ar windshields that don't break during accidents and jets that fly longer without making a refueling stop. Compact discs, credit cards, and pocket calculators. Refrigerator magnets and automatic car window openers. Beach shoes, food packaging, and bulletproof vests made of tough plastics. The quality and range of consumer products have improved steadily since the 1970s. One of the reasons: neutron research.
The fruits of neutron research were publicly proclaimed in the mid-1980s by the Exxon Chemical Company in a two-page advertisement that appeared in science and trade magazines. The ad included an artist's impression of ORNL's George Wignall standing at a neutron beam line from the High Flux Isotope Reactor (HFIR). Exxon and other industrial firms had been invited by ORNL's Wally Koehler to use the new small-angle neutron scattering instrument at the HFIR for their research. Exxon Chemical Company recognized that the HFIR was an inexpensive source of valuable and unique information for its chemists who were developing materials from Exxon's main product petroleum.
Because neutrons, unlike X rays, are deflected by hydrogen and other light atoms, they are useful for determining the arrangements of polymer molecules. In particular, small-angle neutron scattering can reveal how long, threadlike polymer molecules pack to give plastics their diverse properties.
"Industry has been using neutrons from the HFIR and other reactors to verify that certain processes can produce polymer molecules that line up to make some plastics very strong," says John Hayter, scientific director for the Advanced Neutron Source Project at ORNL. "Strong plastic, for example, is used to make the familiar bags of peanuts on airlines that can be torn open only at the precut notch. Some reactors are preferable to the HFIR for these polymer studies because they produce very slow neutrons. These so-called 'cold' neutrons will be produced in the ANS."
The new polymers are much better than the breakable plastics in old toys. These synthetic polymers are tough lightweight materials used in cars and airplanes (e.g., the Boeing 757 jet), bulletproof vests for police officers, beach shoes, packaging for foods and snacks, synthetic threads for wash-and-wear clothes and textiles, and plastic sheets sandwiched between glass sheets to make nearly unbreakable windshields for cars and trucks.
Small-angle neutron scattering can also indicate if there are subtle movements of molecules between materials in a structure, suggesting that the structure is falling apart. For example, it can be used to determine if atoms are migrating between plastic sheets sandwiched together in a laminate.
According to Hayter, neutron scattering is in demand by industry for guiding the development of polymer composites and polymer alloys. "The chemical industry is no longer interested in synthesizing new polymers because of the liability problems of marketing a new material that might prove harmful to human health or the environment," he says. "The emphasis today is on making polymer alloys of known and approved chemicals now being manufactured. The ANS will be needed to guide the development of these materials in the United States."
A colloid is a collection of particles_each about the size of 100 to 1000 atoms_dispersed in a liquid or gas. Particles that size of interest today include viruses and constituents of electronic microchips.
"Twenty years ago," Hayter says, "neutron scientists were studying atoms, while industry was interested in structural detail 100,000 times larger. Today both have converged to study structures the size of 100 to 1000 atoms_the so-called colloidal size range.
"The large surface-to-volume ratio in colloids yields interesting properties," Hayter explains. "Flour, for example, becomes a powerful explosive when reduced to colloidal size in heated air. That's why we hear about occasional explosions in wheat silos."
Complex fluids, such as blood, are examples of colloidal systems. A key feature of such fluids, Hayter says, is self-assembly of molecules into higher structures, such as a micelle (see photograph). A micelle is a model for the submicroscopic structural unit of protoplasm, the important constituent of living cells that is built from polymer molecules.
Self-assembly of molecules into different structures is a phenomenon that intrigues Hayter. At his desk, he conducts a simple experiment to show that molecules have a memory. He pulls out a clear plastic, cylindrical container slightly smaller than a coffee mug. It contains a thick clear liquid called glycerin, which is held against the container wall by a cylinder in the center. Using an eye dropper, Hayter introduces a drop of glycerin colored blue into the clear liquid. He turns the cylinder around more than half way, and a blue ribbon of liquid snakes around the container wall. Then he turns the cylinder back to its original position. The ribbon disappears and a spot of blue slightly larger than the original drop remains. "The molecules remember their original positions as they reassemble themselves," Hayter notes.
Almost all structural information on micelles has come from neutron scattering," Hayter says. The information is obtained by replacing some of the hydrogen atoms in structures with deuterium atoms. Structural details are revealed because hydrogen atoms and deuterium atoms scatter neutrons differently.
Hayter is interested in the effects of applied shear on colloids. Examples of shear are rubbing your hands together to cover them with hand lotion and pushing a roller up a wall to spread paint. He has conducted neutron scattering experiments that show that, contrary to intuition, structures in colloidal fluids may take on even more crystalline order under modest applied shear. The colorful computer-generated results of one of his experiments on a colloidal crystal made the cover of the October 18, 1985, issue of Science magazine. Recently, he and his associates were the first to measure shear-induced ordering in a complex fluid flowing past a surface; their results were published in a paper in the April 4, 1994, issue of Physical Review Letters.
Neutron scattering is useful for understanding the effect of shear on oil, Hayter says. A lubricant is effective only if it can ooze between engine parts gliding past other components, such as a piston in a cylinder. Hayter says that some oils laced with a detergentlike chemical called a surfactant act both as a liquid and solid that can be stretched when shear is applied.
"Such fluids in which molecules self-assemble into larger structures can behave in complex ways," Hayter says. He shows a photograph of a slightly tilted bottle of a thick hydrocarbon oil containing 2% aluminum dilaurate, which is a surfactant. The liquid appears to be pouring easily with normal viscous flow. But a closer examination of the bottle tilt angle reveals that the flow is induced not by gravity but by "self-siphoning."
Neutron scattering can be used to detect structural changes that may be responsible for complex behavior under various conditions. In fact, Hayter says, in 1986 at the HFIR (a few months before it was shut down for more than 3 years), he proved to biochemists at the University of Michigan that neutron scattering could show a structural change in lipopolysaccharides, helping explain why these cells containing lipids and sugars behaved strangely at a certain pH (acidity level). Such information can be useful for both basic understanding, such as relating structural changes in living cells to odd behavior, as well as for practical applications, such as the development of improved lubricants for future engines.
To prevent such a nightmarish scenario, the Air Force examines its fighter aircraft using reactor neutrons and neutrons from robot-held portable sources containing californium-252, which is produced in the HFIR. These neutrons fly by aluminum atoms, but they are scattered by the hydrogen atoms present in water. In this way, neutrons detect and map the locations of moisture_signs of microscopic cracking and early corrosion. These results indicate the structural parts of aircraft wings that should be replaced.
Neutrons can reveal details about the magnetic structures of certain materials that cannot be obtained any other way. Such information has been vital to the creation of high-density recording media such as audiotapes, videotapes, and computer disks.
"Neutron scattering," Hayter says, "helps scientists determine the positions of atoms in material having a certain magnetism. This information helps industry develop and refine processes to manufacture materials with desired magnetic properties."
Using neutron scattering research at the University of Missouri, the Ford Motor Company has developed compact, lightweight magnets made of neodymium, iron, and boron. These small permanent magnets are used in compact motors in cars for automatically adjusting seats and opening windows.
Information on magnetic materials obtained by neutron scattering has made it easier to buy consumer products. It has enabled the addition of high-tech magnetic lines to plastic sheets to make the credit card.
Neutron scattering is also being used to determine the number and positions of oxygen atoms in high-temperature superconducting ceramics, such as yttrium-barium-copper oxides (YBCO), which are also magnetic materials. The number and positions of oxygen atoms are possible keys to high-temperature superconductivity and the ability of a material to carry useful amounts of electrical current. Neutron scattering produces the type of information necessary to understand these materials, including the relationships between their structures and properties. Once they are understood, it will be possible to predict which materials will make the best superconductors.
One of the unfortunate events in American science, Hayter notes, is that two of the best neutron sources in the United States--the HFIR and DOE's High Flux Beam Reactor at Brookhaven National Laboratory--were shut down in 1987. That's the year when high-temperature superconductivity studies were being conducted at a frenetic pace worldwide. Herb Mook of ORNL's Solid State Division, for example, had to perform his neutron research on superconducting materials at the Oak Ridge Electron Linear Accelerator (ORELA) and the reactor at NIST. Other American scientists simply could not find a reactor available for superconductivity studies, while scientists in Europe and Japan forged ahead in their research in this exciting new field.
Neutron scattering can measure residual stresses formed within materials during their manufacture or within welds during their formation. How? Stresses on a crystalline material cause strains in the material, or changes in the distances between the planes of atoms in a lattice. Compressive stresses cause these distances to shrink, and tensile stresses cause them to expand compared with lattice spacings in a stress-free material.
To measure these stresses, a sample is placed in a neutron beam so that the stress-free part of the material will scatter neutrons at 90_. If the separation between the atomic planes is smaller or larger than that in the stress-free part of the material, the neutrons will be scattered at slightly different angles. From these diffraction angles, the lattice spacings and, therefore, the strains are measured. This information can be used, for example, to determine if a weld will hold if operating conditions are changed or if a weld is stronger after heat treatment.
A reactor at the Chalk River Laboratory in Canada has been used to measure strain in welds in oil pipelines. A robot holds a piece of welded pipe in a neutron beam. The results of neutron scattering measurements are used to determine whether part of the pipeline should be replaced or heat-treated to prevent oil leaks to the environment.
In France, the Institut Laue-Langevin reactor is used to measure residual stress in rails for France's high-speed trains. At the HFIR in Oak Ridge, residual stresses were recently measured in a multipass weld for the first time. These reactors could also be used to measure stresses in oil well casings and ceramic-coated turbine blades.
"Strains in various materials subjected to various stresses are predicted by finite element analysis computer programs," Hayter says. "Neutron scattering measurements of sample materials determine whether the predictions of these programs are correct. If the predictions are proven wrong, then the computer programs are altered to reflect the true situation."
Recently at the HFIR and ORNL's High Temperature Materials Laboratory, a Neutron Residual Stress User Facility has been established. ORNL researchers are working with researchers from companies such as General Motors, General Electric, Cummins Engine, and Alcoa to develop and apply neutron residual stress mapping to industrial processes. For example, maps are being made of stresses in automobile gears and silicon carbide fibers in fiber-reinforced titanium alloys. Neutron measurements of residual stress can be used to improve manufacturing quality and uniformity.
Several years ago, a newly developed Rolls-Royce Gem jet helicopter kept seizing up when operated. The lubricating oil apparently was not flowing properly through the engine. So the British company turned to the Harwell Laboratory in England to locate the source of the oil flow problem. A crane was used to position the helicopter engine for real-time neutron radiography outside the containment at Harwell's DIDO reactor. As the engine was operated, it was irradiated with neutrons. The neutrons breezed past the metal atoms of the engine and interacted with the hydrogen atoms in the flowing oil. The emitted neutrons were captured by a television camera fitted with a scintillator, and images of the flowing oil were displayed on a TV as the engine ran. Viewers could see that the oil was not flowing correctly around a particular corner of a pipe. As a result of this neutron research, Rolls-Royce redesigned the helicopter engine and solved the lubricant flow dynamic problem.
Rolls-Royce also uses an accelerator source of neutrons to help the company improve the efficiency of its aircraft engines. By operating each newly developed engine in the presence of neutrons from this source, Rolls-Royce has measured the temperature, strain, and tip clearances of its spinning turbine blades. This information helps it redesign new engines to make them even more efficient.
Hayter has proposed that neutrons from ORELA be used to measure temperature, strain, and tip clearances of spinning turbine blades of operating jet engines made by U.S. aerospace companies. Neutron resonance studies can be conducted at ORELA by painting turbine blades with tantalum, which absorbs neutrons. Because neutron absorption is related to temperature and stretching of the material, it can provide information to help improve the thermodynamic efficiency of American-made jet engines. In this way, American jet engines could become as efficient or more efficient than Rolls-Royce engines, giving the United States the competitive edge in aircraft engine efficiency. Any competitive edge that advanced neutron research could give American industry may be the best fruit of all.--Carolyn Krause
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