Th. Leventouri

Neutron powder diffraction studies of silicon-substituted hydroxyapatite.

The incorporation of a small amount of silicon in the hydroxyapatite (HAp) lattice is known to improve the bioactivity of the material. The effect of silicon substitution was studied by comparing samples of pure and 0.4wt% silicon-substituted HAp prepared by an aqueous precipitation method. High-resolution neutron powder diffraction and Rietveld refinement methods were used to investigate the effect of silicon substitution on the crystal structure parameters of the HAp lattice from room temperature down to 20K. Small structural changes in the lattice constants, interatomic distances, site occupancies and distortion of the phosphate tetrahedron were found. Modifications of the Fourier transform infrared spectra as well as appearance of new modes were observed in the silicon-substituted sample. Heat treatment and silicon substitution also affected the morphology and crystallinity of this bioapatite.

Crystal Structure Studies of Human Dental Apatite as a Function of Age

Studies of the average crystal structure properties of human dental apatite as a function of age in the range of 5–87 years are reported. The crystallinity of the dental hydroxyapatite decreases with the age. The a-lattice constant that is associated with the carbonate content in carbonate apatite decreases with age in a systematic way, whereas the c-lattice constant does not change significantly. Thermogravimetric measurements demonstrate an increase of the carbonate content with the age. FTIR spectroscopy reveals both B and A-type carbonate substitutions with the B-type greater than the A-type substitution by a factor up to ~5. An increase of the carbonate content as a function of age can be deduced from the ratio of the ν 2CO3 to the ν 1PO4 IR modes.

Comparison of crystal structure parameters of natural and synthetic apatites from neutron powder diffraction

A systematic behavior in the crystal structure parameters of natural, synthetic, carbonate, and non-carbonate apatite is revealed from Rietveld refinements of neutron powder diffraction experiments. The results of this work on synthetic carbonate hydroxyapatites (CHAps) are consistent with the mechanism of carbonate substitution on the mirror plane of the phosphate tetrahedron, as it was introduced for the natural carbonate fluorapatite (CFAp). The present comparison shows that the tetrahedral bond lengths P–O1 and P–O2 decrease by 3–4% in all carbonate apatites. The atomic displacement parameters (ADPs) of the tetrahedral (T) and the O3 sites are greater in the carbonate than in the non-carbonate apatites. The atomic positional disorder of the T site (P/C site) is greater in the CFAp than in the CHAps, while the opposite happens at the O3 sites. Finally, the room-temperature ADPs of all of the atoms in the CFAp and CHAps show the same behavior as in the corresponding non-carbonate materials.

Powder Neutron Diffraction Studies of a Carbonate Fluorapatite

Atomic positional disorder of a single-phase natural carbonate fluorapatite (francolite) is revealed from analysis of the atomic displacement parameters (ADPs) refined from neutron powder diffraction data as a function of temperature and carbonate content. The ADPs of the francolite show a strong disturbance at the P, O3, and F sites. When it is heat treated to partially or completely remove the carbonate, the ADPs as well as the other structural parameters resemble those of a fluorapatite (Harding pegmatite) that was measured under the same conditions. The various structural changes are consistent with a substitution mechanism whereby the planar carbonate group replaces a phosphate group and lies on the mirror plane of the apatite structure. (c) 2000 Materials Research Society.

Growth and orientation of crystallites in Y‐Ba‐Cu‐O superconductors

We have studied the orientation of crystallites in samples of the ceramic superconductor YBa2Cu3Ox using x‐ray diffraction and optical microscopy. Although none of the special techniques for developing texture in this material were employed in the synthesis of our samples, it appears from our data that there is considerable orientation of the crystallites and that the pattern of orientation extends throughout the sample. The degree of orientation increases in a predictable way with the amount of processing. We describe a mechanism for explaining these observations.

Improvement of the fracture toughness of hydroxyapatite (HAp) by incorporation of carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and nylon

The potential of improving the fracture toughness of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and polymerized ε-caprolactam (nylon) was studied. A series of HAp samples with CfSWCNTs concentrations varying from 0 to 1.5 wt.%, without, and with nylon addition was prepared. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were used to characterize the samples. The three point bending test was applied to measure the fracture toughness of the composites. A reproducible value of 3.6±0.3 MPa.√m was found for samples containing 1 wt.% CfSWCNTs and nylon. This value is in the range of the cortical bone fracture toughness. Increase of the CfSWCNTs content results to decrease of the fracture toughness, and formation of secondary phases.

Bulk preferred orientation in superconducting YBa2Cu3Ox

Abstract We have studied the orientation of grains in a series of YBa2Cu3Ox pellets that were fabricated with the normal processing steps used in the solid state reaction method. Using x-ray diffraction, we show that the crystallites near the surface to which pressure is applied tend to be oriented with their c-axes parallel to the direction of pressure, and that the degree of orientation increases with the number of processing steps the pellet has undergone until saturation is reached. The orientations of crystallites inside the pellets are studied using polarized Raman spectroscopy on a phonon line that involves vibrations of the O(1) oxygen atom along the c-axis. These orientations are found to track the surface orientations almost exactly, demonstrating that the crystallites are aligned throughout the bulk of the pellets.

Magnetic properties of Fe–Co catalysts used for carbon nanofiber synthesis

The magnetic properties of Fe–Co alloys used as catalysts for vertically aligned carbon nanofiber growth are presented at several stages in the formation process: from thin films to dewetted islands to carbon-encapsulated particles. Electron microscopy shows their morphological properties as a function of the alloy ratio. The magnetic properties are investigated by superconducting quantum interference device magnetometry in a field range of |H|≤20 kOe and temperatures between 2 and 330 K. Magnetization measurements illustrate a composition dependence of the magnetic properties. In addition, there is a significant amount of metal that is not incorporated in the resultant catalyst particles. This metal is superparamagnetic (SPM) and features of the magnetization curve shed light on the magnetic moment distribution of these SPM clusters.

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

Abstract The effects of Fe-substitution of YBa 2 Cu 3 O y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa 2 (Cu 1 − x Fe x ) 3 O y with different dopant concentration (0 ⩽ x ⩽ 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T c , is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x =0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x .

Raman and far-infrared reflectance investigation of microcrystal orientation in YBa2Cu3O7−δ superconductors

Abstract Raman and far-infrared reflectance measurements were carried out on three samples of YBa 2 Cu 3 O 7−δ prepared under various thermal and mechanical treatment conditions. From the changes in the reflectance phonon spectra and the emission anisotropy of the characteristic line at 502 cm -1 the partial orientation of the microcrystal with the consecutive steps of sample treatment is inferred.