Chul Am Kim

A yield stress scaling function for electrorheological fluids

The yield stress dependence on electric field strength for electrorheological (ER) fluids is examined. A proposed scaling function incorporates both the polarization and conductivity models. Proper scaling allows yield stress data for ER fluids to collapse onto a single curve for a broad range of electric field strengths.

An exponential decay function for polymer degradation in turbulent drag reduction

The mechanical degradation of high molecular weight polymers under turbulent flow conditions was investigated using a rotating disk apparatus. The validity of the empirical exponential decay function, which has been used to represent the degradation phenomenon in polymer induced turbulent drag reduction in a pipe flow, has been investigated. Results show that the single exponential decay model is not universally suitable for all polymeric drag reducers, but it effectively represents shear resistant, drag reducing agents, including the various polysaccharides, and is also applicable in describing short time degradation behavior.

Viscosity of magnetic particle suspensions

A rheological approach was used to study the effects of microstructure on the viscosity of magnetic particle suspensions as functions of concentration and shear rate. Empirical formulas derived from mean-field theory and the Mooney equation were used to relate viscosity with particle concentration for both rod- and plate-like particles. The Casson equation was employed to investigate the shear rate dependence of the viscosity.

Rheology of perfluoropolyether lubricants

The rheological properties, including melt viscosity, storage modulus and loss modulus of perfluoropolyether (PFPE) lubricants with different molecular weights and chain-end functionalities, were measured using a rotational rheometer. The melt viscosities of fractionated PFPEs exhibited Newtonian behavior, and increased linearly with molecular weight. For PFPE containing functional, polar endgroups there is a sharp transition in the slope for the viscosity at a critical molecular weight. We also found that the intrinsic viscosity or Huggins coefficient can be used to examine media noise developed in the manufacturing stage.

Effect of reactive end groups on the rheology of disk lubricant systems

We systematically investigate the properties of perfluoropolyethers with different weights and chain-end functionalities, which are used as lubricants in hard disk drive systems. The temperature dependence of the viscosity yields an Arrhenius form; the activation energy and hydrodynamic volume are determined for different molecular weights. The activation energy depends on both end group functionality and molecular weight. Also, modified Cole-Cole plots for storage modulus vs. loss modulus reveal the presence of microstructures due to the interaction of polymer chain-end groups in the bulk. The viscosity-molecular weight relationship exhibits a remarkable crossover behaviour at a critical molecular weight (M/sub c/) for ZdolTX (Zdol with C/O end group ratio of 1.7). Fly stiction (tribology) data also exhibit an M/sub c/ and an excellent correlation with viscosity (rheology) is obtained.

An array of inductively coupled plasma sources for large area plasma

An array of 2 £ 2 inductively coupled plasma (ICP) sources has been built by modifying the conventional reactive ion etching (RIE) type LCD etcher. Each ICP has its own planar circular antenna and quartz dielectric window to the process chamber. One RF power supply and only one impedance matching network are used for delivering the power to all four ICP sources. Distribution of ion and electron densities and electron temperature are measured in terms of the number of acting ICP sources, chamber pressure and RF power. An uniform oxygen plasma density in the order of 10 15 m 23 can be obtained at RF power of 600 W in process chamber of 620 £ 620 mm 2 cross-section, which is higher than that of RIE plasma. By adjusting the power distribution among the four ICP units, uniformity of the plasma can be improved to 10% on substrate stage. Photoresist etching by the oxygen plasma is performed on 320 £ 400 mm glass plates. q 1999 Elsevier Science S.A. All rights reserved.

Synthesis and characterization of polysaccharide phosphates based electrorheological fluids

Abstract. Polysaccharide phosphate particles were synthesized via an esterification process with various molar concentrations of phosphoric acid using both cellulose and potato starch. The particles were characterized with thermogravimetric analysis, scanning electron microscopy, and an inductively coupled plasma-mass spectrometer. These particles were then dispersed in a silicone oil to make anhydrous electrorheological (ER) fluids. Each sample of polysaccharide particles used in this study exhibits thermal stability and possesses excellent ER properties when measured as functions of both particle concentration and electric field strength by a rheometer equipped with a high voltage generator. A difference in the flow behavior for both potato starch and cellulose-phosphates-based ER fluids was also identified.

Electrical and chemical properties of photo-cross-linked polymeric insulating materials

Photosensitive polymeric insulating films, which were prepared by mixing host polymers and a photosensitizer, exhibit high chemical resistance and excellent electrical properties. These polymer films formed by the spin-coating method were exposed through a mask to patterned radiation of UV light. The nonexposed areas of these films were dissolved by an appropriate solvent. Leakage current densities for these films are very low, approximately 5 ×10-7 ~3 ×10-8 A/cm2 at a bias of 10.0 V. These polymer insulators are particularly suitable for plastic-based electronic device applications.

Organic–inorganic hybrid gate dielectric for solution-processed ZnO thin film transistors

The preparation of a hybrid dielectric film was carried out by blending sol–gel-derived sodium beta alumina (SBA) and poly(4-vinylphenol) (PVP) to enhance the capacitance of a gate dielectric film. PVP-SBA was cured at a temperature sufficiently low enough to apply to a plastic substrate, while maintaining good electrical properties and uniformity. Addition of sol–gel-derived SBA improved the film density, resulting in good PVP-SBA thermal stability. The prepared PVP-SBA was used for high-performance aqueous solution-based ZnO transistors at 200 °C.

Abnormal scattering of polymer in binary solvent

The behavior of a high molecular weight polymer (polyethylene-oxide, PEO) in a binary liquid mixture (nitroethane/3-methyl-pentane, NE/MP) is studied at the one-phase temperature of NE/MP by static and dynamic light scattering methods. We found that the scattering intensity increased abruptly near the critical composition of NE/MP although the sample was very far from the critical temperature of NE/MP. Explanations in terms of critical opalescence and wetting layer inversion are discussed.