<p> Fluctuations are one of the main probes of the physics of the new state of hot and dense nuclear matter called the Quark Gluon Plasma (QGP) which is created in the ultra-relativistic heavy ion collisions. In this dissertation we extend and improve upon the existing descriptions of heavy ion collisions in three different directions: we study the new signatures of initial state fluctuations, the propagation of perturbations in the early stages of the collision, and the effect of thermal fluctuations on the hydrodynamic expansion of the QGP. </p><p> First, in Chapter 3 we study initial state fluctuations by examining the complete statistical information contained in the two-particle correlation measurements in hydrodynamic simulations of Pb+Pb collisions at the CERN Large Hadron Collider (&radic;<i>s_NN</i> = 2.76 TeV). We use Principal Component Analysis (PCA) to decompose the spectrum of harmonic flow, v_n(p_T) for <i>n</i> = 0&ndash;5, into dominant components. The leading component is identified with the standard event plane <i> v_n</i>(<i>p_T</i>), while the subleading component describes additional fluctuations in the two-particle correlation function. We find good geometric predictors for the orientation and the magnitude of the leading and the subleading flows. The subleading <i>v</i>₀, <i>v</i>₁, and <i>v</i>₃ flow harmonics are shown to be a response to the radial excitation of the corresponding eccentricity &epsiv;<i>_n</i>. In contrast, for <i>v</i>₂ the subleading flow in peripheral collisions is dominated by the nonlinear mixing between the leading elliptic flow and radial flow fluctuations. Nonlinear mixing also plays a significant role in generating subleading <i>v</i>₄ and <i>v</i>₅ harmonics. The PCA gives a systematic way of studying the full information of the two-particle correlation matrix and identifying the subleading flows, which we show are responsible for factorization breaking in hydrodynamics. </p><p> Second, in Chapter 4 we study the thermalization and hydrodynamization of fluctuations at the early stages of heavy ion collisions. We use leading order effective kinetic theory, accurate at weak coupling, to simulate the pre-equilibrium evolution of transverse energy and flow perturbations. For the short evolution we can use a linear response theory to construct the pre-equilibrium Green functions. Then the energy-momentum tensor at a time when hydrodynamics becomes applicable can be expressed as a linear convolution of response functions with the initial perturbations. We propose combining effective kinetic theory with weak coupling initial state models, such as IP-Glasma, to model the complete pre-thermal evolution from saturated nuclei to hydrodynamics in a weak coupling framework. </p><p> Last, in Chapter 5 we consider out-of-equilibrium hydrodynamic fluctuations in the expanding QGP. We develop a set of kinetic equations for a correlator of thermal fluctuations which are equivalent to nonlinear hydrodynamics with noise. We first show that the kinetic response precisely reproduces the one-loop renormalization of the shear viscosity for a static fluid. We then use the hydro-kinetic equations to analyze thermal fluctuations for a Bjorken expansion. The steady state solution to the kinetic equations determine the coefficient of the first fractional power of the gradient expansion (&infin; 1/(&tau;<i> T</i>)^3/2), which was computed here for the first time. The formalism of hydro-kinetic equations can be applied to more general background flows and coupled to existing viscous hydrodynamic codes to incorporate the physics of hydrodynamic fluctuations.</p><p>

<p> Digital Holography (DH) is an emerging tool for use in the structural investigation of temperature dependent material processes. DH is able to reveal deformations and topological details at ultrahigh sensitivity (a few tens of nanometers) for particular details such as point-like objects and interfacial structures, allowing for the investigation of a range of processes. However, while DH is able to provide high precision data, the height measurement range is limited by the probe wavelength. Therefore a 'synthetic' wavelength created from the superposition of two or more individual wavelengths is often required in order to increase the measurement range to a suitable value dependent upon the object dimensions. </p><p> The use of multiple wavelengths attached to one system thus allows for surface height measurements over a relatively long range. In addition as the complex wave-front of each wavelength can be captured simultaneously in one digital image, real-time performance is achievable. In this thesis a number of materials processes were investigated at differing temperatures. The structural changes associated with the martensite to austenite phase transformation were measured using dual-wavelength digital holography during thermal cycling of nickel-aluminum-platinum (NiAlPt) and single-crystal Fe-15Cr-15Ni alloys. Real-time in-situ measurements reveal the formation of striations within the NiPtAl alloy at &sim;70&deg;C, and the FeCrNi alloy at &sim;520&deg;C. The results demonstrate that digital holography is an effective technique for acquiring non-contact, high precision information of the 3D surface evolution of alloys at high temperatures.</p>

<p> This thesis presents a scheme for preparation of trapped molecular ions with a high degree of internal state purity by optical pumping with a broadband pulse-shaped femtosecond laser; the internal structure of SiO⁺ permits fast stepwise pumping through the tens of rotational levels populated in a room-temperature distribution. Two analyses, which guided the experimental implementation, are presented: (1) a novel method of quantifying anharmonicity in the trapping potentials, which limits the number of ions that can be trapped, and (2) a rate-equation simulation of the quantum state evolution during pumping. Experimental implementation of pulse shaping and its characterization are discussed, as is the molecular spectroscopy used to reference this light to the rotational cooling transitions. Internal state analysis can be performed using resonance enhanced multiphoton dissociation.</p>

abstract: The current research is based on the principles of three-dimensional discrete element method (3D – DEM) through simulations, by using heat transfer models in EDEM, to investigate the effects of fill level, rotation rate and particle size on the steady-state conduction heat transfer in rotary drums. The high heat and mass transfer rates obtained through rotary drums make them very useful for powder mixing and heating processes in metallurgical, cement, mining, pharmaceutical, detergent and other particulate processing applications. However, these complex processes are difficult to model and operate since the particles can have a wide range of properties, and there is currently no way to predict the optimal operating conditions for a given material. Steady-state heat transfer by conduction forms the basis for understanding other steady-state and unsteady-state heat transfer in a rotary drum – conduction, convection and radiation. Statistical analysis is carried out to determine the effects of these process parameters and find optimal operating conditions, which will thereby improve the heat transfer efficiency in rotary drums. A stainless-steel drum with a diameter of 6 inches and a length of 3 inches was modeled in EDEM with silica beads of sizes 2 mm, 3 mm and 4 mm at fill levels of 10%, 17.5% and 25%, and at rotation rates of 2 rpm, 5 rpm and 10 rpm. It was found that the heating uniformity increased with decreasing particle size, decreasing fill level and increasing rotation rate. This research is the first step towards studying the other heat transfer modes and various other process parameters. Better understanding of the various heat transfer modes, when used in combination for heating the particles, will be beneficial in improving the operating efficiency, reducing material costs and leading to significant energy conservation on a global scale. / Dissertation/Thesis / Masters Thesis Chemical Engineering 2020

No description available.

No description available.

In this work, we have designed, simulated, fabricated and characterized homojunction Hg1-xCdxTe detector for high operating temperature in the MWIR region. The IR photon detectors need cryogenic cooling to suppress thermal generation. The temperature of operation in narrow gap semiconductor devices is limited by the noise due to statistical nature of thermal generation-recombination in narrow gap semiconductors. To make IR systems affordable they have to be operated without cooling or with minimal cooling compatible with low cost, low power and long life. Several fundamental and technological limitations to uncooled operation of photon detectors have been discussed in Chapter-1 of this thesis. Way and means adopted to increase the operating temperature, such as non-equilibrium operation, use of multilayer stacked hetero¬structures, optical immersion etc. have also been discussed. Key to improving the detector performance at any temperature is reduction of dark currents to level below the photocurrent and ultimately to the level where detector noise is determined by the fluctuations in photon flux from the scene (BLIP limit). In addition, design of present generation uncooled Hg1-xCdxTe infrared photon detectors relies on complex hetero-structures with a basic unit cell of type n+/π/p+. Theoretical modeling and numerical simulations on TLHJ device consisting of backside illuminated n+/π/p+ photodiodes have been performed. A numerical model for solving carrier transport equations for Hg1-xCdxTe infrared photodiodes was developed in MATLAB. Finite difference discretization of carrier transport equations and successive over relaxation method have been adopted. Numerical models are more appropriate than analytical models when analyzing multi-layer hetero-structures because we can account for realistic doping profiles, compositional grading and hetero-structures using this model. The model can be suitably modified to accommodate different device architectures, designs, material properties and operating temperature. Such a generalized model is useful to a device designer to customize the detector performance as per the availability of the material to suit the application specific requirements. The present work therefore proposes a more flexible, accurate and generalized methodology to accommodate the user needs by simulating the position dependence of carrier concentration, electrostatic potential and g-r rates and their effect on detector performance vis-à¬vis contact doping, absorber doping and absorber width on device performance. We detail aspects of our simulation model by developing a library of Hg1-xCdxTe properties using analytical and empirical expressions for material parameters (energy band gap, electron affinity, intrinsic carrier concentration, carrier effective mass, carrier mobility, dielectric constant and absorption coefficient). The PDEs were solved using the FDM coupled with SOR method. Behavior of Hg1-xCdxTe diodes (homo/hetero-junction) under different biasing, illumination and non equilibrium situations were modeled. Model has been validated for experimental measured data on n on p Hg1-xCdxTe photodiodes. The numerical computations are next applied to simulation/modeling of MWIR (λc=4.5 μm) n+/π/p+ TLHJ device for operation at T=250K. Several recombination processes occur in Hg1¬-xCdxTe depending on material quality, operating temperature, device design and processing conditions. Detailed mathematical models of radiative, Auger, Shockley Read Hall (SRH), surface recombination and optical g-r are analyzed and their effect on carrier lifetime have been evaluated. Analytical models for dark currents affecting the performance of Hg1-xCdxTe diodes at different temperatures are discussed. The mechanisms contributing to dark current are: (i) the thermal diffusion of minority carriers from the neutral regions (IDiff); (ii) generation-recombination from the space charge region of diode (IG-R) (iii) trap assisted tunneling currents, wherein the traps in the depletion region or the traps in the quasi neutral p region close to the depletion edge participate in the tunneling process(ITAT); (iii) band-to-band tunneling currents (IBTB) and (iv) surface leakage currents due to shunt resistance. Total current of a photodiode is ITOT=IDiff+IG-R+ITAT+IBTB+ISH-IP, where IP is the photocurrent. We evaluate the variation of electrostatic potential, carrier concentration, and electric field and g-r profiles as a function of position. The effect of variation in absorber width, doping and contact doping on D* is also analyzed. The mathematical models of different g-r processes (Auger, SRH, radiative, surface recombination and optical generation) affecting the device performance analyzed and their affect on carrier lifetimes are investigated. Responsivity ~3.25Amp-Watt-1, noise current~2pA/Hz1/2 and D* ~8x109 cmHz1/2watt-1 at 0.1V reverse bias have been calculated using optimized values of doping concentration, absorber width and carrier lifetime. The suitability of the method has been illustrated by demonstrating the feasibility of achieving the optimum device performance by carefully selecting the device design and other parameters. The numerical models provided insight about the operation and performance of Hg1-xCdxTe Auger-suppressed infrared photodiodes. Hetero-junction configuration increases the dynamic resistance, while the heavily doped contacts reduce the contact resistance. Wide gap/heavily doped contacts present a barrier to injection of minority carries into the absorber layer. At the same time they allow collection of minority carriers generated in the absorber region at the contacts. Hg1-xCdxTe hetero-diodes are grown by MOCVD and MBE with precise doping and compositional gradient control to reduce g-r contributions from defects and dislocations to the dark current in order to reap advantages of Auger suppression. Measured dark currents in hetero-junction photodiodes continue to be larger than expected in spite of the advancements in MBE technique. Delineation of an array on hetero-structures involves mesa separation of the diodes thus creating additional surface requiring passivation. Overall, the whole effort of fabricating a hetero Hg1-xCdxTe detector array is disproportionate to the overall gain in the performance. Therefore, we employ a much simpler fabrication process of homo-junction Hg1-xCdxTe detectors. It involves a planar device fabrication approach thus minimizing the surface passivation problem. We have deliberated upon the specific growth, characterization techniques and processing steps employed in our study. We discuss some of the experimental issues. We also presented results on the novel processing techniques developed that are potentially applicable to HOT technology and Hg1-xCdxTe technology in general. Hg1-xCdxTe (x=0.27-0.31) layer of ~ 15×15mm2 area and 15-20µm thickness is grown on CdZnTe substrate by Liquid Phase Epitaxy (LPE) in-house. As grown wafer is vacancy doped p-type with a carrier concentration of ~5×1015-1x1016 cm-3 and hole mobility of ~400cm2V-1s-1@80K. Planar n+/ν/p junction ~2-3µm deep is formed by B+ ion implantation and subsequent annealing; details are outlined in Chapter-4. Hall measurements and differential Hall measurements were used to find the carrier concentration, carrier mobility, resistivity of the wafer. The diodes are formed in the form of a 2D array along with various PEV’s for process characterization. Composition of Hg1-xCdxTe wafers used for the work is in the range of 0.27¬ 0.31 as determined by FTIR, corresponding to cutoff wavelength of 4.5-6.5µm. Junction depth and doping profile of the diodes after ion implantation was characterized by differential Hall technique. Transient minority carrier lifetime in fabricated MWIR n+/ν/p Hg1-xCdxTe (x=0.27) diodes were characterized using diode reverse-recovery technique. We prefer this method because it is a direct indicator of device as well as material quality post processing. By this time the device has undergone all the chemical/mechanical treatments and the measured lifetime is the cumulative of g-r mechanisms operative in bulk, space charge region and surface of diode. The value of lifetime extracted from the measured data lies in the range of 80-160ns. Variable temperature lifetime data was also extracted to determine the prevalent g-r process operative in the device. Diode dark I-V and junction C-V measurements were also made to correlate the observed behavior of the measured lifetime with g-r processes. Evidence of Auger suppression at room temperature is seen in the dark I-V characteristics via observation of negative differential resistance in the homo-junction Hg1-xCdxTe diodes. The experimental data is fitted using the numerical and analytical models developed. Based on this fitting, the current mechanisms limiting the dark current in these photodiodes are extracted. An improved analytical I-V model is reported by incorporating TAT and electric field enhanced Shockley-Read-Hall generation recombination process due to dislocations. Tunneling currents are fitted before and after the Auger suppression of carriers with energy level of trap (Et), trap density (Nt) and the doping concentrations of n+ and νregions as fitting parameters. Values of Et and Nt were determined as 0.78-0.80Eg and ~7-9×1014 cm-3 respectively in all cases. Doping concentration of νregion was found to exhibit non-equilibrium depletion from a value of 2×1016 to 4×1015 cm-3. Quantum efficiency of the diodes was found to ~25-30%. Note, that these are wafer level measurements on unpackaged device without backside AR coating. In addition to junction diodes, we present results on several PEV's such as VADA, MIS/MIM capacitors and TLM structures both at room and low temperature. Variable temperature measurements for a VADA tile and subsequent analysis provide evidence of g-r processes originating from defects, dislocations and dislocation loops, which are non-uniformly distributed across the Hg1-xCdxTe wafer and contributes to TAT current at high temperatures. MIS analysis yielded surface charge density lying between 3×1010-1×1011 cm-2 for ZnS/CdTe surface corresponding to a near flat band condition. Results of low and variable temperature measurements on the devices have also been shown to correlate it with the possibility of operating the device at mid temperatures such as 180-250K.

No description available.

No description available.

No description available.