Thermal Excursions Research Articles

Steady-state heat transfer and thermal zone spreading in gel isoelectric focusing.

The zone spreading caused by a transverse pH profile due to a temperature gradient through the thickness of a gel slab is estimated. The temperature difference (delta T) between the upper and lower gel surfaces can be calculated as a function of the electric power dissipated in the gel and the gel dimensions. It is found that when delta T is only 1 degree C the zone spreading due to this thermal excursion is as high as 0.5 mm. Thus, an admissible delta T is found to be equal to 0.2 degrees C, since this corresponds to a thermal zone spreading of only 0.1 mm, i.e. the same order of magnitude as the spatial resolution of a laser scanner. A thermal zone spreading of 0.1 mm is compatible with a resolving power of 0.01 pH unit, the current limit of conventional isoelectric focusing in amphoteric buffers. A requirement for the thickness of a gel slab is formulated: e.g., at 40 W applied power (over a gel surface area of 25 x 11 cm), a thermal zone spreading limited to 0.1 mm can only be obtained with a gel thickness of approximately 170 microns.

Use of spray techniques to synthesize particulate-reinforced metal-matrix composites

In an attempt to optimize the structure and properties of particulate-reinforced metal-matrix composites, a variety of novel synthesis techniques have evolved over the last few years. Among these, the technique of spray processing offers a unique opportunity to synergize the benefits associated with fine particulate technology, namely microstructural refinement and compositional modifications, coupled within situ processing, and in some cases, near-net shape manufacturing. Spray technology has resurrected much interest during the last decade and there now exists a variety of spray-based methods. These include spray atomization and deposition processing, low-pressure plasma deposition, modified gas welding techniques and high velocity oxyfuel thermal spraying. Spray processing involves the mixing of reinforcements with the matrix material under non-equilibrium conditions. As a result, these processes offer an opportunity of modifying and enhancing the properties of existing alloy Systems, and also developing novel alloy compositions. In principle, such an approach will inherently avoid the extreme thermal excursions, and the concomitant macrosegregation associated with conventional casting processes. Furthermore, the spray processing technique also eliminates the need to handle fine reactive particulates associated with powder metallurgical processes. In this paper, recent developments in the area of spray synthesis or processing of discontinuously reinforced metal-matrix composites are presented and discussed with particular emphasis on the synergism between processing, microstructure and mechanical properties.

Time-temperature responses of tomato cells during high- and low-temperature inactivation

Precise time and temperature dependences of the decrease of metabolism of cultured cells of tomato (Lysopersicon esculentum (L.) Mill. L. peruvianum (L.) Mill.) resulting from exposures to high and low temperatures were determined. Equations of the form Ln (activity)= C +1 [A+(T-Tm)(N)+B] describe thermal inactivation and allow prediction of activity loss following any thermal excursion beyond limits of temperature stability. The experimental parameters A, B, C and N derived from these equations allow precise comparison of temperature sensitivities of cells. Analysis of metabolic heat rates, O2-consumption rates and CO2-evolution rates demonstrated simultaneous shifts in metabolic pathways and metabolic activities towards more anaerobic metabolism below about 12° C and at high temperatures that stress growth of tomato cells.

Oxidation Instability of SiC and Si3 N 4 Following Thermal Excursions

The effect of thermal excursion and thermal cycling on the oxidation stability of chemical vapor‐deposited (CVD) and was studied at 1350°C. Thermal cycling alone produced no noticeable change in oxidation kinetics. However, transmission electron microscopy showed that oxide scales grown in cycles consist of alternating layers of and . When the oxidation of CVD or at 1350°C was interrupted with a 1.5‐h annealing in Ar at 1500°C, the kinetics of reoxidation at 1350°C were found to be drastically increased. The and then oxidized essentially at the same rate, which is over 50 times the preannealing rate, and comparable to the expected oxidation rate of these materials at 1500°C. This loss of passivity induced by excursion to higher temperature has also been reported for Si, and is considered an intrinsic instability in materials that oxidize to silica.

Spray-atomized and codeposited 6061 Al/SiCp composites

Spray atomization and codeposition processes have received considerable attention for the synthesis of discontinuously reinforced metal-matrix composites. This methodology involves the mixing of reinforcements and matrix under thermal conditions such that the matrix contains both solid and liquid phases. In principle, such an approach avoids the extreme thermal excursions, with concomitant degradation in interfacial properties and extensive macrosegregation, normally associated with casting processes. Furthermore, this approach also eliminates the need to handle fine reactive particulates normally associated with powder metallurgical processes. To investigate the utility of this process for the preparation of metal-matrix composites, several silicon carbide particulate-reinforced 6061 Al composites were prepared. The spray-atomized and codeposited materials exhibited attractive combinations of strength, elastic modulus and elongation, although further work is needed to optimize their properties.

Evaluation of runaway-electron effects on plasma-facing components for NET

Runaway electrons which are generated during disruptions can cause serious damage to plasma facing components in a next generation device like NET. A study was performed to quantify the response of NET plasma facing components to runaway-electron impact. For the determination of the energy deposition in the component materials Monte Carlo computations were performed. Since the subsurface metal structures can be strongly heated under runaway-electron impact from the computed results damage threshold values for the thermal excursions were derived. These damage thresholds are strongly dependent on the materials selection and the component design. For a carbonmolybdenum divertor with 10 and 20 mm carbon armour thickness and 1 degree electron incidence the damage thresholds are 100 MJ/m 2 and 220 MJ/m 2. The thresholds for a carbon-copper divertor under the same conditions are about 50% lower. On the first wall damage is anticipated for energy depositions above 180 MJ/m 2.

Failure Problems In Composites

AbstractFiber reinforced polymer matrix composites have become very useful in chemical processing systems, in transportation applications such as automobiles, aircraft and boats, in electrical hardware and in sports equipment. Failures in these rarely involve gross cohesive fracture; usually leaks develop, the stiffness decreases, local delamination occurs, the dielectric properties degrade or the strength declines. Almost all of these failures can be traced to local cracking, either at the fiber matrix interface or within the matrix itself. The cracks are generated by mechanical loads, by thermal excursions or by fluid absorption, because the relevant properties of the fiber and the matrix often differ by orders of magnitude. Current technology attempts to avoid the cracks by maximizing the fiber-matrix adhesion and great progress has been made in this area. An alternative interposes a thin compliant layer, rubber, between the two constituents thereby reducing the stress concentrations which exist because of their greatly different properties. Cracking is inhibited, composite strength is increased and its energy absorption also rises; if the rubber layer is thin (a few thousand Angstroms) no loss of stiffness or heat resistance is evident.

Delayed hydride cracking behavior for zircaloy-2 plate

The delayed hydride cracking (DHC) behavior for Zirealoy-2 plate was characterized at temperatures ranging from 300 to 550 °F. Specimens with a longitudinal ( T-L) orientation exhibited a classic two-stage DHC response. At Kvalues slightly above the threshold level ( K th ), crack-growth rates increased dramatically with increasing K values (stage I). The K th value was found to be 11 and 14 ksi√in at 400 and 500°F. At high K values (stage II), cracking rates were relatively insensitive to applied K levels. Stage II crack growth was a thermally activated process described by an Arrhenius-type relationship with an activation energy of 65 kJ/mol. This energy level agreed with the theoretical activation energy for hydrogen diffusion into the triaxial stress field ahead of a crack. Above a critical temperature (300°F), an overtemperature cycle was required to initiate DHC. The magnitude of the thermal excursion required to initiate cracking was found to increase at higher test temperatures. Specimens with a transverse ( L-T) orientation showed a very low sensitivity to DHC because of an unfavorable crystallographic orientation for hydride reorientation. Metallographic and fractographic examinations were performed to understand the DHC mechanism.

Performance of high-temperature survivable gallium arsenide concentrator cells

A report is presented on the fabrication and testing of survivable GaAs/AlGaAs double-heterostructure concentrator cells intended for space operation. Air mass zero (AMO) efficiencies of over 22% at 25 suns have been obtained. These cells are designed to survive thermal excursions to temperatures exceeding 600 degrees C without significant change in efficiency. It is found that the change in conversion efficiency is less than 10%, even after 1 h at 650 degrees C, and that the decrease in efficiency is only about 10% after 25 min at 700 degrees C. The cells continue to function after enduring 820 degrees C for 25 min. Details of the testing are reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal Behaviour of a Twin Axial Groove Bearing under Varying Loading Direction

An experimental investigation and numerical model development to predict the behaviour of a journal bearing subjected to different loading direction is described. The numerical model predicts significant thermal excursions under conditions of extreme load and speed, and these trends are confirmed by experimental observations. The experimental results show that the oil grooves are not effective in providing cool lubricant when they are positioned in the loaded part of the film. The numerical model suggests that at a prescribed eccentricity ratio, a very significant load increase can occur (threefold) for a small change in loading direction. Additionally, predicted side flow is very load direction dependent while bush torque reaction is not; these trends are confirmed by experimental observation.

Nucleate pool boiling curve hysteresis for GEWA-T surfaces in saturated R-113

Nucleate pool boiling tests were performed on a number of GEWA-T deformed low fin surface copper tubes in saturated R-113. The gap width, S T, varied between 0.15 and 0.55 mm. A smooth tube was also tested for reference. The boiling curves for each test section exhibited the hysteresis effect commonly observed with liquids of high wettability; however, the largest thermal excursions or temperature overshoots observed were only 1.5–3.3 K. This is much lower than reported for other enhanced surfaces tested under similar conditions. The small excursions are the result of enhanced free convection and the difficulty of spreading the boiling, both effects being attributed to the re-entrant helical channels.

Review of Mechanically Related Failures of Ceramic Capacitors and Capacitor Materials

This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low‐voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed.

Dynamic mechanical and thermal analysis of crystallinity development in Kel-F 800 and TATB/Kel-F 800 plastic bonded explosives: Part I. Kel-F 800

Dynamic mechanical and differential scanning calorimeter measurements were made on Kel-F 800, a copolymer of chlorotrifluoroethylene and vinylidene fluoride, as a function of time. Samples annealed over a 30 day period at 50 ° C showed reduced relaxation strength in the glass transition which directly correlated with the development of between 10 and 15% crystallinity in this polymer. Kel-F 800 samples thermally cycled from −54 to 74 ° C without annealing for 12, 24, and 36 cycles developed only about half the crystallinity of the annealed samples. Kel-F 800 which was annealed 30 days and subsequently subjected to the above thermal cycling retained almost all of the crystallinity which developed during the annealing process. These results indicate that Kel-F 800 copolymer crystallizes at an extremely slow rate, but as long as the thermal excursion remains below the major melting peak most of the crystallinity remains.

Search for Neutrons from a Titanium-Deuterium System

504This paper reports on titanium sponge charged with deuterium using high-pressured gas techniques. The titanium deuteride is then taken through thermal excursions between 77 K and room temperature while evidence is simultaneously sought for neutron count rates above background due to these excursions. No statistically significant evidence for neutron production in the deuterated titanium is found.

Solidification microstructure of supercooled Ti-Al alloys containing intermetallic phases

Titanium-aluminum alloys (45, 50 and 55 at.% Al) containing the intermetallic phases α 2- Ti 3 Al and γ-TiAl were melted, supercooled and solidified in an electromagnetic levitation device. The thermal history was recorded with the aid of a fast-response two-color pyrometer and the supercooling achieved was determined from the thermal excursion during recalescence. Maximum supercoolings were 262, 286 and 348 K for the 45, 50 and 55 at.% Al alloys, respectively. Microstructural analysis reveals that the 45 at.% Al alloy selects the primary β-(Ti) phase at all supercoolings, while the 50 at.% Al alloy changes from primary α-(Ti) to primary β-(Ti) at supercoolings of the order of 85 K. The relative amount of second phase segregate (γ) decreases with increasing supercooling in the 50 at.% Al alloy, and is absent in Ti-45 at.% Al. The 55 at.% Al alloy microstructure consists of primary a dendrites in a matrix of γ segregate, but in this case the amount of γ increases with increasing supercooling. The primary β α dendrites normally transform upon cooling to an α 2 + γ lath microconstituent, although the transformation is largely suppressed by increasing the cooling rate and/or decreasing the aluminum content.

Single residency sintering and consolidation of powder metal alloys, intermetallics, and composites by pulsed homopolar generator discharge

Utilizing inertial energy storage the homopolar generator (HPG) is capable of delivering multimegawatt, megampere current pulses into resistive or inductive loads with high efficiency. (1) Such HPG's have been used for many years as power supplies for research in pulsed processing of metal alloy components and systems. Most of these processes rely on extremely rapid thermal excursions in the workpiece(s) caused by resistive heating during the current pulse. A new application of pulsed HPG's that carries great promise is homopolar pulse consolidation (HPC) of powder metal alloys or components.(2) In HPC, powder metal constituents are loaded into a thermally and electrically insulated die, then precompacted to an initial pressure. the “rams” of the compaction press are also the electrodes for homopolar discharge current. While in single residency, the powder compact is heated uniformly by HPG discharge current, sintering occurs, and consolidation is accomplished by hydraulic control of the consolidation press. The process is completed in approximately one sec, with unaided cooling to room temperature on the order of 100 sec, depending on compact mass. Microstructural control is closely tied to beginning particle size and distribution. Traditionally unsinterable alloys have been consolidated using HPC. Novel intermetallic alloys and phases have been produced. Cermets and other composites can be produced, as long as the continuous matrix is conductive. New tooling designs allow for controlled atmospheres, near-net shapes, and automated manufacturing.

Thermal degradation of sintered diamond compacts

A method was devised to subject sintered diamond compact drill blanks to rapid thermal excursions, and the properties of the compact were studied after heating. It was found that thermal degradation as detected by rock-cutting tests and polished metallographic cross-sections could be correlated with acoustic emissions of the compact during heating. The drill blanks exhibited a delayed failure phenomenon which followed an Arrhenius relation with an activation energy of 327 kJ mol−1, and this behaviour was attributed to the initiation of graphitization of the sintered diamond compact.

Thermally-induced fracture in composites

This paper presents an experimental and analytical investigation of thermally-induced cracking in cross-ply, graphite/epoxy composites. It is shown experimentally that both rates and amplitudes of the thermal excursions affect the extent and the form of damage. The analytical study shows that the early stages of sufficiently slow thermal excursions result in crack patterns that are analogous to mechanical loading effects, and can be assessed by an approximate, two-dimensional micro-cracking model. However, three-dimensional aspects of the spatially non-uniform stress field may have to be included to model crack formation under subsequent temperature excursions or rapid thermal fluctuations. In the latter cases oblique and curved cracks develop and the laminate is susceptible to internal and free-edge delaminations.

Phase selection in electrohydrodynamic atomization of alumina

Electrohydrodynamic atomization'has been adapted to produce Al2O3 powders ranging in size from 10 nm to 300 μm. Microstructural characterization using x-ray diffraction, scanning, and transmission electron microscopy reveals changes in phase selection as a function of particle size, hence supercooling. Amorphous powders are common below 100 nm in diameter. Cubic spinel γ is found in single phase form between 100 nm and 2μm, and partially transformed to δ between 2 and 20 μm. There is also evidence of σ and θ or a precursor of θ forming directly from the liquid above 5 μm. The stable corundum structure is consistently found above 20 μm but exhibits three different morphologies: faceted, dendritic, and cellular. Phase selection is examined on the basis of fundamental thermodynamic and kinetic considerations and results from computer models predicting the thermal history of the powders. It is concluded that metastable phases require the elimination of catalytic sites for the nucleation of a and are thus more likely to form in the smaller powders. Furthermore, submicron powders achieve sufficiently high cooling rates to preserve the metastable phases formed (γ), but those higher than ∼ 1 μm experience a thermal excursion long enough to transform γ to more stable forms of Al2O3.

Runaway-electron simulation by use of an electron linear accelerator

Runaway-electron events in tokamak fusion devices are regarded as a serious threat to the integrity of first wall components. Thermal excursions in the course of such events can cause damage and failure of surface layers and subsurface structures as highly accelerated runaway-electrons may deeply penetrate into the material where they deposit significant amounts of their energy. Especially endangered are the walls of coolant channels of first wall components which may be melted with subsequent coolant leakage into the vacuum vessel. By use of an electron linear accelerator which provided an electron beam of 20 and 30 MeV at fluxes of up to 3.4 × 10 14/s the thermal and structural response of materials to high energy electron impact was tested and a ranking in the sensitivity of materials to runaway-electron damage established. Among the tested bulk materials graphite showed the highest structural resistance whereas metals showed grain growth, cracking, and melting. For an evaluation of actively cooled structures under the runaway-electron aspect graphite-copper compound systems were used. The results indicate that thick graphite volumes are needed for an effective shielding of the metal base against runaway-electron damage.