Academic literature on the topic 'Low-temperature Techniques'

The reaction-diffusion behaviour of bronze process Nb₃Sn wires has been modelled, and good agreement has been obtained between predicted and measured compositions and Nb₃Sn layer thicknesses for a wide range of isothermal heat treatments. Isothermal and two-stage heat treatments have been assessed for uniformity of Nb₃Sn layer thicknesses and predicted critical current behaviour, and suggestions for optimising heat treatments and wire designs have been made. The electrical resistivity of bronze as a function of temperature and composition has been measured, and its annealing behaviour investigated. These results have been used with the diffusion model to calculate the electrical resistivity during isothermal heat treatment. Most features are in good agreement with experiment results, suggesting that this technique has potential for the monitoring and control of bronze process wire heat treatments. Discrepancies and areas for improving in the calculations have been identified, and these will form the subject of future work. A critical current characterisation system using pulsed currents and pulsed magnetic fields, Cryo-BI-Pulse, has been developed with Metis Instruments and Equipment. Excellent agreement with DC measurements has been obtained for a copper-matrix NbTi wire and a nickel-matrix MgB₂ tape, but discrepancies have been found for materials with resistive matrix materials. These discrepancies are mostly caused by current transfer effects when testing very short samples, and can be addressed by testing longer samples and improving the analysis of the measured sample voltages.

In petrochemical plants and power generating utilities, creep behaviour will affect the service life of components operating at elevated temperatures and stresses. Non-destructive techniques which detect creep damage are therefore important for assessing the remaining service life of the affected components. Whilst various non-destructive techniques have been reported, further studies are necessary to refine and assess the potential of these techniques for quantitative measures to be used for residual life-time prediction. In this thesis, the use of ultrasonic techniques to quantify creep cavitation in 2.25Cr-1Mo low alloy steel was explored. In contrast to conventional methods which only examine surface effects, these approaches should be capable of monitoring both surface and volumetric changes. A computer-based ultrasonic system was developed and testing procedures to characterise the metallurgical variables and creep damage in both ferritic and bainitic 2.25Cr-1Mo steel materials have been established. To evaluate the effects of high temperature exposure to assess microstructural changes and the development of creep damage, a systematic approach with a three-stages experimental work has been undertaken. Firstly, the effects of grain size and phase transformation product on ultrasonic behaviour were evaluated. Samples have been prepared with ferritic and bainitic microstructures. Austenitisation procedures were selected so that samples were produced with average grainsizes in the range 21 to 96 μm. Secondly, ultrasonic techniques were used to characterise microstructural changes due to thermal ageing effects. Materials in ferritic and bainitic microstructures were aged at 700°C for times from 2 to 2000 hours. Thirdly, the assessment of creep damage using ultrasonic techniques was undertaken. This was performed on specimens which had been subjected to creep at various stresses, in a temperature range of 575 to 625°C. In all cases, ultrasonic measurements were supported by optical microscopy, tensile testing and surface hardness measurements to document metallurgical condition.

Vegetable grafting confers soil-borne disease resistance through the use of selected rootstocks. Additional costs associated with grafted vegetable seedling propagation limit grower access to this technology. The use of low temperature storage in grafted seedling production reduces labor costs and allows propagators to meet the seasonal and narrow-window demand of growers. For the first part of the experiment 22 genotypes of Solanaceae or Cucurbitaceae seedlings were evaluated in low temperature storage conditions (5 or 12°C). Seedling performance was better in 12°C storage than in 5°C storage. For the second part of the experiment watermelon, the most low-temperature sensitive species found in our first study, was grafted onto two different commercial rootstocks or on to other watermelon seedlings and stored at 12°C for two and four weeks. Both 'Strong Tosa' and 'Emphasis' rootstocks conferred chilling tolerance during storage to watermelon scions, allowing storage of grafted plants for two weeks.

Le disiliciure wsi::(2) pour la metallisation d'un circuit vlsi (contacts et lignes d'interconnexion) est forme a une temperature aussi basse que possible en utilisant la structure "sandwich" a-si: h(150 a)/w(110 a)/a-si: h(540 a)/c-si implantee par des ions a faible courant et a temperature ambiante. La concentration residuelle d'oxygene dans les couches de w et de a-si:h et a leur interface a une influence decisive sur la formation et la resistivite de wsi::(2). Les plus faibles temperatures de formation de wsi::(2) atteintes sont les plus basses, 550**(o)c apres recuit, 350**(o)c apres implantation de w puis recuit. Les resistivites sont aussi plus faible. Interet et sensibilite de la spectroscopie ir dans la caracterisation des impuretes

La déstabilisation de l'ordre antiferromagnétique de Néel au profit de nouvelles phases quantiques à température nulle à deux dimensions est envisageable grâce au phénomène de frustration magnétique. Le modèle théorique de spins Heisenberg S=1/2 répartis sur le réseau bidimensionnel frustré kagome, constitué de triangles joints uniquement par leurs sommets, est susceptible de stabiliser des phases quantiques originales de liquides de spin, qui ne présentent aucune brisure de symétrie à T = 0. Cette thèse a été consacrée à l'étude expérimentale de deux types de composés de spins S=1/2 (Cu2+) à géométrie kagome à l'aide de techniques spectroscopiques locales, la RMN et la μSR, ainsi que de mesures thermodynamiques (susceptibilité magnétique, chaleur spécifique). Dans Mg-herbertsmithite, la frustration est générée par une interaction d'échange premiers voisins antiferromagnétique J et est responsable d'un comportement liquide de spin jusqu'à des températures de l'ordre de J/10000. Par rapport au composé isostructural antérieur, Zn-herbertsmithite, nous avons montré qu'il possédait des propriétés physiques similaires tout en permettant une caractérisation fine du taux de défauts de substitutions Cu/Mg. Nos expériences réalisées à partir d'échantillons contrôlés permettent d'étudier finement l'origine des plateaux de relaxation observés en μSR à basse température en lien avec l'existence des défauts de spins interplans. La kapellasite et l'haydéite possèdent des interactions ferromagnétiques (J1) et antiferromagnétiques (Jd), offrant la possibilité d'explorer le diagramme de phases générées par la compétition de ces interactions sur le réseau kagome. Pour la kapellasite, nos mesures de μSR démontrent le caractère liquide de spin jusqu'à T ≈ J1/1000. La dépendance en température de la susceptibilité magnétique sondée par RMN du 35Cl ainsi que de la chaleur spécifique permettent d'évaluer le rapport Jd/J1 = 0.85, qui localise classiquement son fondamental au sein d'une phase originale de spins non coplanaires à 12 sous-réseaux appelée cuboc2. Les interactions présentes dans l'haydéite localisent son fondamental au sein de la phase ferromagnétique, en bon accord avec nos mesures qui indiquent une transition partielle à caractère ferromagnétique à T = 4 K. Cette étude confirme la pertinence du réseau kagome frustré pour la stabilisation de phases quantiques originales et démontre l'existence d'une nouvelle phase liquide de spin sur ce réseau, distincte de celle attendue pour des spins couplés antiferromagnétiquement.

Includes abstract.<br>Includes bibliographical references (leaves 158-178).<br>Fischer-Tropsch synthesis is a process that catalytically converts hydrogen and carbon monoxide into a large variety of hydrocarbons and oxygenated products. Over the years many researchers have attempted to describe the full product spectrum (ranging from C1 to C100+) but due to the complexity of the product and shortcomings of certain analytical techniques (or equipment) most researchers were only able to construct product distributions from extrapolations of data recorded from analysis of the C1 to C5 fraction of the Fischer-Tropsch product. With recent advances in analytical technology and the development of comprehensive two-dimensional gas chromatography (GCxGC) it may now be possible to analyze the complex Fischer-Tropsch products in a relatively short time while delivering good separation of even minor compounds such as oxygenates and branched compounds. The aim of this study was to investigate if two-dimensional gas chromatography (GCxGC) really results in improved separation and identification of compounds in the complex Fischer-Tropsch product spectrum and will lead to a more complete product distribution especially of the minor compounds such as branched hydrocarbons, ketones, aldehydes and acids. For this study GCxGC equipment, supplied by Zoex Corporation, was connected to a micro slurry phase reactor system to provide for both on-line gas analysis as well as off-line product analysis. GCxGC methods were developed to analyze the hot tail gas and oil products from Fischer-Tropsch synthesis. Thereafter a test sample (C6 to C30 oil product from Fischer- Tropsch synthesis process) was injected several times into both the GCxGC and 1D GC systems. The purpose of this was to compare the detection ability and accuracy of the two instruments.