1,433 research outputs found
Hysteresis in the Active Oxidation of SiC
Si and SiC show both passive oxidation behavior where a protective film of SiO2 forms and active oxidation behavior where a volatile suboxide SiO(g) forms. The active-to-passive and passive-to-active oxidation transitions are explored for both Si and SiC. Si shows a dramatic difference between the P(O2) for the two transitions of ~10-4 bar. The active-to-passive transition is controlled by the condition for SiO2/Si equilibrium and the passive-to-active transition is controlled by the decomposition of SiO2. In the case of SiC, the P(O2) for these transitions are much closer. The active-to-passive transition appears to be controlled by the condition for SiO2/SiC equilibrium. The passive-to-active transition appears to be controlled by the interfacial reaction of SiC and SiO2 and subsequent generation of gases at the interface which leads to scale breakdown
Active Oxidation of SiC
The high temperature oxidation of silicon carbide occurs in either a passive or active mode, depending on temperature and oxygen potential. Passive oxidation forms a protective oxide film which limits attack of the SiC:SiC(s) + 3/2 O2(g) = SiO2(s) + CO(g.) Active oxidation forms a volatile oxide and leads to extensive attack of the SiC: SiC(s) + O2(g) = SiO(g) + CO(g). The transition points and rates of active oxidation are a major issue. Previous studies are reviewed and the leading theories of passive/active transitions summarized. Comparisons are made to the active/passive transitions in pure Si, which are relatively well-understood. Critical questions remain about the difference between the active-to-passive transition and passive-to-active transition. For Si, Wagner [2] points out that the active-to-passive transition is governed by the criterion for a stable Si/SiO2 equilibria and the passive-to-active transition is governed by the decomposition of the SiO2 film. This suggests a significant oxygen potential difference between these two transitions and our experiments confirm this. For Si, the initial stages of active oxidation are characterized by the formation of SiO(g) and further oxidation to SiO2(s) as micron-sized rods, with a distinctive morphology. SiC shows significant differences. The active-to-passive and the passive-to-active transitions are close. The SiO2 rods only appear as the passive film breaks down. These differences are explained in terms of the reactions at the SiC/SiO2 interface. In order to understand the breakdown of the passive film, pre-oxidation experiments are conducted. These involve forming dense protective scales of 0.5, 1, and 2 microns and then subjecting the samples with these scales to a known active oxidation environment. Microstructural studies show that SiC/SiO2 interfacial reactions lead to a breakdown of the scale with a distinct morphology
Further Observations on the Behavior of Vanillin Substitution Products: (a) The Perkin Reaction; (b) The Preparation of Substituted Vanillic Acids
When the carbonyl group is joined directly to carbon and hydrogen (aldehydic) or to carbon and carbon (ketonic) it reacts with phenylhydrazine to give the corresponding phenylhydrazone. When the radical is joined directly to nitrogen and hydrogen, as in formanilide, the products are aniline and formylphenylhydrazine. Substitution productions of formanilide behave similarly. If the radical is joined directly to nitrogen and nitrogen, as in diphenylurea, the products are aniline and a semicarbazide. The corresponding thiourea behaves similarly, and gives a thiosemicarbazide. If the reaction with the thioureas is carried out in the presence of a desulphurizing agent, guanidine derivatives are obtained
Magnetic order in GdBiPt studied by x-ray resonant magnetic scattering
Rare earth (R) half-Heusler compounds, RBiPt, exhibit a wide spectrum of
novel ground states. Recently, GdBiPt has been proposed as a potential
antiferromagnetic topological insulator (AFTI). We have employed x-ray resonant
magnetic scattering to elucidate the microscopic details of the magnetic
structure in GdBiPt below T_N = 8.5 K. Experiments at the Gd L_2 absorption
edge show that the Gd moments order in an antiferromagnetic stacking along the
cubic diagonal [1 1 1] direction satisfying the requirement for an AFTI, where
both time-reversal symmetry and lattice translational symmetry are broken, but
their product is conserved.Comment: 4 pages, 4 figure
Computational Study of Turbulent-Laminar Patterns in Couette Flow
Turbulent-laminar patterns near transition are simulated in plane Couette
flow using an extension of the minimal flow unit methodology. Computational
domains are of minimal size in two directions but large in the third. The long
direction can be tilted at any prescribed angle to the streamwise direction.
Three types of patterned states are found and studied: periodic, localized, and
intermittent. These correspond closely to observations in large aspect ratio
experiments.Comment: 4 pages, 5 figure
Experimental and Theoretical Study of Thermodynamics of the Reaction of Titania and Water at High Temperatures
The transpiration method was used to determine the volatility of titanium dioxide (TiO2) in water vapor-containing environments at temperatures between 1473 and 1673 K. Water contents ranged from 0 to 76 mole % in oxygen or argon carrier gases for 20 to 250 hr exposure times. Results indicate that oxygen is not a key contributor to volatilization and the primary reaction for volatilization in this temperature range is: TiO2(s) + H2O(g) = TiO(OH)2(g). Data were analyzed with both the second and third law methods to extract an enthalpy and entropy of formation. The geometry and vibrational frequencies of TiO(OH)2(g) were computed using B3LYP density functional theory, and the enthalpy of formation was computed using the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)]. Thermal functions are calculated using both a structure with bent and linear hydroxyl groups. Calculated second and third heats show closer agreement with the linear hydroxyl group, suggesting more experimental and computational spectroscopic and structural work is needed on this system
A Mechanistic Study of Halogen Addition and Photoelimination from ?-Conjugated Tellurophenes
The ability to drive
reactivity using visible light is of importance
for many disciplines of chemistry and has significant implications
for sustainable chemistry. Identifying photochemically active compounds
and understanding photochemical mechanisms is important for the development
of useful materials for synthesis and catalysis. Here we report a
series of photoactive diphenyltellurophene compounds bearing electron-withdrawing
and electron-donating substituents synthesized by alkyne coupling/ring
closing or palladium-catalyzed ipso-arylation chemistry. The redox
chemistry of these compounds was studied with respect to oxidative
addition and photoelimination of bromine, which is of importance for
energy storage reactions involving X<sub>2</sub>. The oxidative addition
reaction mechanism was studied using density functional theory, the
results of which support a three-step mechanism involving the formation
of an initial η<sup>1</sup> association complex, a monobrominated
intermediate, and finally the dibrominated product. All of the tellurophene
derivatives undergo photoreduction using 430, 447, or 617 nm light
depending on the absorption properties of the compound. Compounds
bearing electron-withdrawing substituents have the highest photochemical
quantum efficiencies in the presence of an alkene trap, with efficiencies
of up to 42.4% for a pentafluorophenyl-functionalized tellurophene.
The photoelimination reaction was studied in detail through bromine
trapping experiments and laser flash photolysis, and a mechanism is
proposed. The photoreaction, which occurs by release of bromine radicals,
is competitive with intersystem crossing to the triplet state of the
brominated species, as evidenced by the formation of singlet oxygen.
These findings should be useful for the design of new photochemically
active compounds supported by main-group elements
Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic Mg intermetallics with , Eu, Gd, Yb and , Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic (, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized moments arising from the half-filled
shells of Eu or Gd. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.Comment: 12 pages, 7 figures include
Managing Risk and Growth of Nonprofit Revenue
Managers of nonprofit organizations are challenged to manage revenue growth and risk (i.e., volatility) in order to sustain current and future financial operations. Although the negative repercussions of revenue risk are generally perceived as undesirable, not all risk is bad. If higher levels of revenue risk are compensated with a greater amount of revenue growth, then organizations may rationally pursue volatile revenues that produce growth. This article examines the extent to which a reliance on major revenue sources by nonprofit organizations affects the magnitude of total revenue volatility as well as the pace of total revenue growth. A monitoring application is introduced that can be used to compare the effectiveness of revenue management among similar nonprofit organizations. It can also be used to guide nonprofit managers striving to achieve sustainable financial growth for their organizations
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