Compounds Are Conductive Or Not As Necessary

Researchers at power and automation technology company ABB, Baden, Switzerland, have developed special types of electrically-conductive compounds that could find use in a wide range of electrical applications. The compounds rapidly change from being conductive to non-conductive as they heat up, making them particulary (but not uniquely) suitable for anti-surge devices across a wide range of voltages and currents.

The compounds, whose development is virtually complete, were originally intended for applications within ABB. But due to a change of focus within the group, it is now seeking external licensees for the technology. The company’s r&d intellectual property office is working through a Web-based technology transfer company, yet2.com.

Project leader Ralf Strümpler says the compounds could interest companies that are injection molding low-power resettable fuses, as they would enable them to move into higher-power applications (up to around 600 V and several amperes). Resettable fuses are becoming a standard requirement in industrial and commercial environments.

The compounds can be formulated for fuses that can withstand higher currents and voltages than current devices and possess lower resistivity, as well. The fuses can handle at least seven or eight current surges at high voltage before they need to be replaced. Used for motor protection, the combination of a resettable fuse and a switch is expected to be between 50 and 65% less expensive than a circuit breaker, says Strümpler. ABB also has received interest in the compounds from a company involved in electrodes for fuel cells.

ABB has produced compounds based on several thermoplastics and thermosets. Strümpler says thermoplastics need to be semicrystalline in order to provide the necessary expansion characteristics. He says polyethylene is a good example, but the company has worked with high-temperature materials such as polyphenylene sulfide and fluoropolymers, as well as epoxies.

The compounds contain commercially available conductive fillers, typically a doped ceramic powder at levels of 25 to 45%. This gives them a resistivity at ambient temperature of 10 to 30 mOhm-cm. But if there is a surge in the circuit, the polymer matrix expands, increasing the distance between the conductive particles and rapidly decreasing the conductivity of the component. The current is partially or wholly interrupted. The part then cools down and shrinks, and the circuit is re-established.

The process can be constantly repeated without immediate loss of performance. The transition from conductivity to non-conductivity takes a few milliseconds, with the resistivity of the material increasing by up to eight orders of magnitude.

For high-voltage applications, compounds containing two fillers have been developed. One filler is conductive while the other is a varistor (its resistance decreases with voltage). If the voltage across the component increases too strongly during a surge, the particles of the varistor material form percolating paths, which dissipate the voltage above the specified limit.

ABB also has patents covering compounds that incorporate core shell-type fillers. This eases compounding of the material. And, by varying the shell and core combinations, a wide range of electrical characteristics and conductivity/temperature relationships can be obtained.

Yet2.com says that what makes this technology attractive is its ability to provide inexpensive, yet versatile and sophisticated surge protection for high- and low-power applications. It works equally well in highly-sensitive equipment, such as computers and test and measurement instrumentation, as well as heavy-duty industrial controls and facility-management systems. Other applications include temperature regulation and sensing.

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