Thermocycleur | EPFL Graph Search

thumb|250px|Le Mastercycleur 5330, un thermocycleur produit par Eppendorf Le thermocycleur (aussi appelé cycleur thermique ou machine PCR) est un appareil automatisant la réaction de PCR « classique » ou « point final ». L'appareil est muni d'un bloc thermique (« thermal block ») avec des trous où l'on peut insérer les tubes contenant le mélange réactionnel de la PCR. Certains modèles permettent d'effectuer des gradients thermiques, d'un côté du bloc thermique à l'autre. Cette fonction est utilisée lorsqu'on veut trouver la meilleure température d'hybridation pour un couple d'amorces donné. Les appareils modernes sont équipés d'un toit chauffant qui presse sur les capuchons des tubes, évitant l'évaporation du mélange réactionnel et donc de devoir mettre une couche d'huile minérale dans le tube, au-dessus du milieu réactionnel. Un clavier composé de quelques touches et un écran à cristaux liquides permettent d'entrer des programmes dans la mémoire de la machine. Le premier prix pour un thermocycleur est d'environ 500 US$. Programme exemple : 95 °C pendant 10 à 15 minutes (en cas d'enzyme Hot Start) environ 30 cycles composés de : 95 °C pendant 30 secondes (étape de dénaturation) Tm -2 °C (par exemple 55 °C) pendant 1 minute (étape d'hybridation ou annealing) 72 °C pendant 1 minute (étape d'élongation) 72 °C pendant 5 minutes 40 °C pendant 1 minute (cooling) Note : l'étape d'élongation est définie en fonction de la longueur attendue du fragment à amplifier. On considère que la Taq polymérase synthétise environ 1 000 bases/minute. Pour une longueur attendue de 500 pb, 30 secondes d'élongation sont suffisantes. Ne comprend généralement qu'une vingtaine de cycles. Ce programme est généralement appelé « DENAT ». Il consiste en une unique étape à 95 °C pendant 10 ou 15 minutes. À la fin du cycle, il faut être présent pour sortir rapidement les barrettes d'échantillons pour les plonger rapidement dans la glace. Attention, le bloc du thermocycleur est chaud, il ne faut pas se brûler.

High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

The role of reinforcing phase contiguity in high volume fraction particulate composites is investigated using model composites of pure aluminium reinforced with α-Al2O3 particles. To produce the composites, alumina particle (5µm) preforms were either loosely packed or, alternatively, packed and CIPed; these were then sintered, varying the sintering time and temperature, and infiltrated with pure liquid aluminium using gas pressure infiltration. The microstructure of the resulting composites was characterized using image analysis in terms of reinforcement phase contiguity (β), defined as the ratio of particle surface that is in contact with other particles to that in contact with the matrix. The influence of this parameter on several characteristics of the composites and the preforms they were made from was then studied, using theoretical models in the literature to isolate the influence of this parameter from that of other variables, such as the volume fraction ceramic. To this end, the thermal conductivity of the sintered preforms, basic composite mechanical properties (hardness, tensile strength, and damage evolution during straining) and the thermal expansion of the composites were characterized and compared with theory; furthermore, quenching and neutron diffraction were used to probe in more depth the thermal expansion behaviour of the composites. The results show that the thermal conductivity of the preform increases, as expected, upon increasing the relative density (volume fraction) and the contiguity of alumina particle after sintering. Comparison of data with theory shows that, while experimental values for the CIPed samples agree with the prediction by Argento and Bouvard for the thermal conductivity of powder preforms as a function of densification, neither this model, nor that of Montes et al. , fits all experimental values. Rather, when pooled the data show that the thermal conductivity of powder preforms, normalized for relative density using the differential effective medium model, obeys relatively well a simple linear relation between contiguity and conductivity. The hardness of the composite increases with increasing relative density and increasing contiguity. Here again, after normalization for the dependence of hardness on ceramic volume fraction using current theory, a nearly linear relation is found between hardness and contiguity. Tensile testing results show that the strength of the composite increases with increasing fraction ceramic and increasing contiguity while the elongation decreases, composites with an interconnected ceramic phase rapidly becoming quite brittle, with elongations falling below half a percent. No clear relationship is found between contiguity and Young's modulus after normalization for the volume fraction ceramic. Measurements of internal damage evolution show that damage increases very rapidly with deformation in the present interconnected composites, in agreement with their brittle character. The thermal expansion results show that the CTE of the present composites often varies with temperature at a rate higher than that predicted to result from physical parameter variations by existing theoretical models. With regard to their thermal expansion, the composites can be separated into three classes that are characterized by the level of contiguity of the low expansion phase: low contiguity (i.e. β ≤ 0.045), medium contiguity (i.e. 0.05 ≤ β < 0.074) and high contiguity (i.e. β ≤ 0.074 ). The CTE values during heating for the low contiguity composites lie between Schapery's upper bound and the rule of mixtures, starting upon heat-up from the Schapery upper bound at low temperature and transiting above roughly 150°C to the rule of mixture prediction; upon cool-down the same behaviour is observed in reverse. The CTEs of the composites with medium contiguity at low temperatures are somewhat below the Schapery upper bound, increasing somewhat faster than predicted for this bound with increasing temperature to reach Shapery's upper bound at around 200°C. For the composites with high contiguity, the CTEs lie between elastic and plastic bounds predicted using the self-consistent model, the latter model being obtained by assigning a zero shear stiffness to the matrix. Physical reasoning and comparison of data with theory indicate that these transitions in expansivity of the composites are mostly a result of matrix yield following fully elastic deformation past a certain thermal excursion, this transition being prevented by increased ceramic phase contiguity. This interpretation is confirmed by neutron diffraction results. Increased reinforcement contiguity in the composites also affects the amplitude of strain hysteresis during thermal cycling, the magnitude of hysteresis decreasing in nearly linear fashion with increasing contiguity, reaching zero near β = 15%. Samples that were quenched into liquid nitrogen after a relaxation heat treatment and reheated to room temperature yielded data globally similar to those of as-processed samples. Nitrogen-quenched samples with higher contiguity (β ≥ 0.05) exhibit furthermore signs of ratcheting upon cycling, which was not observed in the composites with low contiguity.

EPFL2008

High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

EPFL2008