V. Perdikatsis

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.

X-ray powder crystallography with vertex instrumentation

Abstract An X-ray Diffractometer for Powder Crystallography is described along with experimental results and future plans. This is an intermediate instrument toward a long linear array system. Three channels of a silicon microstrip detector, are the detecting elements in the present instrument. Each detector channel is followed by a VLSI readout chain, which consists of a charge preamplifier with pulse shaping circuitry, a discriminator, and a 16-bit counter. Control and data acquisition is performed with a custom made PC readout card. A motorized goniometer scans the angle range of interest. Calibration of the system is done with reference samples and data which are captured with a one-channel conventional NaI detector.

Effect of Ca substitutions on the properties of YBa2Cu3O6+d

Abstract We study the effect of Ca substitutions on the structural and superconducting properties of the system of nominal composition YCa x Ba 2−x Cu 3 O 6+ d . The system remains superconducting up to x=1.00 and the critical temperatures vary between 92 and 80K. A new, non superconducting phase of stoichiometry Y 1.14 Ca 1.58 Cu 3.27 O 6.45 appears with concentrations that increase with the nominal concentration of Ca. For x=0.80 and x=1.00 we observe a substitution of the Ba with Ca in crystallites of the form YBa 1.8 Ca 0.2 Cu 3 O 6.5 .

Ca doped YBCO on the Ba site

The effect of partial, nominal replacement of Ba with Ca on the properties of the YBa2Cu3O6+gd superconductor varies with the concentration of Ca. For nominal concentrations in Ca up to 50% a two phase system is formed: the known superconducting phase Y1−yCayBa2Cu3O6+δ and a new, non superconducting phase of composition Y1.14Ca1.58Cu3.27O6.45. The x-ray diffraction pattern of this new phase matches the Bragg peaks, as well as the commensurate and incommensurate peaks of a partially indexed pattern of the phase Y2−xCa2+xCu5O10. For nominal Ca concentrations 40 and 50%, microprobe analysis shows that the superconducting phase becomes a mixture of Y1−yCayBa2Cu3O6+° and YBa1.8Ca0.2Cu3O6.5, where Ca enters the structure in the Ba site. The microraman spectra indicate a small substitution of Ba by Ca, while the new phase shows novel characteristics.

Processing and Properties of Bulk Textured YBa2Cu3Ox

We present our work on the preparation and studies of bulk samples of the YBa2Cu3Ox superconductor that have controlled grain orientation. The effect of the degree of grain alignment on physical properties with emphasis on the critical current densities is described. First, the processing was based on the standard solid state reaction method. We find that repeating the steps of this method leads to higher degree of orientation, until a saturation is reached. X-ray measurements show the preferred orientation of the grains on the surface of the samples with their c-crystallographic axis parallel to the compression axis. Scanning electron microscopy photographs of internal surfaces of samples that were broken indicates that this preferred orientation extends throughout the bulk of the samples with up to ~45% of the microcrystallites aligned. We measure the preferred orientation of the grains inside the samples with a polarized Raman spectroscopy technique that we have developed. Critical current (Jc) experiments show the dependence of the critical current densities on the degree of alignment. At 77K and ambient field, Jc increases with the preferred orientation by a factor of three. Application of melt-growth on pre-oriented samples results to an order of magnitude increase in the average size of the microcrystals.

Processing and Properties of Bismuth Superconducting Phases

We have prepared various compositions of the bismuth superconductor with or without Pb substituting for Bi, according to the solid state reaction method. A.C. magnetic susceptibility and x-ray diffraction measurements show the 80K phase (2212) for the samples with nominal composition Bi4Sr3Ca3Cu6Oy, whereas the samples with nominal compositions Bi1.8Pb0.2Sr2Ca2Cu3Oy, and Bi0.7Pb0.3SrCaCu1.8Oy show transition temperatures at 80K and at 110K (phase 2223). The ratio of the 80K phase (2212) to the 110K phase (2223) is a function of the processing parameters. The preferred orientation of the microcrystallites is studied with x-ray powder diffraction and rocking curve measurements. A close dependence of the degree of the alignement from the fabrication procedure is confirmed.

X-RAY DIFFRACTION AND ELECTRON PARAMAGNETIC RESONANCE STUDIES OF A Bi4Sr2.5Ca2.5Cu4Ox SUPERCONDUCTING PHASE

We report a study of a low Tc phase of the bismuth superconductor. A nominally Bi4Sr2.5Ca2.5Cu4Ox compound was produced by the solid state reaction method with a Tc = 80 K. X-ray diffraction studies show that the structure is very similar to the structure of the 4:3:3:4 composition with indications of small amounts of other phases. Electron paramagnetic resonance spectroscopy gives a strong Cu2+ signal at temperatures above Tc. We make a comparison between the local symmetry of the Cu2+ ion complexes in this Bi sample and in the Y–Ba–Cu–O superconductor we had studied before.