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Jan. 17, 2005 -- Some of this week's stories: Glass gives up secrets under high pressureBy Evelyn Brown Glass is a mysterious material, but when researchers apply pressure, it reveals secrets. Using a variety of techniques, researchers at Argonne have seen, for the first time, the atomic structure of a dense, purely octahedral glass that has eluded scientists for decades. They also witnessed a continuous structural change in the glass, disproving the theory that tetrahedral glasses go through a distinct transition between low- and high-density phases. "Little is known about the structure of glass under pressure," said materials scientist Chris Benmore, "even though it is quite important. We put it in our cars and homes, and use it in many industrial applications, but how does the atomic structure react to extreme pressures?" Benmore is a researcher in Argonne's Intense Pulsed Neutron Source Division (IPNS). This division operates the IPNS, which provides neutrons for condensed matter physics, the study of atomic arrangements and motions in liquids and solids. IPNS is open to researchers from industry, academia and other national laboratories. Glass is difficult to study because it is disordered and lacks a periodic crystal structure. Also, it needs to be studied under pressure, because, as Benmore said, "the glass structure pings right back when the pressure is lessened." Researchers designed original pressure cell geometries for the research. Scientist Chris Tulk from Oak Ridge National Laboratory created novel pressure cells for both instruments in conjunction with other pressure-cell experts from the Carnegie Institution. "Silica is the most important and most widely used glass," said Benmore, "but we studied the softer germania (Ge) because it is a structural analog to silica and transforms to the octahedral form at much lower pressures than silica. Germania also provides a greater contrast in the neutron and the X-ray studies, so the details appear more clearly." At ambient pressure, Ge has tetrahedral network glass structure. Four oxygen atoms enclose a germanium atom and share corners to create cages that are only a nanometer across. Neutron revelations The researchers began their experiments at the IPNS. Neutrons reveal structural and dynamic properties of materials, and they are sensitive to lighter elements such as oxygen. Two piston anvils inside the IPNS's Glass, Liquid, and Amorphous Materials Diffractometer squeezed a 100-cubic millimeter sample of germania dioxide (GeO 2 ) to pressures up to five gigapascals, or 50,000 times atmospheric pressure. IPNS revealed the mechanism of how GeO 2 's tetrahedra collapse under pressure. Oxygen atoms were seen being squeezed into the sides of neighboring tetrahedra as the cages collapsed and the glass density increased. In contrast to the IPNS, the APS reveals germanium atoms more clearly and can test smaller samples, which allows studies at higher-pressures. As a one-cubic-millimeter GeO 2 sample was pushed from 60,000 to 100,000 times ambient pressure, researchers witnessed the tetrahedral cages collapsing and an average of five oxygen atoms organizing around the germanium atom before the octahedral glass was formed. This average coordination number of five still did not clearly resolve the question of whether this phase change in germania is continuous or discontinuous. Researchers thought they may have seen a gradual mixture of five- and then six-fold germanium atoms in the structure as the pressure increased, but the result was still unclear. So they called on their colleagues at the National Research Council of Canada to perform molecular dynamic simulations in which a computer calculates molecular structure and behavior from first principles. "The simulations agreed with our data and revealed a germanate anomaly that allows a distorted five-fold coordination of germanium to exist over a limited pressure range," Benmore explained. "This provided evidence that germania glass transforms continuously, which disagrees with the popular two-state model." As researchers pressurized a GeO 2 sample to 150,000 times ambient pressure, they witnessed a dense, disordered octahedral — eight-sided — structure inside glass for the first time. The angles of the internal structures were not the 90 and 180 degrees of a perfect octahedron; instead, the angles were near 90 and 165 degrees. "We'll continue to study this dense glass since it has never before been characterized," said Benmore. "It is a challenge because of the pressures needed. Also, some glass scientists thought the glass would immediately crystallize if it became octahedral." This research, which has appeared in Physical Review of Letters, Vol. 93, No. 11, was highlighted in the Editors' Choice section in the Oct. 1, 2004 issue of Science. Glass first studied in 1957By Evelyn Brown Argonne has a long-standing history of neutron research in amorphous materials. After the GeO 2 research was published, nuclear physicist Paul Persiani (NE) contacted Benmore to share some history. Persiani published an article about his neutron studies of the structure of vitreous silica (SiO 2 ) in Physical Review in 1957. Back then, Persiani was among the pioneers using neutron radiation to study material structure instead of X-rays. "Neutrons proved to be more direct and a much more effective research tool than X-rays," he said. "We used a slow-neutron velocity selector or `chopper' and CP-3 Prime, a heavy-water natural uranium research reactor, in our initial attempts to see how liquid and amorphous materials would scatter neutrons at room temperature." "Paul and his colleagues were doing work that had never been done with neutrons and their conclusions agreed with the standard glass model," Benmore said. "They determined the technique of neutron scattering to determine structure, and we use the technique day in and out. Nowadays we get a lot more data points than they did."
$424,271 donated during ACA's 2004 campaignDuring the 2004 Argonne Combined Appeal (ACA) campaign, employees and retirees contributed $424,271 to the ACA's 20 health and welfare agencies. This is $2,200 more than was pledged in 2003. "This year's goal was to increase the number of participants to 2,004," said Carol Quinn, ACA co-chair. "We met that goal as 2,247 employees and retirees contributed." "In the past four years, Argonne employees have pledged almost $1.7 million to help people in need," said ACA Co-Chair Cheryl Drugan. "Argonne employees should be proud to show that they care about others in the community, and it is fitting that the 2004 campaign slogan was `Caring is Sharing.' We showed that we cared." Once again, the three agencies receiving the most donations were United Way of Metro Chicago, the American Cancer Society and the American Heart Association. Argonne employees pledged nearly $171,000 to these agencies. ACA agencies receiving the most support were:
Details about donations to all of the agencies can be found on the ACA Web site.. Researchers to discuss results of LDRD workA mini-symposium on the results of Laboratory-Directed Research and Development (LDRD) projects will be held from 2 - 3:30 p.m., Tuesday, Jan. 25, in Argonne-East's Building 201 in Conference Room 190A and B. The topics covered will be:
Town hall' meeting set for Jan. 27 Laboratory employees are invited to the second in a series of "town hall" meetings with Argonne Chief Operations Officer Adam Cohen Thursday, Jan. 27. The one-hour meeting will begin at 11 a.m. in Building 362, Conference Room F108. The meeting will give employees an opportunity to discuss the laboratory's recent cost-cutting measures and other topics of interest in an informal, question-and-answer format. Space is limited, and seating will be first-come, first-served. Additional sessions will be scheduled based on demand. Questions and answers from the first town hall meeting are online. Party to help beat the chillThe winter blues has inspired the Argonne Club to plan a "Think Hot-Hot-Hot" party Thursday, Jan. 20, in the Building 617 Lower Level from 4:30 to 8 p.m. Prizes will be awarded to the best tropical attire. Adult beverages will be available, including tropical drinks. All visitors must be badged by the Argonne Information Center prior to 4 p.m. For more information, contact the Argonne Club at argonneclub@anl.gov or visit the Argonne Club Web site. Menopause, childbearing lectures setA two-part lecture series on women's health will discuss concerns about menopause and the childbearing years. Part one of "The Menopause Years" will be held at noon Wednesday, Jan. 19. Part two will begin at noon Tuesday, Jan. 25. Both lectures will be held in Argonne-East's Building 213 Cafeteria. Part one of "The Childbearing Years," another two-part lecture series, will begin at noon Wednesday, Feb. 2. Part two will begin at noon Tuesday, Feb. 8. Both lectures will be held in the Building 213 Cafeteria. Both series are free and require a minimum 20 participants. For more information or to register, contact the Medical Department at ext. 2-2800.
First Marv Weseley memorial scholarship awardedThe first of a planned series of annual American Meteorological Society (AMS) travel scholarships named in honor of the late Argonne scientist Marv Wesely has been awarded to a graduate student from the University of Colorado. Joel Gratz, a second-year graduate student, will attend the 85th annual meeting of the society in San Diego using the $500 award. The scholarship is awarded from contributions to the Marv Wesely Travel Scholarship Fund of the AMS. Gratz is a member of the AMS Student Conference Committee and has been involved in preparing the program for the meeting. Wesely died in 2003 from a rare type of heart cancer. He began his career at Argonne in 1973, centering on atmospheric science, and wrote or co-wrote more than 150 publications. He was a strong supporter of student participation in research efforts and was a mentor to many undergraduate and graduate students. Additional contributions to the Marvin L. Wesely Student Travel Fund can be made to American Meteorological Society, Attention: AMS Development Office, 45 Beacon Street, Boston, MA 02108-3693. |
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