A. Paúl

Micron-scale analysis of SiC/SiCf composites using the new Lisbon nuclear microprobe

Abstract A new nuclear microprobe has now been commissioned at ITN, Lisbon. This paper first describes the layout and modes of operation of the new microprobe, which is located on a 3.1 MV single-ended Van de Graaff accelerator. Within two days of first commissioning the microprobe, a spatial resolution of ∼1.5 μm was achieved for backscattering analysis, and a resolution of ∼1.0 μm for transmission work. The steps taken to produce this resolution, and the remaining factors, which further limit it are discussed. The first results from this microprobe for the spatially resolved analysis of SiC/SiCf ceramic composites are presented here. These materials have applications in fusion technology and structural changes were investigated after exposure to lithium orthosilicate and lithium titanate breeder materials in fusion relevant conditions. Ti and Cr rich precipitates could be found in the samples that were exposed to lithium titanate.

Young's modulus of (Ti,Si)N films by surface acoustic waves and indentation techniques

c ¨ Abstract Ti Si N films with Si contents up to 17.5 at.% and N contents close to 50 at.% were prepared by r.f. reactive magnetron 1y xx y sputtering. Film densities are within the range 3.4-5.1 g ycm .X-Ray diffraction patterns indicated the formation of two crystalline 3 phases.In the case of low surface mobility, a metastable (Ti, Si)N phase was formed, where Si atoms occupied Ti positions. With increasing surface mobility, a crystalline TiN phase was observed.This behaviour may be explained by the occurrence of Si N segregation, leading to the formation of a nanocomposite film of the type nc-TiNync-(Ti,Si)Nya-Si N , although the 34 34 presence of Si N phase is difficult to prove.In some of the films, a mixture of the (Ti,Si)N metastable phase with the TiN phase 34 was observed, which indicates that the segregation of both TiN and Si N phases is not complete.The Youngs modulus, E ,o f 34 f each coating was evaluated using both indentation tests and the surface acoustic waves (SAW) method.For most samples, the results obtained by these two methods are in good agreement.Some differences were observed in films prepared with a bias voltage of y50 V and Si contents higher than 5.9 at.%. For these samples, indentation values of approximately 10-20% higher than those obtained from SAW were found.This discrepancy is related to the nanostructure of these coatings, and it should be pointed out that the SAW results are strongly correlated with the density of the material. 2002 Elsevier Science B.V. All rights reserved.

Influence of preoxidation and annealing treatments on the isothermal oxidation in air at 1173 K of cerium-implanted EN-1.4301 stainless steel

Improvement of the high temperature oxidation behavior of conventional EN-1.4301 (AISI type 304) stainless steel was achieved by implanting cerium ions. Implantation was performed in samples with two different surface treatments: commercial 2B mill finishing and preoxidation in synthetic air. One set of samples was annealed after implantation in order to recover the damage induced in the crystalline structure. Results show that the implantation of Ce greatly improves the oxidation resistance at 1173 K of the EN-1.4301 grade stainless steels and that the annealing treatment after the implantation is beneficial during the subsequent oxidation. Experiments confirm the presence of spinel-type compounds and Cr2O3 and Fe–Cr mixed oxides with high Cr content in the scale, and martensite being the dominating metallic phase. Chromium depletion in the near-surface layers is also described. Results are compared to previously published data on surface modification using La and Ce by pyrolysis of aerosols.

Fischer-tropsch catalyst deposition on metallic structured supports

Structured catalysts for Fischer-Tropsch synthesis (FTS) have been obtained by deposition of thin 20 wt.Co/γ-A12O3 catalyst layers washcoats onto the metallic surface of monolithic supports applying a preparation procedure involving: (a) preparation of metallic monoliths, (b) preparation of stable slurry of catalyst, (c) dip-coating of monolithic supports. The quality of catalyst coatings was evaluated in terms of loading, homogeneity and adhesion force. Through this procedure, Fecralloy® monoliths pretreated by thermal oxidation have been loaded with a ≈ 450 mg well-adhered catalyst coating layer and tested in FTS at 250 °C, 10 bar, under synthesis gas composition H2/CO = 2. Both the coated monoliths and catalyst powder tested in FTS under similar operating conditions revealed rather comparable results in terms of activity and selectivity as a consequence of using thin layers and small particle size of powder (below 250 µm). Diffusion distances obtained for the catalytic materials resulted to be appropriate for minimizing the mass transfer effects or diffusion limitations.

Development of a modified CVD coating process for the enhancement of the high temperature oxidation resistance of Cr2O3 and Al2O3 forming alloys

Abstract In this work, a new deposition method for coating Cr2O3 and Al2O3 forming alloys has been set up. This is a modified CVD deposition method based on the pyrolisis of an aerosol produced by an ultrasonic wave focussed on the surface of a solution containing a reactive element soluble salt that will later transform to an oxide. The deposition takes place in a reaction chamber maintained at 473 K. To test the efficiency of the deposition method, commercial AISI-304 stainless steel samples were coated directly on the mill finish surface and on a pre-formed oxide layer with lanthanum nitrate. Specimens were then oxidised at 1173 K in synthetic air atmosphere during 25 h in a thermobalance. Oxidation kinetics of coated samples was compared with that of untreated ones. The results show that coated samples improve their high temperature oxidation resistance by decreasing their parabolic rate constant by one order of magnitude. The deposition method developed proves to be an easy, cheap and efficient way of increasing the service temperature of conventional stainless steel.

X-Ray Diffraction Study of Oxide Scales Formed at High Temperatures on AISI 304 Stainless Steel After Cerium Deposition

High-resolution X-ray diffraction has been usedto characterize the structure of oxide scales grownduring oxidation at 1173 K of cerium-modified stainlesssteels. The oxide scales consist of a mixture of Fe2O3 andCr2O3, as well as spinels. Severaloxidation treatments prior to cerium deposition havebeen applied. New phases, which are cerium-related,appear, depending on the preoxidation treatment. Bycomparing these results with previous ones onlanthanum-modified AISI 304 stainless steel, somepossible explanations for the reactive-element effectare proposed.

Low cost rare earth elements deposition method for enhancing the oxidation resistance at high temperature of Cr2O3 and Al2O3 forming alloys

Abstract In this work a deposition method for coating Cr 2 O 3 and Al 2 O 3 forming alloys with rare earth elements is described. This is a modified CVD deposition method based on the pyrolysis of an aerosol produced by ultrasonic waves focused on the surface of a solution containing a soluble salt of the desired element that will later transform to oxide. To test the efficiency of the deposition method, commercial AISI-304 stainless steel samples were coated with lanthanum and cerium nitrates. Oxidation experiments have been performed in a thermobalance at 1173 K in synthetic air atmosphere for 25 h. Oxidation kinetics of coated samples was compared with that of an untreated one. Results show that samples coated with La and Ce improve their high temperature oxidation resistance by decreasing their parabolic rate constant of one order of magnitude. During the oxidation treatment LaCrO 3 perovskite forms in the scale of La-coated specimens while CeO 2 is the crystalline phase detected in the Ce-coated ones.

Phase Transformation and Structural Studies of EUROFER RAFM Alloy

High temperature phase transformations in EUROFER reduced activation ferritic martensitic (RAFM) steel were studied in-situ by means of X-ray diffraction. Results show that, during slow cooling, the austenite to ferrite transformation takes place around 755 oC. Full transformation of the austenitic phase into pure martensite is observed for cooling above 5 oC/min. This transformation was found in samples annealed at 950 oC for 3 h and quenched in liquid nitrogen. TEM analyses reveal a high concentration of carbides along the grain boundaries of the martensitic structure. The thermal expansion coefficient derived from the measurements was 12.7x10-6 K-1.

In situ DRIFTS study of the adsorption–oxidation of CH3OH on V2O5

Abstract The oxidation of CH3OH on V2O5 has been studied from room temperature to 250°C. The reaction products were analyzed by on-line gas chromatography (GC) and the adsorbed species were characterized by “in situ” diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Both isothermal and non-isothermal experiments were carried out in order to identify adsorbed species in the oxidation of CH3OH on V2O5 catalysts. The interaction of CH3OH with theV2O5 surface results in the formation of methoxy groups and V–OH species as stated by DRIFTS. The infrared spectra suggest that more than one catalytic site are involved in the adsorption process. The formation of V–OH and the resulting reduction of the vanadia catalyst leading to V4+⎓O and V3+ groups is inferred from DRIFTS data and X-ray diffraction patterns of the used catalysts showing the presence of reduced oxide phases. As the reaction proceeds, the adsorbed methoxy species are oxidized to H2CO, formate species, partial oxidation products, CO and CO2.

Anomalous scattering study of oxide scales formed at 1173 K on surface modified stainless steel

The effect of lanthanum deposits on AISI 304 grade stainless steels has been studied by anomalous scattering, X-ray diffraction, Rutherford backscattering spectrometry (RBS) and scanning electron microscopy coupled with a energy dispersive spectrometer (SEM-EDX). The lanthanum deposits were obtained by the pyrosol method which implies the interaction at 473 K of an aerosol of lanthanum nitrate with the steel surface. The deposits enhance the corrosion resistance of the stainless steels specimens under synthetic air at 1173 K by a factor of three with respect of the non-treated steels. The main crystalline component of the protective oxide scale is demonstrated to be Fe2O3 . This result is explained on the basis of the oxide crystallinity and the chemical reactions at the alloy surface during the deposition procedure.