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Topic Name: Researchers use unique diamond anvils to view oxide glass structures under pressure

Category: Advanced Materials

Research persons: Chris Benmore, Qiang Mei

Location: Argonne National Laboratory, Department of Energy, United States

Details

Researchers at the U.S. Department of Energy's Argonne National Laboratory have used a uniquely constructed perforated diamond cell to investigate oxide glass structures at high pressures in unprecedented detail.

Argonne physicist Chris Benmore and postdoctoral appointee Qiang Mei, along with colleagues at the University of Arizona, used microscopic laser-perforated diamond anvil cells to generate pressures of up to 32 gigapascals (GPa) – roughly one-tenth the pressure at the center of the Earth. By "squashing" vitreous (glassy) arsenic oxide samples between the anvils, the researchers were able to determine the mechanism behind the structure's atypical behavior under high-pressure.

This research may have far-reaching affects in the geophysical sciences, Benmore said, because oxide glasses and liquids represent a significant percentage of the materials that make up the Earth. For example, knowing the atomic structure of oxide materials at high pressures may give scientists a window on the behaviors of magma during the formation of the early Earth and moon. "We now have a technique where we can look a lot of different silicate glasses that are relevant to the Earth's process and at the complex behaviors of the melts that formed the Earth's mantle," he said.

During their investigation, Benmore and Mei noticed that if arsenic oxide was subjected to high pressures the material underwent an unusual transformation at about 20 GPa, as the color of the compound changed from transparent to red. However, they did not know the atomic cause for this behavior.

By performing X-ray pair distribution function experiments at Argonne's Advanced Photon Source (APS), however, Benmore and Mei were able to see the atomic reconfiguration that produced the color change. Arsenic oxide, at normal pressures, typically exists in isolated molecular "cages" in which four arsenic atoms are surrounded by three oxygen atoms apiece – each of the six oxygen atoms is bounded to two arsenic atoms. When the pressure rose above 20 GPa, however, many of these molecular cages collapsed, creating new isomers in which each arsenic atom was bonded to six oxygen atoms.

Regular diamond anvils could not be used because they caused a great deal of background scattering that obscured the signal from the material. Previous experiments on vitreous materials had used mechanically drilled diamond anvil cells to create the high pressures, but these routinely failed at pressures above 15 GPa. This experiment involved one of the first-ever uses of laser-perforated diamond anvils combined with micro-focused high energy X-ray diffraction techniques, which have the ability to generate high pressures without also producing background noise.

Benmore hopes to extend his research to liquid oxides and silicates by heating them pass their melting points. By doing so, he expects to gain a better understanding of the structural transition, which is expected to occur more abruptly and be reversible in the liquid phases of these materials.

Benmore and Mei's research was funded by the DOE Office of Basic Energy Sciences.

Note for diamond anvil cell 

A diamond anvil cell (DAC) is a device used by physicists to exert extreme pressures on a material. It consists of two opposing cone-shaped diamonds squeezed together. The resultant high pressures — in excess of a million atmospheres — are produced when force is applied to small areas of the opposing diamond culets.

The device has been used to simulate the extreme pressures existing in the hearts of planets, creating new materials in the process. Notable examples include the non-molecular ice X , polymeric nitrogen  and MgSiO3 perovskite, thought to be the major component of the Earth's mantle.

Note for Silicate glasses

Silicate glasses have been commonly used in the field of semiconductor device fabrication as an insulator between active layers of the semiconductor device. Also, some airbags in cars react SiO2 with harmful byproducts of nitrogen gas producing reactions to produce Silicate glass to remove the harmful substances (K2O and Na2O) .

These materials have relatively low melting temperatures, and can be flowed by heating in order to "planarize" the semiconductor layers. There will typically be contact holes or vias etched into the glass layers using wet etching or dry etching, and the silicate glasses can then be reflowed by heating in order to make smoother tops to the contact holes or vias, which makes the metal connections into the contact holes or vias more durable.

The silicate glasses are typically formed of phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG). The boron and/or phosphorus impurity levels used can be adjusted to affect the silicate glass's melting point.

Note for Earth's mantle

Earth's mantle extends to a depth of 2890 km, making it the largest layer of the Earth. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales. Convection of the mantle is expressed at the surface through the motions of tectonic plates. The melting point and viscosity of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 1021 and 1024 Pa·s, depending on depth. In comparison, the viscosity of water is approximately 10-3 Pa·s and that of pitch 107 Pa·s. Thus, the mantle flows very slowly.

About Researchers:

Dr. Chris Benmore

Chris obtained his PhD in physics in 1993 from the University of East Anglia, England. He then worked as a postdoctoral fellow at the University of Guelph in Canada for four years. In 1997 he returned to England as the deputy instrument scientist on the SANDALS liquids diffractometer at the ISIS spallation source. In February 2000 he took up the position of GLAD instrument scientist at IPNS, USA. Chris's scientific interests span the structure and dynamics of disordered materials. In particular, the isotopic substitution technique in neutron diffraction, high energy x-ray diffraction, inelastic neutron Brillouin scattering and more recently molecular dynamics simulations.

Contact Information:
telephone: (630) 252-7665
fax: (630) 252-4163
Address:
Argonne National Laboratory
9700 S. Cass Ave.
Bldg. 360 
Argonne, IL 60439
USA

Dr. Qiang Mei

Hometown: Beijing, China
Graduate School: Iowa State University
Major: Material Science Engineering
Graduated: May 2003, Ph.D.
Current Employer: Argon National Lab

About DOE Office of Basic Energy Sciences

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, providing more than 40 percent of total funding for this vital area of national importance. It oversees – and is the principal federal funding agency of – the Nation’s research programs in high-energy physics, nuclear physics, and fusion energy sciences. 

The Office of Science manages fundamental research programs in basic energy sciences, biological and environmental sciences, and computational science. In addition, the Office of Science is the Federal Government’s largest single funder of materials and chemical sciences, and it supports unique and vital parts of U.S. research in climate change, geophysics, genomics, life sciences, and science education.

The Office of Science manages this research portfolio through six interdisciplinary program offices: Advanced Scientific Computing Research, Basic Energy Sciences, Biological and Environmental Research, Fusion Energy Sciences, High Energy Physics and Nuclear Physics. In addition, the Office of Science sponsors a range of science education initiatives through its Workforce Development for Teachers and Scientists program.