François Gitzhofer

Morphological study of hydroxyapatite nanocrystal suspension

Nanometer size hydroxyapatite (HA) crystals are prepared by a wet chemical precipitation method at different synthesis temperatures and with various reactant addition rates. The resulting aqueous suspensions are studied in terms of morphology (transmission electron microscope, specific surface area), phase (X-ray diffraction, electron diffraction and infrared spectroscopy) and rheological properties. This work shows that shape, size and specific surface area of the HA nanoparticles are very sensitive to the reaction temperature and also to the reactant addition rate. The measured pH at the end of synthesis, which is strongly linked with the reactant addition rate, is a key parameter which can be used to determine the purity of the synthesized HA nanocrystal and also for the stabilization (dispersion) of the suspension. HA nanoparticles synthesized at low temperature (T < 6°C) are monocrystalline. A transition temperature (T=60 °C) can be defined as a limit for the synthesis of monocrystalline HA nanocrystals, above this critical temperature nanocrystals become polycrystalline. HA monocrystals adopt a needle shape and are oriented following the c-axis of the hexagonal HA structure. The as-synthesized suspension is then concentrated and the effect of a dispersing agent addition, which is needed to get a high solid/liquid ratio coupled with good flowability of the suspension, is also shown, because this suspension is used in the suspension plasma spraying process.

Mechanisms underlying the limited injectability of hydraulic calcium phosphate paste.

Calcium phosphate (CaP) cements are being increasingly used for minimally invasive hard tissue implantation. Possible approaches to improve the bad injectability of hydraulic calcium phosphate pastes have been discussed and investigated in a number of recent publications. However, the liquid-phase separation mechanism leading to the limited injectability has not yet been addressed. Liquid-phase separation means that the liquid-to-powder ratio (LPR) of the extruded paste is higher than the LPR of the paste left in the syringe. The goal of this paper was to remedy this situation by looking at the liquid-phase migration occurring during the injection of a paste from a syringe through a cannula. Experimentally, it was seen that the liquid content of both the syringe paste and the extrudate decreased during the paste injection. Moreover, a high extrusion velocity, small syringe size, short cannula and high LPR favored a good injectability. These results could be partly explained in light of rheological measurements performed with the investigated paste.

Induction plasma synthesis of ultrafine SiC powders from silicon and CH4

Ultrafine SiC powders have been synthesized from elemental silicon and methane using induction plasma technology. The powder products were characterized by X-ray diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, electron probe microanalysis, infrared spectroscopy, and surface area measurement. The powders collected from various sections of the reactor system showed different features reflecting different compositions and powder morphologies. The purest SiC powder was collected in the metallic filter. It was composed of both α-and β-phase of SiC with small levels of free silicon and carbon. The reaction route used is based on the evaporation of the injected pure silicon starting powder, followed by carburization of the silicon vapour using methane. The silicon evaporation rate was found to depend strongly on the particle size of the silicon powder. Using silicon powder with a mean particle diameter of 100 μm, at a plasma power level of P=43.2 kW, the conversion of silicon to SiC and the overall SiC content in the product powder was 44.2% and 50.8 wt%, respectively. The injection probe position was Z=9.3 cm, the silicon feed rate was 4 g min−1, and the C/Si molar ratio was 0.7. Using silicon particles with a mean diameter of 45 μm, the conversion and overall content of SiC increased to 70.4% and 73.9 wt%, respectively, under the same plasma operating conditions and powder feed rates. By appropriate selection of experimental conditions, ultrafine SiC powder of high quality was achieved.

Suspension plasma spraying for hydroxyapatite powder preparation by RF plasma

Numerous techniques have been developed to synthesize ceramic powders with improved physical and chemical characteristics. This paper describes a new process called suspension plasma spraying (SPS), based on the use of radio frequency (RF) plasma technology. The objective of SPS is to prepare dense and spherical powders from a suspension of fine (<10 pm) or even ultrafine (<100 nm) powders. The precursor for SPS is a colloidal suspension (or physical gel) which is gas atomized in the plasma. Liquid evaporation, consolidation, and sintering occur during the plasma heat treatment. Results concerning the preparation of a bioceramic (hydroxyapatite, HA) powder from an aqueous suspenslon precursor are reported. Process variables are studied as a function of phase structure morphology and crystallinity of the obtained powder. The plasma power was kept in the range 35-45 kW; the plasma gas was a mixture of Ar/H/sub 2/ or Ar/O/sub 2/. Investigations by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy were performed on the suspension as well as on the produced powders. Atomized HA particulates are pure, dense, and well spheroidized, with an average particle diameter of 20 /spl mu/m.

Connected Three-Phase Boundary Length Evaluation in Modeled Sintered Composite Solid Oxide Fuel Cell Electrodes

A numerical methodology for evaluating the three-phase boundary length (TPBL) in sintered composite solid oxide fuel cell electrodes is developed. Three-dimensional models of a representative volume element of sintered composite electrodes are generated for which the mean particle diameter, composition, and total porosity may be specified as input parameters. Tomographic methods are used to reconstruct the modeled electrode and the percolation for each phase is evaluated. The connected TPBL is calculated for a range of electrode designs and comparisons are made with calculated TPBL values available in the literature. The maximum connected TPBL occurred at a porosity of 0.21 and at equal solid volume fractions of ionic and electronic conducting phases for particles having the same mean diameter and particle size variance. A cubic envelope having a minimum length of 14 times the mean particle diameter was necessary to adequately represent the electrode structure.

Preparation of perovskite powders and coatings by radio frequency suspension plasma spraying

Perovskite-type LaMnO3 powders and coatings have been prepared by a novel technique: reactive suspension plasma spraying (SPS) using an inductively coupled plasma of approximately 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility, and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO2 powder in a saturated ethanol solution of LaCl3 with a 1 to 1 molar ratio of lanthanum and manganese. A low reactor pressure was helpful in diminishing the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70 to 90%. After a posttreatment with an 80% oxygen plasma, an almost pure LaMnO3 deposit was achieved in the center of the incident plasma jet.

Mechanisms underlying the limited injectability of hydraulic calcium phosphate paste. Part ІІ: Particle separation study

Calcium phosphate cements (CPCs) are of great interest for bone augmentation procedures. In these a hydraulic calcium phosphate paste is injected through a small bore needle into the bone. The injectability of these pastes is relatively poor, resulting into partial injection only. In earlier studies we have shown that phase separation brings the injection process to a halt. Phase separation is characterized by a faster flow of the liquid than of the solid during paste extrusion. So far it is unclear whether or not particle separation contributes to the poor injectability of such hydraulic pastes. It is hypothesized that fine particles behave like a liquid and thus separate under the injection pressure, leaving larger particles behind. A factorial experimental design was used to examine this hypothesis. The particle size distribution (PSD) of the extrudate was measured over the course of each injection experiment using laser diffraction. The solid content of the paste was further inspected using scanning electron microscopy. A total of 48 experiments covering four factors at two levels each were performed. One factor was the ultrasound exposure duration, to ensure the dispersion quality of the particles during the PSD measurements. Another factor was the location of the samples over the course of the injection, so as to compare the extrudate with the PSDs remaining in the syringe. The liquid:powder ratio (LPR) in the injected paste was another factor investigated. Specifically, two different pastes with 40% and 50% LPR were examined. The dispersal medium was a fourth factor investigated, to ensure adequate dispersion of the particles during the PSD measurements. Analysis of variance showed that sample location did not significantly affect PSD. No apparent PSD change for the extruded paste and the paste remaining in the syringe could be detected by scanning electron microscopy. In conclusion, the present study did not show any evidence suggesting that particle separation occurred over the course of injection and thus that phase separation remains the main phenomenon leading to the poor injectability of CPCs.

Study of the performance of TBC under thermal cycling conditions using an acoustic emission rig

An experimental rig based on the use of infrared quartz lamps has been developed to monitor the degradation mechanisms causing failure of thermal barrier coatings (TBC) under thermal-cycling conditions. An acoustic emission (AE) technique monitored these degradation mechanisms, and advanced signals processing identified the key parameters that classify the AE signals according to the long-term behavior of the TBC. The AE technique enabled the localization of degradation sources inside the TBC with a linear resolution of ∼5 mm by the use of two transducers fixed at both ends of the sample. Furthermore, sample zones of high AE activity showed typical vertical cracks at the surface and delaminations at the interface between the ceramic and the bond-coat layer. Vertical cracks were induced preferentially during the heating period of the thermal cycles when the ceramic coating was in a tensile-stress state, while delaminations were induced during the cooling period when the TBC was in a compressive-stress state.

Monitoring and control of RF thermal plasma diamond deposition via substrate biasing

In a RF induction thermal plasma chemical vapour deposition system the substrate is used as an electrical probe to monitor the diamond film evolution in situ. The evolving electron emission current allows us to identify the transition from the initial nucleation stage to the diamond growth stage. A direct-current bias voltage is applied to the substrate, and the polarity is adjusted in situ according to the changing growth requirements, providing a tool for controlling the diamond formation independent of the plasma source.

Statistical design of experiments for the spheroidization of powdered alumina by induction plasma processing

The principal factors controlling the spheroidization process of Al2O3 powder in the induction plasma are the position of the powder injector, the powder feed rate, and their interactions. A higher level of powder feed rate (4.2 kg/h) has been achieved at the r.f. plate power of 40 kW with the application of response surface methodology (RSM). Under these loading conditions, the spheroidization of the Al2O3 powder of −44+15 µm size attained 94.9%, while for Al2O3 of −89 + 44 µm size, 83.3% spheroidization was achieved.