Samuel Graham

Bio: Samuel Graham is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Thermal conductivity & Thermal resistance. The author has an hindex of 48, co-authored 347 publications receiving 9774 citations. Previous affiliations of Samuel Graham include Merck & Co. & United States Military Academy.

Annealing Effects on Mechanical and Transport Properties of Ni and Ni-Alloy Electrodeposits

Abstract: The effect of annealing at temperatures up to 600 degC on the mechanical properties and the thermal and electrical transport characteristics of nickel and a nickel-manganese electrodeposits are presented. The samples include Ni plated from sulfamate salt with dodecyl sulfate surfactant and from NiSO4 with saccharin additive and a NiMn alloy deposited from a nickel sulfamate bath with added MnCl2. Recrystallization and grain growth, induced by annealing, are shown to strongly affect the mechanical and transport properties. Relatively coarse-grained Ni-sulfamate electrodeposits yielded properties closest to bulk Ni. The incorporation of sulfur (from saccharin additions to the plating electrolyte) or Mn into electrodeposited Ni produces materials with exceptionally fine grain size and with very high yield and ultimate strength. At the same time, the thermal and electrical conductivities are smaller than bulk Ni. Thermal annealing leads to a reduction in strength and an enhancement of the transport properties. The Ni-Mn alloy shows the best temperature stability of the mechanical and transport properties among the tested samples. The observed trends are explained in terms of the influence of microstructure on the mechanical and transport properties

Application of the Taylor polycrystal plasticity model to complex deformation experiments

Abstract: The extended Taylor assumption of uniform deformation gradient among grains was applied in 3-D polycrystal plasticity simulations for complex loading paths at finite strain for OFHC Cu using the Los Alamos polycrystal plasticity (LApp) code (Kocks et al., 1994). Comparisons of both stress-strain behavior and texture evolution, with and without the inclusion of latent hardening effects, show that the theory overpredicts the rate of development of texture in both torsion and compression. Compression stress-strain behavior was accurately predicted, but the effect of the prestrain, either compressive or torsional, on subsequent nonproportional deformation response was inadequately modeled. Some possible sources of the discrepancies are discussed, including the low order nature of the extended Taylor model for intergranular interactions as compared to self-consistent models, low order formulation of slip system hardening, lack of accounting for formation of dislocation substructure within grains, and the possible role of anisotropic elasticity. Deformation-induced anisotropy and accommodation of intergranular constraint afforded by geometrically necessary dislocation substructure formation is viewed as the key neglected element of the formulation.

UV Thermal Imaging of RF GaN Devices with GaN Resistor Validation

Abstract: Shrinking features and growing device complexity in advanced microwave devices has increased the challenge of fully understanding device thermal behavior on the sub-micron scale. Predicting the device static and dynamic thermal behavior is essential for ensuring optimal tradeoffs between performance and device reliability. Thermal imaging based on the Thermoreflectance Principle can meet the challenges imposed by these compact, high power density RF devices by providing submicron spatial resolution and temporal resolution in the picosecond range. This technique overcomes the limitations of traditional thermal imaging techniques such as IR and Micro-Raman and some of the specific challenges in measuring GaN devices. In the past, Thermoreflectance Imaging has been shown to accurately estimate the temperature rise of metals using visible wavelength excitation sources. This paper presents a novel method to estimate the temperature directly on GaN surfaces using UV wavelengths. These UV thermoreflectance measurements were verified with measurement of an on-chip GaN mesa resistor. Temperature measurements on top of the field plate and inside the GaN channel were compared for a commercial GaN HEMT both on Si and SiC substrate. The advantages and disadvantages will be presented for the thermoreflectance technique for thermal imaging.

Measuring the Thermal Resistance in Light Emitting Diodes Using a Transient Thermal Analysis Technique

Abstract: We explore a modified thermal resistance analysis by induced transient method applied to light emitting diodes (LEDs) to discretize the junction-to-package thermal resistance. The temperature response of LED and package configuration is evaluated for discrete contributions from identifiable spatial domains in the multilayered device and package structure to obtain their thermal resistances and thermal capacitances using a Laplace transform-based method. The technique successfully extracts the junction-to-package thermal parameters of a variety of LED package configurations from the experimental temperature transient measurements of the LED junction and provides a straightforward method by which these parameters can be obtained.

Lattice boltzmann and discrete ordinates methods for phonon transport modeling: A comparative study

Abstract: The description of heat transport at small length scales is very important in understanding a wide range of micro and nanoscale systems. In systems where coherent phonon transport effects are negligible, the Boltzmann transport equation (BTE) is often employed to describe the distribution and propagation of thermal energy in the lattice. The phonon distribution function depends not only on the temporal and spatial coordinates, but also on polarization and wave vector, making fully-resolved simulations very expensive. Therefore, there is a need to develop accurate and efficient numerical techniques for the solution of the BTE. The discrete ordinates method (DOM) and more recently, the lattice Boltzmann method (LBM) have been used for this purpose. In this work, a comparison between the numerical solution of the phonon BTE by the LBM and DOM is made in order to delineate the strengths and weaknesses of these approaches. Test cases are chosen with Knudsen (Kn) numbers varying between 0.01–100 to cover the full range of diffusive to ballistic phonon transport. The results show that solutions obtained from both methods converge to analytical results for the 1 dimensional phonon transport in a slab. Solutions obtained by two methods converge to analytical solutions of 2 dimensional problems at low Kn. However, solution accuracy is strongly determined by angular resolution for moderate to high Kn. Since the number of propagation directions in LBM are limited, significant errors are engendered in multi-dimensional acoustically-thin problems. DOM also suffers errors at low angular resolutions for high Kn, but yields accurate solutions when sufficient angular resolution is employed.Copyright © 2011 by ASME

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Interface engineering of highly efficient perovskite solar cells

Abstract: Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

An electron acceptor challenging fullerenes for efficient polymer solar cells.

Abstract: A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion effi ciencies of up to 6.8%, a record for fullerene-free PSCs.