Person:

Graham, Samuel

Permanent Link

https://hdl.handle.net/1853/71304

Associated Organization(s)

Organizational Unit

George W. Woodruff School of Mechanical Engineering

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Publication Search Results

Now showing 1 - 10 of 21

Conformally coating vertically aligned carbon nanotube arrays using thermal decomposition of iron pentacarbonyl

(Georgia Institute of Technology, 2012-05) Hildreth, Owen ; Cola, Baratunde A. ; Graham, Samuel ; Wong, C. P.

Conformally coating vertically aligned carbon nanotubes (v-CNT) with metals or oxides can be difficult because standard line-of-sight deposition methods, such as dc sputter coating and electron-beam evaporation, are hindered by the low mean-free-path with
Effects of nonframework metal cations and phonon scattering mechanisms on the thermal transport properties of polycrystalline zeolite LTA films

(Georgia Institute of Technology, 2010-03) Greenstein, Abraham ; Hudiono, Yeny ; Graham, Samuel ; Nair, Sankar

We present a systematic study to investigate the effects of nonframework cations and the role of phonon scattering mechanisms on the thermal transport properties of zeolite LTA, via experiment and semiempirical lattice dynamics calculations. Our study is motivated by the increasing interest in accurate measurements and mechanistic understanding of the thermal transport properties of zeolite materials. The presence of a nanostructured pore network, extra-framework cations, and tunable framework structure and composition confer interesting thermophysical properties to these materials, making them a good model system to investigate thermal transport in complex materials. Continuous films of zeolite LTA with different nonframework cations (Na⁺, K⁺, and Ca⁺²) were synthesized and characterized. The thermal conductivity was measured using the three-omega method over a wide range of temperature (150–450 K). These are the first thermal conductivity measurements performed on bulk LTA, so they are more accurate than previous measurements, which involved the use of compacted zeolite powders. Our data showed significant dependence of the thermal conductivity on the extra-framework cations as well the temperature. The thermal conductivities of the zeolite LTA samples were modeled with the relaxation time approximation to the Boltzmann transport equation. The full phonon spectra for each type of LTA zeolite were calculated and used in conjunction with semiempirical relaxation time expressions to calculate the thermal conductivity. The results both validated, and suggested the limitations of, this modeling approach. Optical phonons dominated the thermal conductivity and boundarylike scattering was found to be the strongest phonon scattering mechanism, as also observed in MFI zeolite.
A hybrid encapsulation method for organic electronics

(Georgia Institute of Technology, 2009-04) Kim, Namsu ; Potscavage, William J., Jr. ; Domercq, Benoit ; Graham, Samuel

We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOₓ or SiNₓ (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al₂O₃ (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1) x 10 ⁻⁵ g/m² day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C ₆₀ solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.
Specific contact resistance at metal/carbon nanotube interfaces

(Georgia Institute of Technology, 2009-01-05) Jackson, Roderick ; Graham, Samuel

In this report, the specific contact resistance between a thin film single wall carbon nanotube electrode and a deposited silver contact was measured. The specific contact resistance was found to be 20 mΩ cm², which is an order of magnitude higher than typically observed in standard Si photovoltaic technology. We demonstrate that when utilized as the transparent anode in organic photovoltaics, the specific contact resistance has the potential to induce non-negligible resistive power losses. Thus, specific contact resistance will adversely affect the performance of these systems and should therefore be addressed. © 2009 American Institute of Physics.
Micro-Raman thermometry in the presence of complex stresses in GaN devices

(Georgia Institute of Technology, 2008-06) Beechem, Thomas ; Christensen, Adam ; Graham, Samuel ; Green, D.

Raman thermometry is often utilized to measure temperature in gallium nitride (GaN) electronics. However, the accuracy of the technique is subject to errors arising from stresses which develop during device operation as a result of both thermoelastic and inverse piezoelectric effects. To assess the implications of these stresses on Raman thermometry, we investigate the use of the Stokes peak position, linewidth, and Stokes to anti-Stokes intensity ratio to estimate the temperature of GaN devices during operation. Our results indicate that only temperature measurements obtained from the intensity ratio method are independent of these stresses. Measurements using the linewidth, meanwhile, were found to correspond well with those obtained from the intensity ratio through the use of a reference condition which accounted for the stress dependency of this spectral component. These results were then compared to a three dimensional finite element model which yielded a correlation to within 5% between the computational and experimental methods. The peak position method, in contrast, was found to underpredict temperature in all circumstances due to the stress distribution which is present during device operation.
Temperature and doping dependence of phonon lifetimes and decay pathways in GaN

(Georgia Institute of Technology, 2008-05) Beechem, Thomas ; Graham, Samuel

The lifetimes of polar optical phonons are known to affect both the electrical and thermal performances of gallium nitride (GaN) based devices. Hence, understanding the dynamical behavior of these phonons in GaN is integral to the elucidation of carrier drift velocities, hot phonon effects, and temperature localization in these nitride semiconductors. To investigate this dynamic behavior, temperature dependent phonon lifetimes were acquired through utilization of the linewidth of the Raman response for GaN samples having various doping types and concentrations. The temperature dependent lifetimes of the four examined phonon modes were then correlated with the Klemens decay model modified to account for four-phonon processes to deduce the decomposition of the zone center phonons. A graphical method that maps this decomposition in the high symmetry directions of the Brillouin zone is also presented. From the variation in lifetime with free carrier concentration, dominant scattering mechanisms are subsequently found for each of four different phonon modes. It is observed that the phonon-carrier interaction directly determines the lifetimes of the polar optical A1 and E1(LO) modes, while the transverse modes into which these longitudinal phonons decay are independent of this interplay. These results indicate that temperature localization likely arises due to the continual emission and reabsorption between the LO phonon modes and the free carriers rather than the persistence of lattice/carrier interaction throughout the entirety of the energy cascade.
Feasibility study on nanowriting for tracking

(Georgia Institute of Technology, 2008-01-15) Graham, Samuel
Stress relaxation in GaN by transfer bonding on Si substrates

(Georgia Institute of Technology, 2007-12-17) Hsu, S. C. ; Pong, B. J. ; Li, W. H. ; Beechem, Thomas ; Graham, Samuel ; Liu, C. Y.

The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1 to 40 µm, the high compressive stress state in GaN layer was relieved. A 10 µm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼ 85 meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.
Effects of composition and phonon scattering mechanisms on thermal transport in MFI zeolite films

(Georgia Institute of Technology, 2007-09) Hudiono, Yeny ; Greenstein, Abraham ; Saha-Kuete, Carine ; Olson, Brandon ; Graham, Samuel ; Nair, Sankar

We report a systematic study that reveals the effect of composition (silicon-to-aluminum ratio) and the role of different phonon scattering processes on thermal transport in the nanoporous zeolite MFI. This is accomplished via synthesis of a series of films with graded compositions, thermal property measurements, and lattice dynamical modeling in the framework of the Boltzmann equation. MFI films with different Si/Al ratios (from infinity to 26) and constant (h0l) out-of-plane orientation were successfully synthesized by a seeded hydrothermal process. Three-omega measurements on these films allowed us to obtain comprehensive information on the thermal conductivity of MFI as a function of temperature (150-450 K) and Si/Al ratio. Detailed atomistic simulations (energy minimization and phonon dispersion calculations) were carried out for the MFI crystal structure with different Si/Al ratios and incorporated into a Boltzmann transport model along with approximate theoretical expressions for describing the rate of phonon scattering through umklapp, defect, and boundary scattering processes. The model predicts the observed thermal conductivity behavior very well across the entire range of temperature and composition investigated, with only a small number of fitting parameters of physical significance which allow us to distinguish the contributions of the different phonon scattering mechanisms. In particular, our results strongly suggest that the upper limit of thermal conductivity is defined by boundary-like scattering associated with the pore structure of the material. Below this limit, silicon substitution with aluminum allows considerable suppression of thermal conductivity by point defect scattering and a decrease in phonon velocity. These findings are important from the point of view of developing a robust platform for understanding thermal transport in complex crystalline materials with nanostructural features (such as an ordered nanopore network), which in turn serve as model systems for tuning of phonon transport properties in complex materials.
Thermal metrology of silicon microstructures using Raman spectroscopy

(Georgia Institute of Technology, 2007-06) Abel, Mark R. ; Wright, Tanya L. ; King, William P. ; Graham, Samuel

Thermal metrology of an electrically active silicon heated atomic force microscope cantilever and doped polysilicon microbeams was performed using Raman spectroscopy. The temperature dependence of the Stokes Raman peak location and the Stokes to anti-Stokes intensity ratio calibrated the measurements, and it was possible to assess both temperature and thermal stress behavior with resolution near 1µm. The devices can exceed 400 C with the required power depending upon thermal boundary conditions. Comparing the Stokes shift method to the intensity ratio technique, non-negligible errors in devices with mechanically fixed boundary conditions compared to freely standing structures arise due to thermally induced stress. Experimental values were compared with a finite element model, and were within 9% of the thermal response and 5% of the electrical response across the entire range measured.