Surface topography of the {0 0 0 1} facet plane of as-grown 6H- and 4H-SiC crystals was studied ex situ by Nomarski optical microscopy (NOM) and atomic force microscopy (AFM). The surface polarity and the polytype of grown crystals largely affect the growth surface morphology of SiC{0 0 0 1} via differences in several thermodynamic and kinetic parameters. NOM observations revealed giant steps of a few micrometers in height on the {0 0 0 1} growth facet, and it was found that a morphological transition of the growth facet occurred when the growth conditions were changed. AFM imaging of the stepped structure of SiC{0 0 0 1} detected steps of height equal to the unit c-lattice parameter (c=1.512 nm for 6H-SiC and 1.005 nm for 4H-SiC). They are fairly straight and very regularly arranged, giving rise to equidistant step trains. Upon nitrogen doping, these regular step trains on the 6H-SiC(0 0 0
)C and 4H-SiC(0 0 0
)C surfaces became unstable: the equidistant step trains were transformed into meandering macrosteps of height greater than 10 nm. In this paper, we discuss the mechanism of macrostep formation (step bunching) on the SiC{0 0 0 1} surfaces through the consideration of the interplay between step energetics (repulsive step interaction) and kinetics (asymmetric step kinetics) on the growing crystal surface and elucidate how they affect the growth surface morphology of the SiC{0 0 0 1} facet. 相似文献
Adsorption isotherms of nitrogen monoxide (NO) and in situ EPR spectra of adsorbed NO on mordenite zeolites (MOR) of different cation types (HM, NaM and CaM) are measured at different temperatures to elucidate the effect of the strong adsorption promoted by the enhancement of potential field in micropore of MOR (micropore filling) as well as the electrostatic interaction in MOR on NO adsorption. The NO molecules adsorb irreversibly and fill up the micropore of MOR at 201 K, above the critical temperature of NO, regardless of the kind of cation species. The NO adsorption takes place even at 273 K. In the adsorption at 273 K, the strength of electrostatic field formed by cation sites affects the adsorptivity and the order of saturation amount of adsorption (Vs) corresponds to that of the electrostatic field strength. EPR results show that NO molecules strongly interact with cation sites in MOR and disproponation reaction of NO take place on CaM. 相似文献
The cis configuration between the hydroxo and the carboxylato and the three amino groups of the tetradentate, tripodal ligand tris(6-neopentylamino-2-pyridylmethyl)amine favors the formation of hydrogen bonds which stabilize the hydroxo–Feiii complex 1 . Thus, its structure closely resembles that of the active center of Feiii –soybean lipoxygenase-1, which also contains a six-coordinate Feiii atom. 相似文献
Benzene rings severely bent and closely stacked face-to-face are revealed in the crystal structure of the [1.1]paracyclophane derivative 1 , which could be isolated thanks to the kinetic stabilization provided by the steric shielding of the bridgehead sites by the substituents. 相似文献
We consider primal-dual pairs of semidefinite programs and assume that they are singular, i.e., both primal and dual are either weakly feasible or weakly infeasible. Under such circumstances, strong duality may break down and the primal and dual might have a nonzero duality gap. Nevertheless, there are arbitrary small perturbations to the problem data which would make them strongly feasible thus zeroing the duality gap. In this paper, we conduct an asymptotic analysis of the optimal value as the perturbation for regularization is driven to zero. Specifically, we fix two positive definite matrices, \(I_p\) and \(I_d\), say, (typically the identity matrices), and regularize the primal and dual problems by shifting their associated affine space by \(\eta I_p\) and \(\varepsilon I_d\), respectively, to recover interior feasibility of both problems, where \(\varepsilon \) and \(\eta \) are positive numbers. Then we analyze the behavior of the optimal value of the regularized problem when the perturbation is reduced to zero keeping the ratio between \(\eta \) and \(\varepsilon \) constant. A key feature of our analysis is that no further assumptions such as compactness or constraint qualifications are ever made. It will be shown that the optimal value of the perturbed problem converges to a value between the primal and dual optimal values of the original problems. Furthermore, the limiting optimal value changes “monotonically” from the primal optimal value to the dual optimal value as a function of \(\theta \), if we parametrize \((\varepsilon , \eta )\) as \((\varepsilon , \eta )=t(\cos \theta , \sin \theta )\) and let \(t\rightarrow 0\). Finally, the analysis leads us to the relatively surprising consequence that some representative infeasible interior-point algorithms for SDP generate sequences converging to a number between the primal and dual optimal values, even in the presence of a nonzero duality gap. Though this result is more of theoretical interest at this point, it might be of some value in the development of infeasible interior-point algorithms that can handle singular problems.