Two years ago I told you about see-through wood. Now the research team headed by Liangbing Hu of the University of Maryland’s Department of Materials Science and Engineering has invented wood that is 10 times stronger than before!
Their findings, reported in the February edition of Nature, detail the process and give some interesting suggestions where this new material may be used. Let’s take a look at what they did but first a review of wood and a few of its important properties.
Structural engineers know that many materials with exceptional mechanical performance suffer from either large weight (alloy steels) or complex manufacturing processes (graphite fibers) and thus require high cost in processing. Natural wood, on the other hand, is a low-cost and quite abundant material that has been around for thousands of years as a structural material for making buildings and furniture. If the stiffness of wood could be increased by a factor of 10 it just may become a substitute for exotic metals such as a replacement for titanium in fighter jets. Wood is also a renewable resource.
The most important structural constituent of wood is cellulose. In organic chemistry we denote any material with a –ose as a sugar, so the cellulose of the cell walls in wood is really a sugar. Trees are made of a high molecular weight, long, linear polymer of the common sugar we call glucose. Jimmy Gilmer would be happy to know that.
Weight for weight, the rigidness, or Young’s Modulus, of cellulose is neck and neck with many metals and its tensile strength is actually higher. Because the loads in a live tree are for the most part compressive, it does not “pay” to use a completely solid structure while growing and nature uses the most advantageous method to spread the energy gathering regions (leaves) around with hollow cells.
A tree grows along its outer surface, the region we call sapwood. This is the living area that contains the water and nutrients that travel both up and down its structure. As the tree grows and expands the inner heartwood stops moving liquids and becomes biologically inactive with the result that its role is now a structural one. The cell cavities that once contained water now only hold air, so a great reduction in density is achieved. This process is what allows mature California redwoods the ability to climb vertically 300 feet or more and become the tallest living things in the world.
Where density is concerned, wood is both strong in compression and tension. When wood is squeezed it does not suffer the sudden compressive failures seen in concrete or brick where 45 degree shear cracks can run together in a sudden and explosive mechanism. Instead, because of the hollow cellular morphology of wood, a crack may run a small distance but then gives in to local buckling of the cell walls with a crease that does not jog at a diagonal but instead travels along the direction of the applied stress. You can envision this compressive mechanism by pushing down on a rolled up front door mat and observing the local deformations.
The fact that wood has great resistance to tensile forces puzzled many experts and was not fully explained until the 1960s when the British botanist Preston discovered that the cellulose fibrils in cell walls were essentially oriented a few degrees off of the grain direction. When a board is placed under tension, such as by bending, the regions that are stretched actually compress the tilted cell walls, picking up the ability to stretch slightly without breaking apart. This allows wood to be used where tension forces are present, unlike pure concrete, which cannot.
Using a two-step process that involves the partial removal of lignin and hemicellulose from the natural wood, the research team at the University of Maryland reports a tenfold increase in strength, toughness and ballistic resistance.
The first step in this process is to boil the wood in an aqueous mixture of lye and sodium sulfite in order to remove the lignin and hemicelluloses from the cells. The second step is listed as a hot-pressing that leads to the total collapse of cell walls and the complete densification of the natural wood with highly aligned cellulose nanofibers.
This results in a processed wood having a specific strength higher than that of most structural metals and alloys, making it a low-cost, high-performance, lightweight alternative.
They are also investigating making a solar cell from leaves.