Amid concerns about a supply shortage of tantalum capacitors and the rising cost of tantalum powder, capacitor manufacturers are touting their latest niobium capacitors as direct replacements for tantalum and aluminum electrolytic devices. Companies such as AVX, Epcos, Kemet, NEC and Vishay are sampling parts now, and most expect to begin manufacturing niobium caps in volume next year for use in mobile communications, telecom and datacom, and consumer electronics.
Capacitor manufacturers have been evaluating niobium material off and on for years because of its potential for higher capacitance and the possibility that niobium capacitors could replace low-voltage aluminum electrolytics. "Companies go back to look at niobium every time the tantalum supply situation becomes unstable, either in terms of quantity or price, but it has only been in the last two to three years that niobium has started to look like a viable alternative to tantalum," said Pete Maden, vice president of strategic planning for capacitors at Vishay Intertechnology Inc. (Malvern, Pa.).
Vishay is offering solid niobium capacitors with capacitance ratings from 10 to 1,000 microfarads, voltage ratings of 6 and 10 V, a -55 degrees C to 85 degrees C operating-temperature range and a derating temperature of 125 degrees C. Its devices are available in several different packages.
AVX Corp. (Myrtle Beach, S.C.) is looking at niobium as a way to fill the technology gaps for both tantalum and aluminum capacitors, said marketing manager Mark Obuszewski. The strategy is to target capacitance ratings from 10 to 1,000 microfarads, with voltage ratings of 4, 6 and 10 V. AVX is sampling devices in the 100- to 470-microfarads range, with an operating temperature range to 105 degrees C.
Epcos AG (Evora, Portugal) is offering samples of 100-, 150- and 220-microfarads devices, all at 10 V, in standard and low-profile D cases, with temperatures to 105 degrees C. The company plans to expand the caps' operating-temperature range to 125 degrees C for automotive and industrial applications.
Kemet Corp. (Greenville, S.C.) is sampling a 150-microfarads, 6-V cap, while NEC Corp.'s Energy Devices Division (Tokyo) offers a 220-microfarads, 2.5-V conductive-polymer type niobium device with a range of -55 degrees C to 85 degrees C. NEC plans to expand the capacitance/voltage (CV) ratings up to 1,000 microfarads and 10 V when volume production begins in mid-2002. It also plans to crank its temperature range to 105 degrees C.
Niobium is both plentiful and vastly less expensive than tantalum. Vishay reports that the selling price for niobium oxide is just $5.50 to $7/pound, against roughly $40 to $80/pound for tantalum oxide. That translates into a better chance of stable supply and stable pricing over the long term.
Nevertheless, niobium poses some technical challenges that capacitor makers are struggling to overcome. Obstacles to a surge in niobium demand have included high dc-leakage current (two to five times worse than for tantalum capacitors) and susceptibility to damage by thermal shock during manufacturing. Low-CV capabilities have also been a problem because of the lack of capacitor-grade powders, but earlier this year materials supplier H.C. Starck (Newton, Mass.) produced niobium powder capable of 100,000 CV/gram, which is comparable to 50,000-CV/gram tantalum powder.
Niobium oxide has a dielectric constant roughly twice as high as that of tantalum oxide, said Werner Lohwasser, executive vice president and general manager of Epcos. The higher dielectric constant makes it possible to build smaller capacitors or to implement higher capacitance in the same package size.
According to Kemet Electronics, niobium pentoxide offers a dielectric constant of 41, compared with 26 for tantalum pentoxide, which means that half again as much CV can be obtained from the niobium material.
The density of niobium is only half that of tantalum, however, so twice as much material is needed per unit volume to provide the same charge.
"The higher dielectric constant of niobium looks good on the surface, but you have to form a thicker niobium oxide dielectric for the same voltage," said Vishay's Maden. "The dielectric thickness and the dielectric constant tend to cancel each other out."
Capacitor makers with an eye on the promise of niobium are changing their manufacturing processes and materials to overcome the challenges of a higher voltage constant as well as susceptibility to damage by thermal shock during manufacturing. For instance, Vishay's new patent-pending technique enables the manufacture of solid niobium capacitors with a conventional manganese dioxide cathode as a drop-in replacement for tantalum devices. The niobium capacitors have about the same dc leakage and equivalent series resistance as comparable tantalum devices.
Yoshitaka Hori, manager of the solution engineering department at NEC Energy Devices, noted that the effects of heat treatment during the manufacturing process remain a big issue for conventional niobium capacitors. Hori said NEC has brought the thermal characteristic to a satisfactory level by improving the oxidization process. The company mitigates the negative effects caused by heat treatment by using polymer for the cathode instead of manganese dioxide, which reduces the heat-treatment process cycle, he said.
Already, suppliers report that they are pricing their niobium capacitors below what their tantalum counterparts sell for. NEC Energy Devices believes it can manufacture niobium devices at half the cost of tantalum capacitors if the capacitor-grade niobium powder is produced in large quantities.
While a plentiful supply of niobium ore exists, Starck is one of the few suppliers-if not the only one-of niobium powder. Not surprisingly, H.C. Starck's niobium product manager, Bill Serjak, sees no problem with that.
"Our major objective is to gear up capacity to meet what we expect to be demand," he said. "We're working very hard to make sure there is plenty of niobium to meet the demand. Our objective is to make niobium into a stable, good material." The company is building a pilot plant in Germany that will meet demand until early 2003, Serjak said.
Starck is also developing higher-capacitance powders to target the aluminum capacitor market. Suppliers that have tested the niobium powder are getting capacitance ranges equivalent to tantalum powder in the 80,000- to 100,000-CV/gram range, according to Serjak.
Since the development of niobium capacitors is still in its infancy, companies are likely to expand product offerings significantly over the next year, with those products reflecting wider capacitance ratings, higher voltages, extended operating temperature ranges and better leakage current.
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