Enrique Manuel López Cuéllar

Engineering of SiO2 Nanoparticles for Optimal Performance in Nano Cement-Based Materials

The reported research examined the effect of 5-70 nm SiO2 nanoparticles on the mechanical properties of nano-cement materials. The strength development of portland cement with nano-SiO2 and superplasticizing admixture was investigated. Experimental results demonstrate an increase in the compressive and flexural strength of mortars with developed nanoparticles. The distribution of nano-SiO2 particles within the cement paste plays an essential role and governs the overall performance of these products. Therefore, the addition of a superplasticizer was proposed to facilitate the distribution of nano-SiO2 particles. Superplasticized mortars with 0.25% of selected nano-SiO2 demonstrated a 16% increase of 1-day compressive strength, reaching 63.9 MPa; the 28-day strength of these mortars was 95.9 MPa (vs. strength of reference superplasticized mortars of 92.1 MPa). Increase of 28-day flexural strength of superplasticized mortars with selected nano-SiO2 was 18%, reaching 27.1 MPa. It is concluded that the effective dispersion of nanoparticles is essential to obtain composite materials with improved performance.

Performance of Cement Systems with Nano-SiO2 Particles Produced by Using the Sol–Gel Method

The effect of 5- to 70-nm SiO2 nanoparticles on the mechanical properties of nanocement materials was examined. The strength development of portland cement with nano-SiO2 and superplasticizing admixture was investigated. Experimental results demonstrate an increase in the compressive and flexural strengths of mortars with SiO2 nanoparticles. Because the distribution of nano-SiO2 particles within the cement paste plays an essential role and governs the overall performance of these products, the addition of a superplasticizer was proposed to facilitate the distribution of nano-SiO2 particles. The application of superplasticizer and high-speed dispergation were found to be effective disagglomeration techniques to improve the strength of superplasticized portland cement mortars, which reached up to 63.9 and 95.9 MPa (Sample 4B3, synthesized by using the sol–gel method in a base reaction medium at ethanol-to-tetraethoxysilane and water-to-tetraethoxysilane molar ratios of 24) at the ages of 1 day and 28 days, respectively. At corresponding ages, the compressive strength of reference portland cement mortars was 53.3 and 86.1 MPa. It is concluded that the effective dispersion of nanoparticles is essential to obtain composite materials with improved performance.

Thermomechanical Analysis on Ti-Ni Shape Memory Helical Springs Under Cyclic Tensile Loads

Shape memory alloys (SMAs) present some characteristics, which make it unique material to be use in applications that require strength and shape recovery. This alloys was been used to manufacture smart actuators for mechanical industry devices and several other applications in areas as medicine, robotics, aerospace, petroleum and gas industries. However it is important to know the actuators response to external stimulus (heat source, electrical current and/or external stress) in these technological applications. This work investigated the thermomechanical behaviors of helical actuators produced from Ti-Ni alloy commercial wires. Initially, the wire was subjected to some heat treatment and characterized by differential scanning calorimeter (DSC), scanning electron microscopy (SEM), optical microscopy (OM) and Energy dispersive spectroscopy. Then two heat treatments were selected to obtain the helical actuators. The actuators were tested in an apparatus developed to apply an external traction stress in helical actuators during thermal cycles. Two wires were tested in a dynamic mechanical analyzer (DMA). The results were analyzed in comparison with thermoplastic properties obtained in thermomechanical tests. The analysis took into consideration the wiring forming process, the precipitates formation, the stress fields generated by dislocations and reorientation of martensite variants during the actuators training process.

Recrystallization of a Ti-45Ni-5Cu cold-worked shape memory alloy characterized by thermoelectric power and electrical properties

The recrystallization of a cold-worked shape memory alloy (SMA) without R-phase transition, i.e. the Ti-45.0Ni-5.0Cu (% at.), has been studied by thermoelectric power (TEP) technique and electrical resistivity change (Dρ) on several heat treated samples after cold working. This study was also supported by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness Vickers (HV) measurements. Transformation temperatures show an increase with heat treatment temperature, while hysteresis decreases. Hardness values decrease with aging temperature for the same heat treatment time. No important changes were observed in electrical resistivity with heat treatment temperature, except at 300oC. TEP results show an important increase with heat treatment temperature, and a linear relationship was found between hardness and thermoelectric power. With this correlation between TEP and microhardness it is possible to predict its mechanical response by knowing its TEP, so this correlation could be important for applications.

Effect of Spun Velocities and Composition on the Microstructure and Transformation Temperatures of TiNi Shape Memory Ribbons

Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloy ribbons were fabricated by melt-spinning method at different circumferential wheel velocities. The effects of wheel velocity, chemical composition and heat treatments on microstructure and Transformation temperature were investigated. Differences in wheel velocity led to differences in cooling rate and sample dimension, as well as in phase transformation temperatures. Two heat treatment conditions were studied, 350°C for 1h and 350°C for 5h. In the samples produced at high wheel velocity and heat-treated at 350°C for 5h, nanosized Ti-rich precipitates were observed in both chemical compositions. Cross-sectional microstructure was studied by optical microscopy; SEM was used to study the nanometric grains and nano precipitation. The transformation temperatures were analyzed by DSC.

Effect of Spun Velocities and Composition on the R‒phase and Thermomechanical Behavior in Ti‒Ni Ribbons Electrically Heated

This work deals with structural, electrical and mechanical characterization of Ti‒50.13Ni and Ti‒49.62Ni (at.%) shape memory alloys (SMAs) fabricated at different circumferential wheel velocities. The effect of wheel velocity, chemical composition and heat treatments are investigated. The characterization of crystallographic phases of the Ti‒Ni ribbons was carried out using X-ray diffraction. Electrical resistance variations as function of temperature (∆R/R %) were analyzed using a non-commercial technique, which consists in a thermal-adjustable bath apparatus revealing the temperatures of B2→R→B19´ two stage transformation, whereby the presence of R‒phase can be definitively confirmed. The Stress-Assisted Two-Way Memory Effect was measured by an own designed apparatus with an Linear Variable Differential Transformer captor and a current controlled heating, and results indicate that the as-spun condition, promotes the Stress-Assisted Two-Way Memory Effect. On the other hand, increments in Ni content tend to decrease transformation temperatures and high wheel velocities help to the R‒phase formation.