Theory and Design of Terahertz Quantum-Cascade Littrow Metasurface External Cavity Lasers

Quantum-cascade vertical-external-cavity surface-emitting-laser (QC-VECSEL) is a recently developed compact and coherent source of THz radiation which has demonstrated excellent beam quality and scalable high-power. The key component of the QC-VECSEL is an amplifying reflectarray metasurface made up of an array of sub-wavelength metal-metal waveguides loaded with quantum-cascade (GaAs/AlGaAs) laser gain material. To further the usefulness of this technology for many applications, including spectroscopy, heterodyne detection, and multispectral imaging, broadband and tunable THz QCLs are required. In this work, I investigate the feasibility of two techniques for tuning THz metasurface-based QCLs. First, A Littrow metasurface external cavity laser (ECL) is modeled and studied. We also propose and evaluate a novel method to implement Littrow ECL based upon blazed metasurface gratings. Electromagnetic simulations show that these metasurfaces can provide up to 15% fractional tunability around the center frequency of the laser at 3.3 THz. Preliminary results on several active resonant-phonon quantum-cascade materials are also obtained. Current progress on actual fabrication and device testing is reported

Effect of Heat Treatment on Microstructure and Hardness of a Worn Rail Repaired Using Laser Powder Deposition

The frequent replacement of worn rails on tracks brings an immense economic burden on the railroad industry, and also causes significant interruptions to railroad operation. Restoration of worn rails via laser powder deposition (LPD) can considerably reduce the associated maintenance costs. This study was focused on the use of LPD to repair the worn profile of a standard U.S. rail. The microstructure of the 304L stainless steel deposits with a minimum hardness of 85 HRB was composed of austenite, δ-ferrite, and sigma. Micropores were dispersed throughout the deposit, and microcracks were found at the rail-deposition interface. The pearlitic rail substrate showed a moderate hardness of 94 HRB. The fine-grain, pearlitic-ferritic heat affected zone had the maximum hardness of 96 HRB, which was still below the minimum required hardness of 97 HRB for a typical rail. To increase the hardness to or above 97 HRB and to mitigate the microstructural defects, the as-repaired rail went through a heat treatment process. The average hardness of the as-treated rail was increased significantly, i.e., to 103 HRB. Besides, the porous and coarse-grain deposition materials were transformed into an impermeable and fine-grain microstructure. However, heat treatment intensified the microcracks at the rail-deposition interface and also led to the formation of martensite and augmentation of the micropores in the parent rail. Isolation of the base rail during heat treatment and preheating were suggested as solutions for the problematic results. The LPD process ultimately was found to be a promising technique for repairing rails

Biochemical Characteristics and Viability of Probiotic and Yogurt Bacteria in Yogurt during the Fermentation and Refrigerated Storage

This research aimed to investigate the viability of probiotic bacteria (Lactobacillus acidophilus LA-5 and Bifidobacterium lactis BB-12) and yogurt bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) in yogurt during the fermentation, immediately after fermentation and during refrigerated storage (21 d, 4˚C). Also the biochemical characteristics of milk as affected by the commercial 4-strain mixed starter culture were investigated. Storage time affected the viability of all bacterial species. The concentration of lactic acid during the fermentation increased in parallel with the titrable acidity, and the concentration of acetic acid was proportional to the viability of Bifidobacterium lactis. The acetaldehyde level was decreased in the yogurt from day 0 up to the end of the storage. Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus were multiplied considerably during the fermentation. Streptococcus thermophilus could maintain its viability to the highest level, but Lactobacillus delbrueckii ssp. bulgaricus lost its viability rapidly during the cold storage compared to Streptococcus thermophilus. The multiplication and viability of probiotic bacteria were also influenced by the associative strains and species of yogurt organisms. Bifidobacteria counts were satisfactory. The loss of viability for bifidobacteria was gradual and steady during the storage, and they showed good stability during the storage as compared to Lactobacillus acidophilus

Polymer adsorption on silica and wettability of graphene oxide surfaces, experiments and simulations

Among the various classifications of polymer composites, studying polymers adsorbed to a surface such as silica is important due to their numerous applications. Adsorbed polymers usually show different properties than their bulk counterparts due to their interactions with the surface. In this study, we observed tightly- and loosely-bound polymer and mobile components in poly(vinyl acetate) (PVAc) on silica both with temperature-modulated differential scanning calorimetry (TMDSC) experiments and computer simulations. The more-mobile component which correlated to the region of low density at the air interface is reported for the first time using TMDSC thermograms. Pore size distribution and pore volume development of adsorbed PMMA samples showed different behavior below and above the tightly-bound amount of the polymer. The amount of tightly-bound polymer was obtained by a linear regression analysis of the ratio of the area under the two glass transitions. The values obtained vary from 0.52 to 0.86 mg PVAc/m2 silica depending upon the molecular mass for the amounts of PVAc and the specific surface area of fumed silica.Direct comparisons of the thermal properties and intermolecular interactions were performed between PVAc and poly(methyl methacrylate) (PMMA) with similar molecular masses and adsorbed amounts on silica. A larger amount of tightly-bound polymer and a greater change in glass transition were observed for adsorbed PMMA compared to adsorbed PVAc. These observations suggested that the interactions between PMMA and silica were stronger than those between PVAc and silica. Molecular modeling of these surface polymers showed that PMMA associates more strongly with silica than does PVAc through additional hydrogen-bonding interactions.Graphene oxide (GO) material surface characteristics make it easy to functionalize, making it a water repellant surface. To test the effect of chemical makeup and size of attached groups on the surface wettability of GO, we performed experimental water contact angle measurements and molecular modeling investigations on functionalized GO surfaces. Experimental and molecular simulation water contact angle measurements showed quantitative agreement for functionalizing groups with the same chain length at a variety of surface coverages

Mobile 3D Printing of Rail Track Surface for Rapid Repai

The frequent replacement of worn rails on tracks brings an immense economic burden on the railroad industry, and also causes significant interruptions to railroad operation. Restoration of worn rails via additive manufacturing, i.e., 3D printing, can considerably reduce the associated maintenance costs.The scope of this dissertation is to utilize metal additive manufacturing (AM) technology to repair worn rails that are used in the U.S. railway transportation system (e.g. 75-lb/yd light transit rails, or 136-lb/yd heavy freight rails). More specifically, this study aims to understand the interactive mechanisms between the deposited materials and the high-carbon rail steel to identify and alleviate the effect of the associated drawbacks, i.e., residual thermal stresses, micro pores, micro cracks, and martensite nucleation, on the resulting mechanical and metallurgical properties of the repaired rail. Submerged arc welding (SAW) and laser powder deposition (LPD) have been selected as two potential AM approaches for rail repair. Various deposition steels such as rutile E71T-1C and Lincore 40-S wire for the SAW process, and 304L stainless steel, 410L stainless steel, Stellite 6, and Stellite 21 in the powder form for the LPD process have been chosen to be deposited on the surface of the railhead. The study starts with the investigation of repairing the 75-lb/yd light transit rail through SAW and LPD processes. It then continues with exploration of LPD- and SAW-repairing of the 136-lb/yd (136RE) heavy freight/passenger rail. A finite element (FE) model is developed in which the thorough process of the AM rail repair is simulated through an element-birth-and-kill method and all of the involved interactions including thermal interactions, mechanical evolutions, and kinetics of phase transformation are defined and given to the model. The FE model development is simultaneously followed by similar experimental lab investigations to verify the model outcomes and hence, validate the developed FE model. This dissertation will try to modify the mechanical and metallurgical properties of the repaired rail through different approaches, i.e., preheating, post heat treatment, AM method alteration, tool path alteration, and deposition material alteration. Regarding the mechanical properties, distribution of hardness and residual stresses are measured via Rockwell hardness test and X-ray diffraction (XRD) stress measurement, respectively. Evaluation of the distribution of micro-pores, micro-cracks, and metallurgical phases is carried out through scanning electron microscope (SEM), optical microscope (OM), and XRD analysis. The tensile/compressive yield and shear strength of the repaired rail are assessed via 3-point bending and tensile tests, respectively. Initial efforts are performed through experimental lab measurements, and further efforts for various deposition materials and AM process parameters are conducted through the validated FE model

Evaluation of coenzyme Q10 addition and storage temperature on some physicochemical and organoleptic properties of pomegranate juice

Today, in parallel to growing in acceptance of functional products, various additives are used to improve the characteristics of functional food products. The coenzyme Q10 plays a vital role in cellular energy production. It also increases the body's immune system via its antioxidant activity. The aim of this study was to evaluate the addition of coenzyme Q10 on physicochemical properties of pomegranate fruit juice. The variables were concentrations of coenzyme Q10 (10 or 20 mg in 300 ml) and storage temperature (25°C and 4°C) and the parameters were pH, titrable acidity, brix, viscosity, turbidity and sensory evaluation during three months of storage. By increasing time and temperature, pH was decreased and with increasing concentration of coenzyme Q10, pH was increased. Time and temperature had direct influence on acidity, and the concentration of coenzyme Q10 had the opposite effect on the acidity. With increasing storage time and concentration of coenzyme Q10, Brix, viscosity and turbidity levels were increased and with increasing time and concentration of coenzyme Q10, the Brix, viscosity and turbidity were increased. The addition of coenzyme Q10 in grape juice showed no negative effect on the physicochemical and sensory properties.