Slow Thermal Cycling Research Articles

Molecular and micro-scale heterogeneities in Raman modes of a relaxing polymer glass

We have used Raman spectroscopy to study relaxation dynamics at two different length scales, molecular level and micro-scale in order to probe the presence of cooperative rearranging regions in a polymer glass. Response to slow thermal cycles and fast quench through the glass transition temperature (T g) is analyzed for film and unprocessed forms of polyvinyl acetate (PVAc). In PVAc film, enhanced disorder and molecular mobility lead to peak broadening by about a factor of 10 compared to unprocessed PVAc. Thermal cycles (10 K min−1) produce hysteresis in integrated Raman peak intensity (loop area ). values of film are two orders of magnitude more than unprocessed, indicating more configurational mosaics with higher interfacial energy dissipations. Ageing after 60 K min−1 quench manifests as heterogeneous molecular dynamics of film Raman modes with significant peak-width variations, differentiating high mobility and low mobility modes. Two-dimensional mapping of film Raman modes after quench reveal micro-scale clusters of average size ≈250 molecules having fractal boundaries with fractal dimension d f = 1.5, resembling d f of percolation clusters below percolation threshold. During thermal cycling and relaxation after a quench, cooperative segmental dynamics with large correlations between skeletal C–C stretch and side branch modes is observed. The observations are analyzed in the context of the random first order transition theory of glasses, which attributes heterogeneous relaxations in glasses to the presence of clusters of variable configurational states.

Microstructures from wire-fed laser welding of high strength steel grades

In welding, wire-feeding enables alteration of the resulting microstructure and, in turn, the mechanical behavior of the welded joint. For pipeline steel grades, very few commercial wires are matching at high strength and simultaneously ensure sufficient toughness. New wire chemistries need to be investigated. Promising consumable chemistries can be studied through metal cored wires. One promising concept is alloys that promote acicular ferrite instead of bainite. Interlocking instead of parallel laths can lead to higher toughness. In the gouge range of 15–19 mm, laser-arc hybrid welding has been studied for pipeline steel grades X80 and X100. For efficient mapping of various weld metal conditions, a simplifying “snapshot” method was developed. A pulse shaped laser beam melts wire pieces in a controlled manner, reproducing thermal cycles in welding. The weld metal tends to form bainite, but under certain conditions, complex microstructures with interlocking laths can be generated. Slow thermal cycles can ...

Investigation of startup, performance and cycling of a residential furnace integrated with micro-tubular flame-assisted fuel cells for micro-combined heat and power

Solid Oxide Fuel Cells (SOFCs) offer advantages for micro-Combined Heat and Power (μCHP), but currently suffer from slow startup (>1 h) and limited thermal cycling which reduces the applications, energy savings and economics. In this work, a micro-Tubular SOFC stack is integrated into a residential furnace to create a micro-Tubular Flame-assisted Fuel Cell (mT-FFC) μCHP system. A high power density of 202 mW cm−2 is reported operating in synthesis gas generated from fuel-rich combustion of natural gas/air. Unlike previous reports, instabilities in the polarization are attributed to low temperature of the oxygen reduction reaction at the cathode. The mT-FFC stack achieved peak power density in 6 min after ignition. 200 thermal cycles at an average heating rate of 215 °C.min−1 and average cooling rate of 176 °C.min−1 were conducted and a low degradation rate of 0.0325 V per 100 cycles per fuel cell was achieved. Low NOx emissions (10 ppm) and high combined efficiency is reported.

Infrared absorption characteristics of solid nitrogen at near-triple point temperatures

The smooth, uniform, and transparent solid nitrogen-molecular film was grown by applying the slow thermal cycles near the triple-point temperature to the growth process in our home-made liquid/solid preparation apparatus. The infrared absorption spectra of solid nitrogen at near-triple point temperature are measured by the infrared spectroscopy system. A broad absorption band can be observed from 2222 to 2439 cm-1 with the strongest peak at 2288 cm-1. This has been well explained theoretically on the basis of the ground-state vibration and the coupling between the ground-state vibration and rotation at low temperatures within the framework of anharmonic approximation.

A study about the contribution of the α-β phase transition of quartz to thermal cycle damage of a refractory used in fluidized catalytic cracking units

The deterioration of refractories used in fluidized catalytic cracking units (FCC-units) is responsible for high costs of maintenance for the petrochemical industry. This is commonly associated with coke deposition during the production of light hydrocarbons. However, other mechanisms responsible for causing damage may also occur, such as the generation of cracks by expansive phase transition. The aim of the work herein was to study the contribution of the a-b phase transition of quartz particles to the deterioration of a commercial aluminosilicate refractory used in a riser by the means of slow thermal cycles. Such damage may occur if the working temperature of the equipment fluctuates around the a-b transition temperature (573 °C). The current study considered the material with and without coke impregnation to evaluate the combined effect of coke presence and phase transition. To evaluate the damage, it was used the Young's modulus as a function of temperature by applying the Impulse Excitation Technique under controlled atmosphere. An equipment recently developed by the authors research group was applied. Specimens were prepared and submitted to slow thermal cycles of temperatures up to 500 °C and up to 700 °C, with a heating rate of 2 °C/min. Part of the specimens was previously impregnated with coke by a reactor using propen. To complete the evaluation, characterization by X-ray diffraction, as well as by dilatometry and scanning electron microscopy were performed. The findings of this study showed that the presence of quartz particles determine the thermo-mechanical behaviour of the material, as well as the thermocycling damage resistance. In spite of the fact that the a-b phase transition stiffens the material during the heating stage, it increases the damage by slow thermal cycling. The coke impregnation increases the resistance to slow thermal cycles, however it decreases the resistance to the damage evolution.

X-ray Reflectivity Evaluation of the Thermal Cycling Effects in Methylcellulose Thin Films

Changes in the layer thickness and roughness of as-prepared methylcellulose (MC) films with thermal cycling were studied using the X-ray reflectivity technique. It was found that thermal stress induced by rapid and slow thermal cycling brings no apparent changes in the surface and interface roughness after thermal cycling. On the other hand, some reduction of layer thickness was observed. With rapid thermal cycling, the relative shrinkage of the films is much higher at a maximum of 5% change at 15 cycles. With slow thermal cycling, it remains at a maximum of 3.6% even after 20 thermal cycles. Such film shrinkage is highly dependent on the initial film thickness and the amount of absorbed water molecules in the film.

Parameter extraction during thermal cycling and optical backreflection measurements of bidirectional optoelectronic modules

In this paper, we discuss the conditions under which we can correctly extract a variety of module parameters of single mode bidirectional optoelectronic duplexer and diplexer modules during relatively slow thermal cycling between -40/spl deg/C and +85/spl deg/C. The obtained values of parameters such as tracking error, optical crosstalk, and the laser diode characteristic temperature have been compared for bidirectional modules from different manufacturers and the impact on these parameters of different interconnect techniques, like microoptic (Mo), silicon optical bench, and planar lightwave circuit technologies is discussed. It is shown that there is a conflict between small threshold currents and good temperature stability for InGaAsP-based Fabry-Perot laser diodes emitting at 1300 and 1550 nm. Then the results of spatially low coherence reflectometry for the different bidirectional module types are presented.

Evidence for a structural change in TSMG Y123 at 225 K

This paper reports on an ultrasonic study at 85 kHz and 485 kHz of melt-textured Y123 (YBa 2Cu 3O 7− δ ) where a possible structural change has been identified in the 220 K region. Work on oxygenated and partially deoxygenated samples has shown that the change is associated with oxygen in the samples and that the activation energy for the process is 110 meV. The conclusions are based on a qualitative and quantitative description of ultrasonic attenuation and ultrasonic velocity changes in the material as a result of slow thermal cycling and quenching experiments.

Strains generated at austenitic:ferritic dissimilar welds during elastic pressure changes at high temperature

Dissimilar joints between ferritic and ausenitic materials are complex in terms of the heterogeneity of the weldment structures and their physical, mechanical and time dependent properties. Recent work studied the effect of slow thermal cycles, incorporating pressure increases and hold periods at the peak temperature, on the strain accumulated in dissimilar welds between 2.25CrIMo low alloy steel and Type 316 stainless steel fabricated with Inconel and Type 316 filler metals. The current paper examines the effect of thermal cycles and the time at peak temperature and stress on the elastic strains developed during pressure loading of the joints. The results suggest that creep damage caused during the hold period at high temperature can influence the loading-unloading strains, which generally increases with increasing damage. It is suggested that such methods could have potential for monitoring of creep damage in dissimilar welds in service and laboratory test situations.

Effect of thermal cycling on creep behaviour of 2·25Cr–1 Mo/type 316 steel dissilllilar Illetal vvelds

AbstractThe high temperature performance of dissimilar metal welds betweenferritic and austenitic materials is directly influenced by a number of material and operational factors. The welds are heterogeneous and metallurgically complex, and their performance is not well understood. The present work specifically examines the effect of slow thermal cycles on the high temperature creep behaviour of two 2·25Cr–1Mo/type 316 alloy welds: one using a type 316 filler and one using a high nickel filler metal. A third, all stainless steel weldment was also tested for comparison purposes. A vessel containing the three test welds was manufactured and strain gauges were positioned axially across the interface of each weld. This vessel was then pressurised and subjected to three different types of test cycle, each including a holding period. Twenty three separate cycles were used and the vessel was destructively examined after completion of the test period. Results indicate that the similar metal weldment exhibited beh...

Applying a physics‐of‐failure model to predicting surface mount solder joint reliability

AbstractTo address the wide variety of solder joint configurations, the authors have developed and applied a physics‐of‐failure model to predicting the service life of solder joints under thermal cycling conditions. The wide variety of solder joint geometries, materials and environments makes it impractical to develop and apply empirical models to predicting the service life of solder joint interconnects. On the other hand, a physics‐of‐failure model that describes the failure mechanisms in solder joints can be applied to a wide range of conditions. The physics‐of‐failure model framework is described and a model is demonstrated for predicting the failure of a leadless surface mount solder joint under slow thermal cycling conditions.

Leakage of Transport Container Seals During Slow Thermal Cycling to −40°C

AbstractMeasurements of gas leakage rates have been made on Viton and silicone O-ring seals during slow thermal cycling between +10°C and −40°C. Additional measurements of the sealing force exerted by the seal on the flange have also been made during the tests. Measured leak rates through Viton seals increase to 1 kPa.cm3.s−1 10−2 bar.cm3.s−1 at −40°C but it is shown that this would not result in loss of particulates through the seal. On increasing the temperature to +10°C, the leak rate decreases to its original value of 5 Pa.cm3.s−1 (5×10−5 bar.cm3.s−1.) Leak rates through silicone seals do not change significantly in the temperature range tested.

Mechanical Modeling of Multilayered Films on an Elastic Substrate—Part I: Analysis

In this paper the basic residual stress problem for multilayered or multiple films on an elastic substrate is considered. The stresses may be caused by a homogeneous temperature variation or slow thermal cycling and by far field mechanical loading. The films are approximated by orthotropic membranes and the substrate is assumed to be an elastic continuum. The interfacial zone is modeled by either an ideal interface or a homogeneous shear layer. The primary interest in the paper is in examining stress concentrations or singularities near the film ends. For the two interface conditions considered, this is done by varying the film/substrate contact angle. Also studied are strain energy release rate for the propagation of an interfacial crack and the direction and the magnitude of the maximum cleavage stress for a possible crack initiation in the substrate. The basic modeling and analysis are considered in Part I. Part II of the paper is devoted to the presentation and discussion of the results.

Unstable high temperature superconductivity and martensitic effects in YBaCuO

We report on a case of a temporary increase of the transition temperature of YBaCuO from 90 K to 220 K, which was achieved by slow thermal cycling below 240 K. Because of the correlation of this effect with lattice instabilities at 240 K and below, we argue that T c depends strongly on the mesostructure of the lattice.

On the chemistry and stability of complex carbides and nitrides in microalloyed steels

Light optical and electron microscopy, and quantitative (STEM-EDX) micro-analysis are employed to characterize a number of different Ti-micro-alloyed steels, containing various alloying additions including high and low levels of nitrogen, and a Nb-steel is used as a comparison. The steels are studied in both the as-received and weld simulated conditions. Particular emphasis is placed on measuring the particle size distributions and chemical compositions of a large number of the complex carbides and nitrides formed in these steels. It is found that while individual particles inevitably reflect the compositions of the steels in which they have formed, the relative amounts of the various metallic elements present in particles are a function of the prior thermal treatment of the alloy. Thus, weld thermal cycling treatments substantially change particle compositions compared to the as-received (rolled or normalized) conditions, and even the energy input of the welding process is of importance in this respect. As a general rule, TiN-containing steels provide for good grain growth control during weld cycling, although the presence of additional alloying elements in the particles may affect particle stability in a complicated way. It is shown that equilibrium thermodynamics can be used to make reasonable predictions of particle compositions provided the prior heat treatment is such to allow the particles to remain near their equilibrium condition, such as in slow thermal cycling. In certain cases, as in low energy weld thermal cycling, this not likely and it is then necessary to take the diffusional kinetics of individual elements in particles into account, if compositional changes are to be understood.

Thermal cracks in unconfined sioux quartzite : Friedman, M; Johnson, B Proc 19th US Symposium on Rock Mechanics, Stateline, Nevada, 1–3 May 1978, V1, P423–430. Publ Reno: University of Nevada, 1978

The morphology, orientation, and abundance of thermal cracks are characterized in unconfined specimens of Sioux quartzite subjected to (1) slow thermal cycling (< 2 C/min) to 385, 560 and 685 C; and (2) slow heating at the same rate followed by quenching from 400 C. In both, all the thermal cracks are microscopic in size and occur primarily along grain boundaries and secondarily as intragranular microfractures. The latter intersect parted grain boundaries. In the slowly cycled specimens the intragranular cracks are strongly oriented at 60 to 90 to the axis of the greatest principal elongation (100 x 10/sup -6/, elongations counted positive) of the average state of residual elastic strain in the rock prior to thermal treatment, as measured by x-ray diffractometry. This geometry suggests a genetic relation between the cracks and the net stresses resulting from superposition of the residual stresses and the thermoelastic stresses developed from differential expansions of anisotropic nearest-neighboring grains. 17 references.

Measurement of low resistances by means of the wheatstone bridge

Dissimilar joints between ferritic and ausenitic materials are complex in terms of the heterogeneity of the weldment structures and their physical, mechanical and time dependent properties. Recent work studied the effect of slow thermal cycles, incorporating pressure increases and hold periods at the peak temperature, on the strain accumulated in dissimilar welds between 2.25CrIMo low alloy steel and Type 316 stainless steel fabricated with Inconel and Type 316 filler metals. The current paper examines the effect of thermal cycles and the time at peak temperature and stress on the elastic strains developed during pressure loading of the joints. The results suggest that creep damage caused during the hold period at high temperature can influence the loading-unloading strains, which generally increases with increasing damage. It is suggested that such methods could have potential for monitoring of creep damage in dissimilar welds in service and laboratory test situations.