Surface Analysis with 1014-1015 W cm2 Laser Intensities

He, Chun; Becker, Christopher H.

doi:10.1002/(sici)1096-9918(199602)24:2<79::aid-sia91>3.0.co;2-8

Cited by 11 publications

(4 citation statements)

References 16 publications

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“…Two components are present in the Sr 3d peak separated by about 2-2.5 eV with a higher intensity found for the higher energy ͑HE͒ component. This HE component has previously been identified as a surface species in La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3−␦ and related samples 41 as well as in La x Sr 1−x MnO 3 films, 42 apparently being concentrated in the topmost oxide layer. In the present experiments, the strong increase of Sr after cathodic polarization is accompanied by a further enhancement of the HE species, whereas after the anodic treatment this HE component is suppressed.…”

Section: Resultsmentioning

confidence: 70%

Strong Performance Improvement of La[sub 0.6]Sr[sub 0.4]Co[sub 0.8]Fe[sub 0.2]O[sub 3−δ] SOFC Cathodes by Electrochemical Activation

Baumann

Fleig

Konuma

et al. 2005

J. Electrochem. Soc.

209

153

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It is shown that the electrochemical resistance of mixed conducting solid oxide fuel cell ͑SOFC͒ model cathodes can be reduced drastically by a short but strong dc polarization of the cell. The samples investigated are dense thin-film microelectrodes of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ on a yttria-stabilized zirconia solid electrolyte. Relative performance improvements of more than two orders of magnitude can be achieved with a cathodic dc bias of the order of 1 V, applied for a few minutes at fuel cell operating temperature. The positive effect on the electrode performance corresponds to an acceleration of the oxygen surface exchange reaction, initially the resistance determining process. For this surface-related resistance, absolute values as low as 0.065 ⍀ cm 2 at 700°C have been obtained. After such an activation, the resistance slowly increases again on a much larger time scale, indicating the possibility of a steady performance enhancement by a periodic activation with short dc pulses. X-ray photoelectron spectra show that a strong cathodic polarization severely changes the cation concentrations within the outermost surface layer of the electrode, and these field-induced surface compositional changes are assumed to be the main cause of the performance improvement.High-temperature solid oxide fuel cells ͑SOFCs͒ receive considerable attention because they enable a highly efficient conversion of chemical into electric energy. A major goal in SOFC research is the development of cathodes with a sufficiently low electrochemical resistance ͑ϳ0.15 ⍀ cm 2 ͒ at operating temperatures substantially below 800°C. 1-3 It is generally accepted that this is hardly achievable with lanthanum strontium manganite ͑LSM͒-based oxygen electrodes, which are the most thoroughly studied and technically developed SOFC cathodes at the moment. Focus has been shifted toward other materials that offer a sufficient electrochemical performance already in the intermediate-temperature range, and particularly the mixed conducting perovskites of the ͑La,Sr͒͑Co,Fe͒O 3−␦ family are considered as promising candidates.In the literature, activation effects upon dc polarization have been reported for porous LSM and LSM/YSZ ͑yttria-stabilized zirconia͒ composite electrodes, 4-11 as well as for Pt electrodes. 11-13 However, a general agreement on the causes of these activation effects has not been achieved yet, and the reported magnitudes of the effect and the suggested mechanisms strongly vary in the literature. Measurements on morphologically and crystallographically well-defined electrodes are not available but are highly desirable in this context. 11 Moreover, to the best of the authors' knowledge, a significant performance improvement by electrochemical polarization has not been observed for the technologically important class of mixed conducting electrode materials.In this work, a strong electrochemical activation effect is reported for mixed conducting La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ ͑LSCF͒ electrodes. The performance improvement is achieved...

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Section: Resultsmentioning

confidence: 70%

Strong Performance Improvement of La[sub 0.6]Sr[sub 0.4]Co[sub 0.8]Fe[sub 0.2]O[sub 3−δ] SOFC Cathodes by Electrochemical Activation

Baumann

Fleig

Konuma

et al. 2005

J. Electrochem. Soc.

209

153

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“…DeWitt and Levis attributed the photoionization time-of-flight mass spectra of some polyatomic molecules to a field ionization mechanism, although for some of the studied molecules the Keldysh parameter was greater than unity (γ = 4.5) at the lower laser power densities used. Alternatively Chun He and Becker in a very recent paper dealing with photoionization of both sputtered and gaseous samples have claimed that for laser intensities <7 × 10 13 W/cm 2 at 532 nm, multiphoton ionization is the dominating process. In the present work, the calculated values for the Keldysh parameter are between 3.5 and 11 for the range of the laser power densities applied.…”

Section: Resultsmentioning

confidence: 99%

On the Fragmentation of Nitrobenzene and Nitrotoluenes Induced by a Femtosecond Laser at 375 nm

Kosmidis

Ledingham²,

Kılıç³

et al. 1997

J. Phys. Chem. A

107

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The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared with the fragmentation by a 10 nanosecond laser at the same wavelength. The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior to the dissociation is also discussed. For nitrobenzene, it is suggested that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH 3 to the NO 2 group (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.

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“…Surface segregation of Sr has been reported by a number of authors. [13][14][15] In contrast to this proposed hindrance of the surface reaction by Sr species, Lee et al 16 reported increased performance of LSM-YSZ porous electrodes, but decreased performance of pure LSM electrodes, upon the addition of Sr to the electrode surface. In addition, Baumann et al 17 attributed an improvement in LSCF-based cathode performance to surface Sr enrichment occurring during polarization.…”

mentioning

confidence: 99%

Performance and Activation Behavior of Surface-Doped Thin-Film La[sub 0.8]Sr[sub 0.2]MnO[sub 3−δ] Cathodes

Vance

McIntosh

2008

J. Electrochem. Soc.

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film electrodes of thickness 100, 450, 650, and 700 nm were fabricated on yttria-stabilized zirconia ͑YSZ͒ electrolyte pellets by ultrasonic spray pyrolysis. In addition, 650 nm thick LSM electrodes with a surface doped with La-, Sr-, or Mn-nitrate were studied. Electrodes were deposited on both sides of the YSZ pellets in a symmetric arrangement. The individual electrode impedance spectra were measured at 973 K in laboratory air using a reference electrode placed on the free electrolyte surface. The open-circuit polarization resistance of the pure LSM and Sr and Mn surface-doped electrodes decreased significantly after initial application of cathodic current. In contrast, the La-doped sample showed no activation, with an initial polarization resistance similar to the activated, undoped LSM film. This indicates that surface La species play an important role in LSM electrode activation. All of the electrodes showed a strong, nonlinear decrease in polarization resistance accompanied by a linear decrease in ohmic resistance with increasing applied cathodic current. These changes are attributed to the introduction of a bulk-ion-transport path in the LSM film due to reduction of LSM under polarization. A minimum polarization resistance of 0.17 ⍀ cm 2 was measured for the La surface-doped 650 nm thick electrode at 250 mA/cm 2 .

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Surface Analysis with 1014-1015 W cm2 Laser Intensities

Cited by 11 publications

References 16 publications

Strong Performance Improvement of La[sub 0.6]Sr[sub 0.4]Co[sub 0.8]Fe[sub 0.2]O[sub 3−δ] SOFC Cathodes by Electrochemical Activation

Strong Performance Improvement of La[sub 0.6]Sr[sub 0.4]Co[sub 0.8]Fe[sub 0.2]O[sub 3−δ] SOFC Cathodes by Electrochemical Activation

On the Fragmentation of Nitrobenzene and Nitrotoluenes Induced by a Femtosecond Laser at 375 nm

Performance and Activation Behavior of Surface-Doped Thin-Film La[sub 0.8]Sr[sub 0.2]MnO[sub 3−δ] Cathodes

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