The melting points and densities of the so-called Noble Metals, coupled with their resistance to chemical attack, sets them apart from most other materials.
Edited by Jean M. Hoffman
Unless you work with precious metal (PM) frequently, your knowledge of these metals is probably limited. The 15-min discussion devoted to PMs in college materials-science classes likely did not do justice to the engineering value for these eight elements. If you did not know there were eight (No, titanium is not one of them.), read on to find out what your professor probably did not know either.
A precious metal is a rare metallic element of high economic value. PMs are also referred to as Noble Metals because they resist most types of environmental and chemical attack. One of the few chemical solutions that attacks them (with the exception of iridium) is aqua regia. Only copper and PMs are found in nature in their metallic state. All other metals are processed from minerals or ores into metals which are inherently unstable and have a tendency to revert to their more stable mineral forms.
PMs, as a group, have a set of physical and chemical properties that are unrivaled by many other materials. If PMs were more available (in both quantitative and economic terms), there would be far more applications overall. Though typical applications use only small amounts, PMs may be used in large quantities when there is no feasible substitute.
The eight PMs gold (Au), silver (Ag), platinum (Pt), iridium (Ir), palladium (Pd), rhodium (Rh), ruthenium (Ru), and osmium (Os) are conveniently grouped together in the periodic table. A subset of this group is called the Platinum Group Metals (PGM), which includes all but Au and Ag. Typically the PGMs are found combined together in rich ore. Chemical processing extracts the individual elements. The short supply (there are only a few major mining locations), economic value, and costly mining and extraction methods have raised the cost of these metals substantially.
In 2004, South Africa produced a total of 5 million troy ounces of Pt (70% of the world’s output) and 8 million troy ounces of PGMs (50% of the world’s output). It takes about a ton of rich ore to produce approximately 1 troy ounce of PGMs, at best. Some mines only produce PGMs on the level of 5 to 25 gm/ton of processed ore.
PM properties typically differ from those of conventional metals in two primary areas: melting point and density. The melting point (MP) for low-alloy steel is about 2,800°F with a density in the range of 7.8 gm/cm3; compare this to Ag, Au, and Pt, for example, with respective melting points of 1,764, 1,947, and 3,216°F and densities of 10.5, 19.3, and 21.5 gm/cm3. These features, coupled with their resistance to chemical attack, set PMs apart from most other materials. The range of applications for PMs are diverse and they serve in applications where other materials won’t work.
Electrical and thermal conductivity: Many of the PMs have excellent electrical and thermal conductivity. Silver has the distinction of having the highest room-temperature conductivity of the PMs, as well as the highest of all metals. It should be no surprise that copper is the metal more predominantly used for electrical wire instead of Ag because it costs much less.
Corrosion resistance: PMs form, in some cases, an almost imperceptible oxide film. Their use as plating materials are effective and very broad. Additionally, these metals and their alloys are used in cathodic-protection systems to protect large-scale systems from the effects of corrosion.
Catalysts: Pt, Pd, Rh and their various alloys are widely used catalysts in large and small chemical reactors such as vehicle exhausts. A rich solution “washed” onto a ceramic substrate can leave a catalytic surface. The surface can also be a robust construction of woven or knit wire that provides a large-scale surface for chemical production. These are the primary applications for PGMs. Pt-based catalysts have been used for nitric acid production for more than 100 years.
High temperature applications: Combined high MP temperatures and low reactivity at elevated temperatures is a key quality of PGMs. Steel melts at 2,800°F, while Pt, Rh, and Ir have MPs of 3,216; 3,560; and 4,429°F, respectively. Vessels made from Pt, Pt-Rh, and Ir are used in the making of fiberglass and silicon ingots, as well as for the melting of other high MP, reactive media. One clever application uses Pt and the zirconium oxide formed during powdered-metal spraying to produce a highly creep-resistant material even when heated close to its melting point. Zirconia-grain-stabilized (ZGS) platinum and Pt-Rh alloys have been used in the glass industry for many years. The addition of zirconium and its subsequent oxidation during metal spraying create a grain structure that limits grain growth and boosts high-temperature creep strength.
Thermocouple devices: Thermocouples made from Pt and Pt-Rh wire pairs are unparalleled at giving accurate and finer temperature measurements. Currently, wire producers are able to make wire diameters small to keep costs low.
High-temperature heating coils: A heating coil can obviously only go as high as the melting point of the material used to construct it. PGM alloys can survive repeated oxidation cycles that can reduce the life of the heating coil. The use of PGM alloys satisfies both the issue of high service temperature and the problem of long-term oxidation attack.
Spark-erosion resistance applications: The development and application of Pt and Ir alloys along with pure Ir (some combinations of PMs are patented) has resulted in spark plugs that last for more than 100,000 miles. Some manufacturers use ball-bearing fabrication equipment to make small Pt alloy spheres that are then resistance welded onto the plug to form the electrode pair. For the more critical applications on aircraft, short pieces of Ir rod-stock material are centerlessly ground to an exacting size and form and then installed in the spark plug. Additionally, electrical contacts with an extended operational life have been made from Pt and Pd strip stock for various devices by high-speed stamping of small crowned circular blanks.
Fuel-cell applications: The electrical output from the fuel cell is made by combining hydrogen (the fuel) and oxygen (from air) over a catalyst such as platinum.
Biocompatibility and radio-opacity: The medical devices produced from PMs include stents, marker bands for angioplasty devices, pacemaker wire, endoscopy tips, and special surgical tools. The material most commonly used is Pt (or alloys of Pt), except for dental applications which use Au and Pd. X-rays don’t easily pass through Pt, Au, and Ir because of their atomic absorption coefficients, as well as their high densities. Thus these materials typically show up as a white area on film or scanning device. This property, referred to as radio-opacity, coupled with their biocompatibility properties, lets doctors see the exact location of these materials when used within the human body.
Pharmaceutical use: Pt-based drugs have been in use to treat cancer for 30 years and are the widely accepted standard-course-of-treatment for testicular cancer. Gold has also been used for the treatment of prostate cancer, whereby small gold “seeds” are irradiated and injected into the cancer site to kill the cancer cells by the slow release of radiation.
Labware, equipment and related devices: Pt and Ag resist attack from many substances. As such, they are used as crucibles, electrodes, inoculating loops, ignition boats, and many other forms of labware. Because these materials are noble, the testing method is not skewed by contamination from the test equipment. Basic forms of material (wire, tube, sheet, and strip) can be fabricated into countless products for industrial use.
Photographic applications: At one time, the Eastman Kodak Co. was the single largest user of Ag in the world. Many films and photo papers used silver compounds as the light-sensitive emulsion. Pt and Pd compounds also were used to produce black and white printing paper, which was and still is considered by many to be the best paper for fine tonal reproduction. These prints are the most resistant to environmental attack.
Coinage, collector’s items, and jewelry: PMs have been used throughout history for currency and jewelry. Our ability to examine past cultures is due, in part, to the nobleness of these materials. They are able to survive hundreds or even thousands of years of concealment and burial and still be viable as a historical record. Jewelry accounts for the second largest demand of PMs. md
John C. Keefe previously worked as the manager of Manufacturing Engineering at Johnson Matthey (Precious Metals Div.) in West Chester, Pa., where he supervised engineering and fabrication of a range of products made from precious metals and their alloys.
A different weight measurement system is used for precious metals. The standard unit is the troy ounce. The troy ounce (to) is different than our common ounce, in that it has a mass of 31.1035 gm versus 28.350 gm for the standard (avoirdupois) ounce (32.1507 to = 1 kg). Even when referring to large amounts of precious metal, the quantity is still expressed in troy ounces.
Gold (Au) has the longest and most storied history of all the precious metals. It is the most malleable of metals and therefore can be worked with simple tools to form complex shapes. The metal’s low MP has made it one of the first metals that could be readily cast. Its excellent corrosion resistance, thermal, and electrical properties have made it a top design choice for many devices. The ability to plate gold in extremely thin layers still allows for more extensive application of the material. The cost of gold is constantly varying; currently it costs about $980/to.
Silver (Ag) has the best room-temperature electrical and thermal conductivity of all metals. The principal sources of Ag are as an associated element from the mining of copper, copper-nickel, Au, lead and, lead-zinc ores obtained from Canada, Mexico, Peru, Australia, and the U.S. Silver has found many applications primarily because of its lower melting point and ease of fabrication. Silver is the most available and least costly of the precious metals at $20/to.
Platinum (Pt) occurs naturally and is accompanied by small quantities of the other PGMs. It is a beautiful silvery-white metal that is malleable. The metal is extensively used in jewelry, wire, vessels for laboratory use, and in many valuable industrial products including thermocouples, medical devices, and anticancer drugs. It is also used for electrical contacts, corrosion-resistant devices, and in dentistry. Pt-cobalt alloys have powerful magnetic properties. One such alloy, made of Pt-23.3 wt.% Co, offers a maximum magnetic field strength almost twice that of AlNiCo V, a strong permanent-magnet material. Resistance wires made from Pt are used in the construction of high-temperature electric furnaces. Pt can be drawn into fine wire and then knit into fabric referred to as gauze. The gauze is then fabricated into a large catalytic surface. It has long been used in the process for producing sulfuric acid (H2SO4) and nitric acid (HNO3). The present cost for Pt is approximately $2,045/to.
Iridium (Ir) is the rarest precious metal and the densest material known. It is more difficult to mechanically work than any other face-centered cubic (FCC) metal. This has been attributed to a reduction in ductility caused by trace element impurities that cause a modification of the grain boundary behavior. Its high tensile strength at elevated temperatures and high MP, makes it viable for crucibles in crystal growing. Hot working is one of the few ways to reasonably work the metal. Precise dimensional cuts are difficult, but are best achieved from either grinding or wire-EDM. Iridium is the most resistant of all metals to corrosion; it is insoluble in mineral acids including aqua regia. Resistance to spark erosion has made this element popular for spark plug applications.
Palladium (Pd) is a steel-white metal which doesn’t tarnish in air but can be attacked by nitric and sulfuric acids. It has the lowest density and melting point of the PGMs. At room temperature the metal has the unique property of absorbing up to 900 times its own volume of hydrogen. Hydrogen readily diffuses through heated palladium and development of this property provides a means of purifying the gas. The metal and its alloys are used in dentistry, watch making, surgical instruments, catalytic converters, jewelry trade, and electrical contacts.
Rhodium (Rh) occurs naturally with other PGMs. It has a high reflectance and is hard and durable. Sputtering targets of Rh are used to make the reflective surface for automobile mirrors and other optical instruments. As a bulk metal, it is mostly used as an alloying agent to harden Pt and Pd. Such alloys go in furnace windings, thermocouple elements, and to make bushings for glass fiber production. The addition of Rh increases both the operation temperature and the mechanical properties of the material. Rhodium also serves in a range of catalyst applications and in alloys and coatings for jewelry. Rhodium has the current distinction of being the most costly precious metal at over $9,300/to.
Ruthenium (Ru) is a hard, white metal mainly used as an alloying agent for platinum. The addition of 0.1% ruthenium to titanium immensely improves the corrosion resistance. It is a versatile catalyst and can help promote the splitting of hydrogen sulfide. Pure Ru is a difficult material to work.
Osmium (Os) is used almost exclusively as an alloying agent and has the distinction of having the highest melting point of the precious metals. Certain forms (tetroxides) are highly toxic. Osmium tetroxide is used in forensic science as a stain for fingerprints, microscope samples, and DNA materials. The metal is lustrous, bluish-white, and extremely hard and brittle even at high temperatures.
The density of gold and platinum is almost twice that of lead, whose density = 11.34 gm/cm3. In most old western movies, and more recently in the George Clooney movie called Three Kings (1999), the “bad guys” are shown loading gold bars. This is a bit far-fetched because many of the bars at the sizes shown would weigh close to 80 or 90 lb. A saddlebag of these plus the rider would be too much for the horse, or a human, to carry. Most people don’t have the opportunity to actually lift a gold bar, so Hollywood perpetuates the myth.