Chromium Source Research Articles

Transporte de crômio trivalente influenciado pelo pH, horizonte do solo e fontes do crômio

Neste trabalho estudou-se qualitativamente a eluviação de Cr(III) em colunas, influenciada: (i) pelo pH, (ii) pelo horizonte e (iii) por fontes de crômio. Utilizaram-se amostras dos horizontes A e B de dois solos, com diferentes teores de matéria orgânica, do Estado de São Paulo: Latossolo Vermelho Eutroférrico, textura argilosa (LVe) e Latossolo Vermelho Amarelo Distrófico, textura média (LVd) em que as amostras do horizonte superficial receberam adição de carbonato de cálcio. Realizaram-se experimentos de lixiviação em colunas, para obtenção das curvas de eluição de três fontes de crômio: a) em solução salina (Cr-SAL); b) complexado (Cr-EDTA) e c) associado a um resíduo de curtume (Cr-RESÍDUO). A eluviação do crômio foi maior no LVd de textura média que no LVe argiloso; já a calagem teve pouca ou nenhuma influência na diminuição da eluviação. No solo LVe, a movimentação de crômio foi maior no horizonte superficial em relação ao subsuperficial, possivelmente em decorrência da formação de compostos solúveis entre o crômio e o ácido fúlvico presente na matéria orgânica. A eluviação foi influenciada pela fonte de crômio, cujos maiores valores foram observados com a aplicação de Cr-EDTA , e os menores com Cr-RESÍDUO.

Cr(VI) sorption by intact and dehydrated Candida utilis cells: differences in mechanisms

The Cr(VI) sorption kinetics by intact and dehydrated Candida utilis cells were studied. Yeast dehydration led to enhanced Cr(VI) sorption activity by cells during the first 15–20 min of rehydration. In experiments with K 2Cr 2O 7 as a chromium source, [Cr 2O 7] 2− was converted into [CrO 4] 2− during incubation with dehydrated biomass. An effect of the considerably enhanced Cr(VI) sorption activity by dehydrated cells in the buffered media at pH≈8.0 can serve as another confirmation of the different sorption mechanisms of intact and dehydrated C.utilis cells. This effect was not revealed in water solution at pH 8.0. This phenomenon will be studied in further investigations.

The impact of chlorine on chromium speciation in a laminar diffusion flame

Measurements of total chromium and hexavalent chromium have been obtained in a laminar methane/air diffusion flame. Chlorine was added to methane in the form of methyl chloride gas. A dilution-quenching probe sampled the emissions from the flame. Particles were collected on a filter and vapor phase species were collected in impingers. All samples were analyzed for hexavalent chromium with a spectrophotometric method. The filter sample was also analyzed for total chromium by X-ray fluorescence. Total chromium added to the flame was determined by gravimetric analysis of the chromium source, chromium hexacarbonyl. Recovery of chromium of up to 64% was achieved. The addition of methyl chloride to the fuel had a dramatic impact on the speciation of the chromium emission. The fraction of hexavalent chromium increased significantly as small amounts of chlorine were added. Partitioning between phases was also altered; much more hexavalent chromium was found in the impingers as the chlorine loading increased. Equilibrium calculations reproduced experimental trends but showed strong sensitivity to the thermochemical properties of the included species, particularly CrO 2Cl 2.

Morphology of chromium emissions from a laminar hydrogen diffusion flame

The morphology and size distribution of chromium oxide particles has been studied in laminar hydrogen diffusion flames. Nitrogen was added to vary the flame temperatures. Two sources of chromium compounds were introduced: chromium nitrate and chromium hexacarbonyl. In order to clarify the morphology of chromium oxide particles that were produced in the flame, a detailed investigation was carried out by scanning electron microscopy (SEM) after sampling with an isokinetic, dilution sampling probe. The morphology of the particles varied with the flame temperature and with the chromium source. The particles were characterized by porous structures, cenospheres, and agglomerated dense particles when chromium nitrate solution was added to the flames. At low to moderate temperatures, porous sintered cenospheric structures were formed, in some cases with a blow hole. At higher temperatures, an agglomerated cluster, which was composed of loosely sintered submicron particles was observed. SEM analysis revealed that the size distribution of chromium oxide particles was bimodal; submicron sizes were generated by the mechanism of cenosphere fragmentation. Chromium oxides formed from adding chromium hexacarbonyl to a hydrogen flame did not exhibit cenospheric structures. At high temperatures, a film of crystalline material was deposited onto a filter; at low temperatures very small agglomerated chains were found. The morphology of the metal particles is strongly dependent on the form in which the metal enters a flame.

Standard solar neutrinos

Abstract The 8B solar neutrino flux predicted by the standard solar model (SSM) is consistent within the theoretical and experimental uncertainties with that observed at Super-Kamiokande. The combined results from the Super-Kamiokande and the Chlorine solar neutrino experiments do not provide a solid evidence for neutrino properties beyond the minimal standard electroweak model. The results from the Gallium experiments and independently the combined results from Super-Kamiokande and the Chlorine experiment imply that the 7Be solar neutrino flux is strongly suppressed compared with that predicted by the SSM. This conclusion, however, is valid only if the neutrino absorption cross sections near threshold in Gallium and Chlorine do not differ significantly from their theoretical estimates. Such a departure has not been ruled out by the Chromium source experiments in Gallium. Even if the 7Be solar neutrino flux is suppressed compared with that predicted by the SSM, still it can be due to astrophysical effects not included in the simplistic SSM. Such effects include spatial and/or temporal variations in the temperature in the solar core, rotational mixing in the solar core, and dense plasma effects which may strongly enhance p-capture by 7Be relative to e-capture. The new generation of solar observations, which already look non stop deep into the sun, like Super-Kamiokande through neutrinos, and SOHO and GONG through acoustic waves, may be able to point at the correct solution; astrophysical solutions if they detect unexpected temporal and/or spatial behaviour, or particle physics solutions if Super-Kamiokande detects characteristic spectral distortion or temporal variations (e.g., the day-night effect) of the 8B solar neutrino flux. If Super-Kamiokande will not discover spectral distortions or time dependence of the 8B solar neutrino flux then, only future solar neutrino experiments such as SNO, BOREXINO and HELLAZ, will be able to find out whether the solar neutrino problem is due to neutrino properties beyond the minimal standard electroweak model or whether it is just a problem of the too simplistic standard solar model.

Implications of the GALLEX results after the Chromium source experiment

Since May 1991, the GALLEX experiment — installed in the Gran Sasso Underground Laboratory — is continuously monitoring the solar neutrino signal by measuring the 71 Ge production rate in a target of 30.3 tons of nat Ga. The measured signal 77.1 ± 8.5 (stat) +4.4 – 5.4 SNU (1 σ ) [1] is well below the Standard Solar Model (SSM) prediction: ⋍ 120–130 SNU. To exclude unknown systematic effects as responsible for the observed deficit, an experiment with a (62.5 ± 0.4) PBq 51 Cr neutrino source has been performed 2. , 3. . The ratio R=0.97 ± 0.11 between measured and expected Cr-produced 71 Ge demonstrates that the deficit cannot be ascribed to unknown experimental systematics. Although astrophysical reasons could still remain as a possible explanation of the “solar neutrino puzzle”, we recall that the GALLEX result, when considered together with the results of the other solar neutrino experiments, would allow to strongly constrain the neutrino masses and mixing angles in the framework of the MSW effect.

Surface hardening of ductile cast iron using stainless steel

A surface hardening technique for improving the wear resistance of ductile cast iron (DI) is proposed by fabricating molten DI with stainless steel. The essential idea is concerned with using the chromium source in stainless steel and the carbon source in DI to form chromium carbides. It was demonstrated that the surface of the fabricated DI consisted of firstly a layer of high chromium white cast iron ((Cr,Fe) 7C 3 eutectic carbides), which was originally stainless steel, and then a layer of medium low-chromium white cast iron ((Fe,Cr) 3C eutectic carbides), which was originally DI. Thus the hardness of the surface layer became 2–3 times that of the baseline ductile cast iron. Applying austenitic stainless steel produces a much better hardening effect than applying ferritic stainless steel. The depth of the hardened surface layer depends on the pouring temperature of the iron melt, the wall thickness of the iron casting and the cooling rate of the casting mold. The present strategy is, in principle, possibly applied to complete as well as local surface hardening of various kinds of ductile iron castings.

Limits on electron-neutrino oscillations from the GALLEX 51Cr source experiment

The recent result from the chromium source experiment carried out by the GALLEX collaboration implies interesting limits on the parameters Δm 2 and sin 2 2 θ describing neutrino oscillations. Values of Δm 2 > 0.17 eV 2 for maximal mixing and of sin 2 2 θ > 0.38 for Δm 2 > 1 eV 2 are ruled out at 90% C.L. This result improves by more than an order of magnitude previous limits on Δm 2 derived from electron-neutrino oscillation experiments at accelerators.

Development and validation of the US EPA hexavalent chromium source emissions measurement method

The US Environmental Protection Agency (EPA) has validated a sampling and analytical method for measuring hexavalent chromium, Cr(VI), in source emissions at municipal sewage sludge incinerators and hazardous waste incinerators. The sampling method uses an impinger train that is designed to continuously recirculate an alkaline impinger solution to the sampling nozzle, where the incoming Cr(VI) is stabilized. Recirculation of an alkaline solution in the sample nozzle and through the probe avoids dry surfaces common to the traditional EPA Method 5 and Method 13-type sampling trains. Hot, dry surfaces in a sampling train can contribute to the conversion of Cr(VI) to trivalent chromium, Cr(III), during sampling. Additional post-sampling procedures to minimize the conversion of Cr(VI) include purging the sample train with nitrogen to remove reactive gases, followed by filtration of the impinger solution to remove particulate matter and insoluble chromium (Cr) compounds. The impinger solution is then analyzed by ion chromatography coupled to a post column reactor (IC/PCR), employing a diphenyl carbazide (DPC) reagent. Depending on the sample matrix, an in situ preconcentration technique can be employed resulting in an in-stack detection limit of 10 ng/m 3. During the validation of this test method, a radioactive isotope of Cr(VI) was used to monitor any conversion that occurred to Cr(VI) during sampling and preparation prior to analysis. EPA-sponsored laboratory experiments and field tests of this Cr(VI) sampling train typically demonstrated over 80% recovery of Cr(VI), compared to the Method 5-type train where recoveries were less than 50%. This paper describes EPA's current measurement method for Cr(VI) source emissions, discusses results obtained with this method at municipal sewage sludge incinerators and a hazardous waste incinerator, and presents sampling techniques for sources with elevated acid gas levels.

Evaluation of tetra-alkylchromium precursors for organometallic chemical vapor deposition II: Unusual low temperature chromium carbide deposition from Cr[C(CH 3) 3

The tetra- tert-butylchromium compound Cr[C(CH 3) 3] 4 was found to be appropriately volatile and sufficiently thermally stable to be used as a chromium source in organometallic chemical vapor deposition. Thin films of chromium carbide were deposited using this precursor in the exceptionally low temperature range 100–150 °C. The films were X-ray amorphous and the atomic ratio of carbon to chromium was in the range 1.2–1.4, even using hydrogen as carrier gas. Without stringent precautions the films may be partially oxidized owing to the reactivity of this precursor towards traces of oxygen and moisture. Chemical bonding analysis using X-ray photoelectron spectroscopy data revealed carbon incorporation both in carbidic form and free carbon form (about 50%). The β-elimination mechanism probably initiates the decomposition but carbon incorporation may be due to side heterogeneous reactions involving carrier gas molecules because the carbon contamination of films grown under dynamic vacuum was very low.

Role of chromium sublayers in the growth of highly crystalline (111)-oriented gold films on sapphire

Highly crystalline (111)-oriented gold films have been vacuum deposited on chromium sublayers evaporated on sapphire (0001) substrates. The degree of a preferred orientation of gold films remarkably depends on the chromium layer thickness and is almost independent on substrate temperatures in the region of 150–325°C. In particular, chromium sublayers with a thickness of less than 5 nm gives rise to a significant improvement in the alignment of the gold films. It is evident from TEM observations that the vacuum evaporated sublayers, by heating a chromium source, are composed of a well-defined (0001) epitaxial Cr2O3 thin films grown on (0001) sapphire. The formation of the Cr2O3 layers allows growing easily epitaxially (111)-oriented gold films.

Diffusion chromizing of ferrous alloys

The results of an extensive study of the mechanism of formation of chromium diffusion coatings on ferrous alloys are described. The effects of the significant coating variables on the composition, thickness and microstructure of the coatings are discussed. These variables include the pack geometry, substrate composition, type of halide activator, type of “inert filler”, time, temperature and the type of the chromium source material. The diffusion of chromium and chromium plus vanadium are considered. The chromium concentration profiles are seen to deviate from those predicted from the Fe-Cr phase diagram. Mechanisms for the coating formation are described.

Abstract: Partial pressure analysis of chromium source material

Abstract: Partial pressure analysis of chromium source material