Numerical treatment of pebble contact in the flow and heat transfer analysis of a pebble bed reactor core

This paper studies the numerical treatment of the inter-pebble regions in the modeling of a packed bed geometry for the computational fluid dynamics (CFD) analysis of a pebble bed reactor (PBR) core, where the pebbles are physically in contact with each other. In some studies, the inter-pebble regions have been approximated with gaps, in consideration of the problems on mesh quality or economy of the CFD calculation. To examine such a methodology, a sensitivity analysis for the gap size was conducted with two spherical pebbles, where the inter-pebble region was modeled by means of two kinds of inter-pebble gap and two kinds of direct contact. The cases of direct contact showed numerous differences in the results of the flow regime around the pebbles as well as in the wake, compared to the cases of the inter-pebble gap. No large differences were found between the two cases of direct contact. Based on the result of the sensitivity analysis, the two cases of inter-pebble modeling, i.e., the 1-mm gap and area-contact, were applied to the PBR simulation. It was concluded that the flow regimes and their relevant flow-induced local heat transfer were significantly dependent on the modeling of the inter-pebble region.     

Influence of laser treatment on the electrical properties of plasma-enhanced-atomic-layer-deposited TiO2 thin films

TiO₂ dielectric films with 38 nm thickness were grown on Si (100) substrates at 200°C by plasma-enhancedatomic-layer deposition. Laser-irradiated TiO₂ films maintained an amorphous phase similar to as-grown films and showed an increase in permittivity and leakage current density with increasing laser powers and the number of laser shots at constant laser power. Laser-irradiation of TiO₂ films at room temperature produced oxygen vacancies at the film surface and new Ti³⁻ valences. The electrons and space charges produced through the defect chemistry increased the leakage current density and permittivity in laser-irradiated TiO₂ films, respectively. The dielectric and electrical properties of the laser-irradiated TiO₂ films were completely recovered to correspond with those of as-grown films by post-annealing at 300°C for 5 min in O₂ ambient.     

Formation and Size Control of ZnO Nanowires on Al-Zn-Si-Fe Alloy by Directed Melt Oxidation Process

Tetrapod like ZnO nanowires were formed by the directive melt oxidation of Al-Zn-Si-Fe alloy at the temperature over 1000°C in air. X-ray diffraction patterns revealed that the ZnO nanowires had wurtzite structure with the c-axis and a-axis lattice constants of 0.520 and 0.325 nm, respectively. The lattice constants are similar with those of ZnO single crystal. The size control of ZnO nanowires was achieved by varying the oxidation temperature and the amount of Zn in alloy. The lower the oxidation temperature was, the smaller was the diameter of the ZnO nanowires. And with decreasing the amount of Zn, it was diminished the diameter of the formed ZnO nanowires. The diameter was as small as 50 nm at the amount of Zn of 5 wt%.     

3D-printed wood-polylactic acid-thermoplastic starch composites: Performance features in relation to biodegradation treatment

In view of the current public concern about microplastics, biodegradability of 3D-printed polymeric materials is an important issue. In this work, we studied the effects of soil-burial biodegradation treatment on wood-polylactic acid (PLA) blends with three different thermoplastic starch (TPS) contents (i.e., 0, 10, and 20 wt%). The wood-PLA-TPS composites were 3D-printed through the fused deposition modeling technique. The use of TPS led to reduced mechanical properties and thermal stability of the composites. Biodeterioration of the composites was clearly observed for the composite with high TPS content after the soil burial test. Thermal decomposition temperature, activation energy, and degree of crystallinity of biodegradation treated composites increased for composites with higher TPS content. The observed behavior was due to decomposed TPS and wood in the composites from biodegradation treatment, resulting in increase in the relative PLA content level. Also, the weight loss percentage of composites increased from 3.2% to 18.5% after the soil burial test as the TPS content in the composites increased from 0 to 20 wt%. The presence of TPS and wood helped improve the biodegradation of the 3D printed PLA composites, while PLA itself is not easily bio-degraded.     

Effects of deposition temperature on the crystallinity of Ba0.5Sr0.5TiO3 epitaxial thin films integrated on Si substrates by pulsed laser deposition method

Epitaxial Ba_0.5Sr_0.5TiO₃ (BSTO) thin films were grown on TiN buffered Si (0 0 1) substrates by PLD method and the effects of deposition temperature on their crystallinity and microstructure were studied. BSTO thin films were prepared with substrate temperature ranging from 350 to 650 °C. The BSTO films grown at below 400 °C showed amorphous phase and the film grown at 450 °C showed mixed phase of crystalline and amorphous, where crystalline phase was observed only at the top surface portion of the film. The BSTO films with fully crystalline phase were obtained in the samples deposited at above 500 °C. The (0 0 l) preferred orientation and the crystallinity of the BSTO films were improved with increasing the temperature. The dielectric constant, measured at 100 kHz and at room temperature, of the BSTO film grown at 650 °C was measured to be as high as 1129.     

A thermal-fluid assessment of a cooled-vessel concept for a VHTR

Flow distribution and thermal analyses of a conceptual design of a cooled vessel for a very high temperature reactor (VHTR), which has a forced vessel cooling with an internal coolant path through a permanent side reflector, have been performed. A computational fluid dynamics (CFD) code was employed to investigate flow distributions at inlet and upper plenums of the proposed cooled-vessel concept. Thermal-fluid analyses of the cooled vessel during a normal operation were carried out by using the CFD code with the boundary conditions provided by the GAMMA system analysis code. The transient analyses during postulated accidents were conducted by the GAMMA code itself. According to the results, the flow deviation at the riser holes due to a change of the inlet flow path to the core inlet is about ±20% which results in about a 3-7% core flow deviation from the average value depending on the upper plenum height. The pressure drops in the inlet and upper plenums are estimated to be from 13 to 25 kPa with a change of the upper plenum height. A cooling flow of more than 4 kg/s is sufficient to maintain the RPV temperature within the required limit during a normal operation. Transient analysis reveals that the reactor vessel is exposed to a temperature above its limit of 371 °C but this duration is shorter than the allowable time for a creep region with a sufficient safety margin. The results suggest that the cooled-vessel concept considered in this paper has the potential to be used for a VHTR but further and more detailed studies are required to realize the proposed concept.     

A Robust Joint Frequency Offset Estimation for the OFDM System Using Cyclic Delay Diversity

Cyclic delay diversity (CDD) is a low-complexity transmit diversity technique for orthogonal frequency division multiplexing (OFDM), which transforms a multiple-input channel into an equivalent single-input channel with a large delay spread. Consequently, high frequency selectivity makes channel and frequency estimation a challenging task. This paper proposes an improved joint estimation of carrier frequency offset and sampling frequency offset for the OFDM system using CDD technique. By finding the amount of a transmit-antenna specific delay which reduces the variance of the frequency estimation scheme, these parameters improve the robustness of the joint frequency estimation scheme against the frequency selectivity of the channel. Computer simulation shows that the joint frequency-offset estimation scheme with properly designed cyclic delay parameters performs robustly, with the performance better than the conventional scheme.     

Formation and characterization of nanoparticles based CuInS2 absorbing layer for solar cell

CuInS2 powders have been successfully prepared by simple heating a mixture of copper nitrate, indium nitrate and thiourea in ethylene glycol. The prepared powders were fully characterized by SEM, XRD and UV-Vis spectroscopy. The particle shape of the powders obtained with an optimal condition showed spherical shape with about 50 nm in size. The XRD results showed more enhanced crystallized CuInS2 with chalcopyrite structure with increasing reaction time. The values in band-gap energy of nano-sized CuInS2 powders would be estimated 1.63 eV, blue-shifted from that of micron-sized powders due to size quantization effect.     

Crystalline characterization and photodecomposition properties of rod-shaped Na2Ti6O13 powder prepared by molten salt process

To prepare one-dimensional nanostructured Na2Ti6O13 powder, the starting materials of TiO2, NaCl and Na2CO3 were mixed and then heat-treated at 1000 degrees C for 2 hrs in air under molten state of NaCl. Changes in shape and phase, photo absorbance and photocatalytic ability of TiO2 particle were observed controlling added amount of Na2CO3 under constant weight ratio of TiO2 to NaCl using SEM, X-ray diffractometer, Raman spectroscopy, and UV-Vis spectroscopy. The TiO2 particle was changed into rod-shape Na2Ti6O13 with the addition of Na2CO3, showing increase in optical energy band-gap of the powder as well as gradual decrease of the photo-decomposition ability.     

Excess Bismuth-Dependent Characteristics of Chemical Solution Deposited Bi3.15La0.85Ti3O12 Thin Films

The phase formation and electrical properties of (Bi_3.15La_0.85)Ti₃O₁₂ (BLT) thin films prepared by the chemical solution deposition method on Pt/Ti/SiO₂/Si substrates have been investigated. It was observed that the microstructure and electrical properties of BLT thin films dramatically varied with the excess Bi content. The crystallographic orientation of BLT films was varied with excess Bi content and the intermediate rapid thermal annealing (RTA) process. While BLT thin films prepared without intermediate RTA process have ?117? orientation irrespective of excess Bi content, BLT thin films with RTA process at 450°C have an orientation change with excess Bi content. The leakage current of BLT thin films slightly increased with increasing excess Bi content up to 6.5% and then considerably decreased in BLT film with 10% Bi, where was revealed to be almost stoichiometric composition.