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.

Investigating the thermal response of a micro-optical shutter

Abstract: This paper discusses the thermal analysis of a fully integrated micro-switch for surety applications. Specifically, this study focuses on the temperature increase of a micromachined optical shutter with spot heating from a micro-laser. To analyze the shutter response, a 'Design-to-Analysis' interface has been built that generates an accurate 3-D solid geometry from the 2-D mask layout. Besides performing analysis, engineers can also use this solid modeler to virtually prototype and verify a design before fabrication. A parametric study is performed to determine the effects of thermal conductivity and contact resistance on the thermal response of this passively cooled device.

Self-Heating and Quality Factor: Thermal Challenges in Aluminum Scandium Nitride Bulk Acoustic Wave Resonators

Abstract: Understanding the thermal properties of piezoelectric thin films is essential in studying the performance and ultimate dissipation limits of bulk acoustic wave resonators. Here, we present the experimental and modeled results of thermal conductivity of the in-demand piezoelectric material aluminum scandium nitride (Al 1-x Sc x N), with x = Sc/(Sc+Al) ratio. We construct the three-dimensional (3D) finite-element modeling (FEM) of a back-side etched thin-film bulk acoustic wave resonator (FBAR) with aluminum nitride (AlN) and Al 0.7 Sc 0.3 N thin films. Comparison reveals a 26% more temperature rise in Al 0.7 Sc 0.3 N FBAR with equal input surface heat density of 2 W/mm2. The trend is consistent with the drastic decrease of thermal conductivity with increasing x in Al 1-x S cx N. Consequently, as we study the upper limit of the frequency (f), quality factor (Q) product (f. Q) under phonon interactions, Al 1-x S cx N exhibits a greater amount of degradation due to self-heating. This work reports the first comparison of thermal properties of AlN and Al 1-x S cx N resonators, critical in material selection for resonator operation under high power levels.

Thermal Characterization of Discrete Device Layers in AlxGa1−xN Based Ultraviolet Light Emitting Diodes

Abstract: The temperature rise in LEDs is an important parameter that must be determined for both thermal management and device lifetime/reliability assessment. Commonly used indirect methods of measuring the device temperature either estimate the multiple quantum well (MQW) temperature based on measuring temperature dependent device characteristics (e.g., forward voltage and electroluminescence methods), or they measure the average temperature across the device structure using optical methods such as infrared (IR) thermography and thermoreflectance. However, none give true insight into the vertical distribution of temperature in these structures. In this study, Raman spectroscopy is applied for the first time to operating UV LEDs to give the temperature rise in discrete layers within the LED device structure, going from the growth substrate to layers adjacent to theMQWs. Comparisons are made with IR themography to contrast with this depth sensitive measurement technique. It was observed that the peak temperatures in the device were much higher than the temperatures indicated by IR while the averaged temperatures through the structure compared favorably. Additional comparisons to electroluminescence measurements were also made which compared favorably with the peak temperatures found by Raman Spectroscopy.Copyright © 2012 by ASME

Estimating the Effects of Interface Disorder on the Thermal Boundary Resistance Using a Virtual Crystal Approximation

Abstract: An analytical method is presented to estimate the effects of structural disorder on the thermal boundary resistance (TBR) between 2 materials. The current method is an extension of the diffuse mismatch model (DMM) where the interface is modeled as a virtual crystal of finite thickness with properties derived from those of the constituent materials. Using this virtual crystal extension, the predictive capabilities of the diffuse mismatch method are greatly increased with added insight into the sensitivity of materials to interface quality.Copyright © 2006 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.