Pulse tube refrigerators (PTR) are robust, rugged cryocoolers that do not have a moving component at their cold ends. They are often employed for cryogenic cooling of high performance electronics in space applications where reliability is paramount. Miniaturizing these refrigerators has been a subject of intense research interest because of the benefits of minimal size and weight for airborne operation and because miniature coolers would be an enabling technology for other applications. Despite much effort, the extent of possible PTR miniaturization is still uncertain. To partially remedy this, an investigation of the miniaturization of pulse tube refrigerators has been undertaken using several numerical modeling techniques. In support of these models, experiments were performed to determine directional hydrodynamic parameters characteristic of stacked screens of #635 stainless steel and #325 phosphor bronze wire mesh, two fine-mesh porous materials suitable for use in the regenerator and heat exchanger components of miniature PTRs. Complete system level and pulse tube component level CFD models incorporating these parameters were then employed to quantitatively estimate the effects of several phenomena expected to impact the performance of miniature PTRs. These included the presence of preferential flow paths in an annular region near the regenerator wall and increased viscous and thermal boundary layer thicknesses relative to the pulse tube diameter. The effects of tapering or chamfering the junctions between components of dissimilar diameters were also investigated. The results of these models were subsequently applied to produce successively smaller micro-scale PTR models having total volumes as small as 0.141 cc for which sufficient net cooling was predicted to make operation at cryogenic temperatures feasible. The results of this investigation provide design criteria for miniaturized PTRs and establish the feasibility of their operation at frequencies up to 1000 Hz with dimensions roughly an order of magnitude smaller than those that have recently been demonstrated, provided that challenges related to their regenerator fillers and compressors can be addressed.

Identifying the matter in the universe is one of the main challenges of modern cosmology and astrophysics. An important part of this matter seems to be made of non-baryonic particles. EDELWEISS is a direct dark matter search using cryogenic germanium bolometers in order to look for particles that interact very weakly with the ordinary matter, generically known as WIMPs. An important challenge for EDELWEISS is the radioactive background and one of the ways to identify it is to use a larger variety of target crystals. Sapphire is a light target which can be complementary to the germanium crystals already in use. Spectroscopic characterization studies have been performed using different sapphire samples in order to find the optimum doping concentration for good low temperature scintillation. Ti doped crystals with weak Ti concentrations have been used for systematic X ray excitation tests both at room temperature and down to 30 K. The tests have shown that the best Ti concentration for optimum room temperature scintillation is 100 ppm and 50 ppm at T = 45 K. All concentrations have been checked by optical absorption and fluorescence.<br />After having shown that sapphire had interesting characteristics for building heat-scintillation detectors, we have tested if using a sapphire detector was feasible within a dark matter search. During the first commissioning tests of EDELWEISS II, we have proved the compatibility between a sapphire heat-scintillation detector and the experimental setup.

Porous tungsten is used to manufacture dispenser cathodes due to it refractory properties. Surface porosity is critical to functional performance of dispenser cathodes because it allows for an impregnated ceramic compound to migrate to the emitting surface, lowering its work function. Likewise, surface roughness is important because it is necessary to ensure uniform wetting of the molten impregnate during high temperature service. Current industry practice to achieve surface roughness and surface porosity requirements involves the use of a plastic infiltrant during machining. After machining, the infiltrant is baked and the cathode pellet is impregnated. In this context, cryogenic machining is investigated as a substitutionary process for the current plastic infiltration process. Along with significant reductions in cycle time and resource use, surface quality of cryogenically machined un-infiltrated (as-sintered) porous tungsten has been shown to significantly outperform dry machining. The present study is focused on examining the relationship between machining parameters and cooling condition on the as-machined surface integrity of porous tungsten. The effects of cryogenic pre-cooling, rake angle, cutting speed, depth of cut and feed are all taken into consideration with respect to machining-induced surface morphology. Cermet and Polycrystalline diamond (PCD) cutting tools are used to develop high performance cryogenic machining of porous tungsten. Dry and pre-heated machining were investigated as a means to allow for ductile mode machining, yet severe tool-wear and undesirable smearing limited the feasibility of these approaches. By using modified PCD cutting tools, high speed machining of porous tungsten at cutting speeds up to 400 m/min is achieved for the first time. Beyond a critical speed, brittle fracture and built-up edge are eliminated as the result of a brittle to ductile transition. A model of critical chip thickness (hc) effects based on cutting force, temperature and surface roughness data is developed and used to study the deformation mechanisms of porous tungsten under different machining conditions. It is found that when hmax = hc, ductile mode machining of otherwise highly brittle porous tungsten is possible. The value of hc is approximately the same as the average ligament size of the 80% density porous tungsten workpiece.

Liquefied natural gas (LNG) is stored at a cryogenic temperatures ≤ -160°C and around atmospheric pressure to insure the minimum storage volume in tanks. The demand for LNG has been increasing as a primary source of energy. Therefore, there is significant interest in the construction of LNG tanks to achieve low cost and safe storage. Three systems are typically used to store LNG: single containment, double containment, and full containment. Concrete is used in these containment systems, and consequently, understanding concrete behavior and properties at cryogenic temperatures is important. The research documented in this thesis deals with computational analysis of the behavior of concrete subjected to cryogenic temperatures. The analysis focuses on the effect of aggregate sizes, coefficient of thermal expansion, volume fraction, and the shape of aggregate on damage of concrete subjected to cryogenic temperatures. The analysis is performed by developing a computational model using the finite element software ABAQUS. In this model, concrete is considered as a 3- phase composite material in a meso-scale structure: mortar matrix, aggregate, and interfacial transmission zone (ITZ). The Concrete Damage Plasticity model in ABAQUS is used to represent the mortar and ITZ phases of concrete. This model has the advantage of accounting for the effect of temperature on material properties. The aggregate phase is modeled as a linear-elastic material. The model parameters are selected based on comprehensive literature review of material properties at different temperatures. The finite element results provide very useful insight on the effects of concrete mixture design and properties on resistance to damage. The most important factor that affected damage development was the difference in the coefficient of thermal expansion between the mortar and aggregates. Models in which the mortar and aggregate had close values of positive coefficients experienced less damage. The model with irregular shape particles experienced more localized damage than the model with circular shape particles. The model was successful in demonstrating the effect of using air entrained concrete in reducing damage. The damage results predicted by the model for air entrained and non-air entrained concrete are validated by comparing them with experimental data from the literature. The analysis validated the capabilities of the mode in simulating the effect of reduction in temperature on damage. The modeling results and the findings from the literature review were used to put forward recommendations regarding the characteristics of concrete used in LNG storage.

High signal-to-noise ratio (SNR) is typically required for higher resolution and faster speed in magnetic resonance imaging (MRI). Planar microcoils as receiver probes in MRI systems offer the potential to be configured into array elements for fast imaging as well as to enable the imaging of extremely small objects. Microcoils, however, are thermal noise dominant and suffer limited SNR. Cryo-cooling for the microcoils can reduce the thermal noise, however conventional cryostats are not optimum for the microcoils because they typically use a thick vacuum gap to keep samples to be imaged to near room temperature during cryo-cooling. This vacuum gap is typically larger than the most sensitive region of the microcoils that defines the imaging depth, which is approximately the same as the diameters of the microcoils. Here microfluidic technology is utilized to locally cryo-cool the microcoils and minimize the thermal isolation gap so that the imaging surface is within the imaging depth of the microcoils. The first system consists of a planar microcoil with microfluidically cryo-cooling channels, a thin N2 gap and an imaging. The microcoil was locally cryo-cooled while maintaining the sample above 8°C. MR images using a 4.7 Tesla MRI system shows an average SNR enhancement of 1.47 fold. Second, the system has been further developed into a cryo-cooled microcoil system with inductive coupling to cryo-cool both the microcoil and the on-chip microfabricated resonating capacitor to further improve the Q improvement. Here inductive coupling was used to eliminate the physical connection between the microcoil and the tuning network so that a single cryocooling microfluidic channel could enclose both the microcoil and the capacitor with minimum loss in cooling capacity. Q improvement was 2.6 fold compared to a conventional microcoil with high-Q varactors and transmission line connection. Microfluidically tunable capacitors with the 653% tunability and Q of 1.3 fold higher compared to a conventional varactor have been developed and demonstrated as matching/tuning networks as a proof of concept. These developed microfluidically cryo-cooling system and tunable capacitors for improving SNR will potentially allow MR microcoils to have high-resolution images over small samples.

Kompleksinės sistemos pasižymi naujomis savybėmis, kurias neįmanoma prognozuoti tiriant atskirus tų sistemų elementus. Buvo pastebėta, kad žinomi nesudėtingų sistemų analizės metodai yra neadekvatūs naujiems uždaviniams, neatsako į užduodamus klausimus, todėl kompleksinėms sistemoms analizuoti reikia kurti naujus metodus. Kompleksinių sistemų teorija yra palyginti nauja teorija ir viena iš pagrindinių jos taikymo sričių yra žmogaus organizmo funkcinės būklės vertinimo, mokslinių hipotezių tikrinimo, naujų žinių paieškos uždaviniai. Pasirinkta darbo tematika neabejotinai yra aktuali šiandienos teorijai ir praktikai. Darbe buvo nagrinėjami šeši elektrokardiografiniai parametrai: trukminiai RR-DQRS-DJTp ir amplitudiniai AR-AT1-ATS2. Iš parametrų skirtumų sudaromos matricų sekos ir skaičiuojami invariantiniai parametrai. Trečios eilės charakteringosios lygties didysis ir mažasis diskriminantai atvaizduojami jungtinėje fazinėje plokštumoje. Duomenims buvo pasirinkti kriogeninės terapijos elektrokardiogramų parametrai. / Complex systems have new features that can not be predicted testing of the individual elements of the system. It was noted that the known systems of simple analytical methods are inadequate for new tasks, not respond to questions, therefore, to analyze complex systems need to develop new methods. Complex systems theory is a relatively new theory, and one of its major application areas is human body's functional status assessment, scientific hypothesis testing, and the new knowledge-based search tasks. Selected topics of work are certainly actual today's theory and practice. Were analyzed six electrocardiographic parameters in this work: durational RR-DQRS DJTp and amplitudical-AR-AT1-ATS2. Differences between parameters of the sequence of matrices drawn up and calculated invariantical parameters. Third-order characteristic equations, great and small discriminants are represented in the united phase plane. Electrocardiograms cryogenic therapy parameters have been chosen for data.

The development of a sustainable manufacturing process requires a comprehensive evaluation method and fundamental understanding of the processes. Coolant application is a critical sustainability concern in the widely used machining process. Cryogenic machining is considered a candidate for sustainable coolant application. However, the lack of comprehensive evaluation methods leaves significant uncertainties about the overall sustainability performance of cryogenic machining. Also, the lack of practical application guidelines based on scientific understanding of the heat transfer mechanism in cryogenic machining limits the process optimization from achieving the most sustainable performance. In this dissertation, based on a proposed Process Sustainability Index (ProcSI) methodology, the sustainability performance of the cryogenic machining process is optimized with application guidelines established by scientific modeling of the heat transfer mechanism in the process. Based on the experimental results, the process optimization is carried out with Genetic Algorithm (GA). The metrics-based ProcSI method considers all three major aspects of sustainable manufacturing, namely economy, environment and society, based on the 6R concept and the total life-cycle aspect. There are sixty five metrics, categorized into six major clusters. Data for all relavant metrics are collected, normalized, weighted, and then aggregated to form the ProcSI score, as an overall judgment for the sustainability performance of the process. The ProcSI method focuses on the process design as a manufacturer’s aspect, hoping to improve the sustainability performance of the manufactured products and the manufacturing system. A heat transfer analysis of cryogenic machining for a flank-side liquid nitrogen jet delivery is carried out. This is performed by micro-scale high-speed temperature measurement experiments. The experimental results are processed with an innovative inverse heat transfer solution method to calculate the surface heat transfer coefficient at various locations throughout a wide temperature range. Based on the results, the application guidelines, including suggestions of a minimal, but sufficient, coolant flow rate are established. Cryogenic machining experiments are carried out, and ProcSI evaluation is applied to the experimental scenario. Based on the ProcSI evaluation, the optimization process implemented with GA provides optimal machining process parameters for minimum manufacturing cost, minimal energy consumption, or the best sustainability performance.

In recent years, there has been a substantial growth in the application of carbon fiber reinforced plastic (CFRP) composite materials in automobile and aerospace industries due to their superior properties such as lightweight, high strength, excellent corrosion resistance, and minimal fatigue concerns. The present study evaluates the drilling performance of woven carbon fiber reinforced plastics under both dry and cryogenic cooling conditions using uncoated solid carbide drill with a through-hole for coolant application. The effects of the cooling conditions and the cutting parameters on drilling performance in drilling CFRP were evaluated in terms of generated thrust force, torque, cutting edge radius, outer corner flank wear, hole quality (including surface roughness, diameter error, roundness, delamination, burr formation, sub-surface quality). Both cooling conditions and cutting parameters were found to influence the thrust force and torque at different levels. The thrust force and the torque are higher in cryogenic cooling under all cutting parameters. In most of the cases, cryogenic drilling gives better bore-hole quality with lower surface roughness, more accurate diameter, less burr generation, better sub-surface quality, etc. Also, the tool-wear rates measured in drilling shows that cryogenic drilling produces less tool-wear than dry drilling does.

Astronomy large-scale surveys require instrumentation to minimize the time required to complete observations of large sections of the sky. Optimizing receiver systems has been achieved through reducing the system temperature primarily by advances in low-noise amplifier technology to a point that the internally generated noise is now fast approaching the quantum limit. Instead, reflector-coupled focal plane arrays are now used to increase the field of view (FoV) by employing either multi-element horn feeds or phased array feeds. Widely spaced (2-3 wavelengths diameter) horn feeds inefficiently sample the available focal plane radiation, thus requiring multiple imaging passes. Alternatively, a more efficient method is to use a narrow element (0.5 wavelengths diameter) phased array feed with a beamformer to produce overlapping beams on the sky, fully Nyquist sampling the focal plane with a single pass. The FoV can be further increased with additional phased array feed (PAF) antenna-receiver modules adding to the contiguous fully sampled region. A 5 x 5 K-band (18 - 26 GHz) single polarization modular PAF incorporating an antenna array of planar axially symmetric elements is designed, simulated, manufactured and tested. Each narrow width tapered slot antenna element has an independent receiver chain consisting of a cryogenic packaged monolithic microwave integrated circuit (MMIC) GaAs amplifier and a packaged MMIC down converting mixer. Synthesized beams and beamformer characteristics are presented. The PAF imaging system performance is evaluated by survey speed and compared to the industry standard, the single pixel feed (SPF). Scientifically, K-band is attractive because it contains numerous molecular transitions, in particular the rotation-inversion lines of ammonia. These transitions are excited in dense gas, and can be used to directly measure kinetic temperatures and velocities of protostars throughout the Galaxy. Depending on the line detected, gas of different temperatures can be probed. It is concluded that even with a higher system temperature, a PAF with sufficient number of synthesized beams can outperform a SPF in imaging speed by more than an order of magnitude. / Graduate

L'observation spatiale exige l’utilisation de systèmes à très haute résolution avec de grandes dimensions. La conception de ces systèmes implique la détection et associe des systèmes d'actuation pour assurer un positionnement très stable de composants optiques. Toutefois, les matériaux actuellement utilisés pour l’actionnement piézoélectrique (PZT) montrent des propriétés fortement diminuées à très basse température. Ce travail explore les performances de composés piézoélectriques PMN-xPT à des températures cryogéniques. La dépendance en température des céramiques PMN-xPT est comparée à la dépendance thermique de PZT dur (PZT-4) et doux (PZT5H) sous forme de céramiques de la température ambiante à 10 K. Les compositions en PMN-PT de structure quadratique présentent des constantes piézoélectriques stables dans la plage de température 250 - 100 K. Les plus hautes valeurs de constantes piézoélectriques sont observées pour le composé PMN-38PT dans la gamme de température 200 à 50 K. Toutes les céramiques présentent des performances décroissantes en fonction de la température. Ce comportement est attribué à un «freezing out» des contributions extrinsèques du phénomène de piézoélectricité, mais est également dû à une contribution d’effets quantiques. Le facteur de qualité mécanique (Qm) augmente très rapidement à partir de 50 K pour tous les composés. Ceci pourrait être dû à de faibles processus de relaxation à très basses températures. Les transducteurs acoustiques actuels fonctionnent avec des céramiques à base de plomb, et plus particulièrement avec des céramiques PZT. Ce matériau a de fortes propriétés électromécaniques. Cependant à cause de la toxicité du plomb, nous devons trouver des composés sans plomb afin de remplacer le PZT. Dans ce travail de recherche, un oxyde mixte synthétisé par voie classique, (K, Na, Li) (Nb, Ta, Sb) O3 sous forme de céramiques est étudié. Ce composé possède d’excellentes densités (98% dth) et de bonnes propriétés électromécaniques à température ambiante. L’étude de la texturation de ce composé montre que par la méthode de coulage en bande, nous pouvons obtenir une certaine orientation des céramiques (60%). / Space borne observation requires extremely high resolution systems with large dimensions. The design of such systems implies associating sensing and actuation systems to insure a highly stable positioning of optical components. However, the currently used PZT materials for piezoelectric actuation show strongly diminished strain at cryogenic temperatures. This work explores the performances of PMN-xPT piezoelectric materials at cryogenic temperatures. Temperature dependence of PMN-xPT ceramics is compared with hard PZT (PZT-4) and soft PZT (PZT5H) ceramics from RT to 10 K. PMN-PT tetragonal compositions exhibit stable piezoelectric constant in the 250-100 K range. The highest values of piezoelectric constant are observed for PMN-38PT in the 200-50 K range. All ceramics exhibit decreasing performances versus cooling temperature. This behaviour is attributed to a “freezing out” of extrinsic contributions to piezoelectricity [12] but is also due to a contribution of quantum effects [13, 14]. The mechanical quality factor (Qm) increases very quickly from 50 K to 10 K for all samples. This fact could be due to low relaxation processes at very low temperatures. Today acoustic transducers operate with lead-based ceramics, and more particularly lead–zirconate–titanate (PZT). This material is very powerful thanks to these high electromechanical properties. However cause of lead toxicity we have to find new materials without lead in order to replace PZT. In this paper, main guidelines to develop lead-free ceramics are given specially for acoustic transducer. A conventional mixed oxide and carbonate route to synthesis (K,Na,Li)(Nb,Ta,Sb)O3 lead-free ceramics is presented. Excellent density (98% dth) and good electromechanical properties were observed at room temperature.