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Kim, Taehwan

Publications and source records attributed to Kim, Taehwan.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Effect of rice husk ash-derived activator on the structural build-up of alkali activated materials

Highlights: • Filtered and unfiltered sodium silicate solutions were synthesised from RHA. • The dissolution efficiency of RHA in NaOH solution was 76.3%. • RHA-derived activator showed a structural build-up comparable to the commercial activator in AAM pastes. • The undissolved residues in the unfiltered RHA-derived activator increased the yield stress. • Comparable compressive strength and early age gel formation were observed from both commercial and RHA-derived solutions. Waste-derived sodium silicates (SSs) were synthesised from rice husk ash (RHA) to produce alkali activated materials (AAMs) using fly ash and slag as precursors. The effects of these SSs on the structural build-up were investigated by monitoring the evolution of the viscoelasticity, ultrasonic pulse waves, setting time, and infrared spectra. Two types of RHA derived SSs (filtered SS and unfiltered SS) were prepared to investigate the effect of the undissolved residue on the hardening process. The derived SS contained a large portion of monomer silicate species. The results showed that the structural build-up mechanism was comparable between the AAMs activated by the filtered SS and a commercial SS. The unfiltered SS increased the yield stress of the paste by up to 10 times. However, the residues in the unfiltered SS had insignificant impacts on the gel formation and compressive strength when using identical SiO{sub 2}/Na{sub 2}O and Na{sub 2}O/binder ratios in the mix design.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chemo–mechanical properties of carbon fiber reinforced geopolymer interphase

Geopolymers, as a potentially environmentally friendly alternative to Portland cement, are increasingly attracting attention in the construction industry. Various methods have been applied for customizing the properties of geopolymers and improving their commercial viability. One of the promising methods for refining the properties of geopolymers such as their toughness is the use of short fibers. The effectiveness of a high-strength short fiber in the geopolymer matrix is largely dependent on the interfacial bonding between the fiber and its surrounding matrix. While the importance of this interfacial chemistry is highlighted in the literature, the characteristics of this bonding structure have not been fully understood. In this paper, we aim to investigate the bonding mechanism between the carbon fiber and metakaolin-based geopolymer matrix. For the first time, the existence and nature of the chemical bonding at the interfacial region (interphase) between carbon fiber and geopolymer matrix has been revealed. X-ray pair distribution function computed tomography (PDF-CT), field emission-scanning electron microscopy imaging, and nanoindentation techniques are employed to discern the chemo-mechanical properties of the interphase. PDF-CT results show the emergence of a new atom-atom correlation at the interfacial region (around 1.82 angstrom). This correlation is a characteristic of interfacial bonding between the fiber and its surrounding matrix, where the existence of chemical linkages (potentially Al-V-O-C) between fibers and the matrix contributes to the adhesion between the two constituents making up the composite. Due to such chemical bonding, the nanomechanical properties of the interfacial region fall between that of the carbon fiber and geopolymer. The combination of advanced techniques is proved useful for enhancing our understanding of the interfacial chemistry between fibers and the binding matrix. This level of knowledge facilitates the engineering of composite systems through the manipulation of their nanostructure.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Distinctive rheological and temporal viscoelastic behaviour of alkali-activated fly ash/slag pastes: A comparative study with cement paste

This paper presents the results of the investigation on the fundamental differences in the viscoelasticity between alkali-activated fly ash/slag materials (AAMs) and cement pastes. The effects of the precursor and the activator on the rheological behaviour of AAM pastes were studied. Given the specific precursor, the activator viscosity significantly affected the AAM paste viscosity. The high viscous activator increased the plastic viscosity of AAM pastes by 4–8 times higher than that of cement paste and drastically decreased the yield stress due to viscous effects and weak colloidal interactions. Temporal changes in viscoelasticity showed that the negligible colloidal interactions between particles in AAM paste made the system non-percolated until the initial setting. This resulted in very different viscoelastic behaviour compared to the one from the well-percolated network in cement paste. Considering all results obtained in this study, the paper describes the short-term evolution of the AAM paste from the fresh condition to the initial setting.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Performance of fly ash concrete with ferronickel slag fine aggregate against alkali-silica reaction and chloride diffusion

Ferronickel slag (FNS) is an industrial by-product of ferronickel alloy production at a high temperature which can be a promising potential to be used as fine aggregate to produce more sustainable concrete. In this study, the performance of concrete containing ferronickel slag sand and fly ash relating to alkali-silica reaction (ASR) and chloride contamination was investigated. ASR-induced expansion, chloride diffusion resistance, and chloride binding capacity of FNS concrete were determined through concrete prism tests (CPT), accelerated diffusion test, and bulk diffusion test. Thermogravimetric analysis (TGA) was conducted to measure the amount of Portlandite and Friedel's salt in concrete. Concrete with 50 wt% FNS sand as fine natural aggregate replacement and 25 wt% of cement replacement by fly ash showed a remarkable potential to be used not only as a low-carbon concrete with comparable mechanical properties to conventional concrete but also with a better performance against ASR and chloride contamination.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrical injection and transport in Teflon-diluted hole transport materials

Diluting small molecule organic semiconductor hole transport materials with Teflon AF (TAF) dramatically increases their thermal/morphological stability while counterintuitively also increasing the current density in hole-only diodes. Here, we explore the origin of this current enhancement in co-evaporated blends of TAF and N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPD) and find that it results from improved hole injection and not from a mobility enhancement due to semiconductor dilution effects. Whereas electroabsorption measurements indicate that an interface dipole shift of up to 0.5 eV (depending on TAF concentration) aides hole injection at the indium-tin-oxide anode, time-of-flight and temperature-dependent current-voltage measurements show that the mobility of 25 vol% TAF:NPD blends decreases by roughly an order of magnitude relative to neat NPD despite having slightly lower energetic disorder. For comparison, diluting with 25 vol% high density polyethylene achieves a similar reduction in energetic disorder but only decreases the mobility by a factor of two. Furthermore, these results highlight the tradeoff between reducing energetic disorder and frustrating percolation in diluted organic semiconductors, and represent a step toward thermally-evaporated polymer-small molecule composites that maintain high mobility while offering improved morphological stability and lower refractive index for organic light emitting diodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Information-rich localization microscopy through machine learning

Recent years have witnessed the development of single-molecule localization microscopy as a generic tool for sampling diverse biologically relevant information at the super-resolution level. While current approaches often rely on the target-specific alteration of the point spread function to encode the multidimensional contents of single fluorophores, the details of the point spread function in an unmodified microscope already contain rich information. Here we introduce a data-driven approach in which artificial neural networks are trained to make a direct link between an experimental point spread function image and its underlying, multidimensional parameters, and compare results with alternative approaches based on maximum likelihood estimation. To demonstrate this concept in real systems, we decipher in fixed cells both the colors and the axial positions of single molecules in regular localization microscopy data.

42 ENGINEERING↗