Marcus Aurélio Ribeiro Miranda

Structural properties of hydroxyapatite obtained by mechanosynthesis

In this paper mechanical alloying has been used successfully to produce nanocrystalline powders of hydroxyapatite (HA) using five different experimental procedures in a pure dry process. The milled HA were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. For four different procedures, HA was obtained after a couple of hours of milling (in average 60 hours of milling, depending in the reaction procedure). This milling process, used to produce HA, presents the advantage that melting is not necessary and the powder obtained is nanocrystalline with crystallite size in the range of 22 nm to 39 nm.

Piezoelectric properties of collagen-nanocrystalline hydroxyapatite composites

In this paper we did a study of the physicochemical, dielectric and piezoelectric properties of anionic collagen and collagen-hydroxyapatite (HA) composites, considering the development of new biomaterials which have potential applications in support for cellular growth and in systems for bone regeneration. The piezoelectric strain tensor element d14, the elastic constant s55, and the dielectric permittivity ɛ11 were measured for the anionic collagen and collagen-HA films. For the collagen-HA composite film (Col-HACOM) the main peaks associated to the crystalline HA is present. For the nanocrystalline composite, nanometric HA powder (103 nm particle size) (HAN), obtained by mechanical milling were used. For the composite film (Col-HAN) the HA and CaH(PO4)2H2O phases were detected. One can see that the HA powder (HAN) present the main peaks associated to crystalline HA. The IR spectroscopy measurements on HA-COM and HAN powders, Col-HACOM and Col-HAN composite films and collagen film (Col) presents the main resonances associated to the modes of (PO4)3−, (CO3)2−. The IR spectra of Collagen Film (Col) shows the bands associated to amide I (C=O), amide II (N–H) and amide III (C–N) vibrational modes. The scanning electron photomicrography of the Col-HACOM and Col-HAN films, respectively, shows deposits of HA on the surface of collagen. It also shows that HACOM crystals has a dense feature, whereas the HAN crystals has soft porous surface. Energy-dispersive spectroscopy (EDS) analysis showed that the main elements of the hybrid sponge were carbon, oxygen, calcium, and phosphorus. The EDS of HACOM crystal, present in the Col-HACOM composite showed a molar ratio Ca/P = 1.71, whereas the Col-HAN composite the molar ratio of calcium and phosphorus (Ca/P = 2.14) and the amount of carbon were greater. The piezoelectric strain tensor element d14 obtained for the anionic collagen was around 0.102 pC/N. The collagen composite with nanocrystalline HA crystals (Col-HAN) present a better result (d14 = 0.040 pC/N) compared to the composite with the commercial ceramic (d14 = 0.012 pC/N). This is an indication that the nanometric particles of HA present little disturbance on the organization of the collagen fibers in the composite. In this situation the nanometric HA are the best candidates in future applications of these composites.

Piezoelectric coefficients of l-arginine hydrochloride monohydrate obtained by X-ray multiple diffraction using synchrotron radiation

In this paper, a very useful development to improve the application of a method based on the multiple-diffraction technique to determine the piezoelectric coefficients of crystals is described. The idea is to choose any specific crystallographic direction (plane) through the secondary peaks in the Renninger scan instead of those directions related to the electric field by the piezoelectric tensor. For a set of multiple-diffraction planes, it is possible to obtain an equation system based on the ordinary peak position for each H secondary plane that appears in the Renninger scan. This useful development was successfully applied to the amino acid L-arginine hydrochloride monohydrate in order to provide the d 21 , d 22 , d 23 and d 25 piezoelectric coefficients by applying the electric field along the [010] crystallographic direction.

Characterization by X ray diffraction of mechanically alloyed tripotassium sodium sulfate

Nanocrystalline powder of tripotassium sodium sulfate (K3Na(SO4)2 - KNS) was successfully obtained by mechanical alloying a mixture of potassium sulfate (K2SO4) and sodium sulfate (Na2SO4) with a proportion of 3:1 in a planetary mill. X ray powder diffraction (XRPD) was used to characterize this material. Powders produced with high milling times: 15, 30, 45 and 60 hours, have shown a single phase. For 60 hours of milling time, the powder was formed with very small grains (60 nm in average). We also prepared several samples with low milling times: 30, 60, 120, 150 and 180 minutes. The results for this series show that 120 minutes of milling is enough to produce a single crystalline phase of KNS. Therefore, we showed that a nanocrystalline powder of tripotassium sodium sulfate is easily obtained by mechanical alloying and that the grain size can be controlled by the amount of milling time.

The limit of application of the Scherrer equation

The Scherrer equation is a widely used tool to obtain crystallite size from polycrystalline samples. Its limit of applicability has been determined recently, using computer simulations, for a few structures and it was proposed that it is directly dependent on the linear absorption coefficient (μ0) and Bragg angle (θB). In this work, a systematic study of the Scherrer limit is presented, where it is shown that it is equal to approximately 11.9% of the extinction length. It is also shown that absorption imposes a maximum value on it and that this maximum is directly proportional to sin θB/μ0.

X-ray scattering studies of the metastable ferroelectric phase in KDP induced by static electric field

Potassium dihydrogen phosphate presents a ferroelectric phase below 122 K and a paraelectric phase for T > 122 K. X-ray scattering measurements were performed in order to verify the mechanism of the phase transition from point group C2v19 to C2v¹19 at 119 K induced by DC electric field; in other words, a change of the local site symmetries of the phosphate group occurs from C2 site in the C2v19 point group to Cs in the C2v¹19 point group. This phenomenon was shown by analyzing the behavior of the integrated intensity of the (800), (080), (1600), (0160), (400) and (040) reflections. The curve fitting of these reflections showed the occurrence of a rotation of the phosphate group around the [010] crystallographic direction during the phase transition. This study confirms the observation of this metastable phase obtained by Raman spectroscopy.

Synchrotron radiation X-ray multiple diffraction in the study of KDP phase transition induced by electric field

In this work, the application of the X-ray multiple diffraction technique using synchrotron radiation at Laboratorio Nacional de Luz Sincrotron (LNLS) to study KDP phase transition induced by electric field is discussed. A Huber three-axis (w, f, 2q) diffractometer mounted on a table which rotates around the incident beam allows to measure Renninger scans (RS) at the XRD station of LNLS using the adequate polarization for the experiments. Prior results indicated (080) as the best choice for the KDP primary reflection and the (440)( 40) four-beam case for the secondary. Two Bragg-surface diffraction (BSD) reflections, where the secondary beam is propagated parallel to the sample surface, are involved in this multiple diffraction case. A RS interval of 3.3° around f = 0° position was measured while the electric field was increased up to 6 kV/cm. At this value, a phase transition was observed and the secondary peaks became narrower and changed their RS position indicating possible occurrence of a metastable ferroelectric phase.