T. Pradell

Crystallisation kinetics and microstructure development in metallic systems

The primary crystallisation of a highly undercooled/supersaturated liquid is considered, and the application to nanocrystallisation by heat treatment of metallic glasses is studied from the thermodynamic, kinetic and microstructural point of view. The thermodynamic evolution is modelled assuming transformation rates low enough to ensure thermal equilibrium to be almost achieved. A mean field approximation is used, which allows us to determine the time evolution of the kinetic variables governing the transformation. The interplay between interface and diffusion controlled growth rate is studied, and both nucleation and crystal growth changes within the transformation are considered as soft mechanisms. The kinetics of the transformation is described in the framework of the Kolmogorov, Johnson and Mehl and Avrami (KJMA) model, which is adequately generalized for primary transformations. The microstructural evolution is described by a populational model, also based on KJMA. The predicted kinetic evolution results are compared to the experimental data on the primary nanocrystallisation of a FINEMET alloy.

Diffusion controlled grain growth in primary crystallization: Avrami exponents revisited

The anomalous behaviour of the Avrami exponents found in the primary crystallization of amorphous alloys leading to nanostructured materials is considered. A kinetic model able to adequately treat such phase transformation has been formulated by means of the implementation of a soft-impingement diffusion mechanism after a transient interface controlled growth. A decrease in the nucleation rate as crystallization proceeds has also been considered. Comparison of the model with experimental data is performed, giving excellent agreement. The soft-impingement diffusion mechanism is demonstrated to be responsible for the anomalous behaviour of the Avrami exponent, the decreasing nucleation rate being a second-order effect.

The colours of Ca-rich ceramic pastes: origin and characterization

The colour changes found in the wall thickness of archaeological calcareous ceramics are analysed. The main colour change from creamy to reddish is due to the incorporation of Fe3+ in several calcium silicates and aluminosilicates developed during firing. Iron appears as iron oxides (haematite and maghemite) and as structural iron, either in tetrahedral coordination related to the dehydrated clay minerals and to the melilites, or in octahedral coordination related to the pyroxenes of fassaite–diposide type. The creamy colour is related to the complete decomposition of calcite, a significant development of pyroxenes, and to the low presence of iron oxides. Haematite is determined across the creamy and reddish layers except in the outer surface, where maghemite is determined. The presence of maghemite and Fe2+ in the outer surfaces agrees with the use of a low reducing atmosphere at the end of the firing process.

Identification of copper-based green pigments in Jaume Huguet's Gothic altarpieces by Fourier transform infrared microspectroscopy and synchrotron radiation X-ray diffraction

The scientific investigation of ancient paintings gives a unique insight into ancient painting techniques and their evolution through time and geographic location. This study deals with the identification of the green pigments used by one of the most important Catalan masters in Gothic times, Jaume Huguet. Other pigments and materials have also been characterized by means of conventional techniques such as optical microscopy, scanning electron microscopy and Fourier transform infrared spectroscopy. Synchrotron radiation X-ray diffraction has been used to produce maps of phases at a spatial resolution of 100 microm across chromatic layers.

Identification of reaction compounds in micrometric layers from gothic paintings using combined SR-XRD and SR-FTIR

Synchrotron radiation X-ray diffraction (micro-SR-XRD) and Fourier transform infrared spectroscopy (micro-SR-FTIR) are used in the non-destructive identification of reaction and aging compounds from micrometric ancient painting layers. The combination of the micrometer size and non-destructive nature of the techniques together with the high resolution and brilliance of the synchrotron radiation has proved to be a procedure most advantageous for the study of reaction, aging and degradation processes. Copper, lead and calcium carboxylates and oxalates are determined in the chromatic, preparation and alteration layers from 15th century egg tempera and oil paintings. Their nature and crystallinity have been assessed. Some hypothesis about the mechanisms of development of both carboxylates and oxalates are presented.

Lustre Recipes from A Medieval Workshop in Paterna

The discovery of the workshop ‘Les Olleries Xiques’ in Paterna has provided a unique opportunity to analyse the raw materials, and in particular the recipes used in the production of the lustre decorations. Chemical and phase analyses of lustre raw materials and of lustre decorations belonging to the workshop pottery are shown. A comparison with existing ancient documentation on lustre recipes is also presented.

Non-random nucleation and the Avrami kinetics

Abstract The theory available for obtaining the time evolution of the transformation for a nucleation and growth process, the well-known Kolmogorov-Johnson-Mehl and Avrami kinetic equation, is not accomplished for non-random nucleation processes. However, non-random nucleation protocols are very common; for example in first-order phase transitions where position-dependent nucleation protocols arise from the modification of the nucleation rates in the regions around the growing grains. The present paper attempts to study the effect of these position-dependent nucleation protocols in the overall kinetics of the transformation. Monte Carlo simulations of such processes are performed, and the deviations of the resulting kinetics of the transformation from the Avrami equation are analysed in detail. Consideration of the different effects resulting from the nucleation protocol allows us to model the transformation. A modified Avrami equation is obtained and compared with Monte Carlo simulations, giving excellent agreement. The modelling procedure enables us to extend the validity of the Avrami formulation beyond its limits of applicability.

The growth of sanidine crystals in the lead of glazes of Hispano-Moresque pottery

Abstract The nature and the formation of minute sanidine crystals which appear in the sine of the glaze of some Hispano-Moresque pottery from a 14th century A.D. medieval workshop at the Testar del Moli, in Paterna (Valencia, Spain), were studied by scanning electron microscopy. The identification and refinement of the lattice parameters were carried out from the X-ray diffraction spectra that coincide with the above-mentioned feldspar. The chemical composition of the small crystals was studied by means of electronic microprobe, which revealed the presence of 5% lead joined to the lattice substituting the potassium positions. In conclusion, we deduce some technological implications concerning the process of manufacture, especially as regards the cooling of the melt phase.

KINETICS OF MICROSTRUCTURAL DEVELOPMENT IN NANOCRYSTALLINE MATERIALS

Abstract A quantitative determination of the microstructural evolution resulting from nucleation and growth of a primary phase is yielded. Recently developed kinetic models constitute a powerful tool when an appropriate physical description of the growth mechanisms which drive the process is implemented. To account for the high grain density developing in nanocrystalline materials, diffusion controlled growth with soft impingement has been modeled. Comparison between computed microstructures obtained with hard and soft impingement shows that the latter explains the delay of the transformation observed experimentally. Application to the nanocrystallization of a Fe-Si-B-Cu-Nb amorphous alloy (FINEMET) is presented. The observed kinetic evolution of both transformed fraction and microstructural quantities agree with computed values when soft impingement is considered. The influence of the annealing temperature on the resulting volume and surface grain size distributions is discussed.

The red–orange patina developed on a monumental dolostone

Orange and reddish patinas developed on a dolostone of the cathedral of Lleida (Spain) have been analysed and characterised by XRD, SEM/EDX, ICP/MS, optical microscopy and Mossbauer spectroscopy. From the experimental data, a biologically induced transformation of the dolomite rock forming crystals, which form a dolomitic patina of micrite size, can be stabilised. Furthermore, the iron oxides as determined by Mossbauer spectroscopy play a role as colouring patina, together with organic matter from the bioactivity. Erosive and constructive processes, as determined through the SEM observations, lead to aesthetical and microstructural changes in the stone surface.