Wafers that go bang

Chemists at the University of California, San Diego have discovered that silicon wafers, the raw starting material for computer chips, can be easily made into tiny explosives that might be used one day to chemically analyse samples in the field or serve as power sources for tiny electronic sensors the size of a speck of dust.

The UCSD scientists provide the technical details for some of these futuristic applications in a paper featured on the cover of the January issue of Advanced Materials, a scientific journal based in Germany.

'Most people are familiar with silicon as the material that's used in computer chips for circuits,' says Michael J. Sailor, a professor of chemistry and biochemistry who headed the research project. 'This is the same material, but we're making it into a very finely divided form of silicon - a nanocrystal - that has such a high surface area that it burns very quickly. The faster the burn, the bigger the bang.'

Like gunpowder - a mixture of carbon, potassium nitrate and sulphur—the UCSD scientists knew previously that a silicon-based explosive would explode when mixed with potassium nitrate. However, Frederic V. Mikulec, a postdoctoral researcher in Sailor's laboratory, discovered by accident while working with a porous wafer of silicon that substituting potassium nitrate with gadolinium nitrate had the same effect.

'When he tried to cleave the wafer with a diamond scribe, it blew up in his face,' recalls Sailor. 'It was just a small explosion, like a cap going off in a cap gun. But it really surprised us, so we started looking more closely at it, because the gadolinium produced a very clean burning flame.'

The absence of chemical impurities, the UCSD scientists say, makes the gadolinium- and silicon-based explosive ideal for use in a device that could perform rapid chemical analysis of toxic metals and other elements in the field.

'If you want to look for lead or other toxic metal ions in a sample of groundwater, typically what's done is that the sample is taken back to a laboratory and analysed in an emission spectrometer,' explains Sailor. 'In that spectrometer, the groundwater is mixed with other chemicals and burned - and the flame produces a characteristic set of colours that correspond to certain chemicals. What we did in this experiment is to show that you can miniaturise this analytical laboratory using porous silicon and gadolinium nitrate into something that's as small as the diameter of a human hair. So that when you're out in the field, you can do this flame emission spectrometry instantaneously with a device that fits in the palm of your hand.'

Another, more futuristic, application of this miniature silicon explosion might be to use it as the propulsion source for micro-electrical mechanical systems, or MEMS. These dust-sized devices built on silicon could be designed to search for explosives, toxic compounds or biological agents - and powered by little rocket engines built into the silicon. One advantage of silicon explosives over conventional explosives, which must be ignited by mechanical means, is that they can be ignited electronically, allowing the military to build safer explosives, which won't detonate when dropped.

'Let's say you have a computer chip collecting information on the ground and 10 minutes later you want it to flip over, or self destruct, or ignite so that it will show up on an infrared or night-vision camera,' says Sailor. 'What we've shown in this paper is that a small voltage can be used to ignite this chip, so you don't need any sophisticated devices other than the tiny voltages you already have flowing through the electronic circuits of the chip. You just need to send them through the part of the chip that contains the blasting cap.'

Other possible security or military applications of this explosive might be the construction of information-collecting devices that self-destruct. 'Let's say you've built a secret electronic device that you don't want someone to take apart or to find out how it works,' says Sailor. 'You could build a self-destruct mechanism into the computer chip that would basically destroy it and any information that had been stored on it.'

What makes all of these futuristic applications plausible is that current manufacturing processes for computers could be easily adapted to produce such smart explosive or propulsion systems. 'One of the things we've shown is that the construction of this explosive is compatible with conventional silicon fabrication techniques,' adds Sailor. 'In other words, the same tools that right now can put transistors on a chip could be used to build a rocket motor using the same material, silicon, that forms the chip as the power source.'

Joseph D. Kirtland, an undergraduate from Union College in Schenectady, NY, working in Sailor's laboratory over the summer, also contributed to the study. The project was financed by the National Science Foundation and the Defense Advanced Research Project Agency's Tactical Sensors Program.