In this investigation strontium aluminate phosphors (SrAl2O4:Eu,Dy) were synthesized through two modified routes, sol-gel method (route of Pechini) and molten salts (activated by mechanosynthesis). ...The slight modifications allow to use lower temperatures and shorter times of thermal treatments than those typically used. Moreover, the synthesis is performed under a simple air atmosphere. The intermediates identified during the synthesis by sol gel are: polyesters, probably coordinated to metallic ions, the cubic PbNO32-type structure of Sr(NO3)2, and the orthorhombic aragonite-type structure of SrCO3. The intermediates identified during the synthesis by molten salts are: highly crystalline NaNO3 (obtained by the manual milling of reactants), the monoclinic bayerite-type structure of Al(OH)3 and the orthorhombic aragonite-type structure of SrCO3. The results of the characterization of these intermediates provided worthy information to propose the experimental conditions of reaction. The XRD characterization of products, obtained by both methods, shows the monoclinic stuffed tridymite-type structure of SrAl2O4 as predominant polycrystalline phase with a calculated mean particle size of ~26 nm. Transmission electron micrographs reveal particles with different morphology and broad distribution of sizes. HRTEM images discard the presence of amorphous phases and SAED patterns corroborate the presence of the crystalline phases previously identified by XRD. The persistent afterglow emission is centered close to the white light with a dominant wavelength of 550 nm and purity color of 0.45. The persistent emission lifetime is similar, at around 7 min, value that is sufficient for bio-imaging or security codes applications.
Albeit the thermoluminescent properties of both Al2O3 and natural topaz have been documented, owing to a controlled composition, the prospective of synthetic topaz for dosimetry applications is ...expected to be further promising. In this contribution, the thermoluminescent (Tl) properties of synthetic topaz (Al2F1.44(OH)0.56SiO4) containing materials are reported for the first time. Synthesis tests by chemical vapor deposition (CVD) were conducted using aluminum oxide (Al2O3) and sodium hexafluorosilicate (Na2SiF6) as solid precursor, varying temperature, time, Al2O3 compact angle position, and atmosphere during dwell stage (with and without nitrogen). Subsequently, samples were thermally treated at 500, 600, 700, 750, 800, 850 and 900 °C. Results indicate that samples treated at 800 °C show a standard deviation of 1.2% (SD %) of the area under the glow curve in the range 53–300 °C. Irradiation with 90Sr beta-source of sample A4 (800 °C), from 0.022 Gy to 66 Gy, revealed a nonlinear response, with intensity increasing as doses increase. In addition to the fact that synthesized topaz-containing material possesses a similar Tl intensity compared with that of the commercial dosimeter TLD-100, incidence of the greatest fading (<22%) during the first 20 h suggests its potential application in dosimetry.
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•Topaz (Al2F1.44(OH)0.56SiO4) has been synthesized by CVD.•Synthesized topaz-containing materials were irradiated with 90Sr beta-source.•Tl properties of synthetic topaz containing materials are reported for first time.•A fading <22% in the topaz-containing materials occurs during the first 20 h.•Synthesized topaz-containing materials suggest its potential dosimetry application.
A first study on the radioluminescence (RL) response of topaz synthesized is reported. Using three alternative and different compact reactants separately, i. e., aluminum oxide, aluminum hydroxide ...and kaolinite, synthesis tests by chemical vapor deposition (CVD) were conducted varying systematically processing temperature and time, incidence angle, and atmosphere. Results show the feasibility to form topaz using the three types of reactants, in varying amounts of 60%, 100% and 47%, respectively. Synthesized topaz exhibits a variety of morphologies: fibers and rectangular bars using Al2O3, cross needle shape compacts using Al(OH)3 and needle-shape phases using Al2Si2O5(OH)4. Irradiation with 90Sr beta-source revealed that only samples prepared using aluminum hydroxide exhibit RL response. The emission band centered at 390 nm of thermally untreated samples and the lack of RL emission of samples treated thermally at 500 °C confirm that the RL response is due to the (H3O4)0 color centers. The RL response showing the highest RL intensity at 390 nm is promoted by processing at 700 °C for 90 min, used for sample H3. A set of reaction pathways for topaz formation using the three alternative compact reactants is proposed.
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•Three different pathways for topaz synthesis by CVD are reported.•Three alternative reactant compacts for topaz synthesis: aluminum oxide, aluminum hydroxide and kaolinite.•Processing conditions that promote RL response: Al(OH)3 reactant compact, at 700 °C for 90 min.•Emission band centered at 390 nm of untreated samples and lack of emission of samples treated thermally are key facts.•RL emission is ascribed to the existence of (H3O4)0 color centers.
The presence of deleterious Al4C3 phase has been for long a major challenge in the manufacture of Al matrix composites reinforced with carbide phases. In this contribution, the hydrophilic Al4C3 ...phase has been successfully eliminated from the surface of B4C/Al composites prepared by pressureless infiltration, using a post-processing approach by the hybrid system chemical vapor deposition (HYSY-CVD) route. B4C/Al composites containing intentionally formed Al4C3 were first prepared by pressureless infiltration and subsequently treated by HYSY-CVD, varying temperature (400 and 500°C), time (30 and 60min) and amount of solid precursor Na2SiF6 (10 and 20g). In conformity with reaction thermodynamic predictions using the HSC 6.1® and FactSage® programs and databases, analysis by XRD and SEM confirms the superficial elimination of Al4C3 and the seedless formation of passivating AlF3 and Na5Al3F14. Optimal conditions to maximize the amount of AlF3 deposited were established by ANOVA and the effectiveness of the fluoride phases was evaluated in hardness and corrosion tests, confirming the stability of fluoride phases and the integrity of the composites. Microstructure characterization and hardness tests of specimens after three years substantiate the efficacy of AlF3 and Na5Al3F14 phases. A hydrophobic and impermeable fluoride coating multiphase layer is thus obtained.
Microstructure evolution of B4C/Al composites before and after post-processing. Display omitted
•The hygroscopic and deleterious Al4C3 phase was successfully eliminated by a post-processing approach.•A hydrophobic and impermeable fluoride coating multiphase layer is deposited on the composites.•The stability of fluoride phases and the integrity of the composites are confirmed.•Optimal conditions to maximize the amount of AlF3 deposited were established.
The optically stimulated luminescence (OSL) properties under beta radiation of synthetic topazes produced from three different reactants are reported for the first time. Topazes were synthesized by ...chemical vapor deposition using sodium hexafluorosilicate (Na2SiF6) compacts as solid precursor and either aluminum oxide, aluminum hydroxide or kaolinite compacts as reactants. The OSL dosimetric characteristics of the most promising composition, namely, topaz synthesized using aluminum oxide, were determined and analyzed. The alumina-derived topaz exhibited a linear response in the dose range from 0.2 up to 20 Gy, a satisfactory repeatability and a fading of approximately 30% in the first hours, after which the OSL response remained constant. These characteristics, together with a minimum detectable dose (MDD) of 0.01 Gy – corresponding to a 3σ background signal – suggest the potential of synthesized topaz as an OSL dosimeter. The OSL mechanism might be explained by at least two trap types, possibly related to topaz shallow traps. A decreasing behavior – featuring a single exponential decrease –, similar to that of OSL signal was observed in ultraviolet and visible light bleaching measurements.
•OSL properties of synthetic topaz were investigated for the first time.•Synthetic topaz showed an excellent linear response from 0.2 up to 20 Gy.•A minimum detectable dose of 0.01 Gy was found.•The OSL signal decreases up to 70% and then remains constant.•The potential of this topaz for radiation dosimetry was evaluated.
A novel synthesis route for topaz (Al{sub 2}SiO{sub 4}(OH,F){sub 2}) by chemical vapor deposition (CVD) using Na{sub 2}SiF{sub 6} as solid precursor was developed. Synthesis tests were conducted with ...and without a flow of nitrogen, positioning the Al(OH){sub 3} substrate at 0° and 90° with respect to the gas flow direction, at 700 and 750 °C, for 60 and 90 min, respectively. It was found that topaz is synthesized through two pathways, directly and indirectly, involving a series of endothermic and exothermic, heterogeneous and homogeneous reactions between Al(OH){sub 3} and SiF{sub 4}(g). Analytical structural determination confirmed existence of orthorhombic polycrystals with lattice parameters of a =4.6558 Å, b=8.8451 Å and c=8.4069 Å. According to ANOVA, while temperature, time and interaction of substrate angular position with atmosphere (P×A) are the parameters that most significantly influence the variability in the amount of topaz formed – equivalent contributions of 31% – topaz lattice parameters are mostly impacted by the same factors (T, t, P, A), but without the interaction factor. The projected amount of topaz is in good agreement with that obtained in confirmation tests under optimal conditions: Al(OH){sub 3} substrate compact placed at 0°, treated at 750 °C for 90 min in the absence of N{sub 2}. - Highlights: • Topaz synthesis as a unique phase by CVD, using solid precursor Na{sub 2}SiF{sub 6} is feasible. • Two pathways, a series of endothermic/exothermic, heterogeneous/homogeneous reactions. • Crystal structure, orthorhombic polycrystals: a =4.6558 Å, b=8.8451 Å, c=8.4069 Å. • Anova: amount of topaz formed and lattice parameters are impacted by same factors. • Projection of topaz quantity in good agreement with those from confirmation tests.
A first study on the radioluminescence (RL) response of topaz synthesized is reported. Using three alternative and different compact reactants separately, i. e., aluminum oxide, aluminum hydroxide ...and kaolinite, synthesis tests by chemical vapor deposition (CVD) were conducted varying systematically processing temperature and time, incidence angle, and atmosphere. Results show the feasibility to form topaz using the three types of reactants, in varying amounts of 60%, 100% and 47%, respectively. Synthesized topaz exhibits a variety of morphologies: fibers and rectangular bars using Al2O3, cross needle shape compacts using Al(OH)3 and needle-shape phases using Al2Si2O5(OH)4. Irradiation with 90Sr beta-source revealed that only samples prepared using aluminum hydroxide exhibit RL response. The emission band centered at 390 nm of thermally untreated samples and the lack of RL emission of samples treated thermally at 500 °C confirm that the RL response is due to the (H3O4)0 color centers. The RL response showing the highest RL intensity at 390 nm is promoted by processing at 700 °C for 90 min, used for sample H3. A set of reaction pathways for topaz formation using the three alternative compact reactants is proposed.
This work concerns the optimization of furnace brazing conditions for joining micro-multiport aluminum tubes and fins made with AA4343/AA3003/AA4343 brazing sheet in mini-assemblies mimicking the ...core of an automotive heat exchanger. Taguchi method was used for design of experiment, considering five process parameters with two levels of values. The aim was to maximize the fillet size of the brazed joints, which has an important influence on the thermal integrity and mechanical properties. Fillet length measurements of brazed joints were performed with a metallographic microscope. The statistical analysis allowed to obtain the optimum values of process parameters (peak temperature, residence time, heating rate, microchannel tube type and flux). At a 95% confidence level, the variability of fillet length is most significantly affected by the peak brazing temperature (77%), residence time (15%) and heating rate (7%). The predicted maximum fillet length was (152±11) µm, which was corroborated by confirmation trials. The microstructural analysis of tube−fin joints showed that variations in peak temperature and residence time affect only the size of the eutectic zone of fillet formed, but not the nature or composition of the constituent phases.
This article describes a new method for 3D QEEG tomography in the frequency domain. A variant of Statistical Parametric Mapping is presented for source log spectra. Sources are estimated by means of ...a Discrete Spline EEG inverse solution known as Variable Resolution Electromagnetic Tomography (VARETA). Anatomical constraints are incorporated by the use of the Montreal Neurological Institute (MNI) probabilistic brain atlas. Efficient methods are developed for frequency domain VARETA in order to estimate the source spectra for the set of 103–105 voxels that comprise an EEG/MEG inverse solution. High resolution source Z spectra are then defined with respect to the age dependent mean and standard deviations of each voxel, which are summarized as regression equations calculated from the Cuban EEG normative database. The statistical issues involved are addressed by the use of extreme value statistics. Examples are shown that illustrate the potential clinical utility of the methods herein developed.
