NIST Tech Beat - January 27, 2009

Your refrigerator’s humming, electricity-guzzling cooling system could soon be a lot smaller, quieter and more economical thanks to an exotic metal alloy discovered by an international collaboration working at the National Institute of Standards and Technology (NIST)’s Center for Neutron Research (NCNR).*

The alloy may prove to be a long-sought material that will permit magnetic cooling instead of the gas-compression systems used for home refrigeration and air conditioning. The magnetic cooling technique, though used for decades in science and industry, has yet to find application in the home because of technical and environmental hurdles—but the NIST collaboration may have overcome them.

Magnetic cooling relies on materials called magnetocalorics, which heat up when exposed to a powerful magnetic field. After they cool off by radiating this heat away, the magnetic field is removed, and their temperature drops again, this time dramatically. The effect can be used in a classic refrigeration cycle, and scientists have attained temperatures of nearly absolute zero this way. Two factors have kept magnetic cooling out of the consumer market: most magnetocalorics that function at close to room temperature require both the prohibitively expensive rare metal gadolinium and arsenic, a deadly toxin.

But conventional gas-compression refrigerators have their own drawbacks. They commonly use hydrofluorocarbons (HFCs), greenhouse gases that can contribute to climate change if they escape into the atmosphere. In addition, it is becoming increasingly difficult to improve traditional refrigeration. “The efficiency of the gas cycle has pretty much maxed out,” said Jeff Lynn of NCNR. “The idea is to replace that cycle with something else.”

The alloy the team has found—a mixture of manganese, iron, phosphorus and germanium—is not merely the first near-room-temperature magnetocaloric to contain neither gadolinium nor arsenic—rendering it both safer and cheaper—but also it has such strong magnetocaloric properties that a system based on it could rival gas compression in efficiency.

Working alongside (and inspired by) visiting scientists from the Beijing University of Technology, the team used NIST’s neutron diffraction equipment to analyze the novel alloy. They found that when exposed to a magnetic field, the newfound material’s crystal structure completely changes, which explains its exceptional performance.

“Understanding how to fine-tune this change in crystal structure may allow us to get our alloy’s efficiency even higher,” says NIST crystallographer Qing Huang. “We are still playing with the composition, and if we can get it to magnetize uniformly, we may be able to further improve the efficiency.”

Members of the collaboration include scientists from NIST, Beijing University of Technology, Princeton University and McGill University. Funding for the project was provided by NIST.

* D. Liu, M. Yue, J. Zhang, T.M. McQueen, J.W. Lynn, X. Wang, Y. Chen, J. Li, R.J. Cava, X. Liu, Z. Altounian and Q. Huang. Origin and tuning of the magnetocaloric effect for the magnetic refrigerant MnFe(P_1-xGe_x). Physical Review B. Vol. 79, 014435 (2009).

Researchers at the National Institute of Standards and Technology (NIST) have discovered that a carefully built magnetic sandwich that interleaves layers of a magnetic alloy with a few nanometers of silver “spacer” has dramatically enhanced sensitivity—a 400-fold improvement in some cases. This material could lead to greatly improved magnetic sensors for a wide range of applications from weapons detection and non-destructive testing to medical devices and high-performance data storage.

Those applications and many others are based on thin films of magnetic materials in which the direction of magnetization can be switched from one orientation to another. An important characteristic of a magnetic film is its saturation field, the magnitude of the applied magnetic field that completely magnetizes the film in the same direction as the applied field—the smaller the saturation field, the more sensitive the device.

The saturation field is often determined by the amount of stress in the film—atoms under stress due to the pull of bonds with neighboring atoms are more resistant to changing their magnetic orientation. Metallic films develop not as a single monolithic crystal, like diamonds, but rather as a random mosaic of microscopic crystals called grains. Atoms on the boundaries between two different grains tend to be more stressed, so films with a lot of fine grains tend to have more internal stress than coarser grained films. Film stress also increases as the film is made thicker, which is unfortunate because thick films are often required for high magnetization applications.

The NIST research team discovered that magnetic film stress could be lowered dramatically by periodically adding a layer of a metal, having a different crystal structure or lattice spacing, in between the magnetic layers. Although the mechanism isn’t completely understood, according to lead author William Egelhoff Jr., the intervening layers disrupt the magnetic film growth and induce the creation of new grains that grow to be larger than they do in the monolithic films. The researchers prepared multilayer films with layers of a nickel-iron-copper-molybdenum magnetic alloy each 100 nanometers (nm) thick, interleaved with 5-nm layers of silver. The structure reduced the tensile stress (over a monolithic film of equivalent thickness) by a factor of 200 and lowered the saturation field by a factor of 400.

The work has particular application in the design of “flux concentrators,” magnetic structures that draw in external magnetic field lines and concentrate them in a small region. Flux concentrators are used to amplify fields in compact magnetic sensors used for a wide variety of applications.

* W.F. Egelhoff, Jr., J. Bonevich, P. Pong, C.R. Beauchamp, G.R. Stafford, J. Unguris, and R.D. McMichael. 400-fold reduction in saturation field by interlayering. J. Appl. Phys. 105, 013921 (2009). Published online Jan. 13, 2009. DOI:10.1063/1.3058673

Engineers at the National Institute of Standards and Technology (NIST) are patenting a method that is expected to double the service life of concrete. The key, according to a new paper*, is a nano-sized additive that slows down penetration of chloride and sulfate ions from road salt, sea water and soils into the concrete. A reduction in ion transport translates to reductions in both maintenance costs and the catastrophic failure of concrete structures. The new technology could save billions of dollars and many lives.

Concrete has been around since the Romans, and it is time for a makeover. The nation’s infrastructure uses concrete for millions of miles of roadways and 600,000 bridges, many of which are in disrepair. In 2007, 25 percent of U.S. bridges were rated as structurally deficient or functionally obsolete, according to the Federal Highway Administration. Damaged infrastructure also directly affects large numbers of Americans’ own budgets. The American Society of Civil Engineers estimates that Americans spend $54 billion each year to repair damages caused by poor road conditions.

Infiltrating chloride and sulfate ions cause internal structural damage over time that leads to cracks and weakens the concrete.

Past attempts to improve the lifetime of concrete have focused on producing denser, less porous concretes, but unfortunately these formulations have a greater tendency to crack. NIST engineers took a different approach, setting out to double the material’s lifetime with a project called viscosity enhancers reducing diffusion in concrete technology (VERDICT). Rather than change the size and density of the pores in concrete, they reasoned, it would be better to change the viscosity of the solution in the concrete at the microscale to reduce the speed at which chlorides and sulfates enter the concrete. “Swimming through a pool of honey takes longer than making it through a pool of water,” engineer Dale Bentz says.

They were inspired by additives the food processing industry uses to thicken food and even tested out a popular additive called xanthum gum that thickens salad dressings and sauces and gives ice cream its texture.

Studying a variety of additives, engineers determined that the size of the additive’s molecule was critical to serving as a diffusion barrier. Larger molecules such as cellulose ether and xanthum gum increased viscosity, but did not cut diffusion rates. Smaller molecules—less than 100 nanometers—slowed ion diffusion. Bentz explains, “When additive molecules are large but present in a low concentration, it is easy for the chloride ions to go around them, but when you have a higher concentration of smaller molecules increasing the solution viscosity, it is more effective in impeding diffusion of the ions.”

The NIST researchers have demonstrated that the additives can be blended directly into the concrete with current chemical admixtures, but that even better performance is achieved when the additives are mixed into the concrete by saturating absorbant, lightweight sand. Research continues on other materials as engineers seek to improve this finding by reducing the concentration and cost of the additive necessary to double the concrete's service life.

A non-provisional patent application was filed in September, and the technology is now available for licensing from the U.S. government; the NIST Office of Technology Partnerships can be contacted for further details (Contact: Terry Lynch, terry.lynch@nist.gov, (301) 975-2691).

* D.P. Bentz, M.A. Peltz, K.A. Snyder and J.M. Davis. VERDICT: Viscosity Enhancers Reducing Diffusion in Concrete Technology. Concrete International. 31 (1), 31-36, January 2009.

The National Institute of Standards and Technology (NIST) has issued a new standard—a certified reference material—to aid in the detection and measurement of the potent carcinogen hexavalent chromium in soil. Developed in collaboration with the Environmental Protection Agency (EPA) and the New Jersey Department of Environmental Protection (NJDEP), the new reference material will provide a crucial benchmark for the high-quality chemical measurements needed to guide and assess cleanup efforts.

Chromium, familiar to most people as the shiny, highly polished metal known simply as chrome, is found in products ranging from automobiles to stainless steel. Chromium can occur in a number of different chemical forms including trivalent chromium, a micronutrient, and hexavalent chromium. The processing of chromium ores can extract portions of these species for use in other industrial processes, as an anti-corrosion agent, and in paints, dyes and leather tanning. Reprocessed waste, consisting of a mix of trivalent and hexavalent chromium containing compounds, was dumped in sites around the country during the 1940s and 1950s. In New Jersey alone, authorities have identified more than 160 sites in need of cleanup.

However, cleanup efforts are complicated by the fact that interactions with components in the soil such as organic carbon and iron, as well as and the acidity of the soil can cause either of these two forms of chromium to convert to the other. This complicates laboratory measurements and makes it difficult to accurately quantify the original concentration of each species in the soil. The uncertainty hampers regulatory agencies in making decisions about which sites need to be cleaned up—if a site is misidentified as contaminated, expensive remediation efforts will be wasted, and if a site is misidentified as uncontaminated, the health of local residents could be at risk.

To aid in reducing the uncertainty surrounding these important measurements, NIST scientists have prepared and assigned certified values for Standard Reference Material (SRM) 2701, Hexavalent Chromium in Contaminated Soil, High Level (https://www-s.nist.gov/srmors/view_detail.cfm?srm=2701). The source material for the new SRM was collected from a waste site in Hudson County, N.J. After milling, blending, and sterilizing, scientists analyzed the samples for hexavalent chromium using several EPA-approved methods designed to minimize and compensate for the chameleon-like chemical properties of chromium.

The contaminated soil SRM has been assigned certified values for hexavalent chromium, total chromium, iron and manganese as well as reference values for other elements of environmental interest including aluminum, calcium, magnesium, nickel, silicon, sulfur, titanium and vanadium.

Standard Reference Materials are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes more than a thousand different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields. For more information, see NIST’s SRM Web page at http://ts.nist.gov/measurementservices/referencematerials.

Physicist Deborah S. Jin, a Fellow of the National Institute of Standards and Technology (NIST) and JILA, a joint institute of NIST and the University of Colorado at Boulder, will receive Sigma Xi's 2009 William Procter Prize for Scientific Achievement, the research society announced this week.

The Procter Prize, the society's highest honor, has been presented annually since 1950 to an outstanding scientist or engineer who is known for effective communication of complex ideas. Past recipients include prominent researchers in diverse fields, such as physics Nobel Laureate Murray Gell-Mann, animal behaviorist Jane Goodall, geologist Stephen Jay Gould, and oceanographer Robert Ballard.

In the citation, Sigma Xi noted that “Jin's technical innovations in the field of ultra cold Fermionic (atomic) gases have led to discoveries that define this new area of physics research. Her research has been described as the crucial first step in developing superconductors that work at room temperature, which could lead to faster computers and other advances. A MacArthur Fellow, Jin is one of only a handful of women physicists elected to the National Academy of Sciences.”

The Procter Prize and the research society's other top annual awards will be presented at Sigma Xi's annual meeting in the fall. Additional information is available at http://www.sigmaxi.org/about/news/2009awards.shtml.

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Editor: Michael Baum

Date created: January 27, 2009
Date updated: January 27, 2009
Contact: inquiries@nist.gov