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Lighter atoms create more friction
RESEARCH NEWS
Dislocations and defects aren’t what they’re cracked up to be MECHANICAL BEHAVIOR
Irradiating materials with high-energy particles has a dramatic effect on their microstructural properties, but the process is not well understood. Getting a better picture of what happens is essential for nuclear fusion or fission, as well as ion beam processing. Two teams have independently reported observations of different dislocation and defect behavior that call our current understanding into question. Kazuto Arakawa from Osaka University together with colleagues from Shimane and Tohoku Universities in Japan have shown, using in situ transmission electron microscopy (TEM), that nanometer-sized dislocation loops undergo onedimensional diffusion in Fe even in the absence of stress [Arakawa et al., Science (2007) 318, 956]. “In one sentence, our work is the first observation of the diffusion of dislocations,” says Arakawa. The dislocation loops that show the effect (as shown) are 6–20 nm in size (with a Burgers vector of ½<111>) – significantly larger than expected from molecular dynamics simulations. The researchers suggest that the diffusion mechanism is linked to Cottrell atmospheres, whereby interactions with interstitial impurity atoms control the diffusivity of the loops. “[The results] will lead to the precise prediction of the
Schematic view of the observation of the onedimensional glide motion of a nanometer-sized dislocation loop by TEM. The red ring is a dislocation loop. The direction of the motion of the loop is parallel to its Burgers vector. (© 2007 AAAS.)
lifetime of the radiation-resistant materials,” believes Arakawa. Simultaneously, Yoshitaka Matsukawa, now at the University of Illinois at Urbana-Champaign, and Steven J. Zinkle of Oak Ridge National Laboratory have observed the one-dimensional fast migration of nanometer-sized clusters of vacancies in face-centered cubic (fcc) Au [Science (2007) 318, 959]. When a material is irradiated by energetic ions and neutrons, crystal lattice vacancies and self-interstitial atoms
(SIAs) are formed, which can form clusters. “We [have] discovered that the migration mode of vacancy clusters is different from that of single vacancies in Au,” says Matsukawa. In fact, vacancy-type dislocation loops exhibit onedimension diffusion that is faster than single vacancies and comparable to SIAs. “The findings confound existing theories of irradiation damage microstructure evolution in metals under nuclear reactor environments,” he says. Even more surprisingly, the researchers also find that the highly mobile vacancy loops can transform into a sessile configuration (stacking fault tetrahedron), resulting in self-organization of the vacancy cluster arrays. This could offer a means of controlling or removing defects, not just from metals but also semiconductors. These findings, which have been independently confirmed at the University of Oxford, UK, and Argonne National Laboratory, provide a vital insight into the irradiation of materials. “The new discoveries, in terms of their significance and potential impact on material science, are similar to the first observation of live bacteria in biology,” says Sergei L. Dudarev, senior principal scientific officer of the EURATOM/UKAEA Fusion Association at Culham Science Centre, UK. Cordelia Sealy
Lighter atoms create more friction MECHANICAL BEHAVIOR The friction between two sliding bodies decreases on increasing the mass of one of the contact surfaces, say a team of scientists from IBM Research, Switzerland, Argonne National Laboratory, and the Universities of Houston, Pennsylvania, and Wisconsin–Madison [Cannara et al., Science (2007) 318, 780]. They used atomic force microscopy (AFM) to study friction at surfaces with similar chemical properties, but varying masses. As the AFM tip slides over the surface, kinetic energy from the tip is absorbed by the surface atoms of the sample and converted to vibrations. If the surface atoms are light, they vibrate faster than heavy atoms and dissipate energy more rapidly. The team recorded friction data for Si and diamond substrates, each modified with H or with its heavier isotope, D. H and D are chemically similar, but, unlike H, D contains a neutron that contributes to its mass. All the hydrogenated surfaces studied by the team exhibit higher friction than any of the surfaces modified with D.
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Graham Leggett of the University of Sheffield told Materials Today, “Understanding the pathways involved in energy dissipation is vital if we are to understand what friction force microscopy tells us about materials and their surface interactions. This study provides a very clear and, importantly, quantitative insight into what must be a major pathway for energy dissipation in any molecular material – the excitation of bond vibrations. The use of D substitution is a very elegant and unambiguous way of modifying the dissipative properties of the material without changing the basic chemistry, and provides some very important insights into where the energy goes in friction force microscopy.” Robert W. Carpick, now of the University of Pennsylvania, is already looking forward to the next step, “We are planning to see if the effect occurs if H is substituted for D in different chemical and molecular groups attached to surfaces. Measuring the temperature dependence will also be a key test for any model that describes the result. And we are
JAN-FEB 2008 | VOLUME 11 | NUMBER 1-2
Chemical vapor deposition chamber used to put H or D onto diamond surfaces. (Courtesy of Robert W. Carpick.) undertaking collaborations with modelers to see if they can explain our results using theory or simulation, because right now no theory exists to explain this effect in detail.”
Katerina Busuttil
Dislocations and defects aren’t what they’re cracked up to be MECHANICAL BEHAVIOR
Irradiating materials with high-energy particles has a dramatic effect on their microstructural properties, but the process is not well understood. Getting a better picture of what happens is essential for nuclear fusion or fission, as well as ion beam processing. Two teams have independently reported observations of different dislocation and defect behavior that call our current understanding into question. Kazuto Arakawa from Osaka University together with colleagues from Shimane and Tohoku Universities in Japan have shown, using in situ transmission electron microscopy (TEM), that nanometer-sized dislocation loops undergo onedimensional diffusion in Fe even in the absence of stress [Arakawa et al., Science (2007) 318, 956]. “In one sentence, our work is the first observation of the diffusion of dislocations,” says Arakawa. The dislocation loops that show the effect (as shown) are 6–20 nm in size (with a Burgers vector of ½<111>) – significantly larger than expected from molecular dynamics simulations. The researchers suggest that the diffusion mechanism is linked to Cottrell atmospheres, whereby interactions with interstitial impurity atoms control the diffusivity of the loops. “[The results] will lead to the precise prediction of the
Schematic view of the observation of the onedimensional glide motion of a nanometer-sized dislocation loop by TEM. The red ring is a dislocation loop. The direction of the motion of the loop is parallel to its Burgers vector. (© 2007 AAAS.)
lifetime of the radiation-resistant materials,” believes Arakawa. Simultaneously, Yoshitaka Matsukawa, now at the University of Illinois at Urbana-Champaign, and Steven J. Zinkle of Oak Ridge National Laboratory have observed the one-dimensional fast migration of nanometer-sized clusters of vacancies in face-centered cubic (fcc) Au [Science (2007) 318, 959]. When a material is irradiated by energetic ions and neutrons, crystal lattice vacancies and self-interstitial atoms
(SIAs) are formed, which can form clusters. “We [have] discovered that the migration mode of vacancy clusters is different from that of single vacancies in Au,” says Matsukawa. In fact, vacancy-type dislocation loops exhibit onedimension diffusion that is faster than single vacancies and comparable to SIAs. “The findings confound existing theories of irradiation damage microstructure evolution in metals under nuclear reactor environments,” he says. Even more surprisingly, the researchers also find that the highly mobile vacancy loops can transform into a sessile configuration (stacking fault tetrahedron), resulting in self-organization of the vacancy cluster arrays. This could offer a means of controlling or removing defects, not just from metals but also semiconductors. These findings, which have been independently confirmed at the University of Oxford, UK, and Argonne National Laboratory, provide a vital insight into the irradiation of materials. “The new discoveries, in terms of their significance and potential impact on material science, are similar to the first observation of live bacteria in biology,” says Sergei L. Dudarev, senior principal scientific officer of the EURATOM/UKAEA Fusion Association at Culham Science Centre, UK. Cordelia Sealy
Lighter atoms create more friction MECHANICAL BEHAVIOR The friction between two sliding bodies decreases on increasing the mass of one of the contact surfaces, say a team of scientists from IBM Research, Switzerland, Argonne National Laboratory, and the Universities of Houston, Pennsylvania, and Wisconsin–Madison [Cannara et al., Science (2007) 318, 780]. They used atomic force microscopy (AFM) to study friction at surfaces with similar chemical properties, but varying masses. As the AFM tip slides over the surface, kinetic energy from the tip is absorbed by the surface atoms of the sample and converted to vibrations. If the surface atoms are light, they vibrate faster than heavy atoms and dissipate energy more rapidly. The team recorded friction data for Si and diamond substrates, each modified with H or with its heavier isotope, D. H and D are chemically similar, but, unlike H, D contains a neutron that contributes to its mass. All the hydrogenated surfaces studied by the team exhibit higher friction than any of the surfaces modified with D.
10
Graham Leggett of the University of Sheffield told Materials Today, “Understanding the pathways involved in energy dissipation is vital if we are to understand what friction force microscopy tells us about materials and their surface interactions. This study provides a very clear and, importantly, quantitative insight into what must be a major pathway for energy dissipation in any molecular material – the excitation of bond vibrations. The use of D substitution is a very elegant and unambiguous way of modifying the dissipative properties of the material without changing the basic chemistry, and provides some very important insights into where the energy goes in friction force microscopy.” Robert W. Carpick, now of the University of Pennsylvania, is already looking forward to the next step, “We are planning to see if the effect occurs if H is substituted for D in different chemical and molecular groups attached to surfaces. Measuring the temperature dependence will also be a key test for any model that describes the result. And we are
JAN-FEB 2008 | VOLUME 11 | NUMBER 1-2
Chemical vapor deposition chamber used to put H or D onto diamond surfaces. (Courtesy of Robert W. Carpick.) undertaking collaborations with modelers to see if they can explain our results using theory or simulation, because right now no theory exists to explain this effect in detail.”
Katerina Busuttil