In a new study recently published in Nature Nanotechnology, researchers from Columbia Engineering, Cornell, and Stanford have demonstrated heat transfer can be made 100 times stronger than has been predicted, simply by bringing two objects extremely close–at nanoscale distances–without touching. Led by Columbia Engineering’s Michal Lipson and Stanford Engineering’s Shanhui Fan, the team used custom-made ultra-high precision micro-mechanical displacement controllers to achieve heat transfer using light at the largest magnitude reported to date between two parallel objects.

“At separations as small as 40 nanometers, we achieved almost a 100-fold enhancement of heat transfer compared to classical predictions,” says Lipson, Eugene Higgins Professor of Electrical Engineering and professor of applied physics. “This is very exciting as it means that light could now become a dominant heat transfer channel between objects that usually exchange heat mostly through conduction or convection. And, while other teams have demonstrated heat transfer using light at the nanoscale before, we are the first to reach performances that could be used for energy applications, such as directly converting heat to electricity using photovoltaic cells.”

All objects in our environment exchange heat with their surroundings using light. This includes the light coming at us from the sun, the glowing red color of the heating element inside our toaster ovens, or the “night vision” cameras that enable image recording even in complete darkness. But heat exchange using light is usually very weak compared to what can be achieved by conduction (i.e., by simply putting two objects in contact with each other) or by convection (i.e., using hot air). Radiative heat transfer at nanoscale distances, while theorized, has been especially challenging to achieve because of the difficulty of maintaining large thermal gradients over nanometer-scale distances while avoiding other heat transfer mechanisms like conduction.

Lipson’s team was able to bring objects at different temperatures very close to each other–at distances smaller than 100 nanometers, or 1/1000th of the diameter of a strand of human hair. They were able to demonstrate near-field radiative heat transfer between parallel SiC (silicon carbide) nanobeams in the deep sub-wavelength regime. They used a high-precision micro-electromechanical system (MEMS) to control the distance between the beams and exploited the mechanical stability of nanobeams under high tensile stress to minimize thermal buckling effects, thus keeping control of the nanometer-scale separation even at large thermal gradients.

Using this approach, the team was able to bring two parallel objects at different temperatures to distances as small as 42 nm without touching. In this case they observed that the heat transfer between the objects was close to 100 times stronger that what is predicted by conventional thermal radiation laws (i.e. “blackbody radiation”). They were able to repeat this experiment for temperature differences as high as 260oC (500oF) between the two objects. Such high temperature difference is especially important for energy conversion applications since, in these cases, the conversion efficiency is always proportional to the thermal difference between the hot and the cold objects involved.

“An important implication of our work is that thermal radiation can now be used as a dominant heat transfer mechanism between objects at different temperatures,” explains Raphael St-Gelais, the study’s lead author and postdoctoral fellow working with Lipson at Columbia Engineering. “This means that we can control heat flow with a lot of the same techniques we have for manipulating light. This is a big deal since there are a lot of interesting things we can do with light, such as converting it to electricity using photovoltaic cells.”

St-Gelais and Linxiao Zhu, who co-authored the study and is a PhD candidate in Fan’s group at Stanford, note that the team’s approach can be scaled up to a larger effective area by simply arraying several nanobeams–on top of a photovoltaic cell, for example–and by individually controlling their out-of-plane displacement using MEMS actuators. The researchers are now looking at applying their same approach for ultra-high-precision displacement control, this time with an actual photovoltaic cell to generate electricity directly from heat.

“This very strong, non-contact, heat transfer channel could be used for controlling the temperature of delicate nano devices that cannot be touched, or for very efficiently converting heat to electricity by radiating large amounts of heat from a hot object to a photovoltaic cell in its extreme proximity,” Lipson adds. “And if we can shine a large amount of heat in the form of light from a hot object to a photovoltaic cell, we could potentially create compact modules for direct conversion of heat to electrical power. These modules could be used inside cars, for instance, to convert wasted heat from the combustion engine back to useful electrical power. We could also use them in our homes to generate electricity from alternative energy sources such as biofuels and stored solar energy.”

Scientists at Nanyang Technological University, Singapore (NTU Singapore) have developed a chip that allows new radar cameras to be made a hundred times smaller than current ones.

With this NTU technology, radar cameras that usually weigh between 50 kg and 200 kg and are commonly used in large satellites can be made to become as small as palm-sized.

Despite being small, they can produce images that are of the same high quality if not better compared to conventional radar cameras. They are also 20 times cheaper to produce and consume at least 75 per cent less power.

Developed over the past three years at NTU, the promising technology has already secured S$2.5 million in research funding from Singapore government agencies.

The radar chip has attracted the attention of several multinational corporations, and is now being researched for use in Unmanned Aerial Vehicles (UAVs) and satellite applications.

Assistant Professor Zheng Yuanjin from NTU’s School of Electrical and Electronic Engineering who led the research, said that the size and effectiveness of the chip will open up new applications not possible previously.

“We have significantly shrunk the conventional radar camera into a system that is extremely compact and affordable, yet provides better accuracy. This will enable high resolution imaging radar technology to be used in objects and applications never before possible, like small drones, driverless cars and small satellite systems,” said Asst Prof Zheng.

Advantages over current technology

Current radar camera systems are usually between half and two metres in length and weigh up to 200 kg. They cost more than US$1 million on the market and can consume over 1000 watts in electricity per hour, the energy equivalent of a household air-conditioning unit running for an hour.

Known as Synthetic Aperture Radar (SAR), these large radar cameras are often carried by large satellites and aircrafts that produce detailed images of the Earth’s surface. Objects longer than a metre, such as cars and boats, can be easily seen by the radar camera mounted on an aircraft flying at a height of 11 kilometres.

Unlike optical cameras which cannot work well at night due to insufficient light or in cloudy conditions, a radar camera uses microwaves (X-band or Ku-band) for its imaging, so it can operate well in all weather conditions and can even penetrate through foliage.

These detailed images from radar cameras can be used for environmental monitoring of disasters like forest fires, volcano eruptions and earthquakes as well as to monitor cities for traffic congestions and urban density.

But the huge size, prohibitive cost and energy consumption are deterrents for use in smaller unmanned aerial vehicles and autonomous vehicles. In comparison, NTU’s new radar chip (2mm x 3mm) when packaged into a module measures only 3cm x 4cm x 5cm, weighing less than 100 grams.

Production costs can go as low as US$10,000 per unit, while power consumption ranges from 1 to 200 watts depending on its application, similar to power-efficient LED TVs or a ceiling fan.

It can also capture objects as small as half a metre which is twice as detailed as the conventional radar camera used in large aircrafts or satellites.

Potential applications of the new radar chip

Asst Prof Zheng said that when mounted on UAVs, it can take high quality images on demand to monitor traffic conditions or even the coastlines for trespassers.

“Driverless cars will also be able to better scan the environment around them to avoid collisions and navigate more accurately in all weather conditions compared to current laser and optical technologies,” he added.

“Finally, with the space industry moving towards small satellite systems, such as the six satellites launched by NTU, smaller satellites can now also have the same advanced imaging capabilities previously seen only in the large satellites.”

Large satellites can weigh up to 1,000 kg, but microsatellites weigh only 100 to 200 kg.

Recognized internationally with strong market interest

NTU’s new radar chip was presented and published at the prestigious International Solid-State Circuits Conference (ISSCC) 2016. Commonly referred to as the “Olympics of Integrated Circuits Design,” ISSCC is the world’s top forum for presenting advances in solid-state circuits and systems and is attended by major industry players.

The chip was developed by Asst Prof Zheng’s team of five at NTU’s VIRTUS IC Design Centre of Excellence. The group was the first from Singapore to publish in ISSCC and is also the most published local group, with seven papers to date.

NTU’s new technology has attracted the attention of many multinational corporations, such as US aerospace company Space X; Netherlands semiconductor company NXP; Japanese electronics giant Panasonic, and French satellite maker Thales.

The next phase will be research in space applications to be carried out at the Smart Small Satellite Systems – Thales in NTU (S4TIN), a joint laboratory between NTU and Europe’s largest satellite manufacturer Thales Alenia Space.

Game changer for Singapore

Associate Professor Low Kay Soon, Director of NTU’s Satellite Research Centre, said the new radar chip will be a game changer in the space industry, which will bolster Singapore’s growing reputation as a satellite building nation.

“Monitoring the environment with a clear image using a traditional optical camera is always very challenging due to clouds and changing light conditions,” said Assoc Prof Low.

“This is especially the case for the tropics where the sky is always cloudy. With a miniature radar-on-chip system, it cuts down the required weight and size of the payload that a satellite needs to carry.

“More significantly, the lower power consumption makes it very suitable for microsatellites such as the X-SAT or VELOX-CI which NTU has launched. For small satellites, there is a limited area to mount the solar panels, which limits its power generation. Consequently the conventional SAR systems cannot be used due to its high power requirements.”

Asst Prof Zheng says it will take another three to six years before NTU’s new radar chip is ready for commercial use. He is now working with NTU’s innovation and enterprise company, NTUitive to find industry partners to license the technology or to spin off a company.

Director of VIRTUS, NTU Professor Joseph Chang added: “Singapore is one the very few select countries in the world with advanced technical capabilities to design complex microchips for space applications.”

“NTU professors associated with VIRTUS have received research funding of over S$5 million from Singapore and various countries like the United States, to design microchips for space applications. Recently, two patents have been filed for the novel design of these microchips.”

VIRTUS filed ten patents in the last year alone, for various innovative microchips with applications ranging from image processing to computing.

According to Markets and Markets global forecasts and analysis, the global market for radar systems is estimated to grow to US$24 billion by 2020.