Silver’s common name originated from the Anglo-Saxon word for the material: selofor or siolfur.  The somewhat puzzling atomic symbol of Ag is short for the Latin: “argentum,” meaning “gray.”

Of course, the bright luster of pure, polished silver can reflect the full spectrum of visible colors.  In this condition, silver is the most optically reflective of all metals, although it’s an oddly poor reflector of the invisible ultraviolet light spectrum.

Even though silver is considered a noble metal that is resistant to chemical attacks, airborne contaminants such as ozone and hydrogen sulfide (H₂S) will tarnish exposed silver surfaces over time.  The tarnishing effect reduces silver’s reflectance and introduces other surface defects that can cause problems for electric circuits.

The First Recorded Semiconductor

More than a century before the silicon transistor would be invented, the first-ever recorded semiconducting material: silver sulfide (Ag₂S) was documented by Michael Faraday in 1833.

Faraday’s silver sulfide samples were crystalline—and like other semiconductors—it becomes a better electrical conductor when heated.  This is the reverse of how a metal conductor behaves, and it captivated the attention of Faraday, who published his findings. 

This wasn’t the only strange property to catch Faraday’s attention, as he also noticed the same material could conduct silver ions (charged particles containing silver) despite being solid.  He also observed similar effects in lead fluoride (PbF₂).  The idea of ions being able to flow through a solid material radically changed how people think about electrochemistry. This helped spin off the study of solid ionic electrolytes that are still applied in the 21st century in advanced batteries.

Silver in Photographic Processing

In the past, the compound silver nitrate (AgNO₃, also called lunar caustic), was extensively used in film and in photographic fixers in the film-making process.  The rise of digital imaging since the 1990s has greatly diminished this end-use.  However, the silver halides are still used in medical and industrial X-ray film.

Silver in Touchscreen-Compatible Gloves

Much like copper, gold, and palladium; silver can also be drawn out into long strands and used as electrical wire.  Special clothing can leverage this property to hide silver wires within the woven fibers of gloves to make them compatible with capacitive touch screens.  The idea is to let users interact with touchscreens without needing to withdraw their hand from the glove, and without needing a touchscreen that is strictly pressure-sensitive.  Patents for this process have used silver combined with a polymer carrier such as nylon, polyester or vinyl resin.

Silver vs. Copper

In electronics: many of silver’s primary end-uses revolve around its extremely high value as a conductor.  At room temperature, metallic silver surpasses all other metals (even copper) in electrical conductivity.

Conductivity is the reciprocal of resistance.  That’s to say, silver doesn’t resist the flow of electric charge as much as other materials.  When an electric current is forced to flow through a resistance, a certain amount of electric power will be converted to heat.  This is the effect called Joule Heating and Ohmic Heating.

When heating is undesirable, or if efficiency is important, silver has an advantage over every other metal.  But this also presents a dilemma, since silver’s spot price is often several times more expensive than copper.  Therefore, applications typically use copper except in niche situations.

Silver Epoxy and Plating

One exception is when silver is applied as a constituent of paint, ink, or epoxy.  According to the U.S. Geological Survey, silver-based inks are used to make the miniature antennas in Radio Frequency Identification (RFID) systems.  These tiny RFID tags are used in hundreds of millions of products to prevent theft, and to allow for inventory control.

Certain types of printed circuit boards (PCBs) may have conductive circuit traces made of conductive silver paint. This material is an alternative to the more-common copper foil.  In 2015, the circuit board printing system: Voltera V-One garnered crowd-funded support using this very concept.  Likewise, other companies such as Voxel 8, and several universities have used similar conductive materials to print circuit board traces directly onto insulating substrates.

Silver in Solder

Solder is a metal alloy used for joining components to a circuit board, or for joining two or conductors (such as electrical wire). Electrical-grade alloys of solder will often include a small percentage of silver that has been combined with other metals like lead, tin, and copper.

Silver in Thermistors

Despite all the fanfare about silver’s excellent conductivity, silver can be combined with other metals like palladium, zinc, or tin to produce a thermally-controlled resistor, i.e. a thermistor.  As the name implies, the thermistor’s job is to have an electrical resistance that responds to temperature.  These silver-based classes of thermistors increase their resistance with increasing temperature, making it a Positive Temperature Coefficient thermistor, or a “PTC” for short.

Silver-based PTCs are known to tarnish when exposed to the atmosphere.  So, silver PTCs often use a protective layer of glass or a similar encapsulating material.

Silver in Thermal Interface Materials

Silver isn’t just good at conducting electricity.  It’s also the best metal when it comes to thermal conductivity.  Power electronics including high-power Light Emitting Diodes (LEDs), lasers, and large transistors can use silver-based thermal interface materials (TIM) to transfer heat to a heat sink and other cooling structures.

For LEDs, this heat transfer is especially important for optimizing the efficiency of the light source.  Light emission and heat for LED systems are closely-intertwined.

The TIM may be a colloidal mixture of silver in epoxy, silicone grease, or even paraffin.  21st-century TIM materials are even using silver nanoparticles with carbon nanotubes to improve performance, lifespan, and reliability.

Silver Migration and Other Unwanted Effects

Despite its excellent properties, silver can sometimes be a material to steer clear of.

When metallic silver is exposed to sulfur-containing chemicals in the atmosphere, it forms a layer of dense, black, solid silver sulfide that is difficult to get rid of.  This is especially true around hydrogen sulfide (H₂S).  Silver components exposed to the air near paper mills, sewers, oil and gas facilities, or other polluted environments are especially at-risk.  Worse still, once a thick layer of silver sulfide is formed, the surface may grow long silver filaments called silver whiskers that can cause short circuits.  This effect has been observed since the 1920s, and it has caused a number of violent equipment failures.

Silver is also known to grow conductive, snowflake-shaped crystals.  These are silver dendrites.  Inside the dendrites, tiny charged silver ions will seek out anything nearby with an opposing electric charge.  This growth mechanism is called electrochemical migration (ECM).  On circuit boards, this basically means the dendrites of migrating silver will grow toward opposing conductors.  Much like with the silver whiskers, the root cause is chemical contamination from the environment.  Common contaminants may include acidic or alkaline electrolytes, salt electrolytes, and especially chlorine.

Because of these environmental factors, printed circuit board assemblies will often be coated with a protective layer called conformal coating.  The coating will block reactive gasses, as well as humidity that may otherwise attack exposed silver.

The environment may affect silver, but silver after affects the environment.  Silver is classified as being “very toxic to aquatic life” with both acute and long-lasting effects.  In short, it’s smart to keep silver waste away from our much-needed waterways. 

Silver-Oxide (Silver-Zinc) Cells and Batteries

In battery technology: the silver-oxide cell/battery (also called the silver-zinc battery) operates by reacting metallic zinc with silver oxide to produce electricity.  The electrolyte was typically potassium hydroxide (KOH) or sodium hydroxide (NaOH).  In the 20th century, a small amount of mercury was typically included as a way to prevent early oxidation of the zinc.  This practice fell out of favor due to the many health hazards and harmful environmental effects of mercury. Fortunately, mercury-free alternatives became popular around the year 2004.

This form of battery is a primary battery, meaning they are non-rechargeable.  But they have a 40% longer run time than comparable lithium-ion batteries, and a high energy-to-weight ratio. As you might imagine, the high cost of silver is the main barrier to wider adoption.  Still, the silver-oxide cell can be found in hearing aids, aerospace applications, and other niche applications where high energy density is an acceptable trade-off for the high cost of silver.

Future Implications

Silver Nanoparticles in Thin-Film Solar Cells

The renewable energy research field is already leveraging the properties of nanoparticles.  These particles can make thin-film solar cells more efficient thanks to the effect called light trapping.  This allows the thin film to collect visible and near-infrared energy from a wider range of incident angles.  The approach has been performed on amorphous silicon solar cells, and silver stands out because of its strong resonance and low absorption in the visible and near-infrared spectrum.  The use of widely-sized nanoparticles (20-200 nm) has been seen to improve solar conversion efficiency by 5.2 % to 16.2 %.

Silver in Artificial Muscles

As of 2019, mechanical systems like robots, vehicles, and prosthetic limbs depend largely on electromotors for movement.  These arrangements can become awkwardly large and heavy due to the need for permanent magnets.  Some emerging fields have a need for smaller, light-weight motors that mimic natural animal muscle.

To solve this: materials scientists are hard at work using materials like silver to create artificial muscle fibers.  These fibers often consist of a polymeric fiber that has been metalized with silver.  The metalized fibers can then be heated by an electric current, causing the fiber to contract like a muscle.  When the electric power stops, the fiber cools, and the fiber relaxes.  Individual fibers can braided or woven into intricate patterns to increase overall strength.  The result is called a Twisted-Coiled polymer fiber actuator (TCA).

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