Thermal expansion of the V(5)Si(3) and T(2) phases of the V-Si-B system investigated by high-temperature X-ray diffraction

The thermal expansion anisotropy of the V(5)Si(3) and T(2)-phase of the V-Si-B system were determined by high-temperature X-ray diffraction from 298 to 1273 K. Alloys with nominal compositions V(62.5)Si(37.5) (V5Si3 phase) and V(63)Si(12)B(25) (T(2)-phase) were prepared from high-purity materials through arc-melting followed by heat-treatment at 1873 K by 24 h, under argon atmosphere. The V(5)Si(3) phase exhibits thermal expansion anisotropy equals to 1.3, with thermal expansion coefficients along the a and c-axis equal to 9.3 x 10(-6) K(-1) and 11.7 x 10(-6) K(-1), respectively. Similarly, the thermal expansion anisotropy value of the T(2)-phase is 0.9 with thermal expansion coefficients equal to 8.8 x 10(-6) K(-1) and 8.3 x 10(-6) K(-1) along the, a and c-axis respectively. Compared to other isostructural silicides of the 5:3 type and the Ti(5)Si(3) phase, the V(5)Si(3) phase presents lower thermal expansion anisotropy. The T(2)-phase present in the V-Si-B system exhibits low thermal expansion anisotropy, as the T(2)-phase of the Mo-Si-B, Nb-Si-B and W-Si-B systems. (C) 2009 Elsevier Ltd. All rights reserved.

Modification of surface properties of Ti-16Si-4B powder alloy by plasma immersion ion implantation

Results of the surface modification of Ti-16Si-4B powder alloy by nitrogen ion implantation are presented, together with the experimental description of the preparation of that powder by high-energy ball milling and hot pressing. The phase structure, chemical composition and morphology of sample surfaces were observed by utilizing X-ray diffractometer (XRD), atomic force microscope (AFM) and scanning electron microscopy (SEM). A tribological characterization was carried out with a ball-on-disc tribometer and an SEM. Friction coefficient is compared with the one obtained for Ti-6Al-4V alloy and the wear scars characterized by SEM/EDS (energy dispersive spectroscopy). The concentration profile of the detected elements have been investigated using Auger electron spectroscopy (AES) depth profiling. Our results show that a shallow implanted layer of oxygen and nitrogen ions were obtained at the Ti-16Si -4B alloy surface, sufficient to modify slightly its tribological properties. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.

Topological instability, average electronegativity difference and glass forming ability of amorphous alloys

In this paper, we report the remarkable agreement of the glass forming ability of binary alloys with a new criterion that combines the topological instability parameter (lambda) and the average electronegativity difference among the elements of an alloy, assuming both exert a synergetic effect. The best glass forming compositions for Zr-Cu and Ti-Ni systems are well predicted by this new approach. Although the new criterion needs further refinement, it is concluded that the proposed approach is a promising and simple tool to guide and reduce the tedious and labour intensive work to find good glass former compositions in metallic systems. (C) 2008 Elsevier Ltd. All rights reserved.

Preparation and characterization of ordered intermetallic platinum phases for electrocatalytic applications

Ordered intermetallic phases of Pt with several transition metals have been prepared and their electrocatalytic properties studied. In light of these tests it is proposed that these catalysts could be used as electrodes in fuel cells, as they combine an excellent capacity to adsorb organic fuels at the Pt sites with low susceptibility to being poisoned by intermediates and reaction products at the transition-metal sites. An experimental procedure used to obtain the four intermetallic phases Pt-M (M = Mn, Pb, Sb and Sn) is described. The phases thus produced were characterized by X-ray diffraction, scanning electron microscopy with surface analysis by energy-dispersive X-ray spectrometry, scanning tunneling microscopy and X-ray photoelectron spectroscopy. The data thus obtained support the conclusion that the method described here is highly effective for the preparation of Pt-M phases featuring a range of structural and electronic modifications that will allow a useful relation to be established between their physicochemical properties and predicted electrocatalytic activity. (C) 2007 Elsevier Ltd. All rights reserved.

Analysis of relative humidity sensors at the WMO Radiosonde intercomparison experiment in Brazil

The quality of the vertical distribution measurements of humidity in the atmosphere is very important in meteorology due to the crucial role that water vapor plays in the earth's energy budget. The radiosonde is the humidity measurement device that provides the best vertical resolution. Also, radiosondes are the operational devices that are used to measure the vertical profile of atmospheric water vapor. The World Meteorological Organization (WMO) has carried out several intercomparison experiments at different climatic zones in order to identify the differences between the available commercial sensors. This article presents the results of an experiment that was carried out in Brazil in 2001 in which major commercial radiosonde manufacturers [e.g., Graw Radiosondes GmbH & Co., KG (Germany); MODEM (France); InterMet Systems (United States); Sippican, Inc. (United States); and Vaisala (Finland)] were involved. One of the main goals of this experiment was to evaluate the performance of the different humidity sensors in a tropical region. This evaluation was performed for different atmospheric layers and distinct periods of the day. It also considers the computation of the integrated water vapor (IWV). The results showed that the humidity measurements achieved by the different sensors were quite similar in the low troposphere (the bias median value regarding the RS80 was around 1.8%) and were quite dispersed in the superior layers (the median rms regarding the RS80 was around 14.9%).

Preparation and characterization of ordered intermetallic platinum phases for electrocatalytic applications

Ordered intermetallic phases of Pt with several transition metals have been prepared and their electrocatalytic properties studied. In light of these tests it is proposed that these catalysts could be used as electrodes in fuel cells, as they combine an excellent capacity to adsorb organic fuels at the Pt sites with low susceptibility to being poisoned by intermediates and reaction products at the transition-metal sites. An experimental procedure used to obtain the four intermetallic phases Pt-M (M = Mn, Pb, Sb and Sn) is described. The phases thus produced were characterized by X-ray diffraction, scanning electron microscopy with surface analysis by energy-dispersive X-ray spectrometry, scanning tunneling microscopy and X-ray photoelectron spectroscopy. The data thus obtained support the conclusion that the method described here is highly effective for the preparation of Pt-M phases featuring a range of structural and electronic modifications that will allow a useful relation to be established between their physicochemical properties and predicted electrocatalytic activity. (C) 2007 Elsevier Ltd. All rights reserved.

Modification of surface properties of Ti-16Si-4B powder alloy by plasma immersion ion implantation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Measurements of Humidity in the Atmosphere and Validation Experiments MOHAVE-2009; overview of campaign operations and results

The Measurements of Humidity in the Atmosphere and Validation Experiment (MOHAVE) 2009 campaign took place on 11–27 October 2009 at the JPL Table Mountain Facility in California (TMF). The main objectives of the campaign were to (1) validate the water vapor measurements of several instruments, including, three Raman lidars, two microwave radiometers, two Fourier-Transform spectrometers, and two GPS receivers (column water), (2) cover water vapor measurements from the ground to the mesopause without gaps, and (3) study upper tropospheric humidity variability at timescales varying from a few minutes to several days. A total of 58 radiosondes and 20 Frost-Point hygrometer sondes were launched. Two types of radiosondes were used during the campaign. Non negligible differences in the readings between the two radiosonde types used (Vaisala RS92 and InterMet iMet-1) made a small, but measurable impact on the derivation of water vapor mixing ratio by the Frost-Point hygrometers. As observed in previous campaigns, the RS92 humidity measurements remained within 5% of the Frost-point in the lower and mid-troposphere, but were too dry in the upper troposphere. Over 270 h of water vapor measurements from three Raman lidars (JPL and GSFC) were compared to RS92, CFH, and NOAA-FPH. The JPL lidar profiles reached 20 km when integrated all night, and 15 km when integrated for 1 h. Excellent agreement between this lidar and the frost-point hygrometers was found throughout the measurement range, with only a 3% (0.3 ppmv) mean wet bias for the lidar in the upper troposphere and lower stratosphere (UTLS). The other two lidars provided satisfactory results in the lower and mid-troposphere (2–5% wet bias over the range 3–10 km), but suffered from contamination by fluorescence (wet bias ranging from 5 to 50% between 10 km and 15 km), preventing their use as an independent measurement in the UTLS. The comparison between all available stratospheric sounders allowed to identify only the largest biases, in particular a 10% dry bias of the Water Vapor Millimeter-wave Spectrometer compared to the Aura-Microwave Limb Sounder. No other large, or at least statistically significant, biases could be observed. Total Precipitable Water (TPW) measurements from six different co-located instruments were available. Several retrieval groups provided their own TPW retrievals, resulting in the comparison of 10 different datasets. Agreement within 7% (0.7 mm) was found between all datasets. Such good agreement illustrates the maturity of these measurements and raises confidence levels for their use as an alternate or complementary source of calibration for the Raman lidars. Tropospheric and stratospheric ozone and temperature measurements were also available during the campaign. The water vapor and ozone lidar measurements, together with the advected potential vorticity results from the high-resolution transport model MIMOSA, allowed the identification and study of a deep stratospheric intrusion over TMF. These observations demonstrated the lidar strong potential for future long-term monitoring of water vapor in the UTLS.