TUNGSTEN

RICK LOWDEN, OAK RIDGE NATIONAL LABORATORY

Tungsten is an incredible material. It is dense and hard, and it has the lowest vapor pressure and highest melting temperature of all metals. This combination of properties makes tungsten extremely valuable for a myriad of applications, while at the same time creates great challenges in the processing of the metal.

As a child, I was fascinated by how things work and spent a lot of time taking things apart. As with most budding engineers, I rarely reassembled them. Incandescent bulbs were one of my first quarries, carefully disassembled to reveal a hidden treasure: a tungsten filament. It was amazing that this tiny wire could be heated to white-hot temperatures to produce light.

Also at an early age, I was introduced to vacuum tubes, and to this day they are magical in my eyes. When a tungsten filament is heated in a vacuum, the electrons near the surface become energetic enough to be emitted into the surrounding space. Additional tungsten conductors, in the form of grids and plates, can be added to the bulb, and the electrons can then be manipulated to switch, rectify, and amplify. These electronic switches were crucial in the development of modern electronics.

Transistors and integrated circuits have almost entirely displaced tubes; however, some researchers are going "retro," exploring tungsten-containing miniaturized vacuum triodes or "nanotriodes" that may one day be used as miniature electronic switches. But one has to ask, "Do they glow?"

	TAKING AIM Lowden prepares to test the form and function of 9-mm ammunition fabricated from a tungsten-containing composite replacement for lead.
	ORNL PHOTO

Later, I went to Oak Ridge National Laboratory while I studied for a graduate degree in metallurgy. I specialized in vapor-phase processing of high-temperature materials, primarily structural coatings and composites. Once again, tungsten entered my life when I became involved in a project to develop a small fiber-reinforced ceramic can with a tungsten layer deposited on its outer diameter. The cans were to be used as thermionic emitters in advanced space power systems.

I became intimately involved with tungsten during the development of powder-metal replacements for lead in small-arms ammunition, in other words, during the investigation of "green bullets." A perusal of the periodic table reveals very few candidates for replacing lead. Bismuth, tin, and zinc are interesting, but each has deficiencies. Tungsten is heavy and commonly used in ordnance but unfortunately is much too hard for most small-arms applications.

One approach to the problem seemed quite straightforward: Build a composite that combined the properties of different elements to produce a leadlike material. A light, ductile metal like tin could be used as the binder with tungsten included for mass. But tungsten is not easy to process, and bullets had to be cheap. A review of the tungsten binary-phase diagrams revealed fewer than 40 systems, with few intermetallic compounds. Most of the soft metals with low melting points do not wet tungsten, and, in addition, significant differences in density make casting impossible. There had to be a method to combine tungsten and a ductile metal binder.

After World War I, resourceful window and curtain manufacturers sprinkled tungsten particulates onto the surface of tin sheets, which were subsequently fed through a rolling mill to produce high-density, pliable sheets that could be used to make weights. The solution to the lead-replacement problem was actually very simple. Powder blends without additives were pressed at room temperature to produce dense compacts. It wasn't rocket science, but it worked, and the tungsten-tin composite is a leading candidate for replacing lead in small-caliber bullets and for a variety of other applications.

Although we may not realize it, tungsten continues to be a part of our lives because it is still used in lighting and electrical contacts; in electronics, including cell phones and pagers; in cutting tools and engine components; in radiation shielding; and now in sporting goods such as golf balls and shot.

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