An atomistic Green’s function method is developed to simulate phonon transport across a strained germanium (or silicon) thin film between two semi-infinite silicon (or germa-nium) contacts. A plane-wave formulation is employed to handle the translational sym-metry in directions parallel to the interfaces. The phonon transmission function and thermal conductance across the thin film are evaluated for various atomic configurations. The contributions from lattice straining and material heterogeneity are evaluated sepa-rately, and their relative magnitudes are characterized. The dependence of thermal con-ductance on film thickness is also calculated, verifying that the thermal conductance reaches an asymptotic value for very thick films. The thermal boundary resistance of a single Si/Ge interface is computed and agrees well with analytical model predictions. Multiple-interface effects on thermal resistance are investigated, and the results indicate that the first few interfaces have the most significant effect on the overall thermal resistance. DOI: 10.1115/1.270965

Phonon transport across a non-polar nanowire situated between two semi-infinite contacts is simulated in this paper using the atomistic Green's function method. Abrupt geometric changes between the nanowire and bulk contacts are handled by self-energy matrices obtained from bare surface Green's functions. Transport properties such as phonon transmission functions and thermal conductances are calculated, and their dependencies on the interatomic potential, length, diameter, shape, and lattice orientation are investigated. The results reveal that the overall thermal conductance of the nanowire-bulk-contact structure increases with nanowire diameter while the normalized thermal conductance approaches an asymptotic value. Thermal conductance decreases significantly with increasing nanowire length and converges to that of the single-contact case. This method can be generalized to study phonon transport through a variety of nanostructures between bulk contacts.

Polycrystalline diamond and carbon nanotubes exhibit excellent vacuum field emission properties, characterized by low turn-on voltage and high current density. Their atmospheric field emission and ionization capabilities are reported in this paper. Highly graphitic polycrystalline diamond (HGPD) film was grown in a plasma-enhanced chemical vapor deposition process, and its ability to ionize atmospheric air was characterized and compared against carbon nanotubes.The HGPD sample was activated by applying a moderate voltage bias (340V) for an extended period across a 10 µm electrode gap. After activation, a turn-on voltage of 20 V and a sustainable current of 10 µA was observed with the same gap. Results also indicate that field emission helps to create a moderate ionization effect without catastrophic air breakdown. A hydrogen plasma treatment is shown to restore emission current back to or even exceeding the original level, which suggests an important role of surface termination in electron emission process. Carbon nanotubes were grown and tested but did not perform as well under similar conditions.

Abstract. Adding nanophase defects to YBa2Cu3O7-δ (YBCO) superconductor thin films is wellknown to enhance flux pinning, resulting in an increase in current density (Jct). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. This paper compares the effect of the addition of two insulating, nonreactive phases of barium zirconium oxide (BZO) and yttrium oxide Y2O3, both as a single addition of BZO and as a double addition in conjunction with Y2O3. Processing parameters vary the target composition volume percent of BZO from 2-6 vol. %, while maintaining 3 vol. % Y2O3, and the remaining vol. % YBCO. Pulsed laser deposition produced thin films on LaAlO3 (LAO) and SrTiO3 (STO) substrates at various deposition temperatures. Comparison of strong and weak flux pinning mechanisms, current densities, critical temperatures, and microstructures of the resulting films will be presented. Films produced from the 2 vol. % BZO + 3 vol. Y2O3 doped YBCO target at a deposition temperature of 825 °C attained the highest current density. The addition of second phase Y2O3 impacted the film microstructure, resulting in more isotropic behavior when compared to the YBCO films doped with only BZO. . Liu utilized a 1.5 vol. % BZO and YBCO target to produce films on LAO decorated with Y2O3 nano-islands [5]. Optimization of film thickness for films consisting of YBCO with BZO and Y2O3 has also been studied, utilizing a 5 mol. % BZO and 5 mol. % Y2O3 target [6]. It is hypothesized that the combination of BZO and Y2O3 with YBCO influences the lattice strain in the film. This research seeks to explore the effects on current density of doping a YBCO target with 2, 4, and 6 volume percent BZO along with 3 vol. percent Y2O3, with the remaining volume percent YBCO.