Setting electronics in artificial stone

Concrete slabs generally don’t do much except support structures and enclose space. If some researchers have it their way, however, those slabs may double as components in electronic circuits.

Materials scientists at the State University of New York at Buffalo have made crude electronic devices from Portland cement, the widely used gray powder that forms concrete when it’s mixed with sand, gravel, and water.

With minor modifications of the standard formula, Sihai Wen and Deborah D.L. Chung poured soap-bar-size blocks of cement that act as one-way electricity valves known as diodes. The blocks also function as heat-sensitive sources of electric current known as thermocouples, the team reports in the July Journal of Materials Research.

“The idea is to use the concrete structure itself to manipulate electronic signals,” Chung says.

If such components work when sand and stone are included, they could have a wide variety of applications, such as temperature sensing, says Thomas O. Mason of Northwestern University in Evanston, Ill.

Because the devices may also exhibit the so-called thermoelectric effect (SN: 9/6/97, p. 152),

Chung is most excited by the possibility of using the new concrete to heat and cool structures.

“When the current flows one way, it gives heating. The other way, it gives cooling,” she explains. Such temperature controls could prove useful, for example, in roadways where a little heat in the winter could melt dangerous ice.

In making their prototype electronic cement, the Buffalo researchers took cues from everyday semiconductor components. Those devices have junctions where an electron-rich material meets another material with an electron shortage. Voltage differences, which are the basis of transistor action, spontaneously develop at these junctions.

To create cement with excess electrons, Wen and Chung added ingredients such as steel fibers.

For a deficit of electrons, they added substances such as carbon fibers. The researchers then fabricated cement blocks with junctions by filling a mold half with the electron-rich cement and half with the electron-poor variety.

The new concrete may have pitfalls, says Paul J. Tikalsky of Pennsylvania State University in State College. For instance, electric currents in concrete might hasten corrosion of reinforcing steel bars. Also, he suspects that heating and cooling a building with thermoelectric concrete would be inefficient.

Chung remains optimistic. Electronic functions could stay in parts of slabs away from steel bars, she claims. And she suspects that by increasing the concrete’s conductivity she might make heating and cooling efficient enough to compete with more conventional means of climate control.

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