Vanessa Livramento

Consolidation of Cu-nDiamond Nanocomposites: Hot Extrusion vs Spark Plasma Sintering

Due to their interesting properties copper-based materials have been considered appropriate heat-sinks for first wall panels in nuclear fusion devices. The concept of property tailoring involved in the design of metal matrix composites has led to several attempts to use nanodiamond (nDiamond) as reinforcement. In particular, nDiamond produced by detonation has been used to reinforce copper. In the present study, powder mixtures of copper and nDiamond with 20 at. % C were mechanically alloyed (MA) and consolidated via hot extrusion or spark plasma sintering (SPS). The hardness evolutions as well as the structural characterization of as-milled nanocomposite powders and consolidated samples are reported. Density measurements indicate that the consolidation outcome varies significantly with the process used. Transmission electron microscopy (TEM) inspection of the extrusion consolidated sample revealed bonding at the interface between copper and nDiamond particles. The nDiamond size distribution was determined from TEM observations. The results obtained are discussed in terms of consolidation routes.

Bulk Copper-Nanodiamond Nanocomposites; Processing and Properties

Copper has widespread use as engineering material, because of its structural and functional properties, notably high thermal and electrical conductivity. A major drawback of this base metal and its alloys is a relatively low hardness. This precludes its utilization in applications in which both high conductivity and high strength/hardness are needed, e.g. in injection moulds for plastics. Nanostructured metals and nanocomposites are ways to address the low hardness problem, provided the nanostructured material is thermally stable during processing and service. In the present research, composite powders, with 5 to 30 at % nanodiamond, were consolidated into bulk samples. The copper-nanodiamond composite powders were vacuum encapsulated and extruded at 600°C. A significant proportion of the initial hardness in the powders is retained after extrusion. Transmission electron microscopy (TEM) of the extruded material indicates good bonding between the nanodiamond particles and the copper matrix. Raman spectroscopy on the consolidated samples evidences the presence of graphite, possibly due to partial disintegration of ultradisperse nanodiamond agglomerates.

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.

Novel Approach to Plasma Facing Materials in Nuclear Fusion Reactors

A novel material design in nuclear fusion reactors is proposed based on W-nDiamond nanostructured composites. Generally, a microstructure refined to the nanometer scale improves the mechanical strength due to modification of plasticity mechanisms. Moreover, highly specific grain- boundary area raises the number of sites for annihilation of radiation induced defects. However, the low thermal stability of fine-grained and nanostructured materials demands the presence of particles at the grain boundaries that can delay coarsening by a pinning effect. As a result, the concept of a composite is promising in the field of nanostructure d materials. The hardness of diamond renders nanodiamond dispersions excellent reinforcing and stabilization candidates and, in addition, diamond has extremely high thermal conductivity. Consequently, W-nDiamond nanocomposites are promising candidates for thermally stable first-wall materials. The proposed design involves the production of WAV-nDiamondAV-Cu/Cu layered castellations. The W, W-nDiamond and W-Cu layers are produced by mechanical alloying followed by a consolidation route that combines hot rolling with spark plasma sintering (SPS). Layer welding is achieved by spark plasma sintering. The present work describes the mechanical alloying processsing and consolidation route used to produce W-nDiamond composites, as well as microstructur al features and mechanical properties of the material produced Long term plasma exposure experiments are planned at ISTTOK and at FTU (Frascati).

Copper–micrometer-sized diamond nanostructured composites

Reinforcement of a copper matrix with diamond enables tailoring the properties demanded for thermal management applications at high temperature, such as the ones required for heat sink materials in low activated nuclear fusion reactors. For an optimum compromise between thermal conductivity and mechanical properties, a novel approach based on multiscale diamond dispersions is proposed: a Cu–nanodiamond composite produced by milling is used as a nanostructured matrix for further dispersion of micrometer-sized diamond (μDiamond). A series of Cu–nanodiamond mixtures have been milled to establish a suitable nanodiamond fraction. A refined matrix with homogeneously dispersed nanoparticles was obtained with 4 at.% μDiamond for posterior mixture with microdiamond and subsequent consolidation. Preliminary consolidation by hot extrusion of a mixture of pure copper and μDiamond has been carried out to define optimal processing parameters. The materials produced were characterized by x-ray diffraction, scanning and transmission electron microscopy and microhardness measurements.

W-Diamond/Cu-Diamond nanostructured composites for fusion devices

A novel material design for nuclear fusion reactors is proposed based on Cu-Diamond and W-Diamond nanocomposites. The proposed design involves the production of W/W-Diamond/CuDiamond/Cu functionally graded material. W, W-Diamond, Cu-Diamond and Cu nanostructured composite powders were produced independently by mechanical alloying and subsequently consolidated/welded through spark plasma sintering. Modulation of processing parameters allowed controlling the extent of unfavourable conversion of Diamond into tungsten carbide, as well as to overcome the Diamond intrinsically difficult bonding to copper. Microstructural features and microhardness of the as-produced materials are presented.

Preparation of dispersion-strengthened coppers with niobium carbide and niobium boride by mechanical alloying

The present study examines nanocomposites prepared by mechanical alloying of copper with other transition elements, which will produce a dispersion of stable boride and carbides reinforcement particles within a nanostructured copper matrix, at room temperature. Copper, niobium, boron and graphite powder mixtures were mechanically alloyed for several hours in a planetary ball-mill, in argon atmosphere and using a stainless steel container. The powder mixtures were produced with nominal composition of 10-30 vol.% NbC and 7-10 vol.% NbB2, using powders of pure elemental Cu, Nb, synthetic graphite and crystalline boron. The microstructural changes during milling of these powder mixtures were studied using X-ray diffraction, optical microscopy, scanning electron microscopy and microhardness measurements.

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].

Studies on deuterium retention in W-Ta based materials

** *IST/ITN, Instituto Superior Tecnico, Instituto Tecnologico e Nuclear, Universidade Tecnica de Lisboa, Estrada Nacional 10, P-2686-953 Sacavem, Portugal **Associacao Euratom/IST, Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal ***LNEG, Laboratorio Nacional de Energia e Geologia, Estrada do Paco do Lumiar, 1649-038 Lisboa, Portugal ****ICEMS, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal *****National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan ******CENIMAT-I3N, Departamento de Ciencia dos Materiais, Faculdade de Ciencias e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal The high melting point, high sputtering threshold and low tritium inventory rendered W as a potentially suitable material in fusion devices [1-4]. The major problem associated with presently available tungsten grades as structural materials is its brittleness at lower temperatures. This is further worsened by irradiation embrittlement. A strategy for ductility improvement is producing a composite, with the brittle W matrix being reinforced by short fibres of tantalum [5]. As Ta is more ductile than W it can therefore divert or stop cracks propagating in the W matrix. In the present research Ta short fibres and powder were used as reinforcement component for W [6] by alloying Ta short fibres or powder in a W powder matrix. The composites were subsequently irradiated with deuterium to assess the retention of this hydrogenic species in the materials. The irradiated composites, with Ta contents of 10 or 20 at%, were produced from pure elemental powders (W-Ta powder composites), and pure W powder and Ta fibre (W-Ta fibre composites) with 100 μm in diameter by low energy ball milling in argon atmosphere. These materials were consolidated via spark plasma sintering (SPS) in the temperature 1200 to 1600 oC range. Pure W and Ta plates (controls) and W-Ta composites were irradiated with He

Effect of Milling Energy Modulation on the High Temperature Synthesis of FeTi

Depending on the energy level used during mechanical alloying, the constitution of the resulting products can vary extensively. With high energy input, full transformation to the equilibrium phase, FeTi, is achieved. In contrast, for low levels of energy input, the process is akin to mixing without any phase transformation even for extended milling periods. In the present work, nanostructured FeTi powders were produced by mechanical alloying, avoiding the unfavourable agglomeration problem, by using a relatively low level of energy (e.g. 300 rpm) to mill the pure metallic constituents, Fe and Ti, followed by subsequent heat treatment at 800°C. A major achievement of this research was to show that, by modulating the milling intensity and total milling time, the high temperature synthesis reaction of FeTi (1100°C) can be partially or totally suppressed, reverting instead to a metastable reaction path at low temperature (650°C). The mechanical “activation” modifies the reactivity of the system, producing a very thin Ti /Fe layers. That in conjunction with a high level of defects induced mechanically may be responsible for the metastability. Partial substitution of Fe with Ni (10%) resulted essentially in the same phase constitution, indicating solid solution of Ni in FeTi replacing partially Fe lattice positions.