Valentina Medri

Insights into the macroporosity of freeze-cast hierarchical geopolymers

Geopolymer monoliths with controlled lamellar macroporosity and total porosity ranging from 60% to 70% were prepared by ice-templating a partially geopolymerized slurry. Both the maturation treatment of the starting mixture and the water specifically added for freeze-casting were tailored to modify both the geopolymerization and viscosity of the slurry, and, consequently, its freezing behavior, in order to optimize the final lamellar architecture. Following a room temperature maturation treatment, a 50% water content added for freezing developed thick lamellae and wide pores. A lower water content (30%) and curing at 80 °C after maturation at room temperature (for both 50% and 30% H2O) was conducive to a narrow lamellar pore width distribution in the 30–130 μm range. However, the consumption of water due to geopolymerization in samples cured at 80 °C led to a decreased length and thickness of the lamellae. Lastly, the interparticle meso- and macropores (0.003 to 1 μm) within the geopolymer lamellae were only slightly modified by the maturation treatment.

Sr, Mg cosubstituted HA porous macro-granules: Potentialities as resorbable bone filler with antiosteoporotic functions

Porous macro-granules of nanostructured apatite with Ca ions partially cosubstituted with Mg and Sr ions in different ratios (SrMgHAs), were synthesized at 37°C and compared with Mg and/or Sr free apatites (MgHAs and HA). Strontium improved the Mg substitution extent in the apatite and the chemical-physical and thermal stability of the resulting cosubstituted apatite. Porous macro-granules of 400-600 micron with selected composition were tested for the ionic release in synthetic body fluid and the data were related with the results of preliminary cell investigation in vitro. As compared to the corresponding Sr-free granulate, the SrMgHA could be exploited to prolong the beneficial Mg release during the bone regeneration process. In addition the contemporary in situ supply of Sr, an antiosteoporotic and anticarie ion, could influence the quality of new hard tissues. The ionic multirelease created a more favorable environment for human osteoblasts, demonstrated by a proliferative effect for each dose tested in the range 0.1-10 mg/mL.

Doped calcium–aluminium–phosphate cements for biomedical applications

Calcium–aluminium–phosphate cements (CAPCs) for biomedical applications, mainly intended for applications in the dental field as non-resorbable fillers, were obtained by reacting Ca-aluminates compounds, i.e. CaO·Al2O3 (CA) and CaO·2 Al2O3 (CA2), with Al(H2PO4)3 aqueous solution. Hydroxyapatite was also introduced as a bioactive dispersed phase. Suitable elements like Sr and La were used to increase the radiopacity of the set yielded pastes towards X-ray wavelength used in clinical diagnostic radiographic equipments. La and Sr doped Ca-aluminates powders have been synthesized by solid state reaction at 1,400°C from a mixture of CaCO3, Al2O3, La2O3 and SrCO3. The characteristics of the obtained powders were analyzed and related to the starting compositions and synthesis procedures. The microstructure, setting time, radiopacity and compressive strength of the CAPCs have been investigated and discussed.

Oxidation effects on mechanical and electrical properties of electroconductive ceramic composites

The effects of long term oxidation (up to 100 h) in the temperature range 600-1500°C on mechanical strength and electrical resistivity are examined in different types of composites: AlN-SiC, AlN-SiC-MoSi 2 , Si 3 N 4 -MoSi 2 and Si 3 N 4 -TiN. The degradation of strength and of electrical conductivity are related to the characteristics of the oxide scale and of the sub-surface damage after oxidation.

Corrosion of Electroconductive AlN-SiC-MoSi2 Composite in NaOH Solution

The corrosion behaviour of an electroconductive AlN-SiC-MoSi2 ceramic composite, hot pressed with the addition of Y2O3 as sintering aid, was studied in 4 M NaOH solution at R.T., 40°C and 70°C up to 400 hours. Both the measured weight loss and the degree of dissolution of each element (Si, Al, Mo, Y), analysed in the solutions after the tests indicate that the corrosion follows linear kinetics, i.e. the rate controlling step is a chemical reaction. At each testing temperature, grain boundary silicatic compounds are the first and foremost leached phases. Among the main constituents of the composite, AlN is the phase that mainly reacts, particularly at 40 and 70°C. MoSi2 and SiC are less affected by corrosion. Corrosion doesn’t influence the electrical resistivity of the composite. Introduction A promising approach to develop high performance functional/structural materials is to prepare ceramic composites using matrix materials with good mechanical properties and oxidation resistance, with the addition of electronductive phases. Recent studies highlighted that composites of MoSi2 with SiC and/or AlN exhibit an improvement of strength, toughness and oxidation resistance compared to monolithic MoSi2 [1,2]. Outstanding properties were found in an electroconductive composites constituted by an AlN-SiC matrix with the addition of MoSi2 [3]. In this study, the corrosion behaviour of an hot pressed AlN-SiC-MoSi2 composite, with Y2O3 as a sintering aid, was investigated in 4 M NaOH aqueous solution at room temperature, 40°C and 70°C up to 400 hours. The corrosion kinetics were determined and the mechanism governing the corrosion was defined. The electrical resistivity of the corroded samples was also evaluated in relationship with the microstructural changes which are the consequences of the corrosion. Experimental Procedure The dense ceramic composite, with the composition 98 wt% (55 vol.% AlN + 15 vol.% SiC + 30 vol.% MoSi2) + 2 wt.% of Y2O3, was prepared by hot pressing at 1800°C, with an applied pressure of 30 MPa and holding time 10 minutes, in vacuum [3]. Rectangular plates 12.0x8.0x1.0 mm 3 were cut and polished up to 6 μm. The specimens were sealed in a polyethylene tube containing 50 ml of 4 M NaOH solution, previously thermostated at the test temperature. The tests were performed at room temperature (∼20°C), 40°C and 70°C up to 400 h. After the tests, the specimens were immersed in 30 ml of 8 M HCl and rinsed in boiling deionizated water. The remaining HCl solution was added to the NaOH solution. A Varian Liberty 200 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used for the determination of the Al, Si, Mo and Y cations released into the corrosive solutions. The microstructure of the corroded specimens were analyzed through X-Ray diffraction, scanning electron microscopy (SEM) and microanalysis EDS. The electrical resistivity (ρ) was measured at room temperature by a four probe DC method. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 945-948 doi:10.4028/www.scientific.net/KEM.264-268.945