Growth Angle and Melt Meniscus of the RF-heated Floating Zone in Silicon Crystal Growth

This article presents a direct measurement of the growth angle during the growth of a cylindrical 2" silicon crystal using a radio-frequency heated floating zone process. From the high-resolution pictures taken during the process, this growth angle was evaluated to be 11{\deg}{\pm}2{\deg}. Furthermore, the free surface of the melt was modeled using the Laplace-Young equation. This model has to include the electromagnetic pressure calculated by the surface ring currents approximation. The results were compared to the experimental free surface derived from video frames. It could be shown that the calculated free surface will only fit the experimentally determined one if the right growth angle is considered

The Zwicky Transient Facility Camera

The Zwicky Transient Facility Camera (ZTFC) is a key element of the ZTF Observing System, the integrated system of optoelectromechanical instrumentation tasked to acquire the wide-field, high-cadence time-domain astronomical data at the heart of the Zwicky Transient Facility. The ZTFC consists of a compact cryostat with large vacuum window protecting a mosaic of 16 large, wafer-scale science CCDs and 4 smaller guide/focus CCDs, a sophisticated vacuum interface board which carries data as electrical signals out of the cryostat, an electromechanical window frame for securing externally inserted optical filter selections, and associated cryo-thermal/vacuum system support elements. The ZTFC provides an instantaneous 47 deg^2 field of view, limited by primary mirror vignetting in its Schmidt telescope prime focus configuration. We report here on the design and performance of the ZTF CCD camera cryostat and report results from extensive Joule-Thompson cryocooler tests that may be of broad interest to the instrumentation community

Particles Confined by Fluid Interfaces: Imaging Particle Motion, Interface Deformation and Capillary Forces

Small solid particles, confined in two-dimensions by fluid interfaces, were studied by a variety of experimental methods to understand particle motion, menisci shapes near interface-supported particles, and capillary interactions among such particles. Unwanted evaporation was circumvented by adopting non-volatile ionic liquids to create the fluid interfaces. A related application, employment of ionic liquids to float cryo-microtomed polymer sections, was also developed. The Brownian motions of nanospheres and nanorods in free-standing ionic liquid films were visualized in situ by high resolution scanning electron microscopy, which images features almost 100× smaller than possible in an optical microscope. For suspensions that are dilute and films that are thick compared to the particle diameter, the translational and rotational diffusion coefficients determined by single-particle tracking agreed with existing theoretical predictions. In thinner films, a striking and unexpected dynamical pairing of nanospheres was observed, suggesting a balance of capillary and hydrodynamic interactions. Nanospheres at high concentration displayed subdiffusive caged motion and hexagonal-lattice crystallization. Concentrated nanorods in the thinner films transiently assembled into finite stacks but did not achieve high tetratic liquid crystalline order, perhaps because of spherical impurities. A small spherical microparticles on a cylindrically curved liquid interface, to maintain constant contact angle about its wetted periphery, locally induces a quadrupolar interface deformation. Measured by optical profilometry, this deformation was compared to a recent theoretical expression, and good agreement was noted. The interface quadrupoles lead to particle capillary interactions in analogy to a 2d electrostatic quadrupoles, and as one consequence, spheres on a cylindrical interface assemble tetragonally. The assembly was monitored in the optical microscope, with particles driven to assembly as predicted, into a tetragonal lattice aligned with the underlying cylindrical axis. Lastly, ionic liquids and their mixtures with low molecular weight solvents were applied as flotation liquids for cryo-ultramicrotomy. With control of glass transition temperature and liquid viscosity, flat and ultra-thin sections were reliably floated onto transmission electron microscopy grids at cryogenic temperature. Compared to established flotation media for soft polymer systems, the required time and skill are significantly reduced, and the operator was not exposed to noxious fumes

Index to 1983 NASA Tech Briefs, volume 8, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1983 Tech Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Tactile 3D probing system for measuring MEMS with nanometer uncertainty : aspects of probing, design, manufacturing and assembly

Measurement underpins manufacturing technology, or in more popular terms: when you cannot measure it, you cannot manufacture it. This is true on any dimensional scale, so for microand nanotechnology to deliver manufactured products it must be supported by reliable metrology. Component miniaturization in the field of precision engineering and the development of micro electromechanical systems (MEMS) thus results in a demand for suitable measurement instruments for complex three-dimensional components with feature dimensions in the micrometer region and associated dimensional tolerances below 100 nm. As will be discussed in the first chapter of this thesis, several ultra precision coordinate measuring machines (CMMs) are developed. These CMMs are suitable for measuring complex threedimensional products, like MEMS and other miniaturized components. From a discussion on available probe systems in the first chapter it is apparent that, with respect to measurement uncertainty and applicability of measurements on MEMS and other miniaturized components, the performance of ultra precision CMMs is currently limited by the performance of available probe systems. The main reason is that the measurement using a probe system is not purely influenced by work piece topography, but also by interaction physics between probe tip and work piece. As the dimensional scale of the measurement decreases, the problems associated with this interaction become increasingly apparent. Typical aspects of this interaction include the influence of contact forces on plastic deformations in the contact region, surface forces and geometric and thermal effects. The influence of these aspects on the measurement result is discussed in the second chapter. This chapter will combine results from literature, simulation and experimental results to discuss the aspects that influence the measurement result in tactile probes. From these results it will become apparent that these aspects underlie the limitation for precision measurements on miniaturized components using tactile CMM metrology. As a result, these interaction aspects are the main challenge when designing ultra precision probes. The analysis of the interaction physics is used in the design of a novel silicon probing system with integrated piezo resistive strain gauges to measure a displacement of the probe tip. The result is a probe system with a colliding mass of 34 mg and an isotropic stiffness at the probe tip with a stiffness down to 50 N/m. The measurement range of the probing system is 30 µm, but in most measurements a range of 10 µm is used which slightly improves the signal to noise ratio. Calibration results using the planar differential laser interferometer setup as discussed in chapter 1 show a standard deviation of 2 nm over 2000 measurement points taken in a 6 hour time frame over a repeated 5.5 µm displacement. The combined 3D uncertainty of the probing system is estimated to be 17.4 nm. In order to measure micrometer scale structures, including holes and trenches, the probing system can be equipped with micrometer scale probe tips. The main limitation is the relative stiffness between the stylus and the suspension of the probing system. By design optimization, a ratio between the length and radius of the measurement part of the stylus of 50 can be obtained, making the probing system highly suitable for measuring these micrometer scale structures. So far, probe tips with a radius of 25 µm have been manufactured and work is being done to decrease this radius even further. The probing system is implemented on a high-accuracy coordinate measuring machine and is suitable for three-dimensional tactile measurements on miniaturized components with nanometer uncertainty. A main limitation when manufacturing the probe is assembly of the probe tip, stylus and chip which is discussed in chapter 4. Assembly of the probe is investigated in a series of experiments on an automated assembler. Based on these results, the design of the probe is optimized for assembly and the automated assembler is made suitable for assembly of the probe by implementation of a novel suction gripper. This resulted in an improvement in placement uncertainty at the tip by a factor of 10 and an increase in yield during assembly from 60 - 80% initially, to over 95%. In chapter 5 several experimental results with the probe system are discussed, including a quantification of the effects of surface forces on tactile measurements. It is shown that these effects are highly repeatable and result in an attraction of 40 µN and 60 µN in the xy- and z-direction, respectively. Moreover, it is shown that the influence of surface forces on a measurement in the xy-plane can be observed for a separation of 500 µm or less. Finally, conclusions and recommendations for further research are discussed in chapter 6

Understanding sol–gel processing: Hierarchical silica monoliths towards applications in chemical reaction engineering

Hierarchically structured, porous materials in the form of macro–mesoporous silica monoliths represent ideal support materials for a variety of applications such as chemical separation, heterogeneous catalysis, thermal insulation, electrochemical processes and CO2 adsorption. They are well suited for this purpose since the macropores enable fast, advection-dominated transport through the material whilst the mesoporous skeleton provides a large external surface area for mass transfer between the macro- and the mesoporous domains, as well as a large, internal surface area for possible functionalization. In this context, this thesis focuses on the generation of hierarchy in sol–gel based porous silica materials, as well as the determination and interpretation of their properties with respect to applications in reaction technology. For the entire preparation process each individual step is fine-tuned in terms of practicability, time-effectiveness and simplicity to obtain robust and straightforward synthetic routes towards stable, hierarchically structured sol–gel monoliths. The understanding of the chemical and physical processes involved in these steps allows the precise control of the material characteristics. Novel experimental methods and strategies are presented, which minimize the laboratory workload and additionally highlight the superior properties of these materials. The potential of hierarchically structured sol–gel monoliths is demonstrated by applications in heterogeneous catalysis and biocatalysis. In the following, the respective concepts of this thesis are briefly summarized. Chapter 1 examines the concept of hierarchy itself and highlights the advantages of a hierarchically structured pore system in comparison with monomodal pore systems, especially with regard to its mass transport properties. A model system based on silica membranes is presented, which discloses the advantages and disadvantages of purely mesoporous, purely macroporous and hierarchical pore structures. The monolithic support materials are synthesized using the sol–gel process, as this technique is considered as highly variable in the generation of different pore structures and sizes and it enables post-synthetic functionalization of the silica surface. In order to prepare mechanically stable and comparable sol–gel membranes, a novel and simplified drying method is presented. By varying the synthesis compositions, synthesis conditions and post-synthetic treatments, a variety of silica membranes are produced, which differ only in their porous properties. Monomodal structured materials with mean pore sizes in the range from 20 to 40 nm (mesoporous) and 350 to 3250 nm (macroporous) are produced as well as hierarchically structured materials combining these pore size ranges. All surfaces are functionalized post-synthetically with the bifunctional reagent 3-(gylcidoxypropyl)-dimethylmethoxysilane to realize the covalent immobilization of the enzyme acethylcholinesterase (AChE) under ring opening of the epoxy group of the silane. In the following, the three different pore systems are compared in terms of their enzyme loading capacity and their response times. Due to the pore-size-dependent specific surface area, the loading capacities of the representative pore systems differ significantly, resulting in a loading of 4.1 µg of AChE per membrane with a macropore size of 3250 nm and an AChE loading of 38.5 µg per membrane for a mesopore size of 20 nm, whereby the hierarchically structured pore system with equivalent pore size ranges has a loading capacity of 15.5 µg AChE per membrane. The response time of the enzyme-catalyzed substrate degradation reaction of acetylcholine to choline and acetic acid is used to determine the apparent reaction rate, which is used to describe the efficiency of the individual pore structure and thus allows conclusions on intrinsic diffusion limitations. These investigations are conducted for all three pore systems at a constant enzyme loading to ensure comparability. At a loading of 4.1 µg AChE per membrane, the purely macroporous pore system exhibits a slight advantage over the hierarchically structured material, as it causes the fastest reaction, which is due to the lowest mass transfer limitations. By increasing the enzyme loading to 12.9 µg per membrane, it is evident that the hierarchically structured pore system shows a significant reduction of the response time, and thus is superior to the purely mesoporous material, as it combines the advantages of both monomodal systems, the improved mass transport and the higher enzyme loading. In conclusion, this study demonstrates systematic investigations to highlight the advantages of pore space hierarchy, which is characterized by combined functionality and transport efficiency. Chapter 2 presents the urea-controlled synthesis of hierarchically structured silica monoliths, their transfer into a suitable column system and subsequent functionalization of the support surface in order to use them as flow microreactors. The hierarchy is generated by polymer-induced phase separation, which is an important step in the sol–gel process. The silica gels were synthesized using urea to create a mesoporous domain and to control the macroporous system. The influence of urea on the mean mesopore size is based on the base releasing property of urea by decomposition to ammonia and carbon dioxide under elevated temperature and has already been described in the literature. The resulting raise in pH increases the solubility of silica, whereby dissolution and deposition processes ensure that the initially microporous silica skeleton is expanded, resulting in a mesoporous domain. A common scientific approach is to add urea to the aqueous starting solution for a direct incorporation into the hydrogel to ensure a homogeneous pore expansion in the hydrothermal treatment. However, it is found that urea also has a strong influence on the macropore size and skeleton thickness of the obtained sol–gel monolith. With increasing urea content, the average macropore size is significantly reduced down to the submicron range. Urea influences the time sequences of gelation and phase separation due to an increase in the pH of the sol, which already occurs before substantial decomposition and additionally influences its polarity. Therefore, the gelation point occurs at an earlier stage of spinodal decomposition, which fixes a smaller macropore system. The synthesized monoliths with macropores in the submicron range are well-suited for the investigation of intrinsic reaction kinetics, since external and internal diffusion limitations are eliminated and hydrodynamic backmixing is reduced to a degree that a hydrodynamic plug flow behavior can be reached. For the application as flow microreactor, a novel cladding procedure is presented, which enables seamless housing of the sol–gel monolith in stainless steel tubing to withstand column backpressures >100 bar. By a special stop-flow functionalization method, aminopropyl groups are coated onto the silica surface generating a catalytically active column. The acquisition of reaction data for the reaction kinetics of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate to trans-α-ethyl cyanocinnamate, which is a well-known test reaction for basic catalysts, is realized with a two-dimensional HPLC setup. The first dimension automates the adjustment of the desired reaction parameters for the microreactor and the second dimension allows the complete quantification of the reaction data by online HPLC. The entire reaction kinetics of the amino-catalyzed Knoevenagel condensation for five different reaction times at seven different reaction temperatures each is recorded in only about 400 minutes. The reaction data results in a pseudo first order reaction kinetics, which is due to the two-step reaction mechanism. The reaction data reveal a reaction behavior under quasi-homogeneous conditions, which confirms the absence of any transport limitations. In conclusion, hierarchically structured sol–gel monoliths are synthesized using urea as pore size controlling agent to obtain catalytic microreactors with a high active surface area, which allow for the investigation of intrinsic reaction kinetics without transport limitations. Chapter 3 describes a new approach for the preparation of hierarchically structured, sulfonic acid modified silica monoliths based on the sol–gel process, whereby the functionalization is introduced into the pore system in situ during gelation. By using the co-condensation method, an alkoxysilane with a propylthiol function, ((3-mercaptopropyl)trimethoxysilane, MPTMS), is added together with the unfunctionalized silica precursor for sol formation. The synthesis of such organic-inorganic hybrid materials is widely used in the literature, but not for hierarchically structured sol–gel materials, where the hierarchy is generated via polymer-induced phase separation. Here, the incorporated polymer is usually removed from the material at high temperatures by an additional step, called calcination, to obtain the pure silica product. This treatment pyrolyzes any organic matter, which would also result in a loss of incorporated functionality. Therefore, this study presents an extraction of the polymer to avoid the loss of the covalently bound functionalization on the support surface, which simultaneously converts the introduced thiol groups into sulfonic acid functions, resulting in a time-efficient synthesis route. For this purpose, an extracting agent consisting of hydrogen peroxide and nitric acid is used. Macropore size control is demonstrated by the variation of polymer and functionalization reagent compositions, whereby the functionalization reagent has a significant influence on the onset of phase separation and consequently on the final macropore size. Furthermore, the widening process to form the mesoporous domain is strongly affected, resulting in very narrow mesopore distributions <10 nm with specific surface areas of up to 576 m2 g–1. The efficiency of the extraction procedure and the successful generation of sulfonic acid modified silica is extensively investigated and characterized to evaluate the presented approach. It is shown that, as the amount of MPTMS increases, the sulfur content and thus the loading of homogeneously distributed sulfonic acid groups is increased up to 1.2 mmol g–1 without significant loss due to the extraction. The polymer, however, is removed to a high degree during the extraction process. The covalent binding of the functionalization and the successful oxidation of the sulfur to form sulfonic acid functions is demonstrated by IR as well as 13C and 29Si MAS NMR spectroscopy. Moreover, investigations using inverse gas chromatography allow to investigate surface interactions and acid strength. The functionalized organic-inorganic hybrid monoliths have significantly higher surface energies, with the specific (polar) component being more dominant as the dispersive component. Furthermore, it is shown that these materials gain acid strength, which is generated by the incorporated sulfonic acid groups. In summary, a novel and efficient synthesis route for the preparation of hierarchically structured sol–gel monoliths with homogeneously distributed sulfonic acid functionalization is introduced. In conclusion, this thesis improves the understanding of the individual steps of the sol–gel process for the preparation of hierarchically structured silica-based materials. These results are presented in the context of different possible applications, since their variability allows them to be adapted to diverse requirements and problems