Porous carbons (PCs) are versatile materials with a wide range of applications, including sensors, actuators, thermal insulation, and energy conversion. [1] Some examples of PCs are graphene and carbon nanotube-based aerogel. Physical properties that make PCs unique are their low density, high conductivity, mechanical flexibility, and stability in extreme environments. [2]
To ensure durability of PCs, mechanical properties are important to study. Elaborate efforts have been made for studying compressive brittleness of porous carbon materials. In 1999, Iizuka, et al. studied the mechanical properties of wood ceramics, a type of porous carbon material. [3] Stable medium-density fiber was used as the base material of wood ceramics and phenol resin was impregnated into the board. [3] Starting at 300 °C, Young's modulus and the compressive strength first decreased with increasing temperature, but at 500 °C the strength increases sharply until it reaches 800 °C and plateaus. [3] The effects of temperature were due to microstructural changes in the resin during carbonization. Effects of impregnates phenol resin at 800 °C were also investigated. Results showed that Young's modulus increased with phenol resin impregnation (Figure 1). The maximum Young's modulus was 5 MPa and the maximum compressive strength was 80 MPa. [3] Wall-bending mechanical test were also performed and it was found that cell wall is breakage was correlated to relative density on compressive strength and Young's modulus.
Another type of compressive porous carbon consisting of cellulose and graphene aerogels was studied by Mi, et al. Modified cellulose/graphene aerogels (MCGA) was synthesized via bidirectional freeze drying and grafting of long carbon chains through chemical vapor deposition (Figure 2). [4] [2] The final product had a bulk density of 5.9 mg/cm3 and surface area of 47.3 m2/g with flexible cellulose nanofibril and stiff graphene components. [2] After optimizing the concentration of graphene oxide concentration and anisotropic porous structure, tensile tests were performed. It was found that MGCA could recover 99.8% and 96.3% when compressed to 60% and 90% strain, respectively. [2] SEM images showed that due to its unique structure, MCGA pore walls were able to wrinkle and fold during compression. Another unique characteristic of this material is its absorption capacity of 80-197 times its weight towards hydrophobic compounds, such as oils and chemical solvents. [2]
On the contrary, less effort has been made to study the stretchability of porous carbons. Gao, et al. synthesized a long-range lamellar scaffold composed of chitosan and graphene oxide via bidirectional freezing, freeze drying, and annealing. [5] The result is a material with density of 11 mg cm−3 and porosity of about 99.4%. Various tensile tests were conducted, and it was found that carbon spring could revert to its original shape upon 80% compression strain and -60% stretching strain with a Poisson's ratio between 0.05 and 0.1. [5] The narrow hysteresis loop of the stress-strain curve indicates a low energy dissipation (energy loss coefficient of about 0.2) because of its negligible interior friction, localized buckling, or cracks during deformation processes. [5] The stretchable mechanical properties of this material allow for great candidates for vibrational and magnetism sensors.
Wood is a structural tissue found in the stems and roots of trees and other woody plants. It is an organic material – a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or more broadly to include the same type of tissue elsewhere, such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients between the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, woodchips, or fiber.
Young's modulus is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Young's modulus is defined as the ratio of the stress applied to the object and the resulting axial strain in the linear elastic region of the material.
Ultimate tensile strength is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate tensile strength is close to the yield point, whereas in ductile materials, the ultimate tensile strength can be higher.
A binder or binding agent is any material or substance that holds or draws other materials together to form a cohesive whole mechanically, chemically, by adhesion or cohesion.
Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm) or structures having nano-scale repeat distances between the different phases that make up the material.
Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity, etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler that are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.
The mechanical properties of carbon nanotubes reveal them as one of the strongest materials in nature. Carbon nanotubes (CNTs) are long hollow cylinders of graphene. Although graphene sheets have 2D symmetry, carbon nanotubes by geometry have different properties in axial and radial directions. It has been shown that CNTs are very strong in the axial direction. Young's modulus on the order of 270 - 950 GPa and tensile strength of 11 - 63 GPa were obtained.
Solid is one of the four fundamental states of matter along with liquid, gas, and plasma. The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas. The atoms in a solid are bound to each other, either in a regular geometric lattice, or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because the molecules in a gas are loosely packed.
Exfoliated graphite nano-platelets (xGnP) are new types of nanoparticles made from graphite. These nanoparticles consist of small stacks of graphene that are 1 to 15 nanometers thick, with diameters ranging from sub-micrometre to 100 micrometres. The X-ray diffractogram of this material would resemble that of graphite, in that the 002 peak would still appear at ~26o 2 theta. However, the peak would appear considerably smaller and broader. These features indicate that the interplanar distance in exfoliated graphite is similar to that of the parent graphite, but the stack size is small. Since xGnP is composed of the same material as carbon nanotubes, it shares many of the electrochemical characteristics, although not the tensile strength. The platelet shape, however, offers xGnP edges that are easier to modify chemically for enhanced dispersion in polymers.
Nanocellulose is a term referring to nano-structured cellulose. This may be either cellulose nanocrystal, cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC), or bacterial nanocellulose, which refers to nano-structured cellulose produced by bacteria.
Impregnation resins are slightly viscous, organic liquids that are used in the forest products industry for wood modification. They typically contain formaldehyde and are composed of dimers and trimers of the main molecule. These can become polymer solutions upon curing inside of a wood substrate, imparting stabilizing properties. Impregnation of these resins involves a vacuum chamber procedure that completely disperses the resin into the wood. Once inside of the wood, the resin can diffuse into the cell wall and enhance the physical strength of the wood even further.
Ultralight materials are solids with a density of less than 10 mg/cm3, including silica aerogels, carbon nanotube aerogels, aerographite, metallic foams, polymeric foams, and metallic microlattices. The density of air is about 1.275 mg/cm3, which means that the air in the pores contributes significantly to the density of these materials in atmospheric conditions. They can be classified by production method as aerogels, stochastic foams, and structured cellular materials.
A metallic microlattice is a synthetic porous metallic material consisting of an ultra-light metal foam. With a density as low as 0.99 mg/cm3 (0.00561 lb/ft3), it is one of the lightest structural materials known to science. It was developed by a team of scientists from California-based HRL Laboratories, in collaboration with researchers at University of California, Irvine and Caltech, and was first announced in November 2011. The prototype samples were made from a nickel-phosphorus alloy. In 2012, the microlattice prototype was declared one of 10 World-Changing Innovations by Popular Mechanics. Metallic microlattice technology has numerous potential applications in automotive and aeronautical engineering. A detailed comparative review study among other types of metallic lattice structures showed them to be beneficial for light-weighting purposes but expensive to manufacture.
Aerographite is a synthetic foam consisting of a porous interconnected network of tubular carbon. With a density of 180 g/m3 it is one of the lightest structural materials ever created. It was developed jointly by a team of researchers at the University of Kiel and the Technical University of Hamburg in Germany, and was first reported in a scientific journal in June 2012.
Aerographene or graphene aerogel is the least dense solid known to exist, at 160 g/m3. The material reportedly can be produced at the scale of cubic meters.
Reversibly assembled cellular composite materials (RCCM) are three-dimensional lattices of modular structures that can be partially disassembled to enable repairs or other modifications. Each cell incorporates structural material and a reversible interlock, allowing lattices of arbitrary size and shape. RCCM display three-dimensional symmetry derived from the geometry as linked.
Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.
In materials science, a polymer matrix composite (PMC) is a composite material composed of a variety of short or continuous fibers bound together by a matrix of organic polymers. PMCs are designed to transfer loads between fibers of a matrix. Some of the advantages with PMCs include their light weight, high resistance to abrasion and corrosion, and high stiffness and strength along the direction of their reinforcements.
Microscale structural metamaterials are synthetic structures that are aimed to yield specific desired mechanical advantages. These designs are often inspired by natural cellular materials such as plant and bone tissue which have superior mechanical efficiency due to their low weight to stiffness ratios.
A graphene morphology is any of the structures related to, and formed from, single sheets of graphene. 'Graphene' is typically used to refer to the crystalline monolayer of the naturally occurring material graphite. Due to quantum confinement of electrons within the material at these low dimensions, small differences in graphene morphology can greatly impact the physical and chemical properties of these materials. Commonly studied graphene morphologies include the monolayer sheets, bilayer sheets, graphene nanoribbons and other 3D structures formed from stacking of the monolayer sheets.
