For generations, zinc has been the metal of choice in applications spanning energy distribution and energy storage.  Present in weather-resistant metals, its uses have been carefully refined over the years to make it one of the most-used metals after iron and steel.  But as its demand grows, will its natural sources be sufficient to keep up?

Zinc’s Background

Below the very thin copper plating on the United States penny is one of the most ubiquitous metals used today: zinc.  The word zinc has an obscure origin.  However, it’s chief ore: “sphalerite” is named from a Greek word for “deceitful,” as it was often mistaken for the lead ore: galena.  Centuries before it was identified as a unique element, zinc was used in the production of brass (an alloy of copper and zinc).  Metallic zinc was produced in the 13^th century A.D. in India by reducing calamine with organic substances such as wool.  The metal was reused centuries later in Europe where calamine was used again, and the zinc was reduced using charcoal.

Two important natural minerals for zinc sulfide (ZnS) are zinc blende (also called sphalerite) and wurtzite.  Being crystalline: both minerals have a repeating (i.e.: periodic) atomic structure called a crystal lattice.  Many commercially-important semiconductors also have similar crystal lattice structures, even though zinc may not literally be a part of it.  Because of the similarity: many books and published papers on electronic materials will use “zinc-blende,” or “wurtzite” synonymously with the crystal lattice structure they are referencing.

In science, zinc has unusual electrical, thermal, optical, and solid-state properties that have yet to be fully realized.  Zinc is only a fair conductor of electricity: being about 71 % less conductive than annealed copper.  Many of its applications in industry will combine it with other metals such as copper or iron.  The automotive, electrical, and hardware industries use zinc extensively in die casting.

Zinc Compounds for Surge Arrestors

The electrical power industry and the devices it serves are arguably the foundation of any modern economy.  Yet these assets are also exposed to damaging voltage surges that can originate from sudden load changes and electrical faults.  To protect these investments, engineers use many high-voltage surge-arresting technologies.  One such tool is the zinc oxide (ZnO) arrestor.

The ZnO arrestor is a type of variable resistor called a varistor.  In the presence of a normal and safe voltage, the arrestor is in a pre-breakdown state, where it electrically resembles a very large-valued resistor.  This essentially means it has little or no impact on the rest of the circuit.  But in the presence of an abnormally-high voltage, its resistance falls immensely, and the arrestor enters breakdown mode (also called the working area or non-linear area).  Due to its excellent nonlinear voltage-current characteristics, and its capacity for absorbing energy, the ZnO varistor is a core component of overvoltage suppression systems.

The ZnO inside them is a polycrystalline semiconductor ceramic material with small amounts of Bi₂O₃, Sb₂O₃, Co₂O₃, MnO₂, Cr₂O₃, SiO₂ and other metals and non-metal oxides added.  For that reason, the devices are also called metal oxide varistors (MOV).  The approach differs from many other semiconductor devices because the tiny crystalline grains comprising the ceramic behave like countless Schottky barrier diodes connected back-to-back.  Unlike ordinary PN-junction diodes, Schottky diodes have a metal-semiconductor junction that allows very fast switching and recovery.  A pair of back-to-back diodes will be mostly non-conductive until a sufficiently high voltage drives them into conduction.  The MOV is better viewed as a multitude of small diodes in this configuration.

To simulate a complete MOV device, modeling frameworks such as SPICE can approximate an MOV as being a combination of simpler components.

Taken to the extreme, multiple ZnO arrestors can be linked together to form large arrestor blocks that can suppress voltages greater than any single arrestor could alone.

The ZnO suppressor is a vital component, but the technology still has its limitations.  A typical MOV will age each time it successfully suppresses a surge.  The deterioration mechanism may be due to electron trapping, dipole orientation, oxygen desorption, or ion migration within the ZnO crystalline structure during surge events.  Regardless of the details, this aging results in lower performance:

• Devices that become aged may gradually conduct leakage currents when there is no surge to suppress.
• Its voltage-current behavior may change.
• The threshold voltage to trigger them may change; typically by decreasing.

Moreover, if the operating voltage is continuously present, these surge arresters may go into thermal runaway.

Zinc for Corrosion Protection

Protecting power infrastructure from electrical damage is only one of zinc’s many uses.  Preventing steel structures and facilities from corrosion is another topic entirely.

The rusting that happens to iron is a complex chemical reaction that also involves water and oxygen.  Iron dissolves in regions where it can enter the water as the Fe²⁺ ion.  Correspondingly, two negatively-charged electrons travel through the bulk metal to counter-balance the loss of iron’s +2 charge.  The electrons will arrive at locations where they can reduce oxygen to form two negatively-charged hydroxide (OH)- ions.  The dissolved hydroxide ions and Fe²⁺ ions then combine to form rust (Fe₂O₃ · xH₂O).

In electrochemistry: any site where oxidation (an increase in electric charge) occurs is called the anode.  In the above example: the anode was the combination of iron and water.  Zinc can mitigate the rusting process by replacing iron as the anode in these reactions.  This effectively means if corrosion occurs, iron will be protected from oxidation while zinc is given up first because zinc is more anodic than iron. 

In this configuration, the zinc is called the sacrificial anode.  Periodically, such sacrificial anodes will need to be replaced as they corrode.

But steel objects that must withstand the weather can also be completely coated in a layer of zinc.  This process is galvanization.  Zinc is used extensively in industry in the galvanization process.  Galvanized steel is a popular option for everything from steel power transmission towers, to sheet metal, duct work for cooling systems, and chain-link fences.

There are multiple ways to galvanize metals using zinc.  One type is the hot dip method.  This name references the step of immersing the steel in a bath of molten zinc along with other metals such as aluminum, lead, or antimony.  Metals that are galvanized with this method can develop a striking visible pattern of very large zinc crystal grains called spangle.  The spangle is the result of the solidification of molten zinc after the hot dip process, and occurs in shiny, feathery, and dull varieties.

In summary, galvanized steel sheets are characterized by their superior corrosion resistance, thanks to the anodic role of zinc.

Zinc Whiskers and Reliability Effects

Of course, materials can have unintended side effects, depending on the circumstance.  In the case of zinc that is electroplated: this can come in the form of zinc whiskers.

Zinc whiskers are tiny hair-like filaments that are typically a few millimeters long and only a few thousandths of a millimeter wide.  Because of their size, they are difficult to see without magnification, but their electrical effects can be dreadful!

The filaments can form on steel that has been electroplated with zinc.  Luckily, zinc plating is uncommon on electrical components, but it does commonly appear on structural steel parts; such as the raised floor supports common in data centers.  The process is spontaneous, and after an uncertain delay (the incubation period), the whiskers form from the metal surface.  When these whiskers become dislodged, they may be blown around by building cooling systems and drawn into the power supplies of electronic equipment.  From there, they run the risk of creating short circuits and hardware faults. 

Many metals beside zinc can also create unwanted whiskers (such as tin and cadmium), however the ubiquity of electroplated zinc makes it a widescale problem.  Even NASA data centers weren’t immune from the problem in the early 21^st century, where zinc whiskers were once implicated in at least 18 catastrophic power supply failures.  To assist the scientific community and the public, the NASA Goddard Space Flight Center (as of this writing) maintains a web site dedicated to documenting metal whiskers and their effects.

Zinc Anodes in Batteries

Galvanized metals are not the only place where zinc serves as an outstanding anodic material.  In the corrosion example above, there was an electric current in the form of mobile electrons moving through the bulk iron.  This same electrochemical behavior plays a critical function in many batteries.

A battery is built up of galvanic cells.  Each cell has an anode where oxidation occurs and a cathode where reduction occurs.  When the cell is connected to an external load (such as a smart phone, or an electromotor, etc.), electrons flow through the electric circuit.  The direction of electron flow here is always from anode to cathode.

A useful mnemonic is “FAT CAT” for: “From Anode to CAThode.”

Zinc-carbon batteries are a type of galvanic cell that has been in existence for over a hundred years.  One type of zinc-carbon system is the Leclanché cell—developed in 1866 and improved over the years to remain in use today.  Georges-Lionel Leclanché developed the technology in his pioneering work for the telegraph office.  His zinc-carbon system was unique at the time because it was the first practical system to utilize a low-corrosive electrolyte, rather than the strong mineral acids popular at the time.

Today, zinc-carbon batteries are sold in retail stores as AA cells, AAA cells, and 9V batteries just to name a few.  The core features have remained unchanged.  The zinc-carbon cell uses zinc as the anode material and manganese dioxide as the cathode   Carbon (as acetylene black) will be mixed with the manganese dioxide to improve conductivity and retain moisture.  The metallic zinc will be oxidized at the anode, and then go into the battery’s electrolyte solution as the positively-charged Zn²⁺ ion.  The manganese dioxide will be reduced: forming hydrated manganese (III) oxide (Mn₂O₃ or sometimes written: Mn₂O₃ · H₂O).

Many other metals can function as anodes, but zinc is an attractive choice because it is relatively stable in alkaline electrolytes.  In rechargeable metal/air batteries it is also the most active metal that can be electro-deposited from an aqueous electrolyte.

As a general-purpose cell: Leclanché systems apply where intermittent, low-rate, and low-cost energy is required.  These devices use ammonium chloride (NH₄Cl) as an electrolyte, and a starch paste as a separator (to keep the anode and cathode apart).

Other battery chemistries using zinc include:

• The Gelaire cell
• Zinc-chloride cells
• Zinc-air cells
• Nickel-zinc cells
• Zinc-silver oxide cells

Where Zn is the anode material.  Even the emerging zinc-bromine flow battery uses its anodic properties.

The zinc-bromine battery may play a future role in the renewable energy grid as a Battery Energy Storage System (BESS).  These large-scale batteries can provide—among other benefits—storage for excess energy generated from the power grid. 

Zinc and the Photoelectric Effect

One extremely impactful discovery from the early 20^th century was the discovery of the photoelectric effect.  This was the subject of Albert Einstein’s famous paper of 1905 (translated to English by D. T. Haar).  It was this paper that—among other revelations—proposed that the energy of a photon (“energy quanta” in) depends on the photons frequency.  Moreover, his paper revealed the photoelectric effect: where a photon penetrating into a surface can transfer a portion of its energy to an electron in the material.  If electrons at the surface receive the right amount of energy, and at the proper angle, they could leave the bulk material.

This required energy is the “work function” of the surface material.  If there were no forces at the surface tending to hold the electrons in the material, then even photons with low energy (i.e.: long wavelengths and low frequencies) would eject electrons from the surface.  This is not the case.  Therefore, there is a finite amount of energy preventing this that must be overcome.

Paired with proper amplifiers the photoelectric effect of zinc compounds can be amplified to control larger machines.  Not many materials have strong photoelectric effects, but the crystalline structure of zinc blende and the non-metal: selenium were documented exceptions by the 1930’s.  Due to their photoelectric properties, zinc and selenium are used together in other light-based technologies.

Zinc in Photoelectric Devices

Zinc also plays a role in the conversion of electricity to light: including phosphors and electroluminescent (EL) light materials.

ZnS types phosphors, including the green-emitting ZnS:Cu,Al (zinc sulfide with copper and aluminum activator) and the blue-emitting ZnS:Ag,Cl (zinc sulfide with silver and aluminum activators) were very important materials in cathode ray tubes.  These phosphors have a long history dating back to the 19^th century, when prepared crystals of ZnS were found in 1866 glowing in the dark. 

Alternatively, electroluminescence happens when light is generated by applying an electric field to a crystalline material.  Historically, the EL phenomenon was first observed in 1936 from ZnS power phosphors suspended in castor oil when a strong electric field was applied.  This type of EL is known as high-field EL.  High-field EL systems commonly use ZnS in the presence of electric fields on the order of 10⁶ V cm^-1.

An alternate form of EL—called injection EL—was reported in 1952, when the junctions of germanium (Ge) and silicon (Si) diodes were observed emitting infrared radiation.  Unlike with high-field EL, the light is generated when electrons conducted across the LED junction lose their energy in a process called electron-hole recombination.  This energy escapes in the form of light energy.

Ordinary silicon diodes have a crystal structure with a very low probability of generating visible light.  This is due to silicon having an indirect band gap: a property that describes the energy states of electrons in the material.  But usually, crystal lattices that are structured like zinc blende or wurtzeite have direct band gaps.

• ZeSe is used in some LEDs that appear white to the human eye; for lighting applications and in high-speed visible light communication.  (Note: White LEDs do often use other materials, however)
• Zinc telluride (ZnTe) is a promising direct bandgap semiconductor with applications in thin film solar cells.
• Gallium Nitride (in the zinc blende and wurzite form) is another material for blue and ultraviolet light sources.

Zinc sulfide is being used in the making of luminous dials, X-ray screens and fluorescent lights.

Future Implications

From its applications in surge arresting, galvanizing, battery technology, and optoelectronics; zinc is a material that is constantly in high demand.

In short, zinc is one of the most widely-used non-ferrous metals in the world.  To keep up with global demand, recycled zinc is of great long-term interest for many industries.  Targets for zinc recovery include spent batteries and electronic wastes (E-waste), waste from steelmaking, and even wastewater.  Hence, new extraction methods are being researched to deal with multiple scenarios.

In the consumer electronics industry: portable battery recycling is an attractive target due to the short lifespan and high popularity of the zinc-carbon battery.  A 1.5-volt single-use battery of this type will typically be around 16 % zinc in composition, and zinc-air batteries can approach 35 % zinc.  Because of the historic use of hazardous metals such as mercury in batteries, recycling is a sensible option; and the same holds true for other types of electronic wastes.

References

[1]	U.S. Department of the Treasury, “History of the Lincoln Cent,” U.S. Department of Treasury, [Online]. Available: https://www.treasury.gov/about/education/pages/lincoln-cent.aspx. [Accessed 14 Feb. 2019].
[2]	C. R. Hammond, “The Elements,” in CRC Handbook of Chemistry and Physics, 88th ed., D. R. Lide, Ed., Boca Raton, FL: CRC Press, 2008, pp. 1-44.
[3]	“Sphalerite | Definition of Sphalerite by Merriam-Webster,” Merriam-Webster, Inc., [Online]. Available: https://www.merriam-webster.com/dictionary/sphalerite. [Accessed 10 Feb. 2019].
[4]	X.-H. Luan, C. Feng, H.-B. Qin and et. al., “The electronic properties of zinc-blende GaN, wurtzite GaN and pnma-GaN crystals under pressure,” in 18th International Conference on Electronic Packaging Technology (ICEPT), Harbin, China, 2017.
[5]	T. Potlog, N. Maticiuc, A. Mirzac and et. al., “Structural and optical properties of ZnTe thin films,” in CAS 2012 (International Semiconductor Conference), Sinaia, Romania, 2012.
[6]	L. H. Hemmings, “Grounding, shielding, and filtering,” in The Electrical Engineering Handbook, 2nd ed., R. C. Dorf, Ed., Boca Raton, FL, USA, CRC Press, 1998, p. 1007.
[7]	R. C. Dorf, “Electrical and Computer Engineering,” in CRC Handbook of Engineering Tables, Boca Raton, FL, USA, CRC Press, 2004, p. 1.74.
[8]	R. E. Hummel, “Appendix 4, Tables,” in Electronic Properties of Materials, 4th ed., Springer, 2011, p. 456.
[9]	Q. Yang and Y. Zhang, “Lightning Invaded Overvoltage Monitoring Technology in Substation Based on the Principle of ZnO Lightning Arrester Block Voltage Dividing,” in 2018 34th International Conference on Lightning Protection (ICLP), Rzeszow, Poland, 2018.
[10]	M. Hassanzadeh and R. Metz, “Life cycle assessment of medium surge arresters in the context of size‐reduction design,” Journal of Industrial Ecology, vol. 16, no. 2, p. 276, 2012.
[11]	J. G. Zola, “Simple model of metal oxide varistor for Pspice simulation,” IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, vol. 23, no. 10, pp. 1491-1494, 2004.
[12]	S. Santoso and W. H. Beaty, “Lightning and overvoltage protection,” in Standard Handbook for Electrical Engineers, 17th ed., McGraw-Hill Education, 2018, pp. 1468-1476.
[13]	J. He, J. Lin, L. Zhang and et. al., “Indication of initial degradation of ZnO varistor ceramics,” in 1st International Conference on Electrical Materials and Power Equipment (ICEMPE), Xi’an, China, 2017.
[14]	B. M. Wilamowski, “Semiconductor Diode,” in The Industrial Electronics Handbook: Fundamentals of Industrial Electronics, 2nd ed., Boca Raton, FL, USA, CRC Press, 2011, p. 8.12.
[15]	T. Tada, “Degradation of ZnO varistors as estimated by aging tests,” Electrical Engineering in Japan, vol. 170, no. 2, pp. 1-18, 2010.
[16]	J. E. Brady, J. W. Russell and J. R. Holum, “Electrochemistry,” in Chemistry: Matter and Its Changes, John Wiley & Sons Inc., 2000, pp. 873-874.
[17]	R. A. Painter, “The hot-dip galvanizing of structural steel sections,” Proceedings of the IEE – Part III: Radio and Communication Engineering, vol. 99, no. 58, p. 95, 1952.
[18]	S. K. Shukla, B. B. Saha, B. D. Triathi and et. al., “Effect of process parameters on the structure and properties of galvanized sheets,” Journal of Materials Engineering and Performance, vol. 19, no. 5, pp. 650-655, 2009.
[19]	S. Peng, J. Lu, C. Che and et. al., “Morphology and antimony segregation of spangles on batch hot-dip galvanized coatings,” Applied Surface Science, vol. 256, no. 16, pp. 5015-5020, 2010.
[20]	J. Brusse and M. Sampson, “Zinc whiskers: hidden cause of equipment failure,” IT Professional, vol. 6, no. 6, pp. 43-47, 2004.
[21]	G. T. Galyon, “Annotated tin whisker bibliography and anthology,” IEEE Transactions on Electronics Packaging Manufacturing, vol. 28, no. 1, pp. 92-122, 2005.
[22]	L. Wu, M. Ashworth and G. Wilcox, “Investigation of whisker growth from alkaline non-cyanide zinc electrodeposits,” Journal of Electronic Materials, vol. 46, no. 2, pp. 1114-1128, 2016.
[23]	NASA Goddard Space Flight Center, “Zinc Whiskers: Could Zinc Whiskers Be Impacting Your Electronics?,” 2 Apr. 2003. [Online]. Available: https://nepp.nasa.gov/whisker/reference/tech_papers/Brusse2003-Zinc-Whisker-Awareness.pdf. [Accessed 10 Feb. 2019].
[24]	D. Linden and T. B. Reddy, “Basic Concepts,” in Linden’s Handbook of Batteries, McGraw-Hill, 2011, pp. 1.6-1.8.
[25]	S. S. Zumdahl and D. J. DeCoste, Study Guide for Zumdahl/DeCoste’s Chemical Principles, Cengage Learning, 2012.
[26]	B. Schumm, “Zinc-carbon batteries-Leclanché and zinc chloride cell systems,” in Linden’s Handbook of Batteries, 4th ed., T. B. Reddy and D. Linden, Eds., McGraw-Hill, 2011, pp. 9.1-9.41.
[27]	T. B. Atwater and A. Dobley, “Metal/air batteries,” in Linden’s Handbook of Batteries, 4th ed., T. B. Reddy and D. Linden, Eds., McGraw-Hill, 2011, pp. 33.1-33.13.
[28]	J.-U. Lim, S.-J. Lee, K.-P. Kang and et. al., “A modular power conversion system for zinc-bromine flow battery based energy storage system,” in IEEE 2nd International Future Energy Electronics Conference (IFEEC), Taipei, Taiwan, 2015.
[29]	A. Einstein, “Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt,” Annalen der Physik, vol. 322, no. 6, pp. 132-148, 1905.
[30]	D. T. Haar, The Old Quantum Theory, Oxford: Pergamon Press Ltd., 1967.
[31]	A. L. Hughes, “Fundamental Laws of Photoelectricity,” Transactions of the American Institute of Electrical Engineers, vol. 53, no. 8, pp. 1149-1153, 1934.
[32]	S. Shionoya, “Principal phosphor materials and their optical properties,” in Phosphor Handbook, Boca Raton, FL, USA, CRC Press, 1999, pp. 231-238.
[33]	S. Tanaka, H. Kobayashi and H. Sasakura, “Fundamentals of luminescence,” in Phosphor Handbook, Boca Raton, FL, USA, CRC Press, 1999, pp. 123-126.
[34]	B. M. Wilamowski and D. J. Irwin, Eds., “Devices,” in The Industrial Electronics Handbook: Fundamentals of Industrial Electronics, Boca Raton, FL, USA, CRC Press, 2011, p. 8.14.
[35]	S. Coffa, “Light From Silicon,” IEEE Spectrum, 1 Oct. 2005.
[36]	S. Nara and I. Sumiaki, “Fundamentals of luminescence,” in Phosphor Handbook, S. Shionoya and W. M. Yen, Eds., CRC Press, 1999, pp. 21-22.
[37]	S. J. Chang, T. K. Lin, Y. Z. Chiou and et. al., “ZnSe based white light emitting diode on homoepitaxial ZnSe substrate,” IET Optoelectronics, vol. 1, no. 1, pp. 39-41, 2007.
[38]	P. H. Binh and N. T. Hung, “High-speed visible light communications using ZnSe-based white light emitting diode,” IEEE Photonics Technology Letters, vol. 28, no. 18, pp. 1948-1951, 2016.
[39]	S. Chen, X. Li and X. Zhang, “Sustainable Utilization of Zinc Resources in America,” Acta Geologica Sinica (English Edition), vol. 88, no. s2, pp. 1231-1232, 2014.
[40]	K. S. Ng, I. Head, G. C. Premier and et. al., “A multilevel sustainability analysis of zinc recovery from wastes,” Resources, Conservation and Recycling, vol. 113, pp. 88-105, 2016.
[41]	“Wurtzite | mineral | Britannica.com,” Encyclopedia Britannica, Inc., [Online]. Available: https://www.britannica.com/science/wurtzite. [Accessed 10 Feb. 2019].

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