Search

The evolution of HTS device technologies will benefit from the development of MOCVD (Metal-organic Chemical Vapor Deposition) routes to high quality HTS films as well as to those of insulators with low dielectric loss and close HTS lattice matches. Reviewed here are research efforts at precursor design focusing on Ba sources. A novel low pressure TGA technique is used to compare volatilities of MOCVD precursors and to quantify the role of gas phase diffusion in film growth. To form high quality Tl2Ba2Can-1CunO4+2n(n = 2.3) films. BaCaCuO(F) films are first deposited by MOCVD using the liquid precursors Ba(hfa)2[MATH]mep, Ca(hfa)2[MATH]tet, and solid Cu(dpm)2(hfa = hexafluoroacetylacetonate, dpm = dipivaloylmethanate. mep = methylethylpentaglyme, tet = tetraglyme). The film growth psocess is shown to be mass transport-limited, and an interesting ligand exchange process is identified. The superconducting TBCCO phase is formed following an ex-situ anneal in the presence of Tl2O at temperatures from 820-900°C. Transport properties of TBCCO-2223 films include a Tcas high as 115K. Jcof 2x105A/cm2(77K), and Rsof 0.35m[MATH] (5K, 10 GHz). The MOCVD growth of low loss, lattice-matched dielectric NdGaO3, PrGaO3, Sr2AlTaO6, and SrPrGaO4films is also discussed. High quality YBa2Cu3O7-xfilms have been grown upon MOCVD-derived PrGaO3substrates.

Synthetic nanopore membranes can be used to mimic ion channels provided that molecular transport through membranes is precisely gated with selective and reversible chemical interactions. Aligned nanotubes of carbon or other inorganic materials can be assembled to construct higher-order supramolecular architectures using polymer films to force chemical flux through hollow cores. Open tips of carbon nanotubes (CNT) can be activated to have carboxylic groups, which can be easily derivatized with a molecule that binds to a bulky receptor that can open/close the pore entrance. In particular, the core entrances of an aligned CNT membrane were functionalized with a desthiobiotin derivative that binds reversibly to streptavidin, thereby enabling a reversible closing/opening of the core entrance. Ionic flux through the CNT membrane was monitored using optically absorbing charged marker molecules. The flux is reduced by a factor of 24 when the desthiobiotin on the CNT is coordinated with streptavidin; release of streptavidin increases the flux, demonstrating a reversible ion-channel flow. Analysis of solutions of released streptavidin shows approximately 16 bound streptavidin molecules per CNT tip.

A vertically aligned array of multiwalled carbon nanotubes (MWCNTs) impregnated with polystyrene is utilized to independently functionalize each end of the MWCNTs. The presence of the polystyrene matrix prevents sidewall oxidation of the CNTs, resulting in carboxylate derivatization at the CNT tips during processing via plasma oxidation. The membrane is subsequently dissolved in toluene, resulting in a suspension of CNTs with carboxylate‐derivatized tips. The CNT tips are further functionalized using carbodiimide‐mediated linking of carboxylate at the CNT tips with an amine of 2‐aminoethanethiol. This treatment results in thiol functionality and Fourier‐transform infrared (FT‐IR) studies confirm amide‐bond formation. Gold nanoparticles that are readily observed using transmisison electron microscopy (TEM) are then covalently linked to the thiol functional groups. Estimates of the average nanoparticle density are observed to decrease from ∼ 526 particles μm–1near the CNT tips to negligible values (< 7 particles μm–1) at locations beyond 700 nm from the CNT tips. This is consistent with a membrane geometry where CNTs tips are above the polystyrene surface owing to differing oxidation rates. Bifunctional CNTs (with different chemical functionality at either end of each CNT) is achieved by thiol functionalization on only one side of the oxidized CNT membrane floating on top of a 2‐aminoethanethiol functionalization reaction solution. After dissolution of the polystyrene matrix, TEM analysis shows gold‐nanoparticle decoration at the thiol‐functionalized end of the CNT.

Selective area growth of carbon nanotubes (CNT) has been used to control the diameter of CNTs. Narrow lines of SiO2 (12−60 nm) are formed at the cleaved face of a Si/SiO2/Si multilayer structure. CNTs are then grown by a chemical vapor deposition process with a ferrocene/xylene/H2/Ar mixture at 700 °C. CNTs are observed to grow only on the exposed SiO2 surface at the edge of the “mesa” structure with a diameter equal to the thickness of the SiO2 layer.

In early 1992 we identified an outbreak of cryptosporidiosis in Oregon and sought to identify and control its source. We used a series of studies to identify risk factors for illness : (i) a case-control study among employees of a long-term-care facility (LTCF); (ii) a matched case-control study of the general community; (iii) a cohort study of wedding attendees; and (iv) a cross-sectional survey of the general community. Drinking Talent water was associated with illness in the LTCF (OR = 22·7, 95% CI = 2·7–1009·0), and in the community (matched OR = 9·5, 95% CI 2·3–84·1). Drinking Talent water was associated with illness only among non-Talent residents who attended the wedding (P &lt; 0·001) and in the community (RR = 6·5, 95% CI 3·3–12·9). The outbreak was caused by contaminated municipal water from Talent in the absence of a discernible outbreak among Talent residents, suggesting persons exposed to contaminated water may develop immunity to cryptosporidiosis.

Abstract A component of the direct operating cost of aircraft is that associated with the manufacturing cost. This affects depreciation, interest, insurance and maintenance charges. By relaxing the requirements for aerodynamic surface smoothness the manufacturing cost can be reduced at the expense of an increase in drag and corresponding fuel costs. This work is part of a study to examine this multidisciplinary problem. Only isolated turbofan nacelles are considered. The costs associated with assigning different tolerance levels to the feature dimensions on nacelles are assessed. A statistical procedure is employed to estimate the cost-tolerance relationship for eleven features involving gaps, steps, surface profile and fastener flushness. This procedure requires actual manufacturing and cost source data. A knowledge of the cost-tolerance relationships is useful in a concurrent engineering context. It will allow aerodynamicists to optimise surface smoothness in consultation with production engineers, thus achieving the best compromise between cost and drag.

A method for the design of garments at a workstation is proposed. An underlying body form analogous to a trade mannequin is first represented in the computer. Garment panels are considered to be surfaces of complex shape, whose fit with respect to body form may vary over the surface. A method of easy entry of data for representing panels is proposed, whereby fit around panel edges can be defined. The problems of controlling fit in the interior of panels is considered and a scheme is proposed which accomplishes design with economy of effort from the designer.

In the high speed machining of metals it has been established that the shearing process is frequently non-uniform resulting in a serrated chip with highly localised shearing. This type of chip formation is promoted solely by the properties of the material being machined and is not initiated by vibrations or resonances of the machine structure. It has been found that this type of unsteady, shear localised chip form can be duplicated when polyethylene is machined at low speeds with araldite cutting tools. By means of a photoelastic technique the variations in tool stress associated with these non-uniform chip forms has been revealed during the machining process. The photoelastic results clearly show that the tool stresses oscillate in a regular manner during the formation of each chip segment. The results obtained have lead to a better understanding of the chip formation process and provide a more complete description of the stress conditions experienced by the cutting tool.

A regular perturbation expansion is applied to desorption governed by a Freundlich isotherm in a continuous-flow well-stirred vessel. A power series solution is presented, whose convergence is improved by the Shanks transformation. With a symbolic manipulation computer program, the first few temporal moments are derived exactly in closed form from the perturbation scheme.

In many spectroscopic studies, particularly by X-ray photoelectron spectroscopy, XPS, it has been generally assumed that the existence of sub-oxide transition regions at Si&z.sbnd;SiO 2 interfaces were not strongly dependent on synthesis chemistries and processing conditions. This paper presents experimental evidence for a kinetically-limited phase transition at Si&z.sbnd;SiO 2 interfaces at approximately 900°C that can minimize and to a large degree eliminate significant interfacial sub-oxide transition regions. The paper emphasizes a connection between (i) differences in photoluminescence from SiO x bulk films before and after 900°C annealing and (ii) differences in electrical performance of Si&z.sbnd;SiO 2 interfaces and optical second harmonic generation from Si&z.sbnd;SiO 2 interfaces, also before and after 900°C annealing

A method is presented in which a finite-difference model is used to predict the temperature distribution in a solid under a moving heat source. A coordinate system centered on the heat source is adopted so that the temperature field relative to the source is taken to be fully developed and static. As the method uses a nonuniform grid and allows for the variation of thermophysical properties to be specified, solutions can be determined for actual processes. Two processes are considered here: plasma arc heating in a hot machining process (where heating of the metal is carried out upstream of the cutting operation) and internal heating during chip formation in the machining.

Transition regions at SiSiO2interfaces contain su☐ide bonding arrangements which contribute to interface roughness and may give rise to electronically active defects. Interfacial transition regions with su☐ide bonding are a direct result of thermal and plasma-assisted oxidation at temperatures up to at least 800°C, but sub-oxide bonding is significantly reduced following a 30 s, 900°C RTA. The kinetics of annealing are essentially the same as those for separation of homogeneous sub-oxide thin films (SiOx, x< 2) into silicon nanocrystals and stoichiometric SiO2.

In the apparel field, there is the problem of creating a design with respect to an underlying standard form. Instead of applying general-purpose three-dimensional computer aided design systems to such tasks it is proposed that design can be carried more efficiently with bespoke design systems. These can be targeted to deal with the problems of accurately defining the offset surfaces which comprise the garment. Such a system is introduced and the methods of data entry are discussed. Position on the three-dimensional form is determined using a digitizer tablet and fit with respect to the form is specified by an offset dimension. It is shown that complex garment pieces with variable fit can be quickly specified within the system.

In situ measurements of the sizes and concentrations of dust particles generated by the detonation of high explosives in clay soil near Leesville, La., sandy clay soil near Huntsville, Ala., and sandy soils near Orogrande, N.M. are reported. Measurements were generally made within 1 m of the surface (in one case 10 m) at distances ranging from 10 to ~50 m from the detonation point with a combination of Knollenberg light-scattering counters (for particles with equivalent radius in the submicron to 15-µm range) and a Knollenberg optical array probe (for particles of 10–150 µm). Measurements were made for periods of several tens of seconds following detonation. All dust size distributions, irrespective of soil or explosive type, exhibit a bimodal character with mass mean radii of ~7 and 70 µm. Peak aerosol mass loadings inferred from the distributions have values ranging from 0.05 to 10 g gm^-3 with the larger mode of particles contributing most to the mass loading. Predictions of dust extinction coefficients at visible (0.55-µm) and IR (10.4-µm) wavelengths were made using the measured size distributions together with estimates of dust refractive indices. These predictions suggest that extinction should be nearly neutral (wavelength independent) in agreement with transmission measurements made during some of the tests. Predicted mass extinction coefficients, under the assumption of dust material density of 2.5 g cm^-3, are of the order of 0.05 m^2 g^-1 at both visible and IR wavelengths. This value is also in good agreement with a test-averaged measured value of 0.03 m^3 g^-1 (at λ = 10.6 µm) obtained using a short path transmissometer and hi-vol sampler.

Measurements of the size distribution, mass concentration, morphology, and infrared extinction coefficient of dust particles generated by vehicular traffic on unpaved roadways in the arid southwestern U.S. are reported. Measurements were made within a few meters of the point of closest approach of the various tracked and rubber-tired vehicles used in the experiment. For sandy soil, all vehicular dust size distributions reveal a bimodal character with mode mass mean radii about 4 μm and 45 μm, regardless of vehicle type and vehicle speed (for speeds ranging from 5 to 12 ms-1). The larger mode of particles appears to be characteristic of the parent soil, but not the smaller mode. The smaller mode likely derives from large soil grains through some process involving disaggregation. A limited number of measurements made in silty soil also display bimodal size distributions, although with slightly larger mean size and broader distribution for the smaller mode. The shapes of the dust particles generated from sandy soil can be modeled by a bimodal shape distribution of spheroids, with spheroids ranging from 2:1 prolates to spheres to 2:1 oblates most commonly occurring. The dust particles are composed of quartz, the clay minerals montmorillonite, kaolinite, and illite, and calcite and aragonite. Many consist of aggregates of fine particles. Measured mass extinction coefficients (the ratio of extinction coefficient to mass concentration) at the 9.504 μm (CO2 laser) wavelength are in rough agreement with a theoretical estimate of 0.11 m2g-1, obtained by modeling the dust as a polydis-persion of particles having a shape distribution of both prolate and oblate spheroids. The sizes and composition of vehicular dust are similar to those of wind-blown dust.

The synthesis and characterization of a series of polyethers and volatile, low-melting polyether complexes of bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)barium having the general formula Ba(hfa)2·RO(CH2CH2O)nR‘ where R = R‘ = CH3, n = 3; R = CH3, R‘ = C2H5, n = 3; R = R‘ = H, n = 5, 6; R = R‘ = CH3, n = 4; R = CH3, R‘ = C2H5, n = 5; R = CH3, R‘ = n-C4H9, n = 5, 6; R = CH3, R = C5H11O, n = 3; R = CH3, R‘ = n-C6H13, n = 4, 5; R = C2H5, R‘ = n-C4H9, n = 5; R = n-C4H9, R‘ = n-C4H9, n = 4, 6; R = n-C4H9, R‘ = n-C6H13, n = 5 are reported. The complexes are conveniently synthesized by reaction of n-propylammonium⁺hfa^- in DMF with an aqueous solution of Ba(NO3)2 and the polyether or, alternatively, by reaction of the polyether with Ba(hfa)2 in toluene. These new complexes were characterized by elemental analysis, FT-IR, ¹H, ¹³C, and ¹⁹F NMR, MS, X-ray diffraction (Ba(hfa)2·CH3O(CH2CH2O)3CH3·H2O, Ba(hfa)2·CH3O(CH2CH2O)5C2H5), and thermogravimetric analysis. The melting points of the complexes are strongly dependent on the architecture of the polyether chain and dimensions of the terminal polyether substituent, with the lowest melting points corresponding to the longest polyethers having the largest terminal groups. The volatility of the Ba(hfa)2·polyether compounds is dependent on molecular weight and molecular structure; however, there is little direct correlation between melting point depression and enhanced volatility. The applicability of these complexes in metal−organic chemical vapor deposition is demonstrated by the successful growth of phase-pure BaTiO3 thin films using Ba(hfa)2·CH3O(CH2CH2O)5C2H5 as the Ba source. Phase composition and epitaxy in these films is analyzed by energy-dispersive X-ray spectroscopy and X-ray diffraction &THETAV;−2&THETAV;, ω, and φ scans.

A synopsis of four reports on the occurrence of blowing dust and dust storms in the boundary layer over selected continental areas of the Earth is presented.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.