Jorge Cruz Fernandes

Weibull statistical analysis of flexure breaking performance for alumina ceramic disks sintered by solar radiation heating

Densified alumina ceramic disk specimens were prepared through sintering compacted alumina powders by concentrated solar beam in inert Ar gas atmosphere and their flexure breaking behaviour was evaluated by the ring-on-ring jig test under application of equibiaxial stress. Slope m of Weibull plot for MOR (modulus of rupture) values of these specimens was comparable to that of the reference alumina disk specimens prepared by sintering in an electric furnace under comparable conditions (1600°C for 30 min) whereas mean MOR level of the former was slightly lower than that of the latter. Thus, the failure mechanism of the specimens sintered by solar radiation heating and that of the specimens sintered in the electric furnace were concluded to be practically indistinguishable. This evidence guarantees that alumina ceramic sintered bodies, with quality comparable to those obtained through traditional sintering process, can be prepared by solar heating in spite of quite fast rate of heating and cooling realised in the solar furnace.

Fracture toughness of solar-sintered WC with Co additive

A tungsten carbide (WC) cylinder specimen containing 10 mass% Co sintering aid was prepared under concentrated solar radiation (measured maximum temperature no higher than 1500 °C) in dynamic primary vacuum and its fracture toughness was evaluated by Vickers indentation method. In spite of very fast rates of heating and cooling applied in the solar-sintering process, fracture toughness of the prepared WC cylinder specimen was comparable to that of WC sintered piece prepared through a conventional industrial process in electric furnace under a slow heating and cooling condition. Present results together with the previously reported results for solar-sintered alumina ceramic disk appear to suggest the promising possibility of using solar radiation heating for manufacturing sintered ceramic components.

Photochemical Effects in Carbide Synthesis of d-group Transition Metals (Ti, Zr; V, Nb, Ta; Cr, Mo, W) in a Solar Furnace at PSA (Plataforma Solar de Almerı́a)

During the course of our recent series of solar carbide and carbonitride synthesis work During the course of our recent series of solar carbide and carbonitride synthesis work for d-group transition elements (Ti,Zr;V,Nb,Ta;Cr,Mo,W), we detected several intriguing evidences apparently indicative of involvement of photochemical effects in some reactions under solar radiation. 1) Acceleration of graphitization of amorphous carbon during carburization of W under solar radiation. 2) Promoted formation of high remperature mono-carbide phase of Mo under solar radiation at a temperature lower than the one indicated in equilibrium phase diagram. 3) Formation of sub-carbide Ta 2 C besides mono-carbide TaC in the carburization of Ta with amorphous carbon under solar radiation. Only mono-carbide TaC formed by the carburization of Ta with graphite under solar radiation in accordance with the equilibrium phase diogram. These features were already published individually elsewhere. This article is the brief summary of the reported facts in our earlier publications. Although the results so-far-obtained are rather primitive and our earlier publications. Although the results so-far-obtained are rather primitive and still lacking quantitative characterization, there seems to be no doubt that solar materials processing would open new routes for unique products which cannot be prepared through traditional industrial manufacturing processes.

Carbide formation of Va-group metals (V, Nb and Ta) in a solar furnace

Abstract In our recent series of solar carbide synthesis for Si and d-group transition metals (Ti, Zr; Cr, Mo, W) using graphite (G) or amorphous carbon (aC) as carburising medium, we observed some evidences indicative of involvement of photochemical effects in the reaction process. For example, (1) graphitisation of aC was accelerated during carburisation of W under solar radiation; (2) high-temperature mono-carbide phase of Mo (α-MoC1−x) formed at a temperature appreciably lower than the lower-threshold temperature of the stability range of the α-MoC1−x phase given in the available equilibrium phase diagram for Mo–C system. In this work, we investigated solar carburisation reactions for Va-group metals (V, Nb, Ta) in Ar or N2 gas environment. For this group of metals, only carbide phases formed even in N2 atmosphere without forming carbonitride like the cases with VIa-group metals (Cr, Mo, W). For carburisation of Ta with aC in Ar, an unexpected formation of sub-carbide Ta2C besides mono-carbide TaC took place, while Ta2C phase did not form through carburisation of Ta with aC in N2 or carburisation of Ta with G in either Ar or N2 atmosphere. No graphitisation promotion for aC was detected during the carburisation of none of the Va-group elements.

A Comparison between X-Ray Diffraction and Petrography Techniques Used to Determine the Mineralogical Composition of Granite and Comparable Hard Rocks

Petrography and X-ray diffraction techniques are used in this work to determine the mineralogical composition of a granite, a gabbro and a quartzite. The experimental difficulties and the results obtained by both methods are described and discussed. The semi-quantitative procedure of XRD analysis used in this work allows an estimate of the relative mineral phase composition of each type of rock. Petrography was found to be crucial in terms of textural analysis.

An adaptive temperature control law for a solar furnace

This paper describes the development of an adaptive control law based on exact feedback linearization and Lyapunov adaptation of the process dynamics applied to a solar furnace. The controller is tested on a 6kW solar furnace model that represents a plant installed at the Odeillo Processes Materials and Solar Energy Laboratory (Oriental Pyrenees in the South of France). The adaptive features allow to tackle the problems posed by knowledge uncertainty about furnace dynamics. It is concluded that the specifications related to material testing are met.

Corrosion Behaviour of Environmentally Friendly Treatments for Aluminium Alloys

In this paper an alternative anodising procedure, that confers a good base for painting and adhesive bonding of aluminium, is envisaged to replace the chromic acid anodising The substrate consisting of Al 2024-T3 is immersed in a cerium solution in order to create a cerium-containing film that covers the copper rich precipitates, followed by anodising in a boric acid-sodium borate/sulphuric acid solution. The results showed that this procedure confers good corrosion protection to the substrate and can potentially constitute an alternative to the Cr-VI based treatment. Introduction Painting and adhesive bonding of aluminium are essential technologies in many industrial sectors (e.g. aerospace industry) and each process depends on the generation of a strong and stable interface between an organic layer and the surface consisting of naturally formed metal oxide. Pre-treatments on the substrates, which leave the metal with an oxide structure that is stable and compatible with the organic layer, are vital to achieve the desirable levels of strength and reliable long-term performance and have been used for decades. However, they involve a range of environmentally objectionable chemicals, such as solvents and chromates, thus needing to be replaced. Several attempts have been made to replace the Cr (VI) – based treatments. In the present work the traditional chromic acid anodising was replaced by a boric acid-sodium borate/sulphuric acid bath. The alloy was pre-treated in a cerium nitrate solution, in order to form a film that covers the copper rich precipitates of the alloy, which impair good anodising. Experimental Aluminium alloy 2024-T3 coupons were used. The specimens were degreased with tricloroethylene (4 min), followed by etching in 3 g.l NaOH solution (8 min) and desmutting in 50% v/v HNO3 solution (30 sec). The anodising bath (ABS) consisted of a mixture of H2SO4 (15%) with a solution containing 0.5M H3BO3 and 0.05 M Na2B4O7.10H2O, in the proportion 70/30 (v/v). The optimised temperature and current density were respectively 40oC and 2 A.dm and the anodising time was 30 minutes. The choice of 40oC came from the fact that lower temperatures lead to higher coating thickness, which should be deleterious for fatigue resistance. The anodising was performed on as-etched surfaces and on surfaces also pre-treated by immersion in 0.01M Ce(NO3)3 solutions for 4 hours. After anodising the specimens were sealed in boiling distilled water for 30 minutes. However, before sealing the specimens were observed on a field emission scanning electron microscope (SEM), on the surface and in a section almost perpendicular to the surface. This latter observation also allowed the determination of the coating thickness. Since the fatigue is an important property of these materials, fatigue tests were carried out on anodised specimens. For comparison, fatigue resistance was also obtained on as received and chromic acid anodised specimens. The alloy was submitted to tensile-tensile fatigue tests in air and at the environmental temperature, in order to obtain S-N plots. The tests were carried out according to ASTM E 466-82 [1], using an INSTRON 8502 servo-hydraulic machine. The corrosion behaviour was studied in 3% NaCl solution by Electrochemical Impedance Spectroscopy, using a Solartron 1250 frequency response analyser connected to the cell via a Solartron 1286 electrochemical interface. Results and Discussion Figure 1a shows a SEM photograph of the transverse section of the specimen anodised in the ABS. It allows the determination of the coating thickness that is approximately 2.5 μm. Higher magnification micrograph (160 000 ×, Fig. 1b) shows in more detail the structure of this coating. Linear continuous pores perpendicular to the specimen surface, as those reported in the literature for anodised aluminium, cannot be seen. The coatings have a grain like structure with the grains separated by pores. The grains seem to have grown preferentially in one direction, revealing a lamellar shape. Coatings formed after pre-treatment in cerium solutions were also observed by SEM. The structures observed were similar to those previously reported, apart a finer structure with smaller grains. The presence of a barrier layer in contact with the metallic surface could not be detected in both cases, which indicates that it should be very thin. a) b) Figure 1. Micrographs of transverse sections of anodised Al 2024-T3 in ABS: a) 20 000×.; b)160 000 × The impedance results for 1 day and 7 days immersion in 3% NaCl solution for ABS anodising are shown in Fig. 2a. The impedance values do not change significantly with time, which reveals a good corrosion performance of the specimen. As a plateau is not usually observed at low frequencies for anodised samples that show even pitting or pronounced corrosion, practical criteria have been established to assess the corrosion behaviour of this material [2-4]. Most of them are based on the value of the impedance in the low frequency region. Thus, according to Mansfeld and Kendig [2], a damage function, D, which is defined as

Comparison between Cobalt and Niobium as a Matrix Component for Diamond Impregnated Tools Used for Stone Cutting

This investigation examines the possibility of adding niobium (Nb) to the Fe-Cu system in order to obtain, after powder compaction and sintering, a binder material (bonding matrix) with the adequate wear properties needed for the fabrication of diamond impregnated tools for stone cutting. The main objective is the replacement of cobalt (Co) by Nb in the Fe-Cu-Co alloys that are currently used in diamond tools. For this purpose, a comparative study has been conducted on some selected compositions of matrices integrated in cutting discs. Results of diamond impregnated 25wt%Fe-50wt%Cu-25wt%Nb and 25wt%Fe-50wt%Cu-25wt%Co matrices are directly compared because all tests were conducted under similar conditions. Two types of diamonds were used: without coating and Ti-coated. After the hot-pressing cycles, the main mechanical properties of the sintered bodies were evaluated. Cutting tests under real conditions, in Porriño granite, were carried out to compare the performance of the tools. Although the mechanical properties of the matrices with Nb were different from those found in the matrices with Co, the results obtained from the cutting tests have revealed that the tools made of 25wt%Fe-50wt%Cu-25wt%Nb and 25wt%Fe-50wt%Cu-25wt%Co matrices, both with 50/60 mesh diamonds, show quite similar behaviour during the cutting operations, thus indicating that replacement of Co by Nb is a promising route to be followed in the near future.

Evaluation of the Relationship between Diamond Tool Wear Performance and the Mechanical Properties of the Individual Metallic Binders

A test methodology is employed to investigate the cutting behaviour of five different diamond impregnated tools for cutting hard materials, such as rocks. A set of parameters, which characterise the tool wear (specific wear loss in the tool), and its relationship with the cutting force of the examined tools, are established to evaluate the tool wear performance. The procedure established in this work describes the specifications of the equipment used to carry out the tests, the different cutting conditions, the format of the output parameters and the characterisation of the different materials used (binders and rocks), hence allowing to discuss the overall cutting wear behaviour of the tools. The methodology presented indicates a universally applicable procedure for measuring the wear performance of the diamond tools as, at the same time, it establishes a relationship between wear and the mechanical parameters of the different metallic binder materials used to manufacture the correspondent tools. The procedure is proven to be an indispensable instrument for correctly carrying out wear performance tests and for reliably interpreting the wear mechanisms of the tool.

Surface Singularity Upon Solar Radiation Heating of Graphite/Tungsten Powder Mixture Compacts to Temperatures in Excess of 1600°C

Synthesis of single-phase tungsten sub-carbide W2C was attempted by heating pellets made out of a source of carbon (graphite-G) and W powders with G/W atom ratio between 0.35 and 0.50 to two target temperatures, namely 1600°C and 1900°C in an argon atmosphere using a solar furnace at PROMES-CNRS in Odeillo (France). The results showed that synthesis of single-phase W2C phase was difficult at either target temperature yielding the W2C co-existed with free metallic W. It was noted that the thin top surface layer of the solar-synthesised tungsten carbide pellets heated to 1900°C was distinguishable from the rest of the bulk specimen showing localised growth of nano-meter scale WC whiskers over W2C grains. Detailed XRD (X-ray diffraction) results on the effect of both G/W ratio and temperature on W2C lattice parameters are discussed.