Cluster impurity

Type of defects vacancies, bivacancies, complexes, clusters, impurity atoms, dislocations, isotopes and so on. 

An important approach to the study of nucleation of solids is the investigation of small droplets of large molecular clusters. Years ago, Turnbull showed that by studying small droplets one could eliminate impurities in all except a few droplets and study homogeneous nucleation at significant undercoolings [13]. 

One of the first cluster embedding schemes was put forth by Ellis and co-workers [172]. They were interested in studying transition metal impurities in NiAl alloys, so they considered a TMAl cluster embedded in a periodic self-consistent crystal field appropriate for bulk p -NiAl. The field was calculated via calculations, as was the cluster itself The idea was to provide a relatively inexpensive alternative to supercell DET calculations. 

Ellis D E, Benesh G A and Byrom E 1978 Self-consistent embedded-cluster model for magnetic impurities p -NiAl J. Appl. Phys. 49 1543... 

Asay and Gupta [25] measure elastic precursor amplitudes as functions of propagation distance for two different divalent impurity concentrations in <100)-loaded LiF. It is shown that not only does the presence of divalent ions affect the precursor amplitude, but also that the state of the dispersion plays an important part. It is concluded that, for a given concentration of defects, the rate of precursor attenuation is reduced if the defects are clustered. 

J.R. Asay and Y.M. Gupta, Effect of Impurity Clustering on Elastic Precursor Decay in LiF, J. Appl. Phys. 43, 2220-2223 (1972). 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

Floe Clumps of bacteria and particulate impurities or coagulants that have come together and formed a cluster. Found in aeration tanks and secondary clarifiers. Flocculation The process of forming floe particles when a chemical coagulant or... 

Impurity-cluster size nearest-neighbor approximation... 

Fig.l The 20-atoin cluster used in tlie present calculations. All the atoms are on fee sites two impurity sites (J, 2) and tlie neighboring site.s (3-20). 

The LDOS have been calculated using 10 exact levels in the continued fraction expansion of the Green functions. For clean surfaces the quantities A Vi are the same for all atoms in the same plane they have been determined up to p = 2, 4, 6 for the (110), (100) and (111) surfaces, respectively, and neglected beyond. The cluster C includes the atoms located at the site occupied by the impurity and at al the neighbouring sites up to the fourth nearest neighbour. 

P.J.Braspenning,R.Zeller,A.Lodder and P.H.Dederichs, Self-consistent cluster calculations with correct embedding for 3d, 4d and some sp impurities in copper , Phys.Rev. B29 703 (1984) ... 

Figure 4.28. STM image of a PtRh(lOO) surface. Although the bulk contains equal amounts of each element, the surface consists of 69% of platinum (dark) and 31 % of rhodium (bright), in agreement with the expected surface segregation of platinum on clean Pt-Rh alloys in ultrahigh vacuum. The black spots are due to carbon impurities. It is seen that platinum and rhodium have a tendency to cluster in small groups of the same elements.

SIMS intensities from the "clean" Cu/Ni surfaces cannot be used to determine Cu/Ni surface concentrations, or relative change in concentration from one surface to another. This is because trace impurities (of very low but unknown concentration) preferentially bond to Ni sites and therefore the Ni containing SIMS cluster ions are preferentially enhanced, leading to an erroneously high determination of Ni concentration. 

Observation of Au2H impurity in pure gold clusters and implications for the anomalous Au-Au distances in gold nanowires. Journal of Chemical Physics, 121, 8231-8236. 

In pharmaceutical analysis the detection of impurities under a chromatographic peak is a major issue. An important step forward in the assessment of peak purity was the introduction of hyphenated techniques. When selecting a method to perform a purity check, one has the choice between a global method which considers a whole peak cluster (from the start to the end of the peak), and evolutionary methods, which consider a window of the peak cluster, which is... 

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