G. Carotenuto

A Review of Ceramic Sintering and Suggestions on Reducing Sintering Temperatures

Theories and applications associated with sintering of ceramics and reducing of sintering temperature are reviewed. The whole sintering process is divided into three sub-processes as powder preparation, compaction and sintering, and each sub-process is discussed in terms of its possible contribution to the reducing of sintering temperature. New approach for practical optimization of sintering process is investigated. The application of above in the Low Temperature Co-fired Ceramics (LTCC) is discussed. Meanwhile, many successful applications in reducing the sintering temperatures are presented.

Degradation of SiC particles in aluminium-based composites

The technological process used for the production and processing of metal matrix composites (MMCs) can require contact over extended periods of time between matrix and the ceramic reinforcement or at least the permanence at high temperature and pressure of the two parts in contact. During the contact a chemical interaction takes place in the interfacial zone as a consequence of the free-energy difference existing between ceramic compound and metal. Aluminium and aluminium alloys reinforced with silicon carbide are widely utilized materials. The chemical interaction between matrix and reinforcement is not very fast but the reaction equally occurs, and a harmful layer of interfacial compound (Al4C3) is developed after a sufficiently long time. At present, the degradation of the reinforcement produced by molten matrix is the major problem for some production technologies. This problem has only been solved partially by using a coating or changing the chemical nature of the matrix. In particular, the technological problem of interfacial reaction in the SiC-Al system can be solved by adding elemental silicon to the matrix to achieve the eutectic composition. However, this expedient gives rise to a consequent significant lowering of the melting point. The problem can be overcome and the production process improved without changing the characteristics of the material by the control of processing parameters. The interfacial reaction also produces elementary silicon and this has been found as aluminium-silicon eutectic segregated at the aluminium grain boundary. An accurate description of the kinetic process can be obtained by determining the silicon content present in the matrix by original derivations obtained by means of calorimetric analysis.

A method for the preparation of PMMA-SiO2 nanocomposites with high homogeneity

It is possible to optimize the performance of the inorganic-organic composites dispersing the inorganic component in the organic matrix on a nanomiter length scale. If dry the inorganic phase cannot be intimately dispersed during the incorporation in the matrix. When the particle surface is organically modified, and the incorporation is made starting from a liquid dispersion (particles in polymer solution), the resulting composites exhibit an excellent homogeneity. Here, monolithic [poly(methyl methacrylate)/monodisperse silica particles] nanocomposites have been prepared and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), micro-hardness, and differential scanning calorimetry (DSC).

Fabrication and properties of carbon fibre-reinforced copper composite by controlled three-step electrodeposition

Fabrication and properties of continuum carbon fibre-reinforced copper materials by controlled three-step electrodeposition are described. The effects of processing parameters, i.e. hot-pressing temperature, pressure and duration, and reinforcement content, on the properties of composites are discussed. An alloying element, nickel, was introduced into the C/Cu interface to enhance the composite strength. Finally, the relatively optimum production parameters (i.e. 30 wt% fibre, 700°C, 40 min and 10 MPa) are suggested.

Synthesis and characterization of new polymer-ceramic nanophase composite materials

Optically transparent nanocomposites are of special interest since they have great potential in optical applications. The synthesis of poly(methacrylic methylester)/monodispersed Cu2(OH)2CO3 particles, a new nanocomposite material with a refractive index similar to glass, is here described. The system can be easily deposited on glass plates by spin-coating technology, yielding homogeneous, low-defect thin films of high transparency, that can be used as color filters for liquid crystal displays. Differential scanning calorimetry (DSC), dynamic light scattering (DLS), scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersive spectroscopy (EDS) have been used to characterize the composite material.

Dense/Porous Layered Hydroxylapatite Ceramic for Orthopedic Device Coating Prepared by Tape Casting Technique

A possible surgical technique in the replacement of a traumatized hip joint by a prosthesis system is to connect the acetabular component of the implant directly with pelvis bone tissue, without use of bone cement. It is possible to improve the osteointegration process and to ensure a better connection with bone tissue by coating the outside implant surface with a biocompatible ceramic. The best choice for a bioceramic coating is porous hydroxylapatite because its surface shows bonding-osteogenesis properties much higher than other materials. Here, a double HAp layer has been made by tape casting technology. The first layer was a high porous HAp ceramic with high osteophilic-osteoconductive characteristics. Because the scale of porosity was relatively insensitive to slurry composition and sintering temperature such a microstructure was produced using a particular technique described here. The second layer was dense HAp ceramic that resulted a substrate able to improve the mechanical properties of the brittle porous HAp layer. Several microstructure-designed ceramic coatings having the porous part with a controlled porosity can be obtained by tape casting using the same technique.

Preparation of ?-alumina precursors for metal reinforcement

In the metal matrix composite field, both fine (<0.5 µm) and coarse, crystalline, calcined aluminas with narrow or broad particle size distributions, are being used as reinforcement phase. The effect was studied of the preparation method of fine boehmite (γ-AlOOH), precursor of the γ-Al2O3, on the product morphology. The material was produced by heating three kinds of hydrothermal precursors at different pH, and for 2 h at 200°C, using constant stirring. Under the conditions investigated (i.e., 0.12 mol/dm3 of Al(OH)3 and only diluted NaOH and HNO3 solutions used to adjust the pH of slurry), the pH of hydrothermal slurry influenced the product morphology; in contrast, the three different hydrothermal precursors, namely dry aluminum hydroxide gel, fresh aluminum hydroxide precipitate and gibbsite reagent powders, had only a little effect on the product morphology. The dehydration/transformation mechanism from Al(OH)3 to γ-AlOOH is believed to be dissolution/reprecipitation rather than a direct dehydration.

Stability of nickel coatings on carbon fiber preforms: A SEM investigation

Metal Matrix Composites (MMCs) reinforced with continuous fibers were generally fabricated by a foil-sandwich technique or by liquid metal infiltration. Liquid metal infiltration may be used to cast final shapes in molds containing fiber preforms. It is also used to make composite wire from which may be fabricated panels and shapes by hot-press diffusion bonding or pultrusion. The major drawback of this method is that the molten matrix must wet the fiber for successful infiltration to occur, requiring special fiber surface treatments or matrix additives, and that, molten metals generally dissolve or degrade the fibers, necessitating a barrier coating on the fibers. All these problems can be solved using carbon fibers coated with metallic layers, e.g. nickel. This work analyses an easy method to produce modified carbon fibers by electroplating and the process of its recristallization. The topography of the growth front of the deposit has been studied. At temperatures higher than about 300° C an annealing under vacuum is required, because of the high reactivity of metal coating, nevertheless the heat treatment of metal deposit produces always an embrittled material.

Fractal Analysis and Simulation of Surface Roughness of Ceramic Particles for Composite Materials

An analytical model is presented for the determination of the fractal dimensions of particles which are widely used as reinforcement in nanocomposites. The model is used to characterize the surface irregularity or roughness. It was found that fractal dimensions of both the contour and surface of particles depend only on the relative particles size ratio between secondary particles and subunits. It is proposed that, in practical applications, the fractal dimension of a certain reinforcement particle can be obtained by a combination of this model and a state-of-the-art instrument that can determine the sizes of primary and secondary particles by image analysis. It is possible to relate the fractal dimension with the adhesion and other physical and chemical properties at the interface between particles and matrix.

Erosion of reinforcement particles in SiC/aluminum composites

The granulometrical and morphological variations of eroded reinforcement particles during annealing for different times at several temperatures have been studied for SiC-pure Al system. The changes produced have been analysed by scanning electron microscope (SEM) after particle extraction.