Superconduction At Nanowire Levels

Passing electricity through any conductor generates heat. Even the best conductor such as a copper wire offers some resistance to electrons passing through it. As electrons move through the ordinary conductor, they occasionally collide with its atoms and this releases energy as heat.

Cooling ordinary materials to very low temperatures changes the scenario drastically. Cold temperature dampens the Brownian motion of their atoms, allowing electrons to zip past with very few collisions. Therefore, very low voltage difference is required to pump electrons through ordinary materials when they are at cryogenic temperatures.

For example, Niobium Nitride, which is the base for several superconducting circuits, has a relatively high operating temperature of 16 degree Kelvin. This is equivalent to -257 degree Celsius and is achieved with liquid Helium. Within a superconducting chip, the liquid Helium circulates through a system of pipes in an insulating housing, much like Freon circulating inside a household refrigerator.

Although superconducting materials offer huge benefits, cooling to extremely low temperatures is very expensive and many researchers are working across the globe to make the process commercially viable. Researchers at MIT claim to have developed a circuit design that can help to make simple superconducting devices with extremely low electrical resistance much cheaper.

According to the researchers at MIT, chips made using the technology would make them 50-100 times more energy efficient compared to today’s chips. Although their working would not top the speed of current chips, recovering results of calculations that Josephson junctions perform would be made much simpler.

The current research at MIT has the cryotron as its basis. Cryotron or the Cryotron Computer, an experimental computing circuit, was developed by the MIT professor Dudely Buck in the 1950s. Although, the cryotron attracted a great deal of interest at the time as the possible future for a new generation of computers, the Integrated Circuit eclipsed it.

Current research at MIT in this field has resulted in the development of the nanocryotron. Researchers have tested superconducting circuits made with nanocryotron in light detectors and have been successful in registering the arrival of a single photon or light particle. They also wired several of these circuits together to produce the half-adder, a fundamental component of digital arithmetic. This square-centimeter chip has the nTron adder and performs computations using the new superconducting circuit.

A system using liquid-Helium for cooling is sure to increase the power consumption of a superconducting chip. However, given that this increase starts at about one percent of the energy required for a conventional chip, the overall savings can potentially be enormous. For example, making single-photon detectors would become very cost-effective – this being an essential component in any information system exploiting the computational speedup promised by quantum computing.

The nanocryotron or, as the researchers prefer to call it, the nTron, is an individual layer of Niobium Nitrate on an insulator. The device gets its name from its pattern that looks much like a capital ‘T’. The junction of the base and the crossbar tapers to a narrow region, forming a switch to control the current flow through the crossbar by injecting a current in the base.

Imagine an easy and non-polluting way of disposing of your old electronic gadgets that have outlived their usefulness. E-waste or waste from electronic products is a ticking time bomb that threatens to engulf us. For instance, about 85% of e-waste is discarded as landfills, releasing several toxins into the environment. Although only 2% of America’s trash in landfills is e-waste, it equals 70% of the overall toxic waste, with lead as the major element. Every year, worldwide, disposal of e-waste is nearing 50 million metric tons of which, only 12.5% is currently recycled.

To combat the menace of e-waste, SINTEF, a research organization in Scandinavia, is making progress in developing electronic components that can dissolve in water. The components are printed on silicon wafer and they contain extremely thin circuits, which are only a few nanometers thick. Being made of a combination of silicon, magnesium or silicon with magnesium additives, these circuits are water-soluble and disappear after a few hours.

Final working products are usually protected with a coating that prevents external fluids from reaching the inside of the packaging and degrading the circuit. Therefore, the requirement is that the circuit be designed to complete its job before that can happen. For example, a circuit package designed to operate in seawater and fitted with sensors to detect oil spills may have a film that remains in place for the few weeks when detection is due.

At present, SINTEF is not manufacturing final products, but only demo products that demonstrate how electronic components can have properties that make them degradable. As their project enters its second year, SINTEF is searching for an active industry partner and additional funding to carry their research further. However, they are confident eco-friendly electronics has a future of its own.

Apart from eco-friendly electronics, researchers are also working on electronic devices that are biodegradable. Such a device, when implanted in the body for different uses such as pain management or for combating infection, will dissolve over time after its objective is met. While several countries, especially America, has made colossal contributions towards resolving the issue of waste and building relations to medical applications, SINTEF s trying to find alternative approaches to this problem.

Other researchers are also working along similar lines. For example, the world’s only ‘biodegradable’ drone, built with biodegradable material, starts to break down upon impact in the event of a crash – eventually leaving no evidence of its existence. This drone was designed and built by a team of students from the Spelman college, Brown University and Stanford University, in collaboration with Ecovative Design for IGEM, a New York based biomaterials company.

Such an aerial vehicle, unmanned and made from biological materials, is ideal for venturing into sensitive areas, while leaving no trace of its existence in the event of a crash. Scientific expeditions with such drones will not contaminate the environment. It will be easy for covert military drones to conceal the fact that they have been spying.

In fact, the biological prototype drone may use a plant-root-like material such as mycelium. This is a part of a fungus, often used as a lightweight and sustainable material for packaging wine or for use in surfboard cores. Several other biological materials are being developed for making all parts of the drone biodegradable – including the sensors.

The introduction of Li-ion batteries and brushless DC or BLDC motors has opened up a new market for battery powered motor driven products. You will find brushless motors powered with rechargeable batteries being used in products such as uninterruptible power supplies, wheelchairs, e-bikes and other small electric vehicles and in small tools such as leaf blowers, chainsaws and drills. To take advantage of the integration of BLDC motors with Li-ion batteries for providing power requires updated MOSFET bridge drivers.

Although batteries such as lead-acid, Ni-MH and Ni-Cd are more popular, Li-ion batteries with their high energy density offer significant advantages over other battery technologies. Li-ion batteries typically offer two to three times the energy density as compared to what other conventional battery technologies currently offer. With higher energy density, users can make do with smaller battery packs that lead to lighter and more compact hand-held tools. Wheelchairs and e-bikes can operate for longer times without any increase in the physical size of their original battery pack.

However, there are some disadvantages associated with the high energy density of Li-ion batteries. It is customary to think of batteries as voltage sources, but for Li-ion batteries, this is not the case. Li-ion batteries have a significantly high internal inductance that generates considerable ripples on its voltage as a consequence of driving the motor with PWM or Pulse Width Modulation methods. Although this can be easily offset by adding sufficient capacitance across the MOSFET bridge, there can be enclosure limitations leading to prohibitive cost increases.

Low capacitance on the MOSFET bridge can lead to significant voltage ripples. For example, the ripple voltages found on a typical 18-20V Li-ion battery under heavy load can range from 5V at minimum to 36V at the maximum. Additionally, the battery voltage is likely to fall to an abysmally low value when the motor is overloaded to a stall or locked rotor condition. Therefore, presence of a controller is necessary to decide on how to react to such extreme operating conditions.

Compared to conventional brushed DC motors, BLDC motors offer significant advantages. For example, brushes limit the speed of a brushed DC motor, but the BLDC motor has no such limitations; the design of its rotor decides its maximum operating speed or RPM. Most applications do not require the full speed of the motor and a transmission with a gear reduction is used to bring down the motor speed to the desired RPM. A BLDC motor can rotate at significantly higher RPMs compared to the speed of a brushed motor. Therefore, the required torque at the output of the device can be achieved easily with a smaller BLDC motor and a corresponding transmission gear ratio.

As BLDC motors do not have brushes, they do not produce EMI as the brushed motors do. Additionally, the absence of brushes leads to lower maintenance and an increase in the efficiency of BLDC motors. On average, a BLDC motor is 1.5 times or more efficient than a brushed motor is. However, the drive electronics adds complexity to the application of a BLDC motor, requiring ICs to reduce component count, real estate and BOM costs, especially where space is a constraint.