Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013. / "June 2013." Cataloged from PDF version of thesis. / Includes bibliographical references (p. 29-30). / This research investigates the copolymerization of divinylbenzene and 4- vinylpyridine into organic thin films that exhibit conformal, stable, and uniform surface properties. Thin films were grown using initiated chemical vapor deposition, a variant of hot-wire deposition using a chemical initiator. Readily variable monomer flow into the active stage of the reactor allows for directly tunable copolymer composition. This tunability extends onto the control of material surface properties of a substrate that is coated with these organic thin films. The conditions of iCVD allow a variety of delicate substrates to be coated and for the full retention of pendant functional groups. This leads to their application to many industries including water desalination membranes, microfluidics, photolithography, sensors, among many others. The focus of this paper is on the facilitated control of surface modification using iCVD techniques and some of its future applications are also discussed. / by Ernesto Martinez. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / Electrostatic fiber formation, or electrospinning, offers a particularly simple and robust method to create polymeric nanofibers of various sizes and morphologies. In electrospinning, a viscoelastic fluid is charged so that a liquid jet is ejected from the surface of the fluid (typically in the form of a drop supplied by a needle or spinneret) and collected on a grounded plate, creating a nonwoven fiber mat. Modification of the diameter of the fibers as well as the porosity, specific surface area, and mechanical properties of the mat allows one to tailor electrospun mats for specific applications. Despite the widespread and rapidly growing use of electrospinning in the fabrication of novel nanomaterials, there are no simple, universal methods of predicting, a priori, the properties of electrospun fibers from knowledge of the polymer solution properties and electrospinning operating conditions alone. Changing a single fluid or processing parameter can affect the jet and fiber formation through several mechanisms. For example, using a different solvent can change several properties of the electrospinning fluid, such as the dielectric constant, conductivity, surface tension, and solute-solvent interaction. The work in this thesis seeks to develop a simple relation for predicting terminal jet diameter during electrospinning, which accounts for solution viscoelasticity as well as solution conductivity and operating parameters that can be easily measured and controlled. The mechanical and tribological properties of electrospun fiber mats are of paramount importance to their utility as components in a variety of applications. Although some mechanical properties of these mats have been investigated previously, reports of their tribological properties are essentially nonexistent. In this thesis, electrospun nanofiber mats of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) and poly(hexamethylene adipamide) (PA 6,6) are characterized mechanically and tribologically. Post-spin thermal annealing was used to modify the fiber morphology, inter-fiber welding, and crystallinity within the fibers. Morphological changes, in-plane tensile response, friction coefficient, and wear rate were characterized as functions of the annealing temperature. The Young's moduli, yield stresses and toughnesses of the PA 6(3)T nonwoven mats improved by two- to ten-fold when annealed slightly above the glass transition temperature, but at the expense of mat porosity. The mechanical and tribological properties of the thermally annealed PA 6,6 fiber mats exhibited significant improvements through the Brill transition temperature, comparable to the improvements observed for amorphous PA 6(3)T electrospun mats annealed near the glass transition temperature. The wear rates for both polymer systems correlate with the yield properties of the mat, in accordance with a modified Ratner-Lancaster model. The variation in mechanical and tribological properties of the mats with increasing annealing temperature is consistent with the formation of fiber-to-fiber junctions and a mechanism of abrasive wear that involves the breakage of these junctions between fibers. A mechanically robust proton exchange membrane with high ionic conductivity and selectivity is an important component in many electrochemical energy devices such as fuel cells, batteries, and photovoltaics. The ability to control and improve independently the mechanical response, ionic conductivity, and selectivity properties of a membrane is highly desirable in the development of next generation electrochemical devices. In this thesis, the use of layer-by-layer (LbL) assembly of polyelectrolytes is used to generate three different polymer film morphologies on highly porous electrospun fiber mats: webbed, conformal coating, and pore-bridging films. Specifically, depending on whether a vacuum is applied to the backside of the mat or not, the spray-LbL assembly either fills the voids of the mat with the proton conducting material or forms a continuous fuel-blocking film. The LbL component consists of a proton-conducting, methanolimpermeable poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6-dimethyl 1,4- phenylene oxide) (PDAC/sPPO) thin film. The electrospun fiber component consists of PA 6(3)T fibers of average diameter between 400 and 800 nm, in a nonwoven matrix of 60-90% porosity depending on the temperature of thermal annealing utilized to improve the mechanical properties. This thesis demonstrates the versatility and flexibility of this fabrication technique, since any ion conducting LbL system may be sprayed onto any electrospun fiber mat, allowing for independent control of functionality and mechanical properties. The mechanical properties of the spray coated electrospun mats are shown to be superior to the LbL-only system, and possess intrinsically greater dimensional stability and lower mechanical hysteresis than Nafion under hydration cycling. The electrochemical selectivity of the composite LbL-electrospun membrane is found to be superior to Nafion, which makes them a viable alternative proton exchange membrane for fuel cell applications. The composite proton exchange membranes fabricated in this work were tested in an operational direct methanol fuel cell, with results showing the capability for higher open circuit voltages (OCV) and comparable cell resistances when compared to Nafion. / by Matthew Marchand Mannarino. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 159-169). / Block copolymers (BCPs) are a class of soft materials consisting of two (or more) different chains joint together by covalent bond. This special chemical structure leads to microphase separation and consequently a variety of highly controllable self-assembly patterns. Directed self-assembly (DSA) of BCPs has therefore emerged as one of the most promising technologies to fabricate functional nanostructures and is able to produce patterns with ultra-small resolution (sub-10 nm) while maintaining high throughput and order. However, existing DSA methods depend mostly on carbon or silicon-based BCPs, thus lack functionality for sophisticated applications. This work aims at expanding the capability of DSA techniques by exploring new ways of incorporating functional materials into the BCP matrix and by imposing non-native symmetries on the BCP patterns. First, we focused on constructing nanocomposite thin films composed of BCPs and various types of functional materials (i.e., inorganic ions, inorganic-organic complex, organic compounds and nanoparticles). Based upon this methodology, we developed novel ways of fabricating mesoporous thin film structures with rectangular, triangular and quasicrystalline symmetries by means of graphoepitaxial post array templates. On the other hand, we also examined the limits of DSA by introducing artificial noise to mimic fabrication errors and studied the corresponding responses from BCP. This study demonstrates the potential of DSA of BCP in building thin film nanostructure of unconventional symmetries with functional components. / by Yi Ding. / Ph. D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1990. / Includes bibliographical references (leaf 41). / by Susan M. Kuhn. / B.S.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 60-61). / Alloy anodes form a promising path to the use of multi-cation electrolytes by increasing chemical stability. In this study, a lithium-magnesium alloy anode was developed such that lower cost and lower melting temperature multi-cation electrolytes could be incorporated in liquid metal batteries (LMBs). In a first part of this work, Lithium-magnesium was proven to be a viable anode in a standard uni-cation (Li+) Li-Mg/LiCl-LiF-LiI/Sb-Pb battery. SEM and EDS confirmed the stability of this anode with respect to the cathode (Sb-Pb) and the standard uni-cation electrolyte. Performance metrics (voltage, efficiencies, etc.) for the Li-Mg anode cell were found to be comparable to the analogous pure Li anode system. In a second part of this work, using the alloyed Li-Mg anode, we demonstrated successful cycling of cells using multi cation electrolytes in Li-Mg/LiBr-KBr/Sb-Pb and Li-Mg/LiCl-KCl/Sb-Pb LMBs. Each of these multi-cation electrolyte systems boasted an active materials energy cost of (<150$/kWh), which is less expensive than the metric cost to implement storage batteries in the electrical grid.[1] These results open the door to incorporating lower cost and lower melting temperature electrolyte candidates in LMBs by using alloyed anodes. / by Allan Blanchard. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 150-157). / Previous studies conducted at MIT showed that the power performance of an inverted pressurized water reactor (IPWR) conceptual design, i.e. the coolant and moderator are inverted such that the fuel is the continuous medium and the moderator flows through coolant channels, has potential to outperform a traditional pressurized water reactor (PWR). Similar to the traditional PWR, the IPWR design involves a tradeoff between core pressure drop and the minimum departure from nucleate boiling ratio (MDNBR). In order to increase the power density of the IPWR, Ferroni [231 examined the possibility of inserting multiple short-length twisted-tapes (MSLTTs) in the cooling channels. For a fixed coolant mass flow rate, the swirling flow produced by the MSLTTs allows the IPWR to have a higher operating heat flux while maintaining the design criteria of MDNBR as compared to either the traditional PWR or IPWR without swirl promoters. However, the addition of each twisted-tape increases the core pressure drop which limits the coolant flow rate due to pumping power limitations of existing reactor coolant pumps (RCPs). In order to better characterize the critical heat flux (CHF) enhancement caused by the addition of MSLTTs, this study performed a critical analysis of existing CHF correlations and models. Initially a phenomenological model was sought to describe the mechanisms of CHF for tubes containing MSLTTs; however, the full-length twisted-tape (FLTT) model that was selected for modification was found to have terms that could not be reconciled for the transition from fully developed swirl to decaying swirl. The existing CHF correlations for swirling flow were also found to be unsatisfactory. Therefore, the insights gained through working with the phenomenological model were used to develop a new empirical correlation to describe the departure from nucleate boiling (DNB) using existing swirling flow DNB data as well as an existing swirl decay model. In order to allow for more flexibility in the placement of the MSLThs, an existing FLTT pressure drop correlation was modified to account for the form pressure drop at the entrance to each twisted-tape insert as well as the friction pressure drop in the decaying swirl region downstream from the exit of each MSLTT. A sensitivity analysis of the new pressure drop correlation was also performed to determine if the complete methodology could be simplified. Design insights were presented that help to narrow the design space for the IPWR. These steps should be followed in order to find the maximum power density possible by the IPWR design. Finally, the existing swirl flow CHF data and correlations are presented in the appendices of this thesis. / by Tyrell Wayne Arment. / S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 54-55). / Thermalization losses drastically reduce the efficiency of silicon solar cells. In an age where we need sustainable energy production more than ever, silicon is the best material to target due to its high stake in the sustainable energy market. An organic-inorganic solar cell hybrid of tetracene-covered silicon can reduce thermalization losses through the downconversion process of using a high energy photon to generate two lower energy electron-hole pairs. This occurs through the singlet-triplet fission process that excitons can undertake in tetracene. The effect of the interface quality between tetracene and silicon on successful triplet energy transfer is investigated. RFPCD (Radio Frequency Photoconductive Decay) is used to measure the bulk lifetime as well as the surface recombination lifetime of minority carriers in both n- and p-type Silicon of various doping concentrations. The surface recombination velocity was calculated from the measurement of surface recombination lifetime and analyzed after the silicon underwent RCA clean, RCA clean followed by an HF dip, tungsten nitride ALD, and tetracene evaporation using various combinations of these steps to form appropriate process flows. It was found that the highest surface quality was obtained by the lowest doped wafers. Additionally, similar doping levels were affected similarly by the various processing steps outlined above while the type of dopant did not seem to dictate the surface quality response. Triplet energy transfer was not fully confirmed from tetracene to silicon, but the surface quality turned out to be a very important indication for whether or not this energy transfer could occur. / by Eveline Postelnicu. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2007. / Includes bibliographical references (leaf 31-34). / Introduction: Introduction: The world today contains an estimated 6.7 billion humans, and our population is growing at an unprecedented rate, consuming an ever-increasing amount of global resources. According to United Nations projections, the majority of this growth will occur in the third-world nations of Africa, and, to a lesser extent, Asia, among those peoples least able to afford the increasing burden on their resources. Clearly, what is needed in these African nations in the near future are more efficient, low-cost methods of using those resources they already have. Foremost among the problems faced by African developing nations is a lack of a reliable, sufficient, and nutritious food supply. Much of the African population survives on malnourished diets irregularly supplied by subsistence agriculture. In addition, crop loss due to both pests and post-harvest spoilage is much higher than in first world nations, with cold-storage technologies and modem pesticides. Equally important are the lives lost each year to food-borne disease. In the United States alone, food-borne infections cause an estimated 76 million cases of illness and 323,000 hospitalizations annually, for an estimated annual treatment cost of $6.7 billion and a death toll of thousands. In developing nations, of course, these casualty figures are much higher. It is precisely this crop loss and food-borne disease which this thesis proposes to address, by both proposing and evaluating a method, namely, food irradiation, to diminish crop loss in African villages and small-towns. As stated by Fritz Kaferstein in the Journal of Public Health Policy, "In developing countries with warm climates, with non-grain staples, vegetables and fruits, the pos-tharvest loss is believed to exceed 50%. With commodities such as dried fish, insect infestation is reported to result in a loss of 25% of the product with an additional 10% lost due to spoilage. While not all of these losses can be prevented by food irradiation, the technology does offer unique potential to destroy insect infestation and reduce spoilage." / by Nishaal Jitendra Parmar. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages [143]-155). / This thesis explores bulk and interface defects in germanium (Ge) and silicon (Si) with a focus on understanding the impact defect related bandgap states will have on optoelectronic applications. Optoelectronic devices are minority carrier devices and are particularly sensitive to defect states which can drastically reduce carrier lifetimes in small concentrations. We performed a study of defect states in Sb-doped germanium by generation of defects via irradiation followed by subsequent characterization of electronic properties via deep-level transient spectroscopy (DLTS). Cobalt-60 gamma rays were used to generate isolated vacancies and interstitials which diffuse and react with impurities in the material to form four defect states (E₃₇, E₃₀, E₂₂, and E₂₁) in the upper half of the bandgap. Irradiations at 77 K and 300 K as well as isothermal anneals were performed to characterize the relationships between the four observable defects. E₃₇ is assigned to the Sb donor-vacancy associate (E-center) and is the only vacancy containing defect giving an estimate of 2 x 10¹¹ cm-³ Mrad-¹ for the uncorrelated vacancy-interstitial pair introduction rate. E₃₇ decays by dissociation and vacancy diffusion to a sink present in a concentration of 10¹² cm-³. The remaining three defect states are interstitial associates and transform among one another. Conversion ratios between E₂₂, E₂₁, and E₃₀ indicate that E₂₂ likely contains two interstitials. The formation behavior of E₂₂ after irradiation in liquid nitrogen indicates that E₃₀ is required for formation of E₂₂. Eight defect states previously unseen after gamma irradiation were observed and characterized after irradiation by alpha and neutron sources. Their absence after gamma irradiation indicates that defect formation requires collision cascades. We demonstrate electrically pumped lasing from Ge epitaxially grown on Si. Lasing is observed over a ~200nm bandwidth showing that this system holds promise for low-cost on-chip communications applications via silicon microphotonics. The observed large threshold currents are determined to be largely a result of recombination due to threading dislocations. We estimate that recombination by threading dislocations becomes negligible when threading dislocation density is </~ 4 10⁶ cm-². We developed a process for incorporation of colloidal quantum dots (QD) into a chalcogenide glass (ChG) matrix via solution based processing in common solvents. Observation of photoluminescence (PL) comparable to QD/polymethyl methacrylate (PMMA) films shows potential for this material to form the basis for low cost light sources which can be integrated with ChG microphotonic systems. We investigated the impact of surface recombination on the benefit of combining a singlet fission material (tetracene) with a Si solar cell. Our simulations show that for efficiency gains, surface recombination velocity (SRV) for the tetracene/silicon interface must be less than 10⁴ cm s-¹. Characterization via radio frequency photoconductivity decay (RFPCD) measurements show that tetracene does not provide a sufficient level of passivation thus requiring another material which passivates the interface. Using thin films fabricated by atomic layer deposition (ALD), we showed the first direct evidence of triplet energy transfer to Si via magnetic field effect (MFE) PL measurements. / by Neil Sunil Patel. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 205-210). / This thesis presents the theory, design, and experimental demonstration of coherent microwave generation at 2.4 GHz in a metamaterial loaded waveguide using a 490 keV, 84 A, one microsecond pulse length electron beam that produced more than 5 MW of microwave power. Three different metamaterial structure designs named MTM1, MTM2, and MTM3 were tested with design frequencies of 2.8, 2.4, and 3.7 GHz, respectively. The waveguides were loaded with two metamaterial plates that were machined with complementary split ring resonators with periods ranging from 5 to 10 mm. The metamaterial waveguides supported two distinct modes: a symmetric mode that occurs when the two metamaterial plates were excited in phase, and an antisymmetric mode that occurs when the metamaterial plates were excited out of phase. The electron beam propagated on axis between the metamaterial plates. The output radiation was studied for solenoid magnetic field values in the range 350 to 1600 G and for beam voltages from 350 to 500 kV. The best results were found in a 370 mm long structure using the MTM2 design, where output power levels of up to 5 MW were obtained at 400 G in the antisymmetric mode at a frequency near 2.39 GHz. The frequency tuning vs. magnetic field for operation at a power level exceeding 1 MW was consistent with that predicted by an anomalous Doppler shifted resonance condition, [omega] = [kappa]z[upsilon]z - [Omega]c/[gamma]. At magnetic fields above 750 G, the microwave output switched to the symmetric mode at a frequency near 2.44 GHz, but the power level dropped drastically to below 100 W. In contrast to the antisymmetric mode, the frequency tuning of the symmetric mode was consistent with that predicted by a normal Cherenkov resonance, [omega] = [kappa]z[upsilonl]z. CST PIC simulations predict the observed output frequencies and the switch between modes at 750 G. However, the CST simulations also predict multi-megawatt power levels in both modes, which was observed in the antisymmetric mode, but not the symmetric mode. The discrepancy between the symmetric mode output power of the simulations and experiment Is unexplained. To the authors knowledge, these are the first reported experimental results of high power (> 1 MW) microwave generation from an electron beam interacting with a metamaterial structure. The results are important for the development of new microwave sources and novel devices which utilize active metamaterials / by Jason Samuel Hummelt. / Ph. D.