|Precious Metals: Materials of Diversity |
|By N. Baltzer|
Cendres+Metaux, Biel-Bienne, Switzerland
The properties of precious metals and their alloys make them suitable for an expanding number of applications. Their great capacity to be cold worked and other valuable characteristics for medical device applications are outlined.
For a material scientist, precious metals are those elements that, independent of their aggregate state, are stable in air and do no corrode. These are ruthenium (Ru), gold (Au) platinum (Pt), palladium (Pd), iridium (Ir) and rhodium (Rh). Osmium is also defined as a precious metal, but it is not extensively used because it is toxic at certain temparatures. Silver (Ag) is considered to be a semi-precious metal, because it is affected by sulphur. Pt has the highest electrical potential in the electrochemical series, which makes it the standard for measuring other precious metals.
Pure precious metals are soft and can be easily deformed. To achieve higher mechanical strength they are alloyed. Au, Pt, Pd and Ag are combined with small amounts of nonprecious metals such as copper, zinc, tin and indium to create a number of alloys with varying properties. It is this variety that makes precious metals interesting materials. Better mechanical properties also require a fine “crystal” structure and this is achieved during casting. To gain this effect, Ir, Ru and /or rhenium, which have high melting points (above 2300 °C), are added to common precious metals with lower melting points (1000-2000 °C) such as Au, Pt and Pd. Those with a higher melting point act as nuclei onto which metal crystals grow before the liquid as a whole solidifies.
Use and value
Applications of precious metals include dentistry (dental attachments), permanent implantable devices such as cochlea implants/hearing aids, connectors and marking pins when taking X-rays. Production costs represent a small amount of the value of precious metals. The desirability associated with these materials together with speculation on future price increses their worth. Prices can rise dramaticaly and increase the cost of medical devices. For example, dental systems are fabricated from Pd-based alloys; between 2001 and 2004 the price of Pd rose from US$ 5000- 60000 per kg. Today, the cost of precious metals is slightly higher than production costs. The high material costs are compensated for by the properties they offer medical device designers.
The range of properties achievable with precious metals stems from their bonding characteristics. The electrons on the outer shell of the atoms (the conductive band) form a free electron gas, the so-called Fermi lake.This free electron gas builds the background bonding force of the atoms in a metal. The atoms are relatively mobile, yet highly ordered in crystal structures. This gives the materials their ability to undergo strain similar to that achieved with plastic materials. Precious metals are good conductors because they have many electrons in the conductive band. The electric configuration of precious metals and their alloys prevents the exhange of electrons with atoms such as oxigene, which is the reason for their low corrosion rates. Their resistance to oxidisation means they offer high biological compatibility. This makes them suitable for use in implantable electronics, which is a growing area of application.
The figure shows corrosion measurements on three precious metal alloy; even after 80 days the corrosion rate is below 1 µg/cm2. If a dental restoration has suface of 1 cm2 and a weight of 1g, it would take 200000 years to completely corrode away this restoration.
The figure shows stress-strain curves for one alloy in diferent conditions: annealed and cold worked at 25%, 50% and 75%. Precious metals have a great capacity to be cold worked, particularly in the annealed condition. With repeated cold work and annealing, an homogenous can be obtained. This is the basic requirement for obtaining parts of high precision and small dimensions. Precious metals and their alloys offer high mechanical strength at the same time as high deformability. There are alloys with srengths of more than 1000 Mpa that show strain values of 10%.
Colour and conductivity
Precious metal alloys offer a diversity of colour, from golden to white, and electrical conductivity and X-ray opacity.