Material of the Month - Silver

Materials World magazine
,
12 Jun 2014

This month, Anna Ploszajski investigates the properties of silver.  

Long before the discovery of this lustrous transition metal’s superior physical properties, silver was highly sought after for its aesthetic value. Recently, silver’s properties have seen its use spread from the pockets and dining tables of the wealthy to a diverse range of technological applications. The dawn of the nanotechnology era will undoubtedly secure its status as an indispensable element for centuries to come.

For almost 5,000 years, most of the silver produced globally was mined in modern day Turkey, Greece and Spain, and the industry grew steadily in Europe until the discovery of the Americas in 1492. The enormous deposits found in Latin America enabled production in Bolivia, Mexico and Peru to dwarf that of the European industry, accounting for 85% percent of global production by 1800. Improvements in mining technologies and new discoveries of deposits throughout the world facilitated even greater extraction, particularly in the USA. According to the World Silver Survey, 30,000 tonnes of silver were produced worldwide in 2012.

Silver deposits can be found in the metal’s pure form, as a gold alloy called electrum, or in ores containing sulphur, arsenic, antimony and chlorine. Industrial processes such as electro-refining of other metal ores yield silver as a by-product, and today the majority of silver produced industrially comes from electrolytic copper, gold, nickel and zinc refining, as well as lead refining using the Parkes process.

Silver’s precious metal status is a result of its rarity and attractiveness, and has given it high-value applications throughout history, including coins, jewellery, ornaments, tableware and bars for investment. In fact, the name Pound Sterling for the British currency refers to its traditional value of one Tower pound (350g) of sterling silver. The £ symbol we use today is an old-style letter L, with a crossbar to indicate that it is an abbreviation of libra, from the Latin name of a Roman unit of weight. The Tower pound standard was used until 1526, when Henry VIII issued a royal proclamation that the Troy pound be used instead. The name pound has remained to this day, although unfortunately one pound sterling is worth a lot less than its weight in silver these days. Nevertheless, silver’s intrinsic and permanent value makes it a secure and affordable investment, wellinsulated from the volatility of the financial markets.

Highly charged
Nowadays, silver isn’t only valued for being easy on the eye. Of all the elements, it has the highest electrical conductivity (6.30x107 Sm-1 at room temperature), exceeding even that of copper (5.96×107S/m). Although its higher value limits silver’s mainstream use as a conductor, niche applications exploiting this exceptional property exist in radiofrequency engineering, high-quality audio products and motor vehicles. In these systems, wires and parts are silver plated for optimal electrical conductivity, and yield superior performance. Silver also has the highest thermal conductivity of any metal (429 Wm-1K-1 at room temperature), surpassed only by diamond (up to 3,320Wm−1K−1, depending on purity) and superfluid helium (>100,000Wm−1K−1 at 2.2K). This is due to the high degree of atomic order in silver’s face-centred cubic molecular structure, allowing high thermal conductivity via phonons. The combination of excellent thermal and electrical conductivity makes silver a superb choice for high-quality heat exchangers and electronics where the importance of superior functionality outweighs the cost.

Silver also boasts excellent low surface frictional properties. Steel bearings in jet engines electroplated with silver have greater strength, durability and high-temperature longevity, and improve the engine’s overall performance. In an entirely different field, this low surface friction makes sterling silver the material of choice for high-quality musical instruments, such as flutes.

Silver’s favourable physical properties are also significant in the quest for sustainable energy. As an effective absorber of free neutrons, silver alloys containing 15% indium and 5% cadmium make excellent control rods used to regulate nuclear fission chain reactions in pressurised water nuclear reactors. Additionally, silver’s high conductivity is harnessed in the silver paste used in the electronics of most silicon photovoltaic (PV) cells, helping to enhance their efficiency. Furthermore, a highly reflective thin layer of silver on glass produces high-quality mirrors for use in solar reflectors, which concentrate the sun’s energy to produce heat or augment PV systems. Finally, transparent nano scale silver coatings applied to windows reflect up to 95% of the sun’s rays, significantly reducing the need for energy intensive air-conditioning systems to cool buildings in sunny climes.

Industrially, silver is indispensable. It is the only catalyst suitable for converting ethylene to ethylene oxide, a precursor for ethylene glycol used in textiles and computer keyboards, and as the active ingredient in antifreeze. Unaffected by the reaction, silver used in this process is completely recoverable, which favours its use economically.

Picture perfect
Photosensitive silver halide crystals dispersed in a gelatin emulsion are used in photography to record a latent image, which can later be amplified into an image by chemical development. When the silver halide crystals absorb photons, electrons are promoted into the conduction band. These electrons are attracted by sensitivity specks – shallow electron trap sites, such as crystal defects or dopants. Combined with an interstitial silver ion, these sensitivity specks become a small speck of metallic silver, which appears black on the film. This forms a latent image, since the areas struck by the most photons have the highest density of dark metallic specks. Silver bromide, silver chloride and mixed-phase halide crystals can grow in a large variety of morphologies, dependent on the number and orientation of twin planes in their face-centred cubic crystal lattices, and on the growth conditions. Heat treatments with small amounts of sulphur and gold compounds chemically sensitise the silver halide crystals, introducing more sensitivity specks and, therefore, increasing the crystals’ photographic speed. Cyanide dyes applied to the crystal surface extend the UV and blue light sensitivity to other colours in the visible spectrum. Colour photography is made possible by layering emulsion films treated to be selective to blue, green or red light.

A particularly hot new topic for this diverse metal is in nanotechnology, with applications from hospital beds to space exploration. Silver was used to prevent infection more than 2,500 years ago, in ancient Macedon, and now, new silver nanomaterials incorporated into modern wound dressings are believed to reduce external infection.

Furthermore, silver nanomaterials are used in water purification systems in hospitals, on the International Space Station, and in developing nations worldwide to provide safe drinking water to millions. These applications make use of the oligodynamic effect, whereby the bioactive silver ions readily kill bacteria, algae and fungi in vitro by irreversibly damaging key enzyme systems in the cell membranes. This antibacterial effect can be enhanced by applying an electric field, due to the increased release of silver ions from nanostructured silver electrodes.

Silver nanoparticles also boast unique optical properties. They are extremely efficient at absorbing and scattering light, and their colour depends upon their size and shape. This strong interaction is due to surface plasmon resonance, whereby conduction electrons on the nanoparticle surface collectively oscillate when excited by an incident photon of a specific wavelength. Therefore, silver nanoparticles have significantly larger absorption and scattering intensities compared to non-plasmonic nanoparticles of the same size. Surface plasmons enhance the sensitivity of spectroscopic measurements such as fluorescence and Raman scattering. Despite showing great promise, safety concerns surrounding silver nanoparticles have given the technology some bad press.

On human exposure, silver nanoparticles have been reported to accumulate primarily in the liver, but also show toxicity in the brain and aid the formation of dangerous free radicals. Further research is required to fully understand the risk levels involved in working with silver nanomaterials, but it is hoped that with sufficient protection in place, the full potential of these unique materials can be realised for tremendous benefit.

The Inca people called silver the tears of the moon, and this association also appears in folk law, alchemy and the arts. The relationship suggests a mystery and beauty surrounding this precious metal. Although science seeks to solve mysteries and explain the beauty of the world we see around us, new mysteries and technologies of beauty are constantly being created by the solutions to the old.