Concept of spinsonde for multi-cycle measurement of vertical wind profile of tropical cyclones

Tropical cyclones and cyclogenesis are active areas of research. Chute-operated dropsondes jointly developed by NASA and NCAR are capable of acquiring high resolution vertical wind profile of tropical cyclones. This paper proposes a chute-free vertical retardation technique (termed as spinsonde) that can accurately measure vertical wind profile. Unlike the expendable dropsondes, the spinsonde allows multi-cycle measurement to be performed within a single flight. Proof of principle is demonstrated using a simulation software and results indicate that the GPS ground speed correlates with the wind speeds to within +/-5 km/h. This technique reduces flying weight and increases payload capacity by eliminating bulky chutes. Maximum cruising speed (Vh) achieved by the spinsonde UAV is 372 km/h.Comment: arXiv admin note: substantial text overlap with arXiv:1407.845

Journey to the Typhoon

Abstract Application of UAVs (unmanned aerial vehicles) for tropical cyclone missions is an emerging area of research and recent advances include the concept of spinsonde for multi-cycle measurement of vertical wind profile within the storm. This work proposes the design of a typhoon UAV as part of a cost-effective approach for acquiring atmospheric data to improve prediction and refine models. Land-and carrier-based flight schemes are proposed in this study and computer simulations are carried out to investigate the flight performance. Results suggest that the UAV achieves a maximum cruising speed in excess of 350 km·h −1 with excellent spinsonde performance. Furthermore, the UAV is capable of performing high-alpha maneuvers as well as vertical landing, thus rendering it suitable for space-efficient operation whether on land or aircraft carrier

Graded heterojunction for improved performance and stability of organic solar cells

Research Doctorate - Doctor of Philosophy (PhD)The photoactive layer of an organic solar cell is typically 100 to 200 nm thick. At such a low dimensional thickness the light interference effect becomes significant. There are regions within the photovoltaic structure where the localized light intensity is low. In some instances, a relatively transparent material such as one made from titanium dioxide has been engineered into the solar cell structure so that the photoactive layer is strategically positioned at a location where the light intensity is high. The experimental work in this thesis begins by observing the existence of the interference effect in a P3HT-PCBM BHJ solar cell. An optical simulation package is employed to calculate the light intensity developed in the photoactive layer when its thickness is varied. Actual solar cells with the same varying thicknesses are fabricated. The physical solar cells are noted for their short circuit currents and the open circuit voltages. The open circuit voltages are found to vary from 0.617 to 0.642 V, depicting a linear correlation to the calculated field intensity in the photoactive layer. Additionally, the experimental short circuit current changes according to the expected profile obtained from the optical simulation. The observations demonstrate that the performance of the physical solar cell can be influenced by the optical interference effect arising from the low dimensional nature of its photoactive layer. Consequently, the next phase of investigation involves incorporating an optically transparent region into the solar cell so that the thickness and the position of the photoactive layer can be independently optimized and therefore allowing more light to dissipate within that layer. The PCBM component which is widely used in the construction of organic solar cells and with a light absorption at a low 345 nm wavelength is seen as an attractive candidate for this purpose. The standard BHJ morphology from which the photoactive layer of the organic solar cell is normally constructed is found to be inadequate in this case as the morphology involves no extended region composing purely of the transparent PCBM. On the other hand, the graded heterojunction architecture is found to be an elegant structure in the pursuit of the concept of using the naturally occurring PCBM in the solar cell as an optical spacer. The graded structure offers a region of interdiffused zone consisting of the P3HT and the PCBM, and adjacent to that is a layer of rather pristine PCBM. According to optical simulation a 20 nm PCBM region in combination with a photoactive layer (the interdiffused zone) of 60 nm will put the photoactive layer into alignment with an intense band of light which has developed within the solar cell structure. However, the intermixed zone appears to be only 40 nm thick for the as-fabricated graded structure, while the pristine PCBM region is estimated to be 30 nm in thickness. Fortunately, it is noted that thermal interdiffusion can be used to transform this semi-optimized graded structure into a structure whose thickness parameters are as required by the optical model to achieve optimum light harvesting. The as-fabricated graded solar cell structure is placed on a digitally controlled hot plate held at 140 °C. The rate of interdiffusion at the set temperature is initially unknown. However, after a series of experiments, the depletion rate of the PCBM layer is estimated to be 0.5 nm per minute, representing a rather fine structural control at the nano-scale. At this rate of depletion, a heating time of 20 minutes is required to reduce the intrinsic PCBM layer to 20 nm thick. After the graded structure has been tuned, the obtained conversion efficiency is 3.4 %. This contrast well with 3.2 % observed earlier with the bulk heterojunction morphology. When the graded structure is used, i.e. when there is a layer of the transparent PCBM as optical spacer incorporated into the solar cell structure, the open circuit voltage is observed to increase to 0.666 V. The highest open circuit voltage obtained is 0.676 V when a slightly thicker interdiffused region is chosen, however the thicker version of the photoactive region only leads to poorer fill factor (due to higher series resistance) and thus the overall conversion efficiency has remain unchanged at 3.4 %. Thus, the proof of concept effort in this thesis demonstrates that the PCBM is capable of acting as an optical spacer, leading to increased open circuit voltages observed with the graded solar cells. Apart from its natural capacity to accommodate an optical spacer, the graded architecture is also noted to have longer operating lifetime in ambient air, compared to the solar cells having the bulk heterojunction morphology. The improved stability is thought to be the added PCBM layer, which made the solar cell more robust to the diffusions of the metal species from the neighboring metal electrode. With some modification, it is anticipated that gold nanoparticles can also be incorporated into the graded solar cell and therefore offering an extended absorption spectrum for the P3HT-PCBM solar cell. Test with a bilayer P3HT-C₆₀ solar cell indicates that the structure has improved EQE at 395 and 675 nm wavelengths. These enhancements can be attributed to the optical absorption of gold and light scattering in the presence of the nanoparticles

Characterization of Pd nano-thin films for high-speed switchable mirrors

We fabricated Pd thin films from 2 to 35 nm thick via thermal evaporation, and a hermetically sealed hydrogen optical cell was used to characterize the films for properties such as hydrogen fractional ratio, optical switching contrast (Weber contrast), and response and recovery times. An atomic force microscope with a high resolution scanning tip was used to study the evolution of the film morphology

Organic solar cells: evaluation of the stability of P3HT using time-delayed degredation

Despite the fact that the performance of organic solar cells is generally susceptible to degradation by moisture exposure, there has been suggestion that the photoactive layer (P3HT) is surprisingly resilient. This work attempts to confirm the stability of P3HT as an organic solar cell material by deliberately introducing water into the photoactive layer. A dramatic step drop in device performance during cell characterization is observed approximately one day after the device has been fabricated. The time-delayed step drop in output efficiency strongly suggests that moisture has little effect on the P3HT conducting polymer

Plasmonic nanostructure embedded within photoactive layer for enhanced power conversion efficiency of organic solar cells

Gold quantum dots have been successfully embedded into the photoactive layer of a P3HT:C60 bilayer solar cell. I-V and EQE measurements confirm that the demonstrated bilayer device shows a 67% increase in conversion efficiency

FDTD modeling to enhance the performance of an organic solar cell embedded with gold nanoparticles

Optical enhancement is demonstrated in a bilayer P3HT-C60 solar cell by embedding gold nanoparticles directly into the P3HT layer of the photovoltaic device. FDTD simulations are used to model the observed performance gain. A qualitative agreement between the experimental and numerical results is achieved. This validates the numerical model and the simulation is subsequently extended to predict the performance gain of the bilayer device constructed with thinner P3HT layer. The numerical results reveal that the plasmonic structure has even larger effect on such thinner bilayer device. The enhancement is expected to be most significant when the p-n interface is allowed to assume the conformal hemispherical profile of the metal particles

Influence of calcination and GGBS addition in preparing β-hemihydrate synthetic gypsum from phosphogypsum

Transforming phosphogypsum (PG) to β-hemihydrate PG with cementitious properties at low calcination temperature is an effective method for large-scale utilization of by-product PG. Nevertheless, due to the poor water resistance of PG, its application and recycling rate is limited. This study aims to produce water-resistant β-hemihydrate synthetic gypsum (SG) with minimum cost and energy consumption, using calcined PG as the raw material. The effect of PG calcination temperature (100 °C, 140 °C, 180 °C, and 220 °C) on the properties of SG was evaluated, and it was found that 180 ºC calcination temperature was able to fully convert the PG to β-hemihydrate SG. Additionally, SG prepared using 180 ºC calcined PG exhibited the highest compressive strength (9.15 MPa). On the other hand, various contents of the mineral admixture, namely, GGBS (5 %, 10 %, 15 %, 20 %, and 25 %), were incorporated in SG to evaluate the viability of GGBS to improve the compressive strength and water resistance of SG. However, the results revealed that adding GGBS has little effect on the performance of SG. Especially, when 25 % GGBS was added, the compressive strength of SG was reduced by 66.3 %. Consequently, further investigation into how to enhance the effect of GGBS is required