The Activity of Silicon Carbide Particles in Al-Based Metal Matrix Composites Revealed by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is used to image variations in electrochemical activity over the surface of an aluminum-based metal matrix composite (MMC) in contact with buffered or unbuffered neutral solutions. The composite consists of an Al - 13.5% Si - 9% Mg alloy matrix and reinforcing silicon carbide particles (SiCp). Feedback mode SECM imaging using ferrocenemethanol as a redox mediator in 0.1 M NaCl solution and in buffer solution (pH 6.8) revealed that the SiC particles are electrochemically active. The data suggest that the electronic conductivity at these sites is higher than that of the Al2O3 film covering the alloy matrix surface. The reduction of dissolved oxygen on the silicon carbide particles was investigated by in situ SECM images of samples and current vs. tip-substrate distance curves. The results with samples of SiCp/Al composites immersed in distilled water alone or in either 0.1 M NaCl or boric acid/borax buffer containing ferrocenemethanol as mediator demonstrate that the silicon carbide particles are conductive and act as local cathodes for the reduction of oxygen

Phase Transitions for Al-base Alloy on Themicro Structural Experimental Measurements

ABSTRACT:The ternary Al-base alloys were re-melted and processed with a rapid extraction of thermal energy. In an effort to find the influences of the temperature on the solid transition, the materials were annealing at 500 °C during 48 hours. The effects of the rapid extraction of thermal energy plus the phase transition on the morphology were studied by SEM. Microstructural morphology has been related to mechanical properties thought-out the high cooling rate. In order to provide a means of determine their mechanical properties,the microhardness, and tensile tests have been characterized. The microstructure refinement of ternary Al-base alloy was observed due to melt undercooling associated with the cooling rate. The transition phase process from melt to solid by conventional methods of melting and casting has been approached by cooling rate parameter.A successful approach to finer microstructure is correlated with the grain size measured. Reduced grain size results depends on increased cooling rates because nucleation events become more frequent by establishing a positive temperature gradient in the melt ahead of the advancing solid-liquid interface. The purpose of the present study in this paper, is evaluatethe relationship between microstructure, rapid extraction of thermal energy, and the mechanical properties as hardness and tensile tests.The present investigation clearly compared the hardness measurements inthe Al-VIIB-VIB alloy processed at high cooling rateswith the less microhardness values from the annealed condition. Correlation between tensile tests results and fracture type indicate that a transgranular + intergranular mode depend on the solidification route compared with the microstructural changes associated with an increment on grain-size refinement.In both cases, in as-spun and annealed condition grain size was found to depend significant on the magnitude of the cooling rate parameter obtained by the wheel speed velocity

Topaz synthesis using Al2O3, Al(OH)3 or Al2Si2O5(OH)4 and color centers promoting its radioluminescence response

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.Fil: Trujillo Vazquez, E.. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados; MéxicoFil: Pech Canul, M. I.. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados; MéxicoFil: Marcazzo, Salvador Julian. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Fisica Arroyo Seco; Argentin

Thermoluminescent characterization of Al2O3-derived synthetic topaz

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.Fil: Trujillo Vazquez, E.. Instituto Politécnico Nacional; MéxicoFil: Pech Canul, M. I.. Instituto Politécnico Nacional; MéxicoFil: Marcazzo, Salvador Julian. Universidad Nacional del Centro de la Pcia.de Bs.as.. Centro de Investigaciones En Física E Ingeniería del Centro de la Provincia de Buenos Aires. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones En Física E Ingeniería del Centro de la Provincia de Buenos Aires. Provincia de Buenos Aires. Gobernación. Comision de Invest.científicas. Centro de Investigaciones En Física E Ingeniería del Centro de la Provincia D; Argentin

Cinética de formación del Al₂O₃ en capas de aluminio estudiada mediante mediciones eléctricas

In this work a simple method for monitoring the low temperature oxidation of aluminum thin films, which is based on measurements of electrical resistance using the four-probe technique, is proposed. Kinetic growth data of the aluminum oxide layer, obtained using different values of applied current on as-deposited metallic films is presented. The temperature gradient measured on the films were 2 °C and 9 °C when the corresponding applied currents were 0.1A and 0.2 A. The obtained data show a good agreement with the Cabrera-Mott`s model for low temperature oxidation of metals. From kinetic curves, values of the energy barrier (U) and of the electrostatic potential (V) were obtained, which are reasonable with the values reported in the literature. The sample with a larger current applied, exhibited a higher oxidation rate due to a thermal effect and by the larger electrostatic potential established across the oxide layer. This simple method can be interesting to monitoring and controlling the oxidation process.<br><br>En este trabajo se propone un método sencillo para el seguimiento del proceso de oxidación de capas delgadas de aluminio, basado en mediciones de la resistencia eléctrica usando la técnica de cuatro puntas en línea. Se presentan datos de cinética de crecimiento de la capa de óxido de aluminio para dos muestras recién depositadas a las que se aplicaron valores distintos de corriente. Los cambios de temperatura medidos fueron de 2 °C y 9 °C para corrientes aplicadas de 0.1 A y 0.2 A. Los datos muestran buen acuerdo con el modelo de Cabrera-Mott para oxidación de metales en baja temperatura. A partir de las curvas de crecimiento se obtuvieron valores de la barrera de energía (U) y del potencial electrostático (V), razonables con los reportados en la literatura. La muestra con mayor corriente aplicada se oxidó más rápido debido al efecto térmico ya que se estableció un mayor potencial electrostático a través de la capa de óxido. Este sencillo método de análisis puede ser de gran interés para el control del proceso de oxidación en metales

OSL dosimetric properties of synthetic topaz

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.Fil: Camargo Jimenez, Luis Antonio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Fisica Arroyo Seco; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; ArgentinaFil: Trujillo Vázquez, E.. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados; MéxicoFil: Pech Canul, M. I.. Instituto Politécnico Nacional. Centro de Investigación y de Estudios Avanzados; MéxicoFil: Marcazzo, Salvador Julian. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Fisica Arroyo Seco; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires. - Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; Argentin

Sinterización por plasma de particulas ultrafinas de cromita de lantano dopado con clacio obtenidas por síntesis hidrotermal

[EN] Lanthanum chromite nano-particles, with a composition of La0.9Ca0.1CrO3 and La0.8Ca0.2CrO3, were produced by 1 h of hydrothermal reaction at 400 and 425°C respectively. The sintering of the powders was conducted using a spark plasma apparatus over the temperature range 1300-1550ºC for 1 min with a constant loading pressure of 45 MPa. Additional sintering experiments using conventional firing were carried out for comparison. Fully densified (98 % r.d.) lanthanum chromite pellets with fine equiaxial grains 2.3 μm in size were obtained using the SPS (spark plasma sintering) method. In contrast, a maximum relative density of 97 % was produced using La0.8Ca0.2CrO3 sintered conventionally at 1400ºC for 300 min, and the average grain size of the resulting sintered sample was 6 μm.[ES] Partículas ultrafinas de cromita de lantano, con una composición de La0.9Ca0.1CrO3 y La0.8Ca0.2CrO3, se obtuvieron después de 1 hora de síntesis hidrotermal a las temperaturas de 400 y 425°C respectivamente. Los compuestos obtenidos, con un tamaño de partícula de ~ 200 nm, se caracterizaron utilizando las técnicas de DRX, MEB y MET. La sinterización de estos polvos se efectuó en un equipo de chispa de plasma en el rango de temperatura de 1300-1500°C durante 1 min, y a una presión de compactación de 45 MPa. Ambos polvos también se sinterizaron siguiendo un tratamiento térmico convencional, en aire, con el propósito de comparar ambos métodos de sinterización. Las muestras de cromita de lantano sinterizadas por plasma presentaban una densidad relativa del 98 % (ρ/ρt); y una microestructura monofásica con granos equaxiales con un tamaño medio de grano menor de 2.3 μm. En contraste, la composición La0.8Ca0.2CrO3, sinterizada a 1400°C/300 min, por métodos convencionales alcanzó una densidad relativa máxima del 97 % y su microestructura estaba formada por una sola fase con un tamaño medio de grano de 6 μm.The authors acknowledge the support of CONACyT through the research grant (Project 34830-U) and CINVESTAV (Mexico). L.P.R.V. is indebted to the CONACyT for financial support in the form of a PhD scholarship.Peer reviewe