C. Gau

Ordered bulk heterojunction solar cells with vertically aligned TiO2 nanorods embedded in a conjugated polymer

Vertically aligned TiO2 nanorods were prepared by its sol-gel which is spun coat onto aluminum anodic oxide template pregrown on an indium tin oxide glass substrate. The poly(3-hexylthiophene) (P3HT), a conjugate polymer infiltrated into the nanorod arrays, and the combined system were used as the active layer. The nanorods/P3HT solar cell with cell size of 0.06cm2 demonstrated a power conversion efficiency of 0.512% while the bilayer TiO2 film/P3HT cell was 0.12%. The current work provides fabrication method for a stable well aligned nanorods/polymer hybrid solar cell production.

MELTING AND SOLIDIFICATION OF A METAL SYSTEM IN A RECTANGULAR CAVITY

The role of natural convection on solid-liquid interface motion during melting and solidification of Lipowitz metal in a rectangular cavity was studied. The measurements of both temperature distributions and temperature fluctuations were used as a qualitative indication of the natural convection flow regimes and structures of melt during phase transformation. The measured and predicted solid-liquid interface positions during solidification from below and above, as well as melting from above and below, show reasonably good correspondence. The suppression of natural convection in the Lipowitz metal, which is not taken into account in the model, leads to a slower rate of melting and a higher rate of freezing than that predicted.

Three-dimensional natural convection heat transfer of a liquid metal in a cavity

The paper describes a numerical algorithm for the solution of the steady-state Navier-Stokes equations in three dimensions for the problem of natural convection in a rectangular cavity as a result of differential side heating. Numerical results for two- and three-dimensional models are reported for a cavity filled with a low-Prandtl-number fluid. Supporting experiments using gallium as a working fluid are described. Measured temperatures are compared with predictions of the three-dimensional model. Agreement between data and predictions is only fair and reasons for the discrepancy are identified.

Impingement cooling flow structure and heat transfer along rib-roughened walls

Abstract Slot-air-jet impingement cooling flow structure and heat transfer along rib-roughened walls is studied experimentally. The flow structure along the ribbed wall is visualized with smoke that is generated by vaporizing the oil coated over a heated resistance wire. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 3 and 4, respectively. During the experiments, the Reynolds number varies from 2500 to 11 000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16. Due to the protrusion of the rib, the formation of an air bubble enclosing the cavity occurs which can prevent the jet from impinging on the wall and reduce the heat transfer. However, some portion of the jet flow in the downstream region, especially when it becomes turbulent, can penetrate the air bubble and impinge, and recirculate inside the cavity, which significantly increases the heat transfer. In general, the flow structure and the heat transfer observed are significantly different from the results of a flat plate. A comparison and correlations of the stagnation point Nusselt number under different conditions are presented and discussed.

Piezoresistive characteristics of MWNT nanocomposites and fabrication as a polymer pressure sensor

Polyimide (PI)-carbon nanotube composites were fabricated by in situ polymerization using multi-wall carbon nanotubes (MWNT) as fillers. The composite film was characterized by some analytical instruments to ensure its structure and good dispersion of the MWNTs in the composites. The electrical resistivity of this composite was found to vary significantly with both the temperature and the stress in the material. The PI-MWNT composites possess a very linear piezoresistive nature which can be used as a good pressure sensor material, provided with proper temperature compensation. Fabrication of a micropolymer pressure sensor using this nanocomposite sensing material is demonstrated and sensor performance is evaluated. The sensor has a higher sensitivity than a polysilicon sensor, rapid response, and is thermally stable. The sensor is suitable for mass production, and can be widely applied or integrated in a microfluidic system or biochip where pressure information is required.

Effect of natural convection on solidification from above and melting from below of a pure metal

Abstract The role of natural convection on the solid-liquid interface motion during melting from above and solidification from below of a pure metal was studied. The measurements of both temperature distributions and temperature fluctuations were used as a qualitative indication of the natural convection flow regimes and structures of melt during phase transformation. The photographs of the solid-liquid interface morphology (after draining the melt from the test cell) revealed complicated time-dependent flow structure in the liquid. The measured (average) and predicted solid-liquid interface positions during solidification from above and of melting from below show reasonably good correspondence.

Nano Temperature Sensor Using Selective Lateral Growth of Carbon Nanotube Between Electrodes

This paper presents lateral growth of carbon nanotube (CNT) between two electrodes and its use as nano temperature sensor. Fabrication of electrodes is made by MEMS techniques. The CNT is grown selectively by microwave plasma chemical vapor deposition between two electrodes. After wire bonding, the grown CNT is tested and calibrated. The growth conditions of CNT, such as the flow rate of CH4 or N2 gas, are varied to obtain a high-quality CNT sensor. SEM is used to observe the shape and structure of CNT, while Raman spectrum analysis is used to indicate the degree of graphitization or amorphous structure in the CNT. The I-V curve of CNT is measured at different temperature, and a linear relationship between the electric resistance of CNT and the temperature is obtained. This result indicates that CNT can be used as a temperature sensor

Arrangement of band structure for organic-inorganic photovoltaics embedded with silicon nanowire arrays grown on indium tin oxide glass

Silicon nanowires (SiNWs) arrays grown vertically on an indium tin oxide glass substrate by the vapor liquid solid process are covered with poly (3-hexylthiophene) (P3HT) mixed with [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) to make into a hybrid solar cell. With proper control of the nanowire size and arrangement of the band structure suitable for charge carrier transport, the P3HT/SiNWs solar cell can have a much better energy conversion efficiency than the P3HT+PCBM solar cell. Poor band structure arrangement can lead to band barrier and enhanced electron-hole pair recombination and much lower efficiency even with more light absorption.

Flow and impingement cooling heat transfer along triangular rib-roughened walls

Abstract Experiments are performed to study slot air jet impingement cooling flow and the heat transfer along triangular rib-roughened walls. Both flow visualization and local heat transfer measurements along the ribbed wall are made. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 2 and 4, respectively. The widely opened cavity between neighboring ribs make more intense transport of momentum between the wall jet and cavity flow so that recirculation cell in the cavity is hardly observed. This leads to a higher heat transfer around the cavity wall than in the case with rectangular ribs. However, in the region of laminar wall jet, a number of air bubbles enclosing the cavities are formed which prevent penetration of the wall jet into the cavities. This leads to a significant reduction in the heat transfer. The geometric shape of the triangular ribs is more effective in rebounding the wall jet away from the wall than in the case with rectangular ribs. The rebound of the jet away from the wall causes a significant reduction in the heat transfer. A comparison and correlations of the stagnating point Nusselt number under different conditions are presented and discussed. During the experiments, the Reynolds number varies from 2500 to 11,000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16.

MIXED CONVECTION FLOW AND HEAT TRANSFER IN A HEATED VERTICAL CONVERGENT CHANNEL

Experimental studies of the mixed convection flow and heat transfer in a vertical convergent channel have been carried out. One of the side walls which maintains the vertical position is heated uniformly, and the opposite wall which has a convergence angle of 3° is insulated. The ratio of the height to the width at the inlet of the channel is 15. During the experiments, the Reynolds number ranges from 100 to 4000 and the buoyancy parameter, Gr/Re2, ranges from 0.3 to 907. Flow structure inside the channel is visualized with a heated smoke wire. For both assisted and opposed convection, flow reversal occurs only in the upstream of the channel but not in the downstream where rapid acceleration of flow occurs and the transport process is dominated by forced convection. For assisted convection, the reversed flow forms a steady recirculation cell along the insulated wall, and has no effect on the heat transfer. For opposed convection, the reversed flow occurs along the heated wall, and is unstable and sometimes generates a number of vortices and becomes turbulent. This flow can significantly enhance the heat transfer along the heated wall. Temperature fluctuations at different locations are measured and used to indicate oscillations and fluctuations of the reversed flow. The effect of the buoyancy parameter on the reversed flow structure and the Nusselt number is presented and discussed. The Nusselt number results are correlated in terms of relevant nondimensional parameters for both pure forced and mixed convection, respectively. For the purpose of comparison, the Nusselt numbers for parallel-plate channel are also measured and presented.