Work Reports Experiments Research Articles

Polycyclic Aromatic Hydrocarbons and Dust Particle Surface Interactions: Catalytic Hydrogenation of Polycyclic Aromatic Hydrocarbon Molecules under Vacuum Conditions

This work reports experiments on the catalytic interaction occurring between polycyclic aromatic hydrocarbon (PAH) molecules and TiO2 dust grain surfaces under vacuum conditions. The investigation ...

Contributions of vacancies and self-interstitials to self-diffusion in silicon under thermal equilibrium and nonequilibrium conditions

Since many years, the contribution of vacancies ($V$) and self-interstitials ($I$) to silicon (Si) self-diffusion is a matter of debate. Native defects and their interaction among themselves and with foreign atoms influence the processes taking place during device fabrication, starting with the growth of Si single crystals and ending with doping of nanosized electronic devices. Considering this relevance, it is remarkable that present data about the properties of native point defects in Si are still limited and controversial. This work reports experiments on self-diffusion in Si for temperatures between 650${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C and 960${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C to verify recent results of Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] that give rise to inconsistencies in $V$-mediated self- and dopant diffusion. Two different structures of isotopically controlled epitaxial layers of Si are used for the diffusion study. One structure consisting of 20 bilayers of ${}^{29}$Si/${}^{28}$Si was grown by molecular beam epitaxy (MBE). The other structure with a ${}^{28}$Si layer sandwiched between natural Si was grown by means of chemical vapor deposition. Self-diffusion in (${}^{29}$Si/${}^{28}$Si)${}_{20}$ multilayers (ML) was analyzed by means of secondary ion mass spectrometry (SIMS) and neutron reflectometry, whereas self-diffusion in ${}^{\mathrm{nat}}$Si/${}^{28}$Si/${}^{\mathrm{nat}}$Si sandwich (SW) structures was measured with SIMS only. Analysis of the experimental profiles reveals an enhanced self-diffusion in ML compared to SW structures. The enhanced diffusion is ascribed to the dissolution of $V$- and $I$-related defect clusters grown-in during MBE. On the other hand, self-diffusion in the SW structures accurately confirms the data of Shimizu et al. that are considered to represent data for thermal equilibrium conditions. The temperature dependence of self-diffusion is described by $V$- and $I$-mediated contributions with temperature-dependent thermodynamic properties of $V$. This interpretation can solve the inconsistency between self- and dopant diffusion in Si, but further experiments are required to verify this concept.

Impact of online handwriting recognition performance on text categorization

Today, there is an increasing demand of efficient archival and retrieval methods for online handwritten data. For such tasks, text categorization is of particular interest. The textual data available in online documents can be extracted through online handwriting recognition; however, this process produces errors in the resulting text. This work reports experiments on the categorization of online handwritten documents based on their textual contents. We analyze the effect of word recognition errors on the categorization performances, by comparing the performances of a categorization system with the texts obtained through online handwriting recognition and the same texts available as ground truth. Two well-known categorization algorithms (kNN and SVM) are compared in this work. A subset of the Reuters-21578 corpus consisting of more than 2,000 handwritten documents has been collected for this study. Results show that classification rate loss is not significant, and precision loss is only significant for recall values of 60–80% depending on the noise levels.

Sound control by temperature gradients

This work reports experiments showing that airborne sound propagation can be controlled by temperature gradients. A system of two heated tubes is here used to demonstrate the collimation and focusing of an ultrasonic beam by the refractive index profile created by the temperature gradients existing around the tubes. Numerical simulations supporting the experimental findings are also reported.

Doped semiconductors as half-metallic materials: Experiments and first-principles calculations ofCoTi1−xMxSb(M=Sc, V, Cr, Mn, Fe)

This work reports experiments and first-principles calculations on the substitutional semiconducting $C{1}_{b}$ compound $\mathrm{Co}{\mathrm{Ti}}_{1\ensuremath{-}x}{M}_{x}\mathrm{Sb}$. Diluted magnetic semiconductors have been prepared by substituting titanium in the semiconducting compound CoTiSb by other $3d$ transition elements $M$. Self-consistent calculations of the electronic structure predict some of the materials to be half-metallic ferromagnets. The structural, electronic, electric, and magnetic properties of the pure and substituted materials have been investigated. It is found from the experiments that substitution of up to 10% Ti by Fe, Mn, Cr, and V does not affect the crystalline structure and the lattice mismatch is less than 0.5% in the substituted compounds. The electric properties can be tuned from semiconducting to metallic by using different dopants. The $M=\mathrm{Cr}$-doped compound exhibits a metal to semiconductor transition. Photoemission spectroscopy and conductivity measurements agree well with the calculated electronic structure. The measured Curie temperature of the Fe-substituted alloy is far above room temperature $(g700\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. This fact and the very low lattice mismatch between the substituted and the pure compound make this material a serious candidate for future electronic applications, in particular for magnetoelectronics and spintronics.

Visualization of pool boiling on enhanced surfaces

Abstract This work reports experiments to visualize nucleate boiling on an enhanced tubular surface having sub-surface tunnels and surface pores. The finned copper tube had 1575 fins/m (40 fins/in.) and 0.8 mm fin height. The fins are covered by a thin foil sheet having 0.23 mm pores at 1.5 mm pore pitch along each interfin region. Data are provided for two foil cover sheets, one copper and the other a transparent plastic. The uniqueness of this work is that the visualization method allowed direct observation of the boiling process in the subsurface tunnels. Use of a high speed camera with 30 × magnification allowed detailed observation of the evaporation process in the tunnels and of the vapor bubbles emerging from the pores. The experiments were conducted for saturated and subcooled boiling in the horizontal and vertical orientations. For the vertical tube, the saturated boiling experiments showed that all of the tunnels were vapor filled except for liquid menisci in the corners. This was also true for the horizontal tube at high heat flux. For the horizontal tube at low heat flux, portions of the tunnel length was liquid filled. A large portion (70–90%) of the region was vapor filled except for liquid menisci in the corners, and the remaining part of the region had oscillating menisci. These experiments provide conclusive proof that the heat transfer mechanism in the subsurface tunnels is evaporation on the menisci in the corners.

Catalytic cooperation between MoO 3 and Sb 2O 4 in N-ethyl formamide dehydration II. Nature of active sites and role of spillover oxygen

The present work reports experiments designed to further investigate the role of MoO 3 and Sb 2O 4 in the cooperative effect they exhibit in N-ethyl formamide dehydration. Temperature-programmed desorption of NH 3 showed that Bronsted acid sites are responsible for activity. TPD of CO 2 gave no indication of basic sites. The Brønsted sites are sensitive to reduction and reoxidation. Thermoreoxidation of previously reduced molybdenum oxide and oxidation of carbon deposited on MoO 3 are more rapid when α-Sb 2O 4 is admixed. The interpretation of the phenomenon is that α-Sb 2O 4 dissociates molecular O 2 to an oxygen spillover species which can maintain MoO 3 at a high degree of oxidation and, if necessary, reoxidize it or remove deposited carbon. Brønsted sites necessitate a high degree of oxidation and their number is consequently promoted by spillover oxygen. Mechanisms are proposed for the dissociation of oxygen, the reoxidation of pre-reduced MoO 3, and the formation of Brønsted acid sites. The importance of the reported results for selective oxidation or multicomponent catalysts is discussed.

Anisotropic action of cetyl pyridinium chloride on rat heart mitochondria

This work reports experiments that show that in rat heart mitochondria, the alkyl cation cetyl pyridinium chloride induces inhibition of the electron transport with NAD-dependent substrates. It also induces an enhancement of oxygen uptake with succinate as substrate, stimulation of adenosine triphosphatase activity, release of Ca 2+ that have been accumulated, and inhibition of the energy-dependent uptake of ethidium bromide; these findings suggest that cetyl pyridinium chloride induces a collapse of membrane potential. The experiments carried out with submitochondrial particles showed that this reagent inhibits the oxidation of NADH, provided an uncoupler is added to the system. According to these data it is proposed that the latter effect is due to the binding of cetyl pyridinium chloride to the inner mitochondrial membrane in a site that faces the cytosol.

Binding of hydrophobic compounds to apoferritin subunits: Effects on the polymerization state

Apoferritin is a very stable polymer made up by 24 identical subunits forming a nearly spherical, hollow shell in which a ferric hydroxide core is enclosed. Dissociation of apoferritin into subunits occurs only at extremes of pH; thus, in the acid range, dissociation begins below pH 2.8 and the subunits reassociation begins above pH 3.5 [ 11. The importance of hydrophobic interactions at the intersubunit contact regions has been clearly established [ 1,2] and binding of hydrophobic molecules by the subunits could be anticipated. This work reports experiments on the binding of two hydrophobic compounds to apoferritin subunits. ANS was chosen because of its widespread use as a hydrophobic fluorescent probe; BZ, which is an antiinflammatory drug, since it has been shown to bind to hydrophobic sites in serum albumin [3]. Both compounds bind to apoferritin subunits, but not to the native protein, and induce their polymerization.

Distribution of a Gene for Susceptibility to Plasmodium gallinaceum in Populations of Aedes aegypti (L.)

Female A. aegypti that are homozygous for pls, a recessive gene on linkage group 2, cannot develop oocysts of P. gallinaceum. The gene has not been found in strains from Asia or the Americas but was present in all of 8 strains tested from Africa (Ghana, Nigeria, Tanzania, Uganda). Estimated gene frequency ranged from 0.27 to 1.0. Experimental laboratory populations were initiated with different proportions of pls, allowed to breed continuously for 8 months, and sampled periodically. Homozygous populations (all or no pls) remained unchanged; mixed populations showed no strong selection against pls. Most laboratory strains of Aedes aegypti (L.) are uniformly susceptible to Plasmodium gallinaceum Brumpt. Most parasitologists consider this susceptibility as a species characteristic. However, Kilama (1969, 1970) and Kilama and Craig (1969) showed that mosquitoes that are homozygous for the gene pls (plasmodium-susceptibility) cannot develop oocysts of P. gallinaceum. In refractory mosquitoes, the ookinete moves through the gut wall but the newly formed oocyst dies within 40 to 60 hr of the infectious blood meal. The gene pls behaves as a simple Mendelian autosomal recessive factor, located on linkage group 2. The pls factor was originally isolated from two strains of A. aegypti formosus (Walker) from West Africa (Kilama and Craig, 1969). Tests on more than 20 laboratory strains which had originated from Asia or the Americas showed uniform susceptibility; there was no evidence of pls in these parts of the world. These findings stimulated other questions: (1) Is pls restricted to West Africa? (2) Alternatively, could pls be found throughout tropical Africa, associated with the nondomestic subspecies, A. aegypti formosus? (3) What is the frequency of pls in field populations? (4) What is the selective value of pls and can this be tested in artificial populations in the laboratory? The present work reports experiments attempting to answer the above questions. The work was performed in 1969-70. Received for publication 11 November 1971. * Supported in part by U. S. NIH Research Grant No. AI-02753. t Present address: Faculty of Medicine, University of Dar es Salaam, Tanzania. MATERIALS AND METHODS The 8A strain of P. gallinaceum was used in these studies. Methods for maintaining malaria in chickens and for rearing and infecting mosquitoes were those reported by Kilama and Craig (1969). The strains of A. aegypti used are listed in Table I. All strains are currently available at the WHO International Reference Centre for Aedes at the University of Notre Dame. Individual mosquitoes may be refractory to malaria for reasons other than the presence of pls. Hence, the presence and frequency of pls was indirectly determined by using a complementary test; i.e., 2 recessive mutations in a transheterozygote are usually allelic if they produce a mutant phenotype (refractory) and nonallelic if they give wild-type progeny (susceptible). Individuals from the strain to be tested were therefore crossed with individuals from the ILOBI line which was homozygous for pls and therefore refractory. The F1 progeny from single-pair crosses were tested and segregated for susceptibility to P. gallinaceum. Experimental populations were established in the laboratory to follow the frequency of pls over a period of 8 months. Five populations were initiated, with the following proportions of pls to wild-type individuals: 10:0, 9:1, 1:1, 1:9, 0:10. Cages were 61 by 61 by 61 cm. The initial population in each cage consisted of 500 males and 500 females in the proper proportion. The mosquito populations were blood-fed twice per week on anesthetized mice. Each population received a constant supply of apple slices or sugar cubes. A large dishpan containing water and measuring 31 by 39 by 17 cm supplied an oviposition site. The water level in the pans was periodically raised to stimulate hatching. The larvae were fed on a solution of liver powder (Nutritional Biochemical Corporation). The frequency of pls in each population was determined monthly. Eggs were removed from each cage, conditioned, and hatched. The resulting adult females when approximately a week old were fed on an infected chicken, and scored for susceptibility a week later. The number of oocysts per mosquito stomach was counted; mosquitoes showing 0 to 10 oocysts were considered as re-