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Transparent wood composites are novel wood materials which have up to 90% transparency. Some have better mechanical properties than wood itself. They were made for the first time in 1992. These materials are significantly more biodegradable than glass and plastics. [1] [2] [3] Transparent wood is also shatterproof, making it suitable for applications like cell phone screens. [4]
A research group led by Professor Lars Berglund [6] from Swedish KTH University along with a University of Maryland research group led by Professor Liangbing Hu [3] have developed a method to remove the color and some chemicals from small blocks of wood, followed by adding polymers, such as poly(methyl methacrylate) (PMMA) and epoxy, at the cellular level, thereby rendering them transparent.
As soon as released in between 2015 and 2016, see-through wood had a large press reaction, with articles in ScienceDaily , [7] Wired , [8] The Wall Street Journal , [9] and The New York Times. [1]
Actually those research groups rediscovered a work from Siegfried Fink, a German Researcher, from as early as 1992: with a process very similar to Berglund's and Hu's, the German Researcher turned wood transparent to reveal specific cavities of the wood structure for analytical purpose. [10]
In 2021 researchers reported a way to manufacture transparent wood lighter and stronger than glass that requires substantially smaller amounts of chemicals and energy than methods used before. The thin wood produced with "solar-assisted chemical brushing" is claimed to be lighter and about 50 times stronger than wood treated with previous processes. [11] [12] [13]
In its natural state, wood is not a transparent material because of its scattering and absorption of light. The tannish color in wood is due to its chemical polymer composition of cellulose, hemicellulose, and lignin. The wood's lignin is mostly responsible for the wood's distinctive color. Consequently, the amount of lignin determines the levels of visibility in the wood, around 80–95%. [14] To make wood a visible and transparent material, both absorption and scattering need to be reduced in its production. The manufacturing process of transparent wood is based on removing all of the lignin called the delignification process.
The production of transparent wood from the delignification process vary study by study. However, the basics behind it are as follows: a wood sample is drenched in heated (80 °C–100 °C) solutions containing sodium chloride, sodium hypochlorite, or sodium hydroxide/sulfite for about 3–12 hours followed by immersion in boiling hydrogen peroxide. [15] Then, the lignin is separated from the cellulose and hemicellulose structure, turning the wood white and allowing the resin penetration to start. Finally, the sample is immersed in a matching resin, usually PMMA, under high temperatures (85 °C) and a vacuum for 12 hours. [15] This process fills the space previously occupied by the lignin and the open wood cellular structure resulting in the final transparent wood composite.
While the delignification process is a successful method of production, it is limited to its laboratory and experimental production of a small, and low-thickness material that is unable to meet its practical application requirements. [16] However, at Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Resources in 2018, Xuan Wang and his colleagues developed a new production method of infiltrating a prepolymerized methyl methacrylate (MMA) solution into delignified wood fibers. By utilizing this new technique, large-size transparent wood with any thickness or any measure can be easily made. [16] Yet in spite of this success in the manufacture, challenges still exist with regard to mechanical stability and adjustable optical performance. [14]
Wood is a natural growth material that possesses excellent mechanical properties, including high strength, good durability, high moisture content, and high specific gravity. [15] Wood can be classified in two types of wood, softwood and hardwood. While each type is different—e.g., the longitudinal cells in softwood are shorter in length when compared to hardwood—both types have a similar hierarchical structure, meaning the orientation of the cells is identical in the wood. [15] This unique anisotropic structure, the properties with distinctive values when measured in several directions, allows it to pump ions and water for photosynthesis in the wood. [15] Similarly, in transparent wood composites, removing the lignin and maintaining the cellulose fiber tubes it allows it to become a clear wood that can get soaked in a glue-like epoxy that makes it a robust and transparent material. [17] An excellent raw material with high transmittance and enhanced mechanical properties.
Researchers have successfully tested an eco-friendly alternative: limonene acrylate, a monomer made from limonene, into an acrylate. [18] Limonene is a common cyclic terpene that can be extracted from industrial waste, via isomerization of α‐pinene (from wood) or from citrus peel oil. The bio-based polymers can offer advantages compared to conventional non‐renewable polymers from fossil resources, and still retain a high mechanical performance and it is lightweight, stemming from its porous and anisotropic cellulosic structure; and is of great interest for large-scale sustainable nanotechnologies. Succinylation of the delignified wood substrate using succinic anhydride results in a nanostructured and mechanically strong biocomposite. The polymer matrix usually accounts for ≈70 vol%, results in nanostructured biocomposites combining an excellent optical transmittance of 90% at 1.2 mm thickness and a remarkably low haze of 30%, with a high mechanical performance (strength 174 MPa, Young's modulus 17 GPa). [19]
Transparent wood derives its mechanical properties and performance primarily from its cellulose fiber content and the geometric orientation of the fiber tube cells (radial and tangential) structure, providing the structural base for the design of advanced materials applications. [15]
One aspect of the transparent wood mechanical property is the strength of the material. According to Zhu and his colleagues, transparent wood in the longitudinal direction has an elastic modulus of 2.37 GPa and strength of 45.38 MPa (both which are lower than for pure PMMA [20] ) and twice as high as those perpendicular to the longitudinal direction, 1.22 GPa and 23.38 MPa respectively. [3] They conclude that longitudinal to transverse properties decreased for transparent wood, which they expected as the presence of the polymer resin suppresses the cavity space. [3] Also, the plastic nature of transparent wood composite provides advantages compare to other brittle materials like glass, meaning it does not shatter upon impact. [17]
The transparent wood, tightly packed and perpendicularly aligned cellulose fibers operate as wideband wave-guides with high transmission scattering losses for light. This unique light management capacity results in a light propagation effect. [21] By measuring its optical properties with an integrated sphere, Li and her colleagues found that transparent wood exhibits a high transmittance of 90% (lower than for pure PMMA) and a high optical haze of 95%. [21] As a result, transparent wood as an energy efficient material could be used to decrease the daytime lighting energy usage by efficiently guiding the sunlight into the house while providing uniform and consistent illumination throughout the day. [21]
Similarly, the transparent wood's thermal conductivity is attributed to the alignment of the wood cellulose fibers, which has been preserved after lignin removal and polymer infiltration. Transparent wood has a thermal conductivity of 0.32 W⋅m−1⋅K−1 in the axial direction and 0.15 W⋅m−1⋅K−1 in the radial direction respectably. [21] Based on the study done by Céline Montanari of the KTH Royal Institute of Technology in Stockholm, the transparent wood's thermal conductivity, which transforms from semi-transparent to transparent when heated, could be used to make buildings more energy-efficient by capturing the sun's energy during the day and releasing it later at night into the interior. [22]
Although the development of transparent wood composites is still at a lab-scale and prototype level, their potential for energy efficiency and operational savings in the building industry are very promising. An essential advantage with transparent wood is its combination of structural and functional performance for load-bearing structures that combine optical, heat-shielding, or magnetic functionalities. [23] Transparent wood is also researched for potential use for touch-sensitive surfaces. [13] [24]
Such is the case in building applications where artificial light can be replaced by sunlight through a light transmittance design. Based on research and simulation performed by Joseph Arehart at the University of Colorado Boulder, transparent wood as a glass glazing system replacement could reduce the space conditioning energy consumption by 24.6% to 33.3% in medium (climate zone 3C, San Francisco, CA) and large office spaces (climate zone 4C, Seattle, Washington) respectably. [25] These are relevant insights in transparent wood's potential functionality because it shows lower thermal conductivity and better impact strength compared to popular glass glazing systems.
Another direction for transparent wood applications is as a high optical transmittance for optoelectronic devices as substrates in photovoltaic solar cells. Li and her colleagues at the KTH Royal Institute of Technology studied the high optical transmittance that makes transparent wood a candidate for substrate in perovskite solar cells. They concluded that transparent wood has high optical transmittance of 86% and long term stability with fracture of toughness 3.2 MPa⋅m1/2 compared to glass substrate fracture of toughness 0.7–0.85 MPa⋅m1/2, which meets the substrate's requirements for solar cells. [26] These are relevant information for transparent wood's possible application because it is a suitable and sustainable solution to the substrate for solar cell assembly with potential in energy-efficient building applications, as well as replacements for glass and lowering the carbon footprint for the devices. [26]
Transparent wood could transform the material sciences and building industries by enabling new applications such as load-bearing windows. These components could also generate improvements in energy savings and efficiency over glass or other traditional materials. A lot of work and research is needed to understand the interaction between light and the wood structure further, to tune the optical and mechanical properties, and to take advantage of advanced transparent wood composite applications
Cellulose is an organic compound with the formula (C
6H
10O
5)
n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose content of cotton fiber is 90%, that of wood is 40–50%, and that of dried hemp is approximately 57%.
Fiber or fibre is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate fibers, for example carbon fiber and ultra-high-molecular-weight polyethylene.
Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.
Engineered wood, also called mass timber, composite wood, human-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of wood, together with adhesives, or other methods of fixation to form composite material. The panels vary in size but can range upwards of 64 by 8 feet and in the case of cross-laminated timber (CLT) can be of any thickness from a few inches to 16 inches (410 mm) or more. These products are engineered to precise design specifications, which are tested to meet national or international standards and provide uniformity and predictability in their structural performance. Engineered wood products are used in a variety of applications, from home construction to commercial buildings to industrial products. The products can be used for joists and beams that replace steel in many building projects. The term mass timber describes a group of building materials that can replace concrete assemblies.
Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O by weight. Oxygen-saturated compositions are so typical that unsaturated compositions are termed oxygen-deficient ITO. It is transparent and colorless in thin layers, while in bulk form it is yellowish to gray. In the infrared region of the spectrum it acts as a metal-like mirror.
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a polymer mixture of two ionomers. One component in this mixture is made up of polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.
LIGA is a fabrication technology used to create high-aspect-ratio microstructures. The term is a German acronym for Lithographie, Galvanoformung, Abformung – lithography, electroplating, and molding.
Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose and hemicellulose, and an aromatic-rich polymer called lignin. Any biomass rich in cellulose, hemicelluloses, and lignin are commonly referred to as lignocellulosic biomass. Each component has a distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms. These carbohydrate-containing polymers contain different sugar monomers and they are covalently bound to lignin.
As the world's energy demand continues to grow, the development of more efficient and sustainable technologies for generating and storing energy is becoming increasingly important. According to Dr. Wade Adams from Rice University, energy will be the most pressing problem facing humanity in the next 50 years and nanotechnology has potential to solve this issue. Nanotechnology, a relatively new field of science and engineering, has shown promise to have a significant impact on the energy industry. Nanotechnology is defined as any technology that contains particles with one dimension under 100 nanometers in length. For scale, a single virus particle is about 100 nanometers wide.
Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. A promising low cost alternative to conventional solar cells made of crystalline silicon, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.
An organic solar cell (OSC) or plastic solar cell is a type of photovoltaic that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Most organic photovoltaic cells are polymer solar cells.
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.
Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films.
Cellulose fibers are fibers made with ethers or esters of cellulose, which can be obtained from the bark, wood or leaves of plants, or from other plant-based material. In addition to cellulose, the fibers may also contain hemicellulose and lignin, with different percentages of these components altering the mechanical properties of the fibers.
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.
There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be categorized into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as more efficient energy converters from light energy into electric current or light absorbers and charge carriers.
A luminescent solar concentrator (LSC) is a device for concentrating radiation, solar radiation in particular, to produce electricity. Luminescent solar concentrators operate on the principle of collecting radiation over a large area, converting it by luminescence and directing the generated radiation into relatively small photovoltaic solar cells at the edges.
A. R. Forouhi and I. Bloomer deduced dispersion equations for the refractive index, n, and extinction coefficient, k, which were published in 1986 and 1988. The 1986 publication relates to amorphous materials, while the 1988 publication relates to crystalline. Subsequently, in 1991, their work was included as a chapter in “The Handbook of Optical Constants”. The Forouhi–Bloomer dispersion equations describe how photons of varying energies interact with thin films. When used with a spectroscopic reflectometry tool, the Forouhi–Bloomer dispersion equations specify n and k for amorphous and crystalline materials as a function of photon energy E. Values of n and k as a function of photon energy, E, are referred to as the spectra of n and k, which can also be expressed as functions of the wavelength of light, λ, since E = HC/λ. The symbol h represents Planck’s constant and c, the speed of light in vacuum. Together, n and k are often referred to as the “optical constants” of a material.
A solid-state electrolyte (SSE) is a solid ionic conductor and electron-insulating material and it is the characteristic component of the solid-state battery. It is useful for applications in electrical energy storage (EES) in substitution of the liquid electrolytes found in particular in lithium-ion battery. The main advantages are the absolute safety, no issues of leakages of toxic organic solvents, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability. This makes possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte thanks to the property of lithium dendrite suppression in the presence of a solid-state electrolyte membrane. The use of a high capacity anode and low reduction potential, like lithium with a specific capacity of 3860 mAh g−1 and a reduction potential of -3.04 V vs SHE, in substitution of the traditional low capacity graphite, which exhibits a theoretical capacity of 372 mAh g−1 in its fully lithiated state of LiC6, is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. Moreover, this allows the reach of gravimetric and volumetric energy densities, high enough to achieve 500 miles per single charge in an electric vehicle. Despite the promising advantages, there are still many limitations that are hindering the transition of SSEs from academia research to large-scale production, depending mainly on the poor ionic conductivity compared to that of liquid counterparts. However, many car OEMs (Toyota, BMW, Honda, Hyundai) expect to integrate these systems into viable devices and to commercialize solid-state battery-based electric vehicles by 2025.
A polymer electrolyte is a polymer matrix capable of ion conduction. Much like other types of electrolyte—liquid and solid-state—polymer electrolytes aid in movement of charge between the anode and cathode of a cell. The use of polymers as an electrolyte was first demonstrated using dye-sensitized solar cells. The field has expanded since and is now primarily focused on the development of polymer electrolytes with applications in batteries, fuel cells, and membranes.