An Opportunistic Directional MAC Protocol with Pulse/Tone Exchange in Wireless Ad-Hoc Networks

Wireless Personal Communications - This paper proposes a Medium Access Control (MAC) protocol using directional antennas in wireless ad-hoc networks, which achieves frame-collision reduction,...     

Equilibrium and crystallographic measurements of the binary tetrahydrofuran and helium clathrate hydrates

Clathrate compounds are crystalline materials formed by a physical interaction between host and relatively light guest molecules. Various types of nano-sized cages surrounded by host frameworks exist in the highly unique crystalline structures and free guest molecules are entrapped in an open host-guest network. Recently, we reported two peculiar phenomena, swapping and tuning, naturally occurring in the hydrate cages. Helium, one of the smallest light guest molecules, must be the challengeable material in the sense of physics and moreover possesses versatile applications in the field of superconductivity technology and thermonuclear industry. In this regard, we attempted for the first time to synthesize helium hydrates at moderate temperature and pressure conditions. According to inclusion phenomena, helium itself normally cannot form clathrate hydrates due to being too small molecularly without the help of hydrate former molecules (sI, sII, and sH formers). In this study, the hydrate equilibria of the binary clathrate hydrate containing tetrahydrofuran, helium, and water were determined at 2, 3, 5.56 THF mol%. Direct volumetric measurements were also carried out to confirm the exact amount of helium captured in the hydrate cages. Finally, the crystalline structure of the formed mixed hydrates was identified by powder X-ray diffraction, resulting in structure II.     

Development of an SH-SAW sensor for the detection of DNA hybridization

We have developed SH (shear horizontal) surface acoustic wave (SAW) sensors for the detection of DNA (deoxyribonucleic acid) hybridization on the gold-coated delay line of transverse SAW devices. DNA hybridization experiments were performed with 15-mer oligonucleotides (probe and complementary target DNA). The sensor consists of twin SAW delay line oscillators (sensing channel and reference channel) operating at 100 MHz fabricated on 36° rotated Y-cut X-propagation LiTaO₃ piezoelectric single crystals. The relative change in the frequency of the two oscillators was monitored to detect hybridization between the target DNA and probe DNA immobilized on the delay line of the SAW sensor. The measurement results showed a good response of the sensor to the mass loading effects of the DNA hybridization with a sensitivity level up to 1.55 ng/ml/Hz.     

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state ¹³C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The ¹³C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.     

Dynamics of hydrogen molecules in the channels of binary THF-H2 clathrate hydrate and its physicochemical significance on hydrogen storage

We have studied the dynamics of the H₂ loaded THF (tetrahydrofuran) clathrate hydrate by using an inelastic neutron scattering technique. At 2 K, a well defined single peak indicative of the ortho (J = 1) to para (J = 0) hydrogen transition is observed at 13.6 meV. The neutron spectrum undergoes two distinct changes as the temperature increases. First, additional peaks appear from splitting of the rotational transition above 10 K. Upon further heating, a quasi-elastic neutron scattering (QENS) signal due to hydrogen molecular diffusion through the THF hydrate crystalline lattice grows significantly above 65 K. These results indicate that H₂ molecules trapped inside the THF hydrate crystalline structure undergo two distinct transformations in their dynamical behavior.     

Tuning clathrate hydrates: Application to hydrogen storage

Gas hydrates represent an attractive way of storing large quantities of gases such as methane, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we proposed the peculiar cage occupancy dynamics observed at the mixed hydrates simultaneously containing both one gas and one liquid guest. Liquid guests participating in forming hydrates must be water-soluble and possess the sufficient interaction with surroundings in order to make strong host-guest networks. However, it seems to be almost impossible for small molecules such as hydrogen and methane to replace the relatively large liquid guest molecules and thus an appropriate condition needs to be made for small guests to be enclathrated in large cages. By lowering the concentrations of liquid guests the small gas guests are found to enter the large cages feeling like their home. We also tried to explain this natural phenomenon with solid solution theory for clathrate hydrates. Special types of water-soluble liquid materials, such as THF, 1,4-dioxane, and t-BuNH₂, form sII double hydrates upon reacting with gas guest molecules. We expect that the storage capacities of gas guests largely depend on both the chemical nature of liquid guests and their relative concentration in host water. This tuning mechanism occurring in the mixed clathrate mixtures appears to be quite general and can be applied to energy and environmental systems including gas storage and transportation fields.     

Phase and kinetic behavior of the mixed methane and carbon dioxide hydrates

Large amounts of CH₄ are stored as hydrates on continental margins and permafrost regions. If the CH₄ hydrates could be converted into CO₂ hydrate, they would serve double duty as CH₄ sources and CO₂ storage sites in the deep ocean sediments. As preliminary investigations, both the phase behavior of CH₄ hydrates and kinetic behavior of CO₂ hydrate were measured at versatile conditions that can simulate actual marine sediments. When measuring three-phase equilibria (H-LW-V) containing CH₄ hydrate, we also closely examined pore and electrolyte effects of clay and NaCl on hydrate formation. These two effects inhibited hydrate nucleation and thus made the hydrate equilibrium line shift to a higher pressure region. In addition, the kinetic data of CO₂ hydrate in the mixtures containing clay and NaCl were determined at 2.0 MPa and 274.15 K. Clay mineral accelerated an initial formation rate of CO₂ hydrate by inducing nucleation as initiator, but total amount of formed CO₂, of course, decreased due to the capillary effect of clay pores. Also, the addition of NaCl in sample mixtures made both initial formation rate and total amount of CO₂ consumption decrease.     

Effect of photosensitivity in acrylic photoreactive electrode pastes on line width uniformity for large‐sized plasma display panels

Ag electrodes with line width uniformity for large‐sized plasma display panels were successfully fabricated through a photolithographic process using photosensitive Ag pastes with optimized photosensitive properties. The photosensitivity of the Ag electrode pastes in the photolithographic process was investigated as a function of the types and contents of photoinitiators, the molecular weights and acid values of acrylic binders with carboxylic acid groups, and the process variables, such as the UV‐light intensity and dose, with a step tablet. This study revealed that the photoinitiator was a major parameter for the photosensitivity of the Ag electrode pastes. With the photosensitivity of the photosensitive Ag electrode pastes optimized by the study of the photoinitiator contents, Ag electrodes with line width uniformity were achieved with an HSP‐188 photoinitiator content of 15 wt % on the basis of the reactive monomers, regardless of the variation of the light dose from 250 to 350 mJ/cm² and intensity from 15 to 25 mW/cm². © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A Delay-Aware Congestion Control Protocol for Wireless Sensor Networks

In wireless sensor networks, congestion leads to buffer overflowing, and increases delay. The tradi-tional solutions use rate adjustment to mitigate congestion, thus increasing the delay. A Delay-aware congestion con-trol protocol (DACC) was presented to mitigate congestion and decrease delay. In order to improve the accuracy of the existing congestion detection model which is based on the buffer occupancy of a single node, DACC presents a new model considering both the real-time buffer occupancy and the average transmission time of packets. DACC uses the untapped bits in the IEEE 802.11 Distributed coordination function (DCF) frames header to carry congestion infor-mation. During the congestion alleviation period, DACC presents a channel occupancy mechanism which is based on the real-time buffer occupancy for the purpose of decreas-ing delay and preventing packet loss. Simulation results indicate that in terms of delay, packet delivery ratio, col-lision and buffer load, DACC has comparative advantages than those of 802.11 DCF, Priority-based congestion con-trol protocol (PCCP) and Decoupling congestion control and fairness (DCCF).     

Surface plasmon resonance spectroscopic comparison of spiroxazine monolayer and polymeric thin film

Photochromic dye has recently attracted much interest in optical applications because of its reversible photoswitching properties by light irradiation. To apply those aspects of photochromism to actual photonic devices, the form of the thin film must be favorable to the photochromic molecular system. In this study, a polymeric thin film and self‐assembled monolayer were constructed with a photochromic spiroxazine derivative that is well known for its good fatigue properties. Their photoisomerization properties were characterized through surface plasmon resonance (SPR). As a result of ultraviolet irradiation to these thin films, the SPR angle shift clearly appeared as the optical thickness (Refractive index × Geometrical thickness) change in both of the films. The Fresnel calculation based on measurement results showed that the angle shift of the polymeric thin film mainly corresponded to the change of the refractive index. For the spiroxazine monolayer, the geometrical thickness change caused the resonance angle to shift dominantly. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3459–3465, 2003