High Temperature Excursions Research Articles

Nickel-catalyzed autothermal reforming of jet fuel surrogates: n-Dodecane, tetralin, and their mixture

The potential of a nickel-ceria-zirconia based catalyst was examined for autothermal reforming of n-dodecane, tetralin and their mixture into a hydrogen-rich product stream suitable for high-temperature fuel cells. n-Dodecane and tetralin were chosen as representative compounds for alkanes and bicyclic compounds in jet fuel. It was possible to reach conversions greater than 90% for both components, as long as sufficiently high oxygen concentrations were maintained in the feed. Tetralin gave larger yields of reforming products than n-dodecane. During the transient start-up phase of the reactor, large temperature excursions were observed, suggesting that the reaction starts with complete combustion of fuel, giving way to autothermal reforming after a few minutes on stream. These high-temperature excursions during reactor start-up are large enough to change the catalyst surface areas. Interestingly, the mixture of tetralin and n-dodecane did not behave as a linear combination of the two pure components, but showed reforming characteristics similar to pure tetralin. The non-linear behavior of the mixture provides a caveat that investigations of single component model compounds may not be able to capture the reforming behavior of more complex fuel mixtures.

Assessment of structural steel from the World Trade Center towers, part IV: Experimental techniques to assess possible exposure to high-temperature excursions

Recovered structural steel from the World Trade Center was examined as part of the National Institute of Standards and Technology investigation to provide data on potential temperature excursions seen by the steel for input and validation of the fire and thermal finite element models. While numerous experimental techniques were appraised for use during this study, two proved to be practical: assessment of the primer paint on the structural elements and examination of the steel microstructure. Results from these two techniques are presented. Evaluation of primer paint from 21 exterior panel sections, which represent approximately 3% of the panels from fire-involved floors, was conducted and indicated that only three locations may have experienced temperatures over 250°C. Steel microstructures taken from these and other areas on exterior panels exposed to pre-collapse fires showed no evidence of exposure to temperatures exceeding 625°C for times longer than the detectable lower limit of 15 min. The lack of high-temperature excursions observed during this analysis may be related to the protection afforded by intact spray-applied fire-resistant material on the components at the time of exposure.

Dynamics of gold clusters on amorphous carbon films induced by annealing in a transmission electron microscope

The change of the size distribution of Au clusters induced by annealing was studied in situ by transmission electron microscopy. Starting from statistically distributed Au clusters on a thin amorphous carbon film, “islands” are formed within a few months storage at room temperature, which consist of Au clusters with sizes <4 nm embedded in a thin Au film. These islands cover originally areas with sizes around 25 × 70 nm 2. If the temperature is increased in the transmission electron microscope two different processes can be clearly distinguished that lead to the coarsening of the cluster size distribution: cluster coalescence and (contactless) Ostwald ripening. The degree and rate of the coarsening are found to depend on the underlying surface (Au film or amorphous carbon) and the exposure to the high-flux high-energy electron beam, which can be estimated to lead to high-temperature excursions in a cluster on a 10 −12 s time scale. The experimental findings are confirmed by Monte-Carlo simulations using the many-body Gupta potentials in order to calculate the Au/Au interaction. Moreover, the results of MC simulations suggest an electron-beam induced formation of a “quasi-two-dimensional gas” of small highly mobile Au species on the Au film, which promotes Ostwald ripening.

An Electrochemically Driven and Electrochemically Regenerated NOx Trap

Absorption traps are used in a wide variety of different applications to prevent potentially harmful chemicals from entering the environment and/or from coming into contact with sensitive downstream equipment. Many of the materials in use cannot be regenerated so that concerns relating to sustainable usage and environmentally acceptable disposal are important; in other cases, regeneration can only be accomplished with relative difficulty. Herein we describe a new principle that can be used to address both these issues. A key application where chemical traps have found widespread use is in the control of NOx emissions generated by fuel-efficient automotive engines which, by reducing the atmospheric CO2 burden, can significantly mitigate the impact of road transport on global warming. Unfortunately, such engines produce much higher concentrations of nitrogen oxides (NOx) than conventional gasoline engines [1] with attendant adverse impact on human health and the environment. As a consequence, conventional “three-way” catalytic converters are inadequate and a post-converter NOx trap is required. These traps consist of an alkaline-earth component (usually barium) that is able to store NOx species as various nitrates and nitrites under the oxygen-rich conditions typical of “lean” engine operation. Precious metals, usually platinum, are also used as they aid in the uptake of NOx by providing sites for the adsorption and catalytic oxidation of NO to NO2. To restore the trap the engine is momentarily switched to “rich operation”, generating a large concentration of reducing species that cause reduction of the adsorbed nitrate/nitrite species to nitrogen thus regenerating the active BaO/BaCO3 component. A disadvantage of this procedure is that relatively high temperatures ( 900 K) are required for complete regeneration. Although the current generation of NOx traps is very effective when run under these conditions, a number of significant problems remain, especially the need for regular high-temperature excursions, the requirement of periodic rich engine operation (which partially defeats the object of lean operation), and susceptibility to poisoning by sulfur compounds present in the fuel. Herein we report a novel electrochemically driven NOx trap that can operate effectively over a range of temperatures and can be regenerated as required in a controlled manner, without the requirement for temperature excursions or changes in gas-phase composition. Although NOx trapping is used herein to illustrate the technique, the general method employed could be applicable in a wide variety of different applications, including, for example, removal oxides of carbon and sulfur. The trap consists of a thin porous layer of a precious metal (Pt, Pd, or Rh) deposited onto the surface of a solid electrolyte wafer that is biased to deliver the active species to the metal surface where it encounters the adsorbed NOx. We used sodium and potassium ionic conductors although a variety of other solid electrolytes could be used, depending on the application. NOx-trapping and trap-regeneration measurements were performed in a well-mixed microreactor operated at atmospheric pressure and described in detail elsewhere. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG ADES 400 spectrometer system and in situ infrared spectroscopy measurements were performed with a Perkin Elmer GX2000 spectrometer utilizing a Harrick Refractor Reactor specular reflection accessory. The performance of the trap is illustrated in Figure 1 which shows the effects of 1) electrochemically trapping NOx

The Effect of Rapid High Temperature Excursions on the Moisture Absorption and Dynamic Mechanical Properties of Carbon Fibre Epoxy Composite Materials

AbstractThe effect of elevated temperature excursions (thermal spiking) on the moisture absorption characteristics and dynamic mechanical properties of Cycom 8 HS carbon fibre epoxy laminates was investigated. Cured laminate samples were preconditioned (65d̀C, 95%R.H.) and these samples were exposed to various thermal spiking (150d̀C/2min) programmes. Dynamic mechanical thermal analysis (DMTA) techniques measured the changes in glass transition temperature (Tg) storage modulus (log E') and damping (Tan δ max) of the laminates as a result of exposure to these environments. The thermal spiking programme was shown to cause an increase in both the amount and rate of moisture absorption of the laminates. These increments were accompanied by a significant decrease in Tg, log E', and Tan δ max. Scanning Electron Microscopy (SEM) analysis also showed the progressive growth of both interlaminar and translaminar micro‐cracks as a result of thermal spiking.

Enhanced adhesive shear strength of cyanate Ester electrical insulation for fusion magnets

In many Next-Step Option (NSO) fusion magnet designs the shear strength of the electrical insulation and its adhesion to the conductor is a performance limiting factor, especially during high temperature excursions. Significant design compromises, such as the need for large compressive loads or increased part cross-sections, must be made in order to accommodate the low shear strength of the insulation. Improving the shear strength of the insulation/conductor system will reduce the negative impact on system design and allow for higher temperature operation. Typical adhesive shear strength of conventional epoxy-based insulation systems on clean copper ranges from 76 MPa in liquid nitrogen to only 17 MPa at 100 /spl deg/C. These values are insufficient for the planned applications. On the other hand, cyanate ester based insulation has been shown to exhibit enhanced properties that will allow for magnet operation at 100 /spl deg/C and possibly even higher. A new cyanate ester resin has been developed by Composite Technology Development exhibiting improved adhesive strength to copper, especially at elevated temperatures. It is expected that the high temperature value can be further increased with optimization. In addition, the enhanced radiation resistance of cyanate ester resins implies that the new insulation systems will out-perform traditional epoxy-based insulations and lead to reduced costs for future fusion energy devices.

High-Yield Butane to Maleic Anhydride Direct Oxidation on Vanadyl Pyrophosphate Supported on Heat-Conductive Materials: β-SiC, Si3N4, and BN

Both the preparation and the characterization of vanadyl pyrophosphate (VPO) supported on new heat-conductive ceramic supports (β-SiC, Si3N4, and BN) are described. These catalysts provide a very significant gain in maleic anhydride yield by reacting air and butane in a fixed-bed reactor, because of control of the catalyst surface temperature. The stable high maleic anhydride selectivity at high temperature, ensured by the presence of the support, allows the use of significantly high partial pressures of butane. In addition, the β-SiC support protects the catalyst against high-temperature accidental excursions.

Internal strain evolution during heating of Ti-6Al-4V/SCS-6 composite

The characteristics of the residual stresses and their effects on the properties in continuous SiC fiber reinforced Ti-6Al-4V matrix composites (TMCs) have been extensively studied. However, to date, few experimental studies (e.g. Ti-14Al-21Nb/SCS-6) have characterized the thermal residual strain in TMCs at elevated temperatures. Therefore, the authors investigated the evolution of the thermal residual strain during heating of Ti-6Al-4V/35vol% SiC composite. In this study the authors used in situ high temperature neutron diffraction to measure strains: (1) in the matrix ({alpha} and {beta} phases) and in the fiber, (2) for several lattice reflections in each phase and (3) from both axial and the transverse directions. One distinguishing feature is the wide temperature range (from room temperature up to 1,170K) over which the study was performed. Although the proposed application temperature is typically less than 800K, TMCs are subject to higher temperatures during fabrication and may experience high temperature excursions while in service. Therefore, the authors extended the study to the high temperature regime where the matrix starts to undergo a phase transformation between {alpha}{minus} and {beta}{minus}Ti. Measurements from this regime (800{approximately}1,170K) provide insights on; (1) the inelastic relaxation of the residual strains through matrix yielding and creep, (2) the effectmore » of the phase transformation on the residual strains and (3) the effect of the presence of SiC on the matrix phase evolution.« less

Solvent Vapor Recovery by Pressure Swing Adsorption. I. Experimental Transient and Periodic Dynamics of the Butane-Activated Carbon System

ABSTRACT An experimental investigation was carried out for the separation and recovery of butane vapor (10 to 40 vol%) from nitrogen using Westvaco BAX activated carbon in a twin-bed pressure swing adsorption (PSA) system utilizing a 4-step Skarstrom- type cycle. Twenty-four runs, covering a broad range of process and initial column conditions, were performed to investigate the transient and periodic process dynamics. In all cases the approach to the periodic state was very slow, taking up to 160 cycles depending on the initial condition of the beds; and peak bed temperatures of up to 105°C were observed depending on both the initial condition of the beds and the process conditions. Also, the periodic state of each run was unique when approaching a new periodic state from less contaminated beds. The uniqueness of the periodic states, together with the exceedingly high peak temperatures, inferred much about the practice of preconditioning beds to avoid high temperature excursions. The periodic enriched butane vapor concentration histories also gave considerable insight into new cycle designs for improved solvent vapor enrichment.

Electron beam induced small particle transformations: temperature

Real time high resolution microscopy of Au, Ag and Pb particles on C and SiO substrates was performed to determine the role of the electron beam on the morphology transformations. The data conclusively rule out high temperature excursions in the small particles due to the beam.

FCOB reliability evaluation simulating multiple rework/reflow processes

Flip-chip assembly [direct chip attach (DCA) or flip-chip on board (FCOB)] on printed wiring boards (PWBs), in conjunction with conventional leaded device surface mount technology, is beginning to proliferate in compact and portable systems. DCA with conventional controlled collapse chip connection (C4) bumps requires solder coated bond pads to allow joining in typical surface mount technology (SMT) reflow cycles. A flip-chip device on a typical FCOB/SMT board will usually experience no high temperature excursions after the die joining and underfill encapsulant cure unless the board undergoes a rework cycle. FCOB single chip packages and multichip modules are now in development with standard C4 bumps, and a new Motorola E-3 bump which requires no solder on bond pads. These solder interconnects must be stable through multiple heat treatments expected in subsequent system level assembly and repair operations. Flip-chip plastic ball grid arrays (FC-PBGAs) will typically undergo three solder reflow, or reheat, cycles to /spl sim/220/spl deg/C subsequent to initial flip-chip reflow assembly. The multiple reheats are for BGA ball attach, board level BGA SMT assembly, second side BGA SMT assembly, and possible rework operations. In this paper, the effect of multiple reheats on the solder connection microstructure and strength (before, and after, underfill encapsulation), and the integrity of the underfill encapsulant adhesive and cohesive strength is reported, using both FCOB single chip packages and multiple chip modules. The effect of multiple reheats on electrical resistance of daisy chain nets, and die stress (radius of curvature), is also reported. Hot air gun rework (before underfill) is simulated and standard belt furnace reflows are utilized. Cross sections of bump connections and underfill interfaces were studied to assess changes induced by the temperature exposures. The reliability of the FCOB assemblies was assessed via temperature cycle, thermal shock, and autoclave tests.

Evaluation of PS 212 coatings under boundary lubrication conditions with an ester-based oil to 300 °C

High friction and wear of turbine engine components occur during high temperature excursions above the oxidation threshold of the liquid lubricant. This paper reports on research to study the use of a high temperature self lubricating coating, PS 212 for back-up lubrication in the event of failure of the liquid lubricant. Pin on disk tests were performed under dry and boundary-lubricated conditions at disk temperatures up to 300 °C. The liquid lubricant was a formulated polyol ester qualified under MIL L-23699. At test temperatures above the oil's thermal degradation level, the use of PS212 reduced wear, providing a back-up lubricant effect.

Evaluation of PS 212 coatings under boundary lubrication conditions with an ester-based oil to 300 °C

High friction and wear of turbine engine components occur during high temperature excursions above the oxidation threshold of the liquid lubricant. This paper reports on research to study the use of a high temperature self lubricating coating, PS 212 for back-up lubrication in the event of failure of the liquid lubricant. Pin on disk tests were performed under dry and boundary-lubricated conditions at disk temperatures up to 300 °C. The liquid lubricant was a formulated polyol ester qualified under MIL L-23699. At test temperatures above the oil's thermal degradation level, the use of PS212 reduced wear, providing a back-up lubricant effect.

Comparison of Activated Carbon and Zeolite 13X for CO2 Recovery from Flue Gas by Pressure Swing Adsorption

For the recovery of high purity CO[sub 2], two adsorbents, zeolite 13X and activated carbon, are examined. Pressure swing adsorption (PSA) cycle simulations are performed to compare the performance of different adsorbents in separation from two feed mixtures: 16/84 (v/v) CO[sub 2]/N[sub 2] and 26/74 CO[sub 2]/N[sub 2]. Despite high-temperature excursions in the adsorption step, zeolite 13X is a better adsorbent than activated carbon in nonisothermal, adiabatic PSA process due to favorable isotherm shape and equilibrium selectivity. Moreover, a higher temperature excursion for 13X during the product (CO[sub 2]) purge step reduces the purge gas amount and this actually helps PSA performance. High purity of CO[sub 2] (over 99%) can be produced by zeolite 13X at higher recoveries (53% from low-CO[sub 2]-concentration flue gas and 70% from high-CO[sub 2]-concentration flue gas) and higher productivities than carbon. In selecting an appropriate adsorbent, high working adsorption capacity, high equilibrium selectivity, and low purge gas requirement dominate the separation by PSA process whereas the heat effects play a secondary role. A simple criterion is proposed for a priori adsorbent selection.

Metallurgy of open-bath plasma processes

Abstract

Effect of high temperature spikes on a carbon fibre-reinforced epoxy laminate

Carbon fibre-reinforced epoxy composite laminates were exposed to programmes of temperature and humidity intended to represent the high temperature excursions (thermal spikes) caused by ground-reflected engine efflux experienced by vtol aircraft. Measurement of the laminate weight change during the tests indicated a change in the laminate moisture kinetics resulting in increased moisture equilibrium levels at spike temperatures up to 175°C. Spiking at temperatures of 200–300°C showed a marked loss of laminate weight which cannot be attributed to loss of water alone. Permanent damage such as cracking was evident in some cases. The contribution of moisture to laminate degradation is discussed.

L'influence des thermal spikes sur le comportement d'absorption d'humidite de CFC

AbstractFor the judgement of mechanical properties of carbon fibre reinforced plastics (CFRP) the knowledge of the moisture absorption behaviour is of essential importance. The influence of constant environmental conditions of CFRP has already been the subject of extensive investigations, but the effects of rapid high temperature excursions (“thermal spikes”) are not yet very well understood. These thermal spikes are encountered on the surface of aircraft components during supersonic flights.This paper describes a laboratory method for the simulation of thermal spikes. Investigations have been carried out on four different epoxy systems. The influence of thermal spikes with different heating rates on the moisture absorption behaviour of CFRP is reported.

Hydrogen and sulfur from hydrogen sulfide—IV. Quenching the effluent from a solar furnace

Solar thermochemical production of H 2 and sulfur from H 2S were studied, using a 4.2 m solar furnace as a source of process heat. We used two reactor configurations. The independent variables were temperature, feed rate and pressure. Hydrogen production rate, yield, and the quench fraction (fractional yield) were measured. High yields (of the order 0.5) and quench fractions (of the order 0.7) were obtained over a range of temperatures and feed rates. Yields were a monotonically increasing, almost linear, function of the temperature. The observations are rationalized by a kinetic model that concludes that gas phase reaction rates are too low to account for the observed yields; a good reactor should embody the heating of the feed stream in contact with a surface to facilitate decomposition of H 2S, and the product stream should be quenched in such a way as to minimize surface reactions. The reactors are described. Mullite, Al 2O 3, Fiberfrax and ZrO 2 are suitable materials of construction from a chemical standpoint. Mullite underwent some sintering, deformation and thinning in the hottest parts of the reactor, where it may have been exposed to high temperature excursions and some cracking, apparently due to thermal stress, where it came in contact with a separator.

The Dynamic Compaction of Metallic Powders

ABSTRACTThe Dynamic Compaction of powdered materials becomes a particularly attractive technique for consolidating hard materials, because of the ease with which high pressures are achieved, and for metastable materials, because of the possibility of avoiding large-scale, high temperature excursions. The shock-wave conditions necessary for achieving high densities and interparticle bonding are evaluated experimentally. The bonding and melting taking place can be identified by post-consolidation studies, and the inter-relation between compact integrity and melting is shown. An attempt is made to develop a “process window” analogous to that of explosive bonding, to describe the conditions necessary for consolidation. Finally, in view of the necessity for localisation of plastic flow during consolidation, an indication of the influence of material deformation parameters is given.

Overview of the solid-solid interface: Mechanical stability

Photothermal and photovoltaic device structures require large-area thin film multilayers that must be uniformly processed and will withstand a hostile service environment for long periods without degradation. Our present experience with demanding thin film applications resides in microcircuits and optical applications. Thus little background for predictive capability exists for high cyclic temperature excursions, oxidation-corrosion ambients and abrasive winds to be expected for reflectors and collectors. Long-term mechanical stability will be governed by control of the stresses produced during growth followed by stabilization of the microstructure. Low temperature transport has long been known in films but data for many incorporated impurity species including H 2 and OH are not common. The stress relief that accompanies such recovery processes has been studied in detail in only one soft low melting film. Reliable creep data for films do not exist. At high temperatures, even diffusion into the bulk may be an important sink for material originally at interfaces. Surface morphology changes after annealing have been identified. At fast deposition rates, grain boundaries may become even wider and posses large quantities of impurities. Many dielectric films contain microscopic voids which give rise to reversible and irreversible behavior. Disorder is quenched in at low effective atom mobilities with the formation of amorphous or metastable phases. Even amorphous films have their own scale of structure, lower elastic constants and doubtful high temperature stabilization. There is a parallel with laser glazing and near-surface modification by ion beams that should be exploited. The mechanical strains that are produced in a film during growth are a result of the microstructure as well as of thermal expansion. Some control is possible but is often limited as a result of the (gaseous) impurities which may not be stable at high temperatures. Elastic stress distributions may be calculated but are useless without knowledge of plastic flow. Modifications of an interface by grading, stress compensation and topological interlocking are known to prevent failure. New techniques for the measurement of “adhesion” and the localized characterization of mechanical properties together with the application of fracture toughness concepts are necessary to understand interfacial strengths. Mechanical integrity requires the solution to many of these problems. Recent developments in high strength films, the prevention of grain growth by modulating the structure and the production of a specific microstructure for a desired optical property show that progress is possible. Opportunities for surface science will be to comprehend atomic mobilities in the formation and stabilization of real microstructures in solar environments, to produce new materials and to develop new techniques, especially in situ and non-destructive, to give early detection and predictive capability of failure.