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Abstract The hybrid composite films containing nanodiamonds (ND) coated with polydopamine (pDA) (ND-pDA) and reduced graphene oxide (rGO) converted from graphene oxide (GO) films are designed for achieving extremely high thermal conductivity. The ND-pDA/rGO films are successfully fabricated using the vacuum-filtration process followed by a heat treatment at 800 °C. The thermal conductivities of the films in the in-plane (K //) and through-plane (K ⊥) directions are measured by the laser flash method to better understand how the addition of dopamine, the amount of ND-pDA, and the test temperature affect the thermal properties of the hybrid films. The experimental results show that the addition of dopamine results in dense structure of the ND-pDA/rGO hybrid films, which is favorable for phonon transport, and thus remarkably increase the thermal properties of the film. Additionally, films with higher ND-pDA loading possess lower in-plane but higher through-plane thermal conductivity. K // and K ⊥ of 1406 and 0.677 W m−1 K−1, respectively, for 20ND-pDA/20rGO measured at 25 °C are achieved. Both the K // and K ⊥ of 20ND-pDA/20rGO increase with test temperature. Maintaining such high thermal conductivities at high temperature, the hybrid films are believed to be suitable for lightweight thermal management materials with high heat transfer properties in a specific direction. [ABSTRACT FROM AUTHOR]

The investigation on the augmentation of convection heat transfer of water rod bundle flow was performed experimentally. The 4 × 4 tightly arrayed rod bundle with narrow gap was tested and its P / D (Pitch-to-Diameter ratio) was ∼1.08. As the spacer grid, the twist-vane spacer grid was used. The axial mean bundle-flow velocity was U ∼ 1.0 m/s, and the corresponding Reynolds number was ∼10,400. Circumferential and axial distributions of rod-wall temperatures were measured. Upstream of twist-vane spacer grid, the rod-wall temperature at the subchannel center was measured to be lower than that at the rod-gap center. Downstream of twist-vane spacer grid, the rod-wall temperature toward the tip of twist-vane was lowest, which might be due to the deflected flow caused by twist-vane. The twist-vane spacer grid enhanced significantly the convection heat transfer in the tight-lattice rod bundle, and its maximum enhancement value was ∼77% near the downstream of spacer grid. Based on the present experimental data, the previous correlations of Nusselt number enhancement were compared and assessed. Then, the enhancement correlation of convection heat transfer for the twist-vane spacer grid in the tightly arrayed rod bundle was proposed. [ABSTRACT FROM AUTHOR]

In the present experimental study, the effect of change in surface condition induced by oxidation on the transient pool boiling heat transfer was investigated using vertical SS (Stainless Steel) and copper rodlets. The quenching method was applied, and pure water was used as a quenchant. Various kinds of surface characterizations (i.e., photograph, microscopic image, surface roughness, and water contact angle) together with boiling visualizations were provided and discussed in detail. The surface conditions of the copper test specimen were more sensitively influenced by the repeated quenching tests and 2 h-oxidation, as compared with those of a SS test specimen. The copper test specimen showed a shorter quenching duration than the SS test specimen, owing to its smaller heat capacity. Through a 2 h-oxidation of the test specimens, the surface conditions and quenching curves of the SS test specimen were not altered much. On the other hand, the copper had a very rough surface with unique feather-like structures, which considerably enhanced the quenching performance. This may be because the flaky feather-like structures kept disrupting the vapor film. This result implies that the transient pool boiling heat transfer can be improved and controlled by the functionally well-designed surface structure. Through the boiling visualizations of 2 h-oxidized SS and copper test specimens, the vapor film collapse modes were observed to be different. In SS, the propagative collapse mode was mainly observed. On the other hand, in copper, the vapor film seemed to begin to be collapsed through a coherent collapse mode. Based on this study, it was found that the quenching performance and vapor film collapse mode are strongly influenced by changes in the surface conditions induced by oxidation. [ABSTRACT FROM AUTHOR]

The aqueous extract of Hibiscus sabdariffa L., which is inherently non-toxic, environmentally friendly and cost effective, was used as a green reducing agent to chemically reduce graphene oxide (GO). The GO and reduced graphene oxide by the aqueous extract of Hibiscus sabdariffa L. (HRGO), prepared under different conditions, were characterized. The characterization was conducted to better understand the evolution of HRGO from GO. The flexible graphene film electrodes were fabricated using the synthesized HRGO as a primary material that exhibited a good specific capacitance of 133.07 F g −1 , which attributed to the surface modification of the HRGO using the extract of Hibiscus sabdariffa L. Furthermore, over four orders of magnitude in electric conductivity (while the sheet resistance decreased from 0.63 MΩ/sq. to 36.50 Ω/sq.) were achieved with an excellent specific capacitance of 204.38 F g −1 when the HRGO was treated in a household microwave oven. [ABSTRACT FROM AUTHOR]

The influence of heated surfaces and fluids on pool boiling heat transfer was investigated experimentally. As the test specimen, Plain Surface (PS) and Nano-Structured Surface (NSS) were used. As the test fluid, pure water, an aqueous Lithium Bromide (LiBr) solution, and an aqueous Sodium Dodecyl Sulfate (SDS) solution were tested. Under the saturated water and aqueous LiBr solution conditions, the NSS showed a lower Onset of Nucleate Boiling (ONB) and higher Nucleate Boiling Heat Transfer Coefficient (NBHTC) than the PS. On the other hand, under aqueous SDS boiling conditions, the NBHTC was increased on the PS, but decreased on the NSS, as compared with under pure water boiling conditions. It was found that the interaction and combination between the heated surface and fluid can be an important parameter affecting the boiling heat transfer performance, and both the surface tension and surface wettability (i.e., contact angle) should be taken into account. The change of water contact angle induced by the chemical reaction between the heated surface and fluid was examined and discussed. As the test surface, copper PS, titanium PS, and titanium dioxide NSS were prepared. Then, they were boiled under saturated water and butanol conditions for 8 h, and their water contact angles were compared with the fresh (i.e., pre-boiled) surfaces. The water-boiling process remarkably decreased the water contact angles for all surfaces, as compared with the butanol-boiling process. This implies that the chemical reaction between the heated surfaces and boiling fluids affects the surface wettability, which can strongly influence the pool boiling heat transfer performance. [ABSTRACT FROM AUTHOR]

Thin ultrananocrystalline diamond (UNCD) films were evaluated for use as hermetic and bioinert encapsulating coatings for implantable microchips, where the reaction to UNCD in vitro and in vivo tissue was investigated. Leakage current tests showed that depositing UNCD coatings, which were conformally grown in (1% H2) Ar/CH4 plasma, on microchips rendered the surface electrochemically inactive, i.e. with a very low leakage current density (2.8×10−5 Acm−2 at −1V and 1.9×10−3 Acm−2 at ±5V) ex vivo. The impact of UNCD with different surface modifications on the growth and activation of macrophages was compared to that of standard-grade polystyrene. Macrophages attached to oxygen-terminated UNCD films down-regulated their production of cytokines and chemokines. Moreover, with UNCD-coated microchips, which were implanted subcutaneously into BALB/c mice for up to 3months, the tissue reaction and capsule formation was significantly decreased compared to the medical-grade titanium alloy Ti–6Al–4V and bare silicon. Additionally, the leakage current density, elicited by electrochemical activity, on silicon chips encapsulated in oxygen-terminated UNCD coatings remained at the low level of 2.5×10−3 Acm−2 at 5V for up to 3months in vivo, which is half the level of those encapsulated in hydrogen-terminated UNCD coatings. Thus, controlling the surface properties of UNCDs makes it possible to manipulate the in vivo functionality and stability of implantable devices so as to reduce the host inflammatory response following implantation. These observations suggest that oxygen-terminated UNCDs are promising candidates for use as encapsulating coatings for implantable microelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: The turbulent mixing coefficient is an essential parameter to describe the energy and momentum transports between adjacent subchannels in a nuclear fuel assembly and affect the thermal–hydraulic characteristics in nuclear reactor core. Hence, its estimation is in close connection with the design and performance–prediction of nuclear reactor. In the present experimental study, the effect of gap width on the flow pulsation and turbulent mixing was investigated using PIV (Particle Image Velocimetry). The W/D (the ratio of distance to wall to rod diameter) covered from 1.03 to 1.14, and the single cylindrical rods with five different diameters were installed in a square channel to adjust the gap width. Among the W/D cases tested, in the W/D =1.03, 1.06, 1.08, and 1.10, the flow pulsation, a quasi-periodic oscillating flow motion, appeared at the rod gap center, while it was not observed for W/D =1.14. As an increase in Reynolds number and/or a decrease in W/D, the peak frequency and power spectral density of flow pulsation became higher and larger, respectively. On the other hand, the W/D cases with a flow pulsation exhibited higher RMS (Root–Mean–Square) values of lateral velocity than the case without a flow pulsation. The change in the RMS values of lateral velocity was more sensitive to the W/D conditions, particularly for the small W/D cases (i.e., W/D ⩽1.10) showing the flow pulsation. This implies that the RMS values of lateral velocity are strongly influenced by the flow pulsation, which in turn, can determine the turbulent mixing between subchannels. To propose the turbulent mixing coefficient for nuclear reactor core application, the W/D ranges were divided into two groups dominated by (i) the large eddy motion of the flow pulsation, and (ii) the typical fluctuation of the turbulent flow. Finally, the empirical correlations of the turbulent mixing coefficient considering the effects of W/D and the Reynolds number were proposed using the RMS value of lateral velocity, which were in agreement with previous studies within a reasonable degree of accuracy. [Copyright &y& Elsevier]

Abstract: Cyclic voltammetry deposition (CV-D) was applied to deposit ZnO films on indium-tin-oxide (ITO) glass. Delineated in details are the optimizations of CV-D parameters which bestow enhanced nucleation of ZnO grains. The result is much superior coverage of the CV-D thin films as compared to those obtained by conventional electrochemical deposition. Memristive devices in the stacks of Pt/ZnO/ITO were prepared and tested. The devices in which ZnO prepared by CV-sweeping within ±0.9V for 6 cycles then fix-potential-deposited at −0.75V for 300s show reproducible forming-free bipolar switching operation at voltages ≤±1V. The cycle-life in DC sweeping mode is at least 200 cycles. The electrical conduction belongs to space-charge-limited-current conduction mechanism, which is fitted to extract carrier mobility 0.97cm2/Vs and carrier concentration 8.6×1018 cm−3. Chemical bonding status and concentration profiles of oxygen and zinc were examined by electron spectroscopy and an Auger electron nanoscope. Gradient oxygen bonding status in ZnO film, typical of cyclic voltammetry deposition, facilitates the electrical switching at low voltages. [Copyright &y& Elsevier]

Abstract: In order to investigate nucleate pool boiling heat transfer characteristics in saturated water, a plain aluminum alloy surface (6061, untreated) and a uniquely fabricated aluminum oxide Nano-Porous Surface (NPS) sample were prepared. Generally, the NPS exhibited a lower wall superheat at the onset of nucleate boiling and a higher nucleate boiling heat transfer coefficient than the plain surface. It was also noted that the nucleate boiling heat transfer coefficient decreased by 30% on the plain surface and by 37% on the NPS after five repeated boiling tests. It was found that such performance-degradation in both of the test samples was due primarily to the formation of aluminum hydroxide on the boiling surfaces. It is our belief that the aluminum hydroxide, formed on both boiling surfaces, significantly altered the surface morphologies as shown in the microscopic images of the post-boiling surfaces, which resulted in the decrease of the active nucleation sites. Although NPS is an excellent tool to enhance the nucleate pool boiling heat transfer coefficient, its life span and usefulness can be impeded by undesirable surface reactions with working fluids. [Copyright &y& Elsevier]

Abstract: The characteristics of water droplet evaporation on three different hydrophobic surfaces, PCu (Plain Copper, θ =115°), MSCu (Micro-Structured Copper, θ =126°) and NSCuO (Nano-Structured Copper Oxide, θ =159°) with coating of the same SAM (Self-Assembled Monolayer) material, were experimentally investigated. For industrial heat transfer applications, copper material was used as the substrate, and the simple and cost-effective fabrication technique to prepare the superhydrophobic surface, NSCuO, was introduced. Based on the observations, the behavior of droplet evaporation was divided into three stages: Stage I (constant contact area stage), Stage II (constant contact angle stage) and Stage III (mixed stage). When studying the PCu surface, the Stages I, II, and III were observed, consistent with previous reports. For the MSCu surface, Stages I and III appeared without Stage II, and the pinning period of contact line was the longest among the test samples due to the formation of Wenzel state droplet. In the case of the superhydrophobic NSCuO surface, only Stage III occurred, and the contact line moved freely during the entire evaporation time because of the formation of Cassie state droplet. The total evaporation time of the NSCuO was the longest out of all the samples tested. At the last stage of evaporation, the edge of the droplet shrank at a much faster rate in all surfaces. On the other hand, the shrinking velocity of the droplet height drastically increased only on the NSCuO, which was considered as the unique behavior of superhydrophobic surface. In this experiment, it was found that the surface structure determines the motion of the contact line on the surface, which, in turn, strongly influences the characteristics of the droplet evaporation. [Copyright &y& Elsevier]

Abstract: Anisotropic electrical conduction measurements have been carried out for thin films of vertically-aligned single-walled carbon nanotubes (VA-SWCNTs) grown by an alcohol catalytic CVD process. Combined with controlled synthesis and structure characterization by optical spectroscopy, the influence of the aligned structure on the electrical conduction has been identified. The out-of-plane conductivity of the films was measured to be about 0.56S/mm, independently of the film thickness. On the other hand, the in-plane conductivity was found to be more than an order of magnitude smaller, which gives rise to highly anisotropic electrical conduction, reflecting the high degree of alignment in the VA-SWCNT films. The in-plane conductivity decreases with increasing film thickness, in contrast to the film of random SWCNT networks, which exhibit thickness-independent in-plane resistance. The thickness-dependent in-plane conductivity can be expounded by a growth model of vertically aligned SWCNT films in which a thin layer of nanotube networks form on top of films at the initial stage of the growth. Such electrical anisotropy of VA-SWCNT films can be useful in miniaturized sensing devices. [ABSTRACT FROM AUTHOR]

Abstract: In the present experimental study, the pressure drop of the two-phase dry-plug flow (dry wall condition at the gas portions) in round mini-channels was investigated. The air–water mixtures were flowed through the round mini-channels made of polyurethane and Teflon, respectively, with their inner diameters ranging from 1.62 to 2.16mm. In the dry-plug flow regime, the pressure drop measured became larger either by increasing the liquid superficial velocity or by decreasing the gas superficial velocity due to the increase of the number of the moving contact lines in the test section. In such a case, the role of the moving contact lines turned out to be significant. Therefore, a pressure drop model of dry-plug flow was proposed through modification of the dynamic contact angle analysis taking account of the energy dissipation by the moving contact lines, which represents the experimental data within the mean deviation of 4%. [Copyright &y& Elsevier]

• Phenomena of a single water droplet impact on various liquid surfaces were investigated. • Water, ethanol, methanol, and n-heptane were used as liquids. • Through impact visualizations, pattern maps were constructed for all liquid surfaces. • Previous correlations of regime transitions were compared and assessed. • New correlations for the regime transitions were proposed. The interaction between an impacting droplet and liquid surface is complex and is strongly influenced by the liquid surface type and impact droplet factors such as its size and velocity. In this experimental study, the impacts of a single water droplet on various liquid surfaces were investigated. The diameter and velocity of the single water droplet were 2.0–3.2 mm and 0.26–3.73 m/s, respectively. Water, ethanol, methanol, and n-heptane were used as liquids. Through impact visualizations, at the water surface, patterns of coalescence, single jet, and splash without secondary jet were observed. In contrast, at the other surfaces, patterns of coalescence, single jet, splash with secondary jet, and canopy were observed. Based on the experimental observations, pattern maps were constructed for all liquid surfaces and the previous correlations of regime transitions were compared and assessed. According to the present experimental data, it was found that the previous correlations cannot accurately predict the regime transitions for some liquid surfaces. Hence, using the present experimental data and previous correlation form, new correlations with a dimensionless parameter (K), expressed as a function of the density, viscosity, and surface tension ratios, were proposed for the regime transitions between coalescence and single jet patterns and between single jet and splash patterns. The proposed correlations showed good predictions for the present and previous experimental data. [ABSTRACT FROM AUTHOR]

Abstract: In the present experimental study, the pressure drop of two-phase plug flows in round mini-channels was investigated for three different tube materials, i.e., glass, polyurethane and Teflon, respectively, with their inner diameters ranging from 1.62 to 2.16mm. Air and water were used as the test fluids. In the wet-plug flow regime (wet wall condition at the gas portions), the pressure drop was reasonably predicted by the homogeneous flow model or by the correlations of Mishima and Hibiki [K. Mishima, T. Hibiki, Some characteristics of air-water two-phase flow in small diameter vertical tubes, Int. J. Multiphase Flow 22 (1996) 703–712] and Chisholm [D. Chisholm, A theoretical basis for the Lockhart-Martinelli correlation for two-phase flow, Int. J. Heat Mass Transfer 10 (1967) 1767–1778]. On the other hand, in the dry-plug flow regime (dry wall condition at the gas portions), the role of the moving contact lines turned out to be significant. To take into account the effect of the moving contact lines, a modified Lockhart–Martinelli type correlation was proposed, which fitted the measured pressure-drop data within the mean deviation of 6%. [Copyright &y& Elsevier]

Abstract: Ultra-nano-crystalline diamond (UNCD) nano-emitters were prepared by coating UNCD films on the tip of silicon nanowire (SiNW) templates by microwave plasma-enhanced chemical vapor deposition process. The electron field emission properties of the UNCD/SiNW nano-emitters varied markedly with the pre-seeding process for the SiNW-templates. The direct ultrasonication process is more efficient in the formation of the diamond nuclei than the carburization/ultrasonication process, yielding UNCD/SiNWs nano-emitters with better electron field emission properties. The electron field emission can be turned on at (E 0)UNCD/SiNW4 =3.75 V/µm, yielding a large electron field emission current density of (J e)UNCD/SiNW4 =11.22 mA/cm2 at an applied field of 9.75 V/µm. These characteristics are significantly better than those of bare SiNWs or planar UNCD films. [Copyright &y& Elsevier]

Abstract: Nano-sized cubic Pb2Ti2O6 and monoclinic PbTi3O7 were obtained as the first stage products, by sol–gel process using anhydrous lead acetate and titanium(IV) isopropoxide as precursors; then, as the powder was heated, Pb2Ti2O6 gradually transformed to perovskite PbTiO3 around 673K. [Copyright &y& Elsevier]

Highlights • Transient CFD analysis using DES turbulence model was performed against Vattenfall T-junction test. • Applicability of DES model to turbulent thermal mixing was evaluated by comparing with experimental data. • The DES turbulence model has a capability to simulate turbulent thermal mixing phenomenon in a mixing tee. Abstract Turbulent thermal mixing is one of the major degradation mechanisms of thermal fatigue, called high cycle thermal fatigue, and a mixing tee has been known as typical component susceptible to high cycle thermal fatigue. From a numerical analysis point of view, accurate prediction of turbulent flow and associated thermal fields in a T-junction is an essential task; that requires computational fluid dynamics (CFD) with advanced turbulence modeling. The detached eddy simulation (DES) model is hybrid turbulence model which combines classical Reynolds Averaged Navier-Stokes (RANS) formulations with elements of large eddy simulation (LES) method. The DES model has a benefit from computational cost point of view, but the studies of its applicability to industrial problems seem to have been conducted relatively insufficiently. Therefore, in this study, transient CFD analysis using the DES model was performed against Vattenfall T-junction test, and the applicability of DES model to turbulent thermal mixing was evaluated by comparing with its experimental data. For the comparison of velocities, the DES results were in good agreement with the experimental data. For the comparison of temperature, the calculated results were generally in good agreement, but at separation region, a large difference of mean temperature was observed. For the locations where the wall temperature variation is large in which the risk of thermal fatigue is expected to be higher, it was seen that the low frequency oscillations are dominant and the energy begins to decrease from ∼4 Hz. In conclusions, it was confirmed that the DES turbulence model has a capability to simulate turbulent thermal mixing phenomenon in a mixing tee and the CFD analysis using that model can provide reliable results for the assessment of the structural integrity of such a piping system. [ABSTRACT FROM AUTHOR]