Doctoral Theses

Permanent URI for this collection

https://ir.iitj.ac.in/handle/123456789/10

Browse

Browsing Doctoral Theses by Subject "Applications"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Design, Development and Characterization of Low and High Temperature PCMs for Thermal Energy Storage Applications
    (Indian Institute of Technology Jodhpur, 2017-10) Dixit, Ambesh
    Solar energy is attracting attention as a renewable energy source due to the rapid depleting conventional energy resources and current environmental problems such as global warming, removal of ozone layer, escalating pollution, and also the increasing cost of fossil fuels. The use of solar energy is two folds: (i) energy conversion and its utilization and (ii) energy conversion, storage and use it later on demand. The later will rely on efficient energy storage materials and systems for probable applications such as electricity generation, solar cookers, solar water heaters, building heating/cooling, heat packs for therapeutic/body comfort applications etc. Thermal energy can be stored either as a sensible or latent heat or in both forms, depending on the storage media and operating conditions. Phase change materials (PCMs) are used as the thermal energy storage media, where heat is stored as latent heat of the storage material. A wide number of organic and inorganic PCMs are developed for such applications. However, the latent heat of fusion is comparatively low for commonly available PCMs in 15-20 0C range of melting temperature. Further, very little data are available for thermophysical properties of PCMs. The different solar thermal applications require relatively higher melting temperature and high latent heat. Additionally, the therapeutic applications require small crystallite PCMs in contrast to conventional hard crystallites, causing body discomfort, shape adaptability problem, damage to the heat packs in conjunction with low discharge time. PCMs also suffer with low thermal conductivity, which may hinder effective heat transfer during charging/discharging. This compels to design and develop suitable PCMs for such applications. Furthermore, the conventional differential scanning calorimeter (DSC) does not provide reliable data on thermophysical properties of PCMs because of its small sample size limitation and temperature-history system, suitable for PCM characterization is not available commercially. The present thesis work aims: (i) design and development of low and high temperature phase change materials, (ii) improvement of SAT PCM thermophysical properties for heat pack applications, (iii) development of temperature – history (T-history) measurement setup, and (iv) optimization of exfoliated graphite process and development of high thermal conductivity PCM-exfoliated graphite composites to enhance thermal conductivity of PCMs. The fatty acids (capric, lauric, myristic and palmitic acids) and 1-deodcanol based binary eutectic compositions are developed with suitable melting temperature 17-20 ºC and sufficiently high latent heat of fusion 175-190 kJ kg-1. The sodium acetate trihydrate (SAT), water and ethylene glycol based composite PCM is developed showing enhanced heat retention time (~10%), small crystallites, enhanced thermal stability against spontaneous nucleation. Further, NaNO3, KNO3, LiNO3 and NaCl derived homogeneous high temperature eutectics are synthesized at room temperature using an innovative approach and their thermophysical properties are evaluated. The chemical exfoliation process is optimized and volume of graphite flakes is enhanced ~ 250 times. Further, the work is extended to develop PCM- ExG composites with different wt% of ExG (5, 10, 15 and 20%) to enhance thermal conductivity of PCMs. The maximum thermal conductivity of LiNO3/NaCl-ExG composite is ~ 13.8 W m-1 K-1 at ~1400 kg m-3 density with 20 wt% ExG.
  • Thumbnail Image
    Item
    Quantum Communication Under Noisy Environment: From Theory to Applications
    (Indian Institute of Technology Jodhpur, 2018-07) Banerjee, Subhashish
    Quantum information is based on the laws of quantum mechanics to solve the issues related to information theory. Any communication system based on the principles of the quantum information technology deploys microscopic information carriers which have comparatively less signal to noise ratio. From the technology point of view, the construction and real eld implementation of such systems would be complex and challenging as one cannot neglect noise in practical applications. To make such systems reliable and e cient, the designer needs expertise in the elds of quantum electronics, communication theory, semiconductors, optoelectronics, photonics in addition to quantum optics. This doctoral thesis is the analysis of various quantum cryptography protocols under various noisy models in addition to quantum-based satellite communication under atmospheric turbulence. The research includes the calculation of delity, mutual information, key generation rate, quantum bit error ratio (QBER), entropy and engineering the communication distance. The noisy conditions between the transmitter and receiver are crucial and unavoidable for real eld applications. For this reason, we have to deal with such situations in the case of remote state preparation and various other quantum cryptography protocols. Along with this, another challenging task is atmospheric turbulence which degrades the performance of quantum-based satellite communication. The factors to deal with under such turbulence phenomena are maintaining proper synchronization between the sender and receiver, alignment of the optical instruments and remotely controlling all these complex setups for low earth orbit (LEO), medium earth orbits (MEO) and between inter-satellite communication for tracking the laser beam in case of uplink and downlink scenarios to maintain the optimum and accurate use of the whole setup.
  • Thumbnail Image
    Item
    Small Molecule Based Solution Processed Organic Field-Effect Transistors and Applications
    (Indian Institute of Technology Jodhpur, 2017-02) Tiwari, Shree Prakash
    Organic field-effect transistors (OFETs) have gained tremendous attention from researchers due to its suitability for applications in large area flexible electronics. Solution processing of organic semiconductors, which act as active layer in these devices, is preferred for its attractive advantages of low cost and simplicity. The quality of active layer and dielectric-semiconductor interface are some of the crucial factors which regulate the performance of OFETs. However, improvement in these aspects is quite challenging due to numerous intricate issues related with material properties and processing conditions. In this work, the organic semiconductor TIPS-pentacene is explored for demonstration of high performance OFETs in bottom gated top contact (BGTC) architecture, which has become the conspicuous choice for OFETs due to its inherent high mobility and air stability. Successively, two types of flexible OFETs have been fabricated which are used to demonstrate high electromechanical stability and sensing phenomenon depending on their suitability. Firstly, the effect of structural dissimilarity of the additive solvent from the main solvent on the properties of the resulting crystals of TIPS-pentacene and corresponding device performance on rigid Si/SiO2 substrates is comprehensively studied. With toluene as the main solvent, benzene, cyclohexane, and hexane were used as additives for making solutions of TIPS-pentacene. It was found that a higher structural dissimilarity of the additive in the binary solvent mixture promotes a better molecular aggregation and higher crystallinity in the active layer and improved electrical characteristics in the corresponding OFETs. OFETs fabricated from toluene/hexane solvent resulted in improved field-effect mobility higher than 0.1 cm2 V-1 s-1 compared to 0.05 cm2 V-1 s-1 for toluene solvent. Subsequently, blending of insulating polymer binder polystyrene (PS) with TIPS-pentacene was studied for phase separation and resulting high performance and electrically stability in OFETs. Drop casting of blend solution and solvent evaporation on Si/SiO2 rigid substrates resulted in a vertically phase separated, tri-layer semiconductor-polymer-semiconductor structure. Though the dielectric capacitance density decreased from 10.6 nF cm-2 (300 nm SiO2) to 3.1 nF cm-2 due to phase separated additional PS dielectric layer, the average process transconductance parameter (product of mobility and capacitance density) improved by a factor of ~4, with maximum mobility as high as 2.6 cm2 V-1 s-1 in saturation region for -30 V operation. These devices exhibited average mobility of 1.5 cm2 V-1 s-1, threshold voltage of 1.4 V, and high current on-off ratio of ~8×106 with better stability under bias stress compared to neat devices. For future flexible electronics, low voltage operated and high performance OFETs on flexible substrate are required, for which two device strategies were employed on PET substrates. First type of devices were fabricated with an active layer of neat TIPS-pentacene on HfO2-PVP hybrid gate dielectric, whereas the other type of devices were fabricated with TIPS-pentacene:PS blend films on HfO2. At an operating voltage of -10 V, neat TIPS-pentacene devices exhibited average and maximum mobility of 0.11 and 0.23 cm2 V-1 s-1, average threshold voltage of 0.1 V and current on-off ratio of ~105, whereas TIPS-pentacene:PS blend devices showed average and maximum mobility of 0.44 and 0.93 cm2 V-1 s-1, average threshold voltage of -0.3 V and on-off ratio of ~105. Blend devices outperformed neat devices due to a better quality of dielectric-semiconductor interface, which was developed because of vertical phase separation. Due to their better electrical performance, TIPS-pentacene:PS blend OFETs were further explored for demonstration of low voltage operation and high electro-mechanical stability. At further reduced operating voltage of -5 V, an average and maximum mobility of 0.5 and 1.1 cm2 V-1 s-1, average threshold voltage of -0.5 V, current on-off ratio of ~105, and low sub-threshold swing of 0.3 V/dec. were achieved. In addition, low drain current decay of 10% and a very small threshold voltage shift of 0.3 V were observed for 1 hr bias stress at VDS = VGS = -5 V, indicating to very high bias stress stability in these devices. For a bending radius of 5 mm, high stability in electrical characteristics was found with increasing duration of mechanical strain, where average mobility changed from 0.43 to 0.30 cm2 V-1 s-1 for a very long strain duration of 2 days. Upon application of 100 cycles of tensile and compressive strain, mobility of representative device changed from 0.32 to 0.29 cm2 V-1 s-1, indicating very high electromechanical stability in these devices. Further, flexible OFETs with active layer of neat TIPS-pentacene on HfO2-PVP hybrid gate dielectric were used to examine the photo-sensitivity of TIPS-pentacene to visible and UV lights because of relatively simplified device structure and purity of semiconductor layer. Photo-sensitive OFETs exhibited maximum response to blue light illumination with intensity of 1.7 mW/cm2, showing a current modulation as high as ~105 at low operating voltage of -5 V. Enhancement in the photo-response was observed with increasing time of visible illumination and gate bias during illumination. However, increasing UV irradiation time resulted in an enhanced positive threshold voltage shift and reduced mobility. The saturation drain current at biasing conditions of VGS = -10 V and VDS = -5 V was found to rise slightly for smaller values of irradiation time, however decreased for higher values of illumination time. A similar trend of positive shifting of VTH and mobility roll-off was observed when gate bias during UV irradiation was increased.