Haeng-Ki Lee

Bio: Haeng-Ki Lee is an academic researcher from KAIST. The author has contributed to research in topics: Fly ash & Compressive strength. The author has an hindex of 45, co-authored 251 publications receiving 6513 citations. Previous affiliations of Haeng-Ki Lee include University of Miami & Chosun University.

Shrinkage characteristics of alkali-activated fly ash/slag paste and mortar at early ages

Abstract: The purpose of this study is to investigate the shrinkage characteristics of alkali-activated fly ash/slag (henceforth simply AFS) and the factors affecting it. A series of tests were conducted to determine the chemical shrinkage, autogenous shrinkage and drying shrinkage. The microstructures and reaction products were also characterized through XRD and SEM/EDS analyses. An increase in the slag content from 10% to 30% resulted in a denser matrix and showed a higher Ca/Si ratio of C–N–A–S–H in the microstructure. Higher sodium silicate and slag contents in a mixture caused more chemical, autogenous, and drying shrinkage, but led to a higher compressive strength. From the test results, it can be concluded that the autogenous shrinkage of AFS mortar occurs mainly due to self-desiccation in hardened state rather than volume contraction by chemical shrinkage in fresh state. The AFS paste showed higher drying shrinkage than ordinary Portland cement (OPC), which may be caused by the higher mesopore volume of the AFS paste compared to that of OPC paste.

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.

American Society for Testing and Materials

Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

Polymer/carbon based composites as electromagnetic interference (EMI) shielding materials

Abstract: The extensive development of electronic systems and telecommunications has lead to major concerns regarding electromagnetic pollution. Motivated by environmental questions and by a wide variety of applications, the quest for materials with high efficiency to mitigate electromagnetic interferences (EMI) pollution has become a mainstream field of research. This paper reviews the state-of-the-art research in the design and characterization of polymer/carbon based composites as EMI shielding materials. After a brief introduction, in Section 1, the electromagnetic theory will be briefly discussed in Section 2 setting the foundations of the strategies to be employed to design efficient EMI shielding materials. These materials will be classified in the next section by the type of carbon fillers, involving carbon black, carbon fiber, carbon nanotubes and graphene. The importance of the dispersion method into the polymer matrix (melt-blending, solution processing, etc.) on the final material properties will be discussed. The combination of carbon fillers with other constituents such as metallic nanoparticles or conductive polymers will be the topic of Section 4. The final section will address advanced complex architectures that are currently studied to improve the performances of EMI materials and, in some cases, to impart additional properties such as thermal management and mechanical resistance. In all these studies, we will discuss the efficiency of the composites/devices to absorb and/or reflect the EMI radiation.