Analytical and experimental investigation on the operational characteristics and the thermal optimization of a miniature heat pipe with a grooved wick structure

A mathematical model for heat and mass transfer in a miniature heat pipe with a grooved wick structure is developed and solved analytically to yield the maximum heat transport rate and the overall thermal resistance under steady-state conditions. The effects of the liquid-vapor interfacial shear stress, the contact angle, and the amount of initial liquid charge have been considered in the proposed model. In particular, a novel method called a modified Shah method is suggested and validated; this method is an essential feature of the proposed model and accounts for the effect of the liquid-vapor interfacial shear stress. In order to verify the model, experiments for measuring the maximum heat transport rate and the overall thermal resistance are conducted. The analytical results for the maximum heat transport rate and the total thermal resistance based on the proposed model are shown to be in close agreement with the experimental results. From the proposed model, numerical optimization is performed to enhance the thermal performance of the miniature heat pipe. It is estimated that the maximum heat transport rate of outer diameter 3 and 4 mm heat pipes can be enhanced up to 48% and 73%, respectively, when the groove wick structure is optimized from the existing configurations. Similarly, the total thermal resistance of these heat pipes can be reduced by 7% and 11%, respectively, as a result of optimization.     

Graphene based materials: Past, present and future

Graphene, a two dimensional monoatomic thick building block of a carbon allotrope, has emerged as an exotic material of the 21st century, and received world-wide attention due to its exceptional charge transport, thermal, optical, and mechanical properties. Graphene and its derivatives are being studied in nearly every field of science and engineering. Recent progress has shown that the graphene-based materials can have a profound impact on electronic and optoelectronic devices, chemical sensors, nanocomposites and energy storage. The aim of this review article is to provide a comprehensive scientific progress of graphene to date and evaluate its future perspective. Various synthesis processes of single layer graphene, graphene nanoribbons, chemically derived graphene, and graphene-based polymer and nano particle composites are reviewed. Their structural, thermal, optical, and electrical properties were also discussed along with their potential applications. The article concludes with a brief discussion on the impact of graphene and related materials on the environment, its toxicological effects and its future prospects in this rapidly emerging field.     

Effect of Ga doping concentrations,substrate temperatures and working pressures on the electrical and optical properties of ZnO thin films

We fabricated Ga-doped ZnO (GZO) thin films on glass substrate by RF magnetron sputtering method with different conditions of Ga₂O₃ concentration, substrate temperature and working pressure. Next we investigated the electrical, optical and structural properties of the GZO thin films. At a substrate temperature of 300 °C, a working pressure of 1 mTorr, and a Ga₂O₃ concentration of 3 wt%, the GZO thin films showed the lowest resistivity of 3.16 × 10^?4 Ω cm, a carrier concentration of 7.64 × 10²⁰ cm^?3 and a Hall mobility of 25.8 cm²/Vs. Moreover, the GZO thin films exhibited the highest (002) orientation under the same conditions and the full width at half maximum of X-ray peak was 0.34°. All GZO thin films showed the optical transmittance of more than 80 % in the visible range regardless of working conditions. The Burstein–Moss effect was observed by the change of doping concentration of Ga₂O₃. The GZO thin films were fabricated to have the good electrical and optical properties through optimizing doping concentration of Ga₂O₃, substrate temperature, working pressure. Therefore, we confirmed the possibility of application of GZO thin film as transparent conductive oxide used in flat panel display and solar cell.     

Practical switch based state-space modeling of DC-DC converterswith all parasitics

All parasitics such as switch conduction voltages, conduction resistances, switching times, and ESRs of capacitors are counted in a proposed DC-DC power convertor state-space modeling based method on nonideal switching functions. An equivalent simplified model is derived from the complex circuit with parasitics. The modeling procedure is shown for the buck-boost converter as the general converter among the buck, boost, and buck-boost converters. The pole frequency, DC voltage gain, and efficiency are analyzed and verified by experiments that show good agreement with theory. The procedures for determining the gain margin of the controller, the turn ratio of an isolation transformer, the optimum duty factor, and the switching frequency are given for an example flyback converter     

PWSCC of thermally treated alloy 600 pulled from a Korean plant

Three tubes of alloy 600 were pulled out from a Korean nuclear power plant. The microstructure was analyzed using an optical microscope and TEM. Information on the crack length and depth was obtained by metallography, and crack detection and evolution were evaluated by analyzing the eddy current data obtained from each in-service-inspection (ISI). The carbon content in the pulled tubes was higher (around 0.03 wt.%) than that (around 0.015 wt.%) of Alloy 600 tubing used in other operating nuclear power plants. Most carbides in the pulled tubing were distributed in the grains rather than along the grain boundaries. The poor microstructure might come from high carbon contents, low temperature annealing, or high residual stresses during tube straightening. Mill annealing temperature should be high enough to dissolve all carbon in order to decorate the grain boundaries with semi-continuous carbide precipitation during 700 °C thermal treatment. Shot peening seemed to suppress the growth of the axial cracks, while it was analyzed to play a role in increasing crack growth in the wall thickness direction.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

The bulk piezoresistive characteristics of carbon nanotube composites for strain sensing of structures

The bulk piezoresistivity of carbon nanotube (CNT) in polymer matrix was discussed to develop a strain sensor for engineering applications. The polymer improves interfacial bonding between the nanotubes and the CNT composite and that enhances the strain transfer, repeatability, and linearity of the sensor. The largest contribution of piezoresistivity of the sensor may come from slippage of overlaying or bundled nanotubes in the matrix, from a macroscopic point of view. Nano interfaces of CNTs in a matrix polymer also contribute to the linear strain response compared to other micro size carbon filler. The strain sensor had a low bandwidth and adequate strain sensitivity. The nanocomposite strain sensor is particularly useful for detecting large strains which can monitor strain and stress on a structure with simple electric circuit for strain monitoring of structures.     

Synthesis and characterization of Y-shape electron donor–acceptor type organic dyes for dye-sensitized solar cells

Three Y-shape organic dyes, (Z)-3-(5-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)thiophen-2-yl)-2-cyanoacrylic acid (OD-1), (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-2,2′-bithiophen-5-yl)-2-cyanoacrylic acid (OD-2) and (Z)-3-(5′-(3,5-bis(4-(9H-carbazol-9-yl)styryl)-4-methoxyphenyl)-3,4′-4″-trithiophenyl-5-yl)-2-cyanoacrylic acid (OD-3) were synthesized and used as sensitizers in nanocrystalline dye-sensitized solar cells (DSSCs). The introduction of the bis(carbazolylstyryl) units as an electron donor group and oligothiophene units as a both electron donors and π-spacers increased the conjugation length of the sensitizers and thus improved their molar absorption coefficient and light harvesting efficiency. DSSCs with the configuration of SnO₂:F/TiO₂/organic dye/liquid electrolyte/Pt devices were fabricated using these OD-1, OD-2 and OD-3 as a sensitizers. Among the devices, the DSSC composed of OD-3 exhibited highest power conversion efficiency of 3.03% under AM1.5G (100 mW cm⁻²).     

Adsorption of CO<Subscript>2</Subscript> and CO on H-zeolites with different framework topologies and chemical compositions and a correlation to probing protonic sites using NH<Subscript>3</Subscript> adsorption

Adsorption of CO₂ and CO at 25 °C has been conducted using commercially-available (Y, ZSM-5) and laboratory-synthesized (SSZ-13, SAPO-34) H-zeolites with different framework topologies and chemical compositions, and their textual and surface properties have been characterized by N₂ sorption and NH₃ adsorption techniques. All the zeolites were microporous, although ZSM-5 and SSZ-13 apparently showed a mesoporous sorption behavior due to the interparticle spaces. The zeolites had Si/Al values in the order of SSZ-13 (16.44) > ZSM-5 (16.08) ? Y (2.82) ? SAPO-34 (0.19). Regardless, high CO₂ adsorption capacity was obtained for SSZ-13 and SAPO-34 with a CHA framework. The FAU zeolite Y with the highest micropore volume showed less CO₂ adsorption than the CHA zeolites and the MFI-type ZSM-5 yielded the poorest performance. Probing acid sites in the H-form zeolites using NH₃ disclosed that these all contain both weak and strong acid sites with significant dependence of their strengths and amounts on the topology. The acid strength of the weak acid sites in the CHA zeolites was the weakest, which might allow a stronger interaction with CO₂. The H-zeolites gave CO₂/CO selectivity factors that were in the range of 4.61–11.0, depending on the framework topology.     

In situ Lubrication Dispersion of Multi‐Walled Carbon Nanotubes in Poly(propylene) Melts

An in situ lubrication dispersion method is developed to achieve electrical conductivity in PP containing a small amount of MWCNTs. Good dispersion of the MWCNTs in PP is observed even after a short mixing time because the interactions between the entangled nanotubes are reduced. By in situ lubrication dispersion, the electrical percolation threshold of the PP nanocomposite can be as low as 0.5–0.7 wt% MWCNT. Rheological data also support percolation at 0.5 wt% MWCNT. With 0.5 wt% MWCNT, the slope of G′ at low frequency approaches unity and shows non‐terminal behavior. The proposed dispersion method enhances the wetting of MWCNTs and improves MWCNT dispersion compared to both direct mixing of MWCNT powder with a polymer melt and conventional master batch dilution.