J.S. Moya

The antibacterial and antifungal activity of a soda-lime glass containing silver nanoparticles

The antibacterial and antifungal activity of a low melting point soda-lime glass powder containing silver nanoparticles has been studied. Nano-Ag sepiolite fibres containing monodispersed silver nanoparticles (d(50) approximately 11 +/- 9 nm) were used as the source of silver. This powder presents a high antibacterial (against gram-positive and gram-negative bacteria) as well as antifungal (against I. orientalis) activity. The observed high activity against yeast has been explained by considering the inhibitory effect of the Ca(2+) lixiviated from the glass on the growth of the yeast colonies.

Elastic/plastic indentation in ceramics: a fracture toughness determination method

An extensive literature survey of the evolution of the indentation technique is made. The elastic/plastic solution for the spherical cavity expansion is used to obtain a physical explanation for the quantitative measurement of the radial cracks that occur at the indentation impression corners. Finally an expression based on the mentioned theory is given. Experimental data from a serie of brittle materials are well fitted by this expression.

Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO2 composites with increasing amount of ZrO2 (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheKIC values increase from 2.1 to 3.2 MN m−3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO2 increases to reach ∼0.1 in the sample with 0.2 volume fraction of ZrO2. The presence of ZrO2 enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO2 composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (∼0.5 wt %) of ZrO2 in mullite.

Kaolinite-mullite reaction series: a TEM study

The kaolinite-mullite reaction series in single crystal kaolinite has been characterized by transmission electron microscopy. The exotherm observed at 980° C is attributed to the formation of a spinel phase. Mullite crystallites have also been observed with the spinel phase and both phases have a composition near that of 3∶2 mullite. Subsequent heat treatment leads to the growth of mullite crystallites on the original kaolinite plates with the c-axis perpendicular to the plate.

Multicomponent toughened ceramic materials obtained by reaction sintering: Part 1 ZrO2-Al2O3-SiO2-CaO system

Fully dense zirconia toughened ceramics with a mullite matrix from the basis of information on the quaternary system ZrO2-Al2O3-SiO2-CaO, in a temperature range as low as 1425 to 1450° C, have been obtained by reaction sintering of zircon/alumina/calcium carbonate mixtures. The shrinkage, advance of reaction, microstructure and mechanical properties of different compositions are reported. The results are explained in terms of a transitory liquid phase that appears at temperatures lower than 1400° C.

Powder processing of mullite/Mo functionally graded materials

Graded or layered materials have been used for many years in specific applications to improve reliability for bonds between dissimilar materials, for improved coating adherence and to obtain better compatibility between fibers and matrices in composites. Recently, the concept has been formalized, and such materials entitled FGMs. Molybdenum and mullite are two ideal candidates to fabricate ceramic/metal composites and FGMs because of their similarity in thermal expansion, lack of reactivity at high temperature and high refractoriness. In the present work a systematic investigation of powder processing for the fabrication of mullite/Mo FGMs is described. It was found that the processing of mullite/Mo in water resulted in a formation of a homogeneous composite, whereas mixtures processed in ethyl alcohol produced FGMs with continuous gradient. Compositional gradients, fracture toughness, hardness and electrical conductivity of the FGMs were also evaluated.

Effect of ZrSiO2 and MgO additions on reaction sintering and properties of Al2TiO5-based materials

Two sets of Al2TiO5-based composites were prepared by reaction sintering of (a) Al2O3/TiO2/ZrSiO4 and (b)Al2O3/TiO2/ZrSiO4/MgO powder mixtures. The influence of the variation of ZrSiO4 content (0 to 10wt%) and the addition of 2 wt% MgO on the reaction-sintering process, microstructure, mechanical and thermal properties, were evaluated. ZrSiO4 addition shifted the Al2TiO5 formation to higher temperatures, whereas MgO accelerated both Al2TiO5 formation and ZrSiO4 decomposition. The presence of ZrSiO4 and an excess of Al2O3 generated a dispersion of ZrO2 and mullite particles in the grain boundaries and enhanced simultaneously the densification process. After sintering in the temperature range 1350 to 1500 ° C, the obtained composites exhibited significantly higher bending strength than the monophasic aluminium titanate (up to 80 M Pa). Al2TiO5 (80wt%)-mullite-ZrO2 composites which combined good mechanical strength (55MPa), low thermal expansion (α20−1000C < 1 × 10−6 K−1) and excellent thermal stability were obtained by reaction and sintering of powder mixtures containing both ZrSiO4 and MgO.

Mullite formation in kaolinite-α-alumina

Abstract The primary and secondary mullite formation in high purity kaolinite-α-alumina have been studied by conventional processing, XRD, TEM, AEM, and optical microscopy. After 4 h firing at 1300°C or below, only monosized primary mullite was formed in platy kaolinite, leaving α-alumina largely inert. Bimodal mullite crystals demonstrating secondary mullite nucleation and priamry mullite growth were observed after 4 h firing at 1400°C. At 1400°C and above, the rapid kinetics of secondary mullite formation, the characteristics of the co-existing glassy phase and the microstructure, as well as the evidence of liquid phase sintering strongly that solution-precipitation via a transitory liquid phase was the major mechanism for secondary mullite formation. Secondary mullite formation via inter-diffusion, though also observable, was considered as a minor route in the kaolinite-α-alumina reaction.

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO2-Al2O3-SiO2-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at ≤1425° C and ~ 1450° C respectively.

Synthesis of nanocrystalline yttrium disilicate powder by a sol-gel method

Abstract In the present work a new route for the synthesis of α-Y2Si2O7 is reported. The method consists in the gelation of an acid solution composed by Y(NO3)3 and tetraethylorthosilicate (TEOS) in the adequate molar ratio to form a precusor complex gel containing the Y3+ and Si4+ cations in the 1:1 ratio, which yields the stoichiometric α-Y2Si2O7 after calcining. The nanosize powder of Y2Si2O7 obtained at 1060°C was characterised by XRD and TEM. The conditions and main parameters such as temperature, pH or time for the hydrolysis of the Y(NO3)3/TEOS solution in HCl medium was investigated and optimised to obtain the precursor gel which was analysed by DTA and TGA.