Nanocomposites With Carbon Nanostructures

One of the ways to solve the problem of increasing the physical and mechanical properties of materials is to create composites that are formed from structurally heterogeneous substances by various methods, depending on the initial structure of the matrix.

Currently, the most active studies are being conducted for metal-based composites such as aluminum; copper; iron, etc. A significant amount of research has been done for polymer-based composites. In recent years, active research has been conducted for materials based on cement binders.

Despite the principal difference in the matrix basis, the uniformity of the distribution of reinforcing nanoscale structures in the bulk of the material and the chemical interaction between the filler and the matrix are the most important principles that ultimately determine the properties of the composite being created, which can be achieved by a one-stage process with the best performance in Tornado technology.

Creation of composite materials begins with the preparation of the original components. Of particular interest are nanocomposites, the dimensions of the structural elements of which (at least one of the components) do not exceed 100 nm.
In this dimensional region, the hardening particles acquire a special structure and exhibit unique mechanical, electronic, thermal properties.
Such objects include carbon nanostructures discovered over the last decades — fullerenes, nanotubes, astralenes, nanodiamonds.

As already noted, a significant problem in creating nanocomposites is the uniform distribution of nanoparticles in the volume of the material. It is due to the propensity of nanoparticles to agglomerate. Due to the positive features of the “Tornado” technology, polymer, mineral and astringent materials do not lose their original physicochemical properties (no heating) when grinding, and the input of nanoscale components is controlled at the initial stage of grinding with the necessary proportions in the final product — composition .

Before adding to the composite material, carbon nanostructures are subjected to special treatment in the Tornado activation device, which leads to an increase in the interaction energy of the particles with the matrix, contributing to homogeneity of the particle distribution.

The use of functionalized nanoparticles makes it possible to significantly change the properties of the resultant composite material. The introduction of various nanoscale carbon nanostructures can significantly improve the mechanical properties of ceramics, mixtures and other materials in comparison with the original unmodified formulations.

At the same time, there is an increase in the service properties, such as heat resistance, thermal conductivity, and others.
It was found that fullerenes and multi-walled nanotubes most effectively influence the properties, then carbon nanotubes previously activated in the Tornado, that is, with the defects introduced.

CNS (carbon nanostructures) acquire chemical activity and can interact with both polymer macromolecules and polymer degradation products.
In the same way, according to the developed technology, CNS behaves in cement and concrete composites with predetermined properties.  Since the CNT functional group (carbon nanotube) is separated from the electron, the CNT surface becomes more active and acquires a positive charge, as a result of which the probability of interaction with hydrocarbon ions increases.  The interaction energy, for example, is about 136 kcal/mol on polypropylene.

Building composites and concrete

At present, the improvement of physical and mechanical properties and the imparting of new functional properties to building composites based on cement binders are possible due to the modification of the matrix by carbon nanotubes.  It is known that CNTs possess unique chemical, mechanical and electrically conductive properties, while under the action of van der Waals forces, CNTs tend to form agglomerates, which is the main difficulty in their use as fillers for materials with a matrix of any chemical composition.  The main task of using CNT as a reinforcing component in concrete is the uniform distribution of individual nanoparticles in the volume of the composite, which is achieved by two methods developed:

  • «Dry method» in «Tornado» in the preparation and grinding of any component for concrete bearing CNT, be it sand or cement, or a finished composition of all components of concrete with the already activated activated CNTs. The properties of concrete with the addition of CNTs largely depend on the aspect ratio and surface functionalization of nanostructures, as well as on the method of dispersing CNTs in the volume of the material.
  • Dispersion of carbon nanotubes (CNTs) by hydrodynamic cavitation.

The «Tornado” technology, today is the most advanced and effective in comparison with other methods existing in the world. In both cases, multi-walled CNTs were used as the dispersion additive, which consist of several layers of nanotubes with an outer diameter of 10 to 15 nm, a length of 1 to 15 μm, and an average density of 5-150 kg/m3.  Nanotubes are usually prone to agglomeration, so the granular CNT powder of various manufacturers (with an average particle size of 400 μm) was pre-functionalized (particle breakage, agglomeration dispersion, defect insertion) in the “Tornado” installation.

The disintegration beams and large agglomerates of CNTs caused by synthesis and ensuring their stabilization in aqueous suspensions and stability of suspensions of nanotubes in storage is of great importance, in the case of the second method.

In addition, the activation of cement matrices by grinding in the “Tornado” was investigated, as was the activation of quartz prior to its amorphization.

Prior to the research of TORtec GROUP on the Tornado installation, M.Konsta-Gdoutos, S.P. Shah et al. Studied the effect of the CNT aspect ratio on the mechanical properties of a cement composite.  It was found that long CNTs are less dispersed in the bulk of the material compared to short ones.  Therefore, CNTs are preliminarily ground with introduction of defects for activation in the «TORNADO» installation.  The tensile strength is maximal at a concentration of 0.08% by weight of the binder for short CNTs and 0.048% for long ones.  However, when the concentration of long CNTs is increased by more than 0.048%, an increase in strength is not observed.  Thus, the use of CNT crushed in a Tornado is confirmed not only by the creation of defects (activation), but also for the process of dispersion.  A team of SRI TP scientists, investigating the structure of concrete with the addition of short and long CNTs, concluded that short nanotubes form grids along the edges of individual crystallites of the C-S-H gel, binding them together, thereby promoting hardening of the cement stone.

It is noted that short CNTs are effective provided that the distance between individual crystallites is not more than one micron.  While long CNTs can overlap voids with dimensions exceeding 1 micron.

According to the work of Li, the modification of the structure of the cement composite by functionalized (treated in the «Tornado») CNT leads to a decrease in the porosity of the material and increase its mechanical properties.    Microstructural studies of concrete with the addition of functionalized CNTs show the presence of separate tubular structures covered with hydration products, while pure CNTs form three-dimensional spatial grids in microcracks of cement stone.

Hydration products on the surface of pure CNTs were not detected.  The data of mechanical tests show the best results for concrete with the addition of functionalized CNTs.  Thus, the aspect ratio, functionalization, the method of introducing CNTs determine the main effect from the use of nanomodifier.

Conclusions

  • Fullerene has the greatest influence on the increase in the yield strength of the NCM, followed by flake graphite, carbon nanotubes and ultradisperse nanodiamonds, previously functionalized in the Tornado.
  • With the activation selected in the Tornado, flake graphite is crushed to a nanoscale state and represents clusters consisting of 7-10 graphene layers. In the process of activation in Tornado, CNT undergoes destruction, defectiveness affects positively on the bond with the matrix.
  • New technologies and new materials — composites for use in various industries — will be created on the basis of Tornado.

In comparison with ultrasonic cavitation for the intensification of technological processes in the manufacture of concretes, the hydrodynamic cavitation method requires 10 times less energy, and with the «dry method» TORNADO preparation of suspensions is 20 times less. The optimal content of carbon nanotubes in the preparation of a concrete mixture was 0.006% of the cement mass. The compressive strength reached 36.33 MPa (18.46 MPa for the control sample), which represents a strength increase of 96.8%. The tensile strength at bending reached 3.35 MPa (in the control sample — 2.31 MPa), which gives an increase in strength by 45.1%. Investigation of the microstructure of cement concrete has shown that the introduction of carbon nanotubes leads to a radical change in the morphology of crystalline hydrate neoplasms in the cement matrix by structuring the cement matrix to form a dense, defect-free shell over the surface of solid phases, including cement and aggregate particles, providing better adhesion to their surface. In this case, spatial framework cells from calcium hydrosilicates are formed in the structure of the modified cement matrix. Analysis of frost resistance of concrete showed an increase in the index from F200 for control samples to F300 for prototypes of concrete modified by the dispersion of multilayered carbon nanotubes.