Why Combine Two Elements with the Exact Same Density?
Admittedly, our subtitle is a bit misleading. It is true that by a quirk of nature, gold and tungsten have identical densities. However, the two also have radically different mechanical properties, including their respective melting temperatures. The most common situation in which gold and tungsten are combined is when gold is plated onto tungsten. Because gold is so expensive, the plating is often very thin — generally, as thin as possible while remaining functional for the application.
But, why would anyone want to plate tungsten with gold except for nefarious purposes?
Gold-Tungsten’s Controlled Discharge Wire Act
The most popular — and legitimate — application for gold-plated tungsten is as a corona discharge wire used to produce controlled electrical discharges in a variety of filtration, printing, and other processes. In addition to its use in air ionizers and photocopying, interesting commercial and industrial applications of corona discharge include the following:
- Removal of unwanted electric charges from aircraft surfaces
- Manufacture of ozone for oxidation use
- Sanitization of pool water
- Scrubbing of particles from air conditioning systems
- Removal of unwanted volatile organics (such as pesticides, solvents, or chemical weapons) from the atmosphere
- Surface treatment of polymer films for use with adhesives or printing inks
- Production of photons for Kirlian photography film exposure
- Electrohydrodynamic (EHD) thrusters, lifters, and other ionic wind devices
- Nitrogen laser
- Surface treatment for tissue culture (polystyrene)
- Ionization of samples for mass spectrometry or ion mobility spectrometry
- Solid-state cooling components for computer chips
Contrary to what you see in the old Frankenstein movies, the key to effective corona discharge is control — and gold-plated tungsten wire has proven to be very effective for precise, high repetition cycles where every discharge needs to be the same.
Clearing the Air with Gold-Tungsten Wire
With the continued and growing worldwide concern about air quality, the global standard for acceptable fine particulate matter (PM2.5) is driving the demand for tungsten wire in air filtration systems. In addition to reducing visibility and creating a haze when levels are elevated, PM2.5 air pollutants are a general health concern and are potentially dangerous for people with breathing and heart problems, children, and the elderly.
Gold-plated tungsten wire is often used in air ionizers (also called negative ion generators) — devices that use high voltage to electrically charge air molecules that attract airborne particles and thus purify the air in the surrounding environment. Most commercial air purifiers are designed to generate negative ions (anions); airborne positively charged particles are attracted and “stick” to the ions in an effect similar to static electricity, and are then passed through a filter. In addition, air ionizers are sometimes used in electronic work environments to reduce or eliminate the buildup of potentially damaging static electricity buildup on insulators, conductors, and other equipment.
Gold-Standard Quality for Photocopying
Using corona discharge to generate charged surfaces is vital to electrostatic copying, or photocopying — and gold-plated tungsten wire plays a special role in fixing the ink in the corona discharge process in photocopiers. In general terms, most plain photocopiers use electrostatic charges on a light-sensitive photoreceptor to attract and then transfer toner particles onto paper in the form of an image; heat, pressure, or a combination is used to then fuse the toner to the paper. The electrostatic charges within a photocopier are produced by using the corona discharge process.
Although unplated electropolished tungsten wire can perform the same fundamental physical task of photocopying, gold-plated tungsten wire is literally the gold standard for the process. Because it dramatically reduces the incident of defects that are viewed as small dots on the paper’s surface, gold-plated tungsten wire results in better print quality for high-speed photocopying.
Not Just a Veneer of Quality
In a completely different field — medicine — plating of tungsten wire with a very thin layer of gold allows tungsten to be substituted for precious metal wire in important medical device applications, providing quality at a price that cannot be beat. In an age of extreme cost consciousness in healthcare, the use of pure precious metals such as platinum/iridium can be prohibitively expensive, especially for single-use devices.
For example, the traditional approach of adding 10% or more of iridium to platinum is an expensive method of achieving strength. Tungsten wire alone has the necessary strength to meet or exceed that requirement; the addition of gold plating increases resistance to corrosion, among other beneficial properties. Round tungsten wire can be made to remarkably small diameters — well smaller than even the current needs for medical devices — as well as made into ribbon that can be produced very thin and then plated with gold, all while maintaining the wire’s straightness and shape. In addition, the fluoroscopic benefits of gold plating are substantial, with the combination of these two dense materials in the form of gold-plated tungsten wire delivering radiopacity properties similar to those of platinum/iridium, but at a much lower cost.
So once again, we find that tungsten wire excels in a growing range of applications, in electronics, medicine, and beyond.