Payzant; Ryan Dehoff; Chad Duty; William Peter; Craig Blue; and Craig Brice. Neutron Characterization for Additive Manufacturing. Advanced Materials & Processes, Volume 171, Issue 3

• Dudley A. Raine lII, Camden R. Hubbard, Paul M. Whaley, and Michael C. Wright (1999) Designs for Neutron Radiography and Computed Tomography at Oak Ridge National Laboratory.