Single-walled carbon nanotubes (SWNTs) were synthesized by the laser-heated ACCVD method on the sample stage of an environmental AFM with Raman scattering measurement capabilities. Fe/Co or Co/Mo metal particles, which were supported on zeolite particles or silicon substrates, were used as catalyst, and ethanol vapor was used as the carbon source. The catalysts on the AFM sample stage were heated (to about 800°C) in ethanol vapor (0.01∼1 Torr) by Ar-ion laser irradiation, and SWNTs grew from the catalyst particles. Though this laser-heated ACCVD method was simple, Raman scattering spectra, AFM images, and SEM images showed that high-quality SWNTs were generated. By using the laser-heated ACCVD method, in-situ Raman scattering, which was caused by the heating laser irradiation, was measured during the entire CVD process. The G-band from SWNTs and the silicon peak appeared in in-situ Raman scattering spectra. The intensity of the G-band showed the growth of SWNTs, and the temperature dependence of the Raman shift of the silicon peak was used to determine the sample temperature. In-situ Raman scattering elucidated the lifetime of the catalyst and the existence of an incubation time before the onset of SWNT growth. SWNTs started to grow rapidly after the incubation time and the growth rate gradually decreased. The incubation time was strongly dependent on the pressure of the ethanol gas.
Two-phase flow nozzles are used in the total flow system for geothermal power plants and in the ejector of the refrigerant cycle, etc. One of the most important functions of a two-phase flow nozzle is to convert the thermal energy to the kinetic energy of the two-phase flow. The kinetic energy of the two-phase flow exhausted from a nozzle is available for all applications of this type. There exist the shock waves or rarefaction waves at the outlet of a supersonic nozzle in the case of non-best fitting expansion conditions when the operation conditions of the nozzle are widely chosen. Those waves affect largely on the energy conversion efficiency of the two-phase flow nozzle. The purpose of the present study is to elucidate the character of the rarefaction waves at the outlet of the supersonic two-phase flow nozzle. The high pressure hot water blow down experiment has been carried out. The decompression curves by the rarefaction waves are measured by changing the flow rate of the nozzle and inlet temperature of the hot water. The back pressures of the nozzle are also changed in those experiments. The divergent angles of the two-phase flow flushed out from the nozzle are measured by means of the photograph. The experimental results show that the recompression curves are different from those predicted by the isentropic homogenous two-phase flow. The regions where the rarefaction waves occur become wide due to the increased outlet speed of two-phase flow. The qualitative dependency of this expansion character is the same as the isotropic homogenous flow, but the values obtained from the experiments are quite different. When the back pressure of the nozzle is higher, these regions do not become small in spite of the super sonic two-phase flow. This means that the disturbance of the down-stream propagate to the up-stream. It is shown by the present experiments that the rarefaction waves in the supersonic two-phase flow of water have a subsonic feature. The measured expansion angles become larger by increasing the flow rate of the two-phase flow and by decreasing the back pressures.
Nanocrystalline silicon particles with a grain size of at least less than 10 nm are widely recognized as one of the key materials in optoelectronic devices, electrodes of lithium battery, bio-medical labels. There is also important character that silicon is safe material to the environment and easily gets involved in existing silicon technologies. To date, several synthesis methods such as sputtering, laser ablation, and plasma enhanced chemical vapor deposition (PECVD) based on low-pressure silane chemistry (SiH4) have been developed for precise control of size and density distributions of silicon nanocrystals. We explore the possibility of microplasma technologies for the efficient production of mono-dispersed nanocrystalline silicon particles in a micrometer-scale, continuous-flow plasma reactor operated at atmospheric pressure. Mixtures of argon, hydrogen, and silicon tetrachloride were activated using very high frequency (VHF = 144 MHz) power source in a capillary glass tube with a volume of less than 1 μ-liter. Fundamental plasma parameters of VHF capacitively coupled microplasma were characterized by optical emission spectroscopy, showing electron density of approximately 1015 cm-3 and rotational temperature of 1500 K, respectively. Such high-density non-thermal reactive plasma has a capability of decomposing silicon tetrachloride into atomic silicon to produce supersaturated atomic silicon vapor, followed by gas phase nucleation via three-body collision. The particle synthesis in high-density plasma media is beneficial for promoting nucleation process. In addition, further growth of silicon nuclei was able to be favorably terminated in a short-residence time reactor. Micro Raman scattering spectrum showed that as-deposited particles were mostly amorphous silicon with small fraction of silicon nanocrystals. Transmission electron micrograph confirmed individual silicon nanocrystals of 3-15 nm size. Although those particles were not mono-dispersed, they were well separated and not coagulated.
The previously proposed linear stability theory of vapor film in subcooled film boiling on a sphere was generalized to take account of the interaction between the base flow and the perturbed component. A disturbance of standing wave type was assumed to be superimposed on the base flows of the surrounding liquid and the vapor film. For the surrounding liquid, the wave equation was applied to the whole region including the boundary layer and the energy equation was solved analytically by introducing a simplifying assumption. For the vapor film, the basic equations were solved by the integral method. By use of the compatibility conditions at the liquid-vapor interface, the solutions for the surrounding liquid and the vapor film were combined to yield an algebraic relation among the vapor film thickness, the order of disturbance and the complex amplification factor of the disturbance. The numerical solutions of the critical vapor film thickness at which the real part of the complex amplification factor was equal to zero were obtained for the disturbances of the orders of 0, 1 and 2. The numerical results indicated that the vapor film was most unstable for the disturbance of the 0-th order (i.e., uniform disturbance). The calculated value of the critical vapor film thickness for the uniform disturbance compared well with the average vapor film thickness at the minimum-heat-flux point obtained from the immersion cooling experiments of spheres in water at high liquid subcoolings.
In this study, the natural cooling of electronic equipments has been studied experimentally. The natural convection between two vertical walls modeling printed circuit boards was investigated. Velocity fields between the walls were measured using a PIV and a cooling capacity was estimated as functions of the board spacing and a heating power. It was found that the cooling capacity was small when the board spacing was less than 10mm under the condition examined. Measured velocity profiles were compared with available analytical and experimental data and the validity of the PIV measurement was confirmed.
In previous study, the characteristic of the condensation heat transfer on the dispersed vertical surface were investigated experimentally for the application of the finned surface to the thermoelectric generator utilizing boiling and condensation as the electrodes of the thermoelectric module. A prediction model for this diapered finned surface was proposed, based on Adamek-Webb model of the condensation on a finned tube. In this study, a condensation heat transfer experiment on a vertical dispersed finned surfaces using FC5312 was carried out, in order to enhance the condensation heat transfer coefficient by optimizing the fin size on a dispersed heat transfer surface. The object of the experiment was limited to the rectangular fin with the height of 3 mm. Experimental parameters were the temperature difference, the fin groove width, the fin thickness and the dispersing size on the vertical direction. As the results, it was found from the experiment that the dependence of the condensation heat transfer coefficient on the dispersed size is controlled by the fin groove width. That is, the condensation heat transfer coefficient will increase for a smaller fin groove width and will decrease for a larger fin groove width, with decreasing of the dispersing size. Moreover, there is an optimum fin thickness at which the condensation heat transfer coefficient becomes the maximum in the case of constant fin groove width for both size of the fin groove width. This effect of the fin thickness is more significant for the smaller fin groove width. Further, the prediction values exhibit a good agreement with the experimental data in the present experiment.
The magnetic convection of paramagnetic fluid is studied in a strong magnetic field. The fluid in a cubic enclosure is heated from one vertical wall and cooled from the opposite one. The fluid is the 80% mass aqueous solution of glycerol with 0.8 mol/kg concentration of gadolinium nitrate hexahydrate to make the working fluid paramagnetic. The small amount of liquid crystal slurry is added to the fluid in order to visualize the temperature profiles in a vertical cross-section. This system is placed directly below the solenoid of the superconducting magnet which is oriented vertically. The temperature of cold wall is constantly controlled by the water flowing from a thermostating bath. On the other hand, the hot wall is heated by a nichrome wire from a DC power supply. In the numerical computations, the configuration of the system is modeled to be as close as possible to the real system. The physical properties of the working fluid are used to compute dimensionless parameters in the numerical model and the computations are carried out for corresponding cases. Later, the numerical and experimental results are compared with each other.
Cells and living tissue slightly but always generate metabolic heat as long as they are alive. Thus, biological activity can be measured through the observation of metabolic heat, which has been developed as “bio-calorimetry”. On the other hand, further improvements in thermal sensing ability can be expected with use of the MEMS (Micro Electro Mechanical System) technology. The purpose of this study is to develop the monitoring technique of the metabolic heat of cells in as small number as possible with the MEMS technology. If the monitoring technique of metabolism of a few cells or even a single cell is made available, it plays very important rolls in bio- and medical- engineering, pharmaceutical sciences, and so on. In this study, a bio-calorimeter with a MEMS thermopile sensor was made, and its performance and metabolism monitoring of Yeast were tested. The thermopile sensor consisted of 350 thin film thermocouples of Cr and Ni strips of 20 μm width on a 150 μm thick glass plate. The thermopile sensor composed a calorimetric cell as a bottom plate with thick aluminum frame. The calorimetric cell was placed in a triple thermostatic chamber which employs a proportional control with a Peltier device and PID control with heater. The calorimeter showed a sensitivity of 0.62 V/W under the condition of including culture solution, time constant of the calorimetric cell of 90 sec, and a noise equivalent power of 60 nW, which corresponds to metabolic heat of 3 × 103 cells of Yeast. In the growth experiments of Yeast, growth thermograms for 105∼107 cells can be measured with reasonable generation times. It was demonstrated that the detectable number of Yeast cells of the MEMS calorimeter is much smaller than that for the traditional bio-calorimeter.