Joe Wong

Manganese speciation in exhaust particulates of automobiles using MMT-containing gasoline.

Manganese speciation in exhaust particulates of automobiles using MMT-containing gasoline monochromator, 50% detuned, was used. Multiple scans varying from 4 sweeps for XANES and 8 sweeps for EXAFS were performed to yield optimal S/N ratios. All spectra were collected in the QEXAFS mode (Frahm, 1988). A manganese metal standard foil located in front of a reference ion chamber was measured simultaneously with each sample for in-situ energy calibration.

Analysis of heat-affected zone phase transformations using in situ spatially resolved x-ray diffraction with synchrotron radiation

Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed (hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50 mm X-ray probe that could be positioned with 10-µm precision on the surface of a quasistationary gas tungsten arc weld (GTAW). The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool, respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of the HAZ, combined with a conduction-based thermal model of the weld, were correlated with the SRXRD results. Finally, analysis of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information about the microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first directin situ mapping of phase boundaries in fusion welds.

A time-resolved diffraction study of the Ta--C solid combustion system

The formation of TaC and Ta[sub 2]C by combustion synthesis from their elemental constituents has been studied by time-resolved x-ray diffraction (TRXRD) using synchrotron radiation. The reactions have been followed with a time resolution down to 50 ms. Since the adiabatic temperatures for both reactions are well below any liquidus temperature in the Ta--C phase diagram, no melting occurs and these combustion reactions occur purely in the solid state. The phase transformations associated with these reactions are followed by monitoring the disappearance of reactant and appearance of product powder diffraction peaks in real time as the reaction front propagates through the combusting specimen. In the synthesis of TaC, the results show the formation of the subcarbide (Ta[sub 2]C) phase as an intermediate. In the synthesis of Ta[sub 2]C, the reaction proceeds directly to the product with no discernable intermediate Ta--C phase within a 50 ms time frame. The chemical dynamics associated with the combustion synthesis of TaC may be described by an initial phase transformation to hexagonal Ta[sub 2]C arising from carbon diffusion into the Ta metal lattice. As more carbon is available this intermediate subcarbide phase, which has one-half of its octahedral interstices occupied by the carbon, further transformsmorexa0» to the cubic TaC final product, in which all octahedral sites are now occupied. The time-resolved data indicate that the rate of formation of Ta[sub 2]C is a factor of two faster than that of TaC.«xa0less

Dynamics of phase transformations and microstructure evolution in carbon-manganese steel arc welds using time-resolved synchrotron X-ray diffraction.

Phase transformations that occur in both the heat-affected zone (HAZ) and the fusion zone (FZ) of a carbon-manganese steel spot weld have been investigated using time-resolved X-ray diffraction (TRXRD) with time resolutions down to 50 ms. It is found that in both zones the gamma(f.c.c.) --> alpha(b.c.c.) transformation on cooling is twice as fast as the forward transformation of alpha --> gamma on heating. Profile analysis of the major Bragg reflections recorded in the TRXRD patterns reveals similarities and differences in the microstructural evolution with time in the HAZ and in the FZ. The latter undergoes melting and solidification in addition to solid-state transformations. With increasing temperature, the (110) d-spacing of the alpha phase prior to and during the alpha --> gamma transformation and the (111) d-spacing of the gamma phase just after the same transformation exhibit a decrease. The observed (and unusual) lattice contraction with temperature rise may be attributed to chemical effects, such as carbide precipitation in the alpha matrix, and/or mechanical effects due to stress relief. In the FZ, the gamma-Fe that forms has a preferential (200) texture on solidification of the liquid, whereas, on cooling in the HAZ, the gamma-Fe retains largely a (111) texture that is induced in the alpha --> gamma transformation on heating. On cooling in the HAZ, the width of the gamma(111) reflection increases initially, which is indicative of microstrain developing in the f.c.c. lattice, but decreases as expected, with a reduction of thermal disorder, on further cooling until the completion of the gamma --> alpha transformation. In the FZ, however, the microstrain in the gamma phase increases steadily on solidification and more rapidly for the duration of the gamma --> alpha transformation on further cooling. The final microstructure of the FZ is likely to consist of a single alpha phase dispersed in two morphological entities, whereas in the HAZ the alpha phase persists in one morphological entity in the final microstructure.

Spatially resolved diffraction using a Soller Collimator–Imaging plate assembly

Abstract A series of multiple foil collimators have been constructed to perform spatially resolved diffraction using a synchrotron radiation X-ray source and a two-dimensional X-ray detector. Slit gap sizes of 0.75, 0.25 and 0.125xa0mm were evaluated. A thin vanadium foil (BCC) on bulk titanium (HCP) was studied as a test system. This system simulates an α-Ti (HCP)→β-Ti (BCC) transformation in the heat affected zone of Ti fusion welds. Diffraction lines from both V(2xa00xa00) and Ti(1xa00xa02) were measured in the gap aligned with the V/Ti boundary. With improvements in the mounting of the slits, a spatial resolution of at least 0.12xa0mm will be possible.

Real-Time Probe of Reaction Centers in Solid Combustions by QEXAFS on the Sub-Second Time Scale

A QEXAFS (quick-scanning EXAFS) technique has been devised to follow the local atomic coordination changes about selected reactants in a class of highly exothermic solid combustion reactions. Real-time EXAFS measurements during the combustion were made in the time frame of a few seconds. By tuning the monochromator to a specific energy, at which maximum changes occur in an EXAFS feature of an element transforming from the reactant to the product phase, a time resolution of 20 ms was achieved. The Ni + Al → NiAl reaction has been investigated in some detail in light of a possible intermediate phase in the so-called after-burn region. The present QEXAFS findings together with the recent time-resolved diffraction data on the same system lends, for the first time, experimental insights into the structural macrokinetics of this class of combustion systems.

Time‐resolved diffraction studies of fast solid combustion reactions at high temperature

Real time synchrotron diffraction has been used to monitor the phase transformatins of highly exothermic, fast self‐propagating solid combustion reactions on a subsecond time scale down to 50 milliseconds. A specially designed reaction chamber was constructed to enable simultaneous in situ diffraction and IR thermal imaging to be measured. A position‐sensitive photodiode array detector capable of a full scan of 1024 pixels in 4 ms was used to record the time‐resolved diffraction patterns from a fixed location of the specimen during passage of the combustion wave front. The detector was triggered by an inframetric camera or a thermocouple placed upstream from the area illuminated by the x‐rays. The phase transformations and chemical changes in a number of simple binary systems of the type A+B→AB are reported here. These include Ti+C→TiC, Ni+Al→NiAl, Ta+C→TaC, and 2Ta+C→Ta2C. This new experimental approach can be used to study the chemical dynamics of high‐temperature solid‐state phenomena and to provide th...

The combustion synthesis of the ferroelectric material, BaTiO{sub 3}, studied by time-resolved X-ray diffraction

The combustion synthesis of the common ferroelectric material, BaTiO{sub 3}, was developed using the stoichiometry: BaO{sub 2}+0.2thinspTi+0.8thinspTiO{sub 2}{r_arrow}BaTiO{sub 3}+0.3thinspO{sub 2}. An adiabatic temperature, T{sub ad}, of the reaction was calculated from known thermodynamic data to be 1917thinsp{degree}C. Real time chemical changes in the formation of BaTiO{sub 3} during the reaction have been monitored using time-resolved X-ray diffraction with synchrotron radiation as the X-ray source. A time resolution of 250 ms was achieved. The combustion synthesis of BaTiO{sub 3} was followed by observing the intensities of reactant and product Bragg diffraction peaks in order to qualitatively identify the phases present. Because BaTiO{sub 3} forms initially as a cubic phase, X-ray diffraction of the product was monitored for a period of 20 min after the reaction to observe the phase transformation to the tetragonal form. This transformation is evident in these post-reaction scans as the cubic 110 and 220 peaks are split to the tetragonal 101/110 and 202/220 ones, respectively. {copyright} {ital 1999 International Centre for Diffraction Data.}