July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 3 5 (b) (a) copper standard defined as 100%, and the electrical conductivity of any ma- terial is still expressed as percent IACS (International Annealed Copper Stan- dard), equal to 100 times the ratio of the volume resistivity of the annealed cop- per standard (0.017241 μΩ m) at 20°C (68°F) to the value measured for the material concerned. The highest purity copper (99.999% Cu) has been found to be 103% IACS. Electrical conductivity is sensi- tive to temperature: for copper it drops from 800% IACS at –240°C (–400°F) to 38% IACS at 425°C (800°F). The conduc- tivity of copper is independent of its crystal orientation and does not vary significantly with grain size. Cold work- ing an annealed copper to about 90% reduction can cause a drop of 2 to 3% IACS. All additives to pure copper reduce its electrical conductivity, depending upon the element and amount in sol- id solution. Only small decreases are caused by elements added in excess of solubility. There is a cumulative effect when more than one element is add- ed. The drop in electrical conductivity caused by additions of commonly used alloying elements is illustrated by Fig. 1, which shows the strongly detrimental effects of phosphorus and iron and the relatively mild decreases caused by sil- ver and zinc additions. Oxygen in stan- dard-grade copper reacts with many impurities, yielding insoluble oxides and thereby greatly reducing the harm- ful effects. Where oxygen-free or deox- idized copper is used, impurity levels must be reduced below those in cath- ode copper to achieve >100% IACS. As with other metal systems, cop- per is intentionally alloyed to improve strength without unduly degrading ductility or workability. However, it should be recognized that additions of alloying elements also degrade electri- cal and thermal conductivity by various amounts, depending on the alloying element and the concentration and Fig. 1 — Effect of alloying elements on the conductivity of oxygen-free high conduc- tivity copper. Fig. 2 — Electrical conductivity as a function of tensile strength for (a) annealed and (b) 60% cold-reduced copper alloy strip. listed infection-causing bacteria when tested under three separate EPA protocols. This EPA registration was pioneering be- cause it was the first time that a solid material achieved an EPA Public Health Registration. Previously, this distinction was held only by liquids and gasses. Every time a copper alloy enters into commerce with antimicrobial claims, the following statement must appear on the label: “Laboratory testing shows that, when cleaned regularly, antimicrobial copper surfaces kill greater than 99.9% of the fol- lowing bacteria within 2 hours of exposure: MRSA, VRE, S taphylococcus aureus, Enterobacter aerogenes, Pseudomonas aeru- ginosa, and E. coli O157:H7. Antimicrobial copper surfaces are a supplement to and not a substitute for standard infection control practices and have been shown to reduce microbial contamination, but do not necessarily prevent cross contamina- tion or infections; users must continue to follow all current infection control practices.” This EPA Public Health Registration should not be confused with the lower level EPA Treated Article Exemption which al- lows claims related to inhibiting the growth of microbes, acting as a preservative including preventing deterioration, discol- oration, or odor. No mention of the effect of the product on specific organisms is permitted. The Treated Article Exemption is typically associated with metallic and non-metallic coatings, as well as plastics and textiles. We live in a changing world and it is highly likely that COVID-19 is not the last pandemic that will emerge, due in part to the ease of international travel. It is essential that we get ready for the next one. Antimicrobial copper alloys will be useful in controlling newly emerging infections from a variety of causal agents. The data in the ASM Specialty Handbook: Copper and Copper Alloys contains a wealth of useful information for designers, engineers, specifiers, and manufacturers that will facili- tate the deployment of components made from antimicrobial copper in hospitals, nursing homes, medical and dental offices, schools, mass transit systems, cruise ships, commercial aircraft, public buildings, eating facilities, and everywhere that con- tains surfaces that humans touch. Harold T. Michels, consultant and retired senior vice president, Copper Development Association, Manhasset, New York.