Luís Guerra Rosa

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.

Temperature control of a solar furnace for material testing

A solar furnace is a thermodynamic device that concentrates sun radiation in order to achieve high-temperatures at a focus, where a sample of the material to be tested is located. This article address the problem of designing a control architecture for solar furnaces. It is motivated by the use of a solar furnace as an instrument in material science research to perform two types of experimental tests. In the first type, samples are tested in high-temperature stress cycles. In these experiments it is important to control the temperature profile. The second test type is characterised by imposing a solar radiation flux profile. On both experiment types there are strong nonlinear effects, a significant degree of parametric uncertainty and disturbances, such as solar radiation fluctuation due to clouds and to suns apparent movement. Although the actuator (shutter) is much faster than the thermic system, it may destabilise the overall controlled system. The contribution of this article consists in the design of a control architecture for solar furnaces and the application of singular perturbation methods in order to derive conditions in the form of bounds for the speed of actuator dynamics so that the closed-loop system is stable.

Effect of Paraffin Oil Used as a Lubricant in Tensile Tests of Carbon Fibre Bundles

The failure of carbon fibre bundles of Tenax 5131 HTA with 6000 filaments is characterised using the European Standard EN 1007-5. It is shown that the addition of liquid paraffin to the bundle causes a global change of the load-displacement curve and the effects of the presence of a liquid lubricant are discussed.

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.

Comparison of Young’s Moduli of Engineered Stones Using Different Test Methods

This work reports the results of Young’s modulus of elasticity obtained for 3 types of engineered stones. Using parallelepiped specimens with dimensions 150  30  20 mm3, Young’s modulus is determined by different methods: static and dynamic. Via quasi-static deformation tests: - uniaxial tension, - uniaxial compression, and, - pure bending (i.e. symmetrical four-point bending), determination of Young’s modulus was carried out by means of the conventional electric-resistance extensometry with strain-gauge strips glued to the specimens. The results obtained from these quasi-static deformation tests are compared with the results of dynamic Young’s modulus obtained with RFDA equipment (Resonant Frequency & Damping Analyser) using the parallelepiped specimens in a flexural vibration mode. Dynamic Young’s modulus was also evaluated through measurements of ultrasonic pulses velocity. Composition and microstructures of the materials under investigation are also presented and discussed.

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.