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New Materials for Public Safety

It’s easy to forget about the materials that make up the tools that we use in daily law enforcement work. Kevlar was originally intended as a material to make tires stronger. Most officers have no idea what kind of metal is in their side arms, other than “steel.” There is some exciting work being done in materials science that promises new and better equipment for law enforcement officers within a few years.

The U.S. Army is working on a materials application they call “liquid armor.” Liquid armor is actually a “shear thickening fluid” or STF that is combined with Kevlar to improve wearability and effectiveness. The liquid component of STF is polyethylene glycol (a first cousin of good old antifreeze coolant), which is non toxic and stable over a wide range of temperatures.

Very hard nanoparticles of silica (basically, glass) are suspended in the polyethylene glycol, and then the mixture is soaked into the conventional Kevlar layers. In normal handling, the Kevlar/STF material is highly deformable, as the STF flows like a liquid. However, when struck with a high-speed projectile, the STF doesn’t flow so well, and makes for a rigid barrier that slows or stops the projectile. When the projectile has been slowed or stopped, the STF returns to its flowable liquid state.

The mechanics of STF are easily demonstrated by stirring some of the light blue fluid in a vial. When stirred slowly, the liquid behaves like thick oil, moving smoothly. But accelerate the stirring rod, and the liquid seizes up, holding the rod fast. The glass particles can get out of one another’s way when the liquid is moving slowly, but they bump into one another and become rigid when the liquid velocity increases.

Liquid armor has shown to be far more stab resistant than conventional Kevlar, so it could make for a revolution in the design of body armor worn by corrections personnel, whose greatest threat is from pointed and edged weapons. An ice pick in a man’s hand can penetrate four layers of untreated Kevlar, but the same ice pick is stopped when the layers are soaked with STF. The greater flexibility of the material may also make it possible to provide better protection for knees, elbows, and the lower body, where the stiffness of conventional body armor makes it difficult to protect and still allow for full mobility.


First liquid armor, and now liquid metal? These two technologies with similar names were developed independently, the latter by a private firm called Liquidmetal Technologies and a professor of engineering and applied science at Caltech. Liquidmetal is an alloy with properties unlike any other metal. Metals normally exist in either a solid or a liquid phase—liquid when very, very hot, then crystallizing to a solid form as it cools. The crystalline form is hard and malleable (although both hardness and malleability depend on the type of metal), but can also be brittle, breaking when stressed too much or too often.

The crystalline form of metal can be described as “periodic,” where the crystal patterns repeat over an extended range. Liquidmetal has an amorphous (“without shape”) structure that more closely resembles metal in the liquid phase. The difference is that Liquidmetal retains this amorphous structure as it cools to room temperature and below, making the material far more adaptable than conventional metals.

Zirconium-base and titanium-base Liquidmetal has over twice the yield strength (yield strength is the amount of stress necessary to deform a material) of stainless steel and titanium alloy and almost three times the elastic limit. The material also has superior resistance to corrosion and wear.

One of the first applications of Liquidmetal in the consumer market has been in the manufacture of golf clubs and tennis racquets. Liquidmetal is resistant to the absorption of energy, so more of the effort supplied by the golfer or tennis player is transmitted to the ball. This is illustrated in a video clip that is downloadable from the Liquidmetal web site.

Three glass tubes, side by side, each have a metal plate on the bottom end—one stainless steel, one titanium, and one Liquidmetal. A stainless steel ball is dropped from the top of each tube onto the metal plate at the other end. The ball bouncing off of the stainless steel continues to rebound for about 18 seconds; the one hitting titanium goes a few seconds longer. After a full minute, the ball is still bouncing off of the Liquidmetal plate.

What kinds of applications are likely for the public safety market? Guns might be a likely candidate. Guns with Liquidmetal surfaces would be far more resistant to rust, corrosion, and wear. The metal could be machined to tolerances not presently attainable with conventional metal alloys.

Electronics companies are already seeing the utility of Liquidmetal casings to house the delicate innards of cell phones and PDAs, which are often stuffed into a pocket and subjected to high external stresses (the first and second most common causes of cell phone breakage are dropping the phone and stuffing the phone into pants that are too tight).

Liquidmetal casings are more durable, scratch-resistant, and thinner than casings of other materials that provide less protection. Liquidmetal may find its way into prosthetic devices for orthopedic surgery and to make longer-lasting and sharper scalpels. And in the defense sector, Liquidmetal technology is being developed to replace depleted (although still slightly radioactive) uranium projectiles as Kinetic Energy Penetrators (KEP) to pierce tank armor and destroy missiles and satellites in flight.

Carbon Nanotube Update

The next big thing in protective materials for the military and public safety may be based on carbon nanotubes. Nanotubes are, as their name implies, sub-microscopic tubes made up of soccer ball-shaped molecules called fullerenes or buckyballs. Buckyballs (both the names “buckyball” and “fullerene” stem from Buckminster Fuller, the inventor of the geodesic dome that the molecules resemble) are structures of 60 carbon atoms that are immensely strong and stable.

When these molecules are linked together, which is a neat trick in itself, they form tubes that are essentially one huge molecule, and possess a tensile strength unlike anything yet devised. The manufacturing problem to date has been in making the tubes into a practical length. Most of the nanotubes produced have been only tenths of a millimeter long, and they have to be lined up end to end to produce anything useful. The bonds between the ends of the tubes are the weak link, limiting the effectiveness of any material composed of them.

Researchers at Los Alamos National Laboratory and Duke University have succeeded in creating nanotubes that appear limited only by the size of the chamber used to make them. By flowing ethanol vapors at 900° C over an iron catalyst set on a silicon wafer, the scientists were able to grow nanotubes up to four centimeters long. That might not seem like much, but if the nanotube was the diameter of a typical human hair, that four centimeters would translate to a length of almost two miles. Nanotubes woven into a thread or fiber could be as strong as any material known to man, and composed solely of one of the most plentiful materials on the earth.

There will be a few bumps in the road before you will be able to buy a tactical vest made of carbon nanofiber, however. Buckyballs tend to suck up loosely bound electrons from neighboring molecules. In some applications, this is a good thing, but in the body, this releases free radicals that wreck one’s cell chemistry. A paper presented at an American Chemical Society meeting in March 2004 described how bass fish exposed to fullerenes developed brain damage.

Because of the widespread number of possible applications of fullerenes (including drug delivery systems, superconductors, chemical sensors, and fuel cells), research is continuing on a large scale. One Japanese manufacturer is producing 40 metric tons of fullerenes annually and has plans to expand its manufacturing capacity to 1,500 metric tons. You will be hearing more about buckyballs and carbon nanotube materials.

Tim Dees is a former police officer who writes and consults about applications of technology in law enforcement. He can be reached at (509) 585-6704 or by e-mail at tim@timdees.com.

Published in Law and Order, May 2005

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Buckyballs, as shown, are structures of 60 carbon atoms.
Buckyballs, as shown, are structures of 60 carbon atoms.
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