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1. Influence of multi-walled carbon nanotubes on the hydration products of ordinary Portland cement paste

   https://doi.org/https://doi.org/10.1016/j.cemconres.2020.106197  

   Jiaxin Chen, Ange-Therese Akono (November 2020, Cement and concrete research)

   null (Ed.)

   We elucidate the mechanisms by which multi-walled carbon nanotubes (MWCNTs) influence the microstructure, fracture behavior, and hydration of cement paste. We disperse MWCNTs using a multi-step approach that involves high-energy pre-dispersion using ultrasonic energy followed by low-energy dispersion using un-hydrated cement particles. In turn, the low-energy dispersion step involves high-shear mixing and mechanical stirring. High-resolution environmental scanning electron microscopy of cement+0.2 wt% MWCNT, cement+0.5 wt% MWNCT, and of cement+1 wt% MWCNT show that MWCNTs bridge air voids, thereby refining the pore size and strengthening the C-S-H matrix. The fracture toughness increased by 9.38% with the addition of 0.2 wt% multi-walled carbon nanotubes, and by 14.06% with the addition of 0.5 wt% multi-walled carbon nanotubes and ligament bridging was the dominant toughening mechanism. Moreover, for all reinforcement levels, MWCNTs induced a conversion of low-density C-S-H into high-density C-S-H along with a drastic drop in the capillary porosity: adding 0.1–0.5 wt% MWCNT resulted in a 200% increase in the volume fraction of high-density C-S-H. Thus, our experiments show that MWCNT enhances the mechanical properties and transport properties by: (i) promoting high-density C-S-H formation, (ii) promoting calcium hydroxide formation, (iii) filling microscopic air voids, (iv) reducing the capillary porosity, (v) increasing the fraction of small gel pores (1.2–2 nm in size), and (vi) by bridging microcracks. 

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2. Multiwalled carbon nanotubes as hard templates to yield advanced geopolymer-based self-assembled nanostructured ceramics

   https://doi.org/10.1016/j.mechrescom.2023.104216  

   Xu, Yunzhi; Lee, Haklae; Buettner, Nathanial; Akono, Ange-Therese (December 2023, Mechanics Research Communications)

   Novel multifunctional construction materials are needed to promote resilient infrastructure in the face of climate change and extreme weather. Nanostructured materials such as geopolymer reinforced with carbon-based nanomaterials are a promising way to reach that goal. In recent years, several studies have investigated the influence of nanomaterials on the physical properties of geopolymer composites such as compressive strength and fracture toughness. Yet, a fundamental understanding of the influence of nanomaterials on the nanoscale and micron-scale structure has been elusive so far. Our research objective is to understand how multiwalled carbon nanotubes (MWCNT) can help tailor the microstructure of geopolymers to yield architected multifunctional nanocomposites. We synthesized geopolymer nanocomposites reinforced with 50-nm thick multiwalled carbon nanotubes with mass fractions in the range of 0.1 wt%, 0.2 wt%, and 0.5 wt%. Our major finding is that MWCNTs act as hard templates that promote geopolymer formation via self-assembly. Geopolymer nanoparticle growth is observed along the walls of MWCNTs. A refinement in grain size is observed: increasing the fraction of MWCNTs by 0.5 wt% leads to a reduction in grain size by 54%. Similarly, increasing the mass fraction of MWCNTs leads to a densification of the geopolymer matrix as demonstrated by the Fourier transform infrared spectroscopy results and the statistical deconvolution analysis. Mercury intrusion porosimetry shows a nanoscale tailoring of the pore size distribution: a 26% decrease in porosity is observed as the fraction of MWCNTs is increased to 0.5 wt%. As a result of these nanoscale structural changes, a greater resistance to long-term deformation is observed for MWCNT-reinforced geopolymers, as the creep modulus increases both locally and macroscopically. At the macroscopic level, a 42% increase in the macroscopic logarithmic creep modulus is observed as the fraction of MWCNTs is increased to 0.5 wt%. These findings and the supporting methodology are important to understand how to manipulate matter below 100 nm. This research also paves the way for the design of resilient infrastructure materials with tailored microstructure and mechanical properties. 

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3. Topology-Engineered Piezoresistive Lattices with Programmable Strain Sensing, Auxeticity, and Failure Modes

   https://doi.org/10.1039/D5MH00884K  

   Schneider, Johannes; Utzeri, Mattia; Krishnamurthy, Vinayak Raman; Akleman, Ergun; Kumar, S (January 2025, Materials Horizons)

   This study investigates the programmable strain sensing capability, auxetic behaviour, and failure modes of 3D-printed, self-monitoring auxetic lattices fabricated from in-house engineered polyetheretherketone (PEEK) reinforced with multi-walled carbon nanotubes (MWCNTs). A skeletally-parametrized geometric modelling framework, combining Voronoi tessellation with 2D wallpaper symmetries, is used to systematically explore a vast range of non-predetermined topologies beyond traditional lattice designs. A representative set of these architectures is realized via fused filament fabrication, and multiscale characterization—including macroscale tensile testing and microstructural analysis—demonstrates tuneable multifunctional performance as a function of MWCNT content and unit cell topology. Real-time resistance measurements track deformation, damage initiation, and progression, with the sensitivity factor increasing from below 1 in the elastic regime (strain sensitivity) to as high as 80 for PEEK/MWCNT at 6 wt.% under inelastic deformation (damage sensitivity). Implicit architecture-topology tailoring further allows fine-tuning of mechanical properties, achieving stiffness values ranging from 9 MPa to 63 MPa and negative Poisson’s ratios between –0.63 and –0.17 using ~3 wt.% MWCNT at a relative density of 25%. Furthermore, a novel piezoresistive finite element model, implemented in Abaqus via a user-defined subroutine, accurately captures the electromechanical response up to the onset of ligament failure, offering predictive capability. These results demonstrate how architecture-topology tuning can be leveraged to customise strain sensitivity and failure modes, enabling the development of multifunctional piezoresistive lattice composites for applications such as smart orthopaedic implants, aerospace skins, and impact-tolerant systems. 

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4. Joining of metal and non-polar polypropylene composite through a simple functional group seeding layer

   https://doi.org/10.1016/j.jmapro.2022.11.022  

   Khan, A.S.; Liu, F.; Dong, P. (January 2023, Journal of Manufacturing Processes)

   Polypropylene (PP) and its composites are one of the hardest to directly join with metals due to their inherent chemical incompatibility. This paper presents a simple, efficient, and cost-effective method for joining PP composite to aluminum alloy in spot welding configuration by seeding the functional groups via an insert layer of PA6 thin film without requiring surface or material pre-treatment. The resulting joint loading capacity is shown to be sufficiently high to consistently develop failures in PP substrates in lap shear tensile tests away from the bonded area. Joint interface microstructure features are examined in detail. Bonding mechanisms are then described based on the detailed observations obtained in this study. 

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5. Enhanced joining strength in additive-manufactured polylactic-acid structures fused by embedded heated metallic meshes

   https://doi.org/10.1016/j.jmapro.2024.04.089  

   Cao, Dongyang; Bouzolin, Dan; Lu, Hongbing; Griffith, D Todd (July 2024, Journal of Manufacturing Processes)

   Additively manufactured thermoplastic polymers, such as polylactic acid (PLA), hold significant promise for sustainable engineering structures, including wind turbine blades. Upscaling these structures beyond the limitations of 3D printer build volumes is a challenge; fusion joining presents a potential solution. This paper introduces a displacement-controlled resistance welding process for PLA, as an alternative to the typical force controlled methods. We investigated the bonding quality of resistance-welded and adhesive-bonded PLA beams through three-point bending and measured the surface deformations using digital image correlation. Different metal meshes (30 %/0.11 mm Ni–Cu, 34 %/0.07 mm Ni–Cu, and 36 %/0.25 mm Co–Ni) served as heating elements. The process parameters were varied for the 34 %/0.07 mm Ni–Cu mesh to identify an optimum set of parameters. Results showed that this optimized displacement-controlled welding achieved 94 % of the original strength of monolithic samples. This indicates that the new welding process not only ensures high quality bonding and fine surface finishing but also promotes sustainability, recyclability, and economic efficiency in various polymer and composite structural applications. 

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