Energy-saving silicon carbide could give Japan a technical edge
Post Date: 15 May 2013 Viewed: 385
They’re tough, durable and have been described as a possible “magic wand” that saves power and fights global warming. But next-generation silicon carbide (SiC) semiconductor chips are hampered by high production costs, and they are expected to be initially used only in inconspicuous areas.
Tsunenobu Kimoto, a professor of semiconductor engineering at Kyoto University, told an international symposium in Belgium in June 2012 that his team had produced an SiC semiconductor device that withstood a world record of 21,700 volts. Kimoto said he studied localized electric charges that made device interfaces vulnerable to breakdown.
To block the localized interface charge, he came up with a disk-shaped structure consisting of multiple rings with openings in between to produce the ultra-high voltage device.
One of its potential uses is in "pole transformers," the heavy, bucket-like structures on utility poles. They lower the high voltages along transmission lines to 100 volts for household use.
Copper coils currently used in the appliances cannot withstand high voltages, so they reduce the voltages in three to four steps. During the process, about 30 percent of the power that turns into heat is wasted.
An ultra-high voltage SiC device could do that in a single step--with only a 1-2 percent loss. SiC devices also do not require separate cooling systems, which could help reduce the size and weight of the apparatus.
"SiC can make a tough semiconductor that combines the characteristics of diamond, the toughest substance in the world, and of silicon, the central player in all semiconductor chips," Kimoto said.
Carbon, a constituent element in SiC, has a small atom size, and its strong interatomic ties are behind the toughness of diamond. SiC, a compound of silicon and carbon, is hard enough to be used as an abrasive agent and is also resistant to high temperatures.
Pioneering studies by Kyoto University professor emeritus Hiroyuki Matsunami paved the way for SiC's use in semiconductors. Researchers hope the next-generation material will replace silicon in the field of devices called "power semiconductors."
"If memory chips and large-scale integrated circuits are the 'brains' in charge of memory and thinking, then power semiconductors are tantamount to the 'muscles,'" said Tatsuo Oomori, senior manager of the SiC Device Business Development Center at Mitsubishi Electric Corp.'s Power Device Works in Fukuoka.
Power semiconductors play a role in power conversion, such as changing direct current to alternating current, modifying frequencies and altering voltages.
Major power conversion processes lead to a 10-percent power loss. These processes include reducing high transmission voltages for household use, and altering frequencies in the mutual supply of electricity between a utility in the 60-hertz zone in western Japan and a utility in the 50-hertz zone in eastern Japan.
That loss is estimated to total about 80,000 gigawatt-hours annually across Japan, equivalent to the annual output of 10 1-gigawatt nuclear reactors at nearly full capacity, sources said. In addition, small power losses in household electric appliances, which do not need to be adapted to high voltages, can add up to large figures.
The use of SiC power semiconductors could raise the conversion efficiency from the current 96 percent to 98 percent, when direct current powered by solar cells is turned into alternating current before use.
The idea has been put to practical use.
Motors in air conditioners, trains and electric vehicles contain power semiconductor appliances called "inverters." They make fine adjustments to motor rotation speeds and other quantities by regulating the short intervals at which the electric current is turned off and on.
Mitsubishi Electric's SiC inverters are used in train cars that Tokyo Metro Co. introduced to its Ginza subway line on a trial basis. Although the inverters by themselves have only limited energy-saving effects, they have produced unforeseen windfalls.
A mechanism called a regenerative brake allows power to be generated by the rotational force of train wheels when the brakes are applied. That power can be used to drive other trains.
Silicon inverters, which are vulnerable to large currents, allow that mechanism to be used only at low speeds. But SiC inverters, which can withstand both large currents and high temperatures, have made it usable at higher speeds, resulting in a 38.6-percent reduction in overall power consumption.
Unfortunately, SiC power semiconductor devices still cost many times more than their silicon counterparts. Oomori said he and his colleagues expect their use to spread dramatically if the price difference is narrowed to half.
They are expensive partly because they have yet to benefit from economies of scale. But the high cost is due largely to a difficulty in the manufacturing process. Silicon can be made into well-ordered crystals after being melted into liquid form.
Heated SiC, however, evaporates directly into gas form without going through a liquid phase. Depositing crystals from the gas form requires delicate temperature control under seal.
"But that technical difficulty constitutes expertise that cannot be imitated easily," said Tatsuo Fujimoto, a senior researcher at Nippon Steel & Sumitomo Metal Corp.'s Advanced Technology Research Laboratories in Chiba Prefecture.
In many product categories, such as memory chips and TV screen materials, Japanese manufacturers opened the door to high-quality products but ended up having their markets taken by late-starting industrial nations. The power semiconductor sector is considered a potential field where Japan can still maintain its competitive edge.
"To avoid repeating the same mistakes, it is essential to strategically map out which part of Japan's patents and technologies will be allowed to go public and which part should be held back," said Koji Asano, an official at the Research and Development Division of the industry ministry's Industrial Science and Technology Policy and Environment Bureau.