Low Ion Diffusion Research Articles

폴리머 시멘트 고화체에 대한 구조적 건전성 평가

폴리머 시멘트 고화체는 일반 몰타르 내의 시멘트 수화물을 폴리머 개질제를 이용하여 부분적으로 대체함으로써 그 기능을 강화시킨 복합재료로써, 특히 시멘트 몰타르에 폴리머를 첨가하는 것은 그 화학적 내구성을 향상시킨다고 알려져 있다 따라서 본 연구에서는 고화재료로서의 폴리머 시멘트에 대한 낮은 침투성 및 낮은 이온 확산도 등과 같은 향상된 화학적 내구성을 확인하기 위하여 폴리머 시멘트 시편들을 제조하였다. 이때 폴리머의 함량은 0 에서부터 30%까지 변화시켰으며, 물에 대한 시멘트 비 (W/C)를 33%와 50%로 각각 유지 시켰다. 충분히 경화시킨 후에, 제조된 시편들에 대한 구조적 건전성을 압축강도와 수침법에 의한 공극도를 통하여 평가하였다. 그 결과, W/C 비가 33%이고, 폴리머 함량이 약 10%인 폴리머 시멘트 시편에서 가장 향상된 개질변화를 얻을 수 있었다. 끝으로 이 최적의 조합비를 가지는 시편에 대하여 ANS 16.1에 따르는 침출시험을 수행하였으며, 그 결과를 일반 시멘트 고화체와 비교하였다. Polymer-modified cement is the composite material made by partially replacing and strengthening the cement hydrate binders of conventional mortar with polymeric modifiers such as polymer latexes and redispersible polymeric modifiers. It is known that the addition of polymer to cement mortar leads to improved quality, which would be expected to have a high chemical resistance. Therefore, the purpose of this study is to identify the improved chemical resistance, such as low permeability and low ion diffusivity, of the polymer-modified cement as a solidification agent for the radwaste. First, polymer-modified cement specimens by latex modification were prepared according to the polymer content from 0% to 30% to select the optimized polymer content. At those specimens, the water-to-cement (W/C) ratio was maintained to 33% and 50% respectively. After the much curing time, the structural integrity of specimens was evaluated through the compressive strength test and the porosity evaluation by the water immersion method. From the results, 10% of the polymer content at 33% of the W/C ratio was shown to have the most improved quality. Finally, the leaching test referredfrom ANS 16.1 for the specimens having the most improved quality was conducted. Dedicated specimens for the leaching test were then mixed with radioisotopes of <TEX>$^{60}Co$</TEX> and <TEX>$^{137}Cs$</TEX> at the specimen preparation.

Ultralong-CNTA-Supported Pd-Based Anodes for Ethanol Oxidation

A three-dimensional ultralong-CNTA-supported Pd-based anode (Pd/CNTA) has been designed for ethanol oxidation. Nano-size Pd particles were uniformly attached to the surface of CNTs within CNTA by using pulse electrodeposition technique. The as-prepared Pd/CNTA anode is featured with low ion diffusion resistance, vertical electron conduction, and large surface area, which offer enhanced active sites for ethanol oxidation reaction and allow fast ion adsorption/desorption from the electrode. The performance of the Pd/CNTA anode is tested and compared with that of Pd/C and Pd/MWCNT (multiwall carbon nanotube) electrodes for ethanol oxidation in alkaline media. The mass activity of the current Pd/CNTA electrode was 17.5 and 10.3 times higher than that of Pd/C and Pd/MWCNT electrodes, respectively. This novel electrode can provide efficient ethanol oxidation and may form a new promising platform for both fundamental investigation of the effects of electrode structure and the development of high-performance alkaline direct ethanol fuel cells.

Kinetics and Initial Stages of Bismuth Telluride Electrodeposition

The kinetics and initial stages of electrodeposition on gold from an aqueous electrolyte composed of bismuth and telluryl ions and nitric acid were investigated using voltammetry and chronopotentiometry. For tellurium alone, it is found that the reduction of tellurium ions to tellurium occurs with four exchanging electrons at vs ). At , tellurium reduces to under kinetic control and the following simultaneous diffusion-controlled reaction occurs with one exchange electron per tellurium atom: . Bismuth telluride deposition starts at more positive potential than bismuth and tellurium deposit individually. It is suggested that the electrodeposition of proceeds via a Stransky–Krastanov mechanism. From the Bard correction to the Sand equation, the diffusion coefficients of bismuth and telluryl ions were determined to be and , respectively, in the investigated solutions. Due to the low diffusivity and solubility of telluryl ions, the limiting current density for deposition in the investigated solution is low; it is only . Above the limiting current density, a smooth deposit cannot be obtained without agitation. A smooth deposit can be achieved when the depletion of anions is eliminated.

First Measurements of Ionisation Clusters on the DNA Scale in a Wall-less Sensitive Volume

A nanodosimetric technique and first ionisation cluster spectra on the DNA scale are presented. Single radiation-induced ions are extracted through a small aperture from a wall-less sensitive volume formed in a tissue-equivalent gas; after acceleration in vacuum, the ions are counted by an electron multiplier. Low ion diffusion pernilts the formation of well defined tissue-equivalent sensitive volumes, down to the scale of 0.1 nm; the upper limit is of the order of a few tens of nm. Distributions of ionisation clusters deposited by a particles in sensitive volumes down to 1.1 nm X I nm (diameter X length) are presented, with the relevance to nanodosimetric studies on the DNA scale. Present limitations and possible improvements are discussed.

Effects of long-range interactions on the dynamics of ions in aqueous solution

Molecular dynamics simulations have been carried out for a Br − ion in aqueous solution in order to establish the effect of truncation of long-range interactions on the dynamical properties of the ion. Simulations using smooth truncation of the potential at different cutoff radii were carried out and compared to results using the Ewald summation method. It is shown that when small cutoffs are applied (i.e. R c  8Å), the calculations yield low ionic diffusion coefficients relative to experiment, as well as short-time dynamical behavior which is inconsistent with the Ewald calculations. As the cutoff is increased, the results approach both the Ewald and the experimental results. In contrast to the results with the truncated potential, the short-time dynamical behavior of the hydrated bromide ion obtained with the Ewald method can be described by simple Langevin dynamics.

Porosity and Ion Diffusivity of Latex-Modified Cement

ABSTRACTLatex-modified Portland cement, which would be expected to have low permeability and ion diffusivity, was studied for possible application as a solidification agent for radioactive wastes generated from nuclear power plants. In order to predict the leaching ratio of radionuclides from the cementitious waste forms, the effect of water and latex content in the fresh cement paste on total porosity and ion diffusivity of hardened paste was quantitatively estimated.Total porosity of hardened cement paste decreased with the reduction of water content in the fresh paste and it was also reduced by latex addition. This latter effect could be attributed to the latex emulsion forming a water-proof film and filling the capillary pores. Also Cs ions diffusivity, which is the ratio of the diffusion coefficient in pore water to that in bulk water, showed an exponential correlation with total porosity for both cement materials. An empirical equation, expressing ion diffusivity as a function of total porosity, was derived from the consideration that the water constrictivity in this porous medium could cause an increase of the apparent viscosity of pore water. These results suggested a possibility that the transport behavior of radionuclides through the cementitious matrix could be estimated from the mixing parameters of the original cement pastes.

Cathodic Polarization Phenomena of Perovskite Oxide Electrodes with Stabilized Zirconia

The cathodic polarization of the electrodes sputtered on yttria stabilized zirconia electrolyte was studied at 800°C in air. The system showed high electrode activity for oxygen reduction. The study of the electrode resistance as functions of and temperature revealed that the rate‐determining steps for oxygen reduction were as follows: the charge transfer process for , the dissociation of oxygen molecules on the surface for and , and the oxygen diffusion on the electrode surface for . The marked electrode activity of was explained by the large effective reaction surface area, which is caused by the high oxide ion diffusivity and the high dissociation ability of oxygen molecules. In , the ability of the dissociation of oxygen molecules was considered to be lower than that in . The low activity of Cr‐perovskite electrode was explained by the low diffusivity of oxygen ions.

Mineral–water interactions and their influence on the physical behavior of highly compacted Na bentonite

Na bentonite is the active component of one of the engineered barriers that are being considered in Sweden to surround the final disposal sites of radioactive wastes from nuclear reactors. It has been chosen because of its low permeability and ion diffusivity and of its self-healing ability through swelling. These properties are due to mineral–water interactions that yield a microstructural pattern with very narrow passages and a low average mobility of interparticle water molecules at high bulk densities.