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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Article
  • Peer Reviewed
UC Davis Previously Published Works (2016)
In this paper we consider integers in base 10 like $abc$, where $a$, $b$, $c$ are digits of the integer, such that $abc^2 - (abc \cdot cba) \; = \; \pm n^2$, where $n$ is a positive integer, as well as equations $abc^2 - (abc \cdot cba) \; = \; \pm n^3$, and $abc^3 - (abc \cdot cba) \; = \; \pm n^2$ We consider asymptotic density of solutions. We also compare the results with ones with bases different from 10.
    • Article
    • Peer Reviewed
    UCOP Previously Published Works (2009)
    Powder metallurgy is a useful route to forming particulate composite materials; however, the densification of hard and soft powder mixtures is usually inhibited by the more refractory phase. The Bi-W powder compacts were uniaxially pressed at room temperature and the compaction behavior and mechanical properties were evaluated. Pressing was performed in incremental steps from ~1 to 540 MPa. After each step, the pressure was relieved and the thickness and sound-wave transit time were measured in situ (in the die), in order to determine the density and sound-wave velocity in the compact. The data show that the unreinforced Bi powder compacts to ~98 pct density at 540 MPa. The W reinforcement inhibits the densification process, resulting in increased levels of residual porosity. The compaction behavior was evaluated using a modified Heckel equation, while the porosity dependence of the ultrasonically determined elastic modulus was described by a site percolation approach. Postcompaction sound-wave velocity and Vicker’s hardness measurements show <5 pct anisotropy between the axial (pressing) and radial directions. The mechanical characterization illustrates the competing effects of the W reinforcement and the associated residual porosity.
      Cover page: Compaction Behavior and Mechanical Properties of Uniaxially Pressed Bi-W Composites
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2016)
      Obliteration of matter by pulsed laser beams is not only prevalent in science fiction movies, but finds numerous technological applications ranging from additive manufacturing over machining of micro- and nanostructured features to health care. Pulse lengths ranging from femtoseconds to nanoseconds are utilized at varying laser beam energies and pulse lengths, and enable the removal of nanometric volumes of material. While the mechanisms for removal of material by laser irradiation, i.e., laser ablation, are well understood on the micrometer length scale, it was previously impossible to directly observe obliteration processes on smaller scales due to experimental limitations for the combination of nanometer spatial and nanosecond temporal resolution. Here, we report the direct observation of metal thin film ablation from a solid substrate through dynamic transmission electron microscopy. Quantitative analysis reveals liquid-phase dewetting of the thin-film, followed by hydrodynamic sputtering of nano- to submicron sized metal droplets. We discovered unexpected fracturing of the substrate due to evolving thermal stresses. This study confirms that hydrodynamic sputtering remains a valid mechanism for droplet expulsion on the nanoscale, while irradiation induced stress fields represent limit laser processing of nanostructured materials. Our results allow for improved safety during laser ablation in manufacturing and medical applications.
        Cover page: High speed direct imaging of thin metal film ablation by movie-mode dynamic transmission electron microscopy
        • Article
        • Peer Reviewed
        LBL Publications (2003)

        New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. We find the copper segregation to be site specific, changing the structure of the boundary by unexpectedly occupying interstitial sites. The calculated energy for segregation was found to be sufficient for essentially all of the interstitial sites to be filled. Minor elemental constituents in materials can have profound effects on their engineering performance, often through segregation to grain boundaries in the host material. One important example is the great resistance to electromigration damage in microelectronics imparted by small additions of copper to aluminum interconnects.

          Cover page: Study by atomistic theory and high-resolution electron microscopies of Cu atoms at an Al grain boundary
          • Article
          Lawrence Berkeley National Laboratory (2003)

          New insight into the atomic segregation of copper to an aluminum grain boundary has been obtained using multiple, complementary atomic resolution electron microscopy techniques coupled with ab-initio electronic structure calculations. The copper segregation is site specific and changes the structure of the boundary by occupying interstitial sites. Minor elemental constituents in materials can have profound effects on their engineering performance. This change in structure can be associated with these strong effects. The observed structural change will alter the mass transport behavior of the boundary and has implications for the understanding of electromigration mechanisms.

            Cover page: Copper segregation to the Sigma5 (310)/[001] symmetric tilt grain boundary in aluminum
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