Isabel M. Martins

Manufacture of Ceramic Products Using Inertized Aluminum Sludges

The aim of the present research work was to study the effect of mixing aluminum hydroxide sludges with wastes resulting from the cutting and polishing of dimensional stones, which have high content of alumino-silicates, in order to develop mullite. This study shows that, after sintering different mixtures to temperatures up to 1300oC, there is an important increase of secondary mullite phase in the resultant material, particularly in the mixes 2:1 (alumina:silica ratio) simultaneously with a significant decrease of silica phases. The presence of important quantities of -alumina was also detected with the increasing of sintering temperature. Therefore the product obtained after sintering was a composite of mullite and alumina. The mullitisation behaviour was studied using X-ray diffraction and microstructural analysis, which have confirmed the increase of mullite attaining a maximum when the sintering temperature was 1270oC. The composite formed during sintering was responsible for a flexure modulus higher than 100 MPa, with a Weibull Modulus typical of technical ceramics, without degrading other properties, like water absorption. The new developed material was found to be inert after leaching tests carried out according to DIN standard 38414.

Ni4Ti3 Precipitation during Ageing of MARES NiTi Shape Memory Alloys Studied by FEG-SEM

Martensitic transformation determines almost all important properties of NiTi shape memory alloys (SMAs), including shape memory effect. Moreover, any compositional inhomogeneities in the matrix of NiTi SMAs affect the martensite start temperature, Ms, because it depends on the concentration of Ni in the matrix: for Ni content exceeding 51.5 at.% Ms is lower than -200 oC [1]. There is a very good way to adjust the transformation temperature of NiTi alloys for Ni-rich alloys even after the alloys have been made. It is possible to use ageing treatment to rectify the transformation temperature due to incorrect initial composition of Ni-rich NiTi SMAs. The principle behind this method is the (metastable)-equilibrium between TiNi and Ni4Ti3 precipitates since the formation of those Ni-rich precipitates affects the composition of the retained matrix [1-2]. Although Ni4Ti3 is considered as a metastable phase compared with the equilibrium Ni3Ti precipitate, it is quite stable at temperatures below 600 oC and under normal ageing condition only Ni4Ti3 is observed. The ageing temperature and time dependence comes from the evolution of the density and size of Ni4Ti3.

X-Ray Diffraction Study of NiTi Produced by Mechanically Activated Reactive Extrusion Synthesis (MARES)

This study reports the use of X-ray diffraction quantitative phase analyses in NiTi alloys produced by MARES (Mechanically Activated Reactive Extrusion Synthesis). These analyses were performed with the PowderCell 2.4 software. The mechanically activated powders heated in a DTA furnace at 500 °C had as main phases Ni (27 wt %) and Ti (30 wt %) and the major intermetallic phase was Ni3Ti (20 wt %). Above 500 °C the intermetallic phases were predominant. At 600 °C the major phase was Ni3Ti (29 wt %) and at 700 °C was NiTi2 (32 wt %). In this temperature range the NiTi was a minor intermetallic phase (14-20 wt %). No changes in the constitution or in the amount of the phases were detected between the degassed powder samples and the extruded materials. The intermetallic phases were always predominant and the major was Ni3Ti (27-32 wt %). The NiTi phase content was in a range of 15-22 wt %. The weighted residual error, Rwp, of the fittings ranged between 17 and 27. Using the Williamson and Hall plot, crystallite sizes within the range of 26-53 nm and of 12-25 nm were evaluated for the metallic and intermetallic phases, respectively. Vickers micro-hardness measurements were virtually unchanged with the extrusion parameters but increased relatively to the mechanically activated powders.

Role of oxygen and nitrogen in mechanical alloying mechanism of Ni–Ti powder mixtures

Abstract In this study, the critical milling behaviour of Ni–Ti powder mixtures was evaluated in relation to the effect of atmospheric gases, more specifically to oxygen and nitrogen. Within the experimental conditions used, it is shown that both gases play an important role in the alloying process and that not only oxygen gas reacts with the mechanically alloyed powders but also nitrogen. The most effective mixing occurred for the mixtures with the highest contaminant contents.

Magnesium Removal from TiC-TiB2 SHS-Powders by Controlled Hot Acid Leaching

TiC and TiC–TiB2 powder mixtures obtained directly from titanium (TiO2) and boron (B2O3 or B4C) raw-materials by reduction with magnesium by self propagating high temperature synthesis (SHS) contain, as impurity, large quantities of MgO under its periclase form, together in some cases with unreacted magnesium. Several leaching agents, namely hydrochloric acid, sulphuric acid, acetic acid and EDTA were tested aiming at removal of magnesium from these powders as required characteristic for further work. Several parameters as leachant concentration, pH, reaction time and temperature were evaluated. Alternative leaching methodologies were compared in order to achieve magnesium removal yields over 98% and minimising at the same time the expected high matrix losses due to TiC and/or TiB2 co-solubilisation. As main conclusion it was established that strong hydrochloric acid (6M) is the most efficient medium to remove magnesium from these particular TiC-TiB2 SHS mixtures. The leaching methodology used (controlled hot acid leaching under close conditions) allowed to minimise TiC and TiB2 solubilisation losses. Sulphuric acid is not an effective leaching medium and contaminates the resulting powder mixture with unfriendly sulphur. Using acetic acid, magnesium removal yield is low and titanium losses are considerable. On the other hand, low aggressive EDTA complexant leaves TiC-TiB2 matrix unalterable but residual MgO remains over 6%. The proposed process seems to have potential for application in the general field of semi-micro materials purification.

New Ceramic Products from Slate and Aluminium Sludge Wastes

Previous studies had shown that a promising material could be obtained by mixing aluminium-rich sludge with slate powder (with Al2O3:SiO2 molar proportion of 2:1), shaping by uniaxial pressing and sintering at 1300oC, in order both to detoxify the sludge by fixing the metals in a leach-resistant ceramic matrix and to yield a new material from the reactions of the above compounds at high temperature. In view of potential applications, since this material was also found to be inert after leaching tests, a further systematic study, reported in the present paper, was carried out, in order to determine its mechanical properties and discuss them in function of the microstructure. The average values of Vickers hardness HV3 (683), bend strength (126 MPa), Youngs modulus (161GPa) and fracture toughness (3.35 MPa.m1/2) are significantly higher than those found for the as sintered slate powders, which is attributed to the increased fraction of α- alumina and reduced content of glassy phase in the microstructure of the material with sludge addition.

Characterization of Cu2ZnSn(SSe)4 monograin powders by FE-SEM

The design and synthesis of high-efficiency materials to convert solar to electrical energy is an increasingly important research field. Within the photovoltaic technologies, crystalline Si have an 80% share while the remaining 20% are mostly thin film solar cells based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe [1,2]. However, the cost, the abundance and the environmental impact of the elemental components cannot be neglected. For these reasons, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and their solid solutions CZTSSe has attracted much attention recently since they can provide the development of cost competitive solar cells. The CZTS-based solar cells consist of earth abundant and relatively inexpensive elements and represent an environmentally friendly alternative compared to the above mentioned systems [3]. The energy conversion efficiency of the CZTS-based solar cells has increased from 0.66% in 1996 to 11.1% recently [4].

Powder Metallurgical Processes for NiTi Shape Memory Alloys

Two promising powder metallurgy (PM) processes were used for the fabrication of NiTi shape memory alloys (SMA): Mechanically Activated Reactive FOrging Synthesis (MARFOS) and Mechanically Activated Reactive Extrusion Synthesis (MARES). In these two processes, equimolar powder mixtures of elemental Ni and Ti are first mechanically activated and then forged/extruded at relatively low temperature. Afterwards, heat treatments are used to promote homogenization and to adjust the composition of the NiTi matrix. When MARFOS and MARES processes are compared some differences have been observed but only in relation to the extent of phase transformation and to the degree of densification. The crystallite size was less than 100 nm for all the phases which indicates nanostructured materials and multi-step martensitic transformations could be observed in heat treated materials.

Sintering M3/2 High Speed Steel Powder by DMLS Process

The Direct Metal Laser Sintering (DMLS) technology uses a mixture of metal powders with different melting points to build objects layer by layer, directly from the geometric digitalised information. This process allows the manufacturing of prototype and production tools. In this study, the M3/2 high speed steel powder blended with 20 wt.% Cu3P and 0.25 wt.% graphite was laser-sintered, using two scan speeds (100 and 200 mm/s), keeping constant both hatching (0.30mm between two consecutive lines) and laser power (180W). The powder was spread in uniform layers of about 20m over a steel plate (100x60x6mm). The laser beam scanned small areas (12x15mm) in a single direction (OX). The surface morphology of the laser-sintered material shows that all material melted, but for 200mm/s scan speed, strings are well defined. This is probably due to a lower level of energy supplied to the material. The microstructure of the sintered material was studied in the longitudinal and transverse sections, to evaluate the consolidation process and layer growth. The material showed porosity and cracks formed during the process.