Papers by Author: Paola Palmero

Abstract: Several hydroxyapatite (HAp) synthesis routes, such as solid state reactions, hydrothermal methods, wet procedures have been widely investigated. At the same time, in the last years many alternative methods to produce HAp, exploiting fish, bovine or pig bones, have been developed; among them the most used are thermal processes as well as subcritical water or alkaline hydrothermal routes. This paper deals with a natural HAp nanopowder extracted from pig bones by a treatment with hot NaOH solution. It was demonstrated that the decomposition phenomena were avoided when the extracted HAp powder was calcined in CO₂ atmosphere or by inducing the formation of CaO at 700 °C and then eliminating it through a washing procedure. The washed HAp was then exploited for producing macroporous components having controlled porosity features in terms of pore shape, size and volume percentage. For this aim, a combination of gelcasting and sacrificial template methods was applied as forming procedure, using agar as natural gelling agent and polyethylene (PE) spheres as pore former.

Abstract: Al2O3-YAG-ZrO2 composites have been produced by surface modification of a commercial nano-crystalline alumina powder with inorganic precursors of the desired second phases. The doped powders were calcined at various temperatures and for different times: as a function of the thermal treatment, zirconia directly crystallized on the alumina surface, while YAG phase was yielded by solid state reaction among an amorphous yttrium-rich precursor and alumina powder. Several compositions, with increasing second phases volume fractions, were investigated, precisely, Alumina-5vol.%YAG-5vol.%ZrO2, Alumina-20vol.%YAG-20vol.%ZrO2 and Alumina- 33vol.%YAG-33vol.%ZrO. Slip cast bodies were produced by aqueous suspensions of calcined and well-dispersed powders; free-sintering performed at 1500°C for 3 h allowed to reach full densification. The role of the second phases amount on the microstructural features and on some mechanical data preliminary evaluated was established.

Abstract: Wet-chemical syntheses have been applied to the production of ceramic powders, with the aim of tailoring compositional and micro/nanostructural features, as an imperative requirement toward the elaboration of ceramic components with improved functional or even structural properties. Three syntheses are here presented and discussed, respectively concerning a purephased, nanostructured YAG powder, a biphasic Al2O3-YAG composite and three-phased Al2O3- YAG-ZrO2 material. In particular, this paper is aimed to illustrate the path followed from the set-up of the easier synthesis of the mono-phased system to the definition of the advanced procedures for the production of more and more complex compositions.

Abstract: An innovative gel-casting process was developed in order to obtain macro porous ceramics scaffolds of hydroxyapatite to be used in regenerative medicine for bone tissue reconstruction. Mechanical investigation was carried out on different formulations of dense hydroxyapatite samples in order to evaluate the effect of the gel casting process parameters on the density, the elastic modulus, the tensile and the compressive strength. The fracture critical stress intensity factor (KIC) was also evaluated by means of microhardness measurements. The correlations between KIC and tensile and compressive strength were examined taking into account the average and maximum size of porosity. The mechanical properties of macro and micro-porous HA are in agreement with the model of Gibson and Ashby.

Abstract: The elaboration of 50 vol.% Al2O3/YAG (Y3Al5O12) nanocomposites was pursued by comparing two processing routes. In one case, a composite powder was firstly synthesized via reverse-strike co-precipitation and then submitted to an optimized thermal and extensive milling pre-treatment prior to sinter. In the second case, a pure-alumina precursor was prepared via reverse-strike precipitation and then doped with an yttrium salt solution; such doping procedure was performed on alumina samples submitted to several thermal and/or mechanical pre-treatments carried out to yield more or less relevant α-phase amounts. The optimization of the thermal/mechanical pre-treatment of the former powder led to a very homogeneous, dense and fine microstructure, made of α-alumina and YAG grains of about 300 nm in size. On the contrary, sintered bodies characterised by a larger mean grain size and a lower homogeneity in microstructure were obtained by using the second starting powder, even if, also in this case, a suitable control of the powder processing can allow a promising improvement of the microstructural features.

Abstract: In this paper, five powdered samples, containing different amounts of nanosized α-alumina, were obtained by controlled thermal pre-treatments of a transition alumina, and then densified by both free-sintering in air at 1500°C for 3 h and by Spark Plasma Sintering in the temperature range 1150°C - 1400°C with different soaking times at the maximum temperature. A comparative study of the influence of the phase composition in the starting alumina powders on both sintering behavior and microstructural features of the densified bodies is presented, showing a relevant influence of powder thermal pre-treatment on the SPS process, in a strict analogy to natural sintering.

Abstract: Two α-Al2O3/YAG composite powders have been prepared by reverse-strike precipitation, starting from chlorides aqueous solutions, the former containing 50 vol% of the two phases (labelled as AY50) and the latter made of 90 vol% of alumina and 10 vol% of YAG (AY90). The as-prepared powders were characterised by DTA/TG simultaneous analysis as well as by XRD analysis performed after calcination at different temperatures. A systematic TEM analysis was performed on AY50 powders pre-treated at different temperatures, in order to investigate the crystallites size evolution as a function of the temperature. After that, samples were compacted by uniaxial pressing and sintered at 1600°C for 3h. SEM observations revealed a homogeneous microstructure made of micronic alpha-alumina and YAG grains. For limiting grain growth through the decreasing of the maximum sintering temperature, an innovative activation procedure by coupling suitable thermal and mechanical treatments of the powders was performed. After that, high densification (>95% of the theoretical density) was easily achieved by performing a free sintering in the temperature range between 1320° and 1420°C, with different soaking times at the maximum temperature. The resulting sintered bodies showed an effective retention of the nano-size of the primary particles. By SEM, highly-homogeneous nanostructures, with an average grains size of about 200 and 300 nm for AY50 and AY90, respectively, were observed.