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1. 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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2. 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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3. Nanostructure and Fracture Behavior of Carbon Nanofiber-Reinforced Cement Using Nanoscale Depth-Sensing Methods

   https://doi.org/10.3390/ma13173837  

   Akono, Ange-Therese ( September 2020 , Materials)

   In recent years, carbon nanofibers have been investigated as a suitable reinforcement for cementitious composites to yield novel multifunctional materials with improved mechanical, electrical, magnetic, and self-sensing behavior. Despite several studies, the interactions between carbon nanofibers and Portland cement hydration products are not fully understood, with significant implications for the mechanical response and the durability at the macroscopic lengthscale. Thus, the research objective is to investigate the influence of carbon nanofibers on the nanostructure and on the distribution of hydration products within Portland cement paste. Portland cement w/c = 0.44 specimens reinforced with 0.0 wt%, 0.1 wt%, and 0.5 wt% CNF by mass fraction of cement are cast using a novel synthesis procedure. A uniform dispersion of carbon nanofibers (CNF) via a multi-step approach: after pre-dispersing carbon nanofibers using ultrasonic energy, the carbon nanofibers are further dispersed using un-hydrated cement particles in high shear mixing and mechanical stirring steps. High-resolution scanning electron microscopy analysis shows that carbon nanofibers fill nanopores and connect calcium–silicate hydrates (C–S–H) grains. Grid nano-indentation testing shows that Carbon nanofibers influence the probability distribution function of the local packing density by inducing a shift towards higher values, η = 0.76–0.93. Statistical deconvolution analysis shows that carbon nanofibers result in an increase in the fraction of high-density C–S–H by 6.7% from plain cement to cement + 0.1 wt% CNF and by 10.7% from plain cement to cement + 0.5 wt% CNF. Moreover, CNF lead to an increase in the C–S–H gel porosity and a decrease in both the capillary porosity and the total porosity. Based on scratch testing, adding 0.1 wt% CNF yields a 4.5% increase in fracture toughness and adding 0.5 wt% CNF yields a 7.6% increase in fracture toughness. Finally, micromechanical modelling predicts an increase of respectively 5.97% and 21.78% in the average Young’s modulus following CNF modification at 0.1 wt% CNF and 0.5 wt% CNF levels. 

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4. Size Effect on Structural Strength of Lego Beams

   Santamarina, A. ; Almeida, L. ( November 2021 , ASME IMECE 2021)

   LEGOs are one of the most popular toys and are known to be useful as instructional tools in STEM education. In this work, we used LEGO structures to demonstrate the energetic size effect on structural strength. Many material's fexural strength decreases with increasing structural size. We seek to demonstrate this effect in LEGO beams. Fracture experiments were performed using 3-point bend beams built of 2 X 4 LEGO blocks in a periodic staggered arrangement. LEGO wheels were used as rollers on either ends of the specimens which were weight compensated by adding counterweights. [1] Specimens were loaded by hanging weights at their midspan and the maximum sustained load was recorded. Specimens with a built-in defect (crack) of half specimen height were considered. Beam height was varied from two to 32 LEGO blocks while keeping the in-plane aspect ratio constant. The specimen thickness was kept constant at one LEGO block. Slow-motion videos and sound recordings of fractures were captured to determine how the fracture originated and propagated through the specimen. Flexural stress was calculated based on nominal specimen dimensions and fracture toughness was calculated following ASTM E-399 standard expressions from Srawley (1976). [2] The results demonstrate that the LEGO beams indeed exhibit a size effect on strength. For smaller beams the Uexural strength is higher than for larger beams. The dependence of strength on size is similar to that of Bažant’s size effect law [3] . Initiation of failure occurs consistently at the built-in defect. The staggered arrangement causes persistent crack branching which is more pronounced in larger specimens. The results also show that the apparent fracture toughness increases as the specimen size decreases. Further ongoing investigations consider the effects of the initial crack length on the size effect and the fracture response. The present work demonstrates that LEGO structures can serve as an instructional tool. We demonstrate principles of non-linear elastic fracture mechanics and highlight the importance of material microstructure (architecture) in fracture response. The experimental method is reproducible in a classroom setting without the need for complex facilities. This work was partially supported by the National Science Foundation (NSF) under the award #1662177 and the School of Mechanical Engineering at Purdue University. The authors acknowledge the support of Dr. Thomas Siegmund and Glynn Gallaway. [1] Khalilpour, S., BaniAsad, E. and Dehestani, M., 2019. A review on concrete fracture energy and effective parameters. Cement and Concrete research, 120, pp.294-321. [2] Srawley, J.E., 1976, January. Wide range stress intensity factor expressions for ASTM E 399 standard fracture toughness specimens. In Conf. of Am. Soc. for Testing and Mater., Committee E-24 (No. E-8654). [3] Bažant, Z.P., 1999. Size effect on structural strength: a review. Archive of applied Mechanics, 69(9), pp.703-725. 

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5. Thermoelectric, Magnetic, and Mechanical Characteristics of Antiferromagnetic Manganese Telluride Reinforced with Graphene Nanoplates

   https://doi.org/10.1002/adem.202000816  

   Hooshmand Zaferani, Sadeq ; Ghomashchi, Reza ; Vashaee, Daryoosh ( October 2020 , Advanced Engineering Materials)

   Mechanical and thermal stability are the two challenging aspects of thermoelectric compounds and modules. Microcrack formation during material synthesis and mechanical failure under thermo‐mechanical loading is commonly observed in thermoelectric materials made from brittle semiconductors. Herein, the results of graphene‐nanoplates (GNPs) reinforcement on the mechanical and thermoelectric properties of MnTe compound are reported. The binary antiferromagnetic MnTe shown promising thermoelectric characteristics due to the paramagnon–hole drag above the Néel temperature. In this study, different bulk MnTe samples are synthesized with the addition of GNPs in a small quantity (0.25–1 wt%) by powder metallurgy and spark plasma sintering. The thermoelectric factors, magnetic behavior, microstructure, and mechanical properties of the samples are evaluated and analyzed. Nearly 33% improvement is observed in the fracture toughness of MnTe reinforced with 0.25 wt% GNPs compared to the pristine structure. The Néel temperature remains approximately unaffected with the GNP inclusion; however, the low‐temperature ferromagnetic phase impurity is significantly suppressed. The thermal conductivity and power factor decrease almost equally by ≈34% at 600 K; hence, the thermoelectric figure‐of‐merit is not affected by GNP reinforcement in the optimized sample.

    

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