THE search for limitless clean energy tends to bring out the ridiculous in people, whether it鈥檚 perpetual motion, cold fusion or even hot fusion in the shape of multimillion-dollar machines that come and go without generating a watt of consumable power. The quest seems to have become a modern form of alchemy, yet there鈥檚 something deeply appealing about it. If we succeeded we could solve so many problems, from breaking our addiction to fossil fuels and sorting out global warming to making a large dent in global poverty.
The latest contender in the limitless clean energy stakes arrived in early March in the unlikely shape of gas bubbles in a beaker of acetone bathed in sound waves. The hydrogen in the acetone had been switched with deuterium 鈥� hydrogen with an added neutron 鈥� while the sound waves alternately expanded the bubbles and then forced them to collapse.
The researchers, led by Rusi Taleyarkhan of Oak Ridge National Laboratory in Tennessee, argued that as the bubbles imploded, the temperature inside them would rise high enough to force hydrogen and deuterium nuclei to fuse, generating heat. Sure enough, in a paper in Science (vol 295, p 1868), Taleyarkhan and colleagues reported finding neutrons and tritium 鈥� that鈥檚 hydrogen with two neutrons 鈥� both of which are telltale signs of fusion.
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The decision by Science to publish the work was a brave one in the light of the cold fusion debacle of 1989. The claim that an electrochemical cell with palladium and platinum electrodes dipped in 鈥渄euterated鈥� water could generate more energy than it consumed had seriously damaged the process of research and academic freedoms.
Journals refused to publish electrochemistry research that mentioned deuterated liquids and palladium in the same article. Mainstream scientists overreacted to the controversy, ostracising anyone pursuing research in the area 鈥� even if it had nothing to do with fusion per se. Younger researchers learned that their careers would stall if they showed interest in the field. As a result, solving the riddles of what really happens in a cold-fusion cell has largely been left to a few privately owned labs and an underground army of researchers who鈥檝e set up their own journals and peer-review systems.
Science鈥檚 decision was all the more brave because another research group at Oak Ridge had repeated Taleyarkhan鈥檚 experiment and concluded that there was nothing in it. Their work, which was not peer-reviewed, argued that Taleyarkhan鈥檚 telltale neutrons and tritium were artefacts of his experimental set-up. In this atmosphere, evidence that Taleyarkhan鈥檚 group had really stumbled across something would only come when others repeated the experiment.
Compared with the global frenzy of research that followed the announcement of cold fusion, the response to 鈥渟onofusion鈥� has been lethargic to say the least. But this week comes the first shot across the bows from an independent research group (see 鈥淏urst bubble鈥�). Yuri Didenko and Kenneth Suslick of the University of Illinois at Urbana-Champaign have made the first direct measurements of reactions happening inside a single air bubble in water as it collapses.
They found that as a bubble implodes, some of the gases trapped inside, including oxygen, nitrogen and hydrogen, begin to react. These reactions absorb heat, acting as a damper on the rising temperature of the collapsing bubble. In a paper in this week鈥檚 Nature, Didenko and Suslick conclude that these reactions would make it highly unlikely that the temperatures needed for fusion could ever be reached in a bubble. Worse, because Taleyarkhan鈥檚 acetone is more volatile than water, the dampening effect on it would be even greater.
Of course, there are still uncertainties to be cleared up. We cannot yet measure the temperature in a bubble directly, so there鈥檚 still room for hope. (Didenko and Suslick also suggest that fusion might be possible in less volatile liquids such as molten salts). It was clear from the start that sonofusion would have a steep climb to gain acceptance. That hill just got steeper.
There is one very positive outcome to this episode. Thirteen years after cold fusion first reared its head, both Nature and Science have broken their silence on table-top fusion. These are no longer dirty words. Let鈥檚 hope that this is the first step in the rehabilitation of innovative electrochemistry. If there is anything strange or useful about palladium electrodes in deuterated water, we should find out about it 鈥� not try to stop people finding out.