Pedro L. Guzzo

A comparative study on ultrasonic machining of hard and brittle materials

Precision abrasive processes are commonly employed to machine glasses, single crystals and ceramic materials for various industrial applications. Until now, precision machining of hard and brittle solids are poorly investigated in Brazil from the fundamental and applied point of views. Taking into account the major technological importance of this subject to the production of functional and structural components used in high performance systems, the present study investigated the ultrasonic abrasion of different workpiece materials - alumina, zirconia, quartz, glass, ferrite and LiF - by using a stationary ultrasonic machine. Experiments were conducted using a rectangular shaped cutting toll and SiC particles with mean grain size of 15mm. The machined surfaces were characterized by surface profilometry and scanning electron microscopy. In the case of alumina, zirconia and quartz, the rates of material removal decrease with the depth of machining. The rate of material removal remained constant for the others materials. The micrographs showed that brittle microcracking was the primary mechanism involved with material removal. The rates of material removal and the machined surface topographies were discussed as a function of intrinsic stiffness, hardness and fracture toughness of workpiece materials.

Impurity content characterization of brazilian quartz lascas

Lattice impurities (Al, Li, Fe) free from the influence of cations in inclusion liquid were evaluated by atomic absorption spectrophotometry (AAS) in quartz lascas taken from 10 deposits with known localities in three states, Minas Gerais, Tocantins and Bahia. Levels of Al content and associated Li content depend on the localities of deposits. Quartz deposit regions with high Al content are composed of granite, gneiss, and pegmatite as surrounding rocks. It was concluded that the charge compensation for Al-Si substitution in quartz lascas was realized substantially by the formation of Al-Li and Al-OH centers which were confirmed from the linear relation of Al-Li plots by AAS data and IR spectra due to Al-OH center. The variety in Al-Li and Al-OH correlations was attributed to the fluctuation of growing conditions of quartz in nature.

Effect of high gamma doses on the sensitization of natural quartz used for thermoluminescence dosimetry

This article reports the sensitization of the glow peak at 300 °C in crystalline quartz by using high doses of gamma radiation. Five lots of samples were prepared from single crystals extracted from different geological occurrences in Brazil. The specimens were submitted to high doses of 60Co radiation starting at 25 kGy and going up to a cumulative dose of 350 kGy. After each high dose application, the samples were submitted to a thermal treatment and then irradiated with gamma radiation doses ranging from 2 to 20 mGy. The thermoluminescence (TL) response for each quartz lot was then determined. The results showed a remarkable increase in the TL peak at 300 °C for samples from three of the lots after the administration of a dose of 25 kGy. The pre-dose necessary to obtain the maximum in sensitization was dependent on the origin of the quartz samples and on their Al, Li and OH impurity contents.

Dating of fossil human teeth and shells from Toca do Enoque site at Serra das Confusões National Park, Brazil

This work reports the dating of a fossil human tooth and shell found at the archaeological site Toca do Enoque located in Serra das Confusões National Park (Piauí, Brazil). Many prehistoric paintings have been found at this site. An archaeological excavation unearthed three sepulchers with human skeletons and some shells. Two Brazilian laboratories, in Ribeirão Preto (USP) and Recife (UFPE), independently performed Electron Spin Resonance (ESR) measurements to date the tooth and the shell and obtain the equivalent dose received by each sample. The laboratories determined similar ages for the tooth and the shell (~4.8 kyBP). The results agreed with C-14 dating of the shell and other samples (charcoal) collected in the same sepulcher. Therefore, this work provides a valid inter-comparison of results by two independent ESR-dating laboratories and between two dating methods; i.e., C-14 and ESR, showing the validity of ESR dating for this range of ages.

OSL and photo-transferred TL of quartz single crystals sensitized by high-dose of gamma-radiation and moderate heat-treatments.

This study investigates the optically stimulated luminescence (OSL) and the photo-transferred thermoluminescence (PTTL) signals in quartz single crystals showing a strong TL peak near 300°C after being sensitized by irradiation with 25kGy of gamma rays and heating at 400°C. Natural and sensitized samples were prepared from two crystals with different sensitivity levels in the 300°C TL region. Continuous-wave (CW) and linearly-modulated (LM) OSL signals were stimulated with blue light-emitting diodes during 40 and 1000s, respectively. Two components were isolated from the CW-OSL signals of sensitized samples. These components were clearly seen in LM-OSL measurements together with two long-term components. LM-OSL showed that the sensitization process considered in this study sensitized an ultrafast OSL component of these crystals. The similar behavior found for the thermal stabilities of OSL and TL signals and the dependence of these signals with sample origins suggested that the trapping site related to the ultrafast component is also related to the TL process of the sensitized peak. The PTTL signal induced by blue LEDs increased the intensity of the sensitized glow peak. On the other hand, a remarkable reduction in the intensity of this peak as a result of the accumulated effect of blue light exposure was clearly seen in both kinds of crystals. These results were explained by a mechanism of competition between optically unstable deep traps and trap levels responsible to the sensitized TL peak.

Two-dimensional small-angle X-ray scattering from as-grown and heat-treated synthetic quartz

The small-angle X-ray scattering (SAXS) in as-grown and heat-treated quartz crystals was investigated as a function of the azimuth angle around the primary beam. For this, samples parallel to (10\bar 10) were extracted from Z- and −X-growth sectors of a synthetic quartz bar which had the OH content evaluated by infrared spectroscopy (IRS). In addition, SAXS and IRS were independently recorded as a function of heating temperature. As a result, the two-dimensional SAXS images revealed an anisotropic pattern randomly decorated by low-intensity Kossel lines. The intensities were projected along specific directions or were axially integrated around the primary beam. It was observed that the Porod invariant (Q) increased and the Kossel lines moved slightly to higher q values with increasing temperature. The effect of the sample orientation on the Q value and the lack of a clear relationship between Q and OH content suggested that the diffuse scattering due to the periodicity of the crystal lattice played an important role in the small-angle scattering of quartz. The net scattering intensities produced by heat-treatment at 873 K were attributed to molecular water aggregates created by the diffusion of as-grown OH defects.

Distribution of aluminum and hydroxyl impurity-centers in synthetic quartz grown from cylindrical seeds

This study investigates the distribution of Al and OH defects in synthetic quartz grown from cylindrical seeds parallel to X, Y and Z crystallographic axes. Prismatic habit crystals were grown with m-, R-, r- and x- faces. The growth sectors were identified by radiation-induced coloration and their morphologies were characterized by X-ray topography. Optical and infrared spectroscopies were adopted to evaluate the occurrence of Al-hole, Al-OH and as-grown OH centers. It was found that the relative content of Al-hole and Al-OH centers were dependent on the growth sector and the orientation of the seed crystal. The ratio of Al-hole to Al-OH centers between +X- and -X-sectors grown from the Z-axis seed-crystal is opposite to that found in cylindrical Y-axis and standard Y-bar synthetic quartz crystals. The incorporation of Al-Li centers was responsible for the intense lattice strains observed in r-growth sectors appearing around the X-axis.

Characterization of heterogeneities in new-generation synthetic quartz crystals by small-angle X-ray scattering

Small-angle X-ray scattering was employed to investigate heterogeneities in growth sectors of a large Z-bar synthetic quartz. The measurements were performed in as-grown and heat-treated samples. As a result, X-ray scattering was observed in all samples. The Guinier approximation revealed a polydispersive distribution of heterogeneities in as-grown samples with a radius of gyration ranging from 80 to 250 A. Infrared spectroscopy and X-ray fluorescence analysis were used to elucidate the nature of these heterogeneities. Considerable variations in the intensity of scattered X-rays due to heat treatments were only noticed in growth sectors with high OH-content. The results suggest that tiny aggregates of molecular water are the main reason for the scattering patterns observed in the range 0.006 < q < 0.1 A -1 .

Effect of particle size in the TL response of natural quartz sensitized with high gamma dose

The aim of this study is to investigate the effect of particle size in the thermoluminescence (TL) response of natural quartz sensitized with high gamma dose. For this, fragments of a single crystal taken from the Solonopole district (Brazil) were crushed and classified into ten size fractions ranging from 38 μm to 5 mm. Aliquots of each size fraction were sensitized with 25 kGy of gamma dose of 60Co and heat-treated in a muffle furnace at 400oC. The non-sensitized samples were exposed to test doses between 50 Gy and 5 kGy and the sensitized samples were exposed to a unique test dose equal to 50 mGy. For non-sensitized samples, the TL peak near 325 °C increases with the particle size decreasing. However, in the case of sensitized samples, the TL output near 280 °C increases with the increasing of particle size up to mean grain size equal to 308 μm. Above 308 μm, an abrupt reduction in the TL intensity was noticed. These effects are discussed in relation to the specific surface area and the different interaction of high gamma doses with fine and coarse particles of quartz.

Espectroscopia infravermelha à baixa temperatura em quartzos e ametistas com altas concentrações de OH e H2O

The aim of this study is to characterize OH-related defects in natural quartz and amethyst grown in geodic cavities formed during basaltic flow. For this, infrared (IR) spectroscopy at low temperature (-175°C) was carried out in amethyst and quartz crystals taken from two deposits located in the State of Rio Grande do Sul in Brazil. For one specimen, IR spectra were recorded directly upon the growth sectors r{101} and z{011} appearing in (0001) plates. IR spectra recorded from 3800 to 3000 cm-1 showed an intense broad band superposed upon sharp bands assigned to [H4O4]0 point defects. These bands were observed in all samples and growth sectors independently from the intensity of its violet color. On the other hand, sharp bands assigned to [AlO4/H]0 defects were observed only in translucent and pale citrine quartz and also in the colorless regions of r and z growth sectors in amethyst. Contrarily to previously reported data, it was observed that z growth sectors have the highest content of OH-related defects and the darkest violet color when compared to r sectors. This reversal relation was discussed taking into account the malformed habitus of the amethyst specimen probably caused by the anisotropic solution flow inside the geode. It was concluded that the amethyst color is independent of the concentration of OH-related defects. Besides the intense broad OH-band, the IR spectra of such crystals were characterized by the presence of a sharp band near 3595 cm-1.