Superficies y Vacío

Photoluminescent carbon colloids prepared by laser fragmentation of carbon from waste coffee grounds

2024-08-30T16:51:34-06:00

Colloidal suspensions of carbon nanostructures (CNSs) were prepared by laser fragmentation in various liquid media using heat-treated coffee grounds as carbon precursor. A study by calorimetry was done in powder of waste coffee grounds to determine the temperature of obtaining carbon. The experiments were carried out in two stages, the first one consisted in obtaining the carbon source, for which powder of waste coffee grounds was thermally treated in air. The as-obtained carbon was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy and infrared spectroscopy. In the second step the as-obtained carbon was separately dispersed in four liquid media (acetone, toluene, methanol and isopropyl alcohol) to be fragmented by using a ns-pulsed Nd:YAG laser at its 1064 nm fundamental emission. The morphological features of the carbon nanostructures were obtained by transmission electron microscopy, while the optical properties of the colloidal suspensions were characterized by UV-Vis and photoluminescence spectroscopies. Results indicate that carbon nanostructures are successfully obtained in the four liquid media after the fragmentation process. The four colloidal suspensions show photoluminescent properties, which are seen to depend on the liquid medium nature. We found that the liquid medium also influences the efficiency of the laser fragmentation.

Method for measuring the setting process of cement-water mixtures by electrical impedance

2024-01-11T10:11:58-06:00

The setting and hardening of cement paste can be taken as the progressive hydration reaction of cement. This paper reports the findings on setting time of cement paste at early hydration by using a novel method named relative electrical impedance (REI). The novel REI method measures the relative electrical impedance between electrodes (ring shaped) embedded in the cement paste. REI measurements were conducted on cement paste samples with a water-cement ratio of 1:2 over 24 hours. REI measurements were compared with VICAT and heat latent methods for indirect validation. It was found that the proposed REI technique determined the 4 periods of the setting time process (sleeping, hydration, deceleration, and diffusion) and allowed the measurement of the sleeping period in detail. It was concluded that the novel REI method could be used as a new method to measure the setting time of cement paste in early hydration.

Tailoring optical response in nanostructured bilayers: Effects of surface roughness and layer thickness on NbN/Nb and TiN/Ti

2023-10-13T13:24:43-06:00

Numerical calculations were performed on NbN/Nb and TiN/Ti nanostructured bilayers. Discrete dipole approximation
implemented in the DDSCAT code was utilized. In order to design a TUC (Target Unit Cell) with realistic surfaceroughness
pattern, Atomic Force Microscope images of 100 ±10 nm Nb thin films were used. The optical properties of nanostructured bilayers (60-100 nm thickness with a ±10 nm surface roughness) were calculated, keeping the interface
between the layers as smooth flat surfaces. For the NbN/Nb nanostructure with a thickness between 100 ±10 nm, and 70 ±10 nm, in a wavelength range between 200 and 350 nm, the reflectance and absorptance show a nearly flat spectrum
with an intensity of 20% and 80%, respectively. For larger wavelengths, absorptance decays smoothly. In contrast, for the 60 ±10 nm bilayer, the absorptance decays faster. For the TiN/Ti nanostructure, at 415 nm, a relative maximum in the reflectance, and a minimum in absorptance, are observed. The critical value of the absorbed light spectra begins to shift to higher wavelengths as the thickness of the TiN layer increases. We demonstrate that adding a rough nitride layer on top of its metallized layer drastically modifies the optical response of the nanostructured bilayer, an interesting result for
plasmonics and nanoelectronics applications.

Study of electronic and optical properties of Sm3xY3(1-x)Ba5Cu8O18 superconductor using the LAPW+lo method

2024-05-17T13:32:46-06:00

This work presents a calculation of the electronic and optical properties of the family of high critical temperature superconductors (T_c > 90 K) Sm_3xY_3(1-x)Ba₅Cu₈O₁₈ (Sm_3xY_3(1-x)58), para 0 < x < 1, as well as those of Y₁Ba₂Cu₃O₇ (Y123) and those of Sm₁Ba₂Cu₃O₇ (Sm123), carried out using the first principles method LAPW+lo (linearized augmented plane wave + local orbitals), within the DFT theory (density functional theory). The Density of Electronic States (DOS) and optical conductivity are calculated for four crystal structures generated by varying x (for x = 0; 1/3; 2/3; 1). The objective was determined. DOS (partial and total) and optical conductivity at low energies. In particular the effect of the f orbitals of Sm near the Fermi energy (E_f). It was found that by substituting Sm atoms for Y atoms, the total DOS is affected around E_f due to the contribution of the Sm f orbitals. When the system has two atoms of Y and one of Sm (Sm₁Y₂58). The value of the total DOS in the E_f increases 547% with respect to the compound Y358. When the system has 2 atoms of Sm and one of Y (Sm₂Y₁58), the value of the total DOS in the E_f increases 1043% with respect to the compound Y358. While in the case of Sm358, it increases 1432%. For Sm123 and Sm358 the optical conductivity presents a peak near the origin. While for Y358 it does not present any peak in conductivity and presents very low values near the origin. For the cases x = 1/3 and 2/3 we find that the conductivity is modified near the origin. The highest value occurs for the system containing two Sm atoms, this was at 0.14 eV.