Search Results (9)

Sustainability, safety and service life expansion in the construction sector have gained a lot of scientific and technological interest during the last few decades. In this direction, the synthesis and characterization of smart cementitious composites with tailored properties combining mechanical integrity and self-sensing capabilities have been in the spotlight for quite some time now. The key property for the determination of self-sensing behavior is the electrical resistivity and, more specifically, the determination of reversible changes in the electrical resistivity with applied stress, which is known as piezoresistivity. In this study, the mechanical and piezoresistive properties of mortars reinforced with carbon nanotubes (CNTs) and carbon micro-fibers (CMFs) are determined. Silica fume and a polymer with polyalkylene glycol graft chains were used as dispersant agents for the incorporation of the CNTs and CMFs into the cement paste. The mechanical properties of the mortar composites were investigated with respect to their flexural and compressive strength. A four-probe method was used for the estimation of their piezoresistive response. The test outcomes revealed that the combination of the dispersant agents along with a low content of CNTs and CMFs by weight of cement (bwoc) results in the production of a stronger mortar with enhanced mechanical performance and durability. More specifically, there was an increase in flexural and compressive strength of up to 38% and 88%, respectively. Moreover, mortar composites loaded with 0.4% CMF bwoc and 0.05% CNTs bwoc revealed a smooth and reversible change in electrical resistivity vs. compression loading—with unloading comprising a strong indication of self-sensing behavior. This work aims to accelerate progress in the field of material development with structural sensing and electrical actuation via providing a deeper insight into the correlation among cementitious composite preparation, admixture dispersion quality, cementitious composite microstructure and mechanical and self-sensing properties. Full article

Electroactive microfiber-based scaffolds aid neural tissue repair. Carbon microfibers (CMFs) coated with the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly[(4-styrenesulfonic acid)-co-(maleic acid)] (PEDOT:PSS-co-MA) provide efficient support and guidance to regrowing axons across spinal cord lesions in rodents and pigs. We investigated the electrical and structural performance of PEDOT:PSS-co-MA-coated carbon MFs (PCMFs) for long-term, biphasic electrical stimulation (ES). Chronopotentiometry and electrochemical impedance spectroscopy (EIS) allowed the characterization of charge transfer in PCMFs during ES in vitro, and morphological changes were assessed by scanning electron microscopy (SEM). PCMFs that were 4 mm long withstood two-million-biphasic pulses without reaching cytotoxic voltages, with a 6 mm length producing optimal results. Although EIS and SEM unveiled some polymer deterioration in the 6 mm PCMFs, no significant changes in voltage excursions appeared. For the preliminary testing of the electrical performance of PCMFs in vivo, we used 12 mm long, 20-microfiber assemblies interconnected by metallic microwires. PCMFs-assemblies were implanted in two spinal cord-injured pigs and submitted to ES for 10 days. A cobalt–alloy interconnected assembly showed safe voltages for about 1.5 million-pulses and was electrically functional at 1-month post-implantation, suggesting its suitability for sub-chronic ES, as likely required for spinal cord repair. However, improving polymer adhesion to the carbon substrate is still needed to use PCMFs for prolonged ES. Full article

This paper investigates the mechanical properties and the durability implications of innovative cement-based mortars doped with carbon microfibers. In particular, mixes with different amounts of carbon additions are produced, and the properties of fresh and hardened samples are analyzed through workability, water absorption, and compressive and flexural tests under specific environmental conditions. These composites can be employed to enhance construction performance or provide structures with strain-monitoring capabilities. However, the analysis of their mechanical properties and their durability behavior is needed before extensive structural use. In this work, the preparation procedure is defined for the various mix designs, considering different amounts of carbon microfibers; then, fresh properties are evaluated, and different types of samples formed. After various curing times, the specific rheological and hardened properties of the specimens are tested in different conditions to consider the durability of the composites, essential for the real-scale adoption in structural elements. Preliminary electrical and sensing tests are first conducted to evaluate the monitoring potential of the investigated composites. The findings highlight the impact of carbon inclusions on the performance of cement-based mortars, offering valuable insights for their utilization in masonry construction or for repairing concrete structures. In particular, sensing capabilities are found to be highly enhanced by the presence of CMF. Additionally, the results of this research pinpoint key areas for further analysis in the material’s development process. Full article

Wind energy is considered a clean energy source and is predicted to be one of the primary sources of electricity. However, leading-edge erosion of wind turbine blades due to impacts from rain drops, solid particles, hailstones, bird fouling, ice, etc., is a major concern for the wind energy sector that reduces annual energy production. Therefore, leading-edge protection of turbine blades has been an important topic of research and development in the last 20 years. Further, there are critical issues related to the amount of waste produced, including glass fiber, carbon fiber, and various harmful volatile organic compounds in turbine fabrication and their end-of-life phases. Hence, it is vital to use eco-friendly, solvent-free materials and to extend blade life to make wind energy a perfect clean energy source. In this study, cellulose microparticles (CMP) and cellulose microfibers (CMF) have been used as fillers to reinforce water-based polyurethane (PU) coatings developed on glass fiber reinforced polymer (GFRP) substrates by a simple spray method for the first time. Field emission scanning electron microscopy images show the agglomerated particles of CMP and fiber-like morphology of CMF. Fourier transform infrared spectra of CMP, CMF, and related coatings exhibit associated C–H, C=O, and N–H absorption bands of cellulose and polyurethane. Thermal gravimetric analysis shows that CMP is stable up to 285 °C, whereas CMF degradation is observed at 243 °C. X-ray photoelectron spectroscopy of C 1s and O 1s core levels of CMP, CMF and related coatings show C–C/C–H, C–O, C–OH, and O–C=O bonds associated with cellulose structure. The solid particle erosion resistance properties of the coatings have been evaluated with different concentrations of CMP and CMF at impact angles of 30° and 90°, and all of the coatings are observed to outperform the PU and bare GFRP substrates. Three-dimensional (3D) profiles of erosion scans confirm the shape of erosion scars, and 2D profiles have been used to calculate volume loss due to erosion. CMP-reinforced PU coating with 5 wt.% filler concentration and CMF-reinforced PU coating with 2 wt.% concentration are found to be the best-performing coatings against solid particle erosion. Nanoindentation studies have been performed to establish a relation between H³/E² and the average erosion rate of the coatings. Full article

The synthesis and characterization of fibrous materials with a hierarchical structure are of great importance for materials sciences. Among this class of materials, microfibers of different natures coated with carbon nanofibers attract special interest. Such coating modifies the surface of microfibers, makes it rougher, and thus strengthens its interaction with matrices being reinforced by the addition of these microfibers. In the present work, a series of hierarchical materials based on carbon microfibers, basalt microfibers, and fiberglass cloth coated with up to 50 wt% of carbon nanofibers was synthesized via the catalytic chemical vapor deposition technique. The initial items were impregnated with an aqueous solution of nickel nitrate and reduced in a hydrogen flow. Then, the catalytic chemical vapor deposition process using C₂H₄ or C₂H₄Cl₂ as a carbon source was carried out. A simple and cost-effective technique for the preparation of the samples of hierarchical materials for transmission electron microscopy examination was developed and applied for the first time. The proposed method of sample preparation for sequential TEM visualization implies an ultrasonic treatment of up to four samples simultaneously under the same conditions by using a special sample holder. As was found, the relative strength of carbon nanofibers coating the surface of microfibers decreases in the order of CNF/CMF > CNF/BMF > CNF/FGC. Two effects of the ultrasonic action on the carbon coating were revealed. First, strongly bonded carbon nanofibers undergo significant breakage. Such behavior is typical for carbon and basalt microfibers. Secondly, carbon nanofibers can be completely detached from the microfiber surface, as was observed in the case of fiberglass cloth. In the case of CNF/CMF material, the graphitized surface of carbon microfiber is coherent with the structure of carbon nanofiber fragments grown on it, which explains the highest adhesion strength of the carbon nanolayer coated on carbon microfibers. Full article

This work investigates the interplay of carbonization temperature and the chemical composition of carbon microfibers (CMFs), and their impact on the equilibration time and adsorption of three molecules (N₂, CO₂, and CH₄). PAN derived CMFs were synthesized by electrospinning and calcined at three distinct temperatures (600, 700 and 800 °C), which led to samples with different textural and chemical properties assessed by FTIR, TGA/DTA, XRD, Raman, TEM, XPS, and N₂ adsorption. We examine why samples calcined at low/moderate temperatures (600 and 700 °C) show an open hysteresis loop in nitrogen adsorption/desorption isotherms at −196.15 °C. The equilibrium time in adsorption measurements is nearly the same for these samples, despite their distinct chemical compositions. Increasing the equilibrium time did not allow for the closure of the hysteresis loop, but by rising the analysis temperature this was achieved. By means of the isosteric enthalpy of adsorption measurements and ab initio calculations, adsorbent/adsorbate interactions for CO₂, CH₄ and N₂ were found to be inversely proportional to the temperature of carbonization of the samples (CMF-600 > CMF-700 > CMF-800). The enhancement of adsorbent/adsorbate interaction at lower carbonization temperatures is directly related to the presence of nitrogen and oxygen functional groups on the surface of CMFs. Nonetheless, a higher concentration of heteroatoms also causes: (i) a reduction in the adsorption capacity of CO₂ and CH₄ and (ii) open hysteresis loops in N₂ adsorption at cryogenic temperatures. Therefore, the calcination of PAN derived microfibers at temperatures above 800 °C is recommended, which results in materials with suitable micropore volume and a low content of surface heteroatoms, leading to high CO₂ uptake while keeping acceptable selectivity with regards to CH₄ and moderate adsorption enthalpies. Full article

The isosteric enthalpy of adsorption (${\Delta }_{ads}\dot{h}$) of CO₂ in three different micro and mesoporous materials was evaluated in this work. These materials were a microporous material with functional groups of nitrogen and oxygen (CMFs, carbon microfibers), a mesoporous material with silanol groups (SBA-15, Santa Barbara Amorphous), and a mesoporous material with amine groups (SBA-15_APTES, SBA-15 amine-functionalized with (3-Aminopropyl)-triethoxysilane). The temperature interval explored was between 263 K and 303 K, with a separation of 5 K between each one, so a total of nine CO₂ isotherms were obtained. Using the nine isotherms and the Clausius–Clapeyron equation, the reference value for ${\Delta }_{ads}\dot{h}$ was found. The reference value was compared with those ${\Delta }_{ads}\dot{h}$ obtained, considering some arrangement of three or five CO₂ isotherms. Finally, it was found that at 298 K and 1 bar, the total amount of CO₂ adsorbed is 2.32, 0.53, and 1.37 mmol g⁻¹ for CMF, SBA-15, and SBA-15_APTES, respectively. However, at a coverage of 0.38 mmol g⁻¹, ${\Delta }_{ads}\dot{h}$ is worth 38, 30, and 29 KJ mol⁻¹ for SBA-15_APTES, CMFs, and SBA-15, respectively. So, physisorption predominates in the case of CMF and SBA-15 materials, and the ${\Delta }_{ads}\dot{h}$ values significantly coincide regardless of whether the isotherms arrangement used was three or five. Meanwhile, in SBA-15_APTES, chemisorption predominates as a consequence of the arrangements used to obtain ${\Delta }_{ads}\dot{h}$. This happens in such a way that the use of low temperatures (263–283 K) tends to produce higher ${\Delta }_{ads}\dot{h}$ values, while the use of high temperatures (283–303 K) decreases the ${\Delta }_{ads}\dot{h}$ values. Full article

This paper aims to demonstrate the self-protection and self-sensing functionalities of self-compacted concrete (SCC) containing carbon nanotubes (CNT) and carbon microfibers (CMF) in a hybrid system. The ability for self-sensing at room temperature and that of self-protection after thermal fatigue cycles is evaluated. A binder containing a high volume of supplementary mineral additions (30%BFSand20%FA) and different type of aggregates (basalt, limestone, and clinker) are used. The self-diagnosis is assessed measuring electrical resistivity (ER) and piezoresistivity (PZR) in compression mode within the elastic region of the concrete. Thermal fatigue is evaluated with mechanical and crack measurements after heat cycles (290–550 °C). SCC withstands high temperature cycles. The protective effect of the hybrid additive (CNT+CMF) notably diminishes damage by keepinghigher residual strength and lessmicrocracking of the concrete. Significant reductions in ER are detected. The self-diagnosis ability of functionalized SCC isconfirmed with PZR. A content of the hybrid functional additive (CNT+CMF) in the percolation region is recommended to maximize the self-sensing sensitivity. Other parameters as sample geometry, sensor location, power supply, and load level have less influence. Full article

Carbon microfibers (CMF) has been used as an adsorbent material for CO₂ and CH₄ capture. The gas adsorption capacity depends on the chemical and morphological structure of CMF. The CMF physicochemical properties change according to the applied stabilization and carbonization temperatures. With the aim of studying the effect of stabilization temperature on the structural properties of the carbon microfibers and their CO₂ and CH₄ adsorption capacity, four different stabilization temperatures (250, 270, 280, and 300 °C) were explored, maintaining a constant carbonization temperature (900 °C). In materials stabilized at 250 and 270 °C, the cyclization was incomplete, in that, the nitrile groups (triple-bond structure, e.g., C≡N) were not converted to a double-bond structure (e.g., C=N), to form a six-membered cyclic pyridine ring, as a consequence the material stabilized at 300 °C resulting in fragile microfibers; therefore, the most appropriate stabilization temperature was 280 °C. Finally, to corroborate that the specific surface area (microporosity) is not the determining factor that influences the adsorption capacity of the materials, carbonization of polyacrylonitrile microfibers (PANMFs) at five different temperatures (600, 700, 800, 900, and 1000 °C) is carried, maintaining a constant temperature of 280 °C for the stabilization process. As a result, the CMF chemical composition directly affects the CO₂ and CH₄ adsorption capacity, even more directly than the specific surface area. Thus, the chemical variety can be useful to develop carbon microfibers with a high adsorption capacity and selectivity in materials with a low specific surface area. The amount adsorbed at 25 °C and 1.0 bar oscillate between 2.0 and 2.9 mmol/g adsorbent for CO₂ and between 0.8 and 2.0 mmol/g adsorbent for CH₄, depending on the calcination treatment applicated; these values are comparable with other material adsorbents of greenhouse gases. Full article