Michael H. Check

Electrostatic Quadrupole Plasma Mass Spectrometer Measurements during Thin Film Depositions using Simultaneous Matrix Assisted Pulsed Laser Evaporation and Magnetron Sputtering

A hybrid plasma deposition process, combining matrix assisted pulsed laser evaporation (MAPLE) of carbon nanopearls (CNPs) with magnetron sputtering of gold was investigated for growth of composite films, where 100 nm sized CNPs were encapsulated into a gold matrix. Composition and morphology of such composite films was characterized with x-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy (TEM) analysis. Carbon deposits on a gold magnetron sputter target and carbon impurities in the gold matrices of deposited films were observed while codepositing from gold and frozen toluene-CNP MAPLE targets in pure argon. Electrostatic quadrupole plasma analysis was used to determine that a likely mechanism for generation of carbon impurities was a reaction between toluene vapor generated from the MAPLE target and the argon plasma originating from the magnetron sputtering process. Carbon impurities of codeposited films were significantly reduced by introducing argon-oxy...

Scanning Tunneling Microscopy Observation of Phonon Condensate

Using quantum tunneling of electrons into vibrating surface atoms, phonon oscillations can be observed on the atomic scale. Phonon interference patterns with unusually large signal amplitudes have been revealed by scanning tunneling microscopy in intercalated van der Waals heterostructures. Our results show that the effective radius of these phonon quasi-bound states, the real-space distribution of phonon standing wave amplitudes, the scattering phase shifts, and the nonlinear intermode coupling strongly depend on the presence of defect-induced scattering resonance. The observed coherence of these quasi-bound states most likely arises from phase- and frequency-synchronized dynamics of all phonon modes, and indicates the formation of many-body condensate of optical phonons around resonant defects. We found that increasing the strength of the scattering resonance causes the increase of the condensate droplet radius without affecting the condensate fraction inside it. The condensate can be observed at room temperature.

Synthesis of zinc fulleride (ZnxC60) thin films with ultra-low thermal conductivity

The structure and physical properties of doped fullerene materials were investigated for their interesting thermal properties. The synthesis and thermal properties of ZnxC60 thin films are reported. Thin films of ZnxC60 were found to have an exceedingly low thermal conductivity of 0.13 Wm−1 K−1. Differential scanning calorimetry results suggested that a temperature of 357 °C is needed in order to fully intercalate the Zn with the C60. Both charge transfer and covalent bonding (between Zn and C60) should be considered when attempting to understand the Raman spectra observed. Moreover, the ZnxC60 thin films created represent an interesting class of materials that could find use in several thermal applications. Furthermore, in the present case, the exceptionally low thermal conductivity is accompanied by a substantial increase in the electrical conductivity, suggesting interesting thermal and electrical transport.

Multidimensional nanoscopic approaches to new thermoelectric materials

The advantages of thermoelectric energy conversion technologies are briefly summarized. Recent material advances are discussed, with the focus on one-dimensional (1-D) self-assembled molecular materials as building blocks for new thermoelectric materials. The preparation, doping, and thermal characterization of phthalocyanine based materials are presented. The thermal conductivity of the doped material is lower than the undoped material even though the electrical conductivity of the doped material is orders of magnitude higher than the undoped material. This is counter intuitive against the backdrop of the Wiedemann-Franz treatment of thermal conductivity in electrical conductors from which one would expect thermal and electrical conductivity to both increase with introduction of additional charge carriers. These unusual results can be understood as a competition between the generation of an increased number of charge carriers and enhanced phonon scattering resulting from the introduction of chemical dopants. The thermal conductivity of the undoped phthalocyanines has been found to be small and only modestly temperature dependent in the 50-300 C range, but it is larger than a previous, indirect measurement.

Correlation of Electrochemical Impedance to Pool Boiling Behavior on Copper

Pool boiling studies were performed on roughened copper in a dilute saline solution while incorporating a three electrode electrochemical monitoring system. Electrochemical impedance at a frequency of 1000 Hz was monitored during nucleate boiling and through the transition point from nucleate to film boiling. During nucleate boiling, a steady increase in impedance was observed as the solid-vapor contact area increased. Impedance monitoring lead to the discovery of a critical impedance vector magnitude (|Z|) between the range of 1.85 kΩ-cm2 and 2.35 kΩ-cm2 where critical heat flux was most likely to occur. At the critical point just after nucleate boiling and beginning transition film boiling, an impedance vector magnitude spike was observed from approximately 2 kΩ-cm2 to 100 kΩ-cm2 , due to the formation of a current impeding vapor layer encompassing the substrate surface. A phase angle decrease at this boiling regime transition point from −2.25 degrees to −30 degrees provides insight into a change in electrochemical circuit phenomena from a purely resistive to a somewhat capacitive electrochemical system.Copyright

Effects of UV-Visible Irradiation on Pool Boiling Behavior of Copper

Roughened copper substrates were exposed to a broadband UV-VIS light source during nucleate boiling at a heat flux of 60–70% of the amount expected to result in critical heat flux (CHF) without exposure to a light source. The surface temperature decreased by 0.5–1.0°C within minutes after the UV-VIS light exposure began. CHF occurred after less than 20 minutes of exposure to the light source. Nanoscale features were observed in the light-exposed region of the copper surface after boiling, which were primarily associated with formation of Cu2 O. The induced CHF likely occurred due to surface oxide formation, a resultant decrease in wettability of the surface.Copyright