Fernando Ramirez

Functionally graded Cu-SiC-MWCNTs composites were fabricated via mechanical alloying of copper, silicon carbide, and multi-walled carbon nanotube powders, followed by uniaxial hot pressing. Alternative stacking sequences were implemented to investigate the effects of constituents on the copper matrix and to engineer through-thickness gradation of microstructure and properties. This work addresses whether combining SiC particles and MWCNTs in a graded Cu matrix can simultaneously enhance densification and stiffness, and tests the hypothesis that such hybrid gradation yields superior flexural performance that can be captured by the computational model. The materials were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy mapping, X-ray diffraction and Vickers microhardness testing. The results showed a monotonic increase of discrete, well-dispersed SiC through the thickness, while MWCNTs appeared as localized clusters. FGM composites (Cu-SiC) exhibited residual qualitative porosity; however, hybrid FGMs showed no oxygen signals in compositional maps, and XRD identified Cu-SiC-MWCNTs without secondary oxides or carbides. Hardness increased with SiC content and was systematically higher in the hybrids, consistent with MWCNT-assisted densification. Numerical four-point bending simulations compared an explicit ten-layer graded beam with a homogenized equivalent, enabling extraction of layer-wise effective properties and directly linking graded architecture to flexural response. The results demonstrate that SiC-driven stiffness combined with MWCNT-enabled densification yields graded copper composites with enhanced predicted flexural performance.

Frequency spectra of free standing, traction free sphere- and cube-shaped Si, C, and Ge nanoparticles are investigated using both local and non-local elasticity theories. Different particle sizes, internal material lengths, internal-material-length to particle-size ratios, and weighting factors, are considered. Results confirmed that non-local natural frequencies exhibit a non-linear dependence on particle size, indicating that the continuum frequency scale invariance is not valid at the nanoscale. As the l / R ratio decreases, frequencies increase and approach the classical elasticity values. However, for the present non-local formulation, setting l = 0 alone is not sufficient to recover the local elasticity frequencies, the non-local factor must also approach zero. Compared to local elasticity results, nonlocal frequencies are lower, with reductions of up to 44% observed for large non-local weighting factors. In addition, modal shapes and frequency degeneracy are identical for both approaches. Local elasticity formulations provide accurate estimates for the lowest frequencies when the characteristic particle dimension is at least 50 times larger than the internal material length. Finally, polynomial relations are proposed to estimate non-local frequencies from known local elasticity values.

Manufacturing of commercialized silicate is a highly polluting source in geopolymer production. To reduce these emissions, rice husk ash has been considered as an option for synthesizing alternative silicate. However, available research has been focused on the study of geopolymer pastes and mortars, but unknowns remain on its reliability to manufacture plain geopolymer concrete, motivating this research. Goals were synthesizing an unfiltered-alternative sodium silicate from rice husk ash at room conditions (21 °C − 65 ± 5%RH − atmospheric pressure) for manufacturing plain ambient-cured geopolymer concrete and assess its engineering properties. Results shown that the proposed concrete reached 21 MPa, 17959 MPa, 0.183, and 2.13 MPa for 28-day compressive strength, modulus of elasticity, Poisson’s ratio, and splitting tensile capacity, respectively. Results also portrayed that all these properties of the proposed concrete were comparable to those of geopolymer concrete with commercial silicate and traditional Portland-cement concrete. Its elasticity modulus was 44% higher than that of the geopolymer concrete with commercial silicate, which is considered as serviceability proxy. Based on the results, three equation models were developed to predict the engineering properties of the proposed concrete. Results demonstrate that the use of alternative silicate from rice husk ash provides an innovative-cheaper and eco-friendlier geopolymer concrete exhibiting suitable properties for structural applications.

This article presents a pressure sensor composed of a set of three Archimedean spiral inductors. One of them is below the other two concentric spirals, where the upper coils are over a flexible membrane. When external pressure is applied over the sensor, the membrane deforms and changes the distance between the lower spirals and the upper inside coil, the capacitance, and its resonance frequency. However, once the sensor is in contact with a conductive medium, a change in the effective permittivity surrounding the sensor takes place reshaping the resonant frequencies. To improve the sensor response, we introduce a reader resonator sensor system. We present the comparative response of the system as a prototype, an analytical model, and a finite element model (FEM). The prototype system is fabricated using a photolithographic process for the elements of the sensor over a polymethylsiloxane and glass substrate, while the reader is fabricated over a FR4 substrate. COMSOL Multiphysics is used for the FEM simulation of the system to evaluate its behavior. The analytical model involves the integration of techniques to obtain the effective permittivity for each element plane and a coupled magnetic resonance to represent the interaction of the parts. We obtain a correspondence between the resonances of the different elements, with a maximum displacement frequency of 240 MHz for the proposed model, and 80 MHz in the FEM case related to the implemented prototype. The accuracy of this model allows a fast analysis to improve the design of this type of topology without high computational processing.

Contactless sensors with CMOS on-chip antennas use only the top thick metal for the polarization antenna due to the low inductance contribution of the bottom (thin) CMOS metals adding only resistance to the antenna. In this letter, a twolayer inductor (thick top metal layer inductor in series with the bottom thin four-layer metal inductor) configuration was proposed for wireless polarization (by electromagnetic induction). The design proposed was simulated and fabricated in a 0.18-μm CMOS process. The on-chip inductor powers a chip with a 1-mW load at a distance of 2 mm coupled with an external wireless power transmitter at 13.5 MHz with a 30-dBm output. The two-layer inductor configuration reduces the area required for the on-chip inductor by 38% and increases the on-chip inductor quality factor by 25% compared with a single-layer configuration, thus reducing the cost of fabricating fully integrated contactless sensors using only CMOS technologies.

This paper describes an approach to generating the geometry in 3D of composite materials, taking the particular case of concrete. Concrete is presented as a particle array (aggregates) embedded in a matrix (mortar plus fine aggregates). Most of the earliest models consider the aggregates as spheres or ellipsoids, this is a fair approximation considering that the aggregates are oval-shaped but not good enough for crushed aggregates. For this reason, a realistic approach is proposed in which the aggregates are polyhedron-shaped taking into account the geometric description of a particular real sample. The method used to accomplish the generation is based on the Voronoï diagram, the algorithm used is presented and geometries up to 34% of aggregate fraction are included in the results and compared to actual concrete samples.